STDMA analyse report:

Aviation and maritime administrations are looking for alternative ways to replace or complement current radar systems for surveillance purposes. The use of Global Navigation Satellite Systems (GNSS) together with an efficient, digital, radio link, appears to be a feasible method. This thesis looks at systems which combines a digital radio, a computer and a GNSS receiver, together with an algorithm for organization of a digital radio link.

The described system, combines the line-of-sight characteristics of VHF frequencies, very accurate timing from the GNSS receiver (or some other timing) and a self-organizing transmission algorithm, to create an efficient data link. The data link can be shared among a large number of stations. The algorithm is called Self-Organized Time-Division Multiple Access - STDMA. The purpose of the algorithm is to allow short transmissions (called bursts), from several stations, to be organized in time, so that transmission conflicts are minimized. The algorithm is self-organizing, which means that controlling, or polling functions, via master control, are not needed.

The thesis studies an environment, with geographically distributed and moving stations (such as airplanes or ships), all equipped with STDMA stations. Each station operates in its own VHF cell limited by line-of-sight. The STDMA algorithm can only provide partial organization of the communication link since the VHF cells only partially overlap. Some of the communication will create conflicts, which may reduce the overall throughput. A model, describing the dynamics of this system, is derived. This model is used in order to analyze capacity and throughput.

The STDMA system works very well in this dynamic environment, ensuring safe delivery of safety critical information to surrounding stations. It also handles overload very well, with gracefully reduced VHF cell size as a result.

The features, which make the system feasible for surveillance applications, are:

The support, from the following individuals, has been a great asset during the research work:

In the age of information it becomes important to efficiently communicate information. A wide variety of ways exist. One can use cable, fiber, satellites or why not regular mail? Information must generally travel over great distances. Satellites provide an efficient radio link for that. The radio medium becomes more and more important for digital communication. Take cellular mobile phones, for example. But, the radio medium is a very limited resource and therefore frequencies must be reused over and over around the globe. Depending on which frequency the communication uses, different characteristics apply to the way the information travels in the air. Some frequencies limit the information to travel by line-of-sight, thus enabling reuse quite often. Other frequencies allow it to travel around the globe. The application decides which characteristic is the most useful.

This thesis deals with digital radio communication in a line-of-sight environment. Information is communicated between geographically distributed stations, which generally also are moving at great speeds. Basic knowledge of frequency sharing using time division, multiple access, is an aid when reading the material.

Aviation and maritime administrations are currently investigating new technologies for use in modern surveillance systems.

Traditionally radar (RAdio Detection And Range) has been the cornerstone in surveillance systems. Two types of radar exists: primary and secondary. Primary radar sends out a high-powered radio signal, which may bounce back, after hitting a target. The time, between transmission and receipt of the return signal, indicates the distance to the target. Secondary radar requires equipment to be installed onboard participating vessels. This equipment will send a digital signal back to the transmitter when it detects the radar signal (generally a secondary signal, from a transmitter piggybacked on the primary radar antenna). The digital signal can contain an identifier and altitude, for example. Radar gives very little and often inaccurate information. The update rate is generally slow, since it is equal to the rotation rate of the radar dome. Partly due to the shortcomings of radar, aircraft are required to fly with great spacing. This introduces traffic delays, which is quite costly for commercial airlines.

A surveillance system, based on other means than radar, can introduce new benefits. Assume that each vessel can continuously determine its own position. A position sensor, such as a GNSS receiver can do just that. This position is then broadcast, in an orderly fashion, on a globally shared digital radio link. In this context, a digital radio link is one, or more, frequency channels, shared among several users. The sharing may be done, by dividing time into time slots, each slot being an opportunity to access the channel and transmit data. This type of system is independent of ground infrastructure (i e radar stations) and will work equally well all over the globe. It is a new application of digital radio communication. The demands on such an application are yet to be fully explored. The number of participating stations is unknown and constantly changing. In addition, the stations are constantly moving. The digital radio link must be able to handle overload situations without collapsing. Safe communication is important, since it shall be used in surveillance applications.

All of these environmental requirements must be considered prior to implementing an operational system. There have been made some attempts to specify the requirements on such systems, from a user point of view [RTCA 186].

This thesis explores the environmental requirements based on the current technological developments within the maritime and aviation fields. Terms, such as capacity and throughput, are defined and quantified with respect to the use of Self-Organized Time Division Multiple Access (STDMA) in the surveillance application.

The results can be an aid in the continued work of developing standards and legislation for broadcast surveillance systems.

The thesis is structured in a main section and appendices. The main section presents some basic concepts, results, conclusions and discussion. The rational behind the results and conclusions can generally be found in the appendices.

The purpose of this thesis is to…

The problem definition is:

Taking into account the application environment, transmission characteristics and the ability to organize transmissions using STDMA, what capacity and throughput can be expected on the STDMA data link in surveillance applications?

This question requires a definition of the terms capacity and throughput.

The problem domain deals with:

..the sharing of a digital VHF Data Link (VDL) for the purpose of surveillance applications.

When analyzing a VDL, a number of features can be taken into account.

Propagation of radio waves, packet protocol, modulation techniques, just to mention a few. It becomes necessary to narrow the problem domain. Emphasis is on the STDMA algorithm and the effects it has on capacity and throughput.

The method is described, using terms and definitions from [Holme et al].

This work is based on practical experiences from the application environment (maritime and aviation) and anticipated future demands on surveillance systems.

Several different VDL systems compete to be the future core of surveillance systems. Due to this, it can be assumed that available documentation is only partly objective in its character. Commercial and national interests appear to play a large role and thus influence the written material. A humanistic view is applied, in order to deal with this.

A quantitative scientific method is used, though the results often have to be post-analyzed. Some source material must, however, be viewed using a qualitative scientific method.

Case studies, desktop analysis and simulations are used in the research work. A case study is merely a way to test derived equations and models in a scenario modeled after real life. The desktop analysis is based on radio communication theory and experiences from tests on existing equipment. Simulations are performed in order to study a specific behavior or feature. The simulations are thus not performed on a complete system model. All simulations are done using the C/C++ programming language.

Since this field is little explored, primary data is predominately used. Primary data is derived by analysis and simulation. Secondary data is used in those cases when it can be verified to be applicable to the problem under study.

Primary data dominates, and it is thus possible to state that the source is known. Personal experience from the application environment, as well as the technical environment should aid in establishing reliability. But, due to the fact that I am working at a company involved in STDMA technology, it is important, that all results are scrutinized, by others too. Results are therefore verified with two persons, both experts in radio communication. These persons are independent in the context that they have no relation to each other. Mr Lars Zetterberg is a professor at the department of signals, sensors and systems, at the Royal Institute of Technology. Mr Lennart Dreier is technical director at GP&C Sweden AB, a company working with STDMA technology (and the company asking for this analysis).

Validity is determined when the results are checked to fulfill purpose and problem definition.

The work method is described in figure 3.6.a below:

The Swedish Civil Aviation Authorities (SCAA) has conducted tests, on a limited scale, as a proof of concept for the STDMA system. The SCAA has also performed simulations on a European traffic scenario [Frisk et al] [PMEI].

It was discovered, however, that no one had made an analysis, simulation or test, which modeled the STDMA system in high density, traffic situations. Since the system is a candidate for future surveillance applications, this scenario is the foundation for the analysis work. The idea is to describe the behavior of the STDMA system in the surveillance application. The primary work was to be done as a desktop analysis, supported by simulations of isolated features, requiring more thorough investigation.

The idea is transferred into a purpose and problem definition. The problem domain is defined and narrowed. These initial results are then communicated to the university, for approval.

The research started with a literature study. Books and articles about communication theory were studied, in order to find commonalties with the STDMA system. Documentation on surveillance systems, in general, was examined, for example [S. R. Jones]. STDMA related documentation, such as analyses, simulation reports and descriptive texts, were also part of the literature study. With own personal experiences and new basic knowledge about the problem domain, the analysis was divided into areas of interest. Each area was then examined separately and presented in an appendix. A desktop analysis was conducted. Output from the analysis was verified by simulation in C/C++ unless, the output could be verified by reasoning, or comparison with documentation.

The documentation is produced by first documenting each interest area in an appendix. Based on purpose and problem definition, the chapter on results, is derived. The rest of the thesis was then assembled progressively.

This chapter, gives a brief description of surveillance systems, defines commonly used terms, and describes some different technologies which are used in surveillance systems.

Traditionally, surveillance systems have relied heavily on ground infrastructure. This is the case with radar systems for aviation ( i e Air Traffic Control - ATC) and marine Vessel Traffic Service centers (VTS). A surveillance system can encompass radar, voice communication, automated information services and navigational aids.

The main purpose for having surveillance systems, is enhanced safety for personnel and materiel, but environmental issues are also important. Transportation of humans and goods have an impact on the environment, which must be minimized. A well functioning surveillance system will help to avoid traffic conflicts and possible disasters. It may also allow vessels to travel a more direct route and thus using less fuel. Unfortunately, due to the shortcomings of current systems, the opposite often occurs. The vessels must follow assigned detours, since the surveillance system lacks coverage in certain regions.

New technology is readily available for implementation as a replacement or complement to radar. Global Navigational Satellite Systems (GNSS), such as the GPS (controlled by the USA) or GLONASS (controlled by Russia) allow a vessel to establish its own position anywhere on the globe. A position sensor, such as a GNSS receiver, is needed. A vessel can be equipped with a system which continuously receives its own position, from the position sensor, and then repeatedly broadcasts it on a VHF data link. This concept works globally, regardless of ground infrastructure.

In the aviation environment this is called Automatic Dependent Surveillance Broadcast, or ADS-B. In the maritime environment, this is called Automatic Identification System, or AIS. These systems are called Broadcast Systems in this document.

A broadcast system supplies communication, navigation and surveillance (commonly called the CNS concept) to the user.

Both the aviation and maritime community has acknowledged the need for new technologies in order to meet the traffic demand of the next century. Great effort is spent to derive standards which define the behavior and use of broadcast equipment.

In the International Civil Aviation Organization (ICAO), the Aeronautical Mobile Communication Panel (AMCP) is working on a draft Standards and

Recommended Practices (SARPS) called VDL Mode 4 [ICAO AMCP].

The International Maritime Organization (IMO) is the maritime equivalent of ICAO. IMO has specified draft performance standards [IMO Nav 43] for a Shipborne Universal AIS.

In both of these processes, the GP&C system (see appendix A) has had great influence. Contributions, well worth mentioning, are Time Division Multiple Access (TDMA), for transmission synchronization, and the self-organizing algorithm (STDMA) for sharing and organization of the VHF data link.

For the purpose of data link communication, the term organization has a special definition:

The process, in which stations select their transmission schedule, adhering to other stations, while avoiding and resolving transmission conflicts.

This definition implies that the data link is shared among several independent stations.

The data link can be categorized based on its mode of operation, which is defined as:

the method used in order to create a transmission schedule for repetitive transmissions.

The figure below illustrates the different modes of operation:

Figure 4.3.a

The mode of operation applies to transmission of repeatable character, such as position information.

It means that, while one mode is used for continuously transmitting position reports, another mode can be used for transmission of other information.

In the autonomous mode of operation, each individual station is responsible for organization, and thus determines its own transmission schedule. Two techniques can be distinguished in autonomous mode: self-organized and random mode.

A station, which determines its own transmission schedule, based on some intelligent algorithm, is operating in a self-organized mode. This algorithm schedules transmissions, based on prior knowledge, of future activities, on the data link. It will organize the transmissions in order to avoid transmission conflicts, and to quickly resolve conflicts when they do occur. The primary mode of operation, in the STDMA system, is self-organized mode but, it can also be put into an assigned mode (see section 4.3.2.2).

A station, which determines its own transmission schedule on some algorithm, disregarding current and future data link traffic, is operating in random mode. In this mode, the station will schedule its transmissions purely based on randomization around some given criteria. It will transmit even though other stations might be doing the same simultaneously. Mode S Squitter [V. Orlando et al], Pure Aloha and Slotted Aloha [G. Maral et al] are examples of systems which operates in random mode. Conflict resolution is based on random behavior which statistically ensures that, at least some transmissions, will be successfully received.

In the controlled mode, a master is responsible for scheduling transmissions. Some systems may use a quasi-controlled mode, in which a station assumes control, based on some criteria, in lack of a master. Even though advocates of such systems may argue that they operate in autonomous mode, they are in fact operating in controlled mode since scheduling is controlled by one station; the current master. One example of a quasi-control system, is the AIS CS/TDMA system [MCC].

The polled mode is a controlled mode, where the master interrogates other stations. One interrogation results in one, or more responses. If a station is not interrogated, it remains silent. The radar system operates in polled mode since the radar interrogates and receives a response in the form of an echo and a secondary radar response.

The assigned mode is a controlled mode, where the master assigns a transmission schedule to other participating stations. This schedule is generally valid for a limited time, after which the master must re-assign the schedule. The assigned mode is more efficient than polled mode since one transmission, from the master, results in a transmission schedule, which will operate for quite a while. The STDMA system can operate in assigned mode.

The GSM system, for cellular mobile phones, is an example of a system, which operates in assigned mode.

The STDMA system shares the data link by dividing time into slots (Time Division Multiple Access - TDMA). A position report fits into one slot. A set of slots is grouped in a frame. A frame is one minute long. The term synchronization is thus defined as:

The process, in which a station aligns itself in time, with other stations, in order to enable Time Division Multiple Access to the data link.

A very accurate source, for synchronization, is the 1PPS pulse from a GNSS receiver (see Appendix A). Other sources can also be used. Stations can synchronize their clocks to transmissions from semaphore stations. The semaphore station is responsible for providing timing simply by transmitting some synchronization message at specific intervals.

The semaphore scheme uses elaborate algorithms in order to select semaphore and to maintain synchronization [IALA].

Capacity is defined as:

Measurement of how many stations the system can handle, and how well it handles overload.

In a TDMA system, it is quite easy to calculate how many stations that can be simultaneously handled. Assume that each station has a given report rate (RR), in transmissions per minute. One minute is equivalent to a set of slots (SL). The number of possible stations is then SL/RR. This, however, is only partly true. The reality is more complex.

A surveillance system can not have restrictions which specifies a maximum number of stations. The system must be able to handle overload (i e more slots required than currently available), and to adapt to the traffic, in a controlled and safe manner.

A TDMA system divides the communication channel in time, by first specifying a frame. The STDMA frame is one minute long. The start of a frame can be different between stations, but all stations strive to coordinate the frame start to Coordinated Universal Time (UTC).

The frame is divided into time slots. The start of each slot is an opportunity for a station to transmit. The slot start shall be the same for all participating stations. The current time slot, is the slot in which a station currently is receiving or transmitting. All stations are thus always at the same current slot, though they may have numbered it differently because the frame start differs.

Each station will transmit its position at some given time interval. The report rate is specified as the number of transmitted reports per minute. The report rate may differ between stations. Depending on factors such as speed or change of course.

There are two types of report rates:

Nominal report rate, which is defined as the report rate currently used by a station (or transmission attempts in a given time interval).

Net report rate, which is the experienced report rate (or successfully received transmissions) at a receiving station. Due to slot reuse, and other disturbances, a station may receive less reports than actually transmitted.

The net report rate may vary even though the nominal report rate remains fixed.

Net throughput is defined as:

The relation between a nominal report rate, output by a transmitter, and the experienced report rate, from that same transmitter, at a receiver.

The net throughput can be derived, by dividing the number of successfully received transmissions, with the number of transmission attempts. This assumes that one transmission occupies exactly one slot.

The Self-Organized Time Division Multiple Access (STDMA) algorithm enables a self-organized autonomous mode of operation. A brief description follows below, based on information from [IALA].

Each station will continuously broadcast its position using a transmission packet, which occupies one time slot. The time slot is selected using STDMA. In addition to the position, the transmitted packet will also contain a Comm State.

The Comm State contains information, which states the intention of the STDMA algorithm:

When a station first powers up, it will begin an initialization phase, which takes one minute. During this minute, the station will monitor the data link to determine channel activity, other, participating, stations and current slot assignments. After the first minute, the station enters the Network Entry Phase.

During the network entry phase the station selects its first transmission slot and prepares to make itself visible on the data link.

It first determines its Nominal Increment (NI). The NI is equal to the number of slots per minute divided by the desired report rate.

To select the first transmission slot, the station selects a Nominal Start Slot (NSS). The NSS is randomly selected from the current slot and NI slots ahead in time.

After this, a Selection Interval (SI) is determined. The SI is always 20% of NI and placed so that the NSS is in the middle.

Figure 4.7.3.a

A slot is randomly selected within the SI. This slot is checked to see if it is available for use. If not, the adjacent slot to the left and right is checked. This process is continued until an available slot is found within SI. There shall always be a set of available slots. If the link is approaching maximum capacity, slots are reused from stations at the greatest distance, from the own position.

The selected slot becomes the Nominal Transmission Slot (NTS).

Upon reaching the NTS, the First Frame Phase is entered.

During the first frame phase, the station continuously allocates its nominal transmission slots (NTS) and transmits its position. Each NTS is allocated a unique slot time-out between 4 and 8 minutes.

Upon reaching the first NTS, a new Nominal Slot (NS) and NTS is selected for the following transmission. The NS is selected by adding NI to the NTS (or the latest NS, as applicable).

Figure 4.7.4.a

A new SI is placed around the NS and the NTS is allocated as described previously.

This process is continued until one minute has elapsed and the initial NSS is approached again. At this stage the station enters the Continuous Operation Phase.

In the Continuous Operation Phase, the station transmits in the allocated NTS’s and decrements the slot time-out. When the time-out reaches zero, a new NTS is selected within the SI as described above. The relative offset (slot offset) is inserted into the position report packet and transmitted so that receiving stations are made aware of the intentions.

This phase is maintained until the system is shut down, enters assigned mode (see section 4.3.2.2) or changes reporting rate.

The results, presented in this chapter, are described in details in chapter 8, Appendices.

This section is based on the analyses described in Appendix B.

This analysis examines the factors, which affect the range of radio waves in space (for details, see 8.2.2), omitting such factors as terrain and atmospheric effects. This is a desktop analysis based on available theory on the subject.

Given the characteristics of the GP&C transponder system (see 8.2.2.3), the following equation is derived:

Table 5.1.1.a

The following values are inserted into the equation:

Table 5.1.1.b

The resulting range (R) then becomes 1125 km (608 nm). With a gain of -15 dB, the range becomes 113 km (61 nm) or 1/10 of the range at -5 dB.

The gain greatly affects the result. It shall be noted that only gain in a horizontal direction is considered. The gain in vertical direction is generally much less, but considering the surveillance application, that is of little importance. Stations are generally spread in the horizontal plane to such an extent, that the vertical plane can be neglected. This is especially true in maritime environments, where the vertical separation is virtually nil.

This analysis examines how the radio waves propagates through the atmosphere, considering frequency characteristics and terrain (for details, see 8.2.3).

Radio waves, in the Very High Frequency (VHF) domain (30 to 300 MHz), primarily travel as space waves (i e the most prominent path is a direct line between transmitter and receiver).

The radio waves thus have line-of-sight reach. To calculate the farthest distance between two stations, the following equation is derived:

In the aviation environment, the coverage is studied depending on a sub-environment (see section 8.2.3.5.5) as listed below:

Table 5.1.2.1.a

Each sub-environment has a minimum and maximum altitude. It can be assumed, that the height of an aircraft, is equal to the height of the antenna.

A base station, positioned at 20 meters MSL, will have the following range:

Table 5.1.2.1.b

Mobile stations (i e aircraft) flying at equal altitudes, will have the following range:

In the maritime environment, altitude can only be affected by the height of the antenna. For these reasons, it is assumed that a base station antenna is positioned at 150 meters above mean sea level (MSL). The antenna, of a ship, is positioned at 30 meters MSL. Based on this:

This analysis examines how well transmissions are received in situations where conflicts occur (i e several stations use the same time slot). For details, see section 8.2.4. The analysis is based on laboratory and practical tests described in appendix B.

The analysis show that the power ratio (i e the difference in power between two interfering stations) affect the capability of a receiver to successfully receive one of the transmissions.

If it is assumed that, all participating stations, radiate the same transmit power, The power ratio can be converted into a distance ratio. The following equations are used for this:

PR = 20 log( DR ) [Eq B.11]

Table 5.1.3.a

The radio signal is modulated, using Frequency Modulated, Gaussian Minimum Shift Keying (FM/GMSK). This modulation technique results in a worst case power ratio of 5 dB (see 8.2.4.5.1). This equates to a distance ratio of 1.8.

For the purpose of further analysis, the power ratio will be assumed to be 6 dB, and thus the distance ratio is 2.0.

This can be illustrated with the figure below:

Figure 5.1.3.a

The term garble means, that neither station is successfully received by Rx. When one station is successfully received, that station is said to have discriminated the other.

This section is based on the analyses described in Appendix C and is a desktop analysis, supported by simulation in C/C++, of specific behavior. See the appendix, for details.

Radio waves propagates through the air, roughly at the speed of light (300 meters per microsecond). This is accounted for in the time slots, by a propagation buffer. This buffer ensures integrity, for the transmitted packet (called guard distance), for up to 370 km (200 nm). This is approximately 1/3 of the theoretical range in free space (see section 5.1.1). The STDMA system allows a transmission to overlap slot boundaries, but it could be a cause of transmission conflicts with stations, transmitting in adjacent slots.

When conflicts do occur due to propagation overlap, they will always result in discrimination (see section 8.3.6.2), whereby the closer station is successfully received.

When the required capacity approaches the theoretically maximum available, the cell size is reduced, and stations outside the guard distance are always discriminated.

This section is based on the analyses described in Appendix D, and is a desktop analysis, supported by simulations of specific behavior. See the appendix for details.

It describes the dynamics of a system, which resorts to other means of synchronization, than the primary source (such as the 1PPS from a GNSS receiver). The assumptions are based on requirements found in the VDL Mode 4 draft SARPS [ICAO AMCP].

It is assumed that base stations always have access to a primary time source with an accuracy of 1 microsecond and, that they can provide own position with high accuracy.

The analysis show that a synchronization jitter of approximately 13 us is introduced, when synchronizing to transmissions from a base station. This is equivalent to 3.9 km. The guard distance is thus reduced by approximately 1%.

In the maritime AIS system, the requirement on timing sources is less stringent. The cell size is always restricted by line-of-sight distances, which are around 64 km or less (see 5.1.2.2 above). This is only 20% of the guard distance.

It is assumed that mobile stations always have access to a primary time source with an accuracy of 1 microsecond, but only have access to own position with low accuracy.

The analysis show that a maximum jitter of 15 us is introduced. This is equivalent to 4.5 km. The guard distance is thus reduced by approximately 1.2%. This is only slightly less than when synchronization is derived from base stations.

A floating network exists when all participants on the data link have lost their primary time source and must adopt to some sort of fallback mode for time synchronization.

It can be assumed that base stations know their position (it is always fixed and has been surveyed), and also have access to an independent primary timing source. If mobiles can receive base stations, the situation is identical of the one found in 5.3.1 and thus those result apply.

When a population of mobile stations have lost primary time source and are unable to receive transmissions from base stations (such as over an ocean) all sources of time are lost. This is a true floating network.

If a station has been fully synchronized to the primary time source and then suddenly loses it, it will maintain a very accurate synchronization for quite some time. A design which drifts approximately 3 us per minute is realistic to achieve [GP&C/S 2]. If a station maintains a lost sync state for an extended period of time, the cell radius will gracefully be reduced. Assuming, in worst case, that transmissions, in adjacent slots, drift in opposite directions, the system will still be operable for an additional 3 hours, or more.

If it is assumed that the system lacks the very accurate, primary time source (such as the 1PPS provided by a GNSS receiver), the system must rely totally on some sort of fallback mode. Appendix D examines a method, whereby all stations perform arrival time measurements of received transmissions. The arrival times are then used in order to derive an average transmission time, which is then used for own transmission.

The analysis show that, for one population, all stations will strive towards a common transmission start time, and thus float in time with very little timing jitter and small overlaps in time slots.

Even if two separate populations are allowed to merge, they will tend to strive towards a common, floating time base.

This section is based on analyses described in appendix E and is a pure desktop analysis, based on the results from previous sections. The method of organization, provided by the STDMA algorithm, is examined to see how well it operates in an overloaded system and how it works together with other features of the STDMA system (such as discrimination).

For purpose of analysis, the VHF cell is approximated as a cylinder, with its radius extending to the horizon or, to the range given by link budget.

Figure 5.4.1.a

The horizontal distribution is far greater than the vertical. Therefore, the vertical distribution is disregarded.

Since the stations are distributed geographically, each VHF cell is unique. This reduces the ability of the STDMA algorithm to maintain organization.

In a cellular system, the frequency resource is shared and reused. The sharing is in time, using time slots. Reuse is performed when two or more stations use the same time slots for transmission. Two types of slot reuse can be distinguished in the STDMA system:

Automatic slot reuse, where two or more stations, unintentionally attempt to use the same time slot for transmission. The stations are unaware of each other’s action.

Intentional slot reuse, where stations reuse time slots, known to be reserved by other stations. The decision to reuse the time slots, is based on distance from own position.

Intentional and automatic slot reuse can produce one, of two results, at a receiving station:

Garble, where none, of a number of transmissions in a time slot, was received correctly at a receiver.

Discrimination, where one, of a number of transmissions in a time slot, was received successfully at a receiver.

Distance ratio is used when determining whether slot reuse results in garble or discrimination. The distance ratio can be converted to a power ratio, which is assumed to be 6 dB. This equates to a distance ratio of ½ (see also Appendix B).

In remote areas, there is little, or none, data link communication. Studying two vessels approaching each other, shows, that the probability of transmission conflicts, is extremely small.

Figure 5.4.4.a

The probability of n consecutive transmission conflicts is:

Assuming that the Selection Intervals (SI), of both stations, overlap completely.

When the overlap is partial, the expression below can be used (courtesy of Professor Lars Zetterberg):

In an example, with 2250 slots per minute, and an update rate of 12 reports per minute, the probability of 3 consecutive transmission conflicts is 2 * 10^-7, or less. In other words: the probability of detection during this time is 99.99998%.

The example above shows that, even though two stations are within VHF coverage, the actual detection may be delayed somewhat. This can be visualized by a modified VHF cell. At high speeds, the cell is compressed in front of the stations.

Figure 5.4.4.b

A transmission conflict is resolved after 4 minutes, on average. The conflict is isolated to a slot. Therefore, the receiving station will experience a reduced report rate, rather than 100% garble.

The data link is saturated when the required number of time slots is equal to, or higher than the available. Under these circumstances more slots may generally be needed than currently available, and slot reuse will take place.

The primary results, presented in this section, are based on the assumption that participating stations are evenly distributed in space. Should this not be the case, the results will be affected (see also section 5.4.5.3).

A receiving station (R) is in the middle of a population of transmitting stations. Based on the distance between R and a transmitting station, the net throughput will vary, resulting in varying net report rate.

Three throughput zones can be identified:

the Aloha Zone,

the Discrimination Zone, and

the Protected Zone.

The Aloha Zone (column A in the table in figure 5.4.5.1.a) stretches from r to r/2. Any time stations, within this zone attempts to reuse the same time slot, garbling will occur at R. The receiving station (R) will experience, that the transmitting stations, exercise slotted Aloha behavior. With an increase in required capacity, the time slot reuse is increased. Since this results in garble, the net throughput is decreased proportionally. Figure 5.4.5.1.a shows how net throughput is decreased with an increase in required capacity (see table at bottom of figure).

The Discrimination Zone (column D in the table in figure 5.4.5.1.a) stretches from r/2 to r/3. Within this zone, transmissions, in reused time slots, may result in garble or discrimination. The net throughput is increased, at R, as a transmitting station moves from r/2 towards r/3. Stations within the Discrimination Zone are able to receive each other, and are thus STDMA organized. Slot reuse then only occurs in time slots used by stations in the Aloha Zone. In figure 5.4.5.1.a, the table does not indicate a net throughput, but only a dash. The dash simply means that the net throughput is changing from the value in column A towards the value in column P.

The Protected Zone (column P in the table in figure 5.4.5.1.a) stretches from r/3 to 0 (zero). Within this zone, all stations are STDMA organized. Slot reuse will occur in slots used by stations in the Aloha Zone. Each time slot reuse occurs, it will result in discrimination, thus allowing closer stations to have 100% net throughput. The protected zone ensures good throughput between stations, which are close to each other.

It is assumed that a station will apply intentional slot reuse when all time slots appear to be reserved. Since some automatic slot reuse result in garble, a station will not detect that the data link is used to its maximum capacity until it has been overloaded to a certain amount.

There is a relationship between the required capacity and the perceived load. If it is assumed that all stations are uniformly distributed, the table below shows this relationship. The calculations are based on the throughput analysis in appendix E. It can be noted that the data link would have to be overloaded by approximately 700%, before intentional slot reuse starts to occur.

Table 5.4.5.2.a

At 700% required capacity, the effective cell radius has, since long, shrunk to r/2 (the net throughput, in the Aloha Zone, is only 1% at 500% required capacity already).

In the example above, the Aloha Zone will contain 75% of all stations, the discrimination zone will contain 14% and the protected zone 11%. If the distribution is the opposite, the perceived load would be 100% already when the required load is approximately 110%.

The hidden user scenario, assumes that a station, has been isolated from all other traffic, and then suddenly reappear. This could be the case when a station is behind an obstruction where the VHF signals can not reach. When it appears, it is completely unorganized. Assuming that the appearing station is inside the protected zone, it will cause garble when reusing time slots from others within r/2 of R. It will discriminate others.

At the receiving station, the net throughput will be approximately 75% of nominal, assuming 100% required capacity. This reasoning is based on the fact that the protected and discrimination zones cover 25% of the whole VHF cell.

The results, so far, have assumed autonomous operation using the STDMA algorithm. It is possible to achieve 100% throughput within all throughput zones, by allowing R to become a controlling base station. As a controller, R will assign a transmission schedule to all transmitting stations. This way, R will be responsible for resolving transmission conflicts. There are penalties associated with this.

The controlling station must be allocated data link capacity for controller management. It must be able to transmit an assignment message, containing the transmission schedule, for each individual station.

If the data link is used to its maximum capacity, a new station, entering the cell, will be invisible to R until it reaches r/2.

Figure 5.4.6.2.a

The reason being that all stations are now organized. The arriving station will reuse slots for each of its transmissions. While inside the Aloha Zone, the reuse results in garble 100% of the time. Important to notice, though, is that the station will be visible to other stations in its own vicinity.

This situation can be avoided by always having some spare capacity available.

Since a controlled scenario must have spare capacity, overload is difficult to accommodate. The only way to handle that, would be to assign a lower reporting rate, as the required capacity is increased. This does not make sense, since a high report rate is desired when the traffic density is high.

Assuming that a ground infrastructure exists, with controlling base stations. The base stations have partially overlapping VHF cells.

Figure 5.4.6.4.a

Coordination is necessary, if 2 or more controlling base stations are involved. Otherwise, stations in the overlapping region, could not be assigned slots so that both R1 and R2 successfully can receive them. Furthermore, stations in the vicinity will experience reduced throughput.

The only way to handle the overlapping area, will be to split the time slots between R1 and R2 (R1 would have access to 50% and R2 the other 50%). If a third base station would be involved, the split would have to be three ways. The result is, that there will be available capacity, which can not be used.

A control center, such as a VTS or an ATC, wants good throughput within its coverage area. This can be achieved in two ways, using stacked stations, or satellite stations. The two, can be combined as feasible.

By stacking stations, at the control center, and giving each stations a sector, the frequency resource can be reused. The sectors are created by the use of directional antennas.

Figure 5.4.7.1.a

Each station will only receive transmissions from the sector covered by the directional antenna. There will be some overlap in adjacent sectors. This overlap adds redundancy, and thus safety, to the system.

The output, from the stacked stations, is input to a concentrator, which removes duplicate information prior to forwarding it to the control center. Each stacked station will have improved throughput in its sector. The net result is overall better coverage and throughput at the control center.

By connecting, geographically distributed stations, via a wide area network (WAN), the frequency resource can also be reused.

Figure 5.4.7.2.a

This scenario results in redundancy, since the VHF cells overlap partially. The concentrator removes any duplicates prior to sending the information to the control center. Each satellite station will have good throughput in its vicinity. The net result is better overall coverage and throughput at the control center.

The purpose of this thesis has been fulfilled since:

the terms capacity and throughput have been defined,

the dynamics and features, of the STDMA system, have been explored to great extent,

descriptive models for capacity and throughput, have been derived.

The problem definition has been answered since it is now possible to determine capacity and throughput, given a specific scenario. It is important, though, to consider the assumptions made in order to derive the analysis model.

The limiting factor when determining cell radius, restricted by radio wave propagation, is line-of-sight. In the results, the power dissipation resulted in a theoretical range of 1125 km. The maximum line-of-sight distance was 1018 km (2 aircraft at 20,000 meters). Two aircraft would have to fly higher than 20,000 meters for the cell radius to be restricted by power dissipation due to radio propagation.

Discrimination is a key feature in an autonomous, cellular system and it cooperates with the line-of-sight characteristic to promote cellular operation.

The STDMA system has been analyzed with a power ratio of 6 dB (equates to a distance ratio of 2.0). This discrimination behavior depends on the following features:

Frequency Modulation (FM). FM is a robust method of modulating a radio signal, compared to Amplitude Modulation (AM), for example.

Gaussian Minimum Shift Keying (GMSK). GMSK is a robust method of modulating data. One symbol equals one digital bit. It works very well with FM to produce a robust radio signal. Forward error correction is not needed.

Packet Protocol. The packet protocol is based on the High Level Data Link Communication (HDLC) packet structure. The use of unique flags and bit stuffing enables the system to easily detect the start of a packet even while another packet is being received. The bit stuffing removes the need for bit scrambling.

Data Encoding. Non-Return to Zero Indication (NRZI) is used when encoding data. This works by giving a change in polarity when a digital zero is encountered in the data stream. NRZI allows the radio signal to average around a stable carrier.

Time Division Multiple Access (TDMA). TDMA ensures transmission integrity over large distances and reduces the probability of transmission conflicts (compare pure and slotted Aloha [G. Maral et al]).

Bit Rate. Assuming that the channel bandwidth shall remain at 25 kHz [IALA] [ICAO AMCP], the used bit rate affects discrimination. A higher bit rate will reduce the ability of strong discrimination.

The STDMA system can handle transmissions from stations at distances greater than the guard distance (370 km). As the required capacity increases, more and more of the distant transmissions are discriminated, thus reducing the cell size of the receiving station.

A conflict, due to slot overlap, caused by propagation delay, will always result in discrimination of the farther station(s).

This fact is desirable in a cellular system such as STDMA.

Using fallback modes for synchronization, allows stations which lack accurate timing, to participate in the STDMA system anyway.

When the GNSS system fails, the STDMA system will degrade gracefully, allowing the participating stations to enter into a floating network gradually.

The analysis, in appendix D, examines two merging populations. This may very well work fine, but the scenario becomes quite complicated when additional, independent populations merge.

The STDMA system thrives on cellular operation and slot reuse. The STDMA algorithm cooperates with strong discrimination to provide a protected zone close to each individual station. This zone ensures slot integrity and safe delivery of time critical information.

In remote areas, with little data link traffic, the probability of transmission conflicts, is very small. If a conflict should occur, it will be isolated to one slot. Since position reports are repeatedly transmitted, the same, or similar, information is transmitted over and over again, in other slots. This makes an isolated transmission conflict non-critical.

The three throughput zones (Aloha, Discrimination and Protected zone) automatically adjust the net report rate. At long distances the net report rate is low. As the distance is shortened, the net report rate is increased. This is achieved without any change of nominal report rate. This behavior is desired, since a distant station is less of a threat than a close station.

As the data link overloads, the VHF cells shrink. The size, of protected zone, is maintained until the data link overloaded by almost 700% (assuming even distribution of stations). When intentional slot reuse is applied, the protected zone shrinks to r/9.

Assuming that all stations report 12 times per minute, 700% equates to 2625 stations within the region covered by the VHF cell (2 channels using 2250 slots per minute). Since the throughput from the Aloha zone, is zero, the cell radius is down to r/2 with 656 stations visible to the receiving station (25% of the whole region). Of these 656 stations, 291 stations have a net report rate equal to the nominal report rate (i e 12 reports per minute).

A surveillance system must allow overload, by gracefully degrade in a safe and controlled manner. By using autonomous mode, all stations can maintain their report rate, even when the data link is overloaded.

The ground infrastructure should be designed and based on autonomous mode. Factors, to take into account, are traffic density, geography and desired throughput. A control center can monitor a large area, not limited by a single VHF cell. Mobile stations can operate within the control area, regardless of how the ground infrastructure is designed.

During 1994, the Swedish CAA conducted a live test at the Landvetter Airport. The test included approximately 35 vessels, equipped with GP&C transponders, and one base station. The test proved that autonomous operation was possible through the STDMA algorithm. In 1994, autonomous operation was a fairly unknown concept and met with some skepticism. In 1997, the IMO [IMO Nav 43] specified that the primary mode of operation shall be autonomous. It appears as if the importance of autonomous operation is now widely accepted. The results, from this thesis, underline that.

The analysis model assumes a smooth surface with evenly, geographically distributed stations. Altitude variations, between stations, are neglected. In reality, the traffic density will vary. Some areas will have very little traffic (in the middle of the ocean, for example), other areas will have a concentration of traffic (a harbor or an airport, for example). This will affect the dynamics of capacity and throughput.

The only way to study the dynamics, would be simulations where the traffic situation is allowed to vary over time. The net throughput will then vary with the traffic situation. The analysis model is still quite useful, though. When studying a specific region, based on a documented traffic density, that density could be increased. By doing that, a worst case concentration, of vessels, can be assumed in the whole region. The results from such an analysis will be very conservative and certainly applicable in the whole region.

When stations are partially organized, the system will behave as a slotted Aloha system within the Aloha Zone. This behavior is desired since it allows transmissions to be successfully received even in an overloaded system.

Some other studies, such as [V. A. Orlando] and [R. E. Boisvert] have come to other conclusions. Orlando and Boisvert argue that a transmission conflict (i e a reused slot) will inhibit reception of the involved stations for 4 to 8 minutes. This, regardless of data link load. It appears as if they believe that a transmission conflict is propagated to all time slots, occupied by the involved stations.

Orlando and Boisvert also present results, showing that all transmissions, in the Aloha zone, result in garble, regardless of data link load. This is contrary to the results presented in this thesis. The Aloha system is well documented and understood. Maybe the similarities with the STDMA system, were not fully explored.

The propagation buffer provides integrity for up to 202 nm. Should two stations be separated by a greater distance, transmissions will overlap into adjacent slots. Depending on the data link load, these transmissions may still be received. Orlando [V. A. Orlando] indicates that the slot overlap is a problem. On the contrary, this is the desired behavior of a cellular system. The cell size shrinks with increased load. A ground infrastructure will still cover a large area, by the use of interconnected satellite stations. This architecture shall be based on the anticipated traffic density in the region.

The SARPS [ICAO AMCP] specify GMSK/FM and D8PSK for modulation. D8PSK is not a feasible modulation method due to its poor discrimination capabilities [L. Johnsson].

The GMSK/FM modulation is combined with a bitrate of 19.200 bps. It is important to produce equipment, which can operate at 19.200 bps and maintain a 25 kHz channel bandwidth. The anticipated power ratio is 7 dB [ICAO AMCP], but that must be validated. It may be necessary to relax the channel bandwidth requirements somewhat, in favor of strong discrimination. The environment must also be taken into consideration. Unwanted disturbances, such as interfering radio equipment and multi-path, must be considered.

This thesis has only looked at the situation when primary time source is lost. Other abnormal situations may also occur. Transceiver failures which may disturb the communication are not covered, for example. A deeper study of this may result in the need to specify additional system requirements, in order to minimize the probability of abnormal behavior.

This appendix summarizes the GP&C system with respect to the problem it is trying to solve and its basic functionality. The GP&C system is an implementation of a broadcast system using STDMA.

It is assumed that the patent [EPO] describes the key issues and, that current implementations of the system are based on the assumptions and requirements stated in the patent documentation.

The objective of this analysis is to analyze the patent in order to describe:

The GP&C system attempts to satisfy communications requirements found in aviation, maritime and land-mobile environments. The common scenario, in these environments, is a population of independently moving objects with a need to exchange information (primarily position information). Base stations, which monitor the communication, can also be part of the scenario.

Radar (Radio Detection And Range) and voice radio is the traditional solution to this communication need. But, the need to communicate extends farther than the range of radar. Because of the limitations of radar, aircraft, for example, are required to fly dedicated paths, with certain spacing between them. This is expensive for the airlines and introduces delays because of heavy air traffic. The GP&C system operates autonomously, allowing two aircraft to exchange information anywhere on the globe. If this information is position information, the aircraft could monitor the traffic and decide its path based on cost and current traffic in the area.

The maritime and land-mobile environments have similar needs.

This section describes the key concepts of the invention, as written in the Swedish patent.

The system is described as a population of moving object which, all know their position by receiving signals from geographically distributed transmitters (GDT:s). The position of each GDT is known by the moving objects.

Each moving object is equipped with a Very High Frequency (VHF) transceiver which is used in order to communicate the own position and other relevant information. All moving objects share a common frequency channel.

The GDT:s, which are used for positioning, also provide a time base to the moving objects. The time base is used to divide the common frequency channel into time slots.

The communication channel is divided into a frame, consisting of a pre-defined number of time-slots.

Figure 8.1.3.3.4.a

Each moving object is able to autonomously allocate slots within the frame. The slots are then used by each moving object, to communicate the own position, and other, relevant information.

Each moving object operates in a communication cell. An object is primarily interested in other objects within its vicinity. The range of the radio wave shall thus be restricted. The system is recommended to use a frequency, which has line of sight restriction as its primary propagation characteristic. VHF frequencies are in the range 30 to 300 MHz [L Ahlin et al].

Slots can be allocated autonomously or via master control. When an object allocates slots autonomously, it will primarily select unused slots. If the data link is saturated, it will employ a slot reuse algorithm and reuse slots, occupied by objects, which are far away.

When a master is introduced, it decides which slots to use by other objects.

The slots are used in order to broadcast the own position. Each object will have a default transmission rate, which can be changed automatically or via master control. The criteria, for changing transmission rate, is not defined.

This section describes how a GP&C system can be constructed, using existing technology.

Figure 8.1.3.4.a

Figure 8.1.3.4.A illustrates the main components of a GP&C transponder.

Position information is generally provided via a satellite navigation system, such as Global Positioning System (GPS) or GLONASS. GPS and GLONASS are jointly called Global Navigation Satellite Systems (GNSS). A GP&C transponder contains a GNSS receiver as a position sensor.

The GP&C transponder is equipped with a VHF transceiver which is able to operate on channels with 25 kHz spacing. The radio can be keyed and de-keyed within 1 ms.

The communication processor is a computer, which co-ordinates the use of the communication channel.

The GNSS receiver provides a time base through its One Pulse Per Second (1PPS). This pulse is coordinated to Universal Time Coordinated (UTC).

The communication processor is connected to the VHF transceiver and the position sensor. The communication processor holds a virtual image of the time-slot frame in its memory. It will transmit the position information, from the position sensor, based on the timing information from the timing pulse (1PPS).

The VHF transceiver ensures that the cellular concept is fulfilled. Frequencies in the VHF band propagates with the characteristics of line of sight.

Slot allocation is controlled by the communication processor. It continuously updates the virtual frame image and allocates slots from free slots or reuses from stations, which are far away.

The unique feature, which makes the GP&C system possible to patent, is the synchronization mechanism. The GNSS system provides position information all over the globe. In order for the GNSS systems to work, time must be one of the dimensions. The GP&C system states that the time, from the same transmitters which give position, shall be used to synchronize the slots on the data link. Given this, a system scenario evolves. The scenario shows a globally functioning Communication, Navigation and Surveillance (CNS) system. This puts additional demands on the system functionality. The system must be able to operate autonomously, for example. The algorithm which allows an object to enter the system, resolve communication conflicts and share the data link resource, is a very important feature of the GP&C system.

The objective of this analysis is to:

The system claims to be operable globally. A common frequency around the globe supports the global concept. The frequency is called the Global Signaling Channel (GSC). The system would work with a mix of frequencies too.

By implementing a data link administration telegram, objects could be instructed to change to local frequencies. This implies that some sort of controlling station is part of the scenario.

The system provides a globally functioning system for communication, navigation and surveillance. This is commonly called the CNS concept.

Communication is provided by the data link synchronized to UTC.

Navigation is provided by, for example, the GPS system.

Surveillance is the result of participating objects which communicate their position on the data link. The surveillance function can be extended so that an authority and, or company can monitor traffic from a central facility. This is achieved by connecting GP&C base stations in a wide area network (WAN). The connected base stations become a ground infrastructure.

The common frequency channel becomes a shared data link which is divided into time-slots. The data link is thus time divided. Since it also allows multiple access the link is called a TDMA link (Time Division Multiple Access). Access to the data link is given at the start of a slot, based on a slot selection algorithm.

The data link is organized so that a specified number of time-slots make up a repeatable frame. Each station can autonomously allocate and de-allocate slots within the frame. This makes the system independent from a ground infrastructure. The participating objects apply a slot allocation algorithm which is self organizing. This means that it allocates slots, de-allocate slots and resolves communication conflicts by itself. The selection algorithm is thus called Self Organized Time Division Multiple Access (STDMA).

An object, wishing to enter the data link network, will monitor the data link for a time equal to the length of a frame. During this time the object will create a virtual image of the data link traffic. After this, it will select slots for transmission. The slots are randomly selected with a spacing, which will allow the object to hold a pre-defined update rate (for example 12 reports per minute). If a randomly selected slot is busy, the object will look one slot ahead. If that slot is also busy, it will look one slot behind. It will continue this selection process until an available slot is found.

Each selected slot is tagged with a time out. This time out is part of the transmitted telegram. The time out randomly selected and may be different for each slot. For each transmission, in the slot, the time out is decremented. When it reaches zero, the object calculates a new slot to use and adds a slot offset to the transmitted telegram. The slot offset specifies how many slots forward or backward the object intends to move its transmission in the next frame.

By employing the STDMA algorithm, transmission conflicts are eventually resolved without intervention from controlling stations. Even though the data link is synchronized all over the globe, it would not operate unless it had a slot selection algorithm which supported the global concept.

The position information has a central role in the system. This information ensures a graceful degradation in case of heavy data link traffic. When free slots are unavailable, objects will start to reuse slots from distant stations. This allows the system to adjust when the communication traffic is increased.

This appendix presents analyses which look at factors and behavior surrounding the transmission and propagation of radio waves. Three areas are studied:

Power dissipation or link budget. Analysis of the factors affecting the power while the radio waves travel through the air.

Radio propagation. Analysis of the path of the radio waves.

Discrimination. Analysis of how conflicting transmissions discriminate or garble each other.

The purpose of these analyses is to give a background to the input used in the overall analysis of the STDMA system.

The objective of this analysis is to:

A transmitter and receiver are placed in space. The only obstructions, between the two stations, are the vessels in which the transmitter and receiver are installed. The environment at the transmitter is identical to the environment at the receiver.

The analysis shall give an indication of how far radio waves can travel with respect to transmission power and other factors affecting radio wave propagation in the scenario described above.

This is a desktop analysis based on theory regarding calculation of link budget and the assumptions from table 8.2.2.3.a.

The maximum distance (R), in free space, between a transmitting station and a receiving station is sought.

Figure 8.2.2.5.1.a

Equation B.1 below serves as the basis for the calculation [G. Maral et al].

Table 8.2.2.5.1.a

The wave length is calculated based on the equation B.2 below:

Table 8.2.2.5.1.b

According to the assumptions (table B.1), the transmit gain (G_T) and the receive gain (G_R) are equal (labeled G). The basis for further analysis is then the equation below:

The values above give a l of approximately 2 meters.

By applying the assumptions and the derived l , equation B.1 can be simplified as shown below:

Looking at equation B.3, high transmit power, low minimum required receive power, high gain and a long wavelength, improves the range.

These parameters are analyzed in more detail below and also commented at the end of this chapter.

In order to apply equation B.4, the gain must be determined. The gain varies around the antenna. The gain can be positive and thus enhance the output power. It can also be neutral or negative.

The following is assumed regarding gain:

The surrounding environment is simple in its structure.

Only negative gain is considered.

Only transmissions, which travel in a horizontal direction, are considered.

Based on these assumptions, and the assumptions in table 8.2.2.3.a, a gain of -5 dB (10^-0.5) is a realistic figure [Lennart Dreier, GP&C Sweden AB].

The maximum distance that the radio waves can travel is approximately 608 nm. If the gain is -15 dB instead of -5, the distance becomes 61 nm, or 1/10 of 608 nm. Diagram B.1 below illustrates how the distance varies with different values of gain:

This analysis omits factors such as atmospheric effects, multipath and interfering noise.

It does, however, give an indication of the theoretical range with respect to transmit power and the lowest acceptable receive power.

The effect of wavelength is interesting. A low frequency also allows low transmit power. This is in agreement with the GP&C system, which operates in the maritime and aviation VHF domain (107-174 MHz). It generally uses low transmit power (5 Watts). This can be compared to similar systems such as Mode S squitter, which operates on 1030/1090 MHz and uses high transmit power (1500 Watts) to achieve a range of 100 nm. [Orlando et al].

The objective of this analysis is to:

Two stations, a transmitter and a receiver are placed in space with the earth as an obstruction.

Figure 8.2.3.1.1.a

The radio waves from the transmitter propagates through the air to the receiver. Based on how the radio waves behave, the reception will be successful or fail.

The analysis shall result in an equation of how far the radio waves can reach, based on:

This is a desktop analysis of available literature. Equations are derived and compared with results in the literature.

VHF radio waves primarily travel as space waves [J.M. Ferrara]. This means that the most prominent path is a direct line between the transmitter and the receiver. The space wave component generally dissipates quickly. The radio waves can be said to have line-of-sight reach. Atmospheric conditions can cause the range to exceed beyond line-of-sight. This is generally called refraction [L Ahlin et al].

The figure below illustrates two objects in the atmosphere and the distance (D) between them.

Figure 8.2.3.5.2.a

Table 8.2.3.5.2.a

Based on the figure, the following equations are derived:

Further simplifications of equation B.7 results equation B.8:

Equation B.8 uses a constant (k), which equals 3.6. In [L Ahlin et al, page 35] a similar equation is presented. The constant (k) equals 3.6 also (see also comments in section 8.2.3.5.4).

If it can be assumed that h₁ and h₂ are equal, the situation can be illustrated as shown in figure 8.2.3.5.3.2.a below:

Figure 8.2.3.5.3.2.a

Since h₁ = h₂ = h, the following equation can be derived:

The distance (D) can be derived from equation B.9 and results in equation B.10 below:

Equation B.10 produces the same results as equation B.8.

It has been assumed that the radio waves propagates in a straight line. This results in a distance as described in equation B.8. In this equation the constant (k) equals 3.6. If refraction is considered, the constant (k) becomes 4.1 instead [L Ahlin et al, page 35]. However, according to L Ahlin, it is not feasible to use this factor due to destructive interference, caused by multipath, for example.

To study VHF coverage, the aviation environment is selected. Within aviation, three sub-environemnts are defined:

Enroute environment, which is where aircraft are enroute between departure and destination. Flight levels are generally high in this environment.

Terminal environment, which is where aircraft are approaching an airport in order to land.

Airport environment, which is where aircraft are either on ground at the airport, on final approach or, departing the airport.

The table below lists assumptions regarding altitude within the different environments:

Table 8.2.3.5.5.a

The altitudes are chosen based on how airspace is organized according to international civil aviation regulations.

Base stations are generally positioned on the ground, in the vicinity of an airport or, along enroute segments in order to cover common airways. If it is assumed that a base station antenna generally is positioned 20 meters above ground, the following ranges can be expected (employing equation B.8):

The range between mobile stations can be derived by assuming that two stations are at the same altitude.

The results are based on a smooth surface and a globe which has a uniform radius (6370 km). In addition to this, multipath and refraction has been left out. In areas with none or few obstructions, the resulting range is likely to be longer than calculated. In areas with many obstructions the range will be shorter.

The objective of this analysis is to:

A receiver is placed in the middle of a population of constantly moving objects, equipped with transceivers. Some of the objects are un-organized and, as a result, transmission conflicts occur.

The analysis shall result in information which describe the dynamics of discrimination and garbling. Discrimination occurs when one, of two or more conflicting stations, is received. Garbling occurs when none of the conflicting stations are received.

This is a study of experiments performed in laboratory, land-mobile and aviation environments, found in [LFV Card 1].

In all three tests, the setup is similar to the one showed in figure 8.2.4.4.a below:

Figure 8.2.4.4.a

Two transmitters (Tx 1 and Tx 2). Are stationary and separated by a distance (D). A receiver (Rx) is allowed to move back and forth between Tx 1 and Tx 2. The two transmitters output position reports once per second, in the same time slots, thus inducing transmission conflicts. The number of successfully received position reports are measured at the receiver (Rx) while moving from Tx 1 to Tx 2. In the laboratory test, this is simulated by using attenuators.

A successfully received transmission depends on the ratio in power between Tx 1 and Tx 2. This ratio is called power ratio (PR). Tx 1 and Tx 2 use the same nominal transmit power. The power ratio can easily be converted to a distance ratio (DR), which describes the required separation between Rx to Tx 1 and Rx to Tx 2 (d₁/d₂). To convert between PR and DR, the following equations are used [LFV Card 1]:

PR = 20 log( DR ) [Eq B.11]

Power Ratio (PR) is expressed in decibels (dB).

The laboratory test simulated the distance (D) and the position of Rx within D, by the use of a setup as shown in figure 8.2.4.5.1.A below:

Figure 8.2.4.5.1.a

The test was performed using a signal strength of approximately -80 dBm. The test produced the following results:

Table 8.2.4.5.1.a

The land-mobile test was performed using the setup described in figure 8.2.4.4.a above. Two transmitters (Tx 1 and Tx 2) were positioned at opposite ends of an airport runway. A receiver (Rx) was installed in a car and then allowed to travel between Tx 1 and Tx 2.

The aviation test was performed using the setup described in figure 8.2.4.4.a above. Two transmitters (Tx 1 and Tx 2) were positioned at two different airports. A receiver (Rx) was installed in an aircraft and then allowed to fly between Tx 1 and Tx 2.

Only the laboratory test was conducted in a controlled environment. This test resulted in the poorest results (PR = 5 dB). The difference between the laboratory and the land-mobile and aviation tests can probably be explained by environmental causes, such as interfering obstructions, antenna positions a s o. For the purpose of analysis, 5 dB, or more, should be used.

In the ICAO draft SARPS [ICAO AMCP], a second modulation method (Digital Eight Phase Shift Keying - D8PSK) is specified. According to [R. E. Boisvert], this modulation method results in a PR of 18 dB, or a DR of 7.9. In general, a lower power ratio (and thus a lower distance ratio), will promote improved throughput. Therefore, D8PSK is not a feasible modulation method, compared to GMSK/FM.

This appendix focus on how large distances, between communicating stations, introduce delays and, as a result, possible collisions in transmissions. The STDMA system has a built-in guard time (a buffer which protects data) in each slot, which allows stations to be 370 kilometers (200 nautical miles) apart. A station can still receive a transmission farther away, but that transmission will cross slot boundaries and potentially be in conflict with a transmission in an adjacent slot.

The objective of this analysis is to:

study how propagation affects throughput and cell radius when stations are far apart (more than 370 km) and the slot occupancy is less than maximum.

When stations are more than 200 nautical miles (nm) apart, their transmissions will overlap slot boundaries and thus no longer be protected within a slot. There is a probability that transmissions in adjacent slots will be in conflict.

Focus is only on propagation. Conflicts, due to stations not being STDMA organized, are omitted in the analysis.

The figure below shows three circles. The inner circle (A) has a radius of 100 nm. The middle circle (B) has a radius of 200 nm and the outer circle (C) has a radius of 400 nm and is the total area in this analysis. The area of A is 1/16 of the total area. The area encompassed by B is 3/16 of the total area and the area C encompasses 12/16 of the total area.

Figure 8.3.2.1.a

The total number of transmissions occupy all available slots, but initially only 5% of the transmissions are within 200 nm of the receiving station. The receiving station is positioned in the center and the throughput (i e correctly received transmissions divided by actual transmissions) to this station is studied. Any transmission inside the 200 nm radius is protected by the guard time. The stations are then slowly allowed to move into the 200 nm radius until 95% of all transmissions are within.

The analysis shall result in equations describing:

The nature of propagation and discrimination is studied to determine the factors affecting the success of a transmission in a slot. Slots can be in 3 states:

An empirical method is used. A C++ program is created in order to retrieve results. The program constructs a virtual image of a traffic situation with stations randomly spread within a 400 nm radius of a receiving station. Each slot is assigned a distance to the receiver. Based on the distance, an analysis is performed to see if a transmission is in conflict with the previous transmission. If this is the case, the previous transmission is either discriminated or both transmissions are garbled. The program constructs the virtual image 1000 times and then calculates average values of slots which are okay, slots which are garbled and slots which are discriminated.

The results from running the program are input to a spread sheet program. Diagrams are drawn and equations are derived based on the input and the diagrams.

The data link is time divided. This means that each transmission is allocated its own time slot. This analysis only looks at conflicts due to propagation. Therefore it is assumed that only one station transmits in each time slot. Conflicts will then only be possible when transmissions overlap slot boundaries at the receiving station.

When a conflict is a fact it can either cause one transmission to be discriminated (i e 1 of 2 transmissions are received) or it can cause both transmissions to be garbled (i e 0 of 2 transmissions are received). To discriminate, the distance between the conflicting stations must be as described below:

Distance(Distant)/Distance(Close) >= 2 [Eq. C.1]

The figures below show different examples transmissions and propagation time.

 Figure 8.3.6.2.a

In figure 8.3.6.2.a the conflict is avoided since the transmission from the station at 200 nm results in a propagation which allows the station at 400 nm to overlap into the next slot but not overlap the transmission.

 Figure 8.3.6.2.b

In figure 8.3.6.2.b the station at 100 nm overrides the station at 400 nm, thus discriminating that station (400/100> 2).

 Figure 8.3.6.2.c

When the previous transmission is at a closer range than the following one, they are in fact separated and no conflicts exist (figure 8.3.6.2.c).

 Figure 8.3.6.2.d

Using Eq. C.1 would result in garbling in the scenario shown in figure 8.3.6.2.d (400/300 < 2), but as can be seen, the propagation time separates the two transmissions.

Thus, garbling due to propagation time only, can never occur in the STDMA system.

Discrimination is the only factor limiting throughput in the non-saturated scenario with respect to propagation time.

Table 8.3.6.3.a

Table 8.3.6.3.b

The results are shown in the diagram below:

Diagram 8.3.6.3.a

The curved line shows the overall net throughput from areas A, B and C. The straight line shows the throughput from stations in area C.

The presented equations contain constants which have been derived from the empirically collected data. The net throughput can be expressed as shown below:

TP(net) = 0.37*(Pop(AB)-0,50)+0,907 [Eq. C.2]

Table 8.3.6.4.a

The throughput of transmissions outside the 200 nm protected can be expressed as shown below:

TP(C) = 1 - 0,37 * Pop(AB) [Eq. C.3]

The test program keeps statistics about discrimination in relation to distance from the receiving station. The diagram below shows the result of this.

Diagram 8.3.6.5.a

Each curve in diagram 8.3.6.5.a represents a certain population within area A and B. The probability of discrimination is increasing with an increase in distance and an increase in population.

At 300 nm, the angle of the curve is increased by a factor of 3. The reason for this is that between 200 and 300 nm discrimination is a result of stations within area A conflicting with stations in C. B and C are in harmony. Between 301 and 400 nm both A and B may conflict with C. The area of B is 3 times larger than A, thus the factor of 3.

The probability of discrimination between 201 nm and 300 nm can be expressed as:

Table 8.3.6.5.a

The probability of discrimination between 301 and 400 nm can be expressed as:

Table 8.3.6.5.b

In order to study the dynamics of the cell radius, a minimum acceptable throughput must be established. Diagram 8.3.6.5.b shows curves for acceptable throughput ranging from 50% to 90%.

Diagram 8.3.6.5.b

The Y-axis shows the resulting cell radius and the X-axis shows the population within A and B. The table below lists the values used to derive diagram 8.3.6.5.b.

The 200 nm guard distance is not an absolute limit to how far apart a transmitter and receiver can be. In this scenario, if all transmissions would have been inside 200 nm, the net throughput would have been 100%. Likewise, if all transmissions would have been outside the 200 nm radius, the throughput would have been 100%. This is not realistic. The scenario assumes that all transmissions are STDMA organized and within VHF link budget, which is quite unlikely in reality. However, it does show some important points:

Time synchronization analyses focus on the different sources of synchronization to the TDMA data link when the primary synchronization source is lost. The primary source of synchronization is generally required to have an accuracy of 400 nanoseconds [ICAO AMCP] in aviation applications. This source can be provided by the 1PPS pulse generated by a GNSS receiver.

The assumptions listed below are valid for every analysis in this appendix.

Table 8.4.1.2.a

The report rate in the airport environment is a truly worst case. In reality most aircraft are either stationary or moving in non-critical areas of the airport. As a result, the average report rate can be a lot less than 60 times per minute.

The objective of this analysis is to:

…determine the effects of an isolated station losing its primary time source due to loss of GNSS receiver.

A station loses its internal GNSS and thus also its primary time source. The failure is local to that station and there are other stations (mobiles and base stations) in the area of reception. The station reverts to its backup source for positioning, but must derive a new source of time synchronization to the TDMA data link.

The analysis shall show whether loss of primary time synchronization affects the throughput on the TDMA data link, both with respect to own reception and with respect to disturbances to other stations.

This is a desktop analysis based on the basic assumptions (section 8.4.1) and the assumptions from table 8.4.2.3.a.

The criteria for maintaining slot integrity is first analyzed. The timing error is then calculated. These results are finally compared in order to conclude if there are any conflicts due to the less accurate timing source.

In the worst case scenario the receiving station is separated with the transmitting station by 200 nm (the guard distance). This eats up the buffer allocated for guard distance. At the same time the transmitting station, which is poorly synchronized, produces a transmit jitter (G) which is equivalent to the sum of all timing errors.

Figure 8.4.2.5.1.1.a

The start jitter results in a transmission starting either prior to, or after nominal slot start. It will be centered around the nominal slot start with +/- G/2.

The base station uses the high position accuracy and will thus gives the smallest error in distance measurement.

When a station attempts to synchronize to transmissions from another stations it will try to compensate for the distance between itself and the transmitting station. A timing error is introduced as a result in the inaccuracies in position information. The figure below illustrates this:

Figure 8.4.2.5.2.1.a

The worst case distance error between the mobile and a base station can be calculated as shown below:

The internal position source is updated once every second (see general parameters). In the worst case scenario, the synchronization calculations are performed just before a new position is derived. The position is then 1 second old and the station has traveled a distance based on its speed over ground (SOG). The position error can be calculated using the equation below:

Table 8.4.2.5.2.3.a below shows the timing errors common to every aircraft environment (airport, terminal, en route).

Table 8.4.2.5.2.3.a

Table 8.4.2.5.2.3.b below shows the accumulated timing error for the three different environments:

Table 8.4.2.5.2.3.b

Table 8.4.2.5.2.3.b shows that the timing error is less than 13 us for all environments. This jitter affects the buffer allocated to guard time. 13 us is equivalent to 3.9 km (13*300/1000 km). The guard distance is thus reduced by 3.9 km or approximately 1%.

The degradation in performance, due to loss of primary synchronization, only affects the radius protected by guard time. In the analysis this degradation is 1% or a reduction from 370 km to 366 km radius. Stations in this region will only be in conflict with other stations on the same region. The throughput within 366 km is intact.

A station should calculate an average time based on several transmissions. This will smoothen the timing jitter and not simply add the timing error to the own transmission.

The mobile station uses the low position accuracy (in accordance with assumptions) and will thus give higher error in distance measurement than the base station case.

Distance error is derived in the same manner as in section 8.4.2.5.2.1 but the equation looks like below:

In the worst case scenario, both stations are moving away from each other with a relative speed of 2*SOG. If it also is assumed that the transmitted position is 1 second old, the resulting equation looks like below:

The table below shows the timing errors common to every aircraft environment (airport, terminal, en route).

Table 8.4.2.6.a

Table 8.4.2.6.b below shows the accumulated timing error for the three different environments:

Table 8.4.2.6.b

Table 8.4.2.6.b shows that the timing error is less than 15 us for all environments. This is equivalent to 4.5 km, which still is approximately 1% of the distance allocated to guard time.

The degradation in performance, due to loss of primary synchronization, only affects the radius protected by guard time. In the analysis this degradation is 1% or a reduction from 370 km to 366 km radius. Stations in this region will only be in conflict with other stations on the same region. The throughput within 366 km is intact.

A floating network exists when participants on the data link have lost their primary time source and must adopt to some sort of fallback mode for time synchronization.

The objective of this analysis is to:

The GNSS system fails (maybe it is shut down by the operator) and all stations are suddenly without primary time source. All stations reverts to their backup sources for positioning. The only source left for time synchronization is the transmission from other un-synchronized stations, or possibly base stations with a backup system for time synchronization. The TDMA data link becomes a floating network of stations continuously attempting to synchronize to each other.

The analysis shall show whether loss of primary time synchronization affects the throughput on TDMA data link, both with respect to own reception and with respect to disturbances to other stations.

This is a desktop analysis, combined with simulations in C/C++, based on the basic assumptions (section 8.4.1) and the assumptions from table 8.4.3.3.a.

The criteria for maintaining slot integrity is first analyzed. The timing error is then calculated. These results are finally compared in order to conclude if there are any conflicts due to the less accurate timing source.