How to choose a broadband modem for unmanned aerial vehicle (UAV) or robotics
The task of transferring large amounts of data from an unmanned aerial vehicle (UAV) or ground-based robotics is not uncommon in modern applications. This article discusses the criteria for selecting broadband modems and related problems. The article is written for developers of BLA and robotics.
The main criteria for choosing a broadband modem for UAV or robotics are.
Communication range depends not only on the modem, but also on antennas, antenna cables, radio wave propagation conditions, external interference and other causes. In order to separate the parameters of the modem itself from other parameters affecting the communication range, consider the distance equation [Kalinin, AI, Cherenkova, EL. The propagation of radio waves and the work of radio lines. Connection Moscow. 1971]
Where
$ inline $ r $ inline $ - the desired communication range in meters;
$ inline $ F $ inline $ - frequency in Hz;
$ inline $ P_ {TXdBm} $ inline $ - modem transmitter power in dBm;
$ inline $ G_ {TXdB} $ inline $ - transmitter antenna gain in dB;
$ inline $ L_ {TXdB} $ inline $ - cable loss from modem to transmitter antenna in dB;
$ inline $ G_ {RXdB} $ inline $ - receiver antenna gain in dB;
$ inline $ L_ {RXdB} $ inline $ - cable loss from modem to receiver antenna in dB;
$ inline $ P_ {RXdBm} $ inline $ - modem receiver sensitivity in dBm;
$ inline $ | V | _ {dB} $ inline $ - attenuation factor, taking into account additional losses due to the influence of the Earth's surface, vegetation, atmosphere and other factors in dB.
From the distance equation it can be seen that the range depends only on two parameters of the modem: transmitter power $ inline $ P_ {TXdBm} $ inline $ and receiver sensitivity $ inline $ P_ {RXdBm} $ inline $ or rather from their difference - the modem's energy budget
The remaining parameters in the range equation describe the signal propagation conditions and the parameters of the antenna-feeder devices, i.e. to the modem are not related.
So, in order to increase the communication range, you must choose a modem with a large value $ inline $ B_m $ inline $ . Zoom $ inline $ B_m $ inline $ in turn, by increasing $ inline $ P_ {TXdBm} $ inline $ or by reducing $ inline $ P_ {RXdBm} $ inline $ . In most cases, UAV developers are looking for a modem with high transmitter power and pay little attention to the receiver sensitivity, although you need to do exactly the opposite. A powerful onboard broadband modem transmitter entails the following problems:
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The first two problems are related to the fact that modern methods of transmitting large amounts of information over a radio channel, for example OFDM, require a linear transmitter. The efficiency of modern linear radio transmitters is low: 10–30%. Thus, 70–90% of the precious energy of the power source of the UAV is converted into heat, which must be effectively removed from the modem, because otherwise it will fail or its output power will drop due to overheating at the most inopportune moment. For example, a 2 watt transmitter will consume 6–20 watts from a power source, of which 4–18 watts will be converted to heat.
Energy stealth radio is important for special and military applications. Low stealth means that the modem signal is relatively likely to be detected by the receiver of the jamming station. Accordingly, the probability of suppressing a radio with a low energy stealth is also high.
The sensitivity of a modem receiver is characterized by its ability to extract information from received signals with a given quality level. Quality criteria may vary. For digital communication systems, the probability of error per bit (bit error rate - BER) or the probability of error in the information package (frame error rate - FER) is most often used. Actually, sensitivity is the level of the very signal from which information should be extracted. For example, a sensitivity of −98 dBm with BER = 10 −6 indicates that information with such a BER can be extracted from a signal with a level of −98 dBm or higher, and no longer a signal from a level with, say, −99 dBm. Of course, the decrease in quality with a decrease in the signal level occurs gradually, but it is worth bearing in mind that most modern modems are inherent in the so-called. The threshold effect at which a decrease in quality with a decrease in the signal level below sensitivity occurs very quickly. It is enough to reduce the signal by 1−2 dB below the sensitivity so that the BER increases to 10 −1 , which means that you will not see video from the board of the BLAH. The threshold effect is a direct consequence of the Shannon theorem for a channel with noises; it cannot be eliminated. The destruction of information when the signal level drops below the sensitivity is due to the influence of noise, which is formed inside the receiver itself. The internal noise of the receiver cannot be completely eliminated, but it is possible to reduce its level or learn how to efficiently extract information from a noisy signal. Modem manufacturers use both of these approaches, making improvements to the receiver's RF units and improving digital signal processing algorithms. Improving the sensitivity of the receiver modem does not lead to such a dramatic increase in power consumption and heat dissipation, as an increase in transmitter power. There is, of course, an increase in energy consumption and heat generation, but it is rather modest.
We recommend the following algorithm for selecting a modem in terms of achieving the required communication range.
The sensitivity values depend on the data transfer rate according to the rule: higher speed - worse sensitivity. For example, a sensitivity of −98 dBm for a speed of 8 Mbps is better than a sensitivity of −95 dBm for a speed of 12 Mbps. You can only compare modems for sensitivity at the same data rate.
Data on transmitter power is almost always available in the specifications of modems, but data on the sensitivity of the receiver is not always or in insufficient volume. At a minimum, this is a reason to be wary, because the beautiful numbers hardly make sense to hide. In addition, by not publishing sensitivity data, the manufacturer deprives the consumer of the opportunity to estimate the communication range by calculation before purchasing a modem.
The choice of modem for this parameter is relatively simple, if the speed requirements are clearly defined. But there are some nuances.
If the problem being solved requires ensuring the greatest possible range of communication and at the same time it is possible to allocate a sufficiently wide frequency band for a radio link, then it is better to choose a modem that supports a wide frequency band (bandwidth). The fact is that the required information speed can be provided in a relatively narrow frequency band by using dense modulation types (16QAM, 64QAM, 256QAM, etc.), or in a wide frequency band by using low density modulation (BPSK, QPSK ). The use of low density modulation for such tasks is preferable due to the higher noise immunity. Therefore, the sensitivity of the receiver turns out better, respectively, increases the energy budget of the modem and, as a consequence, the communication range.
Sometimes, manufacturers of UAVs set the information speed of a radio link much higher than the speed of the source, literally 2 or more times, arguing that sources such as video codecs have a variable bit rate and the modem speed should be selected taking into account the maximum bitrate emissions. The communication range is naturally reduced. Do not use this approach without extreme necessity. Most modern modems have a capacious buffer in the transmitter that can smooth out bit-rate emissions without packet loss. Therefore, a margin of more than 25% is not required. If there is reason to believe that the buffer capacity in the purchased modem is insufficient and a significantly greater increase in speed is required, then it is better to refuse to buy such a modem.
When evaluating this parameter, it is important to separate the delay related to the transmission of data via radio link from the delay created by the source encoding / decoding device, such as a video codec. The delay in the radio link consists of 3 quantities.
According to the author’s experience, the type 1 delay is in the range from tens of microseconds to one millisecond. Type 2 delay depends on the communication range, for example, for a 100 km link it is equal to 333 µs. Type 3 latency depends on the TDD frame length and on the ratio of the transmission cycle length to the total frame duration and can vary from 0 to frame duration, i.e. it is a random value. If the transmitted information packet is at the transmitter input when the modem is in the transmission cycle, the packet will be transmitted with type 3 zero delay. If the packet is slightly late and the reception cycle has already started, it will be delayed in the transmitter buffer for the duration of the reception cycle . Typical TDD frame lengths range from 2 to 20 ms, respectively, the type 3 delay in the worst case does not exceed 20 ms. Thus, the total delay in the radio link will be within 3−21 ms.
The best way to know the delay in a radio link is a full-scale experiment using utilities to evaluate network performance. It is not recommended to measure the delay by the request – response method, since the delay in the forward and reverse directions may be different for TDD modems.
The choice of onboard modem unit by this criterion does not require special comments: the smaller and lighter, the better. Do not forget also about the need to cool the onboard unit, additional radiators may be required, respectively, the weight and dimensions may also increase. Preference here should be given to light, small-sized blocks with low power consumption.
For the ground block, mass-dimensional parameters are not so critical. At the forefront ease of use and installation. The ground unit must be a device that is reliably protected from external influences and has a convenient system for fastening to a mast or a tripod. A good option when the ground unit is integrated in the same package with the antenna. Ideally, the ground unit should be connected to the control system through one convenient connector. This will save you from strong words when you need to carry out work on the deployment at a temperature of −20 degrees.
On-board units, as a rule, are released with the support of a wide range of supply voltages, for example, 7–30 V, which covers most of the voltage options in the UAV energy network. If you have a choice of several supply voltages, then give preference to the lowest value of the supply voltage. As a rule, internal modem power is supplied from 3.3 and 5.0 V voltages via secondary power supplies. The efficiency of these secondary power sources is the higher, the smaller the difference between the input and internal voltage of the modem. Increased efficiency means reduced power consumption and heat dissipation.
On the other hand, ground blocks must maintain power from a source with relatively high voltage. This allows the use of a power cable with a small cross-section, which reduces the weight and simplifies installation. With other things being equal, give preference to ground blocks with PoE (Power over Ethernet) support. In this case, to connect the ground unit with the control station will need only one Ethernet cable.
An important opportunity in those cases when there is no space left for the UAV to install a separate command-telemetric modem. If there is space, a separate broadband modem control / telemetry channel can be used as a backup. When choosing a modem with this option, pay attention to that the modem supports the desired protocol for communication with UAVs (MAVLink or proprietary) and the possibility of multiplexing control / telemetry channel data into a convenient interface at a ground station (NS). For example, the onboard unit of the broadband modem is connected to the autopilot via an RS232, UART or CAN interface, and the ground unit is connected to the control computer via the Ethernet interface through which it is necessary to exchange command-telemetry and video information. In this case, the modem should be able to multiplex the command-telemetric flow between the RS232, UART or CAN interfaces of the onboard unit and the Ethernet interface of the ground unit.
The presence of duplex mode. Broadband modems for UAVs support either simplex or duplex modes of operation. In the simplex mode, data transmission is allowed only in the direction from the UAV to the National Assembly, and in the duplex mode - in both directions. As a rule, simplex modems have a built-in video codec and are designed to work with video cameras that do not have a video codec. To connect to an IP camera or to any other devices that require an IP connection, the simplex modem is not suitable. In contrast, a duplex modem is usually designed to connect the onboard IP network of the UAV with the NS network of the NS, i.e., it supports IP cameras and other IP devices, but may not have a built-in video codec, since IP cameras typically have your video codec. Ethernet interface support is possible only in duplex modems.
RX diversity. The presence of this feature is mandatory to ensure uninterrupted communication throughout the flight distance. When propagating above the Earth’s surface, radio waves arrive at the reception point by two rays: in a straight path and with reflection from the surface. If the addition of waves of two rays occurs in phase, then the field at the point of reception increases, and if in antiphase, it is weakened. Attenuation can be very significant - up to the complete loss of communication. The presence of two antennas on the NA, which are located at different heights, helps to solve this problem, because if at the location of one antenna the rays are in antiphase, then at the location of the other there is none. As a result, you can achieve a stable connection throughout the course.
Supported network topologies. It is advisable to choose a modem that provides support not only for point-to-point (PTP) topology, but also for point-to-multipoint (PMP) and relay (repeater) topologies. The use of relaying through an additional UAV allows you to significantly expand the range of the main UAV. PMP support will allow receiving information simultaneously from several UAVs on one NS. Please also note that support for PMP and retransmission will require an increase in modem throughput compared to the case of communication with one UAV. Therefore, for these modes it is recommended to choose a modem with support for a wide frequency band (at least 15-20 MHz).
Availability of noise immunity enhancement tools. Useful option, given the tense interference situation in the places of use of the UAV. Interference immunity refers to the ability of a communication system to perform its function when there is interference of artificial or natural origin in the communication channel. To combat interference, there are two approaches. Approach 1: design the modem receiver so that it can confidently receive information even if there is interference in the communication channel band at the cost of some reduction in the information transfer rate. Approach 2: suppress or mitigate interference at the receiver input. Examples of the implementation of the first approach are spread spectrum systems, namely: frequency hopping (FH), spectrum expansion with a pseudo-random sequence (DSSS) or their hybrid. FH technology is widely used in UAV control channels due to the small amount of required data transfer rate in such a communication channel. For example, for a speed of 16 kbps in the 20 MHz band, it is possible to organize about 500 frequency positions, which allows you to reliably protect against narrowband interference. The use of FH for a broadband communication channel is problematic because of the resulting large bandwidth. For example, to get 500 frequency positions when working with a signal with a 4 MHz bandwidth, you will need 2 GHz of free band! Too much to be a reality. The use of DSSS for a broadband UAV link is more relevant. In this technology, each information bit is duplicated simultaneously on several (or even on all) frequencies in the signal band and, in the presence of narrowband interference, can be isolated from the spectral regions not affected by interference. The use of DSSS, as well as FH, implies that if a channel is interfered with, a data transfer rate will be required to decrease. Nevertheless, it is obvious that it is better to receive video from the side of the UAV in a lower resolution than nothing at all. Approach 2 uses the fact that the interference, unlike the internal noise of the receiver, enters the radio link from the outside and, if there are certain means in the modem, can be suppressed. Interference suppression is possible if it is localized in the spectral, temporal or spatial regions. For example, a narrowband interference is localized in the spectral region and can be “cut out” from the spectrum using a special filter. Similarly, the impulse noise is localized in the time domain, to suppress it, the affected area is removed from the receiver input signal. If the interference is not narrowband or impulse, then a spatial suppressor can be used to suppress it, since the interference gets into the receiving antenna from the source from a certain direction. If the zero of the receiving antenna pattern is located in the direction of the interferer, the interference will be suppressed. Such systems are called adaptive beamforming control (adaptive beamforming & beam nulling) systems. In the well-known broadband modems for UAVs, such systems are not used, although nothing prevents their occurrence in the future.
Used radio protocol. Modem manufacturers can use a standard (WiFi, DVB-T) or a proprietary radio protocol. This parameter is rarely indicated in the specifications. The use of DVB-T is indirectly indicated by the supported frequency bands 2/4/6/7/8, sometimes 10 MHz, and the mention in the text of the specification of coded OFDM technology in COFDM in which OFDM is used in conjunction with noise-resistant coding. At the same time, we note that COFDM is a purely advertising slogan and does not have any advantages over OFDM, since OFDM without noise-resistant coding is never used in practice. Equal COFDM and OFDM when you see these abbreviations in the radio modem specifications.
Modems that use a standard protocol, as a rule, are based on a specialized chip (WiFi, DVB-T), working in conjunction with a microprocessor. The use of a specialized chip removes from the modem manufacturer a lot of headaches associated with the development, simulation, implementation and testing of its own radio protocol. The microprocessor is used to give the modem the necessary functionality. Such modems have the following advantages.
Disadvantages are also available.
The low stability of supply is due to the fact that chip manufacturers focus primarily on mass markets (TVs, computers, etc.). Modem manufacturers for UAVs are not a priority for them and they cannot in any way influence the decision of the chip manufacturer to discontinue production without adequate replacement for another product. This feature enhances the tendency to package radio interfaces into specialized chips of the System on Chip type (System on Chip), which is why individual radio interface chips are gradually being washed out of the semiconductor market.
Limited opportunities in the provision of technical support are due to the fact that the development teams of modems based on the standard radio protocol are well-equipped with specialists primarily in electronics and microwave engineering. There are no radio communication specialists at all, since there are no tasks for them to be solved. Therefore, UAV manufacturers looking for solutions to non-trivial radio communication tasks may be disappointed in terms of consultation and technical assistance.
Modems using a proprietary radio protocol are built on the basis of universal analog and digital signal processing chips. The stability of the supply of such chips is very high. True, the price is also high. Such modems have the following advantages.
Disadvantages.
The table shows the technical parameters of some UAV modems available on the market.
Please note that although the 3D Link modem has the lowest transmitter power compared to the Picoradio OEM and J11 modems (25 dBm vs. 27-30 dBm), the 3D Link power budget is higher than those of these modems, due to the high sensitivity of the receiver (with the same data transfer rate of the modems being compared). Thus, the communication distance when using 3D Link will be longer with better energy stealth.
Table. Specifications of some broadband modems for UAV and robotics
* n / d - no data.
Alexander Smorodinov [a.smorodinov@geoscan.aero] is a leading specialist of Geoscan LLC in the field of wireless communications. From 2011 to the present, he has been developing radio protocols and signal processing algorithms for wideband radio modems for various purposes, as well as the implementation of the developed algorithms based on programmable logic chips. The author’s interests include the development of synchronization algorithms, channel properties estimation, modulation / demodulation, noise-resistant coding, as well as some algorithms of the medium access level (MAC). Before joining Geoscan, the author worked in various organizations, developing non-standard wireless communication devices. From 2002 to 2007, he worked at Protey LLC as a leading specialist in the development of communication systems based on the IEEE802.16 (WiMAX) standard.From 1999 to 2002, the author was engaged in the development of noise-tolerant coding algorithms and modeling of radio link paths at the FSUE Central Research Institute "Granit". The author received a Ph.D. in technical sciences from St. Petersburg University of Aerospace Instrumentation in 1998 and a radio engineer degree from the same university in 1995. Alexander is an active member of the IEEE and the IEEE Communications Society.
Criterias of choice
The main criteria for choosing a broadband modem for UAV or robotics are.
- Communication range
- Maximum data transfer rate.
- Delay in data transfer.
- Mass-dimensional parameters.
- Supported information interfaces.
- Power Requirements.
- Separate control / telemetry channel.
Communication range
Communication range depends not only on the modem, but also on antennas, antenna cables, radio wave propagation conditions, external interference and other causes. In order to separate the parameters of the modem itself from other parameters affecting the communication range, consider the distance equation [Kalinin, AI, Cherenkova, EL. The propagation of radio waves and the work of radio lines. Connection Moscow. 1971]
$$ display $$ R = \ frac {3 \ cdot 10 ^ 8} {4 \ pi F} 10 ^ {\ frac {P_ {TXdBm} + G_ {TXdB} + L_ {TXdB} + G_ {RXdB} + L_ {RXdB} + | V | _ {dB} -P_ {RXdBm}} {20}}, $$ display $$
Where
$ inline $ r $ inline $ - the desired communication range in meters;
$ inline $ F $ inline $ - frequency in Hz;
$ inline $ P_ {TXdBm} $ inline $ - modem transmitter power in dBm;
$ inline $ G_ {TXdB} $ inline $ - transmitter antenna gain in dB;
$ inline $ L_ {TXdB} $ inline $ - cable loss from modem to transmitter antenna in dB;
$ inline $ G_ {RXdB} $ inline $ - receiver antenna gain in dB;
$ inline $ L_ {RXdB} $ inline $ - cable loss from modem to receiver antenna in dB;
$ inline $ P_ {RXdBm} $ inline $ - modem receiver sensitivity in dBm;
$ inline $ | V | _ {dB} $ inline $ - attenuation factor, taking into account additional losses due to the influence of the Earth's surface, vegetation, atmosphere and other factors in dB.
From the distance equation it can be seen that the range depends only on two parameters of the modem: transmitter power $ inline $ P_ {TXdBm} $ inline $ and receiver sensitivity $ inline $ P_ {RXdBm} $ inline $ or rather from their difference - the modem's energy budget
$$ display $$ B_m = P_ {TXdBm} -P_ {RXdBm}. $$ display $$
The remaining parameters in the range equation describe the signal propagation conditions and the parameters of the antenna-feeder devices, i.e. to the modem are not related.
So, in order to increase the communication range, you must choose a modem with a large value $ inline $ B_m $ inline $ . Zoom $ inline $ B_m $ inline $ in turn, by increasing $ inline $ P_ {TXdBm} $ inline $ or by reducing $ inline $ P_ {RXdBm} $ inline $ . In most cases, UAV developers are looking for a modem with high transmitter power and pay little attention to the receiver sensitivity, although you need to do exactly the opposite. A powerful onboard broadband modem transmitter entails the following problems:
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- high power consumption;
- the need for cooling;
- the deterioration of electromagnetic compatibility (EMC) with the rest of the onboard equipment of the UAV;
- low energy stealth.
The first two problems are related to the fact that modern methods of transmitting large amounts of information over a radio channel, for example OFDM, require a linear transmitter. The efficiency of modern linear radio transmitters is low: 10–30%. Thus, 70–90% of the precious energy of the power source of the UAV is converted into heat, which must be effectively removed from the modem, because otherwise it will fail or its output power will drop due to overheating at the most inopportune moment. For example, a 2 watt transmitter will consume 6–20 watts from a power source, of which 4–18 watts will be converted to heat.
Energy stealth radio is important for special and military applications. Low stealth means that the modem signal is relatively likely to be detected by the receiver of the jamming station. Accordingly, the probability of suppressing a radio with a low energy stealth is also high.
The sensitivity of a modem receiver is characterized by its ability to extract information from received signals with a given quality level. Quality criteria may vary. For digital communication systems, the probability of error per bit (bit error rate - BER) or the probability of error in the information package (frame error rate - FER) is most often used. Actually, sensitivity is the level of the very signal from which information should be extracted. For example, a sensitivity of −98 dBm with BER = 10 −6 indicates that information with such a BER can be extracted from a signal with a level of −98 dBm or higher, and no longer a signal from a level with, say, −99 dBm. Of course, the decrease in quality with a decrease in the signal level occurs gradually, but it is worth bearing in mind that most modern modems are inherent in the so-called. The threshold effect at which a decrease in quality with a decrease in the signal level below sensitivity occurs very quickly. It is enough to reduce the signal by 1−2 dB below the sensitivity so that the BER increases to 10 −1 , which means that you will not see video from the board of the BLAH. The threshold effect is a direct consequence of the Shannon theorem for a channel with noises; it cannot be eliminated. The destruction of information when the signal level drops below the sensitivity is due to the influence of noise, which is formed inside the receiver itself. The internal noise of the receiver cannot be completely eliminated, but it is possible to reduce its level or learn how to efficiently extract information from a noisy signal. Modem manufacturers use both of these approaches, making improvements to the receiver's RF units and improving digital signal processing algorithms. Improving the sensitivity of the receiver modem does not lead to such a dramatic increase in power consumption and heat dissipation, as an increase in transmitter power. There is, of course, an increase in energy consumption and heat generation, but it is rather modest.
We recommend the following algorithm for selecting a modem in terms of achieving the required communication range.
- To determine the magnitude of the data transfer rate.
- Choose the modem with the best sensitivity for the desired speed.
- Determine the range of communication by calculation or during the experiment.
- If the communication range is less than necessary, then try to use the following measures (arranged in order of decreasing priority):
- reduce antenna cable loss $ inline $ L_ {TXdB} $ inline $ , $ inline $ L_ {RXdB} $ inline $ by applying a cable with a lower attenuation at the operating frequency and / or reducing the length of the cables;
- increase antenna gain $ inline $ G_ {TXdB} $ inline $ , $ inline $ G_ {RXdB} $ inline $ ;
- increase the power of the modem transmitter.
The sensitivity values depend on the data transfer rate according to the rule: higher speed - worse sensitivity. For example, a sensitivity of −98 dBm for a speed of 8 Mbps is better than a sensitivity of −95 dBm for a speed of 12 Mbps. You can only compare modems for sensitivity at the same data rate.
Data on transmitter power is almost always available in the specifications of modems, but data on the sensitivity of the receiver is not always or in insufficient volume. At a minimum, this is a reason to be wary, because the beautiful numbers hardly make sense to hide. In addition, by not publishing sensitivity data, the manufacturer deprives the consumer of the opportunity to estimate the communication range by calculation before purchasing a modem.
Maximum data transfer rate
The choice of modem for this parameter is relatively simple, if the speed requirements are clearly defined. But there are some nuances.
If the problem being solved requires ensuring the greatest possible range of communication and at the same time it is possible to allocate a sufficiently wide frequency band for a radio link, then it is better to choose a modem that supports a wide frequency band (bandwidth). The fact is that the required information speed can be provided in a relatively narrow frequency band by using dense modulation types (16QAM, 64QAM, 256QAM, etc.), or in a wide frequency band by using low density modulation (BPSK, QPSK ). The use of low density modulation for such tasks is preferable due to the higher noise immunity. Therefore, the sensitivity of the receiver turns out better, respectively, increases the energy budget of the modem and, as a consequence, the communication range.
Sometimes, manufacturers of UAVs set the information speed of a radio link much higher than the speed of the source, literally 2 or more times, arguing that sources such as video codecs have a variable bit rate and the modem speed should be selected taking into account the maximum bitrate emissions. The communication range is naturally reduced. Do not use this approach without extreme necessity. Most modern modems have a capacious buffer in the transmitter that can smooth out bit-rate emissions without packet loss. Therefore, a margin of more than 25% is not required. If there is reason to believe that the buffer capacity in the purchased modem is insufficient and a significantly greater increase in speed is required, then it is better to refuse to buy such a modem.
Data transfer delay
When evaluating this parameter, it is important to separate the delay related to the transmission of data via radio link from the delay created by the source encoding / decoding device, such as a video codec. The delay in the radio link consists of 3 quantities.
- Delay due to signal processing in transmitter and receiver.
- The delay due to the propagation of the signal from the transmitter to the receiver.
- Delay due to data buffering in the transmitter in time division duplex modems (TDD - time division duplex).
According to the author’s experience, the type 1 delay is in the range from tens of microseconds to one millisecond. Type 2 delay depends on the communication range, for example, for a 100 km link it is equal to 333 µs. Type 3 latency depends on the TDD frame length and on the ratio of the transmission cycle length to the total frame duration and can vary from 0 to frame duration, i.e. it is a random value. If the transmitted information packet is at the transmitter input when the modem is in the transmission cycle, the packet will be transmitted with type 3 zero delay. If the packet is slightly late and the reception cycle has already started, it will be delayed in the transmitter buffer for the duration of the reception cycle . Typical TDD frame lengths range from 2 to 20 ms, respectively, the type 3 delay in the worst case does not exceed 20 ms. Thus, the total delay in the radio link will be within 3−21 ms.
The best way to know the delay in a radio link is a full-scale experiment using utilities to evaluate network performance. It is not recommended to measure the delay by the request – response method, since the delay in the forward and reverse directions may be different for TDD modems.
Mass-dimensional parameters
The choice of onboard modem unit by this criterion does not require special comments: the smaller and lighter, the better. Do not forget also about the need to cool the onboard unit, additional radiators may be required, respectively, the weight and dimensions may also increase. Preference here should be given to light, small-sized blocks with low power consumption.
For the ground block, mass-dimensional parameters are not so critical. At the forefront ease of use and installation. The ground unit must be a device that is reliably protected from external influences and has a convenient system for fastening to a mast or a tripod. A good option when the ground unit is integrated in the same package with the antenna. Ideally, the ground unit should be connected to the control system through one convenient connector. This will save you from strong words when you need to carry out work on the deployment at a temperature of −20 degrees.
Power Requirements
On-board units, as a rule, are released with the support of a wide range of supply voltages, for example, 7–30 V, which covers most of the voltage options in the UAV energy network. If you have a choice of several supply voltages, then give preference to the lowest value of the supply voltage. As a rule, internal modem power is supplied from 3.3 and 5.0 V voltages via secondary power supplies. The efficiency of these secondary power sources is the higher, the smaller the difference between the input and internal voltage of the modem. Increased efficiency means reduced power consumption and heat dissipation.
On the other hand, ground blocks must maintain power from a source with relatively high voltage. This allows the use of a power cable with a small cross-section, which reduces the weight and simplifies installation. With other things being equal, give preference to ground blocks with PoE (Power over Ethernet) support. In this case, to connect the ground unit with the control station will need only one Ethernet cable.
Separate control / telemetry channel
An important opportunity in those cases when there is no space left for the UAV to install a separate command-telemetric modem. If there is space, a separate broadband modem control / telemetry channel can be used as a backup. When choosing a modem with this option, pay attention to that the modem supports the desired protocol for communication with UAVs (MAVLink or proprietary) and the possibility of multiplexing control / telemetry channel data into a convenient interface at a ground station (NS). For example, the onboard unit of the broadband modem is connected to the autopilot via an RS232, UART or CAN interface, and the ground unit is connected to the control computer via the Ethernet interface through which it is necessary to exchange command-telemetry and video information. In this case, the modem should be able to multiplex the command-telemetric flow between the RS232, UART or CAN interfaces of the onboard unit and the Ethernet interface of the ground unit.
Other options that need attention
The presence of duplex mode. Broadband modems for UAVs support either simplex or duplex modes of operation. In the simplex mode, data transmission is allowed only in the direction from the UAV to the National Assembly, and in the duplex mode - in both directions. As a rule, simplex modems have a built-in video codec and are designed to work with video cameras that do not have a video codec. To connect to an IP camera or to any other devices that require an IP connection, the simplex modem is not suitable. In contrast, a duplex modem is usually designed to connect the onboard IP network of the UAV with the NS network of the NS, i.e., it supports IP cameras and other IP devices, but may not have a built-in video codec, since IP cameras typically have your video codec. Ethernet interface support is possible only in duplex modems.
RX diversity. The presence of this feature is mandatory to ensure uninterrupted communication throughout the flight distance. When propagating above the Earth’s surface, radio waves arrive at the reception point by two rays: in a straight path and with reflection from the surface. If the addition of waves of two rays occurs in phase, then the field at the point of reception increases, and if in antiphase, it is weakened. Attenuation can be very significant - up to the complete loss of communication. The presence of two antennas on the NA, which are located at different heights, helps to solve this problem, because if at the location of one antenna the rays are in antiphase, then at the location of the other there is none. As a result, you can achieve a stable connection throughout the course.
Supported network topologies. It is advisable to choose a modem that provides support not only for point-to-point (PTP) topology, but also for point-to-multipoint (PMP) and relay (repeater) topologies. The use of relaying through an additional UAV allows you to significantly expand the range of the main UAV. PMP support will allow receiving information simultaneously from several UAVs on one NS. Please also note that support for PMP and retransmission will require an increase in modem throughput compared to the case of communication with one UAV. Therefore, for these modes it is recommended to choose a modem with support for a wide frequency band (at least 15-20 MHz).
Availability of noise immunity enhancement tools. Useful option, given the tense interference situation in the places of use of the UAV. Interference immunity refers to the ability of a communication system to perform its function when there is interference of artificial or natural origin in the communication channel. To combat interference, there are two approaches. Approach 1: design the modem receiver so that it can confidently receive information even if there is interference in the communication channel band at the cost of some reduction in the information transfer rate. Approach 2: suppress or mitigate interference at the receiver input. Examples of the implementation of the first approach are spread spectrum systems, namely: frequency hopping (FH), spectrum expansion with a pseudo-random sequence (DSSS) or their hybrid. FH technology is widely used in UAV control channels due to the small amount of required data transfer rate in such a communication channel. For example, for a speed of 16 kbps in the 20 MHz band, it is possible to organize about 500 frequency positions, which allows you to reliably protect against narrowband interference. The use of FH for a broadband communication channel is problematic because of the resulting large bandwidth. For example, to get 500 frequency positions when working with a signal with a 4 MHz bandwidth, you will need 2 GHz of free band! Too much to be a reality. The use of DSSS for a broadband UAV link is more relevant. In this technology, each information bit is duplicated simultaneously on several (or even on all) frequencies in the signal band and, in the presence of narrowband interference, can be isolated from the spectral regions not affected by interference. The use of DSSS, as well as FH, implies that if a channel is interfered with, a data transfer rate will be required to decrease. Nevertheless, it is obvious that it is better to receive video from the side of the UAV in a lower resolution than nothing at all. Approach 2 uses the fact that the interference, unlike the internal noise of the receiver, enters the radio link from the outside and, if there are certain means in the modem, can be suppressed. Interference suppression is possible if it is localized in the spectral, temporal or spatial regions. For example, a narrowband interference is localized in the spectral region and can be “cut out” from the spectrum using a special filter. Similarly, the impulse noise is localized in the time domain, to suppress it, the affected area is removed from the receiver input signal. If the interference is not narrowband or impulse, then a spatial suppressor can be used to suppress it, since the interference gets into the receiving antenna from the source from a certain direction. If the zero of the receiving antenna pattern is located in the direction of the interferer, the interference will be suppressed. Such systems are called adaptive beamforming control (adaptive beamforming & beam nulling) systems. In the well-known broadband modems for UAVs, such systems are not used, although nothing prevents their occurrence in the future.
Used radio protocol. Modem manufacturers can use a standard (WiFi, DVB-T) or a proprietary radio protocol. This parameter is rarely indicated in the specifications. The use of DVB-T is indirectly indicated by the supported frequency bands 2/4/6/7/8, sometimes 10 MHz, and the mention in the text of the specification of coded OFDM technology in COFDM in which OFDM is used in conjunction with noise-resistant coding. At the same time, we note that COFDM is a purely advertising slogan and does not have any advantages over OFDM, since OFDM without noise-resistant coding is never used in practice. Equal COFDM and OFDM when you see these abbreviations in the radio modem specifications.
Modems that use a standard protocol, as a rule, are based on a specialized chip (WiFi, DVB-T), working in conjunction with a microprocessor. The use of a specialized chip removes from the modem manufacturer a lot of headaches associated with the development, simulation, implementation and testing of its own radio protocol. The microprocessor is used to give the modem the necessary functionality. Such modems have the following advantages.
- Low price.
- Good mass-dimensional parameters.
- Low power consumption.
Disadvantages are also available.
- The inability to change the characteristics of the radio interface by changing the firmware.
- Low stability of supply in the long term.
- Limited opportunities in providing qualified technical support in solving non-standard tasks.
The low stability of supply is due to the fact that chip manufacturers focus primarily on mass markets (TVs, computers, etc.). Modem manufacturers for UAVs are not a priority for them and they cannot in any way influence the decision of the chip manufacturer to discontinue production without adequate replacement for another product. This feature enhances the tendency to package radio interfaces into specialized chips of the System on Chip type (System on Chip), which is why individual radio interface chips are gradually being washed out of the semiconductor market.
Limited opportunities in the provision of technical support are due to the fact that the development teams of modems based on the standard radio protocol are well-equipped with specialists primarily in electronics and microwave engineering. There are no radio communication specialists at all, since there are no tasks for them to be solved. Therefore, UAV manufacturers looking for solutions to non-trivial radio communication tasks may be disappointed in terms of consultation and technical assistance.
Modems using a proprietary radio protocol are built on the basis of universal analog and digital signal processing chips. The stability of the supply of such chips is very high. True, the price is also high. Such modems have the following advantages.
- Wide possibilities of adapting the modem to the needs of the customer, including the adaptation of the radio interface by changing the firmware.
- Additional features of the radio interface, interesting for use in UAVs and not in modems built on the basis of standard radio protocols.
- High stability of supply, incl. in the long run.
- High level of technical support, including non-standard tasks.
Disadvantages.
- High price.
- Mass-dimensional parameters may be worse than that of modems on standard radio protocols.
- Increased power consumption of the digital signal processing unit.
Technical data of some UAV modems
The table shows the technical parameters of some UAV modems available on the market.
Please note that although the 3D Link modem has the lowest transmitter power compared to the Picoradio OEM and J11 modems (25 dBm vs. 27-30 dBm), the 3D Link power budget is higher than those of these modems, due to the high sensitivity of the receiver (with the same data transfer rate of the modems being compared). Thus, the communication distance when using 3D Link will be longer with better energy stealth.
Table. Specifications of some broadband modems for UAV and robotics
| Parameter | 3D Link | Skyhopper pro | Picoradio OEM (done on Microhard pDDL2450 module) | SOLO7 (see also SOLO7 receiver ) | J11 |
|---|---|---|---|---|---|
| Manufacturer, Country | Geoscan, RF | Mobilicom, Israel | Airborne Innovations, Canada | DTC, United Kingdom | Redess, China |
| Communication range [km] | 20−60 | five | n / a * | n / a * | 10-20 |
| Speed [Mb / sec] | 0.023−64.9 | 1.6-6 | 0.78–28 | 0.144–31.668 | 1.5−6 |
| Data transfer delay [ms] | 1-20 | 25 | n / a * | 15−100 | 15-30 |
| Dimensions of the onboard DhShhV block [mm] | 77x45x25 | 74x54x26 | 40x40x10 (without body) | 67x68x22 | 76x48x20 |
| Airborne block weight [gram] | 89 | 105 | 17.6 (without case) | 135 | 88 |
| Information Interfaces | Ethernet, RS232, CAN, USB | Ethernet, RS232, USB (optional) | Ethernet, RS232 / UART | HDMI, AV, RS232, USB | HDMI, Ethernet, UART |
| Power onboard unit [Volt / Watt] | 7-30 / 6.7 | 7−26 / n / d * | 5−58 / 4.8 | 5.9−17.8 / 4.5−7 | 7-18 / 8 |
| Ground power supply [Volt / Watt] | 18−75 or PoE / 7 | 7−26 / n / d * | 5−58 / 4.8 | 6-16 / 8 | 7-18 / 5 |
| Transmitter power [dBm] | 25 | n / a * | 27-30 | 20 | thirty |
| Receiver sensitivity [dBm] (for speed [Mbps]) | −122 (0.023) −101 (4.06) −95.1 (12.18) −78.6 (64.96) | −101 (n / a *) | −101 (0.78) −96 (3.00) −76 (28.0) | −95 (n / d *) −104 (n / d *) | −97 (1.5) −94 (3.0) −90 (6.0) |
| Modem power budget [dB] (for speed [Mbit / s]) | 147 (0.023) 126 (4.06) 120.1 (12.18) 103.6 (64.96) | n / a * | 131 (0.78) 126 (3.00) 103 (28.0) | n / a * | 127 (1.5) 124 (3.0) 120 (6.0) |
| Supported frequency bands [MHz] | 4-20 | 4.5; 8.5 | 2; four; eight | 0.625; 1.25; 2.5; 6; 7; eight | 2; four; eight |
| Simplex / duplex | Duplex | Duplex | Duplex | Simplex | Duplex |
| Diversity Reception Support | Yes | Yes | Yes | Yes | Yes |
| Separate channel for control / telemetry | Yes | Yes | Yes | not | Yes |
| Supported UAV control protocols in control / telemetry channel | MAVLink, proprietary | MAVLink, proprietary | not | not | MAVLink |
| Control / telemetry multiplexing support | Yes | Yes | not | not | n / a * |
| Network topologies | PTP, PMP, relay | PTP, PMP, relay | PTP, PMP, relay | PTP | PTP, PMP, relay |
| Noise Enhancements | DSSS, narrowband and impulse noise suppressors | n / a * | n / a * | n / a * | n / a * |
| Radio protocol | proprietary | n / a * | n / a * | DVB-T | n / a * |
about the author
Alexander Smorodinov [a.smorodinov@geoscan.aero] is a leading specialist of Geoscan LLC in the field of wireless communications. From 2011 to the present, he has been developing radio protocols and signal processing algorithms for wideband radio modems for various purposes, as well as the implementation of the developed algorithms based on programmable logic chips. The author’s interests include the development of synchronization algorithms, channel properties estimation, modulation / demodulation, noise-resistant coding, as well as some algorithms of the medium access level (MAC). Before joining Geoscan, the author worked in various organizations, developing non-standard wireless communication devices. From 2002 to 2007, he worked at Protey LLC as a leading specialist in the development of communication systems based on the IEEE802.16 (WiMAX) standard.From 1999 to 2002, the author was engaged in the development of noise-tolerant coding algorithms and modeling of radio link paths at the FSUE Central Research Institute "Granit". The author received a Ph.D. in technical sciences from St. Petersburg University of Aerospace Instrumentation in 1998 and a radio engineer degree from the same university in 1995. Alexander is an active member of the IEEE and the IEEE Communications Society.
Source: https://habr.com/ru/post/444898/
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