The task of transmitting a large amount of data from an unmanned aerial vehicle (UAV) or ground robotics is not uncommon in modern applications. This article discusses broadband modem selection criteria and related issues. The article is written for developers of UAVs and robotics.
Criterias of choice
The main criteria for choosing a broadband modem for UAVs or robotics are.
- Communication range.
- Maximum data transfer rate.
- Delay in data transfer.
- Mass-dimensional parameters.
- Supported information interfaces.
- Nutrition requirements.
- Separate control/telemetry channel.
Communication range
The communication range depends not only on the modem, but also on antennas, antenna cables, radio wave propagation conditions, external interference and other reasons. In order to separate the parameters of the modem itself from other parameters that affect the communication range, consider the range equation [Kalinin A.I., Cherenkova E.L. Propagation of radio waves and the operation of radio links. 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$inline$ — required 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 multiplier that takes into account additional losses due to the influence of the Earth's surface, vegetation, atmosphere and other factors in dB.
It can be seen from the range equation that the range depends only on two modem parameters: transmitter power $inline$P_{TXdBm}$inline$ and receiver sensitivity $inline$P_{RXdBm}$inline$, or rather, on 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. have nothing to do with the modem.
So, in order to increase the communication range, it is necessary to choose a modem with a large $inline$B_m$inline$ value. In turn, $inline$B_m$inline$ can be increased by increasing $inline$P_{TXdBm}$inline$ or by decreasing $inline$P_{RXdBm}$inline$. In most cases, UAV designers are looking for a modem with a high transmitter power and pay little attention to the sensitivity of the receiver, although the exact opposite should be done. A powerful broadband modem on-board transmitter causes the following problems:
- high power consumption;
- the need for cooling;
- 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, such as OFDM, require linear transmitter. The efficiency of modern linear radio transmitters is low: 10–30%. Thus, 70-90% of the precious energy of the UAV power supply is converted into heat, which must be efficiently 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 2W transmitter will draw 6-20W from the power supply, of which 4-18W will be converted to heat.
The energy stealth of the radio link is important for special and military applications. Low stealth means that the modem signal is relatively likely to be detected by the jamming station's reconnaissance receiver. Accordingly, the probability of suppressing a radio link with low energy stealth is also high.
The sensitivity of a modem receiver characterizes its ability to extract information from received signals with a given quality level. Quality criteria may vary. For digital communication systems, the bit error rate (BER) or the probability of an error in the information packet (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 at BER=10−6 indicates that information with this BER can be extracted from a signal with a level of -98 dBm or higher, but no more information can be extracted from a signal with a level of, say, -99 dBm. Of course, the decrease in quality with a decrease in the signal level occurs gradually, but it should be borne in mind that most modern modems are inherent in the so-called. threshold effect at which the decrease in quality when the signal level decreases 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 no longer see the video from the UAV. The threshold effect is a direct consequence of Shannon's theorem for a noisy channel and cannot be eliminated. The destruction of information when the signal level drops below sensitivity occurs 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 effectively extract information from a noisy signal. Modem manufacturers are using both of these approaches, making improvements in the receiver's RF blocks and refining digital signal processing algorithms. Improving the modem receiver sensitivity does not lead to such a dramatic increase in power consumption and heat dissipation as an increase in transmitter power. Of course, there is an increase in energy consumption and heat release, but it is rather modest.
The following modem selection algorithm is recommended in terms of achieving the required communication range.
- Decide on the value of the data transfer rate.
- Select a modem with the best sensitivity for the required speed.
- Determine the communication range 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 losses in antenna cables $inline$L_{TXdB}$inline$, $inline$L_{RXdB}$inline$ by using a cable with lower linear attenuation at the operating frequency and/or reducing the cable length;
- increase antenna gain $inline$G_{TXdB}$inline$, $inline$G_{RXdB}$inline$;
- increase the power of the modem transmitter.
Sensitivity values depend on the data transfer rate according to the rule: higher speed - worse sensitivity. For example, -98 dBm sensitivity at 8 Mbps is better than -95 dBm sensitivity at 12 Mbps. You can compare modems by sensitivity only for the same data transfer rate.
Data on transmitter power is almost always available in modem specifications, but data on receiver sensitivity is far from always or in insufficient volume. At the very least, this is a reason to be wary, because there is hardly any point in hiding beautiful numbers. In addition, by not publishing sensitivity data, the manufacturer deprives the consumer of the opportunity to estimate the communication range by calculation. to buying a modem.
Maximum baud rate
Selecting a modem by this parameter is relatively simple if the speed requirements are clearly defined. But there are some nuances.
If the task to be solved requires ensuring the maximum possible communication range and at the same time it is possible to allocate a sufficiently wide frequency band for the radio link, then it is better to choose a modem that supports a wide frequency band (bandwidth). The fact is that the required information rate 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 because of the higher noise immunity. Therefore, the sensitivity of the receiver is better, respectively, the energy budget of the modem increases and, as a result, the communication range.
Sometimes UAV manufacturers set the information rate of the radio link to be much higher than the source rate, 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 bit rate spikes. The communication range, of course, decreases in this case. Do not use this approach unless absolutely necessary. Most modern modems have a capacious transmitter buffer capable of smoothing bitrate spikes without packet loss. Therefore, a speed margin of more than 25% is not required. If there is reason to believe that the buffer capacity in the modem you are purchasing is insufficient and a significantly greater increase in speed is required, then it is better to refuse to purchase such a modem.
Delay in data transmission
When evaluating this parameter, it is important to separate the delay associated with data transmission over the radio link from the delay created by the encoding/decoding device of the information source, such as a video codec. The delay in the radio link consists of 3 values.
- Delay due to signal processing in the transmitter and receiver.
- Delay due to signal propagation from transmitter to receiver.
- Delay due to data buffering in the transmitter in time division duplex (TDD) modems.
Type 1 delay, in the author's experience, ranges from tens of microseconds to one millisecond. Type 2 delay depends on the communication range, for example, for a 100 km link it is 333 µs. Type 3 delay depends on the length of the TDD frame and on the ratio of the duration of the transmission cycle to the total duration of the frame and can vary from 0 to the duration of the frame, i.e. it is a random value. If the transmitted information packet is at the transmitter input at the moment the modem is in the transmission cycle, then the packet will be transmitted on the air with a type 3 zero delay. If the packet is a little late and the receive cycle has already begun, then it will be delayed in the transmitter buffer for the duration of the receive cycle . Typical values for TDD frame lengths are from 2 to 20 ms, respectively, the type 3 delay in the worst case will not exceed 20 ms. Thus, the total delay in the radio link will be in the range of 3–21 ms.
The best way to find out the delay in a radio link is a field experiment using network performance evaluation utilities. It is not recommended to measure latency using the request-response method, since the delay in the forward and reverse directions may not be the same for TDD modems.
Weight and dimensions
The choice of the onboard unit of the modem according to 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 units with low power consumption.
For a ground unit, the weight and size parameters are not so critical. Ease of use and installation comes to the fore. The ground unit should be a device reliably protected from external influences with a convenient system for attaching to a mast or tripod. A good option is when the ground unit is integrated in the same housing with the antenna. Ideally, the ground unit should be connected to the control system through one convenient connector. This will save you the hard words when you need to carry out deployment work at -20 degrees.
Nutrition Requirements
Onboard units, as a rule, are produced with support for a wide range of supply voltages, for example, 7–30 V, which covers most of the voltage options in the UAV power network. If you have a choice of several supply voltages, then give preference to the lowest value of the supply voltage. As a rule, modems are internally powered from 3.3 and 5.0 V voltages through secondary power supplies. The efficiency of these secondary power supplies is the higher, the smaller the difference between the input and internal voltage of the modem. Increased efficiency means less power consumption and less heat.
Ground units, in contrast, must be powered by a relatively high voltage source. This allows the use of a power cable with a small cross section, which reduces weight and simplifies installation. Other things being equal, give preference to ground units with PoE (Power over Ethernet) support. In this case, only one Ethernet cable is required to connect the ground unit to the control station.
Separate control/telemetry channel
An important feature in cases where there is no space left on the UAV for installing a separate command-telemetry modem. If there is room, then a separate control/telemetry channel of the broadband modem can be used as a backup. When choosing a modem with this option, please note that the modem supports the desired protocol for communication with the UAV (MAVLink or proprietary) and the ability to multiplex control/telemetry data into a convenient ground station (GS) interface. For example, the broadband modem on-board unit is connected to the autopilot via an interface such as RS232, UART or CAN, and the ground unit is connected to the control computer via an Ethernet interface through which it is necessary to exchange command-telemetry and video information. In this case, the modem must be able to multiplex the command-telemetry stream between the RS232, UART or CAN interfaces of the on-board unit and the Ethernet interface of the ground unit.
Other parameters to pay attention to
The presence of duplex mode. Broadband modems for UAVs support either simplex or duplex modes of operation. In simplex mode, data transmission is allowed only in the direction from the UAV to the NS, and in duplex mode, in both directions. As a rule, simplex modems have a built-in video codec and are designed to work with cameras that do not have a video codec. To connect to an IP camera or any other device that requires an IP connection, a simplex modem is not suitable. On the contrary, a duplex modem, as a rule, is designed to connect the onboard IP network of the UAV with the IP network of the HC, i.e. it supports IP cameras and other IP devices, but may not have a built-in video codec, since IP video cameras, as a rule, have your video codec. Support for the Ethernet interface is possible only in duplex modems.
Diversity reception (RX diversity). The presence of this possibility is mandatory to ensure continuous communication throughout the flight distance. When propagating above the Earth's surface, radio waves arrive at the receiving point in two beams: along a direct path and with reflection from the surface. If the addition of the waves of two rays occurs in phase, then the field at the receiving point is amplified, and if in antiphase, then it is weakened. The weakening can be quite significant - up to the complete loss of communication. The presence on the NS of two antennas located at different heights helps to solve this problem, because if at the location of one antenna the rays are added in antiphase, then at the location of the other they are not. As a result, you can achieve a stable connection throughout the distance.
Supported network topologies. It is advisable to choose a modem that provides support not only for point-to-point topologies (point-to-point - PTP), but also point-to-multipoint topologies (point-to-multipoint - PMP) and relay (relay, repeater). The use of relaying through an additional UAV allows you to significantly expand the coverage area of the main UAV. Support for PMP will allow you to receive information simultaneously from several UAVs to one NS. Please also note that PMP and relay support will require an increase in modem bandwidth compared to a case of communication with a single 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 means to increase noise immunity. A useful option, given the tense interference environment in the places where UAVs are used. Immunity is understood as the ability of a communication system to perform its function in the presence of artificial or natural interference in the communication channel. There are two approaches to dealing with interference. Approach 1: design the modem receiver so that it can confidently receive information even in the presence of interference in the communication channel band at the cost of some reduction in the information transfer rate. Approach 2: suppress or attenuate the interference at the receiver input. Examples of the implementation of the first approach are spread spectrum systems, namely: frequency hopping (FH), pseudo-random sequence spread spectrum (DSSS) or their hybrid. FH technology has become widespread 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 a 20 MHz band, about 500 frequency positions can be organized, which makes it possible to reliably protect against narrow-band interference. The use of FH for a broadband communication channel is problematic due to the resulting too large bandwidth. For example, to get 500 frequency positions on a 4 MHz signal, you need 2 GHz of free bandwidth! Too many to be real. The use of DSSS for a broadband communication channel with UAVs is more relevant. In this technology, each information bit is duplicated simultaneously at several (or even at all) frequencies in the signal band and, in the presence of narrow-band interference, can be separated from parts of the spectrum not affected by interference. The use of DSSS, as well as FH, implies that when interference occurs in the channel, a decrease in the data rate will be required. Nevertheless, it is obvious that it is better to receive video from the UAV in a lower resolution than nothing at all. Approach 2 uses the fact that interference, unlike the internal noise of the receiver, enters the radio link from the outside and, if certain means are included in the modem, can be suppressed. Interference suppression is possible if it is localized in the spectral, temporal or spatial domains. For example, narrow-band interference is localized in the spectral region and can be "cut out" from the spectrum using a special filter. Similarly, impulse noise is localized in the time domain; to suppress it, the affected area is removed from the input signal of the receiver. If the interference is not narrowband or impulse, then a spatial suppressor can be used to suppress it, since interference from a source from a certain direction enters the receiving antenna. If you place the zero of the receiving antenna in the direction of the interference source, then the interference will be suppressed. Such systems are called adaptive beamforming & beam nulling systems.
The radio protocol used. Modem manufacturers can use a standard (WiFi, DVB-T) or proprietary radio protocol. This parameter is rarely specified 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 COFDM (coded OFDM) technology specification in which OFDM is used in conjunction with error-correcting coding. In passing, we note that COFDM is purely an advertising slogan and does not have any advantages over OFDM, since OFDM without error-correcting coding is never used in practice. Equalize COFDM and OFDM when you see these abbreviations in radio modem specifications.
Modems using a standard protocol are usually built on the basis of a specialized chip (WiFi, DVB-T) that works in conjunction with a microprocessor. The use of a specialized chip relieves a modem manufacturer of a lot of headaches associated with the development, modeling, 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 weight and size parameters.
- Low power consumption.
There are also disadvantages.
- The inability to change the characteristics of the radio interface by changing the firmware.
- Low stability of supply in the long term.
- Limited opportunities to provide qualified technical support when solving non-standard tasks.
The low stability of supplies is due to the fact that chip manufacturers are oriented primarily to mass markets (TVs, computers, etc.). Manufacturers of modems 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 an adequate replacement for another product. This feature is strengthened by the trend of packing air interfaces into specialized chips such as "system on chip" (SoC), in connection with which individual air interface chips are gradually being washed out of the semiconductor market.
Limited opportunities in providing technical support are due to the fact that the development teams of modems based on the standard radio protocol are well staffed with specialists primarily in electronics and microwave technology. There may not be radio communication specialists there at all, because for them there are no tasks that need to be solved. Therefore, UAV manufacturers looking for solutions to non-trivial radio communication problems may find themselves disappointed in terms of advice and technical assistance.
Modems using a proprietary radio protocol are based on 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 range of options for adapting the modem to the needs of the customer, including adapting the radio interface by changing the firmware.
- Additional features of the radio interface that are interesting for use in UAVs and are not available in modems based on standard radio protocols.
- High stability of supplies, incl. in the long run.
- High level of technical support, including solving non-standard tasks.
Disadvantages.
- High price.
- The weight and size parameters may be worse than those of modems based 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.
Note that although the 3D Link modem has the lowest transmit power compared to the Picoradio OEM and J11 modems (25 dBm vs. the same data transfer rate for the compared modems). Thus, the communication range when using 27D Link will be longer with better energy stealth.
Table. Technical data of some broadband modems for UAVs and robotics
Parameter
(performed on the module by Microhard)
(see also )
Manufacturer, country
Geoscan, RF
Mobilicom, Israel
Airborne Innovations, Canada
DTC, UK
Redess, China
Communication range [km] 20−60
5
n/a*
n/a*
10 − 20
Speed [Mbps] 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
Onboard unit dimensions LxWxH [mm] 77x45x25
74h54h26
40x40x10 (without case)
67h68h22
76h48h20
Onboard unit weight [grams] 89
105
17.6 (without case)
135
88
Information interfaces
Ethernet, RS232, CAN, USB
Ethernet, RS232, USB (option)
Ethernet, RS232/UART
HDMI, AV, RS232, USB
HDMI, Ethernet, UART
Onboard unit power supply [Volt/Watt] 7-30/6.7
7-26/n/a*
5-58/4.8
5.9-17.8/4.5-7
7-18/8
Ground Unit Power [Volt/Watt] 18-75 or PoE/7
7-26/n/a*
5-58/4.8
6-16/8
7-18/5
Transmitter power [dBm] 25
n/a*
27 − 30
20
30
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/a*) −104(n/a*)
−97(1.5) −94(3.0) −90(6.0)
Modem energy budget [dB] (for speed [Mbps])
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; 4; 8
0.625; 1.25; 2.5; 6; 7; 8
2; 4; 8
Simplex/Duplex
Duplex
Duplex
Duplex
Simplex
Duplex
Diversity support
Yes
Yes
Yes
Yes
Yes
Separate channel for control/telemetry
Yes
Yes
Yes
no
Yes
Supported UAV control protocols in the control/telemetry channel
MAVLink, proprietary
MAVLink, proprietary
no
no
MAVLink
Support for multiplexing in the control/telemetry channel
Yes
Yes
no
no
n/a*
Network topologies
PTP, PMP, relay
PTP, PMP, relay
PTP, PMP, relay
PTP
PTP, PMP, relay
Means to improve noise immunity
DSSS, Narrowband and Transient Suppressors
n/a*
n/a*
n/a*
n/a*
radio protocol
proprietary
n/a*
n/a*
DVB-T
n/a*
* n/a - no data.
About the Developer
Alexander Smorodinov [[email protected]] 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 broadband radio modems for various purposes, as well as implementing the developed algorithms based on programmable logic chips. The author's area of interest includes the development of algorithms for synchronization, channel property estimation, modulation/demodulation, error-correcting coding, as well as some medium access level (MAC) algorithms. Prior to 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-correcting coding algorithms and modeling of radio link paths at the Federal State Unitary Enterprise Central Research Institute "Granit". The author received a Ph.D. in Engineering from St. Petersburg University of Aerospace Instrumentation in 1998 and a radio engineering degree from the same university in 1995. Alexander is an active member of the IEEE and IEEE Communications Society.
Source: habr.com