Application of OFDM in short wave communication
Satellite communication and short wave (1.5 ~ 30MHz) communication are the two main means of long-distance communication. For military communications, satellites are susceptible to interference or blockage during wars, and even destroyed to lose communication capabilities. Therefore, in terms of communication's stubbornness, mobility and flexibility, short-wave communication has unparalleled advantages . The transmission power is small, the equipment is simple, the communication method is flexible, and the invulnerability is strong. The ionosphere is used as the transmission medium, and the ionosphere is basically indestructible. The transmission distance can reach thousands of companies without forwarding. These advantages make short-wave communications an important tool for long-range communications and command in military departments and other agencies. In addition, short-wave communication plays an important role in maritime communication and airborne communication. Submarines, surface warships, ocean-going merchant ships, fishing vessels and scientific research fleets are usually equipped with short-wave radio stations to establish communication links with the outside world. Moreover, maritime communication requires higher and higher data transmission speeds, effectively launching the development of maritime short-wave communication technology. Airborne short-wave and ultra-short-wave communication is an important means of aviation communication, especially when the aircraft is to carry out low-altitude, over-the-horizon and long-range communication, but lacks modern early warning aircraft and airborne satellite communication systems, airborne short and ultra-short wave communication has become the only Communication channels.
Shortwave communication terminal design
1 Modulation technology for transmitting high-speed data signals in short-wave communications
Shortwave transmission is divided into sky wave and ground wave. For the sky wave transmission method, the short wave channel is a time-varying dispersion channel, which uses the reflection of the ionosphere to transmit information. Since the ionosphere is a layered, non-uniform, anisotropic, random, and space-time medium, the short-wave channel has a multipath delay, fading, and space-time medium, so the short-wave channel has multipath delay, fading, A series of complex phenomena such as Doppler frequency shift, frequency shift diffusion, white noise with approximate Gaussian distribution and radio interference. In addition, for modern shortwave communication systems, most of the channels have frequency selectivity. Multipath transmission produces coherent fading of signals and symbol interference. The performance of shortwave communication depends largely on the effect of system design on channel transmission compensation. The short-wave channel is usually a slowly changing channel, with a typical multipath delay of 2 to 8 ms, a typical value of Doppler frequency expansion of 0.1 Hz, and a Doppler frequency shift of 0.01 to 10 Hz, in high latitudes. Multipath delay can reach more than 13ms, and Doppler expansion can reach 73Hz.
The time-domain expansion caused by the multipath effect is the main factor limiting the data communication rate. At present, there are two systems of parallel system and serial system in short wave communication. The parallel system is to distribute the transmitted data in parallel to multiple sub-channels for transmission, and the serial system uses single carrier modulation to send information. There has been controversy about the superiority of the serial and parallel modulation methods. The document believes that these two modems have been used in low-speed communication for many years, and none of them have shown absolute advantages. At present, serial and parallel modulation systems are considered in the NATO 9.6kbs HF communication standard. The vast majority believe that the serial system is more advantageous. If the bit error rate of the two is compared under the same pass rate, then the serial system is lower than the parallel system.
The characteristic of the serial system is to use a single carrier to serially transmit high-speed data signals within a voice channel bandwidth, thus improving the power utilization of the high-frequency transmitter and overcoming the shortcomings of parallel system power dispersion. Since the serial system adopts efficient adaptive equalization, sequence detection and channel estimation and other combined technologies, it can overcome the symbol interference (ISI) caused by multipath propagation and channel distortion. At present, the most advanced serial system modem adopts 256QAM modulation and applies a technology called "packet decision feedback equalization (BDFE)". The data transmission rate reaches 16 kbps on a 3 kHz bandwidth.
The parallel system has been in existence for decades. Before the mid-1990s, the subcarriers of the parallel system did not overlap with each other in frequency. The orthogonal frequency division multiplexing (OFDM) technology was not used. The three-generation military standards MIL-STD-188-141B and MIL-STD-188-110B define two modes of 16-tone and 39-tone in the parallel modulation mode, and the subcarriers do not intersect.
OFDM (OrthogONal Frequency Division MulTIplexing) is a special multi-carrier transmission method. Due to the orthogonality between the sub-carriers, the spectrum of the sub-channels is allowed to overlap with each other. Compared with the conventional frequency division multiplexing system, OFDM can maximize Limit the use of frequency spectrum resources. At the same time, it converts high-speed data through serial, so that the duration of the data symbol on each subcarrier is relatively increased, which reduces the information rate of the subchannel, and converts the frequency-selective fading channel into a flat fading channel, which has good anti-noise, The ability to resist multipath interference is suitable for high-speed data transmission in frequency-selective fading channels. Introducing cyclic prefix in OFDM overcomes the interference between adjacent blocks of OFDM (IBI) and maintains the orthogonality between carriers. At the same time, the length of the cyclic prefix is ​​greater than the channel extension length, effectively suppressing symbol interference (ISI). At present, OFDM technology is in IEEE8.2.11a, ETSI BRAN HIPERLAN / 2, local multipoint service distribution system (LMDS), digital subscriber line (ADSL / VDSL), data audio broadcasting (DAB), data video broadcasting (DVB), Digital Radio Mondial (DRM) is widely used.
The new-generation parallel system modem currently under development adopts OFDM technology. By adding a guard interval, ISI can be effectively eliminated and the complexity of the balance can be reduced. The following describes the application of OFDM in short-wave communications and several key issues that still need to be resolved.
2 Several specific applications of short wave communication based on OFDM system
With the improvement of baseband signal processing capacity and the increase of users' demand for bandwidth, the HF data transmission rate has greatly increased in the past few years. The test work of the Canadian CRC is the first successful attempt to achieve a transmission rate of 9600bps in a 3kHz bandwidth. Subsequently, Harris Corporation of the United States, General Aerospace Defense Company, Thomson Corporation of France and Daimler-Chrysler Aerospace Department of Germany did a lot of meaningful work in the field of high-speed HF data communication, and currently can provide transmission rates of more than 9600bps.
Increasing the communication rate is a major research direction in the field of HF communication. HIL-STD-188-110B provides transmission services from 3200, 4800, 6400, 8000, 9600, and 12800 bps (without coding) at transmission rates above 2400 bps. STANG5066 also supports high-speed HF data communication services. Finding new transmission waveforms and new coding methods in both parallel and serial modulation methods is the key to improving the HF communication rate. Because OFDM technology has a strong ability to resist multipath interference, it can effectively suppress ISI and subcarrier interference (ICI), and has been successfully used in DRM. It is worth noting that DRM also uses the short-wave band (3 ~ 30MHz) to transmit audio and data information. The following is a few examples of the successful application of OFDM in the field of short wave communication.
2.1 The realization way of British Racal Research Limited
In the tactical radio environment, VHF (30 ~ 300MHz) communication is a commonly used specification; but in complex terrain conditions, VHF communication sometimes has obstacles, you can try to use near normal incidence (NVIS) short wave radio Establish communication. Britain's Racal Research Limited has developed a parallel system modem adapted to the HF NVIS channel. It adopts OFDM technology, the number of subcarriers is 56, the modulation methods of the signals are 250QAM, 64QAM, 16QAM, FSK, PSK, SSB, and the highest transmission rate without coding on the 3kHz bandwidth is 16kbps, which can be extended by Doppler 1Hz and delayed It works normally under the condition of 5ms HF NVIS channel. The modem is implemented on a fast DSP prototype platform. The system uses Motorola's fixed-point DSP56300 processor, and the design complexity is greatly reduced through software radio technology.
In addition, in order to further verify the actual performance of modems using OFDM technology, in June 1999, a three-week field comparison test was conducted on the CRC serial modem and the Racal parallel system modem in DERA Canada. The transmitter is a 10kW DERA Cove radio station, and the receiving sites are located in Malvern (distance 140km) and FunTIngton (distance 45km) of DERA. After field tests, the performance of the two modems is slightly different. At dawn, OFDM performs better than serial modems. The bit error rate performance is directly low throughout the night, and both modems work well during the day. Since neither modem uses FEC encoding, the bit error rate is high.
2.2 The realization way of French Thomson company
Adopt OFDM system, the number of subcarriers is 79, channel coding adopts turbo code coding method based on frame structure, and the data transmission rate reaches 9600bps.
When the semiconductor diode is turned on, it is equivalent to the switch closing (circuit is turned on), and when it is turned off, it is equivalent to the switch opening (circuit cut), so the diode can be used for switching, and the commonly used model is 1N4148. Since the semiconductor diode has a unidirectional conduction characteristic, the PN junction is turned on under a positive bias, and the resistance in the on state is small, about several tens to several hundreds of ohms; in the reverse bias, it is off. The resistance is very large. Generally, the silicon diode is above 10 Μ Ω, and the bismuth tube is also several tens of kilo ohms to several hundred kilo ohms. With this feature, the diode will act as a control for the current to turn on or off in the circuit, making it an ideal electronic switch.
Switching Diode,1N4148 Signal Diode,4148 Diode,1N4148 Diode
Dongguan Agertech Technology Co., Ltd. , https://www.agertechcomponents.com