Digital microwave relay systems, also known as digital microwave relay systems, are multiplex communication systems that transmit digital signals in a long distance by means of microwave relay.
Chinese name
Digital microwave relay communication system
English name
digital microwave relay system
definition
A multiplex communication system for long-distance transmission of digital signals by means of microwave relay.
Applied discipline
Communication technology (first level discipline), wireless transmission and access (second level discipline)
Digital microwave relay communication system is a multi-channel communication system that transmits digital signals in a long distance by means of microwave relay, also known as digital microwave relay system.For analog signals such as telephone and fax, analog signals must be converted into pulse code modulation (PCM) digital signals with a bit rate of 64kbit/s through sampling, quantization, coding and other processes, and the multiplexed signals are formed into baseband signals by time division multiplexing (TDM), and then the intermediate frequency carrier is modulated twice with this baseband signal (such as phase-shift keying PSK),Then it is moved to microwave band through up converter.This system can be recorded as TDM PSK.Digital microwave system can be used to transmit telephone, data, image, TV and other new communication services.
characteristic
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The digital microwave relay communication system has the following characteristics:
① In the digital microwave relay system, the relay station can regenerate the received discrete digital signals to eliminate the accumulation of interference and noise in the transmission process. Therefore, each relay station in the whole relay system needs to adopt baseband transfer mode. ②For long distance microwave relay communication, the cost of digital system per call path km is lower than that of corresponding analog system. If digital transmission and digital switching are considered to form an integrated digital network, analog/digital conversion devices can also be saved, and more costs can be saved. ③The frequency spectrum utilization ratio of voice digital signal is low, and the frequency band occupied by medium and large capacity digital microwave relay communication system is too wide, so it is necessary to consider the technology of effective spectrum utilization, as shown in Figure 1.
Figure 1 Digital modulation efficiency
Main technical problems
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The main technical problems in the digital microwave relay system include:
① Synchronization.In order to realize the transmission of discrete digital signals in digital microwave system, it is necessary to solve the synchronous operation of the receiver and the transmitter and recover the demodulation carrier frequency.Synchronization includes bit synchronization, time slot synchronization and frame synchronization.Bit synchronization keeps the transceiver working at the same bit rate, and other secondary synchronization ensures correct grouping of digital sequences output by the receiver.For this reason, the transmission code type and frame structure of digital signals must be convenient for extracting these synchronous signals, realizing the quasi synchronization between adjacent relay stations and maintaining the correct phase of the demodulation carrier frequency.
② Use spectrum technology effectively.In the digital microwave relay system, in order to improve the utilization rate of the spectrum, multi band modulation, spectrum shaping technology and cross polarization frequency reuse technology can be used, as shown in Figure 2.
③ Forward error correction (FEC).In order to meet the requirements of high transmission quality and low interference tolerance of high-capacity multi ary orthogonal modulation technology, forward error correction technology must be adopted.
④ Equalization and interference cancellation technology.With the continuous increase of communication capacity and the application of effective spectrum utilization technology, equalization and interference cancellation technology against channel distortion caused by multipath fading are required.Diversity reception is an effective method to improve the signal fading caused by multipath effect. In the case of broadband modulation, the diversity combination method with small dispersion can be used.In addition, there are adaptive equalization technology and adaptive cross polarization interference cancellation technology.
⑤ Code collection and descrambling technology.Add pseudo-random scrambling codes at the input end of digital microwave relay equipment to eliminate a large number of "0", "1" code sequences and DC components, so that the binary digital series transmitted in the channel is close to pure random characteristics, so that the bit synchronization system, time domain equalization system and error code monitoring system can work normally as a whole.
⑥ No damage switching.In medium and large capacity digital microwave relay communication systems, the use of direct switching devices will cause the "break" and "dislocation" of the code stream, resulting in the loss of frame signals and the instantaneous interruption of the communication system. Therefore, the medium and large capacity digital microwave relay communication systems must first adopt the ability to adaptively adjust the delay difference between the main and standby channels, wait for the signal codeAfter the timing and the position of the frame signal are corresponding, the main channel of the signal deterioration will be eliminated automatically after parallel operation in diversity synthesis mode.
