Microwave communication technology has been around for more than half a century. It is a wireless communication method for information transmission through the ground line of sight in the microwave frequency band. The original microwave communication system was an analog system. It was an important transmission method for the long-distance transmission trunk line of the communication network with the coaxial cable carrier transmission system at that time. For example, the transmission of television programs between cities in China mainly relied on microwave transmission. Since the 1970s, digital microwave communication systems with medium and small capacity (such as 8Mb/s, 34Mb/s) have been developed, which is the inevitable result of the development of communication technology from analog to digital. In the late 1980s, with the popularization and application of the Synchronous Digital Hierarchy (SDH) in the transmission system, N&TImes; 155Mb/s SDH large-capacity digital microwave communication system appeared. Nowadays, digital microwave communication together with optical fiber and satellite is called the three pillars of modern communication transmission. With the continuous development of technology, in addition to the traditional transmission field, digital microwave technology has attracted more and more attention in the field of fixed broadband access. The LMDS (Local Multipoint Distribution Service) operating in the 28 GHz band has been widely used in developed countries, indicating that digital microwave technology will still have a good market prospect.
two. The main development direction of digital microwave communication technology1. Improve QAM modulation levels and strict band limits
In order to improve the spectrum utilization rate, multi-level QAM modulation technology is generally adopted, and 256 and 512QAM have been achieved at present, and 1024/2048QAM can be realized very quickly. At the same time, very strict requirements are placed on the design of the channel filter: in some cases, the remaining string roll-off factor should be as low as 0.1. It can now be around 0.2.
2. Grid coded modulation and Viterbi detection technology
In order to reduce the system error rate, complex error correction coding techniques must be used, but this will result in a decrease in frequency band utilization. To solve this problem, trellis coded modulation (TCM) technology can be employed. The use of TCM technology requires the use of Viterbi algorithm decoding. In high-speed digital signal transmission, it is difficult to apply such a decoding algorithm.
3. Adaptive time domain equalization
Use high-performance, fully digital two-dimensional time domain equalization to reduce the effects of intersymbol interference, quadrature interference, and multipath fading.
4. Multi-carrier parallel transmission
Multi-carrier parallel transmission can significantly reduce the rate of transmitting symbols and reduce the effects of propagation dispersion. Using dual-carrier parallel transmission can reduce the instantaneous rate to 1/10.
5. Other technology
Such as multiple spatial diversity reception, transmit power amplifier nonlinear pre-correction, adaptive orthogonal polarization interference cancellation circuit.
three. The main application of digital microwave communication system1. Backup and supplement of trunk fiber transmission
Such as point-to-point SDH microwave, PDH microwave and so on. It is mainly used for emergency repair of trunk fiber transmission systems in the event of self-disaster, and for locations and occasions where fiber is not suitable for various reasons.
2. For rural areas, islands and other remote areas and private communication networks to provide basic services for users
In these occasions, microwave point-to-point, point-to-multipoint systems can be used, and wireless subscriber loops in the microwave band also fall into this category.
3. Short-distance feeder connection in the city
Such as between mobile communication base stations, interconnection between base station controllers and base stations, wireless networking between local area networks, and the like. Both small and medium-capacity point-to-point microwaves can be used, as well as microwave digital spread spectrum systems that do not require a frequency.
4. Future broadband service access (such as LMDS) (fiber-optic communication technology).
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