Wireless communication signal interference analysis and solution
With the rapid development of computer and communication technologies, the global information network is rapidly evolving to the ip-based next generation network (ngn). In the future, the trend of broadband and mobile technology for personal multimedia communications in the world, coupled with market demands for flexibility and convenience, make the goal of seamless coverage and wireless connectivity increasingly becoming a reality. At present, various wireless technologies present a situation of blooming and arguing, which accelerates the popularity of wireless applications and faces problems that cannot be ignored due to the inherent frequency interference of wireless technologies.
1. Analysis of frequency interference principle
The generation of wireless interference is various. The original dedicated radio system occupies existing frequency resources, improper network configuration of different operators, transmitter self-setting problems, cell overlap, environment, electromagnetic compatibility (emc), etc. The cause of radio frequency interference in wireless communication networks. Coexistence interference between systems operating at different frequencies is essentially due to the imperfections of the transmitter and receiver. In general, active devices emit unwanted signals, and due to the limitations of the device itself and the out-of-band rejection of the filter, unwanted signals such as spurious, harmonic, and intermodulation are generated outside of its operating band. It interferes with the operating band of other wireless systems.
For wireless systems, the transmitter generates out-of-band radiation when it transmits a useful signal, including adjacent-frequency radiation and out-of-band spurious emissions due to modulation. While receiving the wanted signal, the receiver may fall into the interference signal of the channel, which may cause loss of receiver sensitivity. Interference signals falling within the receiving bandwidth may cause in-band blocking. At the same time, the receiver also has nonlinearity. Imperfect, out-of-band signals (transmitted useful signals) can cause out-of-band blocking of the receiver.
In addition to the quality of the device itself, the intensity of unwanted signals generated by active devices, such as out-of-band spurs, harmonics, and intermodulation, is related to two factors: the greater the output power of itself, the greater the output of unwanted signals; The extent of deviation from the working bandwidth, the farther away from the working bandwidth, the smaller the unwanted signal. The tolerance of the system to external interference is also related to two factors: the strength of the signal itself, the stronger the signal, the less the chance of interference; the smaller the interference signal, the smaller the interference signal level, and the lower the signal interference. In addition, the interference between the transmitter and the receiver is also dependent on factors such as the separation of the operating bands of the two systems and the spatial isolation of the transceiver.
The interferences of wireless and mobile communication systems mainly include co-channel interference, adjacent-frequency interference, out-of-band interference, intermodulation interference and blocking interference.
2. Frequency interference of wireless communication systems
From the actual situation in China, the main wireless communication technologies will be: gsm and narrowband cdma belonging to the second generation cellular mobile communication technology, phs (PHS) and scdma (the big smart) positioned as fixed telephones, and the third party. The tdd system of the cellular mobile communication system td-scdma and the fdd system wcdma/dma2000, wlan/wimax applied to broadband wireless access, uwb based on short-range communication, and frid to be applied to wireless identification. Although the application areas of these technologies overlap, their specific market needs will coexist for a long period of time, so their interference must be considered.
2.1 Existing wireless communication spectrum solutions
The specific allocation of the existing wireless and mobile communication spectrum in China is shown in Figure 1. In addition, wlan uses the unlicensed ism band, uwb uses the 3.5/5.8g band, and wimax and rfid have not yet finalized the band, where wimax may be allocated. In the 2.5g, 3.5g or 5.8g frequency band.
2.2 Basic situation of wireless interference
2.3 Mobile communication system interference
The various interferences in a mobile communication system can be generally classified into interference within a cell, interference between cells, interference between different communication systems, interference between different operators, interference caused by system equipment, and the like.
Interference in the small area mainly includes multipath interference, near-far effect and multiple access interference. The generation of these interferences is determined by the time-varying of the wireless channel and the delay and fading during the propagation of the electromagnetic wave. The interference generated when the adjacent cells use the same frequency is especially serious for the tdd system. The interference between the tdd system and the fdd system is mainly the interference between the tdd channel (including the uplink channel and the downlink channel) and the fdd uplink channel. In addition to the above interference, interference between different operators, interference caused by system equipment, etc. are also issues that need to be considered.
3. Interference solution
Although the interference in the wireless communication system is ubiquitous, according to the analysis of the root cause and interference situation of the interference, combined with computer simulation and a wide range of field tests, some effective methods for reducing and eliminating interference are also found. These methods are mainly divided into two categories: basic technology and engineering construction.
3.1 Basic technical methods
From the perspective of specific technology, intra-cell interference can be designed with multiple orthogonality codes, uplink and downlink synchronization, error correction coding, power control, diversity reception/transmission, joint detection, smart antenna, space-time processing, etc. Processing techniques are improved or resolved. The inter-cell interference and the interference between the tdd and fdd systems can be considered from the physical layer technology, or from the high-level wireless resource management technology. From the perspective of the physical layer, synchronization technology and smart antenna technology are good measures. From the perspective of wireless resource management, dynamic channel allocation is a very effective solution. In addition, it is also necessary to consider the unified coordination of network planning by different operators.
3.2 Engineering Construction Method
The project construction plan is to improve the wireless interference from the engineering point of view in the process of mobile network planning and construction. These methods mainly include: increasing the frequency protection band, improving the filtering accuracy, increasing the site spacing, optimizing the antenna installation, and limiting the device parameters.
The frequency band protection solution is increased by frequency planning so that the transmitting frequency band of the interference system and the receiving frequency band of the interfered system are isolated in the frequency domain. As the isolation increases, the component of the interfering system transmitter signal falling within the received bandwidth of the interfering receiver decreases, and the fading of the receiver receiving filter to the interfering system transmit signal increases, thereby reducing inter-system interference.
Proper frequency protection bands can effectively alleviate interference problems. At the same time, when considering the use of additional filters to limit the interference signal, since the ideal linear filter is difficult to implement, it is also necessary to leave a certain guard band to provide a transition band for the filter. On the other hand, due to the scarcity of frequency resources and the different frequency response characteristics of the transmit and receive filters, other interference solutions should be considered when using guard bands to minimize the size of the protection bandwidth.
The solution to improve the filtering accuracy is to further improve the filtering characteristics of the transmitter or receiver by using a high-precision filter or an additional filter based on the wireless transceiver system of the original device to achieve the isolation required for coexistence between systems. Improving the filtering accuracy is one of the ways to effectively solve the interference, but it also means an increase in cost.
The method of increasing the site spacing can effectively reduce the interference, but this method is limited by the lack of site resources and the coexistence of multiple operators, and the implementation of specific projects is difficult.
Optimized antenna installation includes antenna tilt, azimuth, vertical and horizontal isolation, etc. By adopting some optimization measures, the coupling loss between the antennas is improved and the interference is reduced.
Restricting device parameters is to stipulate sufficient device specifications to ensure the coexistence of adjacent transmission and reception frequencies, mainly to strictly limit the transmission power.
4 Conclusion
Analysis of different wireless interference scenarios, targeted solutions, and in the process of technology evolution, equipment development, network planning, system construction, operation and optimization, reducing or even eliminating interference is an important research area. On the eve of the construction of 3g, it is especially necessary for China's scientific research and engineering and technical personnel to make more efforts to build a boutique network and construct a harmonious communication.
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