Reprinted from: http://m.elecfans.com/article/664619.html
Frequency hopping is a commonly used carrier technology in mobile communications. It has a good anti-interference effect and can effectively improve communication quality.
Frequency hopping means that the carrier frequency hops according to a certain rule within a certain range.
Frequency hopping means that both the mobile phone and the base station send and receive information according to the same frequency sequence. This frequency sequence is the frequency hopping sequence (HSN). A frequency hopping sequence is a given frequency point set (MA) that contains N frequency points. Through a certain algorithm, all (N) are uniquely determined by the frequency hopping sequence number (HSN) and mobile allocation offset (MAIO). An arrangement of frequency points. N channels on different time slots (TN) can use the same frequency hopping sequence, and different channels in the same time slot in the same cell use different mobile allocation offsets (MAIO).
When using tight frequency reuse technology, system interference is the most important factor in determining the frequency reuse ratio. In order to reduce system interference, the technology usually used is power control and discontinuous transmission technology (DTX); and in order to resist interference and improve the communication quality of the system under the same interference conditions, frequency hopping technology is usually used.
Therefore, frequency hopping is an important technology for the GSM system to resist interference and improve frequency reuse. According to the GSM specification, slow frequency hopping can be used in the GSM communication system. Frequency hopping means that the carrier frequency is within a certain range and changes according to a certain rule. The frequency hopping function of each cell channel group can be activated or deactivated individually. BCCH time slot does not participate in frequency hopping, TCH channel, SDCCH channel can use frequency hopping. There are two types of frequency hopping used by base stations, baseband frequency hopping and radio frequency hopping, and their implementation principles are different.
Frequency hopping can reduce various types of interference, such as channel interference, adjacent channel interference, and intermodulation interference. 跳频使移动台所受的连续长时间干扰变成单个突发脉冲的不连续干扰； 而这种干扰通过信道解码、去交织与纠错技术，可以在很大程度上获得纠正。Therefore, the use of frequency hopping technology enables GSM base stations and mobile stations to work in a harsher radio wave environment.
In short, after using frequency hopping, cell planning is more flexible. 均化干扰的效果取决于跳频的模式，如果干扰信号和有用信号使用相互不同的跳频序列即非相关序列，那么通话质量就可获得最佳效果。相关性越小，均化效果越好。If both the interference signal and the useful signal use cyclic frequency hopping, and the frequency hopping sequence is the same, they may be in the same step. The interference signal and the useful signal are always on the same frequency, and the interference to the useful signal is the same as that without frequency hopping. . This is equivalent to two frequency hopping sequences are completely correlated. Therefore, the interference diversity effect of random frequency hopping is better.
In a full-load system, all interference signals are being transmitted at the same time, so there is continuous interference. Even so, the use of random frequency hopping sequences can still obtain a very good improvement in call quality. just,同频率小区要使用不同的跳频序列。跳频的频率越多，效果越好。
Frequency hopping can reduce the influence of Rayleigh fading on signal strength. This effect is equivalent to frequency diversity. After channel coding, a voice code block of GSM is scattered in 8 bursts (BURST) for transmission. Because Rayleigh fading is frequency-based, that is to say, the valley points of signal fading appear in different positions at different frequencies. Therefore, after frequency hopping is used, the modulation frequency of each burst is different. If a certain frequency point When frequency selective fading occurs, the time that the signal received by the mobile phone stays at the valley point will not exceed one burst period, and only a burst pulse is affected. The change in signal strength is divided into small enough to achieve channel coding and interleaving errors. Therefore, the signal valley of Rayleigh fading can be ignored, and the valley of Rayleigh fading signal level tends to be flat. In this way, the slow-moving mobile phone will be in a more uniform radio wave environment. For a fast-moving mobile phone, regardless of its speed, the effect of frequency diversity is the same. Just because the distance between bursts varies greatly, the effect is not very good.
In the GSM system, the intensity and distribution of interference received by each frequency point in the cell are different. The change in the carrier frequency of the burst pulse of the same call reduces the interference received by the signal, and the radio wave interference received by the call is Average, otherwise, if frequency hopping is not used, and the mobile station has been working on a fixed frequency point, every burst of the entire conversation may be subject to constant strong interference. That is to say, the frequency hopping technology is used to disperse the interference to different carrier frequencies that carry burst pulses. This effect is called "equalized interference" or "interference diversity." Cellular networks are frequency multiplexed, and co-channel interference exists. Frequency hopping makes the signal suffer from discontinuous interference rather than continuous interference, and the radio wave environment has been improved. The interference received by each burst is variable, which is conducive to the improvement of the quality of the call, otherwise, the entire call will be greatly interfered. 也就是说干扰分散到了携带突发脉冲的不同的载频上. Of course, the frequency collision caused by frequency hopping will cause strong instantaneous interference. Different definitions of MAIO can solve this problem. During frequency hopping, the more frequency points we use, the smaller the possibility of frequency collision and the higher the frequency hopping gain.