Huawei base station BTS supports two methods at the same time. In the base station system design, the role of frequency hopping in frequency diversity and interference diversity is fully considered. It can support both baseband frequency hopping and radio frequency hopping. It has achieved scale on the Internet. application. From the perspective of actual application, the frequency hopping technology independently developed by Huawei can improve the anti-interference and anti-fading performance of the GSM system, greatly improve the call quality, enhance the networking capability of close multiplexing, increase the system capacity, and have a strong technology Features. The technical difficulties of RF frequency hopping are mainly manifested in how to achieve fast frequency conversion in a wide frequency band and how to ensure the high quality of the signal while performing fast frequency conversion. Fast frequency conversion and high signal quality are contradictory. In the GSM system, the gap between each time slot is only 20 microseconds. To achieve radio frequency hopping, the system must quickly switch from one frequency point to another within the protection time of 20 microseconds between time slots. Frequency. According to the previous technology, the realization of fast frequency hopping will inevitably bring a series of negative effects such as a decrease in modulation accuracy, a deterioration in receiving sensitivity, an increase in spurs, and a decrease in blocking performance. How does Huawei's base station solve this problem? Below we start with an analysis of the RF phase-locked loop. The lock time of the phase-locked loop is mainly determined by the loop bandwidth. The wider the bandwidth, the shorter the lock time. The quality of the local oscillator signal is mainly determined by factors such as the reference clock (phase discrimination frequency), voltage-controlled oscillator, and loop bandwidth. Within the loop bandwidth, the phase noise of the local oscillator depends on the reference clock, and outside the loop bandwidth, it mainly depends on Voltage controlled oscillator. There are only two ways to widen the optimal loop bandwidth. One is to reduce the performance of the voltage-controlled oscillator, which is obviously undesirable; the second is to improve the reference performance. Because the GSM system uses a 200kHz bandwidth, the phase detection frequency cannot be too high, especially for the DCS1800 system, so it is difficult to increase the loop bandwidth in the GSM system, that is, reduce the frequency lock time. In order to overcome the above two difficulties, Huawei adopts a unique set of dynamic loop bandwidth and ping-pong switching technology, which can solve the contradiction between fast frequency conversion and signal quality. Dynamic loop bandwidth technology: The loop bandwidth is not fixed during operation, but changes with the needs of the system. However, when the system is not in operation, the loop bandwidth is guaranteed to change back to the optimal bandwidth to optimize the output signal and ensure the system. The best performance. Ping-pong switching technology: Two identical oscillators are designed on the circuit, and the two local oscillators are selected through the switch. When one local oscillator is working, the other local oscillator quickly locks to the next required frequency point. The middle of the two time slots is switched to another LO circuit through a switch. In this way, instantaneous system performance degradation at the beginning and end of the time slot is avoided. By adopting the unique dynamic loop bandwidth and ping-pong switching technology, any frequency hopping within the 25MHz bandwidth of 900MHz and the 75MHz bandwidth of 1800MHz is achieved, and all frequency hopping indicators exceed the requirements of the GSM protocol. The technical difficulty of baseband frequency hopping lies in how to realize the high-speed exchange of information data to meet the frequency hopping speed of 217 hops / sec and the data transmission rate of 271 kbits / s. Consider the data exchange based on the time slot of the wireless interface. The exchange method can be space division, time division, and data packet exchange. Huawei's base station uses advanced bus technology in its design, and baseband frequency hopping is implemented based on time slot exchange. The specific implementation method is: Each transmitter (TRX) is tuned to a fixed frequency and has a fixed ID number. The encoder of the transceiver encodes the downlink signal to form burst format data. The encoder calculates the frequency channel (ie TRX number) that this burst should modulate according to the frequency hopping algorithm, and adds additional information about power control to form a specific data packet Format, the encoder of the transceiver sends out data packets within a fixed time (sub-slot). The modulator checks the TRX number of the data packet of each sub-slot. If it is different from the ID number of this TRX, the next sub-slot is accepted; if it is the same, the data packet of this sub-slot is received with a delay The slot is then transmitted to the space interface to achieve baseband frequency hopping. The baseband frequency hopping has very high requirements on the real-time ID identification of TRX. At this point, Huawei uses ASIC technology to achieve high-speed and reliable TRX-ID identification. PCB Assembly
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GSM RF frequency hopping and baseband frequency hopping
From the perspective of technical realization, the realization of frequency hopping in GSM is divided into baseband frequency hopping and radio frequency hopping.