Research On Microwave Fast Carrier Synchronization Technique

In the past few decades, the transmission data rate in wireless communication system is increasing very fast according to Moore’s law. Especially in recent years the explosive growth of multimedia information challenges mobile backhaul network’s speed and capacity. Adopting point-to-point microwave wireless backhaul technology is a kind of economic and effective method, better than the traditional optical fiber/coaxial cable technology, especially in the sparsely populated rural or regional superiority complex terrain. However microwave wireless backhaul is facing many technical challenges, especially our country dominated TDD communication mode, high-speed transceiver switch lead to a very high requirement of carrier synchronization time. In this paper, we study a technique to reduce synchronization time.In order to reduce synchronization time, carrier synchronization can’t use digital circuit implementation, so this paper puts forward a kind of analog circuit solutions. Simulation scheme must let QPSK signal go through fourth power circuit in order to remove modulation, obtaining four times of the carrier frequency. This process introduces stray close to the carrier, this requires a very narrow band filter to block the spurious frequency. SIR filter and split ring can realize the function of the narrowband filter, but take actual application under consideration, we choose the open-loop filter as a component in the carrier recovery. The final design of the filter shows that center frequency is 1.2 GHz, relative bandwidth is 3%, insertion loss also under 3dB, achieving good effect in practical application.In order to guarantee the quality of carrier recovery, a self-synchronous circuit is designed using PLL. PLL uses sampling phase locked detector, solves carrier recovery for the signal of high data rate. As self synchronous technology makes the signal go through the same path and generated phase cancellation, thus removed the use of phase shifter. Then use signal source with different symbol rate QPSK signal respectively to test the synchronization performance of the circuit. Test results show that the recovery of carrier phase noise is better than 80 dBc/Hz @ 1 KHz, the time interval of VCO tuning voltage reaching stability state is less than 29 milliseconds.Next is the analysis of the external-synchronization scheme. Compared to the traditional synchronization scheme, its lock threshold is lower, can also keep good loop stability under the condition of low SNR. TDD communication up and down link data rate is not fixed, often times there will be a very big change, in this case the externalsynchronization scheme can trace the changes of the carrier well.