Dqpsk Spread Spectrum Modulation Despreading Technology Research

An new DQPSK spread spectrum (SS) airborne communication system is described in detail in this paper. It's quite different from QPSK SS coherent communication system or DQPSK SS system based on conventional SAW matched finer. The breakthrough lies on the I/Q division of SS code rather than data division as we did before. Therefore this new system has following advantages comparing to conventional system:(1) More flexibility in selecting SS code. In common de spreading of DQPSK ss communication system, the SS code must both have good auto-correlation and cross-correlation character. This may limit the selection of PN code length is less than 32 bits. In this new system, the only concern is on the auto-correlation character of SS code which will be more flexibility in code seletion.(2) The structure of the despread demodulation receiver based on SAW matched filter is much more simple. Since the data rate will not change during SS demodulation, the SAW matched filter is reduced half of the length comparing to that of conventional one.The communication system hardwares are also developed in this paper, such as: DQPSK SS emitter, DQPSK SS despread receiver, 280MHz local frequency source. High performance SAW filter With low loss,high outband rejection and low rectangular coefficient is used intransmitter. The structure is less complex since the frequency translation of the transmitting signal to IF is dirctely reduced . Decimal fraction frequency division design is adopted in 280MHz frequency source, so that the difference between the practical phase noise and the theoretical value is less than 2 dB. DQPSK receiver is composed of AGC main IF amplifier, DQPSK SAW matched filter, SAW IF delay line, IF differential demodulation circuit, clock recovery circuit, data recovery circuit, SS pseudo code synchronous indicating circuit etc. the combination design of DQPSK SAW match filter with SAW IF delay line is firstly presented in china and is utilized in system application for the first time, which has important effects on development of SAW devices and DQPSK SS communication system.Test results giving in the end of the paper show this new system can well meet the requirement of FH/DS mix spread spectrum airborne communication system . It's also proved that the design of this DQPSK is available in ss commumication.