| Summary: | 碩士 === 國立東華大學 === 電機工程研究所 === 87 === There are growing interest in apply quadrature amplitude modulation (QAM) to wireless communications. To satisfy the bandwidth and power limitations of mobile communication, bandwidth efficient modulation techniques, such as M-ary phase shift keying (MPSK) or M-ary quadrature amplitude modulation (MQAM), are needed. The fundamental difficulty in employing such signals is that they are sensitive to channel impairments, especially to rapidly changing multiplicative fast fading. If the channel is relatively benign and fading is not an issue, one may obtain higher bit rates for an equivalent bandwidth by using M-ary quadrature amplitude modulation (MQAM). In addition, with the same bit error rate the signal-to-noise ratio (SNR) performance is better than MPSK.
Quotient coding was designed to removes the channel noise effects from the symbol amplitude as well as its phase. It is applied to QAM, which is termed QQAM, and is effective at suppressing the effects of channel fading with respect to the entire symbol as DPSK at the phase alone. One major disadvantage of QQAM is that it is quite sensitive to frequency offset and the performance of this QQAM receiver can barely meet the requirement of secure transmission when the channel condition is poor. It uses the signal received in the previous symbol interval as a signal reference for the received signal in the current interval. For wireless communications, mobile channels vary from time to time. That may introduce irreducible bit-error rate (BER) in the fading channel which varies fast relative to the symbol rate.
In this thesis, we propose a technique to improve the performance of QQAM by removing the channel effects from symbol phase in advance. We applied the double differential coding scheme to the QQAM technique, which we called Differential-QQAM (DQQAM).
In addition to the DQQAM, we also propose a novel adaptive detection scheme to increase the performance of QQAM in this thesis. The novel adaptive detection scheme can significantly reduce the irreducible BER of QQAM due to Doppler spread by the adaptive linear prediction of the reference signal. The predictor coefficient is adapted to the channel conditions by using the recursive least-square (RLS) algorithm. Signal estimation based on the Viterbi Algorithm (VA) and decision feedback algorithm (DFA) is applied as well. Since the adaptive receiver for QQAM can adapt the reference signal prediction to changing channel conditions, its applications include mobile radio where channels may change from AWGN channels to Rayleigh channels and vice versa according to mobile terminal movement.
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