Research On Demodulation Technology For Multi-Antenna High-Order QAM Signals

As high efficiency utilization of spectrum, high-order quadrature amplitude modulation(QAM) has been widely used in digital microwave communication systems, cable TV network, satellite communications and many other fields. But it's very sensitive to noise interference and channel distortion, requiring high performance demodulation technology. How to improve demodulation performance of high-order QAM in low signal-to-noise-ratio (SNR) and poor channel conditions being a difficult problem in research of QAM. The technique of multi-antenna combining can suppress noise and improve signal quality effectively, bringing a certain combining SNR gain. So utilizing multiple antennas receiving QAM signals from the same source, then demodulating the received signals jointly and combining with optimal weights would improve demodulation performance effectively. Therefore the research on demodulation technology for multi-antenna high-order QAM signals has important practical significance and application value. This paper has mainly researched on the key demodulation technology for multi-antenna high-order QAM signals and then designed demodulation scheme. The main work is listed as follows:1. Study the symbol timing recovery algorithm for high-order QAM signals. First, the forward square timing synchronization algorithm and feedback timing recovery structure of Gardner have been analyzed and simulated. Simulation results show that both of the algorithms perform well in Gaussian channel, but square timing recovery algorithm performs better than Gardner in fading channels. At the same time, there types of interpolation filtering technology have been analyzed, including linear interpolation, parabolic interpolation and cubic interpolation, of which cubic interpolation is the best. So the square timing algorithm is selected for timing recovery of QAM signals and cubic interpolation is chosen for timing error correction.2. Study the phase estimation algorithm for multi-antenna high-order QAM signals. Two phase estimation and symbol detection algorithms based on maximum likelihood EM are proposed for multi-antenna signals with different initial phase offset. The first one has improved phase offset estimation accuracy comparing with existing PL and HA. The second one has realized joint symbol detection of multi-antenna signals, degrading symbol error ratio greatly comparing with symbol detection under single channel. Also a joint carrier phase recovery algorithm based decision-directed carrier recovery loop is proposed for phase estimation of received multi-antenna signals with small frequency offset, which improves the stability of phase locked loop(PLL) in low SNR effectively and greatly reduces the probability of lock loss.3. Study the joint blind equalization algorithm for multi-antenna high-order QAM signals. Considering the equalization for single channel signals with frequency offset, a joint equalization and carrier recovery structure is given based on the study of dual mode blind equalization algorithm. Then its performance has been testified by simulation. Also, a joint blind equalization algorithm is given for multi-antenna signals, improving performance over severe deep fading channel significantly and reducing the steady state error comparing with single channel.4. Based on the research of key technologies, the demodulation scheme is designed for multi-antenna signals in Gaussian channel and fading channel respectively. Then the feasibility of the scheme is analyzed and verified. After that, demodulation software platform is constructed in Matlab environment, achieving complete demodulation functions. At the same time experimental environment of Gaussian channel and fading channel have been built for testing demodulation software. Test results show that the software can implement joint demodulation of two-channel signals effectively, improving performance significantly compared with single channel.