Research On Adaptive Processing Algorithm Under Frequency Selective Fading Wireless Channel

In recent years, with the development of social economy and popularization of Internet, higher communication quality and transmission rate have been required in the data communications, expecially in wireless mobile communication, which can provide convenient communication at anytime and anywhere. From 1G to 3G of the mobile telecom network, from WiFi that is used in wireless local area network (WLAN) area to WiMAX that is used in wireless metropolitan area network (WMAN) in computer network, it has shown that wireless communication is experiencing high-speed development.It is well known that the channel of the wireless communication is very compli-cated. It has limiting bandwidth, transmit waste, time variety, noise, interfere and multi-path characteristic, which restrict the better transmit speed and performance of system in wireless communication. Thus, more advanced technologies need to be used to satisfy the requirements.This dissertation mainly is dedicated in studying adaptive signal processing tech-nology under wireless frequency selective channel. In wireless high transmit rate system, the most important characteristic of the channel is frequency selective, which introduces the inter-symbol interference (ISI). Serious ISI makes impossible to communicate. The conventional method for fighting back ISI is equalization tech-nology, and the design of efficient equalizor is important content in the receiver of wireless system. In high-speed wireless communications systems, the quite large-scale equalizer is required in order to reduce effectively the ISI component of the received signal. Orthogonal frequency division multiplexing (OFDM) is the best ef-ficient technique to eliminate the ISI for high-speed digital transmission over severe multi-path fading channel, in which the delay spread is large than the symbol dura-tion. And now OFDM has been considered to be a promising key technique for fu-ture mobile communication systems. OFDM is used in air interface standard IEEE 802.11 and IEEE802.16 of WLAN and WMAN system.Following the study of the conventional equalization, an efficient decision feed-back equalizer (DFE) is presented for sparse channels with large delay echoes, which are encountered in many high-speed wireless communication applications. Unlike the conventional DFE, this proposed equalizer considers that the channel is sparse and large number of DFE taps lead to noise accumulation problem. The new method can achieve efficient equalization by setting the active group for equaliza-tion taps and clearing the abandoned taps mechanism. As a result, it can both reduce hardware implementation complexity effectively and improve the performance compared to the conventional DFE. Brief analysis about noise accumulation effects in large-scale equalizer and working mechanism description of this effective algo-rithm exhibits us where the performance gain lies. Simulation results show that this modified DFE exhibits considerable computational savings, faster convergence, and better performance and improved tracking capabilities than the conventional ones. It is quite fit for applications in high-speed systems, such as high definition television (HDTV) and broadband mobile communication.The combination between OFDM and multiple input multiple output (MIMO) technology is the keystone in wireless research field. Compared with a single input single output (SISO) system, a MIMO system can improve the capacity linearly. This implies that MIMO system has the channel capacity beyond Shannon limit and has huge potential applications in broadband wireless communications. OFDM can simplified the receiver implement and provide high data rate services. It is self-evident that MIMO-OFDM has higher spectrum efficiency and bigger system capac-ity than any of the above two. MIMO-OFDM allows an impressive increase in data rate in a mobile wireless link without additional power or bandwidth consumption through the use of multiple antennas at both transmitter and receiver and OFDM modulation scheme. So recent industry activities suggest that the use of MIMO-OFDM is particularly promising for new generation wireless communications. By adjusting parameter in transmitter, such as bit allocation and power, the adaptive it-erative modulation is used to track the variety of channel for obtaining better per-formance in fix and wireless communication. It is supported in WiMAX. The con-ventional adaptive modulation is implemented by water-filling to maximum the data rate under fixed transmit power. In this paper, an adaptive minimum transmit power modulation scheme under constant data rate and fixed bit error rate (BER) for the MIMO-OFDM system is proposed. It adjusts the modulation order and allocates the transmit power to each spatial sub-channel by iterative method when meeting the user's QoS requirements at the cost of minimum transmission power. At first, the bit allocation and power can be obtained by Lagrange algorithm. But there is error by round the modulation order, which can be solved by comparation or adaptive itera-tive method. Proof is given at the same time. Computer simulation results present that the proposed method can be meeting the requirement of the system with lower transmission power and lower computational complexity.In general, mobile station is equipped by single antennas for its size. At the same time, base station is equipped by multiple antennas. That is single input multiple output (SIMO) or multiple input single output (MISO). Adaptive antenna array has been widely studied in that system, and it is another effective method to greatly in-crease wireless communication system capacity and performance by suppressing co-channel interference, improving coverage quality and mitigating multi-path interfer-ence. OFDM is the best efficient technique to eliminate the ISI. So combination be-tween OFDM and adaptive antenna array can be used to implement high data trans-mission. For an OFDM system, adaptive antenna array beamforming can be applied to either time domain or frequency domain at the receiver, which are called to"Pre-FFT OFDM adaptive antenna array"and"Post-FFT OFDM adaptive antenna array"respectively. In the paper, different adaptive weight algorithms have been studied under two architechtures, such as MMSE, MBER. At first, a Post-FFT OFDM adap-tive antenna array based MBER is proposed. It is shown that the MBER adaptive beamformer outperforms the MMSE beamformer, since it directly minimizes the BER. In the same situation, the MBER beamform can find accurately desired user and obtain lower BER of system. Compared with Post-FFT adaptive antenna array, much lower computation complexity and shorter training symbols are required in the Pre-FFT adaptive antenna array scheme for having only a FFT processor and Pre-FFT signal processing, at a cost of slight performance degradation. Then the Pre-FFT OFDM beamform based RLS and MBER is studied. The RLS beamform can be quickly converged by using the training sequence well and has good performance in time variety channel. At lase, the MBER Pre-FFT beamformer is applied to obtain better BER performance.