Channel Estimation Algorithms For MB-OFDM UWB System

In recent years, wireless communication technology has been developed rapidly and applied in commercial successfully, which undoubtedly has a profound influence on our daily life. We hope to exchange a large amount of information at any time, from any location. As a result, sharing and efficiency utilization of spectrum resources has been an important issue in wireless communication field. Ultra wideband (UWB) has become an attractive candidate for future short-range and high date rate wireless communication owing to the potential for high data rates, and the potential for low processing power along with low implementation cost. UWB, which is a shared unlicensed system, coexists with other licensed and unlicensed narrowband systems. In spite of this, UWB will change the status of wireless frequency resource shortage and incompatible, and offer attractive solutions for increasingly scarce spectrum resources. As a strong candidate for UWB, MB-OFDM UWB drew more and more attention because of its advantages such as ability to provide high bandwidth efficiency, capability of capturing the multipath energy efficiently, flexible and high wireless data transmission, which can be applied to digital wireless home or office network, personal handy system or military field.To ensure the reliability of data and limitation of power, UWB system usually adopts coherent detection. By this way, unfortunately, the data is inevitably affected by random phase shift and amplitude fluctuation, it's needed for receiver to understand the fading propagation effects in frequency selective channels characteristics and carry out channel estimation. Considering the algorithms complexity and precision simultaneously, we present some novel and practical channel estimation schemes tailored to MB-OFDM UWB applications. The main innovations of the dissertation are as follows:There are important differences between UWB and narrowband channels, especially with respect to the number of resolvable paths and arrival times of multipath components. This paper compares the different transmission characteristics of some classic channel model, such as tap-line delay model,Δ-K model and S-V model, in terms of the statistical characteristics of multi-path delay amplitude and arrival times. Having insight into the in-door channel model of IEEE802.15, this paper probes into multi-path delay time and coefficients statistical characteristics based on tap-line model aiming to MB-OFDM UWB system. In order to analysis the channel estimation method in frequency domain, we demonstrate that the UWB channels can be adequately modeled by low-order autoregressive (AR) models, and discuss the correlation of subcarrier channel frequency response. The traditional data-aided channel estimation algorithm obtains the channel state information by the training sequences or polite symbols. Although this kind of algorithm is relatively simple and easy to implement, it will waste frequency and power efficiency. Based on comparing the algorithms complexity and performance for different estimation citation such as LS and MMSE, This paper presents new date-aided channel estimation algorithm for MB-OFDM UWB system. To make good trade-off between performance and computational complexity under satisfactory synchronization, we employ a simple LS method together with frequency-domain smoothing operation to estimate the channel frequency response using training preamble, then uses this channel estimate result to detect the frame header and refines the channel estimate by using a decision-directed technique. Furthermore, we developed an efficient phase error mitigation scheme. This method employs inter OFDM symbol smoothing technique for achieving further suppression in low SNR regime.Aimed to time-varying channel, this paper utilizes the Kalman filter to track the channel state varying on adjacent symbols. The traditional channel estimation based on Kalman filter is only made use of the channel correlation in time domain, this paper refine the estimation performance considering the channel correlation in frequency domain. The algorithm reduces the complexity by reducing dimension according the different correlation of subcarrier in different environment. Based on this, we combine polite interpolate and phase compensating when existing frequency offset, which improve the tracking performance.To enhance the data rates and transmission ranges of UWB systems, applying the MIMO scheme has attracted considerable interest, which could expand the capacity of UWB system. To estimate the channel state information effectively, this paper proposes channel state information estimation algorithms based on ML-EM with hard-decision receiver and MAP-EM with soft-dicision receiver respectively. The EM algorithm decomposes the MIMO channel estimation problem into a series of single input single output (SISO) problems, avoiding the complex computations. In the situation of multi-users, we adopt Space Alternating Generalized EM to accelerate the algorithm iteration rate. Compared with data-aided channel estimation method, EM channel estimation algorithm needn't transmit training symbols periodically, which reduce the transmission power and improve the bandwidth and spectrum efficiency.