Time Domain Equalization for Multicarrier Systems

• Main Authors: Lee, Chun-Fang, 李俊芳
• Other Authors: Wu, Wen-Rong
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/03207213349971832322

博士 === 國立交通大學 === 電信工程研究所 === 98 === In multicarrier systems, cyclic prefix (CP) is introduced to avoid intersymbol interference (ISI). The CP is an overhead and its size is chosen as a compromise between the transmission efficiency and system performance. If the length of the channel response exceeds the CP range, the ISI is induced and the system performance will be degraded. A simple remedy for this problem is to apply a time-domain equalizer (TEQ) such that the channel response can be shortened into the CP range. This dissertation is aimed to develop new TEQ design methods for two well known multicarrier systems: discrete multitone (DMT) and orthogonal frequency division multiplexing (OFDM). The TEQ is a commonly used device in DMT systems. Many methods have been proposed to design the TEQ with a capacity maximization criterion. An implicit assumption used by existing methods is that circular convolution can be conducted for the noise signal and the TEQ. This assumption is not valid because the noise vector, observed in a DMT symbol, does not have a CP. A similar assumption is also made for the residual ISI signal. Due to these invalid assumptions, the TEQ-filtered noise and residual ISI powers in each subcarrier were not properly evaluated. As a result, the existing optimum solutions are actually not optimal. In the first part of the dissertation, we attempt to resolve this problem. We first analyze the statistical properties of the TEQ-filtered noise signal and the residual ISI signal in detail, and derive precise formulae for the calculation of the TEQ-filtered noise and residual ISI powers. Then, we re-formulate the capacity maximization criterion to design the true optimum TEQ. Simulations show that the proposed method outperforms the existing ones, and its performance closely approaches the theoretical upper bound. A wireless channel typically has the multi-path response, exhibiting a finite impulse response (FIR) characteristic. Thus, the corresponding TEQ will have an infinite impulse response (IIR). The direct application of conventional TEQ designs results in a filter with high computational complexity. In OFDM systems, the criterion for the TEQ design is the average bit error rate (BER) which is a complicated function of the TEQ, and the optimum TEQ is difficult to obtain. In the second part of the dissertation, we develop new methods to overcome the problems. We propose using an IIR TEQ to shorten the CIR. It can be shown that the ideal TEQ exhibits low-order IIR characteristics, and the order of the IIR TEQ can be much lower than that of the FIR TEQ. Simulations show that while the proposed method can reduce the computational complexity significantly, its performance is almost as good as existing methods. We then further propose an OFDM system with a unitary precoding. The precoded OFDM system not only enhances the diversity of subcarriers, but also facilitates the TEQ design. We propose a TEQ design method called the maximum signal-to-interference-plus-noise ratio (MSINR). It is shown that the optimum TEQ, maximizing the SINR of all subcarriers, can be easily derived. To full explore the diversity the channel provides, the detector used at the receiver must be the maximum-likelihood (ML). The computational complexity of the ML detector for the precoded OFDM system can be very high. We then propose a detection method, called the sphere-decoding-and-successive-interference-cancelation (SDSIC). The proposed method can have near-optimal performance but the computational complexity is low.