Study And Design On Equalizers For Wireless Communications

In the field of wireless communications, the most problem is intersymbol interference(ISI) because of multipath transmission. Equalizers are often used to combat the influence of channels for improving communication's quality and decreasing ISI in receivers. Compare to linear transversal equalizers(LTE), decision feedback equalizers(DFE) are very often used to combat the distortion of communication channels because of their many advantages. Since the channels are unknown, the DFE must be implemented in an adaptive way. In a classical DFE, adaptation cannot be started without the transmission of a known training data sequence. Unfortunately, a training sequence is not always possible in channels, which decreases the effective bit rate. That is why unsupervised DFE, without any training sequence, is necessary.On the basis of studying on wireless communication channels, this paper discusses the appearance of ISI and equalization, introduces structures and algorithms of equalizers, presents a unsupervised(blind) adaptive DFE. It can be thought of as the cascade of four devices, whose main components are a purely recursive filter(R) and a transversal filter(T). The basic idea of the cascade is to split the difficult task of unsupervised equalization into several, but easier, subtasks. The four devices of the cascade have decoupled criteria in the sense that adaptive blind equalization can be achieved step by step. With the same computational complexity, the unsupervised equalizer exhibits the same convergence speed, steady-state mean-square error(MSE) and bit-error rate(BER) as the trained conventional DFE, but it requires no training. Moreover, the performances can be improved by implementing the algorithm of adaptive filtering with averaging (AFA), which is one of an recursive least squares(RLS) algorithm. The algorithm has the same convergence speed, but at the same time has easier computational complexity as RLS.Hence, the conclusion-adaptive blind DFE can now skip the training period and improve the communication performance.