A reconfigurable multi-standard channelizer using QMF trees for software radio receivers

The flexibility of a software-defined radio (SDR) depends on its capability to operate in multi-standard wireless communication environments. The most computationally intensive part of wideband receivers is the channelizer, which extracts multiple narrowband signals from adjacent frequency bands. In an SDR receiver, the compatibility of the channnelizer with different communication standards is guaranteed by its reconfigurability. This paper presents an efficient channelizer that has a reconfigurable architecture based on quadrature mirror filter bank (QMF) trees. We show that the channelizer can be efficiently implemented using common subexpression based filter structures. An example of dual-mode Global System for Mobile Communication (GSM)/Personal Digital Cellular (PDC) channelizer is discussed to illustrate the proposed design methodology. Keywords-Software radio, quadrature mirror filter bank, dualmode channelizer, common subexpressions, canonic-signed digit. I. II. A. INTRODUCTION SDR migrates the traditional hard-wired radio platforms to flexible software definable platforms that can support multiple air interface standards [1], [2]. A first order estimate of the resources required to implement the wideband receiver of a software radio shows that the intermediate frequency (IF) processing block to be the most computationally intensive part since it operates at the highest sampling rate [1]. A reconfigurable IF processing architecture for the extraction of individual channels can significantly reduce the cost and complexity of cellular base stations. Efficient implementations of channelizer have been proposed using Discrete Fourier Transform (DFT) filter bank [3], [4]. The basic principle of the DFT filter bank channelizer is to simultaneously extract a number of equally spaced channels using a uniform filter bank derived from the modulation of a given prototype filter as shown in Fig. 1. The filter bank channelizer extracts every channel between [( where is the sampling frequency of the wide-band ADC. The complexity of a filter bank channelizer is independent of the number of received channels. However, DFT filter banks cannot perform channelization for channels with different bandwidth. This is so since DFT filter banks are modulated filter banks with equal bandwidth of all bandpass filters. Therefore, it can only support a given air interface standard. Hence each cellular standard would require a distinct channelizer to extract its channels. Multi-mode receivers discussed in the literature [5], [6] employ distinct filter bank channelizers for each cellular communication mode. This paper presents the implementation of a reconfigurable channelizer using QMF trees and its efficient implementation employing common subexpressions. The paper is organized as follows. Section II presents the reconfigurable architecture of the proposed channelizer. We discuss a hardware efficient implementation of channel filters in Section III. In section IV, we illustrate the realization of a dual-mode GSM/PDC channelizer receiver using a design example. Section V provides our conclusions. )], 2 / ( ), 2 / s s F F − s F TREE STRUCTURED QMF BANK CHANNELIZER It is well known that tree-structured QMF banks are employed in the implementation of discrete wavelet transform [7]. Basically, the QMF tree decomposes the input signal into several frequency subbands. We employ the tree-structured QMF bank shown in Fig. 2 to perform channel extraction from a wideband received signal. The input bandwidth of the IF signal from the radio frequency (RF) module will cover several channels of different communications standards. The QMF analysis banks split the signal into several subbands down the tree. Thus, channel extraction in this multistage channelizer is performed at a stage whose subband corresponds to the channel spacing of the current mode of operation. In conventional multi-mode receivers, many individual channelizers are required to support different communications standards in one single cellular base station. Thus, reconfigurability of such receivers is constrained to switching the operation among distinct channelizers based on the current mode. This is an inefficient approach due to its increased hardware complexity and power consumption. We define reconfigurability as the ability to reprogram the same channelizer to work with different standards. The proposed multi-standard channelizer offers reconfigurability in two levels, namely, architecture level and filter level. Architecture Reconfigurability The reconfigurability of the channelizer architecture can be illustrated using the tree-structured QMF bank shown in Fig. 2. For simplicity, a dual-mode operation is considered. Let the sampling frequency of the input signal be and and represent the channel spacing of modes 1 and 2 , s F 1 cs f 2 cs f respectively, where and n is an even integer. Employing the proposed architecture, the channels of mode 1 can be extracted at the output of stage, and that of mode 2 at the output of stage, in respective mode of operation, where and are give by the expressions: 2 1 cs cs f n f =