Implementation of SDR-based WCDMA Air Protocol Analyzer with an Emphasis on Equalizer for Software Modem

Abstract

This paper presents an implementation of SDR-based Air Protocol Analyzer(APA). APA is a measurement system providing real-time analysis of wireless signals between User Equipment (UE) and Node-B. Since the implemented system proposed in this paper consists of Digital Signal Processors (DSPs) and Field Programmable Gate Arrays (FPGAs), it can be reconfigured through a software exchange for supporting various communication standards. The waveform of Wideband Code Division Multiple Access (WCDMA) has been selected for verification of the proposed system, which simultaneously supports up to 48 users in the frequency band of 3 Frequency Allocations (FA) in real-time. Note that the proposed system exhaustively supports all the channels defined in 3GPP (Release 7) which includes High Speed Packet Access channels as well as normal data channels. After the decoding procedures in Layer1, Layer2, and Layer3, the proposed APA system provides final outputs of audio and video signals through speaker and monitor, respectively. It can also store the wireless signals and decode storage data on off-line basis. In WCDMA system, receiving procedures except some of the Physical (PHY) layer parts, i.e., transport layer part, L2, and L3 parts, are simply reverse of the transmitting procedures like in most other systems as well. So, it could be said in some sense that the entire performance is largely dependent upon PHY layer algorithms. Based on that principle, we designed the software modem using 2 different ways, one with rake method and the other equalization method, to compare the performance to each other. Through extensive experimental tests, we have found that the equalization method outperforms the rake method in most practical signal environments. Especially, in adverse signal environments such as low Signal-to-Noise Ratio (SNR), fast mobility, high-rate modulation like 16QAM, equalizer far outperforms rake receiver. Our experimental tests have shown that throughput provided by equalizer-based modem is nearly 2-10 times higher than that provided by rake-based modem. The main reason being so is that, when SNR is very poor because of too low spreading factor and/or too high other-cell interference like in some cases of HSPA signal environments and/or in cell edge area, rake-based modem performs very badly while equalizer-based modem can still retrieve the transmit data as long as channel estimation remains acceptable, which provides a reasonable estimation of equalizer tap coefficients. The only disadvantage with the equalizer-based software modem is relatively heavy computational complexity which is mainly due to the inversion of channel matrix, which poses as a hindrance in hardware implementation of the APA system. We propose a novel algorithm for simplifying the computational procedure of the equalizer-based software modem and compare the performance with rake-based modem as a function of computational complexity. The key idea of reducing the computational complexity is to exploit the fact that the channel matrix, which is a key part in estimating the equalizer tap coefficients, is sparse and quasi-toeplitz. Proposed algorithm reduced the computational complexity from 3rd order to 2nd order of the tap length. As our system has adopted 60 taps, we have verified in various experimental signal environments that the proposed algorithm of reduced complexity enables real-time processing of the software modem of APA system for WCDMA standards. In this paper, we have shown that real-time processing of WCDMA APA system can be supported by the proposed equalizer-based software modem. In the implemented system, all the signal processing required in L1, L2, and L3 including antennas and Radio Frequency(RF) parts as well are exhaustively realized to support all the channels defined in 3GPP Release 7 together with HSDPA and HSUPA.