An Efficient Circulant MIMO Equalizer for CDMA Downlink: Algorithm and VLSI Architecture
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An Efficient Circulant MIMO Equalizer for CDMA Downlink: Algorithm and VLSI Architecture Yuanbin Guo,1 Jianzhong(Charlie) Zhang,1 Dennis McCain,1 and Joseph R. Cavallaro2 1 Nokia
Research Center, 6000 Connections Drive, Irving, TX 75039, USA of Electrical and Computer Engineering, George R. Brown School of Engineering, Rice University, 6100 Main Street, Houston, TX 77005, USA
2 Department
Received 29 November 2004; Revised 5 June 2005; Accepted 14 June 2005 We present an efficient circulant approximation-based MIMO equalizer architecture for the CDMA downlink. This reduces the direct matrix inverse (DMI) of size (NF × NF) with O((NF)3 ) complexity to some FFT operations with O(NF log2 (F)) complexity and the inverse of some (N × N) submatrices. We then propose parallel and pipelined VLSI architectures with Hermitian optimization and reduced-state FFT for further complexity optimization. Generic VLSI architectures are derived for the (4 × 4) high-order receiver from partitioned (2 × 2) submatrices. This leads to more parallel VLSI design with 3× further complexity reduction. Comparative study with both the conjugate-gradient and DMI algorithms shows very promising performance/complexity tradeoff. VLSI design space in terms of area/time efficiency is explored extensively for layered parallelism and pipelining with a Catapult C high-level-synthesis methodology. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
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INTRODUCTION
Wireless communication is experiencing radical advancement to support broadband multimedia services and ubiquitous networking via mobile devices. MIMO (multiple-input multiple-output) technology [1–3] using multiple antennas at both the transmitter and receiver has emerged as one of the most significant technical breakthroughs for throughput enhancement. On the other hand, UMTS [4] and CDMA2000 extensions optimized for data services lead to the standardization of multicode CDMA systems such as the high-speed downlink packet access (HSDPA) and its equivalent 1X evolution data and voice/data optimized (EV-DV/DO) standards [5]. This leads to an asymmetric capacity requirement, where the downlink even plays a more essential role than the uplink because of the downloading features. The application of the MIMO technology in CDMA downlink receives increasing interest as a strong candidate for the 3G and beyond wireless communication systems. Known as D-BLAST [3] and a more realistic strategy as V-BLAST [2] for real-time implementation, the original MIMO spatial multiplexing was proposed for narrowband and flat fading channels. In a multipath fading channel, the orthogonality of the spreading codes is destroyed. This introduces both the multiple-access interference (MAI) and the intersymbol interference (ISI). The conventional Rake receiver [6] could not provide acceptable per-
formance because of the very short spreading gain to support high-rate data services in multicode CDMA downlink. LMMSE (linear-minimum-mean-squared-error)-based chip equalizer is promising to restore the orthog
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