Time-reversal space-time block coded WCDMA receiver in urban and suburban environments G. White, J. Gil, L. Correia, Y. Zakharov and A. Burr Abstract: Time-reversal space-time block coding (TR-STBC) allows spatial diversity gains to be achieved over time-dispersive channels. The technique can be applied to the WCDMA downlink in order to exploit both spatial and multipath diversity. In the paper, a low-complexity frequency- domain implementation of TR-STBC for a WCDMA chip equalisation receiver with complexity approximately one-third that of the time-domain equivalent, enabling implementation on existing DSP devices is proposed the performance of the proposed receiver is evaluated in a macro-cellular train (suburban) scenario and a micro-cellular street (urban) scenario. The effects of base-station antenna spacing are investigated. It is shown that the proposed TR-STBC receiver can provide significant performance benefits over SISO chip equalisers and TR-STBC RAKE receivers in a rich-scattering urban environment, even for small base-station antenna spacings. 1 Introduction Space-time block coding (STBC) [1, 2] is a MIMO technique that enables spatial diversity of a channel to be exploited, in order to improve system performance over fading channels. The popularity of STBC stems from its ability to offer maximum likelihood decoding with simple linear procesing at the receiver side. Also, unlike MIMO schemes based on statistical multiplexing (e.g. BLAST [3]), STBC can utilise any number of receive antennas, thus simplifying mobile terminal design. However, STBC as proposed in [1, 2] assumes frequency-flat channels and suffers performance degradation over frequency-selective channels (see Section 2). Time-reversal STBC (TR-STBC) was proposed in [4] as an extension of STBC for frequency- selective fading channels. Wideband code-division multiple access (WCDMA) has become the predominant multiple access technique for third generation cellular systems (e.g. UMTS [5]). Down- link transmission in current WCDMA cellular standards employs synchronous transmission and user spreading codes that are orthogonal at the transmitter. However, frequency-selective channels result in loss of orthogonality between spreading codes which may lead to significant levels of multi-access interference (MAI). Chip equalisation (e.g. [6–8]) can restore the orthogonality of the spreading codes and suppress MAI. In [9] , TR-STBC was applied to the WCDMA downlink, incorporating time-domain chip equalisation to mitigate MAI. Training was based on the RLS algorithm applied to a pilot burst. A key disadvantage of time-domain equalisation is the requirement for large-memory FIR equalisation filters for long delay spread channels, resulting in large computational complexity. In particular, TR-STBC equalisation requires longer FIR filters than single-input single-output (SISO) equalisation – typically several times the length of the channel. This problem can be circum- vented through the use of frequency-domain processing, whereby equalisation filtering becomes a point-by-point multiplication. In this paper, we propose a frequency-domain TR-STBC receiver for the WCDMA downlink, incorporating chip equalisation to suppress MAI and in which channel estimation is performed using a 3G-standard compatible code-multiplexed pilot signal (as in UMTS). We evaluate the complexity of the receiver and show that the reduction in complexity for frequency-domain processing would make it realisable on an existing DSP device, in contrast to the time-domain equivalent. A broadband geometrically-based finite scatterer channel simulator [10, 11] has been used to model two propagation scenarios likely to be encountered by a 3G cellular mobile terminal – a macro-cellular train scenario and a micro-cellular street scenario. The simulator has been parameterised so as to match closely measured delay and angular spreads in the literature for these scenarios [12] . The performance of the proposed receiver is determined by simulation in the two propagation scenarios. The effects of transmit antenna spacing, and thereby the effect of correlated channels, is investigated for the two scenarios. 2 Time-reversal space-time block coding 2.1 Background STBC as initially proposed [1, 2] achieves diversity gains over ( frequency-flat) fading channels by performing encoding on pairs, or small groups, of adjacent symbols. For two transmit antennas and one receive antenna, a two-fold diversity gain can be achieved using STBC over a frequency-flat channel, as shown in Fig. 1 (solid line, circles). However, over a frequency-selective channel, inter-symbol interference leads to a deterioration in G. White, Y. Zakharov and A. Burr are with Communications Research Group, University of York, York, YO10 5DD, UK J. Gil and L. Correia are with Instituto de Telecomunica- c * oes (IT)/Instituto Superior T! ecnico (IST), Technical University of Lisbon (TUL), Portugal J. Gil is also with Escola Superior de Tecnologia e Gest * ao (ESTG), Leiria Polytechnic Institute, Portugal E-mail: gpw100@ohm.york.ac.uk r IEE, 2005 IEE Proceedings online no. 20045275 doi:10.1049/ip-com:20045275 Paper first received 2nd November 2004 and in final revised form 21st April 2005 IEE Proc.-Commun., Vol. 152, No. 6, December 2005 1047