Journal of VLSI Signal Processing 39, 147–160, 2005 c  2005 Springer Science + Business Media, Inc. Manufactured in The Netherlands. Flexible Implementation of a WCDMA Rake Receiver LASSE HARJU, MIKA KUULUSA AND JARI NURMI Tampere University of Technology, Institute of Digital and Computer Systems, P.O. Box 553, FIN-33101, Tampere, Finland Received March 20, 2003; Revised May 21, 2003; Accepted May 21, 2003 First online version published in September, 2004 Abstract. This paper presents an ASIC implementation of a WCDMA Rake receiver targeted for mobile termi- nals. The implementation is based on a FlexRake architecture that shares hardware resources between multipath components and uses data-level parallelism for despreading multiple code channels. This approach facilitates the flexibility of multipath operation and improves the receiver hardware efficiency. The architecture was implemented using register-transfer-level VHDL description and logic synthesis with standard cells. Synthesis for 0.18 μm CMOS technology resulted in 0.238 mm 2 area and 45.5 μW power consumption at 1.6 V. Keywords: WCDMA, Rake receiver 1. Introduction Third generation communications systems are based on Wideband CDMA air-interface that employs direct- sequence spread spectrum with 3.84 Mcps chip rate, and QPSK modulation for obtaining a peak data rate of 2.3 Mbps. Variable user data rates are obtained by changing the user spreading factor or by employing multicode transmission [1]. These methods provide the bandwidth-on-demand needed for variable data rate services, but at the same time, they set high require- ments for the receiver flexibility. Rake receivers are used in CDMA systems for ob- taining multipath diversity which is one of the most important capacity improving features of CDMA sys- tems [2]. Typically this involves a number of Rake fin- gers, each receiving a multipath signal, and a maximal ratio combiner for combining the outputs of each fin- ger. Even though the functionality of a Rake receiver is fairly simple, its implementation is of paramount importance considering the overall receiver flexibility, i.e., its controllability under changing radio conditions and variable user data rates. In this article, we present a WCDMA Rake receiver implementation based on the FlexRake architecture presented in [3] and [4]. The proposed architecture pro- vides improvements to the shortcomings of the tradi- tional Rake architecture related to the receiver flexibil- ity. This paper is organized as follows. First the Rake functionality is introduced in general, followed by a comparison between a conventional and the FlexRake architecture. In the next section, two implementation versions of the proposed architecture are studied and their advantages are highlighted. Simulation and syn- thesis results are then presented, and finally, conclu- sions are drawn. 2. Rake Receivers As a result of multipath propagation, several copies of the transmitted signal with different delays, attenua- tion, and phases are picked up by the receiver antenna resulting in multipath fading. In CDMA, multipath fad- ing is mitigated through Rake reception which is a form of space diversity. The essence of Rake reception is that the multipath components are treated as diversity