1536 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 18, NO. 8, AUGUST 2000 A Generalized RAKE Receiver for Interference Suppression Gregory E. Bottomley, Senior Member, IEEE, Tony Ottosson, Member, IEEE, and Yi-Pin Eric Wang, Member, IEEE Abstract—Currently, a global third-generation cellular system based on code-division multiple-access (CDMA) is being developed with a wider bandwidth than existing second-generation systems. The wider bandwidth provides increased multipath resolution in a time-dispersive channel, leading to higher frequency-selectivity. In this paper, a generalized RAKE receiver for interference suppression and multipath mitigation is proposed. The receiver exploits the fact that time dispersion significantly distorts the interference spectrum from each base station in the downlink of a wideband CDMA system. Compared to the conventional RAKE receiver, this generalized RAKE receiver may have more fingers and different combining weights. The weights are derived from a maximum likelihood formulation, modeling the intracell interference as colored Gaussian noise. This low-complexity detector is especially useful for systems with orthogonal downlink spreading codes, as orthogonality between own cell signals cannot be maintained in a frequency-selective channel. The performance of the proposed receiver is quantified via analysis and simulation for different dispersive channels, including Rayleigh fading channels. Gains on the order of 1–3.5 dB are achieved, depending on the dispersiveness of the channel, with only a modest increase in the number of fingers. For a Wideband CDMA (WCDMA) system and a realistic mobile radio channel, this translates to capacity gains on the order of 100%. Index Terms—Code division multiple access (CDMA), interfer- ence suppression, maximum likelihood detection, multipath chan- nels, spread spectrum communication. I. INTRODUCTION A S DEMAND for wireless communications continues to grow, third-generation cellular communications sys- tems are being standardized to provide flexible voice and data services. Standardization bodies around the world are developing systems based on direct-sequence code-division multiple-access (DS-CDMA). In North America, the second generation DS-CDMA standard IS-95 is being used as a basis for a third-generation system (IS-2000) with wider bandwidth [2]. In Japan and Europe, a third-generation wideband CDMA (WCDMA) system [1], [2] is also being developed. Currently, there is significant effort to harmonize and merge these systems into a common, global third generation CDMA standard. From a receiver perspective, a wider bandwidth usually corre- sponds to a smaller chip period, increasing multipath resolution. In the downlink, where user signals from the same base station Manuscript received August 1999; revised February 28, 2000. G. E. Bottomley and Y.-P. E. Wang are with Ericsson Inc., RTP, NC 27709 USA (e-mail: bottoml@rtp.ericsson.se; wang@rtp.ericsson.se). T. Ottosson is with the Department of Signals and Systems, Chalmers University of Technology, SE-412 96 Göteborg, Sweden (e-mail: tony.ot- tosson@s2.chalmers.se). Publisher Item Identifier S 0733-8716(00)06120-5. pass through the same dispersive channel, increased multipath resolution results in interference with significant spectral dis- tortion. Also, the increased multipath resolution leads to loss of orthogonality to interferers within the cell in systems using or- thogonal spreading codes. The purpose of this paper is to develop a downlink receiver approach that takes advantage of these interference properties. As downlink receivers are typically small, battery powered, portable devices, the simple RAKE receiver structure is used, in which despread values produced by RAKE “fingers” are combined to generate a decision statistic. The interference components of the different RAKE fingers are modeled as colored, Gaussian noise to account for multipath dispersion and pulse shaping. The use of orthogonal spreading codes is accounted for when computing noise correlation between fingers and when determining the noise powers on the different RAKE fingers. These noise properties are used in a maximum likelihood (ML) approach to determine combining weights. Finger placement is based on maximizing the signal-to-noise ratio (SNR) of the decision statistic. By contrast, conventional RAKE reception uses finger placement and combining weights corresponding to the channel impulse response of the signal of interest (see, for example, [3, pp. 797–806]). As the proposed approach uses the RAKE receiver structure, but with possibly different finger placement and combining weights, it can be viewed as a generalized RAKE receiver. We will show results for a realistic mobile radio channel that indicate improvements in capacity on the order of 100% with no increase in the number of fingers and some additional complexity in calculating the combining weights. These gains, for a modest increase in complexity, make the proposed receiver a good candidate for improving downlink performance in both the existing IS-95 system and the forthcoming third-generation CDMA systems. In the past, other single-user detection methods have been de- veloped which model interference in a similar way. Noneaker [4] proposed a modified RAKE receiver for a downlink CDMA system where the combining is based on the signal-to-interfer- ence ratio per finger instead of the signal strength, accounting for uneven noise powers between fingers due to the use of or- thogonal spreading codes. However, the color (correlation) be- tween the fingers was not considered. Modeling interference as colored noise due to the pulse shaping was used in the work by Monk et al. [5]–[7], Wong et al. [8], [9], and Yoon et al. [10], [11]. Monk et al. considered frequency nonselective uplink channels with random spreading codes, proposing an approach that exploits the color introduced by the pulse shaping. An alternative structure based on this approach was given by Wong et al. [8]. Yoon et al. extended 0733–8716/00$10.00 © 2000 IEEE