A New Combination of RAKE Receiver and Adaptive Antenna Array Beamformer for Multiuser Detection in WCDMA Systems

Hindawi Publishing Corporation International Journal of Antennas and Propagation Volume 2011, Article ID 208301, 9 pages doi:10.1155/2011/208301 Research Article A New Combination of RAKE Receiver and Adaptive Antenna Array Beamformer for Multiuser Detection in WCDMA Systems Shahriar Shirvani-Moghaddam1 and Hajar Sadeghi2 1 Digital Communications Signal Processing (DCSP) Research Lab., Faculty of Electrical and Computer Engineering, Shahid Rajaee Teacher Training University (SRTTU), 16788-15811 Tehran, Iran 2 Department of Electrical Engineering, Islamic Azad University, Tehran South Branch, 177761365 Tehran, Iran Correspondence should be addressed to Shahriar Shirvani-Moghaddam, sh Received 8 July 2011; Revised 22 August 2011; Accepted 23 August 2011 Academic Editor: Mandeep Jit Singh Copyright © 2011 S. Shirvani-Moghaddam and H. Sadeghi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The aim of this paper is to combine smart antenna beamforming and RAKE receiver in wideband code division multiple access (WCDMA). The proposed method combines spatial diversity as well as temporal diversity to improve the performance and overcome both interferences and multipath fading. This investigation has focused on one of the new proposed blind beamforming algorithms. It is based on constrained constant modulus (CCM) algorithm which is used for deriving a recursive-least-squares (RLS-) type optimization algorithm. We illustrate the comparison of bit error rate (BER) of the proposed receiver with simple correlator and also 1D-RAKE receiver in multiuser detection (MUD) WCDMA. The simulation results show that the proposed 2D-RAKE receiver offers lower BER rather than conventional ones, that is, it is an effective solution for decreasing the effect of interference and increasing the capacity, in a joint state. 1. Introduction In the cellular mobile communications, increasing demand to accommodate more users and new high bit rate data services becomes more and more obvious. Therefore thirdgeneration (3G) communications are oriented towards multimedia message capability [1] rather than voice communications. Wideband code division multiple access (WCDMA) has been adopted as the air-interface technology by the 3G wireless communication systems [2]. WCDMA is the wideband direct-sequence CDMA (DS-CDMA) system, that is, user information bits are spread over a wide bandwidth by multiplying the user data with pseudorandom bits (called chips). Some of the most important features of WCDMA are providing high capacities such as transmitting variable data rates with different mobility and quality of service requirements (multimedia services) and reducing the transmitted power [1]. In this system, receiver is a simple correlator which is not optimal due to its disability for combating the multipath effects. RAKE combiner is a popular and effective receiver utilizing temporal diversity in the presence of multipath effects. But unfortunately its performance is limited by multiaccess interference (MAI) because of disability for separating additive white Gaussian noise (AWGN) from MAI [3–7]. Since in WCDMA system, all of the users share the same frequency through the mobile radio channel, any other active users are cochannel interference to the desired user. Indeed, the system performance in WCDMA is limited by interference. Also one of the worse disadvantages of WCDMA is near-far effect. Recently, different ways have been presented to solve these problems. Some research works focused on introducing and using better receivers and other ones offer combined antenna beamforming and WCDMA receiver. Reference [6] develops an adaptive G-RAKE receiver, employing practical algorithms for finger placement and weight computation. It provides significant gains in performance by suppressing interference. In [8] a RAKE receiver has been proposed which works on chip level equalization on each RAKE finger to cancel MAI in multipath channel. 2 Adaptive antenna array systems combine an antenna array with a digital signal processing (DSP) unit. Using the estimated angle of signals arrival, this system form the main lobe of the antenna toward the desired signal and nulls on the direction of interferences. It enhances the quality of received desired signal, suppresses all interfering signals, increases spectrum efficiency, extends range coverage, and enlarges capacity [8, 9]. Recently, different research works have been presented which combine antenna beamforming and RAKE receiver for WCDMA applications. Due to the main goal of this paper, some of them are mentioned below. Reference [9] uses conjugate gradient (CG) adaptive beamforming for space diversity processing in each RAKE finger in a DS-CDMA. This paper improves network bit error rate (BER) and capacity of the system, using base-station assignment method based on minimizing the transmitter power (BSA-MTP) technique and power control error (PCE) in comparison with the conventional case, and adaptive beamforming passes the desired users’ signal in an arbitrary path. Also it reduces the MAI from other users and cancels the interpath interference (IPI) in other paths in each RAKE finger. Reference [10] develops a new scheme of blind beamformer for each branch of RAKE receiver [11] to suppress MAI, jamming, and multipath before combining. It shows better performance than the pre- and postcorrelation (PAPC) and the filter pair (FP) schemes. In [12] fixed-beam antenna array beamforming is combined with new improved version of root multiple signal classification (Root-MUSIC), direction of arrival (DOA) estimation algorithm. In this way, it works as a spatial filter to reduce the cochannel interference and noise. A beamformer-RAKE receiver is a computationally feasible solution to allow temporal-spatial signal processing for effectively overcoming signal fading and MAI. In [13], the performance of one type of beamformer-RAKE receiver in the geometrically based single bounce (GBSB) circular channel with the uplink WCDMA signal standard has been investigated. The least mean squares (LMS) and the recursive least squares (RLS) beamforming techniques and also maximal ratio combining (MRC) coherent RAKE receiver with different number of fingers are considered for simulation. The performances of the aforementioned beamforming techniques are compared in terms of BER against the number of users in different antenna elements and RAKE fingers. In [14–18] adding spatial processing achieved by digital beamforming antenna array systems to the RAKE receiver is proposed. This idea is one of the most promising techniques for increasing capacity, radio resource management (RRM), canceling the interferences such as MAI, and near-far effect in WCDMA systems. In [19], a code-CCM design criterion for linear receivers of DS-CDMA is investiga (...truncated)