FREQUENCY DOMAIN EQUALIZER FOR MULTIRATE UMTS-TDD SYSTEMS Luis Gonc ¸alves, Atilio Gameiro Instituto de Telecomunicac ¸˜ oes, Campus Universit´ ario de Santiago, 3810-193 Aveiro, Portugal Departamento de Electr´ onica e Telecomunicac ¸˜ oes, Universidade de Aveiro, Portugal email: lgoncalves@av.it.pt, amg@det.ua.pt Abstract— Direct Sequence Spread Spectrum (DS-SS) signals exhibit cyclostationary properties which imply a redundancy between frequency components separated by multiples of the symbol rate. In this paper we present a Multiple Access In- terference Canceller that explores this property and applies to UMTS-TDD. This linear frequency domain canceller operates in the spreaded signal in such way that the interference and noise at its output is minimized (Minimum Mean Squared Error Criterium). The performance is evaluated in two configurations: one including the Frequency Shift Canceller (FSC) and the other concatenated with Parallel Interference Canceller (PIC). The results are benchmarked against the performance of the conventional RAKE detector and the conventional PIC detector. Index Terms— cyclostationary, redundant , frequency shift, PIC, DS-CDMA I. I NTRODUCTION Direct sequence spread-spectrum (DS-SS) code division multiple access (CDMA) has emerged as one of the most promising techniques to implement various radio communi- cation systems. It presents significant advantages over Time Division Multiple Access (TDMA), namely frequency diversity, multipath diversity and more spectrum efficiency on multicell systems [1], which led to its choice as the technology for third generation cellular systems. The first version of third generation CDMA systems will be based on the conventional RAKE receiver, which is known to be limited by the multiple access interference (MAI) and require a very precise power control. To overcome these limitations and therefore enhance the capacity of CDMA systems, joint detection of the received DS-SS signals has been proposed to be used at the base station (BS) or at the user equipment. The optimum joint detector [2] although well known requires however a prohibitively high computational complexity, and consequently effort has been made to devise suboptimum algorithms with good compromise between performance and complexity that can be implemented without prohibitive costs in near future CDMA systems. This communication fits in this approach, and aims at presenting a moderate complexity MAI canceller operating on the broad- band DS signal. The Frequency Shift Detector can be used either as standalone unit or it can be used prior to a PIC where it is intended to produce signals clean enough so that the first decisions of the PIC can be considered reliable enough to be used by the subsequent stages. This work is being supported by a PhD Grant from PRAXIS XXI, Fundac˜ ao para a Ciˆ encia e Tecnologia and the projects ASILUM and VISEF The DS-SS signal is a particular case of a stationary random pulse amplitude modulation. This kind of signals are known to have cyclostationary properties [3], [4]. Those properties imply redundancy between frequency components separated by multiples of the symbol rate. It is this characteristic that we explore to propose a new MAI canceller. In previous work [5] the Frequency Shift Canceller (FSC) was defined and implemented for a fixed spreading factor. The results have shown that considerable improvement is achieved against the RAKE and the FSC even outperforms the PIC for moderate to high values of E b /N 0 . The FSC was also tested as a precanceller to be used prior to a PIC and the results have shown that nearly complete interference cancellation in a UMTS-TDD scenario with up to eight simultaneous users with spreading factor in the same slot. In the present paper the FSC was implemented to comply with full UMTS-TDD norms. This enables the transmission of services with different rates by varying the spreading factor, but the structure of the spreading which is a composite operation involving channelization and scrambling, in UMTS-TDD, leads to codes that extend over several symbols. The paper is outlined as follows. In section two we show that in a DS signal non overlapping frequency bands separated by a multiple of the baud rate are linearly related. This result is used to present in section three the architecture and design principles of a MAI canceller that explores this redundancy. In section four we present simulation results that illustrate the performance provided by the new canceller. Finally in section five the main conclusions of this work are outlined. II. THEORETICAL BACKGROUND A DS-SS signal with spreading factor Q max (maximum spreading factor allowed in the system) is represented by s(t)=  k a k g(t - kT ) (1) where {a k } is the sequence of information symbols, 1 T the symbol rate and g(t) is the signature waveform. The Fourier Transform of this signal is S(f )=  k a k G(f )e -j2πfkT = G(f )A(f ) (2) with A(f )=  k a k e -j2πfkT (3) brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by Repositório Institucional da Universidade de Aveiro