An enhanced widely linear CDMA receiver with OQPSK modulation

Abstract

Abstract-This paper studies an enhanced widely linear (WL) receiver for direct-sequence code-division multiple-access (DS-CDMA) systems that employ aperiodic spreading sequences with offset quadrature phase-shift keying (OQPSK) modulation. The modulation scheme generates improper complex multiple-access interference (MAI) and necessitates the use of WL receivers. Focus is on bandlimited pulse shapes and the inherent cyclostationarity (CS) of the uncoded received signal. The enhanced WL receiver replaces the conventional chip-matched filter with new filters that exploit the CS of the received signal through frequency shifting. The proposed WL receiver is shown to outperform the classical strictly linear (SL) receiver when the interfering users are (quasi-)synchronous with respect to the user of interest. High-powered interfering users, that may exist to support high data rates, increase the performance improvement delivered by the WL receiver. Moreover, it is shown that MAI can become proper, either identically or asymptotically, when users are asynchronous and equally powered. This is despite the fact that individual interfering signals are improper. Numerical results demonstrate that the WL receiver can outperform the SL receiver by 1-3 dB under the examined scenarios with current CDMA standards settings. In asynchronous or quasi-synchronous transmission modes, performance gain of the WL receiver degrades unless the number of high-powered active users remains small. An example for implementation of the WL receiver is proposed and compared with that of the SL receiver when minimum-shift keying modulation, a special case of OQPSK, is used. The implementation is based on a fractionally spaced equalizer whose taps are updated by an adaptive algorithm. It is shown that the proposed structure is capable of delivering the maximum signal-to-noise ratio predicted by theory.Index Terms-Bandlimited pulses, code-division multiple-access (CDMA), cyclostationarity (CS), fractionally spaced equalizer (FSE), improper noise, linear/conjugate-linear (L/CL) filtering, minimum-shift keying (MSK), offset quadrature phase-shift keying (OQPSK), widely linear (WL) receiver.

Widely linear MMSE precoding and equalization techniques for SC-FDE systems

“…The division by 2 in (41) is because the final symbol decision is only done on the real estimate [18]. Since [14] does not specify well A 1 and A 2 , (41) clarifies the calculation of the SINR for a SC-FDE system using widely linear MMSE equalization in the SISO case.…”

Adaptive-FRESH Filtering

“…This procedure requires knowing, at least, the cyclic spectrum of the interference. On the other hand, much work has been done when only the SOI cyclic spectrum is known, mainly with the aim of robust signal detection and estimation (Gardner, 1993;Mirbagheri et al, 2006;Zhang et al, 1999). The approach in this section is significantly different from these works, since it exploits the cyclostationarity of those signals to be removed, i.e.…”

“…This optimality can be understood in the sense of Signal-to-Noise Ratio (SNR) maximisation or in the sense of Minimum Mean Squared Error (MMSE, the one used in this chapter), where the optimal LAPTV filter may differ depending on the criterion used. Therefore, LAPTV filters find application in many signal processing areas such as signal estimation, interference rejection, channel equalization, STAP (Space-Time Adaptive Processing), or watermarking, among others (see (Gardner, 1994) and references therein, or more recently in (Adlard et al, 1999;Chen & Liang, 2010;Chevalier & Blin, 2007;Chevalier & Maurice, 1997;Chevalier & Pipon, 2006;Gameiro, 2000;Gelli & Verde, 2000;Gonçalves & Gameiro, 2002;Hu et al, 2007;Li & Ouyang, 2009;Martin et al, 2007;Mirbagheri et al, 2006;Ngan et al, 2004;Petrus & Reed, 1995;Whitehead & Takawira, 2004;2005;Wong & Chambers, 1996;Yeste-Ojeda & Grajal, 2008;Zhang et al, 2006;). This chapter is devoted to the study of LAPTV filters for adaptive filtering.…”

Multicarrier waveforms with I/Q staggering: uniform and nonuniform FBMC formats

“…This modulation format creates the basis for efficient utilization of spectrally shaped subchannels of the filter bank multicarrier (FBMC) aggregations, also known as OFDM/OQAM that can reduce the adjacent channel interference, at least without the presence of nonlinear distortions, whose significance has been discussed in [1] (The cosine-modulated multitone (CMT) therein is actually a form of OFDM/OQAM with odd-spaced subchannels). On the other side, this modulation format offers an additional degree of freedom, enabling suppression of the kindred co-channel interference based on the quasi-rectilinear constellation and the intrinsically real domain orthogonality by means of application of the minimum mean square error widely linear filtering (MMSE WLF) equalization [2][3][4]. Starting from the time-frequency duality between standard OFDM/OQAM formats and TLO multicarrier formats, we introduce uniform (even-and odd-spaced) FBMC-FLO and FBMC-TLO configurations.…”