Subblock Equalization and Code Averaging for DS-CDMA Receivers

• Published in: IEEE transactions on vehicular technology 2010-09, Vol.59 (7), p.3321-3331
• Main Authors: Bottomley, G E, Wang, Y.-P E
• Format: Article
• Language: English
• Subjects: Applied sciences; Channels; Code averaging; code-division multiple-access (CDMA); Coding, codes; decision feedback equalization; Decision feedback equalizers; Design engineering; Detection, estimation, filtering, equalization, prediction; Dispersion; Equalization; Exact sciences and technology; Field-flow fractionation; Filtering; Formulations; Information, signal and communications theory; interference suppression; Intersymbol interference; linear equalization; Multiaccess communication; Nonlinear filters; Pulse shaping methods; Radiocommunications; Receivers; Shape; Signal and communications theory; Signal, noise; Spreading; Studies; Symbols; Systems, networks and services of telecommunications; Telecommunications; Telecommunications and information theory; Transmission and modulation (techniques and equipments); Transmitters. Receivers; Transversal filters
• ISSN: 0018-9545; 1939-9359
• DOI: 10.1109/TVT.2010.2050349

As third-generation (3G) cellular communication systems evolve to higher data rates, channel equalization becomes an important tool in maintaining receiver performance in dispersive channels. Because these systems employ direct-sequence code-division multiple-access (DS-CDMA), they pose unique challenges in equalizer design. In this paper, linear equalization (LE) and decision feedback equalization (DFE) designs are developed based on subblock equalization, in which a limited form of maximum-likelihood (ML) joint detection is used to detect symbols within a subblock. A pseudo-ML formulation is used, treating interference from symbols outside the subblock of interest as noise. This formulation leads to a feedforward filter (FFF) whose coefficients depend on the spreading codes of the symbols. These spreading codes change every symbol period, due to long-code scrambling, requiring constant recomputation of the FFF coefficients. To reduce complexity, code averaging is used, resulting in a code-independent periodically varying transversal filter. Another design is obtained by constraining the transversal filter to be time invariant. Simulation results for the high-speed packet-access uplink show that code averaging introduces a small loss in subblock equalization performance and that interesting gains over conventional LE can be obtained.