Soft-input/soft-output multiuser detection for iterative decoding of asynchronous CDMA systems with convolutional codes
ABSTRACT The optimal decoding scheme for asynchronous code-division multiple access (CDMA) systems that employ convolutional codes results in a prohibitive computational complexity. To reduce computational complexity, iterative receiver structures have been proposed for decoding multiuser data. At each iteration, extrinsic information is exchanged between a soft-input/soft-output (SISO) multiuser detector (MUD) and a bank of single-user SISO channel decoders. For asynchronous cases (see Alexander, P.D. et al., IEEE Trans. Commun., vol.47, p.1008-14, 1999), an optimal SISO MUD was derived based on the chip-synchronous assumption which is in general not valid for asynchronous CDMA systems. We propose an algorithm for an optimal SISO MUD without the chip-synchronous assumption. A direct implementation of the optimal SISO MUD also has exponential computational complexity in terms of the number of users, which is still prohibitive for channels with medium to large numbers of users. A low-complexity SISO MUD is proposed based on the decision-feedback scheme. In it, tentative hard decisions are made and fed back to the SISO multiuser from previously decoded output. The computational complexity of this detector can be linear in terms of the number of users and can be adjusted according to the complexity/performance trade-off. Simulations show that the performance of the low-complexity SISO MUD approaches that of the single-user system for moderate to high signal-to-noise ratios (SNR), even for a large number of users.
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ABSTRACT: We consider synchronous code-division multiple-access (CDMA) communications incorporating an error-control code (ECC) for improved performance. We describe a reduced-complexity multiuser sequence detector and develop a simple method for computing reliability values for the bit decisions produced by this detector. This reliability information is used by a soft-input Viterbi algorithm to decode the ECC. We compare the uncoded and coded performance of this reduced-complexity detector to those of certain decision-feedback equalizer (DFE) multiuser detectors. Simulation shows that the reduced-complexity sequence detector works better than the DFE detectorsIEEE Transactions on Vehicular Technology 09/1998; DOI:10.1109/25.704848 · 2.64 Impact Factor
Conference Paper: Near Shannon limit error-correcting coding and decoding: Turbo-codes. 1[Show abstract] [Hide abstract]
ABSTRACT: A new class of convolutional codes called turbo-codes, whose performances in terms of bit error rate (BER) are close to the Shannon limit, is discussed. The turbo-code encoder is built using a parallel concatenation of two recursive systematic convolutional codes, and the associated decoder, using a feedback decoding rule, is implemented as P pipelined identical elementary decodersCommunications, 1993. ICC 93. Geneva. Technical Program, Conference Record, IEEE International Conference on; 06/1993
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ABSTRACT: A code-division multiple-access system with channel coding may be viewed as a serially-concatenated coded system. In this paper we propose a low complexity method for decoding the resulting inner code (due to the spreading sequence), which allows iterative (turbo) decoding of the serially-concatenated code pair. The per-bit complexity of the proposed decoder increases only linearly with the number of users. Performance within a fraction of a dB of the single user bound for heavily loaded asynchronous CDMA is shown both by simulation and analytically.European Transactions on Telecommunications 09/1998; 9(5). DOI:10.1002/ett.4460090504 · 1.35 Impact Factor