Superposition coded modulation

Abstract

Coded modulation (CM) is an effective high-rate transmission technique that has attracted tremendous attention. The revolutionary trellis coded modulation (TCM) scheme can achieve high spectrum- and power-e ciencies based on the joint design of coding and modulation. However, it requires a specially-tailored code for each particular transmission rate. This issue can be tackled by multi-level codes (MLC) and bit-interleaved coded modulation (BICM) where the on-shelf binary codes can be directly applied. All these CM schemes can yield signi cant coding gains. However, they still su er from capacity loss, high decoding and design complexities. Superposition coded modulation (SCM) is an alternative CM scheme recently proposed. Compared with conventional schemes which usually employ non-linear bit-to-symbol mapping, SCM produces transmit signals by superimposing independent binary coded sequences (each referred to as a layer). By properly choosing the weighting factors, the same binary component code can be used at all layers. This greatly simpli es the search of good component codes. Furthermore, thanks to the linearity involved, a successive-interference cancelation (SIC)-type receiver can be applied to reduce complexity. SCM has other advantages such as capability of achieving shaping gain, diversity gain and unequal error protection. However, as a newly proposed CM scheme, SCM is still limitedly explored. In this thesis, we make a comprehensive study on the theoretical and practical aspects of SCM. In the rst contribution, the analysis and design of SCM over memoryless channels is investigated. The basic features of SCM are described, followed by the information-theoretic analysis. Different encoding/decoding strategies are compared from the capacity point of view, which also provides insights into the design of capacity-approaching schemes. Error-bounding analysis is then discussed to predict the asymptotic performance with low-complexity codes. In order to examine the convergence property of iterative decoding, a mutual-information-based (MI) evolution technique is proposed subsequently. These analysis techniques provide convenient tools for performance evaluation and optimization. In the second contribution, we study the power e ciency of SCM with high data rates. In particular, we consider the peak-to-average power ratio (PAPR) problem. In SCM with very large constellations, the transmit signal exhibits a high PAPR, which may degrade the e ciency of the radio frequency power amplier. We apply a straightforward but effective clipping technique to handle this problem. We show that the capacity loss is marginal for clipping depth of practical interests if the optimal receiver is used. We also devise a low-cost, iterative soft compensation receiver for real implementations. Numerical examples are provided to demonstrate that SCM with clipping can still provide e cient high-rate transmissions. In the third contribution, we consider the application of SCM in orthogonal frequency-division multiplexing (OFDM) systems. As well known, OFDM also suffers from high PAPR. We apply clipping to solve this problem for general coded OFDM schemes and devise a novel soft compensation method that outperforms conventional approaches. Our focus is on the comparison of the performance with different signaling schemes. The major nding is that SCM-OFDM schemes are more robust to the clipping effect than other alternatives such as those based on conventional BICM. This is veri ed by both theoretical proofs and simulation studies. We also extend the MI evolution method to clipped SCM-OFDM systems with soft compensation for performance analysis and optimization. In the fourth contribution, we discuss SCM in communication systems suffering from interferences, such as the inter-symbol interference (ISI) in multi-path channels and the cross-antenna interference (CAI) in multiple-input multiple-output (MIMO) channels. A generic linear vector channel is used to model such systems. We rst outline an iterative linear minimum mean square error (LMMSE) receiver to provide a uni ed solution. We then study a semi-analytical evolution technique to provide quick prediction of the system performance. We show that SCM has a signi cant advantage in canceling interferences and can greatly improve the overall performance when compared with other conventional CM schemes. In the nal contribution, we consider the applications of SCM in multi-user communications. In particular, we incorporate SCM into the OFDM interleavedivision multiple-access (OFDM-IDMA) scheme. Analysis and design issues are discussed. We show that SCM-based OFDM-IDMA schemes are more robust in fading channels than other alternative multiple-access schemes. In particular, it provides a simple and e ective means of achieving multi-user gain with low PAPR. In summary, this thesis presents a comprehensive study of SCM over a variety of channels including memoryless and multi-path/multi-user/multi-antenna channels. Both single- and multi-carrier transmissions are examined. The theoretical and simulation studies show that SCM o ers a very attractive option for high-rate transmissions.

Date of Award	15 Jul 2009
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ping LI (Supervisor)