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CTH14-5: Probability Density Functions of Logarithmic Likelihood Ratios in Phase Shift Keying BICM | IEEE Conference Publication | IEEE Xplore

CTH14-5: Probability Density Functions of Logarithmic Likelihood Ratios in Phase Shift Keying BICM


Abstract:

Bit-interleaved coded modulation (BICM) is a coded-modulation scheme where the output of the channel encoder and the input of the modulator are separated by a bit- level ...Show More

Abstract:

Bit-interleaved coded modulation (BICM) is a coded-modulation scheme where the output of the channel encoder and the input of the modulator are separated by a bit- level interleaver. From the decoder's point of view, the modulator, the transmission channel, and the demodulator (calculating bits' reliability metrics) become a memoryless BICM channel with binary inputs and real outputs. The BICM channel's outputs (reliability metrics) are known to be Gaussian for binary-or quaternary phase shift keying but no expression for their probability density function (PDF) is known for higher-order modulation. We fill this gap presenting closed-form expressions for PDF of reliability metrics in BICM based on M-PSK with gray mapping. Such probabilistic description of BICM channel is necessary to calculate the channel's information-theoretic parameters such as capacity, or cut-off rate but it is also a tool required to analyze the performance of coded transmission.
Published in: IEEE Globecom 2006
Date of Conference: 27 November 2006 - 01 December 2006
Date Added to IEEE Xplore: 16 April 2007
ISBN Information:
Print ISSN: 1930-529X
Conference Location: San Francisco, CA, USA

I. Introduction

Bit-interleaved coded modulation (BICM) is a flexible modulation/coding scheme which allows the designer to choose a modulation constellation independently of the coding rate [1] [2]. In BICM, the output of the channel encoder and the input to the modulator are separated by a (pseudo-)random bit-level interleaver. At the receiver's side, the reliability metrics for the coded bits are calculated according to the maximum likelihood principle. These logarithmic likelihood ratios (LLR), or simply -values, are then deinterleaved and further used by the soft-input channel decoder.

References

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