TRELLIS CODED MODULATION AND BIT INTERLEAVING: A REVIEW

: This research article represents the literature review on study of coded convolution codes like trellis coded modulation (TCM) and bit interleaved coded modulation (BICM) for robust error correction of codes. In wireless communication error correction codes and interleaving have an important application due to fading. Reviewed research papers have TCM and BICM with different techniques and methods used in it. Papers show different modulation schemes used with TCM for various applications. Different algorithms are used with BICM for improving bit error rate and improving bandwidth efficiency in reviewed papers. Different techniques like Iterative decoding and Constellation mapping are used with BICM and TCM to improve Bit error rate (BER) and signal to noise ratio (SNR).


INTRODUCTION
In wireless communication, signal is transmitted wirelessly (without wires) from source to destination. But the signal at the receiver end, we get, is distorted due to different obstacles in non line of sight that is called fading. To overcome fading error detection and correction codes are used. Error detection is the recognition of error due to niose or other deterioration during transmission. Error correction is the recognition of errors and reformation of the original data with no error. Error detection and correction can be done by simply adding some extra bits to a message which receiver can use to check compactness of the delivered message. Error correction codes are differentiated asconvolution codes and block codes. Block codes are processed on a segment basis and convolution codes on symbol basis. Convolution codes are the error correcting codes in which parity symbols are generated by sliding application (convolution of encoder over data) of a Boolean polynomial function to a data steam. Convolution codes are used to attain reliable data transfer in fields like mobile communication, satellite communication etc. So in today's wireless communication convolution codes can be used in different schemes like in trellis coded modulation.

A. Trellis coded modulation (TCM)
Trellis coded modulation is the composition of coding which is done digital and modulation which is done in analog as a single function. 'Trellis' stands for the use of trellis (also called convolution code). It has non linear nature of performance and its performance is measured by coding gain over an uncoded signal. By the use of convolutional coding TCM is bandwidth efficient modulation.The trellis coded modulation function contains a Trellis code and a Constellation Mapper. The function of Convolution coder of rate R = N/N+1 and a M-ary signal mapper that maps M = 2 N input points into bigger constellation of M = 2 N+1 constellation points are combined in TCM.

B. INTERLEAVING:
Interleaving is a process of making system more efficient, fast and reliable by reorganising a group of symbols. Interleaving can be used in arranging storage in RAM, error correction in data and in multidimensional data structure.

C. BIT INTERLEAVING:
In bit interleaving the data is shuffled so that the burst errors are converted into random error. Time division multiplexing (TDM), telecommunication and computer memory uses bit interleaving.

D. BIT INTERLEAVED CODED MODULATION:
Bit interleaved coded modulation is considered as the powerful strategy to achieve high data rates with general signal constellation. It uses powerful family of binary codes with any modulation format. [1] BICM for a binary code can be obtained by sequencing an encoder (ENC) with a memory less modulator over a specific signal set (x) throughout a bit interleaver π and one to one binary tagging map (µ). The code sequence is interleaved firstly and again it is subdivided into sub sequences of specific bits. In the end the resulting signal is transmitted on the vector channel. In BICM [10] channel encoder and modulator are distinguished by a bit level interleaver, increasing the flexibility in choosing the code rate and constellation individually.

LITERATURE SURVEY
This section provides a brief summary of work held on Trellis coded modulation and Bit interleaving coded modulation.
In [2] Lee -Fang Wei suggested the use of multidimensional TCM schemes like four, eight, or 16dimensional constellation instead of two -dimensional with same complexity but better tolerance to phase ambiguities. In [3] author analysed the performance of trellis coded MPSK in the faded environment. In the analysis the effect of fading is overcome by techniques like phase locked loop, pilot tone calibration, interleaving and deinterleaving. Analysis states the increase in gain depends upon lower bit error rate, size of interleaver and Doppler frequency. In [4] author gives an idea of multiple trellis coded modulation ( MTCM) with two -state trellis codes for symmetric Multiple phase shift keying modulation and Analog modulation gives three db gain without any loss in information bit rate compared to conventional trellis coded modulation , though some additional computations in twostate MTCM is needed. With advancements number of states and modulation schemes can be extended. In [5] Michel L. Moher and John H. Lodge describes a technique TCMP (trellis coded modulation with pilot sequences) to overcome the drawback of TCM that is TCM need coherent detection for achieving full optimization. In TCMP trellis coded data and pilot sequences are multiplexed together for coherent detection to provide robust performance in presence of fading.
G.Caire, G.Taricco, E.Biglieri in [6] give a brief idea about Bit interleaving coded modulation (BICM) and in what ways it is better than trellis coded modulation. Paper is focussed on evaluation of its performance on different fading channel like AWGN and Rayleigh fading. Comparative study results shows superiority of bit interleaved codes over Viterbi's orthogonal Convolutional codes (OCC). BICM specifications involve minimum hamming distance, minimum Euclidean distance and number of nearest neighbours which indicates the performance of BICM. Vincent K.N Lau [7] proposed an error correcting method, Adaptive Bit interleaved coded modulation for bandwidth efficiency in wireless system. Bit interleaving is preferred over TCM due to bit by bit interleaving instead of symbols. In ABICM according to channel conditions code rates and modulation level are varied. By less sufficiency feedback link -in good channel state modulation level and code rate are increased and vice versa in band channel state to maintain targeted error level. ABICM focused on multilevel puncturing and interleaving. In terms of signal to noise ratio and output significant performance improvement is obtained. Author of [8] proposed a bandwidth efficient method over fading channels that is BICM -ID. Compared to trellis modulation soft decision iterative decoding has large diversity with small Euclidean distance. According to results Performance efficiency of BCIM-ID depends on error free feedback. Naghi H. Tran and Ha H. Nguyen [9] investigated different mapping techniques for 8-ary constellation in BICM -ID. Main is put on Euclidean distance and Mutual information for mapping techniques. Free squared Euclidean distance (FED) distance parameters are also discussed in this paper. After comparing different mapping schemes Io Maximised mapping scheme was found to be most suitable for 8-ary constellation BICM -ID system. Results shown that BER (bit error rate) at high SNR for an uncoded system oven an additive white Gaussian noise channel (AWGN) is minimized by Io mapping in 8-ary constellation. In [10] Hierarchical constellations are used to protect a data stream from channel impairments. BER performance of single data stream is improved by using Constellation. Multiplexer and interleaver are also used for Nakagami-m fading channels. Bit interleaved coded modulation with iterative decoding in two way relaying communication (TWRC) is proposed in [11]. For multiple access (MA) phase iterative decoding is adopted relaying on quaternary code representation. Bit error probability is successfully deducted in this paper with the use of iterative decoding and XOR based network coding at the relay.