ISSN (Print) : 2319-5940 ISSN (Online) : 2278-1021 International Journal of Advanced Research in Computer and Communication Engineering Vol. 1, Issue 10, December 2012 Copyright to IJARCCE www.ijarcce.com 837 A User-Spread Interleave Division Multiple Access System P. Niroopan, Yeon-ho Chung Department of Information and Communications Engineering, Pukyong National University, Busan, South Korea ABSTRACT:In this paper, we propose a User-Spread Interleave Division Multiple Access (US-IDMA) system that employs two way user separation using interleavers and spreading sequences to improve the performance of the system with more secure and reliable communications compared with the conventional IDMA system.These spreading sequences are not only as random as user data and independent of current symbols, but also dynamically changes from one symbol to another according to the user data.Therefore, this system offers enhanced security and privacy, due to the fact that the sequences make unwanted detection of the data by unintended receivers practically impossible.Also, simulation results show that the proposed system improves the bit error rate performance of the system in flat fading channel. Keywords:IDMA, Interleaver, Multiple access, Spreading, User data I. INTRODUCTION A new multiple access scheme called Interleave Division Multiple Access (IDMA) [1], [2], which is a special case of Code Division Multiple Access (CDMA) [3], was recently proposed as a spread spectrum multiple access scheme. Unlike a CDMA system, a user-specific spreading sequence is not used in the IDMA system. Instead, user-specific interleavers are adopted as the only mechanism for user separation where interleaver that gives the name to IDMA has an important role in the system architecture. As such, the IDMA utilizes a low- complexity iterative multiuser detection process in the receiver. In this paper, we propose a User-Spread IDMA (US- IDMA) system with user spread data. The conventional IDMA system uses repetition code only for improving the bit error performance of the system. Without the repetition code, IDMA systems give poor performance over the propagation channel and also affected by multiple access interference. Thereby without avoiding redundancy by the repetition code, we replace using the spreading sequence which is different for each user that also helps to mitigate multiple access interference in the IDMA system. Here we use randomly generated user data as a spreading sequence [4]–[6]. These dynamically changing spreading sequences make unwanted detection of the data from an unintended receiver practically impossible. To recover the dispreading sequences in the receiver, we use the genetic search algorithm and Markov chain analysis that help to refine dispreading sequences and updates in an optimal manner [7]. The remainder of this paper is organized as follows. In Section II, we present a brief introduction of transceiver structure in the IDMA system. Then, US-IDMA is described in Section III. In Section IV, numerical simulation results are presented and Section V concludes with remarks. II. IDMA SYSTEM The transmitter and receiver structures of an IDMA system with K-simultaneous users are shown in Figure 1. At the transmitter, the block size of N-length information bits from each user-k is denoted as d k = [d k (0),………, d k (N-1)] T , k =1,2,…,K. The data sequence is encoded using a convolutional code into b k = [b k (0),..……, b k (N C - 1)] T . That is, the code rate is defined as R 1 =N/N C . Then each bit of b k is again encoded using a low rate code such as a spread encoder with a rate of R 2 =1/S k , where S k is a spreading factor. Thus, the overall code rate is R 1 R 2 , which produces a chip signal. The second encoder output is fed into the user specific interleaver (π 1 , π 2 ,.….., π K ) for user separation, which generates x k (j), j=1, 2, ….., J, where J is the user frame length. The resultant signal is then transmitted through the multiple access channel.