WIRELESS COMMUNICATIONS AND MOBILE COMPUTING Wirel. Commun. Mob. Comput. (2010) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/wcm.1055 RESEARCH ARTICLE Interleave-division multiple access (IDMA) using low-rate layered LDGM codes Francisco J. V ´ azquez-Ara ´ ujo 1∗ , Miguel Gonz ´ alez-L ´ opez 1 , Luis Castedo 1 and Javier Garcia-Frias 2 1 Department of Electronics and Systems, Universidade da Coru˜ na, 15071 A Coru˜ na, SPAIN 2 Department of Electrical and Computer Engineering, University of Delaware, DE 19716, U.S.A. ABSTRACT We propose the use of low-rate layered serially concatenated low-density generator matrix (SCLDGM) codes in interleave- division multiple access (IDMA) systems to approach the multiuser capacity. We study the behavior of the soft interference cancellation (SIC) detector employed in IDMA systems and design the channel codes using EXtrinsic Information Transfer (EXIT) evolution, aiming at optimizing the system performance. Simulation results show that the designed codes approach the theoretical limits and outperform previous IDMA schemes based on Turbo-Hadamard codes. Copyright © 2010 John Wiley & Sons, Ltd. KEYWORDS LDGM codes; SCLDGM codes; IDMA * Correspondence Francisco. J. V´ azquez-Ara ´ ujo, Facultade de Informatica, Campus de Elvi ˜ na s/n, 15071 A Coru˜ na, Spain. E-mail: fjvazquez@udc.es 1. INTRODUCTION In a multiple access channel (MAC), data are transmitted simultaneously by several users and arrives at the destina- tion as superimposed signals corrupted by noise. Optimum A Posteriori Probability (APP) detection in MACs involves an exhaustive search over all the possible transmitted streams, which has a complexity that is exponential with the number of users. There exist, however, several MAC transmission schemes that allow detection at the receiver with affordable complexity. Traditionally, these schemes have been based on separating the users by orthogonally dividing a common resource, such as time (TDMA), fre- quency (FDMA), or code space (CDMA). Nevertheless, such a resource division is not necessary if channel coding is correctly performed: MAC capacity can be achieved just by dividing the total capacity among the users, assigning to each user a fraction of the total [1] by directly encod- ing such user with a low-rate code of the desired rate [2,3]. In fact, orthogonal schemes, such as TDMA and FDMA, cannot approach the capacity of many channels such as the ergodic Rayleigh fading MAC. Interleave-division multiple access (IDMA) is an evo- lution of CDMA with forward error correction (FEC). A CDMA with FEC system consists of the concatenation of a FEC code, a spreading operation (unique for each user) and an interleaver (common to all users). Thus, FEC and spectrum spreading are carried out independently with different codes. As indicated in Reference [4], traditional CDMA performs far from the capacity limits because the MAC capacity is only achievable when the entire band- width expansion is devoted to coding. In contrast, in IDMA the spreading operation is the same among all users and a different interleaver is used by each user. Since the serial concatenation of FEC and spreading can be seen as a ‘whole’ low-rate FEC code [4--6], all users employ the same ‘whole’ FEC code, but the interleaver is different for each user. It is natural then to substitute the concatenation of code plus spreading by a single low-rate code. In Reference [5], the use of Turbo-Hadamard [7] codes is proposed, show- ing that the obtained performance is better than that of a conventional medium-rate Turbo code concatenated with a spreading operation. In this paper, we propose the use of a low-rate lay- ered LDGM code [8] as the FEC code of an IDMA scheme. Layered LDGM codes present several advan- tages over Turbo-Hadamard codes. First, their structure remains simple, since it only includes single-parity checks instead of Hadamard check constraints, so that the stan- dard sum-product algorithm can be applied for decoding. Copyright © 2010 John Wiley & Sons, Ltd.