Interleave Division Access
Chapter 5: Interleave Division Multiple Access (IDMA)
5.1 Introduction
In Interleave Division Multiple Access (IDMA) communication, all users share the same transmission bandwidth at all times; interleavers are used to identify different users on the multiple access channel i.e. each user has its own unique interleaving pattern. IDMA is sometimes viewed as the special form of CDMA systems. Direct Sequence CDMA (DS-CDMA) uses signature sequences (PN codes) for user separation and identification but IDMA achieves the same goal through the use of different interleavers. IDMA uses a very simple chip-by-chip detection algorithm at the receiver side, for retrieving each user's data.
The characteristic feature of CDMA systems is the use of Pseudo Noise (PN) code for user separation. An interleaver is used between FEC encoder and Spreader to reduce the effect of burst errors and then spread signals from all users are super imposed on each other to yield a composite signal. CDMA receiver uses MUD (Multi User Detection) techniques to over come the problem of MAI (Multiple Access Interference) which is caused when signals from different users are superimposed on each other. The complexity of the MUD technique increases as the number of users in the system increase.
On the other hand, IDMA architecture outlines a very simple chip-by-chip detection technique to overcome the problem of MAI. IDMA receiver consists of an ESE (Elementary Signal Estimator) and FEC decoders which work in turbo (iterative) type manner. Moreover the complexity of chip-by-chip detection is independent of the number of users, which is the main advantage of IDMA scheme over CDMA.
IDMA also inherits many advantages [ref] of CDMA architecture like robustness against fading, dynamic channel sharing and ease of cell planning.
5.2 IDMA Architecture
5.2.1 Function of FEC Encoder
Data signal from each user is encoded by rate ‘R' FEC encoder. The function of FEC encoder is to spread the data over a large bandwidth i.e. a low rate code is used. (Bandwidth expansion is achieved thorough the FEC code, on the contrary CDMA performs this function through the use of signature sequence). The differentiating factor between CDMA and IDMA is that, CDMA achieves spreading through PN codes which are also used for user separation, while IDMA achieve bandwidth expansion (spreading) through FEC code and users are separated on the basis of Interleaver.
5.2.2 Function of Interleaver
After FEC encoding, encoded data is passed through a Pseudo Random Interleaver (for user identification). Every user has different interleaver, and all interleavers are generated independently. Moreover, the interleavers are known a priori at the receiver.
5.2.3 Elementary Signal Estimator (ESE)
ESE calculates initial course estimates [ref] for the decoding process; it jointly processes the signals for all users. The input to the ESE is the received (composite) signal and a prior probability of occurrence of a symbol. ESE does not consider FEC encoding constraint; it considers channel's impact and tries to estimate a chip by calculating mean and variance of the input. ESE produces soft output, which is given as input to the FEC decoder.
5.2.4 FEC Decoder
FEC Decoder performs APP (A posterior Probability) decoding in order to improve the estimates provide by the ESE. ESE and FEC Decoder work in iterative manner to improve the results. They are based on soft in soft out principle. Each FEC decoder handles data for a single user only, hence the complexity of a decoder is independent of the number of users [ref].
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