18-04-2014, 12:23 PM
Orthogonal Frequency Division Multiplexing–Interleave Division Multiple Access with User Grouping
Abstract
Frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA) are the main technologies used in first- to third-generation mobile cellular networks. Orthogonal frequency division multiplexing (OFDM) and the related orthogonal frequency division multiple access (OFDMA) are used in beyond third generation and fourth generation mobile cellular networks.
OFDMA can achieve inter-symbol interference (ISI) free transmission in multi-path channels. The related receiver cost is independent of the number of resolvable paths. OFDMA allows each user to select subcarriers with high channel gains. Multiple access interference (MAI) free transmission can be achieved by allocating different subcarriers to different users. Plain OFDMA is prone to fading in subcarriers. One solution to this problem is the use of forward error control (FEC) coding across subcarriers. Another solution is OFDM-CDMA, in which each information bit is spread and transmitted over a number of subcarriers. Since spreading results in rate loss, OFDM-CDMA allows multiple users to share common subcarriers. This avoids rate loss but at the cost of reintroducing MAI.
Multi-user detection (MUD) is a potential solution to the MAI problem. The interleave division multiple access (IDMA) scheme allows a very low-cost chip-by-chip (CBC) MUD algorithm to be used. The related complexity is independent of the number of users.
The main disadvantage of IDMA is that its receiver complexity increases linearly with the number of paths, which can be a concern for very wideband systems.