EXIT chart analysis for turbo LDS-OFDM receivers

In this paper, the mutual information transfer characteristics of turbo Multiuser Detector (MUD) for a novel air interface scheme, called Low Density Signature Orthogonal Frequency Division Multiplexing (LDS-OFDM) are investigated using Extrinsic Information Transfer (EXIT) charts. LDS-OFDM uses Low Density Signature structure for spreading the data symbols in frequency domain. This technique benefits from frequency diversity besides its ability of supporting parallel data streams more than the number of subcarriers (overloaded condition). The turbo MUD couples the data symbols´ detector of LDS scheme with users´ FEC (Forward Error Correction) decoders through the message passing principle. The effect of overloading on LDS scheme´s performance is evaluated using EXIT chart. The results show that at E_b/N₀ as low as 0.3, LDS-OFDM can support loads up to 300%.