Blind Receiver Design for OFDM Systems Over Doubly Selective Channels

We develop blind data detectors for orthogonal frequency-division multiplexing (OFDM) systems over doubly selective channels by exploiting both frequency-domain and time-domain correlations of the received signal. We thus derive two blind data detectors: a time-domain data detector and a frequency-domain data detector. We also contribute a reduced complexity, suboptimal version of a time-domain data detector that performs robustly when the normalized Doppler rate is less than 3%. Our frequency-domain data detector and suboptimal time-domain data detector both result in integer least-squares (LS) problems. We propose the use of the V-BLAST detector and the sphere decoder. The time-domain data detector is not limited to the Doppler rates less than 3%, but cannot be posed as an integer LS problem. Our solution is to develop an iterative algorithm that starts from the suboptimal time-domain data detector output. We also propose channel estimation and prediction algorithms using a polynomial expansion model, and these estimators work with data detectors (decision-directed mode) to reduce the complexity. The estimators for the channel statistics and the noise variance are derived using the likelihood function for the data. Our blind data detectors are fairly robust against the parameter mismatch

Performance Analysis of OFDM-CDMA Systems with Doppler Spread

6 pagesInternational audienceMulti carrier modulation are very sensitive to rapid time-varying multi-path channel characterized by Doppler spread. Although progress has been made in the description of the time variation, there is still considerable gaps in its effect especially on diversity gain acquired by time selectivity. This paper models a general case of time-varying channel effect on the OFDM-CDMA performance. This performance is measured through the Signal to Interference and Noise Ratio SINR at the output of the detector and the Bit Error Rate BER at the output of the channel decoder. The originality of the paper is twofold. First, we propose a simple tool to evaluate an analytical expression of the SINR independently on the spreading codes while taking into account their orthogonality. Second, we adapt a new technique to predict the BER at the output of the channel decoder from the link level simulation expressed in terms of the SINRs. We show by simulation the validity of our analytical models. We show also that the time variation of the channel would be favourable for system performance in MC-DS-CDMA system and QPSK constellation however it is destructive with other simulation assumptions