Multistream differential modulation for cyclic delay diversity in OFDM

A cyclic delay diversity in OFDM with noncoherent detection is considered. Delay diversity is done in a cyclic manner in order not to exceed the guard interval. We give a robust design criterion for the cyclic delay which enables to pick up the full spatial diversity in frequency selective channels with unknown delay spread. However, the effective channel coefficients of neighbouring subcarriers appear to be uncorrelated which makes differential modulation in frequency direction impossible. Therefore, we propose a multistream scheme which allows to noncoherently exploit the full spatial diversity in combination with standard differential modulation techniques. The new scheme is less complex than differential unitary space-time modulation. The overhead due to reference symbols is minimized and the detection delay is reduced compared to differential modulation in time direction.