To cope with unknown multipath propagation in wireless multiuser communications, existing blind CDMA approaches rely on the received data and the user-codes in order to suppress multi-user interference (MUI) at the receiver. Starting with a multirate equivalent model, this project takes a fresh look at designing appropriate codes that lead to low-complexity quasi-synchronous CDMA systems capable of eliminating MUI deterministically in the presence of unknown and even rapidly varying multipath. Judicious design of user codes converts the effect of frequency selective channels into flat fading. Residual flat fading effects are handled with differential encoding, and deep channel fades via a user code hopping technique at the transmitter -- an idea permeating benefits of frequency-hopping to direct-sequence CDMA. Orthogonal frequency division multiplexing (OFDM) follows as a special case and optimal user codes are investigated when carrier frequency errors, Doppler effects, time- and frequency-selective multipath, oversaturation, and different priority users are involved. Comparisons and performance studies are conducted using analytical tools, simulations, and real data collected from a software radio testbed.
