Spatially Multiplexed CDMA Multiuser Underwater Acoustic Communications

Direct-sequence (DS) code-division-multiple-access (CDMA) signaling allows multiple users to communicate simultaneously over the same bandwidth at the expense of a reduced rate. Due to severe cochannel interference (CoI) in multipath rich underwater channels, no more than 4 users have been demonstrated. Multiple-input-multiple-output (MIMO) communication also allows multiple users. Using parallel/sequential interference cancellation schemes, experimental results show similar limitations on the number of users. To extend the number of users beyond four (as required for certain applications), it is proposed to combine CDMA with MIMO, utilizing the spatial-temporal processing gain of passive-phase conjugation, and the approximate-orthogonality of the spreading codes to remove the (increased) CoI. The receiver algorithm uses simple matched filters without explicit channel-equalization and CoI cancellation. The algorithm performance is studied as a function of input-SNR, using simulated data based on experimentally measured channel impulse responses. M-sequences with 127, 255 and 511 chips are used. One finds that 255/511 chips are generally required to support 8 users, and 127/255 chips are needed to support 6 users for the channel studied. Multiuser communications are demonstrated with at-sea data for 8 users, with spreading codes of 511 chips, yielding zero bit errors. The data rate is ~ 8 bits/s/user, given 4 kHz bandwidth.