Combined Code Division and Space Division Multiple Access for Broadband Acoustic Networks

We investigate the design of an acoustic communication network where multiple users, distributed across space, transmit simultaneously in the same band to a common base station. Specifically, we focus on a system in which the users transmit in an asynchronous fashion, employing orthogonal frequency division multiplexing (OFDM) as a modulation method. To distinguish between the users, the base station uses a combination of code-division and space-division multiple access. The base station iteratively steers a beam to each stable propagation path of the desired user's channel while placing nulls in the direction of other paths as well as in the directions of interesting users. Finally, the multiple paths of the desired user are recombined before data detection. Broadband beamforming is used to account for the broadband nature of acoustic signals. The beamformer coefficients on each of the OFDM carriers depend on the angles of signal arrivals, which are estimated using a dedicated procedure. The design concepts are demonstrated using a simulated shallow water channel, as well as experimental over-the-air transmissions in an indoor environment of an acoustic communications testbed.