Orthogonal signal-division multiplexing (OSDM) is an attractive alternative to conventional orthogonal frequency-division multiplexing (OFDM) due to its enhanced ability in peak-to-average power ratio (PAPR) reduction. Combining OSDM with multiple-input multiple-output (MIMO) signaling has the potential to achieve high spectral and power efficiency. However, a direct channel equalization in this case incurs a cubic complexity, which may be expensive for practical use. To solve the problem, low-complexity per-vector and block equalization algorithms of MIMO-OSDM are proposed in this paper for time-invariant and time-varying channels, respectively. By exploiting the channel matrix structures, these algorithms have only a linear complexity in the transformed domain. Simulation results demonstrate their validity and the related performance comparisons.

Orthogonal signal-division multiplexing (OSDM) is a recently emerging modulation scheme which, compared to conventional orthogonal frequency-division multiplexing, can effectively lower the peak-to-average power ratio and introduce intra-vector frequency diversity. In this paper, a time-domain oversampled OSDM system for underwater acoustic (UWA) communications is designed, where each OSDM vector is equivalently transmitted over multiple virtual channels, and thus an enhanced frequency diversity gain can be achieved. Moreover, at the receiver, zero vectors and frequency-shifted Chu sequences are used for Doppler compensation and channel estimation, respectively, while low-complexity per-vector equalization is performed based on the composite channel matrix factorization. Finally, the performance of the proposed OSDM system is evaluated through both numerical simulations and a short-range field experiment, and its effectiveness over time-varying UWA channels is confirmed.