  In this paper, we propose an optimal but low-complexity optimization method
for nonsquare differential massive MIMO. While a discrete nonlinear
optimization is required for the conventional nonsquare differential coding, we
newly modify it to perform a low-complexity continuous linear optimization.
This novel method exhibits immediate convergence as compared to the
conventional method. Additionally, the proposed differential coding can be
regarded as a differential counterpart of the coherent generalized spatial
modulation. Our numerical comparisons demonstrate that the proposed method
achieves the best coding gain for any number of transmit antennas, although the
optimization cost is nearly negligible.
