A MIMO channel perturbation analysis for robust bit loading

In narrowband multiple-input multiple-output (MIMO) communication systems, when the channel state information (CSI) is known perfectly at the transmitter and the receiver, techniques such as waterfilling may use the singular value decomposition to separate the MIMO channel into independent single-input single-output subchannels. The signal-to-noise ratios of these subchannels are easily found, and, therefore, so are the subchannel bit allocations. In practice, perfect CSI is difficult to obtain. Imperfect CSI results in subchannel coupling and co-channel interference. In this paper, simple first-order expressions are presented for the signal and interference/noise powers for each subchannel for the imperfect CSI case. These expressions may be used to obtain more realistic subchannel bit allocations, allowing for fewer channel outages. Numerical simulations demonstrate the applicability and usefulness of the derived expressions.