Delay-Limited Feedback of Channel State Information in OFDM Systems

In the work at hand the problem of transmitting channel state information (CSI) from the receiver to the transmitter of a multicarrier system is addressed. Transmission of CSI takes place over a fading multicarrier feedback link with strict delay constraints. The goal is to minimize a minimum squared error (MSE) distortion measure indicating the difference between the forward channel state and the CSI provided to the transmitter. An upper bound on performance is derived that implicitly considers the effects of the delay constraint by limiting the amount of diversity to that available in one transmission window. The optimum linear analog transmission approach is found for a case of practical importance. For such case, optimality of linear analog transmission is shown in the low signal-to-noise ratio (SNR) regime. In the high SNR regime delay constrained digital transmission is shown to be able to leverage the available diversity degree in terms of distortion decay rate. This is in contrast to linear analog transmission schemes, which show a distortion decay rate of 1 regardless the amount of diversity available in the feedback link.