1 research outputs found
Analysis of Feedback Overhead for MIMO Beamforming over Time-Varying Channels
In this paper, the required amount of feedback overhead for multiple-input
multiple-output (MIMO) beamforming over time-varying channels is presented in
terms of the entropy of the feedback messages. In the case that each transmit
antenna has its own power amplifier which has individual power limit, it has
been known that only phase steering information is necessary to form the
optimal transmit beamforming vector. Since temporal correlation exists for
wireless fading channels, one can utilize the previous reported feedback
messages as prior information to efficiently encode the current feedback
message. Thus, phase tracking information, difference between two phase
steering information in adjacent feedback slots, is sufficient as a feedback
message. We show that while the entropy of the phase steering information is a
constant, the entropy of the phase tracking information is a function of the
temporal correlation parameter. For the phase tracking information, upperbounds
on the entropy are presented in the Gaussian entropy and the von-Mises entropy
by using the theory on the maximum entropy distributions. Derived results can
quantify the amount of reduction in feedback overhead of the phase tracking
information over the phase steering information. For application perspective,
the signal-to-noise ratio (SNR) gain of phase tracking beamforming over phase
steering beamforming is evaluated by using Monte-Carlo simulation. Also we show
that the derived entropies can determine the appropriate duration of the
feedback reports with respect to the degree of the channel variation rates.Comment: 22 pages, 8 figure