Buffer State Information: Two-Level Water-Filling for Fixed Rate Applications

When transmitting at a fixed-rate to a wireless mobile terminal with strict latency requirements channel inversion is often used but at the cost of a significant loss in efficiency. Further, many such systems utilize limited packet buffering at the mobile terminal. In this paper we propose a method to recapture the lost spectral efficiency by exploiting the local buffer. Specifically, in severe channel conditions, latency and buffer state information (BSI) affords the transmitter to wait for good channel conditions rather than attempt to waste power through channel inversion. We propose a scheme that has nearly the spectral efficiency of optimal water-filling in time, yet is able to guarantee Quality of Service by ensuring a probabilistic buffer stability criterion. The solution is a cross-layer approach that utilizes event triggered BSI.