Delay-Constrained Transmission over Flat Fading Channels in the low SNR Range

When the channel state information is known to both the transmitter and the receiver, opportunistic transmission over fading AWGN channels can lead to increased rates in the low SNR region. Constraints on the transmission delay limit this gain and result in non-zero outage probability. In this article, in addition to a delay constraint, it is assumed that, while the transmitter and the receiver can estimate the channel perfectly, the transmitter can not know a priori the future channel states. Two transmission schemes are examined. The first scheme performs waterfilling with virtual future channel gains, whereas the second one only allocates power when the channel gain exceeds a threshold. The performance of the two schemes is evaluated in terms of the trade-off between the attainable average rate and the probability of outage for flat i.i.d. Rayleigh fading.