Achieving Delay Rate-function Optimality in OFDM Downlink with Time-correlated Channels

TL;DR

This paper investigates delay-optimal scheduling policies for OFDM downlink channels with time correlation, demonstrating that OPF policies are optimal under certain conditions related to channel correlation and OFF period distribution.

Contribution

It establishes delay rate-function optimality of OPF policies for time-correlated channels with specific conditions, extending previous results to more realistic channel models.

Findings

OPF policies are delay rate-function optimal for non-negatively correlated channels.

Optimality holds when the OFF period distribution is geometric.

Simulations support the theoretical findings.

Abstract

There have been recent attempts to develop scheduling schemes for downlink transmission in a single cell of a multi-channel (e.g., OFDM-based) cellular network. These works have been quite promising in that they have developed low-complexity index scheduling policies that are delay-optimal (in a large deviation rate-function sense). However, these policies require that the channel is ON or OFF in each time-slot with a fixed probability (i.e., there is no memory in the system), while the reality is that due to channel fading and doppler shift, channels are often time-correlated in these cellular systems. Thus, an important open question is whether one can find simple index scheduling policies that are delay-optimal even when the channels are time-correlated. In this paper, we attempt to answer this question for time-correlated ON/OFF channels. In particular, we show that the class of oldest packets first (OPF) policies that give a higher priority to packets with a large delay is delay rate-function optimal under two conditions: 1) The channel is non-negatively correlated, and 2) The distribution of the OFF period is geometric. We use simulations to further elucidate the theoretical results.