Cross-Layer Design of FDD-OFDM Systems based on ACK/NAK Feedbacks

TL;DR

This paper proposes a low-complexity, asymptotically optimal cross-layer resource optimization method for FDD-OFDM systems using ACK/NAK feedbacks, eliminating the need for explicit channel state information at the base station.

Contribution

It introduces a simple closed-form solution for cross-layer optimization based on ACK/NAK feedbacks, suitable for real-time implementation and outperforming systems relying on unreliable CSIT.

Findings

Achieves significant performance gain over non-ACK/NAK based systems.

Provides a low-complexity, asymptotically optimal solution for small PER.

Demonstrates robustness without explicit CSIT.

Abstract

It is well-known that cross-layer scheduling which adapts power, rate and user allocation can achieve significant gain on system capacity. However, conventional cross-layer designs all require channel state information at the base station (CSIT) which is difficult to obtain in practice. In this paper, we focus on cross-layer resource optimization based on ACK/NAK feedback flows in OFDM systems without explicit CSIT. While the problem can be modeled as Markov Decision Process (MDP), brute force approach by policy iteration or value iteration cannot lead to any viable solution. Thus, we derive a simple closed-form solution for the MDP cross-layer problem, which is asymptotically optimal for sufficiently small target packet error rate (PER). The proposed solution also has low complexity and is suitable for realtime implementation. It is also shown to achieve significant performance gain compared with systems that do not utilize the ACK/NAK feedbacks for cross-layer designs or cross-layer systems that utilize very unreliable CSIT for adaptation with mismatch in CSIT error statistics. Asymptotic analysis is also provided to obtain useful design insights.