[Paper Review] Analysis and Throughput Optimization of Selective Chase Combining for OFDM Systems

Taniya Shafique, Muhammad Zia+1 more|arXiv (Cornell University)|Mar 19, 2015

Advanced Wireless Communication Techniques17 references3 citations

TL;DR

This paper proposes cross-layer multiple selective Chase combining (MSCC) and Chase combining with selective retransmission (CCWS) for OFDM systems, optimizing throughput by retransmitting only poor-quality subcarriers based on SNR thresholds. It achieves significant throughput gains—especially in low SNR—by jointly decoding selective retransmissions, with analytical bounds closely matching simulation results.

ABSTRACT

In this paper, we present throughput analysis and optimization of bandwidth efficient selective retransmission method at modulation layer for conventional Chase Combining (CC) method under orthogonal frequency division multiplexing (OFDM) signaling. Most of the times, there are fewer errors in a failed packet and receiver can recover from errors receiving partial copy of original frame. The proposed selective retransmission method at modulation layer for OFDM modulation requests retransmission of information corresponding to the poor quality subcarriers. In this work, we propose cross-layer multiple selective Chase combining (MSCC) method and Chase combining with selective retransmission (CCWS) at modulation level. We also present bit-error rate (BER) and throughput analysis of the proposed MSCC and CCWS methods. In order to maximize throughput of the proposed methods under OFDM signaling, we formulate optimization problem with respect to amount of information to be retransmitted in selective retransmission in the event of packet failure. We present tight BER upper bounds and tight throughput lower bounds for the proposed selective Chase combining methods. The simulation results demonstrate significant throughput gain of the optimized selective retransmission methods over conventional retransmission methods. The throughput gain of the proposed selective retransmission atmodulation layer are also holds for conventional for hybrid automatic repeat request (HARQ) methods.

Motivation & Objective

• To address inefficiency in conventional Chase Combining (CC) that retransmits entire packets despite only partial errors.
• To exploit frequency-selective fading in OFDM to selectively retransmit only subcarriers with poor channel quality.
• To optimize the threshold on channel norm to maximize system throughput under selective retransmission.
• To develop tight analytical bounds for BER and throughput to validate performance without relying solely on simulations.
• To demonstrate that selective retransmission at the modulation layer outperforms conventional ARQ and HARQ methods in low-to-moderate SNR regimes.

Proposed method

• Proposes a cross-layer selective retransmission strategy at the modulation layer, targeting only subcarriers with low signal-to-noise ratio (SNR).
• Introduces multiple selective Chase combining (MSCC) to jointly decode multiple selective retransmissions from different subcarriers.
• Uses the norm of channel gain per subcarrier as a channel quality indicator to determine which subcarriers require retransmission.
• Optimizes the threshold τ on channel norm to maximize throughput, derived via exhaustive search for optimal SNR points.
• Derives tight upper bounds for bit-error rate (BER) and lower bounds for throughput to enable performance analysis.
• Validates the proposed methods using Monte Carlo simulations with half-rate LDPC codes over Rayleigh fading channels.

Experimental results

Research questions

• RQ1 How does selective retransmission of only poor-quality subcarriers in OFDM improve system throughput compared to full-packet retransmission?
• RQ2 What is the optimal threshold τ on channel norm that maximizes throughput in selective Chase combining schemes?
• RQ3 How does joint decoding of multiple selective retransmissions (MSCC) affect BER and throughput performance?
• RQ4 To what extent do analytical bounds for BER and throughput match simulation results in practical OFDM systems?
• RQ5 Does the proposed method maintain throughput gains when integrated with hybrid ARQ (HARQ) protocols?

Key findings

• The proposed MSCC method achieves significant throughput gain over conventional SCC and CCWS, especially in low and moderate SNR regimes.
• Throughput of CCWS with optimal threshold τ₀ exceeds that of CCWS with threshold τ_f (set to match full retransmission BER), indicating that optimal threshold selection enhances performance.
• There is a marginal gap between analytical throughput bounds and Monte Carlo simulation results, validating the accuracy of the derived lower bounds.
• BER performance does not always correlate with throughput; lower BER at τ_∞ does not yield higher throughput, as τ₀ maximizes throughput despite higher BER.
• The throughput of optimized selective retransmission methods exceeds that of conventional Chase Combining (CC) and ARQ, even when combined with HARQ protocols.
• Simulation results show that CCWS-HARQ with optimal threshold τ₀ achieves BER performance very close to that of conventional CC-HARQ (full retransmission), while offering higher throughput.