[Paper Review] Random Linear Network Coding For Time Division Duplexing: When To Stop Talking And Start Listening

Daniel E. Lucani, Milica Stojanovic+1 more|ArXiv.org|Sep 13, 2008

Cooperative Communication and Network Coding19 references4 citations

TL;DR

This paper proposes a random linear network coding scheme for time division duplexing (TDD) channels that optimizes the number of coded packets transmitted back-to-back before listening for an ACK, minimizing mean completion time. The key contribution is showing that selecting the optimal number of coded packets—dependent on latency, packet erasure probability, and ACK erasure—achieves performance close to full-duplex systems, with significant gains over ARQ schemes under high latency and error conditions.

ABSTRACT

A new random linear network coding scheme for reliable communications for time division duplexing channels is proposed. The setup assumes a packet erasure channel and that nodes cannot transmit and receive information simultaneously. The sender transmits coded data packets back-to-back before stopping to wait for the receiver to acknowledge (ACK) the number of degrees of freedom, if any, that are required to decode correctly the information. We provide an analysis of this problem to show that there is an optimal number of coded data packets, in terms of mean completion time, to be sent before stopping to listen. This number depends on the latency, probabilities of packet erasure and ACK erasure, and the number of degrees of freedom that the receiver requires to decode the data. This scheme is optimal in terms of the mean time to complete the transmission of a fixed number of data packets. We show that its performance is very close to that of a full duplex system, while transmitting a different number of coded packets can cause large degradation in performance, especially if latency is high. Also, we study the throughput performance of our scheme and compare it to existing half-duplex Go-back-N and Selective Repeat ARQ schemes. Numerical results, obtained for different latencies, show that our scheme has similar performance to the Selective Repeat in most cases and considerable performance gain when latency and packet error probability is high.

Motivation & Objective

• To address the lack of network coding schemes optimized for time division duplexing (TDD) channels where nodes cannot transmit and receive simultaneously.
• To minimize the mean completion time for transmitting M data packets over a TDD link with packet erasures.
• To determine the optimal number of coded packets to send before stopping to listen for an ACK, balancing transmission and feedback delays.
• To compare the proposed scheme’s performance with standard half-duplex ARQ protocols (Go-Back-N and Selective Repeat) in terms of throughput and delay.
• To demonstrate that the scheme guarantees low per-packet delay and outperforms ARQ under high latency and high error probability.

Proposed method

• The sender transmits a block of N_i randomly linear coded packets back-to-back before pausing to receive an ACK from the receiver.
• The ACK conveys the number of remaining degrees of freedom (dofs) required for successful decoding at the receiver.
• An optimization framework is developed to find the optimal N_i that minimizes mean completion time, considering latency, packet erasure probability, and ACK erasure probability.
• The optimal N_i is computed via a recursive one-dimensional search over M values, avoiding a complex M-dimensional search.
• The scheme uses a block-by-block transmission model where each block of M data packets is fully transmitted and acknowledged before the next block begins.
• Optimal values are pre-computed and stored as look-up tables, minimizing real-time computational load at the nodes.

Experimental results

Research questions

• RQ1 What is the optimal number of coded packets to transmit before pausing to listen for an ACK in a TDD network coding system?
• RQ2 How does the optimal transmission block size depend on latency, packet erasure probability, and ACK erasure probability?
• RQ3 How does the proposed scheme compare to Go-Back-N and Selective Repeat ARQ in terms of throughput and delay under high-latency and high-error conditions?
• RQ4 Can the proposed scheme achieve performance close to that of a full-duplex system in half-duplex TDD environments?
• RQ5 Does the scheme guarantee low per-packet delay compared to ARQ protocols that may reorder or delay individual packets?

Key findings

• The proposed scheme achieves throughput performance very close to that of a full-duplex system, especially when the optimal number of coded packets is used.
• Choosing a suboptimal number of coded packets can cause significant performance degradation, particularly under high latency and high packet error probability.
• For a 10 Mbps link with 80% packet error probability and high RTT, the scheme’s throughput was more than three times higher than that of Selective Repeat ARQ.
• The scheme outperforms Go-Back-N significantly under high error conditions, especially when latency is high.
• The scheme guarantees low delay per data packet, unlike Selective Repeat ARQ, which may delay individual packets indefinitely.
• The computational overhead is negligible at runtime, as optimal transmission block sizes are pre-computed and stored as look-up tables.