[Paper Review] Cooperative Encoding and Decoding of Mixed Delay Traffic under Random-User Activity

Homa Nikbakht, Michèle Wigger+2 more|arXiv (Cornell University)|Jun 2, 2021

Cooperative Communication and Network Coding12 references4 citations

TL;DR

This paper proposes a cooperative coding scheme for Wyner's symmetric interference network with randomly activated transmitters and receivers, supporting both delay-sensitive (fast) and delay-tolerant (slow) traffic. By enabling joint transmitter and receiver cooperation over D rounds, the scheme achieves full multiplexing gain for fast messages with negligible penalty on sum multiplexing gain, even for finite D, outperforming scheduling-only approaches that incur linear penalties.

ABSTRACT

This paper analyses the multiplexing gain (MG) achievable over Wyner's symmetric network with random user activity and random arrival of mixed-delay traffic. The mixed-delay traffic is composed of delay-tolerant traffic and delay-sensitive traffic where only the former can benefit from transmitter and receiver cooperation since the latter is subject to stringent decoding delays. The total number of cooperation rounds at transmitter and receiver sides is limited to $\\D$ rounds. We derive inner and outer bounds on the MG region. In the limit as $\\D\ o \\infty$, the bounds coincide and the results show that transmitting delay-sensitive messages does not cause any penalty on the sum MG. For finite $\\D$ our bounds are still close and prove that the penalty caused by delay-sensitive transmissions is small.

Motivation & Objective

• To analyze the multiplexing gain (MG) region in Wyner's symmetric interference network with random user activity and mixed-delay traffic.
• To investigate the impact of cooperation on MG when only delay-tolerant (slow) messages can benefit from cooperation due to strict delay constraints on delay-sensitive (fast) messages.
• To derive inner and outer bounds on the MG region under limited cooperation rounds D.
• To demonstrate that joint transmitter and receiver cooperation eliminates the MG penalty associated with fast messages, unlike scheduling-only schemes.

Proposed method

• Design a two-phase coding scheme: one for scheduling non-interfering fast transmissions using first Tx-cooperation round to precancel interference.
• Use the first Rx-cooperation round to forward decoded fast messages to neighboring receivers, enabling interference mitigation before slow message decoding.
• Apply Coordinated Multipoint (CoMP) reception over remaining D−2 cooperation rounds to jointly decode slow messages.
• Derive an inner bound via time-sharing between a fast-message-only scheme and a slow-message-only scheme.
• Establish an information-theoretic converse to derive outer bounds on the MG region.
• Analyze the MG region in the limit D→∞ and for finite D, showing tightness of bounds under full cooperation.

Experimental results

Research questions

• RQ1 What is the achievable multiplexing gain region for mixed-delay traffic in a Wyner network with random user activity and limited cooperation rounds D?
• RQ2 How does joint transmitter and receiver cooperation affect the multiplexing gain penalty for delay-sensitive (fast) messages compared to scheduling-only strategies?
• RQ3 Can the sum multiplexing gain be preserved when transmitting fast messages if both transmitters and receivers cooperate over D rounds?
• RQ4 What is the impact of finite cooperation rounds D on the MG region, especially for sparse user activity (ρ < 1)?
• RQ5 How does the proposed scheme compare to prior work relying only on receiver cooperation, which incurs a linear penalty in fast MG?

Key findings

• When D→∞ or all transmitters are active (ρ=1), the inner and outer bounds on the MG region coincide, proving that transmitting fast messages incurs no penalty on the sum multiplexing gain.
• For finite D and ρ=1, the achievable slow multiplexing gain is S^(S) = (D+1)/(D+2), approaching 1 as D increases.
• For ρ∈(0,1), the achievable slow MG is S^(S) = ρ − (1−ρ)ρ^(D+2)/(1−ρ^(D+2)), which approaches ρ as D→∞.
• The penalty on sum MG caused by fast messages is negligible under joint Tx-Rx cooperation, in contrast to scheduling algorithms that suffer a linear penalty in fast MG.
• The proposed scheme achieves full multiplexing gain for each fast transmission by precanceling interference at the transmitter using cooperation, enabling interference-free decoding.
• The results confirm that joint cooperation at both transmitter and receiver sides is essential to mitigate interference and eliminate MG penalties for fast traffic.