[Paper Review] Joint Uplink and Downlink Relay Selection in Cooperative Cellular Networks

Wei Yang, Lihua Li+3 more|arXiv (Cornell University)|Sep 1, 2010

Cooperative Communication and Network Coding5 references3 citations

TL;DR

This paper proposes Joint Uplink and Downlink Relay Selection (JUDRS), a novel relay selection scheme in cooperative cellular networks that jointly optimizes relay selection for uplink and downlink transmissions to minimize weighted total energy consumption. By considering channel quality and asymmetric traffic loads, JUDRS achieves full spatial diversity and demonstrates significant energy efficiency gains over existing schemes like best-worst and harmonic mean selection, particularly under asymmetric traffic conditions.

ABSTRACT

We consider relay selection technique in a cooperative cellular network where user terminals act as mobile relays to help the communications between base station (BS) and mobile station (MS). A novel relay selection scheme, called Joint Uplink and Downlink Relay Selection (JUDRS), is proposed in this paper. Specifically, we generalize JUDRS in two key aspects: (i) relay is selected jointly for uplink and downlink, so that the relay selection overhead can be reduced, and (ii) we consider to minimize the weighted total energy consumption of MS, relay and BS by taking into account channel quality and traffic load condition of uplink and downlink. Information theoretic analysis of the diversity-multiplexing tradeoff demonstrates that the proposed scheme achieves full spatial diversity in the quantity of cooperating terminals in this network. And numerical results are provided to further confirm a significant energy efficiency gain of the proposed algorithm comparing to the previous best worse channel selection and best harmonic mean selection algorithms.

Motivation & Objective

• To address the inefficiency of traditional relay selection schemes that treat uplink and downlink separately, increasing selection overhead and failing to account for asymmetric traffic loads.
• To minimize the weighted total energy consumption of MS, relay, and BS by jointly selecting relays for both uplink and downlink transmissions.
• To achieve full spatial diversity in the number of cooperating terminals, ensuring robust performance in fading environments.
• To design power allocation policies that enhance overall system energy efficiency under varying traffic and channel conditions.

Proposed method

• Proposes a joint relay selection framework that selects a single relay for both uplink and downlink transmissions, reducing selection overhead compared to independent selection.
• Introduces a weighted energy consumption metric that incorporates channel gains (|h_i|² for MS-relay, |g_i|² for relay-BS) and traffic load asymmetry via a factor ζ.
• Uses a relay selection function based on minimizing the sum of weighted energy costs, where weights reflect traffic load differences between uplink and downlink.
• Applies information-theoretic diversity-multiplexing tradeoff (DMT) analysis to prove that JUDRS achieves full spatial diversity with N cooperating nodes.
• Derives outage probability bounds for uplink and downlink using exponential channel gain distributions and applies asymptotic analysis to derive DMT performance.
• Validates the scheme through simulations comparing energy consumption per bit against best-worst and harmonic mean selection under varying relay numbers and traffic asymmetry.

Experimental results

Research questions

• RQ1 Can joint uplink and downlink relay selection reduce system overhead while maintaining or improving energy efficiency in cooperative cellular networks?
• RQ2 How does traffic asymmetry between uplink and downlink impact optimal relay selection and energy consumption?
• RQ3 What is the diversity-multiplexing tradeoff (DMT) performance of a jointly selected relay in a dual-hop cooperative network with multiple relays?
• RQ4 To what extent does JUDRS outperform existing relay selection schemes like best-worst and harmonic mean selection in terms of energy efficiency?
• RQ5 How does the energy saving of JUDRS scale with the number of available relays and the degree of traffic asymmetry?

Key findings

• JUDRS achieves full spatial diversity in the number of cooperating terminals, as proven by the DMT analysis showing d_JUDRS(r) = (N+1)(1 - (2N+1)/(N+1)r)+.
• The scheme reduces relay selection overhead by selecting one relay for both uplink and downlink, eliminating the need for separate selection processes.
• Numerical results show that JUDRS reduces total transmission energy per bit by up to 30% compared to the best-worst channel selection and up to 20% compared to the harmonic mean selection under asymmetric traffic.
• Energy savings increase with higher traffic asymmetry (lower ζ), with the relay's energy consumption being the primary contributor to savings.
• The energy efficiency gain is more pronounced when the direct MS-BS link is weak, as in the 1200m scenario, where JUDRS shows superior performance.
• With 8 active relays and asymmetric traffic (ζ < 0.3), JUDRS achieves significant energy savings, especially in downlink-heavy scenarios, demonstrating adaptive efficiency.