Energy Efficiency of Cooperative SIMO Schemes -Amplify Forward and Decode Forward

In this paper, we quantify and compare the en- ergy efficiency of Amplify-and-Forward (AF) and Decode-and- Forward (DF) schemes on a basic three node cooperative network. Total energy is calculated considering circuit energy as well as transmission energy. For AF and DF schemes, we set a target Symbol Error Probability (SEP) and evaluate the minimum transmission energy for achieving the target SEP. In this process we first derive an approximation for SEP at high SNR for DF with possible error in relays. Next, we formulate energy calculation as a convex optimization problem and present the theoretical solution. The result is used to compare the total energy consumption of AF and DF for various relay positions: relay near source and relay near destination. The results illustrate significant dependence on (1) circuit energy for short distances where DF has lower energy efficiency and, (2) transmission energy at larger distances (between source and destination) where AF has lower efficiency. I. INTRODUCTION Recently, user-cooperative MIMO systems (1), (2) have been generating significant interest due to their capac- ity/performance gains over SISO networks. In cooperative MIMO architectures, individual nodes with single antennas collaborate with each other to act as a MIMO unit. As a result, the individual node complexity associated with tradi- tional MIMO implementations is alleviated. This feature is especially beneficial in sensor networks and celluar systems where individual node energy, size and cost are important constraints. Additionally, cooperative MIMO schemes provide all the benefits of traditional MIMO systems (3). Cooperation among nodes can be implemented in multiple ways. Typically, the goal of conveying information from a source to a destination is aided by a relay node that can either Amplify-Forward(AF) or Decode-Forward(DF) the informa- tion. At a higher protocol level, the rules of cooperative trans- mission can be further choreographed to implement a MIMO, MISO and SIMO schemes as presented in (4). The BER performance, capacity and outage probability calculations for AF and DF based cooperation strategies have been extensively studied (4), (5). Prior work in cooperative communicatons focus on efficient power allocation strategies for source and relay(s) in order to minimize BER/Outage probability or maximizing capacity(assuming limited total power) (6), (7). However, only a few papers have looked into the energy efficency of a cooperative communication system. By energy efficiency we mean: "What is the minimum (optimal) energy required by the network to transmit one bit from a source to a destination subject to a target performance criterion?". Fig. 1. Basic user-cooperative MIMO model This is unlike power allocation where we divide available power between source and relay(s) to maxmize/minimize some criterion(capacity/BER). Analyzing the energy efficiency of AF and DF based cooperative MIMO scheme will provide insights into which of the two modes can be used to meet a specific performance criterion when nodes are in power starved situation. This may in turn help in increasing the