14-11-2012, 02:24 PM
Optimal Power Allocation Algorithm for AF BAT Relaying
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Abstract
The Ampli¯er-and-Forward (AF) Bi-directional Ampli¯cation of Throughput (BAT)
relaying is one of the promising techniques to enhance the throughput of the relay-based bi-
directional transmission. However, the AF BAT protocol only works e®ectively at symmetric
channels in the bi-directional scenarios. In order to further enhance the system performance in
various channel propagations, we study the optimal power allocation algorithm for the AF BAT
protocol in this paper. The numerical results show that signi¯cant performance improvement is
achieved for the AF BAT relaying after the power allocation algorithm is applied, especially for
the transmission in asymmetric channels. Furthermore, compared with the DF BAT relaying,
the AF BAT protocol with optimal power allocation achieves higher spectrum e±ciency and is
more robust to the various channel propagations.
INTRODUCTION
The relay technique [1], which takes the advantage of cooperative diversity, is agreed to be one of
the most attractive and practical way to enhance the system performance with higher spectrum
e±ciency [2] and low interference. The chief limitation of the relay protocol is the shortage at
throughput, since the relay node cannot transmit and receive simultaneously. Compared with
the traditional bi-directional relay (TBR) protocol, the Decode-and-Forward (DF) Bi-directional
Ampli¯cation of Throughput (BAT) [3] is an e®ective way to improve the transmit rate by utilizing
the network coding algorithm. To further improve the peak normalized throughput, later in [4],
P. Popovski and H. Yomo proposed the Ampli¯er-and-Forward (AF) BAT, which utilize the inherent
packet combining from multiple access channel. However, the AF BAT algorithm only works
e®ectively when the tra±c intensity is symmetric in both S1-R and S2-R channels. When these
two channels are asymmetric, the throughput of the AF BAT relaying will be reduced signi¯cantly.
Thus proper modi¯cation should be concerned for the AF BAT algorithm.
PROTOCOL DESCRIPTION
We consider a three-node wireless network as the system model, which includes two terminals
S1 and S2 and the relay node R, and assume each node is equipped with a single antenna. As
illustrated in Figure 1, the parameters GS1;R, GS2;R and GS1; S2 represent the large scaled path
fading of links S1-R, S2-R, and S1-S2 respectively. During the bi-directional relaying transmission,
the signals x1 and x2 are destined from S1 to S2 and S2 to S1, respectively.
The TBR is illustrated in Figure 2(a). During the ¯rst two slots, S1 and S2 transmit signals
x1 and x2 to R separately. Then R forwards x1 and x2 to S2 and S1 during the next two slots.
Thus the TBR relaying spends 4 time slots for a complete circuit of transmission, and the transmit
e±ciency for the TBR is only 1/2.
CONCLUSION
Compared with the traditional relaying protocol, the AF BAT relaying is a promising way to
improve the spectrum e±ciency during the bi-directional transmission. However, the performance
of the AF BAT relaying is greatly attenuated in asymmetric channels. In order to enhance the
robustness of AF BAT protocol, we study the O-PA algorithm in this paper. Numerical results
show that the O-PA AF BAT relaying not only achieves signi¯cant PA gain at asymmetric channels,
but also has slight performance improvement at symmetric channels.