Threshold-Based Relay Selection for Detect-and-Forward Relaying in Cooperative Wireless Networks
EURASIP Journal on Wireless Communications and Networking
Hindawi Publishing Corporation
Threshold-Based Relay Selection for Detect-and-Forward Relaying in Cooperative Wireless Networks
Furuzan Atay Onat 1 2
Yijia Fan 0
Halim Yanikomeroglu 2
H. Vincent Poor 0
George Karagiannidis
0 Department of Electrical Engineering, Princeton University , Princeton, NJ 08544 , USA
1 ASELSAN Inc. , 06172 Ankara , Turkey
2 Department of Systems and Computer Engineering, Carleton University , Ottawa, ON , Canada K1S 5B6
This paper studies two-hop cooperative demodulate-and-forward relaying using multiple relays in wireless networks. A threshold based relay selection scheme is considered, in which the reliable relays are determined by comparing source-relay SNR to a threshold, and one of the reliable relays is selected by the destination based on relay-destination SNR. The exact bit error rate of this scheme is derived, and a simple threshold function is proposed. It is shown that the network achieves full diversity order (N + 1) under the proposed threshold, where N is the number of relays in the network. Unlike some other full diversity achieving protocols in the literature, the requirement that the instantaneous/average SNRs of the source-relay links be known at the destination is eliminated using the appropriate SNR threshold.
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1. Introduction
1.1. Background. Cooperative relaying can induce spatial
diversity in wireless networks without the need for
multiple antennas on a single terminal. Various
decode-andforward protocols have been proposed based on selective
relaying, distributed space-time coding, and relay selection
and have been shown to achieve full diversity [
1–5
]. Recently,
detection aspects of cooperative relaying have been analyzed
[
5–10
]. These works study the detect-and-forward (or
demodulate-and-forward) cooperative relaying protocols, in
which the relaying does not rely on any error correction or
detection codes. Such protocols are particularly attractive for
systems that do not use error detection/correction codes due
to tight energy constraints. One possible application is sensor
networks, which typically function under extremely limited
battery-supplied energy. Most coding schemes can consume
significant energy, and thus their use reduces sensor and
network lifetime if each relay decodes the data. Moreover,
the messages transmitted in sensor networks are usually
very short while coding is usually efficient only for long
messages.
Another relaying scheme that does not rely on any error
correction or detection is the amplify-and-forward protocol,
in which the relay amplifies and forwards the received
waveforms to the destination. The main disadvantage of this
scheme is noise amplification, which cannot be avoided due
to the physical presence of the thermal noise at the relay
receiver. The focus of this paper is on detect-and-forward
relaying.
1.2. Related Work. The detect-and-forward protocol has the
well-known disadvantage of error propagation. Unlike in
ideal decode-and-forward relaying, in detect-and-forward
relaying the relays can forward erroneous information, and
with a conventional combining scheme such as Maximal
Ratio Combining (MRC), these errors propagate to the
destination, causing end-to-end (e2e) detection errors. Existing
techniques for mitigating error propagation can be classified
into two groups. The first of these comprises selective and
adaptive relaying techniques, which include link adaptive
relaying (LAR) [
6
] and threshold digital relaying (TDR) [
11–
13
]. Both techniques use link SNRs to evaluate the reliability
of the data received by the relay. In TDR a relay forwards
the received data only when its received SNR is above a
threshold value. In LAR the relay transmits with a fraction
α of its maximum transmit power, where α depends on the
source-relay and relay-destination SNRs. In [
6
], a function
for calculating α is provided, and the resulting scheme is
shown to achieve full diversity if the relays are capable of
adjusting their transmit powers continuously. However, the
proposed function cannot provide diversity if reduced to two
power levels, that is, on/off power adaptation. TDR can also
be viewed as on/off power adaptation, and it is shown in [
13
]
that it can achieve full diversity in the single relay case. In
[
5
], a relay selection scheme similar to ours is studied. In
this paper an approximate expression for bit error probability
is derived as a function of relay threshold assuming that
MRC is performed at the destination. It is observed that the
performance of threshold based relay selection is sensitive to
the value of threshold.
The second approach to mitigate error propagation is
to develop better combining schemes for the destination.
These schemes take the possibility of error propagation
into account and require the relays to send their
sourcerelay link SNRs (average or instantaneous) to the
destination. In [
8
], Wang et al. ass (...truncated)