BEST RELAY SELECTION IN DECODE AND FORWARD
COOPERATIVE COGNITIVE RADIO RELAY NETWORKS
OVER RAYLEIGH FADING CHANNELS
Yan Li
1
, Zhongshan Zhang
2
, Xu Zhang
3
, Bin Zhong
4
, Keping Long
5
Institute of Advanced Network Tech. and New Services (ANTS) and Beijing Key Laboratory of Knowledge Engineering for
Materials Science, USTB Beijing China, 100083
Keywords: cognitive radio, cooperative networks, relays,
decode and forward.
Abstract
The performance of the best relay selection on the decode-
and-forward (DF) mode cognitive radio relay networks is
studied. Some important factors, such as the outage
probability, the bit error ratio (BER), and the average channel
capacity, are analysed in this paper. As compared to the
conventional relay selection schemes, the impact of spectrum
sensing process as well as the spectrum utilization efficiency
of primary user on the performance of DF-based cognitive
radio relay networks is considered. In particular, this paper
has derived exact closed-form expressions for the outage
probability, BER, and average capacity over independent and
identically distributed (i.i.d.) Rayleigh fading channels,
respectively. The proposed scheme is validated by
simulations, and the analyses of closed-form expressions for
some figures of merit, e.g., the outage probability, the BER
and the average channel capacity, are all consistent with the
outcomes of simulations. The simulations also prove that the
performance of the proposed scheme is impacted
considerably by some other critical parameters, such as the
number of relays.
1 Introduction
Cognitive Radio networks exploit the existed SH to
communicate with destination, and the efficiency of spectrum
utilization is promoted consequently [1]-[4]. Depending on
cooperative relays, the cognitive radio networks has the
potential of offering a number of significant performance
benefits, which includes expanded coverage, better immunity
against signal fading, more system-wide power saving, and
the increased throughput of the whole system[5]-[9]. The
diversity order can be increased by employing multiple relays
transmit replicas to destination.
However, in the presence of multiple relays, a smart resource
allocation algorithm which guarantees the orthogonal
channels (in carrier frequencies, time slots or codes) are
allocated to each relay is required to defend against
interferences of all the relays between each other [10], [11].
As the number of relays increases, the cost of bandwidth
penalty may even deteriorates the overall performance of the
cooperative system.
Relay selection is one of the most attractive methods to solve
the complicated interference-mitigation issue met in the
multi-relay systems. Selecting the best relay to forward data is
an ideal method to maintain a full diversity gain and improve
the spectral efficiency at the same time [12], [13].
Furthermore, only two time slots or two orthogonal channels
(regardless of the number of relays) are required [14].
Currently, most of the studies about relay selection have been
focused on the primary user network [15], [16]. Besides, other
significant factors, such as mutual interference between
primary users and secondary users, are also considered in
cognitive radio relay network [17], [18]; the impact of
interference threshold on the amplify-and-forward model of
underlay cognitive networks has been studied in [19]. To the
best of the our knowledge, the impact of spectrum sensing
process and spectrum utilization efficiency of primary users
on the overlay cognitive radio relay networks with best relay
selection has not been studied considerably in prior works.
In this paper, best relay selection in DF cooperative cognitive
radio relay networks is studied. One main contribution of this
paper is to derive the closed-form expressions of some figures
of merit, whose contents are the outage probability, BER and
the average channel capacity over i.i.d. Rayleigh fading
channels for the proposed partial relay selection mode. In
particular, the derived results expose the dependence of
performance on the parameters of spectrum sensing process
as well as the spectrum utilization efficiency of primary users.
The validity of the proposed theoretical analysis is also
proven by simulations.
The remainder of this paper is organized as follows. Section 2
introduces the system model of best relay selection in
cognitive radio relay networks. The probability density
function (PDF) and cumulative distribution function (CDF) of
received SNR at destination are introduced in section 3. The
closed-form expressions of some critical parameters,
including outage probability, BER, and channel capacity, are
derived in section . Section gives out the numerical results.
Finally, Section concludes this paper.
Notation:
ab
represents the SNR of link a
ė
b link. f
X
(.) and
F
X
(.) represent the PDF and CDF of the random variable X,
respectively. Pr(.) denotes the probability.