Photon Netw Commun (2017) 33:143–151
Relay selection in mixed RF/FSO system using DF relaying
· Ankur Bansal
· Parul Garg
Received: 27 November 2015 / Accepted: 2 May 2016 / Published online: 17 May 2016
© Springer Science+Business Media New York 2016
Abstract In this paper, we carry out the performance analy-
sis of relay selection in decode-and-forward (DF)-based
mixed radio frequency/free-space optical (RF/FSO) system.
We consider two relay selection schemes, namely max-
select and distributed switch and stay, and compare their
performance with the system having all relays active. The
selected DF relay decodes the received RF signal from
source and converts it into an optical signal using the sub-
carrier intensity modulation scheme for transmission over
FSO links. The RF links follow generalized η − μ distribu-
tion, while the FSO links are subjected to pointing errors and
are assumed to follow gamma–gamma distribution. Novel
analytical expressions for cumulative density function and
the moment generating function of the equivalent end-to-end
signal-to-noise ratio are derived. Capitalizing on the derived
statistics, we provide the new closed-form expressions of the
outage probability for different relay selection schemes.
Keywords Mixed RF/FSO · Relay selection · Decode-and-
forward · Outage probability
The dual-hop relaying technology has been extensively used
in context of RF system [1–3]. However, the traditional
RF dual-hop system suffers from some crucial limitations
like scarcity of licensed spectrum and limited data rates.
To overcome these limitations, the free-space optical (FSO)
Division of Electronics and Communication Engineering,
Netaji Subhas Institute of Technology, Sector 3 Dwarka,
New Delhi 110078, India
communication system [4,5] is considered to be used in the
relaying with the RF system due to its desirable features [6,7].
The performance analysis of decode-and-forward (DF) relay-
ing in gamma–gamma fading channel is observed in , the
channel statistics, namely probability density function (PDF)
and cumulative distribution function (CDF), are evaluated,
and average BER of the considered system is investigated.
The mixed RF/FSO system provides high data rates, sup-
ports long distance transmission and also avoids signiﬁcant
interference as the RF and FSO links operate on different fre-
quency bands. Thus, to above-mentioned merits, the mixed
RF/FSO system has gained interest of various researchers [9–
The performance of mixed RF/FSO has been extensively
observed; however, these pioneer studies assume Nakagami-
m or Rayleigh fading channels for the RF link. These
distributions do not provide good ﬁt to experimental data 
and do not incorporate actual fading statistics of the RF link.
On the contrary, η − μ distribution  is a more gener-
alized fading model and outperforms some widely known
fading distributions.Also η−μ distribution provides a uniﬁed
framework and characterizes various well-known RF channel
models, Rayleigh (η → 0, k → 0,μ = 1), Nakagami-
m(η → 0, k → 0,μ= m) and Rician (μ = 1).
All the literature mentioned till now focuses on the sin-
gle relayed mixed RF/FSO communication system. Further
improvement in the system performance can be observed by
using the multiple relay system due to the spatial diversity;
however, the multiple relayed system suffers from limitation
of power distribution, i.e., either we need more transmitting
power or it is equally divided into all the relays, and also it
requires strict synchronization at the receiver. Thus to over-
come these limitations, we go for relay selection [14–16].
In relay selection, based on a performance metric the single
best relay is selected for data transmission.