Performance and optimization of hybrid FSO/RF communication system in varying weather

Radio frequency (RF) spectrum is already dense enough and hard to add more broadband channels to meet the current user demands. Optical free-space communications could be an excellent alternative to the RF communications system, and it provides additional benefits, e.g., large bandwidth, high data rates and reliable communication link. Therefore, free-space optical (FSO) communication system becomes more attractive for the deployment of additional broadband channels, and it fulfils the current user demands of bandwidth-hungry applications. FSO communication links are susceptible to numerous meteorological conditions such as fog, snow, dust, smoke, scintillation and smog. Achieving better connectivity under the above-mentioned severe conditions is a crucial research question. Joint optical-RF communication system is developed to overcome the problems as mentioned earlier. The proposed adaptive optical-RF transmission system is optimized so that the system performance is maximized under all channel conditions. Optimization is achieved over the respective channel mappings, and the total required power by exploiting the proposed algorithm. The mapping schemes of each link are optimally chosen such that the total mutual information is maximized while distributing optimal power to the individual channel. Simulations are performed and verified with the analytical results to validate the proposed design. A comparison of the adaptive joint system (i.e., hybrid FSO-RF) over the non-adaptive system under various weather conditions is provided. From simulation results, the performance gain of more than 1 dB is achieved under the minimum power level. It is, therefore, recommended that the adaptive hybrid FSO-RF communication system is always an optimum solution for all weather conditions.

APD:

Avalanche photodiode

APSO:

Accelerated particle swarm optimization

BPSK:

Binary phase-shift keying

DEC:

Decoder

ENC:

Encoder

E/O:

Electrical to optical

EXIT:

Extrinsic information transfer

FSO:

Free-space optical

GGD:

Gamma–Gamma distribution

IR:

Infrared

IIGN:

Input-independent Gaussian noise

IM/DD:

Intensity modulation with direct detection

JPC:

Joint power constraints

LDPC:

Low-density parity check

LED:

Light emitting diodes

LOS:

Line-of-sight

MI:

Mutual information

OWC:

Optical wireless communication

OOK:

On–off keying

PIN:

Positive-intrinsic-negative

pdf:

Probability density function

4-PAM:

4-Level pulse amplitude modulation

RF:

Radio frequency

SDGN:

Signal-dependent Gaussian noise

SNR:

Signal-to-noise ratio

VLC:

Visible light communication

k :

Information bits

\(b_{\mathrm{r}}\) :

Encoded bits over the RF channel

\({\mathcal {S}}\) :

FSO channel input constellations

\({\widehat{s}}\) :

RF channel symbols

\(s_{\mathrm{H}}\) :

Hybrid symbol

\(m_{\mathrm{H}}\) :

Modulation rate

\({\widehat{f}}\) :

RF channel bit fraction

b :

Encoded bits

\(b_{\mathrm{o}}\) :

Encoded bits over the FSO channel

\(\widehat{{\mathcal {S}}}\) :

RF channel input constellations

s :

FSO channel symbols

\(\tau\) :

Codeword duration

\(P_{\mathrm{o}}\) :

Transmit power

f :

Optical channel bit fraction

The author would like to thank Prof. Bill Cowley and Dr. Khoa D. Nguyen from the Institute for Telecommunication Research, South Australia, for providing helpful support and useful suggestions during the course of investigation.

Authors and Affiliations

1. Department of Electrical Engineering, The University of Lahore, Lahore, Pakistan

   Muhammad Nasir Khan, Hasnain Kashif & Abdul Rafay

Corresponding author

Correspondence to Muhammad Nasir Khan.

Conflict of interest

The authors declare that they have no conflict of interest.

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