Pritam Keshari Sahoo* Error Rate Analysis of Phase Sampled RZ-GMSK over Turbulent FSO Channel https://doi.org/10.1515/joc-2018-0179 Received September 30, 2018; accepted October 07, 2018 Abstract: In this paper authors have studied the error performance of an ISI compensating modulation technique called phase sampled RZ-GMSK. The numerical results are derived suitably for optical wireless back-haul networks under log-normal turbulence fading. It is seen that the proposed modulation technique outperforms other techni- ques at higher turbulence and for high speed data transmission. Keywords: FSO, RZ-GMSK, log-normal turbulence, phase sampling 1 Introduction Optical wireless communication (OWC) also known as free-space optical communication (FSO) has recently gained advantages due to its cost effectiveness and license free spectrum. Further FSO overcome the drawback of network traffic bottleneck in RF transmission [1], with bit rate comparable to optical fiber communication. The objective of this research is to study numerically the error performance of a power efficient, continuous phase modulation technique called return to zero coded Gaussian minimum shift keying (RZ-GMSK) for the back- haul network. The Phase sampling technique in addition to coherent detection is a cost effective and inter-symbol interference (ISI) less reception that has been implemented in this paper. Finally authors have compared the error performance of RZ-GMSK with earlier proposed modula- tion techniques like PPM [2] and PPM-MSK [3] for the back- haul networks application. 2 System model RZ-GMSK is a continuous phase modulation which can be generated using Gaussian low pass filter followed by frequency modulator and the corresponding coherent receiver followed by phase sampling structure is shown in Figure 1. RZ source coding is used to consider a narrow pulse duration symbol with objective to reduce the symbol spreading caused by Gaussian filter leading to ISI. Bandwidth-Time duration product (BT parameter) of the Gaussian filter has control over the bit error rate (BER) as in [4] and side lobe power. Thus the authors have opted a compromising value of BT = 0.6 throughout this paper. The output response of Gaussian filter (g(t)), to a RZ coded pulse results a symbol of duration greater than Bit duration (T). 3 BER performance Let’s consider coherent receiver in presence of receiver shot noise and thermal noise. The error probability can be written as [5] p ec = 1 2 erfc d min 2 ffiffiffiffiffi N 0 p   (1) where, d min is the distance between two signal points on the constellation diagram. d min =2 ffiffiffiffiffi E b p sin Φ t ðÞ ð Þ j j =2 ffiffiffiffiffi E b p sin 2πp ð t 0 gt ðÞdt           (2) m is the modulation index having value equals to 0.5. g(t) = h(t) * rect(  ). h(t) = time response of pulse shaping Gaussian filter [4]. rect(.) = RZ coded binary sym- bol. If the power transmission is same for each symbol irrespective of the pulse width, the dispersed pulse g(t) is gt ðÞ = 1 2T dc ð Þ erfc ffiffi 2 p πBT t - T *dc ð Þ T ffiffiffiffiffiffiffiffiffiffi ln 2 ðÞ p  ! - erfc ffiffi 2 p πBT t + T *dc ð Þ T ffiffiffiffiffiffiffiffiffiffi ln 2 ðÞ p  ! " # (3) where, dc is the amount of duty cycle and the phase part of eq. (2) is computed and sampled at ‘T’, which can be written as eq. (3). *Corresponding author: Pritam Keshari Sahoo, Electronics and Communication Engineering Department, MNNIT, Allahabad, India, 211004, E-mail: sahoo.pritamkeshari@gmail.com http://orcid.org/0000-0001-5509-6431 J. Opt. Commun. 2018; aop Brought to you by | Göteborg University - University of Gothenburg Authenticated Download Date | 10/17/18 5:41 PM