Hybrid direct-detection differential phase shift keying-multipulse pulse position modulation techniques for optical communication systems Ahmed E. Morra a,b,d,n , Hossam M.H. Shalaby b,1 , Salem F. Hegazy c,d , Salah S.A. Obayya d a Faculty of Electronic Engineering (FEE), Menoufia University, Menouf 32952, Egypt b Egypt-Japan University of Science and Technology (E-JUST), Alexandria 21934, Egypt c National Institute of Laser Enhanced Sciences, Cairo University, Giza 12613, Egypt d Zewail City of Science and Technology, Sheikh Zayed District, Giza 12588, Egypt article info Article history: Received 25 July 2015 Received in revised form 26 August 2015 Accepted 27 August 2015 Available online 5 September 2015 Keywords: DD-DBPSK DD-DQPSK MPPM Optical amplifier noise limited systems abstract In this paper, a hybrid differential phase shift keying-multipulse pulse position modulation (DPSK–MPPM) technique is proposed in order to enhance the receiver sensitivity of optical communication systems. Both binary and quadrature formats are adopted in the proposed systems. Direct-detection DPSK schemes that are based on an asymmetric Mach–Zehnder interferometer with a novel ultrafast discrete delay unit are presented to simplify the receiver implementation. Expressions for the bit-error rate (BER) of the proposed hybrid modulation techniques are derived taking into account the effect of the optical amplifier noise. Under the constraints of the same transmitted data rate, bandwidth, and average received optical signal-to-noise ratio, the BER performances of the proposed schemes are then evaluated numerically and compared with that of tra- ditional differential binary phase shift keying (DBPSK), differential quadrature phase shift keying (DQPSK), and MPPM schemes and with that of recent hybrid schemes. Furthermore, a comparison between the proposed systems and the traditional ones is held in terms of the bandwidth-utilization efficiency. Our results reveal that the proposed hybrid schemes are more energy-efficient and have higher receiver sensitivity compared with the traditional ones while improving the bandwidth-utilization efficiency. The proposed DPSK–MPPM system is ready to accommodate adjustable (or variable) bit rates, by virtue of the programmable delay integrated to the receiver system. & 2015 Elsevier B.V. All rights reserved. 1. Introduction The sensitivity of the receiver is one of the most important issues for many optical communication systems. Higher receiver sensitivity implies less number of transmitted signal photons per bit at the same bit-error rate (BER) [1]. Among the preeminent modulation schemes of optical communication systems featuring high receiver sensitivities are differential binary phase shift keying (DBPSK) and differential quadrature phase shift keying (DQPSK) [2]. On the other hand, DQPSK is one of the most popular receivers for multilevel phase-modulated optical communication systems and is more bandwidth efficient than DBPSK. Optical differential phase shift keying (DPSK) signals can be directly detected using an asymmetric Mach–Zehnder interferometer. This simplifies the receiver implementation and avoid the need for optical local oscillators and microwave carrier recovery circuits. Recently, several ideas for optical hybrid-modulation systems have been suggested so as to enhance the sensitivity of the receiver. Liu et al. presented a combination of m-ary pulse position modulation (PPM) or m-ary frequency-shift keying (FSK) with additional polar- ization and/or phase modulation [1, 3]. The first experimental realiza- tion was then carried out in [4] of hybrid polarization-multiplexed- 2PPM-quadrature phase-shift keying (PM-2PPM-QPSK ) modulation for long-haul transmission at a data rate of 42.8 Gbit/s. As a good step to improve the performance of both traditional binary phase shift keying (BPSK) and multipulse pulse position modulation (MPPM) techniques in optical fiber communications, Selmy et al. proposed hybrid BPSK–modified MPPM, which surpasses the traditional BPSK and MPPM techniques [5]. A hybrid orthogonal frequency-divi- sion multiplexing-pulse-position modulation (OFDM-PPM) technique was then proposed in [6] for free-space optical communications (FSO). Furthermore, Shi et al. proposed a hybrid polarization-division- multiplexed quadrature phase-shift keying-MPPM (PDM-QPSK- MPPM) for FSO [7]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications http://dx.doi.org/10.1016/j.optcom.2015.08.081 0030-4018/& 2015 Elsevier B.V. All rights reserved. n Corresponding author at: Faculty of Electronic Engineering (FEE), Menoufia University, Menouf 32952, Egypt. Fax: þ20 483660716. E-mail addresses: ahmed.morra@el-eng.menofia.edu.eg (A.E. Morra), shalaby@ieee.org (H.M.H. Shalaby), shegazy@zewailcity.edu.eg (S.F. Hegazy), sobayya@zewailcity.edu.eg (S.S.A. Obayya). 1 On leave from the Electrical Engineering Department, Alexandria University, Alexandria 21544, Egypt. Optics Communications 357 (2015) 86–94