1680 J. Opt. Soc. Am. A / Vol. 29, No. 8 / August 2012 Faridzadeh et al.

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1 1680 J. Opt. Soc. Am. A / Vol. 29, No. 8 / August 2012 Faridzadeh et al. Hybrid pulse position modulation and binary phase shift keying subcarrier intensity modulation for free space optics in a weak and saturated turbulence channel Monire Faridzadeh, 1 Asghar Gholami, 1 Zabih Ghassemlooy, 2 and Sujan Rajbhandari 2, * 1 Department of Electrical and Computer Engineering, Isfahan University of Technology, Isfahan , Iran 2 Optical Communications Research Group, School of Computing, Engineering and Information Sciences, Northumbria University, Newcastle upon Tyne, UK *Corresponding author: sujan.rajbhandari@northumbria.ac.uk Received February 23, 2012; revised June 22, 2012; accepted June 24, 2012; posted June 25, 2012 (Doc. ID ); published July 25, 2012 In this paper a hybrid modulation scheme based on pulse position modulation (PPM) and binary phase shift keying subcarrier intensity modulation (BPSK-SIM) schemes for free-space optical communications is proposed. The analytical bit error rate () performance is investigated in weak and saturated turbulence channels and results are verified with the simulation data. Results show that performance of PPM-BPSK-SIM is superior to BPSK-SIM in all turbulence regimes; however, it outperforms 2-PPM for the turbulence variance σ 2 1 > 0.2. PPM-BPSK-SIM offers a signal-to-noise ratio (SNR) gain of 50 db in the saturation regime compared to BPSK at a of The SNR gain in comparison to PPM improves as the strength of the turbulence level increases Optical Society of America OCIS codes: , , , , INTRODUCTION In the last decade, we have seen a growing level of research and development activities in the emerging field of free-space optical (FSO) communication systems. The FSO technology is an attractive complementary alternative to the radio frequency (RF) wireless and wired (including optical fiber) for the last mile communications as it does not require the right of way and digging trenches. High security, a license-free frequency spectrum, low installation costs, and the immunity to electromagnetic interference are some of other key features of FSO technologies [1 4]. The availability of the FSO link is very much dependent on the atmospheric conditions. Atmospheric factors such as fog, haze, rain, and aerosols attenuate the optical intensity by photon absorption and scattering, thus degrading the FSO link performance. Fog with particle sizes that are comparable to optical wavelengths ( nm) contribute the highest levels of attenuation [1]. In heavy fog conditions, the FSO link experiences attenuation greater than 300 db km, which results in a substantial link range reduction from a few kilometers to less than a hundred meters [5]. In such circumstances, one solution would be to utilize a hybrid FSO/RF (40 GHz) scheme. Using such a scheme increases the link availability from 96.8% to 99.92% [6,7]. However, this is achieved at the cost of reduced data rate and loss of data during switch-over from FSO to RF or vice versa [2,8]. In a clear atmospheric condition, the link attenuation is very low (<0.43 db Km) and a link span greater than a few kilometers is easily achievable. However, inhomogeneities in the temperature and pressure variation will lead to fluctuations in the refractive index structure of the air, thus resulting in the random fluctuation of the received optical beam intensity (scintillation) and the phase that will lead to severe degradation of the link performance [9 11]. In FSO links the atmospheric turbulence induced fading normally last μs, which implies the loss of up to 10 9 consecutive bits in a high data rate system [12,13]. A number of mitigation techniques have been reported, including optical beam profile manipulation [14], spatial diversity, error correction coding, adaptive modulation schemes, adaptive optics, aperture averaging, and modulation schemes [3,12,15 19]. On-off keying (OOK) is the most widely adopted modulation scheme in commercial FSO systems due to its simplicity and resilience to the inherent nonlinearity of the laser source. However, optimal FSO systems employing OOK in the atmospheric turbulence environment will require an adaptive thresholding scheme at the receiver. This leads to increased complexity due to continuous tracking of the channel noise and fading [2,20]. Digital pulse modulation schemes such as PPM offer excellent power efficiency at the cost of a relatively poor bandwidth efficiency and a high system complexity compared to OOK [1]. In [1,2,20] it has been shown that the SIM scheme offers a significant resilience to the turbulence compared to OOK employing a threshold detection scheme. In this paper, a new hybrid PPM-BPSK-SIM modulation scheme is proposed, which combines the advantages of /12/ $15.00/ Optical Society of America

2 Faridzadeh et al. Vol. 29, No. 8 / August 2012 / J. Opt. Soc. Am. A 1681 PPM and BPSK-SIM and has the potential to offer improved performance in a turbulence channel. The performance of the proposed scheme is analytically investigated and verified using the simulated data, and compared to BPSK-SIM and PPM. The rest of paper is organized as follows. The lognormal and negative exponential distribution models are introduced in Section 2. BPSK-SIM, PPM, and PPM-BPSK-SIM are outlined in Section 3, and closed-form expressions for unconditional error probabilities in a turbulence channel are also shown in this section. The simulation results are depicted in Section 4, and finally Section 5 presents the concluding remarks. 2. ATMOSPHERIC TURBULENCE MODEL One of the notable features of the atmospheric channel is the turbulence. Actually the absorption of solar radiation by the earth makes the surface air warmer than higher altitudes, and consequently rising up and mixing with the surrounding cooler air, thus resulting in turbulence [2]. Inhomogeneities in air due to turbulence act as discrete refractive prisms with different indices of refraction size varying from 0.1 to 10 m. Consequently, the propagating optical beam experiences a random variation in its irradiance and phase, thus leading to the system performance deterioration. The strength of irradiance fluctuation in a turbulent medium depends on the link span, the optical wavelength, and the refractive index structure parameter C 2 n [1,9,10]. The latter is the most widely used parameter as a measure of the strength of turbulence, which describes the temperature atmospheric pressure dependency of the turbulence. A number of models for atmospheric turbulence have been proposed [9,21]. Here, we use both lognormal and negative exponential distribution models to assess the proposed FSO system performance. A. Lognormal Model This is the most widely used model to describe the atmospheric turbulence, which is based on Rytov approximation and characterized by the log intensity variance σ 2 l as given by [22] q σ 2 l 1.23C 2 6 n k 7 L 11 p ; (1) where L p is the link length and k is the wavenumber. The lognormal distribution model is valid only for weak turbulence where the log intensity l N σ 2 l 2; σ2 l. Thus, the probability density function (PDF) of the laser intensity I I 0 expl is given as [2] 8 1 >< ln I I 0 σ2 l 2 p I q exp I 2πσ 2 >: 2σ 2 l l 2 9 >= >; I 0; (2) where I 0 is received optical intensity in the absence of turbulence. B. Negative Exponential Model Beyond the strong turbulence regime, the number of independent scattering becomes large. Theoretical and experimental results have shown that the irradiance of the received signal in the saturation regime is best described by the negative exponential distribution given as [9] p I 1 I 0 I exp 3. MODULATION SCHEMES I 0 I 0: (3) In FSO communications the average emitted optical power is limited due to the skin and eye safety constrains. Hence, modulation schemes are often compared in terms of the average received optical power required to achieve the required performance. Although both power and bandwidth efficiencies are important factors in selecting a modulation scheme, the simplicity in implementation is as equally important. Complex modulation schemes might render it impracticable, regardless of the power and bandwidth efficiencies [2]. In the next section, we first introduce PPM and BPSK-SIM followed by the proposed PPM-BPSK-SIM scheme. A. PPM PPM is an orthogonal pulse time modulation technique with an inherent power efficiency that makes it a prime candidate for a number of applications, including optical fiber and deepspace optical communications. However, PPM suffers from poor bandwidth efficiency due to its short pulse duration (i.e., higher bandwidth) and stringent synchronization requirements, which introduce more system complexity [23]. In PPM a block of M log 2 L data bits is mapped to a symbol that consists of L-slots and a single pulse of one slot duration T s T b M L, where T b is the bit duration. The position of the pulse within a symbol (T symb T b M) corresponds to the decimal value of M-bit. Therefore, the electrical signal at the output of the photodetector in one symbol period is defined as [2,24] it RI XL 1 k0! C k gt kt s nt; (4) where R is the photodetector responsivity and C k is codeword of PPM. gt is the rectangular pulse shape of one time slot duration, nt is the additive white Gaussian noise (AWGN) with zero mean and variance σ 2 L ppm N 0R b L 2M, N 0 is the double-sided power spectral density of the Gaussian noise, and R b is the bit rate. I is the intensity of the received signal with a peak intensity in the absence of turbulence I 0 LP av, P av is the average transmitted optical power [25]. At the receiver, detection is carried out by choosing the index of the slot containing the maximum received signal in a symbol; hence, the conditional slot error rate (SER) is obtained as [23,24] 0 1 P SER Q@ I qa: (5) 0 2σ 2 L ppm By averaging SER over the turbulence duration, the unconditional symbol error rate (SYER) for L 2 is obtained as follows: 0 1 Z P SYER Q@ I qapidi: (6) 2σ 2 2 ppm

3 1682 J. Opt. Soc. Am. A / Vol. 29, No. 8 / August 2012 Faridzadeh et al. B. BPSK-SIM SIM is a technique adopted from the RF communication in which the RF subcarriers pre-modulated with the data are used to modulate the optical carrier signal. Each subcarrier modulated by any standard RF digital/analog based modulation schemes could be employed for subcarrier modulation. Here, we have used BPSK with a coherent demodulation, which offers high immunity to the intensity fluctuation and does not require adaptive thresholding scheme at the receiver as information is contained in the phase of the carrier signal [1]. In addition, the coherent demodulation scheme is relatively less complex when using a low noise RF oscillator [26]. However, the requirement for an additional DC bias level to avoid signal clipping and the fact that the optical source is ON during the transmission of both digital 1 and 0, makes BPSK-SIM less power efficient [2]. By using a direct detection scheme at the receiver, the received signal can be modeled as [1] it RI1 ζmt nt; (7) where ζ is the optical modulation index while I 0 P av. The unconditional bit error probability of BPSK-SIM is obtained as [1] P e Z 0 Q p I 2σ 2 pidi: (8) C. Hybrid PPM-BPSK-SIM This scheme combines the power efficiency and the low sensitivity to the irradiance fluctuation of PPM and BPSK-SIM, respectively. The block diagram of the proposed scheme is shown in Fig. 1. The transmitter M-bit input data is first converted into the PPM format. Following parallel-to-serial conversion and a standard RF modulator, PPM codewords are applied to the BPSK modulator. A DC bias is added to the BPSK signal prior to intensity modulating the light source. By employing a direct detection scheme at the receiver, the photodetector output current for one symbol stream is obtained as! it R XL I k 1 ζ cosωt C k πgt k 1T s nt: k1 In order to limit the noise contribution, the regenerated electrical signal is passed through a bandpass filter with a (9) minimum bandwidth of 2R s (slot rate) centered at the RF subcarrier frequency. The output of the filter is multiplied with the reference carrier signal cosωt followed by a low pass filter, a match filter, and a sampler. The signal for one symbol period at the sampler output is given by i D t R 2 X L k1 I k n D t; (10) where n D t is AWGN with the variance of σ 2 ppm 2. Finally, signal recovery is carried out by finding the index of the largest sample within one symbol. In order to evaluate the performance of the proposed modulation technique, the is calculated for L 2, which is equal to the slot error probability for 2-PPM. Assuming transmitting a bit 1 sequence, the received signal in one symbol period at each slot at the sampler output is given as i 1 t 1 2 RI 1 n 1 t; i 2 t 1 2 RI 2 n 2 t: (11) The probability of choosing the wrong slot is P eslot Pi 1 >i 2. Hence, the conditional for the 2-PPM- BPSK-SIM scheme is obtained as I1 I P e Q 2 : (12) 2σ 2 ppm D. 2-PPM-BPSK-SIM in Lognormal and Negative Exponential Atmospheric Turbulence Assuming z I 1 I 2 I 3 I N expu is the sum of N lognormal random variables with log intensity variance σ z and mean μ z, PDF of z is defined as [2] where 1 lnz pz p z μu 2 exp 2πσ 2 2σ 2 ; (13a) u u μ u lnn 0.5 ln 1 expσ2 l 1 ; (13b) N Fig. 1. Block diagram of L-PPM-BPSK-SIM FSO communication system: (a) transmitter and (b) receiver.

4 Faridzadeh et al. Vol. 29, No. 8 / August 2012 / J. Opt. Soc. Am. A 1683 Table 1. Parameters Simulation Parameters σ 2 u ln 1 expσ2 l : (13c) N Therefore, the unconditional is obtained by averaging (12) over the atmospheric turbulence with PDF of pz given by Z I0 z p 2 ppm bpsk Q pzdz: (14) o 2σ 2 ppm Under negative exponential atmospheric turbulence, z is the sum of N negative exponential random variables with PDF of [2] pz I 0 N z N 1 exp z I 0 ΓN z 0: (15) The unconditional is calculated using (14) and (15). 4. RESULTS AND DISCUSSION Values Responsivity R 1 Bit rate R b 1 Mbps Carrier frequency F c 4 MHz Sampling frequency F s 20 MHz Modulation index ξ 1 PRBS length 20 To evaluate the performance of the PPM-BPSK-SIM modulation scheme, we have carried out analytical and simulation error probabilities for L 2 at different turbulence levels and compared the results with PPM and BPSK-SIM modulation schemes. In order to make fair comparisons between modulation schemes, the average transmitted optical power and the data rate are made to be constant. The most significant simulation parameters are listed in Table 1. The theoretical and simulation results for 2-PPM-BPSK-SIM, 2-PPM, and BPSK-SIM in the weak turbulence channel σ 2 l f0; 0.3g (db) SNR gain compared to BPSK-SIM compared to 2-PPM Fig. 3. SNR gain against the turbulence strength at a of 10 6 for 2-PPM-BPSK-SIM when compared with 2-PPM, BPSK-SIM in the lognormal atmospheric turbulence channel. are depicted in Fig. 2. Results illustrate a good agreement between theoretical and simulation data, thus validating the accuracy of the analysis and results obtained. As expected, increasing the strength of atmospheric turbulence decreases the system performance. The SNR gain of 2-PPM-BPSK-SIM compared with 2-PPM and BPSK-SIM schemes under the lognormal model for a range of log intensity variance at a desired of 10 6 is plotted in Fig. 3. Notice a 3 db gain for 2-PPM compared to 2-PPM-BPSK- SIM in the absence of turbulence, which is expected as a pulse occupies one of two possible slot durations in 2-PPM. However, the signal is transmitted in both possible slot durations in 2-PPM-BPSK. BPSK-SIM shows identical performance to BPSK in the absence of turbulence. However, the clear advantage of PPM-BPSK-SIM over BPSK and PPM can be observed at higher turbulence levels (σ 2 l > 0.2) with the SNR gain increasing with the turbulence strength. For example, for σ 2 l 1.2 there is 7 db and 10 db SNR gain using 2-PPM-BPSK-SIM in comparison with 2-PPM and BPSK SIM schemes, respectively. Accordingly, the performance of the proposed modulation scheme is investigated in the saturation regime under the negative exponential model, and plots for 2PPMBPSKSIM, 2-PPM, and BPSK-SIM are shown in Fig. 4. As expected, 2-PPM-BPSK- SIM offers enhanced performance in comparison to other modulation schemes in the strong atmospheric turbulence regime with the SNR gain of 50 db at a of simulation results for 4-PPM-BPSK-SIM under the lognormal atmospheric turbulence for σ 2 1 f0.5 and 0.9g, is plotted in Fig. 5. The proposed modulation shows enhanced performance in the σ l 2 σ l 2 =0.3 2PPM-BPSK-theory PPM-sim PPM-BPSK-SIM-sim PPM BPSK-SIM BPSK-SIM-sim BPSK-SIM 2-PPM 2-PPM-BPSK 2-PPM-BPSK-sim 2-PPM-sim BPSK-SIM-sim No turbulence Fig. 2. Theoretical and simulated against the SNR (db) for 2- PPM-BPSK in the lognormal and negative exponential turbulence channel Fig. 4. against the SNR (db) for 2-PPM-BPSK, 2-PPM, and BPSK-SIM in the saturation regime.

5 1684 J. Opt. Soc. Am. A / Vol. 29, No. 8 / August 2012 Faridzadeh et al. σ l 2 =0.5 BPSK 4-PPM-BPSK 4-PPM σ l 2 = Fig. 5. against the SNR (db) for 4-PPM-BPSK, 4-PPM, and BPSK-SIM in the lognormal atmospheric turbulence channel. strong turbulence regime for the higher bit resolution as well. Figure 6 shows the simulation results under the negative exponential atmospheric turbulence. As illustrated in Fig. 6, 35 db reduction in the required SNR is achieved compared with 4-PPM at a of Finally, Fig. 7 compares the simulated BPSK 4-PPM 4-PPM-BPSK sim Negative exponential Fig. 6. against the SNR (db) for 4-PPM-BPSK, 4-PPM, and BPSK-SIM in the negative exponential atmospheric turbulence channel σ l 2 =0.3 2-PPMBPSK 4-PPMBPSK 8-PPMBPSK Negative exponential Fig. 7. Simulated against the SNR (db) for 2-, 4-, and 8- PPM-BPSK-SIM in the lognormal and saturation turbulence regime. results for the L-PPM-BPSK-SIM modulation scheme for L 2; 4, and 8 under lognormal (σ ) and negative exponential atmospheric turbulence conditions. As shown, by increasing L the system performance decreases whereas the bandwidth increases. 5. CONCLUSION In this paper, a new modulation scheme so called the hybrid PPM-BPSK-SIM has been proposed, and its performance was investigated for different turbulence levels. The analytical unconditional under the lognormal and negative exponential models were calculated for 2-PPM-BPSK-SIM and verified using computer simulations. Comparing the predicted results of 2-PPM-BPSK-SIM with BPSK and 2-PPM showed that in the turbulence-free channel, 2-PPM-BPSK-SIM has the same performance as BPSK-SIM, but is inferior to 2-PPM. In the atmospheric turbulence channel, 2-PPM-BPSK-SIM offers performance superior to that of other modulation schemes investigated in this paper, and its superiority is enhanced for higher turbulence levels. In the saturation regime, 50 db SNR gain was achieved in comparison with BPSK-SIM at a of Hence, the proposed modulation is recommended for long haul FSO links. Simulated performance for the L-PPM-BPSK-SIM modulation scheme for L 2; 4, and 8 under lognormal (σ ) and negative exponential atmospheric turbulence conditions were also carried out, showing performance degradation for higher values of L. REFERENCES 1. Z. Ghassemlooy and W. O. Popoola, Terrestrial free-space optical communications, in Mobile and Wireless Communications Network Layer and Circuit Level Design, S. A. Fares and F. Adachi, eds. (InTech, 2010), pp W. O. Popoola, Subcarrier intensity modulated free space optical communication systems, Ph.D. dissertation (Northumbria University, 2009). 3. A. A. Farid and S. Hranilovic, Diversity gain and outage probability for MIMO free-space optical links with misalignment, IEEE Trans. Commun 60, (2012). 4. N. Perlot, E. Duca, J. Horwath, D. Giggenbach, and E. Leitgeb, System requirements for optical HAP-satellite links, in 6th International Symposium on Communication Systems, Networks and Digital Signal Processing, Graz, Austria, 25 July, 2008 (2008), pp B. Braua and D. Barua, Channel capacity of MIMO FSO under strong turbulent conditions, Int. J. Comput. Sci. 11, 1 5 (2011). 6. N. Letzepis, K. Nguyen, A. Guillen i Fabregas, and W. Cowley, Outage analysis of the hybrid free-space optical and radiofrequency channel, IEEE J. Sel. Areas Commun. 27, (2009). 7. F. Nadeem, V. Kvicera, M. S. Awan, E. Leitgeb, S. Muhammad, and G. Kandus, Weather effects on hybrid FSO/ RF communication link, IEEE J. Sel. Areas Commun. 27, (2009). 8. I. I. Kim, B. McArthur, and E. Korevaar, Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications, Proc. SPIE 4214, (2001). 9. W. O. Popoola and Z. Ghassemlooy, BPSK subcarrier intensity modulated free-space optical communications in atmospheric turbulence, J. Lightwave Technol. 27, (2009). 10. X. Zhu and J. M. Kahn, Free-space optical communication through atmospheric turbulence channels, IEEE Trans. Commun. 50, (2002). 11. W. Gappmair, S. Hranilovic, and E. Leitgeb, OOK performance for terrestrial FSO links in turbulent atmosphere with pointing errors modeled by Hoyt distributions, IEEE Commun. Lett. 15, (2011).

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