OSA's Digital Library

Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 17 — Aug. 26, 2013
  • pp: 20346–20362
« Show journal navigation

Ergodic capacity comparison of optical wireless communications using adaptive transmissions

Md. Zoheb Hassan, Md. Jahangir Hossain, and Julian Cheng  »View Author Affiliations


Optics Express, Vol. 21, Issue 17, pp. 20346-20362 (2013)
http://dx.doi.org/10.1364/OE.21.020346


View Full Text Article

Acrobat PDF (915 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

Ergodic capacity is investigated for the optical wireless communications employing subcarrier intensity modulation with direct detection, and coherent systems with and without polarization multiplexing over the Gamma-Gamma turbulence channels. We consider three different adaptive transmission schemes: (i) variable-power, variable-rate adaptive transmission, (ii) complete channel inversion with fixed rate, and (iii) truncated channel inversion with fixed rate. For the considered systems, highly accurate series expressions for ergodic capacity are derived using a series expansion of the modified Bessel function and the Mellin transformation of the Gamma-Gamma random variable. Our asymptotic analysis reveals that the high SNR ergodic capacities of coherent, subcarrier intensity modulated, and polarization multiplexing systems gain 0.33 bits/s/Hz, 0.66 bits/s/Hz, and 0.66 bits/s/Hz respectively with 1 dB increase of average transmitted optical power. Numerical results indicate that a polarization control error less than 10° has little influence on the capacity performance of polarization multiplexing systems.

© 2013 OSA

1. Introduction

Terrestrial optical wireless communication (OWC) systems are vulnerable to atmospheric turbulence and suffer random irradiance fluctuation, also known as scintillation, due to the random refractive index variation caused by inhomogeneities in both temperature and pressure. Scintillation is a major source of performance degradation, since it affects both the reliability and data rate of the OWC systems [1

1. V. W. S. Chan, “Free-space optical communications,” J. Lightwave Technol. 24, 4750–4762 (2006). [CrossRef]

]. Several fading mitigation techniques have been proposed for OWC systems operating in atmospheric turbulence. These techniques include error control coding with interleaving, maximum-likelihood sequence detection, and spatial diversity using multiple apertures for transmission and/or reception. Adaptive transmission technique that varies basic transmission parameters according to the channel fading is also a promising fading mitigation solution. The main advantage of the adaptive transmission is the provision of high spectral efficiency without sacrificing bit-error rate (BER) requirements. Adaptive transmissions employing phase-shift keying (PSK) and quadrature amplitude modulation (QAM) were studied for the outdoor OWC systems [2

2. N. D. Chatzidiamantis, A. S. Lioumpas, G. K. Karagiannidis, and S. Arnon, “Adaptive subcarrier PSK intensity modulation in free space optical systems,” IEEE Trans. Commun. 59, 1368–1377 (2011). [CrossRef]

, 3

3. Md. Z. Hassan, Md. J. Hossain, and J. Cheng, “Performance of non-adaptive and adaptive subcarrier intensity modulations in Gamma-Gamma turbulence,” IEEE. Trans Commun. 61, 2946–2957 (2013). [CrossRef]

].

The motivation of this work is to analyze and compare the ergodic capacity of the subcarrier IM/DD and coherent OWC systems with and without POLMUX using different adaptive transmission strategies when the CSI is available at both transmitter and receiver. In particular, we consider VPVR, CCIFR, and TCIFR adaptive transmission policies over the Gamma-Gamma turbulence channels. For all the adaptive transmission schemes, we assume the receiver can accurately estimate the channel, and an accurate channel estimation is available at the transmitter via a reliable feedback channel. Highly accurate ergodic capacity expressions are derived using a series expansion of the modified Bessel function and the Mellin transformation of the Gamma-Gamma random variable. Unlike [16

16. A. Laourine, M.-S. Alouini, S. Affes, and A. Stéphenne, “On the capacity of generalized-K fading channels,” IEEE Trans. Wireless Commun. 7, 2441–2445 (2008). [CrossRef]

], our series solutions for VPVR and TCIFR schemes work well when both parameters of the Gamma-Gamma turbulence channels take non-integer values. In addition, we carry out the asymptotic analysis of our ergodic capacity expressions and reveal some interesting insights into the capacity performance of the considered systems over the Gamma-Gamma turbulence channels. We also present a channel capacity comparison among the subcarrier IM/DD, and coherent OWC systems with and without POLMUX under an average transmitted optical power constraint. Although for non-adaptive transmission, the ergodic capacity is achievable with the assumption of independent turbulence/fading across time, capacity associated with adaptive transmission schemes namely, VPVR, CCIFR, and TCIFR is achievable without such independent assumption. While the detailed reason can be found in [34

34. A. J. Goldsmith and P. P. Varaiya, “Capacity of fading channels with channel side information,” IEEE Trans. Info. Theory 43, 1986–1992 (1997). [CrossRef]

], for completeness of this paper, we discuss it here briefly. For the VPVR transmission, the time-varying channel can effectively be reduced to a set of time-invariant additive white Gaussian noise (AWGN) channels in parallel [34

34. A. J. Goldsmith and P. P. Varaiya, “Capacity of fading channels with channel side information,” IEEE Trans. Info. Theory 43, 1986–1992 (1997). [CrossRef]

]. For each of these time-invariant channels, a capacity achieving encoder/decoder pair can be used in a time multiplexed fashion. As such there is no need to have the channel turbulence to be independent. The average capacity of the time-varying channel corresponds to the sum of capacities of the time-invariant channels weighted by the probabilities of having these time-invariant channels over time. For CCIFR transmission, since the transmitter adjusts the transmit signal power according to the channel turbulence in order to maintain a constant received SNR and transmits at a fixed rate, the channel appears to the encoder/decoder pair as a time-invariant AWGN channel. The capacity achieving encoder/decoder pair for the AWGN channel can be used and there is no need to have the channel turbulence to be independent. For TCIFR transmission, the transmitter adjusts the transmit signal power according to the channel turbulence in order to maintain a constant received power and transmits at a fixed rate as long as the received SNR is above a certain threshold, otherwise, the transmission is suspended. Similar argument as CCIFR can be made for TCIFR transmission. Therefore, our derived capacity expressions for the above mentioned three adaptive transmission schemes are valid without independent assumption of atmospheric turbulence over time. Since OWC systems transmit in the range of Gbits/sec, ergodicity assumption for such systems may not be valid. However, this is a standard assumption used in the literature [8

8. C. Liu, Y. Yao, Y. Sun, and X. Zhao, “Average capacity for heterodyne FSO communication systems over Gamma-Gamma turbulence channels with pointing errors,” Electron. Lett. 46, 851–853 (2010). [CrossRef]

12

12. K. P. Peppas, A. N. Stassinakis, G. K. Topalis, H. E. Nistazakis, and G. S. Tombras, “Average capacity of optical wireless communication systems over I− K atmospheric turbulence channels,” J. Opt. Commun. Netw. 4, 1026–1032 (2012). [CrossRef]

, 20

20. J. A. Anguita, I. B. Djordjevic, M. A. Neifeld, and B. V. Vasic, “Shannon capacities and error-correction codes for optical atmospheric turbulent channels,” J. Opt. Netw. 4, 586–601 (2005). [CrossRef]

, 21

21. A. García-Zambrana, C. Castillo-Vázquez, and B. Castillo-Vázquez, “On the capacity of FSO links over Gamma-Gamma atmospheric turbulence channels using OOK signaling,” EURASIP Journal on Wireless Communications and Networking 2010. Article ID 127657, 9 pages, 2010. doi: [CrossRef]

]. This assumption corresponds to an ideal scenario and obtained results based on this assumption can be considered as theoretical upper bounds on the systems’ performance. Yet these results allow us to compare different systems under consideration and reveal insight into system’s performance improvement as well as design.

2. Channel and system models

2.1. Atmospheric turbulence models

2.2. Coherent OWC system

In a coherent OWC system, the information is modulated on the optical carrier amplitude and phase for transmission. After being transmitted through a free-space channel, the received optical signal is coherently mixed with the optical beam generated by a local oscillator (LO), and the combined optical beam is incident on the photodetector. We assume the phase noises from the turbulence and narrow-linewidth lasers change slowly, and they can be compensated using a phase locked loop (PLL) aided phase noise compensation mechanism [26

26. M. Niu, “Coherent optical wireless communications over atmnospheric turbulence channels,” Ph. D. Thesis, The University of British Columbia (2012).

]. This assumption is reasonable because atmospheric turbulence channel is a slowly varying fading channel with channel coherence time on the order of milliseconds [6

6. A. Belmonte and J. M. Kahn, “Capacity of coherent free-space optical links using diversity-combining techniques,” Opt. Express 17, 12601–12611 (2009). [CrossRef] [PubMed]

], and the narrow-linewidth lasers have linewidths on the order of 10 kHz [27

27. M. Niu, J. Schlenker, J. Cheng, J. F. Holzman, and R. Schober, “Coherent wireless optical communications with predetection and postdetection EGC over Gamma-Gamma atmospheric turbulence channels,” J. Opt. Commun. Netw. , 3, 860–869 (2011). [CrossRef]

]. Consequently, the accumulated phase noises introduced by the atmospheric turbulence and/or lasers have millisecond variation on the timescales. In order to compensate such phase noises the PLL circuits should have a loop bandwidth (also known as the response rate of the PLL circuit) on the order of KHz [27

27. M. Niu, J. Schlenker, J. Cheng, J. F. Holzman, and R. Schober, “Coherent wireless optical communications with predetection and postdetection EGC over Gamma-Gamma atmospheric turbulence channels,” J. Opt. Commun. Netw. , 3, 860–869 (2011). [CrossRef]

]. Note that a practical PLL can work successfully up to 10 MHz [28

28. D. Banerjee, PLL Performance, Simulation and Design (National Semiconductor, 2006).

]. Consequently, the accumulated phase noises due to the atmospheric turbulence and/or lasers can be tracked and corrected almost perfectly following the photodetection [18

18. N. Cvijetic, D. Y. Qian, J. J. Yu, Y. K. Huang, and T. Wang, “Polarisation-multiplexed optical wireless transmission with coherent detection,” J. Lightwave Technol. 28, 1218–1227 (2010). [CrossRef]

, 29

29. E. J. Lee and V. W. S. Chan, “Diversity coherent and incoherent receivers for free-space optical communication in the presence and absence of interference,” J. Opt. Commun. Netw. 1, 463–483 (2009). [CrossRef]

]. Therefore, in the subsequent analysis we do not consider the impact of phase noises on the coherent detection. The optical power incident on the photodetector can be expressed as
Pr(t)=Ps+PLO+2PsPLOcos(ωIFt+ϕ)
(3)
where Ps is the received optical signal power, PLO is the LO power, ϕ is the phase information associated with the modulation order, and ωIF = ωcωLO is the intermediate frequency, where ωc and ωLO denote the carrier frequency and the LO frequency, respectively. The photocurrent generated by the photodetector can be written as ir,c(t) = iDC + iAC(t) + nc(t) where iDC = R(Ps + PLO) and iAC(t)=2RPsPLOcos(ωIFt+ϕ) are the DC and AC terms, respectively, R is the responsivity of phtotodetector, and nc(t) is a shot noise limited AWGN process with variance σc2. In a practical coherent OWC receiver, we have PLOPs, and hence, the DC term of the photocurrent is dominated by the term RPLO. The shot noise generated by the LO is dominant compared to the background irradiance generated shot noise and/or receiver thermal noise. The shot noise variance can be written as σc2=2qRPLOΔf, where q is the electronic charge, and Δf is the noise equivalent bandwidth of photodetector. The signal-to-noise ratio (SNR) at the input of an electrical demodulator can be written as the ratio of the time-averaged AC photocurrent power to the total noise variance. The received optical power Ps can be written as Ps = AI where A is the photodetector area, and I is the received optical irradiance. Assuming the mean of optical irradiance I is unity, the instantaneous SNR at the input of a electrical demodulator can be obtained as [33

33. G. P. Agrawal, Fiber-Optical Communication Systems (Wiley, 2002). [CrossRef]

]
γc=iAC2(t)2qRPLOΔf=RAqΔfI=γc¯I
(4)
where γc¯ is the average SNR per symbol. The average received power, Ps¯, can be expressed as Ps¯=gPt¯ where g is a constant path-loss factor, and Pt¯ is the average transmitted optical power. The average SNR of the coherent OWC system can be written in terms of the average transmitted optical power as
γc¯=RqΔfP¯s=RqΔfgP¯t.
(5)

2.3. POLMUX OWC system with coherent detection

In a coherent POLMUX OWC system, the optical signal from a continuous wave laser source is splitted into two orthogonal polarizations by a polarization beam splitter (PBS). The outputs of PBS are amplitude and/or phase modulated and then combined by a polarization beam combiner (PBC) before being transmitted. Since atmospheric turbulence channel is typically a weak depolarizing channel [30

30. X. Zhao, Y. Yao, Y. Sun, and C. Liu, “Circle polarisation shift-keying with direct detection for free-space optical communication,” J. Opt. Commun. Netw. 1, 307–312 (2009). [CrossRef]

], the fluctuation of the state of polarization (SOP) of the transmitted optical beam is assumed to be in a controllable order [19

19. G. D. Xie, F. X. Wang, A. Dang, and H. Guo, “A novel polarisation-multiplexing system for free-space optical links,” IEEE Photon. Technol. Lett. 23, 1484–1486 (2011). [CrossRef]

]. The electric field of the received optical signal at the front end of optical receiver can be written as Er(t) = [Ex,r (t), Ey,r(t)]T where the subscripts x and y denote the two orthogonal channels, and where
Ex,r(t)=Ps2ej(ωct+ϕ)Ey,r(t)=Ps2ej(ωct+ϕ).
(6)
At the receiver, a polarization controller (PC) is employed to adjust the SOP of the received optical beam. The adjusted optical beam is splitted into two orthogonal channels using a second PBS. The output of the second PBS can be written as [19

19. G. D. Xie, F. X. Wang, A. Dang, and H. Guo, “A novel polarisation-multiplexing system for free-space optical links,” IEEE Photon. Technol. Lett. 23, 1484–1486 (2011). [CrossRef]

, Eq. 3]
[Ex(t)Ey(t)]=[cosεsinεsinεcosε][Ex,r(t)Ey,r(t)]
(7)
where ε is the polarization control error which describes the SOP difference between the light after PBC at the transmitter and the light after PBS at the receiver. Following [18

18. N. Cvijetic, D. Y. Qian, J. J. Yu, Y. K. Huang, and T. Wang, “Polarisation-multiplexed optical wireless transmission with coherent detection,” J. Lightwave Technol. 28, 1218–1227 (2010). [CrossRef]

, Fig. 5], a post photodetection digital signal processing (DSP) aided coherent receiver is employed to recover the transmitted symbols from the two outputs of the PBS. For a fair comparison, we assume the LO power is equally distributed between two orthogonally polarized modes. We express the instantaneous SNR per orthogonal channel as [18

18. N. Cvijetic, D. Y. Qian, J. J. Yu, Y. K. Huang, and T. Wang, “Polarisation-multiplexed optical wireless transmission with coherent detection,” J. Lightwave Technol. 28, 1218–1227 (2010). [CrossRef]

, Eq. 20]
γx=R2PsPLO(cosε+sinε)2qRPLOΔf=λ1γp¯Iγy=R2PsPLO(cosεsinε)2qRPLOΔf=λ2γp¯I
(8)
where γp¯=γc¯2=RA2qΔf is the normalized average SNR per channel, λ1 = cosε + sinε, and λ2 = cosε − sinε. For a system with fully equalized cross-polarization interference, we have ε = 0° or λ1 = λ2 = 1. The POLMUX systems with coherent detection are widely used for the fiber-optic applications since the need of significant high data rate predominates the implementation cost and complexity. The major impairments of the fiber-optics POLMUX systems are the polarization mode dispersion (PMD), polarization dependent loss (PDL), and depolarization [31

31. I. B. Djordjevic, L. Xu, and T. Wang, “Beyond 100 Gb/s optical transmission based on polarization multiplexed coded-OFDM with coherent detection,” J. Opt. Commun. Netw. 1, 50–56 (2009). [CrossRef]

]. However, such impairments are absent and/or negligible in atmospheric optical communications, and consequently, the atmospheric channels are more favourable to the coherent POLMUX systems. Atmospheric channel offers zero PMD (since atmosphere is not a birefringent propagation medium), negligible PDL, and depolarization [32

32. J. Grosinger, “Investigation of polarisation modulation in optical free space communications through the atmosphere,” Master thesis, Technical University of Vienna (2008).

]. Polarization rotation is the only effect that needs to be considered for the coherent POLMUX OWC systems. However, the polarization rotation is a slowly varying process, and the resultant cross-polarization interference can be equalized by a post-photodetection DSP. Consequently, coherent POLMUX OWC system is less complex compared to the fiber-optics POLMUX system with coherent detection. In addition, the recent development of integrated coherent receivers and post-photodetection high speed DSP have made this approach practically feasible [18

18. N. Cvijetic, D. Y. Qian, J. J. Yu, Y. K. Huang, and T. Wang, “Polarisation-multiplexed optical wireless transmission with coherent detection,” J. Lightwave Technol. 28, 1218–1227 (2010). [CrossRef]

]. However, a detailed study on the practical implementation of POLMUX system for OWC applications is beyond the scope of current paper and the interested readers are referred to [32

32. J. Grosinger, “Investigation of polarisation modulation in optical free space communications through the atmosphere,” Master thesis, Technical University of Vienna (2008).

] for further reading.

2.4. OWC system with subcarrier intensity modulation/direct detection

In a subcarrier IM/DD OWC link, an RF signal, s(t), pre-modulated with data source, is used to modulate the irradiance of a continuous wave optical beam at the laser transmitter after being properly biased. For an atmospheric turbulence channel, the received photocurrent after direct detection using photodetector can be expressed as
ir(t)=RI(t)A[1+ξs(t)]+n(t)
(9)
where ξ is the modulation index satisfying the condition −1 ≤ ξs(t) ≤ 1 in order to avoid overmodulation, I(t) is assumed to be a stationary random process for the received irradiance fluctuation caused by atmospheric turbulence, and n(t) is the noise term caused by background radiation (i.e., ambient light) and/or thermal noise, and it is modeled as an AWGN process with variance σn2. The sample I = I(t)|t=t0 at a time instant t = t0 gives the RV I. Normalizing the power of s(t) to unity, the instantaneous SNR at the input of electrical demodulator can be written as [33

33. G. P. Agrawal, Fiber-Optical Communication Systems (Wiley, 2002). [CrossRef]

]
γs=(RAξ)22Δf(qRIb+2kbTk/RL)I2=CsI2
(10)
where Ib is the background light irradiance, kb is Boltzman constant, Tk is the temperature in Kelvin, RL is the load resistance, and Cs=(RAξ)22Δf(qRIb+2kbTk/RL) is a multiplicative constant. Assuming unit mean received irradiance, i.e., E[I] = 1, and using (2), we define the average SNR of the subcarrier IM/DD OWC system as γs¯=CsE[I2]=Cs(α+1)(β+1)αβ, and relate γs¯ to the instantaneous SNR γs by
γs=αβγs¯(α+1)(β+1)I2.
(11)
The average SNR can also be expressed as γs¯=(Rξ)22Δf(qRIb+2kbTk/RL)E[Ps2]. Using (2) and the relation P¯s=gP¯t, we write average SNR of the subcarrier IM/DD systems in terms of the average transmitted optical power as
γs¯=(Rξ)2(1+1α)(1+1β)2Δf(qRIb+2kbTk/RL)g2P¯t2.
(12)

3. Ergodic capacity of coherent OWC system

3.1. Variable-power, variable-rate adaptive transmission

In a VPVR adaptive transmission scheme the transmitter simultaneously adapt the power and data rate in order to maintain a target BER at the receiver for all SNRs. With this adaptive transmission policy, more power and higher data rates are allocated when the channel condition is good, and the transmission is terminated when the received SNR falls below a cutoff level γo. For a VPVR adaptive scheme, the ergodic capacity (in bits/s/Hz) of a fading channel can be calculated by [34

34. A. J. Goldsmith and P. P. Varaiya, “Capacity of fading channels with channel side information,” IEEE Trans. Info. Theory 43, 1986–1992 (1997). [CrossRef]

, Eq. 7]
CVPVRcoh=γolog2(γcγo)fγc(γc)dγc=1ln2γoln(γcγo)fγc(γc)dγc.
(13)
Under an average transmit power constraint, the cutoff level γo must satisfy [34

34. A. J. Goldsmith and P. P. Varaiya, “Capacity of fading channels with channel side information,” IEEE Trans. Info. Theory 43, 1986–1992 (1997). [CrossRef]

, Eq. 6]
γo(1γo1γc)fγc(γc)dγc=1
(14)
where in Eqs. (13) and (14) fγc (γc) is the PDF of the SNR γc. Using a series expansion of the modified Bessel function of second kind [25

25. Md. Z. Hassan, X. Song, and J. Cheng, “Subcarrier intensity modulated wireless optical communications with rectangular QAM,” J. Opt. Commun. Netw. 6, 522–532 (2012). [CrossRef]

, Eq. 15], we obtain the PDF and the cumulative distribution function (CDF) of γc, respectively, as
fγc(γc)=Λ(α,β)p=0[ap(α,β)(γcγc¯)p+βγc1ap(β,α)(γcγc¯)p+αγc1]
(15)
and
Fγc(γc)=Λ(α,β)p=0[ap(α,β)p+β(γcγc¯)p+βap(β,α)p+α(γcγc¯)p+α].
(16)
where Λ(α,β)Γ(αβ)Γ(1α+β)Γ(α)Γ(β) and ap(x,y)(xy)p+yΓ(px+y+1)p!.

3.1.1. Computation of Cutoff SNR γo

Equation (14) can be rewritten as
Fγc(γo)+γo(0γc1fγc(γc)dγcr1,c(α,β,γc¯)0γoγc1fγc(γc)dγcr2,c(α,β,γo,γc¯))+γo=1.
(17)
Substituting γc=γc¯I into the first integral of Eq. (14), we can write r1,c(α, β, γc¯) as
r1,c(α,β,γc¯)=1γc¯0I1fI(I)dI=1γc¯E[I1]
(18)
where E[I−1] is the first negative integer moment of the Gamma-Gamma RV I. The Mellin transformation of a positive RV X provides all moments of X including positive integer, negative integer, and fractional moments [35

35. N. Cressie, A. S. Davis, J. L. Folks, and G. E. Policello II, “The moment-generating function and negative integer moments,” The American Statistician 35, 148–150 (1981).

]. Using the Mellin transformation of the Gamma-Gamma RV (see Appendix), we obtain r1,c(α, β, γc¯) as
r1,c(α,β,γc¯)=1γc¯Γ(α1)Γ(β1)αβΓ(α)Γ(β).
(19)
Substituting Eq. (15) into the second integral of Eq. (14), after some algebraic manipulation, we can evaluate r2,c(α, β, γo, γc¯) as
r2,c(α,β,γo,γc¯)=Λ(α,β)p=0[ap(α,β)(p+β1)γo(γoγc¯)p+βap(β,α)(p+α1)γo(γoγc¯)p+α].
(20)
Finally, applying Eqs. (16), (18), and (20) to Eq. (17), one obtains a series expression involving the cutoff SNR γo. For min{α, β} > 1, such expression can be numerically solved along with Eq. (14) in order to compute the cutoff SNR γo for a given average SNR. From Eqs. (16), (19) and (20), when γc¯ we have Fγc (·) → 0, r1,c(α, β, γc¯) → 0, and r2,c(α, β, γo, γc¯) → 0. As a result, when the average SNR in Eq. (17) approaches infinity, γo approaches unity. Therefore, for the Gamma-Gamma turbulence channels, the value of the cutoff SNR, γo, is restricted to [0, 1].

3.1.2. Computation of ergodic capacity

In order to facilitate the computation of ergodic capacity of the VPVR adaptive scheme, we rewrite Eq. (13) as
CVPVRcoh=1ln2[0ln(γc)fγc(γc)dγcg1,c(α,β,γc¯)0γoln(γc)fγc(γc)dγcg2,c(α,β,γo,γc¯)]log2(γo)(1Fγc(γo)).
(21)
Here g1,c(α, β, γc¯) in (21) is the expectation of lnγc, i.e., E[lnγc]. In order to estimate the expected value of lnγc, we first define Z = lnγc. The moment generating function of Z is
MZ(s)=E[esz]=E[eslnγc]=E[γcs].
(22)
The expected value of lnγc can be obtained as
E[lnγc]=E[Z]=dMZ(s)ds|s=0=dE[γcs]ds|s=0.
(23)
It can be easily shown that if γc=γc¯I, the k-th moment of γc is E[γck]=γc¯kE[Ik], where E[Ik] is the k-th moment of RV I and it is given by [24

24. N. Wang and J. Cheng, “Moment-based estimation for the shape parameters of the Gamma-Gamma atmospheric turbulence model,” Opt. Express 18, 12824–12831 (2010). [CrossRef] [PubMed]

, Eq. 3]
E[Ik]=Γ(α+k)Γ(β+k)Γ(α)Γ(β)(1αβ)k.
(24)
Using [36

36. The Wolframe Function Site. [Online]: http://functions.wolfram.com.

, Eq. 06.05.20.0001.01] we can derive g1,c(α, β, γc¯) as
g1,c(α,β,γc¯)=ψ(α)+ψ(β)+lnγc¯ln(αβ)
(25)
where ψ(·) is the Euler’s digamma function [23

23. I. S. Gradshteyn and I. M. Ryzhik, Table of Integrals, Series, and Products (Academic Press, 2000).

, Eq. 8.360(1)]. In order to evaluate g2,c(α, β, γo, γc¯) in Eq. (21), we first recall the fact that limz→0za ln z = 0 for any real non-negative z and a. Substituting Eq. (15) into the second integral of Eq. (21) and using [23

23. I. S. Gradshteyn and I. M. Ryzhik, Table of Integrals, Series, and Products (Academic Press, 2000).

, Eq. 2.723(1)], we obtain g2,c(α, β, γo, γc¯) as
g2,c(α,β,γo,γc¯)=Λ(α,β)p=0[ap(α,β)wp(α,β,γo)(γoγc¯)(p+β)ap(β,α)wp(β,α,γo)(γoγc¯)(p+α)]
(26)
where wp(x,y,γo)lnγop+y1(p+y)2. To evaluate Eq. (26) numerically, we consider only a summation of finite K + 1 terms. Consequently, a truncation error is introduced due to elimination of infinite terms after the first K + 1 terms. Using a similar method described in [25

25. Md. Z. Hassan, X. Song, and J. Cheng, “Subcarrier intensity modulated wireless optical communications with rectangular QAM,” J. Opt. Commun. Netw. 6, 522–532 (2012). [CrossRef]

], we can derive an upper bound for the truncation error, and show that the truncation error rapidly decreases with increasing K values and/or average SNR γc¯. Finally, using Eqs. (25) and (26) one obtains the ergodic capacity for the coherent VPVR adaptive OWC system as
CVPVRcoh=1ln2(g1,c(α,β,γc¯)g2,c(α,β,γo,γc¯))log2(γo)(1Fγc(γo)).
(27)

3.1.3. Asymptotic high SNR capacity

When the average SNR γc¯ approaches infinity, the value of the cutoff SNR γo approaches unity, and consequently, log2(γo) approaches zero. Also, it can be shown that in large SNR regimes g2,c(α,β,γo,γc¯)g1,c(α,β,γc¯) since in large SNR regimes (1γc¯)βlnγc¯. Hence, for asymptotically high average SNR we can evaluate the ergodic capacity of a coherent VPVR adaptive OWC system as
CVPVRcoh,asym=ψ(α)+ψ(β)ln2log2(αβ)+3.3log10γ¯c=ψ(α)+ψ(β)ln2log2(αβ)+log2(RgqΔf)+0.33(10log10P¯t)
(28)
where we have used Eq. (5) to obtain the last equality. Equation (28) revels that the high SNR ergodic capacity of the coherent VPVR system gains 0.33 bits/s/Hz with 1 dB increase of average transmitted optical power.

3.2. Complete channel inversion with fixed rate

Under this adaptive transmission scheme, the transmitter adapts the transmit power (under an average transmit power constraint) according to the channel fading state in order to maintain a constant SNR at the recevier, i.e., inverts the channel fading while maintaining a constant transmission rate. The ergodic capacity (in bits/s/Hz) of a CCIFR adaptive transmission scheme is given by [34

34. A. J. Goldsmith and P. P. Varaiya, “Capacity of fading channels with channel side information,” IEEE Trans. Info. Theory 43, 1986–1992 (1997). [CrossRef]

, Eq. 9]
CCCIFRcoh=log2(1+10γc1fγc(γc)dγc).
(29)
The integral 0γc1fγc(γc)dγc has already been evaluated in (19). As a result, we can express the ergodic capacity of the CCIFR scheme as
CCCIFRcoh=log2(1+(α1)(β1)αβγc¯).
(30)
It can be shown that for a given channel capacity the CCIFR schemes over the Gamma-Gamma turbulence channels exhibit SNR penalty of 10log10(αβ(α1)(β1)) dB from the AWGN channel. Equation (30) is similar to the ergodic capacity of the CCIFR scheme over the generalized-K fading channel given by [16

16. A. Laourine, M.-S. Alouini, S. Affes, and A. Stéphenne, “On the capacity of generalized-K fading channels,” IEEE Trans. Wireless Commun. 7, 2441–2445 (2008). [CrossRef]

, Eq. 29]. However, our derivation approach is different from [16

16. A. Laourine, M.-S. Alouini, S. Affes, and A. Stéphenne, “On the capacity of generalized-K fading channels,” IEEE Trans. Wireless Commun. 7, 2441–2445 (2008). [CrossRef]

] and our capcity expression in Eq. (30) does not require that β is an integer. The asymptotic capacity in large SNR regimes can be obtained as CCCIFRcoh,asym=log2((α1)(β1)αβ)+log2(RgqΔf)+0.33(10log10P¯t). Hence, CCIFR scheme also gains 0.33 bits/s/Hz with 1 dB increase of average transmitted optical power.

3.3. Truncated channel inversion with fixed rate

Since the CCIFR scheme exhibits a large channel capacity penalty in severe fading, a modified channel inversion scheme is proposed where only the transmit power is adaptive according to the channel fading state provided that the received SNR is above a certain cutoff SNR γcoh. The channel will not be used if the received SNR falls below γcoh. For this adaptive transmission policy, the ergodic capacity (in bits/s/Hz) is given by [34

34. A. J. Goldsmith and P. P. Varaiya, “Capacity of fading channels with channel side information,” IEEE Trans. Info. Theory 43, 1986–1992 (1997). [CrossRef]

, Eq. 12]
CTCIFRcoh=log2(1+1γcohγc1fγc(γc)dγc)(1Pocoh(γcoh))
(31)
where Pocoh(γcoh) is the outage probability given by Pocoh(γcoh)=Fγc(γcoh). We can evaluate the integral γcohγc1fγc(γc)dγc as
γcohγc1fγc(γc)dγc=r1,c(α,β,γc¯)r2,c(α,β,γcoh,γc¯)
(32)
where r1,c(·, ·, ·) and r2,c(·, ·, ·, ·) are obtained from Eqs. (18) and (20), respectively. Here, the cutoff level γcoh is selected in order to maximize the channel capacity in Eq. (31) for a given average SNR. Our numerical result shows that the cutoff SNR that maximizes the channel capacity of a coherent TCIFR system increases with an increase of average SNR. Recall that the cutoff SNR of a coherent VPVR system is restricted to the range [0, 1]. Hence, due to increasing cutoff SNR values, coherent TCIFR systems exhibit higher outage probability compared to the coherent VPVR systems. Also note that, for coherent systems, Eqs. (30) and (32) are valid when min{α, β} > 1, which is typically satisfied for the Gamma-Gamma turbulence channels [26

26. M. Niu, “Coherent optical wireless communications over atmnospheric turbulence channels,” Ph. D. Thesis, The University of British Columbia (2012).

].

4. Ergodic capacity of coherent POLMUX OWC system

4.1. Variable-power, variable-rate adaptive transmission

In a coherent VPVR POLMUX scheme the transmit power on each channel is adapted subject to an average transmit power constraint per channel. For a fair comparison it is assumed that the total average transmit power of a coherent OWC system with and without polarization are same, and the total average transmit power is equally allocated between the two orthogonal channels in a POLMUX system. Under this assumption, the ergodic capacity of a coherent POLMUX VPVR scheme can be expressed as
CVPVRpol=γalog2(γxγa)fγx(γx)dγxCVPVRpol,x+γblog2(γyγb)fγy(γy)dγyCVPVRpol,y
(33)
where fγx (γx) and fγx (γx) are respectively PDFs of γx and γy, and γa and γb are the cutoff SNRs for the two orthogonal channels satisfying
γa(1γa1γx)fγx(γx)dγx=1
(34)
and
γb(1γb1γy)fγy(γy)dγy=1.
(35)
An asymptotic expansion of Eqs. (34) and (35) reveals that both γa and γb are confined to the range [0, 1]. Using a similar method applied to the coherent OWC system, it can be shown
CVPVRpol,x=1ln2(g1,c(α,β,λ1γp¯)g2,c(α,β,γa,λ1γp¯))log2(γa)(1Fγx(γa)).
(36)
and
CVPVRpol,y=1ln2(g1,c(α,β,λ2γp¯)g2,c(α,β,γb,λ2γp¯))log2(γb)(1Fγy(γb)).
(37)
where Fγx (·) and Fγy (·) are respectively the CDFs of γx and γy, and they can be obtained by substituting γc¯ in Eq. (16) with λ1γp¯ and λ2γp¯, respectively. Using a similar argument for obtaining Eq. (28), the asymptotic capacity at high SNR can be expressed as
CVPVRpol,asym=2ψ(α)+2ψ(β)ln22log2(αβ)+log2(λ1λ2)+2log2(Rg2qΔf)+0.66(10log10P¯t).
(38)
From Eq. (38), we observe that the high SNR ergodic capacity of coherent POLMUX VPVR adaptive scheme gains 0.66 bits/s/Hz with 1 dB increase of average transmitted optical power.

4.2. Complete channel inversion with fixed rate

The ergodic capacity of coherent POLMUX CCIFR scheme is obtained as
CCCIFRpol=log2(1+(α1)(β1)αβλ1γp¯)+log2(1+(α1)(β1)αβλ2γp¯).
(39)
The asymptotic capacity at high SNR is expressed as
CCCIFRcoh,asym=2log2((α1)(β1)αβ)+log2(λ1λ2)+2log2(Rg2qΔf)+0.66(10log10P¯t).
(40)

4.3. Truncated channel inversion with fixed rate

The ergodic capacity of coherent POLMUX TCIFR scheme is obtained as
CTCIFRpol=log2(1+1r1,c(α,β,λ1γp¯)r2,c(α,β,γpol,x,λ1γp¯))(1Fγx(γpol,x))+log2(1+1r1,c(α,β,λ2γp¯)r2,c(α,β,γpol,y,λ2γp¯))(1Fγy(γpol,y)).
(41)
For a given average SNR or average transmitted power the cutoff SNRs γpol,x and γpol,y are selected in order to maximize log2(1+1γpol,iγi1fγi(γi)dγi) (1 − Fγi (γpol,i)) where i = x, y. Similar to the coherent OWC, Eqs. (39) and (41) are valid for min{α, β} > 1.

5. Ergodic capacity of subcarrier IM/DD OWC system

5.1. Variable-power, variable-rate adaptive transmission

The ergodic capacity of a subcarrier IM/DD VPVR adaptive OWC system is given by
CVPVRIM=1ln2[0ln(γs)fγs(γs)dγsg1,s0γeln(γs)fγs(γs)dγsg2,s]log2(γe)(1Fγs(γe))
(42)
where γe is the cutoff SNR satisfying the condition
γe(1γe1γs)fγs(γs)dγs=1.
(43)
In Eqs. (42) and (43), fγs (γs) and Fγs (·) are respectively the PDF and CDF of the SNR of the subcarrier IM/DD OWC systems, and they are given by
fγs(γs)=Λ(α,β)2p=0[bp(α,β)(γsγs¯)p+β2γs1bp(β,α)(γsγs¯)p+α2γs1]
(44)
and
Fγs(γs)=Λ(α,β)p=0[bp(α,β)p+β(γsγs¯)p+β2bp(β,α)p+α(γsγs¯)p+α2].
(45)
where in Eqs. (44) and (45) bp(x,y)ap(x,y)[(x+1)(y+1)xy]p+y2. Similar to the coherent VPVR OWC systems, an asymptotic expansion of Eq. (43) reveals that for subcarrier IM/DD VPVR adaptive system the value of γe is also confined to the range [0, 1]. Using a similar approach in deriving Eq. (25), we can evaluate g1,s as
g1,s=2ψ(α)+2ψ(β)+lnγs¯2ln(αβ).
(46)
Following Eq. (26), we obtain g2,s as
g2,s=Λ(α,β)2p=0[bp(α,β)vp(α,β,γe)(γeγs¯)p+β2bp(β,α)vp(β,α,γe)(γeγs¯)p+α2]
(47)
where vp(x,y,γe)2lnγep+y4(p+y)2. Finally, using Eqs. (46) and (47) we can evaluate the ergodic capacity of the subcarrier IM/DD VPVR adaptive OWC system as
CVPVRIM=1ln2(g1,sg2,s)log2(γe)(1Fγs(γe)).
(48)
Using a similar argument in obtaining Eq. (28), we obtain the ergodic capacity of the subcarrier IM/DD VPVR system at high SNR as
CVPVRIM,asym=2ψ(α)+2ψ(β)ln22log2(αβ)+log2((Rξg)2(1+1α)(1+1β)2Δf(qRIb+2kbTk/RL))+0.66(10log10P¯t).
(49)
We observe from Eq. (49) that the ergodic capacity of subcarrier IM/DD VPVR system gains 0.66 bits/s/Hz with 1 dB increase of average transmitted optical power. In high SNR region with 1 dB increase of average transmitted optical power, the subcarrier IM/DD systems provide higher spectral efficiency improvement than the coherent OWC systems without POLMUX. This can be explained by the following analytical arguments. The average SNR of the coherent OWC systems is proportional to the average transmitted optical power whereas the average SNR of the subcarrier IM/DD systems is proportional to square of the the average transmitted optical power, as observed from Eqs. (5) and (12) respectively. Therefore, the high SNR ergodic capacity of the subcarrier IM/DD VPVR scheme has a steeper slope (2 times) with increasing average transmitted optical power compared to the high SNR ergodic capacity of the coherent VPVR scheme. Due to this steeper slope, in high SNR region subcarrier IM/DD systems provide higher spectral efficiency improvement with 1 dB increase in average transmitted optical power than the coherent detection systems. As a result, although the coherent systems significantly outperform the subcarrier IM/DD systems, the capacity performance gap between the coherent and subcarrier IM/DD systems is reduced with the increase of average transmitted optical power. Such a observation is also depicted by our numerical results.

5.2. Channel inversion with fixed rate

The ergodic capacity of a subcarrier IM/DD CCIFR scheme is given by
CCCIFRIM=log2(1+10γs1fγs(γs)dγs).
(50)
Substituting γs=αβγs¯(α+1)(β+1)I2 into Eq. (50) and using Mellin transformation of Gamma-Gamma RV, the ergodic capacity of the subcarrier IM/DD CCIFR scheme can be expressed as
CCCIFRIM=log2(1+Γ(α)Γ(β)αβ(α+1)(β+1)Γ(α2)Γ(β2)γs¯).
(51)
An asympttoic analysis of Eq. (51) reveals that ergodic capacity of subcarrier IM/DD CCIFR scheme gains 0.66 bits/s/Hz with 1 dB increase of average transmitted optical power. The ergodic capacity of a subcarrier IM/DD TCIFR scheme is given by
CTCIFRIM=maxγth>0log2(1+1γthγs1fγs(γs)dγs)(1PoIM(γth))
(52)
where γth is the cutoff SNR below which no power adaption is accomplished, and PoIM(γth) is the outage probability of the IM/DD OWC systems, and it is given by PoIM(γth)=Fγs(γth). The integral γthγs1fγs(γs)dγs can be expressed as
γthγs1fγs(γs)dγs=0γs1fγs(γs)dγsr1,s0γthγs1fγs(γs)dγsr2,s(γth)
(53)
where
r1,s=(α+1)(β+1)γs¯Γ(α2)Γ(β2)αβΓ(α)Γ(β)
(54)
and
r2,s(γth)=Λ(α,β)p=0[bp(α,β)(p+β2)γe(γthγs¯)p+β2bp(β,α)(p+α2)γe(γthγs¯)p+α2].
(55)
For the subcarrier IM/DD systems, Eqs. (51), (54), and (55) are valid when min{α, β} > 2.

6. Numerical results

In this section, we compare the ergodic capacity of the subcarrier IM/DD, and coherent OWC systems with and without POLMUX using our series solutions with the exact ergodic capacity obtained using numerical integration. All series solutions are calculated using the first 31 terms. We consider the following turbulent OWC scenarios where the path loss factor is empirically expressed in terms of visibility, and the turbulence strength is assumed to increase with propagation distance [26

26. M. Niu, “Coherent optical wireless communications over atmnospheric turbulence channels,” Ph. D. Thesis, The University of British Columbia (2012).

] : 1) a 2 Km haze optical channel (in strong turbulence with α = 2.04 and β = 1.10) with 4.35 dB/Km path loss; 2) a 900 m light smoke optical channel (in moderate turbulence with α = 2.50 and β = 2.06) with 9.56 dB/Km path loss; 3) a 700 m light fog optical channel (in weak turbulence α = 4.43 and β = 4.39) with 11.5 dB/Km path loss. In order to generate the numerical plots of capacity versus the average transmitted optical power, we also make the following assumptions [26

26. M. Niu, “Coherent optical wireless communications over atmnospheric turbulence channels,” Ph. D. Thesis, The University of British Columbia (2012).

]: the modulation index ε = 0.85, the photodetector responsivity R = 0.75 A/W, load resistance RL = 50 Ω, the bit duration T = 1ns with approximate transmission bandwidth 1 GHz, thermal noise variance 3.3 × 10−13 Amp2, and the background noise variance 10−15 Amp2. Assuming a typical LO power 10−2 W [37

37. J. M. Hunt, F. Holmes, and F. Amizajerdian, “Optimum local oscialltor levels for coherent detection using photodetectors,” Appl. Opt. 27, 3135–3141 (1988). [CrossRef] [PubMed]

], the local oscillator-induced shot noise variance is 5 ×10−12 Amp2. Also, in our numerical results we have selected the range of the average transmitted optical power from −12 dBm to 6 dBm so that the assumption PLOPs remains valid.

Figures 1(a) and 1(b) respectively present the ergodic capacity of the coherent and coherent POLMUX OWC systems over a strong Gamma-Gamma turbulence channel. In both figures, excellent agreement is observed between the exact channel capacity and capacity obtained by our series approximations. From these figures, it is obvious that the VPVR scheme achieves the largest channel capacity for both the coherent and coherent POLMUX OWC systems. From Fig. 1(b) it is depicted that due to the polarization control error the ergodic capacity of a coherent POLMUX system gets reduced. However, a polarization control error with ε = 10° has no noticeable influence on the ergodic capacity performance. Asymptotic channel capacities for the VPVR and CCIFR schemes are also shown in both figures, and they converge to the exact capacity at large SNR values, as expected.

Fig. 1 Ergodic capacity of coherent and coherent POLMUX OWC systems in Gamma-Gamma turbulence.

Figures 1(c) and 1(d) plot the ergodic capacities of the coherent and coherent POLMUX OWC systems over a moderate Gamma-Gamma turbulence channel. Excellent agreement is also observed between the exact channel capacity and the capacity obtained by our series approximations. In both figures we observe that the performance gap between different adaptive transmission schemes gets decreased with a decrease of turbulence strength. From Figs. 1(c) and 1(d), the SNR gap between the VPVR and CCIFR schemes is approximately 3 dB for both the coherent and coherent POLMUX OWC systems when α = 2.50 and β = 2.06. This result agrees with the predicted SNR gap of 3.06 dB between the VPVR and CCIFR schemes from our asymptotic ergodic capacity analysis. Asymptotic capacities are also shown in this figure, and they converge to the exact ergodic capacity when γc¯20dB or γp¯20dB.

Figure 2(a) compares the ergodic capacities of the subcarrier IM/DD and coherent POLMUX VPVR schemes over a strong Gamma-Gamma turbulence channel, while Fig. 2(b) compares the ergodic capacities of the subcarrier IM/DD and coherent POLMUX CCIFR schemes over a weak turbulence channel with the same average transmitted power constraint. Figs. 2(a) and 2(b) clearly demonstrate that the coherent POLMUX schemes significantly outperform the coherent and subcarrier IM/DD schemes for the same average transmitted optical power. For example, at an average transmitted optical power of −12 dBm, the coherent POLMUX VPVR scheme achieves 26.28 bits/s/Hz and 25.37 bits/s/Hz channel capacity with polarization control error ε = 0° and ε = 30°, respectively. On the other hand, for the same average transmitted optical power, the coherent and subcarrier IM/DD VPVR schemes attain channel capacity of 14.19 bits/s/Hz and 5.03 bits/s/Hz, respectively. From Fig. 2(b), at an average transmitted optical power of −4 dBm coherent POLMUX CCIFR scheme achieves 32.74 bits/s/Hz and 31.83 bits/s/Hz channel capacity with polarization control error ε = 0° and ε = 30°, respectively. On the other hand, for the same average transmitted optical power, the coherent and subcarrier IM/DD CCIFR schemes obtain channel capacity of 17.42 bits/s/Hz and 9.738 bits/s/Hz, respectively. Both Figs. 2(a) and 2(b) illustrate that the performance gap between the coherent and the subcarrier IM/DD systems gets narrowed with an increase of transmitted optical power. However, the performance gap between coherent POLMUX and subcarrier IM/DD systems remains constant with an increase of transmitted optical power. This is because both coherent POLMUX and subcarrier IM/DD systems have the same slope of 0.66 bits/s/Hz whereas the coherent systems have a slope of 0.33 bits/s/Hz with respect to the dB values of average transmitted optical power.

Fig. 2 Ergodic capacity comparison among the subcarrier IM/DD, and coherent OWC systems with and without POLMUX subject to the same average transmitted optical power.

7. Conclusion

We have developed highly accurate series ergodic capacity expressions for the subcarrier IM/DD and coherent OWC systems with and without POLMUX employing different adaptive transmission schemes. Our ergodic capacity analysis reveals that the coherent POLMUX OWC systems significantly outperform the coherent and subcarrier IM/DD OWC systems even in the presence of polarization control error.

Appendix

Mellin transformation of a RV I is defined as MI(z)=E[Iz1]=0Iz1fI(I)dI. Assuming I is a Gamma-Gamma RV, one can show I = XY, where X and Y are two independent Gamma RVs with PDFs respectively given by fX(x)=ααxα1Γ(α)exp(αx) and fY(y)=ββyβ1Γ(β)exp(βy). Since the Mellin transformation of a product of two independent RVs is a product of Mellin transformations of the two RVs [38

38. P. Galambos and I. Simonelli, Products of Random Variables: Applications to Problems of Physics and to Arithmetical Functions (Marcel Dekker Inc., 2004).

], we obtain MI (z) = MX (z)MY (z). Using an integral identity [23

23. I. S. Gradshteyn and I. M. Ryzhik, Table of Integrals, Series, and Products (Academic Press, 2000).

, Eq. 3.326(2)], we obtain the Mellin transformations of RVs X and Y, respectively, as MX(z)=Γ(α+z1)Γ(α)1αz1 and MY(z)=Γ(β+z1)Γ(β)1βz1. Finally, the Mellin transformation of I becomes
MI(z)=Γ(α+z1)Γ(β+z1)Γ(α)Γ(β)(1αβ)z1.
(56)
It follows that E[I−1] and E[I−2], which are required by Eqs. (18) and (54), can be obtained by letting z = 0 and z = −1 in Eq. (56) [35

35. N. Cressie, A. S. Davis, J. L. Folks, and G. E. Policello II, “The moment-generating function and negative integer moments,” The American Statistician 35, 148–150 (1981).

].

References and links

1.

V. W. S. Chan, “Free-space optical communications,” J. Lightwave Technol. 24, 4750–4762 (2006). [CrossRef]

2.

N. D. Chatzidiamantis, A. S. Lioumpas, G. K. Karagiannidis, and S. Arnon, “Adaptive subcarrier PSK intensity modulation in free space optical systems,” IEEE Trans. Commun. 59, 1368–1377 (2011). [CrossRef]

3.

Md. Z. Hassan, Md. J. Hossain, and J. Cheng, “Performance of non-adaptive and adaptive subcarrier intensity modulations in Gamma-Gamma turbulence,” IEEE. Trans Commun. 61, 2946–2957 (2013). [CrossRef]

4.

F. Lazarakis, G. S. Tombras, and K. Dangakis, “Average channel capacity in a mobile radio environment with Rician statistics,” IEICE Trans. Commun. E77-B, 971–977 (1994).

5.

S. M. Haas and J. H. Shapiro, “Capacity of wireless optical communications,” IEEE J. Sel. Areas Commun. 21,pp. 1346–1357 (2003). [CrossRef]

6.

A. Belmonte and J. M. Kahn, “Capacity of coherent free-space optical links using diversity-combining techniques,” Opt. Express 17, 12601–12611 (2009). [CrossRef] [PubMed]

7.

A. Belmonte and J. M. Kahn, “Capacity of coherent free-space optical links using atmospheric compensation techniques,” Opt. Express 17, 2763–2773 (2009). [CrossRef] [PubMed]

8.

C. Liu, Y. Yao, Y. Sun, and X. Zhao, “Average capacity for heterodyne FSO communication systems over Gamma-Gamma turbulence channels with pointing errors,” Electron. Lett. 46, 851–853 (2010). [CrossRef]

9.

H. E. Nistazakis, E. A. Karagianni, A. D. Tsigopoulos, M. E. Fafalios, and G. S. Tombras, “Average capacity of optical wireless communication systems over atmospheric turbulence channels,” J. Lightwave Technol. 27, 974–979 (2009). [CrossRef]

10.

H. E. Nistazakis, T. A. Tsiftsis, and G. S. Tombras, “Performance analysis of free-space optical communication systems over atmospheric turbulence channels,” IET Commun. 3, 1402–1409 (2009). [CrossRef]

11.

H. G. Sandalidis and T. A. Tsiftsis, “Outage probability and ergodic capacity of free-space optical links over strong turbulence,” Electron. Lett. 44, 46–47 (2008). [CrossRef]

12.

K. P. Peppas, A. N. Stassinakis, G. K. Topalis, H. E. Nistazakis, and G. S. Tombras, “Average capacity of optical wireless communication systems over I− K atmospheric turbulence channels,” J. Opt. Commun. Netw. 4, 1026–1032 (2012). [CrossRef]

13.

G. Pan, E. Ekici, and Q. Feng, “Capacity analysis of log-normal channels under various adaptive transmission schemes,” IEEE Commun. Lett. 16, 346–348 (2012). [CrossRef]

14.

I. B. Djordjevic, “Adaptive modulation and coding for free-space optical channels,” J. Opt. Commun. Netw. 2, 221–229 (2010). [CrossRef]

15.

I. B. Djordjevic and G. T. Djordjevic, “On the communication over strong atmospheric turbulence channels by adaptive modulation and coding,” Opt. Express 17, 18250–18262 (2009). [CrossRef] [PubMed]

16.

A. Laourine, M.-S. Alouini, S. Affes, and A. Stéphenne, “On the capacity of generalized-K fading channels,” IEEE Trans. Wireless Commun. 7, 2441–2445 (2008). [CrossRef]

17.

W. Gappmair, “Further results on the capacity of free-space optical channels in turbulent atmosphere,” IET Commun. 5, 1262–1267 (2010). [CrossRef]

18.

N. Cvijetic, D. Y. Qian, J. J. Yu, Y. K. Huang, and T. Wang, “Polarisation-multiplexed optical wireless transmission with coherent detection,” J. Lightwave Technol. 28, 1218–1227 (2010). [CrossRef]

19.

G. D. Xie, F. X. Wang, A. Dang, and H. Guo, “A novel polarisation-multiplexing system for free-space optical links,” IEEE Photon. Technol. Lett. 23, 1484–1486 (2011). [CrossRef]

20.

J. A. Anguita, I. B. Djordjevic, M. A. Neifeld, and B. V. Vasic, “Shannon capacities and error-correction codes for optical atmospheric turbulent channels,” J. Opt. Netw. 4, 586–601 (2005). [CrossRef]

21.

A. García-Zambrana, C. Castillo-Vázquez, and B. Castillo-Vázquez, “On the capacity of FSO links over Gamma-Gamma atmospheric turbulence channels using OOK signaling,” EURASIP Journal on Wireless Communications and Networking 2010. Article ID 127657, 9 pages, 2010. doi: [CrossRef]

22.

A. Al-Habash, L. C. Andrews, and R. L. Phillips, “Mathematical model for the irradiance probability density function of a laser beam propagating through turbulent media,” Opt. Eng. 40, 1554–1562 (2001). [CrossRef]

23.

I. S. Gradshteyn and I. M. Ryzhik, Table of Integrals, Series, and Products (Academic Press, 2000).

24.

N. Wang and J. Cheng, “Moment-based estimation for the shape parameters of the Gamma-Gamma atmospheric turbulence model,” Opt. Express 18, 12824–12831 (2010). [CrossRef] [PubMed]

25.

Md. Z. Hassan, X. Song, and J. Cheng, “Subcarrier intensity modulated wireless optical communications with rectangular QAM,” J. Opt. Commun. Netw. 6, 522–532 (2012). [CrossRef]

26.

M. Niu, “Coherent optical wireless communications over atmnospheric turbulence channels,” Ph. D. Thesis, The University of British Columbia (2012).

27.

M. Niu, J. Schlenker, J. Cheng, J. F. Holzman, and R. Schober, “Coherent wireless optical communications with predetection and postdetection EGC over Gamma-Gamma atmospheric turbulence channels,” J. Opt. Commun. Netw. , 3, 860–869 (2011). [CrossRef]

28.

D. Banerjee, PLL Performance, Simulation and Design (National Semiconductor, 2006).

29.

E. J. Lee and V. W. S. Chan, “Diversity coherent and incoherent receivers for free-space optical communication in the presence and absence of interference,” J. Opt. Commun. Netw. 1, 463–483 (2009). [CrossRef]

30.

X. Zhao, Y. Yao, Y. Sun, and C. Liu, “Circle polarisation shift-keying with direct detection for free-space optical communication,” J. Opt. Commun. Netw. 1, 307–312 (2009). [CrossRef]

31.

I. B. Djordjevic, L. Xu, and T. Wang, “Beyond 100 Gb/s optical transmission based on polarization multiplexed coded-OFDM with coherent detection,” J. Opt. Commun. Netw. 1, 50–56 (2009). [CrossRef]

32.

J. Grosinger, “Investigation of polarisation modulation in optical free space communications through the atmosphere,” Master thesis, Technical University of Vienna (2008).

33.

G. P. Agrawal, Fiber-Optical Communication Systems (Wiley, 2002). [CrossRef]

34.

A. J. Goldsmith and P. P. Varaiya, “Capacity of fading channels with channel side information,” IEEE Trans. Info. Theory 43, 1986–1992 (1997). [CrossRef]

35.

N. Cressie, A. S. Davis, J. L. Folks, and G. E. Policello II, “The moment-generating function and negative integer moments,” The American Statistician 35, 148–150 (1981).

36.

The Wolframe Function Site. [Online]: http://functions.wolfram.com.

37.

J. M. Hunt, F. Holmes, and F. Amizajerdian, “Optimum local oscialltor levels for coherent detection using photodetectors,” Appl. Opt. 27, 3135–3141 (1988). [CrossRef] [PubMed]

38.

P. Galambos and I. Simonelli, Products of Random Variables: Applications to Problems of Physics and to Arithmetical Functions (Marcel Dekker Inc., 2004).

OCIS Codes
(010.1330) Atmospheric and oceanic optics : Atmospheric turbulence
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: June 14, 2013
Revised Manuscript: July 26, 2013
Manuscript Accepted: August 2, 2013
Published: August 22, 2013

Citation
Md. Zoheb Hassan, Md. Jahangir Hossain, and Julian Cheng, "Ergodic capacity comparison of optical wireless communications using adaptive transmissions," Opt. Express 21, 20346-20362 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-17-20346


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. V. W. S. Chan, “Free-space optical communications,” J. Lightwave Technol.24, 4750–4762 (2006). [CrossRef]
  2. N. D. Chatzidiamantis, A. S. Lioumpas, G. K. Karagiannidis, and S. Arnon, “Adaptive subcarrier PSK intensity modulation in free space optical systems,” IEEE Trans. Commun.59, 1368–1377 (2011). [CrossRef]
  3. Md. Z. Hassan, Md. J. Hossain, and J. Cheng, “Performance of non-adaptive and adaptive subcarrier intensity modulations in Gamma-Gamma turbulence,” IEEE. Trans Commun.61, 2946–2957 (2013). [CrossRef]
  4. F. Lazarakis, G. S. Tombras, and K. Dangakis, “Average channel capacity in a mobile radio environment with Rician statistics,” IEICE Trans. Commun.E77-B, 971–977 (1994).
  5. S. M. Haas and J. H. Shapiro, “Capacity of wireless optical communications,” IEEE J. Sel. Areas Commun.21,pp. 1346–1357 (2003). [CrossRef]
  6. A. Belmonte and J. M. Kahn, “Capacity of coherent free-space optical links using diversity-combining techniques,” Opt. Express17, 12601–12611 (2009). [CrossRef] [PubMed]
  7. A. Belmonte and J. M. Kahn, “Capacity of coherent free-space optical links using atmospheric compensation techniques,” Opt. Express17, 2763–2773 (2009). [CrossRef] [PubMed]
  8. C. Liu, Y. Yao, Y. Sun, and X. Zhao, “Average capacity for heterodyne FSO communication systems over Gamma-Gamma turbulence channels with pointing errors,” Electron. Lett.46, 851–853 (2010). [CrossRef]
  9. H. E. Nistazakis, E. A. Karagianni, A. D. Tsigopoulos, M. E. Fafalios, and G. S. Tombras, “Average capacity of optical wireless communication systems over atmospheric turbulence channels,” J. Lightwave Technol.27, 974–979 (2009). [CrossRef]
  10. H. E. Nistazakis, T. A. Tsiftsis, and G. S. Tombras, “Performance analysis of free-space optical communication systems over atmospheric turbulence channels,” IET Commun.3, 1402–1409 (2009). [CrossRef]
  11. H. G. Sandalidis and T. A. Tsiftsis, “Outage probability and ergodic capacity of free-space optical links over strong turbulence,” Electron. Lett.44, 46–47 (2008). [CrossRef]
  12. K. P. Peppas, A. N. Stassinakis, G. K. Topalis, H. E. Nistazakis, and G. S. Tombras, “Average capacity of optical wireless communication systems over I− K atmospheric turbulence channels,” J. Opt. Commun. Netw.4, 1026–1032 (2012). [CrossRef]
  13. G. Pan, E. Ekici, and Q. Feng, “Capacity analysis of log-normal channels under various adaptive transmission schemes,” IEEE Commun. Lett.16, 346–348 (2012). [CrossRef]
  14. I. B. Djordjevic, “Adaptive modulation and coding for free-space optical channels,” J. Opt. Commun. Netw.2, 221–229 (2010). [CrossRef]
  15. I. B. Djordjevic and G. T. Djordjevic, “On the communication over strong atmospheric turbulence channels by adaptive modulation and coding,” Opt. Express17, 18250–18262 (2009). [CrossRef] [PubMed]
  16. A. Laourine, M.-S. Alouini, S. Affes, and A. Stéphenne, “On the capacity of generalized-K fading channels,” IEEE Trans. Wireless Commun.7, 2441–2445 (2008). [CrossRef]
  17. W. Gappmair, “Further results on the capacity of free-space optical channels in turbulent atmosphere,” IET Commun.5, 1262–1267 (2010). [CrossRef]
  18. N. Cvijetic, D. Y. Qian, J. J. Yu, Y. K. Huang, and T. Wang, “Polarisation-multiplexed optical wireless transmission with coherent detection,” J. Lightwave Technol.28, 1218–1227 (2010). [CrossRef]
  19. G. D. Xie, F. X. Wang, A. Dang, and H. Guo, “A novel polarisation-multiplexing system for free-space optical links,” IEEE Photon. Technol. Lett.23, 1484–1486 (2011). [CrossRef]
  20. J. A. Anguita, I. B. Djordjevic, M. A. Neifeld, and B. V. Vasic, “Shannon capacities and error-correction codes for optical atmospheric turbulent channels,” J. Opt. Netw.4, 586–601 (2005). [CrossRef]
  21. A. García-Zambrana, C. Castillo-Vázquez, and B. Castillo-Vázquez, “On the capacity of FSO links over Gamma-Gamma atmospheric turbulence channels using OOK signaling,” EURASIP Journal on Wireless Communications and Networking2010. Article ID 127657, 9 pages, 2010. doi: [CrossRef]
  22. A. Al-Habash, L. C. Andrews, and R. L. Phillips, “Mathematical model for the irradiance probability density function of a laser beam propagating through turbulent media,” Opt. Eng.40, 1554–1562 (2001). [CrossRef]
  23. I. S. Gradshteyn and I. M. Ryzhik, Table of Integrals, Series, and Products (Academic Press, 2000).
  24. N. Wang and J. Cheng, “Moment-based estimation for the shape parameters of the Gamma-Gamma atmospheric turbulence model,” Opt. Express18, 12824–12831 (2010). [CrossRef] [PubMed]
  25. Md. Z. Hassan, X. Song, and J. Cheng, “Subcarrier intensity modulated wireless optical communications with rectangular QAM,” J. Opt. Commun. Netw.6, 522–532 (2012). [CrossRef]
  26. M. Niu, “Coherent optical wireless communications over atmnospheric turbulence channels,” Ph. D. Thesis, The University of British Columbia (2012).
  27. M. Niu, J. Schlenker, J. Cheng, J. F. Holzman, and R. Schober, “Coherent wireless optical communications with predetection and postdetection EGC over Gamma-Gamma atmospheric turbulence channels,” J. Opt. Commun. Netw., 3, 860–869 (2011). [CrossRef]
  28. D. Banerjee, PLL Performance, Simulation and Design (National Semiconductor, 2006).
  29. E. J. Lee and V. W. S. Chan, “Diversity coherent and incoherent receivers for free-space optical communication in the presence and absence of interference,” J. Opt. Commun. Netw.1, 463–483 (2009). [CrossRef]
  30. X. Zhao, Y. Yao, Y. Sun, and C. Liu, “Circle polarisation shift-keying with direct detection for free-space optical communication,” J. Opt. Commun. Netw.1, 307–312 (2009). [CrossRef]
  31. I. B. Djordjevic, L. Xu, and T. Wang, “Beyond 100 Gb/s optical transmission based on polarization multiplexed coded-OFDM with coherent detection,” J. Opt. Commun. Netw.1, 50–56 (2009). [CrossRef]
  32. J. Grosinger, “Investigation of polarisation modulation in optical free space communications through the atmosphere,” Master thesis, Technical University of Vienna (2008).
  33. G. P. Agrawal, Fiber-Optical Communication Systems (Wiley, 2002). [CrossRef]
  34. A. J. Goldsmith and P. P. Varaiya, “Capacity of fading channels with channel side information,” IEEE Trans. Info. Theory43, 1986–1992 (1997). [CrossRef]
  35. N. Cressie, A. S. Davis, J. L. Folks, and G. E. Policello, “The moment-generating function and negative integer moments,” The American Statistician35, 148–150 (1981).
  36. The Wolframe Function Site. [Online]: http://functions.wolfram.com .
  37. J. M. Hunt, F. Holmes, and F. Amizajerdian, “Optimum local oscialltor levels for coherent detection using photodetectors,” Appl. Opt.27, 3135–3141 (1988). [CrossRef] [PubMed]
  38. P. Galambos and I. Simonelli, Products of Random Variables: Applications to Problems of Physics and to Arithmetical Functions (Marcel Dekker Inc., 2004).

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1 Fig. 2
 

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited