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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 13 — Jun. 21, 2010
  • pp: 13915–13926
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Exact error rate analysis of equal gain and selection diversity for coherent free-space optical systems on strong turbulence channels

Mingbo Niu, Julian Cheng, and Jonathan F. Holzman  »View Author Affiliations


Optics Express, Vol. 18, Issue 13, pp. 13915-13926 (2010)
http://dx.doi.org/10.1364/OE.18.013915


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Abstract

Exact error rate performances are studied for coherent free-space optical communication systems under strong turbulence with diversity reception. Equal gain and selection diversity are considered as practical schemes to mitigate turbulence. The exact bit-error rate for binary phase-shift keying and outage probability are developed for equal gain diversity. Analytical expressions are obtained for the bit-error rate of differential phase-shift keying and asynchronous frequency-shift keying, as well as for outage probability using selection diversity. Furthermore, we provide the closed-form expressions of diversity order and coding gain with both diversity receptions. The analytical results are verified by computer simulations and are suitable for rapid error rates calculation.

© 2010 Optical Society of America

1. Introduction

Free-space optical (FSO) communications has drawn considerable interest. Lower costs, larger bandwidths, better security, and greater flexibility are all FSO benefits [1

1. M. M. Ibrahim and A. M. Ibrahim, “Performance analysis of optical receivers with space diversity reception,” IEE Proc. Commun. 143, 369–372 (1996). [CrossRef]

]. FSO systems with diversity reception, in particular, achieve performance improvements by mitigating atmospheric turbulence. This has been shown for irradiance modulation and direct detection (IM/DD) system with OOK using optimal combining, equal gain combining (EGC), and selection combining [1

1. M. M. Ibrahim and A. M. Ibrahim, “Performance analysis of optical receivers with space diversity reception,” IEE Proc. Commun. 143, 369–372 (1996). [CrossRef]

, 2

2. T. A. Tsiftsis, H. G. Sandalidis, G. K. Karagiannidis, and M. Uysal, “Optical wireless links with spatial diversity over strong atmospheric turbulence channels,” IEEE Trans. Wirel. Commun. 8, 951–957 (2009). [CrossRef]

], and coherent detection has been successful in further improving the sensitivity and spectral efficiency [3

3. E. Lee and V. Chan, “Diversity coherent receivers for optical communication over the clear turbulent atmosphere,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2007).

].

With the benefits of coherent FSO in mind [3

3. E. Lee and V. Chan, “Diversity coherent receivers for optical communication over the clear turbulent atmosphere,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2007).

], exact BER expressions have been developed for differential phase-shift keying (DPSK) over K-distributed [4

4. K. Kiasaleh, “Performance of coherent DPSK free-space optical communication systems in K-distributed turbulence,” IEEE Trans. Commun. 54, 604–607 (2006). [CrossRef]

] and Gamma-Gamma distributed [5

5. T. A. Tsiftsis, “Performance of heterodyne wireless optical communication systems over Gamma-Gamma atmospheric turbulence channels,” Electron. Lett. 44, 373–375 (2008). [CrossRef]

] turbulence. A heterodyne FSO system with pointing errors was studied by Sandalidis et al. [6

6. H. G. Sandalidis, T. A. Tsiftsis, and G. K. Karagiannidis, “Optical wireless communications with heterodyne detection over turbulence channels with pointing errors,” J. Lightwave Technol. 27, 4440–4445 (2009). [CrossRef]

]. Synchronous detection was then studied by Belmonte and Kahn [7

7. A. Belmonte and J. M. Kahn, “Performance of synchronous optical receivers using atmospheric compensation techniques,” Opt. Express 16, 14151–14162 (2008). [CrossRef] [PubMed]

, 8

8. 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 our recent work [9

9. M. Niu, J. Cheng, J. F. Holzman, and L. McPhail, “Performance analysis of coherent free-space optical communication systems with K-distributed turbulence,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2009).

] demonstrated that maximum ratio combining (MRC) can provide excellent performance improvements at practical signal-to-noise ratios (SNRs).

In this work, both equal gain and selection diversity (SD) are analyzed for long-range and high-turbulence conditions with a K distribution [4

4. K. Kiasaleh, “Performance of coherent DPSK free-space optical communication systems in K-distributed turbulence,” IEEE Trans. Commun. 54, 604–607 (2006). [CrossRef]

,10

10. R. L. Phillips and L. C. Andrews, “Measured statistics for laser light scattering in atmospheric turbulence,” J. Opt. Soc. Am. 71, 1440–1445 (1981). [CrossRef]

]. Based on the new combiner SNR statistics, we derive an exact BER expression for binary phase-shift keying (BPSK) and an outage probability expression with EGC. For SD, we obtain outage probability and BER expressions with asynchronous DPSK and frequency-shift keying (FSK). The obtained error rates are highly accurate for all SNR regions. In particular, the asymptotic error rates are obtained based on the development of diversity order and coding gain.

2. The Coherent FSO System Model and Statistics

2.1. Channel Model

For the lth branch of a coherent FSO system using BPSK, with received and local oscillator beams mixed in perfect spatial coherence over a sufficiently small photodetector area. Practical issues such as beam combining and polarization are discussed in [11

11. M. Jafar, D. C. O’Brien, C. J. Stevens, and D. J. Edwards, “Evaluation of coverage area for a wide line-of-sight indoor optical free-space communication system empolying coherent detection,” IET Commun. 2, 18–26 (2008). [CrossRef]

]. The photodetector current is [9

9. M. Niu, J. Cheng, J. F. Holzman, and L. McPhail, “Performance analysis of coherent free-space optical communication systems with K-distributed turbulence,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2009).

]

il(t)=idc,l+iac,l(t)+nl(t)
(1)

where idc,l = R(Ps,l + PLO) and iac,l(t)=2RPs,lPLOcos(ωIFt+ϕ+ϕs,l) are the DC and AC terms, respectively, R is the detector responsivity, and nl(t) is a shot-noise-limited additive white Gaussian noise (AWGN) process. The power PLO is sufficiently large so that shot noise is dominant and can be modeled as AWGN [12

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

]. Here, Ps,l is the received lth optical signal power incident on the beamsplitter, PLO is the local oscillator power assumed to be the same for all branches, and ωIF = ω0ωLO is the intermediate frequency, with ω0 and ωLO denoting the carrier frequency and local oscillator frequency, respectively. The phase information is ϕ ∈ {0, π}, and ϕs,l is the lth branch phase noise. It is assumed that each diversity branch turbulence is independent. For EGC cases, we assume the signal phase is well tracked at the coherent FSO detector and the phase noise can be fully compensated [3

3. E. Lee and V. Chan, “Diversity coherent receivers for optical communication over the clear turbulent atmosphere,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2007).

], as the optical phase varies slowly over 1 or 2 bits at a high data rate.

2.2. SNR Analysis

The SNR of an optical receiver is defined as the ratio of the time-averaged AC photocurrent power 〈i2ac(t)〉 to the total noise variance [12

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

]. For EGC with L diversity branches, the received signals at different branches are co-phased and added with equal weight. In such a case, as shown in the Appendix, the instantaneous SNR at the output of the combiner becomes

γEGC=R(Σl=1LPs,l)2LqΔf=2RPLO(Σl=1LIN,l)2L2EbN0
(2)

where q denotes the electronic charge, Δf is the noise equivalent bandwidth of the photodetector, IN,l is the normalized K turbulent coefficient (E[IN,l] = 1) of the lth branch, N0/2 = qRPLO is the noise power spectral density, and Eb = η2ATb denotes the energy per bit. Here, E[·] denotes the expectation operation, η2 is the mean irradiance, A denotes the detector area, and Tb is the bit duration. With the relationship Ps,l = AIs,l where Is,l is the instantaneous received optical irradiance at the lth branch, we can rewrite the instantaneous SNR as

γEGC=RA(Σl=1LIs,l)2LqΔf=g2(Σl=1LIs,l)2
(3)

Given the lth branch instantaneous SNR as γl = RPs,l/(qΔf) [9

9. M. Niu, J. Cheng, J. F. Holzman, and L. McPhail, “Performance analysis of coherent free-space optical communication systems with K-distributed turbulence,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2009).

], the instantaneous SNR for SD is

γSD=max{γ1,γ2,,γL}=RqΔfmax{Ps,l;l=1,,L}.
(4)

We can observe that the instantaneous combiner SNRs are independent of the local oscillator power for large PLO. This is an important property of coherent FSO communication (in contrast to coherent radio frequency literature), as the local oscillator power does not affect coherent FSO system performance.

2.3. Analysis for the Square Root of the Irradiance

To facilitate the performance analysis for the coherent EGC FSO systems, we can make use of the characteristic function (CF) for the square root of the irradiance Is. In this work, we assume a signal irradiance Is with a K-distributed model having a probability density function (PDF) given by [4

4. K. Kiasaleh, “Performance of coherent DPSK free-space optical communication systems in K-distributed turbulence,” IEEE Trans. Commun. 54, 604–607 (2006). [CrossRef]

]

f(Is)=2Γ(α)ηα+1αα+12Isα12Kα1(2ηαIs),Is>0
(5)

σsi2E[Is2](E[Is])21
(6)

one can show

α=2σsi21.
(7)

Scintillation indices below unity correspond to weak turbulence conditions, which is commonly modeled with a log-normal distribution. For FSO applications, the scintillation index σ2si in the K-distributed model is (2,3) [4

4. K. Kiasaleh, “Performance of coherent DPSK free-space optical communication systems in K-distributed turbulence,” IEEE Trans. Commun. 54, 604–607 (2006). [CrossRef]

], and α is in (1,2). In the strong turbulence regime, the scintillation index can be calculated from the Rytov variance [13

13. L. C. Andrews, R. L. Phillips, C. Y. Hopen, and M. A. Al-Habash, “Theory of optical scintillation,” J. Opt. Soc. Am. 16, 1417–1429 (1999). [CrossRef]

]

σR2=1.23Cn2k76d116
(8)

f(z)=4Γ(α)ηα+1αα+12zαKα1(2αηz),z>0
(9)

which finds applications in modeling the displacement of a two-dimensional unbiased random walk. To obtain the CF of z, we let z = xy, where xIx and yIy are found to have Rayleigh and Nakagami (when α ≥ 1/2) PDFs, respectively, as

f(x)=2η2xex2η2,x>0
(10)

and

f(y)=2ααy2α1eαy2Γ(α),y>0.
(11)

Averaging the CF of z conditioned on Ix , one obtains the CF of z as

ϕz(ω)={ϕz(ω)}+j{ϕz(ω)}
(12)

where the real and imaginary parts are respectively

{ϕz(ω)}=2F1(α,1,12;ω2η24α)
(13)

and

{ϕz(ω)}=ωΓ(α+12)Γ(32)Γ(α)(η2α)122F1(α+12,32,32;ω2η24α).
(14)

Here, 2F1(·, ·, ·; ·) is the Gaussian hypergeometric function, ℜ{·} and ℑ{·} denote the real and imaginary operators, respectively. The CF of z can also be obtained from the moment generating function (MGF) of a product of two independent Nakagami RVs [15

15. G. K. Karagiannidis, N. C. Sagias, and P. T. Mathiopoulos, “N*Nakagami: A Novel Stochastic Model for Cascaded Fading Channels,” IEEE Trans. Commun. 55, 1453–1458 (2007). [CrossRef]

,Eq. (3)].

3. Performance Analysis of EGC FSO Systems

3.1. Error Rate Analysis

The average BER of BPSK over a turbulence channel can be derived by averaging the conditional bit-error probability as

Pe=0Pe(zs)f(zs)dzs
(15)

where zs is the sum of L square roots of the irradiance, i.e., zs=Σl=1LIs,l . Here, f(zs) is the PDF of zs and Pe(zs) denotes the conditional bit-error probability given by

Pe(zs)=12erfc(γEGC2)=12erfc(gzs2)
(16)

where erfc(·) is the complementary error function. In general, it is difficult to obtain a closed-form expression for the PDF of zs. Hence, we use the Fourier inversion theorem to obtain the PDF of zs by

f(zs)=12πϕzs(ω)ejωzsdω=12πl=1Lϕzl(ω)ejωzsdω
(17)

where j2 = −1, ϕzs(ω) is the CF of zs and ϕzl(ω) is the CF of zl (with zl=Is,l ). With Eq. (15)–Eq. (17), one can obtain the average BER as

Pe=12πϕzs*(ω)0Pe(zs)ejωzsdzsdω
(18)

where ϕzs*(·) denotes the conjugate of ϕzs(·) . Since the Fourier transform of the complementary error function is known as [16

16. A. Annamalai, C. Tellambura, and V. K. Bhargava, “Exact evaluation of maximal-ratio and equal-gain diversity receivers for M-ary QAM on Nakagami fading channels,” IEEE Trans. Commun. 47, 1335–1344 (1999). [CrossRef]

]

𝘍(ω)=0erfc(zs)ejωzsdzs=1π1F1(1,32,ω24)+jω(1eω24),
(19)

Equation (18) can be rewritten as

Pe=12πl=1Lϕzl*(gω2)𝘍(ω)2dω.
(20)

With a change of variable ω = tanθ, it is possible to rewrite Eq. (20) in terms of a definite integration for easier numerical evaluation

Pe=12ππ2π2sec2θl=1Lϕzl*(gtanθ2)𝘍(tanθ)2dθ.
(21)

Though BPSK is only considered here, it is straightforward to extend our analysis to other coherent modulations such as quadrature phase-shift keying and M-ary quadrature amplitude modulation.

3.2. Outage Probability Analysis

To find the outage probability, we make use of the Gil-Pelaez formula to express the CDF of zs as [18

18. Q. T. Zhang, “Outage probability in cellular mobile ratio due to nakagami signal and interferers with arbitrary parameters,” IEEE Trans. Veh. Technol. 45, 364–372 (1996). [CrossRef]

]

Fzs(zo)=121π0{ϕzs(ω)exp[jωzo]}ωdω.
(22)

Since the turbulence in each branch is independent, ϕzs(ω) can be written directly as ϕzs(ω)=l=1Lϕzl(ω) . To continue, we express the CF of zl in the polar form as

ϕzl(ω)=2{ϕzl(ω)}+2{ϕzl(ω)}exp[jtan1({ϕzl(ω)}{ϕzl(ω)})]
(23)

where the real and imaginary parts of the CF of zl are obtained from Eq. (13) and Eq. (14). The numerator of the integrand in Eq. (22) can now be written as

{ϕzs(ω)exp[jωzo]}=l=1L2{ϕzl(ω)}+2{ϕzl(ω)}sin[Σl=1Ltan1({ϕzl(ω)}{ϕzl(ω)})ωzo].
(24)

With a change of variable, substitution of Eq. (24) into Eq. (22) gives the outage probability as

Po,EGC(γ*)=122π0π2l=1L2{ϕzl(tanθ)}+2{ϕzl(tanθ)}sin2θcsc[Σl=1Ltan1({ϕzl(tanθ)}{ϕzl(tanθ)})tanθωγ*g]dθ
(25)

where γ* is the specified outage threshold. Equation (25) can provide accurate outage calculation with a simple numerical integration algorithm.

4. Performance Analysis of SD FSO Systems

SD has the least complexity when compared to MRC and EGC, as it simply selects the branch with the strongest instantaneous received SNR. In the following, we present exact outage probability and exact error rates expressions for coherent FSO systems with SD reception.

4.1. Outage Probability Analysis

After a change of variable and help from [17

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

,Eq. 6.561(8)], we can obtain the CDF of the optical signal irradiance, Is,l, at the lth branch from Eq. (5) as

FIs,l(I)=12(αlI)α2Γ(αl)ηlαlKαl(2ηlαlI),I>0.
(26)

It is straightforward to show that the outage probability with SD becomes

Po,SD(γ*)=l=1LFIs,l(γ*g2).
(27)

4.2. Error Rate Analysis for Differential and Asynchronous Detection

Unlike the outage probability analysis, the desired error rate analysis depends upon the modulation scheme and requires the PDF of Is,SD ≜ max{Is,l, l = 1, …, L}. For identically distributed turbulence, we can show the PDF of Is,SD as

fIs,SD(Is)=L[FIs,l(Is)]L1f(Is).
(28)

Differential and asynchronous detection are two alternatives for synchronous detection to minimize phase noise effects. In this work, coherent FSO with asynchronous detection refers to coherent photodetection at the receiver front-end, followed by envelope detection for signal recovery [12

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

, P. 490]. The conditional BER with an average SNR of γ¯ is known to be

Pe(Is)=12exp[βγ¯Is]
(29)

where β = 1/2 for DPSK (assuming moderate frequency offset) and β = 1/4 for asynchronous FSK [12

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

]. Hence, the average BER for DPSK or FSK with SD is

Pe=120exp[βγ¯Is]fIs,SD(Is)dIs=120π2sec2θexp[βγ¯tanθ]fIs,SD(tanθ)dθ.
(30)

5. Analysis of Diversity Order and Coding Gain

The analysis in this section allows us to gain valuable insight into BER behavior in the large SNR region for coherent FSO with both EGC and SD reception. The asymptotic BER can be expressed as Pe=.(Gcγ¯)Gd where Gd and Gc respectively denote diversity order and coding gain [9

9. M. Niu, J. Cheng, J. F. Holzman, and L. McPhail, “Performance analysis of coherent free-space optical communication systems with K-distributed turbulence,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2009).

]. Similarly we denote Gdl and Gcl as the diversity order and coding gain of the lth branch.

Using the MGF of a K-distributed RV and following [9

9. M. Niu, J. Cheng, J. F. Holzman, and L. McPhail, “Performance analysis of coherent free-space optical communication systems with K-distributed turbulence,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2009).

], one can show that the diversity order and coding gain offered by the lth branch are Gdl = 1 and Gcl = 2βη2(α−1)/α respectively. Consequently, the diversity order of SD becomes Gd,SD = ∑Ll=1 Gdl = L and the coding gain of SD can be derived as

Gc,SD=[2L1Γ(Σl=1LGdl)Σl=1L(Gdl)l=1LGclGdlΓ(Gdl)Gdl]1Σl=1LGdl.
(31)

For BPSK EGC, the diversity order and coding gain offered by the lth branch can be found to be Gdl = 1 and Gcl = 2η2(α − 1)/α respectively. The overall EGC diversity order is Gd,EGC = L, and the expression for the overall EGC coding gain, Gc,EGC can be found in [19

19. Z. Wang and G. B. Giannakis, “A simple and general parameterization quantifying performance in fading channels,” IEEE Trans. Commun. 51, 1389–1398 (2003). [CrossRef]

,Eq. (12)]. The diversity order here is similar to the diversity order observed in an RF system (with the exception of the Nakagami-m fading model).

6. Numerical Results

Coherent FSO numerical case studies are presented. The average SNR per branch is γ¯ = RAη2/(qΔf). BER curves for BPSK-modulated signals with EGC diversity reception (α = 1.8) are presented in Fig. 1 for average SNRs from 0 to 30 dB. Asymptotic BERs are also shown and agree with our analytical results when the SNR is large. When the number of diversity branches increases, the BER performance improves. At an average SNR=30 dB, a three-branch EGC reception achieves an error rate of 2.5 × 10−8. Though it is not shown in Fig. 1, all analytical results have been verified with computer simulations and they have excellent agreement.

Fig. 1. The exact BERs and asymptotic BERs of BPSK for coherent EGC diversity operating on L-branch K-distributed turbulence channel with α = 1.8.

Figure 2 compares the BER performance of MRC and EGC for BPSK-modulation and shows that coherent FSO systems using EGC offer a close error rate performance to that of MRC. At a BER of 10−7, 1 dB SNR (or less) is required for EGC to achieve the MRC performance. Hence, EGC offers a good balance between system performance and complexity.

To further reduce the system complexity, SD becomes a viable alternative. The received optical signals need to be co-phased for EGC, but this procedure is not required for SD reception. Therefore, SD can reduce the receiver complexity. The SD outage probability is shown in Fig. 3 along with EGC and MRC. It is apparent that the outage probability can be improved by using systems with increased complexity (e.g. MRC) and higher diversity orders. With little knowledge of the turbulence phase noise, asynchronous DPSK and FSK with SD become the preferred choices. BERs for these two modulation schemes are presented in Fig. 4 and Fig. 5. It is apparent that SD reception mitigates turbulence effects for differential and asynchronous modulations. Moreover, the performance improves as the effective aperture size increases. The asymptotic solutions are also shown in both Fig. 4 and Fig. 5. It is found that they agree with the analytical results when the SNR becomes large. Turbulence effects are shown in Fig. 6 for σ2si ranging from 2.1 to 2.9 for both MRC and EGC. As expected, BER performance improves for coherent FSO communication as the value of σ2si decreases.

It is interesting to observe from Fig. 6 that EGC with σ2si = 2.1 outperforms MRC with σ2si = 2.5 for the same number of diversity branches. For practical applications, a modest parametrization error in estimating the scintillation indices or channel parameters will overshadow the benefits provided by MRC. In such a case, EGC may be a better choice due to its reduced complexity.

Fig. 2. Exact BERs of BPSK for coherent MRC and EGC diversity operating on L-branch K-distributed turbulence channel with α = 1.8.
Fig. 3. Outage probabilities of coherent MRC, EGC and SD operating on L-branch K-distributed turbulence channel with α = 1.8.
Fig. 4. The exact BERs and asymptotic BERs of asynchronous DPSK for SD operating on L-branch K-distributed turbulence channel with α = 1.8.
Fig. 5. The exact BERs and asymptotic BERs of asynchronous FSK for SD operating on L-branch K-distributed turbulence channel with α = 1.8.
Fig. 6. The impact of scintillation index σ2si on BPSK BERs for MRC and EGC diversity operating with three diversity branches on K-distributed turbulence channels.

7. Conclusion

In this work, we have studied the performance of diversity reception for coherent FSO communication systems in strong atmospheric turbulence. The exact BER expressions and outage probability with EGC and SD reception have been obtained. Our numerical results show that coherent FSO transmission with EGC outperforms that with SD and gives comparable performance to that of MRC with reduced complexity, while the numerical results have demonstrated that the new analytical expressions are useful in providing highly accurate error rate estimation. In addition, we studied the diversity order and coding gain for coherent FSO combining showing the performance impact of optical systems and channel parameters. Our analytical study can be easily extended to coherent FSO communications under different turbulence conditions.

Appendix

At the output of the lth photodetector, the received photocurrent is given in Eq. (1). Let cl=2RPLOPs,l , and the AC current can be expressed as follows

iac,l(t)=cl[cosϕcos(ωIFt+ϕs,l)sinϕsin(ωIFt+ϕs,l)].
(32)

Then, two real filters are used to implement the complex filtering in the downconverter process. The real and imaginary parts of the baseband signal are, respectively, obtained as

yc,l(t)=2{cl[cosϕcos(ωIFt+ϕs,l)sinϕsin(ωIFt+ϕs,l)]}cos(ωIFt)
(33)

and

ys,l(t)=2{cl[cosϕcos(ωIFt+ϕs,l)sinϕsin(ωIFt+ϕs,l)]}sin(ωIFt).
(34)

After a lowpass filter, the inphase and quadrature components become

y˜c,l(t)=cl2[cosϕcosϕs,lsinϕsinϕs,l]
(35)

and

y˜s,l(t)=cl2[cosϕcosϕs,l+sinϕsinϕs,l].
(36)

Now we combine (35) and (36) to obtain the equivalent baseband signal as

y˜l(t)=y˜c,l(t)+jy˜s,l(t)=cl2[cosϕ+jsinϕ]ejϕs,l.
(37)

The last step is to compensate the phase noise, giving the branch signal before combining as

y˜l(t)ejϕs,l=2RPLOPs,lejϕ.
(38)

Consequently, with the assumption of equal noise variance in each diversity branch, the instantaneous SNR at the output of the equal gain combiner becomes

γEGC=(Σl=1L2RPLOPs,l)2Σl=1Lσn,l2=(Σl=1L2RPLOPs,l)2L(2qRPLOΔf)=R(Σl=1LPs,l)2LqΔf
(39)

where σ2n,l = 2qRPLOΔf,l = 1, …, L, is the noise variance in the lth diversity branch at the receiver side, and it is assumed to be the same for all diversity branches.

Acknowledgments

The research was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC).

References and links

1.

M. M. Ibrahim and A. M. Ibrahim, “Performance analysis of optical receivers with space diversity reception,” IEE Proc. Commun. 143, 369–372 (1996). [CrossRef]

2.

T. A. Tsiftsis, H. G. Sandalidis, G. K. Karagiannidis, and M. Uysal, “Optical wireless links with spatial diversity over strong atmospheric turbulence channels,” IEEE Trans. Wirel. Commun. 8, 951–957 (2009). [CrossRef]

3.

E. Lee and V. Chan, “Diversity coherent receivers for optical communication over the clear turbulent atmosphere,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2007).

4.

K. Kiasaleh, “Performance of coherent DPSK free-space optical communication systems in K-distributed turbulence,” IEEE Trans. Commun. 54, 604–607 (2006). [CrossRef]

5.

T. A. Tsiftsis, “Performance of heterodyne wireless optical communication systems over Gamma-Gamma atmospheric turbulence channels,” Electron. Lett. 44, 373–375 (2008). [CrossRef]

6.

H. G. Sandalidis, T. A. Tsiftsis, and G. K. Karagiannidis, “Optical wireless communications with heterodyne detection over turbulence channels with pointing errors,” J. Lightwave Technol. 27, 4440–4445 (2009). [CrossRef]

7.

A. Belmonte and J. M. Kahn, “Performance of synchronous optical receivers using atmospheric compensation techniques,” Opt. Express 16, 14151–14162 (2008). [CrossRef] [PubMed]

8.

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]

9.

M. Niu, J. Cheng, J. F. Holzman, and L. McPhail, “Performance analysis of coherent free-space optical communication systems with K-distributed turbulence,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2009).

10.

R. L. Phillips and L. C. Andrews, “Measured statistics for laser light scattering in atmospheric turbulence,” J. Opt. Soc. Am. 71, 1440–1445 (1981). [CrossRef]

11.

M. Jafar, D. C. O’Brien, C. J. Stevens, and D. J. Edwards, “Evaluation of coverage area for a wide line-of-sight indoor optical free-space communication system empolying coherent detection,” IET Commun. 2, 18–26 (2008). [CrossRef]

12.

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

13.

L. C. Andrews, R. L. Phillips, C. Y. Hopen, and M. A. Al-Habash, “Theory of optical scintillation,” J. Opt. Soc. Am. 16, 1417–1429 (1999). [CrossRef]

14.

E. Jakeman and R. J. A. Tough, “Generalized K distribution: a statistical model for weak scattering,” J. Opt. Soc. Am. A 4, 1764–1772 (1987). [CrossRef]

15.

G. K. Karagiannidis, N. C. Sagias, and P. T. Mathiopoulos, “N*Nakagami: A Novel Stochastic Model for Cascaded Fading Channels,” IEEE Trans. Commun. 55, 1453–1458 (2007). [CrossRef]

16.

A. Annamalai, C. Tellambura, and V. K. Bhargava, “Exact evaluation of maximal-ratio and equal-gain diversity receivers for M-ary QAM on Nakagami fading channels,” IEEE Trans. Commun. 47, 1335–1344 (1999). [CrossRef]

17.

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

18.

Q. T. Zhang, “Outage probability in cellular mobile ratio due to nakagami signal and interferers with arbitrary parameters,” IEEE Trans. Veh. Technol. 45, 364–372 (1996). [CrossRef]

19.

Z. Wang and G. B. Giannakis, “A simple and general parameterization quantifying performance in fading channels,” IEEE Trans. Commun. 51, 1389–1398 (2003). [CrossRef]

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 30, 2010
Revised Manuscript: June 1, 2010
Manuscript Accepted: June 5, 2010
Published: June 14, 2010

Citation
Mingbo Niu, Julian Cheng, and Jonathan F. Holzman, "Exact error rate analysis of equal gain and selection diversity for coherent free-space optical systems on strong turbulence channels," Opt. Express 18, 13915-13926 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-13-13915


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References

  1. . M. M. Ibrahim and A. M. Ibrahim, “Performance analysis of optical receivers with space diversity reception,” IEE Proc. Commun. 143,369–372 (1996). [CrossRef]
  2. . T. A. Tsiftsis, H. G. Sandalidis, G. K. Karagiannidis, and M. Uysal, “Optical wireless links with spatial diversity over strong atmospheric turbulence channels,” IEEE Trans. Wirel. Commun. 8,951–957 (2009). [CrossRef]
  3. . E. Lee and V. Chan, “Diversity coherent receivers for optical communication over the clear turbulent atmosphere,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2007).
  4. . K. Kiasaleh, “Performance of coherent DPSK free-space optical communication systems in K-distributed turbulence,” IEEE Trans. Commun. 54,604–607 (2006). [CrossRef]
  5. . T. A. Tsiftsis, “Performance of heterodyne wireless optical communication systems over Gamma-Gamma atmospheric turbulence channels,” Electron. Lett. 44,373–375 (2008). [CrossRef]
  6. . H. G. Sandalidis, T. A. Tsiftsis, and G. K. Karagiannidis, “Optical wireless communications with heterodyne detection over turbulence channels with pointing errors,” J. Lightwave Technol. 27,4440–4445 (2009). [CrossRef]
  7. . A. Belmonte and J. M. Kahn, “Performance of synchronous optical receivers using atmospheric compensation techniques,” Opt. Express 16,14151–14162 (2008). [CrossRef] [PubMed]
  8. . 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]
  9. . M. Niu, J. Cheng, J. F. Holzman, and L. McPhail, “Performance analysis of coherent free-space optical communication systems with K-distributed turbulence,” in Proceedings of IEEE International Conference on Communications (Institute of Electrical and Electronics Engineers, New York, 2009).
  10. . R. L. Phillips and L. C. Andrews, “Measured statistics for laser light scattering in atmospheric turbulence,” J. Opt. Soc. Am. 71,1440–1445 (1981). [CrossRef]
  11. . M. Jafar, D. C. O’Brien, C. J. Stevens, and D. J. Edwards, “Evaluation of coverage area for a wide line-of-sight indoor optical free-space communication system empolying coherent detection,” IET Commun. 2,18–26 (2008). [CrossRef]
  12. . G. P. Agrawal, Fiber-Optical Communication Systems, (New York: Wiley, 2002). [CrossRef]
  13. . L. C. Andrews, R. L. Phillips, C. Y. Hopen, and M. A. Al-Habash, “Theory of optical scintillation,” J. Opt. Soc. Am. 16,1417–1429 (1999). [CrossRef]
  14. . E. Jakeman and R. J. A. Tough, “Generalized K distribution: a statistical model for weak scattering,” J. Opt. Soc. Am. A 4,1764–1772 (1987). [CrossRef]
  15. . G. K. Karagiannidis, N. C. Sagias, and P. T. Mathiopoulos, “N*Nakagami: A Novel Stochastic Model for Cascaded Fading Channels,” IEEE Trans. Commun. 55,1453–1458 (2007). [CrossRef]
  16. . A. Annamalai, C. Tellambura, and V. K. Bhargava, “Exact evaluation of maximal-ratio and equal-gain diversity receivers for M-ary QAM on Nakagami fading channels,” IEEE Trans. Commun. 47,1335–1344 (1999). [CrossRef]
  17. . I. S. Gradshteyn and I. M. Ryzhik, Table of Integrals, Series, and Products, (San Diego: Academic Press, 2000).
  18. . Q. T. Zhang, “Outage probability in cellular mobile ratio due to nakagami signal and interferers with arbitrary parameters,” IEEE Trans. Veh. Technol. 45,364–372 (1996). [CrossRef]
  19. . Z. Wang and G. B. Giannakis, “A simple and general parameterization quantifying performance in fading channels,” IEEE Trans. Commun. 51,1389–1398 (2003). [CrossRef]

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