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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 16 — Aug. 2, 2010
  • pp: 17346–17363
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An efficient rate-adaptive transmission technique using shortened pulses for atmospheric optical communications

Antonio Jurado-Navas, José María Garrido-Balsells, Miguel Castillo-Vázquez, and Antonio Puerta-Notario  »View Author Affiliations


Optics Express, Vol. 18, Issue 16, pp. 17346-17363 (2010)
http://dx.doi.org/10.1364/OE.18.017346


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Abstract

In free space optical (FSO) communication, atmospheric turbulence causes fluctuation in both intensity and phase of the received light signal what may seriously impair the link performance. Additionally, turbulent inhomogeneities may produce optical pulse spreading. In this paper, a simple rate adaptive transmission technique based on the use of variable silence periods and on-off keying (OOK) formats with memory is presented. This technique was previously proposed in indoor unguided optical links by the authors with very good performance. Such transmission scheme is now extensively analyzed in terms of burst error rate, and shown in this paper as an excellent alternative compared with the classical scheme based on repetition coding and pulse-position modulation (PPM), presenting a greater robustness to adverse conditions of turbulence.

© 2010 Optical Society of America

1. Introduction

2. Optical communication through turbulent atmosphere

Atmospheric turbulence can be physically described by Kolmogorov cascade theory [12–14

12. L. C. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media, (Bellingham, Washington, 1998).

]. Turbulent air motion represents a set of eddies of various scales sizes. Large eddies become unstable due to very high Reynolds number and break apart, so their energy is redistributed without loss to eddies of decreasing size until the kinetic energy of the flow is finally dissipated into heat by viscosity. The scale sizes of these eddies extend from a large scale size, L 0, called the outer scale of turbulence, to a small scale size, l 0, denoted the inner scale of turbulence, the scale where the energy is dissipated into heat. It is assumed that each eddy is homogeneous, although with a different index of refraction. These atmospheric index-of-refraction variations produce fluctuations in the irradiance of the transmitted optical beam, what is known as atmospheric scintillation. A widely used model with good accuracy to describe the spatial power spectrum of refractive index, Φn(κ) was proposed by Kolmogorov, which assumes the wavenumber spectrum to be:

Φn(κ)=0.033Cn2(z)κ113,0<κ<
(1)

as indicated in [12

12. L. C. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media, (Bellingham, Washington, 1998).

]. In Eq. (1), κ is the spatial wave number and C 2 n is the refractive-index structure parameter. Under the so-called Rytov approximation [14

14. V. I. Tatarskii, The Effects of the Turbulent Atmosphere on Wave Propagation, (Jerusalem: Israel Program for Scientific Translations, 1971).

], the optical field, u(r), of an optical wave propagating at distance L from the source can be expanded as [6

6. X. Zhu and J. M. Kahn, “Free-space optical communication through atmospheric turbulence channels,” IEEE Trans. Commun. 50, 1293–1300 (2002). [CrossRef]

]:

u(r,L)=A(r,L)·exp[jϕ(r,L)]=u0(r,L)·exp[Φ1(r,L)],
(2)

where r is the observation point in the transverse plane at propagation distance L, A(r,L) is the amplitude of the electric vector of the optical wave and u 0(r,L) is the optical field amplitude without air turbulence expressed, from [6

6. X. Zhu and J. M. Kahn, “Free-space optical communication through atmospheric turbulence channels,” IEEE Trans. Commun. 50, 1293–1300 (2002). [CrossRef]

], as u 0(r,L) = A 0(r,L) · exp [ 0(r,L)]. Finally, the exponent of the perturbation factor is:

Φ1(r,L)=log[A(r,L)A0(r,L)]+j[ϕ(r,L)ϕ0(r,L)]=χ+jS.
(3)

I(t)=I0exp(2χ(t)),
(4)

as shown in [12

12. L. C. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media, (Bellingham, Washington, 1998).

]. In Eq. (4), I 0 = ∣A 02 is the level of irradiance fluctuation in the absence of air turbulence that ensures that the fading does not attenuate or amplify the average power, i.e., E[I] = ∣A 02. This may be thought of as a conservation of energy consideration and requires the choice of E[χ] = −σ 2 χ, as was explained in [15

15. J. W. Strohbehn, “Modern theories in the propagation of optical waves in a turbulent medium,” in Laser Beam Propagation in the Atmosphere, J. W. Strohbehn ed. (Springer, New York, 1978), pp. 45–106.

, 16

16. D. L. Fried, “Aperture averaging of scintillation,” J. Opt. Soc. Am. 57, 169–175 (1967). [CrossRef]

], where E[χ] is the ensemble average of log-amplitude, whereas σ 2 χ is its variance. ence, from the Jacobian statistical transformation, the probability density function of the intensity can be identified to have a log-normal distribution typical of weak turbulence regime [6

6. X. Zhu and J. M. Kahn, “Free-space optical communication through atmospheric turbulence channels,” IEEE Trans. Commun. 50, 1293–1300 (2002). [CrossRef]

]. With these considerations taken into account, an efficient channel model for FSO communications using intensity modulation and direct detection was presented in [8

8. A. Jurado Navas, A. García Zambrana, and A. Puerta Notario, “Efficient lognormal channel model for turbulent FSO communications,” Electron. Lett. 43, 178–179 (2007). [CrossRef]

] under the assumption of weak turbulence regime. For these systems, the received optical power, Y(t), can be written as Y(t) = αsc(t)X(t) + N(t), with X(t) being the received optical power without scintillation, whereas αsc(t) = exp[2χ(t)] is the temporal behavior of the scintillation sequence and represents the effect of the intensity fluctuations on the transmitted signal. To generate αsc(t), a scheme based on a lowpass filtering of a random Gaussian signal is implemented as in [8

8. A. Jurado Navas, A. García Zambrana, and A. Puerta Notario, “Efficient lognormal channel model for turbulent FSO communications,” Electron. Lett. 43, 178–179 (2007). [CrossRef]

]. χ(t) is, as was explained above, the log-amplitude of the optical wave governed by Gaussian statistics. Finally, the additive white Gaussian noise, N(t), is assumed to include any front-end receiver thermal noise as well as shot noise caused by ambient light much stronger than the desired signal.

As was indicated in [8

8. A. Jurado Navas, A. García Zambrana, and A. Puerta Notario, “Efficient lognormal channel model for turbulent FSO communications,” Electron. Lett. 43, 178–179 (2007). [CrossRef]

], to obtain the expression of the lowpass filter, Hsc(f), we started with the covariance function for irradiance fluctuations, BI(r), that for a plane wave and homogeneous and isotropic turbulence leads to [17

17. R. Lawrence and J. W. Strohbehn, “A survey of clear-air propagation effects relevant to optical communications,” Proc. IEEE 58, 1523–1545 (1970). [CrossRef]

]:

BI(r,L)=8π2k2L0(1kκ2Lsinκ2Lk)Φn(κ)J0(κr)κdκ
(5)

where k is the wave number, L is the propagation path length, κ is the scalar spatial wave number, Φn(κ) is the spatial power spectrum of refractive index and J 0(·) is the Bessel function of the first kind and order zero. Based on the Kolmogorov spectrum given in Eq. (1) and after some mathematical manipulations indicated in [8

8. A. Jurado Navas, A. García Zambrana, and A. Puerta Notario, “Efficient lognormal channel model for turbulent FSO communications,” Electron. Lett. 43, 178–179 (2007). [CrossRef]

], it is possible to obtain

BI(uτ)σ12exp(τ2τ02),l0rλL,
(6)

being τ 0 the turbulence correlation time, whereas u is the component of the wind velocity vector perpendicular to the propagation direction. Hence, the low-pass filter can be obtained from

Hsc(f)2=σχ2τ0πe(πτ0f)2.
(7)

This will be the channel model considered in this paper in order to include the atmospheric coherence effects in numerical simulations presented in Section 3. As a final remark, the simplified Gaussian channel model differs from the theoretical one obtained from Eq. (5) when the frequency spread is calculated, as can be deduced by inspecting the tails of the different spectrums displayed in [8

8. A. Jurado Navas, A. García Zambrana, and A. Puerta Notario, “Efficient lognormal channel model for turbulent FSO communications,” Electron. Lett. 43, 178–179 (2007). [CrossRef]

]; but there is no difference between Gaussian and Kolmogorov bells at lower frequencies, i.e. there is no shift of the relative frequency content of the irradiance power spectrum, so the temporal cycles of the scintillation sequences obtained from both the Gaussian and the Kolmogorov model have the same temporal variability. In addition, both models are in total agreement when the average frequency shift is obtained, as it is developed in Appendix I.

2.1. Propagation of Pulses Through Atmospheric Turbulence

Furthermore, in atmospheric optical communications, propagating pulses may be influenced by pulse spreading owing to the scattering caused by turbulent inhomogeneities and hydrometeors in the troposphere, and especially sand and dust particles, which are likely present in extreme scenarios such as the one detailed in [18

18. Y. Ruike, H. Xiange, H. Yue, and S. Zhongyu, “Propagation Characteristics of Infrared Pulse Waves through Windblown Sand and Dust Atmosphere,” Int. J. Infrared Millim. Waves 28, 181 (2007). [CrossRef]

], and causing a distortion of the optical pulse shape during propagation [19

19. C. H. Liu and K. C. Yeh, “Propagation of pulsed beam waves through turbulence, Cloud, Rain or Fog,” J. Opt. Soc. Am. 67, 1261–1266 (1977). [CrossRef]

]. This problem was studied in [20

20. L. C. Lee, “Wave Propagation in a Random Medium: A Complete set of the moment equations with different wavenumbers,” J. Math. Phys. 15, 1431–1435 (1974). [CrossRef]

], requiring a solution for the statistical moments of the wave field at different frequencies and at different positions. It is useful to know the two-frequency mutual coherence function (MCF), studied by many researchers [21

21. C.Y. Young, A. Ishimaru, and L. C. Andrews, “Two-frequency mutual coherence function of a Gaussian beam pulse in weak optical turbulence: an analytic solution,” Appl. Opt. 35, 6522–6526 (1996). [CrossRef] [PubMed]

,22

22. I. Sreenivasiah and A. Ishimaru, “Beam wave two-frequency mutual coherence function and pulse propagation in random media: an analytic solution to the plane wave case,” Appl. Opt. 18, 1613–1618 (1979). [CrossRef] [PubMed]

]. The MCF is an important quantity by itself as it provides a measure of the coherence bandwidth [13

13. A. Ishimaru, Wave Propagation and Scattering in Random Media, (Academic Press, New York, 1978).

]. From the MCF, the pulse broadening can be calculated [21

21. C.Y. Young, A. Ishimaru, and L. C. Andrews, “Two-frequency mutual coherence function of a Gaussian beam pulse in weak optical turbulence: an analytic solution,” Appl. Opt. 35, 6522–6526 (1996). [CrossRef] [PubMed]

] for a Gaussian beam. Thus, the estimate of the received pulse half-width is given by:

T2=(T02+8α),
(8)

with T 0 being the input pulse half-width at the 1/e point [23

23. C. Y. Young, L. C. Andrews, and A. Ishimaru, “Time-of-arrival fluctuations of a space-time Gaussian pulse in weak optical turbulence: an analytic solution,” Appl. Opt. 37, 7655–7660 (1998). [CrossRef]

] whereas the remaining parameter is defined by:

α=0.3908Cn2LL053c2,
(9)

3. Rate-Adaptive Links Using Variable Silence Periods as a Promising Alternative

3.1. General Comments

Then, with all those values detailed at the begining of this section, and from Eqs. (8), (9), the received pulse half-width defined by the 1/e point [10

10. A. Jurado-Navas, J. M. Garrido-Balsells, M. Castillo-Vázquez, and A. Puerta-Notario, “Numerical model for the temporal broadening of optical pulses propagating through weak atmospheric turbulence,” Opt. Lett. 34, 3662–3664 (2009). [CrossRef] [PubMed]

,23

23. C. Y. Young, L. C. Andrews, and A. Ishimaru, “Time-of-arrival fluctuations of a space-time Gaussian pulse in weak optical turbulence: an analytic solution,” Appl. Opt. 37, 7655–7660 (1998). [CrossRef]

] is on the order of T 2T 0 = 4.73 ns, being the broadening negligible, as we had anticipated above. To obtain an appreciable broadening on the order of 10% of T 0, we could use transmission rates up to 87.8 Gbps and 278 Gbps for C 2 n = 1.23 × 10−13 and 1.23 × 10−14 m−2/3 respectively with a 100% of duty-cycle (d.c.) and L 0 = 30.34 m (this last value coincident with the height of our emplacement). These values let us utilize very high binary rates or ultrashort transmitted pulses and still neglecting the temporal broadening of the transmitted pulses. Thus, in this paper, we take advantage of this latter fact by adopting pulses with reduced duty cycle. As the criterion of limited average optical power is adopted, the signal amplitude can be increased as the duty cycle is decreased in order to maintain constant the average optical power, and then, the peak-to-average optical power ratio (PAOPR) can be higher. As shown through this section, a technique that increases the PAOPR parameter is preferred as it provides better performance in atmospheric optical links, overcoming the imposed distortion when a system bandwidth constraint is required. The obtained performance for all analyzed signaling techniques are in terms of burst error rate average. Hence, the impact of the atmospheric channel coherence on the behavior of the different signalling schemes can be taking into account, as was indicated in [8

8. A. Jurado Navas, A. García Zambrana, and A. Puerta Notario, “Efficient lognormal channel model for turbulent FSO communications,” Electron. Lett. 43, 178–179 (2007). [CrossRef]

], due to the burst error rate average represents a second order of statistics and so, the temporal variability of the received irradiance fluctuations can influence on such metric of performance; however, this fact is not considered simply by doing a bit error rate analysis since bit error rate does not change with the variable wind speed, i.e., bit error rate is the first order of statistics and, consequently, it is just a function of the lognormal channel variance. Thus, we followed Deutsch and Miller’s [26

26. L. Deutsch and R. Miller, “Burst statistics of Viterbi decoding,” The Telec. and Data Acquisition Progress Report, TDA PR 42–64, 187–193 (1981).

] definition of a burst error with lengths of 192 and 64 bits respectively for each figure, not containing more than 4 consecutive correct bits (Lb = 5 as explained in [26

26. L. Deutsch and R. Miller, “Burst statistics of Viterbi decoding,” The Telec. and Data Acquisition Progress Report, TDA PR 42–64, 187–193 (1981).

]) any sequence of burst error.

3.2. Peak-to-average optical power ratio (PAOPR)

Let x(t) the instantaneous optical power defined by

x(t)=kak·Ppeak·pn(tkTb)kZ,
(10)

where ak is a random variable with values of 0 for the bit “0” (off pulse) and 1 for the bit “1” (on pulse), Ppeak is the pulse peak power, pn(t) is the pulse shape with normalized peak power, and Tb is the bit period. Then, the average transmitted optical power can be expressed as P = Ppeak · Pn · pr(ak = 1), where Pn is the optical power averaged over Tb given by Pn=1Tb0TbPn(t)dt , and pr(ak=1)=12 as we assume equiprobability of binary symbols. Thus, we can write the peak-to-average optical power ratio as

PAOPR=PpeakP=[pr(ak=1)·Pn]1.
(11)

From Eq. (11), it is deduced that we must reduce Pn, depending on the adopted pulse shape, to improve the PAOPR while maintaining the average transmitted optical power at a constant level. Hereafter, to compare the different pulses performance, we define the increase factor in PAOPR as follows:

ΓPAOPR=PAOPRPAOPRref=1Pn,
(12)

where PAOPRref is the PAOPR obtained with non-return to zero (NRZ) signaling and rectangular pulse shape, i.e. Pnref = 1. Note that ΓPAOPR only depends on the pulse shape characteristics.

Now, we obtain the ΓPAOPR for a rectangular pulse with a duty cycle of ξ, where 0 < ξ ≤ 1. In this case, from Eq. (12), it is easily derived that

ΓPAOPR=1ξ.
(13)

Next, the expression for a normalized Gaussian pulse centered in t 0 = ξTb/2 is given by

pn(t)=e(tt0)22σp2,
(14)

where σp = ξ · σ 0 depends on the duty cycle, ξ, and on σ 0 = Tb/n, which characterizes the pulse width in relation to the bit period. In particular, n defines the amount of optical energy contained within Tb, so that for n ≥ 6 at least the 99.8% of the transmitted energy is within Tb. In our analysis, we assume n = 6 and, thus, from Eq. (12) and Eq. (14), the ΓPAOPR for the Gaussian pulse is given by

ΓPAOPR=2π3ξ[erf(123ξ)]1,
(15)

where erf(·) is the error function. Its pulse power spectral density can be easily calculated and, of course, it can be checked that the occupied bandwidth of Gaussian pulse shape is quite larger than rectangular one. However, the higher Ppeak transmitted more than compensates for the higher distortion induced on Gaussian pulse by a limited channel bandwidth.

Table 1 summarizes the increase in PAOPR for the analyzed pulse shapes and several values of the duty cycle, ξ, and, as expected, the ΓPAOPR offered by the Gaussian pulse is substantially higher than the one obtained by the rectangular pulse shape.

Table 1. ΓPAOPR for rectangular and Gaussian pulse shapes and ξ = 1, 0.5 and 0.25

table-icon
View This Table
Fig. 1. Burst error rate versus normalized average optical power using OOK-GS format and duty cycle (d.c.) of 25%. Classic NRZ format and OOK-GS with a 100% d.c. are also displayed for σ 2 χ=0.1(upper subfigures) and σ 2 χ=0.01(lower subfigures). The burst error length is fixed to (a) and (c) 192 bits; (b) and (d) 64 bits. All subfigures show the obtained performance from the theoretical Kolmogorov spectrum and the approximated Gaussian one.

Furthermore, results obtained for the OOK-GS format are illustrated in Fig. 1 for the proposed Gaussian channel developed in [8

8. A. Jurado Navas, A. García Zambrana, and A. Puerta Notario, “Efficient lognormal channel model for turbulent FSO communications,” Electron. Lett. 43, 178–179 (2007). [CrossRef]

,9

9. A. Jurado-Navas and A. Puerta-Notario, “Generation of Correlated Scintillations on Atmospheric Optical Communications,” J. Opt. Commun. Netw 1, 452–462 (2009). [CrossRef]

] together with the results obtained from the theoretical covariance function shown in Eq. (5) including a Kolmogorov spectrum in Φn(κ). It should be further noted that our Gaussian channel approach works perfectly not only for OOK-GS format but also for all the remaining cases in this paper so results obtained from the use of the more precise expression in Eq. (5) are omitted here for the sake of brevity.

Fig. 2. Burst error rate versus normalized average optical power using 4PPM and OOK-GS formats and duty cycle (d.c.) of 25%. Classic NRZ format and OOK-GS with a 100% d.c. are also displayed for σ 2 χ= 0.1 (upper subfigures) and σ 2 χ = 0.01 (lower subfigures). The burst error length is established to (a) and (c) 192 bits; (b) and (d) 64 bits.

Figure 2 shows a comparative between the OOK-GS pulse shapes displayed in Fig. 1 and PPM scheme for σ 2 χ=0.1 and 0.01. Then, an OOK-GS format and a 25%-duty cycle already achieves similar performance than the 4PPM scheme, but with a lower complexity. Again, a classic signalling technique based on variable-rate repetition coding is implemented. The fact that broadening distortion does not affect to transmitted pulses until very reduced widths, and the better performance obtained in Fig. 2 for schemes with a reduced duty cycle are an interesting starting point to apply our rate-adaptive transmission technique as we explain below.

3.3. Rate Adaptive Links Using Variable Silence Periods

ΓPAOPR=RRs2π3ξ[erf(123ξ)]1.
(16)

As we said above, the distortion of transmitted pulses may be neglected until ultrashort pulse widths of 1 ps or even less, so that we can take advantage of this feature to reduce the width of transmitted pulses in order to implement our rate-adaptive transmission technique. The idea is applied to accommodate the transmission rate to the channel conditions using variable silence periods. In this respect, the worse channel conditions we have, the higher the rate reduction RRs we need to provide a significant improvement in terms of burst error rate. Thus, the increase in the PAOPR parameter is then used being the average optical power maintained at a constant level, and for a same system bandwidth constraint.

Figure 3 shows a comparison between this rate adaptive technique using variable silence periods and the classic one proposed and specified in the Advanced Infrared (AIr) standard based on variable-rate repetition coding in a turbulent atmosphere and for σ 2 χ = 0.1 and σ 2 χ = 0.01. In such figure, all results are obtained for OOK-GS formats and a 25%-duty cycle. From Fig. 3, a remarkable improvement in performance can be noted when the rate adaptive technique based on inserting variable silence periods is adopted in relation to the classic variable-rate repetition coding, because there is no alteration in the statistics of occurrence of the transmitted pulses in this latter option and, consequently, ΓPAOPR is not increased. This superiority is even more relevant when the rate reduction factor is increased, showing a greater robustness to the more severe atmospheric turbulence conditions. For instance, the technique based on variable silence periods can achieve an improvement in average optical power requirements above 1 and 2 optical dB at a burst error rate of 10−6 for a rate reduction of 2 and 4 respectively and σ 2 χ = 0.1 in regard to classic variable-rate repetition coding.

3.4. Rate Adaptive Links Using Variable Silence Periods and Memory

Now, this variable silence time periods are used for modifying the statistics of the amplitude sequence regarding the statistics of the message sequence with the purpose of increasing the PAOPR parameter. Thus, a signaling technique based on giving memory by using the pulse position is included on the rate-adaptive technique based on inserting silence periods. The resulting signalling scheme was named OOK-GSc format as was described in [11

11. A. García-Zambrana and A. Puerta-Notario, “Novel approach for increasing the peak-to-average optical power ratio in rate-adaptive optical wireless communication systems,” IEE Proc. Optoelectron.: Special Issue on Optical Wireless Communications 150, 439–444 (2003).

]. OOK-GSc format is designed to avoid the appearance of more than one pulse in sets of two consecutive symbol periods, increasing the peak optical power while maintaining the average optical power at the same constant level, as before. Figure 4 shows a generic scheme where this OOK-GSc format is combined with the rate-adaptive technique based on inserting silence periods, where RRb represents the initial bit rate of the system, RRc is the rate reduction factor inherent to the OOK-GSc scheme, RRs is the rate reduction factor introduced by inserting RRs − 1 silence bit periods after an information bit, as explained above, and RR = RRc · RRs is the total effective rate reduction factor of the system.

Fig. 3. Burst error rate versus normalized average optical power using OOK-GS formats and duty cycle (d.c.) of 25%. Classic adaptive transmission technique with a rate reduction factor of RR is displayed in comparison to the one based on the insertion of RRs − 1 silence bit periods after an information bit, for σ 2 χ = 0.1 (upper subfigures) and σ 2 χ = 0.01 (lower subfigures). The burst error length is established to (a) and (c) 192 bits; (b) and (d) 64 bits.
Fig. 4. Generic OOK-GSc scheme with an extra rate-adaptive transmission technique stage based on inserting variable silence times.

OOK-GSc is based on a Markov chain of three states, providing two coded bits each information bit and thus, representing a rate reduction factor of RRc=2. Then, and when RRs=1, pr(ak=1)=13RR , so from Eq.(12) and Eq.(14), the ΓPAOPR for a Gaussian pulse shape is now given by

ΓPAOPR=32RR2π3ξ[erf(123ξ)]1.
(17)

Nonetheless, the adopted approach to configure the OOK-GSc format can be generalized to avoid the appearance of more than one pulse in sets of more than two consecutive symbol per-iods. For instance, following [11

11. A. García-Zambrana and A. Puerta-Notario, “Novel approach for increasing the peak-to-average optical power ratio in rate-adaptive optical wireless communication systems,” IEE Proc. Optoelectron.: Special Issue on Optical Wireless Communications 150, 439–444 (2003).

], we can adopt the OOK-GScc scheme, where the appearance of more than one pulse in sets of three consecutive symbol periods is avoided. In this case, OOK-GScc format is based on a Markov chain of nine states, providing four coded bits each information bit and thus, representing a rate reduction factor of RRc = 4. Hence, pr(ak=1)=14RR if RRs = 1, so from Eq.(12) and Eq.(14), the ΓPAOPR for a Gaussian pulse is now given by

ΓPAOPR=2RR2π3ξ[erf(123ξ)]1.
(18)

We can employ the same generic scheme shown in Figure 4, but substituting the OOK-GSc stage for an OOK-GScc technique. Then, this OOK-GScc format is combined with the already explained rate-adaptive technique based on inserting silence periods; where, again, RRb represents the initial bit rate of the system, RRc = 4 is the rate reduction factor inherent to the OOK-GScc scheme, RRs is the rate reduction factor introduced by inserting RRs − 1 silence bit periods after an information bit and RR = RRc · RRs is the total effective rate reduction factor.

Fig. 5. Burst error rate versus normalized average optical power using OOK-GS formats with and without memory and duty cycle (d.c.) of 25%. The adaptive transmission technique based on the insertion of RRs − 1 silence bit periods is adopted for σ 2 χ = 0.1 (upper subfigures) and σ 2 χ = 0.01 (lower subfigures). The burst error length is established to (a) and (c) 192 bits; (b) and (d) 64 bits.

Figure 5 shows obtained results for the OOK-GS format using variable silence times with and without memory in a turbulent atmospheric environment for σ 2 χ = 0.1 and σ 2 χ = 0.01. As it is shown, a remarkable improvement in performance can be observed when OOK-GS format with memory is adopted if compared with the OOK format without memory, where excellent agreement can be noted by using different rate reductions. Nevertheless, OOK-GSc and OOK-GScc formats present a very similar behavior in terms of burst error rate for an identical rate reduction factor. This fact may not comply with the ΓPAOPR heuristic law in the sense that the higher magnitude in ΓPAOPR we have the better performance we should expect from a signalling technique. However, the answer to this apparent incongruence raises from the definition adopted in this paper for a burst error in addition to the particular distribution of error bits in the different signalling techniques with memory analyzed in this paper in conjunction with the Viterbi algorithm employed in reception. In this sense, Fig. 6 shows the number of occurrences for different sets of consecutive correctly received bits for both the OOK-GSc scheme with a rate reduction factor of RR=4 (where RR=RRc · RRs, with RRc=2 and RRs=2); and the OOK-GScc format with its inherent rate reduction factor of 4, for different values of additive white Gaussian noise power.We can check that the occurrence of sequences consisting of an association of ten or more consecutive correct bits is higher when using an OOK-GScc scheme than employing an OOK-Gsc format. This fact is effectively confirmed when we change the definition of a burst error. Now, if any burst error can contain until 9 consecutive correct bits (Lb = 10 as explained in [26

26. L. Deutsch and R. Miller, “Burst statistics of Viterbi decoding,” The Telec. and Data Acquisition Progress Report, TDA PR 42–64, 187–193 (1981).

]), then we can obtain the expected results in terms of burst error rate, as displayed in Fig. 7, where the superiority of OOK-GScc format versus OOK-GSc one is again established, as expected from the heuristic analysis of the ΓPAOPR parameter.

Fig. 6. Histograms representing the number of sequences consisting of an association of k consecutive correct bits for different additive noise power of magnitudes: (a)–(b) −22 optical dB; (c)–(d) −19 optical dB; and (e)–(f) − 17 optical dB. These results are generated from the OOK-GSc scheme with RR=4 (i.e., RR=RRc · RRs with RRc=2 and RRs=2) and the OOK-GScc format with its inherent RR=4 (RRc=4, RRs=1), both obtained from Fig. 5(c).
Fig. 7. Burst error rate versus normalized average optical power using OOK-GSc and OOK-GScc formats with duty cycle (d.c.) of 25% for σ 2 χ=0.1 (solid lines) and σ 2 χ=0.01 (dashed lines). The adaptive transmission technique based on the insertion of only 1 silence bit period is adopted for the OOK-GSc format so that the total rate reduction factor is fixed to 4 for all cases. The burst error length, with Lb=10, is fixed to (a) 192 bits; (b) 64 bits.

Fig. 8. Bit error rate versus normalized average optical power using OOK format with memory (OOK-GSc and OOK-GScc) for σ 2 χ = 0.1 and σ 2 χ = 0.01.

4. Concluding Remarks

In this paper, an alternative rate-adaptive transmission technique successfully proposed in indoor optical communications is adapted here to atmospheric optical links. Such rate-adaptive technique is based on the fact that an increase in the peak-to-average optical power ratio involves higher performance. This feature is intensively studied in this paper providing an extremely complete numerical analysis. To increase such figure of merit (PAOPR), pulses having a shortened duty cycle must be employed taking into advantage the fact that it is possible to neglect temporal broadening, unless until various hundred of Gbps if atmospheric conditions are weaker, as was indicated in Section 3. This fact is the central core of the alternative rate adaptive transmission scheme successfully proved in this paper for turbulent environments. Pulses with reduced duty cycle in addition to the possibility of diminishing the presence of transmitted pulses by employing memory on OOK formats with Gaussian pulse shapes increase significantly the PAOPR parameter. In this sense, better performance are obtained for such signalling schemes as it is shown in the different comparatives included in this work as, for instance, versus 4PPM, 4GPPM or versus classic variable rate repetition coding scheme. Thus, a greater robustness is provided with the proposed signalling scheme with a low required complexity. Finally, we are currently working with pulse shapes based on the power pulse shape of the widely known optical soliton, corresponding to the hyperbolic secant square function, whose PAOPR parameter is higher than the one obtained for Gaussian and rectangular pulses [28

28. J. M. Garrido-Balsells, A. Jurado-Navas, M. Castillo-Vázquez, A. B. Moreno-Garrido, and A. Puerta-Notario, are preparing a manuscript to be called “Improving optical wireless links performance by solitonic shape pulses.”

]. Qualitatively, an optical power pulse shape for different normalized pulses, pN(t), with a reduced duty cycle, ξ, is presented in Fig. 10, showing that the solitonic pulse shape is narrower than rectangular and Gaussian pulses, so that we can increase the peak optical power to a higher lever maintaining the average optical power at the same constant level for all the pulse shapes under study. Then, as concluded in this paper, an increase in the peak-to-average optical power ratio involves higher performance. In this case, the higher pulse peak power transmitted, Ppeak, compensates for the higher distortion induced on solitonic pulse by the system limited bandwidth, so a better performance is obtained when increasing the PAOPR parameter. In addition, we are currently researching about mode locking solitons [29

29. T. Brabec, Ch. Spielmann, and E Krausz, “Mode locking in solitary lasers,” Opt. Lett. 16, 1961–1963 (1991). [CrossRef] [PubMed]

, 30

30. F. X. Kärtner, D. Kopf, and U. Keller, “Solitary-pulse stabilization and shortening in actively mode-locked lasers,” J. Opt. Soc. Am. B 12, 486–496 (1995). [CrossRef]

] and the possibility to properly employ them in an atmospheric optical communication environment.

Fig. 9. Burst error rate versus normalized average optical power using 4GPPM with both a TD detection procedure and a ML detector; and OOK-GSc format and duty cycle (d.c.) of 25% using a ML detection procedure. The burst error length is established to (a) and (c) 192 bits; (b) and (d) 64 bits; and σ 2 χ=0.1 (upper subfigures) and σ 2 χ=0.01 (lower subfigures).

Appendix I

In this Appendix, it is shown that the average frequency shift is coincident for both the theoretical covariance function shown in Eq. (5); and the Gaussian spectrum obtained from Eq. (6) and employed in this paper. This average frequency shift is defined, from [31

31. Q. Yao and M. Patzold, “Spatial-temporal characteristics of a half-spheroid model and its corresponding simulation model,” IEEE 59th Vehicular Technology Conference, VTC 2004-Spring , 1, 147–151 (2004).

], as:

Fig. 10. Optical power pulse shape for different normalized pulses, pN(t), with a reduced duty cycle, ξ and transmitted each bit period, Tb.
Bμiμi(1)=1j2π·ddτ(BI(0))·1BI(0).
(19)

For the proposed Gaussian spectrum given in Eq. (7) the following relation holds:

ddτ(BI(uτ))=2σI2ττ02exp(τ2τ02),
(20)

where, for τ = 0, BI(0) = σ 2 1 and ddτ(BI(0))=0 , so Bμiμi(1)=0 . On the other hand, for the theoretical spectrum obtained from Eq. (5), solving the integral for a Kolmogorov spectrum, we obtain

BI(r,L)=8π2·0.033Cn2k2L{0κ113J0(κr)κdκ-0kκ2Lsin(κ2Lk)κ113J0(κr)κdκ},
(21)

where the convergence of Eq. (21) is ensured due to the principle of analytic continuation. From Fried [32

32. D. L. Fried and J. D. Cloud, “Propagation of an infinite plane wave in a randomly inhomogeneous medium,” J. Opt. Soc. Am. 56, 1667–1676 (1966). [CrossRef]

],

0xαJ0(βx12)dx=(4β2)α+1Γ(α+1)Γ(α);
(22)
0xα1sinxJ0(βx12)dx=Γ(α){sin(πα2)Re[1F1(α;1;jβ24)]
cos(πα2)Im[1F1(α;1;jβ24)]}.
(23)

Thus, using Eqs. (22), (23) in Eq. (21), and knowing that r = u τ,

BI(τ,L)=4π2·0.033·Cn2k76L116[Γ(56)Γ(116)(ku2τ24L)56+
+Γ(116)Im{exp(j11π12)1F1(116,1,jku2τ24L)}].
(24)

where, obviously, for τ = 0, the temporal covariance function written in Eq. (21) reduces to BI(0) = 1.23C 2 n L 11/6 k 7/6 = σ 2 1, i.e., the scintillation index. To obtain ddτ(BI(0)) , we use the following property associated with the confluent hypergeometric function of the first kind:

dkdzk(1F1(a;c;z))=(a)k(c)k1F1(a+k;c+k;z),k=1,2,3
(25)

being (a)k the Pochhammer symbol. Thus, to obtain ddτ(BI(0)) , k=1 in Eq. (25). If z=jkr24L and r=u τ, and applying the chain rule that in the Leibniz notation is stated in the form

ddτ(BI(τ))τ=0=dBI(τ)dzdzdττ=0,
(26)

then:

dBI(τ)dz=4π20.033Cn2k76L116[53·Γ(56)Γ(116)(ku24L)56τ23
116Γ(116)Im{exp(j11π12)jku2τ2L1F1(56,2,jku2τ24L)}].
(27)

Therefore, it is possible to obtain that ddτ(BI(0))=0 and, thus again, Bμiμi(1)=0 .

Acknowledgment

This work was fully supported by the Spanish Ministerio de Educación y Ciencia (MEC), Project TEC2008-06598.

References and links

1.

J. C. Juarez, A. Dwivedi, A. R. Hammons, S. D. Jones, V. Weerackody, and R. A. Nichols, “Free-space optical communications for next-generation military networks,” IEEE Commun. Mag. 44, 46–51 (2006). [CrossRef]

2.

A. Belmonte and J. M. Kahn, “Efficiency of complex modulation methods in coherent free-space optical links,” Opt. Express 18, 3928–3937 (2010). [CrossRef] [PubMed]

3.

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]

4.

J. J. Laserna, R. Fernández Reyes, R. González, L. Tobaria, and P. Lucena, “Study on the effect of beam propagation through atmospheric turbulence on standoff nanosecond laser induced breakdown spectroscopy measurements,” Opt. Express 17, 10265–10276 (2009). [CrossRef] [PubMed]

5.

P. W. Nugent, J. A. Shaw, and S. Piazzolla, “Infrared cloud imaging in support of Earth-space optical communication,” Opt. Express 17, 7862–7872 (2009). [CrossRef] [PubMed]

6.

X. Zhu and J. M. Kahn, “Free-space optical communication through atmospheric turbulence channels,” IEEE Trans. Commun. 50, 1293–1300 (2002). [CrossRef]

7.

M. Al Naboulsi and H. Sizun, “Fog attenuation prediction for optical and infrared waves,” SPIE Optical Engineering 43, 319–329 (2004).

8.

A. Jurado Navas, A. García Zambrana, and A. Puerta Notario, “Efficient lognormal channel model for turbulent FSO communications,” Electron. Lett. 43, 178–179 (2007). [CrossRef]

9.

A. Jurado-Navas and A. Puerta-Notario, “Generation of Correlated Scintillations on Atmospheric Optical Communications,” J. Opt. Commun. Netw 1, 452–462 (2009). [CrossRef]

10.

A. Jurado-Navas, J. M. Garrido-Balsells, M. Castillo-Vázquez, and A. Puerta-Notario, “Numerical model for the temporal broadening of optical pulses propagating through weak atmospheric turbulence,” Opt. Lett. 34, 3662–3664 (2009). [CrossRef] [PubMed]

11.

A. García-Zambrana and A. Puerta-Notario, “Novel approach for increasing the peak-to-average optical power ratio in rate-adaptive optical wireless communication systems,” IEE Proc. Optoelectron.: Special Issue on Optical Wireless Communications 150, 439–444 (2003).

12.

L. C. Andrews and R. L. Phillips, Laser Beam Propagation through Random Media, (Bellingham, Washington, 1998).

13.

A. Ishimaru, Wave Propagation and Scattering in Random Media, (Academic Press, New York, 1978).

14.

V. I. Tatarskii, The Effects of the Turbulent Atmosphere on Wave Propagation, (Jerusalem: Israel Program for Scientific Translations, 1971).

15.

J. W. Strohbehn, “Modern theories in the propagation of optical waves in a turbulent medium,” in Laser Beam Propagation in the Atmosphere, J. W. Strohbehn ed. (Springer, New York, 1978), pp. 45–106.

16.

D. L. Fried, “Aperture averaging of scintillation,” J. Opt. Soc. Am. 57, 169–175 (1967). [CrossRef]

17.

R. Lawrence and J. W. Strohbehn, “A survey of clear-air propagation effects relevant to optical communications,” Proc. IEEE 58, 1523–1545 (1970). [CrossRef]

18.

Y. Ruike, H. Xiange, H. Yue, and S. Zhongyu, “Propagation Characteristics of Infrared Pulse Waves through Windblown Sand and Dust Atmosphere,” Int. J. Infrared Millim. Waves 28, 181 (2007). [CrossRef]

19.

C. H. Liu and K. C. Yeh, “Propagation of pulsed beam waves through turbulence, Cloud, Rain or Fog,” J. Opt. Soc. Am. 67, 1261–1266 (1977). [CrossRef]

20.

L. C. Lee, “Wave Propagation in a Random Medium: A Complete set of the moment equations with different wavenumbers,” J. Math. Phys. 15, 1431–1435 (1974). [CrossRef]

21.

C.Y. Young, A. Ishimaru, and L. C. Andrews, “Two-frequency mutual coherence function of a Gaussian beam pulse in weak optical turbulence: an analytic solution,” Appl. Opt. 35, 6522–6526 (1996). [CrossRef] [PubMed]

22.

I. Sreenivasiah and A. Ishimaru, “Beam wave two-frequency mutual coherence function and pulse propagation in random media: an analytic solution to the plane wave case,” Appl. Opt. 18, 1613–1618 (1979). [CrossRef] [PubMed]

23.

C. Y. Young, L. C. Andrews, and A. Ishimaru, “Time-of-arrival fluctuations of a space-time Gaussian pulse in weak optical turbulence: an analytic solution,” Appl. Opt. 37, 7655–7660 (1998). [CrossRef]

24.

A. Christen, E. van Gorsel, and R. Vogt, “Coherent structures in urban roughness sublayer turbulence,” Int. J. Climatol. 27, 1955–1968 (2007). [CrossRef]

25.

A. Christen, M. W. Rotach, and R. Vogt, “Experimental determination of the turbulent kinetic energy budget within and above an Urban Canopy,” Fifth AMS Symposium on the Urban Environment, Vancouver (Canada), 23–27Aug. 2004.

26.

L. Deutsch and R. Miller, “Burst statistics of Viterbi decoding,” The Telec. and Data Acquisition Progress Report, TDA PR 42–64, 187–193 (1981).

27.

A. García-Zambrana and A. Puerta-Notario, “Improving PPM schemes in wireless infrared links at high bit rates,” IEEE Commun. Lett. 5, 95–97 (2001). [CrossRef]

28.

J. M. Garrido-Balsells, A. Jurado-Navas, M. Castillo-Vázquez, A. B. Moreno-Garrido, and A. Puerta-Notario, are preparing a manuscript to be called “Improving optical wireless links performance by solitonic shape pulses.”

29.

T. Brabec, Ch. Spielmann, and E Krausz, “Mode locking in solitary lasers,” Opt. Lett. 16, 1961–1963 (1991). [CrossRef] [PubMed]

30.

F. X. Kärtner, D. Kopf, and U. Keller, “Solitary-pulse stabilization and shortening in actively mode-locked lasers,” J. Opt. Soc. Am. B 12, 486–496 (1995). [CrossRef]

31.

Q. Yao and M. Patzold, “Spatial-temporal characteristics of a half-spheroid model and its corresponding simulation model,” IEEE 59th Vehicular Technology Conference, VTC 2004-Spring , 1, 147–151 (2004).

32.

D. L. Fried and J. D. Cloud, “Propagation of an infinite plane wave in a randomly inhomogeneous medium,” J. Opt. Soc. Am. 56, 1667–1676 (1966). [CrossRef]

OCIS Codes
(010.1330) Atmospheric and oceanic optics : Atmospheric turbulence
(070.4560) Fourier optics and signal processing : Data processing by optical means
(200.1130) Optics in computing : Algebraic optical processing
(290.5930) Scattering : Scintillation
(060.2605) Fiber optics and optical communications : Free-space optical communication
(200.2605) Optics in computing : Free-space optical communication

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 4, 2010
Revised Manuscript: June 16, 2010
Manuscript Accepted: July 1, 2010
Published: July 30, 2010

Citation
Antonio Jurado-Navas, José María Garrido-Balsells, Miguel Castillo-Vázquez, and Antonio Puerta-Notario, "An efficient rate-adaptive transmission technique using shortened pulses for atmospheric optical communications," Opt. Express 18, 17346-17363 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-16-17346


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References

  1. J. C. Juarez, A. Dwivedi, A. R. Hammons, S. D. Jones, V. Weerackody, and R. A. Nichols, “Free-space optical communications for next-generation military networks,” IEEE Commun. Mag. 44, 46–51 (2006). [CrossRef]
  2. A. Belmonte, and J. M. Kahn, “Efficiency of complex modulation methods in coherent free-space optical links,” Opt. Express 18, 3928–3937 (2010). [CrossRef] [PubMed]
  3. 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]
  4. J. J. Laserna, R. Fernández Reyes, R. González, L. Tobaria, and P. Lucena, “Study on the effect of beam propagation through atmospheric turbulence on standoff nanosecond laser induced breakdown spectroscopy measurements,” Opt. Express 17, 10265–10276 (2009). [CrossRef] [PubMed]
  5. P. W. Nugent, J. A. Shaw, and S. Piazzolla, “Infrared cloud imaging in support of Earth-space optical communication,” Opt. Express 17, 7862–7872 (2009). [CrossRef] [PubMed]
  6. X. Zhu, and J. M. Kahn, “Free-space optical communication through atmospheric turbulence channels,” IEEE Trans. Commun. 50, 1293–1300 (2002). [CrossRef]
  7. M. Al Naboulsi, and H. Sizun, “Fog attenuation prediction for optical and infrared waves,” SPIE Optical Engineering 43, 319–329 (2004).
  8. A. Jurado Navas, A. García Zambrana, and A. Puerta Notario, “Efficient lognormal channel model for turbulent FSO communications,” Electron. Lett. 43, 178–179 (2007). [CrossRef]
  9. A. Jurado-Navas, and A. Puerta-Notario, “Generation of Correlated Scintillations on Atmospheric Optical Communications,” J. Opt. Commun. Netw 1, 452–462 (2009). [CrossRef]
  10. A. Jurado-Navas, J. M. Garrido-Balsells, M. Castillo-Vázquez, and A. Puerta-Notario, “Numerical model for the temporal broadening of optical pulses propagating through weak atmospheric turbulence,” Opt. Lett. 34, 3662–3664 (2009). [CrossRef] [PubMed]
  11. A. García-Zambrana, and A. Puerta-Notario, “Novel approach for increasing the peak-to-average optical power ratio in rate-adaptive optical wireless communication systems,” IEE Proc. Optoelectron.: Special Issue on Optical Wireless Communications 150, 439–444 (2003).
  12. L. C. Andrews, and R. L. Phillips, Laser Beam Propagation through Random Media, (Bellingham, Washington, 1998).
  13. A. Ishimaru, Wave Propagation and Scattering in Random Media, (Academic Press, New York, 1978).
  14. V. I. Tatarskii, The Effects of the Turbulent Atmosphere on Wave Propagation, (Jerusalem: Israel Program for Scientific Translations, 1971).
  15. J. W. Strohbehn, “Modern theories in the propagation of optical waves in a turbulent medium,” in Laser Beam Propagation in the Atmosphere, J.W. Strohbehn ed. (Springer, New York, 1978), pp. 45–106.
  16. D. L. Fried, “Aperture averaging of scintillation,” J. Opt. Soc. Am. 57, 169–175 (1967). [CrossRef]
  17. R. Lawrence, and J. W. Strohbehn, “A survey of clear-air propagation effects relevant to optical communications,” Proc. IEEE 58, 1523–1545 (1970). [CrossRef]
  18. Y. Ruike, H. Xiange, H. Yue, and S. Zhongyu, “Propagation Characteristics of Infrared Pulse Waves through Windblown Sand and Dust Atmosphere,” Int. J. Infrared Millim. Waves 28, 181 (2007). [CrossRef]
  19. C. H. Liu, and K. C. Yeh, “Propagation of pulsed beam waves through turbulence, Cloud, Rain or Fog,” J. Opt. Soc. Am. 67, 1261–1266 (1977). [CrossRef]
  20. L. C. Lee, “Wave Propagation in a Random Medium: A Complete set of the moment equations with different wavenumbers,” J. Math. Phys. 15, 1431–1435 (1974). [CrossRef]
  21. C. Y. Young, A. Ishimaru, and L. C. Andrews, “Two-frequency mutual coherence function of a Gaussian beam pulse in weak optical turbulence: an analytic solution,” Appl. Opt. 35, 6522–6526 (1996). [CrossRef] [PubMed]
  22. I. Sreenivasiah, and A. Ishimaru, “Beam wave two-frequency mutual coherence function and pulse propagation in random media: an analytic solution to the plane wave case,” Appl. Opt. 18, 1613–1618 (1979). [CrossRef] [PubMed]
  23. C. Y. Young, L. C. Andrews, and A. Ishimaru, “Time-of-arrival fluctuations of a space-time Gaussian pulse in weak optical turbulence: an analytic solution,” Appl. Opt. 37, 7655–7660 (1998). [CrossRef]
  24. A. Christen, E. van Gorsel, and R. Vogt, “Coherent structures in urban roughness sublayer turbulence,” Int. J. Climatol. 27, 1955–1968 (2007). [CrossRef]
  25. A. Christen, M. W. Rotach, and R. Vogt, “Experimental determination of the turbulent kinetic energy budget within and above an Urban Canopy”, Fifth AMS Symposium on the Urban Environment, Vancouver (Canada), 23–27 Aug. 2004.
  26. L. Deutsch and R. Miller, “Burst statistics of Viterbi decoding,” The Telec. and Data Acquisition Progress Report, TDA PR 42–64, 187–193 (1981).
  27. A. García-Zambrana, and A. Puerta-Notario, “Improving PPM schemes in wireless infrared links at high bit rates,” IEEE Commun. Lett. 5, 95–97 (2001). [CrossRef]
  28. J. M. Garrido-Balsells, A. Jurado-Navas, M. Castillo-Vázquez, A.B. Moreno-Garrido and A. Puerta-Notario, are preparing a manuscript to be called “Improving optical wireless links performance by solitonic shape pulses.”
  29. T. Brabec, Ch. Spielmann, and E. Krausz, “Mode locking in solitary lasers,” Opt. Lett. 16, 1961–1963 (1991). [CrossRef] [PubMed]
  30. F. X. Kärtner, D. Kopf, and U. Keller, “Solitary-pulse stabilization and shortening in actively mode-locked lasers,” J. Opt. Soc. Am. B 12, 486–496 (1995). [CrossRef]
  31. Q. Yao, and M. Patzold, “Spatial-temporal characteristics of a half-spheroid model and its corresponding simulation model,” IEEE 59th Vehicular Technology Conference, VTC 2004-Spring, 1, 147–151 (2004).
  32. D. L. Fried, and J. D. Cloud, “Propagation of an infinite plane wave in a randomly inhomogeneous medium,” J. Opt. Soc. Am. 56, 1667–1676 (1966). [CrossRef]

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