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Optical modulating retro-reflectors enable free-space optical links that have greatly reduced pointing requirements and do not require a laser at one end of the link. However, these types of links can exhibit very high optical scintillation due to the double passage of the beam through the atmosphere. This high scintillation causes fades and surges that can lead to packet errors in the link. It is shown that scintillation can be greatly reduced through a combination of techniques including retro-reflector diversity, aperture averaging and bistatic optical interrogation. Improvements of 20 dB in link performance are demonstrated.
© 2014 Optical Society of America
1. Introduction
Free space optical (FSO) links can enable high bandwidth communication that does not require frequency allocation, and which is resistant to interception and jamming. A conventional, direct, FSO link has an actively pointed laser terminal on both ends of the link, but the size and power consumption of FSO terminals can sometimes be too large for small platforms. In these cases an FSO link using a modulating retro-reflector (MRR) may be advantageous [1–3]. These links require an active terminal on only one end of the link. On the other end they use a small retro-reflecting terminal that operates at low power, and does not require precise pointing or a laser.
As shown in Fig. 1, an MRR couples a passive optical retro-reflector to an optical modulator. A continuous wave beam from an optical interrogator, which generally takes the form of a conventional FSO terminal, remotely illuminates the MRR. The MRR reflects light back to the interrogator without requiring any pointing, as long as the interrogator is within the field of view of the passive retro-reflector. For typical retro-reflectors the field of view can be tens of degrees. Despite this wide field of view, the retro-reflected beam is narrow, with a divergence generally determined by diffraction from the retro-reflector aperture. The modulator in the MRR is driven with an electrical signal carrying data. This data signal is imposed by the modulator onto the retro-reflected optical beam, and is then carried back to the interrogator. MRRs have been based on corner cube and cat’s eye retro-reflectors, and have used a variety of optical modulators [3]. MRRs have been demonstrated for many different applications, including terrestrial, maritime and ground to air links [4–6].
The returned optical power in retro-reflecting links falls off as 1/R4, where R is the link range. As a result, it is difficult to maintain high link margins at long ranges. A principal limiting factor for the range of retro-reflecting links is the effect of optical scintillation. Scintillation is caused by atmospheric turbulence. Small cells of varying index of refraction move through the beam causing its centroid to wander, and, in more extreme cases, causing the beam to break up into a speckle pattern. Scintillation results in surges and fades of the optical power that last a few milliseconds. Maintaining a low packet error rate on the link requires enough margin so that fades below the detector sensitivity are rare. In high turbulence, intensity fluctuations can exceed 40 dB. Under these conditions reducing link range to increase margin, and reduce scintillation, is often the only way to maintain a high quality of service.
The strength of scintillation on an optical link is measured by the scintillation index,
where I is the intensity. Scintillation is generally considered to be weak when σ2I<<1 and strong for values greater than 1. Direct FSO links have scintillation indices that generally do not exceed about 3 even under high turbulence and long range. The scintillation index of retro-reflector links is highly variable, depending on the optical characteristics of the interrogator and retro-reflector, as well as the link range, the terrain and elevation, the solar fluence and other meteorological parameters. Shorter range MRR links, of 1-2 km, over water have shown scintillation indices less than 1.0 [5]. Longer range links of 4 km to aircraft have also shown relatively low scintillation indices because of the path elevation [4]. However, longer range links of 16 km over water [7], and even moderate range links of 1 km or greater near the ground have shown very high scintillation. A 1 km retro-reflecting link near the ground in a desert environment exhibited a saturated scintillation index greater than 10 during mid-day hours [8].The very high level of scintillation for retro-reflecting links occurs for several reasons. First, the light in a retro-reflecting link passes through the atmosphere twice. Therefore, a turbulence induced fade on the retro-reflector aperture is imposed upon the reflected beam, which is further aberrated on its return path, resulting in even deeper fades.
In addition, if the link is monostatic, so that the outgoing and reflected beam paths transit through the same atmospheric path, an additional increase in scintillation occurs through the backscatter enhancement effect [9,10]. Scintillation can also be high because MRR links often use small retro-reflectors. The small retro-reflector aperture means that relatively little aperture averaging occurs at the MRR end of the link.
In this paper it is shown that the scintillation index of MRR links can be reduced, through a combination of techniques. Saturated scintillation can be reduced from greater than 10 to values near 1. This lower value of scintillation can reduce the link margin required for good quality of service by 20 dB or more.
2. Scintillation reduction using retro-reflector arrays
2.1 Retro-reflector arrays
In previous work, the US Naval Research Laboratory (NRL) Transportable Atmospheric Test Suite (TATS) was used to measure scintillation from a single, passive, 1 cm diameter corner cube retro-reflector at a range of 1.1 km near the ground in a desert environment at China Lake, CA. Measurements of the scintillation index showed a consistent pattern of rising from below 0.1 at dawn to a saturated value of about 10 by mid-morning [8]. Scintillation stayed constant at this saturated value throughout the middle part of the day and then declined in the late afternoon.
The high value of scintillation seen in terrestrial retro-reflector links motivates examining techniques for scintillation reduction. Aperture averaging has been shown to be an effective technique to reduce the effects of optical scintillation [11,12]. A larger receive aperture in an FSO system not only increases the amount of received power, it can also reduce signal variation once the aperture size exceeds the coherence radius. As will be shown in section 3, using a larger receive aperture for the interrogator in an MRR link has a similar effect.
A larger retro-reflector can also reduce scintillation, but this approach has limitations. Large aperture MRRs are generally slower and consume more power because larger modulators have higher capacitance [3,13].
An alternative to a single large retro-reflector aperture is to use an array of MRRs all pointed in the same direction and driven with the same data signal. This form of multiple beam spatial diversity has been theoretically and experimentally studied for direct FSO systems using multiple transmitters [14]. Anguita et. al. defined a beam averaging factor, FB, equal to the ratio of σ2I using multiple transmitters to that using a single transmitter. Their models showed that 1/FB approached the number of transmitters as the separation between transmitters exceeded the size of spatial correlations in the atmosphere. For separations smaller than this, 1/FB was reduced.
Mahon et. al. showed that some additional effects must be considered for retro-reflector arrays due to optical coherence [15]. Each retro-reflector acts as a separate source of light, but, at least in low turbulence, the retro-reflectors are coherent with each other. The return beams from the MRRs will interfere on the aperture of the interrogator’s receiver, producing a fringe pattern. If the relative phases of the reflected beams are not controlled, the phase of the fringe pattern will be arbitrary, and will vary if the relative positions of the MRRs change by distances on the order of an optical wavelength. If the interrogator’s receive aperture is smaller than the spacing of the fringe pattern, rapid variations can occur in the received signal. This will produce an increase in the scintillation index that is not caused by atmospheric turbulence. If the receive aperture is larger than the fringe spacing then it will integrate over the fringe pattern greatly reducing the interference effects.
For two retro-reflectors, the number of fringes across the aperture of the receiver is
where Nfringe are the number of fringes across the receive aperture, Sretro is the spacing between the centers of the retro-reflectors, Drec is the diameter of the receive aperture, λ is the optical wavelength and R is the link range.Mahon measured aperture averaging and coherent interference effects with an array of two corner cube retro-reflectors as a function of spacing under low turbulence conditions [15]. It was shown that when the retro-reflectors were close together the scintillation index was higher than that of a single retro-reflector. This was because the signal variation, which the scintillation index measures, was higher due interference effects. When the separation of the retro-reflectors was increased, producing a larger number of fringes across the receiver, the scintillation index of the two corner cubes decreased below that of a single corner cube. The number of fringes on the receive aperture required to suppress the coherent interference contribution to the signal variation below that of atmospheric turbulence varied from about one to three, depending on the strength of the atmospheric turbulence.
Plett et al. also showed that multiple MRRs could be used to reduce scintillation [7]. In that case three retro-reflectors were used. The spacing between retro-reflectors was 30 cm, the receive aperture was 20 cm and the range was 16 km. In this case the number of fringes across the aperture was about 2, and a reduction in scintillation was observed.
2.2 Experimental conditions
In this work, the effects of retro-reflector arrays on scintillation was measured by using the TATS laser interrogator [8] to illuminate an array of passive corner cube retro-reflectors, each with a 2.5 cm diameter. The array included up to 3 corner cubes placed in a line with spacing that could be varied from 3 cm to more than 30 cm.
The TATS laser interrogator consisted of a monostatic terminal in which the transmit and receive apertures were concentric and overlapped. The laser beam was collimated to 0.8 milliradian divergence and emitted through a 0.5 cm diameter hole in a 3 cm minor diameter elliptical flat mirror held at 45° to the optical axis. The beam was aimed at the array of retro-reflectors and reflected back. The return beam was captured by the 3 cm mirror. The mirror was followed by a 2.8 cm diameter lens with a focal length of 10 cm. It was used to couple light by means of a pair of 1.1 cm focal length aspheres into a 100 micron diameter graded-index multimode fiber. The fiber was then coupled to an InGaAs photodetector. The detector was optimized for large dynamic range (40 dB) and low noise, with a bandwidth of 1MHz. Intensities were digitized using a 16-bit depth analog to digital converter at a 5kHz rate.
Measurements were taken at the Fort AP Hill Laser Test Range in Virginia during the months of November and December. The TATS interrogator and the retro-reflector array were separated by 1.7 km, and both were set up at a height of about 1.5 meters above the ground. The terrain between the two ends of the link was rolling; so the height of the link above the ground varied between about 1.5 meters to 5 meters.
Scintillation was measured by recording the retro-reflected intensity for ten second intervals. Typically, three separate recordings were taken for each experimental condition and statistics were determined using 30 seconds of data. The intensity data were monitored to ensure the levels were utilizing the available dynamic range of the receiver. Measurements were periodically recorded with the laser off in order to determine the solar background level.
Several different length scales affect retro-reflector scintillation [16]. These include the Fresnel zone size,
where k is the optical wave-number. For the conditions of the experiment, the Fresnel zone was about 2 cm.The coherence radius, which, in the spherical wave approximation is,
where Cn2 is the optical structure constant. Data were taken at different times of the day so Cn2 had various values. However, for typical daytime terrestrial values of Cn2 that range from the mid 10−14 m-2/3 to the mid 10−13 m-2/3, the coherence radius would have ranged from 2 cm to 0.5 cm.The retro-reflected spot diameter at the interrogator is,
where Dretro is the diameter of the retro-reflector. The retro-reflected spot size was 26 cm for Dretro = 2.5 cm and R = 1.7km.2.3 Measurement of retro-reflector array scintillation
Measurements were first taken of scintillation using a single retro-reflector as the target. During clear weather, the same pattern of rise and fall in scintillation indices over the course of a day was found at the Fort AP Hill range as at the China Lake range [8]. The scintillation was low at dawn and then climbed rapidly in the mid-morning to a saturated value greater than 10. It held relatively constant at the saturated value until late afternoon at which point it declined rapidly towards values near zero at sunset.
General closed-form theoretical expressions for double pass scintillation are not available, but solutions were found by Andrews for a retro-reflector in the point source limit illuminated by a spherical wave under conditions of high turbulence [17]. The predicted scintillation index depends on the root mean square Rytov variance,
where β0 is the Rytov variance.This model also depends on whether the link is monostatic or bistatic. When the outgoing and retro-reflected paths overlap the link is monostatic, and correlation in turbulence increases the scintillation index, as well as the reflected power. This phenomenon is known as the enhanced backscatter effect. The increase in scintillation index occurs near the optical axis of the transmit beam. The size of the enhanced backscatter region is approximately the Fresnel zone in low turbulence, and the coherence radius in high turbulence. Since the TATS interrogator is monostatic, the enhanced back-scatter effect is expected to increase the scintillation index.
The experimental parameters do not fall exactly into the limits of Andrews’ approximation because the size of the retro-reflector is approximately equal to the Fresnel zone, rather than being much smaller, as is assumed in the point source limit. However, the model seems to fit the observed data relatively well. It shows scintillation indices below 1 for low values of the Rytov variance. As the Rytov variance increases the scintillation index rises until it reaches a saturated value of about 10 for monostatic links. It then declines very gradually. The peak scintillation index occurs for values of the root mean square Rytov variance, β0, of about 2 or greater. For the link ranges studied in this paper this will occur whenever Cn2 exceeds about 2x10−13 m-2/3. Typical values of Cn2 for terrestrial links near the surface under clear daytime conditions range into the mid 10−13 m-2/3 [8,10,18], so, as was observed, scintillation is predicted to be near, or at, saturation at mid-day.
Measurements of retro-reflector array scintillation were first taken early in the morning, under conditions of low turbulence, to explore coherent interference effects. From Eq. (2) it can be determined that the number of interference fringes across the receive aperture for an array of two retro-reflectors was about 0.1 per cm of corner cube separation. Therefore a minimum of 10 cm separation was needed to produce one fringe across the receive aperture. This requirement limits the compactness of the retro-reflector array. One way to allow smaller retro-reflector spacing is to suppress coherent interference using polarization. This approach was tested by first measuring scintillation from one retro-reflector and then comparing it to an array of three retro-reflectors each separated by 5 cm. Scintillation from the array was measured both with and without a quarter-wave plate (QWP) in front of the middle retro-reflector. The wave plate served to rotate the polarization of the return from the middle retro-reflector so that it could not interfere with the two outer retro-reflectors. This approach works even if the laser has a time-varying polarization state, as was the case in this experiment. This is because it is only the relative polarization of the corner cubes that matters. Also, in the experiment, the retro-reflectors were illuminated at close to normal incidence. Corner cube retro-reflectors can mix polarization states if illuminated off-axis. This might reduce the benefit of the QWP.
Figure 2(a) shows a sample of the intensity return from the single retro-reflector. The intensity is normalized so that the mean is equal to one. It showed a σ2I = 0.34. Figure 2(b) shows a sample of the intensity return from the array of three retro-reflectors without a QWP. It showed a σ2I = 0.51, higher than that of the single retro-reflector. The return from the array shows signal variations at two time scales. The slower variations are similar to that shown by the single retro-reflector. The faster variations are sinusoidal and are produced by vibration in the array mount. Figure 2(c) shows a sample of the intensity return from the array of three retro-reflectors with a QWP on the middle corner cube. It showed a σ2I = 0.24, lower than that of both the array without the wave plate and the single corner cube. Its signal variations are only at the slower time scale corresponding to atmospheric turbulence. The interference between the two outer retro-reflectors and the middle was suppressed by the QWP. The signal variation due to interference between the two outer corner cubes was suppressed because their separation of 10 cm was sufficient to produce one fringe across the receive aperture.
Fig. 2 Normalized intensity for a 1.7 km link to (a) a single 2.5 cm diameter corner cube, (b) an array of three 2.5 cm diameter corner cubes separated by 5 cm, and (c) an array of three 2.5 cm diameter corner cubes separated by 5 cm with a quarter-wave plate covering the middle corner cube.
Next, data was taken under clear mid-day conditions during which turbulence was high, and at its saturated value. Scintillation was relatively constant when averaged over thirty seconds. This allowed different array configurations to be tested under comparable scintillation conditions. A typical, one second long, normalized intensity sample from a single retro-reflector is shown in Fig. 3. For this data set σ2I was 10.9. The variations of intensity approaches 40 dB under these conditions as compared to the 10-15 dB fluctuations seen in low turbulence.
Fig. 3 Normalized intensity for a link to a single 2.5 cm diameter retro-reflector at a range of 1.7 km under conditions of saturated turbulence.
The effect of retro-reflector arrays on scintillation was measured by varying the number and spacing of retro-reflectors, and then recording intensity data. The data was then analyzed to determine the scintillation index, σ2I, as well as other parameters. Figure 4(a) shows the average scintillation index measured in a 30 second period for an array of two retro-reflectors versus the separation of the retro-reflectors. Data was taken both with and without a QWP in front of one of the corner cubes. Figure 4(b) shows a similar set of data for three corner cubes in the array. In both cases the retro-reflector arrays show lower scintillation than the single retro-reflector. For larger separations, the inverse of the beam averaging factor, 1/Fb is about 1.8 for two retro-reflectors and about 2.5 for three retro-reflectors. This is consistent with Anguita’s results for multiple transmitters in direct beam links; the inverse beam averaging factors are slightly less than the number of retro-reflectors when the separation was larger than the coherence radius [14].
Fig. 4 The scintillation index for a 1.7 km link to an array of (a) two retro-reflectors and (b) three retro-reflectors as a function of the separation between the retro-reflectors. Cases with a quarter wave plate (QWP) and without are shown.
The effects of increasing separation, and of the QWP, are relatively weak in the presence of strong scintillation as compared to weak scintillation. This indicates that coherent interference does not make much of a contribution to the scintillation index under these conditions. In fact, the intensity data that was used to produce Figs. 4(a) and 4(b) does not show the characteristic rapid sinusoidal variation that results from coherent interference. This may be because the intensity variation from atmospheric turbulence is so strong that it masks the effects of interference. It is also true that the optical coherence radius in strong turbulence is smaller than the retro-reflector diameter. This will tend to wash out the interference between corner cubes.
A previous study of the scintillation statistics of links using a single retro-reflector has shown that they are well modeled using a log-normal distribution [8]. The data collected in this experiment was used to determine the scintillation statistics for links to retro-reflector arrays. Figure 5 shows the experimentally measured probability distribution function of the natural logarithm of the intensity for the array of three retro-reflectors. It also shows a log-normal distribution for the parameter σlnI2 = 2.0. The value of σlnI was derived from the statistics of the natural logarithm of the experimental data, not by fitting the log-normal function to the data. The good correspondence between the model and data, without adjustable parameters, indicates that the log-normal function describes the irradiance fluctuations of retro-reflector arrays well.
Fig. 5 Experimentally measured probability distribution function of the irradiance fluctuations of a 1.7 km link to an array of 3 retro-reflectors separated by 10 cm with a quarter-wave plate covering the middle retro-reflector (solid line), and a log-normal function with parameter σlnI2 = 2.0.
The benefit in using a retro-reflector array can be quantified in terms of the required link margin for a given quality of service. Fades due to scintillation can cause data loss on an optical link. A typical fade length exceeds 1 millisecond. Since bit rates for most optical links are much faster than 1 KHz, the effect of a fade is spread over many bits and results in packet loss. When the returned optical power drops below the noise level of the receive detector a packet will be lost.
The expected packet error rate (PER), for packet lengths shorter than the mean fade time, can be estimated by a statistical analysis of the intensity data. A cumulative histogram of the normalized intensity can be used to determine the percentage of time that the intensity falls below a given fraction, f, of the mean intensity. The value of the cumulative histogram at f is then equal to the estimated PER for a link in which the margin over the detector noise floor is 1/f.
Figure 6 shows estimated PER vs. link margin for three different configurations under clear mid-day conditions: a single retro-reflector, two retro-reflectors separated by 20 cm with a QWP on one, and three retro-reflectors with 10 cm between each and a QWP on the middle corner cube. For the single retro-reflector, the analysis shows that more than 24.5 dB of signal margin over the detector noise floor is needed for a packet error rate of 1%. Even this estimate may be optimistic because it assumes that packet errors are only caused by fades. In many cases photodetectors have limited dynamic range on the high end as well. Thus the 40 dB of signal variation present when σ2I is 10 or greater may make it impossible to close a link at a low error rate regardless of margin. The beam averaging that occurs using the array with two retro-reflectors reduces the margin needed for a 1% PER to 21.75 dB, a 2.75 dB improvement over the single retro-reflector. In addition, using two retro-reflectors instead of one will return 3 dB more light, so the total link improvement for the array is 5.75 dB. The three retro-reflector array yields a 5.5 dB improvement from beam averaging and a 5 dB improvement from additional optical return for a net link improvement of 10.5 dB.
Fig. 6 Estimated packet error rate percentage, derived from the statistics of the experimental data, for a 1.7 km link, under clear mid-day conditions, to a single 2.5 cm diameter retro-reflector, an array of two retro-reflectors separated by 20 cm with a quarter-wave plate on one, and an array of three retro-reflectors separated by 10 cm with a quarter-wave plate on the middle retro-reflector.
3. Interrogator aperture averaging and enhanced backscatter reduction
3.1 Experimental conditions
Scintillation can be reduced by changes at the interrogator end of the link as well as the retro-reflector end. Receiver aperture averaging is often used in direct FSO links [11,12], and it has a similar beneficial effect in reducing scintillation in retro-reflector links. For the return portion of an MRR link, the retro-reflector can be viewed as a source, and the interrogator as the receiver. Just as in a direct link, a larger receive aperture will integrate over the scintillated beam pattern and reduce temporal fluctuation in power. In addition, the increase in scintillation for monostatic interrogators due to the enhanced backscatter effects suggests that a bistatic interrogator, with separate transmit and receive apertures, should exhibit lower scintillation than a monostatic system [9,10].
Both these effects can be exploited in an interrogator design to reduce the scintillation index. Aperture averaging should occur when the receive aperture exceeds the coherence radius (between 0.5 and 2 cm). Reduction of the enhanced backscatter effect should occur when the transmit and receive aperture separation exceeds a similar amount.
To test this hypothesis a new bistatic interrogator was constructed. It used a similar laser and transmit optics as the monostatic system. The photodetector for the receiver and the digitizer were also the same. However, the center of the receive aperture for the bistatic system was offset by 12.5 cm from the center of the transmit aperture. The size of the receive aperture could be varied, using an adjustable diaphragm, from a maximum of 7 cm to a minimum of about 1 cm. The focal length of the receive lens was 15 cm. In this case the light falling on the receive aperture was directly focused onto the 300 micron diameter photodetector, instead of using a fiber collimator and multimode fiber to deliver the light to the detector.
The effects of aperture averaging and enhanced backscatter were studied in a field test at Fort AP Hill, Virginia during the month of July. The retro-reflector array described in section 2a was used on one end. At the other end of the link both the TATS monostatic interrogator and the bistatic interrogator were used to simultaneously illuminate the retro-reflector array.
The path for this test was over 2.1 km of flat terrain, a slightly longer range than in the previous test. The retro-reflected beam diameter was 32 cm. The retro-reflector array was situated about 2 meters above the ground. The interrogators were on the second floor of a building about 10 meters above the ground. The interrogators were separated by about 5 meters so that the returns from their respective interrogating beam did not overlap. Procedures for capturing scintillated intensity data were the same as described in section 2.2.
3.2 Measurements of aperture averaging and enhanced backscatter
Measurements were taken under completely clear conditions at mid-day. As in previous testing the scintillation index, σ2I, of a single retro-reflector, interrogated by the monostatic TATS interrogator, had a low value in the early morning and then rapidly climbed to a saturated value of about 13 by late morning. After that it remained relatively constant for many hours, during which data was taken.
Adding retro-reflectors to the array again reduced the scintillation. An array of 3 retro-reflectors separated by 10 cm with no quarter-wave plate exhibited a scintillation index of 5.5 as measured by the monostatic interrogator. This array configuration was used to determine the effects of aperture averaging and enhanced backscatter.
Scintillated intensity patterns were recorded simultaneously by both the monostatic and bistatic interrogators. In addition, the diaphragm of the bistatic interrogator was set to several different values to measure the effects of aperture averaging. Figure 7 shows the scintillation index of the retro-reflector array, measured by the bistatic interrogator, as a function of receive aperture size. Also shown is a theoretical calculation for aperture averaging in the spherical wave approximation [16] fit to the data with a root mean square Rytov variance of 1.8. In addition, the scintillation index of the array, as measured by the monostatic interrogator with its fixed aperture size of 2.8 cm is shown.
Fig. 7 Experimentally measured scintillation index versus receive aperture size for a monostatic and bistatic interrogator illuminating an array of three 2.5 cm diameter corner cubes separated by 10 cm each. Also shown is a theoretical curve, in the spherical wave approximation, calculated with a root mean square Rytov variance of 1.8.
The data shows a large drop in scintillation index in moving from a monostatic to bistatic interrogator. At the comparable aperture of 2.8 cm, the monostatic scintillation index is 5.1 and the bistatic scintillation index is 1.6, a reduction of a factor of about three. Models of the enhanced backscatter effect indicate a smaller difference between the two cases, closer to 2 [10,17,19,20]. In a different set of experiments, that will be published elsewhere, a ratio of bistatic to monostatic scintillation closer to 2 was found, but with a fairly large statistical spread in the values [21].
Increasing the bistatic interrogator aperture from 1.3 cm to 7 cm decreases the scintillation index from 2.5 to 0.65. This is also somewhat larger than predicted by theory, but overall the aperture averaging data matches well to the model.
The net effect of all these techniques is displayed in Fig. 8, which shows packet error rate versus margin, under clear mid-day conditions, for the monostatic interrogator and single retro-reflector versus the bistatic interrogator, with 7 cm receive aperture, and the array of three retro-reflectors. The margin for 1% PER has been reduced from 24.5 db to 10 dB. Including the additional optical return from the array yields a net improvement of 19.5 dB.
Fig. 8 Estimated packet error rate percentage, derived from the statistics of the experimental data, for a 2.1 km link to a single 2.5 cm retro-reflector using a monostatic interrogator with a 2.8 cm aperture and the same link to an array of three 2.5 cm retro-reflectors separated by 10 cm using a bistatic interrogator with a 7 cm aperture.
4. Conclusion
The very high level of scintillation that is observed for terrestrial modulating retro-reflector links under clear, mid-day conditions can severely limit the range and quality of service of these links. However, a variety of techniques can be combined to greatly reduce the scintillation index. These techniques do not have a large impact on the simplicity, low power consumption and loose pointing requirements of MRR systems. This is important, as these are the features that make MRRs attractive in the first place.
In addition to the data presented here, additional measurements of retro-reflector arrays with the bistatic terminal at 7 cm aperture have been made at ranges up to 5 km. The saturated scintillation index does show some increase at larger ranges, peaking at values between 1.0 and 1.5. These values of scintillation are similar to those seen in long range terrestrial direct links.
A single retro-reflector interrogated by a monostatic terminal with a 2.8 cm aperture was shown to exhibit a saturated scintillation index of greater than 10 for links of about 2 km. In this configuration more than 24 dB of margin is needed for a 1% packet error rate. Using an array of 3 retro-reflectors reduced the scintillation index to about 5. Moving from monostatic to bistatic interrogation reduced the scintillation index to 1.6, and increasing the aperture from 2.8 cm to 7 cm reduced the scintillation index to 0.65. The combination of all these techniques reduces the amount of optical power from the interrogator required for a 1% packet error rate by almost 20 dB. For a fixed interrogator power, and neglecting any additional atmospheric transmission losses, this corresponds to a tripling of link range.
By reducing the dynamic range of the fluctuations to a value that can be accommodated by photoreceivers, these techniques can allow a high quality of service that may be impossible to attain otherwise. In addition, by reducing the maximum value of the scintillation index from greater than 10 to around 1, system design is simplified. There is less variation in scintillation between daytime and nighttime operation, and hence less variation in quality of service. Also, because the scintillation index values are reduced to a range similar to that of direct links, protocols developed for direct FSO links, such as FEC with interleaving and temporal diversity, can be more easily adapted to modulating retro-reflector links [22].
Acknowledgment
We acknowledge the support of the Office of Naval Research.
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