Wavelength selection

Solutions for optical LEO downlinks differ in the system design with monostatic and bi-static designs. While bi-static designs use a separate optical path and aperture for receive and transmit beam, monostatic system designs share one optical system for both.

Monostatic system designs provide advantages in the calibration of the system between receive and transmit path and allow for a more compact terminal especially when using a coarse pointing assembly. However, the wavelength selection for monostatic systems is more challenging compared to bi-static systems where due to separate optics no influence from the transmitter on the receiver and tracking sensor has to be expected. Therefore, bi-static systems allow transmit and receive wavelengths to be very close in the spectrum since no back reflections between the paths have to be considered.

In monostatic systems, depending on the system design and link budget, 30 dBm transmit power or more can be assumed while common tracking sensors have sensitivities of -70 dBm or more. Even with anti-reflection coated optical surfaces, the back reflections of the transmit signal from the optical surfaces can reach values up to 10 dBm which requires a filtering between transmit and receive path of up to 80 dB to achieve a complete separation. To allow filters with the required characteristics, a larger separation of transmit and receive wavelength compared to the bi-static design is required, resulting typically in 30nm band separation.

Figure 4 shows a potential band plan for transmit and receive path (also called uplink (UL) and downlink (DL) seen from the optical ground station) in a LEO scenario. Here, the whole optical C-band (1529 – 1568 nm) can be used for the transmit path (downlink), while the optical L-band (1600 – 1600 nm) is used for the uplink beacon (receive path). This usage of the frequency spectrum in future will also allow techniques like Wavelength Division Multiplexing (WDM) in the C-Band with COTS components.

Figure 4:

Typical Band-plan for uplink (UL) and downlink (DL)

Cloud Blockage and OGS network

Link blockage by clouds is one of the major influences on optical communication systems which operate within the atmosphere. In order to achieve a good availability of the optical satellite-to-ground link, a network of Optical Ground Stations including sites with good weather conditions is required. Complete operational OGS networks are not available for optical downlinks yet. The optimization of optical ground station networks is also a current field of research. Investigations have been performed for the optimization of ground station networks for optical GEO feeder links [14] and European networks for OLEODL are presented in [15].

Atmospheric Attenuation

The optical attenuation due to absorption and scattering depends on the elevation angle between optical ground station and satellite, as the length of the atmospheric path is longer for low elevation angles. These effects also depend on the wavelength of the signal. Figure 5 shows the atmospheric transmission vs. elevation for a wavelength of 1550 nm. The values have been generated with a database available at the DLR Institute of Atmospheric Physics for a visibility of 23 km and 1550 nm wavelength.

Figure 5:

Atmospheric transmission vs. elevation angle.

Scintillation Loss

The atmospheric index of refraction turbulence causes strong fluctuations of the received signal in OLEODLs. These can be more than +/- 6 dB, and have durations between one to several milliseconds – mainly depending on link elevation and Rx-aperture size. The strength of this effect is quantified by the receiver Power Scintillation Index (PSI) (the normalized power variance), as depicted in Figure 9. From the PSI values, together with the assumption of lognormal fading of the power at the OGS, according to [21], one can estimate the loss a_sci (Figure 6) that has to be accounted for in the link budget calculation.

Figure 6.

Based on PSI (Power Scintillation Index, including aperture averaging of OGS) and with an allowed loss fraction (here assuming 1E-6 for OLEODL), the according scintillation-loss can be estimated at maximum ~6 dB.

Figure 7:

Link Budgets for satellites with an orbit height of 400 km and 900 km

Figure 8.

The four modes for varying the data rate in an OLEODL system

Figure 9.

Power scintillation index versus link elevation with different receiver aperture diameters, wavelengths of 847 nm and 1550 nm, for an exemplary downlink scenario.

Link Budgets

Figure 7 then shows exemplary link budgets for satellites with orbit heights of 400 and 900 km and different elevation angles. An optical output power of 1 W and an aperture diameter of 40 mm at the spacecraft side are considered. A maximum channel rate of 10 Gbps is considered as can just be received with bulk (multimode) photo detectors. Scintillation loss has been modelled according to Figure 6. A receiver sensitivity of 800 Ph/bit [22] and a standard Reed-Solomon (255,223) code with a gain of about 4 dB at a BER of 1E-6 has been considered. This is a standard code and has a comparably low implementation effort, enabling its use also at higher data rates. The results show very well the need for variable data rates: While a data rate of 10 Gbps is achievable above 10° of elevation for a 400 km orbit, it does not become feasible before 15° of elevation with the 900 km orbit → if elevations below 10° are to be used for data transmission, lower data rates are required.

Summarizing the link budget values from 5° to zenith elevation during one pass (free-space loss, atmospheric loss, and scintillation loss), we find that for the ISS-orbit variations become ~22 dB, and ~20 dB for the 900km orbit. Even higher variations are to be expected in a non-clear atmospheric situation or with smaller aperture diameters than 60cm. This clearly shows the need for rate variation techniques in OLEODL-systems.

Receiver Aperture Averaging for Fading Mitigation

The choice of the suitable receiver aperture size for the ground segment is crucial for the overall system performance and cost. Currently deployed telescope sizes of ground segments from DLR, Tesat, NICT, and NASA-JPL range from 27 cm to 1.5 m [16], [17], [18], [19]. Rx signal fluctuations can be reduced by aperture averaging, with receiver apertures larger than the intensity correlation width. Aside the antenna gain, the specified tolerable fading loss determines the maximum allowed signal fluctuation and therefore minimum aperture size for a given location and atmospheric condition. For an exemplary ground station near Munich, the expected power scintillation index ranges from ~0.004 at 55° elevation to ~0.3° at 5° elevation at a measurement wavelength of 847 nm and 40cm aperture. In terms of fading loss, this translates to between 1 dB and 11 dB at a threshold of 1E-6 (compare Fig. 6). These values are derived from an average of several individual measurements during nighttime satellite overflights [20]. Figure 9 shows the measurements with the 40 cm aperture (orange line and marker) and with an additional 5 cm aperture (blue line and marker). The solid lines denote model calculations for comparison with the measurements and to show the expected power scintillation index for apertures varying from 5 cm to 60 cm. The graph also shows the decrease of the PSI by use of 1550 nm transmission wavelength. It must be mentioned that the model evaluations are only valid above 20° elevation due to the limitation of weak fluctuation theory here. This explains the strong deviation of the 40 cm measurement below 20°. A scaled Hufnagel-Valley-5/7 model is used as C_n²-profile. The scaling factor is directly multiplied with the standard HV_5/7 and selected to obtain minimum residual error with the power scintillation measurements above 20°.

Variable Channel Rate Techniques

One of the most effective ways to cope with variable link budget conditions in space communications is to lower the data rate [23]. This technique maintains the link even under difficult conditions - although at lower data rates - instead of completely loosing communication. Various techniques to vary (lower) the data rate have been investigated and some of the simple techniques are discussed below with the help of Figure 10. Each technique is evaluated in terms of receiver sensitivity calculated as number of photons required per bit for certain BER performance and the result is summarized in Table 1. The sensitivity depends upon whether peak-power limited (PPL) or average-power limited (APL) transmitters and different receiver models namely, shot-noise limited (SNL), avalanche photodiode (APD) and thermal limited (PIN).

Figure 10.

Channel data rate variation by using different methods. DR = maximum data rate, w = pulse duration. Vertical axis shows the relative amplitude of the signal and horizontal axis shows the relative time.

Table 1.

Summary of various options to vary the data rate and its performance [24]. For complexity, 3 is most complex

Variable FEC-Protection and Interleaving

According to [25] a variety of channel codes can be used for standard RF satellite downlink. In a first approach one would apply these also to OLEODL and chose different coding strengths to enable various levels of coding gains. However, as the characteristics of the transmission channel for optical links deviate strongly from the RF domain, existing RF transmission formats might be non-optimal and their use must be carefully considered.

Such bit-level coding may be used for an overall improvement of the link budget w.r.t. uncoded transmission. However, to compensate link outages due to fading – with fade durations of several ms – FEC must be combined with a long interleaver length, i.e. large memories with fast access times are required, increasing the complexity of the system.

Variable Repetition Coding Techniques

Channel rate (and thus the receiver frontend) can stay constant by simply sending the packets two or more times. This reduces the data rate by factor equal to the number of retransmissions. In the atmospheric fading channel, the repetitions must be separated by a delay larger than the typical fade length, a technique called delayed-frame repetition. Additionally, FEC and soft decoding techniques can be combined to further improve the system sensitivity. The received frames can be treated in different ways, while the format stays unchanged: