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1. DLR Propagation Measurement Campaign
In order to model propagation behaviour at Ku-band and above the Institute for Communications Technology at DLR carries out a propagation measurement campaign starting 1990 with the recording of the 20 and 30 GHz beacon signals of the European satellite OLYMPUS. Recording of these beacons has been stopped in 1993 due to a satellite failure whereupon the DLR earth station has been reconfigured for the reception of the 40 GHz beacon of the Italian satellite ITALSAT.
It is the scope of the measurement campaign to provide a detailed description of satellite channels at 20 GHz and above to properly design satellite systems operating in these frequency bands and to develop appropriate fade countermeasures. Thus, long-term measurements and an exhaustive statistical evaluation of the recorded data is required.
The DLR propagation measurement campaign with ITALSAT is embedded in activities of CEPIT (Coordinamento Esperimento Propagazione ITALSAT), a groupe of experimentors of ITALSAT, and the COST project 255 "Radio Wave Propagation Modelling for New SatCom Services at Ku-Band and Above".
Figure 2 shows a block diagram of the receiver for both the 40 and 50 GHz beacons. Each beacon is down converted in two steps from 40 GHz, respectively 50 GHz to 2.2 GHz and from 2.2 GHz to 70 MHz. The 70 MHz output signal is further processed by an I/Q demodulator whose outputs (both I and Q components) are sampled and recorded.
The sampled data is stored in monitoring files and event files with sampling rate 0.15 Hz and 20 Hz, respectively. A monitoring file contains data of a whole day whereas an event file typically is of shorter duration. Event files are created when the beacon level falls below a preset threshold -- set to 5 dB below clear sky level -- and is closed when the beacon level exceed this threshold. This recording method guarantees fading events to be stored at high sampling rates whereas a considerably reduced sampling rate for interfade durations avoids a huge amount of data to be stored. Recording is carried out on a m VAX II under VAXLAB software allowing for real time operations. Both monitoring files and event files are subject to evaluation. They may also be used for simulation or reproduction of fading events using an I/Q hardware channel simulator developed at DLR. The whole equipment is supported by uninterruptable power moduls in order to enable continuous reception and recording.
Evaluation of the reorded data is carried out in order to obtain a deeper understanding of the channel behaviour and to provide reliable data for static and dynamic fade countermeasures. The probability of exceeding a specific attenuation, the probability of outage and the probability of no fade can be used as a basis to appropriately design fade margins. Seasonal as well as diurnal variations are considered. Statistics on fade slopes and 3 dB time durations enable accurate design of dynamic fade countermeasures. Furthermore, the correlation of rain rate and attenuation is examined.
Evaluation software encompasses various programs e.g. for daily plots, for modification and review of recorded data and for the determination of power level, fade duration and fade slope distributions, probability of outage, probability of no fade, and correlation of rain rate and attenuation.
Figure 3 shows the rain rate together with signal attenuation over 14 hours for June 9th, 1995. We see that signal attenuation very much depends on the actual rain rate which has been measured on ground 50 m close to the earth station site. The mean attenuation over the rain rate is given in Figure 4 and has been obtained for the first half of 1995. Mean attenuation increases from about 0.5 dB at a rain rate of 0 mm/h over 6 dB for 5 mm/h to more than 10 dB for rain rates over 10 mm/h.
The cumulative distribution of attenuation for several months in 1995 is shown in Figure 5 revealing strong seasonal variations: In summer attenuation is much more severe than in the winter: In February attenuation of 5.5 dB and more occurs only in 0.1% of the time whereas in July an attenuation of more than 13 dB has been recorded in over 1% of the time.
The distribution of fade durations is shown in Figure 6. E.g. the probability to be in a fading event lasting longer than 100 seconds and being deeper than 18 dB is about 0.03%.
2. A New TDMA Technique with Adaptive Resource Sharing for Frequency Bands Above 20 GHz
Based on the results of the DLR propagation measurement campaign for fixed satellite channels above 20 GHz new techniques have been developed for a meshed TDMA satellite network enhancing link availability and transmission quality. These techniques employ fade countermeasures to compensate for signal attenuation in both up- and downlink.
Uplink power control (UPC) is the simpliest type of dynamic fade countermeasure increasing transmission power to the same extend as signal attenuation increases. However, UPC is of limited use since the overall system design usually does not provide large fade margins. Furthermore, transmitting amplifiers have a non-linear nature and are limited in output power. Since the satellite transponder is operated close to the saturation point it cannot compensate for downlink fades. Therefore, UPC is used only to combat signal attenuation on the uplink where the increase of output power is limited to typically 3 to 8 dB with respect to nominal power.
Data rate reduction and FEC coding have much more potential to compensate for signal attenuation. Both countermeasures yield higher signal-to-noise ratios by reducing the information bit rate but at the same transmission power. A reduced information bit rate is a severe drawback. However, appropriate use of all resources applying adaptive resource sharing (ARS) overcomes this disadvantage in a TDMA system.
A new TDMA technique based on ARS has been developed. System resources are assigned in a flexible way according to the actual fading situation at each earth station location. A common resource (specific slots within the TDMA frame) is available for all users of the TDMA system. Thus, every user is assigned its own slot and -- if required -- slots of the common resource to enable data rate reduction and FEC coding without reducing the user?s information bit rate. Since the common resource is shared by a great number of users which are subjected to signal fading independent to each other the overall system design enables to minimize overhead and fade margins.
In cooperation with the German and Swiss Telekom and within the framework of the GECO (Group of Experimentors of CEPT for Olympus) an experimental fully meshed satellite TDMA system has been set up and run over the German satellite DFS Kopernikus, see Figure 7. Four earth stations in Oberpfaffenhofen, Darmstadt, Berlin and Bern were connected. The new technique has been tested successfully.
The lower part of Figure 8 shows the signal level of both the up- and downlink of the Oberpfaffenhofen - Darmstadt satellite hop on April 29, 1994. Both links are subjected to signal attenuation. The upper part of Figure 8 gives the gain provided by UPC and code/data rate switching. It can be seen that the overall signal attenuation is compensated by appropriate countermeasures. Uplink signal attenuation is mainly compensated by UPC up to a maximum of 8 dB. Deeper fades on the uplink also require appropriate data rate reduction and FEC coding. Downlink fades are compensated for by appropriate data rate reduction and FEC coding.
Figure 9 gives the bit error rates which have been measured during signal attenuation. It can be seen that fade countermeasures compensate for fades and guarantee to maintain the required bit error rate of 10-6. Without any countermeasures transmission quality would have been very poor, cf. theoretical curve. Overall link availability has been increased, respectively outage has been reduced at more than 30%.
References
[1] Hugo, D. von; Wilde, A.: An Adaptive Resource Sharing Strategy for TDMA. Intern. Journal of Satellite Communications 12 (1994), pp. 249-256.
[2] Schnell, M.; Wilde, A.: Adaptive Resource Sharing for Satellite TDMA. Proc. 5th IEEE Intern. Symp. on Personal, Indoor and Mobile Radio Communications, Vol. 4 (1994), pp. 1188-1191.
[3] Fiebig, U.-C.; Dolainsky, F.; Reinel, H.; Schnell, M.: Attenuation Measurements of the 40 GHz Beacon Signal of ITALSAT. Proc. 3rd CEPIT Meeting (1995), pp. 6.1-6.4.
[4] Schnell, M; Hugo, D. von: Fade Countermeasures and Adaptive Resource Sharing for an Experimental TDMA Satellite Communication System Operating at Ka-Band. To be published at ETT