The fade mitigation techniques described by Castanet et al in Interference and fade Mitigation Techniques for Ka and Q/V Band Satellite Communication Systems[1] allow systems with a small static margin to be designed, while real time cloud attenuation, some fraction of rain attenuation, scintillation and depolarization events are overcome. The Adaptive modulation and coding technique is the one technique that is most employed as they allow the performance of individual links to be optimized. Also, the transmission characteristics can be adapted to the propagation channel conditions and to the service requirements for the given link. In particular, these are observed to be promising in a point-to-point service scenario[2].
In the V Band, Fade mitigation techniques are employed with the objectives of increasing system throughput and of improving system throughput. These techniques involve adapting in real time the link budge to the propagation conditions through some parameters like- power, data-rate and coding. The real time adaptively, however, has an impact not only on carrier-to-noise ratios but also on carrier-to-interference ratios and on upper layers. [1] FMTs for the 10GHz to 50GHz spectral region were investigated with emphasis on modeling, prediction methods, and experimental work in the past [3].
The most relevant Fade mitigation techniques take into account operating frequency bands, performance objectives of the system and geometry of the network.
Fade mitigation for the Physical Layer can be comprehensively grouped into:
1. Power Control- through transmitting power level fitted to propagation impairments. This type of FMT can be sub-divided into:
a. Uplink Power control(ULPC) aims in matching Earth Transmit station’s output power to uplink impairments. In case of transparent payloads, ULPC can prevent from reductions of satellite EIRP caused by the decreased uplink power level that would occur in the absence of ULPC.
b. Down-link Power Control (DLPC) adjusts the on-board channel output power to the magnitude of downlink attenuation. DLPC allocates extra power on-board to compensate a possible degradation in terms of downlink C/N due to propagation conditions on a particular region.
The above two techniques can be grouped as Effective Isotropic Radiated Power techniques (EIRP).
c. End-to-End Power Control (EEPC) could be used for transparent configuration only. The output power of a transmitting Earth Station is matched to up-link or down-link impairments. Since in the case of regenerative speakers, up and down link budgets are independent, the concept of EEPC does not exist.
d. On-board Beam Shaping (OBBS) is a technique based on active antennas, allowing spot beam gains to be adapted to propagation conditions. The objective is to radiate extra-power and to compensate rain attenuation only on spot-beams of high rain occurrence.
2. Adaptive waveform- Fade compensation by a more efficient modulation and coding scheme.
These techniques involve Adaptive Coding (AC), Adaptive Modulation (AM) and Data Rate Reduction (DRR).
Adaptive Coding (AC) consists in implementing a variable coding rate matched to impairments originating from propagation conditions.
Adaptive Modulation (AM) aims to decrease the required energy per information bit required corresponding to a given BER translating to a reduction of the spectral efficiency as C/N decreases.
Data Rate Reduction technique reduces the information data rate at constant BER.
3. Diversity- Fade avoided by the use of another less impaired link.
These techniques re-route information in the network in order to avoid impairments due to an atmospheric perturbation.
4. Layer2- Temporal dynamics of the fade have to be taken into consideration.
Some diversity based techniques are [3]:
1. Site Diversity (SD) - If Signal is received via different paths, it is likely that a deep fade occurs only on one of them, leaving the others less affected by it. SD uses this characteristic of convective rain by engaging two or three earth stations to ensure that the probability of attenuation occurring simultaneously on the space paths is significantly on either individual path. Signals are sent to a master station where they are further processed based on combining, selection combining or maximal ratio combining to improve the Carrier-to-Noise ratio (C/N).
The most important factor affecting the diversity gain offered by SD systems is the separation distance D between earth stations. Larger values of D result in higher diversity gains of up to 30dB, highest gain achieved by any FMT.
2. Frequency Diversity - As frequency of operation increases, a satellite link suffers more from precipitation. Use of on-board repeaters in satellites to operate at various frequency bands is favorable, specially at lower bands of frequency. Use of higher bands (Ka or EHF) during normal operation and switches over to spare channels at lower frequency bands is emphasized [3].
3. Time Diversity- This type of FMT can tolerate delays since it involves the repeated transmission of data corrupted by strong fading. Performance is closely related to fade duration to time intervals between fades.
Despite individual differences, all of the above techniques perform the following [3]:
1. Observe/monitor link quality by performing continuous measurements of propagation conditions. This is done by measuring the BER at the output of the receiver where the corresponding Eb+/N¬0 is calculated. This requires the observation of a sufficient number of errors before the fade level can be estimated. Since it is an indirect method, it introduces the drawback of BER increasing very abruptly when the propagation conditions deteriorate.
2. Give a short term estimation and prediction of the behaviour and the relevant duration of the next state of the satellite channel.
3. Set/ change the parameters of the systems based on the above short-term estimate.
Real-time estimation of fades is a far more difficult task due to the random nature of the various physical phenomena. The current trend of the prediction algorithms for the behaviour of satellite channels is the use of specific fade characteristics like- fade depth, slope, duration or fade envelope and fading rate in combination with the appropriate sampling of the measured data.
References:
[1] L. Castenet, A. Bolea-Almanac, M. Bousquet, “Interference And Fade Mitigation Techniques for KA andQ/V band Satellite Communication Systems,” in International Workshop COST Actions 272 and 280, Noordwijk, The Netherlands, 27 May 2003.
[2] COST 255 : "Radiowave propagation modelling for new satcom services at Ku-band and above", COST 255 Final Report, Chapter 5.3, ESA Publications Division, SP-1252, March 2002.
[3] Panagopoulos, A.D.; Arapoglou, P.-D.M.; Cottis, P.G.; , "Satellite communications at KU, KA, and V bands: Propagation impairments and mitigation techniques," Communications Surveys & Tutorials, IEEE , vol.6, no.3, pp.2-14, Third Quarter 2004doi: 10.1109/COMST.2004.5342290
URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5342290&isnumber=5342289
3.7 Fade Mitigation Techniques
