A Rain Fade Mitigation Scheme for the Forward Path of DVB-RCS Systems

Abstract

This paper presents a rain fade mitigation scheme for the forward path transmission of DVB-RCS systems, based on an FDMA approach using a mix of higher availability rain carriers and higher capacity clear sky carriers. The concept relies on the fact that, at any given time, only a portion of the coverage area and a fraction of the user population are affected by significant rain fading. The proposed fade mitigation scheme controls the allocation of a number of rain “hardened” transponders and clear sky transponders to the user population, depending on the extent of the rain occurring in the coverage area. The scheme employs accurate rain detection and appropriate signaling to initiate a dynamic and automatic switchover from clear sky to rain carrier and back again when the rain fading has significantly diminished. INTRODUCTION As is the case of Direct To Home (DTH) satellite Digital Video Broadcasting (DVB), the forward path of DVB-RCS systems utilize a DVB-S carrier. To ensure sufficient availability of the forward transmission, systems designers typically allow a sufficient fixed margin to accommodate the amount of fading corresponding to the target service availability for a particular rain region. It is well known that the geographic and temporal distribution of heavy rain is usually concentrated in highly localized cells, which only affect a small fraction of the user population at any one time. Allocation of the entire fixed margin is actually only required by those users affected by rain fading at a given time, instead of the entire population of users all the time. A portion of this (‘residual fixed margin’) is primarily allocated for more widespread lighter rain and for the operation of the mitigation scheme itself. In general, the allocation of a larger fixed margin effectively translates into a reduction of the available system capacity for a given transponder bandwidth, due to the use of a more power, instead of bandwidth, efficient transmission scheme. A power efficient scheme utilizes some combination of lower level modulations and more coding, for a given satellite EIRP and terminal G/T, to achieve the extra margin for fading protection. This impact on bandwidth efficiency becomes especially significant, as the DVB-RCS systems migrate to higher frequency bands such as ka band, in order to access more spectrum. By avoiding a large portion of the fixed margin, the clear sky carriers can in principle utilize higher level modulations and reduced or no coding at all, thereby supporting either an increased average bandwidth per user or more users for a given bandwidth. Whichever the objective, the net improvement is in increased transponder throughput for user traffic, and hence improved business plan economics. In general, there are two possible approaches to supporting an adaptive transmission scheme on the forward path. The first relies on the use of different carriers, some utilizing power efficient schemes for rain, and others bandwidth efficient schemes for clear sky. This is referred to as the FDMA method. The second approach allocates segments in time (frames) of a single carrier to different transmission schemes for rain and clear sky. This is referred to as the TDMA method. This latter approach, utilized in the ITU-R ISDB standard is not compatible with DVB-S systems. On the other hand, the FDMA method described in this paper, utilizes standard DVB-S carriers as part of the forward interaction channel of a DVB-RCS system. JSAT FORWARD PATH FADE MITIGATION SCHEME In the JSAT forward rain fade mitigation (RFM) scheme, the terminals not affected by rain fading would normally receive clear sky carriers, with reduced residual margin, and those affected by rain would tune to rain carriers with larger rain margins. This approach relies on the fact that, in general, only a fraction of the terminals in a given coverage area are affected by rain at any one time, requiring a limited number of forward carriers with more protection (more coding and possibly lower level modulation). On the other hand, it 21st International Communications Satellite Systems Conference and Exhibit AIAA 2003-2264 Copyright © 2003 by Copyright 2003 by EMS Technologies Limited and JSAT Corporation. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. American Institute of Aeronautics and Astronautics 2 is expected that most carriers would be of clear sky variety, to handle the bulk of the traffic. In general, the proportion of clear and rain carriers in a given coverage area would need to be adjusted quasi-statically to respond to the occurrence of rain. The overall objective of the rain mitigation scheme is to increase the net system capacity (or data rate), by using as many as possible clear sky carriers with increased throughput. In general, the overall system capacity Fb, will be the sum of the improved capacity of N clear sky carriers (Fb-cs) and the reduced capacity of M rain carriers (Fb-rain):