Technique for increasing the rain margin of a satellite communication system

FIG. 5 illustrates a typical K=8, r = J rate convolutional encoder;

FIG. 6 illustrates a preferred arrangement for a TDMA burst modem data processing arrangement which is found at each receiving ground station that 5 might receive encoded burst information in accordance with the present invention;

FIG. 7 illustrates a typical carrier and clock recovery arrangement for use in the arrangement of FIG. 6;

FIG. 8 illustrates a typical arrangement for the slow 10 speed processor in the arrangement of FIG. 6;

FIG. 9 illustrates a typical fade frame synchronization circuit arrangement for use in the slow speed processor of FIG. 8;

FIG. 10 illustrates a typical K = 3, r=£ rate convolu- 15 tional encoder;

FIG. 11 illustrates a state transmission diagram and generated channel symbols for a typical K=3, r = J rate Viterbi decoder; and

FIG. 12 illustrates a typical path metric update proce- 20 dure in a Viterbi decoder for a K=3, r= i rate convolutional code.

DETAILED DESCRIPTION

The present invention is described with relation to a 25 time division multiple access (TDMA) satellite communication system comprising a single scanning up-link beam and a single scanning down-link beam for purposes of simplicity. It is to be understood, however, that such description is exemplary only and is for the pur- 30 poses of exposition and not for purposes of limitation. It will be readily appreciated that the inventive concept is equally applicable to area beam coverage systems, fixed multiple spot beam systems and single or multiple scannable spot beam systems or any combination thereof. 35

As shown in FIG. 1, the single scanning TDMA spot beam satellite communication system which will be used to describe the present invention comprises a satellite 10 including a single transponder 11 coupled to an up-link antenna 12, capable of receiving a single scan- 40 ning up-link beam 13, and a down-link antenna 14 capable of transmitting a single scanning beam 15 which can cover an entire service area. The service area, which is shown as the continental United States, is divided into N spot beam footprints, labeled Fi through Fn- Each 45 footprint can comprise at least one ground station but typically contains several ground stations.

FIG. 2 illustrates an exemplary TDMA switching sequence performed at the satellite to interconnect the various footprints shown in FIG. 1. Within each frame 50 are dedicated time slots used to establish a two-way communication channel between a ground station and itself and each remote station in the network. For example, in the initial subframe in each frame, antenna 12 is directed to receive an up-link burst from ground sta- 55 tions in footprint Fi which include TDMA bursts destined via the down-link scanning beam 15 for ground stations in footprints Fi to Fat in sequence. Therefore, during the initial subframes, antenna 12 is directed toward footprint Fi to receive transmissions from the 60 ground stations disposed therein while antenna 14 directs down-link beam 15 from footprint Fi to Fn in sequential subframes and in synchronism with the proper transmission time for each associated burst. A similar subframe sequence continues in each frame per- 65 iod for up-link transmissions from ground stations in each of footprints F2 to Fn- Included within each subframe are a number of time slots which are used for

transmitting message bursts comprising preamble and data information between two ground stations. For example, in FIG. 2 in the subframe between ground stations in footprint Fi and footprint Fn, thirteen time slots are shown. It is to be understood that such number is exemplary only and not for purposes of limitation in that each subframe may contain any number of time slots dependent on traffic demand between the two footprints. Also included within each frame, but not shown in FIG. 2, are dedicated portions of subframes, e.g., a time slot, which are used to establish two-way signaling channels between a ground station in one of the footprints designated a master ground station and each of the remote ground stations in the network using any suitable technique known in the art. The signaling channels are used to, for example, enable TDMA synchronization, distribute system status information, handle new requests for service, assign time slots, etc. Except for the signaling slots, all the other time slots can, if needed, be assigned upon demand.

Also shown at the end of the frame is a pool of spare or unused time slots. As will be described hereinafter, these slots are to be made available to ground stations experiencing rain attenuation. It is to be understood that the spare or unused time slots can be obtained in the proper sequence by rearranging the time slot assignments in a frame and using the signaling channels to inform each ground station of such rearrangement. The time slots from the pool can be made available to any ground stations experiencing up-link or down-link rain attenuation. However, a more attractive means for combating up-link fades is via up-link power control. For this approach, the up-link power during rain events is adjusted such that a constant incident power is maintained at the satellite. When the rain attenuation exceeds the margin provided by the maximum ground station transmitter power, fading occurs on the up-link. Since up-link power is usually not at a premium, the maximum transmitter power can often be set so as to overcome the fade condition. Thus, up-link power control represents a very attractive means for combating up-link loss of signal while maintaining a constant signal-to-interference ratio at the satellite. However, it is possible that certain ground stations are not capable of handling additional up-link power to overcome a fade condition due to the circuitry employed, in which case the use of reassigned spare time slots or pool time slots in conjunction with encoding techniques, as will be described hereinafter, will become necessary.

When a down-link fade occurs, the carrier-to-noise ratio at the receiving ground station experiencing the fade is no longer sufficient to maintain the desired bit error rate. Thus, the capacity into that ground station is reduced. Suppose, for example, the rain attenuation is such that the signal level falls 8 dB below the value required to maintain a voice grade bit error rate (BER) equal to 10~3. The channel error rate for Gaussian noise is then about 0.1. A lower bit error rate would result if both Gaussian noise and peak-limited interference set the error rate. The BER, however, can still be maintained at 10_3 or lower in accordance with the present invention.

When power measurements at a ground station indicate that attenuation exceeding the built-in power margin is imminent, then such ground station uses the signaling link to notify the master ground station, as well as all transmitting stations communicating with the fade station, that a fade is about to occur. The master ground