Abstract:
An apparatus for reducing the amount of circuitry required to process satellite communication signals uses a beamformer to reduce the number of signals coupled to the communications circuitry, thereby reducing the amount of communication circuitry required to process received signals.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates in general to satellite communications and, more particularly, to an apparatus for reducing satellite payload complexity when using ground based beam forming. 
     (b) Description of Related Art 
     Some satellite communication systems allow multiple ground users to relay data, voice, and other signals over broad geographic regions. In particular, cellular-based satellite communication systems have become especially popular for commercial applications such as telephony because the number of users that can be accommodated within a given geographic region can be greatly expanded beyond that provided by conventional single channel communication systems. 
     Cellular communications utilize multiple beam antennas and multiple satellite channels to sub-divide a geographic region into a plurality of individual cells. Each cell within the geographic region may be assigned a frequency sub-band selected from a predetermined set of frequency sub-bands. Assignment of the frequency sub-bands typically follows a repeating pattern that results in no two adjacent cells having the same frequency sub-band assignment. This reuse of frequency sub-bands greatly expands the number of users that can be accommodated within a given geographic region. In a typical telephony application a bandwidth B may be divided into p sub-bands each having a bandwidth of B/p. If a region is sub-divided into N′ cells then each sub-band can be reused N′/p times. Thus, the total number of users that can be accommodated within the region is (N′/p)*(B/b), where b is the bandwidth of an individual user and the quantity N′/p represents the expansion factor provided by using a cellular-based communication system. For example, If N′=154, p=4, B=30 MHz, and b=6 KHz then the expansion factor equals 38.5 and the total number of possible users equals 192,000. In contrast, a single channel communication system covering the same region would have an expansion factor of 1 and could only accommodate 192,000/38.5 or approximately 5000 users. 
     To meet system interference requirements, cellular-based communication systems depend on low crosstalk between nearby cells that have been assigned the same frequency sub-band. In practice, the multiple beam antennas that are often used in cellular systems utilize a single reflector or lens and a group of tightly packed feeds in the antenna&#39;s focal plane. These tightly packed feeds create an inherent interference problem because the individual feeds have sidelobes exceeding system interference requirements. Several approaches to reducing this interference to acceptable levels are generally known in the art. 
     The effects of inter-cell interference due to imperfect feed characteristics can be reduced to acceptable levels by using beam forming systems. Beam forming systems synthesize an improved cell antenna pattern, having the required interference properties, by combining the weighted outputs of multiple feeds. These beam forming systems are usually on board the satellite or within a ground station of the cellular system. 
     Space-based beam forming systems perform the beam forming on board the satellite. Thus, the hardware associated with spaced-based beam forming systems is carried as a payload on board the satellite. There are several problems with space-based beam forming systems. Namely, conventional beam forming hardware consumes a large amount of electrical power, which is highly undesirable on board a satellite. Furthermore, beam forming hardware is heavy and occupies a significant amount of space, which is also undesirable because it significantly increases satellite production and launch costs. 
     Ground-based beam forming systems place the beam forming hardware within a ground station of the cellular system. These ground-based systems have the inherent advantage of not being subject to the weight and power restrictions found on board the satellite. 
     Conventional ground-based beam forming systems transmit individual feed signals from the satellite&#39;s receive antenna to the ground station via an additional gateway frequency band that contains tightly packed sub-bands. In a similar manner, the feed signals for the satellite&#39;s transmit antenna are sent up through another gateway frequency band after the feed signals are generated in the ground station from the beam inputs. 
     Conventional ground-based beam forming systems have reduced the cost, weight, and power consumption issues associated with having complex, dynamic beam forming hardware on board the satellite by moving this hardware to the ground station. Despite the benefits of ground-based beam forming, conventional systems still require a large amount of transmit and receive hardware to pass the individual feed signals through gateway uplinks and downlinks to and from the ground station. This transmit and receive hardware is carried on board the satellite as payload and is undesirable because of the weight, cost, and power consumption that it entails. The problem of satellite transmit and receive hardware payload complexity is compounded by the fact that using the aforementioned beam forming techniques to compensate for imperfect feed characteristics requires more feeds than would be required if feed patterns were ideal. For example, a circular region divided into N′ cells would require N=({square root over (N 1 )}+{square root over (M)}) 2  cells, where M equals the number of feeds used in synthesizing each beam. Thus, if N′=154 and M=50 then N=380 feeds. 
     Transmit and receive hardware payload complexity, and thus, its weight and cost, is a highly sensitive function of the number of feed signals that are transmitted across the satellite&#39;s gateway links. To properly implement a ground-based beam forming system, each feed signal must be channelized into all of the sub-bands used in the particular cellular system. Furthermore, additional redundant transmit and receive processing channels are typically provided for each feed signal. For example, if the cellular system utilizes four sub-bands and includes two redundant processing channels then each feed signal requires six channels of receive hardware for the user uplinks and six channels of transmit hardware for the user downlinks. Therefore, it can be immediately appreciated that reducing the number of feeds transmitted via the gateway links to and from the beam forming ground station can dramatically reduce the complexity of the satellite&#39;s transmit and receive hardware payload. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a satellite communications payload includes a set of user feeds coupled to a beam forming network. The beam forming network associates the feeds with a set of beamlets such that there are fewer beamlets than feeds. A multi-channel payload coupled to the beam forming network processes the beamlets. 
     The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a satellite communication system within which aspects of the present invention may be embodied; 
     FIG. 2 is a block diagram illustrating a communications payload in accordance with aspects of the present invention; 
     FIG. 3 is a block diagram of a beam forming network in accordance with the present invention; and 
     FIG. 4 is a block diagram of a multi-channel return payload that may be utilized with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Illustrated in FIG. 1 is a satellite communications system  10  within which aspects of the present invention may be utilized. A plurality of users at remotely disposed locations may use telephones  12 - 22  to communicate with each other via a communications satellite  24 . Each of the telephones  12 - 22  may be connected via conventional public switched telephone networks PSTNs  26 - 36  to respective satellite link or ground stations  38 - 48 . For example, a first user using a telephone  12  may converse with a second user at the remotely located telephone  18 . As the first user at telephone  12  speaks, his voice may be transferred, via the PSTN  26 , to a first satellite link or ground station  38 . In a known manner, the first satellite link station  38  encodes and upconverts the user&#39;s voice for uplink to the communications satellite  24  via user uplinks  50 . 
     A communications payload  52  of the satellite  24  processes the information uplinked from the first satellite link station  38  into a bandwidth-efficient format. This conversion process is well known in the current art. After the information is converted, it is downlinked via gateway downlinks  54  to a dynamic beam forming processor  56  located on the ground. The beam forming processor  56  processes the information received via the gateway downlinks  54  using known beam forming algorithms and uplinks the processed information to the satellite  24  via the gateway uplinks  58 . The payload  52  then transmits, via user downlinks  60 , the processed information to a second satellite link station  44 . The second satellite link station  44  performs the function of recovering the information originally sent from the first satellite link station  38 . Once the data is recovered, the second satellite link station  44  transfers the information to the telephone  18 , via the PSTN connection  32 . This satellite communication process takes place from a number of geographic areas using a number of different frequencies. Although the system shown in FIG. 1 is applicable to telephony transmissions in particular, it should be understood that the present invention is applicable to a wide variety of satellite communications systems other than telephony. 
     By way of example only, FIG. 2 provides a more detailed block diagram of the communications payload  52 , within which aspects of the present invention are embodied. The communications payload  52  includes a user uplinks sub-section  62  and a user downlinks sub-section  64 . 
     The user uplinks sub-section  62  includes a plurality of (N) user uplink feeds  70  coupled to low noise amplifiers (LNAs)  72 , a user uplinks beam forming network  74 , a plurality of (N′) intermediate uplink beamlet signals  76 , a multi-channel return payload  78 , and a plurality of gateway downlink feeds  80  that are driven by power amplifiers  82 . 
     Signals are transmitted from the satellite ground stations  38 - 48 , via the user uplinks  50  to the user uplink feeds  70  of the payload  52 . The user uplink feeds  70  couple the transmitted signals through LNAs  72  to the user uplinks beam forming network  74 . The user uplinks beam forming network  74  synthesizes the individual feed signals into the plurality of intermediate uplink beamlets  76 . Multiple feed signals are used to synthesize each of the intermediate uplink beamlet signals  76 , and there are fewer intermediate uplink beamlets  76  than uplink feed signals. For example, if there are N uplink feeds  70  the user uplink beam forming network  74  may synthesize N′ intermediate uplink beamlets  76 . The ratio N/N′ may typically be greater than two to one but could be much higher depending on the particular feed design and/or configuration used. In addition, the intermediate uplink beamlets  76  are synthesized so that the crosstalk between intermediate uplink beamlets  76  is significantly lower than the crosstalk between uplink feed signals. This allows a smaller number of uplink beamlets, having more ideal interference characteristics, to represent the geographic area covered by the user uplink feeds. 
     The intermediate uplink beamlet signals  76  are processed by the multi-channel return payload  78 . The multi-channel return payload channelizes and frequency shifts the intermediate uplink beamlets  76  into tightly a packed bandwidth of gateway sub-bands. The tightly packed signals are coupled through the power amplifiers  82  to the gateway downlink feeds  80  for transmission via the gateway downlinks  54  to the ground-based beam forming processor  56  (shown in FIG.  1 ). 
     The apparatus and operation of the user downlinks sub-section  64  is analogous to that of the user uplinks sub-section  62 . Tightly packed gateway signals are received via the gateway uplinks  58  and are coupled from a plurality of gateway uplink feeds  84 , through LNAs  72 , to a multi-channel forward payload  86 . The multi-channel forward payload  86  recomposes a plurality of intermediate downlink beamlets  88  that contain substantially the same information as contained within the user uplinks  50 . The intermediate downlink beamlets  88  are passed through a user downlinks beam forming network  90  that may be similar or identical to the user uplinks beam forming network  74 . The user downlinks beam forming network  90  recomposes a plurality of user downlink feed signals that are amplified by power amplifiers  82  and coupled through a plurality of user downlink feeds  92 . User downlink feeds  92  transmit the user downlink feed signals, via the user downlinks  60 , to one of the satellite ground stations  38 - 48 . 
     It is important to recognize that, in accordance with the present invention, the multi-channel return payload processes the intermediate beamlets  76  for transmission to the ground-based beam forming station  56  rather than directly processing the signals from the user uplink feeds  70 . Thus, a large amount of channelizing hardware within the multi-channel return payload  78  can be eliminated with respect to the amount of hardware that would be required to directly process and channelize feed signals received from all user uplink feeds  70 . The hardware complexity of the multi-channel forward payload  86  can be similarly reduced because it also processes and de-channelizes beamlets rather than the larger number of feed signals directly. Thus, the user uplink and downlink beam forming networks  74 ,  90  of the present invention interpose between the user feeds  70 ,  92  and the multi-channel return and forward payloads  78 ,  86  of the satellite  24 . These interposing beam forming networks  74 ,  90  reduce the number of signals processed by the multi-channel return and forward payloads  78 ,  86 , which significantly reduces the hardware complexity of these payloads. This reduction in complexity greatly reduces the power requirements, size, cost, and weight of the satellite&#39;s  24  overall hardware payload. 
     Now turning to FIG. 3, a more detailed block diagram of the user uplinks beam forming network  74  is shown. The user uplinks beam forming network  74  includes a plurality of power splitters  100 , a plurality of attenuators  102 , a plurality of phase shift blocks  104 , a signal routing and interconnections block  106 , and a plurality of power combiners  108 , all preferably arranged as shown. The various components of the user uplinks beam forming network  74  are generally known in the art and may be purchased as complete components and/or fabricated using waveguide technology, coaxial technology, or multi-layer printed wiring board technology, which are also known in the art. It should be appreciated that such beam forming networks may be made from passive components that do not require a DC power source for operation, which is a highly desirable characteristic for satellite hardware payloads. Alternatively, a wide variety of passive and/or active components may be utilized to perform the various functions of the beam forming networks  74 ,  90  without departing from the spirit of the invention. 
     The splitters  100  receive amplified user feed signals  110  from the LNAs  72  (shown in FIG. 2) and divide each of these incoming feed signals  110  equally M ways, where M is the number of beamlets that each feed signal is routed to. For example, if a given feed is used in forming ten of fifty total beamlets (i.e. M=10) then the splitter associated with that feed divides the incoming feed signal ten ways. The attenuators  102  and phase shift blocks  104  are used to generate a series of weighted terms based on the split feed signals. The particular weighting coefficients (i.e. phase and amplitude) are preferably selected a priori (but could also be adjusted dynamically) to produce an optimal set of terms that are used to form, via the combiners  108 , the intermediate beamlets  76 . The routings and interconnections block  106  routes the various weighted terms to the appropriate combiners  108  for summation. The routings and interconnections block  106  may be made from waveguide cables, multi-layer printed wiring boards, or other known techniques. The combiners  108  each receive M′ weighted terms for summation, where each of the M′ terms is a weighted term associated with one of the user feed signals  110 . For example, each of the combiners may sum twenty (i.e. M′=20) weighted feed signals to produce each of the intermediate beamlets  76 . Typically, each beamlet is the summation of a plurality of weighted feeds signals such that the interference of the original feed signals is substantially reduced. As is well known in the art, the sidelobes of a given feed signal may be substantially reduced or canceled by using other feeds having a main lobe aligned with the side lobes of the given feed signal. These other feeds may attenuated and phase shifted (e.g. 180° or out of phase) so that the main lobes when added in the combiners cancel the side lobes of the given feed signal. To achieve a desired final interference level, shaping the intermediate beamlets  76  may require the combination of many weighted terms. 
     The user downlinks beam forming network  90  is preferably identical to the user uplinks beam forming network  76 . In operation, signals pass through the user downlinks beam forming network  90  in a direction opposite that of those in the user uplinks beam forming network  76 . The beam forming networks  76 ,  90  may be fabricated using linear passive components, thereby providing reciprocal transfer functions for signals passing through them in one direction or the other. Namely, the intermediate downlink beamlets  88  pass through the user downlinks beam forming network  90  and are recomposed into a plurality of individual feed signals for transmission to the ground stations  38 - 48  via the user downlinks  60 . 
     Illustrated in FIG. 4 is a more detailed block diagram of the multi-channel return payload  78  that may be utilized with the present invention. Although the multi-channel return payload  78  is described below, it should be noted that it is only exemplary of communications hardware that is well known in the art. Also, the forward payload  88  is not described in detail because its apparatus and function are analogous to the return payload  78  and is similarly known in the art. 
     The multi-channel return payload  78  includes power splitters  200 , a first series of amplifier/mixer/filter stages  210 , a plurality of redundancy rings  220 , a second series of amplifier/mixer/filter stages  230 , a series of multiplexer/summer stages  240 , a third set of amplifier/mixer/filter stages  250 , and a final series of summers  260 , all preferably arranged as shown. 
     The intermediate uplink beamlet signals  76  from the user uplinks beam forming network  74  are coupled to the power splitters  200 , which divide the input power evenly among the amplifier/mixer/filter stages  210 . Each of the mixers may be operated at a different local oscillator frequency to allow for the demodulation of any of the possible frequency sub-bands associate with the cellular system  10 . The amplifier/mixer/filter stages  210  are each coupled to the redundancy rings  220 , which are switch networks that allow for selective coupling between the first series of amplifier/mixer/filter stages  210  and the second series of amplifier/mixer/filter stages  230 . The second series of amplifier/mixer/filters stages  230  downconvert the signals to a second intermediate frequency (IF). The downconverted signals are then coupled to the multiplexer/summer stages  240 . The third series of ampifier/mixer/filter stages  250  mixes the signals with another IF for transmission to the ground station via the gateway downlinks  54 . 
     Of course, it should be understood that a range of changes and modifications can be made to the preferred embodiment described above. For example, the present invention may also be utilized in conjuction with phased array antennas without departing from the spirit of the invention. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of this invention.