Abstract:
A frame signal ( 100 ) for communicating payloads ( 104, 110 ) of data includes a first payload field ( 104 ) and a first header field ( 102 ) with a first frame type indicator ( 120 ). The frame signal ( 100 ) also includes a second payload field ( 110 ) and a second header field ( 108 ) smaller than the first header field ( 102 ) that includes a second frame type indicator ( 128 ). The first payload field ( 104 ), first header field ( 102 ), second payload field ( 110 ), and the second header field ( 108 ) are encapsulated in a single frame ( 100 ) to provide multiple payload delivery with reduced overhead compared to individually transmitted single payload frames.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to TRW Docket No. 22-0067, titled “Downlink Transmission and Reception Techniques for a Processing Communication Satellite” filed Sep. 29, 1999 as application No. 09/408,041, by inventors David A. Wright et al., and since issued as U.S. Pat No. 6,512,749 B1. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to satellite communication system downlink waveform structures. In particular, the present invention relates to an efficient satellite downlink waveform including multiple payloads per downlink frame which may be implemented using a hopping beam. 
     Satellites produce spot beam downlinks that communicate information in time division multiplexed (TDM) frames. In general, the frames include an overhead section and a payload section. The overhead section includes, for example, a guard band and synchronization bits while the payload section carries the “billable” or “useful” data bits. Every time the TDM downlink delivers a payload section in a new frame, the overhead section is retransmitted. Thus, reducing the ratio of overhead to billable data provides an opportunity for increasing the net revenue. 
     In general, these beams may be hopped in time such that any particular downlink beam may illuminate different geographical spots on the ground, called cells, at any particular time. Such hopping beams permit a single beam to provide broader geographical coverage with a single satellite transmitter chain, saving spacecraft size and weight. 
     For hopping beams, each beam hop must start with a retransmission of the synchronization sequence. However, any transmission of overhead information necessarily reduces useful data throughput. Such repetition is particularly undesirable in satellite communications, where bandwidth is extremely valuable and useful data throughput is critical to profitability. 
     In addition to reducing the useful information throughput, the duration of the overhead information represents a hand limit on the minimum delay between delivery of payload sections. In other words, delivery of data that must be split across payload sections in multiple frames incurs an additional delivery delay for every frame. Thus, large messages or data transfers invariably incur significant delivery delays according to the number of frames over which the message or data is distributed. 
     An additional issue arises for hopping beams in which the hopping pattern is influenced by the traffic demands. Since the hopping pattern may not be fixed, the ground terminal would need some knowledge of the hopping sequence in order to known which transmissions contain payloads destined for that terminal. One such method would be for a network controller to broadcast hop sequences to all terminals, but this entails significant overhead and control. Another approach would be to require each terminal to estimate received downlink power and process those TDMA hops for which the measured power exceeds some threshold. This method has the disadvantage in that terminals at ground cell boundaries may experience very small differences in received power between hops directed to it and hops directed to the adjacent ground cells. 
     A need exists in the industry for a downlink frame format that addresses the problems noted above and others previously experienced. 
     BRIEF SUMMARY OF THE INVENTION 
     A preferred embodiment of the present invention provides a downlink waveform for communicating payloads of data in a time division multiplexed frame stream. The frame signal includes a first payload field and a first header field with a first frame type indicator. The frame signal also includes a second payload field and a second header field smaller than the first header field that includes a second frame type indicator. The first payload field, first header field, second payload field, and the second header field are encapsulated in a single frame to provide a multiple payload frame with reduced overhead compared to individually transmitted single payload frames. 
     As will be explained in detail below, the first header field may include a hopping beam guard band with a duration encompassing circuit switching delay to hop a downlink beam between geographical areas, a masterframe count, a subframe count, a pseudorandom noise synchronization code, and a payload type indicator. The first and second payloads may be scrambled according to a pseudorandom noise scrambling sequence. The frame signal may be extended to N payloads with N header fields in a single frame that incurs less overhead than N separately transmitted frames carrying a single payload. 
     A preferred embodiment of the present invention also provides a method for transmitting a communication frame. The method steps include transmitting a first header field including a first frame type indicator and a first payload field. The method continues by transmitting a second header field smaller than the first header field and including a second frame type indicator followed by a second payload field. As noted above, the first payload field, first header field, second payload field, and the second header field are encapsulated in a single frame. 
     In another preferred embodiment of the present invention, a downlink frame processing module forms the frames. The downlink frame processing module includes an outer coder, an inner coder coupled to the outer coder, and a downlink frame organizer. The downlink frame organizer packages overlead data and coded data produced by the outer coder and inner coder sequence into a single frame as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary multiple payload frame signal. 
         FIG. 2  depicts a method for transmitting information to form a multiple payload frame. 
         FIG. 3  shows a pseudorandom noise scrambling sequence generator. 
         FIG. 4  shows a pseudorandom noise synchronization sequence generator. 
         FIG. 5  illustrates a downlink frame processing module. 
         FIG. 6  shows an implementation of a downlink hopping waveform transmission system. 
         FIG. 7  illustrates an implementation of a light convolutional encoder. 
         FIG. 8  shows an implementation of a heavy convolutional encoder. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to  FIG. 1 , that figure illustrates a multiple payload frame signal  100 . The frame  100  includes a first header field  102  followed by a first payload field  104  and a first flush field  106 . In addition, the frame format  100  includes a second header field  108  followed by a second payload field  110  and another flush field  112 . The first header field  102 , first payload field  104 , first flush field  106 , second header field  108 , second payload field  110 , and second flush field  112  are all encapsulated into the single frame  100 . 
     Continuing with reference to  FIG. 1 , the first header field  102  is composed of several subfields. In particular, the first header field  102  includes a hopping beam guard band  114 , a first payload pseudorandom noise (PN) synchronization field  116 , and a spare field  118 . The first header field  102  also includes a first frame type field  120 , a masterframe count field  122 , and a subframe count field  124 . 
     The second header section includes a smaller set of subfields, namely, the second PN synchronization field  126  and the second frame type field  128 . 
     Table 1, below, shows the preferred length and modulation of each field. Symbols are preferably transmitted at 196.7 megasymbols per second. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Field 
                 Symbols 
                 Modulation 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 first header 102 
                 368 
                   
               
               
                   
                 hopping beam guard band 114 
                 114 
                 BPSK 
               
               
                   
                 first payload PN synch 116 
                 64 
                 BPSK 
               
               
                   
                 spare 118 
                 62 
                 BPSK 
               
               
                   
                 first frame type 120 
                 32 
                 BPSK 
               
               
                   
                 masterframe count 122 
                 32 
                 BPSK 
               
               
                   
                 subframe count 124 
                 64 
                 BPSK 
               
               
                   
                 first payload 104 
                 7552 
                 QPSK 
               
               
                   
                 first flush 106 
                 16 
                 QPSK 
               
               
                   
                 second header 108 
                 96 
               
               
                   
                 second payload PN synch 126 
                 64 
                 BPSK 
               
               
                   
                 second frame type 128 
                 32 
                 BPSK 
               
               
                   
                 second payload 110 
                 7552 
                 QPSK 
               
               
                   
                 second flush 112 
                 16 
                 QPSK 
               
               
                   
                 TOTAL LENGTH 
                 15600 
               
               
                   
                   
               
             
          
         
       
     
     The hopping beam guard band  114  provides, in the preferred embodiment, approximately 580 ns of guard time. In general, however, the length of the hopping beam guard band  114  is selected to encompasses an expected circuit switching downlink beam hopping delay. The downlink beam hopping delay represents a worst case estimate of the amount of time that the satellite needs to redirect a downlink beam (i.e., “hop” the beam) to a different geographic area. 
     The first PN synchronization field  116  and the second PN synchronization field provide synchronization bits for earth terminals. As will be explained in more detail below, a single PN synchronization sequence generator is used to provide an identical PN sequence for both PN synchronization fields  116 ,  118 . The subframe count field  124  counts individual frames as they are transmitted. Preferably, the subframe count field  124  includes a 16 bit downlink frame count appended with 8 zeros and convolutionally encoded with a relatively heavy (e.g., ⅜ rate) code. When the subframe count field  124  reaches  9328 , for example, the masterframe count field  122  increments. The masterframe count rolls over after reaching its maximum value (0xFFFFFFFF), although it may be reset or preprogrammed at any time. 
     The space field  118  may be drawn from to provide subsequent enhancements to the frame  100  (e.g., additional synchronization bits). Preferably, the spare field  118 , the hopping beam guard band  114 , and first PN synchronization field  116  are filled with PN bits that are generated by a PN synchronization sequence generator discussed below. 
     The first frame type field  120  generally indicates characteristics of the first payload field  104 , while the second frame type field  128  generally indicates characteristics of the second payload field  110 . The frame type field may be coded using a rate ½ block code, such as an ( 8 ,  4 ) Reed-Muller code. Several examples of codes for the first and second frame type fields  120 ,  128  are illustrated below in Table 2. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Frame Type 
                 Uncoded Value 
                 Coded Value 
               
               
                   
                   
               
             
             
               
                   
                 Light Coding 
                 110 
                 00111100 
               
               
                   
                 Heavy Coding 
                 011 
                 10010110 
               
               
                   
                 Frame Gate 
                 001 
                 10100101 
               
               
                   
                 Power Gate 
                 000 
                 11110000 
               
               
                   
                   
               
             
          
         
       
     
     Although the light coding, heavy coding, and power gating options are with reference to a payload itself, the frame gate option indicates power gating of an entire frame (i.e., all 15600 symbols). Each coded value is preferably repeated four times in the frame type field. For example, a frame type of 00111100 00111100 00111100 00111100 in the first frame type field  120  incidates that the first payload field  104  is lightly coded. As another example, a frame type of 11110000 1111000 11110000 11110000 in the second frame type field  128  indicates that the second payload field  110  will be power gated. When a frame or payload field is power gated, only a small fraction of the ordinary output power will be generated in the downlink beam during for the entire frame, or during the identified payload(s). 
     With regard to the heavy coding and light coding, as examples, a lightly coded payload may indicate ¾ rate, constraint length  7 , punctured convolutional coding of 1416 Reed-Solomon block coded bytes. A heavily coded payload may indicate a ⅜ rate, constraint length  7 , punctured convolutional coding of 608 Reed-Solomon block coded bytes. Thus, the first and second payload fields remain the same size (7552 symbols) under both coding rates. 
     The first and second payload fields  104 ,  110  carry the “useful” data to the earth terminals. The first and second payload fields  104 ,  110  are typically concatenated coded using an inner convolutional code. The first and second flush fields  106 ,  112  are provided as a means to flush the last of the “useful” data bits from the spacecraft convolutional encoders, providing the earth terminal convolutional decoders opportunity to successfully decode the entire burst. 
     The frame signal  100  delivers multiple payloads (in the preferred embodiment, two payloads) in a single frame. Although a first header field  102  is provided as well as a second header field  108 , the second header field  108  is smaller than the first header field  102 . In particular, the second header field does not repeat the hopping beam guard band  114  (since the receiver(s) for the first and second payload fields  104 ,  110  are in the same beam spot for the current hop location), space field  118 , masterframe count  122  and subframe count  124  (since only one count is needed for the single multiple payload frame). 
     As a result, the frame  100  delivers two payloads in a single frame with less overhead than would be incurred by transmitting two single payload frames. Throughput is therefore higher. The specific frame format  100  shown in  FIG. 1  may be generalized to a single N payload N header frame, under the general condition that the sum of the overhead arising from the N headers is less than the sum of the overhead arising from N individual single payload frames. 
     In a typical implementation, multiple frames may be grouped into a master frame. This permits allocation of system resources and scheduling of system events on a longer time scale, if desired. In the preferred embodiment, 9328 frames make up a master frame. 
     Turning now the  FIG. 2 , that figure summarizes a method  200  for transmitting a multiple payload frame. The method includes outer coding  202  payload data with a Reed-Solomon code, interleaving  204  the data, scrambling  206  the data, and inner coding  208  the data with a rate ¾ or ⅜ rate convolutional code. While the multiple payload frame  100  does not necessarily require any coding, scrambling, or interleaving, the payload data is preferably thus conditioned for reliable transmission in the frame  100 . 
     A preferred form of the interleaving tables is presented below. Table 3 shows the manner in which the interleaving table is filled with lightly coded payload bytes, while Table 4 shows the manner in which QPSK I and Q bits are read out of that interleaving table. Similarly, Table 5 shows the manner in which the interleaving table is filled with heavily coded payload bytes, while Table 6 shows the manner in which QPSK I and Q bits are read out of that interleaving table. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Byte Input (Light) 
               
             
          
           
               
                   
                 Col 0 
                 Col 1 
                 Col 2 
                 Col 3 
                 . . . 
                 Col 234 
                 Col 235 
               
               
                   
                   
               
             
          
           
               
                 Row 0 
                 0 
                 1 
                 2 
                 3 
                 . . . 
                 234 
                 235 
               
               
                 Row 1 
                 236 
                 237 
                 238 
                 239 
                 . . . 
                 470 
                 471 
               
               
                 Row 2 
                 472 
                 473 
                 474 
                 475 
                 . . . 
                 706 
                 707 
               
               
                 Row 3 
                 708 
                 709 
                 710 
                 711 
                 . . . 
                 942 
                 943 
               
               
                 Row 4 
                 944 
                 945 
                 946 
                 947 
                 . . . 
                 1178 
                 1179 
               
               
                 Row 5 
                 1180 
                 1181 
                 1182 
                 1183 
                 . . . 
                 1414 
                 1415 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 I &amp; Q Output (Light) 
               
             
          
           
               
                   
                 Col 0 
                 Col 1 
                 Col 2 
                 Col 3 
                 . . . 
                 Col 234 
                 Col 235 
               
               
                   
                   
               
             
          
           
               
                 Row 0 
                 I0 
                 Q0 
                 I6 
                 Q6 
                 . . . 
                 I702 
                 Q702 
               
               
                 Row 1 
                 I1 
                 Q1 
                 I7 
                 Q7 
                 . . . 
                 I703 
                 Q703 
               
               
                 Row 2 
                 I2 
                 Q2 
                 I8 
                 Q8 
                 . . . 
                 I704 
                 Q704 
               
               
                 Row 3 
                 I3 
                 Q3 
                 I9 
                 Q9 
                 . . . 
                 I705 
                 Q705 
               
               
                 Row 4 
                 I4 
                 Q4 
                  I10 
                  Q10 
                 . . . 
                 I706 
                 Q706 
               
               
                 Row 5 
                 I5 
                 Q5 
                  I11 
                  Q11 
                 . . . 
                 I707 
                 Q707 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Byte Input (Heavy) 
               
             
          
           
               
                   
                 Col 0 
                 Col 1 
                 Col 2 
                 Col 3 
                 . . . 
                 Col 234 
                 Col 235 
               
               
                   
                   
               
             
          
           
               
                 Row 0 
                 0 
                 1 
                 2 
                 3 
                 . . . 
                 234 
                 235 
               
               
                 Row 1 
                 236 
                 237 
                 238 
                 239 
                 . . . 
                 470 
                 471 
               
               
                 Row 2 
                 472 
                 473 
                 474 
                 475 
                 . . . 
                 706 
                 707 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 I &amp; Q Output (Heavy) 
               
             
          
           
               
                   
                 Col 0 
                 Col 1 
                 Col 2 
                 Col 3 
                 . . . 
                 Col 234 
                 Col 235 
               
               
                   
                   
               
             
          
           
               
                 Row 0 
                 I0 
                 Q0 
                 I3 
                 Q3 
                 . . . 
                 I351 
                 Q351 
               
               
                 Row 1 
                 I1 
                 Q1 
                 I4 
                 Q4 
                 . . . 
                 I352 
                 Q352 
               
               
                 Row 2 
                 I2 
                 Q2 
                 I5 
                 Q5 
                 . . . 
                 I353 
                 Q353 
               
               
                   
               
             
          
         
       
     
     Continuing with reference to  FIG. 2 , the method  200  transmits  210  the first header field  202 , transmits  212  the first payload field  104 , and transmits  214  the first flush bits. Subsequently, the method  200  transmits  216  the second header field  108 , transmits  218  the second payload field  110 , and transmits  220  the second payload flush field  112 . As noted above, each of the fields are encapsulated into a single downlink frame. Thus, after the second payload flush field  112  is transmitted, the second prepares and sends the next multiple payload frame, starting at step  202 . 
     Turning now to  FIG. 3 , that figure illustrates a preferred embodiment of a PN scrambling sequence generator  300 . The generator  300  includes a serially connected set of shift registers (e.g., the shift registers  302 ,  304 ,  306 , and  308 ). The output of registers  302 – 308  are input to an XOR gate  310  which produces PN bits on the pseudorandom noise scrambling sequence output  312 . The sequence output  312  connects to the Q bit XOR gate  314  as well as the I bit XOR gate  316 . Thus, payload data pulled out of the interleaving tables as I and Q bits presented in scrambled form on the scrambled Q output  318  and the scrambled I output  320 . 
     The PN bits on the scrambling sequence output  312 , as illustrated in  FIG. 3 , correspond to the generator polynomial x^16+x^15+x^13+x^4+1. Other generator polynomials may be used, however. In general, the scrambling sequence generator  300  is preloaded at the beginning of each frame  100  to an initial state. The initial state may vary from frame to frame, for example, based on the current downlink beam hop location. In addition, the sequential state may be varied from masterframe to masterframe in a sequence defined by an algorithm controlled by a key. Varying the initial state provides a means to limit system access only to authorized user terminals, i.e., those terminals with the current keys. Note also that when no scrambling is desired, the scrambling sequence generator  300  may be preloaded with zeros. 
     As noted above, the first and second payload PN synchronization fields  116 ,  126  provide the ground terminal with a synchronization reference.  FIG. 4  shows a Gold Code PN synchronization sequence generator  400  that may be used to generate the PN synchronization bits. The sequence generator  400  includes a first PN code generator  402  and a second PN code generator  404 . The first PN code generator  402  implements the polynomial 1+X+X^6, while the second PN code generator  404  implements the polynomial 1+X+X^3+X^4+X^6. The outputs of the first and second PN code generators  402 ,  404  are coupled to the XOR gate  406  which produces PN bits on the PN synchronization sequence output  408 . 
     As with the scrambling sequence generator  300 , the PN synchronization sequence generator  400  may be preloaded at the beginning of each frame  100  to an initial state. The initial state may vary from frame to frame, for example, based on the current downlink beam hop location. Additionally, the first PN code generator  402  may be seeded independently of the second PN code generator  404 . Either or both the first and second PIN code generators  402 ,  404  may be used to provide a particular PN bit output when the downlink beam hops to a first location, and a second PN bit output when the downlink hops to a second location. In the preferred approach, the spacecraft would be programmed to provide orthogonal sequences in frames destined to different hops. The embodiment of the sequence generators  402  and  404  provides for a multitude of possible sequences. Such an approach permits the earth terminals to correlate on the synchronization pattern and to compare the correlated value against a threshold to determine whether a downlink hop is intended for that earth terminal. Using orthogonal synchronization sequences maximizes the difference between correlation results between even and odd downlink hops, and resolves the aforementioned ambiguity which results from a terminal being situated on hop boundaries. Using orthogonal synchronization sequences permits the network to adaptively alter the hopping pattern to reflect traffic demands without complex coordination. An adaptive hopping pattern permits a higher downlink efficiency. 
     Preferably, the sequence generator  400  runs during, and provides PN bits for, the guard band  114 , first payload PN synchronization field  116 , and the space field  118 . The sequence generator  400  is then halted until PN bits are needed for the second payload synchronization field  126 , at which time the sequence generator  400  continues. Once the sequence generator  400  generates the PN bits for the second payload synchronization field  126 , the sequence generator  400  is halted until the next frame. The PN bits for the guard band  114  and the space field  118  ensure that the power spectral density after modulation and transmission, is within acceptable limits. 
     Note that as illustrated, the period of the sequence generator  400  is  63 . Note also that  126  BPSK symbols or 126 bits separate the first payload PN synchronization field  116  and the second payload synchronization field  126  (from the point of view of the sequence generator  400 ). Thus, the sequence generator  400  provides PN bits in the second payload PN synchronization field  126  that are identical to the PN bits provided for the first payload PN synchronization field  116  because it continues to run over the spare field  118 . 
     A single PN sequence generator  400  thereby provides the ground terminals with multiple opportunities to acquire synchronization from a single frame. Only a single seed need be provided on the satellite and on the ground (although additional seeds may be used if desired). An additional benefit is that the earth terminals can distinguish one frame from the next, as the boundaries of each frame will encompass two identical PN synchronization fields. Furthermore, although the downlink beam may hop between two geographic locations (and thereby be absent from a cell for an entire frame time), twice as many synchronization fields are provided. In other words, the average time between synchronization opportunities is the same as with a single header and payload non-hopping downlink beam. 
     Turning next to  FIG. 5 , that figure illustrates a downlink frame processing module  500  that produces the frame  100 . The processing module  500  includes a data memory  502 , an outer coder  504 , and an interleaver  506 . The processing module  500  also includes a scrambler  508 , an I convolutional encoder  510  and corresponding Q convolutional encoder  512 , and a downlink frame organizer  514 . A data and control bus  516  provides signals that control the operation of the processing module  500 , including, for example, selection of convolutional encoder rates and provision of frame overhead information (e.g., frame type) for the downlink frame organizer  514 . 
     The data memory  502  preferably stores 53 byte ATM cells. However, any particular data former may be used to supply the outer coder  504  (e.g., a Reed-Solomon encoder) with data. The interleaver  506 , which accepts outer coded data, operates as shown above in Tables 3–6 to interleave the data bits and reduce the detrimental effects of burst errors that may occur after transmission. 
     After the processing module  500  reads I and Q data bits out of the interleaver  506 , the I and Q data bits are scrambled in the scrambler  508 . The scrambler  508  may be implemented as illustrated in  FIG. 5  and described above. The processing module  500  then convolutionally encodes the scrambled I and Q data bits and presents the coded data to the downlink frame organizer  514  on the coded data outputs  518 ,  520 . 
     The downlink frame organizer  514  packages the coded data into the frame  100  according to the format shown in  FIG. 1 . In addition, the downlink frame organizer  502  may also maintain internally, or receive over the control and data bus  516 , the masterframe count and subframe count. Similarly, the downlink frame organizer  514  may accept the coded first and second frame types for packaging into the frame  100 . The downlink frame organizer  514 , as it builds the frame  100 , passes frame bits to an RF modulator, e.g., a QPSK or staggered QPSK modulator (not shown), that creates the frame waveform for amplification and transmission. 
     With regard to  FIG. 6 , a more specific implementation of a downlink hopping waveform transmission system  600  is shown. The transmission system  600  includes a data scheduler  602 , a data router  604 , and a waveform processing chain including a QPSK modulator  606 , an upconverter  608 , and a traveling wave tube amplifier (TWTA)  610 . A ferrite switch  612  directs the waveform to be transmitted through either the first feed path  601  or the second feed path  603 . 
       FIG. 6  also shows a control output  616  (that may be used to carry, as examples a power gating signal and a beam hopping selection signal), two frequency selection inputs  618  and  620  for the modulator  606 , a feed path selection input  622 , and an intermediate waveform output  624  from the modulator. Preferably, additional ferrite switches  626  and  628  in the feed paths  601 ,  603  provide additional signal isolation (e.g., approximately 20 dB between input and output when the ferrite switch is off). In other words, the additional ferrite switches  601 ,  603  operate in response to the control output  616  to pass or block a waveform to be transmitted through the feed paths  601 ,  603 . For example, if the RF waveform is destined for the feed path  601 , then the ferrite switch  628  is switched to the ground load  632 . If the RF waveform is destined for the feed path  603 , then the ferrite switch  626  is switched to the ground load  630 . 
     During operation, the transmission system  600  accepts baseband data from the router  604  (e.g., an ATM cell router), and creates a waveform to be transmitted using the waveform processing chain. The waveform processing starts by directly converting baseband I and Q data to an intermediate frequency of, for example, 750 MHz. The waveform processing then selects one of F1 (e.g., 3.175 MHz) and F2 (e.g., 3.425) and one of F3 (e.g., 16 GHz) and F4 (e.g., 17.4 GHz) to produce a waveform to be transmitted with a final center frequency at one of 18.425 GHz, 18.675 GHz, 19.825 GHz, and 20.075 GHz. The scheduler  602  monitors the propagation of data through the waveform processing chain and determines when certain frame signals should be power gated. To that end, the scheduler  602  provides a power gating signal on the control output  616  that is active when power gating is to occur. 
     The TWTA  610  amplifiers the waveform to be transmitted, while the switch  612  determines along which feed path  601 – 603  (or additional feed paths) the amplified waveform will propagate. For this reason, the switch  612  includes the feed path selection input  622  responsive to information on the control output  616 . Because the feed paths  601 – 603  are generally (though not necessarily) associated with the feed horns that produce spot beams in geographically distinct terrestrial cells, the feed path selection input  622  acts to determine the hop location of the downlink waveform. Thus, the downlink manifests itself as a beam spot that, typically, provides bandwidth for multiple terrestrial cells by hopping between them. 
     Turning next to  FIG. 7 , that figure illustrates a light coding convolutional encoder  700 . The convolutional encoder  700  provides a ¾ rate, constraint length  7  convolutional code with a puncturing pattern of |g0|g1|g1g0| read right to left, where “|” delimits bit input epochs. The modulo two adders G0 and G1, and shift register  702  implement:
         C01=B1+S5+S4+S3+S0   C11=B1+S4+S3+S1+S0   C12=B2+S5+S4+S2+S1   C03=B3+B2+B1+S5+S2       

     With generators G0=[1111001] and G1=[1011011]. 
     With regard to  FIG. 8 , that figure illustrates a heavy coding convolutional encoder  800 . The convolutional encoder  800  provides a ⅜ rate, constraint length  7  convolutional code with a puncturing pattern of |g1g0|g2g0|g2g1g0| read right to left, where “|” delimits bit input epochs. The modulo two adders G0, G1, G2, and shift register  802  implement:
         C01=B1+S5+S4+S3+S0   C11=B1+S4+S3+S1+S0   C21=B1+S5+S4+S2+S0   C02=B2+B1+S5+S4+S1   C22=B2+B1+S5+S3+S1   C03=B3+B2+B1+S5+S2   C13=B3+B1=S5+S3+S2       

     With generators G0=[1111001]G1=[1011011], G2=[1110101]. 
     While the invention has been described with reference to preferred embodiments, those skilled in the art will understand that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular step, structure, or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.