Frame structure for a QAM system
A novel framing method for a variable net bit rate digital communications system that utilizes a set of different QAM constellations and punctured trellis code combinations, each combination designated as a mode. This frame structure has a variable integral number of QAM symbols per frame depending on the selected mode, but the number of bytes and Reed-Solomon packets per frame is constant. This is achieved even though the number of data bits per QAM symbol for some modes is fractional. Also the number of trellis coder puncture pattern cycles per frame is an integer for all modes. This arrangement simplifies the synchronization of receiver processing blocks such as the Viterbi decoder, de-randomizer, byte de-interleaver, and Reed-Solomon decoder.
1. A method for variable net bit rate digital communications comprising:
generating a forward error correction (FEC) data frame from one or more interleaved Reed-Solomon (RS) packets, each RS packet encoding a plurality of data bytes;
modulating a plurality of bits of the FEC data frame using a punctured trellis coder that is operated at a selected code rate, the code rate being selected based on white noise performance of a digital communications system;
mapping the modulated bits to quadrature amplitude modulation (QAM) symbols using a QAM constellation selected from a set of candidate QAM constellations; and
providing a transmission frame comprising the QAM symbols and a indication of a transmission mode, the transmission mode identifying the selected code rate and the QAM constellation selected from the set of candidate QAM constellations,
wherein a constant number of RS packets is provided in each of a plurality of transmission frames regardless of transmission mode selected for the each transmission frame.
2. The method of claim 1 , wherein each of the plurality of transmission frames comprises a variable integral number of QAM symbols.
3. The method of claim 1 , wherein an integer number of Reed-Solomon packets is provided per transmission frame regardless of the transmission mode.
4. The method of claim 1 , wherein modulating the plurality of bits includes generating an integral number of puncture pattern cycles per frame of data regardless of the transmission mode.
5. The method of claim 4 , wherein a fractional number of data bits per QAM symbol is defined for one or more transmission modes.
6. The method of claim 5 , wherein a number of trellis coder puncture pattern cycles per frame is an integer regardless of transmission mode selected for the each transmission frame.
7. A method for variable net bit rate digital communications comprising:
synchronizing a convolutional byte interleaver to a frame structure;
providing a randomized frame to a punctured trellis code modulator, wherein the randomized frame is generated from an interleaved frame of data provided by the convolutional byte interleaver, and wherein the punctured trellis code modulator produces a trellis coded frame from the randomized frame at a code rate that is selected based on a measured white noise performance of a digital communications system;
causing a quadrature amplitude modulation (QAM) mapper to map groups of bits in the trellis coded frame to modulation symbols, thereby generating a mapped frame;
adding a synchronization packet to the mapped frame; and
communicating the mapped frame using the digital communications system.
8. The method of claim 7 , wherein adding the synchronization packet to the mapped frame includes adding the same synchronization packet to each of a sequence of mapped frames.
9. The method of claim 7 , wherein a portion of the synchronization packet comprises binary sequences for real and imaginary parts of the modulation symbols.
10. The method of claim 9 , wherein a portion of the synchronization packet comprises different binary sequences for the real and imaginary parts of the modulation symbols.
11. The method of claim 9 , wherein a portion of the synchronization packet comprises an identical binary sequence for both the real part and the imaginary part of the modulation symbols.
12. The method of claim 7 , wherein the synchronization packet comprises data that indicates a QAM constellation used to generate the mapped frame and a trellis code rate used to produce the trellis coded frame.
13. The method of claim 7 , wherein the interleaved frame of data is one of a sequence of interleaved frames of data derived from Reed-Solomon (RS) packets, each interleaved frame of data comprising a constant integer number of Reed-Solomon packets regardless of transmission mode.
14. The method of claim 7 , wherein the mapped frame is one of a sequence of mapped frames, each mapped frame comprising a variable integer number of modulation symbols regardless of transmission mode.
15. The method of claim 7 , wherein the punctured trellis code modulator produces trellis coded frames by generating an integer number of puncture pattern cycles per frame of data regardless of transmission mode.
16. A non-transitory processor-readable storage medium having one or more instructions which, when executed by at least one processing device, cause the at least one processing device to:
synchronize a convolutional byte interleaver to a frame structure;
provide a randomized frame to a punctured trellis code modulator, wherein the randomized frame is generated from an interleaved frame of data provided by the convolutional byte interleaver, and wherein the punctured trellis code modulator produces a trellis coded frame from the randomized frame at a code rate that is selected based on a measured white noise performance of a digital communications system;
cause a quadrature amplitude modulation (QAM) mapper to map groups of bits in the trellis coded frame to modulation symbols, thereby generating a mapped frame;
add a synchronization packet to the mapped frame; and
communicate the mapped frame using the digital communications system.
17. The non-transitory processor-readable storage medium of claim 16 , wherein the synchronization packet comprises data that indicates a QAM constellation used to generate the mapped frame and a trellis code rate used to produce the trellis coded frame.
18. The non-transitory processor-readable storage medium of claim 16 , wherein the interleaved frame of data is one of a sequence of interleaved frames of data derived from Reed-Solomon (RS) packets, each interleaved frame of data comprising a constant integer number of Reed-Solomon packets regardless of transmission mode.
19. The non-transitory processor-readable storage medium of claim 16 , wherein the mapped frame is one of a sequence of mapped frames, each mapped frame comprising a variable integer number of modulation symbols regardless of transmission mode.
20. The non-transitory processor-readable storage medium of claim 16 , wherein the punctured trellis code modulator produces trellis coded frames by generating an integer number of puncture pattern cycles per frame of data regardless of transmission mode.