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 system comprising a modem for transmitting frames of data in a digital signal, the modem comprising:
a convolutional byte interleaver that interleaves a frame of data, wherein the interleaver is synchronized to a frame structure;
a randomizer configured to produce a randomized data frame from the interleaved frame of data;
a punctured trellis code modulator operated at a selectable code rate that produces a trellis coded data frame from the randomized data frame, wherein the selectable code rate is adjusted to obtain an optimized net bit rate based on a measured white noise performance of the system;
a QAM mapper that maps groups of bits in the trellis coded data frame to modulation symbols, thereby providing a mapped frame; and
a synchronizer that adds a synchronization packet to the mapped frame.
2. The system of claim 1 , wherein the system generates a constant integer number of Reed-Solomon packets for each frame of data regardless of transmission mode.
3. The system of claim 1 , wherein the system generates a variable integer number of modulation symbols for each frame of data regardless of transmission mode.
4. The system of claim 1 , wherein the system generates an integer number of puncture pattern cycles per frame of data regardless of transmission mode.
5. A system comprising a modem for transmitting frames of data in a digital signal, the modem comprising:
a convolutional byte interleaver that interleaves a frame of data, wherein the interleaver is synchronized to a frame structure;
a randomizer configured to produce a randomized data frame from the interleaved frame of data;
a punctured trellis code modulator operated at a selectable code rate that produces a trellis coded data frame from the randomized data frame;
a QAM mapper that maps groups of bits in the trellis coded data frame to modulation symbols, thereby providing a mapped frame; and
a synchronizer that adds a synchronization packet to the mapped frame,
wherein the same synchronization packet is added to each of a sequence of mapped frames, and
wherein a portion of the synchronization packet comprises binary sequences for real and imaginary parts of the modulation symbols.
6. The system of claim 5 , wherein a portion of the synchronization packet comprises 127 symbols.
7. The system of claim 5 , wherein a portion of the synchronization packet comprises different binary sequences for the real and imaginary parts of the modulation symbols.
8. The system of claim 5 , 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.
9. The system of claim 8 , wherein the synchronization packet comprises data that indicates a transmission mode for the mapped frame.
10. The system of claim 9 , wherein the data indicating transmission mode includes a selected QAM constellation and a selected trellis code rate.