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. An apparatus comprising:
a QAM demapper that extracts a trellis coded data frame from modulation symbols in a signal received by a modem using transmission mode information provided in the signal, the transmission mode information comprising a trellis code rate and a QAM constellation used to produce the trellis coded data frame, wherein the transmission mode information is provided in a synchronization packet that comprises different binary sequences for real and imaginary parts of the modulation symbols; and
a Viterbi decoder that extracts a frame comprising a plurality of Reed-Solomon (RS) packets from the trellis coded data frame using the trellis code rate, wherein each of a sequence of frames extracted by the Viterbi decoder comprises a constant number of RS packets regardless of transmission mode of the each frame.
2. The apparatus of claim 1 , wherein each of the sequence of frames extracted by the Viterbi decoder comprises a variable integral number of QAM symbols.
3. The apparatus of claim 1 , 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.
4. A method for variable net bit rate digital communications comprising:
extracting a trellis coded data frame from modulation symbols in a signal received by a modem using transmission mode information provided in the signal, the transmission mode information comprising a trellis code rate and a QAM constellation used to produce the trellis coded data frame, wherein the transmission mode information is provided in a synchronization packet that comprises different binary sequences for real and imaginary parts of the modulation symbols; and
extracting a sequence of frames, each frame comprising a constant number of Reed-Solomon (RS) packets from the trellis coded data frame using the trellis code rate regardless of transmission mode of the each frame.
5. The method of claim 4 , wherein each of the sequence of frames comprises a variable integral number of QAM symbols.
6. The method of claim 4 , 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.
7. A non-transitory computer-readable medium having instructions stored thereon, wherein the instructions when executed by one or more processors of a processing circuit cause the processing circuit to:
extract a trellis coded data frame from modulation symbols in a signal received by a modem using transmission mode information provided in the signal, the transmission mode information comprising a trellis code rate and a QAM constellation used to produce the trellis coded data frame, wherein the transmission mode information is provided in a synchronization packet that comprises different binary sequences for real and imaginary parts of the modulation symbols; and
extract a sequence of frames, each frame comprising a constant number of Reed-Solomon (RS) packets from the trellis coded data frame using the trellis code rate regardless of transmission mode of the each frame.
8. The computer-readable medium of claim 7 , wherein each of the sequence of frames comprises a variable integral number of QAM symbols.
9. The computer-readable medium of claim 7 , 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.