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True bit level decoding of TTCM (Turbo Trellis Coded Modulation) of variable rates and signal constellations. A decoding approach is presented that allows for decoding on a bit level basis that allows for discrimination of the individual bits of a symbol. Whereas prior art approaches typically perfo

True bit level decoding of TTCM (Turbo Trellis Coded Modulation) of variable rates and signal constellations. A decoding approach is presented that allows for decoding on a bit level basis that allows for discrimination of the individual bits of a symbol. Whereas prior art approaches typically perform decoding on a symbol level basis, this decoding approach allows for an improved approach in which the hard decisions/best estimates may be made individually for each of the individual bits of an information symbol. In addition, the decoding approach allows for a reduction in the total number of calculations that need to be performed as well as the total number of values that need to be stored during the iterative decoding. The bit level decoding approach is also able to decode a signal whose code rate and/or signal constellation type (and mapping) may vary on a symbol by symbol basis.

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What is claimed is: 1. A bit level decoding method, comprising: receiving a signal that includes a symbol having a plurality of bits; extracting I,Q (In-phase, Quadrature) components of the symbol; calculating a plurality of symbol metrics for the symbol using the I,Q components; decomposing the pl

What is claimed is: 1. A bit level decoding method, comprising: receiving a signal that includes a symbol having a plurality of bits; extracting I,Q (In-phase, Quadrature) components of the symbol; calculating a plurality of symbol metrics for the symbol using the I,Q components; decomposing the plurality of symbol metrics into a plurality of bit metrics; wherein the plurality of bit metrics is representative of the individual bits of the plurality of bits of the symbol; performing iterative decoding using the plurality of bit metrics; wherein the plurality of bit metrics is updated during each iteration of the iterative decoding for use in a current decoding iteration, such that each respective bit metric of the plurality of bit metrics is updated using extrinsic information corresponding to that respective bit from a previous decoding iteration; making soft bit decisions that correspond to the individual bits of the plurality of bits of the symbol; and making hard bit decisions, based on the soft bit decisions, that correspond to best estimates of bit values of the individual bits of the plurality of bits of the symbol; and wherein: the signal is a variable code rate signal whose code rate varies on a symbol by symbol basis; the symbol within the signal is encoded according to a first RC (Rate Control); at least one additional symbol within the signal is encoded according to a second RC; the first RC includes a first modulation having a first constellation and a first mapping; and the second RC includes a second modulation having a second constellation and a second mapping. 2. The method of claim 1, wherein the signal is coded using at least one of TCM (Trellis Coded Modulation) and TTCM (Turbo Trellis Coded Modulation). 3. The method of claim 1, wherein the signal is coded using TTCM (Turbo Trellis Coded Modulation); and the performing iterative decoding using the plurality of bit metrics is performed using a top SISO (Soft-In Soft-Out decoder), a bottom SISO, an interleaver, and a de-interleaver. 4. The method of claim 1, wherein the signal is coded using TTCM (Turbo Trellis Coded Modulation); and the performing iterative decoding using the plurality of bit metrics is performed using one SISO (Soft-In Soft-Out decoder) and an interleaver/de-interleaver. 5. The method of claim 1, wherein the iterative decoding includes performing MAP (maximum a posteriori probability) decoding. 6. The method of claim 1, wherein: the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a pseudo bit metric for an LSB (Least Significant Bit) of the plurality of bits; the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a bit metric for the LSB of the plurality of bits; and the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a bit metric for an MSB (Most Significant Bit) of the plurality of bits. 7. The method of claim 1, wherein: the plurality of bit metrics is a plurality of state dependent bit metrics; and the decomposing the plurality of symbol metrics into the plurality of bit metrics involves converting the plurality of state dependent bit metrics to a plurality of state independent bit metrics. 8. The method of claim 1, wherein the signal is received from an AWGN (Additive White Gaussian Noise) communication channel. 9. The method of claim 1, wherein: the signal includes a plurality of symbols arranged in a frame; and the signal is a variable code rate signal whose code rate varies on a symbol by symbol basis within the frame. 10. The method of claim 1, wherein: the method is performed within an HDTV (High Definition Television) set top box receiver. 11. The method of claim 1, wherein: the method is performed within a DWDM (Dense Wavelength Division Multiplexing) line card that is coupled to a fiber-optic communication channel. 12. The method of claim 1, wherein at least one of the first modulation and the second modulation is at least one of a BPSK (Binary Phase Shift Key) modulation, a QPSK (Quadrature Phase Shift Key) modulation, an 8 PSK (8 Phase Shift Key) modulation, a 16 QAM (Quadrature Amplitude Modulation) modulation, and a 16 APSK (Asymmetric Phase Shift Keying) modulation. 13. The method of claim 1, wherein: the first modulation is a QPSK (Quadrature Phase Shift Key) modulation having a QPSK constellation and the first mapping; and the second modulation is a QPSK modulation having a QPSK constellation and the second mapping. 14. The method of claim 1, wherein: the method is performed within a decoder; and the decoder is implemented within at least one of a satellite communication system, an HDTV (High Definition Television) communication system, a cellular communication system, a microwave communication system, a point-to-point communication system, and a TTCM (Turbo Trellis Coded Modulation) communication system. 15. The method of claim 1, wherein the iterative decoding further comprises calculating a plurality of forward metrics (alphas) and a plurality of backward metrics (betas) using the plurality of bit metrics; and wherein each alpha and each beta corresponds to one individual bit of the plurality of bits of the symbol. 16. The method of claim 15, wherein the iterative decoding further comprises calculating a plurality of extrinsic values using the plurality of bit metrics, the plurality of forward metrics (alphas), and the plurality of backward metrics (betas); and wherein each extrinsic value of the plurality of extrinsic values corresponds to one individual bit of the plurality of bits of the symbol. 17. The method of claim 16, wherein the plurality of extrinsic values that are calculated during a first iteration of the iterative decoding are employed as APP (a priori probability) during a second iteration of the iterative decoding. 18. The method of claim 1, further comprising mapping the plurality of bit metrics to a plurality of trellis metrics according to the first RC and the second RC. 19. The method of claim 18, wherein the plurality of trellis metrics are mapped according to an 8 state trellis of a rate 1/2 encoder. 20. The method of claim 19, wherein the rate 1/2 encoder is a convolutional encoder. 21. A bit level decoding method, comprising: receiving a signal that includes a symbol having a plurality of bits; extracting I,Q (In-phase, Quadrature) components of the symbol; calculating a plurality of symbol metrics for the symbol using the I,Q components; decomposing the plurality of symbol metrics into a plurality of bit metrics; wherein the plurality of bit metrics is representative of the individual bits of the plurality of bits of the symbol; performing iterative decoding using the plurality of bit metrics; wherein the plurality of bit metrics is updated during each iteration of the iterative decoding for use in a current decoding iteration, such that each respective bit metric of the plurality of bit metrics is updated using extrinsic information corresponding to that respective bit from a previous decoding iteration; making soft bit decisions that correspond to the individual bits of the plurality of bits of the symbol; and making hard bit decisions, based on the soft bit decisions, that correspond to best estimates of bit values of the individual bits of the plurality of bits of the symbol; and wherein: the signal is a variable code rate signal whose code rate varies on a symbol by symbol basis; the symbol within the signal is encoded according to a first RC (Rate Control); at least one additional symbol within the signal is encoded according to a second RC; the iterative decoding further comprises calculating a plurality of forward metrics (alphas) and a plurality of backward metrics (betas) using the plurality of bit metrics; each alpha and each beta corresponds to one individual bit of the plurality of bits of the symbol; the iterative decoding further comprises calculating a plurality of extrinsic values using the plurality of bit metrics, the plurality of forward metrics (alphas), and the plurality of backward metrics (betas); each extrinsic value of the plurality of extrinsic values corresponds to one individual bit of the plurality of bits of the symbol; the plurality of extrinsic values that are calculated during a first iteration of the iterative decoding are employed as APP (a priori probability) during a second iteration of the iterative decoding; the first RC includes a first modulation having a first constellation and a first mapping; and the second RC includes a second modulation having a second constellation and a second mapping. 22. The method of claim 21, wherein the iterative decoding includes performing MAP (maximum a posteriori probability) decoding. 23. The method of claim 21, wherein: the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a pseudo bit metric for an LSB (Least Significant Bit) of the plurality of bits; the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a bit metric for the LSB of the plurality of bits; and the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a bit metric for an MSB (Most Significant Bit) of the plurality of bits. 24. The method of claim 21, wherein: the plurality of bit metrics is a plurality of state dependent bit metrics; and the decomposing the plurality of symbol metrics into the plurality of bit metrics involves converting the plurality of state dependent bit metrics to a plurality of state independent bit metrics. 25. The method of claim 21, further comprising mapping the plurality of bit metrics to a plurality of trellis metrics according to the first RC and the second RC; and wherein: the plurality of trellis metrics are mapped according to an 8 state trellis of a rate 1/2 encoder; and the rate 1/2 encoder is a convolutional encoder. 26. The method of claim 21, wherein: at least one of the first modulation and the second modulation is at least one of a BPSK (Binary Phase Shift Key) modulation, a QPSK (Quadrature Phase Shift Key) modulation, an 8 PSK (8 Phase Shift Key) modulation, a 16 QAM (Quadrature Amplitude Modulation) modulation, and a 16 APSK (Asymmetric Phase Shift Keying) modulation. 27. The method of claim 21, wherein: the first modulation is a QPSK (Quadrature Phase Shift Key) modulation having a QPSK constellation and the first mapping; and the second modulation is a QPSK modulation having a QPSK constellation and the second mapping. 28. The method of claim 21, wherein: the method is performed within a decoder; and the decoder is implemented within at least one of a satellite communication system, an HDTV (High Definition Television) communication system, a cellular communication system, a microwave communication system, a point-to-point communication system, and a TTCM (Turbo Trellis Coded Modulation) communication system. 29. The method of claim 21, wherein the signal is coded using TTCM (Turbo Trellis Coded Modulation); and the performing iterative decoding using the plurality of bit metrics is performed using a top SISO (Soft-In Soft-Out decoder), a bottom SISO, an interleaver, and a de-interleaver. 30. The method of claim 21, wherein the signal is coded using TTCM (Turbo Trellis Coded Modulation); and the performing iterative decoding using the plurality of bit metrics is performed using one SISO (Soft-In Soft-Out decoder) and an interleaver/de-interleaver. 31. A bit level decoding method, comprising: receiving a signal that includes a symbol having a plurality of bits; extracting I,Q (In-phase, Quadrature) components of the symbol; mapping the symbol to a constellation point; calculating a squared Euclidean distance for the symbol thereby generating an intermediate metric of the symbol; directly calculating a plurality of bit metrics, using the intermediate metric; wherein the plurality of bit metrics is representative of the individual bits of the plurality of bits of the symbol; performing iterative decoding using the plurality of bit metrics; wherein the plurality of bit metrics is updated during each iteration of the iterative decoding for use in a current decoding iteration, such that each respective bit metric of the plurality of bit metrics is updated using extrinsic information corresponding to that respective bit from a previous decoding iteration; making soft bit decisions, using the plurality of bit metrics, that correspond to the individual bits of the plurality of bits of the symbol; and making hard bit decisions, based on the soft bit decisions, that correspond to best estimates of bit values of the individual bits of the plurality of bits of the symbol; and wherein: the signal is a variable code rate signal whose code rate varies on a symbol by symbol basis; the symbol within the signal is encoded according to a first RC (Rate Control); at least one additional symbol within the signal is encoded according to a second RC; the first RC includes a first modulation having a first constellation and a first mapping; and the second RC includes a second modulation having a second constellation and a second mapping. 32. The method of claim 31, wherein the plurality of bit metrics is calculated using min* processing. 33. The method of claim 31, wherein the signal is coded using at least one of TCM (Trellis Coded Modulation) and TTCM (Turbo Trellis Coded Modulation). 34. The method of claim 31, wherein the signal is coded using TTCM (Turbo Trellis Coded Modulation); and the performing iterative decoding using the plurality of bit metrics is performed using a top SISO (Soft-In Soft-Out decoder), a bottom SISO, an interleaver, and a de-interleaver. 35. The method of claim 31, wherein the signal is coded using TTCM (Turbo Trellis Coded Modulation); and the performing iterative decoding using the plurality of bit metrics is performed using one SISO (Soft-In Soft-Out decoder) and an interleaver/de-interleaver. 36. The method of claim 31, wherein the iterative decoding includes performing MAP (maximum a posteriori probability) decoding. 37. The method of claim 31, wherein: the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a pseudo bit metric for an LSB (Least Significant Bit) of the plurality of bits; the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a bit metric for the LSB of the plurality of bits; and the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a bit metric for an MSB (Most Significant Bit) of the plurality of bits. 38. The method of claim 31, wherein: the plurality of bit metrics is a plurality of state dependent bit metrics; and the decomposing the plurality of symbol metrics into the plurality of bit metrics involves converting the plurality of state dependent bit metrics to a plurality of state independent bit metrics. 39. The method of claim 31, wherein the signal is received from an AWGN (Additive White Gaussian Noise) communication channel. 40. The method of claim 31, wherein: the signal includes a plurality of symbols arranged in a frame; and the signal is a variable code rate signal whose code rate varies on a symbol by symbol basis within the frame. 41. The method of claim 31, wherein: the method is performed within an HDTV (High Definition Television) set top box receiver. 42. The method of claim 31, wherein: the method is performed within a DWDM (Dense Wavelength Division Multiplexing) line card that is coupled to a fiber-optic communication channel. 43. The method of claim 31, wherein at least one of the first modulation and the second modulation is at least one of a BPSK (Binary Phase Shift Key) modulation, a QPSK (Quadrature Phase Shift Key) modulation, an 8 PSK (8 Phase Shift Key) modulation, a 16 QAM (Quadrature Amplitude Modulation) modulation, and a 16 APSK (Asymmetric Phase Shift Keying) modulation. 44. The method of claim 31, wherein: the first modulation is a QPSK (Quadrature Phase Shift Key) modulation having a QPSK constellation and the first mapping; and the second modulation is a QPSK modulation having a QPSK constellation and the second mapping. 45. The method of claim 31, wherein: the method is performed within a decoder; and the decoder is implemented within at least one of a satellite communication system, an HDTV (High Definition Television) communication system, a cellular communication system, a microwave communication system, a point-to-point communication system, and a TTCM (Turbo Trellis Coded Modulation) communication system. 46. The method of claim 31, further comprising mapping the plurality of bit metrics to a plurality of trellis metrics according to the first RC and the second RC. 47. The method of claim 46, wherein the plurality of trellis metrics are mapped according to an 8 state trellis of a rate 1/2 encoder. 48. The method of claim 47, wherein the rate 1/2 encoder is a convolutional encoder. 49. The method of claim 31, wherein the iterative decoding further comprises calculating a plurality of forward metrics (alphas) and a plurality of backward metrics (betas) using the plurality of bit metrics; and wherein each alpha and each beta corresponds to one individual bit of the plurality of bits of the symbol. 50. The method of claim 49, wherein the plurality of forward metrics (alphas) and a plurality of backward metrics (betas) are calculated using min* processing. 51. The method of claim 49, wherein the iterative decoding further comprises calculating a plurality of extrinsic values using the plurality of bit metrics, the plurality of forward metrics (alphas), and the plurality of backward metrics (betas); and wherein each extrinsic value of the plurality of extrinsic values corresponds to one individual bit of the plurality of bits of the symbol. 52. The method of claim 51, wherein the plurality of extrinsic values is calculated using min* processing. 53. The method of claim 51, wherein the plurality of extrinsic values that are calculated during a first iteration of the iterative decoding are employed as APP (a priori probability) during a second iteration of the iterative decoding. 54. A bit level decoding method, comprising: receiving a signal that includes a symbol having a plurality of bits; extracting I,Q (In-phase, Quadrature) components of the symbol; mapping the symbol to a constellation point; calculating a squared Euclidean distance for the symbol thereby generating an intermediate metric of the symbol; directly calculating a plurality of bit metrics, using the intermediate metric; wherein the plurality of bit metrics is representative of the individual bits of the plurality of bits of the symbol; performing iterative decoding using the plurality of bit metrics; wherein the plurality of bit metrics is updated during each iteration of the iterative decoding for use in a current decoding iteration, such that each respective bit metric of the plurality of bit metrics is updated using extrinsic information corresponding to that respective bit from a previous decoding iteration; making soft bit decisions, using the plurality of bit metrics, that correspond to the individual bits of the plurality of bits of the symbol; making hard bit decisions, based on the soft bit decisions, that correspond to best estimates of bit values of the individual bits of the plurality of bits of the symbol; and wherein: the signal is a variable code rate signal whose code rate varies on a symbol by symbol basis; the symbol within the signal is encoded according to a first RC (Rate Control); at least one additional symbol within the signal is encoded according to a second RC; the plurality of bit metrics is calculated using min* processing; the iterative decoding further comprises calculating a plurality of forward metrics (alphas) and a plurality of backward metrics (betas) using the plurality of bit metrics; each alpha and each beta corresponds to one individual bit of the plurality of bits of the symbol; the plurality of forward metrics (alphas) and a plurality of backward metrics (betas) are calculated using min* processing; the iterative decoding further comprises calculating a plurality of extrinsic values using the plurality of bit metrics, the plurality of forward metrics (alphas), and the plurality of backward metrics (betas); each extrinsic value of the plurality of extrinsic values corresponds to one individual bit of the plurality of bits of the symbol; the plurality of extrinsic values is calculated using min* processing; the first RC includes a first modulation having a first constellation and a first mapping; and the second RC includes a second modulation having a second constellation and a second mapping. 55. The method of claim 54, wherein the plurality of extrinsic values that are calculated during a first iteration of the iterative decoding are employed as APP (a priori probability) during a second iteration of the iterative decoding. 56. The method of claim 54, wherein the signal is coded using at least one of 1CM (Trellis Coded Modulation) and TTCM (Turbo Trellis Coded Modulation). 57. The method of claim 54, wherein the signal is coded using TTCM (Turbo Trellis Coded Modulation); and the performing iterative decoding using the plurality of bit metrics is performed using a top SISO (Soft-In Soft-Out decoder), a bottom SISO, an interleaver, and a de-interleaver. 58. The method of claim 54, wherein the signal is coded using TTCM (Turbo Trellis Coded Modulation); and the performing iterative decoding using the plurality of bit metrics is performed using one SISO (Soft-In Soft-Out decoder) and an interleaver/de-interleaver. 59. The method of claim 54, wherein the iterative decoding includes performing MAP (maximum a posteriori probability) decoding. 60. The method of claim 54, wherein: the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a pseudo bit metric for an LSB (Least Significant Bit) of the plurality of bits; the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a bit metric for the LSB of the plurality of bits; and the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a bit metric for an MSB (Most Significant Bit) of the plurality of bits. 61. The method of claim 54, wherein: the plurality of bit metrics is a plurality of state dependent bit metrics; and the decomposing the plurality of symbol metrics into the plurality of bit metrics involves converting the plurality of state dependent bit metrics to a plurality of state independent bit metrics. 62. The method of claim 54, wherein the signal is received from an AWGN (Additive White Gaussian Noise) communication channel. 63. The method of claim 54, wherein: the signal includes a plurality of symbols arranged in a frame; and the signal is a variable code rate signal whose code rate varies on a symbol by symbol basis within the frame. 64. The method of claim 54, wherein: the method is performed within an HDTV (High Definition Television) set top box receiver. 65. The method of claim 54, wherein at least one of the first modulation and the second modulation is at least one of a BPSK (Binary Phase Shift Key) modulation, a QPSK (Quadrature Phase Shift Key) modulation, an 8 PSK (8 Phase Shift Key) modulation, a 16 QAM (Quadrature Amplitude Modulation) modulation, and a 16 APSK (Asymmetric Phase Shift Keying) modulation. 66. The method of claim 54, wherein: the first modulation is a QPSK (Quadrature Phase Shift Key) modulation having a QPSK constellation and the first mapping; and the second modulation is a QPSK modulation having a QPSK constellation and the second mapping. 67. The method of claim 54, wherein: the method is performed within a decoder; and the decoder is implemented within at least one of a satellite communication system, an HDTV (High Definition Television) communication system, a cellular communication system, a microwave communication system, a point-to-point communication system, and a TTCM (Turbo Trellis Coded Modulation) communication system. 68. The method of claim 54, further comprising mapping the plurality of bit metrics to a plurality of trellis metrics according to the first RC and the second RC. 69. The method of claim 68, wherein the plurality of trellis metrics are mapped according to an 8 state trellis of a rate 1/2 encoder. 70. The method of claim 69, wherein the rate 1/2 encoder is a convolutional encoder. 71. A bit level decoding method, comprising: receiving a signal that includes a symbol having a plurality of bits; extracting I,Q (In-phase, Quadrature) components of the symbol; mapping the symbol to a constellation point; calculating a squared Euclidean distance for the symbol thereby generating an intermediate metric of the symbol; directly calculating a plurality of bit metrics, using the intermediate metric; wherein the plurality of bit metrics is representative of the individual bits of the plurality of bits of the symbol; performing iterative decoding using the plurality of bit metrics; wherein the plurality of bit metrics is updated during each iteration of the iterative decoding for use in a current decoding iteration, such that each respective bit metric of the plurality of bit metrics is updated using extrinsic information corresponding to that respective bit from a previous decoding iteration; making soft bit decisions, using the plurality of bit metrics, that correspond to the individual bits of the plurality of bits of the symbol; making hard bit decisions, based on the soft bit decisions, that correspond to best estimates of bit values of the individual bits of the plurality of bits of the symbol; and wherein: the decomposing the plurality of symbol metrics into the plurality of bit metrics involves calculating a pseudo bit metric for an LSB (Least Significant Bit) of the plurality of bits; the decomposing of the plurality of symbol metrics into the plurality of bit metrics involves calculating a bit metric for the LSB of the plurality of bits; the decomposing of the plurality of symbol metrics into the plurality of bit metrics involves calculating a bit metric for an MSB (Most Significant Bit) of the plurality of bits; the plurality of bit metrics is a plurality of state dependent bit metrics; the decomposing the plurality of symbol metrics into the plurality of bit metrics involves converting the plurality of state dependent bit metrics to a plurality of state independent bit metrics; the signal is a variable code rate signal whose code rate varies on a symbol by symbol basis; the symbol within the signal is encoded according to a first RC (Rate Control); at least one additional symbol within the signal is encoded according to a second RC; the first RC includes a first modulation having a first constellation and a first mapping; and the second RC includes a second modulation having a second constellation and a second mapping. 72. The method of claim 71, wherein the plurality of bit metrics is calculated using min* processing. 73. The method of claim 71, wherein the signal is coded using at least one of TCM (Trellis Coded Modulation) and TTCM (Turbo Trellis Coded Modulation). 74. The method of claim 71, wherein the signal is coded using TTCM (Turbo Trellis Coded Modulation); and the performing iterative decoding using the plurality of bit metrics is performed using a top SISO (Soft-In Soft-Out decoder), a bottom SISO, an interleaver, and a de-interleaver. 75. The method of claim 71, wherein the signal is coded using TTCM (Turbo Trellis Coded Modulation); and the performing iterative decoding using the plurality of bit metrics is performed using one SISO (Soft-In Soft-Out decoder) and an interleaver/de-interleaver. 76. The method of claim 71, wherein the iterative decoding includes performing MAP (maximum a posteriori probability) decoding. 77. The method of claim 71, wherein the signal is received from an AWGN (Additive White Gaussian Noise) communication channel. 78. The method of claim 71, wherein: the signal includes a plurality of symbols arranged in a frame; and the signal is a variable code rate signal whose code rate varies on a symbol by symbol basis within the frame. 79. The method of claim 71, wherein: the method is performed within an HDTV (High Definition Television) set top box receiver. 80. The method of claim 71, wherein: the method is performed within a DWDM (Dense Wavelength Division Multiplexing) line card that is coupled to a fiber-optic communication channel. 81. The method of claim 71, wherein at least one of the first modulation and the second modulation is at least one of a BPSK (Binary Phase Shift Key) modulation, a QPSK (Quadrature Phase Shift Key) modulation, an 8 PSK (8 Phase Shift Key) modulation, a 16 QAM (Quadrature Amplitude Modulation) modulation, and a 16 APSK (Asymmetric Phase Shift Keying) modulation. 82. The method of claim 71, wherein: the first modulation is a QPSK (Quadrature Phase Shift Key) modulation having a QPSK constellation and the first mapping; and the second modulation is a QPSK modulation having a QPSK constellation and the second mapping. 83. The method of claim 71, wherein: the method is performed within a decoder; and the decoder is implemented within at least one of a satellite communication system, an HDTV (High Definition Television) communication system, a cellular communication system, a microwave communication system, a point-to-point communication system, and a TTCM (Turbo Trellis Coded Modulation) communication system. 84. The method of claim 71, further comprising mapping the plurality of bit metrics to a plurality of trellis metrics according to the first RC and the second RC. 85. The method of claim 84, wherein: the plurality of trellis metrics are mapped according to an 8 state trellis of a rate 1/2 encoder; and the rate 1/2 encoder is a convolutional encoder. 86. The method of claim 71, wherein the iterative decoding further comprises calculating a plurality of forward metrics (alphas) and a plurality of backward metrics (betas) using the plurality of bit metrics; and wherein each alpha and each beta corresponds to one individual bit of the plurality of bits of the symbol. 87. The method of claim 86, wherein the plurality of forward metrics (alphas) and a plurality of backward metrics (betas) are calculated using min* processing. 88. The method of claim 86, wherein the iterative decoding further comprises calculating a plurality of extrinsic values using the plurality of bit metrics, the plurality of forward metrics (alphas), and the plurality of backward metrics (betas); and wherein each extrinsic value of the plurality of extrinsic values corresponds to one individual bit of the plurality of bits of the symbol. 89. The method of claim 88, wherein the plurality of extrinsic values is calculated using min* processing. 90. The method of claim 88, wherein the plurality of extrinsic values that are calculated during a first iteration of the iterative decoding are employed as APP (a priori probability) during a second iteration of the iterative decoding. 91. A bit level decoder that employs a trellis to decode a signal whose code rate varies on a symbol by symbol basis according to a rate control sequence that includes a plurality of RCs (Rate Controls) arranged in a period, the decoder comprising: a metric generator that calculates a plurality of metrics for each symbol of the signal according to a corresponding RC of the plurality of RCs, each symbol includes a plurality of bits; a decompose symbol metrics to initial bit metrics functional block that decomposes the plurality of symbol metrics into a plurality of bit metrics for each symbol of the signal; wherein the plurality of bit metrics are mapped to a plurality of trellis metrics for each symbol of the signal according to the corresponding RC; a top SISO (Soft-In Soft-Out decoder) that, based on a plurality of trellis metrics, calculates a first plurality of extrinsic values for each symbol of the signal according to the corresponding RC; an interleaver, communicatively coupled to the top SISO, that interleaves the first plurality of extrinsic values; a first bit metric update functional block that employs the interleaved first plurality of extrinsic values by employing first extrinsic information of a bit calculated in a previous decoding iteration to update a first bit metric of the bit for use in a current decoding iteration thereby generating a first APP (a priori probability) information, such that each respective bit metric of the plurality of bit metrics is updated using extrinsic information corresponding to that respective bit from a previous decoding iteration; a bottom SISO that, based on the plurality of trellis metrics, calculates a second plurality of extrinsic values for each symbol of the signal according to the corresponding RC; a de-interleaver, communicatively coupled to the bottom SISO, that de-interleaves the second plurality of extrinsic values; a second bit metric update functional block that employs the interleaved second plurality of extrinsic values by employing second extrinsic information of the bit calculated in the previous decoding iteration to update a second bit metric of the bit for use in the current decoding iteration thereby generating a second APP (a priori probability) information, such that each respective bit metric of the plurality of bit metrics is updated using extrinsic information corresponding to that respective bit from a previous decoding iteration; and wherein: each extrinsic value of the first plurality of extrinsic values and the second plurality of extrinsic values corresponds to one individual bit of the plurality of bits of the symbol; the first APP information is fed back to the bottom SISO; the second APP information is fed back to the top SISO; the top SISO and the bottom SISO operate cooperatively to perform at least one iteration of iterative decoding to make soft bit decisions that correspond to the individual bits of the plurality of bits of each symbol of the signal; a first RC, of the plurality of RCs, includes a first modulation having a first constellation and a first mapping; and a second RC, of the plurality of RCs, includes a second modulation having a second constellation and a second mapping. 92. The decoder of claim 91, further comprising an output processor that makes hard bit decisions, based on the soft bit decisions, that correspond to best estimates of bit values of the individual bits of the plurality of bits of each symbol of the signal. 93. The decoder of claim 91, wherein: the decompose symbol metrics to bit metrics functional block calculates a pseudo bit metric for an LSB (Least Significant Bit) of the plurality of bits of at least one symbol of the signal; the decompose symbol metrics to bit metrics functional block calculates a bit metric for an LSB (Least Significant Bit) of the plurality of bits of at least one symbol of the signal; and the decompose symbol metrics to bit metrics functional block calculates a bit metric for an MSB (Most Significant Bit) of the plurality of bits of at least one symbol of the signal. 94. The decoder of claim 91, wherein: the plurality of bit metrics is a plurality of state dependent bit metrics; and the decompose symbol metrics to bit metrics functional block converts the plurality of state dependent bit metrics to a plurality of state independent bit metrics. 95. The decoder of claim 91, wherein decoder recycles a single SISO to perform the functionality of both the top SISO and the bottom SISO. 96. The decoder of claim 91, wherein decoder employs an interleaver/de-interleaver to perform the functionality of both the interleaver and the de-interleaver. 97. The decoder of claim 91, wherein: the plurality of trellis metrics are mapped according to an 8 state trellis of a rate 1/2 encoder; and the rate 1/2 encoder is a convolutional encoder. 98. The decoder of claim 91, wherein: the method is performed within an HDTV (High Definition Television) set top box receiver. 99. The decoder of claim 91, wherein at least one of the first modulation and the second modulation is at least one of a BPSK (Binary Phase Shift Key) modulation, a QPSK (Quadrature Phase Shift Key) modulation, an 8 PSK (8 Phase Shift Key) modulation, a 16 QAM (Quadrature Amplitude Modulation) modulation, and a 16 APSK (Asymmetric Phase Shift Keying) modulation. 100. The decoder of claim 91, wherein: the first modulation is a QPSK (Quadrature Phase Shift Key) modulation having a QPSK constellation and the first mapping; and the second modulation is a QPSK modulation having a QPSK constellation and the second mapping. 101. The decoder of claim 91, wherein: the decoder is implemented within at least one of a satellite communication system, an HDTV (High Definition Television) communication system, a cellular communication system, a microwave communication system, a point-to-point communication system, and a TTCM (Turbo Trellis Coded Modulation) communication system. 102. The decoder of claim 91, wherein: based on the plurality of trellis metrics, the top SISO calculates a plurality of forward metrics (alphas) and a plurality of backward metrics (betas) for use in calculating the first plurality of extrinsic values. 103. The decoder of claim 102, wherein: the top SISO employs min* processing to calculate at least one of the plurality of forward metrics (alphas), the plurality of backward metrics (betas), and the first plurality of extrinsic values. 104. The decoder of claim 91, wherein: based on the plurality of trellis metrics, the bottom SISO calculates a plurality of forward metrics (alphas) and a plurality of backward metrics (betas) for use in calculating the second plurality of extrinsic values. 105. The decoder of claim 104, wherein: the bottom SISO employs min* processing to calculate at least one of the plurality of forward metrics (alphas), the plurality of backward metrics (betas), and the second plurality of extrinsic values.