64/256-QAM：北美有线调制解调器标准的性能分析与编码方案
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更新于2024-07-31
收藏 165KB PDF 举报 本文主要探讨了64/256-QAM调制与联合编码（Concatenated Coding）在北美有线电视调制解调器标准（MCNS，Multimedia Cable Network System）中的应用性能分析。研究者Dojun Rhee和Robert H. Morelos-Zaragoza来自LSI Logic Corporation，他们关注的是一个将Reed-Solomon (RS) 代码作为外层编码，Trellis Coded Modulation (TCM) 作为内层编码的复合编码方案。
在这个编码体系中，外层采用了一种具有三重纠错能力的扩展RS码（128,122,7）在GF(27)域上工作，其目的是提供高度的数据保护。内层则采用了符合ITU-T建议J.83, Annex B标准的Viterbi解码器，这是北美数字视频传输规格的一部分，适用于MCNS系统，确保了信号的高效传输和准确接收。
文章的核心内容围绕着两个关键的信号 constellation, 即64-QAM和256-QAM在添加白噪声（AWGN）的信道上的误码性能分析。作者利用理论方法推导了联合编码的比特错误率边界，并通过计算机模拟进行了验证，以确保这些理论结果的准确性。此外，文中还探讨了量化对输入符号的影响以及Viterbi解码器中的 traceback深度设置对整体性能的影响。
"关键词"部分提到了"Concatenated Coding"、"Reed-Solomon coding"、"Trellis Coded Modulation"、"64-QAM"、"256-QAM"、"AWGN channel"、"Quantization"和"Viterbi decoder"，这些都是理解本文核心研究内容的关键术语。这篇论文提供了对于如何优化这种混合编码方案在实际通信环境中的性能，特别是在高数据速率和复杂信道条件下，以实现更高效和可靠的有线电视数据传输的重要见解。
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“Y” to represents an 8-PAM signal point. For example, if “X” is “000”, then the In-Phase
component in Figure 5 (b) is always “1” and if “Y” is “111”, then the Quadrature component in
Figure 5 (b) is always “-1”. Therefore, at the TCM decoder, the In-Phase and Quadrature soft
outputs from the 64-QAM demodulator can be decoded independently. Also, at the rate-1/2
Viterbi decoder, each In-Phase (or Quadrature) soft output is compared with reference 8-PAM
signal constellation points {-7, -5, -3, -1, 1, 3, 5, 7}.
Since a rate-1/2 16-state convolutional code is used in the TCM encoder, there are four
distinct labels for each branch in the 16-state trellis, i.e., {00, 01, 10, 11}. Therefore, only the
branch metrics for these four distinct branch labels need to be calculated. There are two-bits
for each branch label, and therefore two consecutive soft outputs must be feed into the branch
metric generator. These two bits are used as LSB for the 8-PAM signal constellation mapping.
For each bit, there are four parallel branches with MSB label {00, 01,10,11}. These four
parallel branch labels are compared with four 8-PAM signal points which are decided by the
LSB of the branch label.For example, suppose “01” is a label for a branch in a rate-1/2 Viterbi
decoder trellis. Then, for label “0”, we calculate branch metrics with (000, 010, 100,110) = (1,
5, -7,-3) in the 8-PAM signal constellation and find the label with minimum branch metric.For a
label “1”, we calculate branch metrics with (001, 011, 101,111) = (3, 7, -5,-1) in the 8-PAM sig-
nal constellation and find the label with minimum branch metric. After this, each branch metric
is added, and pairs of two MSB bits are stored from the labels with minimum branch metric.
After Viterbi Decoding, eight consecutive 2-bits pairs for parallel branches and 4
information bits from 10 consecutive In-Phase component of 64-QAM signals are recovered.
Four recovered information bits are fed into the BCC to produce eight LSBs. These eight LSBs
are combined with the eight consecutive 2-bits pair and fed into a puncturing device to
generate five consecutive 3-bits “X”. Five consecutive 3-bits “Y” bits are recovered by the
same procedure as for the quadrature part. Then 6-bit “X” and “Y” are converted into “I” and
“Q” based on Table 2. These recovered five consecutive 2-bits MSB of “I”, five consecutive 2-
bits MSB of “Q”, and pairs of four information bits from each BCC, totalling 28 bits, are
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