| 单词 | TCM (Trellis-Coded Modulation) |
| 释义 | TCM (Trellis-Coded Modulation) A modulation scheme based on quadrature amplitude modulation (QAM), but adds a forward error correction (FEC) mechanism to overcome the increased susceptibility to signal impairments. TCM is so named because the plotting of the signal points resembles the latticework of a trellis, such as that used in a rose garden, only four-dimensional.TCM employs a convolutional (i.e., error-correcting) that involves adding an extra bit to every symbol for error control purposes. For example, the 128-QAM technique yields 128 possible signal combinations, with each symbol representing seven bits (27 = 128). As TCM uses one bit for error control, only six payload bits remain (26 = 64). Therefore, a modem employing TCM accepts six bits at a time.The two least significant bits (LSBs) are separated from the six-bit payload, are analyzed, and a parity bit is added that describes the mathematical value (odd or even) of the sum of the LSBs.The resulting three bits and the other original four bits are recombined into a seven-bit symbol prior to transmission.The receiving modem reverses the process, analyzes the parity bit describing the LSBs, and either accepts the data as correct, adjusts the data to correct for an error if possible, or requests a retransmission. The LSBs, which are the rightmost bits in a byte, change rapidly if the total byte value changes even slightly.Therefore, they are highly sensitive to errors and very telling in the event that errors occur.When the symbols are plotted onto the logical trellis by the receiver, there are only 64 (26 = 64) legitimate states, or positions, plus the two for the error control bit, for a total of 66 states. If the indicated plot point is one of the other 62 (27 = 128 – 66 = 62), the received symbol is assumed to 74570c20.qxd 9/12/07 12:44 AM Page 477 |
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