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quadrature amplitude modulation (QAM)

Quadrature modulation in which the two carriers are amplitude modulated. 2. In analog communications, the representation (i.e., transmission) of digital information by encoding bit sequences of fixed, specified length (number of bits), and representing these bit sequences as a function of (a) the amplitude of an analog carrier; or (b) a phase shift of the analog carrier with respect to the phase that represented the preceding bit sequence, and where the permissible phase shift is an integral multiple of /2 radians (90°, or onequarter unit interval); or (c) both. Note 1: The name quadrature originates from the stipulation that a phase shift, when required, must be an integral multiple of p/2 radians, i.e., onequarter of a cycle (unit interval, or baud). Note 2: A representative QAM table is presented below. If the baud (carrier) rate is 1200 Hz (the usual case in modems used on conventional telephone lines), this QAM scheme permits transmission of a data rate of 9600 b/s, which is the product of the baud rate and the number of permissible phaseamplitude states.
Bit Sequence Represented 
Normalized Carrier Amplitude 
Relative Phase Shift (Degrees) 
000 
1/2 
0 (0°) 
001 
1 
0 (0°) 
010 
1/2 
/2 (90°) 
011 
1 
/2 (90°) 
100 
1/2 
(180°) 
101 
1 
(180°) 
110 
1/2 
3 /2 (270°) 
111 
1 
3 /2 (270°) 
Any continuous bit stream may be represented as a combination of the eight permissible bit sequences in the above table, and transmitted as such. Note 3: According to mathematical theory, there is no limit to the data rate that may be supported by, or associated with, a given baud rate in a perfectly stable, noiseless transmission environment. In practice, the governing factors are the amplitude (and consequently, phase) stability, and the amount of noise present, in both the terminal equipment and the transmission medium (carrier frequency, or communication channel) involved. Nor does the permitted relative (incremental) phase shift necessarily have to be a multiple of
/2 radians, but of course the name "quadrature" would not apply if any other minimum phase shift were specified or permitted.


