Digital Signal Processing Reference
In-Depth Information
Q axis
(1,0,0,0)
(1,0,0,1)
(0,0,0,1)
(0,0,0,0)
(1,0,1,0)
(1,0,1,1)
(0,0,1,1)
(0,0,1,0)
16 QAM
Constellation
I axis
(1,1,1,0)
(1,1,1,1)
(0,1,1,1)
(0,1,1,0)
(1,1,0,0)
(1,1,0,1)
(0,1,0,1)
(0,1,0,0)
Figure 17.2. 16-QAM constellation.
As shown, 16-QAM is the more efficient modulation method.
For simplicity, assume symbols are transmitted at a rate of
1 MHz, or 1 MSPS (normally, this is much higher). Then our
system, if using the 16-QAM modulation, will be able to send
4 Mbits/second. If instead QPSK is used in this system, it will be
able to send only 2 Mbits/second. We could also use 64-QAM
which is an even more efficient constellation. Since there are 64
possible symbols, arranged as eight rows of eight symbols each,
each symbol carries six bits of information, and supports a data
rate of 6 Mbits/second. A few sample constellation types are
shown in
Table 17.2
.
Table 17.2
Constellation size and bit rate
Possible number
of Symbols
Modulation Type
Bits per Symbol
Transmitted Bit Rate
2
2
QPSK
4
¼
2
2 * symbol rate
2
3
8 PSK
8
¼
3
3 * symbol rate
2
4
16-QAM
16
¼
4
4 * symbol rate
64 ¼ 2
6
64-QAM
6
6 * symbol rate
256 ¼ 2
8
256-QAM
8
8 * symbol rate
