Digital Signal Processing Reference
In-Depth Information
Q axis
(1,0)
(0,0)
I axis
QPSK
Constellation
(1,1)
(0,1)
Figure 17.1. QPSK constellation.
as the coordinates of a location in the I-Q plane. In QPSK, there
are four possible symbols, arranged as shown. Since there are four
symbols, the input data are arranged as groups of two bits, which
are mapped to the appropriate symbol. The arrangement of
symbols in the I-Q plane is also called the constellation.
Table 17.1 QPSK symbol mapping
Constellation size and bit rate
Symbol Value (Location
On Complex Plane)
Input Bit Pair
I Value
Q Value
0, 0
1
1
I + jQ ¼
>
1+j
0, 1
1
1
I + jQ ¼
>
1 e j
1, 0
1
1
I + jQ ¼
>
1+j
1, 1
1
1
I + jQ ¼
>
1 e j
Another common modulation scheme is known as 16-QAM
(quadrature amplitude modulation), which has 16 symbols,
arranged as shown in
Figure 17.2
. Again, don't worry about the
name of the modulation. Since we have 16 possible symbols, each
symbol will map to four bits. To put it another way, in QPSK each
symbol carries two bits of information, while in 16-QAM,
each symbol carries four bits of information.
