dot
UNB EE4253 Digital Communications
Department of Electrical and Computer Engineering - University of New Brunswick, Fredericton, NB, Canada
dot

QPSK Modulation

In Quadrature Phase Shift Keying (QPSK) modulation, a cosine carrier is varied in phase while keeping a constant amplitude and frequency.

QPSK Constellation
 

The term "quadrature" implies that there are four possible phases (4-PSK or QPSK) which the carrier can have at a given time, as shown at right on the characteristic constellation for this moduation type. The four phases are labelled {A,B,C,D} corresponding to one of {0,90,180,270} degrees. 

In PSK, information in conveyed through phase variations, since absolute phase cannot be established. In each time period, the phase can change once while the amplitude remains constant. In QPSK there are four possible phases, and therefore two bits of information conveyed within each time slot. The rate of change (baud) in this signal determines the signal bandwidth, but the throughput or bit rate for QPSK is twice the baud rate. 

The CCITT V.22 (like Bell 212A) modem uses QPSK to send data at 1200 bits per second; however, the phases change only 600 times per second, conveying two bits per change - this is a 600 baud modem.		 QPSK Constellation

Transmitting Data Using QPSK

Each of the four possible phase changes is assigned a specific two-bit value, or dibit. For example, in the V.22 modem, the relationship between phase changes and dibits is given by:
 

PHASE CHANGE (Degrees)
Example state change
Dibit
0
A-to-A
01
90
A-to-B
00
180
B-to-D
10
270
D-to-C
11

In the figure below, a carrier is shifted through the phases ADABAADCCA.
 

psk
Two bits of information are conveyed in the transition between time slots.
 

The signal has undergone the following phase transitions:

PHASE
A
D
A
B
A
A
D
C
C
A
...
Change
-
A-to-D
D-to-A
A-to-B
B-to-A
A-to-A
A-to-D
D-to-C
C-to-C
C-to-A
...
Degrees 
-
270
90
90
270
0
270
270
0
180
 
Dibit
-
11 
00 
00 
11 
01 
11 
11 
01 
10 
 

The corresponding information transmitted is therefore: 110000110111110110


Data Scrambling

Scrambling is used in V.22 modems (and in most higher order modulation types). Randomizing the data stream ensures frequent transitions and avoids the potential for transmitting a constant carrier with no phase changes. For example, the data stream 01010101... would produce a constant phase signal (e.g. BBBBBB...).

The polynomial used in V.22 modems is:

X = 1 + X-14 + X-17

Reference:
Hayes Smartmodem 1200 Hardware Reference Manual, Hayes Microcomputer Products, Inc., 1983

dot
Tue Jun 18 20:17:34 ADT 2013
Last Updated: 11 NOB 1998 		Richard Tervo [ tervo@unb.ca ] Back to the course homepage...
dot