ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 83

(a)

1 baud 1 baud 1 baud 1 baud

Symbol: 00 Symbol: 01 Symbol:10 Symbol:11

(b)

01

Dibit Phase

00 0 °

01 90 ° 10

00

10 180°

11 270°

11

Fig. 4.10 QPSK modulation. a Timing diagram, b Constellation diagram

Among different phase shift key (PSK) modulation techniques, quadrature PSK
has the higher bits per sec communication speed. In general, a bit corresponds to a
particular state of the carrier signal in ASK, FSK, OOK, PSK, etc. But in QPSK a
state of the carries represents two binary bits which can be realized from the
Fig. 4.10 a and its corresponding constellation diagram as shown in Fig. 4.10 b.
Each pair of bits (terms as dibit) corresponds to a particular state of the carrier and
it is termed as the symbol or signature. So to represent a byte in QPSK, 4-symbol
or signature is required. The details of QPSK modulation can be found at [ 30 , 31 ].
The purpose of utilizing QPSK modulation is to transmit the dibit symbols over
the entire band of telephone line at half of the bits per sec (bps) speed. Generally,
the telephone line bandwidths for the voice and data communication are

300-3,300 and 600-3,000 Hz, respectively, as shown in the Fig. 4.11 .

Response

Data service

Voice service

300 600 3000 3300

Frequency (in Hz)

Fig. 4.11 Telephone line data bandwidths for voice and data service