TABLE OF CONTENTS A functional description of the most frequently employed QPSK modems is followed by a study of the spectral efficiency of these systems. A probability of error performance analysis in ideal Nyquist shaped additive white Gaussian noise channels concludes this section.
The abbreviations used in the heading of this subsection are not completely new. If the heading of Section 4.3.1 is examined, the following analogies with quadriphase PSK systems can be noted.
| Binary PSK | Quadriphase PSK |
|---|---|
| BPSK | QPSK |
| DEBPSK | DEQPSK |
| DBPSK | DQPSK |
| ? | OKQPSK |
We recall that BPSK systems are binary coherent systems, which require an unambiguous carrier recovery circuit. Differential encoding is introduced into DEBPSK systems to enable operation with 180 phase ambiguous carrier recovery circuits. Finally, DBPSK systems are introduced to provide an alternative demodulation method, a method that does not require carrier recovery circuitry (see Figs. 4.4 and 4.6).
Quadriphase modems (QPSK) are used in systems applications where the 1-b/s/Hz theoretical spectral efficiency of BPSK modems is insufficient for the bandwidth available. The various demodulation techniques used in binary phase-shift-keyed systems also apply to quadriphase PSK systems. In addition to the straightforward extensions of the binary modem techniques, a technique known as offset-keyed or staggered quadriphase modulation (OKQPSK) is also in use.
In QPSK systems the modulated signal has four distinct phase states. These states are generated by a unique mapping scheme of consecutive dibits (pairs of...
