4.3 Quadrature Phase Shift Keying (QPSK)


where the symbol duration: , the bit period.



PSD in this case for rectangular pulses will be similar to PSK where symbol duration has to replace the bit period :


As it can be seen from the figure below, the null-to-null bandwidth is equal to the bit rate , which is half of that of a BPSK signal as desired.


This modified QPSK system is designed to combat the following ill of QPSK systems: The amplitude of a QPSK signal is ideally constant. However, they are normally pulse-shaped for efficiency and then they loose the constant envelope property. Occasional phase shift of can cause the signal envelope to have a Z.C. Any kind of hard-limiting or non-linear amplification of the Z.C. brings back the filtered sidelobes since the fidelity of the signal at small voltage levels is lost in transmission. To prevent the regeneration of sidelobes and spectral spreading, it is imperative that only linear amplifiers, which are costly and very inefficient, are used for amplifying QPSK signals. If the in-phase and quadrature bit streams of QPSK signals are offset in their relative alignment by one-bit period (half-symbol rate), then this modified version of QPSK is called an Offset QPSK (OQPSK) and it results in more efficient amplification process.



p/4 Shifted QPSK offers a compromise between OQPSK and QPSK in terms of the allowed maximum phase transitions.

. That is:




and the following table gives the phase shifts:





are passed through Raised-Cosine roll-off pulse shaping filters before modulation to reduce the bandwidth occupancy.


There are various types of detection techniques used for these signals, including "baseband differential detection, IF differential detection, and FM discriminator detection." Simulations show that all 3 receiver configurations offer similar BER, although there are implementation issues, which are specific to each technique. (Details of these are neatly presented in Rappaport [11]) and we will only present the block diagram of the baseband differential detector here.