Phase Modulation





The basis of phase modulation is that changes are made to the phase of the carrier wave and these changes represent 0 or 1. Simple phase modulation (BPSK) shifts the phase 180deg. By making such a shift, it does limit the amount of data that can be sent per carrier wavelength. More complex techniques are used to deliver faster data rates. Various techniques allow for up to 64 bits to be sent per carrier wavelength. Phase modulation can be used in an analogue and digital environment, mobile phone communications is digital. In this context the methods are called PSK (Phase Shift Key). The "Key" implying that the different changes to the phase represent a key or binary value.

In any form of PSK there are different states used to define different data values. The most basic form BPSK uses phase changes of 180deg to indicate 0 or 1. One issue is that the receiver will have a different clock to the transmitter and at a distance, so receiver needs to find the state changes. One way to make this easier is to use change, no change representing 0 and 1. Also making the changes at 90deg and 270deg can make receiving easier than using 0 and 180deg. (Middle of the wave rather than top and bottom).

QPSK Quadrature PSK - changes of 90deg making 2 bits data transmitted per change. 00, 01, 10, 11

8-PSK 8 valued PSK - changes of 45deg making 3 bits data transmitted per change. 000, 001, .. 110, 111

16-PSK is also available with phase changes of 22.5deg. This is much harder to detect. 32-PSK will require 11.25deg changes in phase. This is really difficult to detect and further developments ar likely to be impossible. The issue is a balance between the number of bits transmitted with the phase change and the ability to receive the data.

Minimum Shift Key (MSK) - there is an issue with "sharp" changes in a wave caused by the phase changes. Theis "sharp" changes makes for sideband interference, so whilst this channel may be fast, adjoining channels will have high levels of interference. MSK aims to smooth out these "sharp" changes. Gaussian MSK is a specific method of MSK. Electronics of a gaussian filter will reduce the sideband issue. These signals can be amplified in a non-linear amplifier which does not distort the waves and is low power; good for mobile phone handsets.