Mobile Phone Technology

Phase modulation involves making changes to the phase of the carrier wave, with these changes representing 0 or 1. In simple phase modulation (BPSK), the phase shifts by 180°. This modulation scheme sends 1 bit per carrier wavelength. Therefore, the maximum data rate is equal to the frequency. Hence the maximum data rate for a 700 MHz wave is 700 million bits. More complex techniques are required to achieve faster data rates. Various techniques allow up to 16 data values to be sent per wavelength of the carrier. Phase modulation is available in analogue or digital environments; mobile phone communications are digital. In the context of mobile communications, these methods are called PSK (Phase Shift Keying). The term 'key' implies that the different changes to the phase represent a key or binary value.

In any form of PSK, different states are used to represent different data values. The most basic form, BPSK, uses phase changes of 180° to indicate 0 or 1. One issue is that the receiver and transmitter will have different clocks, especially over distance, so the receiver needs to detect state changes. One way to make this easier is to use a change to represent 1 and no change to represent 0.

BPSK Binary PSK - changes of 180° making 1 bit data transmitted per change. 0, 1

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

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

16-PSK (and beyond) is also available with phase changes of 22.5°. This is much harder to detect. 32-PSK would require 11.25° changes in phase. This is really difficult to detect and further developments are 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. These "sharp" changes makes for sideband interference, so whilst one 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. The 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 on power; good for mobile phone handsets.

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