Mobile Phone Technology

The demand to deliver more bits on the carrier wave is always present. 8PSK delivers 3 bits (8 values), but this requires phase changes of 45 degrees. Extending this to 16PSK (22.5-degree phase changes) and 32PSK (11.25-degree phase changes) would be impossible.

In QAM, two carrier waves are transmitted: the 'I' wave, which is a sine wave; and the 'Q' wave, which is a cosine wave. These two waves are out of phase by 90 degrees.

In 16QAM, four bits are sent per wavelength. The first two bits are sent on the 'I' wave, and the other two bits on the 'Q' wave. To modulate the wave to send two bits per wavelength, 16QAM uses amplitude and phase shift keying. There are two possible amplitudes, and the wave can have a phase shift of 0 or 180°.

Once the four bits have been split into two groups of two and modulated onto the waves, they are added together for transmission and then split apart by the receiver for simple decoding. In order to use 16PSK to send four bits, the receiver would need to detect phase changes of 22.5 degrees. However, QAM decoding only has to differentiate between two amplitudes and a 180-degree phase shift, which is much easier to achieve without error.

To transmit more bits per wave, we can use 64QAM, which requires six bits split into two groups of three, again using a 180-degree phase difference and four different amplitudes. 64-QAM is used in 4G LTE-Advanced/5G, with the potential to extend to 256-QAM (8 bits). All of this is designed to transmit more error-free bits per carrier wave. However, as QAM includes amplitude modulation, signal amplification needs to be carried out using a linear amplifier, which consumes more power and therefore has a negative impact on mobile phones.

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