5G Modulation Schemes

- the modulation scheme or schemes adopted for 5G will play a major role in determining the performance and complexity of the handsets and other nodes used.

The modulation schemes used for 5G will have a major impact on performance.

Whilst there are requirements to ensure that the data rates needed can be carried and the 5G modulation schemes performance issues including peak to average power ratio, spectral efficiency, and performance in the presence of interference and noise need to be included in any decisions made.

Peak to average power ratio, PAPR

The peak to average power ratio is one aspect of performance that needs to be considered for any 5G modulation scheme.

The peak to average ratio has a major impact on the efficiency of the power amplifiers. For 2G GSM, the signal level was constant and as a result it was possible to run the final RF amplifier in compression to obtain a high level of efficiency and maximise the battery life.

With the advent of 3G, then its HSPA enhancements and then 4G, the modulation schemes and waveforms have meant that the signals have become progressively more 'peaky' with higher levels of peak to average power ratio. This has meant that the final RF amplifiers cannot be run in compression and as the PAPR has increased, so the efficiency of the RF amplifiers has fallen and this is one factor that has shortened battery life.

The opportunity now arises to utilise 5G modulation schemes that can reduce the PAPR and thereby improve efficiency.

Spectral efficiency

One of the key issues with any form of 5G modulation scheme is the spectral efficiency. With spectrum being at a premium, especially in frequencies below 3 GHz, it is essential that any modulation scheme adopted for 5G is able to provide a high level of spectral efficiency.

There is often a balance between higher orders of modulation like 64 QAM as opposed to 16 QAM for example and noise performance. Thus higher order modulation schemes tend to be only sued when there is a good signal to noise ratio.

Accordingly any 5G modulation scheme will need to accommodate high levels of performance under a variety of conditions.

5G modulation schemes

3G and 4G have used modulation schemes including PSK and QAM. These schemes provide excellent spectral efficiency and have enabled the very high data rates to be carried but fall short in terms of their peak to average power ratio.

To overcome the PAPR issue, one option being considered for a 5G modulation scheme is APSK or amplitude Phase Shift Keying.

However in view of the fact that amplitude components of a signal are more subject to noise, which is substantially amplitude based, it is likely that any overall 5G modulation scheme will be adaptive, enabling the system to switch to the most optimum for of modulation for the given situation.

Note on APSK - Amplitude & Phase Shift Keying:

Amplitude and Phase-Shift Keying, APSK, is a digital modulation scheme that uses both the amplitude and the phase changes to modulate the radio frequency carrier. It has the advantage that the constellation diagram can be adjusted to optimise the signal, particaulrly in terms of reducing peak to average power ratios.

Read more about APSK, Amplitude & Phase Shift Keying

Whilst APSK may provide many advantages in terms of reducing the PAPR, it is not the complete answer and may be used as one option in an overall adaptive modulation scheme.

By Ian Poole

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