Although OFDM has been a great success and still has many advantages, there are many ideas for new 5G waveforms that could bring additional advantages to the new cellular system under certain conditions and circumstances. No single waveform provides all the advantages and answers that are needed.
As a result many anticipate that the final outcome for 5G waveforms may include an adaptive solution - using the optimum waveform for any given situation.
Now that 5G is being investigated, there is an excellent opportunity to consider the optimum waveforms for the 5G system that will be used until at least 2040.
5G waveform background
Orthogonal frequency division multiplexing has been an excellent waveform choice for 4G. It provides excellent spectrum efficiency, it can be processed and handled with the processing levels achievable in current mobile handsets, and it operates well with high data rate stream occupying wide bandwidths. It operates well in situations where there is selective fading.
However with the advances in processing capabilities that will be available by 2020 when 5G is expected to have its first launches means that other waveforms can be considered.
There are several advantages to the use of new waveforms for 5G. OFDM requires the use of a cyclic prefix and this occupies space within the data streams. There are also other advantages that can be introduced by using one of a variety of new waveforms for 5G.
One of the key requirements is the availability of processing power. Although Moore's Law in its basic form is running to the limits of device feature sizes and further advances in miniaturisation are unlikely for a while, other techniques are being developed that mean the spirit of Moore's Law is able to continue and processing capability will increase. As such new 5G waveforms that require additional processing power, but are able to provide additional advantages are still viable.
5G waveform requirements
The potential applications for 5G including high speed video downloads, gaming, car-to-car / car-to-infrastructure communications, general cellular communications, IoT / M2M communications and the like, all place requirements on the form of 5G waveform scheme that can provide the required performance.
Some of the key requirements that need to be supported by the modulation scheme and overall waveform include:
- Capable of handling high data rate wide bandwidth signals
- Able to provide low latency transmissions for long and short data bursts, i.e. very short Transmission Tine Intervals, TTIs, are needed.
- Capable of fast switching between uplink and downlink for TDD systems that are likely to be used.
- Enable the possibility of energy efficient communications by minimising the on-times for low data rate devices.
These are a few of the requirements that are needed for 5G waveforms to support the facilities that are needed.
Candidate 5G waveforms
There are several new 5G waveform formats that are being considered. These include:
- FBMC, Filter Bank Multi-Carrier: FBMC has gained a high degree of interest as a potential 5G waveform candidate. This waveform scheme provides many advantages.
In many ways FBMC has many similarities to CP-OFDM, OFDM using a cyclic prefix which is used as the 4G waveform. Instead of filtering the whole band as in the case of OFDM, FBMC filters each sub-carrier individually. FBMC does not have a cyclic prefix and as a result it is able to provide a very high level of spectral efficiency.
The subcarrier filters are very narrow and require long filter time constants. Typically the time constant is four times that of the basic multicarrier symbol length and as a result, single symbols overlap in time. To achieve orthogonality, offset-QAM is used as the modulation scheme, so FBMC is not orthogonal with respect to the complex plane.
- UFMC, Universal Filtered MultiCarrier: This 5G waveform can be considered as an enhancement of CP-OFDM. It differs from FBMC in that instead of filtering each subcarrier individually, UFMC splits the signal into a number of sub-bands which it then filters.
UFMC does not have to use a cyclic prefix, although one can be used to improve the inter-symbol interference protection.
- GFDM, Generalised Frequency Division Multiplexing: GFDM is a flexible multi-carrier transmission technique which bears many similarities to OFDM. The main difference is that the carriers are not orthogonal to each other. GFDM provides better control of the out-of-band emissions and the reduces the peak to average power ratio, PAPR. Both of these issues are the major drawbacks of OFDM technology.
- Filtered OFDM, f-OFDM As the name, f-OFDM indicates, this form of OFDM uses filtering to provide its unique characteristics. Using f-OFDM, the bandwidth available for the channel on which the signal is to be transmitted is split up into several sub-bands. Different types of services are accommodated in different sub-bands with the most suitable waveform and numerology. This enables a much better utilisation of the spectrum for the variety of services to be carried.
In addition to these waveforms, and number of other waveforms are also being considered for use with 5G.
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