5G Waveform: CP-OFDM & DFT-SOFDM

The waveform that has been adopted for the 5G New Radio is based on OFDM but with updates to that used with LTE

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The standard for the 5G New Radio phase one has been released and within this the waveform to be used has been defined.

A number of candidate waveforms were investigated for 5G, and after much discussion it was decided that a waveform based n OFDM would provide the optimum results.

Accordingly cyclic prefix OFDM, or CP-OFDM was chosen as the main candidate with DFT-SOFDM, discrete Fourier transform spread orthogonal frequency division multiplexing being used in some areas.

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.

Cyclic Prefix OFDM: CP-OFDM

The specific version of OFDM used in 5G NR downlink is cyclic prefix OFDM, CP-OFDM and it is the same waveform LTE has adopted for the downlink signal.

Basic concept of OFDM, Orthogonal Frequency Division Multiplexing used in 5G NR, showing how the sidebands from adjacent carriers cancel at the point of the main carriers
Basic concept of OFDM, Orthogonal Frequency Division Multiplexing

The 5G NR uplink has used a different format to 4G LTE. CP-OFDM- and DFT-S-OFDM-based waveforms are used in the uplink. Additionally, 5G NR provides for the use of flexible subcarrier spacing. LTE subcarriers normally had a 15 kHz spacing, but 5G NR allows the subcarriers to be spaced at 15 kHz x 2s with a maximum spacing of 240 kHz. The integral s carrier spacing rather than fractional carrier spacing is required to preserve the orthogonality of the carriers.

The flexible carrier spacing is used to properly support the diverse spectrum bands/types and deployment models that 5G NR will need to accommodate. For example, 5G NR must be able to operate in mmWave bands that have wider channel widths of up to 400 MHz. 3GPP 5G NR Rel-15 specification details the scalable OFDM numerology with 2s scaling of subcarrier spacing that can scale with the channel width, so the FFT size scales so that processing complexity does not increase unnecessarily for wider bandwidths. The flexible carrier spacing also gives additional resilience to the effects of phase noise within the system.

The use of OFDM waveforms offers a lower implementation complexity compared to that which would be needed if some of the other waveforms considered for 5G had been implemented. In addition to this, OFDM is well understood as it has been used for 4G and many other wireless systems.

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