5G Waveforms & Modulation: CP-OFDM & DFT-s-OFDM

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One of the defining elements of any mobile communications system is the waveform used for the radio link in the radio access network.

During the development phase for 5G technology a variety of waveforms and modulation techniques were postulated, but for 5G New Radio, 5G NR, cyclic prefix OFDM, CP-OFDM was chosen as the main candidate with DFT-s-OFDM, discrete Fourier transform spread orthogonal frequency division multiplexing being used in some areas.

OFDM gives good spectral efficiency whilst providing resilience to selective fading and it also enables multiple access capability to be implemented using OFDMA.

5G waveform background

Orthogonal frequency division multiplexing has been an excellent waveform choice for 4G LTE. 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 technology. 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 mobile communications 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.

Within CP OFDM the last part of data of OFDM frame is appended at the beginning of the OFDM frame and length of cyclic prefix is chosen to be greater than channel delay spread.  This overcomes the inter-symbol interference that can result from delays and reflections. In addition to this, the channel delay spread is frequency dependent with the cyclic prefix length chosen to bee long enough to account for both interferences. For this reason the CP length is adaptive according to the link conditions.

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 Release-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.

DFT-s-OFDM

Direct Fourier Transform spread OFDM, commonly abbreviated to DFT-s-OFDM, is an SC or single carrier-like transmission scheme that can be combined with OFDM that gives significant flexibility for a mobile communications system like 5G. It is more commonly known as SC-FDMA.

The transmission processing of SC-FDMA is very similar to that of OFDMA. For each user, the sequence of bits transmitted is mapped to a complex constellation of symbols (BPSK, QPSK or M-Quadrature amplitude modulation). Then different transmitters (users) are assigned different Fourier coefficients. This assignment is carried out in the mapping and demapping blocks. The receiver side includes one de-mapping block, one IDFT block, and one detection block for each user signal to be received. Just like in OFDM, guard intervals (called cyclic prefixes) with cyclic repetition are introduced between blocks of symbols in view to efficiently eliminate inter-symbol interference from time spreading (caused by multi-path propagation) among the blocks.

5G modulation considerations

Within the overall waveform format, different types of carrier modulation can be used. Within the 5G communications system, these are variants of phase shift keying and quadrature amplitude modulation.

There are several considerations when using the different modulation formats:

• 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 communications 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 it's HSPA enhancements and then 4G LTE, 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 order of the modulation is one factor that has a major impact upon the PAPR: the greater the level of "peakyness" the lower the efficiency that can be achieved by RF power amplifier efficiency, although schemes like envelope tracking and Doherty amplifiers enable improvements to be made.
• 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 64QAM as opposed to 16QAM for example and noise performance. Thus higher order modulation schemes tend to be only used when there is a good signal to noise ratio.

5G modulation: PSK & QAM

A variety of different modulation formats are used for 5G technology.<.p>

• Phase shift keying:   5G technology implements quadrature phase shift keying, QPSK as the lowest order modulation format. Although this will provide the slowest data throughput it will also provide the most robust link and as such it can be used when signal levels are low or when interference is high.
  
  Another form of PSK called π/2BPSK is used in conjunction with DFT-s-OFDM on the up link.
  

  Note on PSK - Phase Shift Keying:

  Phase shift Keying, PSK is a form of modulation used particularly for data transmissions. If offers an effective way of transmitting data. By altering the number of different phase states which can be adopted, the data speeds that can be achieved within a given channel can be increased, but at the cost of lower resilience to noise an interference.

  Read more about Phase Shift Keying, PSK.

• Quadrature amplitude modulation:   Quadrature amplitude modulation enables the data throughput to be increased. Formats used within 5G mobile communications system include 16QAM, 64QAM and 256QAM.
  
  The higher the order of modulation, the greater the throughput, although the penalty is the noise resilience. Therefore 256AM is only used when link quality is good, and it reduces to 64QAM and then 16QAM etc, as the link deteriorates. It is a balance between data throughput and resilience.
  

  Note on QAM - Quadrature Amplitude Modulation:

  Quadrature amplitude modulation, QAM is widely used for data transmission as it enables better levels of spectral efficiency than other forms of modulation. QAM uses two carriers on the same frequency shifted by 90° which are modulated by two data streams - I or Inphase and Q - Quadrature elements.

  Read more about Quadrature Amplitude Modulation, QAM.

The waveform and modulation types used with 5G technology has been chosen to provide spectral efficiency, data throughput and resilience needed for the new mobile communications system.

5G mobile communications is able to provide very high data throughput, and therefore the waveforms and modulation need to be able to support this and provide reliable service for the users.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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