In the race to 5G, CP-OFDM triumphs!
Waveform is a core physical layer component of any wireless technology. For 5G New Radio, 3GPP has chosen CP-OFDM waveform from several waveform proposals. This is something we at Ericsson Research fully support. Let’s look at why we believe CP-OFDM is a great choice for NR.
New Radio (NR) and Long Term Evolution (LTE) are integral parts of 5G radio access, as shown in Figure 1. LTE is expected to operate below 6 GHz frequencies, while NR is envisioned to operate from sub-1 GHz up to 100 GHz. Tight integration will enable aggregation of NR and LTE traffic.
Figure 1: 5G Radio Access Vision
Many features are unique to NR compared to LTE, such as the wide range of carrier frequencies, various deployment options (large to small coverage area per base station) and the diverse use cases (human-centric and machine-centric) beyond mobile broadband services. The first step towards NR development is its physical layer design, of which waveform is a core technology component. LTE uses Cyclic-Prefix Orthogonal Frequency Division Multiplexing (CP-OFDM) waveform for downlink transmissions and DFT-Spread OFDM for uplink transmissions.
On the road to 5G, the mobile communications community has witnessed abundant waveform proposals for NR. Most proposals, being multi-carrier waveforms, are either variations on CP-OFDM (e.g., Windowed-OFDM, Filtered-OFDM, Universally Filtered-OFDM, Unique Word OFDM, Pulse Shaped OFDM) or super cases of OFDM, i.e., OFDM becomes a special case of a more complex waveform (e.g., Filter-Bank Multi-Carrier waveforms). The trend has been to tweak OFDM in any possible way – sub-carrier wise filtering or pulse shaping, filtering of groups of sub-carriers, allowing successive symbols to overlap in time, dropping cyclic-prefix, replacing cyclic-prefix with nulls or with another sequence. Various waveforms became strong contenders for 5G – numerous research publications showed CP-OFDM being outperformed. At one point, we felt that every multicarrier waveform was going to be part of 5G, except CP-OFDM.
As the 5G standardization approached, Ericsson Research performed a thorough assessment of waveforms. We realized that CP-OFDM was in fact the most appropriate candidate for NR. CP-OFDM ranks best on the performance indicators that matter most – compatibility with multi-antenna technologies, high spectral efficiency, and low implementation complexity. Moreover, CP-OFDM is well-localized in time domain, which is important for latency critical applications and TDD deployments. It is also more robust to oscillator phase noise and Doppler than other multicarrier waveforms. Robustness to phase noise is crucial for operation at high carrier frequencies (e.g., mmWave band). OFDM has two drawbacks: less frequency localization and high Peak-to-Average Power Ratio (PAPR) like all multi-carrier waveforms. However, there are well-established simple techniques to reduce PAPR (for example with clipping and companding) and improve frequency localization (such as windowing). These techniques can be easily applied to CP-OFDM at the transmitter in a receiver agnostic way.
In Table 1, we assess CP-OFDM for different link types: Downlink (DL), Uplink (UL), Sidelink (D2D link), Vehicle-to-Anything (V2X) link, and Backhaul link. An OFDM assessment ‘High’ in the second column means that OFDM performs generally well for the given performance indicator. A link requirement ‘High’ for a given performance indicator shows that the given waveform performance indicator is important for the given link type in general. Details related to this assessment can be found in the IEEE Communications Magazine article, Waveform and Numerology to Support 5G Services and Requirements, by A. Zaidi, R. Baldemair, H. Tullberg, H. Bjorkegren, L. Sundstrom, J. Medbo, C. Kilinc, I. Silva.
Table 1: Assessment of CP-OFDM for different link types
Ericsson Research presented this waveform assessment within 3GPP in April 2016 (the first 3GPP RAN 1 meeting on 5G New Radio) and proposed CP-OFDM for 5G New Radio. In August 2016, 3GPP agreed to adopt CP-OFDM for both uplink and downlink in NR. Hence, CP-OFDM takes even greater role in 5G New Radio compared to 4G LTE.
Read the full paper
Waveform and Numerology to Support 5G Services and Requirements