Ericsson researchers are happy the 3GPP has chosen CP-OFDM waveform for 5G New Radio (NR), as the company realized early on that CP-OFDM was the most appropriate candidate for NR among the pack.
In a new blog post, senior researcher Ali A. Zaidi and master researcher Robert Baldemair spell out why CP-OFDM is the best choice. A lot of waveform proposals were presented before the 3GPP settled on it and most proposals, being multi-carrier waveforms, were either variations on CP-OFDM or super cases of OFDM.
“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,” they said. “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.”
Ericsson Research did a thorough assessment of waveforms and realized that CP-OFDM ranks best on the performance indicators that matter most—compatibility with multiantenna 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,” the researchers wrote. “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, they added. However, “there are well-established simple techniques to reduce PAPR (for example with clipping and companding) and improve frequency localization (such as windowing),” they said. “These techniques can be easily applied to CP-OFDM at the transmitter in a receiver agnostic way.”
Details related to Ericsson’s 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.
In August 2016, 3GPP agreed to adopt CP-OFDM for both uplink and downlink in NR, giving CP-OFDM an even greater role in 5G NR compared to 4G LTE.
At a plenary meeting in March of this year after Mobile World Congress 2017, 3GPP agreed to a work plan proposal for the first 3GPP 5G NR specification that will be part of Release 15. Ericsson was among those who committed to accelerate the 5G NR schedule by introducing an intermediate milestone for an early completion of a variant called Non-Standalone (NSA) 5G NR.
AT&T has said that decision means it will be able to launch standards-based mobile 5G services to consumers starting as early as late 2018.