5G waveform candidates explored: which will stick?

Telecoms.com periodically invites expert third parties to share their views on the industry’s most pressing issues. In this post, Afshin Haghighat of InterDigital Labs looks at the leading candidates for the 5G waveform and their pros and cons.

Experts are predicting that 5G will be ready for deployment by 2020, but there is still no consensus regarding exactly what this will look like. Before becoming a reality, researchers must establish the standards for the technology, which is easier said than done.

One detail still being ironed out by teams racing to establish these standards is the waveform that 5G will rely on. Our current 4G systems use the orthogonal frequency multiple access (OFDM) waveform, which is incapable of meeting the requirements of 5G’s robust applications. Traffic created by 5G will have radically different qualities and requirements when compared to the technology being used today.

For example, applications related to the tactile Internet, such as conducting a surgery remotely, require nearly instantaneous communication rates. The latency requirements for these applications needs to be approximately one millisecond or lower, as levels above this run the risk of disorienting human senses, thus impairing the usefulness of the application. In the case of remote surgery, this isn’t just an inconvenience, but an unacceptable risk that must be overcome before the technology can move forward. These communication rates and latency levels are not possible with the current OFDM waveform and subframe definitions, ruling it out as a viable candidate for the 5G standard.

For 5G to live up to the hype being generated around the technology, numerous alternatives to the OFDM waveform must be explored. That’s why my colleagues and I recently contributed a thorough review of waveforms and 5G here. Below I’ve outlined the waveform candidates we researched, as well as their key benefits.

Faster-than-Nyquist (FTN)

One waveform being researched is faster-than-Nyquist (FTN) signaling, which is being touted for its ability to increase system capacity by containing more data in the time and / or frequency domains. Within the time domain, increased capacity is achieved by enabling data bearing pulses to be sent faster. This approach removes orthogonality (the property by which signals don’t interfere with each other), but still allows for adequate detection performance through fairly complex interference mitigation techniques applied at the receiver. Additionally, FTN signaling in time and frequency domains enables multi-carrier FTN, which provides an even greater increase in spectrum efficiency.

Filter Bank Multi-Carrier (FMBC)

Another waveform being explored by academics and private researchers is filter bank multi-carrier (FMBC), which employs per-subcarrier filtering. The filtering is performed to suppress sidelobes (the portion of energy that is spread beyond the subcarrier, which wastes energy and creates interference), while using FTT/IFFT blocks in the same way that they are currently being leveraged by OFDM infrastructure. FMBC distinguishes itself with very high frequency containment; exhibiting very low level of out of band interference.

This feature of FMBC is potentially its most attractive, allowing for increased spectrum efficiency over OFDM, as well as expanded flexibility for utilizing white spaces in cognitive radio networks. Furthermore, FMBC’s improved synchronization and resistance to frequency misalignments make the waveform an enticing alternative to OFDM. However, the additional filtering required increases the implementation complexity.

Universal Filtered Multi-Carrier (UFMC)

An alternative solution that alleviates some of the concerns about implementation complexity of FMBC is offered by UFMC. In UFMC, rather than performing filtering on a single subcarrier basis, the filtering is conducted on a block of data subcarriers. As such, it still achieves high frequency containment enabling easy aggregation and scaling of cells. Moreover, a UFMC waveform is more appropriate for burst data exchange envisioned for IoT, as the employed filter does not rely on long filters that would be required for FBMC waveform generation.

Zero-Tail DFT-s-OFDM (ZT DFT-s-OFDM)

Yes, it’s a mouthful. ZT DFT-s-OFDM can be considered a modified version of SC-OFDM that is currently used in uplink of LTE system. A ZT DFT-s-OFDM waveform is an energy efficient waveform as it does not require a CP, however it still manages to mitigate ISI through its very low energy zero tail. The size of the zero tail can be adjusted conveniently based on system requirement. The implementation complexity of ZT DFT-s-OFDM is relatively low, which makes it another serious contender for 5G.

Generalized Frequency Division Multiplexing (GFDM)

On the low-power side of 5G, generalized frequency division multiplexing (GFDM) is being looked into to address broadband and real-time challenges associated with the Internet of things and wireless networks. GFDM is capable of flexible resource and QoS management, by handling modulation for single blocks, where blocks are comprised of subcarriers and subsymbols. This waveform also employs a model that circularly shifts filters for individual subcarriers with the time and frequency domains. This approach leads to a reduction in inter-symbol interference (ISI) and inter-carrier interference (ICI), with any remaining interference being detected and dealt with on the receiver side.

When all is said and done, 5G may end up relying on all of the waveforms outlined above. It might turn out to be the case that 5G relies on none of these waveforms, but that is the nature of standards research. What is clear is that the new waveform(s) selected will have to stand the test of time, and can’t be chosen without thorough investigation. If 5G is expected to last for decades, we must be sure to take an approach to standardization that will ensure the technologies longevity.

Afshin Haghighat is a Member of Technical Staff, InterDigital Labs, at InterDigital where he is involved in the development of 4G/5G cellular systems. Prior to InterDigital, he worked at Harris Corporation (1998 – 2005) and SR Telecom (1997 – 1998) where he participated in and led development of advanced modem and transceiver platforms for backhaul point-to-point and point-to-multipoint systems. His main areas of interest and expertise are RF Systems, Communications and Signal Processing. He received his PhD degree from Concordia University in Montreal.

Discover the latest developments on the road to 5G at 5G World in London 28-30 June 2016