As research into 5G continues in both academia and the private sector, one thing has become certain: there is no single enabling technology that can achieve all of the applications being promised by 5G networking. Some of 5G’s requirements, such as data rate and network capacity, are agreed upon by most experts, while other aspects of 5G, including waveform, are still hotly debated.
Our current 4G systems rely on the orthogonal frequency multiple access (OFDM) waveform, which is not capable of supporting the diverse applications 5G will offer. This is because the traffic generated by 5G is expected to have radically different characteristics and requirements when compared to current wireless technology. For example, applications like augmented reality require nearly instantaneous communication rates. Without the ability to reduce latency to levels approaching one millisecond, our vision isn’t able to match up with the images or user interface being displayed, impairing the usefulness of the application. Current 4G infrastructure isn’t capable of reaching latency levels this low, partly due to the reliance on the OFDM waveform, and the adopted subframe structure.
For 5G to succeed, numerous alternatives must be explored to best meet its various technical requirements. 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.
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.
At the end of the day, 5G may rely on all or none of the waveforms outlined above. Realities of the future of networking are still being formulated, but one thing is clear, when it comes to waveform, one size won’t fit all.
Afshin Haghighat is Senior Staff Engineer at InterDigital.