The Fog Begins to Lift on 5G Cellular

5G mobile communication is widely forecasted to be a market reality seven to ten years from now. As envisioned by the International Telecommunications Union Radiocommunications sector (ITU-R), in their next generation International Mobile Telecommunication or IMT-2020 documents, 5G comprises three top-level goals: enhanced mobile broadband; massive machine type communications; and ultra-reliability and low latency communications. There are eight technical criteria which underpin these goals: peak data rate; user-experienced data rate; area traffic capacity; spectrum efficiency; network energy efficiency; mobility; connection density; and latency. The IMT-2020 manifesto sets daunting performance enhancement goals for each of these criteria that range from the geometric (such as a 3x increase in spectrum efficiency) to the exponential (such as a 100x increase in network energy efficiency).

The 3rd Generation Partnership Project (3GPP) standard setting organization, having successfully driven the 4G LTE / LTE-Advanced standards into more than 450 networks in about 140 countries worldwide, is starting to establish study items, to be followed by work items, for their Release 15 (5G Phase 1) and Release 16 (5G Phase 2) specifications targeted for 2018 and 2019, respectively. Research organizations in the US, Europe and Asia are actively investigating digital wireless communications and networking technology alternatives and enhancements, such as RF channel modeling above 6 GHz, non-orthogonal waveforms, and network slicing.

One area of relative clarity is the technical consensus that carrier frequencies above 6 GHz are almost certainly required to achieve the IMT-2020 peak data rate target benchmark of 20 Gbps and the user-experience data rate target benchmark of 100 Mbps. This month, the World Radiocommunication Conference 2015 is being held in Geneva. US FCC delegates to this conference are proposing five frequency bands for 5G services: 27.5 to 29.5 GHz; 37 to 40.5 GHz; 47.2 to 50.2 GHz; 50.4 to 52.6 GHz; and 59.3 to 71 GHz. This is a total of 22.4 GHz of potential new spectrum, enabling channels bandwidths up to 2 GHz. Obviously, the FCC must work diligently with other major international regulatory organizations to encourage globally harmonized 5G spectrum, in preparation for the next WRC meeting in 2019.

A little math reminds us that 5 GHz corresponds to 6 cm wavelength, and 30 GHz corresponds to 1 cm wavelength. Frequencies above 30 GHz are considered millimeter waves, and mm waves have significantly different propagation characteristics (such as free space loss, diffraction and delay spread) than cm waves below 6 GHz. Both atmospheric water vapor (peaking at about 22 GHz) and oxygen (peaking at about 63 GHz) absorb significant RF energy. While 60 GHz channel modeling has been conducted in the context of 802.11ad WiGIG, the 5G standardization process requires a more comprehensive effort. Recognizing this shortcoming, in March of this year, Nokia Networks put forward a 3GPP Work Item Description (RP-150308) to organize and unify the mm wave channel modeling effort, explicitly acknowledging the opportunity to leverage the research work currently underway outside the 3GPP.

One of the intriguing aspects of mm wave cellular is the potential for user equipment (i.e., smartphone) high order MIMO two dimensional antenna arrays. Cellular MIMO has been impractical on LTE smartphones largely due to the small physical dimensions of handheld devices, despite antenna innovations from SkyCross and others. On the infrastructure side, LTE eNodeB base stations since 3GPP Release 12 have included 2×2, 4×4, 8×8 single-user MIMO radio options with digital beamforming, and Release 14 supports eNodeB “full dimensional” (i.e., elevation and azimuth “pencil” beamforming) supporting dozens of multi-user MIMO client devices. With mm wave cellular, there is the potential to include an 8×8 array antenna in less than 40 square mm of area, small enough for use in 4.7″ screen size smartphones.

The relatively short range of mm wave cellular has led to a consensus view that 5G’s ambitious 5G peak and average data rates will be practically achievable only using small cell base stations addressing indoor high bandwidth use cases, such as streaming UHD video. Complementary to this view is the conception of 5G as overlay to existing 4G LTE-A networks, with enhancements to LTE-A, such as LTE-M (machine-to-machine) and LTE-U (unlicensed band) likely to be positioned by carriers as 4.5G.
There is much to anticipate with respect to cellular transceiver research and development in the coming decade!