From the IEEE:
Hype surrounding 5G wireless networks may be more pervasive than the networks themselves, but new research is suggesting practical ways to overcome issues brought on by the very methods of expanding these networks.
If the research—which addresses two of the most prominent wireless expansion methods, mass femtocells and millimeter-wavelength (mmW) antennas—hold true, wireless developers and carriers may have some valuable tools at their disposals to make 5G more of a widespread reality.
New research into femtocells and millimeter waves is stoking excitement about the future of wireless systems; for me they stoke excitement and something else – nostalgia.
Five years ago, while in graduate school, I prepared a report and case study of wireless propagation schemes in subway environments; the case study involved the then-experimental Help Points now installed throughout the NYC Subway system, as well as the Transit Wireless Wi-Fi project that’s bringing free Wi-Fi service to NYC’s subway stations. At that time, the MTA tested two different systems for their Help Points – fiber (wired) and millimeter-wave (wireless). As I wrote back then:
In the pilot program, a set of Help Points were installed in two stations on the Lexington Avenue Line – 23rd Street and Brooklyn Bridge. The system at 23rd Street uses a wired fiber-optic network, while the Brooklyn Bridge system uses a millimeter-wave DAS; MTA is testing each system to determine which is most reliable.
(DAS = Distributed Antenna System.)
Earlier in my paper, I expounded on millimeter-wave propagation:
Another solution to the high-speed handoff problem prevalent in subway environments, developed recently and standardized in 2009, is millimeter-wave propagation . As outlined in the IEEE 802.15.3c standard, millimeter-wave propagation uses the unlicensed 57 GHz – 64 GHz band and allows data rates as high 1.6 Gbps . It is expected that this standard will allow data rates up to – and potentially exceeding – 3 Gbps in the near future.
The high data rates provided by millimeter waves allow for such applications as uncompressed high-definition (HD) video transmission and streaming; the speeds would allow users to download or stream such video in a matter of seconds – even in subways . Additionally, given that transit systems around the world are using the Internet, text messaging, and email to inform patrons of up-to-the-minute service changes, diversions, and scheduling information, millimeter-wave technology can help patrons access this information nearly instantaneously.
 and  above refer to two of the 42 sources I cited in my 14-page paper (IEEE manuscript format, not the usual 12-point Times Roman double-spaced format, mind you!) – namely, papers by Fokum & Frost (“A Survey on Methods for Broadband Internet Access on Trains” – 2010) and Razavi (“Design of Millimeter-Wave CMOS Radios: A Tutorial” – 2009).
As seen above, the theoretical data rates for millimeter waves specified in the IEEE 802.15.3c standard of 2009 (roughly 1.6 Gbps, with even higher speeds possible) exceed those of today’s 4G LTE systems (average rate 12 Mbps, peak rate 50 Mbps, theoretical max 300 Mbps) by a wide margin.
Clearly, engineers were on to something when looking at millimeter-wave technology as the future of wireless. (I still believe that Li-Fi is the future of wireless, but I’m partial to millimeter waves as well.)
Further in that section of my report, I wrote:
Incidentally, a potential drawback to millimeter-wave propagation also serves as one of its greatest benefits. Because millimeter waves are absorbed by the oxygen in our atmosphere, they tend to attenuate rapidly. Consequently, the maximum distance between millimeter-wave propagators is limited to 1.6 km. However, this same atmospheric absorption gives millimeter waves a very significant property – interference immunity. That is, the interference due to millimeter waves is negligible compared to other propagation schemes, allowing millimeter-wave propagators to be installed in close proximity to one another; the IEEE 802.15.3c standard specifies close spacing of such propagators.
As noted in the previous section, smart antenna systems with beamforming capabilities reduce interference and maintain high QoS and data rates in the direction of propagation.
Engineers have studied millimeter waves in the context of smart antenna systems for many years; even now, that link could be the key to making millimeter wave systems commercially viable. As proof, here’s another excerpt from the IEEE piece first linked:
As for mmW channels, which are still primarily limited to lab work, researchers have turned to a “smart antenna” technique called massive multiple-input multiple-output (MIMO) to incorporate the short pathways of mmW signals into existing wireless infrastructure.
The piece links to two papers (two separate links, both behind paywalls; IEEE Xplore subscription required to view) that explore the femtocell and millimeter-wave approaches to next-gen wireless systems in-depth, respectively.
Needless to say, I’m super-excited about it!