One of the things I have been writing about quite a bit is the challenges to 5G in the mmWave bands. This seems to be bubbling to the top, more and more, as carriers are under pressure to roll out 5G. The hype is that it is the silver bullet for densification. However, using mmWave >6 GHz is problematic, especially for mobile wireless, for a number of reasons.
It seems that some carriers agree. There has been more data coming out that 5G at mmWave frequencies may be more problematic than first thought. Perhaps that is why U.S. Cellular has followed in T-Mo’s and AT&T’s footsteps and to 5G services to rural customer using old 600 MHz 4G spectrum. Both T-Mo and AT&T have been talking about deploying 5G at frequencies at low band, below 1 GHz.
However, this may not be much more than a slightly jazzed-up version of 4G performance. While new bandwidth manipulation technologies have emerged, they are a bit more limited at lower bands than mmWave, where large chunks of spectrum are available, offering hundreds of megahertz of bandwidth. At low band, bandwidth is in the 10s of megahertz.
Millimeter wave is going to play a critical role in 5G networks. There is no question about that. Nevertheless, it is fraught with challenges, they are varied and complex, particularly the pervasiveness of a cellular signal running on mmWave spectrum from a small cell. While this has always been on the top of the list, a recent report by MoffettNathason presented data that mmWave propagation rates are lower than initially expected. The report cited data from Verizon’s fixed 5G network in Sacramento.
That deployment, according to the report, determined that each small cell was serving an average of 27 eligible addresses. That was lower than initial expectations. The exact reasons for that have yet to be determined, but there is discussion that original estimates were too liberal, or that propagation models were flawed. Regardless, it proves the age-old adage of e=mc (squared), plus or minus 10 percent – what you design is not, always, what you get, in spite of what the math says.
The ramifications of that are significant. That implication permeates across multiple sectors, from power to permissions. In the end, it could require a much denser small cell landscape than originally envisioned and network designers have to start looking harder at some of the variables that affect propagation at mmWave frequencies. Therefore, 5G mmWave is not the magic bullet to nationwide coverage, as has been hyped, and 5G will encompass all bands, low, medium and high bands, as well as mmWave.
We are, really, only at the initial stages of large-scale mmWave deployment. We have a lot to learn. There is no doubt the industry will figure it out, eventually, but as we gain knowledge in this area, we have to be careful not to lose sight of what it can, and cannot do.
Therefore, perhaps, now we have some insight into why the carriers have been paying attention to low band spectrum.
While 5G at low frequencies may not have the sex appeal of mmWave, or the bandwidth availability, applying technologies such as multiple-in, multiple-out (MIMO) carrier aggregation, beam steering, network slicing, as well as new modulations schemes can optimize bandwidth utilization. The propagation properties of under 1 GHz frequencies is both well understood and mature. Working these frequencies with 5G technology can be a viable part of the 5G ecosystem.