The Signal #47: You’re holding your 5G handset wrong

from our 9/7/2019 newsletter

Friends – our last post discussed how broad 5G access (in particular, for mm-wave 5G) will likely be driven by mundane but important operational concerns like pole access. In came the question (thanks, Eric) – what about T-Mobile? Would T-Mobile need to get on every lamppost in San Francisco? (Note T-Mobile already has a lot of DAS poles in San Francisco via partner Crown Castle.)

Good question. The answer comes down to how T-Mobile applies its 600 MHz holdings.  

A little background: T-Mobile acquired 600 MHz in the 2017 600 MHz auction, aka the Broadcast Incentive Auction. That was the second auction of former TV spectrum, following the 700 MHz auction in 2008

For spectrum nerds out there, here’s the 600 MHz band plan.

TV to the left, cellular to the right

Now, in this plan, uplink is on higher spectrum than downlink. In cellular, when using paired spectrum, usually uplink is lower and downlink is higher. Handsets are battery powered and lower spectrum travels further.  Thus, having signals sent from the handset up to cell tower on the lower band puts less burden on the battery, which makes for a happier consumer.  In the 600 MHz case, putting uplink on the higher frequency closer to 700 MHz may be for antenna length reasons. Essentially, the crux of this post.

T-Mobile was the biggest winner at the 600 MHz auction.  The propagation characteristics of 600 MHz and their narrower bandwidth would lend it to use as what operators would call a macro overlay, which provides broad coverage of a city without needing a densified network.  As spectrum, it is cost-efficient from a site acquisition perspective.  T-Mobile has fielded 600 MHz for LTE – if you are a T-Mobile customer, check if your LTE supports Band 71. Here’s a list of compatible handsets. If it does, it is capable of using 600 MHz spectrum.  

Now, if used for 5G,  the narrower bandwidth of 600 MHz channels should make it, well, 5G Lite.  Assuming ~23 b/s/hz for 5G, at 5 MHz bandwidth, then theoretical maximum speeds (without factoring in overhead, or interference from a human hand, etc) would be ~115 Mbps (not accounting for any MIMO magic), and significantly slower in reality. Fast for cellular, but not a fiber substitute. 

Recall that when 600 MHz was used for TV, it was received through a loop antenna.  Which makes me wonder if 600 MHz phones would be better off with an external (extensible) antenna, much as early cellphones had

back to the future

Which reminds me of one of the more amusing moments of the early smartphone era – in 2010, Steve Jobs telling iPhone 4 owners they were holding their phones wrong.  Really, this happened, and only Steve Jobs could get away with that. As a thought exercise, imagine Tim Cook saying something similar.  Or saying, hey, just buy a pair of AirPods. 

Here’s a capture from the iPhone 4 keynote.

Look, magic!

And a great breakdown from Anandtech as to whether the iPhone 4 antenna was actually better than the 3Gs.  Eventually, Apple sent iPhone 4 owners plastic bumpers, which put a buffer between the phone and the bag of RF-impeding water that is a human.

I’m somewhat surprised antenna reception issues haven’t come up with T-Mobile 600 MHz customers (Reddit thread here. Generally, Reddit or T-Mobile support posts on 600 MHz seem to be about limited coverage. With the repacking of TV channels likely happening on an MSA by MSA basis, this is to be expected).    A wavelength calculator indicates that half-wavelength at 663 MHz would be about 9 inches. Longer than most smartphones, but not longer than a tablet.  

T-Mobile has tested 5G at 600 MHz and is certainly talking about it. (The Samsung S10 5G supports Band 71, but for LTE, not 5G NR.)  But, at some point, will John Legere tell consumers they’re holding their 600 MHz 5G phones wrong? Or just send them an externally attachable antenna?

Excuse me,  I’m going to take this call next to a window.

(h/t Jay Goldberg)