How does LTE divide up throughput?
June 19, 2019 12:40 PM   Subscribe

How does LTE divide up throughput? I've searched but the explanations I've found are either so general they're devoid of content or so technical I don't understand. Is there a mechanism for the the tower to assign a certain amount of time/certain frequencies/ other means off distributing throughput to each connected device, or does each device just broadcast when it thinks the channel is free à la Wi-Fi?
posted by Tehhund to Technology (5 answers total) 2 users marked this as a favorite
LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) as opposed to the Time Division or Code Division Multiple Access (TDMA or CDMA) used by GSM and legacy US systems, to divide up bandwidth between multiple handsets accessing the same tower.

OFDMA uses subcarriers; each handset gets one or more subcarriers within the block of frequencies being used by the cell tower (the channel), which itself is a subset of the frequencies available within an LTE band. If there are lots of users on a tower, you might only get assigned a couple of subcarriers; if there are few users, you can get a bunch, giving you better throughput.

There's some fairly technical stuff involved in how the subcarriers are arranged so as not to interfere with each other (that's the "orthogonal" part), but it's not really important IMO to understanding how LTE works at a basic level. What's important is that the carriers are spaced as closely as modern signals engineering can get them, without running into each other.

There is a process of negotiation that happens between the handset and the tower to request subcarriers; the handsets don't just start transmitting on subcarriers without 'asking'. In fact, there is a different scheme for the uplink and downlink sides, so a handset can't just start transmitting on a new subcarrier; it has to request and the tower has to start using the new downlink subcarrier. (Generally there's much more downlink traffic than uplink traffic, similar to your home internet connection.)
posted by Kadin2048 at 1:19 PM on June 19, 2019 [12 favorites]

This is excellent and I can finally say that I get it to my satisfaction. One follow-up: what's the smallest subcarrier available - 1.4 MHz? What happens when every subcarrier is accounted for?
posted by Tehhund at 3:07 PM on June 19, 2019

1.4 MHz is the smallest channel width specified in LTE, with 3, 5, 10, 15, and 20 MHz being the other options. The subcarriers are 15 kHz wide.

If every subcarrier is in use, then the eNodeB (cell site equipment) will not be able to assign each piece of user equipment (phone, data card, etc) as many subcarriers as it wants as often as it wants. This will result in slower data transmission for the affected users.

For what it's worth, in 5G NR (the 5G air interface specification), multiple subcarrier widths/spacings are supported: 15, 30, 60, 120 and 240 KHz. I have not read enough about it to understand the significance or purpose of supporting multiple subcarrier widths.
posted by Juffo-Wup at 4:07 PM on June 19, 2019 [2 favorites]

Subcarriers are very narrow—there are a bunch of different flavors of LTE, but I think they are always either 15 kHz or 7.5 kHz wide. Depending on the LTE channel width—which can be anywhere from 1.4 MHz to 20 MHz, although you generally only see the latter on the higher frequency bands—you can thus fit up to 1200 subcarriers in a channel.

That doesn't necessarily map directly to the maximum number of handsets (which in LTE-speak are called "UEs") that a tower can support. There's an abstraction called a "resource block", which is 12 x 15 kHz subcarriers or 24 x 7.5 kHz subcarriers, which is the smallest unit doled out to a handset. And those are divvied up by "frames" which are 10ms long. (So, somewhat contrary to my earlier oversimplification, there is a time-division scheme in LTE, but it occurs on top of the OFDMA scheme.) An idle phone, for instance, only has to transmit frames more occasionally than a phone that has an active data or voice session.

Many cell sites have MIMO radios which can operate multiple channels and even multiple bands, too.

Before you got to every subcarrier on a cell tower being full, I think you'd probably hit other limits of the "node", but I'm not entirely sure. I think you'd be more likely to register on the node, but get a fast-busy or very slow data transmission speed (or dropped data connections) in real scenarios. But perhaps others can chime in on how it works practically; my knowledge is largely theoretical. :)

On edit, Juffo-Wup did!
posted by Kadin2048 at 4:41 PM on June 19, 2019 [3 favorites]

By the way, the upcoming 802.11ax standard will also use OFDMA.
posted by Standard Orange at 10:49 PM on June 19, 2019

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