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Carrier Aggregation Explained - how LTE speeds are about to get much faster

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[Disclosure: I work for Qualcomm in technical marketing for LTE. However, I wrote this post in my personal capacity. This posting only represents my personal opinions and not those of Qualcomm. No one from Qualcomm reviewed or modified this post prior to its posting.]

LTE is about to get much faster. In fact, in many places around the world, LTE networks have either already been upgraded, or are in the process of being upgraded, to offer much faster LTE speeds.

The key to the speed upgrades is a technology called LTE Advanced Carrier Aggregation.

There's been a lot of discussion in tech blogs recently about Carrier Aggregation, and whether it will really have the impact that is supposed to have. So I thought it would be worthwhile to explain how it works.

To understand how Carrier Aggregation (CA) works and how it makes LTE faster, we first have to understand what impacts cellular network capacity, and by extension, cellular network speed. There are a few factors, among them:

  1. The bandwidth of the connection between your phone and the cell tower
  2. The quality of the signal received from the cell tower
  3. The number of people using the cell tower at the same time as you are
  4. The number of 1s and 0s that each transmission from the tower represents

For this discussion, we'll concern ourselves with the first one - the bandwidth of the connection.

If the connection between the cell tower and your phone can be represented by a highway, the bandwidth of the connection is analogous to the width of the highway. The more lanes in the highway, the more cars can fit at the same time. Same thing with bits - the wider the slice of spectrum the operator is deploying their network on (measured in megahertz, abbreviated as MHz), the more 1s and 0s can be sent to your phone at a given time, and the faster your Internet connection.

Here's the thing. Spectrum is a precious resource. Like real-estate, there's only so much of it to go around. Government agencies responsible for dividing up spectrum amongst mobile operators, such as the FCC in the US, continue to free up slices of spectrum at different frequencies, to give operators the chance to add capacity to their networks.

The result is that a single operator will have licenses to multiple slices of spectrum at different frequencies - that is, they are not adjacent to each other. The result is spectrum fragmentation - an operator could be spectrum rich, but their spectrum is divided up piecemeal amongst different bands. For example, an operator could own a license to 20 MHz of bandwidth in the 700 MHz frequency band, and 20 MHz of bandwidth in the 1800 MHz band.

Let's say the operator first deployed an LTE network in the 700 MHz band. That would allow a peak theoretical download speed of 150 Mbps. The operator then proceeded to deploy LTE in the 1800 MHz band as well. That could effectively double the network capacity (to a first degree...capacity is such a complicated topic :) ). But...an LTE phone connected to that operator's network could only connect to one of those bands at a time, and in any circumstance, would enjoy download speeds of 150 Mbps at best. Of course in reality, because of the other factors we mentioned above, real-world speeds would be much less.

But what if the operator wanted to offer even faster speeds? There is usually no way to simply widen an existing slice of spectrum. Back to our highway analogy, the highway is already surrounded by other roads and buildings (in this case other spectrum users, e.g. another mobile operator), and cannot be made any wider. Not only so, but the LTE standard itself only allows a maximum bandwidth of 20 MHz for any single LTE carrier.

[Note: "carrier" here does not refer to a mobile operator. In the LTE standard, carrier refers to the radio wave that "carries" the information from the cell tower to the phone, or vice versa]

That's where LTE Advanced comes in. It's is a collection of LTE technologies that are designed to make LTE networks run faster and more efficiently, using some pretty exotic techniques. But perhaps the most important single technology in LTE Advanced is Carrier Aggregation.

Carrier Aggregation solves the dilemma we discussed above. It allows fragmented LTE connections to be combined into a wider, faster LTE connection.

So back to our example above, the mobile operator can upgrade their network to deploy the 700 MHz LTE carrier and the 1800 MHz carrier in aggregation. But it's not just the network - the phone needs to support carrier aggregation as well. That means the phone needs to have the right hardware. It's not a simple software upgrade.

Let's talk speeds. The speed boost provided by CA is a matter of math. Every 20 MHz of bandwidth translates to 150 Mbps, at least the way that LTE networks are phones are designed today.

[Note: nerds will know that all these numbers are for FDD LTE. TDD LTE, which splits the time of a single LTE carrier between download and upload, will have lower speeds]

So if the operator aggregates two 20 MHz LTE carriers, the peak theoretical download speed of their network is 300 Mbps. If they aggregate one 10 MHz LTE carrier with a 20 MHz LTE carrier, the peak theoretical download speed is 225 Mbps (¾ of 300). And so on.

So far we've discussed 2x CA - the aggregation of two LTE carriers. But 3x CA is also possible, and it is the maximum supported by the best LTE chipsets out there. Aggregating 3x 20 MHz LTE carriers can result in peak downloads speeds of 450 Mbps.

All these peak speeds are mouthwatering, but of course they are almost never experienced in real life (except in South Korea! More on that later). You may be skeptical, and rightly so - if you don't even get anywhere near to 150 Mbps today from your operator, how are you to believe that 300 Mbps, never mind 450 Mbps, would be any more within reach?

Well here's the beauty of CA. Whatever speed you're experiencing right now - say you're getting 15 Mbps instead of 150 Mbps, CA can potentially double or triple it. So if your operator went from a single 20 MHz LTE carrier to 2x20 MHz LTE carriers, you could go from 15 Mbps to 30 Mbps - assuming your phone supports 2xCA of course. If they're deploying 3x20 MHz, your speed could go from 15 Mbps to 45 Mbps.

Think about that. 45 Mbps from thin air, in the palm of your hand. Kind of amazing actually (and when you dig deep into how it all works, it's astounding that it all works at all). Trying to download a big file from cloud storage? The download time would be slashed by up to two thirds. How often do we get to experience such a step function increase in speeds, in anything?

But where CA really shines is not in the best of circumstances - it's actually in the worst of circumstances. Remember at the very beginning of this discussion, we said that one of the factors that impacts your cellular connection speeds is the quality of the signal. A good strong signal is needed for the phone to distinguish the 1s and 0s that the cell tower is sending it. Really in the same way that the farther away you are from someone, the harder it is to hear them and understand what they're saying.

So if you're far away from a cell tower, or deep inside a building, or under ground, then the signal from the tower will be very faint by the time it gets to your phone (reflected in a lower number of bars on your signal indicator). As a result, the maximum speed your phone can receive data at is diminished.

Let's say you're inside a brick building in New York and you have weak cell reception. If you're connected to a mobile network with only one LTE carrier, maybe your speed is only, say, 3 Mbps (out of 150 Mbps peak theoretical!). You can't even stream a medium quality 5 Mbps YouTube video at that speed. But what if instead of being connected to a single LTE carrier network, you were connected to one that aggregated two LTE carriers (with a compatible phone of course!), then you would get 6 Mbps instead of 3 Mbps. It's not much, but it is double. And it would make that YouTube video stream smoothly.

What's surprising is that CA not only benefits people with CA-enabled phones, it even benefits people without CA-enabled phones. That may sound counter-intuitive, but it's true. Because people with CA-enabled phones finish downloading content faster, they get on and off the network faster. So they free up the network faster, for other people without CA-enabled phones to get their business done. That increases the capacity of the network, which can translate to faster speeds for everyone involved. Sweet.

CA has even more benefits. The fact that it can blend LTE carriers that travel far and penetrate buildings well (e.g. 700 MHz band) with ones that don't travel as far, and as a result, have higher capacity (e.g. 1800 MHz band) means that the a user with a CA-enabled phones that connects to both bands simultaneously gets the best experience possible. Outdoors, the 1800 MHz band does the heavy lifting in terms of providing high speeds, and inside buildings, the 700 MHz penetrates better to carry the day. Awesome.

Yes, I love Carrier Aggregation. It's conceptually such a simple idea, yet in reality has layers and layers of benefits.

BTW, carrier aggregation is not just for downlink. Uplink LTE carriers, that carry data from your phone to the cell tower, can also be aggregated. Uplink CA is also about to become a reality. With all the Instagramming and Periscoping we're all doing, not to mention how large smartphone camera images and videos are becoming, it's about time we get a boost in upload speeds as well.

The question remains: how can you determine whether your phone supports CA? To answer that question, we need to talk about Cats. Not the cats of YouTube video fame, but LTE Categories.

The organization that standardized LTE, called the 3GPP, has designated different speed classes for LTE devices. They are called User Equipment (UE) Categories. A UE category defines the maximum download and upload speeds of an LTE device. The most important UE categories are:

  • Cat 4 - 150 Mbps peak download, 50 Mbps peak upload
  • Cat 6 - 300 Mbps peak download, 50 Mbps peak upload
  • Cat 7 - 300 Mbps peak download, 100 Mbps peak upload
  • Cat 9 - 450 Mbps peak download, 50 Mbps peak upload
  • Cat 10 - 450 Mbps peak download, 100 Mbps peak upload

Note that UE category and the number of aggregated LTE bands are actually two separate concepts. The combination of what is supported by the phone with what is supported by the network can lead to many permutations.

This is going to get confusing, so stay with me. If a phone can only aggregate two bands, but the operator deployed 3x CA to achieve 300 Mbps peak speeds (e.g. 20 MHz + 10 MHz + 10 MHz), then that phone would ever only be able to achieve 225 Mbps peak speed, since it can only combine two of the bands at a time (20 + 10 = 30 MHz total, which translates to 150 + 75 = 225 Mbps). So depending on your operator, and how much spectrum they hold, and what they deploy, your choice of mobile phone will have a direct impact on the maximum speed you can enjoy.

The key here is that your choice of phone matters. LTE speed is not just a question of the network - the phone hardware, specifically the processor with integrated LTE, or the LTE modem, inside the phone, will dictate how fast your speed can be. So if you want to get the best speeds, choose both your network and your phone wisely.

I hope you found this helpful.

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Appendix for nerds

Here is some good reading about Carrier Aggregation

  1. The Global Mobile Suppliers Association tracks how many networks around have CA enabled, and how fast the networks are. Here's a recent report.
  2. You don't have to take my word on how fast CA can really be. Here are three reports about real CA networks from Signals Research Group, which specializes in mobile device and network evaluation:
    1. Testing SK Telecom's network in South Korea (225 Mbps peak a the time of publishing)
    2. Testing LG U+'s network in South Korea (300 Mbps peak at the time of publishing - spoiler: they saw 297 Mbps in real testing!)
    3. Testing China Mobile's crazy 3xCA Cat 9 network (320 Mbps peak at the time of publishing)
  3. Google "4G+" to see how some operators have been promoting and explaining CA to their consumers using simpler terminology.