How do cell phones do carrier aggregation with a single 4G radio?


You can go a bit beyond on this one since I have a rough idea of how cell phones work.

What I’m interested in is how carrier aggregation works. The way I understand it is, your phone can connect to, say a 10 MHz channel from band 3, a 15 MHz channel from band 1, a 5 MHz channel from band 20, let’s say, for a combined 30 MHz.

Okay, but how does it do that with a single 4G radio? That’s 3 different frequency bands. On Wi-Fi, you can only connect to a contiguous channel with 1 radio, you can’t split channels up, let alone bands.

So how does 4G do it? And how can a single 4G radio’s tiny antennas connect to such a wide frequency range spanning 700 – 3500 MHz?

I know MIMO is also a thing, but my understanding is that for MIMO to work, the antennas have to be all the same.

In: 1

Its less to do with the antennas and more to do with signal processing and filtering. Think about the radio in your car. It only has one antenna, but you can change what channel is focusing on. The antenna is still picking up all the other channels, but the radio is filtering for the frequency you selected. So long as the antenna is capable of sending and receiving signals across that frequency range, you only need a single antenna. But you have to have equipment attached to it that can process multiple frequency signals at once.

The 4G and 5G protocols were designed to allow this multi-band use. Wi-Fi wasn’t. Its not that Wi-Fi hardware technology can’t, its more that there wasn’t a good reason to. A single Wi-Fi channel is able to supply most users with the bandwidth they require. Limiting each access point to a single channel at time helps prevent channel congestion. Most Wi-Fi access points are also designed to service relatively small areas with a relatively small number of users (compared to a cell tower).