Why are there two LTE antennas?

Question

I have a design with LTE CAT 4 and I wonder why two antennas are needed. I searched around the web but am still kind of confused. Below are my questions:

1. Is the second antenna used for Rx only?
2. Is LTE modem trying to select which antenna gives the best reception? Or the modem will combine LTE signals from two antenna?
3. If only main LTE antenna is used, what's the impact?

Thanks.

Answer 1

Well, first thing to understand is that what you're asking is basically a content of a couple of university courses, so it's a bit difficult to give an in-depth explanation.

In short, you didn't give a link to modem, so we can't be precise, but there are some general statements that can be made:

1. Should be.
2. Modem will combine both antennas.
3. It's complicated!

In short, category 4 LTE is trying to do so-called spatial multiplexing. The idea is that the antennas will be sufficiently distant, so that (oversimplifying here) each antenna can send its own independent data stream. To do that, you need sufficient number of antennas (also called "layers") at both the transmitter and the receiver. If you disconnect one of the antennas on your modem, you've just completely lost a spatial stream.

Cat 4 devices have one uplink spatial stream and two downlink spatial streams. So, if you remove a spatial stream, then you're more or less down to category 1.

I'm also not sure if the modem will try to do anything smart when transmitting, such as trying out which antenna is better for transmission or not. If I remember correctly, in category 4, there shouldn't be any uplink beamforming or anything else that would require more than 1 transmit antenna, but it's been a while.

Answer 2

It is because category 4 LTE devices support 2x2 MIMO on the downlink (base station to mobile).

So:

1. Using the two antennas simultaneously, is only for the downlink (receiving). But for transmitting, only one antenna would be used (could be either one of the two, depending on the device).
2. The modem will combine the signal received on the two antennas, depending on the LTE transmission mode used.
3. It's not clear what you mean. Category 4 LTE devices need to have 2 antennas. It is part of the requirements for Category 4. Otherwise, go for a lower category device, e.g., Category 1 or 2, and be prepared for lower performance, lower data rates, etc.

Answer 3

The second antenna is the diversity antenna. It's designed to reduce errors due to the signal reflecting off various surfaces on it's way from the transmitter to the receiver which would arrive slightly out of phase with the main direct signal. Those high data rates are achieved with phase and amplitude modulation and reflected signals cause errors. You can normally turn off the diversity antenna on an LTE modem. On a Quectel modem you use the following command:

AT+QCFG="diversity",0

Cat1.bis modems don't need a diversity antenna. These are used for IoT applications where designers never design in a second antenna.