Topic #2: What is MIMO and how does it work?
As the demand for mobile data increases, new ways of delivering more data are needed. The amount of available spectrum is limited, and buying new spectrum is extremely expensive for operators. Another way of boosting capacity is by deploying MIMO.
2G and 3G networks transmitted radio signals on 1 antenna. The phones also had 1 antenna, and the radio-frequency (RF) transmission was quite simple. This implementation is known as SISO.
SISO = single input, single output
SIMO = single input, multiple output
MISO = multiple input, single output
MIMO = multiple input, multiple output
With the launch of LTE, the eNodeB is able to transmit and receive signals for a channel using 2 antennas. The phones have 2 antennas as well for receiving (though only 1 of them can be used for transmitting). So the eNodeB is MIMO, but the phone is MISO.
Under good signal conditions, half of the DOWNLINK data is sent on one antenna, and the other half is sent on the other antenna. Because the phone also has 2 receive antennas, it can receive 2 streams of data simultaneously. Thus, the overall throughput is doubled. This technique is known as spatial multiplexing. The picture below is for Wi-Fi, but the concept is the same.
For UPLINK, the phone sends on one antenna, and eNodeB receives on 2 antennas.
In the example above, downlink is 2x2 MIMO, because transmitter (eNodeB) has 2 ports, and receiver (phone) also has 2 ports. Uplink is not MIMO because transmitter is only 1 port. This is one of the reasons why uplink speeds are slower than downlink. Other reasons include lower modulation and lack of CA.
What happens if the phone is at the edge of coverage and has a weak signal? Instead of sending half the data per antenna, the entire data is sent on both antennas, for redundancy. The receiver can selectively combine the best of each signal path to reconstruct the original signal. This technique is known as transmit/receive diversity and effectively increases the cell coverage.
eNodeB dynamically adjusts between diversity and spatial multiplexing for all users under its coverage based on signal conditions to maximize throughput.
Can we use 4x4 or 8x8 MIMO to increase downlink speeds even more?
Yes, higher-order MIMO has already been standardized, and conceptually we can go even higher than that with massive MIMO, but there are practical limitations to consider. Phones are quite small already, and doubling the number of antennas while keeping a certain separation between them is challenging. Phone manufacturers are only now starting to release 4-antenna models capable of downlink 4x4 MIMO, and this is only on certain bands.
For upcoming 5G networks, massive MIMO is currently being developed and tested, and it is common to see 64 transmit ports on the base station side.
