Multiple Input Multiple Output (MIMO)
MIMO, Multiple Input Multiple Output is another major innovation in LTE which is used to improve the data performance by the use of radio path reflections. MIMO has been included as an integral part of LTE. Multiple antennas for reception and transmission at the eNB and in UEs are a key enabler of the high performance offered by 3rd Generation Partnership Project (3GPP) LTE. Adding more antennas in MIMO increases the complexity of handling the signal but however it enables far high data rates to be achieved along with much improved spectral efficiency. MIMO capacity grows linearly when we increase the number of transmit and receive antennas and it is approximately M times(M number of antennas) larger than SISO capacity.
The advantage (in term of capacity) of MIMO systems is mainly due to the exploitation of multipath. MIMO utilizes the multipath signal propagation that is present in all terrestrial communications. The transmitter and receiver have more than one antenna and using the processing power available at both ends, they are able to utilize the different paths that exist between the Tx and Rx antennas to provide improvements in data rate of signal to noise.
In 4x4 MIMO the base station emits 4 data streams separated from each other using the same resources in both frequency and time, separated only through use of different reference signals. In a practical environment, the throughput enhancement can be lower than the maximum gain according to the channel environment.
The effectiveness of MIMO in a real network depends on a number of factors, including antenna separation on the transmitting and receiving devices, the level of scattering and multipath propagation in the radio path, the signal-to-noise ratio of received signals, and the speed of the mobile terminal. First, they allow the receiver to distinguish the different transmitting antennas, and therefore to simultaneously transmit multiple symbols. Then, each path is a replica of the transmitted signal, and therefore carries useful information. We can also say that each path is equivalent to the direct signal emitted by a virtual antenna, which virtually increases the number of transmit antennas.
Advantages of 4x4 MIMO:
Increased UL/DL throughput by increasing the number of TX and RX antennas to 4
Offers high QoS (Quality of Service) with increased spectral efficiency
Increased spatial diversity and multiplexing gain
Minimizes the fading effects on the signal traveling from transmit to receive antenna
Better Signal to Noise Ratio(SINR) from 4 Rx antennas
It helps in achieving reduction in BER (Bit Error Rate)
However we also have some challenges,
Each antenna requires individual RF units for radio signal processing.
Device battery drains faster due to processing of complex signal processing algorithms
Hardware complexity is higher compared to single antenna based system.
4 layer gains require high SINR
There is a way to decode the UE capability to get information about the LTE bands that support 4x4 MIMO. For this we need to check the “UE Capability Information” message.
To find the LTE bands capable of supporting 4x4 MIMO in TM3/TM4 mode, the LTE bands in IE “supportedBandCombination-vxxxx(Item 0, Item 1,........, Item n)” should be mapped to the IE “supportedBandCombination-r10(Item 0, Item 1,........, Item n)” where n is ≤ 128. In the below diagram it is clearly explained with some examples.
In below diagram it is shown with some examples of how to find the LTE bands capable of supporting 4x4 MIMO in TM9 mode. This has to be checked in the "UE Capability Information" message in the IE “supportedBandCombination-r10(Item 0, Item 1,........, Item n)” where n is ≤ 128.