MIMO, Multiple Input Multiple Output is a technology that was introduced into many wireless communications systems including 4G LTE to improve the signal performance.
Using multiple antennas, LTE MIMO is able to utilise the multiple path propagation that exists to provide improvements in signal performance.
LTE MIMO adds complexity to the system, but it is able to provide some significant improvements in performance and spectral efficiency and these more than justify its inclusion in the LTE standard.
LTE MIMO: introduction
MIMO is basically an antenna technology as it utilises a number of antennas to provide the performance improvements.
The basic concept of MIMO utilises the multipath signal propagation that is present in all terrestrial communications. Rather than providing interference, these paths can be used to advantage.
The transmitter and receiver have more than one antenna and using the processing power available at either end of the link, they are able to utilise the different paths that exist between the two entities to provide improvements in data rate of signal to noise.
Note on MIMO:
MIMO is a form of antenna technology that uses multiple antennas to enable signals travelling via different paths as a result of reflections, etc., to be separated and their capability used to improve the data throughput and / or the signal to noise ratio, thereby improving system performance.
Read more about MIMO technology
MIMO is being used increasingly in many high data rate technologies including Wi-Fi and other wireless and cellular technologies to provide improved levels of efficiency. Essentially MIMO employs multiple antennas on the receiver and transmitter to utilise the multi-path effects that always exist to transmit additional data, rather than causing interference.
The use of MIMO technology has been introduced successively over the different releases of the LTE standards.
MIMO has been a cornerstone of the LTE standard, but initially, in releases 8 and 9 multiple transmit antennas on the UE was not supported because in the interested of power reduction, only a single RF power amplifier was assumed to be available.
It was in Rel. 10 that a number of new schemes were introduced. Closed loop spatial multiplexing for SU-MIMO as well as multiple antennas on the UE.
LTE MIMO modes
There are several ways in which MIMO is implemented in LTE. These vary according to the equipment used, the channel function and the equipment involved in the link.
- Single antenna: This is the form of wireless transmission used on most basic wireless links. A single data stream is transmitted on one antenna and received by one or more antennas. It may also be referred to as SISO: Single In Single Out or SIMO Single In Multiple Out dependent upon the antennas used. SIMO is also called receive diversity.
- Transmit diversity: This form of LTE MIMO scheme utilises the transmission of the same information stream from multiple antennas. LTE supports two or four for this technique.. The information is coded differently using Space Frequency Block Codes. This mode provides an improvement in signal quality at reception and does not improve the data rate. Accordingly this form of LTE MIMO is used on the Common Channels as well as the Control and Broadcast channels.
- Open loop spatial multiplexing: This form of MIMO used within the LTE system involves sending two information streams which can be transmitted over two or more antennas. However there is no feedback from the UE although a TRI, Transmit Rank Indicator transmitted from the UE can be used by the base station to determine the number of spatial layers.
- Close loop spatial multiplexing : This form of LTE MIMO is similar to the open loop version, but as the name indicates it has feedback incorporated to close the loop. A PMI, Pre-coding Matrix Indicator is fed back from the UE to the base station. This enables the transmitter to pre-code the data to optimise the transmission and enable the receiver to more easily separate the different data streams.
- Closed loop with pre-coding: This is another form of LTE MIMO, but where a single code word is transmitted over a single spatial layer. This can be sued as a fall-back mode for closed loop spatial multiplexing and it may also be associated with beamforming as well.
- Multi-User MIMO, MU-MIMO: This form of LTE MIMO enables the system to target different spatial streams to different users.
- Beam-forming & MIMO: This is the most complex of the MIMO modes and it is likely to use linear arrays that will enable the antenna to focus on a particular area. This will reduce interference, and increase capacity as the particular UE will have a beam formed in their particular direction. In this a single code word is transmitted over a single spatial layer. A dedicated reference signal is used for an additional port. The terminal estimates the channel quality from the common reference signals on the antennas.
As the LTE standards progressed, so the numbers of antennas being supported increased. For many mobiles the use of MIMO just resulted in improvements in signal performance, whereas for others it was able to provide real data rate improvements.
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