MIMO and TM( Transmission Mode) in LTE

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4G Americas have published whitepaper titled "MIMO and Smart Antennas for Mobile Broadband Systems" . The above picture and the following is from that whitepaper

Actually MIMO operation is Subset of diffirent Transmission Mode(TM)

Below are some terminology used and their meaning 

MIMO :-MIMO is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas  to exploit multipath propagation , 
it is being  used in Wi-Fi , 3G (HSDPA+) , LTE , LTE-A(4G)
MIMO systems are used to improve the robustness of data transmission or to increase data rates. Typically, a MIMO system(MxN) consists of m transmit antennas and n receive antennas

SU-MIMO:- It will provide multiple layers of communications to a handset for increasing of throughput. Each communication layer added to the User will increase the final throughput based on number of layers. Extra layers are been added using extra antennas in space diversity and more Transmitters in Base-station side

In LTE networks , it is wide beams propagate in entire angle of the sector. But in 5G NR, beams are narrow (Massive MIMO using 64T64R antenna array)

Also number of layers can be added to a single user in Massive MIMO 64T64R array is huge. Theoretically, the number of layers are depends on number of Transmitters in base-station side. Massive MIMO AAU has 64 TX channels which means 64 layers can be added. But practically it is not possible as current algorithms cannot reduce co-channel interference of such large number of transmission layers. Current 5G systems only supports maximum 16 layers. But in SU-MIMO, 16 layers to a single user is not possible due to UE side limitations. Currently UEs are only support 2T4R antenna structure . So in SU-MIMO can have only maximum four layers to a single user.

MU-MIMO:-Main difference in MU-MIMO and SU-MIMO is that MU-MIMO improve cell throughput by utilizing same DL resources (OFDM Frequency and time resources or the RBs) shared between multiple users in same beam direction where the case of less throughput required per user. So the Cell throughput will be improved by sending more data to more users in same TTI. This technique will improve spectrum efficiency also. In SU-MIMO, all resources was allocated to single user and will improve single user throughput.

MU-MIMO also supports up to four layers per user and maximum 16 layers per cell means four users with four layers can be paired at a time or eight users with two layers or 16 users with single layer pairings are possible.

Precoding:- All spatial processing that occurs at the transmitter
Spetial Multiplexing :-multiple data (Single stream divided into multiple stream) stream sent over multiple input antennas
Diversity Coding :-single data stream sent over multiple input antennas with different coding e.g. CDMA soft handover
Beamforming :- Weighted signal pointing to the direction of receiver 

Rank Indication(RI):-The rank indication or the RI in LTE is one of the control information that a UE will report to eNodeB on either PUCCH or PUSCH based on uplink scheduling.

The eNodeB configures the RI reporting periodicity for every UE during the attach procedure and UE has to honour it. The RI reporting periodicity can overlap with CQI reporting periodicity in which case the UE will drop CQI and report RI

Rank Indication is one of the important input to eNB , in selection of the transmission layer in downlink data transmission. Even though the system is configured in transmission mode 3 (or open loop spatial multiplexing) for a particular UE and if the same UE report the Rank Indication value 1 to eNB, eNB will start sending the data in Tx diversity mode to UE . If UE report Rank Indication 2 , eNB will start sending the downlink data in MIMO mode (Transmission Mode 3)

Its very essential for the eNodeB to decode a UEs RI report to know the channel conditions so that it can schedule downlink appropriately. The LTE specifications also say that if the UE is not reporting any RI then the eNodeB should assume RANK1

Data Transmission or Simply Mode of Transmission or Transmission Mode (TM-1,2,3 etc…):-

In LTE multiple antenna used for downlink ( UE Cat >=3) , eNB (Antenna ) uses multiple TX antenna and UE use multiple Rx antenna for example 2X2 antenna system (Not exactly 2X2 MIMO)

We can use 2X2 antenna system in many different way

1.  2X2 MIMO

2. 2X2 in Diversity configuration 

3. 2X2 in Special multiplexing configuartion 

4. Beamforming

5. Use 1X1 instead of 2 antenna

Etc….. as shown in decision tree for transmission mode



Source:- Sharetechnote.com/5gworldpro.com


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