MU-MIMO is controlled by the UlVmimoSwitch option UlVmimoSwitch option of the CellAlgoSwitch. CellAlgoSwitch.UlSchSwitch parameter. You can select TM2, TM3, TM4, TM6 as a fixed MIMO transmission mode for all UEs served by an eNodeB by setting the CellMimoParaCfg. CellMimoParaCfg.FixedMimoMode parameter. You can also allow the eNodeB to adaptively select MIMO transmission modes for UEs based on channel conditions by setting MimoA daptiveS witc h parameter. the CellMimoParaCfg. CellMimoParaCfg. MimoA
Codewords Codewords are data formed after channel coding. Different codewords represent different data. B y transmitting different data, MIMO implements spatial multiplexing. To reduce the overhead on channel quality indicator (CQI) and ACK/NACK reporting, LTE supports a maximum of two codewords. In transmit diversity, the number of codewords is 1. In addition, when there is only one antenna at the transmit or receive end, the number of codeword can only be 1. When there are two or m ore antennas at both transmit and receive ends, the number of codewords depends on the radio channel c onditions and UE category. Dual-codeword transmission is mainly used in scenarios with high SINR, low channel correlation, and UE category of 2 or above.
Layers The number of codewords may be different from the number of transmit antenna ports. Therefore, codewords need to be mapped to antenna ports. This is implemented through layer mapping and precoding. In layer m apping, the codewords are mapped to multiple layers according to certain rules. In precoding, the layered data is precoded and m apped to different antenna ports. In transmit diversity, the number of layers is equal to the number of antenna ports for transmitting cell-specific reference signals (CRSs). In spatial multiplexing, the number of layers is equal to the number of scheduled data streams (that is, the rank value). Mul ti-layer transmission requires that the UE be of category 2 or above. Downlink 2x2 MIMO and 4x2 MIMO support a maximum of two layers. Downlink 4x4 MIMO supports a maximum of four layers.
Ranks The rank of transmit diversity is 1, and the rank of spatial multiplexing is equal to the number of layers.
Downlink 2x2 MIMO and 4x2 MIMO supports rank 1 or 2, and downlink 4x4 MIMO support rank 1, 2, 3, or 4. Note that rank x indicates that the rank is equal to x .
Precoding Precoding performs mapping from layers to antenna ports. For details about precoding, see section 6.3.4 "Precoding" in 3GPP TS 36.211 V10.5.0.
Antenna Port Antenna ports mentioned in this document are logical ports used for transmission. They do not have one-to-one relationship with physical antennas. Signals on one antenna port can be transmitted over one or more physical antennas. Different antennas ports are used to transmit different reference signals. The following table provides an example:
Transmission Mode
MIMO Technique Defined in 3GPP Specifications
Description
TM1
Single antenna port (port 0)
The reference signal (RS) pattern corresponding to antenna port 0 is used for transmission.
TM2
Transmit diversity
Open-loop transmit diversity is used.
TM3
Transmit diversity
If only one data stream is transmitted, open-loop transmit diversity is used.
Large-delay CDD spatial multiplexing
If multiple data streams are transmitted, open-loop spatial multiplexing is used.
Transmit diversity
If only one data stream is transmitted without using the PMIs reported by UEs, open-loop transmit diversity is used.
TM4
Transmission Mode
TM6
MIMO Technique Defined in 3GPP Specifications
Description
Closed-loop spatial multiplexing
If one or more data streams are transmitted using the PMIs reported by UEs, closed-loop spatial multiplexing is used.
Transmit diversity
If only one data stream is transmitted without using the PMIs reported by UEs, open-loop transmit diversity is used.
Closed-loop spatial multiplexing If only one data stream is transmitted using the PMIs reported by UEs, closed-loop transmit diversity using a single transmission layer is used. TM9
Transmit diversity
If the PMIs reported by UEs are not used for signal processing at the transmitter and only one antenna port is used for the physical broadcast channel (PBCH) in non-MBSFN subframes, antenna port 0 is used for transmission. Otherwise, transmit diversity is used and only one data stream is transmitted.
Spatial multiplexing
If the PMIs reported by UEs are used for signal processing at the transmitter, spatial multiplexing is used and one or more data streams are transmitted.
Precoding Precoding is a process in which an eNodeB precodes layered data stream and maps the precoded data stream to different antenna ports. Figure 4-5 shows the layer mapping and precoding when two antenna ports are used. Figure 4-6 shows the layer mapping and precoding when four antenna ports are used. Figure 4-5 Layer mapping and precoding when two antenna ports are used
Figure 4-6 Layer mapping and precoding when four antenna ports are used
Fixed Configuration of Transmission Modes If fixed configuration of transmission modes is enabled f or a cell, the eNodeB configures one transmission m ode for all UEs i n the cell. Generally, fixed configuration of transmission modes is used for performance testing before m ultiple-antenna transmission is put into commercial use.
Adaptive Configuration of Transmission Modes If adaptive configuration of transmission modes is enabled fo r a cell, the eNodeB configures different transmission modes for different UEs in the cell.
To adapt to complex and diverse radio channel conditions, it is recommended that adaptive configuration of transmission modes be enabled for multipleantenna transmission. For example:
For a UE with a high SINR and low channel correlation, spatial multiplexing brings higher throughput gains than transmit diversity.
For a UE with a low SINR, spatial multiplexing brings lower throughput gains than transmit diversity.
For a stationary UE or a UE moving at low speed, closed-loop transmit diversity or spatial multiplexing brings higher performance gains than open-loop transmit diversity or spatial multiplexing. For a UE moving at high speed, closed-loop transmission mode may bring no or even negative performance gains and increase system feedback overheads compared with open-loop transmission mode.
The application scenarios of the transmission modes are as follows:
TM2 UEs are moving at high speed and their SINRs are low.
TM3 UEs are moving at high speed and their SINRs are high.
TM4 UEs are stationary or moving at low speed and their SINRs are high.
TM6 UEs are stationary or moving at low speed and their SINRs are low.
MIMO Technique
Description
Open-Loop Transmit Diversity
UEs with low SINRs are moving at high speed.
Open-Loop Spatial Multiplexing
UEs with high SINRs are moving at high speed.
Closed-Loop Transmit Diversity
UEs with low SINRs are moving at low speed.
Closed-Loop Spatial Multiplexing
UEs with high SINRs are moving at low speed.
LTE CQI The LTE CQI stands for Channel Quality Information. It basically includes CQI, PMI,RI components. The requirement for each of these compo nents depend on transmission mode. All transmission modes need UE to provide CQI feedback. As mentioned in the figure, LTE CQI reports can be aperiodic or periodic. Aperiodic reports are transmitted using PUSCH. Periodic reports are transmitted using PUCCH unless a ny reports coincides with the PUSCH channel transmission. Aperiodic CQI reporting is triggered when CQI request field is set to value 1 within PDCCH DCI-0 or Random Access Response Grant on PDSCH. In contrast to aperiodic reporting, the 'type of reporting' is signalled i instead of 'reporting mode' . The LTE UE uses combination of ' type of reporting' and 'transmission mode' to derive the reporting mode.
LTE PMI LTE PMI stands for Precoding Matrix Indicators. They are applicable to closed loop transmission modes: Transmission Mode-4:Closed Loop Spatial Multiplexing Transmission Mode-5:Multi-User MIMO Transmission Mode-6:Closed Loop Spatial Multiplexing using a single layer The LTE UE use PMI information to signal preferred set of weights to be applied during the precoding process. UE does this in order to maximize the downlink S/N ratio. Table-1 mentions complex weights. Based on following configurations one out of 4 is used by LTE UE. • • •
Antenna ports-2, RI =1 , PMI = {0,1,2,3} Codebook Index
0
1
2
3
Weight for antenna-1
0.7071
0.7071
0.7071
0.7071
Weight for antenna-2
0.7071
-0.7071
j*0.7071
-j*0.7071
Table-1: Closed Loop Spatial Multiplexing Weights(single layer & 2 Antenna Ports) Table-2 mentions complex weights for following configurations. This configured is use d by UE to select 1 of 2 sets of complex weights. Antenna ports=2, RI=2, PMI={0,1}. Antenna-1
Antenna-2
Codebook Index
Codeword-1
Codeword-2
Codeword-1
Codeword-2
1
0.5
0.5
0.5
-0.5
2
0.5
0.5
j*0.5
-j*0.5www
Table-2 Closed Loop Spatial Multiplexing weights(2 layers & 2 antenna ports)
For antenna ports of 4, PMI of value {0,1,....14,15} can be used to indicate 1 of 16 sets of complex weights. LTE PMI can be transmitted using PUSCH or PUCCH channel.
LTE RI LTE RI stands for Rank Indicator. RIs are applicable for open loop transmission and closed loop transmission modes. These modes use more than a single layer between layer mapping and
precoding modules. Transmission mode-3 indicates open loop spatial multiplexing Transmission mode-4 indicates closed loop spatial multiplexing
• •
In LTE system, UE uses RI to inform about number of layers required during layer mapping. LTE RI can be transmitted using PUSCH or PUCCH. Number of layers = Number of codewords (for Antenna elements=2), Here UE can signal RI equal to 0/1 to indicate 1 or 2 layers as preferred one Number of layers >= Number of codewords (for Antenna elements=4), Here UE can signal RI e qual to {0,1,2 or 3} to indicate 1,2,3 or 4 layers as preferred one.
Scheduling Request Scheduling request is a single bit flag which is used to request PUSCH resources from eNodeB. In LTE, SR is transmitted using PUCCH channel.
HARQ ACK and NACK HARQ stands for Hybrid Automatic Repeat Request. Uplink HARQ ACK/NACK are used to acknowledge downlink data transmitted on PDSCH. Either o ne or two ACKs can be reported based on no. of codewords transmitted during corresponding PDSCH subframe. Downlink data is received during subframe "N" is ACKed during Subframe "N+4". HARQ ACK /NACK ca n be transmitted using PUSCH or PUCCH.
