UE’s ability to rapidly send detailed reports on channel conditions, an eNodeB can select among different SU-MIMO modes to implement. Different techniques are then enabled:
- Spatial Multiplexing Mode - A Open Loop
- B Closed Loop
- C Closed Loop Rank 1 - Transmit Diversity
2.3
Open, Closed Loop - Spatial Multiplexing
Both Open-Loop and Closed-Loop techniques together with Spatial Multiplexing are the main features to SU-MIMO’s great achievements in LTE throughput. With the adoption of spatial multiplexing techniques, separate data streams on N multiple antennas are created and its fullest potential is almost close to multiplying the highest throughput from UE by the adopted transmission rank, mainly equal to the number of separate streams transmitted. With spatial multiplexing, the eNodeB can group informations to be transmitted to a particular UE on a given sub-channel into multiple streams, called Layers. The number of layers is equal as the rank. Transmission rank is computed accordingly to channel conditions at the user side, as well as many more considerations such as eNodeB’s capabilities. Sending different data on each antenna, spatial multiplexing modes require rich scattering of multipath signals and best channel conditions. Under the right configuration, a UE can separate the signals from multiple transmitters, identified by different RS signals, and reconstruct separate information streams within same frequency block.
The simplest implementation for spatial multiplexing, allows a rank-2 transmission on a 2x2 MIMO antenna set-up will transmitting one layer from each Tx. Each layer reaches each UE along a different path. The UE can then reconstructs the layers using multiple information from all antennas.
With this technique, informations arrives encoded from higher level entities in one or more code-words. Each code-word represents one or more layers. In 2x2 MIMO, each code-word corresponds directly to a layer. Each layer is then mapped onto one or more antennas using a pre-coding matrix.
26 LTE MIMO Techniques
2.3.1
Open Loop
Adopting Open-loop technique no knowledge of the channel has to be shared with the transmitter. As a consequence, open loop implementations occur when the radio network layer cannot collect information or feedback from users to do any form of adjustment or when transmission quality is not good enough. This could happen for example when the UE is moving too fast and it is impossible to collect data regarding the channel state. In this case the eNodeB receives only a Rank Indicator (RI) and a CQI Report. A RI is defined as the number of data stream transmitted over the air in the same f and resource corresponding to the number of layers.
2.3.2
Closed Loop
Closed loop feedback allows a transmitting entity to retrieve information about channel conditions, provided by the users. If feedback is available, the transmitter can adjust its coding and modulation of the transmitted signals to take into account of the main channel characteristics, just to simplify the signal processing required at the receiver side and enable potentially greater performance gains. Closed-loop systems ask for channel knowledge at the transmitter side. On the other hand, unlike open loop, closed loop is adopted when the radio access network performs dynamic adjustment based on feedback from the user. In this case the eNodeB receives RI, Pre-coding Matrix Indicator (PMI) and CQI.
2.3.3
Transmit Diversity and Closed-Loop Rank-1 - Spatial Multiplex-
ing
Transmit Diversity and Closed-Loop Rank-1 Spatial Multiplexing techniques are imple- mented to boost signal to further improve throughput and capacity. A powerful solution could be experienced the best near the cell edge or in areas where user perceive low SINR or multipath conditions.
When using a Transmit Diversity Mode, MIMO the same information are sent from transmitting antennas with the aim to minimize channel interference. The UE receives data streams from both transmitting antennas at both receveing ones and reconstructs a unique stream from all multipath copies of the same signal. Multiple differentiated signals and multiple copies of the same informations decrease the percentage of losing data due to poor channel conditions.
With Closed-Loop Rank-1 Spatial Multiplexing, the eNodeB transmits a single set of data for both transmitting antennas. However, Closed-Loop Rank-1 Spatial Multiplexing
2.4 MU-MIMO 27
uses a linear pre-coding matrix to improve multipath conditions. When decoded at the receiver side, these signals contain the same informations. To combine constructively at the UE multiple signals a pre-coding matrix is adopting. Its role is to shape signala coming from each transmitting antenna. A good performance is achieved by matching a pre-coding matrix to the channel conditions experienced by users . When closed loop is adopted, the eNodeB collects precious information about channel propagation conditions and choose the best among the multiple pre-coding matrices defined for 2x2 MIMO. Taking a look to more conventional Transmit Diversity techniques, Closed-Loop Rank-1 Spatial Multiplexing allows to increase SNR significantly.
2.4
MU-MIMO
In MU-MIMO, multiple different data streams are sent to spatially separated users using the same channel configuration. Each user has been equipped with multiple receivers antennas. Adopting MU-MIMO the whole system capacity achieves a strong gain, even though no increase in UE’s throughput has been demonstrated. To achieve better results rich scattering conditions are mandatory for all UEs to decode data streams.
LTE adopts a pre-coded configuration fo MU-MIMO with several feedback collected from users. Both transmitters and receivers antennas are aware of the set of configured PMIs. Precoding in LTE is implemented maninly for two purposes:
- reduce interference among transmitted signals at the receiver side which is significantly with orthogonal modulations when multiple parallel channels are in use;
- mapping the number of multiplexed streams to the number of transmitting antennas. Usually, the same pre-coding configurations is used for beam forming and precoded spatial multiplexing in LTE.