Single User and Multi User MIMO codewords in LTE
Figure shows how the two codewords are used for a single user in the downlink. It is also possible that the code words to be assigned to different users to create multi-user MIMO (MU-MIMO). Depending on the channel information available in the eNB, modulation and precoding layers may be different to match the performance.
Precoding choices are defined in the search, known as codebook table. Codebook is used to quantize the options available, and thus limits the amount of information fed back from the receiver to the sender. Some precoding choices are clear. For example, Codebook index (CI) 0 is a direct set of codewords to layers and CI 1 applies spatial expansion.
Table 1 shows the choices codebook for one and two layers. Note only the case in two layers employs spatial multiplexing. Precoding with a layer is limited to a displacement 0 °, 90 ° or 180 ° of phase.
In operation, the UE will send a message to the scheduler ENB with the index codebook that is closest to the channel, even if the system can be configured for multiple codebook values, one for each group of resource block. To use this information while it is still valid, the scheduler must respond quickly, in a few milliseconds, depending on the speed of variation of the channel. If the EU is to provide information on a more regular channels, the information will be more accurate, but the percentage of resources used to increase the signal and set more challenging on ENB.
Single User and Multi User MIMO Codewords in LTE
In LTE, MIMO (Multiple Input Multiple Output) technology is used to improve data throughput and reliability by transmitting multiple data streams simultaneously over multiple antennas. The concepts of single-user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO) codewords are key to understanding how data is transmitted in LTE networks. These approaches are designed to optimize the use of available resources based on the number of users and their channel conditions.
Single User MIMO (SU-MIMO) Codewords:
In SU-MIMO, the focus is on a single user equipment (UE). The eNodeB transmits multiple spatial streams (codewords) to the UE using the multiple antennas available. The number of codewords is typically limited to two in LTE systems. In this scenario, the UE receives all the streams, and its receiver is responsible for separating and decoding them. The key aspects of SU-MIMO codewords are:
- The eNodeB sends multiple data streams (codewords) to a single UE.
- The number of codewords depends on the number of spatial layers and the antennas available at both the transmitter and receiver.
- The UE uses the available channel state information (CSI) to decode the multiple codewords.
- The goal of SU-MIMO is to increase the throughput for a single user by utilizing the spatial diversity and multiplexing offered by the multiple antennas.
Multi-User MIMO (MU-MIMO) Codewords:
In MU-MIMO, the eNodeB serves multiple UEs simultaneously, using the same time-frequency resources. Each user is allocated different spatial streams (codewords) based on their channel conditions. Unlike SU-MIMO, where all the streams go to a single UE, MU-MIMO allows for the transmission of different codewords to different UEs. The main characteristics of MU-MIMO codewords are:
- The eNodeB transmits separate data streams (codewords) to multiple UEs simultaneously using the same frequency and time resources.
- Each UE receives a unique set of codewords, which are separated using advanced techniques like beamforming and spatial division multiplexing.
- MU-MIMO improves overall system capacity and throughput by efficiently using the available spectrum and allowing multiple users to share resources.
- The number of codewords is determined by the number of antennas at the eNodeB and the number of UEs being served, with each UE being allocated a specific codeword (or set of codewords).
Key Differences Between SU-MIMO and MU-MIMO:
- SU-MIMO: Used to improve the performance of a single user by transmitting multiple data streams (codewords) to that user. It maximizes the throughput for the UE by exploiting spatial diversity.
- MU-MIMO: Aimed at improving overall network capacity by serving multiple users simultaneously with different codewords. It is more efficient in utilizing the available radio resources by multiplexing data streams across multiple UEs.
- Codewords: In SU-MIMO, codewords are allocated to a single user, whereas in MU-MIMO, each user gets their own codewords that are decoded separately.
How These Concepts Work Together:
- In SU-MIMO, the eNodeB exploits the channel conditions of a single UE to send multiple streams, improving the data rate for that UE.
- In MU-MIMO, the eNodeB sends different codewords to multiple UEs, improving the system’s overall capacity and making efficient use of the available spectrum.
In summary, SU-MIMO and MU-MIMO codewords in LTE play a vital role in maximizing throughput and improving the efficiency of the network. SU-MIMO focuses on enhancing the performance of individual users, while MU-MIMO improves the overall system capacity by serving multiple users simultaneously with distinct codewords.