Precoding, Codebook vs Non codebook based MIMO

Precoding: As an electromagnetic wave moves from its source to its destination, its path tends to alter. Its phase or amplitude may shift, and if it does so excessively, the target will never be able to decode the original signal. In the event of non-codebook transmission, precoding is one method on the transmitting side that uses reference signals like CSI (channel state information) to adjust these transmitting signals appropriately to ensure that it effectively counters the noise.

Codebook Based MIMO Transmission:

When UE transmits PUSCH utilizing codebook-based transmission, precoding weights chosen from a codebook standardized by 3GPP are used. When choosing the weights that the device will use, gNB on PDCCH indicates these precoding metrics.UE does not have to base its uplink precoding weights on downlink measurements, hence it does not depend on the channel reciprocity.

Antenna are categorised as below:

1. Fully-Coherent antennas: These antennas are more costly than other types in the market since the UE controls the antenna phase.
2. Partially Coherent: This indicates that two of the antenna ports are coherent while the others are not, and crucially, that these combinations are not coherent with one another.
3. Non Coherent: These antenna ports lack phase alignment, both phase and amplitude of the signal direction during beamforming cannot be controlled.

Below is one example for coherant pairs and partial coherence.

Each of these precoding matrices is referred to as the Transmitter Precoding Matrix Indicator (TPMI), which allows the gNB to inform the UE about the appropriate one to utilize according to the current conditions of the channel, aimed at mitigating phase shifts or noise present in the channel.

SRS – Sounding reference signal. It is a reference signal utilized by the device in the uplink direction towards the gNB to gather information such as the number of layers, rank, and antenna ports needed for improved transmission quality, which the device and antenna combination will employ.

Each mobile device will have antenna ports oriented in various directions. For instance, in one scenario, there are two antenna panels with a total of eight combined antenna ports. Each port in the panel produces its own SRS reference signal, which the gNB receives, analyzes, and determines which one provides the strongest and most effective signal, and this is then communicated back to the device.

Each SRS resource set can be configured and triggered by 3 ways.

1. Aperiodic – triggered to UE via PDCCH
2. Periodic – RRC signalling
3. Semi persistent way – MAC control element

gNB compares each of the SRS signals from each antenna port and evaluates the best combination of the panel and port and informs the device via one of the ways mentioned by the gNB.

gNB informs UE via PDCCH where it carries DCI0_1 which has the precoding matrix (TPMI) to be used, pusch resources to used in uplink data transmission, SRI flag where it indicates when to send next SRS signalling and layers.

Non codebook based transmission:

Based on the UE capability information shared to gNB, it decides and indicate UE to use either codebook or non-codebook transmission. Here UE by itself has to generate its own precoding weights based on the measurements in downlink from CSI(channel state information). These precoding matrices are not standardised by 3gpp unlike in case of codebook based tx.

here UE evaluates the best antenna panel in case of multiple ones for uplink transmission, and UE will use downlink measurements to derive precoding index for each SRS resources. out of these SRS resources transmitted to gNB which will indicate the best and evaluate the no of layers to be used out of them.

In non-codebook based tx max layers supported are 4. for ex if UE sends four SRS resources 1,2,3 and 4 gNB can inform only 2 are best and ask the UE to use the same.

3gpp 38.211,38.214 and 38.331