CSI RS Codebook in 5G Networks: Channel State Information (CSI) is increasingly becoming a fast-emerging element of the optimum contributing factor in wireless communications, especially in 5G and what is envisioned for 6G.
CSI is an inherent component in 5G to ensure reliable connections between user equipment (UE) and base stations (gNB) for high data throughput, especially MIMO implementations.
Nominally, CSI encompasses information about the wireless communication channel and enables an efficient strategy of transmission and overall improvement in the performance of the network.
This blog will explore the intricacies of CSI, including its acquisition methods, the concept of codebooks, the role of Channel State Information Reference Signals (CSI-RS), and the structure of CSI-RS codebooks.
What is Channel State Information (CSI)?
Channel state information (CSI) in wireless communications refers to known channel feedback parameters like channel quality indicator (CQI), rank indicator (RI), and precoding matrix indicator (PMI).
These parameters are used for downlink precoding at the BS end.
To maximize the performance of contemporary wireless systems and enable sophisticated signal processing techniques, CSI is essential.
The CSI parameters that are dependent on basic wireless channel properties like:
- Multipath propagation: Multipath effects occur when a sent signal travels through many routes to reach the receiver as a result of environmental obstacles’ reflections, scattering, and diffraction. This phenomenon affects wireless channels.
- Path loss: signal attenuation caused by the transmitter and receiver’s distance from one another.
- Fading: Variations in the received signal strength over time resulting from environmental changes, movement, or multipath propagation.
- Doppler shift: The change in frequency of the received signal brought on by the relative motion of the transmitter and receiver.
- Channel impulse response: Time dispersion and frequency selectivity of the channel are captured in the characterization of the channel’s response to an impulse signal.
Typically, the receiver uses known pilot signals or training sequences sent by the sender to estimate the channel response to get CSI. Both the transmitter and the receiver are familiar with these meticulously crafted pilot signals.
Here’s how CSI estimation typically works:
- Pilot signal transmission: Periodically, the transmitter uses the wireless channel to transmit training sequences or recognized pilot signals.
- Channel estimation at the receiver: To estimate the channel impulse response or channel coefficients for each transmit-receive antenna pair (in the case of MIMO systems), the receiver evaluates the received pilot signals and performs channel estimation techniques.
- CSI feedback (for FDD systems): In frequency-division duplexing (FDD) systems, the estimated CSI must be sent back to the transmitter across a feedback channel as the uplink and downlink channels are different.
- CSI acquisition (for TDD systems): The transmitter can estimate the CSI based on the receiver’s uplink signals in time-division duplexing (TDD) systems, where the downlink and uplink channels are reciprocal.
CSI acquisition is done in two stages:
First Stage of CSI Acquisition
The beam management (BM) phase of CSI collection is completed at this level. After reporting CSI parameters to the BS, UE bases BM on CSI-RS.
Another BM technique uses a sounding reference signal (SRS). SRS-based BM uses linear precoders and necessitates extensive CSI (higher overhead).
On the other hand, when grid of beams (GoB) precoders are employed, CSI-based BM is not complicated. More about precoding based on DFT GoB is in the following section.
Second Stage of CSI Acquisition
A CSI report is sent by UE to the BS during the second phase of CSI acquisition. This comes when the UE sends back the channel state information (CSI) parameters, like the rank indicator (RI), precoding matrix indicator (PMI), and channel quality indicator (CQI), as CSI feedback.
Chapter 5.2.2 of 3GPP TS 38.214 defines these parameters. Encoding, modulation, data rate, etc. are all determined by CQI. The rank or maximum number of transmission layers that is practical is determined by RI.
To obtain the best SINR, PMI calculates the precoding matrix (for mapping layers to the resource grid, i.e., giving appropriate weights to each antenna element).
This is the basic process; however, there are currently several complex codebooks available, and codebook-based precoding is an important area of research.
What are Codebooks
Codebooks are complex-valued matrices that are present in tables and are utilized by the BS to precode downlink transmission based on CQI, PMI & RI from the UE’s CSI report.
The steps involved in downlink transmission are:
- Encoding the bitstream and codewords into PDSCH, and PDCCH objects (layer mapped)
- Precoding of codewords having DMRS information
- Generating Beamforming Matrix
- Transforming the beamformed matrix into a Resource Mapper
- Mapping Resource Mapper into CSIRS logical ports
- Mapping CSIRS logical ports into actual antenna ports (having dual polarization)
A simple precoder is the second phase in this process, and it is derived from the pertinent table using PMI and RI.
A codebook table specifies the precoding matrix to which the PMI in a CSI report is mapped in a basic precoder.
The 3GPP technical specifications (3GPP TS 138.214) contain these specifications (Tables).
By using this technique, transmission overhead is decreased, but the BS must consult the codebook to apply a precoding matrix based on the PMI value or values.
Two types of codebooks are defined in the specification: Type 1 (fit for SU-MIMO) and Type 2 (fit for MU-MIMO). Massive MIMO can also be implemented with Type 2.
For the most recent 5G versions (R18, also known as 5G Advanced), a new kind of codebook called eType 2 (enhanced Type 2) has been developed, and it can be expanded to 6G.
Codebooks for Type 2 and improved Type-2 (eType2) are appropriate for scenarios involving huge MIMO as well as MU-MIMO.
More about CSI-RS
In 5G NR, the Channel State Information Reference Signal (CSI-RS) is a reference signal (RS) used for channel sounding in the downlink (DL) direction.
It measures the properties of a radio channel to ensure proper modulation, code rate, beam shaping, and other uses.
These reference signals will be used by UEs to gauge the DL channel’s quality, which they will then report in the UL via CQI Reports.
To report channel status information, such as CSI-RSRP, CSI-RSRQ, and CSI-SINR for mobility processes, gNB sends CSI Reference signals.
It is possible to set up particular CSI reference signal instances for mobility measurements and time/frequency tracking.
CSI-RS in 5G NR Channel mapping
CSI-RS Structure
A CSI-RS that has been configured can have up to 32 antenna ports, each of which represents a channel that has to be audible. There are two alternative configurations based on the number of antenna ports:
- Single-Port: A single port CSI-RS occupies a single Resource element (RE) within a block corresponding to one slot in the time domain and one resource block in the frequency domain
- Multi-Port: Several orthogonally broadcast per-antenna-port CSI-RS that share the total number of REs allotted to the multi-port CSI-RS configuration. Combinations of code domain sharing (CDM), frequency domain sharing (FDM), and time domain sharing (TMD) can be used to share information. CDM stands for distinct orthogonal patterns, while FDM stands for different subcarriers within an OFDM symbol.
Below is an example showing single-port and multi-port CSIRS:
Source – https://info-nrlte.com/2021/05/09/an-introduction-to-csi-rs/
CSI RS Codebook in 5G
To enable effective and precise channel state information (CSI) feedback from the User Equipment (UE) to the base station (gNB), the CSI-RS Codebook is an essential part of 5G.
The optimization of several transmission tactics, such as beamforming, and precoding, to improve communication performance in dynamic radio settings, depends on this feedback.
Two types of Codebook: Type I and Type II
There are two types of codebooks defined in 5G.
- Type I follows the same principles as the LTE codebook. The NR Type I codebook supports a wider variety of matrix types and is slightly more complex. This type essentially uses several pre-configured matrices that are chosen based on the RRC Configuration and the UE report.
- NR Type II is based on a mathematical formula with numerous parameters that have been specially constructed, rather than a predetermined table. The RRC and UE reports determine those formulaic criteria. Unlike the Type I codebook, the Type II codebook does not have as many separate tables. In a Type II codebook, there would only be one table, but the formula determining the precoding matrix would be far more intricate. Type II can use a more sophisticated precoding matrix than Type I thanks to those intricate formulas and numerous parameters.
- Type II is primarily intended for MU-MIMO (Multi-User MIMO) and is based on a more thorough CSI report.
- The Type I and Type II codebooks are built using a 2-D DFT-based beam grid, allowing for PSK-based co-phase combining two polarizations and CSI feedback on beam selection.
- In contrast to Type I reports, which only specify the phase of the selected beam and not its amplitude, Type II codebook-based CSI feedback provides the selected beams’ wideband and sub-band amplitude information.
- As demonstrated below, the distinction between Type I and Type II is visible. The key distinction between Type I and Type II beam selection is that Type I selects a single beam from a group of beams, while Type II selects a group of beams and linearly combines every beam in the group.
Components of CSI RS Codebook
- CSI-RS Patterns: The gNB transmits these particular reference signal patterns. These patterns are measured by the UE to determine the channel quality.
- Codebook Entries: Every codebook entry correlates to a certain precoding matrix. To maximize the quality of the received signal at the UE, these matrices are utilized to modify the sent signal.
- Feedback Mechanism: Based on the measured CSI-RS patterns, the UE chooses the optimal codebook entry and notifies the gNB of this decision. The Precoding Matrix Indicator (PMI), Rank Indicator (RI), and Channel Quality Indicator (CQI) are included in this feedback.
Conclusion
In conclusion, the CSI RS codebook is an essential part of 5G technology that facilitates communication by providing transmitters and receivers with vital wireless channel information.
By understanding the elements and structure of the CSI-RS codebook, network designers and engineers may optimize MIMO technologies to ensure high data rates and enhanced signal quality.
As we get closer to 6G and beyond, the significance of CSI will, underscoring the need for continued investigations as technology moves towards higher frequencies, particularly the millimeter wave bands.
Embracing these changes will enable the development of robust and efficient communication networks, leading to a more technologically advanced and interconnected future.
