Main Content

SRS-Based Downlink Channel Measurements for TDD System

Channel measurements play an important role in optimizing communication performance. In time division duplexing (TDD) systems, you can utilize a sounding reference signal (SRS) for downlink channel measurements because of the channel reciprocity inherent in these systems.

What is SRS?

SRS serves as an uplink reference signal in 5G New Radio (NR) systems, primarily used for measuring uplink channels. Although mainly intended for the uplink direction, TDD systems also utilize SRS to estimate downlink channel characteristics by exploiting the channel reciprocity principle.

While creating a base station node using the nrGNB object from the 5G Toolbox™, you can enable the TDD mode by specifying its DuplexMode property as "TDD". To set the SRS as the downlink channel state information (CSI) measurement signal as SRS, specify the CSIMeasurementSignalDL name-value argument of the configureScheduler object function as "SRS".

Role of SRS in TDD Downlink Channel Measurements

Channel Reciprocity — In TDD, the downlink and uplink transmissions share the same frequency band but occur at different times. This shared frequency enables you to infer the downlink channel from the uplink channel estimate derived from SRS.

Adaptive Transmission — SRS-based downlink channel measurements enable the network to set downlink transmission parameters, such as the modulation and coding scheme (MCS), power levels, and beamforming vectors, enhancing performance and throughput.

How SRS Works in TDD Systems Transmission

  1. A user equipment (UE) node transmits the SRS toward a 5G base station node (gNB).

  2. The gNB node estimates the uplink channel from the SRS.

  3. Using the reciprocity principle, the gNB node computes the downlink channel and performs channel measurements for the downlink direction.

Mathematical Framework for SRS-Based Downlink Adaptive Transmission

Using an SRS for accurate downlink channel estimation and adaptive transmission strategies involves these computations.

SRS Transmission and Channel Model

A UE node transmits the SRS, denoted as x[k], to the gNB node. The representation of the received signal vector at the kth instant is:

y[k] = H x[k] + n[k],

where y[k] is an NRX-by-1 vector, x[k] is an NTX-by-1 vector, H is a matrix of size NRX-by- NTX, and n[k] is an NRX-by-1 noise vector. NTX represents the number of transmitting antennas, and NRX represents the number of receiving antennas.

Apply Channel Reciprocity

The downlink channel matrix HDL is the transpose (or Hermitian transpose for complex signals) of the uplink channel matrix HUL.

Adaptive Transmission Strategy

  1. Determine the maximum number of independent data streams, or maximum rank as Rmax = min(NRX, NTX).

  2. Compute the zero-forcing (ZF) precoder PZF for each rank as PZF = (HDL H HDL)-1 HDLH using these steps.

    1. Calculate the correlation coefficient matrix of the channel matrix.

    2. Extract the lower triangular part of the correlation coefficient matrix, and take the absolute values.

    3. Identify and sort correlation values by finding the maximum correlation value for each row and sorting the columns accordingly.

    4. Choose the columns with the lowest correlation values, ensuring the number of selected columns equals the current rank.

    5. Calculate the pseudo-inverse of the selected submatrix of the channel matrix.

  3. Select the MCS for each rank.

  4. Calculate the spectral efficiency for each rank using the formula SE = r × log2(M) × CR, where r is the rank of the channel, M is the modulation order, and CR is the coding rate.

  5. Choose the rank that offers the highest spectral efficiency for downlink transmission, along with the corresponding ZF precoder and MCS.

Related Topics