Introduction

The Simulink® models described in this example are fixed-point HDL optimized implementations of SSB decoding and MIB recovery for 5G NR frequency range 1 (FR1). This example is one of a related set, for more information see NR HDL Reference Applications Overview.

MIB recovery requires SSB detection, demodulation, and decoding. This example focuses on SSB decoding. SSB detection and demodulation are described in the NR HDL Cell Search example. This example introduces the SSB decoding Simulink model and uses the MATLAB reference to generate test input and verify the behavior of the model. Then, the example describes a Simulink model that combines SSB detection, demodulation, and decoding to recover MIB from a baseband waveform.

After an SSB has been detected and demodulated, it needs to be decoded to extract the MIB contents. SSB decoding requires demodulation reference signal (DMRS) search, channel estimation and phase equalization, and broadcast channel (BCH) decoding steps as shown in the figure below.

File Structure

This example uses these files.

Simulink models

Simulink data dictionary

MATLAB code

NR HDL SSB Decoding

This figure shows the nrhdlSSBDecoding model. The top level of the model reads the signals from the MATLAB base workspace, passes them to the SSB Decoding subsystem, and writes the outputs back to the workspace. The ParseMIB subsystem takes the pbchPayload and interprets the bit fields to produce the MIB parameter outputs.

SSB Decoding Subsystem

The SSB Decoding subsystem references the nrhdlSSBDecodingCore and nrhdlPolarDecodingChainCore models. The subsystem performs DMRS search, channel estimation and equalization, QPSK symbol demodulation, descrambling, rate recovery, polar decoding, and CRC decoding. This processing is split over two models to allow for the nrhdlPolarDecodingChainCore to be shared between the SSB decoding and SIB1 CORESET0 decoding in the NR HDL SIB1 Recovery example. This section describes the inputs and outputs of that model.

Inputs

Outputs

PBCH Status Signal States

BCH Status Signal States

SSB Decoding Model

This diagram shows the top level of the nrhdlSSBDecodingCore model. The input data is 4 OFDM symbols containing the synchronization signal block (SSB), with the values scaled within the range +/-1. The model starts processing when all of the SSB data has been input to the model and startProcessing is asserted. The startProcessing signal also indicates that the NCellID and Lmax inputs are valid.

The SSB Buffer + DMRS Search subsystem performs the DMRS search. Incoming data is stored in a RAM buffer where it is held until startProcessing is asserted, indicating that all required information is available to start the DMRS search process. The DMRS search reads the DMRS symbols from the RAM and correlates with the 8 possible DMRS sequences, selecting the strongest correlation value to determine ibar_SSB. Once the DMRS search has been completed ibar_SSB is used to generate the reference DMRS required for channel estimation. The reference DMRS is output from the model along with the received PBCH symbols and associated DMRS. This is used to drive the nrhdlPolarDecodingChainCore model.

The extract payload subsystem performs descrambling and deinterleaving of the payload bits returned from the nrhdlPolarDecodingChainCore model.

Polar Decoding Chain Model

The nrhdlPolarDecodingChain model performs channel estimation and equalization, QPSK symbol demodulation, descrambling, rate recovery, and polar and CRC decoding. This signal processing chain is common for the decoding of both the BCH and downlink control information (DCI).

The Channel est + equalize subsystem performs channel estimation using the received data and the reference DMRS. The channel estimate applies linear interpolation between DMRS locations within an OFDM symbol, but does not average across time in case of any residual carrier frequency offset. Phase equalization of the QPSK symbols is then performed, followed by QPSK demodulation and descrambling, using the descrambling shift index and NCellID inputs to seed the scrambler. Subsequent processing performs rate recovery, polar decoding, and CRC decoding of the descrambled data. The Polar Rate Recover subsystem includes signal scaling and wordlength reduction to prepare the data for polar decoding. The scaled, rate-recovered soft bits are then passed to the NR Polar Decoder block, which also performs CRC decoding. The err output port from the NR Polar Decoder block indicates if decoding was successful or encountered any errors.

SSB Decoding Simulation Setup

The block diagram shows the simulation setup implemented by this example. 5G Toolbox™ functions are used to generate a test waveform. MATLAB reference code for the SSB detector is then used to search for and demodulate the strongest SSB within the waveform. This result provides test input for the SSB decoding stage. The test data is passed to both MATLAB and Simulink implementations, and the outputs are compared to verify the operation of the Simulink model.

The Test bench options section of the runNRSSBDecodingModel script includes these variables to configure the simulation:

    Simulation Case    SSB Pattern    Subcarrier Spacing Common    PDCCH Config SIB1    SNR dB    Strongest SSB index    Lmax
    _______________    ___________    _________________________    _________________    ______    ___________________    ____

      "SimCase 1"       "Case C"                 30                       164             50               4              8  
      "SimCase 2"       "Case B"                 15                       100              6               3              4  
      "SimCase 3"       "Case A"                 30                         4             20               2              8  
      "SimCase 4"       "Case A"                 15                        84              7               0              4  

SSB Decoding Simulation

Use the runNRSSBDecodingModel script to run an SSB decoding simulation. The script displays its progress at the MATLAB command prompt. The final results of decoding the SSB in MATLAB and Simulink are displayed, showing that they match exactly. Plots of the DMRS search correlation strength and the equalized PBCH QPSK symbols show that the signals from MATLAB and Simulink match closely.

runNRSSBDecodingModel;

Generating test waveform.
Selected Simulation case:

    Simulation Case    SSB Pattern    Subcarrier Spacing Common    PDCCH Config SIB1    SNR dB    Strongest SSB index    Lmax
    _______________    ___________    _________________________    _________________    ______    ___________________    ____

      "SimCase 1"       "Case C"                 30                       164             50               4              8  

Searching for SSBs using the MATLAB reference.
Demodulating the strongest SSB using the MATLAB reference.
Decoding the SSB using the MATLAB reference.
MIB successfully decoded by MATLAB reference
Decoding the SSB using the Simulink model.
Running nrhdlSSBDecoding.slx
### Starting serial model reference simulation build.
### Model reference simulation target for nrhdlPolarDecodingChainCore is up to date.
### Model reference simulation target for nrhdlSSBDecodingCore is up to date.

Build Summary

0 of 2 models built (2 models already up to date)
Build duration: 0h 0m 1.9161s
..........
MIB successfully decoded by Simulink model
 MATLAB decoded information
    pbchPayload: 17637376
       ssbIndex: 4
            hrf: 0
            err: 0
            mib: [1x1 struct]

 Simulink decoded information
    pbchPayload: 17637376
       ssbIndex: 4
            hrf: 0
            err: 0
            mib: [1x1 struct]

 MATLAB decoded MIB parameters
                     NFrame: 0
    SubcarrierSpacingCommon: 30
                      k_SSB: 0
          DMRSTypeAPosition: 3
            PDCCHConfigSIB1: 164
                 CellBarred: 0
       IntraFreqReselection: 0

 Simulink decoded MIB parameters
                     NFrame: 0
    SubcarrierSpacingCommon: 30
                      k_SSB: 0
          DMRSTypeAPosition: 3
            PDCCHConfigSIB1: 164
                 CellBarred: 0
       IntraFreqReselection: 0

MIB Recovery Model

The nrhdlMIBRecovery model connects the SSB decoding and SSB detection models to create a complete MIB recovery implementation. This model can be used to recover MIB from baseband 5G waveforms.

The status signal from the detector is used to start the SSB decoder when it has reached state 8, indicating that demodulation is complete, SSS has been found, and the demodulated grid has been output. When the SSB decoder has the demodulated grid and received the startProcessing signal it will decode the SSB, outputting the PBCH payload which is then parsed to extract the MIB data.

MIB Recovery Operations

The design operates in one of two modes - search or demodulation. A full MIB recovery consists of one or more searches followed by a demodulation operation. The MIB recovery operations extend the cell search operations in the NR HDL Cell Search example.

1. Search Mode

The search mode behavior is unchanged from the cell search design. The SSB detector searches for SSBs at the specified subcarrier spacing and frequency offset by correlating against each PSS sequence. The operation outputs a list of the detected SSBs, defined by a timing offset and PSS sequence ID.

2. Demodulation Mode

The demodulation mode operation is extended from the cell search design by including the SSB decoder. The demodulated SSB grid from the SSB detector is decoded by the SSB decoder. In single demodulation mode, one SSB is demodulated and decoded to return a single MIB result. In repeat demodulation mode, the SSB grid is decoded after each demodulation to return MIB results for each repetition.

MIB Recovery Simulation

Use the runNRMIBRecoveryModel script to run this model and compare against the MATLAB reference.

The Test bench options section of the runNRMIBRecoveryModel script includes these variables to configure the simulation:

HDL Code Generation and Implementation Results

To generate the HDL code for this example, you must have an HDL Coder™ license. Use the makehdl and makehdltb commands to generate HDL code and an HDL test bench for nrhdlSSBDecoding/SSB Decoding or nrhdlMIBRecovery/MIB Recovery subsystems. The resulting HDL code was synthesized for a Xilinx® Zynq®-7000 ZC706 evaluation board. The table shows the post place and route resource utilization results. The design meets timing with a clock frequency of 150 MHz.

Resource utilization for nrhdlSSBDecoding model:

       Resource        Usage
    _______________    _____

    Slice Registers     9113
    Slice LUTs         11619
    RAMB18                 8
    RAMB36                 5
    DSP48                 37

Resource utilization for nrhdlMIBRecovery model:

       Resource        Usage
    _______________    _____

    Slice Registers    43828
    Slice LUTs         32272
    RAMB18                20
    RAMB36                 5
    DSP48                246

To deploy the nrhdlMIBRecovery model to a hardware platform and recover MIB from off the air signals, see the Deploy NR HDL Reference Applications on FPGAs and SoCs example.