# LTE Transmitter Using Software Defined Radio

This example shows how to generate a reference measurement channel (RMC) downlink (DL) LTE waveform suitable for over-the-air transmission. This example also shows how to use a software-defined radio (SDR) to transmit the generated waveform using single or multiple antennas.

### Introduction

This example generates eight frames of a baseband RMC DL waveform. Using SDR hardware such as the Xilinx® Zynq®-Based Radio, this baseband waveform can be modulated for RF transmission. The SDR transmits the waveform by looping transmission of the eight frames for a specified time period.

This example supports these SDRs.

• USRP™ E310/E312 from the Communications Toolbox Support Package for USRP™ Embedded Series Radio

• AD936x/FMCOMMS5 from the Communications Toolbox Support Package for Xilinx® Zynq®-Based Radio

• USRP™ N300/N310/N320/N321/B200/B210/X300/X310 from the Communications Toolbox Support Package for USRP™ Radio

### Example Setup

Before running the example, ensure that you have installed the appropriate support package for the SDR that you intend to use and that you have configured the hardware.

The `TransmitOnSDR` field of the `txsim` structure determines whether the example transmits the generated waveform using an SDR.

`txsim.TransmitOnSDR = false;`

If you select the `TransmitOnSDR` field of the `txsim` structure, configure the variables required for SDR transmission.

```if txsim.TransmitOnSDR txsim.SDRDeviceName = "AD936x"; % SDR that is used for waveform transmission txsim.RunTime = 20; % Time period to loop waveform in seconds txsim.RadioCenterFrequency = 2450000000; % Center frequency in Hz txsim.RadioIdentifier = '192.168.3.2'; % Value used to identify radio, for example, IP Address, USB Port, or Serial Number end```

Configure the other fields in the `txsim` structure for LTE downlink waveform generation.

```txsim.RC = "R.4"; % Base RMC configuration, 1.4 MHz bandwidth txsim.NCellID = 17; % Physical layer cell identity txsim.NFrame = 700; % Initial frame number txsim.TotFrames = 8; % Number of frames to generate txsim.NumAntennas = 1; % Number of transmit antennas```

### Transmitter Design

Follow these steps to understand how the LTE transmitter functions.

1. Generate a baseband LTE signal.

2. Prepare the baseband signal for transmission using the SDR hardware.

3. Send the baseband data to the SDR hardware for upsampling and transmission at the desired center frequency.

Generate Baseband LTE Signal

The `lteRMCDLTool` (LTE Toolbox) function provides the default configuration parameters defined in 3GPP TS 36.101 Annex A.3, which are required to generate an RMC.

Customize the parameters within the configuration structure `rmc`.

```rmc = lteRMCDL(txsim.RC); rmc.NCellID = txsim.NCellID; rmc.NFrame = txsim.NFrame; rmc.TotSubframes = txsim.TotFrames*10; % 10 subframes per frame rmc.CellRefP = txsim.NumAntennas; % Configure number of cell-specific reference signal antenna ports rmc.OCNGPDSCHEnable = "On"; % Adds noise to unallocated PDSCH resource elements```

If using two or more antennas, enable transmit diversity.

```if rmc.CellRefP >= 2 rmc.PDSCH.TxScheme = "TxDiversity"; rmc.OCNGPDSCH.TxScheme = "TxDiversity"; else rmc.PDSCH.TxScheme = "Port0"; rmc.OCNGPDSCH.TxScheme = "Port0"; end rmc.PDSCH.NLayers = txsim.NumAntennas;```

Create the baseband waveform (`eNodeBOutput`), a fully populated resource grid (`txGrid`), and the full configuration of the RMC using the `lteRMCDLTool` (LTE Toolbox) function.

```trData = [1;0;0;1]; % Transport data [eNodeBOutput,txGrid,rmc] = lteRMCDLTool(rmc,trData); txsim.SamplingRate = rmc.SamplingRate;```

Display the resource grid populated with the highlighted channels and the power spectral density of the LTE baseband signal. You can see a 1.4 MHz signal bandwidth at baseband in the spectrum plot.

If using multiple transmit antennas, set `displayAntennaForGrid` to the antenna port you would like to display.

```displayAntennaForGrid = 1; ax = axes; hPlotDLResourceGrid(rmc,txGrid(:,:,displayAntennaForGrid),ax,displayAntennaForGrid); ax.Children(1).EdgeColor = "none"; title("Transmitted Resource Grid");```

Display the power spectral density.

```spectrumScope = spectrumAnalyzer( ... SampleRate=txsim.SamplingRate, ... SpectrumType="power-density", ... Title="Baseband LTE Signal Spectrum", ... YLabel="Power Spectral Density"); spectrumScope(eNodeBOutput); release(spectrumScope);```

Prepare for Transmission

The transmitter plays the LTE signal in a loop. The example splits the baseband signal into LTE frames of data, and the SDR Transmitter object (`sdrTransmitter`) transmits a full LTE frame. The example reshapes the baseband LTE signal into an $M$- by -$N$ array, where $M$ is the number of samples per LTE frame and $N$ is the number of frames generated.

```if txsim.TransmitOnSDR % Scale the signal for better power output and convert to int16, which % is the native format for the SDR hardware. powerScaleFactor = 0.7; eNodeBOutput = eNodeBOutput.*(1./max(abs(eNodeBOutput))*powerScaleFactor); eNodeBOutput = int16(eNodeBOutput*2^15); % LTE frames are 10 ms long samplesPerFrame = 10e-3*txsim.SamplingRate; numFrames = length(eNodeBOutput)/samplesPerFrame; % Ensure you are using an integer number of frames if mod(numFrames,1) warning("Non integer number of frames. Trimming transmission ..."); numFrames = floor(numFrames); end % Reshape the baseband LTE data into frames for simpler transmission fprintf("Splitting transmission into %i frames\n",numFrames) txFrame = reshape(eNodeBOutput,samplesPerFrame,numFrames,txsim.NumAntennas); if matches(txsim.SDRDeviceName, ["AD936x", "FMCOMMS5", "Pluto", "E3xx"]) sdrTransmitter = sdrtx( ... txsim.SDRDeviceName, ... CenterFrequency=txsim.RadioCenterFrequency, ... BasebandSampleRate=txsim.SamplingRate); if matches(txsim.SDRDeviceName, ["AD936x", "FMCOMMS5", "E3xx"]) sdrTransmitter.ShowAdvancedProperties = true; sdrTransmitter.BypassUserLogic = true; sdrTransmitter.IPAddress = txsim.RadioIdentifier; else sdrTransmitter.RadioID = txsim.RadioIdentifier; end else % For the USRP SDRs sdrTransmitter = comm.SDRuTransmitter(... Platform=txsim.SDRDeviceName,... CenterFrequency=txsim.RadioCenterFrequency); [sdrTransmitter.MasterClockRate, sdrTransmitter.InterpolationFactor] = ... hGetUSRPRateInformation(txsim.SDRDeviceName,txsim.SamplingRate); if matches(txsim.SDRDeviceName, ["B200", "B210"]) % Change the serial number as needed for USRP B200/B210 sdrTransmitter.SerialNum = txsim.RadioIdentifier; else sdrTransmitter.IPAddress = txsim.RadioIdentifier; end sdrTransmitter.EnableBurstMode = true; sdrTransmitter.NumFramesInBurst = numFrames; end sdrTransmitter.ChannelMapping = 1:txsim.NumAntennas; end```

Transmission Using SDR Hardware

The example uses a `try` block to transfer the baseband data to the SDR hardware. Using the `try,` `catch` block means that if an error occurs during the transmission, the hardware releases the resources used by the SDR System object™. The `sdrTransmitter` System object transmits a full frame of LTE data.

```if txsim.TransmitOnSDR fprintf("Starting transmission at Fs = %g MHz\n",txsim.SamplingRate/1e6) currentTime = 0; try while currentTime<txsim.RunTime for n = 1:numFrames bufferUnderflow = sdrTransmitter(squeeze(txFrame(:,n,:))); if bufferUnderflow warning("Dropped samples.") end end currentTime = currentTime+numFrames*10e-3; % One frame is 10 ms end catch ME release(sdrTransmitter); rethrow(ME) end fprintf("Transmission finished\n") release(sdrTransmitter); end```

### Further Exploration

• You can use the companion example LTE Receiver Using Software Defined Radio (LTE Toolbox) to decode the broadcast channel of the waveform generated by this example. Try changing the cell identity and initial system frame number and observe the detected cell identity and frame number at the receiver.

• If using a supported multi-channel SDR, try increasing the number of antennas