BLE Modulation Characteristics, Carrier Frequency Offset and Drift Test Measurements

This example shows how to measure Bluetooth® low energy (BLE) radio frequency physical layer (RF-PHY) transmitter tests specific to modulation characteristics, carrier frequency offset and drift using Communications Toolbox™ Library for the Bluetooth Protocol. This example also verifies whether the test measurements are within the specified limits as per the Bluetooth RF-PHY Test Specifications [ 1 ]

Objectives of BLE RF-PHY Tests

The Bluetooth RF-PHY Test Specifications [ 1 ] defined by Bluetooth Special Interest Group (SIG) includes RF-PHY tests for both transmitter and receiver. The objectives of these RF-PHY tests are to ensure interoperability between all BLE devices and to verify that a basic level of system performance is guaranteed for all BLE products. Each test case has a specified test procedure and an expected outcome, which must be met by the implementation under test (IUT).

RF-PHY Transmitter Tests

The main aim of transmitter test measurements is to ensure that the transmitter characteristics are within the specified limits as specified in the test specifications [ 1 ]. This example covers transmitter tests relevant to modulation characteristics, carrier frequency offset and drift. The test case IDs and test purposes of the considered tests are as follows:

Transmitter Test Procedure

This block diagram summarizes the test procedure for transmitter tests relevant to modulation characteristics, carrier frequency offset and drift as specified in [ 1 ].

  • Generate test packets and pass them through bleWaveformGenerator to generate BLE test waveforms. The test waveforms required for different test IDs are:

  • Perform filtering as specified in the Bluetooth RF-PHY Test Specifications [ 1 ].

  • Add carrier frequency offset and drift for tests: RF-PHY/TRM/BV-06-C, RF-PHY/TRM/BV-12-C, RF-PHY/TRM/BV-14-C.

  • FM demodulate and measure test characteristics (frequency deviation or frequency drift).

  • Get the test verdict and display results.

Configure the Test Parameters

Initialize variables such as transmitter test ID, number of samples per symbol, payload length, and maximum carrier frequency drift.

txTestID = "RF-PHY/TRM/BV-09-C";    % Select one from these test cases:
                                    % RF-PHY/TRM/BV-09-C,RF-PHY/TRM/BV-11-C,
                                    % RF-PHY/TRM/BV-13-C,RF-PHY/TRM/BV-06-C,
                                    % RF-PHY/TRM/BV-12-C,RF-PHY/TRM/BV-14-C
payloadLen = 240;    % Payload length in bytes, must be in the range [37,255]
sps = 32;           % Number of samples per symbol, minimum of 32 samples per
                    % symbol as per the test specifications

% Frequency offset and drift for the tests: RF-PHY/TRM/BV-06-C,
% RF-PHY/TRM/BV-12-C, RF-PHY/TRM/BV-14-C.
maxFreqDrift = 5000;     % In Hz, must be in the range [-50e3,50e3]
initFreqOffset = 12000;   % In Hz, must be in the range [-100e3,100e3]

% The function, <matlab:edit(helperBLEModulationTestConfig.m')
% helperBLEModulationTestConfig.m>, can be configured to generate test
% parameters and to design a channel filter based on test case ID and samples
% per symbol.

[filtDesign,testParams] = helperBLEModulationTestConfig(txTestID,sps);

Generate Test Waveforms and Perform Filtering

Based on the test parameters, generate BLE test waveforms using the helper function helperBLETestWaveform. Thereafter, perform filtering on the generated test waveforms.

testWfmLen = (testParams.nonPDULen+testParams.codingFactor*payloadLen*testParams.bitsPerByte)*sps;
[testWfm,filtTestWfm] = deal(zeros(testParams.numOfTestSeqs,testWfmLen));
for wfmIdx = 1:testParams.numOfTestSeqs
    % Generate BLE test waveforms
    testWfm(wfmIdx,:) = helperBLETestWaveform(testParams.testSeqIds(wfmIdx),...
                                        payloadLen,sps,testParams.phyMode);
    % Perform filtering on the test waveforms
    filtTestWfm(wfmIdx,:) =  conv(testWfm(wfmIdx,:),...
        filtDesign.Coefficients.','same');
end

Add Frequency Offset and Frequency Drift

Add frequency offset and frequency drift only for carrier frequency offset and drift tests: RF-PHY/TRM/BV-06-C, RF-PHY/TRM/BV-12-C, RF-PHY/TRM/BV-14-C.

if any(strcmp(txTestID,{'RF-PHY/TRM/BV-06-C','RF-PHY/TRM/BV-12-C','RF-PHY/TRM/BV-14-C'}))
    driftRate = maxFreqDrift/length(filtTestWfm);% Drift rate
    freqOffset = driftRate*(0:1:(length(filtTestWfm)-1))';% Frequency drift
    freqOffset = freqOffset+initFreqOffset;% Frequency offset and frequency drift
    pfo = comm.PhaseFrequencyOffset('FrequencyOffset',freqOffset,'SampleRate',testParams.sampleRate);
    wfmFreqOffset = pfo(filtTestWfm.');
else
    wfmFreqOffset = filtTestWfm;
end

FM Demodulate and Measure Test Characteristics

The function, helperBLEModulationTestMeasurements.m, performs FM demodulation and computes either frequency deviation, or frequency drift and initial frequency offset based on the provided test case ID.

[waveformDiffFreq,fOut1,fOut2] = helperBLEModulationTestMeasurements(wfmFreqOffset,txTestID,testParams);

Display Test Results

The function, helperBLEModulationTestVerdict.m, verifies whether the measurements are within the specified limits or not, and displays the verdict on the command window.

helperBLEModulationTestVerdict(waveformDiffFreq,txTestID,testParams,fOut1,fOut2)

Test sequence: 00001111
    Measured average frequency deviation = 250 KHz
    Expected average frequency deviation = 247.5 kHz to 252.5 kHz
    Result: Pass
Test sequence: 10101010
    Measured average frequency deviation = 244.0016 kHz
    Expected average frequency deviation > 185 kHz
    Result: Pass
Ratio of deviations between two test sequences = 0.97601
Expected Ratio > 0.8 
    Result: Pass

This example demonstrated the BLE transmitter test measurements specific to modulation characteristics, carrier frequency offset and drift as per the Bluetooth RF-PHY Test Specifications [ 1 ].

Appendix

The helpers used in this example are:

Selected Bibliography

  1. Section 4.4 of the Bluetooth RF-PHY Test Specification, Revision: RF-PHY.TS.5.1.0.

  2. Volume 6 of the Bluetooth Core Specification, Version 5.0 Core System Package [Low Energy Controller Volume].