Structure
The model essentially contains three parts:
Transmitter
Receiver impairments
EVM calculation
The following sections of the tutorial contain descriptions for each part of
the model.
Transmitter.The following blocks comprise the transmitter:
The Random Integer Generator block simulates random data generation. The
EDGE standard specifies that the transmitter performs measurements during
the useful part of the burst – excluding tail bits – over at least 200
bursts. In this mode, the transmitter produces 435 symbols per burst (9
additional symbols account for filter delays). The Phase Offset block
provides continuous 3π/8 phase rotation to the signal. For synchronization
purposes, the Upsample block oversamples the signal by a factor of 4.
The Discrete FIR Filter block provides a GMSK pulse linearization, the
main component in a Laurent decomposition of the GMSK modulation [3]. A
helper function computes the filter coefficients and uses a direct-form FIR
digital filter to create the pulse shaping effect. The filter normalization
provides unity gain at the main tap.
The I/Q Imbalance block simulates transmitter impairments. This block adds
rotation to the signal, simulating a defect in the transmitter under test.
The I/Q amplitude imbalance is
0.5 dB, and I/Q phase
imbalance is 1°.
Receiver Impairments.In this model, the Receiver Thermal Noise block represents receiver
impairments. This model assumes 290 K of thermal noise, representing
imperfections of the hardware under test.
EVM Calculation.The EVM calculation relies upon the following blocks:
The EVM measurement block computes the vector difference between an ideal
reference signal and an impaired signal. The output of the FIR filter
provides the Reference input for the EVM block. The
output of the Noise Temperature block provides the impaired signal at the
Input port of the EVM block.
While the block has different normalization options available, the EDGE
standard requires normalizing by the Average reference signal
power. For illustration purposes in this example, the EVM
block outputs RMS, maximum, and percentile measurement values.
Experimenting with the Model
Run the model by clicking the play button in the Simulink model
window.
Examine the output of the EVM block and compare the measurements
to the limits in the EDGE Standard Measurement Specifications
table.
In this example, the EVM Measurement block computes the
following:
Worst case RMS EVM per burst: 9.77%
Peak EVM: 18.95%
95th Percentile EVM:14.76%
As a result, this simulated EDGE transmitter passes the EVM test
for a Mobile Station under extreme conditions.
Double-click the I/Q Imbalance block.
Enter 2 into I/Q Imbalance
(dB) and click OK.
Click the Play button in the Simulink model window.
Examine the output of the EVM block. Then, compare the
measurements to the limits in the EDGE Standard Measurement
Specifications table.
In this example, the EVM Measurement block computes the following
results:
Worst case RMS EVM per burst: 15.15%
Peak EVM: 29.73%
95th Percentile EVM: 22.55%.
These EVM values are clearly unacceptable according to the EDGE
standard. You can experiment with the other I/Q imbalance values,
examine the impact on calculations, and compare them to the values
provided in the table.