Design Generic Parameter Reference

This chapter provides detailed descriptions of design parameters.

The parameter groups and parameters are used in one or more of the design types:

When an exception is not noted, the parameter groups and parameters are used in all of these design types. Otherwise, design type exceptions will be noted as to which types of designs do not apply for the parameter groups and parameters.

Instrument Setup

The generic test design provides a stimulus-response test environment for an RF Device Under Test (DUT). You can create and refine stimulus characteristics and measurement parameters on a mathematical model of your device prior to measuring the device itself using connected instrumentation.

The former is referred to as DUT Model measurement, the latter physical device measurement.

Effectively, the design contains a double-pole double-throw switch controlled by the Instrument Connectivity Enabled parameter. For discussion of the DUT Model, see DUT Model Simulation to Device Measurement.

An instrument configuration is defined for each of the three design types supported:

InstrumentEnabled

When instrument connectivity is disabled, the design uses its internal software model for the test instruments (ESG and VSA) and the DUT (cascade of DUT pre-amplifier, DUT model, and DUT post-amplifier).

When instrument connectivity is enabled, the design uses the external hardware consisting of the test instruments (ESG and VSA) and the physical DUT (cascade of DUT pre-amplifier, physical DUT, and DUT post-amplifier).

ESG Setup

Provides the basic ESG settings for this design.

The signal will be generated for the number of segments specified and sent to the signal generator. The number of samples collected must be in the range 60 samples to 64 Msamples, where 1 Msample = 1,048,576 samples.

This parameter is not used in baseband transmission designs.

The number of samples collected is equal to (two plus the number of segments specified in the Measurement Setup ) time the (number of samples per segment). See the design example documentation for the relationship between the Signal Setup parameters and the number of samples per signal segment.

The number of samples collected must be less than 64 Msamples.

Note
The ESG requires an even number of samples. The last signal sample collected will be discarded if an odd number of signal samples is collected.

You can modify the default filename for the waveform.

The following parameters control the setup and data transfer of the generated signal to the ESG.

ESG_Instrument

Specifies the ESG to be used. To ensure that this field is populated correctly, click Select to display the Instrument Explorer dialog and select an instrument.

For details on the Instrument Explorer , refer to the chapter "Making Measurements, Discovering Connected Hardware" in the User's Guide .

ESG_MinOutputPower

Specifies the minimum output power limit for the ESG.

ESG_MaxOutputPower

Specifies the maximum output power limit for the ESG.

Advanced ESG Setup

The following parameters may be optionally set. The design will typically perform well with the default settings.

ESG_ExportFilename

The name of the waveform that will contain the data downloaded to the ESG.

ESG_ClkRef

Specifies an internal or external reference for the ESG clock generator. If set to External , the frequency of this clock is set by ExtClkRefFreq .

ESG_ExtClkRefFreq

Specifies the reference frequency of the external clock.

ESG_IQFilter

Specifies the cutoff frequency for the reconstruction filter between the DAC output and the dual arbitrary waveform generator output inside the ESG.

ESG_AutoScaling

The ESG driver expects data in the range [-1, 1]. Set to Yes , inputs are scaled to the range [-1, 1]. Set to No , raw simulation data is downloaded to the ESG without scaling, but data outside the range [-1, 1] is clipped to -1 or 1.

ESG_ArbOn

Set to Yes , the ESG generates the signal immediately after simulation data is downloaded. Set to No , waveform generation must be turned on at the ESG front panel.

RFPowOn

Set to Yes , the ESG enables RF power immediately after simulation data is downloaded. Set to No , RF power must be enabled at the ESG front panel.

ESG_EventMarkerType

Specifies which ESG event markers are enabled: Event1 , Event2 , Both , or Neither . Event markers are used for synchronizing other instruments to the ESG. When event markers are enabled, Event1 and/or Event2 are set beginning from the first sample of the downloaded Arb waveform over the range of points specified by ESG_MarkerLength . This is equivalent to setting the corresponding event from the front panel of the ESG.

ESG_MarkerLength

Specifies the range of points over which the markers must be set, starting from the first point of the waveform. Depending on the setting of Event Marker Type , the length of trigger Event1 or Event2 (or both) is set to a multiple of the pulse width that, in turn, is determined by the sample clock rate of the DAC output.

PSG Setup

Provides the basic PSG settings for this design. This category is available only for baseband transmission designs.

The signal generated for the number of segments specified is recorded into I and Q data files. When InstrumentEnabled is disabled (deselected), the data file will automatically be used in the simulated DUT measurement. When InstrumentEnabled is enabled (selected), the design will pause and display a dialog asking you to manually load the data files into the ARB instrument, and click Continue when ready. The signal defined by the data files will then be read into the ARB instrument whose output is routed the PSG input. The PSG output is then routed to the hardware DUT whose output signal is then routed back to the design for measurement.

See the discussion above for the ARB parameters for further information on how the PSG may affect the maximum sample limit and high frequency roll-off.

The following parameters control the setup and data transfer of the generated signal to the PSG.

PSG_Instrument

Specifies the PSG to be used. To ensure that this field is populated correctly, click Select to display the Instrument Explorer dialog and select an instrument.

PSG_MinOutputPower

Specifies the minimum output power limit for the PSG.

PSG_MaxOutputPower

Specifies the maximum output power limit for the PSG.

VSA Setup

Provides the basic VSA settings for this design.

VSA_MinInputPower

Specifies the minimum input power limit for the VSA. This parameter is not available in baseband transmission test designs.

VSA_MaxInputPower

Specifies the maximum input power limit for the VSA. This parameter is not available in baseband transmission test designs.

Advanced VSA Setup

VSA_DisplayMode

Set to Yes , you can observe the output waveform and spectrum in the display window of VSA software when the VSA output is sent to ADS. Set to No , the VSA window will not be shown.

VSA_Trace

Specifies which 89600 trace will provide measurement data. For this test design, waveform data is set to be displayed in Trace B ( VSA_B ) and all measurements provided are based on Trace B data.

VSA_SetupFile

Enter a setup filename to automatically recall a VSA setup during simulation start-up. To save a VSA measurement setup, in the VSA application, click File > Save > Save Setup . To avoid the need to supply an absolute filename on the schematic, save it in the project data directory.

Note
The VSA does not default to saving in this directory the first time. To refine an existing setup file, modify the VSA application while simulating and then save the setup file before restarting the simulation. If VSA_DisplayMode is set to Yes , the setup file will be loaded before the pause dialog is displayed.

VSA_Title

Specifies the title for the VSA display window.

Alternative VSA Setup

For the WiMax 16e BER test, use a DUT with one RF input and one RF output. Alternatively, you can use the Zero IF option (for a DUT with one RF input and IQ baseband output) or the Digital IF option (for a DUT with logic bits output). The next two parameters are used to enable the Zero IF or Digital IF option.

VSA_AlternativeSetupEnabled

Enables and disables the VSA Zero IF or Digital IF option.

VSA_AlternativeSetup_Option

Selects Digital IF by setting to 0 or Zero IF by setting to 1.

VSA_Zero_IF_SetupFile

Enter a setup filename to automatically recall a VSA setup during simulation start-up. This setup file is intended for use with a two-channel VSA connected to the DUT output. The output from the two-channel VSA is a baseband signal with the signal information at a Zero IF frequency. The baseband two-channel VSA output signal is captured by the VSA software and sent to the design for measurement.

To save a VSA measurement setup, in the VSA application, click File > Save > Save Setup . To avoid the need to supply an absolute filename on the schematic, save it in the project data directory.

VSA Zero IF Option

Provides the VSA settings for this design for use with a two-channel VSA. These parameters are only used for BER designs.

When the VSA_Alternative_Setup_ Enabled is set to 1 and VSA_AlternativeSetup_Option is set to 1 , Zero IF option is enabled. When the Zero IF option is enabled, the instrument configuration has baseband I,Q connected to the DUT output. The output from the two-channel VSA is a baseband signal with the I,Q signals information. The baseband VSA output I,Q signal is captured by the VSA software and sent to the design for measurement. The design extracts the signal information at the Zero IF frequency before proceeding with its measurements.

VSA Digital IF Option

Provides the basic VSA settings for this design for use with a Logic Analyzer (LA). These parameters are only used for BER designs.

When the VSA_Alternative_Setup_ Enabled is set to 1 and VSA_AlternativeSetup_Option is set to 0 , Digital IF option is enabled. When the Digital IF option is enabled, the instrument configuration has a logic analyzer connected to the DUT output. The output from the LA is a digitized baseband signal with the signal information at an IF frequency. The baseband LA output signal is captured by the VSA software and sent to the design for measurement. The design extracts the signal information at the IF frequency before proceeding with its measurements.

VSA_Digital_IF_SetupFile

Enter a setup filename to automatically recall a VSA setup during simulation start-up. This setup file is intended for use with a logic analyzer connected to the DUT output. The output from the LA is a digitized baseband signal with the signal information at an IF frequency. The baseband LA output signal is captured by the VSA software and sent to the design for measurement.

To save a VSA measurement setup, in the VSA application, click File > Save > Save Setup . To avoid the need to supply an absolute filename on the schematic, save it in the project data directory.

Note
The VSA does not default to saving in this directory the first time. To refine an existing setup file, modify the VSA application while simulating and then save the setup file before restarting the simulation. If VSA_DisplayMode is set to Yes , the setup file will be loaded before the pause dialog is displayed.

VSA_Digital_IF_NomSamplingRate

Specifies the nominal sampling rate of the digital IF output signal. The actual sampling rate used will be determined by the expression:

VSA_Digital_IF_SamplingRate =
int( SignalSegmentTime * VSA_Digital_IF_NomSamplingRate ) / SignalSegmentTime
where the SignalSegmentTime is for the wireless signal generated.

See the design example documentation for the relationship between the Signal Setup parameters and the SignalSegmentTime.

VSA_Digital_IF_CarrierFreq

Specifies the IF carrier frequency of the digital IF output signal. The signal information is expected to exist at this IF frequency. The design extracts the signal information at this IF frequency as a complex envelope before proceeding with measurements.

Note
An Agilent VSA is used in the preceding examples. Alternatively, you may use an Agilent PSA E444X or MXA N9020A to capture data at the output of the DUT.
Note
Agilent VSA/PSA instruments require installation of the Agilent 89601A VSA/PSA software on the same workstation as ADS. For MXA applications, use Agilent 89601A VSA Version 6.31 build_1003 or higher.
Note
If you replace the VSA with a PSA or MXA, the parameter Range for ADSVSA_89600_Source component must be changed. For example, if the original setting for the parameter Range is − 40 dBm when a VSA is used, to acheive similar accuracy when measuring the EVM with the PSA or MXA, set the VSA_89600_Source Range between − 32 dBm to − 34 dBm.

Oscilloscope Setup

Provides the basic Digital Storage Oscilloscope (DSO) settings for this design. These parameters are only available for baseband transmission designs.

The DUT Model output (pre-amp, DUT, post-amp) is connected to the DSO input. These parameters define the limits for the DSO input power for proper operation. The DSO output is recorded by the VSA software and routed to the design for measurement.

See the discussion above for the ARB parameters for further information on how the DSO may affect the maximum sample limit and high frequency roll-off.

The following parameters control the setup and data transfer of the generated signal to the DSO.

OScopeMinInputPower

Specifies the minimum input power limit for the DSO.

OScopeMaxInputPower

Specifies the maximum input power limit for the DSO.

DUT Model Setup

Configures DUT model parameters based on the DUT, physical connections, and the instrument setup so that test configurations can be simulated before instrument connectivity is enabled to test the physical device.

When InstrumentEnabled is disabled (deselected), these parameters emulate the hardware path and simulates the DUT performance.

When InstrumentEnabled is enabled (selected), the DUT _MirrorSpectrum value is used to achieve proper demodulation of the hardware DUT signal.

The RF gain parameters are also used to determine that proper power levels are set for RF DUT hardware operation with the ESG and VSA (RF transmission designs), ESG and LA (RF BER designs) or PSG and DSO (Baseband transmission designs).

DUT_PreAmpGain

Specifies the pre-amplifier gain in dB used before the RF DUT.

This value is used to determine if the design is setting the ESG (PSG) output power outside its minimum or maximum limits.

DUT_PreAmpNoiseFigure

Specifies the pre-amplifier noise figure in dB used before the RF DUT.

This value is used when InstrumentEnabled is not selected (disabled) and AWGN_Enabled is selected (enabled). It is used in setting the signal noise level into the DUT.

DUT_Gain

Specifies the nominal DUT gain in dB.

This value is used to determine if the combined design and DUT (pre-amplifier, DUT, post-amplifier) gain results in power beyond the VSA (DSO) minimum or maximum input limits.

DUT_NoiseFigure

Specifies the DUT noise figure in dB.

This value is used when InstrumentEnabled is not selected (disabled) and AWGN_Enabled is selected (enabled). It is used in setting the received signal noise level.

DUT_MirrorSpectrum

Select Yes if the DUT output has a mirrored spectrum about the DUT input carrier frequency.

This value sets the received signal mirroring needed for proper demodulation. The received signal mirroring is set when either MirrorModSpectrum or DUT_MirrorSpectrum are set (but not both).

DUT_PostAmpGain

Specifies the post-amplifier gain in dB used after the RF DUT.

This value is used to determine if the combined design and DUT (pre-amplifier, DUT, post-amplifier) gain results in power beyond the VSA (DSO) minimum or maximum input limits.

DUT_PostAmpNoiseFigure

Specifies the post-amplifier noise figure in dB used after the RF DUT.

This value is used when InstrumentEnabled is not selected (disabled) and AWGN_Enabled is selected (enabled). It is used in setting the received signal noise level.

Advanced Signal Setup

RandomSeed

Specifies the seed for the random number generator. Set to zero (0) for a different seed each time the design is run. Set to an integer greater than zero if the same waveform is desired to be generated for each run.

RF Modulator

ModulatorFreq

Sets the RF carrier frequency.

MirrorModSpectrum

Inverts the polarity of the Q envelope of the generated RF signal before it is sent to the RF DUT. Depending on the configuration and number of mixers in an RF transmitter, the signal at the input of the DUT may need to be mirrored. If such an RF signal is desired, set this parameter to Yes .

RF Impairments

RF_ImpairmentsEnabled

Enables and disables RF I/Q impairment parameters GainImbalance , PhaseImbalance , I_OriginOffset , Q_OriginOffset , and IQ_Rotation .

GainImbalance, PhaseImbalanceI, I_OriginOffset, Q_OriginOffset, and IQ_Rotation

These parameters add impairments to the ideal output RF signal. Impairments are added in the order described here.

The unimpaired RF I and Q envelope voltages have gain and phase imbalance applied. The RF is given by:

where A is a scaling factor that depends on the specified signal power (Watts), V I (t) is the in-phase RF envelope, V Q (t) is the quadrature phase RF envelope, g is the gain imbalance

0
and, φ (in degrees) is the phase imbalance.

The signal V RF (t) is then rotated by the value entered for IQ Rotation. The values entered for I_OriginOffset and Q_OriginOffset are then applied to the rotated signal.

Note
The values specified are percentages with respect to the output rms voltage. The output rms voltage is given by sqrt(2 × 50 × SourcePower), where 50 ohms is used at the source resistance.

Multipath Fading

The multipath fading channel is modeled as a tapped-delay line model. These parameters are available for transmission designs only. Select from a set of predefined technology specific multipath profiles or enable the predefined common multipath model. For definitions, see Multipath Fading Definitions.

MultipathFadingEnabled

Enables and disables the Multipath Fading parameter group.

The predefined technology specific multipath profiles are used only when CommonFadingModelEnabled is deselected (OFF). Predefined technology specific multipath profiles are not available for Baseband Transmission Test designs.

CommonFadingModelEnabled

Enables the common multipath profile parameter group instead of the technology specific multipath profiles. This parameter is not used for baseband transmission designs.

For more information, see Common Fading Model Definitions.

FadingAlgorithm

Specifies the fading algorithm as Jakes or Noise Filter . These choices are based on common mathematical descriptions for a multifading channel. Multiple reflections of the transmitted signal may arrive at the receiver at different times, resulting in intersymbol interference. This time dispersion of the channel, called multipath delay spread, can be modeled by a tapped delay line. Any multipath fading process can be expressed as a tapped delay line. Therefore, this algorithm uses a tapped delay line to model the multipath fading.

FadingSeed

Specifies the random number seed for the Jakes algorithm.

FadingPathNumber

Specifies the number of multipath echoes.

FadingPowerArray

Specifies the delay profile's path average relative power in dB. The number of values in this array must be equal to the FadingPathNumber .

FadingDelayArray

Specifies the delay profile's path delay in nanoseconds. The delay value must be entered in increasing order. The number of values in this array must be equal to the FadingPathNumber .

FadingUserType

Specifies the fading type of the first path as Rayleigh or Ricean . You then define the delay profile by setting the FadingPowerArray and FadingDelayArray values.

FadingRiceanFactor

Specifies the power ratio of the direct signal to all other indirect signals for a Ricean first path fading type.

FadingJakesOscNum

Specifies the number of Jakes Algorithm oscillators. The value has a minimum limit equal to FadingPathNumber .

RF Interferers

RF_InterferersEnabled

Enables the RF interferers parameter group to introduce a CW interfering signal.

ModFreqOffset

Specifies the frequency of the RF modulated interferer relative to the primary RF carrier frequency.

ModPower

Specifies the power of the RF modulated interferer.

CW_FreqOffset

Specifies the frequency of the RF CW interferer relative to the primary RF carrier frequency.

CW_Power

Specifies the power of the RF CW interferer.

Additive White Gaussian Noise (AWGN)

AWGN_Enabled

Adds Additive White Gaussian Noise to the stimulus.

AWGN_Type

Specifies the AWGN Specification Type as Noise (dBm) at ESG output , Noise (dBm/Hz) at ESG output , Local Eb/No (dB) at ESG output , Noise (dBm) at DUT input , Noise (dBm/Hz) at DUT input , Local Eb/No (dB) at DUT input , or System Eb/No (dB) . This parameter is available in transmission test designs only.

AWGN_Value

Specifies the noise value based on the setting of AWGN_Type . This parameter is available in transmission test designs only.

Eb/No_Type

Specifies the AWGN Specification Type as Local Eb/No (dB) at ESG output , Local Eb/No (dB) at DUT input , or System Eb/No (dB) . This parameter is available in BER test designs only.

Eb/No_Value

Specifies the noise value based on the setting of Eb/No_Type . This parameter is available in BER test designs only.

Measurement Setup

NumberSegments

Specifies the number of signal segments to be measured.

The NumberSegments applies to all measurements except:

This convention was used since the envelopes have essentially similar information for each frame. Only two envelopes are measured to reduce data collection.

This convention was used since BER is the principal measurement requiring many signal segment, but source signal measurements are typically adequate with less number of signal segments.

See the design example documentation for the relationship between the signal segment time duration and Signal Setup parameters.

DUT_InputPowerStart

Sets the initial DUT input RF power level. This is the initial power level from which a power sweep is defined using the FadingPowerArray parameter.

DUT_InputPowerStep

Sets the power step in dB for swept power stimulus.

DUT_InputPowerNumSteps

Sets the number of power steps in a power sweep. When set to zero, only DUT_InputPowerStart is used. When set larger than zero, the maximum power will be DUT input RF start power (in dBm) + Power sweep step in dB × Number of power steps .

DUT Output Frequency

DUT_OutFreqEnabled

Enable if a frequency conversion exists for the DUT.

DUT_OutputFreq

Specifies the DUT output frequency. If a frequency conversion occurs in the DUT, then this value will be different than the ModulatorFreq parameter in the RF Modulator parameter group.

Data Display Setup

DDSFileName

Enter a name and the measurement display will use the user-defined data display.

If you do not enter a filename, the default design measurement display will determined by the setting for DisableSourceDataDisplay (see below).

Leave DDSFilename blank to use the predefined data display templates for this design. If you saved a data display file (*.dds) using the data display, click Browse to select the previously saved data display file.

After running the simulation, results are displayed in the data display pages for each measurement activated.

DSFileName

Leave this blank to use the predefined data set for this design. If you saved a dataset file (*. ds ) using the Data Display, click Browse and select a previously saved dataset file.

DisableSourceDataDisplay

Enables ( Yes ) or disables ( No ) the signal generator data display.

Select Yes to display:

SourceDisplayRefPoint

Selects the reference point for display of the source signal. Select either RF DUT Input or ESG Output . When set to RF DUT input , the displayed signal is based on the ESG output signal, plus the gain defined by DUT_PreAmpGain .

SourceDisplaySegments

Defined the number of signal segments to use for Source displays. This is independent from the NumberSegments parameter in the Measurement Setup. This parameter is only available in BER test designs.

DUT_MeasDisplayRefPoint

Selects the reference point for display of the DUT output measurement signal. Select either RF DUT output or VSA input . When set to RF DUT Output , the displayed signal is based on the VSA output signal, minus the gain defined by DUT_PreAmpGain . This parameter is only available in transmission test designs.

Spectrum Analysis Data Display Parameters

Four parameters are used to configure spectrum analysis data displays: SpecResBW , SpecWindow , SpecMirrorFreq , and SpecMeasMirrorFreq .

The spectrum measurement calculates the spectrum of the signal. Averaging the spectrum over multiple bursts can be enabled as described in this section.

SpecResBW

The basis of the algorithm used by the spectrum measurement is the chirp-Z transform. The algorithm can use multiple bursts and average the results to achieve video averaging (see below).

In the following, let

SpecMeasResBW sets the resolution bandwidth of the spectrum measurement when SpecMeasResBW > 0.

Depending on the values of SpecMeasStart, SpecMeasStop, SpecMeasBursts, and SpecMeasResBW, the stop time may be adjusted or spectrum video averaging may occur (or both). The different cases are described in Effect of Values for SpecMeasStart, SpecMeasStop, SpecMeasBursts, and SpecMeasResBW.

Referring to Effect of Values for SpecMeasStart, SpecMeasStop, SpecMeasBursts, and SpecMeasResBW, let:

Start = TimeStep!consolwb-8-1-03.gif!
int((SpecMeasStart/TimeStep) + 0.5)
Stop = TimeStep!consolwb-8-1-04.gif!
int((SpecMeasStop/TimeStep) + 0.5)
(This means Start and Stop are forced to be an integer number of time step intervals.)
X = normalized equivalent noise bandwidth of the window

Equivalent noise bandwidth (ENBW) compares the window to an ideal, rectangular filter. It is the equivalent width of a rectangular filter that passes the same amount of white noise as the window. The normalized ENBW (NENBW) is ENBW multiplied by the duration of the signal being windowed. Window Options and Normalized Equivalent Noise Bandwidth lists the NENBW for the various window options.

The Start and Stop times are used for both the generated signal (into the RF DUT) and the signal output from the RF DUT. The RF DUT output signal is delayed in time from the RF DUT input by the value of the RF DUT time delay. Therefore, for RF DUT time delay greater than zero, the RF DUT output and RF DUT input signal are inherently different and some spectrum display difference in the two is expected.

TimeStep and BurstTime are defined in Multipath Fading Definitions.

Case 1 SpecMeasBursts > 0
Case 2 SpecMeasBursts > 0
Window and Default Constant NENBW
none 1
Hamming 0.54 1.363
Hanning 0.50 1.5
Gaussian 0.75 1.883
Kaiser 7.865 1.653
8510 6.0 1.467
Blackman 1.727
Blackman-Harris 2.021

SpecWindow

Specifies the window that will be applied to each burst before its spectrum is calculated.

Different windows have different properties, affect the resolution bandwidth achieved, and affect the spectral shape. Windowing is often necessary in transform-based (chirp-Z, FFT) spectrum estimation in order to reduce spectral distortion due to discontinuous or non-harmonic signal over the measurement time interval.

Without windowing, the estimated spectrum may suffer from spectral leakage that can cause misleading measurements or mask weak signal spectral detail with spurious artifacts.

The windowing of a signal in time has the effect of changing its power. The spectrum measurement compensates for this and the spectrum is normalized so that the power contained in it is the same as the power of the input signal.

Window Type Definitions
None:

where N is the window size
Hamming 0.54:

where N is the window size
Hanning 0.5:

where N is the window size
Gaussian 0.75:

where N is the window size
Kaiser 7.865:

where N is the window size,  α = N / 2, and I 0 (.) is the 0 th order modified Bessel function of the first kind
8510 6.0 (Kaiser 6.0):

where N is the window size, α = N / 2, and I 0 (.) is the 0 th order modified Bessel function of the first kind
Blackman:

where N is the window size
Blackman-Harris:

where N is the window size.

SpecMirrorFreq

Set to Yes to apply spectrum mirroring to the measured RF DUT input signal.

SpecMeasMirrorFreq

Set to Yes to apply spectrum mirroring to the measured RF DUT output signal. This parameter is only available for transmission test designs.

In the following, TimeStep denotes the waveform sampling time step and FMeasurement denotes the measured RF signal carrier frequency.

The waveform sampling time step is dependant on the design Signal Setup parameters.

See the design example documentation for the relationship between the time step and Signal Setup parameters.

Note that the dBm function assumes a 50-ohm reference resistance.

By default, the displayed spectrum extends from FMeasurement - 1/(2 × TimeStep) Hz to FMeasurement + 1/(2 × TimeStep) Hz.

When FMeasurement < 1/(2 × TimeStep), the default spectrum extends to negative frequencies. The spectral content at these negative frequencies is conjugated, mirrored, and added to the spectral content of the closest positive frequency. This way, the negative frequency tones are displayed on the positive frequency axis as would happen in an RF spectrum analyzer measurement instrument. This process may introduce an error in the displayed frequency for the mirrored tones. The absolute error introduced is less than (Spectrum Frequency Step) / 2 (see Effect of Values for SpecMeasStart, SpecMeasStop, SpecMeasBursts, and SpecMeasResBW for the definition of Spectrum Frequency Step).

Multipath Fading Definitions

This section defines terms and relations relevant to multipath and fading.

Definitions:
V = vehicle speed, in m/s
F c = propagation (carrier) frequency, in Hz
ω c = propagation (carrier) frequency, in radian/sec
n = Doppler frequency, in Hz
n m = maximum Doppler frequency, in Hz
S(t) = transmitted (RF) signal
s(t) = complex envelope of transmitted signal
R(t) = received (RF) signal
r(t) = complex envelope of received signal
α n = random amplitude of n th signal echo
γ n = phase retardation of n th signal echo
τ n = time-delay of n th signal echo
G t ( θ, φ ) = directive gain of transmitting antenna as a function of elevation and azimuth angles
G r ( θ, φ ) = directive gain of receiving antenna as a function of elevation and azimuth angles

Radio waves are received not only via direct path but often by scattering off numerous objects. Delay, attenuation and carrier phase shift are some of the alterations the transmitted signal experiences. This process can be modeled as a linear filter with randomly time-varying impulse response.

In a multipath environment, a transmitted RF signal

is received in the form

where n is the number of different echoes, each having a delay.
The received complex envelope is therefore

The (lowpass) impulse response of the discrete channel h(τ,t), is therefore characterized by several discrete paths, each having a specific delay and attenuation.

Signal fading occurs due to destructive or constructive addition of a large number of phasors. If h(τ,t) is modeled as a zero mean Gaussian process, the envelope �(τ,t)� at any time is Rayleigh-distributed. The transform of h(τ,n) with respect to time, gives the spectrum of time variation S(τ,n), generally referred to as delay-Doppler spread function [1]. The variable n represents the Doppler frequency shift due to changes in the electrical path length as a result of mobile movement.

For two vertically polarized transmit and receive antennas and horizontal propagation of plane waves [2], the Doppler spectrum is

where

is the maximum Doppler shift due to vehicle speed. When a direct path exists the spectrum is Ricean and is given by

with k 1, k 2, k 3 constants related to proportion of direct and scattered signal and the direct wave angle of arrival. Assuming the wide sense stationary uncorrelated scattering (WSSUS) [3], the average delay profiles and Doppler spectra information is needed for the simulation of radio channel. Delay profiles [4] P(τ) can be measured (or approximated) as

where

is the power associated with each path.

Assuming a uniform distribution of independent scatterers in the horizontal plane, each with a Doppler shift relative to the velocity of the mobile, the delay-Doppler spread function S( t , n) and the impulse response of the channel can be constructed. A wide-band, frequency selective, multipath fading model can therefore be constructed using a tapped-delay-line filter. The typical tapped-delay-line filter model for simulation is illustrated in Tapped-Delay Line Model for a Wide-Band Channel.

Tapped-Delay Line Model for a Wide-Band Channel

To generate a Rayleigh fading profile for each path, independent AWGN sources (in cascade with a filter representing the effects of Doppler spread) can be used; see Generation of Rayleigh pdf with a Given PSD.

Jakes [5] proposes a more efficient alternative to Generation of Rayleigh pdf with a Given PSD. In Jakes' model, a number of low-frequency oscillators are used to generate signals that are added together. The amplitude and phases of these oscillators are chosen so that the pdf of the resultant phase approximates to a uniform distribution. The spectrum of the resulting complex function approximates the Doppler spectrum.

Generation of Rayleigh pdf with a Given PSD

References

  1. J. D. Parsons, The Mobile Radio Propagation Channel, Halsted Press, 1992.
  2. R. H. Clarke, "A Statistical Theory of Mobile-Radio Reception," The Bell System Technical Journal, July-August 1968.
  3. Raymond Steele, Mobile Radio Communications, Pentech Press, 1992.
  4. GSM 05.05 Recommendation, Radio Transmission and Reception.
  5. W. C. Jakes (Editor), Microwave Mobile Communications, John Wiley & Sons, 1974.
  6. M. Hata, "Empirical Formula for Propagation Loss in Land Mobile Radio," IEEE Trans. VT-29, pp. 317-325, August 1980.
  7. Y. Okumura, "Field Strength and its Variability in VHF and UHF Land Mobile Service," Review of Electrical Communication Laboratory, Vol. 16, pp. 825-873, Sep-Oct. 1968.

Common Fading Model Definitions

The delay spread is modeled via a tapped delay line where the number of taps is based on the size of FadingDelayArray and FadingPowerArray . In each case the input signal is delayed and the carrier phase due to the delay signal is incorporated.

Delay and Doppler Spread and Carrier Phase Shift illustrates this modeling process when connected to a simple antenna.

Delay and Doppler Spread and Carrier Phase Shift

For each tap, Jakes model or noise filter model provides Doppler spectrum as well as the fading channel specifications.

Jakes model uses N 0 low-frequency oscillators to generate a fading waveform.

where

The maximum Doppler frequency offset ω m = 2π V /l, l denotes wave-length of carrier. β n is set as β n = 2π/ N 0 . Original phase shifts θ n are randomized and uniformly distributed over [0, 2π]. N 0 = 80 is used in the component.
Details about Jakes model can be found in reference [1].

Noise filter model feeds white Gaussian noise to a digital filter matched to the respective fading spectrum, i.e., the filter frequency response must be a good approximation to the square root of the (normalized) tap power spectral density. An interpolator with rate conversion factor follows. Changing tap Doppler spreads is done by simply changing the interpolation factor.

This implementation is illustrated in Noise Filter Model Implementation. The AWGN, filter, and interpolator outputs are complex. The filter and AWGN operate at the signal sampling rate. An 8th-order IIR filter is used for classic and flat spectrums whose normalized fading frequencies are 0.05686. A simple linear interpolator often suffices for changing Doppler spreads.

Noise Filter Model Implementation

If speed is zero, the channel is not a time-variant. However, multipath still exists so a static channel is applied and each tap is a complex constant value.The complex constant value is randomly generated and can be changed by changing the Seed parameter.

 

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