Setting Up a Measurement (TD-SCDMA)
When setting up a TD-SCDMA demodulation measurement, there are several things to be considered. This topic explains how to connect your signal to the 89600 VSA software and successfully demodulate TD-SCDMA signals. The information is provided in this order:
Connecting Your Signal to the VSA
The VSA analyzes your carrier signal or, if your measurement hardware has two baseband input channels, the VSA can analyze the raw I (In-phase) and Q (Quadrature-phase) signals.
For all receiver configurations except
and , connect your signal to Channel 1. In this case, the VSA assumes that the carrier signal contains both the I and Q components. If your measurement hardware has two input channels, the VSA ignores the second input channel.If your measurement hardware has a second input channel, the raw I (In-phase) and Q (Quadrature- phase) signals can be connected. To do this, select
, , or . With this configuration, the VSA removes the quadrature mixer from the demodulator's signal flow, which connects the I component of your signal to Channel 1; the Q component to Channel 2.Notes for I+jQ input:
- Selecting an coupling. configuration enables DC
- Use User Correction to correct for any offset or gain imbalance between the I and Q signals.
- Range is set independently for the I and Q inputs, but can be coupled by selecting the Couple Paired Channels parameter.
Connecting to an Keysight ESA VSA
Due to ESA memory limitations and demodulation algorithm latency requirements, the 89600 VSA software is not able to perform TD-SCDMA demodulation at all when the ESA is in the alias-protected mode. When the ESA is in the non-alias protected mode, this also limits the result length to 5 sub-frames with a Start Boundary of Sub-frame and 2 sub-frames with a Start Boundary of "2 Frames". This 2 sub-frame limitation on results synchronized to 2 frame boundaries can be removed by supplying an external 2-frame trigger and setting the Start Boundary to Sub-frame. The trigger delay can then be used to adjust where the boundary is located within the Result Length (see ESA Setup).
Center Frequency
The VSA's center frequency must be set within approximately ±500 Hz of the carrier frequency of the TD-SCDMA ( signal, otherwise the VSA may not be able to lock onto your signal.
Frequency Span
When selecting a frequency span, select the narrowest span that includes all of your signal components (select a span that is slightly larger than the bandwidth of your signal). If a span that is too narrow is selectd, your measurement may have excessive errors or the VSA may lose carrier lock. If the span is less than the chip rate * (1+filter alpha), the VSA displays "DATA?" to indicate that the results may not be accurate.
For best results, the frequency span should be set according to the following formula (filter alpha is the filter characteristic of the Nyquist filter used to demodulate the TD-SCDMA signal):
Frequency Span ³ (1 + Filter alpha) ´ Chip Rate
If a span that is too wide is selected, your measurement may be affected by excessive noise and slower speed.
Preset to Standard will select a default span that should work well.
Input Range
The input range must be set correctly to obtain accurate measurements. Input ranges that are too low overload the VSA's ADC Analog-to-Digital Converter. Input ranges that are too high increase noise, which increases errors reported in error summary data, such as EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format..
Code/ID Parameters
The Code/ID parameters must be set to the system values to demodulate the test signal (see Code Allocation Overview):
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Downlink Pilot code ( SYNC-DL Down Link (forward link: from base station to cell phone) ID) (if active)
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Uplink Pilot code (SYNC-UL Up Link (reverse link: from cell phone to base station) ID) (if active)
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Scramble code
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Basic Midamble code
Traffic Timeslots
The Max Users (K) parameter must be set for each active timeslot to demodulate the test signal.
Chip Rates, Symbol Rates, and Spread Code Length
To successfully demodulate TD-SCDMA signals, the VSA chip rate must match the chip rate of your TD-SCDMA signal. The Preset to Default demodulation property will set the chip rate as required for the TD SCDMA modulation format.
Result Length
Result Length determines the time capture length of the input signal that the VSA demodulates in terms of the number of sub-frames. Make sure that the result length is long enough to capture all the desired data.
Filtering
For TD-SCDMA demod, the measurement filter is a root-raised cosine filter with user definable alpha capability (default alpha value = 0.22). This differs from the filtering capabilities available with either Digital Demod or Video demodulation. With those two demod types, the option of selecting a measurement and reference filter was available but this is not available for TD_SCDMA demodulation (see Digital Demod About Filters).
Mirrored Spectrum
TD-SCDMA demodulation provides a feature that configures the VSA's demodulator to conjugate the complex time-domain waveform. This has the effect of flipping the spectrum around the VSA's center frequency. This feature is useful when the TD SCDMA signal is inverted or flipped due to an IF down conversion.
Triggered Measurements
When making triggered measurements, the demodulation algorithm requires some amount of data prior to the beginning of the desired sub-frame, in order for demodulation to begin on that sub-frame. This is handled internally but if there are delay differences between the sub-frame boundary in the incoming signal and the trigger then the Trigger Delay may need to be adjusted in order to supply enough data before the beginning of the sub-frame. For instance, if an external trigger occurs 20 us after the beginning of the desired sub-frame then Trigger Delay would need to be set to -20 us.
Averaging
Averaging is available only for code domain data and is applied to numeric error data in the symbol table, as follows:
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rms (video)¾computes an average of each rms error update in the table.
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rms exponential¾is like rms (video) except that averaging continues past the average count with an exponential weighting.
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continuous peak hold¾keeps track of the peak rms error of each update in the table.
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Time and time exponential averaging are not available.
The following average types are available for the pre-demodulated spectrum result:
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rms (video)
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rms exponential
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continuous peak hold
See Also