Setting up an 802.11a/g/j/p OFDM Demodulation Measurement
When setting up an 802.11a/g/j/p OFDM Orthogonal Frequency Division Multiplexing: OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative tranmission scheme to DSSS and FHSS. demodulation measurement, there are several things to consider. This topic tells how to connect your signal to an 89600 VSA and successfully demodulate digital signals. The information is provided in this order:
Connecting Your Signal to the Measurement Hardware
The carrier can be connected or, if your measurement hardware has two input channels, the raw I (In-phase) and Q (Quadrature-phase) signals.
For all receiver configurations except , connect the signal to Channel 1. In this case, the VSA assumes that the signal contains both the I and Q components (the carrier signal). 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 individually. To do this, click. With this configuration, the VSA removes the quadrature mixer from the demodulator's signal flow, which allows the I component of your signal to be connected to Channel 1 and the Q component to Channel 2.
Notes for I+jQ input:
- Selecting an configuration enables DC coupling.
- 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.
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Center Frequency
The measurement hardware center frequency must be within 2 * Subcarrier Spacing (625 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second). for 802.11a or HIPERLAN/2) to lock properly.
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Frequency Span
When selecting a frequency , 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 selected, the measurement may have excessive errors or the VSA may lose carrier lock. If the span is less than 18 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second., the VSA displays to indicate that the results may not be accurate.
If the span is too large, it may allow spurs to be aliased into the region of interest. Selecting Show All Frequency Points (Display, Appearance, Trace tab) can help determine if spurs are being included in the result. If so, the span should be reduced to exclude them and provide a more accurate result.
Using the Preset to Standard configuration settings will select the proper default span.
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Input Range
The input range must be set correctly to obtain accurate measurements. Input ranges that are too low overload the measurement hardware'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..
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Search Length
The must be long enough to capture a complete OFDM burst. Using long pulse lengths will increase the measurement time, but make it more likely that a pulse will be detected.
If a pulse is not detected a PULSE NOT detected status message is displayed.
A method to determine when performing a pulse search:
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SL = (2*MaxOn) + MaxOff
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Where:
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SL is search length in seconds.
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MaxOn is the maximum on time of a pulse in seconds.
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MaxOff is the maximum off time between pulses in seconds.
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Subcarrier Spacing
The must match the transmitted signal subcarrier spacing. Use the default value for the specific OFDM format of interest. The value can be adjusted to match a non-standard demod format.
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Symbol-time Offset
Use the default value. Experienced users may adjust this parameter to fine tune the demodulator.
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Guard Interval
The must match the guard interval of the transmitted signal. Use the default value for the specific OFDM demod format of interest.
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Maximum Result Length
Use the default value. This will provide a good starting value for the result length.
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IEEE 802.11g modulation
The IEEE Institute of Electrical and Electronics Engineers. A US-based membership organisation that includes engineers, scientists, and students in electronics and related fields. The IEEE developed the 802 series wired and wireless LAN standards. Visit the IEEE at http://www.ieee.org 802.11g standard defines both single-carrier and OFDM modulation formats. The single-carrier modulation formats are compatible with 802.11b and can be demodulated using the 802.11b/g DSSS Direct sequence spread spectrum. The data transmission scheme (sometimes referred to as a "'modulation" scheme) used in 802.11b WLANs. DSSS uses a radio transmitter operating at a fixed centre frequency, but using a relatively broad range of frequencies, to spread data transmissions over a fixed range of the frequency band. 802.11a and 802.11g (when not operating in 802.11b mode) use Orthogonal Frequency Division Multiplexing (OFDM)./CCK complementary code keying/PBCC packet binary convolutional code demodulation mode (see About 802.11a/g DSSS/CCK/PBCC (Opt B7R).)
IEEE 802.11g defines two different OFDM modulation formats. One, which is mandatory in 802.11g, is an exact copy of the 802.11a format except that the carrier frequencies are in the 2.4 GHz Gigahertz: A frequency measurement which equals one billion hertz. band. This is referred to as "IEEE 802.11g OFDM". The other OFDM format is an optional format called DSSS-OFDM that combines an 802.11b-style single-carrier preamble with an 802.11a-style OFDM data payload. This is referred to as "IEEE 802.11g DSSS-OFDM". The 802.11a/g/j/p OFDM modulation analysis mode can demodulate and analyze either of these OFDM formats.
See Also
Selecting the 802.11a/g/j/p Demodulator
