Hello and welcome to how to overcome key 802.11ax transmitter design and test challenges with
Keysight's 89600 VSA software.
802.11ax introduces various new technologies and features to meet its goal of high efficiency,
capacity and coverage for a better user experience, especially for dense deployments.
The key technologies and features include multi-user transmission using OFDMA and multi-user
(MU)-MIMO in both uplink and downlink, narrow sub carrier spacing, and 1024QAM modulation.
All of these increase design complexities and create a number of new test challenges.
Let's take a look at these new challenges.
In the downlink multi-user transmission, the access point needs to serve multiple users
simultaneously and often times each user will have different channel configurations including
different modulation formats as well as spatial streams.
The standard also calls for the EVM to be computed for each transmitted resource unit separately.
When testing, you want to verify the packets for the different users are scheduled and
transmitted correctly, and the EVM for each resource unit is computed correctly and it
meets the EVM requirements.
In the uplink multi-user transmission, also called trigger-based, each user is assigned
a single resource unit by an access point and multiple users transmit simultaneously.
To prevent interference, the packet transmission from each station must start and end at the
same time and they must remain synchronized.
The other major challenge is emissions into a non-allocated resource units.
When transmitting in the uplink, each user must only occupy the resource unit they are assigned to, and
interference level outside of the transmitted resource unit, which is used by other stations,
must be minimized.
As a result, a new unused tone EVM or in-band emissions requirement is defined for the uplink.
For 1024QAM, the specification calls for 3 dB stricter EVM, and with higher order modulation,
the spacing between the constellation points become much smaller making the system more
sensitive to impairments.
So, it requires lower phase noise and higher amplifier linearity performance for the transceiver
as well as test equipment.
The narrow subcarrier spacing makes the system more sensitive to frequency offset, phase
noise and sampling clock offset.
This requires local oscillators with improved phase noise performance than needed for legacy
WLAN.
The last challenge I want to bring up is modeling the RF channels.
Outdoor channels experience larger delay spreads and more time variations; therefore, different
channel models are used for indoor and outdoor operation.
The Keysight SystemVue Electronic System Level (ESL) platform provides transmit and receive
reference models connected with either an indoor or outdoor wireless channel models
to enable full link-level EVM or throughput type analysis.
You can also simulate the effect of different impairments on the system.
The simulated waveform then can be downloaded to a signal generator to create the test signal and a
signal analyzer with 89600 VSA software can be used for analysis.
You can also link to the 89600 VSA software within the simulation environment and this
allows for simulated versus measured results comparison by using the same VSA measurement
algorithm both in simulation and test.
Let's take a look at two quick examples using the 89600 VSA software.
Starting with 160 MHz wide downlink OFDMA transmission.
The HE-SIG-B field contains resource unit allocations as well as user specific information.
The 89600 VSA decodes HE-SIG-B field and demodulates the multiple resource units simultaneously.
The main goal is to test the access point transmitter to verify the resource unit allocations
are accurate and evaluate the signal quality and perform error vector measurements of each
resource units separately.
The "Multi-User Info" table is a result of what is contained within the HE-SIG-B field.
So you can see all the scheduled resource units, MCS for each one of them, and other
information such as if spatial streams is used or not.
EVM is computed for each resource unit separately; and in this case, some of the resource units
are using 1024QAM and the rest are using 256QAM.
You can also see that in the constellation diagram.
By looking at the EVM results, you can see a big margin in the EVM performance over the
-35 dB limit for the 1024QAM.
The VSA with its in-depth analysis and troubleshooting capabilities helps you identify causes of EVM.
When you look at the "Error Summary" for example, you can see if there are any imperfections
in the IQ modulator and the software also allows you to compensate for some of those
impairments if needed.
You can compensate for IQ mismatch or compensate for sampling clock error.
All the default settings for the EVM measurement are based on the IEEE standard requirements.
Channel estimation or the equalizer training is based on the preamble only.
If I include the data subcarriers as part of the equalization filter, you can see that
I gain about a 3 dB better EVM performance.
All of the measurements that are available for legacy WLAN such as EVM versus subcarrier
and EVM versus symbols are also available for 802.11ax to help you troubleshoot any
design issues.
Let's move on to a quick uplink demo using a trigger based PPDU in an 80 MHz wide channel.
In the uplink, a user is assigned a single resource unit; and in this case, the resource
unit size is 242 tones and the rest of the 80 MHz channel is occupied by other stations.
Therefore, the interference level outside of the transmitted resource unit, must be
kept at a minimum.
The "Resource Unit Info" table provides EVM of the transmitted resource unit; and in this
case, it is resource unit #1 with size of 242 tones.
It also provides the EVM for the unused tones.
The unused tone EVM is computed separately and it is averaged over each 26 subcarriers
of the unassigned resource units.
In addition, the VSA added a new trace called in-band emissions.
The trace has three plots.
The green plot is the average power over each subcarrier.
The yellow is the unused Tone EVM and the red is the limit defined in the standard.
These correspond to "Unused Tone EVM" and the "Limit" columns in the "Resource Unit Info" table.
This allows you to easily evaluate the uplink signal quality and perform error vector measurements
of both the used and unused tone resource units.
In summary, the 89600 VSA software provides a comprehensive set of tools for demodulation
and vector signal analysis.
These tools enable you to view and troubleshoot 802.11ax chipsets and devices regardless of
the operating modes implemented.
To learn more, and to download a free trial, visit us at these websites.
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