Hello everyone,
In this article, we will introduce 802.11ax Test Challenges, Conclusion, NI, and the 802.11ax standard.
802.11ax Test Challenges
Tighter EVM requirement
The 802.11ax standard now mandates support for 1024-QAM. Additionally, the subcarriers are only 78.125 kHz away from each other. This means that 802.11ax devices need to have oscillators with improved phase noise performance and RF front ends with better linearity. The test instruments that measure DUT behavior in turn require their EVM noise floor to be significantly lower than the DUT’s.
The following table lists the EVM levels that 802.11ax-compliant device will likely have to meet.
Table 3. 802.11ax EVM requirements
National Instruments WLAN test systems combine an RF vector signal transceiver (VST) with the NI WLAN Measurement Suite to support generation and analysis of 802.11ax signals. The software supports waveforms ranging from BPSK (MCS0) to 1024-QAM (MCS10 and MCS11). In addition, NI’s VST hardware consistently delivers best-in-class EVM floor measurements for RF characterization and production needs.
Absolute and Relative Frequency Error
OFDMA systems have a very high susceptibility to frequency and clock offsets. Consequently, 802.11ax Multi-user OFDMA performance demands extremely tight frequency synchronization and clock offset correction. This ensures that all STAs operate exactly within their allocated subchannels with minimal spectral leakage. Additionally, the strict timing requirements guarantee that all STAs will transmit simultaneously in response to the AP’s MU trigger frames.
In the case of 4G LTE systems, base stations have the advantage of GPS disciplined clocks to synchronize all associated devices. However, 802.11ax APs likely won’t have that luxury, and they will have to maintain system synchronization using their own built-in oscillators as the reference. STAs will then adjust their internal clock and frequency references by extracting offset information from the trigger frames that they get from the AP.
Frequency and clock offset testing of 802.11ax devices will involve the following tests:
Absolute frequency error: the DUT sends 802.11ax frames and the test instrument measures the frequency and clock offset against a standard reference. This will be similar to what the current 802.11ac specification indicates, with limits around ±20ppm.
Figure 14. Simple setup for absolute frequency error measurement
Relative frequency error: this will test the ability of a non-AP STA participating in uplink multi-user transmission to align itself with the AP’s frequency. The test procedure goes in two steps. First, the test instrument sends a trigger frame to the DUT. The DUT adjusts itself based on the frequency and clock information it derives from the trigger frame. Then the DUT replies with frequency-corrected frames. The test instrument measures the frequency error of these frames. After carrier frequency offset and timing compensation, these limits will likely remain very strict around less than 350 Hz and ±0.4 µs relative to the AP’s trigger frame.
Figure 15. Setup for relative frequency error measurement
STA Power Control
Similar to the requirement of reduced frequency and clock errors, the power that an AP receives during uplink multi-user transmissions should not have large variations across all users. This requires that the AP controls the transmit power of each individual STA. The AP may use a trigger frame that contains transmit power information for each STA. Developers can test this functionality in two steps, in a way similar to the frequency error test.
Access Point Receiver Sensitivity
Testing the receiver sensitivity of 802.11ax APs presents an additional challenge, considering that the AP acts as the clock and frequency reference. Therefore, the test instrument has to lock on to the AP before sending packets to the AP for packet error rate sensitivity tests.
When the AP initiates by sending a trigger frame, the test instrument adjusts its frequency and clock to match the AP and then responds to the AP DUT with a predetermined number of packets with the expected configuration.
The challenge here has to do with the strict relative frequency error limits of 802.11ax. The test instrument has to derive very precise frequency and clock information from the trigger frames that the AP sends. It might be necessary to perform this calculation over multiple trigger frames to ensure proper frequency and clock synchronization. As a result, this procedure could add a significant delay to the test procedure.
One possible solution to speed up the test procedure is for the AP to export its clock reference, such that the test equipment could lock its clock to it. This avoids the initial synchronization procedure based on trigger frames and leads to faster AP receiver sensitivity test times.
Uplink In-band Emissions
When STAs operate in MU-OFDMA mode they transmit up to the AP using the RU allocation that the AP determined. That is, STAs use only a portion of the channel. The 802.11ax standard might specify an uplink in-band emission test to characterize and measure the emissions that occur when the transmitter uses only a partial frequency allocation.
Figure 16. Potential uplink in-band emissions test mask
Multi-user and Higher Order MIMO
Testing 802.11ax devices with up to 8 antennas in MIMO operation can produce very different results than testing each signal chain individually and sequentially. For example, the signals from each antenna might interfere destructively with each other and affect power and EVM performance, with negative and potentially noticeable effects on the throughput.
Test instrumentation would have to support sub-nanosecond synchronization of the local oscillators for each signal chain to ensure proper phase alignment and MIMO performance over many channels. NI’s test solution, based on the NI VST, uses patented hardware and software technology to enable flexible massive MIMO configurations with up to 8, 16, or even 64 synchronized channels.
Conclusion
802.11ax promises to improve the average data throughput per user in dense environments by 4X. One of the biggest enablers of this efficiency is multi-user technology, both in the form of MU-MIMO and MU-OFDMA. This improvement in spectrum use in crowded environments will likely drive 802.11ax market adoption at faster rates than ever before. However, implementing this functionality will present a whole new set of challenges for the scientists, engineers, and technologists in charge of making these engineering marvels a reality.
NI’s flexible and modular platform offers high-performing hardware with clean oscillators and low EVM floor for 1024-QAM measurements with 4X closer subcarrier spacing. The WLAN Measurement Suite stays abreast of the latest developments of the nascent 802.11ax standard to help you design, characterize, validate and test your 802.11ax devices and be ready for the multi-user revolution.
About NI and the 802.11ax standard
NI is the provider of platform-based systems that enable engineers and scientists to solve the world’s greatest engineering challenges. NI works together with standard bodies and leading semiconductor companies in the development of systems and tools to design, characterize, validate and test the latest wireless communication standards, including IEEE 802.11ax (draft 0.1) High-Efficiency Wireless draft standard.
Figure 17. NI 802.11ax 8x8 MIMO Test System with WLAN Measurement Suite and VSTs
National Instruments’ WLAN Measurement Suite and the PXI RF VST combine to deliver a modular and powerful test solution for 802.11ax devices. The WLAN Measurement Suite gives researchers, engineers and technologists the power and flexibility to generate and analyze a wide range of 802.11 waveforms, such as 802.11a/b/g/n/j/p/ac/ah/af. Now, with the measurement suite’s latest update targeting 802.11ax, these users can speed up development work on 802.11ax devices. The software supports key features of 802.11ax including narrower subcarrier spacing, 1024-QAM, and Multi-user OFDMA. The updated measurement suite also includes LabVIEW system design software example code to help engineers automate WLAN measurements quickly and easily.
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