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LitePoint’s Eve Danel is the author of this two-part blog series. Throughout these posts, you’ll learn about the key RF-PHY changes made in Wi-Fi 6/802.11ax compared to Wi-Fi 5 and older generations: the addition of a new frequency band, higher modulation rates, smaller subcarrier spacing, longer guard intervals and the introduction of the OFDMA modulation technique providing capacity improvements and latency reduction.

In my previous blog post, I explored some of the key RF-PHY changes in Wi-Fi 6, including a fundamental change to the way that Wi-Fi 6 operates, which is using orthogonal frequency division multiple access (OFDMA). Let’s take a closer look at Wi-Fi 6 OFDMA.

Resource Units (RUs)

With OFDMA, the channel bandwidth is divided into resource units (RUs) of various sizes. The size of an RU can vary from the smallest 26 subcarriers (or 2 MHz) up to 996 tones (or 77.8 MHz). The size and the location of the RUs is defined for 20 MHz, 40 MHz, 80 MHz channels, and 80+80 or 160 MHz channels.

Figure 1: RU location in 80 MHz channel

RUs are comprised of:

  • Data subcarriers used to carry data information and form the majority of the subcarrier’s assignment
  • Pilot subcarriers used for phase tracking for channel estimation
  • DC subcarriers at the center frequency of the channel
  • Guard band/Null subcarriers used at the band edges to protect from interference from neighboring RUs

Different numbers and sizes of RUs can be allocated for transmissions to different users, based on how much data each station needs. The access point (AP) is responsible for RU assignment and coordination. For example, applications that require a lot of data, like streaming video, can be assigned a large RU, while applications that require very little data can be assigned a small RU. Each RU can use a different modulation scheme, coding rate and level, and RU assignments can vary on a frame by frame basis.

Figure 2: OFDMA transmission showing 80 MHz channel divided into 8 RUs

For a downlink transmission, an HE MU PPDU carrying a mixture of 26-, 52-, 106-, 242-, 484-, and 996-tone RUs is transmitted from the AP to the stations. The HE MU PPDU preamble is transmitted over the whole channel and contains a new field called the SIG-B that provides information to the stations about their RU size and frequency allocation, the modulation MCS and the number of spatial streams allocated by the AP.

Figure 3: HE MU PPDU

The HE-SIG-B field is only included in HE MU PPDU (downlink) and it includes the information for OFDMA and MU-MIMO resource allocation. The HE-SIG-B field includes a common field followed by user specific fields.

The common field of an HE-SIG-B content channel contains information regarding the RU assignment, the RUs allocated for MU-MIMO and the number of users in MU-MIMO allocations. The SIG-B user specific fields contain information for all users in the PPDU on how to decode their payload.

Wi-Fi 6 OFDMA creates a large number of permutations of possible RU sizes, frequency allocations and power levels that need to be validated to ensure the transmitter and receiver performance meets the standard criteria. All of these permutations can have different error vector magnitude (EVM) and spectral performance.

Wi-Fi 6 OFDMA uplink transmissions are even more complex than downlink operation, because in the uplink, the traffic must be transmitted simultaneously from multiple stations to the AP. In the uplink transmission, the AP acts as an operations and transmission coordinator.

First, the AP sends a trigger frame to all the stations that will be involved in the upcoming transmission, and then these stations transmit simultaneously on their respective RUs in response to the trigger frames. Based on the trigger frame, the client station will need to tune its timing, frequency and power levels to participate in this transmission.

Timing synchronization

Client stations participating in an OFDMA transmission must transmit within 400 ns of each other. In order to synchronize the clients, the AP transmits a trigger frame. This frame contains information about the OFDMA sub-carrier’s RU assigned to each station. In response, the participating clients need to start transmission of the uplink signal after a specified time interval short inter-frame space (SIFS) of 16 µs +/- 400 ns after the end of the trigger frame as mandated by the 802.11ax standard.

Figure 4: Timing synchronization of uplink OFDMA

Frequency synchronization

To prevent inter-carrier interference (ICI) between the clients transmitting simultaneously, all stations participating in the transmission need to pre-compensate for carrier frequency offset (CFO). The client stations adjust their carrier frequency based on the trigger frame received from the AP. The 802.11ax standard requires the residual CFO error after compensation to be less than 350 Hz.

Figure 5: Wi-Fi OFDMA frequency synchronization

Power control

When traffic is transmitted between the AP and client stations that are located at various distances, power control is needed to ensure that stations closer to the AP do not drown out users farther from the AP that are transmitting simultaneously.  The 802.11ax standard requires the stations to adjust their power based on the estimated path loss to the AP. Devices closer to the AP transmit less power while devices farther away transmit more power to achieve the same received power at AP receiver considering the path loss. There are two classes of devices defined in the standard based on how accurately they can control their power. Class A devices control their transmit power within ±3 dB and class B devices control their power within ±9 dB.

Figure 6: Wi-Fi OFDMA power control

The accuracy of the synchronization between the AP and the client is critical because a single bad actor (station) that doesn’t perform as expected will degrade performance for all other users that are sharing the same transmission.

Unused Tone EVM

Another concept introduced in Wi-Fi 6 OFDMA is the unused tone EVM metric. As discussed above, when the stations transmit in the uplink direction on their assigned RU, it is important that emissions do not spill over into other RUs, otherwise that will reduce the system capacity for other users.

Figure 7: Unused tone EVM

To evaluate the station’s performance, the 802.11ax standard introduced this new metric, which is a measure of the in-channel emissions generated by the stations. IEEE specifies RU leakage as a staircase mask requirement, which is called unused tone EVM. The unused tone EVM measures the EVM floor of 26 tone RU over the whole channel bandwidth excluding the position(s) for the active RU.

The validation of uplink multi-user transmission in Wi-Fi 6 OFDMA is one of the most challenging areas of 802.11ax testing. For the tester, it requires the involvement of both the signal generation and signal analysis function, which need to be coordinated to run a test sequence to provide nanosecond-level accuracy measurements to ensure the required 400 nanosecond accuracy of the uplink transmission.

Figure 8: Wi-Fi 6 trigger-based testing

When testing an AP design, the tester emulates a station, and it decodes the received trigger frame information in order to generate a trigger-based physical layer protocol data unit (PPDU) with the correct RU allocation. The tester also needs to adjust its power to the AP requirements in real time because it needs to respond to the AP with a nanosecond level of accuracy.

When testing a client station, the tester emulates an AP, generates a trigger frame and will measure the response with the signal analysis function. The tester measures the trigger-based PPDU generated by the station itself. The tester needs to measure the timing, frequency and power accuracy of this transmission from the client to ensure it meets the 802.11ax standard requirements and the RU leakage requirements.

Conclusion

Beyond testing the traditional transmit and receiver metrics that are necessary for any Wi-Fi generation, the chart below shows an overview of the new test areas that are specific to 802.11ax. These test sequences for Wi-Fi 6 OFDMA require a higher level of test complexity compared to previous Wi-Fi generations.

Figure 9: Wi-Fi 6 key test areas

If the number of test cases and complexity of Wi-Fi 6 OFDMA seems overwhelming, I encourage you to explore LitePoint’s IQfact+ automation solution. The software is tailored to control both the tester and the device under test (DUT) and it provides calibration and verification automation. LitePoint already offers IQfact+ packages for Wi-Fi 6 and Wi-Fi 6E chips for AP and client devices.

 

By Eve Danel

May 25, 2021

LitePoint’s Eve Danel is the author of this two-part blog series. Throughout these posts, you’ll learn about the key RF-PHY changes made in Wi-Fi 6/802.11ax compared to Wi-Fi 5 and older generations: the addition of a new frequency band, higher modulation rate, smaller subcarrier spacing, longer guard interval and the introduction of the OFDMA modulation technique providing capacity improvements and latency reduction. 

Drivers for Wi-Fi 6 vs. Wi-Fi 5: Capacity Improvement

In previous generations, updates to the Wi-Fi standard were mainly focused on increasing the raw throughput. Wi-Fi 6 is faster as well, but different in that it also focuses on improving user experience, especially in dense, congested environments with a large number of users, such as airports, stadiums or educational settings. In these environments, the user experience suffers from inefficiencies when too many users are competing for bandwidth, and multiple overlapping networks interfere with each other.

Wi-Fi 6 adds features that improve average throughput per user by up to four times over Wi-Fi 5 in these highly congested environments. Wi-Fi 6 is also very well suited for use in home networks, where phones and tablets join televisions, home automation controls and other systems in competing for bandwidth. Wi-Fi 6 is particularly important in applications such as video conferencing and content streaming that require very low latency.

Wi-Fi 6 vs. Wi-Fi 5: Key Changes to the RF Physical Layer

Wi-Fi 6, also known as high efficiency (HE) Wi-Fi, is based on the 802.11ax standard and operates in the 2.4 GHz and 5 GHz frequency bands. Wi-Fi 6E, also based on the 802.11ax standard, operates in the 6 GHz frequency band.

Wi-Fi 6 vs. Wi-Fi 5 and Wi-Fi 4

Graphical user interface, application Description automatically generated

The chart above summarizes the key RF-PHY changes made to increase capacity and efficiency in Wi-Fi 6 vs. Wi-Fi 5. Some new features are designed to improve Wi-Fi top speeds. Other new features are designed to improve the user experience in outdoor spaces or in high multipath fading environments. Other features are designed to improve capacity and reduce latency.

Other features not listed in the table, but that still have an impact on the network and devices include target wake time (TWT) for better battery savings and BSS coloring that is designed to improve spatial reuse in congested environments.

802.11ax Modulation

Wi-Fi 6 increases the modulation rates to improve peak data rates. The 802.11ax standard uses 1024 – quadrature amplitude modulation (1024QAM) with a peak data rate that is 25% higher than the peak data rate in the 802.11ac standard, which utilized 256QAM. 1024QAM used in Wi-Fi 6 encodes 10-bits of data per subcarrier, while 256QAM used in Wi-Fi 5 encodes 8-bits of data per subcarrier.

Wi-Fi QAM Rate Evolution

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When encoding data at 1024QAM, the main challenge is ensuring high transmitter performance so that signals are correctly demodulated at the receiver. Error vector magnitude (EVM) is the metric that is used to ensure modulation accuracy. EVM measures the difference in dB between the ideal and the actual I/Q positions on a constellation diagram. The IEEE-defined EVM requirements for 1024QAM require a minimum EVM of -35dB, which is 3 dB better than 256QAM. 

When measuring the EVM for a device with a tester, that tester’s EVM floor is critical. To ensure accuracy, the tester’s EVM floor should be 10 dB better than the device that is being measured. For 802.11ax, a tester that can achieve at least -45 dB to -48 dB residual EVM is needed.

OFDM Subcarrier Spacing and Symbol Duration

Another change to the PHY layer is that 802.11ax adds 4 times more subcarriers than 802.11ac, resulting in the subcarrier spacing becoming ¼ of what it was in 802.11ac. For 802.11ax, there is now a spectral spacing of 78.125 kHz between subcarriers. The symbol duration is inversely proportional to carrier spacing and increases four times over Wi-Fi 5, from 3.2 µs in 802.11ac to 12.8 µs in 802.11ax.

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There are a few advantages from this change to the subcarrier spacing.  The first is that with more subcarriers there can be finer resource unit (RU) granularity. The resource units are allocated in OFDMA to divide the channel bandwidth into smaller sub-channels assigned to different users. The smallest RU size contains 26 subcarriers (around 2 MHz), and allows up to 74 users in a 160 MHz channel (each user being assigned a 26 tones RU).

The second advantage is that 802.11ax is designed with a higher ratio of subcarriers that carry data vs. those that don’t carry data (null and pilot). The increase in the number of data subcarriers increases the efficiency of the transmission. This change results in about 10 percent improvement over 802.11ac.

It is important to keep in mind that because subcarrier spacing has decreased and the signal processing required is also more complex, Wi-Fi 6 devices need better frequency accuracy for proper demodulation.

Guard Interval / Cyclic Prefix

The standard also introduces new guard interval options. In Wi-Fi 6, longer guard intervals are designed to provide improved performance in environments with multi-path and delay spread. These longer guard intervals help to prevent inter-symbol interference in outdoor environments and therefore improve coverage and performance. 802.11ac had two Guard Interval (GI) options – long GI (0.8µs) and short GI (0.4µs). 802.11ax has three types of GI – the normal GI (0.8 µs), double (1.6 µs) GI and quadruple (3.2 µs) GI. Overall efficiency is preserved since in 802.11ax the symbol duration is four times longer than in 802.11ac, therefore the overhead generated by even the longest guard interval is the same percentage of the symbol time as in the previous generation, but with the added benefits of longer guard interval.

Graphical user interface, application Description automatically generated

OFDMA

Wi-Fi 6 utilizes orthogonal frequency division multiple access (OFDMA). This is a fundamental change to the way that Wi-Fi operates. In previous OFDM-based Wi-Fi standards, bandwidth could be increased, but a user transmission would take the full channel for each transmission whether or not there was enough data to fill the bandwidth of the entire channel. Other senders would queue for access to the channel; creating a big problem in networks that supported a lot of simultaneous users.

By contrast, in OFDMA the channel is divided into subchannels called resource units (RUs). Each RU is made of a pre-defined number of subcarriers. The smallest RU can have 26 subcarriers, which is a little less than 2 MHz of spectrum and the largest RU can be as wide as 996 subcarriers, which is close to 80 MHz of spectrum. There is one user per RU for OFDMA;

Each RU is assigned to a different client station and the size and number of RUs allocated to each station is determined by the access point (AP), based on the data transmission requirements of each station.

For example, an application that requires a lot of data, such as content streaming, can be assigned a large RU. A device that requires less data, an IoT sensor for example, can be assigned a very small RU. Each RU can use a different modulation scheme, encoding rate and power level. RU assignments can vary on a frame-by-frame basis, based on what the AP determines is the most efficient use of the spectrum.

OFDMA does not directly impact Wi-Fi’s raw link speed, but instead, it increases its efficiency and will reduce delay for users that are sharing the spectrum. This provides benefits, especially in congested environments.
In my next blog post, I will expand on OFDMA and how it works, and will present important test considerations for Wi-Fi 6 vs. Wi-Fi 5. In the meantime, please visit LitePoint’s Wi-Fi 6 page to learn more about our test and measurement solutions for Wi-Fi 6 and click here for a replay of my webinar on this topic.

LitePoint IQxel-MW Validates the Performance of Morse Micro System-on-Chip Family

SAN JOSE, California

—LitePoint, a leading provider of wireless test solutions, today announced that Morse Micro, developer of the smallest Wi-Fi HaLow single-chip solution, has standardized on the LitePoint IQxel-MW for design verification of its Wi-Fi HaLow system-on-chip family.

Customers and manufacturing partners integrating Morse Micro’s Wi-Fi HaLow SoC based on IEEE 802.11ah into their IoT design will be able to use the IQxel-MW to test the wireless functionality of their product, helping bring the design to market.

“The number of connected Internet of Things devices is growing rapidly and the low power and long-range capabilities of Wi-Fi HaLow will open many more possibilities for IoT applications,” said Vahid Manian, Chief Operating Officer of Morse Micro. “Original equipment manufacturers and original design manufacturers can now develop Wi-Fi HaLow IoT products with confidence using the IQxel-MW platform for design validation and manufacturing testing.”

Wi-Fi HaLow is well suited for a variety of IoT applications like video cameras, industrial automation, occupancy sensors, motion detectors and more, offering a longer signal range of approximately 1-kilometer, lower power connectivity and the ability to penetrate obstacles.

“Morse Micro provides one of the leading chipsets in the Wi-Fi HaLow market. We’re pleased to be collaborating with them in this space” said Adam Smith, Director of Product Marketing at LitePoint. “LitePoint’s IQxel-MW platform is the ideal solution for thorough verification of 802.11ah OFDM RF PHY operation in the unlicensed Sub-1 GHz frequency bands. It allows Morse Micro’s customers to accelerate their Wi-Fi HaLow design and ensure optimal performance of their IoT products.”

Technical Details

LitePoint’s IQxel-MW platform is a leading test solution for Wi-Fi connectivity, meeting the needs of product development and high-volume manufacturing. The IQxel-MW includes support for 802.11a/b/g/n/ac/ax and 802.11ah as well as wide range of connectivity and IoT technologies with a frequency range from 400 to 6000 MHz and upgradable to support up to 7300 MHz for future Wi-Fi 6E bands.

For more information, visit LitePoint’s IQxel-MW test solution.

– End –

By Eve Danel
March 10th, 2021

EVM (Error Vector Magnitude) is the key metric used to evaluate RF transmitter performance, because it provides a consistent “yardstick” to characterize the transmitter regardless of the receiver implementation and it encapsulates a wide range of possible impairments on the transmitter chain into a single measurement. New Wi-Fi generations have increased the modulation constellation size with 1024-QAM and 4096-QAM, placing an even higher requirement into transmitter accuracy. In this blog, we will look at what EVM measures in Wi-Fi and how it is measured.

Quick Facts about QAM Constellation

With each new Wi-Fi generation, higher data rates are achieved by encoding more data bits per symbol. Higher-order QAM enables to transmit more data bits while maintaining the same spectrum footprint and therefore achieve better efficiency.

  • 802.11ac (Wi-Fi 5) supports up to 256-QAM where 8 bits of data are encoded per symbol
  • 802.11ax (Wi-Fi 6) supports up to 1024-QAM, where 10 bits of data are encoded per symbol
  • 802.11be introduces 4096-QAM modulation with 12 bits of data per symbol

With a higher-order modulation, the constellation points are closer together and are therefore more susceptible to noise and non-linearities.  The digital communication channel requires a better Signal-to-Noise ratio (SNR) to operate error free compared to using lower modulation rates, because the separation between constellation points is reduced and so is the decision distance in the receiver. The transmitter also needs to perform with significantly better accuracy, the Error Vector Magnitude (EVM) is the metric used to quantify the accuracy.

What is EVM?

Error Vector Magnitude is the most commonly used modulation quality metric in digital communications, it is a measure of the deviation of the actual constellation points from their ideal locations in the constellation diagram. The Root-Mean-Square (RMS) error is averaged over subcarriers, frequency segments, OFDM frames and spatial streams and measured at the symbol clock transitions. It is expressed in %Root-Mean-Square or dB.

EVM is a comprehensive measure of the transmit quality because it reflectssignal defects that affect the magnitude or phase of the transmitted symbol. It captures the sum of imperfections in the device implementation that impact the transmit symbol’s accuracy. Possible impairments can arise at the baseband, IF or RF elements of the transmit chain.

Some common types of corruption are:

  • I/Q gain and phase mismatch: gain mismatch originates from the two baseband modulation input signals (I and Q) having an amplitude difference at the upconverter. Phase (or Quadrature) mismatch occurs when the two modulated baseband signals are not 100% in quadrature (90°) when being up converted by the upconverter.
  • Symbol clock error: originating at the encoder, the symbol clock controls the frequency and timing of the transmission of the individual symbols
  • Phase noise: originating at the LO (RF or IF)
  • PA compression and non-linearity: originating in the power amplifier being operated in its compression region or showing non-linearity

IEEE EVM Requirements

The IEEE defines the maximum allowed transmitter constellation error as part of the standard. The maximum allowed depends on the data rate i.e. constellation size, since higher-order constellation requires a tighter modulation accuracy.

  • For 1024-QAM, the EVM requirement is ≤ -35 dB with amplitude drift compensation disabled in test equipment.  If amplitude drift compensation is enabled in test equipment, the EVM requirement is ≤ -35 dB, and ≤ -32 dB with amplitude drift compensation disabled.
  • For 4096-QAM, EVM requirements as defined in IEEE 802.11be Draft 0.2

How is EVM Measured?

The test is performed over at least 20 frames. For 802.11ax HE-MU PPDU and HE-TB PPDU, if the occupied RU has 26 tones, the PPDUs under test shall be at least 32 data OFDM symbols long. For occupied RUs that have more than 26 tones, the PPDUs under test shall be at least 16 data OFDM symbols long. The frames should contain random data.

For an HE-TB PPDU with an RU smaller than a 2×996-tone RU, the test shall also include transmit modulation accuracy for the unoccupied subcarriers of the PPDU.

Because Wi-Fi operates on 3 frequency bands (2.4 GHz, 5 GHz and 6 GHz) the transmitter EVM performance should be verified at all transmit power levels and frequencies where the transmitter will operate.

Test equipment used for EVM measurements should support converting the transmitted signals into a stream of complex samples at 160 MHz or more, with sufficient accuracy in terms of I/Q amplitude and phase balance, dc offsets, phase noise, and analog-to- digital quantization noise to ensure low error margin in the measurements.

EVM Correction Items

EVM shows a dependency on the analysis options in the test equipment (such as Phase Tracking, Channel Estimate, Symbol Timing Tracking, Frequency Sync, and Amplitude Tracking). Because analysis parameters can improve EVM, they should be chosen carefully. Addition of non-standard EVM correction methods can artificially improve the DUT’s EVM measurement and hide defects that would have otherwise been detected. It is recommended to only apply IEEE standard defined EVM analysis methods to ensure an objective characterization of the DUT’s transmitter performance.

The following lists the EVM measurement and correction methods:

IEEE defined EVM analysis:

  • Frequency correction: Because the receiver and transmitters have separate clocks, this correction removes the frequency error of the transmitter from the receiver.
  • Phase correction: Because the receiver and transmitters have separate clocks, devices must correct for clock misalignment.
  • Preamble channel estimate: The channel between the tester and DUT is calculated and corrected based on Pilot tones.

IEEE optional EVM analysis:

  • Amplitude Drift Compensation: The 802.11ax standard places a different target EVM whether amplitude drift compensation is enabled in the test equipment. Amplitude drift compensation, also called amplitude tracking,removes variations due to amplitude changes between symbols. When EVM is measured with the minimum IEEE required PPDU size of 16 data OFDM symbols, the effects of amplitude drift may not be noticeable. However, when longer PPDUs are used (e.g. A-MPDU), amplitude drift may have a more pronounced effect and result in degraded EVM. If amplitude drift causing defects are present, enabling amplitude tracking in the test equipment will hide these defects resulting in a better EVM measurement. While most modern DUT Wi-Fi receivers are able to compensate for amplitude drift, defects causing amplitude drift like PA thermal performance could get masked by the use of amplitude tracking. During design evaluation and troubleshooting, EVM should be measured with and without amplitude drift compensation. A large delta between the 2 measurements will indicate an underlying condition.

Non-standard EVM corrections:

  • Full Packet Channel Estimate: With this correction, the channel between tester and DUT is calculated using all the data packets. When the channel model is estimated based upon the full packet, the EVM improves because the channel model more closely fits the full packet. However, within a real-world receiver application, the packets are processed in real-time, and the channel is estimated based on the header only. This correction artificially improves EVM measurement and could hide possible phase noise, analog flatness or IQ mismatch issues (bad VCO, XTAL, compression, thermal issues, power supply)
  • Channel estimate equalization: This correction removes phase noise and amplitude response from the channel, it assumes flat channel response.  This correction could hide possible phase noise and analog flatness issues (bad PA, matching).

Using non-standard EVM correction can artificially improve measurements and hide underlying EVM impairments, therefore care should be taken in the selection of these parameters.

Test Equipment and Error Margin

As the requirements for the transmitter modulation accuracy rise, so do the requirements of the equipment necessary to test it. The DUT transmitter’s EVM measurement requires the test equipment’s own EVM floor to perform even better in order to provide a small measurement error.

For measuring the same DUT performance, a larger margin between DUT performance and the tester’s EVM floor will result in a smaller measurement error.

As shown in the chart above:

  • 16dB margin between DUT’s EVM and tester’s EVM floor results in a 0.1dB error uncertainty to the measurement
  • 6dB margin between DUT’s EVM and tester’s EVM floor results in a 1dB error uncertainty to the measurement
  • 0dB margin between DUT’s EVM and tester’s EVM floor results in 3dB error uncertainty to the measurement

For high order modulations 1024-QAM or 4096-QAM, that require stringent transmitter accuracy, selecting test equipment with low EVM floor is critical, otherwise the error uncertainty contributed by the test equipment reduces the confidence in the final measurement. In extreme cases, where the tester’s EVM floor equals that of the DUT, the measurement error is too large to determine if DUT meets the IEEE EVM requirements.

Key Takeaways

EVM provides a concise “one number” summary of the transmitter quality as it encapsulates a wide range of possible impairments on the transmitter chain. EVM is used during the design phase to characterize devices and uncover underlying sources of distortions. Because of its simplicity, it is also used in manufacturing to guarantee that transmitters will operate properly in real-world environments. However, it is important to understand that EVM is a calculated metric and numerous correction terms are possible that modify the measurement. The test equipment’s EVM floor is an equally important factor that affects the accuracy of the measurement. LitePoint’s IQxel family of testers provide best-in-class EVM performance to ensure high confidence in EVM measurements for the latest Wi-Fi generation.

To learn more about EVM: Read our Application Note.

By Eve Danel
November 30, 2020

LitePoint’s Eve Danel has developed this three-part blog series on Wi-Fi 6E and testing challenges. Throughout this series of blog posts, you’ll learn the basics of operating rules for Wi-Fi 6E in the 6 GHz band, the challenges when validating Wi-Fi 6E designs and what testing solutions LitePoint has available for Wi-Fi 6E.

Building and Testing the Next Generation High Performance Wi-Fi 6E Devices

In my previous blog post, I explored the IEEE 802.11ax rules of operation in the 6 GHz band and how they differ from operation in the 2.4 GHz and 5 GHz bands. In my final post as part of this Wi-Fi 6E series, I want to explore the challenges device makers must consider when building the next generation of Wi-Fi 6E devices and some of the testing solutions that LitePoint offers.

With the FCC’s Report on Order establishing rules for unlicensed devices in Wi-Fi 6E and the IEEE 802.11ax rules of operation, Wi-Fi 6E device makers have a lot to consider. As exciting as this new Wi-Fi spectrum availability is, it is critical that new Wi-Fi 6E devices meet the stringent requirements for performance and interoperability.

Challenges to Building the Next Generation, High Performance Wi-Fi

Building on the analogy of Wi-Fi 6E as a new, large freeway that only allows the fastest cars, how can device makers build the next generation of high-performance Wi-Fi devices that can take advantage of this highway of brand new spectrum? It’s exactly like building a high-performance sports car. There are many challenges to overcome, the below are a few that are particularly demanding.

  • 1200 MHz of additional spectrum added to Wi-Fi devices. It is double the frequency range coverage needed in the past for 2.4 GHz and 5 GHz bands. This can be particularly challenging for Wi-Fi 6E design, especially for the RF front end, because there is a need to deliver consistent performance across the entire spectrum from the low channels to high channels. You also need excellent linearity of the power amplifiers. Peak performance typically starts to roll off when you reach the higher frequency, or the edge of the band and devices will need to be tested to the very highest channels to make sure that they can operate at the expected power levels. Calibration of the transmitter power is very important and will be needed for consistent performance.
  • 160 MHz channels were already defined in the previous Wi-Fi 5 (802.11ac) generation, but they were not mandatory and were not often supported or deployed. With Wi-Fi 6E however, deployments will make full use of these wider channels because there is now sufficient contiguous spectrum. With wider bandwidth, you can have more distortions of the OFDM subcarriers as they cover a wider frequency range. The important metric to focus on for these wider channels is the spectral flatness to ensure even distribution of power. Also, wider channels mean lower SNR per carrier, therefore it requires excellent transmitter modulation performance.
  • OFDMA, the multi-user version of OFDM, is part of Wi-Fi 6 in the 2.4 GHZ and 5 GHz band, but in Wi-Fi 6E it will be even more prevalent because there will be no legacy devices operating in the 6 GHz band. This means all the devices in the 6 GHz band will be able to take advantage of OFDMA. For each transmission in highly congested areas, there will be multiple devices able to share the bandwidth to improve capacity and reduce latency. OFDMA is a very powerful feature, but it’s also one of the most challenging aspects of the IEEE 802.11ax standard because it requires all devices participating in the transmission to be synchronized. One bad actor can ruin the transmission for the others. All of the client stations participating in the transmission must be synchronizing time, they must have their frequency aligned and they must transmit power that is accurate.
  • 1024 QAM modulation was introduced in the IEEE 802.11ax standard and carries 10 bits per subcarrier. This improves peak data rates by 25 percent over previous versions, which is how the highest data speed is achieved. Because the high modulation rates can only be used in very good RF conditions, the 6 GHz spectrum will provide a better and cleaner environment with less interference from other devices since it has enough spectrum to avoid adjacent channels or overlapping channel interference. 1024 QAM also requires the highest level of modulation accuracy. This accuracy is usually measured by EVM (error vector magnitude) that measures the deviation of the constellation points compared to their ideal location.
  • Devices operating in the 6 GHz band must coexist with incumbent devices. Therefore, Wi-Fi 6E access points and clients must comply to regulatory defined emissions limits in order to avoid interference with other devices within the band or in adjacent bands. Spectral masks define the limits of the distribution of power across the channel and into the adjacent channels. It’s important to check that the spectral mask can be met for all the channels and especially to identify the worst case scenarios at the channels that output the highest power.

LitePoint Wi-Fi 6E Test Solutions

When developing Wi-Fi 6E devices, compliance verification and performance validation will be imperative to ensure these devices can really take advantage of this new spectrum. LitePoint has innovative, high performance testing solutions that can help device makers validate and accelerate Wi-Fi 6E device development.

IQxel-MW 7G™

IQxel-MW 7G

LitePoint’s IQxel-MW 7G™ is the first fully integrated tester for Wi-Fi 6 and Wi-Fi 6E. This test solution supports a continuous frequency range from 400 MHz to 7.3 GHz and features native support for per-port 160 MHz and 80+80 MHz signal combination. The IQxel-MW 7G has the best in class, residual EVM performancethat’s needed for 1024 QAM testing and supports packet detection and timing requirements needed for Wi-Fi 6E advanced testing of features like multi-user OFDMA.

IQxel-MW 7G supports legacy Wi-Fi standards, thereby ensuring coverage for Wi-Fi 802.11 a/b/g/n/ac and 802.11ax testing in the 2.4 GHz, 5 GHz and 6 GHz bands. In addition to Wi-Fi, the test system delivers high performance verification for the most popular wireless connectivity standards including WLAN legacy, all Bluetooth device standards (1.x, 2.x, 3.0, 4.x, 5.x), including cellular TDD and FDD non-signaling test modes for 2G/3G/4G and 5G cellular technologies.

The test platform is well suited for use in design verification and manufacturing testing.

IQfact+™ Software

IQfact+™ software is a test automation solution that combines device under test (DUT) and tester control. This software provides turnkey testing and calibration for leading Wi-Fi chipsets, enabling thorough design verification and rapid volume manufacturing with minimal customer engineering effort.

Each IQfact+ software is tailored to provide the best test efficiency for a specific chipset and is designed specifically for the LitePoint tester architecture, resulting in drastically reduced test time and engineering effort. IQfact+ encompasses a growing library of over 350 chipsets and supports all key wireless connectivity technologies.

With IQfact+ turnkey solutions developed for Wi-Fi 6 and 6E chipsets, the IQxel-MW 7G tester provides an out-of-the-box calibration and verification solution that can help device makers accelerate time to market.

Success with LitePoint

LitePoint has helped manufacturers deliver over 10 billion Wi-Fi-enabled products to market. Please visit our Wi-Fi 6E page to learn more about LitePoint’s Wi-Fi 6E solutions, and I invite you to view the full replay of my webinar on this topic. Look for more blog posts on Wi-Fi 6E, UWB and other topics from myself and the LitePoint team in the weeks to come.

By Eve Danel

November 11, 2020

LitePoint’s Eve Danel has developed this three-part blog series on Wi-Fi 6E and testing challenges. Throughout this series of blog posts, you’ll learn the basics of operating rules for Wi-Fi 6E in the 6 GHz band, the challenges when validating Wi-Fi 6E designs and what testing solutions LitePoint has available for Wi-Fi 6E.

Wi-Fi 6E Standard and Channels – 802.11ax Operation in the 6 GHz Band

In my previous blog post, I explored the FCC’s decision to open the 6 GHz band for Wi-Fi 6E standard operation, as well as the rules the FCC put in place to protect incumbent users in that space. Today I want to explore the IEEE 802.11ax rules of operation in the 6 GHz band and how they differ from operation in the 2.4 GHz and 5 GHz bands.

Background on Wi-Fi Standards

Two main groups are responsible for shaping Wi-Fi’s evolution. The IEEE 802.11defines the technical specifications of the wireless LAN standard. The IEEE 802.11ax standard for high efficiency (or HE) covers MAC and PHY layer operation in the 2.4 GHz, 5 GHz and 6 GHz bands. It is scheduled to be finalized by the end of 2020.

The Wi-Fi Alliance focuses on certification of Wi-Fi devices for compliance and interoperability, as well as the marketing of Wi-Fi technology. To improve consumer understanding of the various IEEE 802.11 standard generations, the Wi-Fi Alliance decided to create consumer friendly names. The IEEE 802.11ax standard is now referred to as Wi-Fi 6 or the 6th generation of Wi-Fi and operates the 2.4 GHz and 5 GHz bands.  Wi-Fi 6E operates in the 6 GHz frequency band. Thousands of devices have received Wi-Fi 6 certification since the program started and the Wi-Fi 6E certification is planned to start sometime in early 2021.

IEEE Rules of Operation

Decisions made by the IEEE 802.11ax group and added to the standard will make Wi-Fi 6E even more efficient.

Arguably one of the most important decisions made by the IEEE 802.11ax group is that it disallows older generation Wi-Fi devices in the 6 GHz band, which is important because it means that only high efficiency 802.11ax devices will be able to operate in this band.

Historically, newer Wi-Fi standards have always provided backward compatibility with older generations. This proved to be a great strength to win over consumers, since network equipment doesn’t need to be completely overhauled at each new generation. This has also been a source of congestion, since older slower legacy equipment is sharing available resources (i.e. spectrum) with newer devices.  In the 6 GHz however, only new high efficiency devices will be allowed to operate.

When using the analogy of a freeway to describe Wi-Fi, the 2.4 GHz and 5 GHz band can be compared to congested freeways allowing both fast and slow vehicles, while the 6 GHz band is the equivalent of a new, large freeway that only allows the fastest cars.

With 1200 MHz of spectrum and 59 new 20 MHz channels, a station with a dwell time of 100 ms per channel would require almost 6 seconds to complete a passive scan of the entire band.  The standard implements a new efficient process for clients to discover nearby access points (APs). In Wi-Fi 6E, a process called fast passive scanning is being used to focus on a reduced set of channels called preferred scanning channels (PSC). PSCs are a set of 15 20-MHz channels that are spaced every 80 MHz. The APs will set their primary channel to coincide with the PSC so that it can be easily discovered by a client, and clients will use passive scanning in order to just scan PSCs to look for an AP.

 

To further improve the efficiency of the 6 GHz operation, the standard is also segregating most of the management traffic to other bands. So, a multi-band AP that has 2.4 GHz and 5 GHz will be discoverable by scanning the lower bands. The client will first go into the lower bands, discover the AP there and then move to the 6 GHz band. This way, no probe request frames will need to be sent in the 6 GHz band. This will reduce the probe requests that are sent by stations just trying to find APs because it will not be allowed unless it is a PSC channel.

Wi-Fi 6E Channelization

The 802.11ax standard also defines channel allocations for the 6 GHz band. This allocation determines the center frequencies for 20 MHz, 40 MHz, 80 MHz and 160 MHz channels.

Channels begin at the start frequency of 5950 MHz, leaving just 25 MHz of guard band between the first 6 GHz channels and the upper range of the U-NII 4 band.

If a U-NII band is not allowed in a specific regulatory domain or operates under different rules, then the regulatory specs take precedence over IEEE and channels that are falling on frequencies or overlapping on frequencies that are not supported, are not allowed.

The FCC is providing this pristine new highway of spectrum and the Wi-Fi 6E standard’s rules of operation are ensuring that we can remove the slowest vehicles on the highway. The question now is how do you build the next generation, high performance device that can really take advantage of this new spectrum? There are many challenges to overcome.

In the next post, we’ll explore what testing solutions LitePoint has available for Wi-Fi 6E. In the meantime, please visit the replay of my webinar on this topic.