802.11ac Testing with the Vector Signal Transceiver

Publish Date: Mar 04, 2013 | 3 Ratings | 4.67 out of 5 |  PDF

Overview

The most recent 802.11 standard presents some challenges as discussed in The Next Evolution of Wireless LAN white paper. It is not surprising that test engineers have been scrambling to find the right test equipment to test this standard. Many test engineers have now realized that the old method of finding an expensive boxed instrument with the best performance numbers is now dead. Why, you may ask? The answer is simple: test engineers are getting starved for resources, mainly time, money, and space. The modern breed of test engineers is already using intuitive new technologies to reduce space and decrease test and development time all in a reduced budget. National Instruments is helping test engineers address these challenges with user-programmable FPGA-based instrumentation. This paper discusses the benefits of using an open field-programmable gate array (FPGA) for 802.11ac testing specifically.

Table of Contents

  1. Getting Started With WLAN Measurements
  2. Soft Front Panels
  3. Multiuser MIMO (MU-MIMO)
  4. Benefits of a User-Programmable FPGA
  5. Getting Best-in-Class EVM Numbers
  6. Analyzing 160 MHz and contiguous/non-contiguous 80+80 MHz
  7. Phase Tracking
  8. Channel Tracking
  9. Quadrature Skew Compensation
  10. Adding Impairments
  11. Transmit Spectrum Mask
  12. Measurement Speed
  13. Summary

1. Getting Started With WLAN Measurements

The NI PXIe-5644R is the industry’s first vector signal transceiver (VST). This VST features a real-time bandwidth of 80 MHz up to 6 GHz center frequencies. This instrument also features a programmable FPGA that can be used to speed up tests or implement real-time algorithms such as fast Fourier transforms (FFTs), power control, and even modulation or demodulation. This complete WLAN tester is three PXI Express slots wide and includes a programmable digital I/O port for device under test (DUT) control type of applications.

Figure 1. The NI PXIe-5644R is ideal for making WLAN measurements. The programmable FPGA allows users to customize the instrument to their needs.

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2. Soft Front Panels

The NI WLAN Analysis Toolkit provides soft front panels that you can use with the NI PXIe-5644R for quick generation or acquisition capabilities. You can use the analysis soft front panel for modulated or spectral measurements. You can also use both SFPs with up to four NI PXIe-5644Rs for a 4x4 MIMO configuration. 

Figure 2. The NI WLAN Analysis Toolkit allows you to easily make measurements using the NI PXIe-5644R.

Figure 3. The NI WLAN Generation Toolkit allows you to generate 802.11ac signals with an 80 MHz bandwidth.

802.11ac operates in the 5 GHz band with bandwidths of 20, 40, and 80 MHz that are mandatory. Support for 160 MHz is currently optional. There is also an option for a noncontiguous 80+80 MHz TX and RX bandwidth.

 

Figure 4. The 802.11ac Band Allocation

The IEEE draft requires the 802.11ac standard to be backward compatible with 802.11a and 802.11n in the 5 GHz band so that they can coexist. Some of the other mandatory specifications include, 80 MHz bandwidth, 256-QAM modulation, up to eight spatial streams, and multiuser multiple input, multiple output (MIMO).

802.11ac allows for theoretical maximums of 6.93 Gbit/s using a maximum bandwidth of 160 MHz, 8x8 MIMO configuration, 256-QAM, and a short guard interval. The average case for data rates is 1.56 Gbit/s with a bandwidth of 80 MHz, 4 tx channels, and 256-QAM modulation.

The steps below show the calculation of data rates for an 80 MHz bandwidth, 64-QAM signal with 800 ns guard interval, and one spatial stream. Essentially, there are 234 data carriers (242—8 pilot). The symbol rate can be calculated as follows: 256/80 MHz + 800 ns (GI). Plugging the numbers in the data rate formula, you get the following:

Data Rate

Data Rate

Data Rate

Where

NBPSCS = Number of coded bits per subcarrier per spatial stream

NSD = Number of modulated data symbols per frequency segment

R = Coding Rate

TSYM = Symbol Interval|

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3. Multiuser MIMO (MU-MIMO)

MU-MIMO allows a terminal to transmit or receive signals to and from multiple users in the same band simultaneously. MU-MIMO is a set of advanced MIMO technologies that exploit the availability of multiple independent radio terminals to enhance the communication capabilities of each individual terminal. Single-user MIMO only considers access to the multiple antennas that are physically connected to each individual terminal.

Figure 5. MU-MIMO is a unique concept to 802.11ac that allows multiple receivers.

The PXI platform is inherently ideal for MIMO given the synchronization capabilities over the backplane and the synchronization and memory core (SMC) chips embedded in NI PXI instruments. Using NI-TLCK technologies, you can achieve up to 0.1 degrees of phase offset between multiple analyzers or generators (even across multiple connected chassis).

Moreover, the small footprint of new NI PXIe-5644R VST allows you to fit up to five VSTs in a single chassis for a complete 5x5 MIMO system. Implementing such a system using traditional boxed instruments can result in a complicated setup of cables and instruments.

Figure 6. A 4x4 MIMO 802.11ac solution easily fits in an 18-slot PXI Express chassis.

 

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4. Benefits of a User-Programmable FPGA

The use of FPGAs with RF instruments is not entirely a new concept; however, the ability to provide users a programmable FPGA is new and unique to the NI PXIe-5644R. You can use the open FPGA for the following:

  • Servoing
  • Automatic gain control
  • Modulation and demodulation
  • FFTs and averaging
  • Channel emulation

A traditional boxed instrument restricts access to algorithms such as FFTs and even triggering. It can be difficult for a user to customize the FFT or triggering being used on a boxed instrument. However, the new era of software-designed instruments allows engineers to completely customize their instruments to their needs, much like customizing apps on cell phones. 

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5. Getting Best-in-Class EVM Numbers

As modulation schemes get more complex, maintaining a high-quality signal becomes even more important. Table 1 below shows the RMS EVM requirements of the different modulation schemes in 802.11ac.

Modulation Coding Rate RMS EVM
BPSK 1/2 -5 dB
QPSK 1/2 -10 dB
QPSK 3/4 -13 dB
16 QAM 1/2 -16 dB
16 QAM 3/4 -19 dB
64 QAM 2/3 -22 dB
64 QAM 3/4 -25 dB
64 QAM 5/6 -27 dB
256 QAM 3/4 -30 dB
256 QAM 5/6 -32 dB

Table 1. RMS EVM Requirements of Modulations Schemes in 802.11ac

It is important for test equipment to be able to measure at least 10 dB better than the spec (-32 dB for 256 QAM) to give enough headroom for characterization and production test. As you can see in Figure 7, the NI PXIe-5644R provides industry-leading EVM numbers.

Figure 7. 802.11ac EVM in loopback mode using an NI PXIe-5644R

As with all wireless standards and test equipment, there are a few software and hardware tweaks that you can make for the best measurements. Some of the hardware optimizations that you can make on a signal analyzer are discussed in Adjacent Channel Distortion Measurements Using the NI PXIe-5665 VSA.

The effects of other optimizations such as phase tracking, channel tracking, and quadrature skew compensation are discussed below. 

 

Note: For all the images below, an 80 MHz, MCS 9 802.11ac signal is generated and acquired on the NI PXIe-5644R in loopback mode. 

Figure 8. The NI PXIe-5644R can make EVM measurements of -46 dB for an 80 MHz 256-QAM signal.

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6. Analyzing 160 MHz and contiguous/non-contiguous 80+80 MHz

For a non-contiguous 80+80 MHz 802.11ac signal, you can perform spectrum mask measurements using a single analyzer that can sweep over the spectrum, however if you need to demodulate the two carriers, you need either a wide bandwidth analyzer, that can cover the entire 802.11ac spectrum or use multiple analyzers and generators to demodulate the 2 carriers independently.  The image below shows the use of 2 NI PXIe-5644R vector signal transceivers to demodulate these two signals separately.

Figure 1 - NI complete PXI-based WLAN test solution, featuring up to 160 MHz of instantaneous bandwidth.

Two generators with a combiner gives you the flexibility to make complete spectral and modulated measurements on the 80+80 MHz carriers, contiguous or non-contiguous.  To make spectrum mask measurements, you can use the NI WLAN Toolkit with the NI PXIe-5644R or other NI vector signal analyzers.

Figure 2 – Making spectrum mask measurements on an 80+80MHz contiguous 802.11ac signal.

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7. Phase Tracking

Phase tracking is useful for tracking the phase variation over the modulation symbol caused by residual frequency offset and phase noise. If you set the orthogonal frequency division multiplexing (OFDM) phase tracking method property to Standard, the toolkit performs pilot-based common phase error correction over the OFDM symbol, as specified in section 17.3.9.7 of IEEE Standard 802.11a-1999 and section 20.3.21.7.4 of IEEE Standard 802.11n-2009.

If you set the OFDM phase tracking method property to Instantaneous, the WLAN Analysis Toolkit performs pilot-based common phase error correction over the OFDM symbol and also compensates for the phase distortion in each modulation symbol. Such compensation is not defined in the IEEE standard. However, the compensation is useful for determining the modulation distortion in the amplitude and the contribution of phase errors. Using this method of phase tracking, the toolkit computes only the error vector magnitude (EVM), which is the error caused by the variation in magnitude of the complex modulation symbol over the length of the packet and different subcarriers.

The default value is Standard.

Note: The images below show a zoomed in constellation plot of a 256-QAM signal. Only four symbols are shown to better illustrate the effect of changing parameters.

Figure 9. The above image shows the impact of phase tracking on EVM numbers for an 80 MHz 802.11ac signal. The charts show only four symbols on the 256-QAM constellation plot.

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8. Channel Tracking

By enabling channel tracking, the WLAN Analysis Toolkit estimates the channel response over the preamble and the data and then uses this response as the channel frequency response estimate for the entire packet. If you disable channel tracking, the toolkit estimates the channel response only over the long training sequence (LTS) and uses this response as the channel frequency response estimate over the entire packet.

Figure 10. Effect of Enabling Channel Tracking

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9. Quadrature Skew Compensation

The WLAN Analysis Toolkit can also compensate for phase skews caused because of generators/DUTs. Figure 11 shows a signal with quadrature skew. Compensating for quadrature skew works best for modulation schemes with a large number of points such as 256 QAM.

Figure 11. Signal With Quadrature Skew

 

The 256-QAM constellation plot (zoomed into only four symbols) shows the effect of quadrature skew compensation.

 

Figure 12. Effect of Enabling Phase Deskew Compensation

 

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10. Adding Impairments

The NI WLAN Generation Toolkit also allows you to insert impairments in the generated signal, and observe how the DUT reacts. The following impairments can be added using the WLAN Generation Toolkit:

  • Carrier Frequency Offset
  • Sample Clock Offset
  • IQ Impairments
    • Gain imbalance
    • DC offsets
    • Quadrature skew
    • Timing skew
  • Carrier-to-Noise Ratio

 

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11. Transmit Spectrum Mask

802.11ac requires a mandatory 80 MHz spectral mask test. There are also options for 80+80 MHz and 160 MHz spectral mask tests. The 80 MHz segments might be contiguous or noncontiguous (in different bands). 

Figure 13. Spectral Mask Measurement on an 80 MHz 802.11ac Signal

Engineers can use two synchronized generators or analyzers to generate and acquire the 80+80 signal.   If the two segments are in different bands, then the regular 80 MHz spectrum mask applies to each segment, however if the two are in the same band and are contiguous, then an overlap spectrum mask is applied to the signal as shown in Figure 14.

Figure 14. Spectral Mask Measurement on an 80+80 802.11ac Signal

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12. Measurement Speed

Every test engineer is challenged with the task of reducing test times. In characterization environments, engineers have to keep up with testing the steady flow of new products. In production environments, test engineers have to test as many parameters as possible, as fast as possible.

The PXI platform provides a modular approach not only to instruments but also to the processors used.  The simplest way for test engineers to improve test speed is to use the latest, fastest processor. Trying to upgrade processors on a traditional boxed instrument can be cumbersome. Engineers are also heavily dependent on the instrument vendor to provide the latest processors. With PXI systems, engineers can purchase their own high-performance PCs to perform all the processing.

NI RF instruments implement all modulation/demodulation and processing on host PCs that can either be embedded in the PXI chassis or could be an external PC that is controlling the PXI system.

The images in Figure 15 show test times for performing EVM and spectral mask tests on 802.11ac for varying averages. 

Figure 15. Test Times for Performing EVM and Spectral Mask Tests

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13. Summary

The NI PXIe-5644R is the ideal instrument for WLAN testing because of its speed, performance, size, and flexibility. With the open architecture, users can customize the instrument all the way to the FPGA level, thereby enabling complex triggering solutions and even allowing engineers to implement channel emulation on the instrument.

  

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