Rethink traditional development and test when deploying 802.11ac

Individual Client Channel Optimization

The final major performance boost comes from technologies that optimize communications when speaking to a specific client. The first enhancement is a concept known as transmit beamforming (TxBF). With TxBF, the access point communicates with the client devices to determine which types of impairment are present in the environment. Then the access point “precodes” the transmitted frame with the inverse of the impairment such that when the next frame is transmitted and transformed by the medium, it is received as a clean frame by the client. Since no two clients are in the same location, TxBF needs to be applied on a client-by-client basis and constantly updated to reflect the changing environment.

Meeting 802.11ac Technical Challenges: Testing

Delivering breakthrough 802.11ac demands that developers expand the complexity and precision of their designs. It also requires a rethinking of the traditional approach to testing. Traditional testing requires that the RF section is verified using one set of equipment, and then the upper layer functions are tested using a second set of tools. But the overall technical complexity and the introduction of new technologies mentioned previously, such as TxBF, demand coordination and control between the different layers of the protocol stack. Without this coordination, it is difficult to exercise these functions and to quickly pinpoint performance issues to a specific function of the hardware.

The new generation of testing should be able to decode every frame in real-time and determine each frame’s RF characteristics, as well as their frame-level performance, and generate every frame without limitation in real-time to adequately test the receiver performance. It also needs to be able to tightly integrate RF and MAC functionality in 802.11ac, and include integral, real-time channel emulation to address TxBF performance.

Layer 1 Testing

802.11ac brings changes to layer 1 that are extremely challenging for radio designers: 246-QAM and wider bandwidths. Designers are required to deliver performance advances in phase noise performance, noise floor, modulation accuracy--virtually every dimension that impacts digital modulation performance. Verifying the performance of transmitters and receivers require better performance than was sufficient for 802.11n, and characterization must include all new 802.11ac rates plus legacy/802.11n rates. Best-in-class performance means verifying transmit and receive performance of literally hundreds of frame definitions, varied by modulation rate, frame length, bandwidth, frequency, channel model, and power level. The testing must include legacy frames, 802.11n frames and 802.11ac frame in various combinations, and must be representative of the actual diversity, rate, and complexity that actual devices will experience in the field.

To test these highly-integrated and high performance 802.11ac radio systems on a timeline that meets the aggressive market pace requires two critical components: first, the ability to make RF measurements on every frame transmitted at line rate and second, the ability to generate any desired frame quickly and without limitation. When it comes to testing transmit functions, using I/Q analyzers means that a performance assessment is being made on a small sampling of frames. Because these analyzers trigger only on power, it is statistically improbable that the test engineer will ever see frames that are faulty unless the fault exists on virtually every frame. Power supply issues, clock interference issues, and decoupling problems that impact the fidelity of the transmitted signal randomly or at a low repetition rate are missed. Positive verification of quality comes only when RF measurements are made at line rate on every frame transmitted.

When it comes to receiver testing, the same principle applies. Receivers tested with I/Q signal generators see only a small sampling of frames, and almost no diversity from one frame to the next. Positive verification of quality comes when receivers are subjected to continuous frames at line rate, with real-life variation from frame to frame.

The tables below list the measurements required before the radio transmit and receive performance can be declared complete.

Layer 2 Testing

802.11ac is even more stateful than its predecessors, and therefore includes a great deal of complexity at the MAC layer. Underperformance can be due to any number of potential causes including: slow ACK response times, poorly designed aggregation algorithms, internal buss limitations, poor rate adaptation algorithms, poor AP selection algorithms, power save implementations, poorly implemented legacy protection schemes, and so on.

As with RF testing, the approach to layer 2 functionality testing is to start simple and gradually add more complexity. In this case, that means progressing through a series of steps such as:

• Make sure a single client is able to reliably connect/disconnect;
• Perform benchmark tests using a single client to understand any basic system bottlenecks in the upstream and downstream direction;
• Enable more features on the single client to ensure that the basic functions work properly (aggregation, power save, IPv4 and IPv6, QoS, etc.);
• If testing an access point solution, benchmark with multiple clients to make sure that the system can achieve the expected performance under ideal conditions at scale; and
• Test with mixed-mode clients to make sure that the introduction of legacy devices will not adversely impact your 802.11ac solution.

All of the above tests should be run under cabled conditions. The goal is to eliminate stray RF effects so that any unexpected test results can be isolated to the system design and not be attributed to RF issues.

The last two steps in the process are to run the system level tests while simultaneously varying the individual RF channels of each client. First, the test should be run in a cabled environment with a mix of clients in a variety of locations. Note that it is necessary to create a wide variety of conditions without necessarily using numerous client devices in a physical layout. The cost and repeatability of performing this testing prohibits using actual facilities.

IEEE members have developed a set of channel impairment models that can be used, along with adjustments to client power levels, to virtually create a layout and conduct this testing. The last step is rerunning the same tests over the air in an RF-isolated environment to eliminate any unintended interference. This ensures that the integrated system, including antennas, cabling, etc., continue to perform as expected.

Testing Multiple Layers Concurrently

Much more comprehensive testing can be accomplished much faster if you can test at multiple layers of the protocol stack concurrently. With wireless technologies, the underlying medium is likely to experience issues that will affect the performance of the overall system. The challenge is knowing what to attribute performance degradation to – an RF design issue, an upper layer protocol issue, or the result of impairment in the RF spectrum.

In order to expediently address these issues, a single solution should enable testing of each layer individually but also allow visibility into metrics from both the RF layer and the upper layers at the same time. This approach allows rapid identification and isolation of any discovered issues, and provides the confidence that a product is really ready for production release. By using a common set of tools, productive communication is facilitated between these two functions and test results are easily duplicated.

Rethinking Traditional Approaches

802.11ac holds the promise of gigabit+ performance that will enable much broader adoption in key target markets such as enterprise, residential video, and carrier hot spots. To realize the performance and density promise, chip and hardware developers must navigate some significant technical challenges including:

• Ensuring graceful migrations from existing deployed solutions by providing backward compatibility;
• Delivering high performance RF transmission and receive performance with a wide variety of signals;
• Maintaining high performance to multiple clients under the channel conditions that will exist in real deployments;
• Providing the high reliability and feature robustness to enable enterprise and carrier grade 802.11 adoption; and
• Ensuring that the key application traffic, most notably video, can be delivered with quality.

In addition to meeting all of the technical hurdles, developers are also expected to deliver projects on shortened schedules while striving to improve quality. It is clear that 802.11ac demands a rethinking of the traditional approach to development and testing as described above.

About the Author

Joe Zeto serves as a market development manager within Ixia's marketing organization. He has over 17 years of experience in wireless and IP networking, both from the engineering and marketing sides. Joe has extensive knowledge and a global prospective of the networking market and the test and measurement industry. Prior to joining Ixia, Joe was Director of Product Marketing at Spirent Communications running Enterprise Switching, Storage Networking, and Wireless Infrastructure product lines. Joe holds a Juris Doctorate from Loyola Law School, Los Angeles, CA.