The forthcoming IEEE 802.11n standard will specify next-generation Wi-Fi products with performance that greatly exceeds current solutions. One of the key technologies used in 802.11n is multiple-input multiple-output (MIMO) technology.
MIMO has the potential to boost throughput beyond that of traditional wired
Ethernet connections, significantly increase the range of Wi-Fi devices, and
dramatically improve quality of service (QoS). The advances in the draft 802.11n
specification also include beam-forming techniques, new medium-accesscontrol
(MAC) technology, and new power-save modes.
The resulting products will be able to handle new bandwidthintensive applications
at greater distances with improved reliability. In addition, 802.11n products
will expand the capabilities for Wi-Fi platforms and applications already in
widespread use. Realizing the promise of 802.11n, however, hinges on the availability
of interoperable products that can deliver next-generation wireless applications.
The performance, conformance, and certification testing methodologies that are standard fare for legacy Wi-Fi products face new challenges as the industry transitions to next-generation technology. Specifically, using MIMO technology adds layers of complexity that require new testing techniques.
For vendors to truly deliver the performance promises offered by the 802.11n standard with new applications like video over wireless, they must be particularly diligent in testing their products—not only during the quality assurance (QA) process, but also throughout the design, development, and verification cycles.
MULTIPATH, MIMO, AND WI-FI PERFORMANCE
Imagine a busy workplace filled with cubicles, office equipment, furniture,
and employees. Signals from the wireless local-area network (LAN) fill the room
and bounce off these obstacles, causing the transmissions to take different
paths before arriving at their destination (Fig.
1). This phenomenon is known as multipath.
Multipath causes each of the reflected signals to arrive at the receiver(s)
at different times and with different strengths. Typically, each path is characterized
by a delay in time, a change in amplitude, and other more subtle factors (such
as the angle of arrival, angle of departure, or angular spread).
Multipath isn't easy to predict and control because it's affected by everything
from building construction to the movement of people. The performance of Wi-Fi
products based on single-input single-output (SISO) technology is degraded by
multipath, which affects signal quality and the connection's robustness.
On the other hand, the MIMO technology in the draft 802.11n specification takes advantage of multipath to improve network performance. Multiple transmitters send independent data streams at the same frequency (channel) at the same time.
Because of multipath, these multiple transmissions arrive at the multiple receivers at slightly different times, amplitudes and phases. MIMO algorithms use these differences to decorrelate the original signals. This technique, known as spatial multiplexing, can double or even triple a system's throughput.
MIMO's multiple receivers distinguish between multiple signals and allow parallel
signals to be received (Fig. 2). Besides providing
greater throughput, MIMO receiver chains can better reconstruct weak signals
that have traveled a greater distance. This will yield a distinct range advantage
over SISO systems. While multiple antennas and multiple simultaneous data streams
improve MIMO-system performance, they create complexities in MIMO-product performance.
FUNCTIONAL TESTING OF 802.11N PRODUCTS
Controlled, repeatable testing of Wi-Fi products requires an isolated RF environment,
motion emulation, and a test executive that ensures consistent test execution
and results recovery via automated methods and archivability. Thorough performance
characterization should include the controlled emulation of all environmental
variations.
Therefore, to properly evaluate the performance of MIMO-based products in a real-world environment, multipath must be incorporated into the testing process. A high-level plan for functional testing of 802.11n-based products has four components:
- Basic functionality
- Compliance with the 802.11 specification
- Verification of product interoperability and backward-compatibility with
802.11a/b/g
- Characterization of product performance
The first three of these tests can be performed with good results in an almost
entirely unimpaired RF environment (an environment with no multipath reflections).
Since MIMO relies on multipath and other channel impairments to increase throughput,
throughput performance should be measured under well-defined channel conditions
with impairments. This requires an additional test tool—a channel emulator—to
characterize the performance of these products in impaired environments.
BASIC FUNCTIONALITY
Functional tests prove the product design across the full set of features and
functionality. For example, one basic functional test will test device throughput
over distance. The shape of the resulting plot is determined by the algorithm
used by the product to select its transmission data rate. Other important Wi-Fi
functional tests include:
- Roaming and end-user mobility tests that verify a client device's ability
to successfully roam between access points
- Security testing, which focuses on authentication and encryption and measures
the efficiency with which an access point manages simultaneous authentication
requests
- QoS protocols, which ensure that the network properly prioritizes voice
traffic and accounts for roaming speed, jitter, and network delay
- Network behavior tests that measure performance under abnormal network conditions,
such as congestion, overload, and errors
- AP packet forwarding rate, which measures the processing power of the AP
and its affect on data throughput
Basic functionality testing requires equipment that measures results in an
isolated RF environment. It also must be able to emulate devices in motion and
control the performance parameters of interest.
Table 1 summarizes a few of the 77 Modulation
Coding Schemes (MCS) from the latest draft of 802.11n specification. The throughput/range
performance of a product will depend on its implementation of coding schemes.
The mandatory schemes must be implemented for the product to be specification-compliant.
There are 576 possible data-rate configurations in the current draft. Products
will only interoperate at these configurations when vendor implementations of
these configurations match.