Design Article
Wi-Fi channel emulation goes mainstream
Craig Mathias and Fanny Mlinarsky
7/9/2007 4:06 PM EDT
Beyond the significant installed base present in residential settings worldwide, we are seeing major installations in enterprises, often involving thousands of access points, and increasing installations in metropolitan areas providing voice and data services that rival cable, DSL, and other wireless technologies designed to address the personal and business access needs of individuals.
It does not appear at this point that there is any technology that will obsolete Wi-Fi anytime soon, and we consequently expect continual proliferation of Wi-Fi across all of the above three venues. Note that this analysis ignores the embedded systems market, as well as the hundreds of millions of Wi-Fi radios that we expect to see annually built into handsets of various forms from this point forward. Considering these, the market opportunity is exceptional, to say the least.
Along with progress in the market has come a steady stream of improvements to 802.11 and their subsequent consideration in the Wi-Fi specification. While many of these have addressed such elements as security, time-bounded performance, and regulatory requirements, the most visible have been those dealing with throughput. 802.11b advanced throughput from the original one and two Mbps to 11 Mbps peak, and is largely credited with initiating the period of explosive growth in WLANs that continues to this day.
802.11a added OFDM and operation in the 5 GHz bands, and 802.11g moved the technology of .11a into the 2.4 GHz. bands of the original standard and .11b. Users have in general been able to rely on achieving throughput of at least a third of rated speed under typical operating conditions, taking into account the rate-vs.-range behavior inherent in wireless and subject to such other elements as the construction of buildings hosting indoor operations and the ever-present threat of interference from both Wi-Fi and non-Wi-Fi sources in the unlicensed bands.
Throughput can often reach on the order of half of rated, and most users have found this performance acceptable enough for WLANs to become the preferred vehicle for default and even primary access in many cases.
The ever-present rate-vs.-range problem, which also clearly impacts overall WLAN capacity, provides a continual motivation to further exploit radio technology in the quest for greater throughput, range, and capacity. The 802.11n standard is designed as the next step in addressing these elements.
Incorporating MIMO/OFDM, .11n is now far enough along in the standards-development process that the Wi-Fi Alliance has developed a specification for the interoperability testing of products conforming to the Draft 2.0 release of 802.11n. This is a very important development - since users buy Wi-Fi, and not, strictly speaking, 802.11, we expect the floodgates to open over the next few months resulting in a large array of Draft 2.0-based .11n products for both the residential and enterprise markets.
While some are cautioning that the final standard will not be issued until late 2008 or even early 2009, and may include changes creating fundamental incompatibilities with implementations based on Draft 2.0, we do not believe that such is likely, and would be regardless addressed via backwards compatibility to the current Wi-Fi Draft n specification.
It is therefore important to be in the process of developing and testing .11n products today. But this brings up a very critical question " exactly how does one go about testing wireless-LAN products with the fundamental complexity of 802.11n?
The simple answer here, of course, is to use the same test equipment as is applied to current generations of .11 products. There is, however, a key consideration inherent in this strategy, and that is the subject of this article.
Given the complex behavior of MIMO/OFDM signals interacting with the environment, some form of channel emulation is essential in providing an adequate picture of the performance likely to be realized in any given implementation of an 802.11n-based product.
