Key Wireless Trends for 2004, Part 1: Smart Antennas

Today's reality is that even on well-designed systems, most users connect at less than half the maximum data rate because of limited signal strength, which causes systems to automatically drop their data rate. Beyond achieving maximum performance of existing systems, it seems clear that tomorrow's 100 Mbps+ WLAN standard--in the early stages of development in the 802.11n committee--will almost certainly need to incorporate smart antenna technology.

Today's workhorse antenna is the omni-directional dipole with switched antenna diversity. The transmit/receive pattern of this antenna looks like a donut-shaped sphere that emits radio waves more powerfully on the "east-west" axis than on the "north-south." An omni that provides 3dBi of gain adds enough power to theoretically double transmission range. Unfortunately, with most indoor WLANs, it's not just radio power that affects transmission range. The greater problem is often multipath, which occurs as radio signals bounce off walls, doors and furniture and results in multiple instances of a weaker signal arriving at the receive at slightly different points in time.

Smart antennas are designed to maximize effective transmission signal strength, maximize receive sensitivity and cope with--or even positively exploit--multipath. The switched antenna diversity capabilities found in all current enterprise-class WLAN products represent a basic form of smart antenna technology: By using two antennas and constantly sampling each one to determine which is hearing the best signal, receive sensitivity is enhanced. However, the benefits of diversity vary in part based on the distance between the antenna elements. Accordingly, you'll typically get greater benefits on an access point than on a PC Card NIC.

While antenna diversity is a good first step, antennas are becoming much smarter, thanks in large part to advancements in signal processing that allow systems to more intelligently control and decipher complex radio signals. For example, phased-array antennas like those used by Wi-Fi switch manufacturer Vivato use a large number of antennas to "steer" the radio beam between the AP and a standard 802.11 client. These systems can provide significant improvements in range, particularly in outdoor environments with minimal multipath, but they tend to be very large and expensive.

Another approach to smart antenna design is the adaptive array. Rather than optimizing transmission by choosing a specific antenna and beam, an adaptive array transmits and receives signals on all antenna elements simultaneously and uses signal processing to weight the signals of each antenna. This system design is more effective under conditions of high multipath. Motia, which is developing "antenna appliques" that are designed to bolt onto existing Wi-Fi systems, uses this technology.

The most advanced form of smart antenna technology currently available--and the technology that is likely to be integral to the next generation of faster WLANs--is known as MIMO (multiple-input-multiple-output). MIMO is not a single specific technology or standard. Rather, it is an approach that can be applied in many ways. One of the most interesting applications of MIMO technology is found in new chipsets and reference systems from Airgo Networks. Airgo's MIMO implementation goes well beyond minimizing the negative effects of multipath. In fact, its system actually relies on multipath to increase signal level and range, thereby enabling Airgo to deliver a 108 Mbps data rate in a single 20 MHz 802.11 channel.

While there's reason to be excited about Airgo's technology, it's highly unlikely that its system will be adopted by the 802.11n working group as the foundation for a next-generation WLAN standard. Understanding that reality, Airgo has gone to great lengths to ensure backward compatibility with legacy standards using creative approaches similar to those employed by Motia. In a recent demo in our labs, Airgo's products not only delivered the fastest TCP throughput we've ever seen in a single-channel WLAN (in excess of 40 Mbps), it also provided the best range of any 802.11a-compliant access point we've ever tested--significantly outperforming Cisco's 1200 AP as well as others from Airespace and Aruba.

-- Dave Molta, [email protected]