Smart Antennas: A New Boost for Wireless LANs

Like Rodney Daingerfield, most antennas don't get no respect. And rightly so, because most antennas are dumb. They transmit or receive equally well in all directions, boosting "bad" (interfering) signals just as much as "good" ones. Pulling the good signal out of the bad is a job left to a wireless device's increasingly complex circuitry.

Finally, though, antennas are wising up and doing their own share of the work. New "smart" antennas aren't omnidirectional, but instead dynamically boost gain in the direction of good signals and decrease gain toward sources of interference. As a result, they're increasing the range of wireless LANs by a factor of three or four. Some are even doubling data rates and throughput by sending two data streams simultaneously in the same wireless LAN channel.

Figure1:  A smart antenna is basically an array of antenna elements whose outputs are weighted and combined with sophisticated processing to increase gain in the direction of a signal and to decrease gain in the direction of interference. (Illustration courtesy of Motia)

Antennas have gotten smarter by borrowing from the old adage that two heads are better than one. In this case, though, it's two or three or four antennas that are better than one, because a smart antenna is basically an array of antennas (Figure 1). The multiple antennas work together, via sophisticated processing, to do things that a single antenna can't do. Smart antennas are visually obvious on new wireless LAN products from companies such as Belkin, Netgear, and D-Link (Figure 2).

Figure 2:  The Belkin Pre-N wireless router uses technology from Airgo Networks to implement a smart antenna with three antenna elements. The three elements work together as an array antenna, unlike today's prevalent dual diversity antennas that simply get switched in and out of use dynamically according to which one has the stronger signal. (Illustration courtesy of Belkin)

Although not everyone needs the extra benefits of smart antennas, the extended range and coverage are useful in large spaces like office buildings, malls, and even large houses. For example, in tests conducted by Airgo Networks, two wireless LAN access points implemented with Airgo technology covered a typical 100-person office, whereas nine conventional 802.11b access points were required (Figure 3) to cover the same area.

Figure 3:  Nine conventional 802.11b access points (top) were necessary to cover this typical 100-person office, compared with only two access points equipped with Airgo's smart-antenna technology (bottom). (Illustration courtesy of Airgo Networks)

Increased coverage and throughput are just two of smart antennas' advantages, however. Some of a smart antenna's characteristics that contribute to increased coverage and throughput are also performance improvements in their own right. For example, smart antennas can help suppress interference from devices such as microwave ovens and cordless phones. A smart antenna with a certain number, M, of antenna elements can simultaneously suppress as many as M-1 interferers, even when signals from the interferers are arriving from multiple directions via reflections.

Smart antennas can also help prevent the dead spots that result from multipath propagation. Because wireless LANs always operate in areas where signals travel from transmitter to receiver via multiple reflections (Figure 4), sometimes two reflected signals arrive out of phase and cancel each other out, creating a dead spot. Dual diversity antennas on WLAN equipment handle dead spots well enough when you're sending and receiving data, but if you're using an Internet phone while walking around, you can walk into and out of dead spots, disrupting your conversation. Smart antennas are better than switched-diversity antennas at preventing those dead spots.

Figure 4:  Reflected multipath signals that arrive out of phase can cancel each other out, creating dead spots. Smart antennas help eliminate dead spots, and some actually use multiple reflected paths to an advantage. (Illustration courtesy of Airgo Networks)

So how do smart antennas work? Different types work in different ways, but in general, a smart antenna exhibits an enhanced transmit/receive beam in a desired direction and reduced gain in other directions. In an adaptive-array smart antenna, this pattern results from adjusting the phase and the magnitude of signals from each of the individual antennas and then combining the adjusted signals. The process is fast enough that a smart antenna's pattern isn't fixed, as a dumb antenna's is, but instead can change instantaneously. It can adapt to changes in the transmission environment, to sources of interference that come and go, and to multiple wireless users who come and go or just move around.

Figure 5:  In most cases, the processing that turns an array of antennas into a smart antenna is done digitally. (Illustration courtesy of Airgo Networks)

The sophisticated processing that turns an array of dumb antennas into a smart antenna can be performed either digitally or in analog. Most chipsets that add smart-antenna capability to wireless LANs perform digital processing on digitized baseband signals (Figure 5), but the Javelin 802.11 chip from Motia performs analog processing on radio-frequency (RF) signals (Figure 6). The digital approach provides some capabilities that the RF approach doesn't, but it achieves only modest performance improvements without proprietary, nonstandard, multiple-radio transceivers on both ends of a WLAN link. Motia's Javelin technology, on the other hand, piggybacks on standard 802.11 transceivers to greatly improve range, but it can't match some of the proprietary approaches' speed improvements.

Figure 6:  Motia's Javelin "appliqu" chip performs analog RF processing to turn an array of antennas into a smart antenna that works with existing, standard 802.11 transceivers. (Illustration courtesy of Motia)

The best improvements in performance occur when multiple antennas are on both ends of a wireless link. This configuration, called MIMO (for multiple input, multiple output), permits use of a technique called spatial multiplexing to dramatically boost throughput. Spatial multiplexing is available in smart-antenna implementations that use digital processing, but it can't be implemented with RF processing only.

With spatial multiplexing, a data stream that normally would be sent over one antenna gets split into multiple data streams and then transmitted via different radios and different antennas through a single 802.11 channel. On the receive side, different antennas obtain each of the data streams via slightly different paths (including reflected paths) and so can distinguish, with appropriate processing, one data stream from another. The receiver then combines the different data streams to reconstruct the original stream. With M transmit antennas, spatial multiplexing can yield up to an M-fold improvement in throughput. Current MIMO systems using two transmit antennas therefore roughly double the throughput.

The beauty of spatial multiplexing is that it increases throughput without using additional bandwidth. It's not at all like channel bonding, in which some proprietary wireless LAN technologies boost speed by using two channels. Channel bonding increases throughput when no other users are in the additional spectrum it needs, but when that spectrum is occupied, it has to fall back to a single channel and normal speed. Spatial multiplexing always uses a normal channel, so it can always maintain a higher speed.

An additional beauty of spatial multiplexing is that it actually needs multiple, reflected paths to work, whereas before the advent of smart antennas, multipath was a detriment to a good link. The diversity antennas on most current wireless LAN products are there to eliminate reception dead spots caused by destructive multipath interference, but spatial multiplexing turns the multipath disadvantage into an advantage.

Such gains from the use of smart antennas naturally beg the question: If an additional few antennas lead to significant benefits, can a lot of additional antennas lead to even more? The answer is yes. According to Jack Winters, chief scientist at Motia, studies have shown that you can generally increase throughput by about a factor of 10 in an outdoor setting and up to 100 in an indoor setting, just by adding enough antennas. "But, of course," says Winters, "no one right now is talking about putting 100 antennas on anything. If you increase the number of antennas, you increase the number of RF chains you need, and that dramatically increases your cost. What you want to do is have enough antennas so that you can see substantial improvement in performance, but not so many that your costs or your power usage get out of hand."

As a practical matter, today's wireless LAN products are using only a few antennas. Belkin's Pre-N router, with technology from Airgo, uses two transmit antennas and three receive antennas—a so-called 2-by-3. D-Link's Super G with MIMO wireless router, based on technology from Atheros Communications, has four antennas, and a corresponding CardBus client has two. Upcoming products from Netgear, to be based on Video54's BeamFlex technology, will have seven antennas.

Eventually, the number of antennas on wireless devices will increase. "Going forward," says Airgo director of product management Dave Borison, "the more radios you have transmitting on the same channel, the more throughput you can get. If you can cost effectively design a 3-by-6 or a 3-by-4 or something higher, you would be able to increase your throughput and range and reliability." For now, though, Borison says, the sweet spot in terms of price versus performance is 2 by 3—two transmitter antennas and three receive antennas.

But even current products that use only a handful of antennas are delivering good performance. Motia says that its Javelin appliqu chip, which connects as many as four antennas to a standard 802.11 transceiver, produces an 18 dB gain over a single antenna when used on both a WLAN access point and client. In terms of extended range, that translates to a three to four times improvement. With a four-element antenna on only one side of the wireless link, the gain is still 13 dB, a two to three times improvement.

In terms of throughput, WLAN products using two transmit antennas are essentially doubling normal throughput by splitting a data stream in half and sending both streams simultaneously. In theory, that should achieve 108 Mbps data rates, twice the normal rate of 802.11g or 802.11a. However, since normal rates in practice seldom exceed about 20 Mbps, the smart antenna implementations in practice are actually achieving about 40 Mbps at reasonably close range and lower rates at greater distances (Figure 7). It's at those greater distances, however, where smart antennas show the greatest percentage improvement over legacy WLAN devices, because those are the distances that smart antennas can reach and where single antennas begin to fail.

Figure 7:  Airgo's True MIMO smart-antenna technology (top graph) more than doubles the throughput of well known conventional wireless LAN devices (bottom grouping). All data here came from tests performed by Airgo in an actual office environment.

Throughput figures can vary, of course, with different smart-antenna technologies. InterDigital, Motia, and Video54, for example, all claim 50% or greater improvements in throughput, even without making use of spatial multiplexing. Improvements come solely from boosting antenna gain, thereby enabling higher transmission rates and resulting in fewer packet errors.

Other techniques can also work in conjunction with smart antennas to boost throughput even more. VLocity chips from Atheros, for example, use 40 MHz of bandwidth, rather than a standard 802.11 20-MHz channel, to push throughput to around 50 Mbps. Although that might sound like cheating, 40-MHz channels are a possibility for the upcoming 802.11n standard, which has high throughput as one of its main goals. Atheros, naturally, would like to have a head start on the next standard.

The technologies to be employed in the future 802.11n standard are still to be determined, however, and two different groups of companies are vying to set the standard within the IEEE 802.11 Task Group N (TGn). TGn Sync, which includes Agere, Atheros, Intel, InterDigital, and others, prefers including 40-MHz channels (along with 10- and 20-MHz channels) as one technique for increasing data rates. WWiSE (World-Wide Spectrum Efficiency), whose members include, Airgo, Broadcom, Motorola, STMicroelectronics, and Texas Instruments, would also include 40-MHz channels, but only as an option and only in countries where the wider channels wouldn't create regulatory problems. Otherwise, WWiSE would stick with existing 20-MHz channels.

Regardless of which group prevails, however, smart antennas will be a key component of the new standard. WWiSE proposes using MIMO with spatial multiplexing to achieve a maximum rate of 135 Mbps on a 2-by-2 antenna configuration in a 20-MHz channel. With a 4-by-4 configuration, the maximum rate would be 540 Mbps. TGn Sync also proposes using MIMO with spatial multiplexing, with rates up to 140 Mbps in a 2-by-2 system over a 20-MHz channel and up to 315 Mbps over a 40-MHz channel. With a 4-by-4 system and a 40-MHz channel, TGn Sync rates would top out at 630 Mbps.