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Inside On-Chip Phased Arrays

Beamforming: The Best WiFi You’ve Never Seen
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Let’s get a little more specific. You may be familiar with MIMO (multiple-input, multiple-output) technology, first adopted with some 802.11g products and now incorporated into the 802.11n specification. Go back to our pool example. When you drop a stone in the left edge of the pool and the receiver is on the pool’s right edge, some of the waves will travel in a straight path from left to right and arrive at the transmitter via the shortest route possible. However, some waves will bounce off the top, then arrive at the receiver a bit later than the straight-path waves. Some will bounce off the bottom, then up to the top, and then arrive at the receiver. All of these variations emerge from a single stone drop, or radio burst. To a simple receiver, this sounds a bit like confusing, overlapping echoes. This “multipath” effect has traditionally plagued the performance of radio communications.

But what if you used multiple antennas at each end of the pool, applying enough analysis intelligence on each side to turn those echo paths into conduits for different data streams? With multiple antennas on each end, you can send different data streams from different antennas and receive them at the other end in the same manner.

To drag in a different metaphor, think of a highway. If the highway is only one lane, you can have one big truck going full-speed to its destination. However, if you sub-divide that one big lane into three or four narrower lanes, you can have three or four compact cars going to the same destination at the same speed. They just happen to be going there along slightly different paths. When you took good ol’ 54 Mbps 802.11g and its 20 MHz channel highway, divided that highway into multiple sub-channels, and increased the number of antennas, you got 802.11g MIMO.

Specifically, 802.11n typically transmits three data streams and receives two, commonly referred to as a 3x2 antenna array. There are some 3x3 schemes in the works, such as the so-called 450 Mbps WiFi set out by Intel with the launch of Centrino 2, but no access points have arrived yet to support this. Like 802.11g before it, 802.11n can use channel bonding, turning two 20 MHz streams into a 40 MHz pipe. To be totally accurate, you should actually see antenna arrays noted with three numbers: the number of transmit antennas, the number of receive antennas, and the number of spatial streams (or data streams) to use our sub-divided highway analogy. So a 3x3:2 (also noted as 3x3x2) array would have three transmit antennas, three receive antennas, and two spatial streams.

I mentioned earlier that on-chip beamforming was one of the two beamforming methods applicable in WiFi. This works by not only boosting total power gain by having multiple antennas in play, but also phasing the antenna signals so that a higher signal “beam” is cast in the receiver’s direction while less energy can be expended in other directions. With two transmit antennas, you can expend less total energy while quadrupling the transmit signal sent in the beam’s direction. The transmitter/access point only needs to receive a single packet from the client to get a lock on the signal path. Analysis of multiple packets can determine which of the multipath options is optimal at any given time.

The incredible thing is that chip-based beamforming, like MIMO, has been compatible with 802.11a/b/g for years. In fact, the technology is an optional part of the 802.11n standard. Despite its benefits, though, Cisco is the first to deliver on-chip beamforming to market. The enterprise-oriented AIR-LAP1142N access point is Cisco’s first and so far only product to feature beamforming, which it brands as ClientLink. It arrived in the first quarter of 2009, but the firmware that enables beamforming capability didn’t arrive until July. We tested with this firmware literally within days of its release.

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Top Comments
  • 10 Hide
    antiacid , August 17, 2009 1:13 PM
    awesome article! Thanks for exposing us to this great technology :) 
Other Comments
  • 2 Hide
    chinesemafia69 , August 17, 2009 8:16 AM
    wow....this owns

  • -6 Hide
    bucifer , August 17, 2009 8:42 AM
    This article started up pretty good with lots of technical data and the beamforming technology in theory but after that the goodness stopped.

    1.You cannot compare two products by testing them with a in-house developed software. It's like testing ATI vs nVIDIA with nvidia made benchmark.
    2.If you do something get it done, don't just go with half measures. I don't care if you didn't have time. You should have planned this from the beginning. The tests are incomplete, and the article is filled with crap of Rukus and Cisco.
  • -5 Hide
    Mr_Man , August 17, 2009 12:44 PM
    In defense of your wife, you didn't HAVE to use that particular channel to view all the "detail".
  • 2 Hide
    Anonymous , August 17, 2009 12:58 PM
    @Mr_Man: With a name like yours, I'd think that you'd sympathize with Chris a bit more :p  Unless (Mr_Man == I likes men) :D 
  • 10 Hide
    antiacid , August 17, 2009 1:13 PM
    awesome article! Thanks for exposing us to this great technology :) 
  • -4 Hide
    Pei-chen , August 17, 2009 1:17 PM
    Both Tyra and Heidi have personal issues and would be pretty difficult friend/mate.

    The network idea sounds better. I couldn’t get my 10 feet g network to transmit a tenth as much as my wired network without it dropping.
  • 3 Hide
    zak_mckraken , August 17, 2009 2:23 PM
    There's one question that I think was not covered by the article. Can a beamformaing AP can sustain the above numbers on two different clients? Let's say we take the UDP test at 5 GHz. The result shows 7.3 Mb/s. If we had two clients at opposite sides of the AP doing the same test, would we have 7.3 Mb/s for each test or would the bandwidth be sliced in 2?

    The numbers so far are astonishing, but are they realistic in a multi-client environnement? That's something I'd like to know!
  • -1 Hide
    jerther , August 17, 2009 2:28 PM
    There is so much invisible to understand in wireless technology!
  • -6 Hide
    ebattleon , August 17, 2009 3:01 PM
    You can of course get the same effect by having better antenna on both ends of the network. This would increase gain which would improve signal to noise ratio, which would improve data flow speeds. You would also get the same effect by boost in the power but that could get you in trouble with the law.

  • 1 Hide
    Anonymous , August 17, 2009 3:04 PM
    I'm not an expert on beamforming, but I'm surprised that it is useful at these frequencies. After all, the wavelength at 2.4 GHz is ~12 cm (~5 inches). That means that the pockets of constructive interference (the beam) are very small. Moving the receiver a few inches should make a big difference.

    Are you sure the differences you are seeing aren't simply due to higher power output? Couldn't the same improvements be obtained with a directional antenna like a Yagi?
  • 1 Hide
    Rancifer7 , August 17, 2009 4:00 PM
    So far quite an interesting technology. Its nice to know that at someone in the wireless world is striving to make something innovative!

    When all the major players sell items that look almost the same, act similarly, and perform almost the same, there is something wrong with the industry.
  • 1 Hide
    chaohsiangchen , August 17, 2009 4:13 PM
    Beam forming technology have been for a long time, but they are mostly used in military equipments. Phased Array radars, Synthetic Aperture Radar, Inverse Synthetic Aperture Radar, Plane Array Antenna and antenna for data link. Most consumer products still use Yagi or disk antenna. Cost is a major issue in the application. Military don't care that much about the cost.
  • 0 Hide
    scotty123 , August 17, 2009 4:23 PM
    sounds nice - but i won't get excited until it's available at Best Buy!

    One niggling concern, I felt reasonably safe with the unfocused cloud of RF surrounding me wherever I go, but I am not so sure about the tightly focused beam that the Ruckus provides. What happens to the poor joe who sits directly in the path of such a beam for 8 hours a day?
  • 1 Hide
    williamvw , August 17, 2009 4:39 PM
    Mr_ManIn defense of your wife, you didn't HAVE to use that particular channel to view all the "detail".

    LOL! True enough. It honestly was a reference file I had on hand for such testing situations. Angelini obviously showed a bit more wisdom in his choice of in-house test content.
  • -1 Hide
    williamvw , August 17, 2009 4:44 PM
    scotty123sounds nice - but i won't get excited until it's available at Best Buy! One niggling concern, I felt reasonably safe with the unfocused cloud of RF surrounding me wherever I go, but I am not so sure about the tightly focused beam that the Ruckus provides. What happens to the poor joe who sits directly in the path of such a beam for 8 hours a day?

    My absolutely unqualified opinion is "probably nothing." While the conditions are somewhat different, you might want to read a Tech Myths column segment I did over on Tom's Guide that touches on this issue. http://www.tomsguide.com/us/decibels-noise-canceling,review-1338-10.html
  • 2 Hide
    williamvw , August 17, 2009 5:17 PM
    zak_mckrakenThere's one question that I think was not covered by the article. Can a beamformaing AP can sustain the above numbers on two different clients? Let's say we take the UDP test at 5 GHz. The result shows 7.3 Mb/s. If we had two clients at opposite sides of the AP doing the same test, would we have 7.3 Mb/s for each test or would the bandwidth be sliced in 2?The numbers so far are astonishing, but are they realistic in a multi-client environnement? That's something I'd like to know!

    Excellent question, and one I hope to dive into in a later article. For now, I can only give you the anecdote on my opening page, running the same HD stream to two clients. Ruckus states that BeamFlex can sustain a 50 Mbps minimum per access point. Do the math on your client streams accordingly, I suppose.
  • -1 Hide
    bounty , August 17, 2009 5:24 PM
    williamvwLOL! True enough. It honestly was a reference file I had on hand for such testing situations. Angelini obviously showed a bit more wisdom in his choice of in-house test content.


    Honestly honey, it's just a reference file, I swear it's not porn. I challenge you to find HD streaming content from the internet that highlights the subtle nuance of flesh tones.
  • 2 Hide
    williamvw , August 17, 2009 5:28 PM
    bountyHonestly honey, it's just a reference file, I swear it's not porn. I challenge you to find HD streaming content from the internet that highlights the subtle nuance of flesh tones.

    Hey, the VS catalog mails to her, not me. I was merely trying to be a good husband and participate in her interests. ;-) ANYWAY. Back to beamforming, shall we?
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