标题: Beamforming: The Best WiFi You’ve Never Seen[查看完整版帖子][打印本页]
时间: 2011-6-22 10:54
作者: odyssey_2010 标题: Beamforming: The Best WiFi You’ve Never Seen
You should have seen my wife’s face when she found me glued to the Victoria’s Secret Fashion Show. “No, honey, come here!” I said, my face aglow with the bikini-clad pixels of Tyra and Heidi Klum. “You’ve got to see this!”
Arms crossed. Pursed lips. “Mm-hmm. Yes?”
I pointed at the laptop on the counter in front of me. “Not the models. The video. It’s high-def with a 19.2 megabits per second stream rate. Looks perfect, like HDTV, right?”
“Mm-hmm.”
“Now turn around.” I pointed at the plasma screen on the wall pulling a different part of the same video, a second stream at 18.4 Mbps, through our Xbox 360 with an attached 802.11n bridge. “That’s almost 40 megabits streaming over WiFi. I’ve never even been able to do one stream before, and now we’ve got two!”
My wife looked at the screens, looked back to me, and shrugged. “OK, then. I’ll leave you and the girls to it. Have fun.”
She walked away and slammed the front door. I don’t think she actually cared if I was having fun. Strange. Clearly, she didn’t understand that something amazing had fallen into my lap. Actually, let me rephrase that. Something incredible had happened to my network. With an access point clear across the house, transmitting through one floor and three or four walls, coping with literally a dozen interfering WiFi networks surrounding the house, I was getting wireless network performance unlike anything I’d ever seen before.
This was my first experience with beamforming, something I’d only seen vague mention of on long-term WiMAX roadmaps. But here it was in an 802.11n access point from a company I’d never heard of, and it blew away everything I’d ever seen a wireless product do before.
Interested? Then let’s dig in. I may not have runway models to offer you, but I still think you’ll be impressed.
Think of radio transmitters as little stones dropped in a pool. You know from high school physics that a dropped object will send out waves across the water’s surface. If you drop two stones, those waves will overlap with each other in a regular “interference” pattern. Changing the characteristics of a stone will change the amplitude and phase of the waves it emits, as well as the characteristics of the interference pattern generated with waves from other stones.
If you have enough control over the situation, you can have a sensor at the edge of the pool looking for just the right wave pattern, and you can keep changing the stone characteristics until that exact pattern arrives at that particular point. Elsewhere in the pool, the wave pattern will be different, and that’s fine. You’re only looking for that one pattern in that one place. Everything else can be ignored.
In a nutshell, this is the essence of beamforming. You’re controlling the output characteristics of each transmitter within a transmitter array so that the overall signal is optimized to reach a given receiver in a given direction. With an antenna array in which each antenna is transmitting with slightly different characteristics, you have what’s called a phased array. As we’ll see, there are two primary forms of phased array used in wireless access points: on-chip and on-antenna, adopted by Cisco and Ruckus Wireless, respectively.
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.
时间: 2011-6-22 13:13
作者: odyssey_2010 标题: 回复 2# 的帖子
Ever since the days when 802.11a/g grew a second antenna, we’ve had “transmit/receive diversity,” which sends the same data stream out over multiple antennas and simply lets the access point select whichever antenna is receiving the best signal. Applied to 802.11n, transmit diversity used multiple antennas to help increase range and better deal with difficult locations. This is why 11n does a generally better job than 11a/g at eliminating dead spots.
However, 802.11n equipment got another jump in intelligence with the addition of maximal ratio combining (MRC). This technology combines multiple antenna signals in such a way that strong signals are multiplied while weak signals are attenuated. The signals you want get boosted, while those you don’t have their power cut. MRC is built into all 802.11n chips.
Now, as you might expect, the receiving end can play an important role in optimizing chip-based beamforming. With 802.11a/g, access points could listen to the client and use rudimentary MRC analysis to boost power along the best-suited beam, providing a gain of roughly 1 to 2 dB. The catch here is that the access point was doing all the work. There was no active feedback coming from the 802.11a/g clients.
With “implicit beamforming,” wherein an 802.11n AP is communicating with 802.11n clients, you can have some feedback. Rather than having the access point perform all of the signal analysis, it can query the client and see if it agrees that this or that particular beam orientation is optimal. Having this limited two-way communication yields a maximum of 3 dB additional gain, but the bad news is that there are currently no products on today’s market supporting implicit beamforming.
With “explicit beamforming,” feedback between the AP and client happens much more frequently. This way, if a client moves or an antenna gets adjusted or anything happens to alter the dynamics of the signal strengths, the system is able to adapt almost instantaneously to a new, optimized configuration. Again, having the client involved in this way can yield up to a 3 dB benefit with two radios, but there are no products available today offering this capability. Hopefully this will change.
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