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.
发表于 2011-6-22 10:54:39
