April 2012
Intermediate to advanced
144 pages
4h 34m
English
Content preview from 802.11n: A Survival GuideBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Chapter 4. Advanced PHY Features for Performance
Half the transmit power I pump into my antenna is wasted. The trouble is that I don’t know which half.
—John Wanamaker (had he been a wireless network
administrator instead of an early advertiser)
In its most basic conception, MIMO is technology that provides a multi-lane highway for 802.11n transmissions. When a transmitter can send multiple streams across a link, throughput improves. This description of MIMO is fairly passive, and does not provide a full picture of everything that a MIMO system can do. With an antenna array, it is possible to create far more sophisticated transmissions than just sending multiple streams. With advanced radio chips, it is possible to send transmissions from the MIMO antenna array in a particular direction in a process called beamforming. Individual receiving antennas within MIMO arrays can also cross-check each other to improve signal processing at the receiver. Both applications can increase the signal to noise ratio, which improves speed. In addition to developing the underlying technology for antenna arrays, 802.11n includes protocol features that can improve the signal processing gain. MIMO can also be used to spread a single spatial stream across multiple transmitters for extra signal-processing gain at the receiver, and hence, longer range in a process called Space-Time Block Coding (STBC). 802.11n also provides another option to increase signal-processing gain, the Low-Density Parity Check (LDPC) ...
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