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Archive for September 2012

What’s New In the WiFi for iPhone 5

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Yay, a new iPhone!  

So sayeth me, my relatives (one of whom will receive my old iPhone), California (who will receive 8.75% in sales tax on the FULL UNLOCKED PRICE of the phone because California has a ludicrous sales tax law that taxes the pre-discount price of mobile phones) and anyone else who has been waiting for the iPhone to finally support 5 GHz WiFi.  

But wait, there’s more.  The iPad has long supported 5 GHz 802.11n WiFi, but the iPhone 5 does the iPad one better.  How?  Read on, amigos.

Though Apple’s most popular iOS device, the iPhone, has eschewed 5 GHz WiFi until iPhone 5, iOS-based access to 5 GHz channels (numbered 36 through 165) has been available in every iPad model.

The iPad has always been 802.11n, which is good.  But the WiFi adapter in the iPad has always supported the bare minimum 802.11n, which is bad.  (Specifically, 65 Mbps Data Rate bad.)  This meant that an iPad is going to take about three times as much channel time as a MacBook Pro (which has a top data rate of 450 Mbps) to send or receive a single full sized packet.

To achieve a data rate of 65 Mbps, the iPad supports the following technologies:

  • 20 MHz channel bandwidth
  • 64-QAM modulation
  • 52 data carrying OFDM subchannels
  • 5/6 convolutional coding
  • 800 nanosecond guard interval (same as 802.11a/g)
In supporting the 450 Mbps data rate, the MacBook Pro supports the following additional 802.11n technologies.
  • 40 MHz channel bandwidth (typically only in the 5 GHz band)
  • 400 nanosecond “short” guard interval
  • 3 stream spatial multiplexing
Along comes iPhone 5.  It is better than the iPad, but still a ways below the MacBook Pro.
The iPhone 5 uses the 40 MHz channel bandwidth and the 400 nanosecond guard interval, but not 3 stream multiplexing.  That means a top data rate of 150 Mbps.
Why would the iPhone 5 not use 3 stream spatial multiplexing?  Because spatial multiplexing of any kind involves using multiple input/multiple output (MIMO) technology.  MIMO drains battery life (because it involves multiple transceivers being used instead of one), it can lead to heat issues (same reason) and it is hard to do on a small, slick-looking device (because each antenna that is attached to a transceiver has to be at least a half wavelength from each other antenna).
The bump in maximum data rate to 150 Mbps means that an iPhone 5 at top speed will be sucking up a little bit less than twice the channel time that a MacBook Pro would be if both are sending or receiving full sized packets.  Still less than ideal, but a whole heck of a lot better than what we used to have.

Written by sniffwifi

September 19, 2012 at 8:18 pm

Posted in Uncategorized