Guys, learn about RF
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Given the differences in frequencies used for the GSM and CDMA network standards, it is also unsurprising that the two 3G iPads offer slightly different antenna configurations, with the CDMA version offering one more antenna than the GSM version, similar to the iPhone 4.
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This has NOTHING to do with frequency differences.
It has to do with an issue called fast fading, which results in the effective strength of a digital RF signal varying dramatically over distances of a few centimeters and time-scales of a few milliseconds.
The best known way to combat fast fading is a collection of techniques known as "antenna diversity" which all make use of two (or more) independent antennas and RF chains in a device.
Verizon MANDATE that every device on their network use antenna diversity, and this is a large part of the reason they have a reputation as a reliable network.
ATT, because they are run by idiots who are, apparently, completely unaware of any technology invented since 1988, do NOT mandate antenna diversity --- and have the reputation they have.
It is perfectly feasible for a manufacturer to incorporate two antennas (and two independent RF chains, and some logic to combine the two signals appropriately) to improve the performance of their device. This is not the OPTIMAL way of doing things --- if the network is set up properly and knows a receiver has two antennas, it can utilize something called Alamouti coding to slightly increase robustness --- but simply have two antennas and appropriate combining is already a big step.
So why did Apple not do this? Obviously an initial point is that a chip is needed that provides two (appropriate to GSM) RF chains and combining logic. My guess is that the new Qualcomm chip being used does offer this on the GSM side, though I don't know that for sure.
My personal opinion is that this is something Apple will add to the GSM iPhone 5. They will not announce it or make a big deal about it (because there's always going to be some moron submitting YouTube videos of how he covered his iPhone in tin foil and then couldn't get a signal), but they will do it in the hope that people just slowly start to say "You know, this new iPhone5 is drops calls a whole lot less than the old iPhone4 model. I can't say how, exactly, but it just feels really reliable."
I'd peg them as especially likely to do this if they stick with the existing external antenna design, so they can (not publicly, but implicitly) tell the world "screw you a**holes, we knew then and we know now how to design a decent antenna system".
But we shall see. I personally am amazed that NO GSM company in the world (at least of the companies Apple sells to) mandate dual antennas. And these technical ignoramuses are the people who control our digital futures!
(Antenna diversity can ALSO be used for something known as spatial multiplexing, which increases the throughput available to a given wireless channel. This is a big part of 802.11n. The earliest Mac 802.11n macs had two antenna systems, the newest have three --- and get 50% higher throughput under ideal conditions.
Sadly all iDevices currently have only one 802.11n antenna.
In theory multiple 802.11n antennas, and sending data by the fastest means possible, is also the most power efficient mechanism --- in terms of the RF power and the amount of computation required. In practice it is possible, I don't know, that the current chips are inefficient enough that the slower single antenna scheme remains the best overall in terms of energy.
Another possibility is that Apple is holding back on this because they simply don't need to include it yet, given how weak their competition is. Might as well hold back a feature like this until you actually have a worthy competitor, in which case you can announce "iPad 3.5, along with all its other coolness, now also comes with wifi that's twice as fast"?)
