That chip supports both bands, but only the 2.4GHz band has been "turned on" in the past in most smartphones.
Can Apple make any other tweaks to boost Wi-Fi performance?
In their in-depth review last year of iPhone 4S, Anand Lal Shimpi and Brian Klug, of AnandTech.com, found the 4S Wi-Fi/Bluetooth antenna, repositioned to the top corner of the phone, had "slightly better received signal strength ... compared to a [iPhone] 4 side by side, and upon checking the FCC documents learned the 4S' WLAN antenna has a peak gain of -1.5 dBi compared to -1.89 dBi on the 4, making it better than the previous model." The higher gain, loosely understood as sensitivity, improves the signal strength and helps sustain the higher throughput connection.
For iPhone 5, Apple has added two glass "RF windows" on the back, at top and bottom. There might also be antenna improvements. Together, these could improve signal strength for the phone.
The more powerful A6 CPU, Apple's system-on-chip or SoC, in the iPhone 5 may also have an impact. In their review of the 4S, the authors noted: "I'm starting to think that the bigger boost is actually thanks in part to a faster SoC." Apple says the A6 delivers twice the performance for new phone as for the 4S.
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