|802.11n was developed as a range and speed booster, employing multiple antennas and two or more radios to work over greater distances (sending a stronger signal, having better receiver sensitivity) and at greater speeds (improved encoding, multiple spatial paths, double-wide channels). That’s fine for laptops, desktops, and routers, but it’s hard to cram that much radio technology into a battery-powered mobile device without making the time between charges unusably brief.
Glenn Fleishman, Does the iPhone Need 802.11n?
That’s where single-stream 802.11n comes in. With single-stream 802.11n, only a single radio and single antenna are used…
…802.11n’s single stream encoding is 65 Mbps, where 30 to 50 Mbps of throughput is possible. So you lose wide channels, antenna diversity, and multiple streams, but could gain 50 percent or more in net throughput.