What's the reason for the drastic growth of WLAN speed even though we use the same frequencies and channels?

Question

I have recently asked myself, what causes the usual WLAN speed of today's wifi standards to differ so greatly from older ones.

Let's use 802.11b and 802.11ac as examples. The maximal download rate of 802.11ac goes up to 1.6 Gigabit/s, which is roughly 150 times the download rate of 802.11b, which caps at 11 Megabit/s.
They both use the 2.4GHz frequency band with around 11 to 14 channels, depending on where you live.

Now what factor makes the difference here?
Or am I missing something obvious?

Answer 1

We don't use the same frequencies or channels. 802.11ac uses 5GHz exclusively, and typically uses 80MHz-wide channels, whereas 802.11b used 2.4GHz exclusively, and used 20MHz-wide channels.

Basically, speed improvements have come from 3 sources:

• Modulation improvements. Fancier signaling schemes allowing more bits to be sent per "symbol" or per unit of time. After 802.11g (2003) these improvements have been modest compared to the other two below.
• Channel width doubling, quadrupling, and future potential octupling. 802.11n can use 40MHz-wide channels, 802.11ac can use 80MHz-wide channels, and may expand in the future to 160MHz-wide channels. Doubling the channel width also allows efficiency gains so you get slightly better than a 2x speedup.
• Adding a second, third, fourth...and potentially up to eighth radio chain. 2x the radio chains can mean 2x the speed, 3x radios means 3x speed, etc.

Here's a more detailed summary of how each generation of 802.11 has gotten its speed increase:

• 802.11-1997 DSSS: This is the baseline at 1 and 2 Mbps via DBPSK and DQPSK modulation. 20MHz-wide channels in 2.4GHz.
• 802.11b (1999): Added 5.5 and 11 Mbps via modulation improvements (CCK). The spec contained provisions for a 22 Mbps modulation, but it was encumbered by Texas Instruments patents, and very few vendors licensed the 22Mbps mode from TI.
• 802.11a/g (2002/2003): Added several rates up to 54 Mbps. Cheated the Shannon Limit by stuffing 48 orthogonal subcarriers into the same 20MHz-wide channel. Added 16-QAM and 64-QAM modulation.
• 802.11n (2007): Doubled channel width to 40MHz. Added MIMO (multiple transmit and receive radio chains, multiple "spatial streams"). Added a faster coding rate for 64-QAM. Note that when operating in 20MHz channels with only a single radio chain (for maximum comparability to 802.11a/b/g), 802.11n's modulation improvements only allowed it to get to 72.2 Mbps, so not that much higher than a/g's 54 Mbps. 802.11n's big speed advancements were mostly from doubling the channel width, and from doubling, tripling, or potentially quadrupling the number of radios (radio chains) on each end of the link. The spec contained provisions for up to 4 spatial streams (4x4:4) for a 600Mbps PHY rate, but I'm not aware of anyone shipping a 4 spatial stream 802.11n chipset.
• 802.11ac (2012/2013): Doubled channel width again to 80MHz. Added 256-QAM modulation. The spec contains provisions for another channel width doubling to 160MHz, and up to 8 spatial streams (8x8:8), for a nearly 7 Gbps max PHY rate, but I'm not aware of anyone shipping anything better than 4x4:4 in 80MHz-wide channels as of spring 2017. Just like 802.11ac came along and took away the need to do 4x4:4 in 802.11n, it looks like 802.11ax may come along and provide newer/better/more-practical ways to do faster data rates, so I doubt we'll see anyone actually ship 8x8:8 in 160MHz (7 Gbps) 802.11ac. Also note that if we limit 802.11ac to a single radio chain in a 20MHz-wide channel, it only gets 86.7 Mbps, because its only advantage is 256-QAM modulation.