Wi-Fi (802.11) PHY Data Rates

Devices supporting the latest 802.11ac (draft) standard are now being certified by the Wi-Fi Alliance. 802.11ac promises various improvements over 802.11n. Starting from the first IEEE 802.11 standard in 1997 to the latest 11ac standard, there have been improvements in various aspects of 802.11 networks - speed of the network being one of the major improvements. This article attempts to explain the PHY data rates of 11b, 11a/g, 11n and 11ac.

Modulation

The above diagram shows the modulation techniques used in different 802.11 PHYsical layers. 802.11 systems support two (Ignoring the optional and obsolete types) types of modulation - DSSS/CCK and OFDM. This article is not going to cover the details of DSSS/CCK (a good intro can be found here) and OFDM. The below figure gives an overview of both the techniques.

Data Rate

DSSS/CCK (11b Date Rates)

DSSS/CCK data symbol is formed by spreading and modulating the data bits. The data rate depends on : 

• Chips per second (CSec) = 11,000,000
• Chips per symbol (CSym) = 11 (DSSS) or 8 (DSSS with CCK)
• Bits per symbol (NBits) = 1 or 2 (DSSS), 4 or 8 (DSSS with CCK)

Data Rate = (CSec/CSym)*NBits

Chips per second	Chips per symbol	Bits per symbol	Data Rate (Mbps)
11,000,000	11	1	1
11,000,000	11	2	2
11,000,000	8	4	5.5
11,000,000	8	8	11

11b Data Rates 

OFDM

DSSS/CCK uses the entire available bandwidth as one single channel. OFDM divides the channel into multiple(overlapping) sub-channels. The data rate depends on:

•  Symbol duration (in other words symbols per second) (SDur)
• Symbol duration depends on "Guard Interval" between symbols.
•  Bits per symbol (NBits)
•  Coding rate (CRate)
• Some bits are used for error correction and do not carry data 
• Number of sub-channels (NChan)
• Depends on channel width(20/40/80/160)
• 11n/11ac have more sub-channels in the same bandwidth compared to 11a/11g

Data Rate = (1/SDur)*(NBits*CRate)*NChan

11a/11g Rates

Modulation	NBits	CRate	NChan	SDur (micro sec)	Data Rate (Mbps)
BPSK	1	1/2	48	4	6
BPSK	1	3/4	48	4	9
QPSK	2	1/2	48	4	12
QPSK	2	3/4	48	4	18
16-QAM	4	1/2	48	4	24
16-QAM	4	3/4	48	4	36
64-QAM	6	2/3	48	4	48
64-QAM	6	3/4	48	4	54

11a/11g Data Rates

Note: The above table shows date rates for 20MHz channel width. 5,10 MHz channel widths are not shown.

11n/11ac Data Rates

11n and 11ac data rate improvements are due to:

• The number of sub-channels in 11n and 11ac is more than 11a and 11g. 
• Higher coding rate (5/6)
• 11ac also uses 256-QAM which further increases the data rate.
• Wider Channels (40,80,160)
• Multiple Spatial Streams (MIMO) (Nss) 

Modulation	NBits	CRate	NChan	Data Rate (Mbps) SDur=4us	Data Rate (Mbps) SDur=3.6us
BPSK	1	1/2	52	6.5	7.2
QPSK	2	1/2	52	13	14.4
QPSK	2	3/4	52	19.5	21.7
16-QAM	4	1/2	52	26	28.9
16-QAM	4	3/4	52	39	43.3
64-QAM	6	2/3	52	52	57.8
64-QAM	6	3/4	52	58.5	65
64-QAM	6	5/6	52	65	72.2
256-QAM (11ac)	8	3/4	52	78	86.7
256-QAM (11ac)	8	5/6	52	86.7	96.3

11n and 11ac Data Rates (20 MHz, 1 SS)

The entries highlighted are 11ac only rates and the rest are common to 11n and 11ac.
Updated on 20/08/2014: Note: The last entry,i.e, MCS9 (256 QAM, 5/6 rate) is not allowed in 11ac 20 MHz channel.

The maximum data rate is achieved with MIMO and maximum channel width.

Channel Width	NBits	CRate	NChan	Data Rate (Mbps) SDur=3.6us (Nss=1)	11n Max Rate (Nss=4)
20 Mhz	6	5/6	52	72.2	288.8
40 Mhz	6	5/6	108	150	600

11n Max Data Rates

Channel Width	NBits	CRate	NChan	Data Rate (Mbps) SDur=3.6us (Nss=1)	11ac Max Rate (Nss=8)
20 Mhz	8	5/6	52	96.3	577.8 (Nss=6)
40 Mhz	8	5/6	108	200	1600
80 Mhz	8	5/6	234	433.3	3466.4
160 Mhz	8	5/6	468	866.7	6933.6

11ac Max Data Rates 

Updated on 20/08/2014: Max valid spatial streams for 11ac 20 MHz channels corrected to 6.

Summary

The improvement in data rate from 11b to 11ac is due to various factors: OFDM, Wider Channels, QAM, Higher coding rate and MIMO.  The figures below depicts the improvement in data rate due to each factor(excluding MIMO).