Sunday, August 4, 2013

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).

    3 comments:

    1. Now I understand how DataRate values are derived/calculated for 11g and other modes.

      ReplyDelete
    2. Nice work, thank you very much.

      ReplyDelete
    3. Great work in helping us understand how the different modulation, coding, etc. schemes are interwoven to increase data rates.

      ReplyDelete