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).

    1 comment:

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

      ReplyDelete