[WLAN]Q&A: How to Calculate the Link Setup Rate of the Wi-Fi Air Interface?

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The link setup rate of the Wi-Fi air interface is determined by five factors: the number of spatial streams, symbol and guard interval (GI), modulation mode, code rate, and number of valid subcarriers.

  • Number of spatial streams

    Huawei AP4050DN is used as an example. The AP supports 2x2 multiple-input multiple-output (MIMO) on the 2.4 GHz band, so the number of spatial streams is 2. The AP supports 2x2 MIMO on the 5 GHz band, so the number of spatial streams is also 2.

    When an AP communicates with STAs, the actual link setup rate depends on the lower value. For example, if the AP supports 4x4 MIMO and a STA supports 2x2 MIMO, the AP sets up a link with the STA based on 2x2 MIMO.

  • Symbol and GI

    The unit of symbols is us, indicating a duration. Adjacent symbols must have a GI between to avoid interference between each other. The reciprocal of "symbol + GI" is the number of symbols that can be transmitted in a second. The values of symbols and GIs vary according to the fast Fourier transformation (FFT) orders in 802.11ac and 802.11ax.

  • Modulation mode

    The modulation mode enables a symbol to carry as many bits as possible. Since its birth, Wi-Fi standards have been committed to modulating signals. Currently, the most efficient modulation scheme is 1024 quadrature amplitude modulation (QAM), that is, one symbol can contain 10 bits of data.

  • Code rate

    For higher reliability, some error-correcting codes (ECCs) are added to original data streams. The code rate is the proportion of the useful data streams (non-redundant). However, ECCs lead to a decrease in the actual transmission rate. When the signal-to-noise ratio (SNR) of a channel is high, the 5/6 code rate is used to achieve the transmission rate.

  • Number of valid subcarriers

    In 802.11ac and earlier Wi-Fi standards, 64-point FFT is used to divide 20 MHz into 64 subcarriers. In 802.11ax, 256-point FFT is used to divide 20 MHz into 256 subcarriers. Each subcarrier carries one symbol.

    Note: The bandwidth reciprocal of a subcarrier is the length of the symbol.

For example, Huawei's 802.11ax-compliant AP7060DN supports the link setup rate of 4803 Mbit/s on a single 5 GHz band. This value is calculated as follows:

  • The AP7060DN supports eight spatial streams.

  • In 802.11ax, the symbol length is 12.8 us and the GI is 0.8 us. The reciprocal of "symbol + GI" (12.8 + 0.8) is 1/13.6 us.

  • 802.11ax supports 1024-QAM, which supports 10 bits/subcarrier.

  • The 5/6 code rate is used.

  • At the bandwidth of HT80, the number of valid subcarriers is 980.

According to the preceding formula, the air interface link setup rate on a single 5 GHz band is 4803 Mbit/s (8 x 1/13.6 us x 10 bit/subcarrier x 5/6 x 980 = 4803 Mbit/s).

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