And seven 160 MHz channels will be viable in some deployments as well. We have more 80 MHz channels in 6 GHz than we do 40 MHz in 5 GHz. To put things into perspective, we have more 40 MHz channels in 6 GHz than we do non-overlapping 20 MHz channels in 2.4 and 5 GHz combined. The use of wider channel widths – 40MHz, 80 MHz, or in some cases, even 160 MHz – is expected to be more common with Wi-Fi 6E, due to the massive amount of available spectrum and that there will no longer be SNR tradeoffs by using wider channels. Client devices in Wi-Fi 6E, for both low power and standard power operations, are restricted to 6 dB lower than the access point. Note: Standard power APs follow the same constant EIRP rules as existing UNII-1 and 3, which has a limit of 36 dBm. We can convert between PSD and EIRP with this formula:Ģ1 dBm EIRP = 5 dBm/MHz PSD + 10log(40 MHz) Reference of PSD and EIRP for Low Power Operation The constant PSD in 6E ensures the same SNR across channel widths, allowing the use of wider channels without a signal quality penalty. These graphs (below) illustrate the relative SNR for existing constant EIRP we are used to and the new constant PSD in 6E across channel widths. In effect, we’re able to keep a constant SNR and effective EIRP across channel widths, since wider channels are able to enjoy higher EIRP than lower channel bandwidths. Instead, if we keep PSD constant, as we can do with Wi-Fi 6E, every time the channel bandwidth is doubled, we still take the noise floor hit, but we can also double EIRP. 80 MHz channels actually take a 6 dB SNR hit compared to 20 MHz, making it a challenge to use the wider channels for high density deployments (and also limited bandwidth availability). In existing frequency bands, with constant EIRP, we achieve the maximum SNR with 20 MHz channels. One of the tradeoffs in Wi-Fi and RF in general is that each time the channel bandwidth is doubled, the noise floor is also doubled (increased by 3 dB). In the US, Wi-Fi 6E low power indoor devices must comply with a spectral density of 5 dBm/MHz (there is a proposal to increase PSD to 8 dBm/MHz, which is still open as of this writing). Power Spectral Density is the amount of power over a given bandwidth. When the channel bandwidth is doubled, so is the effective power, but PSD remains constant. With 6E, we’re moving to a constant PSD, which means effective EIRP increases and decreases by 3 dB per channel width. In the Wi-Fi industry, we’re used to a constant EIRP across channel bandwidths and haven’t really had to care about PSD. Upon reading this blog, the reader should have a better understanding of PSD and how it relates to EIRP. Regulators typically set EIRP limits for Wi-Fi devices to comply with: among several other requirements, one of which is Power Spectral Density (PSD – the topic of this blog. Whatever the notation, they’re all cut from the same cloth, meaning they measure essentially the same thing, just at different parts in the RF path. Some vendors use EIRP to represent the transmit power of an access point, others use total power out and others still use per chain transmit power. Most Wi-Fi folks are familiar with and understand the concept of EIRP to represent power, although they may not know what exactly it stands for, which is Equivalent Isotropically Radiated Power (EIRP).
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