802.11ac 5GHz RPi 3B+ - different channel widths

Lennart Heim lennart.heim at rwth-aachen.de
Tue Apr 16 06:06:42 PDT 2019

Dear all,

I’m currently experimenting with the newest Raspberry PI 3B+ generation and their WiFi/BT chipset (Cypress43455 or bcm43455c0 [1]) - focusing on 802.11ac in the 5 GHz spectrum.

I’m running into some issues using the 802.11n and 802.11ac and their HT and VHT features. 
Let’s take a simple scenario using 802.11ac and a 20MHz channel width (hostapd.config see [2]). Using this hostapd my connected client gets downgraded to a bare 802.11a using a maximum bitrate of 54 Mbits/s - while 802.11ac has the possibility to increase the MCS by 2 indexes.
Summing up: How can I mix different channel bandwidths (20, 40 and 80 MHz) with 802.11n and 802.11ac? Which role does HT and VHT play? To my knowledge a 40 MHz channel in VHT (therefore 802.11ac) supports less bandwidth than a 40 MHz channel in HT (therefore 802.11n), as 802.11ac supports two more MCS indexes in each bandwidth.

Furthermore why do I need to enable ‘ht_capab’ when I want to run VHT with ‘vht_capab’ - without ‘ht_capab’ hostapd fails.

We’re using a single spatial stream, no MIMO or anything else involved. According to the datasheet all MCS indexes (0-9) are supported.

I’m grateful about any kinds of tips or further resources.


[1] Chip Documentation: https://www.cypress.com/file/358916/download
[2] hostsapd.conf 802.11ac 20 MHz: https://gist.github.com/lheim/d18e01d1adc9e0439471fbdb41e290ad
[3] hostsapd.conf 802.11ac 40 MHz: https://gist.github.com/lheim/9b5025fcf5ea67bbc650bb719e9fe36f

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