80 MHz vs 40 MHz for Wi-Fi CSI: resolution, subcarriers, and a real-world trap

2026-10-11 · 2 min read

The single most consequential decision in a Wi-Fi CSI setup is channel bandwidth. It sets how much frequency information you get per measurement — and, as we discovered the hard way, it can also break a client entirely.

1. What bandwidth actually buys you

Subcarrier counts vary by chip and firmware; always log data_num from your own capture instead of assuming.

Two things bandwidth does not change:

• Doppler/temporal resolution — that depends on your measurement rate (we run ~95 records/s, plenty for walking and breathing);
• Multipath richness — that depends on the environment, not the width.

2. The trade-off nobody mentions: legacy clients

Our Mac (a 2013-era Broadcom NIC) negotiates 40 MHz only. Put it in an 80 MHz BSS and the AP's transmit rate for that client periodically collapses to VHT-MCS0 1SS — 13.5 Mbps PHY, ~7-10 Mbps of actual downlink — while upload stays fine.

Measured, same client, same room, only the radio width changed (6 x 6 s LAN bursts after 60 s idle) — grade E evidence, raw capture and scripts published ^{[1] [2]}:

So the "narrow-band client inside a wide BSS" case is a real firmware/rate-control failure mode — not a signal problem (RSSI was -40 dBm and the client's own receiver handled 400 Mbps in the fast state).

3. How to get both (resolution and usable clients)

4. Measuring it yourself

# is the client stuck?
iw dev <ap> station dump | grep -E "Station|tx bitrate|rx bitrate"
#   tx bitrate = AP -> client (the one that collapses)
#   rx bitrate = client -> AP (usually fine)

# test with LAN traffic, not a public download
# (a public mirror adds a second variable you cannot control)

Report both the PHY rate and the achieved throughput: during our collapse, 12 s of transfer moved 12.4 MB while the AP sent 12.87 MB — i.e. almost no retransmission waste, just a very low rate and an idle channel.

Related: the MCS0 rate-control deep dive · OpenWrt Wi-Fi slow after idle · reproduction guide