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Effects of the bandwidth setting

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w7rej
Effects of the bandwidth setting

Let me start by saying I have a good understanding of networking in general and of wireless networking but I am certainly not an expert. When it comes to the RF aspect of wireless networking I'm not sure how it all works.

I know that when you lower the bandwidth figure of a node from 20 Mhz, the throughput will also be lowered. It was explained to me that if leave the power figure the same and go from 20 MHz to 10 MHz or 5 MHz you will get greater range. Can someone confirm this and provide a technical explanation as well?

AE6XE
AE6XE's picture
Yes,  the physics say that

Yes,  the physics say that reducing the bandwidth in half yields a 3dB gain in SNR when everything else is equal.     This additional SNR would enable longer distances.   There are other factors at play, generally I find the primary factor is what other environmental noise is around a given node--getting away from it (another channel, rf shielding, etc.) should be first option (to avoid unnecessarily lowering the top end throughput rate you can get).    Be careful, and this is a judgment call, to reduce channel bandwidth to get 1 node working, at the cost of lowering overall achievable throughput for the 30 other nodes.

Also these devices are Software Defined Radios--they are not fixed in how data is transmitted--they're very smart.   Every neighbor and packet the radio transmits may be using different protocol rates (modulation methods) and antenna combinations.   One neighbor might be transmitted a packet at 1Mbps DSSS protocol (802.11b) going out a single antenna (the antenna chosen via 'diversity' to be most effective at maximizing throughput).  In the next outgoing packet, another neighbor might be sent a packet at 130Mbps (802.11n LGI MCS15) and split the power on 2 antennas (Vertical and Horizontal polarization at the same time).     

The radio doesn't look to maximize SNR, rather the radio attempts the various combinations for every neighbor, tracks statistics, and transmits whatever combination maximizes the packet throughput rate, after accounting for packet failures.   A higher throughput might be achieved by splitting power on 2 antennas,  which lowers the SNR another 3dB for a given data stream.

Joe AE6XE

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