Monday, July 31, 2017

Appropriate RSSI for WDS Bridge Links

A customer recently approached me with a question on how much the minimum RSSI should be for a WDS bridge link between two locations in order to maximize data throughput.   (If you don't know what WDS bridging is, see my blog entry on Wireless Backhaul Best Practices.)

The short answer:  -40 dBm to -50 dBm for optimal performance.   This RSSI is readily achievable over distances of up to approximately 2500 ft when using high-gain directional antennas on each end.

This RSSI target is to ensure that the signal to noise ratio (SNR) can be safely above 37 dB.  This means that the highest MCS rate (MCS9) can be achieved and maintained with an 80 MHz channel, and the throughput of the link maximized.  (See Andrew von Nagy's Wi-Fi SNR to MCS Chart for further reference.)

For 802.11ac 2x2:2 APs, measured data throughputs of about 350-400 Mbps can be achieved with 80 MHz channels, when the RSSI of each link is consistently in the -40 dBm to -50 dBm range.  

If the RSSI is too strong (i.e. > -35 dBm), the electronic amplifiers in the AP start to get saturated and data throughout will actually decrease.  This generally only occurs in very short distance shots (< 50 feet), and in those instances we recommend turning down transmit power to minimum and, if still necessary, purposely misaligning the antennas.  

If the RSSI is too weak (i.e. < -70 dBm) the link speed will be very slow (low SNR leading to low MCS rates and slow speeds).  In the presence of any interference, the wireless backhaul link itself can often become unstable.

A Simplified Explanation of the Physics

A signal leaving a transmitter experiences free space path loss (FSPL), which is a geometric spread of the RF energy as it travels away from a transmitter; the further away the receiver is from the transmitter, the more the energy from the transmitter has spread out, so the less amount of energy is seen at the receiver. The FSPL goes as the square of the distance between the transmitter and receiver. Increasing the gain of the antennas at either (or both) ends of the wireless link help to focus the transmission energy and/or the receive sensitivity in a particular direction, allowing the distance to increase between the transmitter and receiver.  

Hence, for point-to-point wireless backhaul applications, you always want to use high gain directional antennas on both ends of the link (such as the integrated 19 dBi antenna on an EnGenius EnStationAC) to maximize the practical link distance and speed.

A Simplified Explanation of the Math

If you want to estimate the RSSI of a link, you can use the following formula:

or rearranged to compute link distance:

  • RSSI = Received signal strength indicator [dBm]
  • d = distance of wireless link [m]
  • f =  operating frequency of wireless link [Hz]
  • c = speed of light (i.e. 300 billion m/s) [m/s]
  • PTx = transmit power of transmit radio [dBm]
  • GTx = gain of transmit antenna, less cable losses [dBi]
  • GRx = gain of receive antenna, less cable losses [dBi]
Based on these formulas, a point-to-point link utilizing EnGenius EnStationAC access points could achieve a -50 dBm RSSI on each end for a WDS bridge wireless link length of approximately 2500 feet.