InfiLINK 2x2 / InfiMAN 2x2: Initial Link Configuration and Installation

05

Link optimization, DFS & Instant DFS

Antennas alignment

Task 5
  • Using the azimuth and elevation values computed by the link planning tool, each team will roughly position the antenna (in each location) to detect the opposite system signal.
  • The signal should be tested in both directions, one at a time.
  • After the initial approximate alignment (link up), the antenna with the lowest gain should be locked into position.
Task 6

Both teams will connect to the unit, and from the web GUI will:

  1. Go to the Device Status section
  2. Select the active link in Links Statistics
  3. Check the Antenna Alignment Tool option

Figure - Accessing Antenna Alignment tool

Task 7

The team at the antenna which has the highest gain will click on the "Start Test" button and start to change the azimuth slowly while watching the signal indicators

  • As soon as the best signal has been found (Input Signal stripes must be located in the black area, closer to its center), the antenna must be locked into that position
  • The same action will be performed for the elevation, and the antenna must be locked into the final position where both elevation and azimuth provide the best signal, according to the indicators provided by the antenna alignment tool: 
    • EVM: higher than 21 in absolute value.
    • Signal level to interference and noise: higher than 28 dB. 
    • Retries: lower than 10 %.

WARNING

No contact should be made with the antennas during signal reading because the human body can affect the radiation pattern of the antenna and signal readings.


The main parameters displayed in the alignment tool

  • RSSI - indicates the power level of the received radio signal, optimal parameter value -60 ... -40 dBm.
  • CINR - input signal level to noise + interference indicator, >=28 dB.
  • Crosstalk - indicates how much vertically and horizontally polarized signals influence each other, >20 dB in absolute value.
  • Error Vector Magnitude (EVM) - indicator of the measured input signal quality (it should be as high as possible in absolute value, the recommended level is not less than 21 dB in absolute value. Some old firmware had EVM value positive, but most the firmware has negative value, so for the troubleshooting, evaluate the absolute EVM value) .
  • Retries - percentage of transmit packet retries (measured in %), <10.
  • Tx bitrate - displays the current bitrate for the remote and local units (measured in Kbps).

Figure - Antenna Alignment 

Task 8
  • As soon as the antennas have been precisely alignment, set the "auto" option for Tx power and bitrate at both units, bearing in mind the EIRP limitations.
  • Depending on the values for SNR, RSSI, retries and current bitrate, change the following parameters:
    • Decrease/increase the Tx power level (keeping the auto option checked) to have the SNR at around 25 dB and the RSSI at around -55 dBm.
    • Decrease the bandwidth to lower the noise and to increase the SNR to above 20 dB.

Figure - "Auto" option and max values for Tx power and bitrate

Wireless link statistics

Task 9
  • Let's check the link parameters (for the correct link model according to the link distance, with an optimal antennas alignment, clear working frequency, and Clear-Line-of Sight, the values displayed must be the optimal ones for that particular link).
  • Go to the Device Status section and in the Link Statistics, check the Level Rx/Tx and Retries Rx/Tx:
    • Retries Rx/Tx: maximum 5 %.
    • Level Rx/Tx: minimum 20 dB.

Figure - Rx/Tx level and retries


NOTE

If the two conditions above are not met, decrease the bandwidth from 40 MHz to 20 MHz, or even to 10 MHz, to lower the noise and to increase the SNR to above 20 dB.


Interface statistics

Task 10
  • Let's check the number of Rx/Tx errors for the Ethernet and Radio interfaces are close to “0”, which is the case for an optimal link establishment (no traffic is generated through the wireless link at this stage).
  • Go to the Device Status section and in the Interface Statistics for eth0 and rf5.0 interfaces, check the Errors Rx/Tx.

Figure - Rx/Tx errors

Dynamic Frequency Selection (DFS)

Dynamic Frequency Selection is an interference mitigation technique that aims to:

  • Protect the radar systems by avoiding co-channel operation with the broadband wireless systems.
  • Ensure a uniform distribution of the applications across the spectrum (according to EN 301 893, uniform spreading is required across 5150 MHz to 5350 MHz and 5470 MHz to 5725 MHz).

Standards specify the performance requirements for the wireless systems to avoid interferences with radars but do not define how to implement DFS practically. Vendors have the responsibility to implement DFS using their specific means to comply with the requirements of the standard. Detection requirements:

  • Channel Availability Check - when initially powered on, or in case radar is detected on the operating channel, a frequency scan must be performed to determine the available channels.
  • In-service monitoring - the operating channel is constantly monitored for the presence of radar signals (the detection thresholds are the same as in case of CAC).
  • In-service monitoring is performed in between each data transmission, and its duration is of µs order.
  • Off-channel Channel Availability Check - in addition to CAC, DFS can also decide to select a better channel (less interfered) if the SNR of the operational channel falls below a certain threshold; this operation requires link interruption as the radio module can either transmit/receive data or perform the medium scan.
Task 11 - for regular DFS units only

Let's measure the link establishment time after the unit restart.

At the Slave unit:

  • Go to the Basic Settings section, then to Link settings, set the Frequency to "Auto".
  • In the Custom Frequency Grid, change the range of the available channels for the working bandwidth as well as only 10 frequencies to be available (chose the less interfered part of the range) and apply the configuration.

At the Master unit:

  • Go to the Basic Settings section, then to Link settings and:
    • Select ‘DFS with Radar Detection’ for the DFS parameter and ‘Auto’ for Frequency.
    • In the Custom Frequency Grid, change the range of the available channels for the working bandwidth as well as only 10 frequencies to be available (chose the less interfered part of the range) and apply the configuration.
  • Reboot the unit from the Maintenance section.
  • After boot-up (indicated by the LEDs on the unit cover), measure the elapsed time to link up (indicated by the LEDs):
    • The initial Channel Availability Check takes 3 seconds per each frequency; thus for the 10 available frequencies, the initial CAC must take 30 seconds, so the link is established in between 30 and 40 seconds because the Slave unit scans, too each frequency in 1 second to find the Master unit.
  • After login, analyze the channels list in the DFS section, consulting the legend.

Figure - Customer Frequency Grid

Figure -DFS

Instant DFS

In case of Instant DFS, Off-channel CAC is performed by an additional radio card that scans all the channels (besides the operational channel which is monitored during initial CAC) defined in the frequency grid.

Advantages of Instant DFS against regular DFS:

  • No link interruption if the radar is detected, by using the frequency database maintained during the Off-channel CAC.
  • In case of link quality deterioration, the Master can decide to switch the channel to a better one without link interruption.
  • The Spectrum Analyzer built-in tool can be run without interrupting the link.
Task 12 - for Instant DFS, 5 GHz units only

Let's measure the link establishment time after the unit restart

At the Slave unit:

  • Go to the Basic Settings section, then to Link settings, set the Frequency to ‘Auto’.
  • In the Custom Frequency Grid, change the range of the available channels for the working bandwidth as well as only 10 frequencies to be available (chose the less interfered part of the range) and apply the configuration.

At the Master unit:

  • Go to the Basic Settings section, then to Link settings and:
    • Select ‘DFS with Radar Detection’ for the DFS parameter and "Auto" for Frequency.
    • In the Custom Frequency Grid, change the range of the available channels for the working bandwidth as well as only 10 frequencies to be available (chose the less interfered part of the range) and apply the configuration.
  • Reboot the unit from the Maintenance section.
  • After boot-up (indicated by the LEDs on the unit cover), measure the elapsed time to link up (indicated by the LEDs):
    • The link is established in between 1 and 10 seconds because the Slave unit scans each frequency in 1 second to find the Master unit, and the initial Channel Availability Check doesn't occur thanks to the continuous Off-channel CAC.
  • After login, analyze the channels list in the DFS section, and the constant Off-channel CAC, consulting the legend.



Figure - Instant DFS

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