Page History

Introduction

Scaling and increasing the performance of the wireless communication systems in the 5 GHz frequency range required an increase in the allowed frequency channels number by additionally using the channels that were initially assigned only for military and meteorological radars. To reduce the impact of the wireless broadband systems on radars, the data exchange protocol included requirements for detecting radar signals and immediately clearing the operating frequency.

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• The dynamic frequency selection implementation requires scanning the available frequency channels and selecting the least noisy channel. Thus, DFS performs a uniform distribution of the wireless data transmission systems over the frequency spectrum.
• A radar detection mechanism is necessary to ban the use of the frequency channels on which radars are currently operating. Thus, the DFS technology retains the possibility of using such channels for the broadband wireless systems, as long as they are immediately released when radar is detected.

Regulatory requirements

Regulatory requirements for wireless communication systems vary by country. The most common requirements for the implementation of dynamic frequency selection and radar detection, were created by the Federal Communications Commission (FCC) and by the European Telecommunications Standards Institute (ETSI). It is described by following standards:

• ITU-R M.1652 – describing the performance requirements for the wireless systems in order to meet the detection and response demands;
• ETSI EN 301 893 – Europe standardization based on ITU-R M.1652 describing the detection, response and test plans demands;
• ETSI TR 102 651 – With the scope of presenting a guide for the DFS implementation by providing additional explanations in relation with the EN 301 893;
• ETSI EN 302 502 – Standardization for fixed broadband data transmitting systems operating in the 5.8 GHz bandwidth.

Detection Requirements

CAC(Channel Availability Check)

• When initially powered on or if radar is detected on the operating channel, a frequency scan must be performed in order to determine the available channels.
• Studies have shown that wireless systems operating at 200 mW or less should be able to detect radar levels down to -62 dBm and higher power systems should detect down to -64 dBm as sufficient condition to avoid interferences with the radar systems. This qualifies as the interference threshold detection requirement (standard EN 301 893, table D.2).
• The CAC should not take less than the specified Channel Availability Check Time.

Off-Channel CAC (Off-channel Channel Availability Check)

• Off-Channel CAC may be performed in order to monitor the channels different from the operating channel.
• A number of non continuous checks are performed over a period of time in order to detect radar signals according to the same detection thresholds. If radar is not detected during the Off-channel CAC Time, the channel is declared as available.

In-service monitoring

The process by which the operating channel is 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.

Detection probability

Response requirements

Channel shutdown

• Defined as “the process initiated by the RLAN device on an Operating Channel after a radar signal has been detected during the In-Service Monitoring on that channel”.
• The master device will instruct its associated slave devices to stop transmitting on this channel, which they shall do within the Channel Move Time. Slave devices that perform radar detection shall stop their own transmission also within the Channel Move Time.
• The aggregate duration of all transmissions shall end within the Channel Closing Transmission Time (the link quality should be sufficient to allow the Master to send command to the Slaves and respect the timers).

Non-occupancy period

• The time interval during which no transmission is allowed on a channel that has detected a radar signal.

Time values

* - For channels whose nominal bandwidth falls completely or partly within the band 5 600 – 5 650 МHz, the value shall be 10 minutes.

** - For channels whose nominal bandwidth falls completely or partly within the band 5 600 – 5 650 МHz, the Off-Channel CAC Time shall be within the range 1 h to 24 h.

Infinet approach

The standards specify the performance requirements for the wireless systems in order to avoid interferences with the radars, but do not specify how to practically implement DFS. 

For the Infinet devices, the regulatory limitations are performed by license which includes the set of frequencies and by the presence of the radar detection configuration option. A license may be issued in accordance with the regulations for a particular territory. For the InfiLINK Evolution / InfiMAN Evolution and Quanta 5 families devices restrictions are determined by the selected regulatory domain, which can be changed in the link settings section (if the appropriate license is installed).

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