Radio Frequency Testing

Currently, in the 802.11 series of standards, there are four standards involving the physical layer: 802.11a, 802.11b, 802.11g, 802.11n, and 802.11ac. According to different physical layer standards, wireless LAN devices are usually classified into different categories, as usual Said 802.11b wireless LAN equipment, 802.11a wireless LAN equipment, etc.

802.11a works in the 5GHz frequency band (U-NII frequency band in the United States: 5.15-5.25GHz, 5.25-5.35GHz, 5.475-5.725GHz, 5.725-5.825GHz), and it uses OFDM (Orthogonal Frequency Division Multiplexing) technology. 802.11a supports data rates up to 54Mbps.

Introduction to WiFi DFS

DFS (Dynamic Frequency Selection) dynamic frequency selection. The 802.11a standard uses the 5GHz frequency. This standard has no problem in the United States, but it has encountered strong resistance in Europe. Because the radar system of the European military widely uses this frequency (the radar that detects stealth aircraft uses this frequency among them). If civilian wireless products also use this frequency, it is likely to interfere with military radar and communications.

In order to solve this security concern, WLAN products sold in Europe must have the two functions of TPS and DFS, that is, transmit power control and dynamic frequency selection. TPS is to prevent wireless products from emitting excessive power to interfere with military radars.

DFS is to enable wireless products to actively detect the frequency used by the military and actively select another frequency to avoid the military frequency. In this way, other WLANs can also be avoided, making the most efficient use of the wavelengths. These two functions are mandatory, and products that do not meet the standards will not receive EU marketing authorization.

Test sample type

Master device: master device, which requires radar pulse detection function. That is to say, it must have the ability to actively detect whether there is radar signal occupation on the working frequency, and actively select another non-occupied frequency to work. For example: routers that include 802.11a, etc.

Slave equipment: Slave equipment, divided into two types, one is the maximum transmission power is less than 200mW, does not require the radar pulse detection function. Another type with a maximum transmission power greater than or equal to 200mW requires a radar pulse detection function. For example: include 802.11a wireless network card, etc.

Testing requirements
CE test compliance standard: EN 301 893

The items to be tested on the Master device are:
》4.6.2.1 Channel Availability Check --- Channel Availability Check
》4.6.2.2 In-Service Monitoring --- Online monitoring
》4.6.2.3 Channel Shutdown ---Channel shutdown
》4.6.2.4 Non-Occupancy Period --- Non-occupancy period
》4.6.2.5 Uniform Spreading --- uniform distribution

Slave equipment needs to test the items:
》4.6.2.2 In-Service Monitoring ---On-line monitoring (no testing is required for those without radar pulse detection function)
》4.6.2.3 Channel Shutdown ---Channel shutdown

For devices with DFS function, if radar pulses can be detected, the data transmission on the current operating frequency will be suspended, and other "empty" frequencies will be searched for retransmission. The frequency of abandoning use will not be used within the next 30 minutes. These actions are automatically completed by the device, and the user will not notice it under normal circumstances.

epilogue
In order to maintain security, European countries have formulated mandatory DFS testing standards, and even if it will cause the 802.11a products produced by manufacturers to fail to enter the market in the EU region, strict review must be implemented. Therefore, when selling 802.11a products overseas, you should pay attention to whether your products meet the strict requirements for DFS set by the European Union and North America.