With the rapid development of wireless communication technologies, the number of devices operating in the 2.4 GHz ISM band has grown rapidly, making this frequency band increasingly crowded and inter-device interference becoming more and more serious. Therefore, various manufacturers are competing to develop wireless communications that operate in higher frequency bands. In order to comply with this demand, various governments have gradually opened the 5 GHz unlicensed band.
However, 5.25 to 5.35 GHz and 5.47 to 5.725 GHz are the working frequency bands of the global radar system. To avoid interference of radar systems by wireless communication devices operating in the 5 GHz band, the requirements of these devices in various countries are in addition to conventional items such as power and spectrum. In addition, the DFS (Dynamic Frequency Selection) feature has been added in particular.
Currently, DFS is mandatory in North America, Europe, Canada, Australia, Japan, and South Korea. 5 GHz devices (such as 802.11a and 802.11n devices) that cannot pass the DFS test will not be allowed to enter the market for sale. The article will discuss the requirements for DFS testing in North America and Europe, and provide system solutions for DFS testing.
1 , DFS test requirements
Fundamentally, DFS is a channel allocation scheme. The device dynamically selects or changes the operating frequency through the DFS function to avoid interference with other systems, especially radar systems, or to avoid interference with other systems.
FCC Part 15 Subpart E states that U-NII (Unlicensed National Information Infrastructure) devices operating in 5.25-5.35 GHz and 5.47-5.725 GHz should have a DFS radar detection mechanism. The ETSI EN 301 893 standard also makes the same requirements for devices operating in this band. In addition, Japan, South Korea, Canada, Australia, and Asia have also established corresponding standards and specifications for DFS. Because there are few types of wireless communication devices working in the 5 GHz frequency band, currently, the objects of DFS testing are mainly Wi-Fi devices operating in the 5 GHz band such as 802.11a and 802.11n. At the request of the Wi-Fi Alliance, the FCC has also issued a memorandum FCC 06-96, which details the testing of DFS.
Table 1 List of DFS Test Items in FCC and ETSI Standards
The test content of each test item is described as follows:
(1) Uniform distribution of working channels: UUT (Unit Under Test) is required to select working channels with equal probability in the 5.25 to 2.35 GHz and 5.47 to 5.725 GHz frequency bands.
(2) Radar Signal Detection Bandwidth: Starting from the center frequency of the UUT working channel, increase (or decrease) the radar signal frequency in 1 MHz steps, observe the UUT's response until the UUT can not detect the radar signal to obtain the UUT can detect The frequency range of the radar signal.
(3) Initial channel availability detection time: After the UUT completes the power-on operation, the UUT shall detect the current working channel for 1 minute to observe the UUT's channel detection mechanism. The UUT shall start transmitting RF signals after completing the 1-minute channel detection.
(4) Channel availability detection: After the UUT completes the power-on operation, the current working channel will be detected for one minute. This test item requires that the radar signal be transmitted on the UUT working channel at the first and last moment of the 1-minute channel detection time due to the radar signal. When the UUT completes the 1-minute test, it should not transmit RF signals on the channel.
(5) Channel migration time: When the UUT and its associated equipment perform service transmission, a radar signal is sent on the UUT working channel, and the timing is started when the radar signal stops transmitting. UUT (if the UUT is the master device, all the slaves related to it are included. The device) shall stop service transmission on the channel within a limited time.
(6) Channel stop transmission time: When the UUT and its associated equipment perform service transmission, a radar signal is transmitted on the UUT working channel. Since the radar signal stops transmitting, the sum of the transmission times on the channel cannot exceed the limit.
(7) Channel non-occupancy period: The time limit for service transmission on the channel after the UUT detects the radar signal on a certain channel.
(8) Detection Threshold Statistical performance test: The specified radar signal is sent at the specified level value in the UUT working channel. This test needs to be repeated multiple times under the same conditions to count the probability of the radar signal that the UUT can detect.
It can be seen from Table 1 that ETSI's DFS test project is a subset of the FCC, so is it not necessary for the product to pass the FCC test as long as it does not have to be tested according to the ETSI standard? What is the difference between the FCC and ETSI test? Where is it?
For the first question, the answer is no. The biggest difference between FCC and ETSI's DFS test is reflected in the selection of radar signals. Table 2 and Table 3 give the FCC and ETSI requirements for radar pulse signals.
Table 2 Radar signal requirements for FCC specification DFS test
Table 3 Radar Signal Requirements for ETSI Standard DFS Test
2 , DFS test solution
Due to the late start of the DFS test, there is currently no unified test program in the world. The paper will combine the DFC test requirements of the FCC and ETSI to provide a set of DFS test system solutions that can be used to implement the DFS automatic test.
Although the FCC and ETSI requirements for radar signals are different, the test methods for the same test items are the same. Therefore, this section will focus on the FCC test requirements to elaborate the structure and working principle of the DFS test system. DFS test system connection block diagram shown in Figure 1 and Figure 2.
Fig. 1 DFS test block diagram of main equipment/slave with DFS function
Figure 2 Slave device DFS test block diagram
The type of device under test in the DFS test is divided into three categories. Different types of UUTs use different test connection schemes. The master device (Mas-ter) uses the system solution shown in Fig. 1. During the test, it sends a radar signal to the UUT. The test result shows the time domain waveform changes of the UOT before and after the radar signal appears. For the client without DFS function, the system solution shown in Fig. 2 can be used. During the test, the radar signal is sent to the master device associated with the UUT, and after the master device responds to the radar signal Observe the behavior of the UUT. The test results show that the signal waveforms in the entire channel change before and after the radar signal appears. For a DFS-enabled slave device, select either Figure 1 or Figure 2 to test the system connection, depending on the test project.
No matter what kind of test connection method is used in the test, the role of each device in the block diagram does not change. The role of the vector signal analyzer includes two aspects. One is to verify various radar signals during the stage of radar signal generation during debugging. In particular, whether the frequency domain and time domain waveforms of the Chirp radar signal meet the test requirements; on the other hand, Used to display and record test results. The instrument uses R&S FSQ26.
For radar signal generators, although there are many devices on the market capable of generating pulse signals, due to the special radar pulse signals used in the test, various parameters of the signals need to be randomly selected, and the parameters of the radar signals change very quickly (micro Second-second), but also control the number of pulse signals, at present, no pulse generator can directly generate the signal required for DFS test. The system selected R&S pulse signal generator SMF100A, through its programming control, to generate the radar signal required for testing.
The main control PC is equipped with test software that can control the radar signal generator to send radar signals at a specified time as required, and at the same time control the signal analyzer to correctly record the test results.
The role of the other two control/display computers in the figure is to transmit the specified video service flow with the UUT, monitor the transmission of the video service flow, and track the working status of the device under test in real time. Due to the high time accuracy required by this test, the AO clock can provide a good reference time signal for signal sources and spectrum analyzers to improve test accuracy.
During the test, the output power of the UUT is set to the maximum value, which is usually greater than 20 dBm, while the intensity of the radar interference signal is lower than -60 dBm. To be able to accurately distinguish the radar signal from the UUT signal in the test result graph, The spectrum analyzer, radar signal generator, and device under test are simply connected together for testing. The test system will adjust the attenuation values ​​of each attenuator to make the relationship of the strength of each signal displayed on the spectrum analyzer as: The UUT signal is secondarily followed by the lowest signal strength of the device associated with the UUT.
3 , the current status of DFS testing
The European ETSI EN 301893 standard began with the V1.2.3 version in August 2003. It added the test requirements for the DFS function and started the DFS test in 2004. Japan and Korea in Asia also started DFS in 2005. Tests; The US FCC announced in February 2006 that any U-NII certified device in the 5 GHz band that was applied after July 20, 2006 must meet the DFS requirements specified in FCC Part 15.407; in addition, the Australian standard AS/ NZS4268:2003+A1:2004 also required DFS, while Canada has already proposed DFS requirements in RSS 210, but the specific test methods and types of radar signals used for testing are still being further developed.
4 Conclusion
From the testing requirements of DFS in various countries, it seems that the wireless communication equipment working in the 5 GHz band in the future needs DFS function as long as its working channel falls into the working frequency band of the radar system. At present, the testing of DFS is still at the initial stage in China. People still lack a deeper understanding of DFS' testing content and testing methods. The article deeply analyzed the test requirements and test methods of DFS, and provided the DFS system solution, which has important reference value for the DFS certification testing of 5 GHz band equipment.
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