Wireless network WiMax RF test application

　　WiMAX technology must demonstrate its advantages in specific application scenarios to gain market recognition, which requires application testing to measure system performance parameters. WiMAX testing methods are divided into three parts: protocol analysis, wireless RF analysis, and transmission performance analysis. The comprehensive test results are obtained based on protocol analysis, wireless RF analysis, and transmission performance analysis.

　　WiMAX reception testing

　　When testing WiMAX amplifiers and modules, an ideal test signal needs to be input; when testing the performance of BS (base station), RS (relay station) or SS (terminal) receivers, a test signal transmitted through a spatial channel needs to be input.

　　Digital vector signal source SMU/SMJ/SMATE can generate WiMAX signals that meet specifications or are user-defined, including complete wireless frame settings, MAC (media access layer) settings, channel coding, etc.

　　Wireless frame settings

　　OFDM mode

　　FIG1 is a frame structure of OFDM TDD mode.

　　

　　Figure 1 OFDM frame structure

　　The downlink subframe consists of three parts: Preamble, FCH (frame control header) and downlink data burst.

　　Preamble is located at the beginning of the uplink and downlink subframes and is used for synchronization between transceivers and channel estimation. In terms of symbol structure, it is divided into long preamble and short preamble: long preamble is used for downlink subframes and consists of two symbols, where the first symbol appears once every four subcarriers and the second symbol appears once every two subcarriers. Short preamble is used for uplink subframes and consists of one symbol, which appears once every two subcarriers. If the downlink subframe transmits multiple data bursts, the midamble between each burst is also a short preamble.

　　FCH (Frame control header) is located after Long Preamble, consists of one symbol, contains some system information such as base station ID and DL data burst attributes, and is used for receiver demodulation. DL Burst contains MAC PDU (Protocol Data Unit) and some broadcast information such as DL-MAP, UL-MAP, DCD (Downlink Channel Description), UCD (Uplink Channel Description). A complete PDU should consist of a 48-bit MAC Header, Payload (data segment) and cyclic redundancy check CRC.

　　In addition to the Preamble and UL PDU, the uplink subframe also includes the ranging part. The Ranging process is that the SS sends a request to the BS to adjust the transmit power, delay and frequency deviation.

　　OFDMA mode

　　FIG2 is a frame structure of the OFDMA mode.

　　

　　Figure 2 OFDMA mode frame structure

　　Due to the introduction of logical subchannel-based Access, OFDMA's wireless frame structure is more complicated. Figure 2 shows the frame structure plane composed of symbol number and subchannel number. Preamble, FCH, broadcast information and data burst are all distributed on this plane. This plane consists of Zone and Segment, which are distinguished from each other by symbol offset and subchannel offset.

　　The use of subchannels is divided into PUSC and FUSC, that is, partial use of subchannels and full use of subchannels. The subchannels are divided into six groups, and their number is determined by the FFT Size. FFT Size 2048/1024/512/128 corresponds to 60/30/15/3 subchannels respectively.

　　RS signal source SMU currently supports automatic generation or custom settings of Preamble, FCH, DL-map, UL-map, ranging, MAC PDU (MAC Header; Payload; CRC). For OFDMA (WiBro) mode, it can support configurations of up to 8 Zones and 3 segments.

　　WiMAX signal generation application

　　Preset frame structure

　　The 802.16 test specification does not define a test model similar to 3GPP, but only provides some test messages for receiver sensitivity testing. Three messages of different lengths (288/864/1536 bits) are pre-set in the SMU, and each message provides different modulation methods and coding rates. This application can generate WiMAX signals quickly and easily. [page]

　　Uplink and downlink signals are transmitted synchronously

　　In some base stations, repeaters, modules and other test environments, WiMAX signals are often required to include both uplink and downlink parts to simulate mutual interference. The SMU with two built-in signal paths provides this function. TDD mode: Trigger baseband unit B through baseband unit A, and superimpose in the baseband part, and then adjust the trigger delay between the two, so that a TDD signal containing complete uplink and downlink data can be output with one RF channel. FDD mode: If the uplink and downlink signal carrier frequency interval does not exceed +/-40MHz, the above baseband superposition function can be used, and then the corresponding frequency deviation can be set; if the carrier frequency interval is large, the uplink and downlink signals that are triggered synchronously can be output through two RF channels.

　　Fading simulation application

　　Transmission between transceivers is often carried out in a spatial channel, which not only has line-of-sight propagation, but also includes reflection and refraction caused by environmental influences, as well as Doppler frequency shift caused by movement, etc. SMU provides fading simulators with up to 40 paths, which can simulate a variety of fading properties and dynamic fading environments.

　　The WiMAX specification has not yet provided a standard fading model. Currently, the models provided by the 3GPP specification or SUI1-6 (Stanford University Interim) are generally used for testing. The technical working group of the WiMAX Forum is also discussing whether to derive WiMAX test standards based on these models.

　　WiMAX Transmit Test

　　Power Measurement

　　Power meter test: NRP provides three methods to measure WiMAX signal power

　　Duty cycle: If the frame period and burst length, i.e., duty cycle, are known, this mode can be used to test the average burst power.

　　Scope Mode: By measuring Power vs. Time and performing threshold scanning, the average Burst power can be obtained.

　　Burst Mode: Use the trigger function of the power sensor to capture the burst and obtain the average burst power.

　　The latter two methods do not need to know the specific frame structure information of the WiMAX signal.

　　Spectrum analyzer test

　　Time domain measurements

　　FIG. 3 shows the measurement of the Preamble power of the WiMAX signal in the time domain. In order to obtain an accurate measurement result, the measurement bandwidth needs to cover the WiMAX signal bandwidth.

　　

　　Figure 3: Measurement of the preamble power of WiMAX signals (time domain)

　　Since the filter SF (shape factor) used in the instrument is different, if an analog intermediate frequency filter is used, its bandwidth must be equal to 5 times the signal bandwidth; if a channel filter is used, its bandwidth must be greater than the signal bandwidth.

　　Frequency domain measurements

　　First, the SF of the WiMAX signal should be as close as possible to the SF of the filter. From this set of data, we can see that 10KHz RBW should be selected.

　　WiMAX

　　B3dB=1.798MHz, B60dB=2.248MHz→SF60/3=1.25

　　RBW filter 10kHz

　　B3dB=9.91kHz, B60dB=53.45kHz→SF60/3=5.39

　　RBW filter 200kHz

　　=196.5kHz, B60dB=1.898MHz→SF60/3=9.66

　　Secondly, we need to select an appropriate scanning time, TSweep = NSweep points · TSignal Cyde. Assuming that the number of scanning points of the spectrum analyzer is 625 and the period of the measured signal is 10ms, the minimum scanning time is 6.25s. If the scanning time is too short, each scanning point cannot cover a complete signal period and cannot reflect its true frequency domain information.