Radio Frequency Testing

WiFi (Wireless Fidelity) phone is an emerging VOIP phone based on WiFi technology. Using this kind of phone, the analog voice signal can be converted into the form of data packets, which can be connected to the Internet based on IP protocol through the hotspot (AccessPoint) for transmission, so as to make and receive calls. Compared with traditional fixed-line calls, the tariff of WiFi calls is very cheap, but the premise of using WiFi calls is that calls can only be made within the coverage of hotspots. But hotspots tend to have very limited coverage.

There are two ways to increase the coverage of hotspots: one is to increase the hotspot’s transmitting power and improve the phone’s receiving sensitivity, but the transmitting power of the hotspot cannot be increased without limit, so at the same time, the receiving sensitivity of the phone must be increased to increase the hotspot coverage. According to the free space transmission loss formula: L (dB) = 32.4 + 20 × lgd (km) + 20 × lgf (MHz).

RF reception index and test process

01. Definition of radio frequency receiving index
According to the IEEE802.11b specification, there are three key RF receiving indicators defined as follows:
1) The minimum input level sensitivity of the receiver For the input level of -76dBm measured on the antenna connector, if the length of the PSDU is 1024 bytes, the bit error rate (FER) should be less than 8%;
2) The maximum input level of the receiver For the maximum input level of -10dBm measured on the receiving antenna, if the PSDU length is 1024 bytes, the maximum bit error rate (FER) should be 8%;
3) Receiver adjacent channel suppression The interval of receiver adjacent channel suppression in each channel group is not less than the ratio of adjacent channel interference signal power to useful signal power between any two channels of 25MHz. For a PSDU with a FER value of 8% and a length of 1024 bytes modulated by 11Mbit/sCCK, the adjacent channel suppression must not be less than 35dB.

02. Bit error rate

In the definition of the above three indicators, a very important parameter is mentioned: the bit error rate, that is, the ratio of the number of codes lost and erroneous during transmission to the total number of codes sent. Only by obtaining the correct bit error rate can the above three receiving performance indicators be accurately tested. The receiving performance test platform built in the laboratory is shown in Figure 1

On the test platform in Figure 1, the PC provides the I/Q signal waveform file in a certain code format for the signal source, and the signal source sends out a certain number of codes. At the same time, under the control of the PC, the DUT receives and demodulates these codes to obtain the corresponding bit error rate.

Then adjust the transmit power of the signal source according to the bit error rate until the bit error rate just meets the index requirements, and then the corresponding receiving performance index of the DUT can be obtained. But on this platform, there are two difficulties in obtaining the correct bit error rate:
1) The code format sent by the signal source must meet the requirements of the DUT. Different chip suppliers have different code-to-code format requirements. If the chip-to-code format requirements cannot be met, the DUT will not be able to correctly count the correct number of codes received, resulting in incorrect calculation of the bit error rate;
2) The signal source must be able to ensure a certain number of codes. If the total number of codes sent by the signal source cannot be determined, the bit error rate cannot be calculated.
(Here, the WIFI base station equipment CMW500 is used for packet sending test)