Analyzing mobile phone wireless tests based on spectrum analyzer

This article will propose corresponding solutions to the problems encountered in mobile phone wireless communication. With the issuance of 3G licenses and operating licenses by the state, China has entered the 3G era. Faced with these new opportunities and challenges, both communication operators and mobile phone manufacturers have begun a new round of fierce competition. When mobile phones are communicating, there are problems such as frequency band control, communication quality detection and signal size control. The spectrum analyzer, which is called "RF multimeter" by RF engineers, can help solve these problems with its absolute advantage in spectrum analysis.

Question 1: Each communication operator must control its own communication frequency band

The ITU has strictly defined the frequency bands for communications. Industrial and scientific communications, fixed and mobile services, satellite communications and other communication methods must be carried out in their respective frequency bands. Even in the same frequency band, the communications of various services have strict definitions. If the frequency band bandwidth of a communication exceeds its own allocated range, it will not only interfere with other communications but also affect its own communication energy. Therefore, it is necessary to test the communication bandwidth and signal energy concentration.

Problem 2: Need to ensure the quality of communication signals

When mobile phones communicate wirelessly, various noises will be mixed in the channel, and the noise will cause the quality of the communication signal to deteriorate, so it is necessary to measure the signal-to-noise ratio of the communication channel.

Question 3: There are breathing effect and near-far effect in mobile phone wireless communication systems

In wireless communication systems, when the interference signal in a cell is very strong, the actual effective coverage area of ​​the base station will be reduced; when the interference signal in a cell is very weak, the actual effective coverage area of ​​the base station will become larger. This is the breathing effect. In short, the breathing effect is manifested as the coverage radius shrinking as the number of users increases. Since mobile phone users are randomly distributed in a cell and are constantly changing, the same mobile phone user may sometimes be at the edge of the cell and sometimes close to the base station. If the transmission power of the mobile phone is designed according to the maximum communication distance, when the mobile phone is close to the base station, there must be excess power and harmful electromagnetic radiation will be formed. This is the near-far effect. The solution to this problem is to adjust the transmission power of the mobile phone in real time according to the different communication distances, that is, power control. This requires real-time monitoring of the size of the communication signal.

Spectrum analyzers can solve the above three problems very well. The following uses the DSA1030A spectrum analyzer launched by RIGOL as an example to introduce in detail how to conduct the test. The frequency range of the DSA1030A spectrum analyzer is 9kHz~3GHz, the displayed average noise level DANL is -148dBm, the typical phase noise value is -88dBc/Hz @(offset 10kHz), and the full amplitude accuracy is less than 1.0dB, which can help solve the problems encountered in mobile phone applications. At the same time, the DSA1030A spectrum analyzer also has a wealth of one-button measurement functions to cope with various complex needs. The following takes the test of a TD-SCDMA signal as an example to illustrate the role of the spectrum analyzer.

For problems one and two, the ACP adjacent channel power measurement and OBW occupied bandwidth measurement in the DSA1030A spectrum analyzer can be used to solve them. The leading channel power measurement can measure the power of the main channel, the power of the previous channel and the next channel, and the signal-to-noise ratio between the main channel and the upper and lower adjacent channels. The measurement results are shown in the lower left of Figure 1. Users can flexibly set the main channel bandwidth, adjacent channel bandwidth, and the spacing between the main channel and the adjacent channel according to specific needs. The measurement setting value is shown in the lower right of Figure 1. By using the leading channel power measurement function, the energy size of the channel communication and the signal-to-noise ratio of the communication can be clearly measured. From Figure 1, it can be concluded that the main channel energy is -10.69dBm and is basically concentrated in the bandwidth of 1.6MHz, which meets the requirement that the bandwidth of each carrier of the TD-SCDMA signal is 1.6MHz. The adjacent channel suppression is -51.32dB and -51.66dB, which can meet general needs.

Figure 1. Using ACP to measure signal energy and signal-to-noise ratio

The OBW occupied bandwidth function can detect whether the energy of the part you are concerned about is within a specific bandwidth. By setting the power ratio to 99%, it is measured that the signal contains 99% of the energy within the bandwidth of 1.396666MHz.

Problem 3 can be solved by using the Pass/Fail function in the DSA1030A spectrum analyzer. Users can edit two standard traces, the upper and lower limits of the measurement, in advance according to actual needs. After turning on this function, the pass rate of the measurement results will be statistically analyzed. When the measurement fails, the measurement will be automatically stopped to check the frequency and amplitude of the measurement failure. As shown in Figure 3, the Pass/Fail function is used to monitor the size of the signal.

Figure 3 Using the Pass/Fail function to monitor signal size

For the above measurements, DSA1030A also provides humanized operation for saving measurement results and loading measurement settings. Users can save the measurement settings locally and load them directly the next time they measure. This can reduce operation time, reduce the probability of operation errors, and improve measurement efficiency. The measurement settings can also be saved to a USB flash drive and loaded into other spectrum analyzers to facilitate measurement migration. Users can edit the measurement settings in advance in the laboratory and save them to a USB flash drive and then directly migrate them to the instrument at the measurement site. The measurement results can also be stored from the instrument to the computer via a USB flash drive for later analysis or as report data.

Figure 4 Save and load Pass/Fail settings

The spectrum analyzer can conveniently and quickly manage and monitor the mobile phone wireless communication frequency band, monitor the signal strength in each channel during communication, and detect the communication process signal.