Fading performance testing of receivers in mobile devices

1. Decline

In terms of scope, fading can be roughly divided into three types: large area fading, small area fading and indoor fading. As shown in Figure 1, large area fading refers to the average attenuation of signals when they propagate over long distances (more than several hundred wavelengths), mainly caused by atmospheric loss, buildings or large objects such as hills and trees.

Figure 1 Large-area fading

Figure 2 Small area fading

Figure 3 Indoor fading

Small area fading is mainly caused by multipath propagation and Doppler frequency shift (as shown in Figure 2). The signal sent by the base station may reach the receiver through direct radiation, reflection or scattering. The strength and delay of the signal on different paths will be different. In addition, since the receiver is moving, it will constantly pass through areas with high or low signal intensity. Another effect is that the high-speed movement of the receiver will cause relative frequency shift. If it moves toward the transmitter, the frequency will increase, and away from the transmitter, the frequency will decrease.

The indoor environment is similar to the small area fading situation (as shown in Figure 3). The signal will be affected by reflection, refraction, and scattering during the propagation process. For example, in an office environment, various metal equipment and furniture will produce multipath effects on the signal.

The fading environment will have a great impact on the communication quality of the wireless communication system. The fading in a large area is mainly manifested in the path loss caused by distance, which will lead to a decrease in the system signal-to-noise ratio. The multipath effect and Doppler effect in a small area will cause inter-code interference and also cause synchronization and phase-locking problems.

2. General Methods for Testing Anti-Fading Performance of Wireless Devices

Since fading has a great impact on communications, various measures must be taken to ensure sufficient margin to prevent fading problems when designing mobile receivers. Therefore, during the development and production stages of mobile devices, devices are tested in a simulated fading environment.

The test standards for various mobile terminals have clear provisions for testing in fading environments, as shown in the following table. W-CDMA specifies the following tests in the 3GPP standard TS 34.121 V3.12.0:

7.3 Demodulation in multi-path fading, case 1, 2, 3, 4, 6

7.4 Demodulation in moving propagation

7.5 Demodulation in birth/death propagation

7.6 Demodulation of DCH in downlink

CDMA2000 specifies the following tests in 3GPP standard C.S0011B:

3.3.4 Demodulation performance in multipath fading

3.4.2 Demodulation performance in multipath fading

3.4.7 Demodulation performance in multipath fading with

closed loop power control (FPC_MODE = \'000\')

3.4.8 Demodulation performance in multipath fading with

closed loop power control (FPC_MODE = \'010\')[page]

3.4.9 Demodulation performance in multipath fading with

closed loop power control (FPC_MODE = \'000\', \'001\', and \'010\')

3.4.10 Demodulation performance in multipath fading with

closed loop power control (FPC_MODE = \'000\') and transmit diversity

3.4.11 Demodulation performance in multipath fading with

closed loop power control (FPC_MODE = \'010\') and transmit diversity

Figure 4 Common methods for fading environment simulation

In many current test systems, most of them adopt the method of inserting fading simulation on the RF link (as shown in Figure 4).

This method first down-converts the signal to be faded and then digitizes it. The digitized signal is added to the fading simulation model, and then the digital-to-analog conversion is performed and up-converted into an RF signal. Finally, the corresponding noise signal is added. Here, the noise signal must be added separately from the fading model, because the noise (AWGN) is independent of any multipath channel. Therefore, there are two places that must be paid attention to in this process, one is the conversion loss during analog-to-digital/digital-to-analog conversion, and the other is the calibration of the noise signal amplitude.

Conversion loss occurs every time a signal is sampled from analog to digital, or from digital to analog, which will bring about a certain system error. Only when this system error is reduced to the minimum possible can the accuracy of fading simulation be reliable.

When adding AWGN to a signal, it is critical that the amplitude is accurate, and it must be added after fading to ensure that the background noise is not attenuated by fading. However, adding noise changes the carrier-to-noise ratio (C/N) while also changing the total power amplitude. In order to ensure the noise amplitude is accurate, the carrier power amplitude after fading must be determined, which is time-consuming because the output power must be repeatedly measured to obtain a statistically correct amplitude.

3. Combining baseband simulation with integrated tester to realize CDMA, 1xEVDO, and W-CDMA anti-fading tests

Since the fading system introduced in Figure 4 is relatively complex, the system cost is also relatively high. For many departments that only test the performance of mobile phone receivers in fading environments, the cost burden is relatively heavy. For this reason, Agilent has designed a low-cost and easy-to-use test solution using a low-cost baseband fading simulation card and mobile phone comprehensive tester 8960. The specific solution is shown in Figure 5.

Figure 5 Agilent Technologies baseband fading simulation card and 8960 test solution

Baseband simulation is a new fading simulation method. It consists of the PCI baseband simulation card N5101A installed in the PC and fading simulation software. It completes the simulation of the fading environment in the digital part and calibrates the noise signal. The 8960 can interface with the fading simulation card by installing the 504 option. In this way, the signal that the 8960 wants to send to the mobile phone is first sent to the baseband fading simulation card in the baseband processing part, and after adding the fading environment and noise, it is sent back to the RF link of the 8960. For the mobile phone, the received signal seems to have passed through the fading environment. At the same time, the user can remotely control the 8960 through the PC to complete the automatic test. Agilent's automated test software WTM has defined the fading environment test defined in the standard as a standard test item. If the user runs the WTM test software, he only needs to select the corresponding test item to complete the test conveniently.

Conclusion: Fading will have a great impact on the quality of wireless signals. In order to overcome this impact, the performance of the receiver of the mobile device should be tested in a fading environment. The test solution designed by Agilent, which combines the baseband fading simulator with the mobile phone comprehensive tester, is simple to configure, cheap, and easy to use. It is more suitable for R&D institutions or quality certification departments of various sizes.