Application of RF fading simulator in signal fading test

introduction

The key factor that determines the quality of communication between the base station transmitter and the mobile receiver is the propagation channel of the signal. During the propagation of the signal in the air, fading occurs. This means that obstacles such as buildings, hills or trees may absorb or reflect the signal, which has a significant impact on its amplitude and phase. Due to reflections, diffraction and local scattering, multiple signal transmission paths may be formed between the base station and the receiver (see Figure 1). This so-called multipath propagation phenomenon causes the receiver to receive different copies of the same signal, each of which has a different transmission path length, arrives at the receiver at a different time, and has different amplitudes and phases. For mobile receivers, there may be additional challenges, such as maximum and minimum signal strength and Doppler shift.

Figure 1 Signal fading during transmission

It is well known that wireless devices such as mobile phones should be tested under realistic conditions to ensure their performance. For this purpose, the International Telecommunication Union (ITU) defines fading to simulate various propagation conditions and certain specific reception conditions. However, fading is not limited to mobile wireless networks. Another key application area for fading is modern military communication systems based on software-defined radio (SDR) technology. They use complex waveforms with very tight timing requirements and extremely short synchronization sequences. Airborne radios in particular have to face some extreme conditions. Long distances result in considerable delays in the transmitted information. Radio waves travel at the speed of light, and each 300 km of distance between wireless communication devices results in a time delay of about 1 ms. Therefore, wireless device manufacturers must verify the performance of frequency-hopping wireless systems under worst-case conditions in order to optimize their designs and to verify the compliance of the radio devices with technical specifications by test laboratories.

Using a fading simulator, the actual performance of a receiver can be tested in a repeatable way, before expensive field tests, i.e. during R&D and acceptance tests. Rohde & Schwarz offers a universal test solution for digital mobile radio receivers and frequency hopping radio systems. This article describes how to use the R&S SMU200A vector signal generator with fading option and the R&S FSG (or R&S FSQ) signal analyzer to quickly and cost-effectively build real-world test scenarios.

2 Common methods of fading simulation

There are several ways to implement fading simulation. The best method is usually to generate fading in the digital baseband section of the signal generator used to test the receiver. This method is very widely used, low cost, and efficient, and can ensure the best test performance and repeatability of signal quality. Another method is to implement fading simulation on an RF input/RF output basis. This method is more expensive to implement fading simulation. In addition, in this method, the signal must be converted to an IF and baseband signal, and converted back to the original signal, which may cause the signal quality to deteriorate.

In some applications, RF fading techniques have to be used because baseband signals are not available. For example, fading tests of actual transmission performance of mobile wireless base stations with signaling functions require the use of RF fading simulators. The same is true for military wireless data with frequency hopping functions. And now real TV signals, even simple FM signals, must also be tested under fading conditions.

For testing of mobile wireless devices, the International Telecommunication Union has developed fading specifications, such as channel models for GSM and UMTS/WCDMA standards. GSM defines three propagation models, namely the typical urban model, the mountain model and the rural area model. Based on the three channel models of the International Telecommunication Union, the UMTS/WCDMA channel model is derived from three models, namely the indoor model, the pedestrian model and the vehicle model. All these channel models simulate the propagation conditions in different environments by modeling the expected impact of the environment. The ITU channel model is based on the tapped delay line channel model and varies with, for example, the number and distribution of fading paths and the delay spread of the channel. In addition to the actual fading curves, the relative movement between the receiver and the transmitter is simulated in the form of Doppler frequency shifts.

When the receiver or any reflective object in the receiver environment is moving, the relative speed of the receiver will cause the transmitted signals of each signal path to shift in frequency. Signals on different paths will have different degrees of Doppler frequency shift, corresponding to different phase change speeds.

3. Build a RF fading simulator

An RF fading simulator can be easily built using a signal analyzer with a digital baseband interface (such as the R&S FSQ as a downconverter) and an RF vector signal source with digital baseband input and fading options (such as the R&S SMU200A with appropriate options). The versatile baseband fading and Gaussian white noise functions both act on baseband signals. If the laboratory already has a suitable signal generator and signal analyzer, this solution is more economical and efficient than purchasing a separate RF fading simulator.

The RF signal to be attenuated is connected to the signal analyzer from its RF input. The signal analyzer acts as a downconverter and obtains an intermediate frequency digital signal through analog-to-digital conversion. A real-time bandwidth of up to 28 MHz can be obtained using the R&S FSQ. The digital baseband interface of the signal analyzer transmits a continuous digital data stream that is consistent with the digital baseband input of the signal source. The digital I/Q input of the signal generator is fed into the signal source via a voltage differential signal (LVDS) cable (see Figure 2). This cable is available as an option.

Figure 2. RF fading simulator with a real-time bandwidth of 28 MHz built using R&S FSQ and R&S SMU200A.

According to this method, the signal source sends an RF signal through the RF output port, and the level, modulation mode and frequency of the signal are the same as the RF input signal fed into the signal analyzer. This signal source has a variety of baseband fading functions, including Gaussian noise (AWGN) that can act on the baseband signal before upconversion to RF. The combination of these two instruments can form an RF fading simulator with a real-time bandwidth of up to 28MHz and an RF frequency of up to 6GHz, covering all current digital radio standards including uplink and downlink signals. [page]

(1) Fading test of mobile wireless receiver

The most important fading test for digital mobile radio receivers is to ensure that good communication is maintained between the base station and the mobile receiver even under the most adverse conditions.

Figure 3 shows how a test engineer can perform a fading test on a mobile wireless receiver. The RF signal of the base station is fed to the RF input of the signal analyzer via a power attenuator. The test engineer connects the digital baseband output of the signal analyzer to the digital baseband input of the signal generator. After the signal analyzer and the signal generator form a fading simulator, the output signal of the signal generator must be fed to the input of the mobile wireless receiver at the required level. Depending on the mobile wireless standard used for the test (GSM, 3GPP, LTE, etc.), the user can perform various fading scenarios in the R&S SMU200A.

Figure 3. Test setup for fading testing of a mobile wireless receiver using base station signals.

(2) Fading test of military airborne transceivers

Modern military communication systems based on software-defined radio technology use complex waveforms with very short synchronization sequences. In addition, wideband fast frequency hopping methods are used as electronic protection measures. Such frequency hopping sequences cover a frequency bandwidth of more than 100MHz and the hopping rate is as high as several thousand hops per second. Before secure communication can be carried out, all radio systems involved must be synchronized to a master clock. After that, each radio device relies on a separate internal system clock to execute the same frequency hopping pattern defined by the master.

The synchronization window for establishing a connection between two radios is extremely short. Time delays and the accuracy of the individual system clocks are critical. These system clocks must be frequently resynchronized with the host. However, the radios must also be able to handle time delays and signal characteristics caused by any frequency hopping patterns.

Aircraft radios in particular have to deal with some extreme conditions. When the distance between the radios is too long, signal delays can sometimes be several milliseconds. In the worst case, a communication link can simply not be established. In addition, the Doppler shift caused by the supersonic speed of the aircraft can cause problems for receiving the signal.

Radio equipment manufacturers must verify the performance of frequency hopping radio systems under worst-case conditions to optimize their designs and verify that the radios conform to system specifications. Typically, test labs rent helicopters, airports, antennas, and personnel to perform "real-world" testing. This testing method is extremely costly, time-consuming, and a large number of known and unknown error sources, such as antenna distribution and other parameters, can affect or even severely degrade the test results of this traditional method.

Figure 4 shows a test platform for a military fast frequency hopping airborne transceiver. The fading module of the signal source can be set to introduce a signal delay of milliseconds to the transmitted signal. This signal delay can be used to test the synchronization function between the receiver (lower device) and the transmitter (upper device). In practice, this type of delay may occur when two communicating aircraft are hundreds of kilometers apart. During the test, the reference transceiver sends an RF signal to the R&S FSQ signal analyzer, which downconverts the signal to baseband. The resulting digital I/Q stream is transmitted in real time to the R&S SMU200A vector signal source. The fading option inside the signal source applies preset delays, fading and multi-spectral speed scenarios to the signal to simulate real-life environments such as large speed differences of aircraft. The test signal is upconverted to RF frequency and transmitted to the transceiver under test to demodulate the signal content. Using an oscilloscope to compare the synchronization signal from the transceiver can be used to test whether correct synchronization has been achieved.

Figure 4 Fading test of airborne wireless transceiver. It can simulate the transmission distance of hundreds of kilometers and the high-speed change conditions.

Synchronous transceiver testing With the fading option of the R&S SMU200A, various "environmental" scenarios can be used to quickly detect the performance limitations of the transceiver under test.

The R&S FSQ supports real-time signal flow analysis bandwidths up to 28 MHz. Frequency hopping signals exceeding this bandwidth can be measured using methods provided by wireless device manufacturers to reduce the frequency hopping bandwidth.

This test platform eliminates unknown error sources, allowing manufacturers to optimize the design of their wireless devices and test labs, military radio users or system integrators to verify compliance with international standards and vendor radio specifications under real-world conditions.

4 Conclusion

A universal RF fading simulator can be easily constructed using a signal analyzer with a digital baseband interface (used as a downconverter) and a vector signal source with digital baseband input and fading options. The vector signal source's universal baseband fading capabilities, including Gaussian white noise, can be used to process RF signals for different applications. For engineers who already have a suitable signal analyzer and vector signal source available, the above test platform is an extremely efficient solution compared to purchasing a separate fading simulator.