Analysis of measurement techniques and requirements for wireless interference testing

In wireless systems , interference in the radio channel can cause many problems for users, reducing the reception rate of the intended signal. Interference can come from intentional, unintentional or accidental radiators and occur in both licensed and unlicensed spectrum. As radio spectrum resources become increasingly scarce, manufacturers are constantly striving to improve spectrum utilization in order to obtain the highest capacity and performance (for example, sharing or reuse). As a result, wireless communication systems must operate with limited radio interference. However, as the demand for spectrum increases, so does the interference in wireless systems. Therefore, the identification and reduction of interference is extremely important for all wireless systems to work properly. Performing interference testing in a wireless environment is not an easy task, it requires the use of new measurement techniques and places higher demands on existing measurement instruments. Effectively performing interference testing requires the use of advanced measurement tools - such as high-performance spectrum analyzers - to measure, monitor and manage interference between different wireless systems.

Interference classification

There are many different types of interference in wireless communication systems. Interference is generally divided into the following categories:

●In-band interference - refers to invalid signals emitted from various communication systems or unintentional radiators but falling within the specified system operating bandwidth.

● Co -channel interference – Common radio interference caused by other radios operating in the same wireless system.

● Out-of-band interference - comes from wireless systems operating within the specified frequency band, but due to improper filtering, non-linearities and/or leakage, the interference also transmits energy into the frequency bands of other wireless systems.

● Adjacent channel interference - is the result of transmission in a given frequency channel generating invalid energy in other adjacent channels, usually located in the same system.

● Uplink (reverse) link interference – can affect base station receivers and related communications from mobile devices to the base station.

● Downlink interference - typically can damage downlink communications between a base station and a mobile device.

The interference classification of a wireless system has a decisive influence on the engineer's response. For example, out-of-band interference occurs when harmonics from a simple or poorly filtered transmitter enter the higher frequency band. Properly filtering out the transmitter's harmonics ensures that the wireless system does not affect other systems operating in higher frequency bands.

Interference Measurement Technology

When a wireless system is not operating as expected and radio interference is suspected, modern high-performance spectrum analyzers should be used to identify unwanted signals in the operating frequency channel. These tools are ideal for measuring interfering signal power variations over time, frequency, and location. Since interference testing often requires collecting measurements and data on the wireless system environment, we recommend that users use lightweight, battery-powered instruments that can match the performance of traditional benchtop instruments.

The process of identifying an unwanted signal may reveal details about the signal: when it was transmitted, how many times it occurred, the carrier frequency and bandwidth, and even the physical location of the interfering transmitter. If the system is operating in full-duplex mode, it may be necessary to examine both the uplink and downlink frequency channels of the interfering signal.

Interference measurements—especially over-the-air measurements—often use spectrum analyzers with very low noise floors, or DANL. DANL is directly related to setting the resolution bandwidth (RBW) lower, which reduces noise. Reducing the RBW by a factor of 10 reduces the noise floor by 10 dB. The analyzer’s measurement sweep time is an inverse function of the RBW. Therefore, longer sweep times are required to achieve lower RBW settings. Since the ability to quickly measure and display low-level signals is directly related to the signal-to-noise ratio (SNR) of the analyzer’s detector, signal levels can be improved by reducing the amount of input attenuation on the analyzer. Input attenuation as low as 0 dB has the potential to increase the RBW, thereby reducing sweep time. Using an internal or external preamplifier can also improve the measured signal level at the detector.

Special attention should be paid to the analyzer when reducing input attenuation and measuring large amplitude signals. Large amplitude signals can overdrive the analyzer front end, causing internally generated distortion or instrument damage. The analyzer displays internally generated distortion (which may be from the signal of interest). Under these conditions, the attenuator setting should be optimized for maximum dynamic range.

When measuring pulsed, intermittent, or frequency-hopping interference, the spectrum analyzer display can be configured in a variety of ways to aid in the detection and identification of such signals. In MaxHold mode, the spectrum analyzer display can save and display the maximum trace value over multiple sweeps (Figure 2). This mode is useful when only the maximum amplitude of an intermittent signal is required. If it is necessary to observe how the signal changes over time, the spectrogram or concatenated display modes can provide a deeper analysis of the intermittent signal structure. [page]

Figure. Measurement results of a frequency hopping signal shown in FieldFox's standard Clear/Write (blue trace) and MaxHold (yellow trace) modes. During the measurement, while the two signals in the frequency domain eventually collide, the signal on the left is stationary and represents the source of the interference from the frequency hopping signal.

The spectrogram is a unique way to examine frequency, time and amplitude on the same display. It shows the spectrum over time, with the color scale mapped to the signal amplitude. The cascaded display shows the amplitude level over frequency and time with three-dimensional color coding.

Zero span mode and swept acquisition may be used to measure intermittent interference. In zero span mode, the center frequency of the spectrum analyzer is tuned to a fixed frequency and swept in the time domain. The RBW filter is adjusted to have sufficient bandwidth to capture as much signal bandwidth as possible without increasing the measurement noise floor to unacceptable levels. Swept acquisition can capture low duty cycle pulses or intermittent signals by capturing all the time domain data at once. By setting the appropriate RBW, attenuation, and turning on the preamplifier, difficult to detect interference signals can be captured.

Understand equipment requirements

When conducting field interference testing, several key features of a spectrum analyzer must be considered, including portability and ruggedness. Field testing also places other demands on frequency deviation analyzers: long battery life, quick and easy battery replacement, fast power-up from suspended state, built-in GPS, DC block, and DC voltage source. The DC voltage source is ideal for powering low noise downconverters (LNBs) in satellite applications when used with an external bias tee.

In addition to a high-performance spectrum analyzer, use quality test cables to establish connections between the analyzer and the system test ports or test antennas. Proper maintenance of the cables—protecting and cleaning the connectors on both the analyzer and the cable—is critical to making accurate, repeatable measurements.

The test antenna is another important part of the interference test components. It should be designed to cover the frequency range of interest while being lightweight and portable. Ideally, its characteristics should be similar to the test antenna used for the wireless system under investigation. If the system antenna is a low-gain omnidirectional antenna with vertical polarization, then the antenna that comes with the spectrum analyzer should be the same (Figure 3). When monitoring spectrum over a wide frequency range, a narrowband system antenna can be used instead of a broadband whip antenna. When measuring extremely weak signals or finding the direction of unauthorized transmitters, a high-gain directional antenna should be connected to the analyzer.

Figure. Comparison of the received signal from an omnidirectional antenna (blue trace) and a high-gain antenna (yellow trace) performed over-the-air with FieldFox. The measured amplitude of the unknown signal increases significantly when using the high-gain antenna, but this measurement requires the antenna to be pointed in the direction of the highest signal amplitude.

Summarize

As the demand for spectrum continues to grow, wireless interference is becoming a growing problem. In the best case, interference affects only a small number of users, while in the worst case, it can disrupt communications across the entire wireless system. This has prompted engineers to effectively test for radio interference. Modern high-performance spectrum analyzers play a key role here. Choose a spectrum analyzer that meets the core requirements of field testing and uses it with a variety of measurement techniques to ensure that wireless systems are protected from the adverse effects of interference.