Tips for Performing Precision Interference Measurements in the Field

Operators of wireless transmitters may occasionally send signals maliciously with the intent to disrupt communications or known broadcast signals without proper licensing. Government agencies will penalize offending operators and sometimes confiscate wireless equipment that violates spectrum allocations. Many government agencies prohibit the intentional or malicious operation of “jammers” whose operations have the potential to interfere with wireless communications services. Regulators will log complaints, use wireless direction-finding equipment to locate the source of interference, and may impose fines and confiscate equipment. To expedite identification and location of jammers, equipment operators often use their own equipment, including spectrum analyzers such as FieldFox, to quickly locate malicious transmissions and expedite elimination of malicious interference through appropriate regulatory channels.

As part of testing system performance and ensuring regulatory compliance, spectrum is continuously monitored for known and unknown signals by commercial and non-commercial organizations in industries such as cellular, radio and television, radar, and satellite. Because wireless systems often share or reuse spectrum, interference from other users can quickly become a problem when system transmitters inappropriately radiate energy into their designated or other frequency bands. In all of these situations where spectrum is continually squeezed for maximum capacity and performance, identifying and mitigating interference is critical to the proper operation of all wireless systems.

Signal Interference Classification

When a wireless system reports adequate received signal strength for a given signal but performance is problematic, it is likely that some form of wireless interference is affecting the operation of the receiver. A spectrum analyzer is an extremely useful tool that measures the amplitude of signals in the frequency range around a given channel to verify whether the performance degradation is caused by interference in the operating channel or in an adjacent channel. The presence of interference in a wireless system can be categorized in a number of ways. Interference may affect only a few users, or it may be transmitted in a manner that disrupts all communications in the entire wireless system. The following table lists the classifications commonly used in the wireless industry.

— In-band interference
— Co-channel interference
— Out-of-
band interference — Adjacent channel interference
— Downlink interference
— Uplink interference

Figure 3 shows an ideal spectrum diagram with multiple signals over a wide frequency range. Using channel 1 as the frequency range for the designated signal, other signals introduced in the frequency domain can degrade the performance of this system. As shown in Figure 3, in-band, out-of-band (including associated harmonics), and adjacent channel interference (represented by the overlap between channel 1 and channel 2) can all affect the system performance on channel 1.

Figure 3. There are multiple signals in the wireless environment, causing different levels of interference in channel 1.

Figure 4. Spectrum measurement results of a 24 GHz microwave communication signal. The system performance is lower than expected, and there may be in-band interference.

What is in-band interference?

In-band interference refers to unwanted signals that fall within the specified system operating bandwidth and are emitted by various communication systems or unintentional radiators. This interference can pass through the receiver's channel filter and, if the interference amplitude is greater than the specified signal amplitude, it will cause damage to the specified signal. If the amplitude of in-band interference is close to or lower than the specified signal, it is very difficult to measure, so the transmitter of the specified signal must be temporarily turned off in order to measure the characteristics of the interference. If the target transmitter cannot be turned off, the spectrum analyzer (including the attached antenna) can be moved around the environment to create appropriate signal conditions so that the amplitude of the interference signal is large enough compared to the specified signal to be viewed and measured using the analyzer.

Figure 4 shows an example of a 24.125 GHz point-to-point microwave communication system being measured with potential in-band interference. The system was reporting lower than expected performance, so we used FieldFox to measure the channel conditions at the receiver. As shown, there is a signal with slightly different amplitude near the center of the band. Troubleshooting this system may require shutting down the main system to view and identify the interference. Another approach is to adjust the pointing of a high-gain antenna to increase the amplitude of the interference being measured so that it can be viewed. A high-gain antenna can also be used to determine the physical location of the interference source—point the antenna into the surrounding environment until an amplitude peak is seen on a handheld spectrum analyzer.

What is co-channel interference?

Co-channel interference creates conditions similar to in-band interference, except that the interference comes from other wireless devices in the same wireless system. For example, when cellular base stations are physically far apart, they will reuse the same frequency channels, but energy from one base station will occasionally enter the adjacent cell area and potentially interfere with communications. Wireless LANs are also subject to co-channel interference, and because unlicensed WLAN radios listen for open channels before transmitting, it is possible for two radios to transmit on the same frequency channel at the same time and cause a collision. As system designers try to design systems that can support a large number of wireless users within a small number of available frequency channels, co-channel interference is one of the most common types of radio interference. The simplest way to observe co-channel interference is to turn off the transmitter of a given wireless device and use a spectrum analyzer to tune to the frequency channel of interest to search for other signals in the same system.

What is Out-of-Band Interference?

Out-of-band interference – occurs when a wireless system operates within the specified frequency band, but due to improper filtering, nonlinearities and/or leakage, emits energy into the frequency bands of other wireless systems. Out-of-band interference occurs when harmonics from a simple or poorly filtered transmitter are introduced into higher frequency bands. Figure 3 shows an idealized out-of-band interference represented by the highest amplitude signal, whose second harmonic falls within the bandwidth of Channel 1. Depending on the amplitude of the second harmonic signal relative to the specified signal, the performance of the Channel 1 system may be degraded. Often, a key requirement of regulatory agencies is to properly filter out the harmonics of a transmitter so that the wireless system does not affect other systems operating in higher frequency bands. When measuring harmonic levels, a spectrum analyzer with a frequency range of at least three times the fundamental operating frequency of the system must be used. For example, when verifying the performance of a transmitter operating at 6 GHz, the second and third harmonics must be measured at 12 GHz and 18 GHz, respectively. Here, FieldFox with frequency ranges up to 9, 14, 18 and 26.5 GHz is the best solution.

Figure 5 shows the frequency response of a wireless communication signal that is not output filtered and is centered at 8.1 GHz. FieldFox is configured to display two overlaid measurement traces, one of the transmitter signal after appropriate bandpass filtering (yellow) and the other of the unfiltered signal (blue trace). Typically, the spectra of the two signals are essentially identical, except that the unfiltered signal (blue trace) shows the presence of a second harmonic with energy centered at 16.2 GHz. Once transmitted, this harmonic energy has the potential to interfere with other systems operating in or near the 16 GHz band, such as commercial airport radar airborne SAR systems.

Figure 5. This screen shows the measurement of an 8.1 GHz wireless communications signal bandpass filtered (yellow trace) and unfiltered on the left (blue trace). The unfiltered response shows the second harmonic visible at 16.2 GHz.

To make the measurements shown in Figure 5, the analyzer settings were optimized to provide the highest dynamic range, including reducing the resolution bandwidth (RBW) and internal attenuator settings, and enabling the built-in preamplifier. Narrow RBW settings are typically used to achieve the lowest analyzer noise floor, or displayed average noise level (DANL). Unfortunately, narrow RBW settings increase the analyzer sweep time, especially when sweeping over a wide frequency range during harmonic testing. In this case, since the only signals of interest are the intended signal and its harmonics, the overall measurement time can be significantly improved by adjusting the analyzer’s center frequency and span, and measuring the signals of interest separately, as shown in Figure 6, where the 8.1 GHz fundamental (Figure 6a) and 16.2 GHz harmonics (Figure 6b) are measured using a smaller frequency span. Using FieldFox, the user can quickly switch the analyzer’s center frequency between the fundamental and harmonics by setting the “CF Step” (center frequency step size) to the fundamental value and then using the arrow keys or knob to change the analyzer’s center frequency.

Figure 6 shows the measurement results of a signal that is bandpass filtered (yellow trace) and unfiltered on the left (blue trace).

Figure 6a. Frequency response of 8.1 GHz fundamental signal

Figure 6b. Response of the 16.2 GHz harmonic

What is adjacent channel interference?