Interference Hunting in 5G Networks

Interference hunting is the process we use to detect, locate and deactivate undesired sources of RF energy. Interference can have a negative impact on a mobile network and is one of the largest contributors to poor network performance.

The issues can range from mild performance degradation to poor voice quality, call or connection drops, low data throughput rates, or even complete blocking of a given cell or sector. In cellular networks, most interference issues result from unwanted signals in the uplink.

Interference hunting in 5G FR1 bands is similar to traditional interference hunting. With high frequency 5G FR2 bands, however, present new challenges for network installers and operators.

 

What is an Interferer?

Generally speaking, an interferer is an active source of RF energy that negatively impacts the network. RF interference has increased with the surge in wireless devices, broadcast communications, and other RF sources competing for radio spectrum.

Traditionally, the most common cause of cellular interference is other communication systems that transmit signals in the same spectrum. This often includes bidirectional amplifiers that may either be broadband (Class B), band selective (Class B) or narrowband (Class A). It can also include modern consumer electronics.

 

Figure 1. Road signs can block cellular signals.

 

For FR2, generic RF noise becomes less common because few devices or phenomena create signals at millimeter wave frequencies, the bands are very quiet. Large spectrum spacing means that even harmonics will be less common sources of interference.

In these cases, passive objects, such as a highway sign erected in front of a base station antenna or a large construction crane, can become obstructive interferers versus traditional active interferers. Figure 1 shows a highway sign and cell tower.

 

Traditional Interference Hunting

Interference hunting is both an art and a science. Success is largely a function of using the proper tools and techniques, with experience playing a significant role.

The first step is to detect that interference is occurring, and to determine if the source is internal or external to the cellular site (Figure 2). 

Figure 2. Different types of interferers may be measured and detected.

 

Cellular sites are commonly affected by internal interference. This unintentional RF interference may be caused by the improper or incorrect setup of network communications equipment. Internal interference often occurs after upgrading physical infrastructure, adding new equipment to networks, or changing existing configurations. This includes additions to a tower, relocation of a receiver, or upgrades to radio consoles and hubs. The improper conductivity of passive devices such as connectors, cables, or antennas bring on the trouble. 

In addition, passive intermodulation (PIM) is a common source of internal interference. PIM interference results from the nonlinear mixing of two or more frequencies in a passive circuit. If the interference coincides with the network’s uplink receive frequencies, it can cripple network performance and throughput. PIM sources include the improper conductivity of passive devices such as connectors, cables, or antennas.

 

Causes of External Cellular Interference

External cellular interference is caused by co-channels, adjacent channels or spurious emissions.

• Co-channel interference. Occurs when two or more transmitters operate on the same channel due to improper frequency coordination, deteriorating or malfunctioning equipment, or anomalous propagation.
• Adjacent channel interference. Occurs when a transmitter operates on an adjacent frequency and its energy spills over into a desired receive channel.
• Spurious emission interference. Happens when a transmitter emits on unintended frequencies.

You should verify cellular site interference is external by means of measurements or statistical data. Items such as high Received Signal Strength Indicator (RSSI), call drops, and poor throughput are all key indicators of external interference. 

 

Testing Around the Affected Area

Next, determine the general location of the external interference to within about 100 m. This is done by drive or walk testing around the affected area.

 

Figure 3. Determine the location of interference by walking the areas with a handheld directional antenna and a spectrum analyzer.

 

The third step is to determine the specific location by walking the area, using handheld directional antennas and a spectrum analyzer (Figure 3) to find the specific device generating the RF interference. This might be a wireless camera, a malfunctioning bidirectional amplifier or a cable TV leak.

Finally, with the appropriate permissions, deactivate the device causing the RF interference and verify that the interference has been eliminated.

The vast majority of cases for 2G and 3G networks could be solved by following these steps, with the frequencies of interest in the range of 850 MHz to less than 2.4 GHz and bands generally 5 MHz wide or less. The real difference was related to frequency, with different antennas or bandpass filters required for each one.

 

Interference Hunting in the 4G/LTE Era

Interference hunting changed with the introduction of 4G/LTE. The higher order modulation schemes were more vulnerable to interference. Initial networks were in the 700 MHz band, so interferers could penetrate better and travel farther. Wider bands of up to 20 MHz meant that more spectrum had to be searched. 5G presents even more challenges with respect to modulation, frequencies and bandwidths.

Modulation

Higher order modulation yields higher throughput, but robustness to interference is partially a function of the modulation order. Because lower-order modulation is more robust, higher order modulation schemes require a “cleaner” RF environment to ensure proper performance, regardless of frequency. 

In cellular networks, if the modulation order can be changed dynamically, higher-order modulation can be used under good conditions. If, however, poor conditions exist due to interference or other factors, a “fallback” in modulation that decreases throughput might be needed. 

 

Figure 4. Diagram for QPSK and 16QAM with equivalent signal to noise ratio

 

Higher-order modulation modes, such as 16QAM, 64QAM and 256 QAM are more desirable in 5G applications to support high bandwidth requirements. If, however, channel conditions deteriorate, QPSK provides more interference resilience, at the cost of data throughput.

Weaker or lower-level interferers are a more serious problem in 5G. Therefore, the tools and techniques must be more sensitive and much faster than those used for previous cellular generations. Figure 4 compares QPSK and 16QAM constellation diagrams.

Frequency 

Changes in frequency are even more important. 5G is divided into two categories based on frequency range (FR):

• FR1 extends from 450 MHz to 6 GHz and is often called “sub 6 GHz”
• FR2 extends from 24 GHz to 53 GHz and is called “mm wave” because of the signal wavelength at these very high frequencies 

There are substantial differences in propagation in these two frequency ranges, as shown in Figure 5. This leads to differences in interference hunting techniques and procedures. 

 

Figure 5. FR1 (sub-6 GHz) and FR2 (>6 GHz)

 

FR1 is similar to traditional cellular frequencies and does not represent substantial challenges in terms of propagation. Because most of this spectrum is refarmed for 5G, spectrum clearing is very important to make sure there is nothing to interfere with the new usage. However, weak interferers in pre-5G networks may not be as weak in 5G.

FR2 is quite different, but it can be even easier for interference hunting. Because cell radii in millimeter wave are so small, if a cell is known to be affected by RF interference the source is likely within 100 meters.

Network statistics have replaced the "driving the sector" step, which is usually the most time-consuming and difficult phase. Because millimeter wave propagation is very much "line of sight," the chances of a distant interferer are reduced due to high levels of free space path loss and attenuation from objects.

At higher frequencies, there are fewer sources of RF energy, either intentional or unintentional. Higher frequency signals don’t travel as far or penetrate as well, devices are more expensive and complex, and there are few physical phenomena that create such high-frequency signals.

There are some intentional sources of RF (in government, aerospace and defense applications), but they are usually highly directional and of short duration, limiting their potential for active interference. Finally, these frequency bands are licensed, not shared spectrum, so any incursions should be relatively rare.

Bandwidth

The higher throughput offered by 5G generally requires higher order modulation and/or wider bandwidths, which necessitates these higher frequencies. The wider bandwidths provide better immunity to the effects of narrowband and even wideband interference sources. The bandwidth can be split into subcarriers as needed. 

Wider bandwidths, however, make it more difficult to detect and locate interferers, particularly when using traditional swept (heterodyne) analyzers. Sweep time becomes larger as the channels become larger, so it is easier to miss short duration or pulsed signals. Shortening the sweep time by using large resolution bandwidth fails due to decreased frequency resolution (making it hard to distinguish interferers) and a higher noise floor (making it hard to detect weaker signals).

 

Interference Hunting Tools 

Two types of tools are used in interference hunting: portable handheld analyzers and antennas. An alternative to the traditional swept analyzer is the scanning FFT based receiver, which can help when looking at very wide spans because it is much faster and much more sensitive.

For FR1, existing tools and techniques can be easily applied. Traditional tools may not be suitable for the mmWave bands in FR2, and new or modified tools may be required that are tunable to the higher frequencies in FR2. 

Portable Handheld Analyzers 

Analyzers, either the swept spectrum analyzer or the FFT-based monitoring receiver, must provide the speed and sensitivity critical for 5G. The heterodyne analyzer sweeps a small portion of the spectrum of user-definable width. The FFT receiver takes a larger chunk of spectrum, performs an analog-to-digital conversion, and then does a fast Fourier transform to generate the frequency domain.

Interference hunting is generally an uplink problem. In frequency division duplex (FDD) networks, uplink interferers are relatively easy to recognize. In time division duplex (TDD) networks, uplink and downlink are interleaved in the same frequency band, making it very difficult to recognize the presence of an interferer. This was not a significant issue as TDD was not widely used, but it becomes especially important in FR2. 

 

Figure 6. 10 MHz real time spectrum and waterfall display of partial TDD-LTE signal with a relatively persistent interferer at 2,602 MHz 

 

In TDD networks, a single frequency band is used for both uplink and downlink by assigning time slots to each direction. The distribution of timeslots between uplink and downlink is configurable but remains fixed within a given network.

Downlink timeslots are "always on" and constantly transmitting signals, but uplink time slots may or may not have traffic, so signals are intermittent. Uplink interference can be masked by the network’s own downlink signals.

 

Figure 7. A gated trigger can be used. Here, only those signals appearing during the gate period are displayed.

Ideally, it would be best if instruments could see only the uplink time slots but turning off the downlink time slots is not often practical. A gated trigger (Figure 7) can be used, whereby only those signals appearing during the gate period are displayed. For more information, check out this video on how to set up a gated trigger for 5G measurements.

For example, if the gate is synchronized with the uplink timeslot, then only those signals occurring during uplink are visible (Figure 8). In viewing the TDD spectrum, if the gate trigger is off, there will be a constant since all of the downlinks are included. With the gate trigger on, only the uplink is seen.

Figure 8. With the trace minimum hold function enabled and the measurement time set to 50 ms, both downlink and uplink TDD signals are suppressed and the relatively persistent interferer at 2,602 MHz can be easily identified.

 

Antennas

The antennas used in interference hunting are usually omnidirectional for determining the general location of the interference and directional for the final location phase. Different types of handheld directional antennas such as Yagi or log-periodic have different patterns (beamwidth and bandwidth) and different patterns. Almost any such antenna can be used for FR1. 

 

Figure 9. Small horn antennas provide high gain and high directionality but they are more expensive.

Meanwhile, for FR2, since antenna size is generally proportional to wavelength, standard directional antennas are impractical for the millimeter wave band. Small horn antennas (Figure 9) are often used, which provide high gain and high directionality but are more expensive.

The cable connecting the antenna to the analyzer needs to be very short to minimize losses. In some cases, it is advantageous to connect the antenna directly to the analyzer, if possible. 

 

Automated Interference Hunting in 5G 

Interference hunting is particularly demanding when using traditional manual techniques. It can be greatly assisted by the automated detection of interference signals before the network is switched on. To clear the spectrum before switching on the network: 

• Identify and map the interference sources
• Process the data to detect interference in bands that should be empty 
• Find and eliminate these interference sources 

If this can be automated, it will vastly reduce the time and difficulty of the process.

 

All About Proper Tools and Techniques

Interference hunting in 5G FR1 bands is similar to the traditional process although higher sensitivity and the use of FFT receivers for wide bandwidth are useful. Tools and accessories must often be modified for interference hunting in FR2 networks.

Higher frequencies mean there are fewer undesired sources of RF energy, so obstructions become the interferers in these networks. Proper tools and techniques are the key to effective interference hunting.

 

All images used courtesy of Rohde & Schwarz

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