This is the fourth of a five-part series from Keysight Technologies on using real-time spectrum analysis to overcome RF/microwave interference in the field. The last installment will be posted on Thursday, September 22, 2016. If you don't want to wait, you can download the entire app note here.
Part 4: RTSA dramatically improves efficiency to root out interference issues
Two types of interference are most challenging in the field. One is co-channel interference, the other uplink interference. In this section, we examine both types and explore how RTSA helps detect and locate these interferences.
Co-channel interference refers to interfering signals that are on the same frequency as the serving carrier or are inside its channel bandwidth. This is a good definition for analog systems, but for digital wireless networks, we need to dig a bit deeper. To have a major impact on digital wireless systems, not only do interfering signals need to be on the same frequency, they also need to be synchronized with the baseband frames. Digital systems treat non-synchronized interferers as noise, which may not have a major negative impact on system performance.
Figures 11 and 12 demonstrate the impact of co-channel interference. These were measured on a lab test system to show the concept, since a constellation cannot easily be obtained in a field test.
Figure 11 shows the LTE signal's quality without co-channel interference. We can see synchronization channels with binary phase-shift keying (BPSK), the physical broadcasting channel with quadrature phase-shift keying (QPSK) and the downlink shared channel (traffic channels) with 16 QAM. Sync channels and broadcasting channel modulation forms a circle as shown in the figure.
When a frequency modulated (FM) wireless microphone signal transmits at the center of an LTE channel, where both the sync and broadcasting channels are assigned, the constellation gets blurred, and the control channels are indistinguishable from the traffic channels. This will prevent the mobile from synchronizing with the network, and the call will eventually drop.
Typically co-channel interference impacts the network quality the most on the downlink, because the system has no direct feedback on downlink co-channel interference. For example, when an illegal wireless microphone blasts RF energy into the middle of LTE downlink channel, the mobile only knows that the signal/noise ratio is bad, and it needs to transmit more power on the uplink. The system doesn’t know this is due to downlink co-channel interference.
Co-channel interference detection and troubleshooting is the most challenging task for communication network operators, because interferers can be hidden underneath the serving frequency signal. Typically, the user has to turn off the carrier transmitter to find if any other signal appears in the same frequency channel, then locate them to eliminate or reduce the impact. It is very intrusive and disrupts normal communication services. Under many circumstances, turning off serving transmitters is not a viable solution.
The RTSA density display is a spectrum measurement enhanced to show the frequency of occurrence. The display is coded using color to show trace intensity, and a persistence function can be added to focus attention on more recent events, as older data fades away. The density display shows frequency, power and signal occurrence within a given time. Because interferences have different signal-level distribution than the serving carrier, the display makes it a lot easier to detect multiple signals in the same channel.
Figure 13 shows a W-CDMA signal with a 2-way radio FM signal buried inside the same channel. A spectrum analyzer is not able to find the hidden signal without turning off the serving carrier, whereas the RTSA density display makes it fairly easy to spot the intruder.
RTSA expands signal intelligence from the two dimensions of frequency and power level to the additional dimension of time of occurrences. This capability allows differentiating multiple signals on the same channel.
LTE uplink operation verification and interference
An LTE network is like most broadband wireless systems: its capacity and performance are uplink noise limited. This is because all cell sites and mobile devices operate on the same frequency, making controlling noise coming from inside and outside of the network crucial.
Gap-free capture and density display are essential to evaluate digital wireless signals. Gap-free allows the analyzer to find the time signatures of a particular signal, and the density display makes it very easy to examine the signal’s power statistical distribution. Timing and signal level distribution can help users to separate various signal types, even within the same network.
Figure 14 illustrates that RTSA is able to scan LTE uplink resource block (RB) assignments. The persistence setting enables a user to observe the frequency of RB allocations, which provides a very good indicator of network congestion. If a non-LTE signal shows up in the band, it can be spotted quickly. A traditional spectrum analyzer is only able to show a cumulative noise floor rise. Any external interference is buried in the rise of the noise floor, so it is very difficult to rely on this tool to detect interference.
This is important, for example, because narrowband interference can often knock down an LTE system. An LTE control channel on the downlink is in the center 1.08 MHz of its 10 MHz or 20 MHz channel. On the uplink, however, physical uplink control channels like a random-access channel (RACH), hybrid automatic repeat request (HARQ) and channel quality indicator (CQI) are carried by subcarriers at the edge of the channel (see Figure 15). If any interference happens to be in these two areas — a 700 MHz wireless microphone, for example — it will create interference in the network operation or potentially block the service for the entire cell site.
What needs to be fixed to mitigate or eliminate interference
Interference can be the manifestation of network component failures. In fact, more than 50 percent of interferences are caused by the malfunction of RF subsystems or components in the network (see Figure 16).
The antenna is the single most important component in a wireless network. It is the only interface between the physical network and the radio waves (i.e., over the air). The key performance parameter is return loss or voltage standing wave ratio (VSWR). If a transmitter antenna’s return loss fails, less energy will be transmitted to the coverage area. This will trigger the mobile to increase its transmitting power, as it thinks it is far from the base station. This, in turn, will cause noise to rise at the base station receivers, which could be interpreted as external interference by the base station and lead technicians in wrong direction seeking a solution. So it is strongly recommended to sweep the antenna first if there is any suspicion of external interference.
The cable system also plays a key role keeping the network running. Because feeder lines are exposed to various environmental changes, connectors will corrode and cables will be bent by external forces like winds. These changes lead to higher cable loss from the first installation, and higher loss will reduce the received power level close to the cell edge. This causes signal-to-noise ratio (S/N) deterioration. Routine cable loss measurement against the link budget is a proactive way to avoid interference issues within the network.
Low noise amplifiers (LNA) are widely used in the base station receiver chain, typically installed right behind the base station receive antenna. An LNA is very beneficial for improving reverse link coverage and improving uplink data throughput. Yet an LNA can be blocked when a mobile is too close to the receiving antenna, such as in an indoor system, or when the receiving antenna is installed too close to pedestrian traffic, at downtown streets, for example. A blocked LNA acts like uplink interference, and it also produces intermodulation products (see Figure 17), further interfering with the network. Fixing the issue involves selection of an LNA with a higher compression point, use of a bandpass filter in front of the LNA and minimizing the LNAs by replacing them with a power-controlled repeater or base station.
FieldFox RTSA software, Option 350, is designed for engineers and technicians performing interference hunting and signal monitoring, specifically in surveillance and secure communications, radar, electronic warfare and commercial wireless markets.