Using Real-Time Spectrum Analysis (RTSA) to Address the Challenges of Field RF and Microwave Interference

Today we will introduce practical methods for overcoming RF and microwave interference problems in the field using real-time spectrum analysis (RTSA). Learn about the different types of interference encountered in commercial and aerospace and defense (A/D) industry-specific wireless communication networks. Uncover the shortcomings of traditional interference analysis methods and explain in detail the principles of RTSA and why such analysis is needed to diagnose interference problems caused by bursty and infrequent signals in today's networks.

As more wireless technologies are used in communication networks, interference is an inherent challenge. Regardless of the type of network, its performance is always limited by the level of noise in the system. Noise can be generated internally and/or externally.

The level of interference management determines the quality of service. For example, managing uplink noise in an LTE network can significantly improve its performance. In an enterprise wireless LAN (LAN), proper channel allocation and reuse can ensure expected connection speeds, while optimized antenna positions/patterns at satellite ground stations help ensure reliable communications in a variety of weather conditions.

To detect demanding signals and diagnose network problems, field testing requires the ability to perform real-time signal analysis (RTSA). In this article, we will introduce the interference encountered in various networks, RTSA technology and its key performance indicators, and explore applications designed to solve interference problems in radar, electronic warfare, and communications networks.

Review of RF and Microwave Interference Issues

1. Challenges of wireless interference

The biggest challenge facing commercial digital wireless networks is how to provide the highest possible capacity within the available spectrum. Driven by this design goal, the industry has set out to implement tighter frequency reuse and more extensive channel deployment. Because cell sites are very close to each other and base stations transmit signals at the same time, a lot of noise is generated on the downlink (the direction from the base station or the direction from the base station to the mobile device). This large noise on the downlink of the mobile antenna causes the mobile device to increase its output power to overcome the impact of this noise. This in turn causes an increase in noise on the uplink of the base station antenna (the direction from the mobile device to the base station). Excessive noise on the base station antenna reduces the capacity of the cellular base station. These situations are all classified as internal network interference.

In addition to internal interference, external interference is becoming more and more common; the main causes of external interference are very small frequency protection bands between network operators, lack of network planning and network optimization, and illegal use of spectrum.

1.1 Interference issues in LTE networks

LTE is a noise limited network. It has a frequency reuse ratio of 1, which means that every cell site uses exactly the same channel. In order for LTE to work properly, it must employ a sophisticated and efficient interference management scheme.

On the downlink, LTE base stations rely on CQI (channel quality index) reports from mobile devices to estimate interference in the coverage area. CQI measures the signal-to-interference ratio on the downlink channel or certain resource blocks, and is used by the base station as an important basis for scheduling bandwidth and determining the throughput provided to mobile devices. Interference includes both noise generated internally to the cell site and interference generated by external transmitters. If there is external interference on the downlink, it will cause the CQI to decrease, more data will need to be retransmitted, and the network speed will be greatly reduced. Downlink interference is one of the most difficult problems to deal with because the base station does not directly feedback that there is interference.

Figure 1. LTE power control and resource block allocation

In LTE interference management, accurate power control is very important because the serving cell and the adjacent cell share the same frequency channel. The network needs to minimize interference at the cell edge while providing enough power to edge users so that they can obtain excellent service quality. LTE base stations provide less power throughout the entire frequency band in the center of the cell; and allocate fewer resource blocks (subcarriers) at the cell edge, but provide more power (Figure 1). This approach improves the overall throughput of the cell and minimizes interference.

Regardless of the channel bandwidth of the system, the LTE control channel is always located in the center of the channel with a bandwidth of 1.08 MHz. Key downlink control channels include primary sync, secondary sync, and broadcast channels. The primary and secondary sync channels are used to synchronize mobile devices to the cell and start decoding system information. If there is narrowband interference near the center of the LTE channel, it can have a significant impact on the synchronization process of mobile devices and sometimes even block the entire cell. For example, some analog 700 MHz FM wireless microphones can easily block LTE cells and are therefore banned by the FCC.

1.2 Microwave backhaul interference issues

Approximately 50% of base stations worldwide are connected to backhaul via microwave radios. With the recent development of Gigabit Ethernet over microwaves, microwave radios are very attractive as backhaul solutions for 4G/LTE deployments.

As with other wireless technologies, there is always interference with this type of network. For microwave radio networks, the main interference actually comes from the following areas.

Reflection and refraction

Microwave radio is widely used in mobile networks to achieve point-to-point connections. Radio stations may be deployed in urban areas, and if their transmission path is blocked, the signal will bounce and cancel out some of the energy transmitted to the remote receiver, or the signal may be turned (called refraction). Both of these situations will cause system disruption.

Interference in unlicensed bands

In recent years, mobile backhaul has widely adopted point-to-point Ethernet microwave links, which are easy to operate and low cost. Point-to-point microwave links can operate in licensed or unlicensed frequency bands, such as 5.3 GHz, 5.4 GHz, and 5.8 GHz. In the unlicensed bands, system outages are more related to interference. These bands are very close to the frequencies used by 802.11n or 802.11ac WLANs, and we see interference between the two systems. For example, when a 5.8 GHz microwave radio is operating near a WLAN, the WLAN may increase the power level of the microwave radio receiver, which can trick the microwave radio into thinking that it needs to reduce its transmit power on the link, so it will not transmit enough power to maintain the actual signal level required, causing outages.

2. Interference issues in the aerospace and defense (A/D) and public safety sectors

Most common aerospace and defense communication systems include satellites, radars, electronic warfare (EW) systems, and secure communications (public safety) networks. As wireless technology in the commercial and aerospace and defense industries rapidly advance, more and more interference is creeping into aerospace and defense systems. To address these issues, aerospace and defense systems are moving to higher frequencies, deploying narrower RADAR pulses, and adopting highly encrypted digital wireless systems for communications.

These technologies can effectively resist external interference, but they also make field fault diagnosis more difficult. New tools and measurement techniques are needed to effectively maintain aerospace and defense communication systems.

2.1 Interference issues in public safety/two-way wireless communications

Figure 2. Public safety narrowband and wideband channel allocations in the 700 MHz band

There are two main problems with public safety wireless systems. One is adjacent channel interference, and the other is intermodulation distortion. Public safety wireless communications usually use narrowband systems with bandwidths such as 25/12.5/6.25 kHz, and their transmission power is much higher than commercial systems. It requires 80 to 100 dB of channel suppression. If the duplexer is not tuned correctly, adjacent channel interference will occur between the operating channels of the base station, reducing the coverage area.

Since public safety transmitters operate at higher power levels, if their power amplifiers saturate, intermodulation products will be generated, and their harmonics are likely to fall on adjacent frequency bands. If these harmonic products fall on the LTE control frequency (see Figure 2), network service will be disrupted.

2.2 Interference issues from satellite ground stations

Satellite communication systems are often deployed in aerospace and defense networks. One trend in this area is to provide high-capacity communication links for military organizations. There are two main ways to increase system capacity: one is to increase the operating frequency from C and Ku bands to Ka bands, and the other is to deploy frequency reuse using multiple beams.

The higher the frequency, the smaller the beam. This requires more precise antenna alignment, which can cause co-channel and adjacent channel interference if it is not aligned. Multi-beam frequency reuse allows adjacent areas to share the same frequency plan and polarization. If the system is not properly optimized, strong co-channel, adjacent channel and cross-polarization interference can occur.

Figure 3. Types of interference in satellite ground station operations

3. Problems with traditional interference analysis methods

There is more than one way to classify interference. From a signal interaction perspective, interference can be divided into co-channel interference, adjacent channel interference, and intermodulation (passive and active). From a network operations perspective, interference can be divided into downlink interference (BS to MS), uplink interference (MS to BS), and external interference.