Coexistence grows as radar and satellite systems use the same or nearby frequency bands with 5G, creating the need to assess coexistence and mitigate potential interference as new 5G networks are deployed.
5G cellular promises new applications for military and government communications, including high-definition video; 3D or augmented reality; ultra-reliable, low latency communications; and massive machine-type communications. These capabilities will enhance intelligence, surveillance and reconnaissance, command and control and supply chain procurement and logistics. With new bands specified for 5G, however, coexistence with existing services poses a dual-edged challenge. Radar and satellite systems using the same or nearby frequency bands can reduce the capacity in 5G systems, while 5G can impair radar performance and damage satellite ground stations. Only by assessing and mitigating the potential impact among 5G, radar, satellites and other systems, can all these coexisting systems deliver their intended performance.
For example, in the U.S., the spectrum between 3.1 and 3.5 GHz is shared between federal and non-federal radio location services, with federal services having the primary allocation or priority. Similarly, both C-Band and extended C-Band frequencies are used for fixed satellite services and 5G, with potential interference issues between them.
WHAT IS COEXISTENCE?
Coexistence refers to the situation when two or more signals have the right to occupy the same or nearby spectrum. Usually, one of the services has priority. Radar typically has priority over 5G. If there’s a conflict, the 5G transmitter must shut off or move to a different frequency. With satellite systems, 5G interference can be severe: receiver front-ends in ground stations are highly susceptible to interference from high-power 5G base stations.
5G operating bands are currently grouped into frequency ranges below 6 GHz (FR1) and mmWave spectrum around 28 or 39 GHz (FR2). To provide the bandwidth for 5G, new operating bands have been allocated, with most of the initial deployments in the 3.6 to 3.8 GHz and 26 to 27.5 GHz bands and more bands planned. The 5G services in these bands must coexist with the downlink range used by satellite ground stations, from 3.4 to 4.2 GHz, and the military satellite bands from 27.5 to 29.5 GHz and the fixed satellite service downlinks from 37.5 to 40 GHz.
Some of these coexistence issues are unique to the U.S., according to a report published by the Congressional Research Service.1 The report says, “Although Department of Defense (DOD) uses certain mmWave frequencies for high-profile military applications such as advanced extremely high frequency satellites that provide assured global communications for U.S. forces, it extensively uses sub-6 frequencies—leaving less sub-6 availability in the United States than in other countries. The Defense Innovation Board (DIB) advised DOD to consider sharing sub-6 spectrum to facilitate the build-out of 5G networks and the development of 5G technologies used in the sub-6 band.”
The solution to these challenges is spectrum sharing, which makes coexistence conflicts likely.
IMPACT OF COEXISTENCE
Coexistence is a concern when two or more signals have the right to occupy similar spectrum. However, the signals don’t have the right to interfere with each other. For communication systems, coexistence issues may degrade the service by decreasing the data throughout or totally disrupting the link, which will create a financial problem from higher operating costs and lower revenue. Ensuring coexistence can be challenging, as the respective systems have different functions, designs, signal characteristics and locations.
Several approaches can be used to minimize potential problems: The frequency regulator, such as the FCC, can define guard bands and frequency spacing between services. Services can be required to maintain minimum distances from transmitters. As an example, the minimum separation between shipborne radars and terrestrial 5G base stations can be defined. Transmit power can be restricted—indoors versus outdoors, for example—and antenna type, angle and elevation defined to restrict the level and direction of the radiated power.
Arguably the most challenging is the coexistence of 5G with satellite systems (see Figure 1). Satellite ground stations have sensitive RF front-ends designed to receive the low-level signals from satellites orbiting at 35,785 km. The low noise amplifier in the receiver can be overloaded by nearby terrestrial sources, such as the much higher-power 5G signals from base stations—both operating in C-Band.
HOW TO MEASURE
The potential of coexistence interference can be assessed in the lab using a tailored test system that enables adjusting parameters such as signal strengths, center frequency, frame structure, modulations, etc. (see Figure 2). In the figure, which shows the satellite-5G example, the signal generator provides the satellite DVB-S2X signal, using software to create the digital video that is downloaded to the hardware.
Some common metrics are used to assess signal quality and the impact of coexistence. One is error vector magnitude (EVM), with units of percent or dB. This measures the difference between a measured symbol and a reference (theoretical) symbol in I and Q. As the demodulation of a signal in the receiver becomes poorer, the EVM increases. A perfect signal will have 0 percent EVM.
The 3GPP standard for 5G details the EVM requirements for various modulations, with the modulation changed to maximize what the channel can support. With lower noise and distortion, the channel can support higher-order modulation, which transmits more symbols in a given time. QPSK is the lowest order and accommodates the highest EVM. As the channel quality improves, the modulation steps to 16-, 64- and 256-QAM.
As 5G is deployed, coexistence will remain a prominent concern, extending from consumers to militaries and governments as private 5G networks are rolled out on bases, in government facilities and conflict zones. In addition to satellite networks, the coexistence risk will need to be assessed for military radar and non-5G communications systems.
Typically, coexistence problems cause service disruptions or performance degradation. Often, however, the consequences remain unknown until a problem occurs. To avoid surprises, a best practice is to prototype scenarios in the lab and look for coexistence issues. Once systems are deployed, 24/7 monitoring in the field can help identify sporadic issues and lead to resolution.
Digital twin technologies can be used to plan for and simulate coexistence scenarios. Scalable channel emulators can support up to 64 channels and 400 MHz bandwidth and will cover mmWave bands with external hardware for up- and down-conversion. Emulators work with various software packages to implement 3GPP 5G and custom channel models. These systems can simulate Doppler shift and delay in the channel, which adds more realism to lab tests (see Figure 3).
When designing and deploying a new 5G network, a “crawl-walk-run” approach is recommended to identify and mitigate coexistence issues (see Figure 4). Begin with software to create a digital twin and model the current transmitters and receivers and see the effects from the new system. Hardware prototyping follows, using available devices and systems with commercial off-the-shelf (COTS) hardware emulators to mimic a small-scale system in a lab or anechoic chamber. COTS emulators enable the frequency, bandwidth and power to be varied, which may identify corner cases where coexistence issues arise.
Outside the lab, plan field testing with deployed 5G, tactical or public safety networks and radar or satellite ground stations. Field tests can measure transmit power, signal strength, EVM, throughput with modulation and MIMO, latency, block error rate and beamforming quality. In some cases, drones can be used for fly testing to determine 3D coverage, measuring signal strength, signal quality and throughput.
To assure the performance of a military or government 5G network, coexistence must be planned and assessed up and down the stack from layer 1 to 7. Testing must span from the chipset to the full network and include multiple RF channels, carrier mechanisms, data protocols and waveforms, such as 3GPP 5G New Radio, pre-5G and custom OFDMA. When assessing the impact of coexistence issues, consider these questions:
- How will the interfering waveforms interact?
- How much suppression is required, in-band and out-of-band?
- How much guard band is necessary?
- What metrics should be used to assess impact?
- Is lab testing sufficient or should it be supplement with field test?
With the ability to assess the coexistence of networks and services, issues can be identified and resolved to achieve reliable communications. Many approaches are available from the lab to the field to assess potential issues that may degrade the performance of military and government systems. Once deployed, ongoing monitoring will reveal new coexistence issues, safeguarding the 5G network and, more importantly, the individuals depending on its performance.
1. Congressional Research Service, “National Security Implications of Fifth Generation (5G) Mobile Technologies,” April 5, 2022, Web: sgp.fas.org/crs/natsec/IF11251.pdf.