Base Station Transmits

Testing Processes to Ensure Radar Altimeters Not Impacted by C-band

February 8, 2022

Even before the January 19^th launch of 5G in the United States, the latest generation of wireless technology was getting a lot of press. Not all of it was positive, though, as the Federal Aviation Administration (FAA) voiced concerns that the C-band spectrum used to rollout 5G may cause interference with avionics equipment, specifically radar altimeters. While federal regulators and billion-dollar companies debated the issue, there was one thing without dispute – network testing is a necessity.

Fortunately, the two sides – mobile operators and the aviation community – have been trying to find a solution. It began in late 2021 when carriers agreed to lower the power on their 5G towers for six months. It continued in mid-January with an agreement not to turn on 5G antennas within two miles of airport runways.  

Given the $80+ billion investment by mobile operators, the rollout of 5G was inevitable. Another certainty is that sound testing processes need to be in place to ensure interference does not occur.

Importance of Radar Altimeters

At the crux of the situation is that 5G near airports may interfere with the signals transmitted by radar altimeters. The Federal Communications Commission (FCC) conducted testing previously and concluded the 400 MHz guard band between the 3.8 GHz C-band and 4.2 GHz signal used by altimeters sufficiently addressed the issue. The FAA still has concerns, however.  

The reason for the trepidation is the importance of the altimeter. Data acquired from radio altimeters informs other safety equipment on aircraft, including navigation instruments, terrain awareness, and collision-avoidance systems. Altimeters do this by transmitting an RF signal from the aircraft down to the terrain and receive the reflected signal back. The time delay of the received signal is used to determine Above Ground Level (AGL) altitude.

Radar altimeters are either pulsed or frequency-modulated continuous wave (FMCW). Commercial aviation predominantly uses FMCW. The RF front-end in FMCW altimeters typically uses homodyne architecture, with the received signal mixed with the transmitted signal to measure the frequency difference. With linear FM chirps, this frequency difference is directly proportional to the propagation time delay, and thus altitude.

Total Test Approach  

Inherently wideband systems, radar altimeters have no requirements for front-end rejection currently. So, they may be susceptible to blocking. That is the reason for the interference concern – and why testing procedures need to be continuous.

Effective testing of base stations around airports will require an ongoing process. Proper deployment and installation will ensure that the 5G antennas do not cause interference with airline altimeters as they approach airports for landings. Continual and regular testing to ensure that base station transmissions continue to fall within the specifications and not impact an altimeter’s ability to provide accurate data to the safety systems of a plane must also be done.

All this testing requires a comprehensive toolbox for regulators, mobile operators, members of the aviation community, and/or any other professionals responsible to ensure the 5G transmissions on the C-band do not create interference. For deployment and installation, a real time spectrum analyzer (RTSA) and interference hunting solution are required. When it comes to ongoing maintenance and monitoring, interference hunting tools and remote spectrum monitoring systems are necessary. Fortunately, Anritsu provides a comprehensive portfolio to address all the requirements.

Real Time Spectrum Analysis

Anritsu’s Field Master™ Pro MS2090A (figure 1) RTSA delivers performance never previously available in a compact, handheld instrument. With continuous frequency coverage from 9 kHz to 54 GHz, The Field Master Pro, with a displayed average noise level (DANL) of -164 dBm and Third Order Intercept (TOI) of +20 dBm (typical), makes measurements such as spectrum clearing, harmonics, and distortion more accurate than previously possible.

Figure 1: Field Master Pro MS2090 Real Time Spectrum Analyzer.

A 110 MHz modulation bandwidth coupled with best-in-class phase noise performance maximizes measurement precision. Typical  amplitude accuracy of ±0.5 dB provides confidence when testing transmitter power and spurious – key measurements to prevent interference with altimeters.

Ongoing Spectrum Monitoring

A remote spectrum monitoring system (figure 2) will facilitate the identification of interference from 5G base stations. Patterns of unwanted signal activity can also be examined, providing an efficient way to characterize and locate the source of the interference problem.

Figure 2: Remote spectrum monitoring solutions can help ensure C-band transmissions do not interfere with radar altimeters.

Spectrum monitoring can serve to enforce compliance with the parameters agreed to by aviators, operators, and regulators. Also, remote spectrum monitoring allows manpower to be effectively multiplied over a wider area and when engineers are not scheduled to be on duty. Air traffic patterns and runway activity will be able to be maintained without interference from the 5G antennas efficiently with such a solution.

Anritsu remote spectrum monitors can be used with Vision off-the-shelf software application. Vision continuously monitors selected spectrum bands to locate any spurious and/or interfering signals. Historic records of the spectral activity over time are saved to a database that can be configured to sound alarms when out-of-band signals are detected. The spectrum records are searchable for detailed analysis over an extended period of time.

To learn more, you can visit a technologies page dedicated to interference, causes, and test solutions. 

C-Band ROI Requires Proper Test Tools and Procedures

November 11, 2021

The C-band spectrum auction held by the Federal Communications Commission (FCC) was surrounded with so much fanfare it seemed impossible it would meet the hype. Well, more than $80 billion later, it’s safe to say that all the “noise” seems warranted. The question now becomes, how do mobile operators get the best possible return on investment (ROI)?

Verizon (estimated 3,500 total licenses) led the way, followed by AT&T (1,621 licenses), with T-Mobile (142 licenses) rounding out the top three. Considering the investment in the midband spectrum, which is well positioned to optimize certain 5G use cases, mobile operators must have a tactical approach with deployment. A key part of that strategy must emphasize network quality to maintain customer satisfaction, meet key performance indicators (KPIs), and limit customer churn. A sound test approach from antenna installation to operation helps achieve this goal.  

Understanding C-band and its Impact

Before outlining test procedures, it’s important to consider how C-band (figure 1) fits into the 5G infrastructure. C-band has such appeal because it addresses coverage and bandwidth issues. It is part of Frequency Range 1 (FR1), which operates from 450 MHz to 6 GHz. 3GPP Release 15 stipulates the n77, n78, and n79 bands for 5G operation in the C-band. The potential service bandwidth is up to 100 MHz, making it ideal for 5G enhanced mobile broadband (eMBB) use cases.

Figure 1: C-band spectrum, including Fixed Satellite Service (FSS).

C-band utilizes Time Division Duplexing (TDD), which has spectral efficiency benefits. TDD requires only unpaired spectrum, making the technology advantageous in environments with limited frequency resources.

On the other hand, timing and synchronization challenges are common with TDD. Because downlink (DL) and uplink (UL) share the same spectrum in a TDD system, strict timing restrictions are enforced to limit interference. If synchronization is in error, TDD framing will drift outside the guard period and into adjacent time slots. The result is network performance lags due to interference.

Spectrum Clearing A Necessity

A key consideration is that much of C-band needs to be cleared before mobile operators can use the spectrum. That’s because the frequencies were (and is some cases still are) used for government, public and private broadcast and satellite services. For this reason, extra attention with respect to deploying, installing, and maintaining C-band base stations must be given.

Contractors retained by mobile operators must conduct spectrum clearing as an initial step. Typically, a handheld spectrum analyzer with an omni-directional antenna (figure 2) may be used to determine if the spectrum is clear and ready for 5G usage.

Figure 2: Handheld spectrum analyzer with omni-directional antenna is used in spectrum clearing applications.

In a similar vein, on-going interference monitoring should be conducted. The same spectrum analyzer with a directional antenna configuration can be used when a performance issue is identified to conduct an interference hunt once the network is live.

Conversely, mobile operators will be required to ensure their 5G base stations do not impact satellite transmissions that have migrated to the upper portion of the C-band. This is particularly important, as massive MIMO and beam steering used in 5G can interfere with low noise block (LNB) downconverters of satellite antenna systems. Such an unattractive scenario is possible if the antenna is too close to the satellite earth station.

eCPRI Considerations

eCPRI is a key element of a C-band base station. It enables efficient and flexible radio data transmission over a packet-based fronthaul transport network. Using Ethernet for transport has a few advantages. For one, it is backward compatible with commodity equipment, enabling greater convergence of access networks. It also makes statistical multiplexing possible, which will help lower the aggregate bit-rate requirement.

There are hurdles with eCPRI, though. Unlike CPRI, eCPRI is not a synchronous technology. GPS, precision time protocol (PTP), and synchronous Ethernet are technologies used to address synchronization. Therefore, technicians need to conduct eCPRI tests to measure throughput, delay, and packet jitter when deploying and maintaining C-band.

OTA Factors

Another key aspect of C-band verification is to ensure that Over-the-Air (OTA) network performance and coverage meet the network design specifications published by the operators. Coverage mapping tools can help satisfy this requirement.

Anritsu spectrum analyzers can accurately measure RF power measurements over a wide frequency range, which, when attached with an antenna, become Received Signal Strength (RSSI). With coverage mapping, RSSI and Adjacent Channel Power Ratio (ACPR) measurements are combined with on-screen map displays to create a versatile solution for mapping coverage.

With a touch of a button, coverage mapping can be started, so technicians at any experience level can conduct measurements. The user has full control over the threshold values and measurement settings. Each measurement point is assigned a color correlating to the threshold values, and this fully customizable function allows for quick results.

Improved Finances

For contractors, payment for services requires submission and subsequent approval of close-out reports. Smart Aligner App (figure 3), a unified reporting solution for antenna alignment and line sweep measurements helps expediate payment. With the app, close out reports can be prepared, saved as pdf files, and sent to operators from the site location, saving time and money while providing operators with confidence that new base stations have been installed according to specification.

Figure 3: Smart Aligner allows simplifies the close out report process for more efficient payment.

Reports can be submitted directly as an email attachment sent over the cellular network. Confirmation of acceptance can be given before the crew departs from the site, eliminating the additional costs and time associated with return visits. Anritsu’s handheld field base station test solutions, including Site Master™ cable and antenna analyzers support the close out app.

To learn more about Anritsu’s interference measurement tools, visit a dedicated technologies page.