EIRP field testing in 5G-NR. What? Why? How?
The introduction of active antenna systems in 5G-New Radio (NR) requires installation and maintenance engineers to use alternative test methods, such as Effective Isotropic Radiated Power (EIRP) for transmitter power and beam verification. Gone are the days of simply measuring transmitted power with an absorption power meter or using a direct connection via a “sniffer” port in the antenna feed. A new generation of field solutions must be employed to conduct these measurements. This article covers the fundamentals, the challenges, and the best practices of EIRP testing in installation and maintenance of 3GPP 5G Next Generation base stations (gNB). The key specifications that a spectrum analyzer needs to ensure accurate EIRP measurements are also discussed.
What is EIRP?
Before the advent of 5G, radio systems, whether cellular, PMR, broadcast, or military, all used relatively simple transmitters and antennas. Knowing antenna gain and input power, one could predict signal strengths and hence validate a base station, ensure safety and analyze performance indicators. Today, this is no longer the case. 5G networks involve complex active antenna systems, imposing significant changes in the installation and maintenance of base stations. This complexity is necessary to achieve the efficiency and speeds of 5G. Using traditional methods to conduct measurements has become impractical and, even when possible, the results have little relationship to the network’s performance.
Everyone involved in 5G networks has different priorities: mobile operators need to assess the quality of User Equipment (UE) connections and map field coverage; engineers installing and maintaining 5G base stations must verify transmitted power and beamforming; regulators need to ensure and assess compliance with electromagnetic levels, as well as enforce spectrum clearing. Effective Isotropic Radiated Power (EIRP) – a product of transmitter power and antenna gain in a specific direction relative to an isotropic antenna – is certainly among the indicators that help determine the performance, safety, coverage, and compliance of a 5G network.
Imagine an antenna radiating as a point source equally in all directions. If power were to be measured at a given distance, this would be the same regardless of the direction. Like a pebble dropped into a pool of water, generating waves equally dispersed in all directions, this antenna would be radiating “isotropically” and have unity gain. Now imagine a directional antenna, with its input power varied to get, at the same distance as before, the same power reading. The radiated power in that specific direction is equivalent to an isotropic antenna with a given input power.
At 1 GHz and 100 m, the free-space path-loss is 72 dB. An isotropic antenna fed with 1-W (+30 dBm), results, at 100 m, in a measured power level of -42 dBm (figure 1-left). If the transmitter is a directional antenna with a 6 dBi gain in the boresight direction (figure 1-right), the power reading would be -36 dBm. In other words, the directional antenna has the effective radiated power of an isotropic antenna fed with +36 dBm, or 4-W. In short, it has an EIRP of 4-W.
EIRP is the power that a given antenna can radiate in a specific direction. Given a real antenna, EIRP is the input power that should be injected in an ideal isotropic antenna to radiate, at the same distance and in the same direction, the same power of the given real antenna. EIRP depends on total transmit power, including amplification and coupling losses, and on antenna gain in the beam direction of the receiving area. Getting the correct net radiated power of an antenna element for a 5G gNB is not easy. No test ports are available and active antenna elements are employed. Every element has its own power amplifier for transmitting and its own low noise pre-amplifier for receiving, strongly affecting field tests.
Under free-space conditions, the Friiss transfer formula that relates the transmitted power to the received power is:
EIRP is the product of transmitted power and antenna gain, so it can be expressed as:
Which, in dBm, reads as:
An antenna is always radiating into a 3D sphere. One would need to take an infinite number of power measurements across infinitely small areas around this sphere to get the Total Radiated Power (TRP) of the antenna. Although very complicated and time consuming, using approximations it’s still possible. In fact, TRP is measured in anechoic chambers during 5G-NR antenna testing. Out in the field, one needs a simpler approach, involving cutting through the 3D antenna pattern along a specific 2D plane and defining this as a representative antenna pattern. Usually these 2D planes are in the boresight direction of the azimuth or elevation direction of the antenna.
Why is EIRP a KPI in 5G-NR?
A 5G antenna could well have 128 to 256 individual transceivers (figure 2), integrated with their own radiating elements. Active antennas can steer beams and change their shape, concentrating energy in the desired direction. This is implemented by changing the gain, hence the input power. Accordingly, using traditional methods to measure the antenna input power is impractical at best. Recognizing this a new solution was needed, and the industry has agreed that in a similar field scenario the most useful power measurement is EIRP, as it is a reliable indicator of signal strength anywhere in a 5G cell. To evaluate the effectiveness of beamforming technology, EIRP needs to be measured.
Knowing EIRP helps manufacturers to validate their transmitters’ performance, operators to determine the beam shapes, the location/angles of the nulls, as well as minimize interference to their own and other network services, and regulators to assess the safety of field strengths close to the base station. In addition, EIRP can also be used by commissioning teams to find faults. Essentially, by placing the base station in a test mode, where it transmits a known “test model” signal in each direction and at each strength., they can determine radiation patterns and measure field strength in complex environments. As such, it provides a means of using conformance test methods in the field.
EIRP measurements can be used with any transmitter, regardless of the complexity. In this sense it provides the way forward for network optimization for the foreseeable future. This means that the new generation of field testers must provide practical and convenient measurements of EIRP in the Far-Field.
How is EIRP measured in 5G-NR according to 3GPP?
Virtually all 5G-NR radios employ two polarized antenna arrays to improve diversity and reduce fading. Typically, they would be angled at ±45°. Not knowing the location of UEs in advance, it is likely that both antenna arrays would carry the same information transmitted at similar power levels. Doing this also ensures that reception in a random location/orientation is optimized. For traffic beams the two antenna arrays can be used in a MIMO fashion to achieve multiple paths to UEs, either to maximize reception quality or under clear signal conditions to improve data throughput. A base station might also choose to vary the signals on the two antenna arrays to reduce mutual interference, or to optimize signals reflected off nearby buildings.
This means that a true beam power characterization requires measurements in two orthogonal planes, which are then added together to obtain the total EIRP. This is exactly what the 3GPP base station test specification TS38.141 states. It also clarifies that two orthogonal polarizations may be measured simultaneously or separately. Having this degree of freedom, it is worth pointing out that measuring two orthogonal polarizations separately not only reduces the cost of equipment, but also cuts the number of elements that need to be calibrated.
Making measurements in the field is not as straightforward as it looks on paper. Even a sub-6-GHz 5G transmitter can use bandwidths as wide as 100 MHz. Therefore, any measuring receiver’s amplitude and phase response must be “flat” across the channel bandwidth and needs to adequately reject other signals from adjacent channels. Any lack of flatness in the receiver gain translates directly into a power measurement error. Additionally, at any reasonable distance from gNB, the signal level will be quite low. This puts very strict sensitivity requirements on the spectrum analyzer, which is assumed to be measuring in the boresight of the antenna beam, both in the azimuth and elevation directions. For reliable and repeatable measurements it is wise to use a tripod and a spherical printhead for the measurement antenna. If the field strength also needs to be measured, the receiving antenna K-factor must be known. Finally, to perform measurements on a live gNB, the spectrum analyzer must be able to measure EIRP while decoding the broadcast signaling and use this to control the timing of the measurements.
Conclusions
The new generation of field testers must provide practical and convenient measurements of EIRP in the Far-Field. The Anritsu Field Master Pro MS2090A handheld spectrum analyzer has been designed to meet this new challenge of measuring EIRP according to the 3GPP specifications directly from a 5G base station. This means that it provides enough bandwidth to make accurate measurements on signals occupying 100 MHz or more, while ensuring the necessary sensitivity and low noise floor to record EIRP at realistic distances from an active base station. In addition, it can also lock on to the primary and secondary synchronization signals and use these to measure the EIRP in all the broadcast beams that a 5G base station sweeps across each sector.
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