February 4, 2020
The importance of active antenna systems in 5G networks has significantly changed the installation and maintenance of base stations. Gone are the days of simply measuring transmitter power with an absorption power meter or by using a direct connection via a “sniffer” port in the antenna feed. Due to the vastly more complex transmitter and antenna systems necessary to achieve the efficiency and speeds of 5G, a different set of tests must be performed. Subsequently, a new generation of field solutions must be employed to conduct these measurements.
Engineers installing and maintaining 5G base stations must gather measurements that were different or not required with 4G LTE. Among the parameters that help operators determine the performance and likely coverage achievable with their 5G networks are:
- Reference Signal Receive Power (RSRP) and Signal-to-Interference-plus-Noise Ratio (SINR) – provide a ratio between the wanted signal and any unwanted signal and noise in the channel bandwidth used.
- Reference Signal Received Quality (RSRQ) – provides a measure of the received signal quality and is derived from detected error rates.
- Error Vector Magnitude (EVM) – a quality measurement that analyzes the accuracy of the modulation transmitted relative to an idealized waveform.
- Effective Isotropic Radiated Power (EIRP) – the product of transmitter power and the antenna gain in a given direction relative to an isotropic antenna of a radio transmitter.
These measurements are necessary to provide mobile operators with an indication of how strong a connection should be to user equipment (UE) in the field.
Active Antenna Systems Impact
EIRP is vital to determine transmitter power and beam verification of a 5G base station. The reason is that active antenna systems operate much differently than the isotropic antennas used for many years in traditional cellular applications.
Isotropic antennas are typically designed to radiate in an arc of 60⁰ to 120⁰, depending on the number of sectors a base station might support. Some electro/mechanical adjustment might be included, so carriers could optimize coverage but, essentially, they were fixed. By knowing the antenna gain and input power, operators could predict signal strengths in the field and estimate performance.
5G uses directional antennas that can generate beams in multiple directions simultaneously and be steered dynamically. To achieve this, it is necessary to use a matrix or array of antennas, each being fed with different signals. At a minimum distance from the antenna, known as the far field, these signals combine together in or out of phase to create a beam-like pattern known as a phased array (figure 1).
For 5G, these active antennas typically have an individual transceiver associated with each antenna element. Each element will also have two cross-polarized radiators to improve diversity. In total, a 5G antenna may have 128 to 256 individual transceivers. All of them may be integrated with their own radiating elements, making the traditional method of measuring antenna input power impractical.
Why EIRP is Important
Based upon this antenna design change, the most useful measurement of power is EIRP. It provides the mobile operator with a reliable indicator of signal strength anywhere in a cell once path loss is taken into consideration.
Conducting EIRP measurements in the field, however, is not as straightforward as it might seem. Even sub–6 GHz 5G transmitters have the potential to use bandwidths of up to 100 MHz, therefore any measuring receiver has to be “flat” across the channel bandwidth while adequately rejecting other signals on adjacent channels. At any reasonable distance from the base station, the signal level is going to be quite small.
If measurements on a live base station are required, the field engineer or technician needs to extract the “beamed” transmission in the direction to be evaluated, as well as know the intended EIRP. This means the test instrument must be able to track the on/off periods of the signal and use that information to control the measurement timing.
The last aspect of making an EIRP measurement is to understand the various path loss elements and remove them from the equation. Free-space path loss is significant and can only be determined if the distance between the measuring antenna and the base station is known. This can be calculated using various techniques, including GPS, laser/optical rangefinders, and accurate large scale maps. Other factors include the gain of the measuring antenna and the insertion loss of any connecting cables.
Instruments for Accurate EIRP
A handheld spectrum analyzer with sufficient bandwidth to accurately measure signals occupying 100 MHz or more, as well as enough sensitivity and low noise floor to record EIRP at realistic distances from an active base station, is necessary to test 5G networks. The Anritsu Field Master Pro MS2090A (figure 2) is one such instrument. It also has the ability to lock onto the primary and secondary synchronization signals, so EIRP can be measured in up to eight broadcast beams that 5G base stations step sweep across a sector. This allows mobile devices to acquire synchronization to report their presence and location (figure 3).
It is also possible for fault finding and commissioning teams to place the base station in a test mode where it transmits a known “test model” signal in a given direction and strength. This allows radiation patterns to be established and field strength in complex environments to be measured. It also provides a means of using conformance test methods in the field.
Conclusion - Impact of Beamforming
It is clear that the arrival of beamforming in 5G has made traditional methods of measuring the total radiated power of a base station ineffective to optimize network coverage. Using EIRP measurements in various directions allows mobile operators to determine beam shapes, location/angles of the nulls, and safety of field strengths close to the base station. The result is minimal interference.
To learn more about how to conduct EIRP measurements and their importance when installing and maintaining 5G base stations, download this application note.
