Lars Jacob Foged of Microwave Vision explains why accuracy matters in measurement of spatial-directional power and sensitivity profiles for 5G antenna arrays.
With 5G cellular networks not too far away, the need for a new approach to ensure accurate measurement of forthcoming 5G cellular networks is needed. With flexible radiation patterns which are capable of adapting to changing situations in mobile networks, the full characterisation of the Active Antenna System (AAS) in 3D space has been the focus of attention as a component of these networks. Lars Jacob Foged from MVG (Microwave Vision Group) explains the new approach to measurement and how fast and accurate AAS characterisation can be achieved.
The importance of MIMO
A prominent role in 5G development, for both user and network segments, is Multiple-Input-Multiple-Output (MIMO) antenna arrays or “Massive MIMO”. “Massive” can vary in definition from AAS arrays with relatively few elements through to more conceptual designs involving hundreds of antennas. Distributed amplification is a common denominator of both beam steering and full integration of the densely packed antenna elements. In order to characterise the AAS, the collective performance must be determined in a calibrated Over-the Air (OTA) setup, in which the spatial-directional power and sensitivity profile are measured. Consequently, the tests for much smaller mobile devices and the associated performance parameters are very similar to these new tests.
AAS performance parameters of interest are the directional dependent power and sensitivity performances in Far Field (FF) condition :
* Effective Isotropic Radiated Power, EIRP(θ,φ)
* Total Radiated Power, TRP
* Effective Isotropic Sensitivity, EIS(θ,φ)
* Total Isotropic Sensitivity (TIS) or Total Radiated Sensitivity (TRS)
The EIRP(θ,φ) and EIS(ϴ,ϕ) are directional performance parameters that can be measured for a given direction of the antenna device in a calibrated Over-the-Air (OTA) measurement setup. The directional EIRP(ϴ,ϕ) is the radiated power weighted by the directional gain G(ϴ,ϕ) of the antenna. The TRP can be determined from a full sphere integration of EIRP(ϴ,ϕ) and associating isotropic gain to the antenna. Likewise, directional EIS(ϴ,ϕ) is TIS/TRS weighted by the directional gain G(ϴ,ϕ) of the antenna. TIS/TRS can be determined by integrating the EIS(ϴ,ϕ) over the full sphere and associating isotropic gain to the antenna.
Defining Far-Field measurement
One of the generally accepted criteria is to define the Far Field (FF) distance of an antenna is 2D2/λ, where D is the diameter of the antenna and λ is the free-space wavelength . For electrically small antennas, such as antennas for mobile communication devices, measurement in FF condition is generally satisfied for convenient short measurement distances. However, even for moderate size AAS antennas, the measurement in FF condition puts unrealistic requirements on measurement distance. Fig 1 illustrates the elevation pattern of an 8-element array antenna @2GHz ( BTS1940 from MVG) for different NF distances and the reference FF distance. The elevation pattern is not fully formed for any realistic measurement distance, as you can see.
The far field pattern of a given antenna can be measured directly in a Compact Antenna Test Range (CATR) [1, 2] or determined from near-field to far-field transformation using standard Near Field (NF) techniques . NF measurements are often preferable for 3D performance scenarios, since they require physically smaller measurement setups and are generally considered faster and more accurate. Due to power conservation, AAS performance parameters can be determined at any distance from the device in a calibrated OTA setup. The difference in NF to FF gain of the antenna can be determined and compensated by standard NFFF transformation techniques .
Phase recovery techniques
Since the AAS antenna is an active device with no fixed phase reference, the measurement in FF condition can be accomplished in an FF setup, such as CATR or a NF range, using phase recovery techniques to allow Near-Field to Far-Field transformation. A common phase recovery method is the holographic technique, which uses different combinations of the measured unknown signal with a stable reference signal. The preferred method here is an evolution of this approach based on the simultaneous reception of the reference and measured signals. A Phase Recovery Unit (PRU) has been designed to perform all the necessary amplification, filtering and signal combination for the accurate determination of the phase of the modulated signal.
Measuring phase recovery
When the actual AAS antenna is emulated using a mobile phone with LTE protocol connected to an 8-element passive array (See Fig 1), as external antenna and Fig 2 illustrates the comparison of the measured amplitude and phase of the co-polar near field using phase recovery compared to passive measurement on the same antenna), you can see that the amplitude and phase correlation between the measurements works well.
When the measurement with phase recovery in LTE modulation was performed with the PRU unit in a 10MHz bandwidth around the 1940MHz centre frequency of the BTS antenna, the error introduced by the phase recovery technique was determined to be equivalent to a -45dB noise level.
NF approach validation
To validate the NF approach, a validation device with known EIS(ϴ,ϕ) and EIRP(ϴ,ϕ) is needed. Since the 8-element antenna and LTE device in this example are separable, the reference EIS(ϴ,ϕ) and EIRP(ϴ,ϕ) performance of the combined device can be determined from the antenna gain and the sensitivity / radiated power of the LTE device from a conducted measurement.
LTE protocol using NF techniques for measurement of EIS(ϴ,Φ) of 8-Element array antenna
The EIS(ϴ,ϕ) of the 8-element array antenna @ 1940MHz using the LTE protocol has been measured in NF and compared to the reference scenario to validate the approach. The EIS(ϴ,ϕ) elevation and azimuth pattern of the reference and NF measurement, using the PRU unit in a 10MHz bandwidth around the 1940MHz centre frequency, are compared in Fig 3. As expected, the pattern shapes are very similar in both azimuth and elevation. The ~1dB offset in measured sensitivity by the two methods is justified by the uncertainties relative to the NF measurements and the determination of the reference scenario. Range calibration and the sensitivity search accuracy for EIS measurement are considered the main uncertainty contributor for the near field measurements. Range calibration and sensitivity search accuracy for conducted sensitivity are considered the main uncertainty contributors for a reference scenario of this type.
LTE protocol using NF techniques for measurement of EIRP(ϴ,Φ) of 8-Element array antenna
The EIRP(ϴ,ϕ) of the 8-element array antenna @ 1940MHz has been measured using the LTE protocol and compared to a reference scenario to validate the approach. The EIRP(ϴ,ϕ) elevation and azimuth pattern of the reference and NF measurement, using the PRU unit in a 10MHz bandwidth around the 1940MHz centre frequency, are compared in Fig. 4. As expected, the pattern shapes are very similar in both azimuth and elevation. The ~0.5dB offset in EIRP(ϴ,ϕ) of the two measurements are justified by the uncertainties relative to the NF measurements and the determination of a reference scenario of this type.
The best way forward
Our conclusion is that the near field measurement technique has been demonstrated effectively in the measurement of performance parameters such as EIRP(ϴ,ϕ) and EIS(ϴ,ϕ) for active antennas such as AAS. It has been confirmed experimentally that the implemented phase recovery unit technique can reliably measure the phase in near field for modulated signal with large BW; such as, LTE and allow for accurate NFFF transformation. The intrinsic advantages of NF measurement techniques makes this, in our opinion, for the accurate measurement and testing of 5G devices, this is the best way forward.
 Ericsson contribution, “On radiated testing of AAS BS”, 3GPP R4-132211, May 2013
 ANSI/IEEE Std 149-1979 Standard Test Procedures for Antennas.
 IEEE Recommended Practice for Near-Field Antenna Measurements, IEEE Std, 1720-2012
 L J Foged, A Scannavini, N Gross, F Cano-Facila “Accurate Measurement of Transmit and Receive Performance of AAS Antennas in a Multi-Probe Spherical NF System”, IEEE International Symposium on Antennas and Propagation, Vancouver, British Columbia, Canada, July 19-25, 2015