OTA testing of 5G’s massive MIMO antenna arrays requires 3D perspective and high-performance equipment
01 June 2018
5G will apply multiple antenna systems and combine them with enhanced spatial multiplexing to provide data for multiple users – known as massive MIMO. One consequence is that performance evaluation of radiation patterns cannot be performed using conducted methods, so connection over-the-air (OTA) will be essential. This piece outlines the technical aspects of how to measure three-dimensional antenna patterns using an OTA testing setup.
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The upcoming 5G standard promises more throughput, capacity and implementation flexibility, while delivering lower operational expenses (OPEX). Other goals include enabling ultra-reliable low-latency communications (uRLLC) and massive machine type communications (mMTC). Software-defined networks (SDN) and massive MIMO multiple antenna scenarios are likely technology choices for achieving these goals.
To obtain the wider bandwidths for higher throughput, 5G systems will use frequencies in the cm and mmWave ranges. One drawback to this approach is higher free-space path loss.
Antenna arrays that provide a much higher antenna gain can compensate for free-space path loss. To maintain the same Rx power at a frequency of 28GHz (compared to 900MHz), the antenna gain must be increased by 30dB. Using a high number of antenna elements, known as beamforming, can achieve this goal.
Beamforming also significantly reduces the energy consumption by targeting individual UEs with their assigned signal. In a base station without beamforming, energy not received by the UE can create interference for adjacent UEs, or is simply lost.
Current standards like LTE or WLAN employ multiple-input multiple-output (MIMO) antennas to obtain a higher capacity through spatial multiplexing. Multi-user MIMO extends MIMO by sending data to different UEs simultaneously using beamforming. The term ‘massive MIMO’ describes the combination of beamforming and spatial multiplexing in a dynamic manner, depending on hardware configuration and channel conditions (see Figure 1).
Challenges for massive MIMO
While massive MIMO offers many advantages, there are also several challenges, including:
1) High throughput for fronthaul interface connection
2) Antenna array calibration
3) Mutual coupling between antenna elements
4) Irregular antenna arrays
5) Antenna array complexity
Massive MIMO introduces similar challenges for characterising signals and measuring antenna array power, which cannot be met by the traditional conductive interface with a cable.
Meaningful characterisation can only be accomplished using OTA testing. There are two major reasons: cost, high losses and coupling experienced at higher frequencies make cable testing unfeasible; and massive MIMO systems integrate the radio transceivers into the antennas, which results in the loss of RF test ports. What are the consequences of this paradigm change?
3D OTA measurements
In the past, power was measured as a function of time, spectrum or code (CDMA systems). The addition of beamforming adds another dimension: space or power versus direction of departure. Figure 2 gives an example of a power measurement.
OTA measurement parameters can be divided into two general categories: R&D and certification or conformance testing, for more complete investigation of the DUT radiated properties; and production, for calibration, verification, and functional testing. The primary test parameters for antenna designers include gain patterns, radiated power, receiver sensitivity, transceiver/receiver characterisation and beam steering/beam tracking. Each has its own implications for OTA measurements.
Beam steering/beam tracking is of special interest, however, because of the frequencies employed by massive MIMO. Although static beam pattern characterisation is used for existing cellular technology, mmWave systems will require dynamic beam measurement to characterise beam tracking and beam steering algorithms accurately.
Conformance and production testing has many aspects. Three that are of particular importance include:
• Antenna/relative calibration – To form beams accurately, the phase misalignment between RF signal paths must be less than ±5°. This measurement can be performed using a phase-coherent receiver to measure the relative difference between all antenna elements.
• Five-point beam test – According to 3GPP, the active antenna system (AAS) manufacturer specifies a beam direction, maximum EiRP and an EiRP threshold for each declared beam. In addition to the maximum EiRP point, four additional points are measured at the declared threshold boundary – in other words, a central point with highest EIRP and the remaining four points that declare the left, right, top and bottom boundary (as Figure 3 illustrates).
• Final functional tests – Performed on the completely assembled unit in production, this can consist of a simple radiated test, a five-point beam test, and aggregate transceiver functionality, such as an EVM measurement of all transceivers.
Near and far-field measurements
OTA measurement systems can be classified according to which part of the radiated field is being sampled. Figure 4 illustrates the near and far-fields from a base station antenna array (eight circular microstrip antenna patches at 2.70 GHz with uniform excitation).
The near-field and far-field regions are defined by the Fraunhofer distance R = 2*D2/?, where D is the maximum antenna aperture or size. In the near-field region, at distances less than R, the field consists of both reactive and radiated components; whereas the far-field of an antenna has only the radiated component.
Precise phase and magnitude measurements over a three-dimensional surface surrounding the DUT are required for the mathematical transformation to the far-field region, resulting in the antenna’s 2D and 3D gain patterns. A measurement in the far-field region needs only the magnitude to calculate the beam pattern of the antenna and can be measured at a single point in space if desired.
For small devices (in terms of wavelengths), such as UEs, the required chamber size for far-field conditions is dominated by the measurement wavelength. For larger devices, such as base stations or massive MIMO, the required chamber size may become very large.
Chamber sizes can be reduced significantly as long as the measurement system accurately samples the phase and magnitude of the electromagnetic field on the entire enclosing surface.
Measuring in the far-field region requires a direct measurement of the magnitude of the plane waves: such chambers are generally quite large, where the length is set by a combination of the DUT size and the measurement frequencies.
Although the far field is generally measured at a suitable distance from the DUT, it is possible to manipulate the electromagnetic fields, such that a near-field chamber can be used to directly measure the plane wave magnitudes.
There are two techniques:
• Compact range chambers, which are most often used for large DUTs, such as aircraft and satellites; and,
• Plane wave converter (PWC): a planar wave is created at the DUT by replacing the measurement antenna with an antenna array. Similar to using lenses in an optics system, the antenna array can generate a planar far field at a targeted zone in the region of the DUT.
Measurements in the near-field region require both the field phase and magnitude sampled over an enclosed surface (spherical, linear or cylindrical) in order to calculate the far-field magnitude using Fourier spectral transforms.
This measurement is usually performed using a vector network analyser, such as the R&S ZNBT20, with one port at the DUT and the other port at the measurement antenna. For active antennas or massive MIMO, there are often no dedicated antenna or RF ports; therefore, the OTA measurement system must be able to retrieve the phase in order to complete the transformation into far field. There are two methods of performing phase-retrieval for active antenna systems:
• Interferometric – A second antenna with a known phase is used as a reference. The reference signal is mixed with the DUT signal with unknown phase. Using post-processing, the phase of the DUT signal can be extracted and used for the near-field to far-field transformation.
• Multiple surfaces or probes – A second surface volume is used as the phase reference with at least one wavelength separation between the two measurement radii. Instead of multiple surfaces, two probes with different antenna field characteristics can be used. The two probes need to be separated by at least a half-wavelength to minimise mutual coupling.
When selecting a vector network analyser (VNA), true multiport VNAs such as the R&S ZNBT20 have an additional advantage for measuring coupling between antenna elements. Having multiple receivers – instead of using switches – to perform tests simultaneously reduces test duration, and does a better job of performing complete mutual coupling measurements.
Antenna arrays will play an essential role in future wireless communication. But challenges in their development, design and production make thorough testing essential to achieving optimal performance. The elimination of RF test ports and the use of frequencies in the centimetre and millimetre wavelength region make OTA a vital tool for characterising the performance of – not just massive MIMO arrays – but the internal transceivers as well.
This will drive a high demand for OTA chambers and measurement equipment to measure the strict radiative properties of antennas and transceiver measurements. Rohde & Schwarz, with its wide range of anechoic chambers and measurement equipment expertise, is well situated to deliver solutions – even for future customer requirements.
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