The evolution of wireless communication devices, such as smartphones, with multiple communication protocols (e.g. Long Term Evolution (LTE), near field communication (NFC), Code division multiple access (CDMA)), has resulted in a dramatic increase in the number of tests required to assess compliance of such devices to human exposure limits in the past ten years. This compliance is evaluation is carried out through the measurement of the specific absorption rate (SAR). New measurement systems exist that can rapidly evaluate the SAR using arrays of vector probes in sealed phantoms and therefore solve the growing test matrix challenge. Yet the traceability and uncertainty level for these systems has not been established. Improved metrology for SAR measurements is also needed to support the development of IEC 62209-3. This standard currently under development by IEC TC 106 intends to standardise SAR measurements with vector probes and will be used to assess the majority of wireless telecommunications devices across Europe.
The development of mobile phones is ever-increasing with 1.3 billion smartphones sold worldwide in 2014. The number of telecommunication protocols tested during the production phase of such smartphones to assess SAR is also increasing, particularly with the arrival of LTE-advanced. In fact, SAR measurements using conventional single probe systems, as defined in IEEE/IEC 62209-1 and 2, have become cumbersome in terms of the time required to test a device. For example, a modern smartphone may count today more than 30 transmission technologies and bands embedded, including multiple LTE bands, CDMA, Universal Mobile Telecommunications System, Enhanced Data rates for GSM Evolution, IEEE 802.11, Bluetooth, also potentially involving multiple-input multiple-output (MIMO), carrier aggregation, etc…, Such a high-end device would typically require a five weeks compliance testing over three shifts . Further to this, upcoming and future communications standards such as LTE Release 10 to 12 will incorporate complex MIMO antennas that cannot be efficiently assessed using the systems specified in current published standards, as they do not measure phase. Measurement of carrier aggregation is also a challenge for traditional SAR measurement technologies as all of them do not have the capability to distinguish between frequency contributions to SAR.
In order to improve the time-to-market of smartphones and take on the challenge of measuring more advanced communication modes, time-domain probes and vector probe arrays have been developed and combined with advanced signal processing techniques. The measurement of electric fields by these means is faster (SAR evaluation is shortened by a factor of at least 100 compared to traditional systems using robot scanning) and gives almost instantaneous measurements of the SAR. The systems use arrays of phasor probes that measure the magnitude and relative phase of the electric fields embedded inside a sealed phantom. Also some of the vector probe systems have a spectral analysis capability making them perfectly suitable for measuring communication modes involving multiple frequency carriers.
The overall objective is to develop traceable measurement and characterisation methods for use in the European telecommunications industry, and supporting a sustainable approach for ensuring the highest measurement quality standards for system complying with the IEC 62209-3 standard being developed by IEC TC 106, “Methods for the assessment of electric, magnetic and electromagnetic fields associated with human exposure”.
The JRP shall focus on metrology research necessary to support standardisation in SAR measurement using vector probes.
1. To develop traceable methods for the calibration of time-domain probes and probe arrays up to 6 GHz. In addition to verify the accuracy of such measurement systems after calibration and to determine the properties of associated sealed phantoms.
2. To establish methods for uncertainty propagation through multivariate models, using the principles given in the ‘Guide to the expression of uncertainty in measurement’ (GUM). This should include identifying the sources of measurement uncertainties and their propagation through multivariate transformations and developing single vector probe systems for use on scanning systems.
3. To verify the reliability of measurement systems for a wide range of transmitter types and improves the measurement of telecommunication signals and SAR measurement for a wide range of device types. This should include the development of improved data processing used with time-domain probes.
4. To develop test protocols for MIMO and modular devices using vector probe arrays in order to determine the maximum SAR value (worst case) by combining MIMO signal figures.
5. To facilitate the uptake of the developed measurement systems and contribute to the standards development work of the technical committee IEC TC 106 on the successful adoption of IEC 62209-3 standard vector-based SAR measurement systems in Europe. In addition, to ensure that the outputs of the project are aligned with the needs of IEC TC 106 and in a form that can be incorporated into the standards at the earliest opportunity.
Methods for the calibration of SAR using single probes are well established in IEC62209-1 and 2 and yield calibration expanded uncertainties of approximately 10 % at k = 2. However, these methods cannot be applied to arrays of sensors, mainly due to their physical size, and they do not calibrate the phase measurement for the vector probe, which is important for the accuracy of the field reconstruction. Further to this, it is the performance of the whole array of sensors that is of interest and this cannot be determined from measurements of individual sensors. Therefore, the IEC is developing a new standard IEC62209-3 to standardise SAR measurements with vector probe arrays, however this has highlighted the challenges of establishing traceability and uncertainty analysis for vector-based measurement systems. As a result there is a current reliance on the validation of SAR measurements using reference sources, and although this is suitable for a small range of existing devices with reference sources, it does not translate to new devices under development (and without reference sources).