Millimeter-wave (mmWave) communications promise Gigabit/s data rates thanks to the availability of large swaths of bandwidth between 10-100 GHz. Although cellular operators prefer the lower portions of the spectrum due to popular belief that propagation there is more favorable, the measurement campaigns to confirm this -conducted by ten organizations thus far -report conflicting results. Yet it is not clear whether the conflict can be attributed to the channel itself -measured in different environments and at different center frequencies -or to the differences in the organizations' channel sounders and sounding techniques. In this paper, we propose a methodology to measure mmWave frequency dependence, using the 26.5-40 GHz band as an example. The methodology emphasizes calibration of the equipment so that the measurement results represent the channel alone (and not the channel coupled with the channel sounder). Our results confirm that free-space propagation is indeed frequency invariant -a well understood phenomena but to our knowledge reported nowhere else at mmWave to date. More interestingly, we found that specular paths -the strongest after the line-of-sight path and so pivotal to maintaining connectivity during blockage -are the least invariant compared to weaker diffracted and diffuse paths.

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Material 13 4.3 Calorimeter Output Analysis 13 4.4 Electrical Calibration of the QUV Calorimeters 15 5. CALIBRATION OF LASER ENERGY OR POWER METERS 5.1 Performance Capabilities of the 248 nm Calibration System 5.2 Calibration of Energy Measuring Instruments 5.3 Calibration of Power Reading Meters 5.4 Laser Meters for Calibration 6. MEASUREMENT UNCERTAINTY 6.1 QUV Reference Standard Calorimeter Uncertainty 21 6.2 Calibration System Measurement Uncertainty 7. MEASUREMENT ASSURANCE 7.1 Electrical Calibration History 25 7.2 Laser Beamsplitter Ratio Measurements 7.3 Check Standard Calibrations to Monitor the QUV System 7.4 Revising the Uncertainty Sources from Accumulated Data 8. FUTURE CALIBRATION SYSTEM CHANGES 9. REFERENCES 111 APPENDIX A. Sample Calibration Report 29 APPENDIX B. Calibration Procedure Outline 33 APPENDIX C. Shipping Instructions 34 TRADE NAME DISCLAIMER Certain commercial components used in the calibration system are identified in this report in order to adequately document the design. Such use and identification do not imply recommendation or endorsement by NIST, nor do they imply that the items are necessarily the best available for the purpose.

We describe the use of a silicon photodiode trap detector and digital storage oscilloscope as an absolute laser pulse energy meter, capable of repetition rates of 85 Hz and 5% uncertainty (k = 2). The maximum repetition rate is limited by the decay time of the output pulse of the detector. The technique relies on a straightforward oscilloscope-based integration of the voltage pulse generated by the photodiode trap detector. We highlight the versatility of the technique by comparing it at 1064 nm with our high and low-level calorimeter based pulse energy scales, to which our calibration services are traceable. The good agreement of the results, along with comprehensive uncertainty analysis, validates the approach we have taken. We have demonstrated a basis for establishing laser pulse energy measurement using continuous wave (CW) sources and standard detectors.

This paper describes a wideband synthetic-aperture system and the associated Fourier processing for generating high-resolution spatial and temporal estimates of the signal propagation environment in wireless communication channels at millimeter-wave frequencies. We describe how to configure the synthetic aperture system for high angular resolution by sampling the progression of signal phase across a large planar area in space. We also show how to synthesize discrete measurements of the channel frequency response taken sequentially over a wide bandwidth to create power delay profiles (PDPs) in specified angular directions with high delay resolution. We provide a rigorous uncertainty analysis that can be made metrologically traceable to fundamental physical standards. This uncertainty framework can propagate the errors inherent in the measured signals through to the final channel estimates and derived parameters such as root-mean-square delay or angular spread. We illustrate use of the system in conjunction with two different analysis tools to extract both narrowband and wideband parameter estimates from the synthetic aperture, allowing its use as a stand-alone channel sounder or as a tool for verifying the performance of wireless devices.