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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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into useful information if the signal energy was high enough to overcome naturally and artificially occurring noise and interference. Multi-path reflections may result in significant interference and information loss even if signal power is high. 2.3.2 RF Considerations The following terms and definitions will help the reader to better understand the vocabulary used in wireless technology and have a stronger understanding of wireless networking considerations. 2.3.2.1 General Concepts and Definitions Antenna-A device that converts electrical energy into propagating electromagnetic waves or the reverse. Antenna Polarity-The orientation of the directionality of electromagnetic waves produced by an antenna. Common polarities are vertical, horizontal, and circular. Receive antennas should be oriented such that its polarity matches that of the transmitting antenna polarity. Bandwidth-The amount of spectrum occupied by a signal. For example, a standard IEEE 802.11g transmission will use a nominal 22 MHz of bandwidth. An IEEE 802.15.4 transmission on which ZigBee, WirelessHART, and ISA100 Wireless are designed will use a nominal 5 MHz of bandwidth. Carrier-A single frequency sinusoidal signal represented by a vertical line or spike in frequency. Channel-A term used to identify a physical communications link and includes the characteristics of the entire path of information flow from transmitter to receiver. A channel is defined by electrical and electromagnetic characteristics of the transmission medium such as bandwidth and distortions. Interference-RF power, typically in the RF band of interest, that disrupts communications by inhibiting the ability of a receiver to decode a transmission. Sources of interference could include anything that radiates electromagnetic (EM) energy such as machinery and undesirable radio devices. Signal-to-Noise Ratio (SNR)-Ratio of signal power to naturally occurring emissions such as thermal noise and cosmic background radiation. Maximizing SNR is the primary goal of wireless communications. Signal-to-Noise-And-Interference Ratio (SNIR)-Ratio of signal power to the sum of naturally occurring noise power and interference power. Minimizing the contribution of interference to SNIR is an important goal of a wireless communications system. Power Decibels-A logarithmic representation of a voltage or power relative to a reference. Power is converted to decibels by the equation 10 10 log Pp . The notation dBW and dBm represent power levels relative to 1 Watt and 1 milliwatt, respectively. The notation dB denotes a ratio of two numbers and should not be used to denote power. Link Budget-Calculations that predict the probability that a transmission will be successfully detected and decoded by the receiver. A link budget will account for transmission power, signal formatting, noise, signal distortions, interference, receiver characteristics, and the link reliability requirements. Link Margin-The difference in decibels (dB) between the ability of a receiver to successfully receive a transmission and...

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