Interest to improve spectrum usage gained attention from researchers and a whole new field of research was born: spectrum sensing. Spectrum sensing consists of finding instantaneous holes in the spectrum so different devices can share and use it according to the real, immediate availability.
Government policies around the world regarding EM spectrum are now actively being revised in order to let researchers and industry players adapt new solutions and technologies to our modern world needs. In Australia, for example, the Australian Communications and Media Authority (ACMA) developed “Principles for spectrum management” to guide its decision-making process on a range of significant spectrum management initiatives.
The principles for spectrum management are as follows:
- Principle 1 – Allocate spectrum to the highest value use or uses
- Principle 2 – Enable and encourage spectrum to move to its highest value use or uses
- Principle 3 – Use the least cost and least restrictive approach to achieving policy objectives
- Principle 4 – To the extent possible, promote both certainty and flexibility
- Principle 5 – Balance the cost of interference and the benefits of greater spectrum
The millimeter wave spectrum
In this decade we were given a very particular challenge: never-ceasing growth in the demand for spectrum to meet global communication needs. The technology has evolved; antenna gain capabilities have dramatically improved with the arrival of semiconductor technology (along with other improvements).
But the available usable spectrum has remained the same… or has it?
In the roughly 120 years of wireless telecommunications history, the exploration of new frequency regions has always led to technological advances. Today, research based in the EHF band or millimeter wave band is focused in finding its advantages and exploitability for radio communications. The wavelength (λ) is 1 to 10 mm and the range of frequency spectrum is from 30 to 300 GHz.
High frequencies are very interesting compared to the traditional lower bands for two reasons:
1. Larger bandwidth availability
2. Smaller antenna dimensions for a fixed gain, or higher gain for a given antenna size.
Larger bandwidth is directly proportional to higher data transfer rates. Furthermore, with larger bandwidth, applications like 10 Gbit/s file transfers, near perfect video streaming, and real-time gaming are feasible.
Additionally, a larger bandwidth enables capabilities like wideband spread-spectrum systems (for reduced multipath and clutter) and systems with a high immunity to jamming and interference.
Millimeter waves open up more spectrum but until recently few electronic components were able to generate or receive millimeter waves, so the spectrum remained unused.
Generating and receiving millimeter waves is a challenge, but the biggest and most challenging factor with these high frequencies is the travelling media. Poor foliage penetration has been observed but the biggest challenges are atmospheric and free-space path loss.
Millimeter waves are governed by the same physics as the rest of the radio spectrum and as such, they have limitations related to their wavelength. The shorter the wavelength, the shorter the transmission range for a given power.
Let’s recall some signal properties remain constant, regardless of factors like antenna gain at the transmission and receiver, or reflection, absorption and diffraction during the signal transmission.
· Free space loss – The free space loss in dB is calculated with:
L(Transmission loss) = 92.4 + 20log(f) + 20log(R)
· Atmospheric absorption – The atmosphere absorbs millimeter waves, thus restricting their transmission range. Rain, fog, and moisture in the air make the signal attenuation very high. Oxygen (O2) absorption is especially high at 60 GHz.
· Mechanical resonance – The mechanical resonance frequencies of gaseous molecules also coincide with the millimeter wave signal. For current technology, the important absorption peaks occur at 24 and 60 GHz.
· Scattering – Millimeter wave propagation is also affected by rain. Raindrops are roughly the same size as the radio wavelengths and therefore cause scattering of the signal.
· Non-line of sight issues – When a line-of-sight path between transmitter and receiver isn’t present, the travelling signal still has alternative ways to reach the receiver, be it through diffraction, reflection or bending. Diffraction in millimeter waves is scarce due to the short wavelengths.
· Brightness temperature – When millimeter waves are subjected to absorption by water vapor, oxygen and rain, these molecules absorb high frequency electromagnetic radiation. This absorption subsequently leads the molecules to emit higher frequency EM radiation (closer to the infrared spectrum). This energy emission, when received by a receiver antenna, is called brightness temperature and it degrades system performance. Any Earth-based antenna aimed at a satellite with a high elevation angle, for example, will suffer signal degradation caused by picking up brightness temperature emanating from atmospheric constituents.
Possible applications for 30 to 300 GHz
Communication systems operating at millimeter wave frequencies can take advantage of the propagation effects described in the preceding sections.
The limited range permits a high degree of frequency reuse. Also, millimeter waves are privileged frequencies for point-to-point systems like local area networks and vehicular radar systems. 
In the absorption resonance bands (e.g. 60 GHz) relatively secure communications can be performed. This is useful for high data rate systems where secure communications with a low probability of intercept is desirable. The bands are also useful for services with a potentially high density of transmitters operating in proximity or for applications where unlicensed operations are desirable.
At some point in the next 5 to 10 years we will likely face heavy congestion in wireless networks. Now is the time to find efficient solutions to this problem. The bridge from 4G to 5G will be dramatic in terms of the exponential growth in data rates and latency (a 1 ms range will be required). Aggregate data rate and edge rate will need to meet requirements that are 1000X and 100X the respective current 4G technology ones.
With these requirements in mind, we will need to exploit the millimeter wave spectrum’s increased bandwidth to provide increased data rate and better quality of the received signal. In the past, transitioning to the next generation of wireless technology has taken roughly a decade. Nothing so far seems to indicate this transition will be faster. However, the pace of technology developments is only increasing as time goes on.
Designing millimeter wave systems
To optimally design a millimeter wave wireless system, a primary requirement is to understand the radio channel at the specific frequencies and the relevant use-cases.
Figure 1: Atmospheric absorption of millimeter waves. 
The negligible atmospheric absorption at 28 and 38 GHz, for example, makes it a good candidate for 5G cellular systems. At these frequencies, the millimeter wave bands offer a massive amount of unlicensed spectrum. In fact, 28 and 38 GHz frequencies can be used to employ steerable directional antennas at base stations and mobile devices.
Higher frequencies, with their shorter wavelengths, make the implementation of dense antenna arrays for massive MIMO systems much more practical than the equivalent for lower-frequency systems. With lower frequencies, the same array of multiple antennas would render the physical towers for cellular networks extremely impractical.
The biggest challenge in urban areas will be signal penetration. To provide adequate coverage for users surrounded by tall buildings in big cities, base stations will need to be closer to improve capacity as well as cheaper to remain sustainable.
This article introduced the basic challenges and initial opportunities for millimeter wave engineering, especially in terms of physical properties. We know how we got here but nobody knows where this new challenge will lead us in the upcoming years, so stay tuned!
 http:/http://www.nutaq.com.wikipedia.org/wiki/Cognitive_radio http://www.acma.gov.au/webwr/_assets/main/lib312084/ifc13_2011_toward_2020-future_spectrum_requirements.pdf
 Belcher, Robert. EXTREMELY HIGH FREQUENCY (EHF) LOW PROBABILITY OF INTERCEPT (LPI) COMMUNICATION APPLICATIONS. MS Thesis. Naval Postgraduate School, 1990. PDF file  http://nyuwireless.com/wp-content/uploads/2013/12/Path-Loss-Models-for-5G-Millimeter-Wave-Propagation-Channels-in-Urban-Microcells.pdf  http://www.rfcafe.com/references/electrical/ew-radar-handbook/rf-atmospheric-absorption-ducting.htm