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Wireless communications has emerged as one of the largest sectors of the telecommunications industry. Wireless data usage has increased at a phenomenal rate and demands the need for continued innovations in wireless data technologies to provide more capacity and higher quality of service.
Today there are billions of cell-phone users in the world. This massive volume of wireless phone communication has been possible because of an enormous cost-reduction to cell phones despite their sophisticated hardware and software capabilities. The existing cell phones are also already equipped with advanced digital imaging and sensing platforms that can be utilized for various applications. Faster mobile broadband connections, more powerful smart phones, connected tablets; networked laptops as well as new consumer and enterprise applications are all driving the wireless industry to provide new technical capabilities. Mobile communication has been one of the most successful technology innovations in modern history.
In order to meet this exponential growth, improvements in air interface capacity and allocation of new spectrum are of paramount importance.All cellular mobile communication requires ultra high frequency band of radio spectrum (collection of various types of electromagnetic radiations of different wavelengths is called as spectrum ) The radio frequency spectrum is a limited natural resource. Variety of services like fixed communication, mobile communication, broadcasting, radio navigation, radiolocation, fixed and mobile satellite service, aeronautical satellite service, radionavigational satellite service etc work in the range of radio frequencies below 3 Ghz.
This band of spectrum is therefore becoming crowded because of enormous growth of mobile services. Howere the millimeter wave spectrum at 30-300 GHz can be exploited for commercial applications, in order to meet the growing demand of data traffic at improved efficiency .
For network upgrade the operators require access to additional spectrum, because the capacity in the network is determined by the amount of spectrum. Spectrum is a finite, non-exhaustible common resource which influences the valuation, and some parts of the frequency band are more valuable than others. Almost all mobile communication systems today use spectrum in the range of 300 MHz-3 GHz.
A millimeter-wave mobile broadband (MMB) system has a candidature for the next generation mobile communication system. IT is the region in electromagnetic spectrum usually ranging from 10 millimeter to 1 millimeter. mm waves are longer than infrared waves, however they are shorter than radio waves .Millimetre-wave (mm-wave) band corresponds to 30-GHz ~300GHz, about 270-GHz bandwidth ,which is ten times the bandwidth in Centimetre-wave band (3-GHz~30-GHz).Millimetres wave can be utilized for a variety of applications including transmitting large amounts of computer data, cellular communications, and radar.
In this paper we discuss the feasibility of using radio frequency spectrum above 3 Ghz in supporting applications such as high speed data transmission and video distribution for wireless applications.
History of MM waves
Millimeter wave technology goes back to the 1890's experiment on millimeter wave signals by J.C. Bose
The early research paved the way for applications of mm wave technology in the field of Radio Astronomy. Satellite-based studies of the upper atmosphere, Climate, rainfall and vegetation patterns, and a host of other environmental concerns.
In the late 1970s, a millimeter wave radiometer began service aboard a NASA aircraft, where it monitored storm activity from an altitude of 60,000 feet. Scanning about 5,000 miles of atmosphere per hour, the device recorded the emitted and reflected energy of storms, including the almost infinitesimal amounts of energy emitted by moisture inside a storm.
This was followed by applications in the military. In the 1990's, the advent of automotive collision avoidance radar at 77 GHz marked the first consumer oriented use of millimeter wave frequencies above 40 GHz.
Georgia Tech scientists also achieved a number of firsts in millimeter characterization of clutter and targets - essential data for reliable millimeter radar systems. Since the 1960s, more than a dozen projects have provided millimeter measurements of the ocean, rain, snow-covered ground, desert, foliage and foreign military vehicles.
In the 1980s,researchers conducted a comprehensive study of the image-quality effects of atmospheric turbulence and precipitation on millimeter wave propagation.
More recently developing markets include consumer satellite communications that bring broadband Internet access to businesses and rural consumers, wireless broadband media transfer within the home, automotive radar for tasks such as adaptive cruise control and collision avoidance, and telecommunications links that are approaching the performance of optical fiber but at a fraction of the cost. Millimeter wave security imaging, such as that used to screen airline passengers and personnel at other checkpoints, is undergoing deployment at airports and businesses, where it is used for loss prevention and inventory control. Systems are even commercially available for retail clothing shoppers to conduct body measurements to determine clothing sizes and recommend appropriate products and brands.
Advantages of mm wave transmission
Huge Spectrum Availability
A 250Ghz bandwidth is available in the mm wave band (30Ghz-300Ghz).This is almost 1000 times higher than the frequency range used these days. The availability of such high carrier frequencies facilitate more data rates by using amplitude,phase or frequency modulations.It can also be reliable for data transmission at GBPS rates.
Small Component Size
For MM waves the wavelengths are shorter and therefore the frequencies are high. Therefore, the antenna systems required for the transmission can be of the millimeter size. This also enables densely packed communication link networks integrating high efficiency radiating elements at the millimeter scale, leading to compact, adaptive and portable integrated systems. Even arrays of antennas may be packaged within the area of a quarter for directionally transmitting and receiving radio signals.
Improvement In The Directivity
The other advantage of smaller antenna size and reduced packaging is the improvement in the directivity. About 25dBi of directivity can be comfortably achieved because the nodes have compact form factor as compared to wireless access point.(this is because of small wavelegths)
Radar is an important use of millimeter waves, which takes advantage of another important property of millimeter wave propagation called beamwidth. Beamwidth is a measure of how a transmitted beam spreads out as it gets farther from its point of origin. In radar, it is desirable to have a beam that stays narrow, rather than fanning out. The use of millimeter-length has allowed engineers to overcome antenna problem. For a given antenna size, the beamwidth can be made smaller by increasing the frequency, and so the antenna can be made smaller as well.
Narrow Beam and Frequency Reuse
Millimeter wave links cast very narrow beams, as illustrated in Figure This allows the deployment of many independent links in close proximal distances.
Therefore along with the huge and unexploited bandwidth availability , smaller range and narrow bandwidth facilitate for a higher degree of frequency reuse. mm wave limits the propagation to a few kilometers, thus they are useful for densely packed communications networks Furthermore the high oxygen absorption in mm range of frequencies gives a large frequency reuse factor .Example : For 60 GHz , the working range for a typical fixed service communications link is of the order of 2 km, and therefor the other link could be employed on the same frequency if it were separated from the first link by about 4 km. IN contrast, at 55 GHz, the working range for a typical fixed service link is about 5 km, but a second link would have to be located about 18 km away to avoid interference.
Millimeter wave technology has a strong history and technological evolution behind it. Properties of millimeter wave propagation have been well researched and documented..Millimeter wave technology has reached a level of maturity comparable to older forms of radio technologies Also,the carrier frequency is high, so expensive compound semiconductor technologies such as GaAs were the only choices earlier. But now some companies have demonstrated that the chipsets can be manufactured with silicon-based technologies .
Limitations of mm wave transmission
Atmospheric Gaseous Losses
When the mm waves are transmitted through the atmosphere they are absorbed by molecules of various gasse and water vapours. At the resonant frequencies of the gas molecules these losses are very high and the absorption results in high attenuation of signals. The transmission can be effective if spectral regions between the absorption peaks are used for propagation.
Rain greatly affects the mm wave propagation. The rain drops being same in size with the radio wavelengths causes large but slow changes in strength of radio signal. Example :A rain rate of 2.5mm/hr yields 1 db/km attenuation while a rate of 25mm/hr results in 10 db/km attenuation. During rainy season where rain rate high there can be loss of communication upto tens of dBs per km. In otherwords increasing rain factor reduces the availability of communication signals.
Foliage losses can change the attenuation rate substantially. Example: At 80GHz frequency and 10 meters foliage penetration, the loss can be about 23.5dB which is about 15dB higher compared to the loss at 3GHz frequency. The transition because of heavy foliage can be abrupt and leads to beam broadening (and depolarization) after transition has occured. This can limit impairment for propagation of mm wave transmission. There can be a significant change in attenuation over the same transmission paths, under summer and winter conditions, i.e. with tree in leaf and without leaves
free-space loss and limited communications range
weakening of mm wave signal due to line of sight path through air is termed the free space path loss. Even a shorter distance leads to a high free space loss for mm waves. Small change in range causes 6dB of change in the attenuation. Obstacles like human body causes significant drop in received power of the signal. Moreover for long distances wireless mm wave signal nullifies gain of antennas. Also ability of signal to bend around edges of obstacle is very weak.
Possibility to transmit mm-wave over the fiber-optic links,
Research is being done to tansmit mm waves using fiber optic links.This will exploit the advantages of both optical fibers and mm-wave frequencies . Figure gives the architecture of mm-wave RoF system. Central Station (CS) and distributed Base Stations (BS) can be linked with optical fibers. BS can be designed to communicate with Mobile Terminals (MT) by wireless signals at mm-wave band.
This system requires generation of low noise mm wave signals to overcome the effects of fiber chromatic dispersion.Base station can be made light wave to mm wave converter and the signal processing can be handled at the central station.
outdoor mesh networks with multiGigabit links at relatively short ranges can be designed using mm wave technology. Such mm wave mesh networks can support a high-speed broadband connectivity .Networking can be
-based on multihops operating over lower frequency mm bands . The directional transmission can be incorporated to improve the connectivity of ad-hoc networks by establishing long-range links even without using smart beam steering.
Survival rate of millimeter-wave mesh is limited due to their severe weather conditions, like precipitation and humidity. Spatially correlated links of a mesh network should use routing protocols to route around the failures.This will increase dependability when compared against existing routing methods.
Millimeter-wave CMOS Platform
Since carrier frequency of mm wave is very high expensive GaAs technology was the only option until recently. Evolution of nm technology in CMOS has made possible the designing of low cost 24 - 60 GHz mm wave signals using silicon. . Combining CMOS technology with FR4-based packaging technology ensures successful deployment of ultra high-speed and high capacity 60 GHz WPAN at minimal cost. IBM proved that silicon based technologies could be the solution for manufacturing chips at reduced cost, power and form factor than GaAs technology. For efficient data transmission at small cost and at low power consumption high speed coding techniques and signal processing can be used. . The chip set including antenna can become small and affordable in near future.
IBM's mmWave transmitter and receiver chip
The major attractiveness of the millimeter-wave is the small wavelength This allows deployment of many radiating elements in an array configuration which will occupy limited space. Compact multi-sector phased-array architecture can overcome range limitations of millimeter-wave signal propagation . The sectored design can either be integrated on a one large panel or in a compact module containing an embedded filter and antenna phased array. Liquid Crystal Polymer has emerged as a promising low cost alternative for millimeter-wave module implementation. High Gain Adaptive phased array technology and multi sectored configurations can provide extended range and better elevation coverage. These can be exploited for commercial development of mm wave system.