General description
As opposed to the wired media (with no distinction between copper wire and fiber optic), Wireless media is an unguided media – it doesn’t require to layout cables of any kind, in order to guide the information on its way. The wireless transmission goes through the air, space and even water, using antennas to broadcast and receive. Since wired media have cables to guide the transmission on its way to the receiver (in the form of electromagnetic waves), there is no problem of finding the receiver, because there is no where else to go but forward, to the receiver. That is not the case in wireless media – the transmission may be radiated in every possible direction, not just the wanted one. This fact depends on the frequency of the transmission - lower frequencies will spread everywhere, but higher frequencies can be directed into a beam, as if the transmission is guided.
In the electromagnetic spectrum of frequencies, wireless transmission begins at the frequency of 30 MHz, and ends around 2*1014Hz.Obviously, this is a wide spectrum to cover, and there are many differences between the different parts of the spectrum. A common division of the spectrum is to refer to [1]:
- 30 MHz to 1 GHz as the broadcast radio frequencies range
- 1 GHz to 40 GHz as the microwave frequencies range
- 3*1011 Hz to 2*1014Hz as the infrared frequencies range
Broadcast Radio (30 MHz - 1 GHz)
Broadcast radio operates in the lower frequencies of the wireless spectrum, so its wavelengths are relatively large (there is a reverse linear relationship between the frequency of the wave and its length). Thus, it prevents us from directing the waves in one direction, so broadcast radio electromagnetic waves are omni directional (radiate in all directions). This characteristic makes it perfect for multicast communication, and since the broadcast radio frequencies cover the VHF and some of the UHF bands, it is the natural choice for FM radio and VHF and UHF TV.
Radio waves above 30MHz are not reflected by the ionosphere, so the transmission is limited to the line of sight (there can't be any use of the ionosphere as a mirror, because there is no reflection). This may limit the coverage of the transmission, but prevents Interference between distant transmitters. However, radio waves are not immune to reflections from the land, water and other elements (for example – an airplane) capable of mirroring the paths of the waves. When looking on the other side of the broadcast radio spectrum - radio waves less than 1GHz are less sensitive to attenuation caused by rain, than microwaves.
As we said, broadcast radio is commonly used for FM radio and TV distribution due to its omni directional characteristic. Many more applications use radio in order to deliver information. For example, paging systems providing one way communications need to carry a small amount of information for tens of kilometers. These systems use broadcast radio to transmit the information to their subscribers.
Cordless Telephone (at these frequencies mainly analog phones) also uses the omni directional characteristic of radio, for a two-way communications across areas of tens of meters. One can move the cordless phone anywhere within the range of the base unit, and keep talking as if it was a wired phone.
Cellular telephone (900 MHz with range of kilometers) and wireless LANs (100 meters) also use radio.
Microwaves (1 GHz - 40 GHz)
As mentioned before, microwaves are the electromagnetic waves at the frequencies of 1GHZ to 40 GHz. At these frequencies the electromagnetic waves are unidirectional, and this makes it perfect for unicast communications.
In order to achieve greater data rates, a higher frequency is needed (so the bandwidth could be wider). This is not an easy task, since at higher frequencies there is bigger attenuation and thus a possible loss of information. The loss can be written as [1]:
Where 
is the wavelength, which gets smaller as the frequency increases. From here it is obvious that when is smaller, the loss is greater. The loss also depends on d – the distance between the stations. However, this dependency is much better than in the copper transmission media where loss grows exponentially with the distance between two stations. Thus, microwaves require fewer repeaters on its way. With that said, microwaves are more limited than other media when there is a bad weather. Rain increases the transmission's attenuation, and this is why we experience reception problems when we watch satellite TV in heavy rain. The effect is even greater at higher frequencies. Another imperfection is noticed when more than one source is broadcasting in the same frequencies. The problem is more severe than the corresponding problem in radio waves, due to the number of sources. Radio is easier to regulate, and is not commonly used. Microwaves are much more common, for example there are many digital phones working at 2.4 GHz. One phone transmission may interfere with another nearby phone.
Microwaves are mainly used in two forms: satellites microwaves and terrestrial microwaves.
Another application of terrestrial microwaves is point-to-point communications between buildings. For example, to connect two LANs, or for closed circuit TV.
Microwaves can also be used for point-to-multipoint wireless systems. A beam from a central source (i.e. a central telephone communication office) is transmitted to all the receivers within its range, and from there onward.
Satellite microwaves aren’t used exactly the same as terrestrial microwaves, because of their different working environment. Microwaves bellow a 1GHz frequency suffers a great amount of noise from the sun, space (cosmic) and the atmosphere. Unlike terrestrial microwaves frequency that goes up to 40GHz, satellite microwaves are attenuated above 10 GHz by the atmosphere [1], so they have a more limited bandwidth. The satellites are mainly used in a 4/6 configuration, meaning that the downlink transmission frequency is about 4 GHz, and the uplink transmission frequency is about 6 GHz. There are also 11/14 and 20/30 configurations [2], but as we said, in these frequency ranges there will be a significant attenuation, so a much more accurate and expensive systems will be needed. The satellite must use different uplink and downlink frequencies in order to be able to transmit and receive at the same time, otherwise there will be interferences. One of the noticeable issues in satellites communication is the propagation delay. Since there is a great distance between the ground systems and the satellite, the receiver ground station will experience a delay of about 0.25-0.5 seconds. This delay is significant in real time applications, or even in a simple conversation.
Satellite applications are versatile. Almost every form of communication can be achieved using satellites. Satellites are commonly used for Television distributions. With an extremely high bandwidth satellites can deliver hundreds of stations. Being positioned in space, satellites can cover enormous areas, and connect the most distant places, even in different continents.
Long distance telephone transmission is another common application for satellite communications. Being able to connect distant places, or even people on the most deserted areas, with no other communication coverage, makes it almost perfect for telephone transmission. The only noticeable disadvantage when using a satellite in this application is a voice delay of several tenths of a second, which could be quite disturbing.
Infrared (300 GHz - 200 THz)
Using much higher frequencies than microwaves brings us to the infrared range, which corresponds with a much smaller wavelength (850 nm – 900 nm). Infrared communication is short ranged, so line of sight transmission is used. It modulates non-coherent infrared light in order to transmit information. Infrared can not pass through walls, so interference between different systems in different rooms does not exist. It is mainly used for cordless devices (keyboards, mice), currently with relatively low bit rates (75 Kb/s to 4 Mb/s [2]) and short ranges (1-10 Meters). It is also widely used in remote controls – using a light emitted diode (LED), the infrared radiation is focused into a beam which is transmitted to the receiver. The receiver uses a photodiode and converts the radiation to electrical energy. Most infrared devices follow the standards set by the Infrared Data Association (IrDA).
Antennas
The wireless transmission is done by broadcasting electromagnetic energy (in the form of waves). To do so, the electrical energy from the transmitter is converted to electromagnetic energy. This is done by an antenna, which radiates the signal to the environment after it was converted. The antenna also works in the opposite direction – it can receive electromagnetic energy and convert it to electrical energy, and then pass it to the receiver.
Broadcast radio mainly uses isotropic antennas. This antenna radiates its output in all directions equally, in the pattern of a sphere.
Microwaves usually use a parabolic reflective antenna or a horn antenna. Using the parabolic characteristics and its reflective surface, the parabolic reflective antenna is able to focus the radiation created by a source of electromagnetic energy in its center (called the paraboloid focus [1]), into a beam. The created beam is parallel, with very little dispersion. On the receiver's side, the opposite action occurs – the beam is parallel to the dish's axis, so when it hits the dish, it is reflected and concentrated to the focus of the parabola.
If we use a lager diameter for the antenna, we gain more accuracy resulting with a more focused beam. This means that most of the energy would be concentrated in one direction. The energy level in this direction will be higher than that emitted by an isotropic antenna (broadcasting at the same power) at any given direction. Obviously, in the other directions the signal will be very weak.
Horn antenna [3] is not that different than the parabolic dish antenna, except it uses a certain kind of pipe for transmission. The signal goes through the pipe, and hits a curved head that changes the signal direction. The outgoing signal is now in a form of a parallel beam, just like in the parabolic dish. The receiving end works in a similar way: the parallel beam hits the curvy head, and collected into the pipe.