WIRELESS TRANSMISSION

WIRELESS TRANSMISSION

For unguided media, transmission and reception are achieved by means of an

antenna. For transmission, the antenna radiates electromagnetic energy into the

medium (usually air), and for reception, the antenna picks up electromagnetic

waves from the surrounding medium. There are basically two types of configurations

for wireless transmission: directional and omnidirectional. For the directional

configuration, the transmitting antenna puts out a focused electromagnetic beam;

the transmitting and receiving antennas must therefore be carefully aligned. In the

omnidirectional case, the transmitted signal spreads out in all directions and can be

received by many antennas. In general, the higher the frequency of a signal, the

more it is possible to focus it into a directional beam.

 

Physical Description

The most common type of microwave antenna is the parabolic "dish." A typical size

is about 10 feet in diameter. The antenna is fixed rigidly and focuses a narrow beam

to achieve line-of-sight transmission to the receiving antenna. Microwave antennas

are usually located at substantial heights above ground level in order to extend the

range between antennas and to be able to transmit over intervening obstacles. With

no intervening obstacles, the maximum distance between antennas conforms to

where d is the distance between antennas in kilometers, h is the antenna height in

meters, and K is an adjustment factor to account for the fact that microwaves are

bent or refracted with the curvature of the earth and will, hence, propagate farther

To achieve long-distance transmission, a series of microwave relay towers is

used; point-to-point microwave links are strung together over the desired distance.

Applications

The primary use for terrestrial microwave systems is in long-haul telecommunications

service, as an alternative to coaxial cable or optical fiber. The microwave facility

requires far fewer amplifiers or repeaters than coaxial cable over the same distance,

but requires line-of-sight transmission. Microwave is commonly used for both

voice and television transmission.

Another increasingly common use of microwave is for short point-to-point

links between buildings; this can be used for closed-circuit TV or as a data link

between local area networks. Short-haul microwave can also be used for the socalled

bypass application. A business can establish a microwave link to a longdistance

telecommunications facility in the same city, bypassing the local telephone

company.

Transmission Characteristics

Microwave transmission covers a substantial portion of the electromagnetic spectrum.

Common frequencies used for transmission are in the range 2 to 40 GHz. The

higher the frequency used, the higher the potential bandwidth and therefore the

higher the potential data rate. Table 3.5 indicates bandwidth and data rate for some

typical systems.

As with any transmission system, a main source of loss is attenuation. For

microwave (and radio frequencies), the loss can be expressed as

where d is the distance and h is the wavelength, in the same units. Loss varies as the

square of the distance. In contrast, for twisted pair and coaxial cable, loss varies logarithmically

with distance (linear in decibels). Repeaters or amplifiers, then, may be

placed farther apart for microwave systems-10 to 100 km is typical. Attenuation

increases with rainfall, the effects of which become especially noticeable above

10 GHz. Another source of impairment is interference. With the growing popularity

of microwave, transmission areas overlap and interference is always a danger. As

a result, the assignment of frequency bands is strictly regulated.

Table 3.6 shows the authorized microwave frequency bands as regulated by

the Federal Communications Commission (FCC). The most common bands for

long-haul telecommunications are the 4 GHz to 6 GHz bands. With increasing congestion

at these frequencies, the 11 GHz band is now coming into use. The 12 GHz

band is used as a component of cable TV systems. Microwave links are used to provide

TV signals to local CATV installations; the signals are then distributed to individual

subscribers via coaxial cable. Higher-frequency microwave is being used for

short point-to-point links between buildings; typically, the 22 GHz band is used.

The higher microwave frequencies are less useful for longer distances because of

increased attenuation but are quite adequate for shorter distances. In addition, at

the higher frequencies, the antennas are smaller and cheaper.

Satelit Microwave

Physical Description

A communication satellite is, in effect, a microwave relay station. It is used to link

two or more ground-based microwave transmitter/receivers, known as earth stations,

or ground stations. The satellite receives transmissions on one frequency

band (uplink), amplifies or repeats the signal, and transmits it on another frequency

(downlink). A single orbiting satellite will operate on a number of frequency bands,

called transponder channels, or simply transponders.

Figure 3.4 depicts, in a general way, two common configurations for satellite

communication. In the first, the satellite is being used to provide a point-to-point

link between two distant ground-based antennas. In the second, the satellite provides

communications between one ground-based transmitter and a number of

ground-based receivers.

For a communication satellite to function effectively, it is generally required

that it remain stationary with respect to its position over the earth; otherwise, it

would not be within the line of sight of its earth stations at all times. To remain stationary,

the satellite must have a period of rotation equal to the earth's period of

rotation. This match occurs at a height of 35,784 km.

Applications

The communication satellite is a technological revolution as important as fiber

optics. Among the most important applications for satellites are

Television distribution

0 Long-distance telephone transmission

Private business networks

Because of their broadcast nature, satellites are well suited to television distribution

and are being used extensively in the United States and throughout the

world for this purpose. In its traditional use, a network provides programming from

a central location. Programs are transmitted to the satellite and then broadcast

down to a number of stations, which then distribute the programs to individual

viewers. One network, the Public Broadcasting Service (PBS), distributes its television

programming almost exclusively by the use of satellite channels. Other commercial

networks also make substantial use of satellite, and cable television systems

are receiving an ever-increasing proportion of their programming from satellites.

The most recent application of satellite technology to television distribution is

direct broadcast satellite (DBS), in which satellite video signals are transmitted

directly to the home user. The dropping cost and size of receiving antennas have

made DBS economically feasible, and a number of channels are either already in

service or in the planning stage.

Satellite transmission is also used for point-to-point trunks between telephone

exchange offices in public telephone networks. It is the optimum medium for highusage

international trunks and is competitive with terrestrial systems for many longdistance

intranational links.

Finally, there are a number of business data applications for satellite. The

satellite provider can divide the total capacity into a number of channels and lease

these channels to individual business users. A user equipped with the antennas at a

number of sites can use a satellite channel for a private network. Traditionally, such

applications have been quite expensive and limited to larger organizations with

high-volume requirements. A recent development is the very small aperture terminal

(VSAT) system, which provides a low-cost alternative. Figure 3.5 depicts a typical

VSAT configuration. A number of subscriber stations are equipped with lowcost

VSAT antennas (about $400 per month per VSAT). Using some protocol,

these stations share a satellite transmission capacity for transmission to a hub station.

The hub station can exchange messages with each of the subscribers as well as

relay messages between subscribers.

Transmission Characteristics

The optimum frequency range for satellite transmission is 1 to 10 GHz. Below

1 GHz, there is significant noise from natural sources, including galactic, solar, and

atmospheric noise, and human-made interference from various electronic devices.

Above 10 GHz, the signal is severely attenuated by atmospheric absorption and

precipitation.

Most satellites providing point-to-point service today use a frequency bandwidth

in the range 5.925 to 6.425 GHz for transmission from earth to satellite

(uplink) and a bandwidth in the range 3.7 to 4.2 GHz for transmission from satellite

to earth (downlink). This combination is referred to as the 416 GHz band. Note that

the uplink and downlink frequencies differ. For continuous operation without interference,

a satellite cannot transmit and receive on the same frequency. Thus, signals

received from a ground station on one frequency must be transmitted back on

another.

The 416 GHz band is within the optimum zone of 1 to 10 GHz but has become

saturated. Other frequencies in that range are unavailable because of sources of

interference, usually terrestrial microwave. Therefore, the 12/14 GHz band has

been developed (uplink: 14 to 14.5 GHz; downlink: 11.7 to 12.2 GHz). At this frequency

band, attenuation problems must be overcome. However, smaller and

cheaper earth-station receivers can be used. It is anticipated that this band will also

saturate, and use is projected for the 19/29 GHz band (uplink: 27.5 to 31.0 Ghz;

downlink: 17.7 to 21.2 GHz). This band experiences even greater attenuation problems

but will allow greater bandwidth (2500 MHz versus 500 MHz) and even

smaller and cheaper receivers.

Several properties of satellite communication should be noted. First, because

of the long distances involved, there is a propagation delay of about a quarter second

between transmission from one earth station and reception by another earth

station. This delay is noticeable in ordinary telephone conversations. It also introduces

problems in the areas of error control and flow control, which we discuss in

later lessons. Second, satellite microwave is inherently a broadcast facility. Many

stations can transmit to the satellite, and a transmission from a satellite can be

received by many stations.

Broadcast Radio

Physical Description

The principal difference between broadcast radio and microwave is that the former

is omnidirectional and the latter is directional. Thus, broadcast radio does not

require dish-shaped antennas, and the antennas need not be rigidly mounted to a

precise alignment.

Applications

Radio is a general term used to encompass frequencies in the range of 3 kHz to

300 GHz. We are using the informal term broadcast radio to cover the VHF and

part of the UHF band: 30 MHz to 1 GHz. This range covers FM radio as well as

UHF and VHF television. This range is also used for a number of data-networking

applications.

Transmission Characteristics

The range 30 MHz to 1 GHz is an effective one for broadcast communications.

Unlike the case for lower-frequency electromagnetic waves, the ionosphere is trans

parent to radio waves above 30 MHz. Transmission is limited to line of sight, and

distant transmitters will not interfere with each other due to reflection from the

atmosphere. Unlike the higher frequencies of the microwave region, broadcast

radio waves are less sensitive to attenuation from rainfall.

A prime source of impairment for broadcast radio waves is multipath interference.

Keflection from land, water, and natural or human-made objects can create

multiple paths between antennas. This effect is frequently evident when TV

reception displays multiple images as an airplane passes by.

Infrared

Infrared communications is achieved using transmitterslreceivers (transceivers)

that modulate noncoherent infrared light. Transceivers must be in line of sight of

each other, either directly or via reflection from a light-colored surface such as the

ceiling of a room.

One important difference between infrared and microwave transmission is

that the former does not penetrate walls. Thus, the security and interference problems

encountered in microwave systems are not present. Furthermore, there is no

frequency allocation issue with infrared, because no licensing is required.