Categories of Microwave Communication
Characteristics of Microwave Communications
Microwave transmission is weather and frequency dependent. The frequency band of 10 GHz is in the routine use. Microwave communication is widely used for long distance telephone communication, cellular telephones, television distribution and other uses that a severe shortage of spectrum has developed.
The following are the characteristics of Microwave communications:
a) Microwave is relatively inexpensive as compared to fiber optics system. For example, putting up two simple towers and antennas on each one may be cheaper than burying 50 km of fiber through a congested area or up tower a mountain, and. it may also be cheaper than leasing the telephone line.
b) Microwave systems permit data transmission rates of about 16 Giga (1 giga = 109) bits per second. At such high frequencies, microwave systems can carry 250,000 voice channels at the same time. They are mostly used to link big metropolitan cities where have heavy telephone traffic between them.
Types of Microwave Data Communication Systems
There are two types of microwave data communication systems. These are
a) Terrestrial Microwave Communication
b) Satellite Microwave Communication
Terrestrial Microwave Communication
Terrestrial microwave systems typically use directional parabloic antennas to send and receive signals in the lower giga hertz range. The signals are highly focussed and the physical path must be line-of-sight. Relay towers are used to extend sjgnals. Terrestrial microwave systems are typically used when using cabling is cost-prohibitive.
Terrestrial microwave systems have the following characteristics:
- Frequency range : Most terrestrial microwave systems produce signals in the low giga hertz range usually at 4 to 6 GHz and 21 to 23 GHz.
- Cost: Short distance systems can be relatively inexpensive and they are effective in the range of hundreds of meters. Long distance systems can be very expensive.
- Installation : Line-of-sight requirements for microwave systems can make installation difficult. Antennas must be carefully aligned. Also because the transmission must be line of sight, suitable h-ans-receiver sites could be a problem_ If your organization does not have a clear line of sight between two antennas, you must either purchase or lease a site.
- Bandwidth capacity : Capacity varies depending on the frequency used but typically, data rates are form 1 to 10 Mbps.
- Attenuation : Attenuation is affected by frequency, signal strength, antenna size, and atmospheric conditions. Normally, over short distances, attenuation is not significant. But, rain and fog can negatively affect higher frequency microwaves.
- Electromagnetic interference (EMI): Microwave signals are vulnerable to EMI, jamming and eyes dropping. Microwave systems are also affected by atmospheric conditions.
Typical application :
Because, terrestrial microwave system does not use cables, microwave links often connect separate buildings where cabling would be too expensive, difficult to install or prohibited. For example, if two buildings are separated by a public road, you may not be able to get permission to install cable over or under the road. Microwave links would be a good choice in this type of situation.
Satellite Microwave Communication
Satellite microwave systems transmit signals between directional parabolic antennas.
Like terrestrial microwave systems, they use low giga hertz frequencies and must be in line-of-sight. The main difference with satellite systems is that one antenna is on a satellite in geosynchronous orbit about 36,000 kilometers (22,300 miles) above the equator. Because of this, satellite microwave systems can reach the most remote places on earth and communicate with mobile devices.
A communication satellite is basically a microwave relay station placed precisely at 36,000 km above the equator where its orbit speed exactly matches the earth's rotation speed. Since a satellite is positioned in a geo-synchronous orbit, (i.e. the orbit where the speed of the satellite matches the earth's rotation speed), it appears to be stationary relative to earth and always stays over the same point with respect to earth. This allows a ground station to aim its antenna at a fixed point in the sky. As in Fig. microwave signals at 6 GHz (read as giga hertz = 109 Hz) are transmitted from a transmitter on earth to a satellite positioned in space. By the time this signal reaches the satellite it becomes weak due to distance travelled of 36,000 km.
The transponder in a satellite amplifies the weak signals and sends them back to the earth at a frequency of 4 GHz. These signals are received at a receiving station on the earth. It may be noted that the transmitting frequency is different from the receiving frequency of the satellite. This is done to avoid interference of the powerful re-transmitted signal with the weak incoming signal.
Satellite microwave systems have the following characteristics:
- Frequency range: Satellite links operate in the low giga hertz range typically, 4-6 GHz and 11-14 GHz.
- Cost: The cost of building and launching a satellite is extremely expensive. Many companies such as AT & T, Hughes etc. lease services making them affordable for larger number of organizations. Although satellite communications are expensive, the cost of cable to cover the same distance may be even more expensive.
- Installation: Satellite microwave installation for orbiting satellites is extremely technical and difficult. The earth based systems may require exact adjustments.
- Bandwidth capacity: Capacity depends on the frequency used. Typical data rates are 1 to 10 Mbps.
- Attenuation: Attenuation depends on frequency, power, antenna size, and atmospheric conditions. Higher frequency microwaves are more affected by rain and fog.
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