Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)
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3.4 TERRESTRIAL RADIO
Free space as the medium has the main advantage that the receiver can be fixed or mobile. Free space is called an unguided medium because the electromagnetic waves can travel freely in all directions. Depending on the frequency of the radio waves, the propagation characteristics vary, and different frequencies are used for different applications, based on the required propagation characteristics. Radio is used for broadcasting extensively because a central station can transmit the program to be received by a large number of receivers spread over a large geographical area. In this case, the transmitter transmits at a specific frequency, and all the receivers tune to that frequency to receive the program.
Radio as a transmission medium has the main advantage that it supports mobility. In addition, installation and maintenance of radio systems are very easy.
In two-way communication systems such as for voice, data, or video, there is a base station located at a fixed place in the area of operation and a number of terminals. As shown in Figure 3.3, a pair of frequencies is used for communication—one frequency for transmitting from the base station to the terminals (the downlink) and one frequency from the terminal to the base station (the uplink). This frequency pair is called the radio channel.
| Note | A radio channel consists of a pair of frequencies—one frequency is used for uplink and one frequency is used for downlink. However, in some radio systems, a single frequency is used in both directions. |
Radio as the transmission medium has special characteristics that also pose special problems.
Path loss: As the distance between the base station and the terminal increases, the received signal becomes weaker and weaker, even if there are no obstacles between the base station and the terminal. The higher the frequency, the higher the path loss. Many models are available (such as Egli's model and Okomura-Hata model) to estimate path loss. To compensate for path loss, we need to use high-gain antennas and also develop receivers of high sensitivity.
| Note | Path loss causes a heavy attenuation of the radio signal. Hence, the radio receiver should be capable of receiving very weak signals. In other words, the receiver should have high sensitivity. |
Fading: Where there are obstacles between the base station and the terminal (hills, buildings, etc.), the signal strength goes down further, which is known as fading. In densely populated urban areas, the signal can take more than one path—one signal path can be directly from the base station to the terminal and another path can be from the base station to a building and the signal reflected from the building and then received at the terminal. Sometimes, there may not be a line of sight between the base station and terminal antennas, and hence the signals received at the terminals are from different paths. The received signal is the sum of many identical signals that differ only in phase. As a result, there will be fading of the signal, which is known as multipath fading or Raliegh fading.
| Note | Multipath fading is predominant in mobile communication systems. The mobile phone receives the signals that traverse different paths. |
Rain attenuation: The rain affects radio frequency signals. Particularly in some frequency bands, rain attenuation is greater. When designing radio systems, the effect of rain (and hence the path loss) needs to be taken into consideration.
The radio spectrum is divided into different frequency bands, and each band is used for a specific application. The details of the radio spectrum are discussed in the next section.
| Note | Radio wave propagation is very complex, and a number of mathematical models have been developed to study the propagation in free space. |
3.4.1 Radio Spectrum
Electrical communication is achieved by using electromagnetic waves, that is, oscillations of electric and magnetic fields in free space. The electromagnetic waves have two main parts: radio waves and light waves. Distinguishing between radio waves and light waves reflects the technology used to detect them. The radio waves are measured in frequency (Hz), and the other types of waves in terms of wavelength (meters) or energy (electron volts).
The electromagnetic spectrum consists of the following:
| Radio waves | : 300GHz and lower (frequency) |
| Sub-millimeter waves | : 100 micrometers to 1 millimeter (wavelength) |
| Infrared | : 780 nanometers to 100 micrometers (wavelength) |
| Visible light | : 380 nanometers to 780 nanometers (wavelength) |
| Ultraviolet | : 10 nanometers to 380 nanometers (wavelength) |
| X-ray | : 120eV to 120keV (energy) |
| Gamma rays | : 120 keV and up (energy) |
The radio spectrum spans from 3kHz to 300GHz. This spectrum is divided into different bands. Because of the differences in propagation characteristics of the waves with different frequencies, and also the effect of atmosphere and rain on these waves, different bands are used for different applications. Table 3.1 gives the various frequency bands, the corresponding frequency ranges, and some application areas in each band.
| Frequency band | Frequency range | Application areas |
|---|---|---|
| Very Low Frequency (VLF) | 3kHz to 30kHz | Radio navigation, maritime mobile (communication on ships) |
| Low Frequency (LF) | 30kHz to 300kHz | Radio navigation, maritime mobile |
| Medium Frequency (MF) | 300kHz to 3MHz | AM radio broadcast, aeronautical mobile |
| High Frequency (HF) | 3MHz to 30MHz | Maritime mobile, aeronautical mobile |
| Very High Frequency (VHF) | 30MHz to 300MHz | Land mobile, FM broadcast, TV broadcast, aeronautical mobile, radio paging, trunked radio |
| Ultra-High Frequency (UHF) | 300MHz to 1GHz | TV broadcast, mobile satellite, land mobile, radio astronomy |
| L band | 1GHz to 2GHz | Aeronautical radio navigation, radio astronomy, earth exploration satellites |
| S band | 2GHz to 4GHz | Space research, fixed satellite communication |
| C band | 4GHz to 8GHz | Fixed satellite communication, meteorological satellite communication |
| X band | 8GHz to 12GHz | Fixed satellite broadcast, space research |
| Ku band | 12GHz to 18GHz | Mobile and fixed satellite communication, satellite broadcast |
| K band | 18GHz to 27GHz | Mobile and fixed satellite communication |
| Ka band | 27GHz to 40GHz | Inter-satellite communication, mobile satellite communication |
| Millimeter | 40GHz to 300GHz | Space research, Inter-satellite communications |
The radio spectrum is divided into a number of bands, and each band is used for specific applications.
International Telecommunications Union (ITU) assigns specific frequency bands for each application. Every country's telecommunications authorities in turn make policies on the use of these frequency bands. The specific frequency bands for some typical applications are listed here:
| AM radio | 535 to 1605 MHz |
| Citizen band radio | 27MHz |
| Cordless telephone devices | 43.69 to 50 MHz |
| VHF TV | 54 to 72 MHz, 76 to 88 MHz, 174 to 216 MHz |
| Aviation | 118 to 137 MHz |
| Ham radio | 144 to 148 MHz420 to 450 MHz |
| UHF TV | 470 to 608 MHz614 to 806 MHz |
| Cellular phones | 824 to 849 MHz, 869 to 894 MHz |
| Personal communication services | 901–902 MHz, 930–931 MHz, 940–941 MHz |
| Search for extra-terrestrial intelligence | 1420 to 1660 MHz |
| Inmarsat satellite phones | 1525 to 1559 MHz, 1626.5 to 1660.5 MHz |
The representative terrestrial radio systems are discussed in Chapter 12, "Terrestrial Radio Communication Systems."
| Note | Some frequency bands such as ham radio band and the Industrial, Scientific, and Medical (ISM) band are free bands—no prior government approvals are required to operate radio systems in those bands. |
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