Ultra High Frequency | Microwave (UHF, M/W) | Digital Radio
Brief introduction to UHF, M/W
With the advent of mass scale industrialization in our country, the demand for more communication facilities came up. Several new telephone exchanges have been installed throughout the country for local communication and more and more carrier channels have been provided for carrying the trunk traffic. With the planned introduction of Subscriber Trunk Dialing throughout the country, the number of carrier chls required to interconnect different cities became too high to be accomplished by overhead lines. Thus, U/G Cables Carrier Systems were introduced, the first of them being the symmetrical pair Cable Carrier System between Calcutta and Asansol with an ultimate capacity of 480 channels. Then came the Co–axial Cable Carrier System linking all major cities in the country. With the development of Microwave technique, which can provide large block of circuits at comparative cost, the problem of long distance communication circuits appear virtually solved. A brief description of the Microwave technique is attempted in the following paragraphs.
- Electromagnetic waves can be broadly classified in terms of frequencies as follows :
|0–30 KHz||V.L.F.||Up to 10 km.||Used for long communication. Has limited information. Bandwidth require very high power.|
|30–300 KHz||L.F.||10 km to 1 km|
|0.3–3 MHz||M.F.||1 km to 100 m||Radio Broadcast, Marine Power in KW, ground wave propagation, i.e. follows the curvature of the Earth.|
|3–30 MHz||H.F.||100 m to 10 m||Long haul point to point communication. Propagation is by one or more reflections from ionosphere layers and so subject to variations.|
|30–300 MHz||V.H.F.||10 m to 1 m||Line of sight, Tropo-scatter communication.|
|0.3–3 GHz||U.H.F.||1 m to 10 cm.||–––––– do ––––––|
|3–30 GHz||S.H.F.||10 cm to 1 cm.||Line of sight, terrestrial M/W and Satellite communication.|
|30–300 GHz||E.H.F.||1 cm to 1 mm.||Experimental.|
The term SHF corresponds to “MICROWAVE” Cent metric waves. As a convention frequencies, above 1 GHz and up to 40 GHz are termed as Microwave. However, most of the m/w systems available are in the range of 1 to 18 GHz.
APPLICATIONS: M/W frequency bands are used for the following services :
- Fixed Radio Communication Services.
- Fixed Satellite Services.
- Mobile Services.
- Broadcasting Services.
- Radio Navigation Services.
- Meteorological Services.
- Radio Astronomy Services.
To meet the requirements of all above mentioned services, co–ordination among the users of M/W spectrum is necessary. In this regard (in the national context) the wireless planning and co–ordination wing (WPC) of the ministry of communication has allotted m/w frequencies spectrum, on the basis of various wireless users classified as general users and major users. Wireless users who are permitted to plan their services and take action for the development of the required equipments are major users. BSNL has been nominated as a major wireless user by the WPC in 1981 in the following sub base band of the m/w spectrum for fixed radio communication. Microwave Spectrum Available for BSNL
|Band||Bandwidth Available||Spectrum Space|
|2 GHz||300 MHz||2000–2300 MHz|
|4 GHz||900 MHz||3300–4200 MHz|
|6 GHz||1185 MHz||5925–7110 MHz|
|7 GHz||300 MHz||7425–7725 MHz|
|11 GHz||1000 MHz||10,700–11,700 MHz|
|13 GHz||500 MHz||12,750–13,250 MHz|
In India the first M/w System was completed in December, 1965 between Kolkata and Asansol with a system capacity of 1200 channels. At present many kilometers of M/W systems are scattered throughout the country and further expansion is taking place at a very large rate.
Frequency Characteristics Microwaves are very short frequency radio waves that have many of the characteristics of light wave in that they travel in line–of–sight paths and can be reflected, boomed and focused. By focusing these ultra high radio waves into a narrow beam, their energies are concentrated and relatively low transmitting power is required for reliable transmission over long distance.
Microwave communication systems are used to carry telephony, television and data signals. Majority of the systems, however, carry multi–channel telephone signals. The spectrum of the multichannel telephone signal is shown in Fig.1. This signal is also called base band (Fig. also shows the TV spectrum). Individual telephone channels, 4 KHz wide (300 to 3400 Hz for speech and the remaining for signaling and guard band) are multiplexed together in a multiplex equipment to get the base band. The base band frequency given in Table below :
|Channel capacity||Base band frequency in KHz|
The system capacity of line of sight systems ranges from 60 telephone channels to 2700 channels over a Radio bearer with a few systems of lower capacities varying from 60 to 60 channels. On the same m/w route one can use more than one radio channels, thus getting still larger capacity. As an example one can accommodate 8 go and 8 return RF channels each with a capacity of 1800 telephone channels in a 500 MHz bandwidth. Of course, in such cases usually one or two RF channels are kept as a standby which are switched over automatically on fading or equipment failure. Usually the system with capacities up to 300 channels is called narrow band system and the systems providing more than 300 channels are called wide band system. M/W systems used to provide communication on major trunk routes with high traffic density and serving long distances are classified as long haul m/w systems. 2, 4, and 6 GHz systems are long haul systems. Systems used to provide communication over short distances for trunk routes with light traffic density are classified as short haul system. 7 and 11 GHz systems are short haul systems.
The salient features of various long distance communication systems are summarised below to make a comparative study.
The Department of Telecommunication at the time of formulation of the 7th Five Year Plan took a decision that the long term perspective for the country would be an integrated services digital network. The approach adopted for achieving this objective is to first proceed towards integrated digital network in which both the switch and the transmission media would be of digital type. Subsequently, through further developments and improvements in technology, it was proposed to bring in the other necessary requirements, viz. capability of the switch to handle data, introduction of No.7 common channel signaling and extension of the digital media up to the subscriber premises for converting the network into ISDN.