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.
There were several reasons for the decision to go in for ISDN network. Some of these are :
- The expected growth in data traffic where the source information is in digital form. The main source of this data traffic is from the use of computers. This has been very evident abroad but also been noticed over the last few years in India.
- Over all economics in the use of fully digital environment as compared to the analogue environment. It is to be noted, however, that economy is not feasible in the mixed environment of analogue and digital.
- The technical performance expected from the new digital equipment is superior to the analogue equipment because of the rapid technological developments in the micro–electronics area and in so far as microwave systems are concerned the immunity of signal from noise even in faded conditions.
- Possibility of providing a large variety of new services to the customer.
- Development of optic fibre technology which for all practical purposes is a digital technology and in this form offers revolutionary advantages in the network.
Available Transmission Media
The major reliable terrestrial transmission media which are available today are :
- PCM on copper cable.
- Fibre optic systems.
- Digital radio systems.
The choice of the transmission media depends on the capacity required, the cost economics for the required capacity and distances and the requirement of media diversity for reliability purposes. The choice made also varies depending on application area such as inter–city, intra–city requirements.
Transmission Capacities Available on the Radio Systems
The transmission capacities available on digital radio systems are, of course, integral multiples of PCM hierarchical bit rates and are classified into small, medium and large or high capacity systems. Specifically this categorization covers :
Low capacity – 704 kbps, 2 mbps and 8 mbps
Medium capacity – 34 mbps
High capacity – 140 mbps
Some manufacturers and some administrations have used some other integral multiples also such as 2 x 8 and 2 x 34 mbps systems but these are not being considered in the Indian network. The 704 kbps system is not other wise a standard system but has been proposed in the Indian network context, because for the rural network it is found that a 2 mbps system corresponding to 30 channels was too large and wasteful of frequency resource. This 704 kbps system corresponds to capacity of 10 channels, which is quite adequate in the rural network of the country..
The frequency bands and the capacities which are proposed to be used by digital microwave and UHF systems in the country are given below :
|Small capacity||0.704||10||658–712 MHz (UHF)|
|Small capacity||2.048||30||400 MHz band (UHF)|
|Small capacity||8.448||120||520–585 MHz (UHF)
622–712 MHz (UHF)
|Small capacity||8.448||120||2 GHz band (M/W)
|Medium capacity||34.368||480||7 GHz band (M/W)
|Medium capacity||34.368||480||13 GHz band (M/W)
(12.75–13.25 GHz) band M/W
|Medium capacity||34.368||480||15 GHz band (M/W)
|High capacity||139.264||1920||4 GHz band (M/W)
(3.3–3.8 and 3.8–4.2 GHz)
|High capacity||139.264||1920||6 GHz band (M/W)
(5.925–6.425 GHz; Lower)
(6.430–7.110 GHz; Upper)
|High capacity||139.264||1920||11 GHz band (M/W)
Digital Radio Applications
Small Capacity Digital Radio Systems
- 10 channel systems in the UHF range are being developed indigenously with a view to utilize them in the rural area where channel requirements are very small, for example, linking an RAX to the nearest large exchange. The systems are expected to work in the 1+0 unprotected configuration.
- 2 Mbps system in the UHF band is expected to serve the purpose of linking secondary switching area centre to concentration points in the rural areas of a secondary switching area. These are expected to be manufactured by ITI, BEL and PCL. .
- 2 and 8 Mbps systems are expected to be available in the 18 to 20 GHz range. These are likely to be applied with integral antenna, mounted on a mast and will have point to multipoint application. These will be suitable for business network in large urban centres. The hop lengths are likely to be a few kilometres depending on rainfall statistics in a given area.
- 8 Mbps system in 2 GHz band is suitable for application as a short haul system and will find application in the rural network, for linking either secondary switching areas to their next higher TAXs or linking the secondary switching centre to trunk concentration points in the rural area. The advantage of this system is the possibility of using long hops. The equipment is to be manufactured by ITI and the expected cost per terminal is not yet established. The type of antenna used will be grid paraboloid.
Medium Capacity Digital Systems
The systems being used in the BSNL in the medium capacity range are 2 GHz and
13 GHz 34 Mbps equipments. Their applications are as follows :
- 2 GHz, 34 Mb/s is to be used in the trunk network with longer hops than those feasible in the higher frequency bands.
- 7 GHz, 34 Mbps system is being used in the trunk network to connect primary centres to secondary switching centres. It is possible to use 4 frequency channels with one standby channel but the equipment currently expected to be available in the country is suitable for 1+1 RF bearer.
The modulation method used in 4 PSK, Hop lengths which sometimes tend to be as much as 40 kms requiring space diversity along with frequency diversity.
- 13 GHz 34 Mbps equipment is being used almost exclusively in the junction network in large urban telephone systems. The rain statistics do dictate the hop length but the use being in the urban network does not cause much problem even in cities like Calcutta where the rainfall is heavy. These are operated in N+1 mode with the possibility of N=7. Because of the small hop length no multipath fading problem is observed. The modulation method used is mostly 4 PSK.
High Capacity Digital Radio Systems
The preferred application is as follows :
Presently, 6 GHz band 140 Mbps system is being introduced for long haul trunk routes between major cities. This equipment because of its large capacity requires several specific features in its design. These include the use of adaptive equalisers including base band transversal equaliser to minimize intersymbol interference and IF band resonance equaliser to equalise notch and slope besides using space diversity. Presently, these are being used in the N+1 mode with N=7.
Comparison Between Digital and Analogue Radio Relay System
With reference to radio relay systems in particular, digital systems have certain advantages and disadvantages. The major advantages include :
- The ability to regenerate at each repeater with the result that circuit performance becomes essentially independent of length.
- The plentiful capacity for data traffic and the ability to support an IDN and subsequent potential involvement into an Integrated Services Digital Networks (ISDN).
- A higher immunity to noise and interference which amongst other things allows operation at higher carrier frequencies and in metropolitan areas.
Associated with the use of higher frequencies for digital radio are reductions in spectrum congestion and equipment size making such equipment easy to transport and install.
On the other hand, DRRS have certain disadvantages. These include :
- The sensitivity of high capacity systems to frequency selective fading which can result in reduction of the effective fade margin by some 20 dB below the flat fade margin for typical analogue hop lengths. To restore such systems to acceptable performance, it is necessary to add various combinations of combining space diversity, adaptive equalizers at IF and/or transversal equalizers.
- The absence of sub–baseband which makes it more costly to drop and insert small numbers of circuits typically used for wayside traffic.
- Higher power requirement when compared to currently available low drain IF or RF repeating analogue radio–relay equipment. Making it uneconomic to power by solar cell arrays.
Most of the disadvantages of high capacity DRRS are being eliminated with the second generation of equipment coming onto the market. Major power drain reduction has occurred, more powerful equalizers have been incorporated as a standard part of the equipment and additional drop and insert capacity is being introduced.
The salient characteristics for the analogue bearer are that the basic noise and intermodulation noise from each hop are cumulative, the voice frequency (VF) channel signal–to–noise (S/N) ratio depends on the received input signal level and more particularly, on the carrier–to–noise (C/N) ratio and that co–channel carrier–to–interference ratio of 30 dB makes the circuit quality unacceptable. The salient characteristics of the digital bearer are that the performance is uniform over a wide range of receive input levels and deteriorates rapidly over a small range of C/N ratios near the threshold. In addition, the introduction of even a 30 db C/N ratio has only a marginal effect in worsening performance near the threshold.
Performance Requirements of Digital Microwave System in Comparison to Analogue Microwave Systems
The performance requirements for digital and analogue microwave systems differ because the definitions of quality differ in the two cases. In the case of analog microwave systems, the quality is measured in terms of the signal–to–noise. For digital microwave systems, the quality is measured in terms of bit error rate (BER). For digital microwave systems, the S/N does effect the performance, but the bit by bit faithful reproduction (which is the ultimate objective) is also influenced by other parameters such as coding, modulation scheme, inter symbol interference properties, etc. Thus, a knowledge of S/N alone may not be adequate to determine the actual BER of the system.
As already mentioned in Section 5, for an analog microwave system the quality is more or less a direct function of the fade, i.e., as fade increases, the S/N deteriorates. On the other hand, in the case of digital microwave system, as the fade increases the quality of the equipment, i.e. BER remains nearly constant upto a value close to the threshold at which point the BER rises rapidly and the system performance collapses.
In view of the above, different definitions of the quality, the CCIR definitions for the performance of hypothetical reference circuit (HRC) for analog system and hypothetical reference digital path (HRDP) for digital system are as follows :
Following noise figures are not to be exceeded for the time percentages indicated :
- 7,500 pwop for more than 20% of any month.
- 47,500 pwop for more than 0.1% of any month.
- 10,00,000 pw (unweighted, with an integrating time of 5 ms, for more than 0.01% of any month).
Following BERs not to be exceeded for the indicated time percentage as given below :
- 1 x 10–7 BER for more than 1% of any month.
- 1 x 10–3 BER for more than 0.5% of any month.
For actual paths which differ from the HRDP in composition or are much smaller in length the performance criterion under consideration by the CCIR is as follows :
When a path is established over a link which is less than the HRDP (2500 kms), but greater than 280 kms and which differs in composition from the HRDP, the allowable time percentage should be proportional to the link length L (kms) of the link.
- 1 x 10–7 BER for more than (L/2500) x 1% of any month.
- 1 x 10–3 BER for more than (L/2500) x 0.05% of any month.
When a path is established over a link which is less than 250 kms, it is proposed that BER not to be exceeded for the indicated time percentage as given below :
- 1 x 10–7 BER for more than (280/2500) x 1% of any month.
- 1 x 10–3 BER for more than (280/2500) x 0.05% of any month.
Note : This takes into account fading, interference and all other sources of performance degradation. It does not include BER greater than 1 x 10–3 for periods exceeding 10 consecutive seconds. This condition is included in the availability criterion. The high BERs caused by switching operations are included in the above criterion, but not the ones caused by scheduled switching for maintenance). Availability criterion is 1 x 10–3 BER (measured for 10s time interval) not exceeding 0.3% of a year. It is important to note that during the conditions of fades well above the threshold margin, the system is almost perfect. In interpreting this statement, it should be kept in mind that threshold margin does not necessarily imply flat fade margin.
As stated in the previous paragraph, all the user Departments, like Railways, Civil Aviation, Defense, Telecommunications Department, etc. are members of the SACFA Board. There is a Central Board at Delhi and Regional Boards at Madras, Bombay and Hyderabad, etc.
The main objective of the function of the SACFA Board is to investigate the interference possibilities, etc. and allot the frequency and spectrum for new routes. All types of Microwave routes should be cleared by this body as far as the frequency to be used, the location, the height of tower are concerned. This body takes the safety aspect from Aviation point of view (of civil as well as Defense flights) also. Hence, while clearing the license for a new route, this Body specifically mentions whether night warning or both Day and Night warning are to be provided for the Microwave towers. Night warning is by means of aircraft warning lamps and day warning is by means of painting the tower with alternate bonds of international orange and white. The SACFA Board also considers the distance of tower location from the nearby Airports and ensures that the specified minimum distance is maintained from the airport. The SACFA Board takes the individual clearance from the member Departments, before clearing a particular Microwave route.