Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

2017-07-07 02:10:07

< Day Day Up >

3.6 RADIO SPECTRUM MANAGEMENT

In the electromagnetic spectrum that spans from 0Hz to 10²⁵GHz (cosmic rays), the radio spectrum spans from 3kHz to 300GHz in VLF, LF, MF, HF, VHF, UHF, SHF, and EHF bands. The only portion of the radio spectrum not allocated to anyone is 3 to 9 kHz (rather, it is allocated to every individual for freedom of speech).

The radio spectrum is a limited natural resource, and hence international and national authorities regulate its use.

The International Telecommunications Union (ITU), through the World Administrative Radio Conferences (WARC), allots frequency bands for different application areas. For administrative convenience, the world is divided into three regions. The allocations for these regions differ to some extent. All nations are bound by these regulations for frequency use. The WARC regulations are only broad guidelines, because a centralized authority in every country manages the radio spectrum. In the United States, the frequency spectrum is managed by the FCC (Federal Communications Commission). In India, the agency is WPC (Wireless Planning and Coordination Cell) under the Government of India.

The complexity of radio spectrum management results from several factors:

There are some frequency bands for the exclusive use of governmental agencies and some for nongovernmental agencies. Some frequency bands are shared. When the same band is shared by different agencies for the same application or for different applications, it is necessary to ensure that there is no interference between various systems.

When a frequency band is allocated for a particular application (e.g., cellular mobile communication), the service can be provided by different operators in the same area. Without a coordinated effort in band allocation, the spectrum cannot be used efficiently to support a large number of users for that service in the same frequency band.

Because of higher user demands, the frequency band allocated for a particular service may become congested and new bands need to be allocated. For example, in the case of mobile communications, the 900MHz band got congested, and the 1800MHz band was allocated. This type of new allocation of bands calls for long-term planning of spectrum use.

As new application areas emerge, accommodating these applications along with the existing applications in the required frequency bands is another challenge in spectrum management.

New technologies (better techniques for reducing bandwidth requirements, new frequency reuse technologies, antenna technologies, etc.) lead to better utilization of spectrum. Ensuring that the new technologies are incorporated is important in spectrum management.

Agencies will be allocated fixed frequencies for use. For various reasons, however, these frequencies may not be used at all or may not be used efficiently. A periodic review of the use of the spectrum is also required. A review process needs to be followed by which an application for frequency allotment will be processed, frequencies allocated, and use monitored.

Radio spectrum management ensures that the allotted spectrum is being used efficiently, to ensure that there is no interference between different radio systems and to allocate new frequency bands for new services.

All these aspects make spectrum management a difficult task, and the need for an efficient spectrum management methodology cannot be overemphasized.

3.6.1 Spectrum Management Activities

Radio spectrum management involves three major activities:

Spectrum assignment and selection involves recommending a specific frequency band of operation for use in a given location. For this, extensive databases containing all the information regarding the present uses of radio spectrum have to be developed and maintained so that the effect of the proposed frequency band uses on existing systems can be studied. Depending on the interference considerations, specific frequency bands can be allocated.

Spectrum engineering and analysis involves computations for installations of radio equipment at specific locations and for predicting the system performance in the radio environment.

Spectrum planning involves long-term/emergency planning, keeping in view, among other things, the demands for new services and technological changes.

Because all these activities involve huge computation, various national authorities are deploying computerized spectrum management systems. Expert systems are also being developed to manage the spectrum efficiently.

Radio spectrum management involves spectrum management and selection, spectrum engineering and analysis, and long-term/emergency spectrum planning for new services.

3.6.2 Cost of Spectrum

Radio spectrum is a limited natural resource, and its optimal utilization must be ensured. Government agencies charge users for the spectrum. The charges are generally on an annual basis.

Nowdays, the government agencies are also using innovative methods for making money out of the spectrum. The present trend is to auction the spectrum. The highest bidder will be given the spectrum for a specific application. For the 3rd Generation (3G) wireless systems, this approach has been followed, and it turned out that in most countries the spectrum cost is much higher than the infrastructure (equipment) cost.

Operators that obtain specific frequency bands for radio services need to pay for the cost of the spectrum.

Summary

The details of various transmission media used in telecommunications systems are presented in this chapter. Based on the considerations of cost, data rates required, and distance to be covered, the transmission medium has to be chosen. The transmission media options are twisted copper pair, coaxial cable, optical fiber, and radio. Twisted pair is of low cost, but the attenuation is very high and the data rates supported are low. Because of the low cost, it is used extensively in the telephone network, in PBX, and for LANs. Coaxial cable supports higher data rates compared to twisted pair and is used in cable TV, LANs, and the telephone network. Optical fiber supports very large data rates and is now the preferred medium for LANs and the telephone networks. The main attraction of radio as the transmission medium is its support for mobility. Furthermore, installation and maintenance of radio systems is easy because there is no need to dig below ground. Terrestrial radio systems are used extensively in the telephone network as well as for mobile communications. Wireless LANs also are becoming predominant nowdays. Satellite radio has the main advantage that remote and rural areas can be connected easily. Satellite radio is also used extensively for broadcasting.

Because radio spectrum is a precious natural resource, the spectrum has to be used effectively. ITU allocates the frequency bands for different applications. In each country, there is a government organization that coordinates the allocation and use of the spectrum for different applications. We have studied the intricacies of spectrum management, which include planning, allocation, and monitoring the use of the spectrum.

References

R. Horak. Communications Systems and Networks, Third Edition. Wiley-Dreamtech India Pvt. Ltd., 2002. This book gives comprehensive coverage of many topics in telecommunication and details of the transmission media.

http://www.inmarsat.com Inmarsat operates a worldwide mobile satellite network. You can get the details of the satellites and the services offered from this URL.

http://www.iec.org/tutorials The Web site of International Engineering Consortium. This link provides many tutorials on telecommunications topics.

Questions

List the different transmission media and their applications.

List the frequency bands of the radio spectrum and the applications in each frequency band.

What are the issues involved in radio spectrum management?

Some frequency bands such as for ham radio and the Industrial, Scientific and Medical (ISM) band are not regulated, and anyone can use those bands. Debate the pros and cons of such unregulated bands.

Exercises

1.	A terrestrial radio link is 30 kilometers long. Find out the propagation delay, assuming that the speed of light is 3 × 10⁸ meters/second.
2.	The coaxial cable laid between two telephone switches is 40 kilometers long. Find out the propagation delay if the speed of transmission in the coaxial cable is 2.3 × 10⁸ meters/second.
3.	Calculate the propagation delay in an optical fiber of 100 kilometers. The speed is 2 × 10⁸ meters/second.
4.	Compile the list of frequency bands used for satellites for the following applications: (a) broadcasting, (b) telephone communications, (c) weather monitoring, (d) military applications.
5.	Calculate the propagation delay from one Earth station to another Earth station in a satellite communication network for (a) mesh configuration and (b) star configuration.

Answers

For a terrestrial radio link of 30 kilometers, assuming that speed of light is 3 × 10⁸ meters/second, the propagation delay is 30,000 / (3 * 10 ⁸) seconds = 100 microseconds.

When the cable length is 40 kilometers and the speed of transmission in the coaxial cable is 2.3 × 10⁸ meters/second, the propagation delay is 40,000/ (2.3 * 10⁸) seconds = 173.91 microseconds.

The propagation delay in an optical fiber of 100 kilometers if the speed is 2 × 10⁸ meters/second, is 100,000 / (2 × 10⁸) seconds = 0.5 msec.

In satellite communication, broadcasting and voice communication are done in the C band and Ku band.

Band Designation	Frequency Range
L band	1-2 GHz
S band	2-4 GHz
C band	4-8 GHz
X-band	8-12 GHz
Ku band	12-18 GHz
K band	18-27 GHz
Ka band	27-40 GHz
Q band	33-50 GHz
U band	40-60 GHz
V band	40-75 GHz
W band	75-110GHz

In mesh configuration, the communication is from one Earth station to another Earth station. Hence, the propagation delay is 240 msec. In star configuration, the transmission is from VSAT to the satellite, satellite to the hub, hub to the satellite, and then satellite to the other VSAT. Hence, the propagation delay is 480 msec.

Projects

Develop a database of frequency bands used for different applications given in Table 3.1. Develop a graphical user interface to facilitate the display of the frequency bands for a given application.

Study the frequency bands of operation for Inmarsat satellites. Inmarsat operates a worldwide mobile satellite network.