Defination of Telecom spectrum-What is spectrum in Telecom? Wireless communications make use of the radio portion of the electromagnetic spectrum. Simply put, telecom operators send and receive signals at various frequencies to enable communication. This radio spectrum contains only a limited number of frequencies. Signals can be sent on different frequencies all at the same time, however, the same or too close they can interfere with each other. As the number of frequencies is limited, they need to be allocated to different service providers. Therefore, spectrum is allocated for various communication purposes world-wide. The percentage of annual spectrum charges for GSM operators, for instance, varies with spectrum usage. Currently, the ceiling for annual spectrum charge is 6% of AGR. Trai, though, has recommended that the ceiling be lowered to 4%.
SPREAD SPECTRUM:-A signal that occupies a bandwidth of B, is spread out to occupy a bandwidth of Bss • All signals are spread to occupy the same bandwidth Bss • Signals are spread with different codes so that they can be separated at the receivers. • Signals can be spread in the frequency domain or in the time domain.
Working of telecom spectrum:- • The term ‘spread spectrum’ describes a modulation technique that makes the sacrifice of bandwidth in order to gain signal-to-noise (S/N) performance. Basically,in a spread-spectrum system,the transmitted signal is spread over a frequency much wider than the minimum bandwidth required to send the signal. • Spread-spectrum signals use fast codes that run many times the information bandwidth or data rate. These special ‘spreading’ codes are called ‘pseudo random’ or ‘pseudo noise’ codes. They are called ‘pseudo’ because they are not real Gaussian noise. • The use of special pseudo-noise codes in spread-spectrum communications makes signals appear wide-band and noise-like. It is this very characteristic that makes spread-spectrum signals possess the quality of low probability of intercept. • Direct sequence and frequency hopping are the most commonly used methods for the spread-spectrum technology. • The frequency hopper is more popular and the only way to survive in the 2.45GHz band because the leakages from the microwave oven (from 2.4 to 2.5 GHz) sometimes exceed 10W.
Frequency hopping spread spectrum:- • In frequency hopping system the carrier frequency of the transmitter changes abruptly in accordance with a unique pseudo-random sequency.The pattern of the frequency changes is however known to both the transmitter & receiver.The set of possible carrier frequency is called hopset.Hopping occurs over a frequency band that includes a no. of channels. • A major advantage of frequency hopping is that it is hard to obtain a high processing gain as in DSSS.There is therefore the need for a frequency stabilizer that would be able to perform fast hopping over the carrier frequencies in order to obtain a high processing gain.
Direct sequence spread spectrum:- • The DSSS Process is performed by effectively multiplying an RF carrier with a pseudo-noise(PN)digital signal.First the PN code is modulated on to the information signal using one of several modulation techniques.Then a doubaly balanced mixer is used to multiply the RF carrier and PN modulated information signal. This process causes the RF signal to replaced with a very wide band-width signal with the spectral equivalent of a noise signal.
TELECOM INFRASTRUCTURE- Due to economy of scale property of telecommunication industry, sharing of telecom infrastructure among telecom service providers is becoming the requirement and process of business in the telecom industry where competitors are becoming partners in order to lower their increasing investments. The degree and method of infrastructure sharing can vary in each country depending on regulatory and competitive climate. REGULATORY VIEW- The Federal Communications Commission (FCC), as an independent agency of the United States government, was established by the Communications Act of 1934. As the successor to the Federal Radio Commission, FCC is taking charge of regulating all non-federal government use of the radio spectrum(including radio and television broadcasting), and all interstate telecommunications (broadband, wireless and satellite) as well as all international communications that originate or terminate in the United States. Telecommunications Act of 1996, all incumbent local exchange carriers have the following obligations discused below:-
FOLLOWING OBLIGATIONS- Interconnection—the duty to provide, for the facilities or equipment of any requesting TCC, interconnection with the LEC network.(Section 251 (c)) Access to rights of way—theduty to afford access to the poles, ducts, conduits ... at nondiscriminatory rates consistent with (Section 224) Reciprocal compensation arrangements—the duty to establish reciprocal compensation arrangements for the transport and termination of telecommunications.(Section 251(b)) For the benefit of sharing infrastructure, new entrants are able to enter the market by sharing (leasing) network without building their own ones. Advantages Of Telecom infrastuctures- 1.telecommunication development shows economy of scale and telecom operator spending has been dominated by considerable investment of technology . investments are fixed, sunk and irreversible, they represent a high risk factor. Maintaining and upgrading infrastructure make this risk even higher. .
Advantages of telecom spectrum:- • No crosstalk interference. • Better voice quality/data integrity & less static noise. • Lowered suspectibility to multipath fading. • Inherent security. • Coexistence • Longer operating distance. • Hard to detect. • Hard to intercept or demodulate. • Harder to jam than narrow bands.
Disadvantages of Telecom Spectrum- A positive amperage of the PCM signal will cause the AM carrierwave's peak-to-peak amplitude to increase while a negative amperage [i.e. going below the x-axis when graphed] will cause a decrease the AMcarrier's peak-to amplitude. 2.A increase in frequency of the PCM signal will cause the AM carrier'speak-to-peak amplitude to vary more rapidly while a decrease in the PCMsignal's frequency will cause the AM wave's peak-to-peak amplitude tovary more slowly. 3. In frequency of the PCM signal will cause the AM carrier'speak-to-peak amplitude to vary more rapidly while a decrease in the PCMsignal's frequency will cause the AM wave's peak-to-peak amplitude tovary more slowly. 4. FM demodulation produces a handy 'capture' effect. If aweak and a strong signal are present together, then the recoveredbaseband signal from the weaker FM signal is greatly reduced. This isuseful in rejecting interference from adjacent stations on the samefrequency.
Applications of Telecom spectrum- Wireless local area network (WLAN):-A WLAN is a flexible data communication system implemented as an extension to or an alternative for a wired local area network. WLANs transmit and receive data over the air minimising the need for wired connections.Thus, WLANs combine data connectivity with user mobility and enable movable LANs.Most WLAN systems use spreadspectrum technique (both frequency hopping and direct sequence). WLANs are being used in health care, retail,manufacturing, warehousing, academic and other arenas. These industries have profited from the productivity gains of using handheld terminal. Other Applications are:- 1.The most important advantage of telecom spectrum is its interference rejection capability. 2.It has uniform energy over a large bandwidth. 3.It is reliable.
Techniques- • Techniques known since the 1940s and used in military communication systems since the 1950s "spread" a radio signal over a wide frequency range several magnitudes higher than minimum requirement. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to-point communication at the same data rate. • Resistance to jamming (interference). DS (direct sequence) is better at resisting continuous-time narrowband jamming, while FH (frequency hopping) is better at resisting pulse jamming. • Resistance to eavesdropping. The spreading code (in DS systems) or the frequency-hopping pattern (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it "encrypts" the signal and reduces the chance of an adversary's making sense of it. Moreover, for a given noise power spectral density(PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower — often significantly lower than the noise PSD — so that the adversary may be unable to determine whether the signal exists at all. However, for mission-critical applications, particularly those employing commercially available radios, spread-spectrum radios do not intrinsically provide adequate security; "...
HISTORY OF TELECOM SPECTRUM This is a technique in which a (telecommunication) signal is transmitted on a bandwidth considerably larger than the frequency content of the original information. Frequency hopping is a basic modulation technique used in spreadspectrum signal transmission. Spread-spectrum telecommunications is a signal structuring technique that employs direct sequence,frequency hopping, or a hybrid of these, which can be used for multiple access and/or multiple functions. This technique decreases the potential interference to other receivers while achieving privacy. Spread spectrum generally makes use of a sequential noise-like signal structure to spread the normally narrowband information signal over a relatively wideband (radio) band of frequencies. The receiver correlates the received signals to retrieve the original information signal. Originally there were two motivations: either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to hide the fact that communication was even taking place, sometimes called low probability of intercept (LPI). Frequency-hopping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum (CSS), and combinations of these techniques are forms of spread spectrum. Each of these techniques employs pseudorandom number sequences — created using pseudorandom number generators — to determine and control the spreading pattern of the signal across the allocated bandwidth. Ultra-wideband(UWB) is another modulation technique that accomplishes the same purpose, based on transmitting short duration pulses. Wireless standard IEEE 802.11 uses either FHSS or DSSS in its radio interface.
ALTERNATIVES OF TELECOM SPECTRUM There are narrowband alternatives to telecom spectrum that can provide most of the benefits of telecom spectrum techniques.There are a no. of alternatives:- Discrete Addressing:- These feature has 2 benefits,the first of this is privacy & the second is selective calling. Resistance to multipath propagation:- A primary manifestation of multipath propagation in narrow band communication system is fading.diversity techniques can be used with narrowband signalling to reduce fading. Spectrum efficiency:-Over the years,a no. of measures have been applied to the concept of spectrum efficiency(30-38).The benefits in terms of the network of simultaneous links or users accomoted or the network information throughout ;the costs are normally defined in terms of the amount of spectrum resources occupied. Resistance to interference:-In spread spectrum techniques may offer the most economical approach to mitigating interference.in other cases,narrowband technices,combined with the use of directional antennas,special filter may be preferable.
. Resistance to multipath propagation:- A primary manifestation of multipath propagation in narrowband communication system is fading.Diversity techniques can be used with narrowband signallin to reduce fading.
Conclusions Spread Spectrum techniques are technologically superior to conventional narrowband modulation techniques in a number of important areas. Because they form the datalink layer of the new generation of radio LANs, systems administrators and computer security experts are well advised to gain a good understanding of their strengths and limitations. Future features will look at the more practical implications of radio LAN technology.
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