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TERM PAPER OF ECE-444, APRIL, 2014
TIME HOPPING SPREAD SPECTRUM
Himanshu shekhar
Reg. No. 11002250
Section: E2012
Roll no....
provides a multiple access capability, when the
several different spreading codes are being used
simultaneously. The use o...
IV. TIME HOPPING
A time hopping system is a spread spectrum
system in which the period and duty cycle of a
pulsed RF carri...
Frequency
Hopping
i) Less affected
by Near-Far
problem
ii) Better for
avoiding jamming
iii) Less affected
by multi-ccess
i...
time hopping spread spectrum term paper
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time hopping spread spectrum term paper

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time hopping spread spectrum term paper,Transmission and reception of THSS and its comparison with DSSS and FHSS

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time hopping spread spectrum term paper

  1. 1. TERM PAPER OF ECE-444, APRIL, 2014 TIME HOPPING SPREAD SPECTRUM Himanshu shekhar Reg. No. 11002250 Section: E2012 Roll no. B16 Lovely professional University,Phagwara I. ABSTRACT This paper deals with time hopping spread spectrum. First of all I will explain the spread spectrum technology and its various techniques. From all those techniques I will shower details on time hopping spread spectrum. And then I will analyze this techniques and go through the process of its generation, its comparisons with frequency hopping spread spectrum and direct sequence spread spectrum and its application . II. INTRODUCTION Spread spectrum is a form of wireless communications in which the frequency of the transmitted signal is deliberately varied. This results in a much greater bandwidth than the signal would have if its frequency were not varied. A time hopping system is a spread spectrum system in which the period and duty cycle of a pulsed RF carrier are varied in a pseudorandom manner under the control of a coded sequence. The transmission hops in time. Information is carried by short impulse which position in time denotes the transmitted bit. These positions are not periodic; they are “random” which leads toTime Hopping (the power is spreaded over a larger bandwidth) III. SPREAD SPECTRUM The IEEE Spectrum of August, 1990 contained an article entitled Spread Spectrum Goes Commercial, by Donald L. Schilling of City College of New York, Raymond L. Pickholtz of George Washington University, and Laurence B. Milstein of UC San Diego. This article summarized the coming of commercial spread spectrum: "Spread-spectrum radio communications, long a favorite technology of the military because it resists jamming and is hard for an enemy to intercept, is now on the verge of potentially explosive commercial development. The reason: spread-spectrum signals, which are distributed over a wide range of frequencies and then collected onto their original frequency at the receiver, are so inconspicuous as to be 'transparent.' Just as they are unlikely to be intercepted by a military opponent, so are they unlikely to interfere with other signals intended for business and consumer users -- even ones transmitted on the same frequencies. Such an advantage opens up crowded frequency spectra to vastly expanded use.” Spread spectrum technology was invented in the 1940s, and has been used extensively since then for military and other applications that require robustness and resistance to jamming or eavesdropping. Types of Spread spectrum Based on the kind of spreading modulation, spread spectrum systems are broadlyClassified as- (i) Direct sequence spread spectrum (DS-SS) systems (ii) Frequency hopping spread spectrum (FH-SS) systems (iii) Time hopping spread spectrum (TH-SS) systems. (iv)Hybrid systems The direct sequence spread spectrum (DSSS) approach is based on multiplication of the original data signal with a much faster pseudo random noise code, which is also called the spreading code. This results in a scrambled signal with a much wider spectrum. DSSS significantly improves protection against interfering signals, especially narrowband interference. It also
  2. 2. provides a multiple access capability, when the several different spreading codes are being used simultaneously. The use of DSSS for multiple access is called CDMA, and is used e.g. in the 3th generation mobile communications Fig.1 Direct sequence spread spectrum transmitter Fig.2 Direct sequence spread spectrum receiver In case of frequency hopping spread spectrum (FHSS) the RF frequency of the narrowband transmission is quickly changed within a certain range, according to a pseudo random noise code. Hence, a hopping pattern can be observed in the spectrum. Like DSSS, FHSS also provides a multiple access capability by using orthogonal hopping codes for different (logical) communication channels. FHSS is for instance used by Bluetooth. Bluetooth hopes 1600 times per second between the 79 available channels. Fig.3 Block diagram of a non-coherent frequency-hopping receiver In case of time hopping a train of short duration pulses is transmitted which is derived from the narrowband information carrying signal through scrambling with a pseudo random modulated impulse train. The short pulse duration generates the spread spectrum profile. Time Hopping is used as a technique to generate a certain type of UWB signals. Hybrid systems use a combination of spread spectrum methods in order to use the beneficial properties of the systems utilized. Two common combinations are direct sequence and frequency hopping.. One data bit is divided over several carrier frequencies
  3. 3. IV. TIME HOPPING A time hopping system is a spread spectrum system in which the period and duty cycle of a pulsed RF carrier are varied in a pseudorandom manner under the control of a coded sequence The TH technique, in fact, works in a very similar way as a digital modulation scheme called pulse position modulation (PPM). In other words, time hopping is nothing but a type of pulse position modulation in a sense that a code sequence is used to key the transmitter on and off, as shown in Figure 5. Where the times for the transmitter to switch on and off follow a specific pseudorandom code sequence The major difference between the PPM and the TH lies in the fact that the former uses pulse position patterns to represent the data information symbols, whereas the latter denotes a particular code sequence, which acts as a secret key to further decode the data information hidden therein. A block diagram of a TH system is shown in Figure 5, where the on–off switch logic unit in the transmitter is used to control the positions that the sent pulse will hop from one to the other Fig.5 Time hopping spread spectrum transmitter A typical time hopping receiver is shown in Fig. 6. The PN code generator drives an on-off switch in order to accomplish switching at a given time in the frame. The output of this switch is then demodulated appropriately. Each message burst is stored and re-timed to the original message rate in order to recover the information. Fig.6 Time hopping spread spectrum receiver V. COMPARISON OF FEATURES OF VARIOUS SPREADING TECHNIQUES If compared with other spread spectrum techniques, The THSS technologies are not as popular as other two SS techniques, that is, DSSS and FHSS techniques. The main reason is its implementation difficulty, especially for the pulse generator, which is the core of a THSS system and should be able to produce a train of very narrow impulses of the order of a nanosecond. The impulses train should also provide very good timing accuracy, such that the PPM can be effectively applied to different SS code sequences for multiple access. It remains a challenging task to make such a pulse generator, even today. Below is the summarize table for comparison. Spreading Method Merits Demerits Direct Sequence i)Simpler to implement ii)Low probability of interception iii)Can withstand multi-access interference reasonably well i) Code acquisition may be difficult ii) Susceptible to Near-Far problem iii) Affected by jamming
  4. 4. Frequency Hopping i) Less affected by Near-Far problem ii) Better for avoiding jamming iii) Less affected by multi-ccess interference i) Needs FEC ii) Frequency acquisition may be difficult Time Hopping i) Bandwidth efficient ii) Simpler than FH system i) Elaborate code acquisition is needed. ii) Needs FEC DISADVANTAGE AND ADVANTAGES OF THSS DISADVANTAGE TH may be used to aid in reducing interference between systems in time division multiplexing (TDM).However, stringent timing requirements is placed on the overall system to ensure minimum overlap between transmitters. This is one of the reasons that makes a TH system much harder to implement than other SS systems. A simple THSS system can be blocked by a jammer that uses a continuous carrier at the signal center frequency. ADVANTAGES The primary advantage offered is in the reduced duty cycle. In other words, to be an effective jammer an interfering transmitter has to be forced to transmit continuously (assuming the TH sequence used by the time hopper is unknown to the interferer). The power required by a legitimate time hopper will be less than that of an interfering transmitter by a factor that should be equal to the PG of this TH system. VI. APPLICATION OF THSS The THSS techniques have been found useful in ranging, multiple access, or other special applications introduced later. A typical example for such applications is the UWB technology, which has had tremendous attention recently, due to its many attractive properties, such as its unique capability to mitigate multipath propagation problems based on its very high time resolution VII. CONCLUSION The THSS technique is much less widely used than either DSSS or FHSS technique. The reason is partly because of the fact that it may suffer from serious interference problems if there exists a continuous transmission in the coverage area, as the TH system only works in an on-and-off fashion in a frame. For this reason, the TH technique usually works with other SS techniques, in particular the FH technique, forming a hybrid TH-FH system ACKNOWLEDGMENT Thankful to Assistant Professor Komal Arora for giving this valuable topic for the term paper REFERENCES [1] Spread Spectrum Goes Commercial, by Donald L. Schilling of City College of New York, Raymond L. Pickholtz of George Washington University, and Laurence B. Milstein of UC San Diego [2]Next Generation Wireless Systems and Networks,Hsiao-Hwa Chen,National Sun Yat- Sen University, Taiwan and Mohsen Guizani,Western Michigan University, USA [3] An introduction to spread spectrum,charles e. Cook and howard s. Marsh  Website links [4]http://www.ausairpower.net/OSR-0597.html [5]http://en.wikipedia.org/wiki/Spread_spectrum [6]http://www.telecomabc.com/s/spread- spectrum.html

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