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

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

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

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