THSS

Topics: Randomness, Pseudorandom number generator, Direct-sequence spread spectrum Pages: 5 (1191 words) Published: October 11, 2013

Spread Spectrum
Time Hopping Spread Spectrum

Table of Content
Introduction1-2
Concept and features of THSS3-4
Why was it developed?5
Advantages and Disadvantages6
How does it work?7-8
Use, Importance and Application9
Conclusion10
Reference11

Introduction

Spread spectrum technology was invented in the 1940s, and has been used extensively since then for military and other applications. Spread spectrum is a technique used in radio transmissions whereby a transmitted signal is spread over or occupies a wide frequency band. In the case of radio transmission, the signal spreads over a much larger part of the Radio Frequency spectrum that is much wider than the minimum bandwidth required for transmitting the information being sent. This makes the signal more robust against interference and jamming. During the process of this technique the narrowband signal is manipulated (scrambled) prior to transmission. The manipulation requires a pseudo random noise. Pseudo random noise is similar to noise and although it seems to lack any definite pattern, pseudo random noise consists of a deterministic sequence of pulses that will repeat itself after its period. Pseudo random noise code is basically a method of transmitting messages in the presence of interference or noise. Spread spectrum communications cannot be said to be an efficient means of utilizing bandwidth. However, it does come into its own when combined with existing systems occupying the frequency. There are three types of spread spectrum methods:

Frequency hopping spread spectrum
Direct sequence spread spectrum
Time hopping spread spectrum
When it comes to the case of the time hopping system, in this method a sequence of short duration pulses is transmitted which is generated from the narrowband information that carries signal through scrambling or manipulating 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 Ultra wide band (UWB) signals. Concept and features of THSS

A time hopping system is a spread spectrum system in which the period and duty cycle of a pulsed Radio Frequency (RF) carrier are varied in a pseudo random manner under the control of a coded sequence. The frequency hopping spectrum is achieved by carrying out the on–off keying in the time-domain. Time Hopping Spread Spectrum-CDMA

 In Time Hopping Spread Spectrum (THSS) system, the time axis is divided into duration intervals representing the frames of the duration τframe. As shown in figure, each frame is then further divided into a number of N slots with time duration τslot. In THSS communications a single user uses one slot out of k possible slots within one frame and sends data with k time’s higher data rate in contrast to the situation where the data is transmitted within the whole frame. Each spreading sequence corresponds to a specific arrangement of the time-slot locations.

 

In the figure of the Time Hopping Spread Spectrum seen below, each burst consists of k bits of data and the exact time each burst transmitted is determined by a PN sequence.

In the figure of the Time Hopping Spread Spectrum, Each burst consists of k bits of data and the exact time each burst is transmitted is determined by a pseudorandom number sequence.

Why was it developed?

In some situations it is required that a communication signal be difficult to detect, and difficult to demodulate even when detected and we do this so that unintended users will find it difficult to interpret the message. In other situations a signal is required so that is difficult to interfere with or difficult to be ‘jam’. By having a wide range of band it makes the signal less prone or robust to intentional or unintentional interference. Another major reason for developing THSS is the ability to do multiple accesses. With proper design of pseudo-random sequences,...
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