Different techniques of spread spectrum

Different techniques of spread spectrum CHAPTER 1 INTRODUCTION: The idea of this project is to study different techniques of spread spectrum. These techniques include the two processes of direct-sequence spread spectrum and frequency hopping spread spectrum. Both these techniques will be simulated in software called MATLAB. Also a brief mention will be there about the multiple access property of spread spectrum. This project deals with the implementation of spread spectrum using the techniques- direct sequence spread spectrum and frequency hopping spread spectrum. Spread spectrum is a new method of communication. It provides a jam-free network and prevents any kind on interference. It helps to provide immunity to channels by not allowing any kind of interference or disturbance. We will be discussing more about spread spectrum and its technique in the following chapters. The project has been divided into six chapters. The second chapter deals with spread spectrum. It starts of with the history of spread spectrum followed by a basic definition of spread spectrum. Certain characteristics of spread spectrum are discussed along with a mathematical explanation of advantage of spread spectrum over narrow band communication. It then describes the working of a spread spectrum and finally mentions how a spread spectrum can be demodulated. Chapter three describes different techniques of spread spectrum. There are four techniques of spread spectrum namely direct sequence spread spectrum (DSSS), frequency hopping spread spectrum (FHSS), chirp spread spectrum (CSSS) and time hopping spread spectrum (THSS). Each of these techniques is described, with a brief mention about its basic mechanism.A brief comparison is drawn between FHSS AND DHSS. A mathematical explanation is given for DSSS under the conditions of noise (jammer). Chapter four discusses different multiple accesses of spread spectrum. These are frequency domain multiple access (FDMA), time division multiple access(TDMA) and code division multiple access(CDMA). CDMA is dealt with in a more detail manner and is presented with a mathematical explanation. Chapter five presents a MATLAB simulation of direct sequence spread spectrum (DSSS), Frequency hopping spread spectrum (FHSS) and Code division multiple access (CDMA). It discusses how a signal when multiplied with a pseudo random noise and put on a frequency wave gets spread. This technique is called direct sequence spread spectrum. Also this chapter presents the MATLAB simulation of Frequency Hopping Spread Spectrum where four randomly carrier frequency waves are generated to form a spread spectrum and multiplied with pseudo random generator to determine the frequencies where the information has to be hopped. The information is sent in the form of a BPSK modulated signal. Also a MATLAB simulation on CDMA is presented .It basically shows how CDMA works. Three stations are taken and how they send codes through a single channel by using the property the CDMA. Finally, chapter six winds up the project with the conclusion from each chapter. It mentions in short what has been realized from this project. CHAPTER 2 SPREAD SPECTRUM 2.1 HISTORY OF SPREAD SPECTRUM This chapter talks about spread spectrum. It begins with a brief history behind spread spectrum. It also mentions about the various fields where spread spectrum is being effectively used. Certain characteristics of spread spectrum are also discussed.A basic definition of spread spectrum is discussed in this chapter which basically explains what actually spread spectrum and why is preferred over conventional wireless communication. A brief mechanism of how spread spectrum works is discussed. It also mentions the mathematical reason behind why a spread signal is an effective way to communicate. A comparison is drawn between spread spectrum transmission and fixed frequency transmission. Finally a demodulation technique is discussed mentioning about how demodulation can be achieved. Spread spectrum has become a new commercial communication technique over the past 8-9 years. However the first intentional use of Spread Spectrum came during the period of 1921-1930 by Armstrong. He had used wideband Frequency Modulation. The real use for Spread Spectrum how ever came in World War II. Both the allies and the Axis powers experimented with simple Spread Spectrum systems. The first publically patent on Spread Spectrum came from Hedy Lamarr, the Hollywood movie actress, and George Antheil, an avant gard composer. Lamarr had mentioned to Antheil about her idea for a Secret Communications System that could guide torpedoes to their target without being intercepted by the enemy. This could be done by sending messages between transmitter and receiver over multiple radio frequencies in a varied random pattern. The message would be transmitted at such a high rate that no one would be able to decode it. They sent their invention to National Inventors Council. Instead of developing the patent commercially, they gave it away to the government for the war effort. As a result of which the commercial use of Spread spectrum came many years later [1, 3, 19]. Spread Spectrum was first used for commercial purposes in the 1980s when Equatorial Communications of Mountain View used Direct Sequence for multiple access communications over synchronous satellite transponders [1]. Today, spread spectrum is being used to provide communications in a variety of commercial applications, including mobile communications and interoffice wireless communications. In the coming years hardly anyone will prevent themselves from being involved in one way or the other with spread spectrum communications as it will become an integral part of the communication world. [1, 4]. 2.2 DEFINITION OF SPREAD SPECTRUM Spread-spectrum is a technique by which electromagnetic energy produced over a particular bandwidth is spread in the frequency domain on purpose. Spread spectrum signals are used for transmission of digital information which are characterized by the fact that their bandwidth W is much greater than the information rate R in bits/sec. Hence the bandwidth expansion factor is always greater than unity. This results to provide a signal with a wider bandwidth and a lower power density. These techniques are used for a variety of reasons which include the establishment of secure communications, increasing resistance to natural interference and to limit the power flux density on satellite downlinks. Narrow band signal and spread spectrum use the same amount of power to transmit the same data. However the power density of the spread spectrum is lower as compared to narrow band and it is this property that makes spread spectrum achieve secure and jam free communication. Spread spectrum is a type of wireless communication in which the frequency of the transmitted signal is intentionally varied and changed to achieve a higher bandwidth. This results to provide a much greater bandwidth than that of the signal if its frequency was not altered [2, 5]. A spread spectrum should full fill the following requirements: 1) The message signal should undergo two modulations that produce the wideband spectrum having bandwidth much in excess of the minimum bandwidth needed to send the message. 2) The spreading ,that is the second modulation is caused by means of a spreading signal 3) The receiver should recover the message by the method of de-spreading the signal that is received by using the same spreading signal [23]. Spread spectrum is preferred over conventional wireless communication for many reasons; however there are at least a couple of problems associated with conventional wireless communication. First, a signal whose frequency is constant and not changing is subject to interference. This condition occurs when another signal is sent on, or very near to the frequency of the desired signal. Interference can be accidental (as in the case of amateur-radio communications) or it can be deliberate like during war time. Second, a constant-frequency signal is very easy to intercept or lock and reveal the information it is carrying and hence is not well preferred to applications where the information must remain confidential between the transmitter and receiver. [2]. To minimize troubles and to maintain the confidentiality of the information, transmitted signals frequency can be deliberately varied and changed over the large segment of the electromagnetic radiation spectrum. This variation is done by using a specific and complicated mathematical function. This function is the most important part of the spread spectrum for maintaining the confidentiality. For intercepting the signal, the receiver must be tuned to frequencies that vary exactly to this function. The receiver must have the knowledge of the frequency-versus-time function employed by the transmitter, and must also be provided with the information about the starting-time point of the function. If someone wants to hack or intercept the spread-spectrum signal, that person must possess a transmitter that contains the information about the function and its starting-time point. The spread-spectrum function hence must be well protected for confidential information communication and it this feature of spread spectrum that helps maintaining the secrecy of information. If this function comes in the wrong hands, it can lead to the invasion of secrecy of the information [2]. 2.3 HOW SPREAD SPECTRUM WORKS Spread Spectrum uses wide band which is a noise-like signals and hence are hard to detect. These signals are also hard to lock on to or be tracked by any intruder. Spread signals are made in such a manner that their bandwidth is larger than the information they are transmitting to give them a noise like appearance, hence protecting the information it is carrying and maintain its secrecy and confidentiality [4]. Spread Spectrum signals use fast codes that have a high data rate. These codes are called Pseudo Random or Pseudo Noise codes. These are called Pseudo for the reason being that they are not real Gaussian noise [4]. Spread Spectrum transmitters possess similar transmits power levels to that of narrow band transmitters. Spread Spectrum signals are so wide hence they transmit at a much lower spectral power density which is measured in Watts per Hertz as compared to narrowband transmitters. This characteristic (lower spectral power density)gives spread signals a huge advantage in digital communication and it is this capability that is responsible for the rapid use of Spread Spectrum today [4]. The numerical advantage of wider bandwidth is obtained from the Claude Shannons equation describing channel capacity [4]. (2.1) Where; C=channel capacity of the signal, B=Bandwidth of the signal, S=signal power, and N=noise power. = (2.2) On Applying the MacLaurin series development for (2.3) is usually low for spread-spectrum applications. Assuming a noise level such that