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Note for Digital Communication - DC By Manjunatha P

  • Digital Communication - DC
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Chapter 1 Spread Spectrum Communication System 1.1 Introduction: Principles of Spread Spectrum: Spread Spectrum Communication Systems: • Model of a Spread Spectrum Digital Communication System • Direct Sequence Spread Spectrum Systems • Effect of De-spreading on a narrowband Interference, Probability of error (statement only) • Some applications of DS Spread Spectrum Signals • Generation of PN Sequences • Frequency Hopped Spread Spectrum • CDMA based on IS-95 (Text 2: 11.3.1, 11.3.2, 11.3.3, 11.3.4, 11.3.5, 11.4.2). Text 2: John G Proakis and Masoud Salehi, “Fundamentals of Communication Systems”, 2014 Edition, Pearson Education, ISBN 978-8-131-70573-5. Note: While preparing this material Text 2 (John G Proakis) is used and most of the Figures are redrawn and some of the Figures and material is used from the Internet. Some concepts are directly downloaded from the Internet. All the concepts may be of from Text book or it may be from Internet. 1

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1.2. INTRODUCTION: 1.2 Chapter 1. Spread Spectrum Communication System Introduction: • Main resources of the communication systems are transmitter power and channel bandwidth W. • The major goal of design engineer is the efficient utilization of transmitter power and channel bandwidth W. • Let R denote the information rate at the input to transmitter and W denotes channel bandwidth. • The ratio of W and R is defined as bandwidth expansion factor and is given by Be = W R • By increasing Be the transmitted signal can be reduced to achieve a specific level of performance. • In most practical communication systems Be is in the range of 2 ≤ Be ≤ 5. • In multiple access communication system when more than two transmitters use the same common channel to transmit information, then interference will occurs and this will limits the performance of the system. • To overcome the degradation of the system performance the bandwidth is further expanded and bandwidth expansion factor Be is much greater than unity. • The expanding of bandwidth is called as spread spectrum signal. The spreading of the bandwidth is accomplished by means of a code that is independent of the information sequence. • The code used is usually called psuedorandom code because it appears random signal to receivers other than the intended receiver. • Spread spectrum communication system was originally developed and used for military communication for secure communication system. But later spread spectrum system is used for civilian applications like digital cellular communication, cordless telephones and other applications • There are mainly two methods are used to spread the signal bandwidth those are – Direct Sequence Spread Spectrum (DSSS) system – Frequency hop Sequence Spread Spectrum (DSSS) system . 1.2.1 Model of Spread Spectrum Digital Communication • A type of modulation in which the modulated signal bandwidth is much greater than the message signal bandwidth. The spreading of the message signal spectrum is done by a spreading code called Pseudo Noise Code (PN Code) which is independent of the message signal. The basic elements of spread spectrum digital communication system is as shown in Figure 1.1 • The channel encoder and decoder and the modulator and demodulator are the basic elements of the system. Dr. Manjunatha P Professor Dept of E&CE, JNN College of Engineering, Shivamogga 2

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1.2. INTRODUCTION: Chapter 1. Spread Spectrum Communication System • There are two identical pseudorandom pseudonoise (PN) pattern generator one with modulator at the transmitting end and a second with the demodulator at the receiving end. • PN sequneces are binary valued sequence which are used to spread the signal at the transmitter and to despread at the demodulator. • Time synchronization of the PN sequence generated at the receiver is required in order to despread the received signal. • Synchronization of PN signal is achieved prior to the transmission of information and this is achieved by transmitting a known fixed PN bit pattern to the receiver • After synchronization the information will be transmitted. • In spread spectrum modulation there are two types digital modulation schemes are used those are 1. Phase Shift Keying (PSK) modulation 2. Frequency Shift Keying (FSK) modulation • PSK modulation is used where phase coherence is possible to maintain between transmitter and receiver. • FSK modulation is used where phase coherence is not possible to maintain between transmitter and receiver. • In PSK modulation the phase of the carrier is varied pseudorandomly in accordance with PN codes, and the modulated signal is called as direct sequence spread spectrum (DSSS) signal. • In FSK modulation the frequency of the carrier is changed pseudorandomly in accordance with PN codes, and the modulated signal is called as Frequency hop spread spectrum (FHSS) signal. k bits Information sequence Channel Encoder Modulator Channel Demodulator Channel Decoder Output m bits PN sequence generator PN sequence generator Figure 1.1: Model spread-spectrum digital communication system Dr. Manjunatha P Professor Dept of E&CE, JNN College of Engineering, Shivamogga 3

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1.2. INTRODUCTION: 1.2.2 Chapter 1. Spread Spectrum Communication System Direct Sequence Spread Spectrum (DSSS) System The model of Direct Sequence Spread Spectrum (DSSS) System is as shown in Figure 1.2 Antenna Binary Data sequence {bk} Polar nonreturn to zero level encoder v(t ) Binary PSK modulator x(t ) c(t ) Carrier PN code generator Ac cos(2π f ct ) Figure 1.2: Modulation of DS spread-spectrum signal • The incoming binary data with bit rate of R and the bit interval Tb = 1/R seconds is multiplied by PN sequence with bandwidth of W = Bc is used to spread the binary data to W Hz. • The spreading of data with PN sequence is as shown in Figure 1.3. Data v(t) +1 -1 Tb Spreading Code c(t) +1 -1 Tc NTc 0 0 1 1 1 0 1 Product Signal =v(t)c(t) +1 -1 Figure 1.3: Generation of a DS spread-spectrum signal Dr. Manjunatha P Professor Dept of E&CE, JNN College of Engineering, Shivamogga 4

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