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# Note for Digital Communication - DC By Manjunatha P

• Digital Communication - DC
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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

#### Text from page-5

1.2. INTRODUCTION: Chapter 1. Spread Spectrum Communication System Figure 1.4: Generation of a DS spread-spectrum signal • The information bearing baseband signal is denoted as v(t) and is expressed as v(t) = ∞ X an gT (t − nTb ) n−∞ where an = ±1 and −∞ < n < ∞ • gT (t) is a rectangular pulse of duration Tb • The spreading sequence is expressed as c(t) = ∞ X cn p(t − nTc ) n−∞ where cn = ±1 and −∞ < n < ∞ • p(t) is a rectangular pulse of duration Tc • The multiplication operation between information data sequence and PN codes serves to spread the bandwidth of the data signal of bandwidth R Hz into the wider bandwidth of the PN code signal c(t) to 1/Tc • The details of the spreading bandwidth is as shown in Figure 1.5 Dr. Manjunatha P Professor Dept of E&CE, JNN College of Engineering, Shivamogga 5