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Reasons for Choosing Encoding Techniques Digital data, digital signal Equipment less complex and expensive than Signal Encoding Techniques digital-to-analog modulation equipment Analog data, digital signal Permits use of modern digital transmission and Chapter 6 switching equipment Reasons for Choosing Encoding Techniques Signal Encoding Criteria Digital data, analog signal What determines how successful a receiver will be Some transmission media will only propagate in interpreting an incoming signal? analog signals Signal-to-noise ratio Data rate E.g., optical fiber and unguided media Bandwidth Analog data, analog signal An increase in data rate increases bit error rate Analog data in electrical form can be An increase in SNR decreases bit error rate transmitted easily and cheaply An increase in bandwidth allows an increase in Done with voice transmission over voice-grade data rate lines Factors Used to Compare Factors Used to Compare Encoding Schemes Encoding Schemes Signal spectrum Signal interference and noise immunity With lack of high-frequency components, less Performance in the presence of noise bandwidth required Cost and complexity With no dc component, ac coupling via transformer The higher the signal rate to achieve a given data rate, possible the greater the cost Transfer function of a channel is worse near band edges Clocking Ease of determining beginning and end of each bit position Basic Encoding Techniques Basic Encoding Techniques Digital data to analog signal Amplitude-shift keying (ASK) Amplitude difference of carrier frequency Frequency-shift keying (FSK) Frequency difference near carrier frequency Phase-shift keying (PSK) Phase of carrier signal shifted Amplitude-Shift Keying Amplitude-Shift Keying One binary digit represented by presence of Susceptible to sudden gain changes carrier, at constant amplitude Other binary digit represented by absence of Inefficient modulation technique carrier On voice-grade lines, used up to 1200 bps A cos(2πf c t ) binary 1 Used to transmit digital data over optical s (t ) = fiber 0 binary 0 where the carrier signal is Acos(2πfct) Binary Frequency-Shift Keying Binary Frequency-Shift Keying (BFSK) (BFSK) Two binary digits represented by two different Less susceptible to error than ASK frequencies near the carrier frequency On voice-grade lines, used up to 1200bps Used for high-frequency (3 to 30 MHz) A cos(2πf t ) radio transmission binary 1 s (t ) = 1 A cos(2πf 2t ) binary 0 Can be used at higher frequencies on LANs that use coaxial cable where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts Multiple Frequency-Shift Keying Multiple Frequency-Shift Keying (MFSK) (MFSK) More than two frequencies are used To match data rate of input bit stream, More bandwidth efficient but more susceptible to each output signal element is held for: error Ts=LT seconds s i (t ) = A cos 2 π f i t 1≤ i ≤ M where T is the bit period (data rate = 1/T) So, one signal element encodes L bits f i = f c + (2i – 1 – M)f d f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L L = number of bits per signal element Multiple Frequency-Shift Keying Multiple Frequency-Shift Keying (MFSK) (MFSK) Total bandwidth required 2Mfd Minimum frequency separation required 2fd=1/Ts Therefore, modulator requires a bandwidth of Wd=2L/LT=M/Ts Phase-Shift Keying (PSK) Phase-Shift Keying (PSK) Two-level PSK (BPSK) Differential PSK (DPSK) Uses two phases to represent binary digits Phase shift with reference to previous bit Binary 0 – signal burst of same phase as previous A cos(2πf c t ) binary 1 signal burst s (t ) = A cos(2πf c t + π ) binary 0 Binary 1 – signal burst of opposite phase to previous signal burst A cos(2πf c t ) binary 1 = − A cos(2πf c t ) binary 0 Phase-Shift Keying (PSK) QPSK and OQPSK Four-level PSK (QPSK) Each element represents more than one bit π A cos 2πf c t + 4 11 A cos 2πf ct + 3π s(t ) = 01 4 3π A cos 2πf ct − 00 4 π A cos 2π f c t − 10 4 Phase-Shift Keying (PSK) Performance Multilevel PSK Bandwidth of modulated signal (BT) Using multiple phase angles with each angle having more than one amplitude, multiple signals ASK, PSK BT=(1+r)R elements can be achieved FSK BT=2DF+(1+r)R R R D= = L log 2 M R = bit rate 0 < r < 1; related to how signal is filtered D = modulation rate, baud DF = f2-fc=fc-f1 R = data rate, bps M = number of different signal elements = 2L L = number of bits per signal element Performance Performance Bandwidth of modulated signal (BT) 1+ r 1+ r MPSK BT = log M R R = L 2 MFSK (1 + r )M log M R BT = 2 L = number of bits encoded per signal element M = number of different signal elements Quadrature Amplitude Performance Modulation QAM is a combination of ASK and PSK Two different signals sent simultaneously on the same carrier frequency s (t ) = d1 (t ) cos 2πf c t + d 2 (t )sin 2πf c t Quadrature Amplitude Modulation Reasons for Analog Modulation Modulation of digital signals When only analog transmission facilities are available, digital to analog conversion required Modulation of analog signals A higher frequency may be needed for effective transmission Modulation permits frequency division multiplexing Basic Encoding Techniques Amplitude Modulation Analog data to analog signal Amplitude Modulation Amplitude modulation (AM) s (t ) = [1 + n a x (t )]cos 2π f c t Angle modulation Frequency modulation (FM) cos2πfct = carrier Phase modulation (PM) x(t) = input signal na = modulation index Ratio of amplitude of input signal to carrier a.k.a double sideband transmitted carrier (DSBTC) Spectrum of AM signal Amplitude Modulation Transmitted power n 2 Pt = Pc 1 + a 2 Pt = total transmitted power in s(t) Pc = transmitted power in carrier Single Sideband (SSB) Angle Modulation Variant of AM is single sideband (SSB) Angle modulation s (t ) = Ac cos[2πf c t + φ (t )] Sends only one sideband Eliminates other sideband and carrier Advantages Phase modulation Only half the bandwidth is required Less power is required Phase is proportional to modulating signal Disadvantages φ (t ) = n p m(t ) Suppressed carrier can’t be used for synchronization purposes np = phase modulation index Angle Modulation Angle Modulation Frequency modulation Compared to AM, FM and PM result in a Derivative of the phase is proportional to signal whose bandwidth: modulating signal is also centered at fc but has a magnitude that is much different φ ' (t ) = n f m(t ) Angle modulation includes cos(∅ (t)) which produces a wide range of frequencies nf = frequency modulation index Thus, FM and PM require greater bandwidth than AM Angle Modulation Basic Encoding Techniques Carson’s rule Analog data to digital signal Pulse code modulation (PCM) where BT = 2(β + 1)B Delta modulation (DM) n p Am for PM β = ∆F n f Am B = for FM 2πB The formula for FM becomes BT = 2∆F + 2 B Analog Data to Digital Signal Pulse Code Modulation Once analog data have been converted to Based on the sampling theorem digital signals, the digital data: Each analog sample is assigned a binary can be transmitted using NRZ-L code can be encoded as a digital signal using a code Analog samples are referred to as pulse other than NRZ-L amplitude modulation (PAM) samples can be converted to an analog signal, using The digital signal consists of block of n bits, previously discussed techniques where each n-bit number is the amplitude of a PCM pulse Pulse Code Modulation Pulse Code Modulation By quantizing the PAM pulse, original signal is only approximated Leads to quantizing noise Signal-to-noise ratio for quantizing noise SNR dB = 20 log 2 n + 1.76 dB = 6.02n + 1.76 dB Thus, each additional bit increases SNR by 6 dB, or a factor of 4 Delta Modulation Delta Modulation Analog input is approximated by staircase function Moves up or down by one quantization level (δ) at each sampling interval The bit stream approximates derivative of analog signal (rather than amplitude) 1 is generated if function goes up 0 otherwise Reasons for Growth of Digital Delta Modulation Techniques Two important parameters Growth in popularity of digital techniques Size of step assigned to each binary digit (δ) for sending analog data Sampling rate Repeaters are used instead of amplifiers Accuracy improved by increasing sampling No additive noise rate TDM is used instead of FDM However, this increases the data rate No intermodulation noise Advantage of DM over PCM is the Conversion to digital signaling allows use of simplicity of its implementation more efficient digital switching techniques

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posted: | 1/12/2012 |

language: | Catalan |

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