COE 341: Data & Computer Communications (T061) Dr. Marwan Abu-Amara

COE 341: Data & Computer Communications (T061) Dr. Marwan Abu-Amara
Chapter 5: Data Encoding

Encoding and Modulation Techniques
COE 341 (T061) – Dr. Marwan Abu-Amara

Digital & Analog Signaling
Digital signaling Data source g(t) encoded into digital signal x(t) g(t) may be analog (e.g. voice) or digital (e.g. file) x(t) dependent on coding technique, chosen to optimize use of transmission medium Conserve bandwidth or minimize errors Analog signaling Based on continuous constant frequency signal, carrier signal (i.e. A cos(2fct+) or A sin(2fct+)) Carrier signal frequency chosen to be compatible with transmission medium Data transmitted by carrier signal modulation by manipulating A, fc, and/or COE 341 (T061) – Dr. Marwan Abu-Amara

COE 341 (T061) – Dr. Marwan Abu-Amara
Analog Signaling Modulation Process of encoding source data onto a carrier signal with frequency fc Operation on one or more of three fundamental frequency-domain parameters: amplitude, frequency, and phase Input signal m(t) Can be analog or digital Called modulating signal or baseband signal Modulated signal s(t) is result of modulating carrier signal; called bandlimited or bandpass signal Location of bandwidth on spectrum related to carrier frequency fc COE 341 (T061) – Dr. Marwan Abu-Amara

Baseband vs. Bandpass Signals
Baseband Signal: Spectrum not centered around non zero frequency May have a DC component Bandpass Signal: Does not have a DC component Finite bandwidth around or at fc COE 341 (T061) – Dr. Marwan Abu-Amara

Encoding and Modulation: Remarks
Encoding is simpler and less expensive than modulation Encoding into digital signals allows use of modern digital transmission and switching equipment Basis for Time Division Multiplexing (TDM) Modulation shifts baseband signals to a different region of the frequency spectrum Basis for Frequency Division Multiplexing (FDM) Unguided media and optical fibers can carry only analog signals COE 341 (T061) – Dr. Marwan Abu-Amara

Encoding Techniques Digital data, digital signal Simple and inexpensive equipment Analog data, digital signal Data needs to be converted to digital form Digital data, analog signal Take advantage of existing analog transmission media Analog data, analog signal Transmitted as baseband signal easily and cheaply COE 341 (T061) – Dr. Marwan Abu-Amara

Digital Data, Digital Signal
Sequence of discrete, discontinuous voltage pulses Each pulse is a signal element Binary data encoded into signal elements Unipolar signal All signal elements have same sign (e.g. 0: +5 V, 1: +10 V  DC content) Polar signal One logic state represented by positive voltage & the other by negative voltage (e.g. 0: +5 V, 1: -5 V  ideally Zero DC content) COE 341 (T061) – Dr. Marwan Abu-Amara

Digital Data, Digital Signal …
Mark and Space Binary 1 and Binary 0 respectively Duration or length of a bit (Tb) Time taken for transmitter to emit the bit Data rate, R ( = 1/Tb) Rate of data transmission in bits per second (bps) Duration of a Signal Element (Ts) Minimum signal pulse duration Modulation (signaling) rate (1/Ts) Rate at which the signal level changes with time Measured in bauds = signal elements per second COE 341 (T061) – Dr. Marwan Abu-Amara

Digital Data, Digital Signal …
Data rate = 1/1ms = 1 M bps Signaling Rate for NRZI: = 1/1ms = 1 M bauds Signaling Rate for Manchester: = 1/0.5ms = 2 M bauds Tb Ts Ts COE 341 (T061) – Dr. Marwan Abu-Amara

Interpretation of the Received Signal COE 341 (T061) – Dr. Marwan Abu-Amara

Interpreting Digital Signal at Receiver
Receiver need to know Timing of bits - when they start and end Signal level Sampling & comparison with a threshold value Factors affecting successful interpretation of signals: signal to noise ratio, data rate, bandwidth Increase in data rate increases bit-error-rate (BER) Increase in SNR decreases BER Increase in bandwidth allows for increase in data rate COE 341 (T061) – Dr. Marwan Abu-Amara

Comparison of Encoding Schemes
Mapping from data bits to signal elements Signal Spectrum Lack of high frequencies reduces required bandwidth Lack of dc component allows ac coupling via transformer  providing isolation & reducing interference Transfer function of a channel is worse near the band edges  Concentrate power in the middle of the bandwidth Clocking Synchronizing transmitter and receiver Sync mechanism based on signal can be built into signal encoding COE 341 (T061) – Dr. Marwan Abu-Amara

Comparison of Encoding Schemes
Error detection Can be built into signal encoding Signal interference and noise immunity Some codes are better than others Cost and complexity Higher signal rate (& thus data rate) lead to higher costs Some codes require signal rate greater than data rate COE 341 (T061) – Dr. Marwan Abu-Amara

Encoding Schemes Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Bipolar –AMI (alternate mark inversion) Pseudoternary Manchester Differential Manchester COE 341 (T061) – Dr. Marwan Abu-Amara

Nonreturn to Zero-Level (NRZ-L)
Two different voltages for 0 and 1 bits Voltage constant during bit interval no transition, no return to zero voltage e.g. Absence of voltage for zero, constant positive voltage for one More often, negative voltage for one value (1) and positive for the other (0) Used to generate or interpret digital data by terminals An example of absolute encoding Encoding data as a signal level COE 341 (T061) – Dr. Marwan Abu-Amara

Nonreturn to Zero Inverted (NRZI)
Nonreturn to zero inverted on ones Constant voltage pulse for duration of bit Data encoded as presence or absence of signal transition at beginning of bit time Transition (low to high or high to low) denotes a binary 1 No transition denotes binary 0 An example of differential encoding Info to be transmitted represented as changes between successive signal elements COE 341 (T061) – Dr. Marwan Abu-Amara

NRZ (+)ve (–)ve Transition Denotes one

NRZ pros and cons Pros Easy to engineer Make good use of bandwidth Cons Large dc component Lack of synchronization capability No signal transitions for long strings of 0’s or 1’s Used for magnetic recording Not often used for signal transmission COE 341 (T061) – Dr. Marwan Abu-Amara

Differential Encoding
Data represented by signal transitions rather than signal levels Advantages; With noise, signal transitions are detected more easily than signal levels In complex transmission layouts, it is easy to accidentally lose sense of polarity RX Effect of swapping terminals on: NRZ-L NRZI + _ COE 341 (T061) – Dr. Marwan Abu-Amara

NRZ pros and cons COE 341 (T061) – Dr. Marwan Abu-Amara

Multilevel Binary Use more than two signaling levels Bipolar-AMI (Alternate Mark Inversion) zero represented by no line signal one represented by positive or negative pulse one pulses alternate in polarity No loss of sync if a long string of ones (zeros still a problem) No net dc component Lower bandwidth Easy error detection COE 341 (T061) – Dr. Marwan Abu-Amara

Pseudoternary One represented by absence of line signal Zero represented by alternating positive and negative No advantage or disadvantage over bipolar-AMI COE 341 (T061) – Dr. Marwan Abu-Amara

Bipolar-AMI and Pseudoternary
All Single Pulse Errors- Detected Double Pulse Error- Undetected Adding Canceling Double Pulse Error- Detected COE 341 (T061) – Dr. Marwan Abu-Amara

Trade Off for Multilevel Binary
Not as efficient as NRZ Each signal element only represents one bit Date Rate=R=1/TB In a 3 level system could represent log23 = 1.58 bits Receiver must distinguish between three levels (+A, -A, 0) Requires approximately 3dB more signal power for same probability of bit error COE 341 (T061) – Dr. Marwan Abu-Amara

Theoretical Bit Error Rate for Various Encoding Schemes

Biphase Manchester Transition in middle of each bit period Transition serves as clock and data High to low represents zero Low to high represents one Used by IEEE (Standard for baseband coaxial cable & twisted pair CSMA/CD bus LANs) Differential Manchester Mid bit transition is clocking only Transition at start of a bit period represents zero No transition at start of a bit period represents one Note: this is a differential encoding scheme Used by IEEE (Token ring LAN) COE 341 (T061) – Dr. Marwan Abu-Amara

Manchester Encoding COE 341 (T061) – Dr. Marwan Abu-Amara

Differential Manchester Encoding

Biphase Pros and Cons Pros Synchronization on mid bit transition (self clocking) No dc component Error detection Absence of expected transition Con At least one transition per bit time and possibly two Maximum modulation rate is twice NRZ Requires more bandwidth COE 341 (T061) – Dr. Marwan Abu-Amara

Modulation (Signaling) Rate
Data rate (R) Bits per second, or bit rate 1/Tb, where Tb is bit duration Modulation rate (D) Rate at which signal elements generated Measured in Baud Modulation Rate D = R × k R = data rate in bps M = signaling levels used  L = bits/signal element = log2 M k = signal elements per bit = signal trans./bit trans. = 1/L In General, Modulation Rate D = R × k = R/log2 M COE 341 (T061) – Dr. Marwan Abu-Amara

Modulation (Signaling) Rate …
Tb Ts k=1 k=2 So, for Manchester  D= kR = 2/Tb COE 341 (T061) – Dr. Marwan Abu-Amara

Scrambling Use scrambling to replace sequences that would produce constant voltage Filling sequence Must produce enough transitions to sync Must be recognized by receiver and replace with original Same length as original Not be likely to be generated by noise No dc component No long sequences of zero level line signal No reduction in data rate Error detection capability COE 341 (T061) – Dr. Marwan Abu-Amara

B8ZS Bipolar With 8 Zeros Substitution Based on bipolar-AMI If octet of all zeros and last voltage pulse preceding was positive encode as If octet of all zeros and last voltage pulse preceding was negative encode as Causes two violations of AMI code Unlikely to occur as a result of noise Receiver detects and interprets as octet of all zeros COE 341 (T061) – Dr. Marwan Abu-Amara

HDB3 High Density Bipolar 3 Zeros Based on bipolar-AMI String of four zeros replaced with one or two pulses COE 341 (T061) – Dr. Marwan Abu-Amara

B8ZS and HDB3 1s COE 341 (T061) – Dr. Marwan Abu-Amara

Digital Data, Analog Signal
Transmission of digital data through public telephone network Public telephone system 300Hz to 3400Hz Use modem (modulator-demodulator) Encoding techniques modify one of three characteristics of carrier signal Amplitude => Amplitude shift keying (ASK) Frequency => Frequency shift keying (FSK) Phase => Phase shift keying (PSK) Resulting signal has a bandwidth centered on carrier frequency COE 341 (T061) – Dr. Marwan Abu-Amara

Digital Data, Analog Signal

Amplitude Shift Keying (ASK)
Binary values represented by different amplitudes of carrier Usually, one amplitude is zero i.e. presence and absence of carrier is used For a carrier signal resulting signal is COE 341 (T061) – Dr. Marwan Abu-Amara

Amplitude Shift Keying (ASK)
Inefficient: up to 1200bps on voice grade lines Used to transmit digital data over optical fiber COE 341 (T061) – Dr. Marwan Abu-Amara

Frequency Shift Keying (FSK)
Binary values represented by two different frequencies near carrier frequency Resulting signal is f1 and f2 are offset from carrier frequency fc by equal but opposite amounts fc f1 f2 Dfc COE 341 (T061) – Dr. Marwan Abu-Amara

Less susceptible to error than ASK Up to 1200bps on voice grade lines Used for high frequency (3 to 30 MHz) radio transmission Even higher frequencies on LANs using coaxial cables COE 341 (T061) – Dr. Marwan Abu-Amara

FSK Carrier 2 Data signal Carrier 1 vd(t) v1(t), f1 v2(t), f2 vFSK(t) Signal power Frequency frequency spectrum In FSK, two fixed amplitude carrier signals are used, one for a binary 0 and the other for a binary 1. The different between the two carriers is known as the frequency shift. The modulation operation is equivalent to summing together the outputs of two separate ASK modulators. FSK is the modulation method that was used in all early low bit rate modems. [Halsall p.62] Df f1 fc Df f2 COE 341 (T061) – Dr. Marwan Abu-Amara

Full-duplex transmission over voice grade line In one direction fc is 1170 Hz with f1 and f2 given by =1270 Hz and 1170–100=1070 Hz In other direction fc is 2125 Hz with f1 and f2 given by =2225 Hz and 2125–100=2025 Hz COE 341 (T061) – Dr. Marwan Abu-Amara

Phase Shift Keying (PSK)
Binary PSK: Phase of carrier signal is shifted to represent different values Phase shift of 180o Differential PSK: Two-phase system with differential PSK Phase shift relative to previous bit transmitted rather than some constant reference signal Binary 0 represented by sending a signal burst of same phase as previous signal burst Binary 1 represented by sending a signal burst of opposite phase as previous signal burst COE 341 (T061) – Dr. Marwan Abu-Amara

BPSK COE 341 (T061) – Dr. Marwan Abu-Amara

Differential PSK (DPSK)
Phase shifted relative to previous signal element rather than some reference signal: 1: Reverse phase 0: Do not reverse phase (A form of differential encoding) Advantage: - No need for a reference oscillator at RX to determine absolute phase COE 341 (T061) – Dr. Marwan Abu-Amara

Quadrature PSK (QPSK) More efficient Bandwidth use by each signal element representing more than one bit Shifts of /2 (90o) Resulting signal is Each signal element represents two bits COE 341 (T061) – Dr. Marwan Abu-Amara

Quadrature PSK (QPSK) COE 341 (T061) – Dr. Marwan Abu-Amara

Quadrature Amplitude Modulation (QAM)
An extension of the QPSK just described Combines both ASK and PSK For example, ASK with 2 levels and PSK with 4 levels give 4 x 2 i.e. 8-QAM Up to M=256 is possible Large bandwidth savings But some susceptibility to noise QAM used on asymmetric digital subscriber line (ADSL) and some wireless systems Constellation M=8, L = 3 COE 341 (T061) – Dr. Marwan Abu-Amara

Multilevel PSK (MPSK) Can use more phase angles and even have more than one amplitude! For example, 9600 bps modems use phase angles, four of which have amplitudes Gives 16 different signal elements  M = 16 and L = log2 (16) = 4 bits Every signal element carries 4 bits (Data sent 4 bits at a time) Baud rate is only 9600/4 = 2400 bauds (OK for a voice grade line) Complex signal encoding allows high data rates to be sent on voice grade lines having a limited bandwidth COE 341 (T061) – Dr. Marwan Abu-Amara

Data Rate & Modulation Rate
In general D: modulation rate (signals per second or bauds) R: data rate (bits per second) M: number of different signal elements L: number of bits per signal element With line signaling speed of 2400 baud For NRZ-L, data rate is 1/Tb For PSK, using L=16 different combinations of amplitude and phase, data rate is 9600 bps, R = 4/Tb For bi-phase, Data rate is 2/Tb COE 341 (T061) – Dr. Marwan Abu-Amara

Performance of D/A Modulation Schemes
Performance of digital-to-analog techniques depends on the definition of the bandwidth of the modulated signal Bandwidth of modulated signal depends on factors such as Filtering technique used to create the band-pass signal ASK and PSK bandwidth directly related to bit rate Transmission bandwidth BT for ASK and PSK is R is data rate r is related to filtering technique; 0< r <1 Transmission bandwidth BT for FSK is where the delta for offset from the carrier frequency: COE 341 (T061) – Dr. Marwan Abu-Amara

Performance of D/A Modulation Schemes
With multilevel signaling, bandwidth can improve significantly In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK (as shown in Figure 5.4) COE 341 (T061) – Dr. Marwan Abu-Amara

FSK (wideband F >> R) FSK (narrowband F  fc)
Bandwidth Efficiency Bandwidth efficiency is the ratio of data rate to transmission bandwidth, R/BT r = 0 r = 0.5 r = 1 ASK 1.0 0.67 0.5 FSK (wideband F >> R) FSK (narrowband F  fc) PSK MPSK: M=4, L=2 2.0 1.33 MPSK: M=8, L=3 3.0 2.00 1.5 MPSK: M=16, L=4 4.0 2.67 MPSK: M=32, L=5 5.0 3.33 2.5 COE 341 (T061) – Dr. Marwan Abu-Amara

Bandwidth Efficiency & Bit Error Rate
The bit error rate (BER) can be reduced by increasing Eb/N0 Bit error rate can be reduced by decreasing bandwidth efficiency Increasing bandwidth Decreasing data rate N0 is the noise power density in watts/hertz. Hence, the noise in a signal with bandwidth BT,, N=N0 BT COE 341 (T061) – Dr. Marwan Abu-Amara

Bandwidth Efficiency & Bit Error Rate
For multi-level signaling, replace R with D COE 341 (T061) – Dr. Marwan Abu-Amara

Example What is the bandwidth efficiency for FSK, ASK, PSK, and QPSK for a bit error rate of 10-7 on a channel with an SNR of 12dB ? Recall that Bandwidth efficiency is the ratio of R/BT COE 341 (T061) – Dr. Marwan Abu-Amara

Example … For FSK & ASK, Eb/N0 = 14.2dB  (R/BT)dB = – 2.2 dB, R/BT = 0.6 For PSK, Eb/N0 = 11.2dB  (R/BT)dB = 0.8 dB, R/BT = 1.2 For QPSK, D=R/2 (biphase)  R/BT = 2.4 COE 341 (T061) – Dr. Marwan Abu-Amara

Analog vs. Digital Signaling Bandwidth Req.
For digital signaling, bandwidth requirement is approximated to be For NRZ, D = R COE 341 (T061) – Dr. Marwan Abu-Amara

Analog Data, Digital Signal
Digitization Conversion of analog data into digital data Digital data can be transmitted using NRZ-L Digital data can be transmitted using code other than NRZ-L Digital data can then be converted to analog signal Analog to digital conversion done using a codec Pulse code modulation Delta modulation COE 341 (T061) – Dr. Marwan Abu-Amara

Pulse Code Modulation (PCM)
Sampling Theorem: If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal Signal maybe constructed from samples using a low- pass filter Voice data limited to below 4000Hz Require 8000 sample per second Analog samples (Pulse Amplitude Modulation, PAM) Each sample assigned digital value COE 341 (T061) – Dr. Marwan Abu-Amara

Quantization Levels are numbered 0 to 15
n = 4 bits  24 = 16 Quantization levels PAM Sample Analog signal is band-limited with bandwidth B Quantization Error Transmitted Serial Code representing the PAM Samples: Sampling rate: 2B Each PAM sample is assigned the number of the nearest quantization level and its digital code is transmitted COE 341 (T061) – Dr. Marwan Abu-Amara Must finish sending the n bits of the code before the next sample is due!

Pulse Code Modulation (PCM)
4 bit system gives 16 levels Quantized Quantizing error or noise Approximations mean it is impossible to recover original exactly SNR for quantizing error is For each additional bit used for quantizing, SNR increases by about 6 dB or a factor of 4 8 bit sample gives 256 levels Quality comparable with analog transmission 8000 samples per second of 8 bits each gives (80008) = 64 kbps COE 341 (T061) – Dr. Marwan Abu-Amara

PCM Example Suppose that we want to code an analog signal that has voltage levels 0-5v using 2-bit PCM Then, we divide the the voltage level in four intervals such that the size of each interval is 5/4=1.25 0-1.25, , , We choose the values to be in the middle of each interval Selected values are: 0.625, 1.875, 3.125, 4.375 This guarantees that the maximum quantization error is ½*5/4=0.625 and quantization SNR = 6 x = dB COE 341 (T061) – Dr. Marwan Abu-Amara

Nonlinear Encoding Absolute error for each sample is the same regardless of signal level Lower amplitude values are relatively more distorted Solution is to make quantization levels not evenly spaced Greater number of quantization steps for lower amplitudes and smaller number of steps for higher amplitudes Reduces overall signal distortion COE 341 (T061) – Dr. Marwan Abu-Amara

Effect of Nonlinear Coding

Companding Effect of nonlinear coding can also be reduced by companding Compressing-expanding More gain to weak signals than to strong signals on input Reverse operation at output COE 341 (T061) – Dr. Marwan Abu-Amara

Example (Problem 5-20) Consider an audio signal with spectral components in the range of 300 to 3000 Hz. Assuming a sampling rate of 7000 samples per second will be used to generate the PCM signal. For SNR = 30 dB, what is the number of uniform quantization levels needed? (SNR)dB = 6.02 n = 30 dB n = (30 – 1.76)/6.02 = 4.69 Rounded off, n = 5 bits  25 = 32 quantization levels What data rate is required? R = 7000 samples/sec  5 bits/sample = 35 Kbps COE 341 (T061) – Dr. Marwan Abu-Amara

Delta Modulation Analog input is approximated by a staircase function Move up or down one level () at each sample interval Binary behavior Function moves up or down at each sample interval A bit stream produced approximates derivative of analog signal rather than its amplitude Produce a 1 if stair function is to go up Produce a 0 if stair function is to go down COE 341 (T061) – Dr. Marwan Abu-Amara

Delta Modulation - example

Delta Modulation - Operation
Analog input compared to most recent value of approximating staircase function If value exceeds staircase function, generate a 1 Otherwise generate a 0 Output of DM process is a binary sequence to be used for reconstructing staircase function Reconstructed stair function is smoothed by a low pass filter to reconstruct approximated analog signal COE 341 (T061) – Dr. Marwan Abu-Amara

Delta Modulation - Operation

Delta Modulation Two important parameters in DM scheme Size of step assigned to each binary digit d Must be chosen to produce a balance between two types of errors or noise If waveform changes slowly, quantizing noise increases with increase in d If waveform changes rapidly, slope overload noise increases with decrease in d Increasing sampling rate improves the accuracy of the scheme Increases data rate Principal advantage of DM is implementation simplicity PCM has better SNR at same data rate COE 341 (T061) – Dr. Marwan Abu-Amara

CODEC - Performance Good voice reproduction PCM levels (7 bit) Voice bandwidth 4 KHZ Data rate should be 8000 x 7 = 56 kbps for PCM Bandwidth requirement Digital transmission requires 56 kbps for 4 KHz analog signal Using Nyquist theorem, this signal requires in the order of 28 KHz of Bandwidth, (C/2=56/2) COE 341 (T061) – Dr. Marwan Abu-Amara

CODEC - Performance A common PCM scheme for color TV uses 10-bit codes For bandwidth=4.6 MHz  92 Mbps (i.e. 2*4.6*10) Digital techniques continue to grow in popularity Repeaters used with no additive noise Time-division multiplexing (TDM) is used for digital signals with no intermodulation noise Use more efficient digital switching techniques More efficient codes are used to reduce required bit rate COE 341 (T061) – Dr. Marwan Abu-Amara

Analog Data, Analog Signals
Modulation Combining an input signal m(t) and a carrier at frequency fc to produce signal s(t) with bandwidth centered on fc Why modulate analog signals? Higher frequency may be needed for effective transmission For unguided transmission, impossible to send baseband signals as required antennas would be kilometers in diameter Permits frequency division multiplexing Types of modulation Amplitude Frequency Phase COE 341 (T061) – Dr. Marwan Abu-Amara

Analog Modulation Amplitude Modulation (AM) Angle Modulation: (Phase, PM) Angle Modulation: (Frequency, FM) COE 341 (T061) – Dr. Marwan Abu-Amara

Amplitude Modulation Simplest form of modulation
Accos 2pfct is the carrier, and x(t)= Amcos 2pfmt is the input modulating signal Modulated signal expressed as: na is the modulation index ( 1): Added ‘1’ is a DC component to prevent loss of information – there will always be a carrier Scheme is known as double sideband transmitted carrier (DSBTC) Amplitude of modulated wave Portion of the modulating signal COE 341 (T061) – Dr. Marwan Abu-Amara

Amplitude Modulation - Example
Given the amplitude-modulating signal x(t)=Amcos 2pfmt , find s(t): Resulting signal has three components: At the original carrier frequency fc A pair of additional components each spaced fm Hz from the carrier Envelope of resulting signal is [1+na x(t)] With na <1, envelope is exact reproduction of the modulating signal, So it can be recovered at receiver With na >1, envelope crosses the time axis and information is lost Ac Am/2 Am/2 fm fc fm COE 341 (T061) – Dr. Marwan Abu-Amara

Amplitude Modulation - Examples
MatLab Simulations Modulating Signal Carrier Envelope Modulated Signal na = 0.5/1 = 0.5 =(1+0.5cos2*pi*t) =(1+nacos2*pi*t) COE 341 (T061) – Dr. Marwan Abu-Amara

na = 1/1 = 1 COE 341 (T061) – Dr. Marwan Abu-Amara

na = 2/1 = 2 (>1) COE 341 (T061) – Dr. Marwan Abu-Amara

Spectrum of an AM signal:
Modulating Signal having a single Frequency, fm Am/2 Ac fc fm COE 341 (T061) – Dr. Marwan Abu-Amara

Spectrum of an AM signal
Modulating Signal having a finite Bandwidth, B Spectrum of AM signal is original carrier plus spectrum of original signal translated on both sides of fc Portion of spectrum f > fc is upper sideband Portion of spectrum f < fc is lower sideband Example: voice signal Hz With fc 60 KHz Upper sideband is KHz Lower sideband is KHz Bandwidth Requirement: 2B COE 341 (T061) – Dr. Marwan Abu-Amara

Amplitude Modulation Total transmitted power Pt in modulated s(t) is given by Pc is transmitted power in carrier na should be maximized (but <1) to allow most of signal power to carry information Modulated signal contains redundant information Only one of the sidebands is enough to restore modulating signal Possible ways to economize on transmitted power: SSB: single sideband, eliminates one sideband and carrier, saves on BW (= B) DSBSC: double sideband suppressed carrier, carrier is not transmitted, no saving on BW (= 2B) Suppressing the carrier may not be OK in some applications, e.g. ASK, where the carrier can provide TX-RX synchronization. Am/2 Ac fc fm COE 341 (T061) – Dr. Marwan Abu-Amara

DSBSC: Double Sideband Suppressed Carrier - Example
Signal is expressed as Suppressed Carrier COE 341 (T061) – Dr. Marwan Abu-Amara

Angle Modulation Includes: Frequency modulation (FM) and Phase modulation (PM) as special cases Modulated signal is given by Phase modulation (PM) Instantaneous Phase is proportional to modulating signal: np is phase modulation index Frequency modulation (FM) Instantaneous frequency deviation is proportional to modulating signal: i.e. Derivative of f is proportional to modulating signal nf is frequency modulation index Total Angle COE 341 (T061) – Dr. Marwan Abu-Amara

Angle Modulation The total phase angle of s(t) at any instant is [2pfct+f(t)] Instantaneous phase deviation from carrier is f(t) Phase Modulation (PM): f(t) = npx(t), instantaneous phase deviation from carrier is directly proportional to x(t) Frequency Modulation (FM): Instantaneous angular frequency, , can be defined as the rate of change of total phase So, for the modulated signal, s(t) In FM, f’(t) is proportional to x(t). So, instantaneous frequency deviations from the carrier frequency is proportional to x(t). COE 341 (T061) – Dr. Marwan Abu-Amara

Phase Modulation (PM)- Example
Derive an expression for a phase-modulated signal s(t) with Ac= 5V, given the modulating signal x(t) = 3 sin 2pfmt We know that s(t): For PM, f (t) is given by: Then s(t) is: Instantaneous frequency of s(t) is: COE 341 (T061) – Dr. Marwan Abu-Amara Note: Frequency variations in s(t) lead x(t) amplitude variations by 90

Frequency Modulation: FM
Peak frequency deviation DF is given by: Where Am is the peak value of the modulating signal x(t) An increase in the amplitude Am of x(t) increases DF, which increases the bandwidth requirement BT But average power level of the FM modulated signal is fixed at AC2/2, (does not increase with Am) In Amplitude Modulation, Am affects the power in the AM signal, but does not affect the bandwidth COE 341 (T061) – Dr. Marwan Abu-Amara

Frequency Modulation - Example
Derive an expression for a frequency-modulated signal s(t) with Ac= 5V, given the modulating signal x(t) = 3 sin 2pfmt We know that s(t): For FM, f’(t) is given by: Then s(t) is: We have: Substituting for DF we get: But frequency varies as f’, i.e. as sin not as – cos !! COE 341 (T061) – Dr. Marwan Abu-Amara

Bandwidth Requirement
All AM, FM, and PM result in a modulated signal whose bandwidth is centered at fc Let B be the bandwidth of the modulating signal For AM, BT = 2B Angle modulation includes a term of the form cos(…+cos()) which is a nonlinear term producing a wide range of frequencies fc+fm, fc+2fm, … (the Bessel function) i.e. Theoretically, an infinite bandwidth is required to transmit an FM or PM signal COE 341 (T061) – Dr. Marwan Abu-Amara

Practical Bandwidth Requirement for Angle Modulation
Carson’s Rule of thumb For FM, BT= 2DF + 2B Both FM and PM require greater bandwidth than AM COE 341 (T061) – Dr. Marwan Abu-Amara

Popular analog signaling technique used in asymmetric digital subscriber line (ADSL) Combination of amplitude and phase modulation Two signals transmitted simultaneously on same carrier frequency using two copies of carrier one shifted by 90o Each carrier is ASK modulated Input is a stream of binary digits arriving at a rate of R bps Converted into two separate bits streams of R/2 bps COE 341 (T061) – Dr. Marwan Abu-Amara

One stream is ASK modulated on a carrier of frequency fc Other stream is ASK modulated on a carrier of frequency fc shifted by 90o The two modulated signals are combined together and transmitted Transmitted signal can be expressed as COE 341 (T061) – Dr. Marwan Abu-Amara

QAM Modulator COE 341 (T061) – Dr. Marwan Abu-Amara

Spread Spectrum Can be used to transmit analog or digital data using analog signal Spread data over wide bandwidth Makes jamming and interception harder COE 341 (T061) – Dr. Marwan Abu-Amara

Spread Spectrum Channel encoder receives input and converts it into analog signal with narrow bandwidth around center frequency Signal is further modulated using a pseudorandom sequence Modulation spreads the spectrum (increases bandwidth) of signal to be transmitted Same pseudorandom sequence used to demodulate the spread spectrum signal COE 341 (T061) – Dr. Marwan Abu-Amara

Frequency hoping Spread Spectrum
Signal broadcast over seemingly random series of frequencies Hopping from one to another frequency in split-second intervals Receiver also hops on the same frequencies in synchronization with sender Difficult to catch and jam the signal without knowing the frequencies COE 341 (T061) – Dr. Marwan Abu-Amara

Direct Sequence Spread Spectrum
Each bit is represented by multiple bits in transmitted signal Multiple bits known as Chipping code Chipping code spreads signal across a wider frequency band in direct proportion to number of bits used A 10-bit chipping code spreads signal across a frequency band 10 times larger than 1-bit code Combine digital information stream with pseudorandom bit stream using exclusive-OR COE 341 (T061) – Dr. Marwan Abu-Amara

Direct Sequence Spread Spectrum

COE 341: Data & Computer Communications (T061) Dr. Marwan Abu-Amara

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COE 341: Data & Computer Communications (T061) Dr. Marwan Abu-Amara

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Presentation on theme: "COE 341: Data & Computer Communications (T061) Dr. Marwan Abu-Amara"— Presentation transcript:

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About project

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