Signal Encoding Techniques Chapter 5 Signal Encoding Techniques TUNALI Data Communication

Encoding and Modulation Two techniques for data transmission Encoding A data source is encoded into a digital signal Encoding technique is chosen to optimize transmission medium Modulation The process of encoding source data onto a carrier signal with frequency fc All modulation techniques involve operation on three signal parameters Amplitude Frequency Phase TUNALI Data Communication

Encoding Techniques Digital data, digital signal Less complex Less expensive than digital to analog modulation Analog data, digital signal Sometimes, it is more advantageous to shift to digital domain Digital data, analog signal Some transmission media (unguided media, optical fiber) only propagate analog signals Analog data, analog signal Analog data transmitted as analog signal easily and cheaply: voice transmission over voice grade lines Shift the bw of baseband signal to another portion of the spectrum Multiple signals can share the transmission medium Frequency Division Multiplexing TUNALI Data Communication

Digital Data, Digital Signal Discrete, discontinuous voltage pulses Each pulse is a signal element Binary data encoded into signal elements In the simplest case, one-to-one correspondence between data and signal elements TUNALI Data Communication

Terms (1) Unipolar Polar Data rate Duration or length of a bit All signal elements have same sign Polar One logic state represented by positive voltage the other by negative voltage Data rate Rate of data transmission in bits per second Duration or length of a bit Time taken for transmitter to emit the bit TUNALI Data Communication

Terms (2) Modulation rate Mark and Space Rate at which the signal level changes Measured in baud = signal elements per second Mark and Space Binary 1 and Binary 0 respectively TUNALI Data Communication

Interpreting Signals Need to know Timing of bits - when they start and end Signal levels Factors affecting successful interpreting of signals Signal to noise ratio Data rate Bandwidth TUNALI Data Communication

Facts An increase in data rate increases bit error rate An increase in SNR decreases bit error rate An increase in bandwidth allows an increase in data rate An improvement in perfomance can also be obtained by encoding scheme Encoding scheme is simply mapping from data bits to signal elements TUNALI Data Communication

Comparison of Encoding Schemes (1) Signal Spectrum Lack of high frequencies reduces required bandwidth Lack of dc component allows ac coupling via transformer, providing isolation, reducing interference Concentrate power in the middle of the bandwidth Clocking Synchronizing transmitter and receiver External clock: an expensive solution Sync mechanism based on signal : can be achieved with suitable encoding TUNALI Data Communication

Comparison of Encoding Schemes (2) Error detection Can be built into signal encoding This permits errors to be detected more quickly Signal interference and noise immunity Some codes are better than others Performance is expressed in BER Cost and complexity Higher signal rate (& thus data rate) lead to higher costs Some codes require signal rate greater than data rate TUNALI Data Communication

Encoding Schemes Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Bipolar -AMI Pseudoternary Manchester Differential Manchester B8ZS HDB3 TUNALI Data Communication

Nonreturn to Zero-Level (NRZ-L) Two different voltages for 0 and 1 bits Voltage constant during bit interval no transition I.e. no return to zero voltage e.g. Absence of voltage for zero, constant positive voltage for one More often, negative voltage for one value and positive for the other This is NRZ-L TUNALI Data Communication

Nonreturn to Zero Inverted NRZI: Nonreturn to zero invert 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 TUNALI Data Communication

NRZ TUNALI Data Communication

Differential Encoding Data represented by changes rather than levels More reliable detection of transition rather than level In complex transmission layouts it is easy to lose sense of polarity E.g. if the leads from an attached device to the twisted pair accidentally inverted, all NRZ-L bits will be inverted TUNALI Data Communication

NRZ pros and cons Pros Cons Used for magnetic recording Easy to engineer Make good use of bandwidth Cons dc component Lack of synchronization capability Used for magnetic recording Not often used for signal transmission TUNALI Data Communication

Multilevel Binary Use more than two levels Bipolar-AMI 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 1s (zeros still a problem) No net dc component Lower bandwidth Easy error detection TUNALI Data Communication

Pseudoternary One represented by absence of line signal Zero represented by alternating positive and negative No advantage or disadvantage over bipolar-AMI AMI: string of 0s, pseudoternary: string of 1s insert additional bits to force transitions Expensive at high data rates TUNALI Data Communication

Bipolar-AMI and Pseudoternary TUNALI Data Communication

Trade Off for Multilevel Binary With suitable modification, multilevel binary schemes overcome the problems of NRZ Not as efficient as NRZ Each signal element only represents one bit In a 3 level system could represent log23 = 1.58 bits Receiver must distinguish between three levels (+A, -A, 0) Requires approx. 3dB more signal power than a two-valued signal for same probability of bit error TUNALI Data Communication

Bit Error Rate for Various Encoding Schemes Fig 5.4 TUNALI Data Communication

Biphase Manchester Differential Manchester Transition in middle of each bit period Transition serves as clock and data Low to high represents one High to low represents zero Used by IEEE 802.3 Differential Manchester Midbit 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 802.5 TUNALI Data Communication

Manchester Encoding TUNALI Data Communication

Differential Manchester Encoding TUNALI Data Communication

Biphase Pros and Cons Con Pros At least one transition per bit time and possibly two Maximum modulation rate is twice NRZ Requires more bandwidth Pros Synchronization on mid bit transition (self clocking) No dc component Error detection Absence of expected transition TUNALI Data Communication

Modulation Rate TUNALI Data Communication

Remark Data rate and modulation rate can differ. Recall that D=R/b=where D = modulation rate (baud) R = data rate (bps) b = #bits per signal element TUNALI Data Communication

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 No dc component No long sequences of zero level line signal No reduction in data rate Error detection capability TUNALI Data Communication

Remark Note that biphase techniques are mostly used in LANs. Since high signaling rate is required relative to data rate, these techniques are not suitable for long distance transmission. Scrambling techniques improve this rate by replacing long sequences of zeros by a fixed pattern. TUNALI Data Communication

B8ZS Bipolar With 8 Zeros Substitution Based on bipolar-AMI If octet of all zeros and last voltage pulse preceding was positive encode as 000+-0-+ If octet of all zeros and last voltage pulse preceding was negative encode as 000-+0+- Unlikely to occur as a result of noise Receiver detects and interprets as octet of all zeros Commonly used in North America TUNALI Data Communication

Number of Bipolar Pulses (ones) since Last Substitution HDB3 High Density Bipolar 3 Zeros Based on bipolar-AMI String of four zeros replaced with sequences containing one or two pulses Number of Bipolar Pulses (ones) since Last Substitution Polarity of Preceeding Pulse Odd Even - 000- +00+ + 000+ -00- TUNALI Data Communication

B8ZS and HDB3 TUNALI Data Communication

Digital Data, Analog Signal Public telephone system 300Hz to 3400Hz – voice frequency range Use modem (modulator-demodulator) Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PK) TUNALI Data Communication

Modulation Techniques TUNALI Data Communication

Amplitude Shift Keying (1) Two binary values represented by different amplitudes of carrier frequency Usually, one amplitude is zero i.e. presence and absence of carrier is used Susceptible to sudden gain changes Inefficient Up to 1200bps on voice grade lines Used over optical fiber TUNALI Data Communication

Amplitude Shift Keying (2)  Acos (2fct) if 1 s(t) =  0 if 0   For LED this operation is valid. For laser transmitters normally have a fixed current that causes the device emit low level light. TUNALI Data Communication

Binary Frequency Shift Keying (1) Most common form is binary FSK (BFSK) Two binary values represented by two different frequencies (near carrier) Less susceptible to error than ASK Up to 1200bps on voice grade lines Commonly used for high frequency radio (3 to 30 MHz) Used at even higher frequency on LANs using co-ax TUNALI Data Communication

Binary Frequency Shift Keying (2)  Acos (2f1t) if 1 s(t) =  Acos (2f2t) if 0 TUNALI Data Communication

Multiple FSK More than two frequencies used More bandwidth efficient More prone to error Each signalling element represents more than one bit Each signal element is held for a period of LT seconds, L: number of bits per signal element, T: bit period TUNALI Data Communication

FSK on Voice Grade Line TUNALI Data Communication

Phase Shift Keying Phase of carrier signal is shifted to represent data Differential PSK Phase shifted relative to previous transmission rather than some reference signal  Acos (2fct + ) if 1 s(t) =  Acos (2fct) if 0   TUNALI Data Communication

Differential PSK TUNALI Data Communication

Quadrature PSK (1) More efficient use of bw by each signal element representing more than one bit e.g. shifts of /2 (90o) Each element represents two bits Can use 8 phase angles and have more than one amplitude 9600bps modem use 12 angles , four of which have two amplitudes Offset QPSK (orthogonal QPSK) Delay in Q stream TUNALI Data Communication

Quadrature PSK (2) Example:shifts of /2 (90o)  Acos (2fct + /4) if 11  Acos (2fct)+ 3/4) if 10 s(t) =  Acos (2fct + 5/4) if 00  Acos (2fct + 7/4) if 01 In general, D=R/b = R/log2L where D is the modulation rate, R is the data rate, L is the number of different signal elements and b is the number of bits per signal element   TUNALI Data Communication

QPSK and OQPSK Modulators TUNALI Data Communication

Examples of QPSF and OQPSK Waveforms TUNALI Data Communication

Bandwidth For ASK and PSK the bandwidth is given as BT = (1 + r) R, where R is the bit rate and r is a constant between 0 and 1.   For multilevel FSK, the bandwidth is given as BT = ((1 + r)M/ log2 M)R., where For multilevel PSK, bandwidth can be given as BT = ((1 + r)/ log2 M)R. L: number of bits encoded per signal element M: number of different signal elements TUNALI Data Communication

Performance of Digital to Analog Modulation Schemes Bandwidth ASK and PSK bandwidth directly related to bit rate In the presence of noise, bit error rate of DPSK and BPSK are about 3dB superior to ASK and BFSK R/BT: bandwidth efficiency For MFSK, error probability decreases as M increases, while opposite is true for MPSK Bandwidth efficiency of MFSK decreases as M increases, while opposite is true of MPSK TUNALI Data Communication

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 12 dB? Solution: Recall that Eb/N0 = S/(N0R). Since the noise in a signal with bandwidth BT is N = N0 BT , we have Eb/N0 = (S/N)(BT/R). Or in dB, (Eb/N0)dB = (S/N)dB + (BT/R)dB or (Eb/N0)dB = (S/N)dB - (R/BT)dB  From Figure 5.4, for FSK and ASK, Eb/N0 = 14.2 dB  And substitution to above formula yields  14.2 dB = 12 dB – (R/BT)dB yielding  R/BT = 0.6.  For PSK, from Fig. 5.4, Eb/N0 = 11.2 dB. Repeating the above computation yields R/BT = 1.2.  In QPSK, since baud rate is D = R/2, we have R/BT = 2.4.   TUNALI Data Communication

Quadrature Amplitude Modulation QAM used on asymmetric digital subscriber line (ADSL) and some wireless Combination of ASK and PSK Logical extension of QPSK Send two different signals simultaneously on same carrier frequency Use two copies of carrier, one shifted 90° Each carrier is ASK modulated Two independent signals over same medium Demodulate and combine for original binary output TUNALI Data Communication

QAM Modulator TUNALI Data Communication

QAM Levels Two level ASK Each of two streams in one of two states Four state system Essentially QPSK Four level ASK (four different amplitude levels) Combined stream in one of 16 states 64 and 256 state systems have been implemented The greater the number of states, the higher the data rate Increased potential error rate TUNALI Data Communication

Analog Data, Digital Signal Digitization Conversion of analog data into digital signals After analog data is converted into digital data: Digital data can then be transmitted using NRZ-L Digital data can then be transmitted using code other than NRZ-L (thus an extra step is required) Digital data can then be converted to analog signal Analog to digital conversion done using a codec Pulse code modulation Delta modulation TUNALI Data Communication

Digitizing Analog Data TUNALI Data Communication

Pulse Code Modulation(PCM) (1) Sample 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 Voice data limited to below 4000Hz Require 8000 sample per second Analog samples (Pulse Amplitude Modulation, PAM) Each sample assigned digital value TUNALI Data Communication

Pulse Code Modulation(PCM) (2) 4 bit system gives 16 levels Quantized Quantizing error or noise Approximations mean it is impossible to recover original exactly 8 bit sample gives 256 levels Quality comparable with analog transmission 8000 samples per second of 8 bits each gives 64kbps TUNALI Data Communication

PCM Example TUNALI Data Communication

PCM Block Diagram TUNALI Data Communication

Nonlinear Encoding Quantization levels not evenly spaced Reduces overall signal distortion Can also be done by companding SNR for quantizing noise SNR = 6.02 n + 1.76 dB, where n is the number of bits representing each voltage level For voice signals, with nonlinear encoding, an improvement of 24 dB to 30 dB have been achieved. TUNALI Data Communication

Effect of Non-Linear Coding TUNALI Data Communication

Typical Companding Functions TUNALI Data Communication

Delta Modulation One of the most popular alternatives to PCM 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 TUNALI Data Communication

Delta Modulation - example TUNALI Data Communication

Delta Modulation - Operation TUNALI Data Communication

Performance Good voice reproduction PCM - 128 levels (7 bit) Voice bandwidth 4khz Should be 8000 x 7 = 56kbps for PCM Digital techniques continue to grow in popularity Repeaters instead of amplifiers TDM instead of FDM (intermodulation noise) Efficient digital switching techniques Data compression can improve codecs e.g. Interframe coding techniques for video TUNALI Data Communication

Performance Use of a telecommunication system will result in both digital-to analog and analog-to-digital processing Local terminals into comm. network is analog and network uses a mixture of analog and digital techniques Analog signals represents voice and digital data Voice signals tend to be skewed lower portion of the bandwidth Because of the presence of higher frequencies, PCM-related techniques is preferable to DM TUNALI Data Communication

Analog Data, Analog Signals Why modulate analog signals? Higher frequency may be needed for efficient transmission Permits frequency division multiplexing (chapter 8) Types of modulation Amplitude Frequency Phase TUNALI Data Communication

Amplitude Modulation s(t) = [1 + na x(t)] cos 2fct where,   2fct is the carrier, x(t) is the input signal, na is the modulating index This is Double Side Band Transmitted Carrier , DSBTC TUNALI Data Communication

AM Fig 5.15 TUNALI Data Communication

Amplitude Modulation, Example Derive an expression for s(t) if x(t) = cos(2fmt)   Solution: We have s(t) = [1 + na cos(2fmt)] cos 2fct. Since cos(a)cos(b) = (1/2)cos(a+b) + (1/2)cos(a-b) s(t) = cos 2fct + (na/2) cos 2(fc – fm) t + (na/2) cos 2(fc + fm) t. If Pt is the total power transmitted in s(t) and Pc is the power transmitted in the carrier, we have Pt = Pc (1 + na2 / 2) TUNALI Data Communication

Spectrum of AM Fig 5-16 TUNALI Data Communication

Single Side Band (SSB) Each sideband of AM contains complete spectrum of the modulating signal In fact, only one sideband is enough to retrieve the complete signal SSB contains only one sideband Bandwidth is halved Less power is required Since carrier is lost, synchronization at receiver might be difficult Vestigial sideband (VSB): uses one sideband and reduced power carrier TUNALI Data Communication

Angle Modulation s(t) = Ac cos [2fct + (t)]   PM and FM are special cases of angle modulation. For PM (t) = np m(t) and for FM (t) = nf m(t) (frequency is the rate of change of phase). TUNALI Data Communication

Analog Modulation TUNALI Data Communication

Bandwidth For AM the bandwidth is given as BT = 2B (where B is the bandwidth of the modulating signal),   For FM, the bandwidth is given as BT = 2B + 2F, where F is the peak frequency deviation. Both FM and PM require higher bandwidth than AM. TUNALI Data Communication

Power and Bandwidth In Amplitude Modulation In Frequency Modulation Increase in m(t) amplitude increases power but does not change bandwidth In Frequency Modulation Increase in m(t) amplitude increases F and hence the bandwidth but does not change the power TUNALI Data Communication

Required Reading Stallings chapter 5 TUNALI Data Communication