EE734

1. EE734/834Network Data Communications Data Encoding

2. Encoding and Modulation Encoding Collection of data bits, or data bit stream, represented as logical symbol stream st [n] Modulation Logical symbol stream st [n] used to create continuous time physical signal st(t) which can be transmitted Demodulation Logical symbol stream sr [n] extracted from measurements of received physical signal sr(t) = Function{st(t)} + noise Decoding Collection of data bits, or data bit stream, extracted from logical symbol stream sr [n] This breakdown of encoding versus modulation is logical, but not followed consistently

3. Encoding and Modulation In general, the modulator/demodulator in “b” would exist between the encoder and decoder in “a”

4. “Classic” Encoding Schemes Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Bipolar -AMI Pseudoternary Manchester Differential Manchester B8ZS HDB3 … conclusion: very many different schemes reflecting variations in transmission media and technology available for encoding and modulation

5. Comparison of Encoding Schemes (1) Signal Spectrum Lack of high frequencies reduces required bandwidth for a specific symbol rate Lack of dc component allows ac coupling via transformer, providing isolation Concentrate power in the middle of the channel bandwidth Clocking Synchronizing transmitter and receiver External clock Sync mechanism based on signal

6. Comparison of Encoding Schemes (2) Error detection and/or error correction Can be built in to signal encoding Signal interference and noise immunity Some codes are better than others Cost and complexity Higher symbol rates (& thus data rates) lead to higher costs Complex encoding/decoding leads to higher costs

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

8. 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

9. NRZ

10. Differential Encoding NRZI is one example of differential encoding based on changes in signal level In general, differential encoding involves data represented by changes in transmitted values rather than specific values. Changes in magnitude, changes in phase, changes in frequency, etc. More reliable detection of transition rather than level In complex transmission layouts it is easy to lose sense of polarity

11. NRZ pros and cons Pros Easy to engineer – Advantage for low cost signaling Advantage for very high speed signaling Cons DC component Lack of synchronization capability - long string of 0’s or 1’s can produce segment with no transitions These can be overcome by additional encoding steps Used for some magnetic recording Used for some optical signaling

12. Biphase Phase encoding – Data represented by clock phase rather than voltage level Manchester Phase Encoding Transition in middle of each bit period Transition serves as clock and data Low to high (one cycle of in-phase clock) represents one High to low (one cycle of 180o out-of-phase clock) represents zero Used by IEEE 802.3 – 10Mbs Ethernet

13. Level Vs. Phase Encoding

14. Biphase Pros and Cons Con 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 Signal detection Absence of expected transitions means no signal

15. Encoding and Modulation Data in all cases: d[n] = 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 1 NRZ-L: s[n] = +V, -V, +V, +V, -V, -V, +V, +V, +V, -V, -V NRZI: s[n] = -V, +V, +V, +V, -V, +V, +V, +V, +V, -V, +V Manchester: s[n] = 0, 180, 0, 0, 180, 180, 0, 0, 0, 180, 180 deg Diff. Manch. s[n] = 180, 0, 0, 0, 180, 0, 0, 0, 0, 180, 0 deg NRZ-L and Manchester s[n] = A, B, A, A, B, B, A, A, A, B, B logical symbols NRZI and Differential Manchester s[n] = B, A, A, A, B, A, A, A, A, B, A logical symbols Where logical symbols are defined as: NRZ: A = +V B = -V Manchester: A = 0 deg B = -180 deg THUS: NRZ-L & Manchester (and NRZI & Diff. Manch.) can be interpreted as the SAME encoding with different modulation (base band level modulation versus carrier phase modulation).

16. Sinusoidal Modulation Collection of data bits, or data bit stream, represented as logical symbol stream s[n] Symbol stream s[n] used to define continuous time signal s(t) Transmitting s(t) directly is called “base-band” signaling s(t) can also be used to modulate a sinusoidal carrier Translates signal spectrum from base-band (DC to SBW) to an upper band (fc-SBW to fc+SBW for AM) Amplitude shift keying (ASK) (also AM) Frequency shift keying (FSK) (also FM) Phase shift keying (PSK) (also PM)

17. Modulation Techniques

18. Modulation Techniques

19. Amplitude Shift Keying Symbol values represented by different amplitudes of carrier Sometimes (e.g. optical fiber), one amplitude is zero i.e. presence and absence of carrier is used Susceptible to line attenuation, sudden signal strength changes, or additive noise, particularly for multi-bit symbols (multiple amplitudes) Widely used over optical fiber

20. Frequency Shift Keying Symbol values represented by different frequencies (near carrier) Less susceptible to noise than ASK Relatively insensitive to signal attenuation, reception strength

21. Phase Shift Keying Phase of carrier signal is shifted to represent symbol Differential PSK More often used than straight PSK Phase shifted relative to previous symbol transmitted, rather than relative to some absolute reference signal Phase is advanced smoothly by temporarily increasing frequency Phase is retarded smoothly by temporarily decreasing frequency PSK has similar properties to FSK In FSK frequency deviations occur during symbol periods while in PSK frequency deviations occur during transitions between symbols

22. Bandwidth of ASK, FSK, & PSK Collection of data bits, or data bit stream, represented as symbol stream s[n] Symbol stream s[n] used to define continuous time signal s(t) with bandwith SBW s(t) is source of modulation (ASK, FSK, or PSK) of sinusoidal carrier of frequency fc. Bandwidth of ASK signal is well known to be ASKBW=2*SBW Bandwidth of FSK and PSK signals can be approximated as FSKBW=PSKBW=2*(SBW+fdmax) where fdmax is the maximum deviation of the frequency from the nominal carrier frequency fc

23. Efficiency of ASK, FSK, & PSK Bandwidth of ASK signal is well known to be ASKBW=2*SBW Bandwidth of FSK and PSK signals can be approximated as FSKBW=PSKBW=2*(SBW+fdmax) From Nyquist, the maximum symbol rate is SMAX=2*SBW Thus, for ASK modulation SMAX= ASKBW For FSK and PSK modulation SMAX= FSKBW- 2*fdmax For 1 bit symbols, the maximum encoding efficiency for ASK is 1.0, and for FSK and PSK is significantly less than 1.0 Remember: required bandwidth is related to symbol rate, which is independent of the number of bits per symbol. Thus, multi-bit symbol encoding are more efficient than single (or partial) bit symbol encodings.

24. 2-D Phase & Amplitude Modulation (PAM) Both phase and amplitude modulated to represent multi-bit symbols Ex. - 4 phases and 2 amplitudes could provide 8 unique symbols, supporting 3 bits per symbol Ex. – standard 56K telephone modem uses 128 discrete 2-D phase/amplitude pairs to represent 7 bit symbols Constellation pattern – plot of discrete symbol locations on polar graph of phase and amplitude

25. 2-D Constellation Pattern Constellation pattern – plot of discrete symbol locations on polar graph of phase and amplitude Distance of point from center indicates amplitude Angle of point from vertical axis indicates phase Each of the 8 symbols is labeled with the 3 data bits that symbol represents Goal is to separate the points as much as possible in the 2-D space for a given maximum amplitude (transmit power)

26. Quadrature Amplitude Modulation PAM often called QAM (Quadrature Amplitude Modulation) as the result of how PAM is implemented Trig. Identity: sin(A+B) = sin(A)*cos(B) + cos(A)*sin(B) Thus: m*sin(2*pi*f*t + p) = m*sin(p)*cos(2*pi*f*t) + m*cos(p)*sin(2*pi*f*t) Or: m*sin(2*pi*f*t + p) = m1*cos(2*pi*f*t) + m2*sin(2*pi*f*t) m = amplitude modulation p = phase modulation m1 = m*sin(p) = amplitude modulation for cos() m2 = m*cos(p) = amplitude modulation for sin() Can implement 2-D phase and amplitude modulation (PAM) using sum of two amplitude modulators which are one-quarter cycle out of phase (QAM)

27. QAM Bandwidth Properties QAM implements 2-D phase and amplitude modulation (PAM) using the linear sum of two amplitude modulators which are one-quarter cycle out of phase (QAM) Bandwidth characteristics for QAM are the same as for ASK (AM) rather than PSK (PM), since the transmitted signal is the linear combination of two amplitude modulated sinusoidal signals For QAM modulation the maximum symbol rate is SMAX= QAMBW For 1 bit symbols, the maximum encoding efficiency for QAM is 1.0