1. Baseband Data Transmission & Digital Modulation Techniques
Chapter 4
Baseband Data Transmission
&
Digital Modulation Techniques

2. Chapter Overview Baseband transmission Line coding
Intersymbol Interference (ISI)
Nyquist Wave
Digital Modulation techniques

3. Baseband Transmission
The original band of frequencies of a signal before it is modulated for transmission at a higher frequency.
A type of data transmission in which digital or analog data is sent over a single unmultiplexed channel, such as an Ethernet LAN.
Baseband transmission use TDM to send simultaneous bits of data along the full bandwidth of the transmission channel.

4. Cont’d... Baseband Center Point Detection
The detection of digital signals involve two processes:
Reduction of each received voltage pulse
(i.e. symbol) to a single numerical value.
Comparison of this value with a reference voltage (or for multisymbol signaling, a set of reference voltages) to determine which symbol was transmitted.

6. Line Coding
Pulse modulation applied to binary symbol, the resulting binary waveform is called PCM waveform.
In telephony applications : line codes
Pulse modulation is applied to nonbinary symbol, the resulting waveform called M-ary pulse modulation waveform.

7. Some basic nomenclature
Information source: can be either analog or digital
binary sequence: sequence of {1, 0} to describe information source
Bit symbol: a symbol represents k bits (M=2k)
Symbol alphabet: s0,s1,…,sM-1 (alphabet size M)
symbol stream: sequence of symbols selected from the alphabet
Data rate
Bit rate Rb in bits/sec (bps).
Symbol rate Rs in symbols/sec (sps)
Bit interval Tb: duration of a bit (sec/bit)
Symbol interval Ts: duration of a symbol (sec/symbol)

8. Important relation

9. Illustration and example

10. Cont’d...
Line codes
Converting standard logic level to a form more suitable to telephone line transmission.
Four main groups
Non return to zero (NRZ)
Return to Zero (RZ)
Phase encoded
Multilevel binary

11. Cont’d... Non Return To Zero (NRZ) Subgroups : NRZ-L NRZ-M NRZ-S
Used extensively in digital logic circuit
Binary 1 one represented by one voltage level
Binary 0 is represented by another voltage level.
NRZ-M
Used in magnetic tape recording.
The 1 (mark) is represented by change in level
The 0 (space) is represented by no change in level
Differential encoding
NRZ-S
Complement of NRZ-M
1 is represented by no change in level
0 is represented by a change in level

12. Cont’d... Return To Zero (RZ) Subgroups: Unipolar RZ Bipolar RZ RZ-AMI
1 is represented by a half bit wide pulse.
0 is represented by the absence of pulse.
Bipolar RZ
1 & 0 are represented by opposite level pulses that are one half bit wide.
Pulse present in each bit interval.
RZ-AMI
1 is represented by equal amplitude alternating pulses.
0 is represented by the absences of pulse.

13. Cont’d... Phase Encoded Multilevel Binary Subgroups:
Manchester coding
Bi-phase-mark
Bi-phase-space
Delay Miller coding
Multilevel Binary
Used three levels to encode the binary data.
Dicode and duo binary

15. Cont’d... Parameters : DC components Self-clocking Error detection
Bandwidth Compression
Differential encoding
Noise immunity

16. Cont’d... ι ≥ log 2 (1/2p) bits PCM Word size
Magnitude of the quantization distortion error
|e| < pVpp
|emax| = q/2
= Vpp
2(L-1) 2L
L = number of quantization levels, large enough
Number of bits/sample, ι
2ι = L ≥ (1/2p) levels
ι ≥ log 2 (1/2p) bits

17. Cont’d... M-ary Pulse Modulation waveforms
Three basic ways to modulate information on to a sequence of pulse
PAM
PPM
pulse duration modulation (PDM)
M-ary pulse modulation
When the information samples are first quantized, yielding symbols from an M-ary alphabet set, then modulated on to pulses, the resulting pulse modulation is digital.

18. Cont’d... M-ary PAM M-ary PPM M-ary PWM
one of M allowable amplitude levels are assigned to each of the M possible symbol values.
M-ary PPM
modulation is effected by delaying or advancing a pulse occurrence.
M-ary PWM
modulation is effected by varying the pulse width by an amount that corresponds to the value of the symbols.

19. M-ary pulse modulation block-diagram

20. Illustration of PCM signals

21. Bandwidth efficiency
How much date rate can be supported by the system with each unit frequency band
The higher bandwidth efficiency, the better.

22. Intersymbol Interference (ISI)
Tx – the information symbols characterized as impulse or voltage levels, modulate pulses that are then filtered to comply with some bandwidth constraint.
Baseband system – the channel has distributed reactance that distorts the pulses.
Bandpass system – characterized by fading channels that behave like undesirable filters manifesting signal distortion.
Rx – equalizing filter is configured to compensate for the distortion caused by Tx & channel.

23. Cont’d... Equivalent system transfer function H(f) = Ht(f) Hc(f) Hr(f)
Where
Ht(f) – transmitting filter
Hc(f) – filtering within the channel
Hr(f) – equalizing filter

24. Cont’d...
Due to effect of system filtering, the received pulses can overlap one another.
Tail of pulse can smear into adjacent symbol intervals , thereby interfering with the detection process and degrading the error performance - Intersymbol interference (ISI)
effects of filtering
Channel-induced distortion

25. ISI measured by eye pattern

26. Cont’d...
Nyquist filter is one whose frequency transfer function can be represented by a rectangular function convolved with any real even-symmetric frequency function
Nyquist pulse is one whose shape can be represented by a sinc (t/T) function multiplied by another time function
Most popular of Nyquist filter
Raised-cosine filter
Root-raised cosine filter

27. Cont’d... Pulse Shaping to reduce ISI
Pulse that spread in time will degrade the system’s error performance due to increase ISI.
Reduce the required system bandwidth.
Compress the bandwidth of the data impulse to some reasonably small bandwidth greater than the Nyquist minimum – pulse shaping with Nyquist filter.
Zero ISI is only when the sampling is performed at exactly the correct sampling time when the tails of pulses are large.

28. Cont’d... Raised-cosine filter
One frequently used H(f) transfer function belonging to the Nyquist class (zero ISI at the sampling time).
It can be expressed as

29. Amplitude response of raised-cosine filter with various roll-off factors

30. Impulse response of raised-cosine filter with various roll-off factors

31. Consecutive raised-cosine impulses, demonstrating zero-ISI property

32. Cont’d... Impulse response for the H(f) Minimum required bandwidth
DSB bandwidth

33. Cont’d... Two types of error-performance degradation
Due to a decrease in received signal power or an increase in noise or interference power, giving rise to a loss in signal-to-noise ratio, Eb/No
Due to signal distortion such as ISI

34. Nyquist’s Wave Nyquist pulse Square-root Nyquist pulse
Impulse response of the raised-cosine filter
Faster transition
Square-root Nyquist pulse
Impulse response of a root-cosine filter
Does not exhibit zero ISI

35. Digital Modulation Techniques
The process by which digital symbols are transformed into waveforms that are compatible with the characteristic of the channel.
Bandpass modulation
The shaped pulses modulate a carrier wave
Radio transmission – the carrier is converted to EM field for propagation to the desired destination

36. Cont’d...
Carrier Signal
If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK)
If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK)

37. Cont’d...
If the phase, θ of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK)
If both the amplitude,V and the phase, θ of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM)

38. Cont’d...

39. Amplitude Shift Keying (ASK)
A binary information signal directly modulates the amplitude of an analog carrier.
Where vask (t) = amplitude shift keying wave
vm(t) = digital information signal (volt)
A/2 = unmodulated carrier amplitude (volt)
ωc = analog carrier radian frequency (rad/s)

41. Frequency Shift Keying (FSK)
Called as BFSK
The phase shift in carrier frequency (∆f) is proportional to the amplitude of the binary input signal (vm(t)) and the direction of the shift is determined by the polarity
Where vfsk(t) = binary FSK waveform
Vc = peak analog carrier amplitude (volt)
fc = analog carrier center frequency (Hz)
∆f = peak shift in analog carrier frequency (Hz)
vm(t) = binary input signal (volt)

43. FSK Bandwidth

44. FSK Transmitter

45. Phase Shift Keying (PSK)

46. BPSK Transmitter

47. BPSK Receiver

48. BPSK Bit value 1 – sine wave Bit value 0 – inverted sine wave
Very simple PSK
Low spectral efficiency
Robust , used in satellite system

49. Output phase vs time relationship for a BPSK modulator
Binary
input
BPSK
output
Time 1

50. QPSK

51. QPSK 2 bits coded as one symbol Symbol determines shift of sine wave
Needs less bandwidth compared to BPSK
More complex

52. QPSK signal in the time domain
The modulated signal is shown below for a short segment of a random binary data-stream.
The two carrier waves are a cosine wave and a sine wave, as indicated by the signal-space analysis above.
Here, the odd-numbered bits have been assigned to the in-phase component and the even-numbered bits to the quadrature component (taking the first bit as number 1). The total signal — the sum of the two components — is shown at the bottom. Jumps in phase can be seen as the PSK changes the phase on each component at the start of each bit-period. The topmost waveform alone matches the description given for BPSK above.

53. The binary data that is conveyed by this waveform is: 1 1 0 0 0 1 1 0.
The odd bits, highlighted here, contribute to the in-phase component:
The even bits, highlighted here, contribute to the quadrature-phase component:

54. End of Chapter 4