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- 1. TELE3113 Analogue and Digital Communications – Digital Band-pass Modulation Wei Zhang w.zhang@unsw.edu.au School of Electrical Engineering and Telecommunications The University of New South Wales29 Sept 2009
- 2. Passband Communication• In baseband date transmission, a sequence of bits is represented in the form of discrete pulse modulated wave (PAM, PPM, PWM) or digital baseband modulated wave (PCM, DM, DPCM) that are transmitted over a low-pass channel (e.g., a coaxial cable).• For a bandpass channel, e.g., wireless radio, microwave, satellite, optical fibre, we have to resort to the use of a modulation strategy configured around a carrier signal.• Passband signals are generated by modulating a baseband signal onto a carrier.• The frequency band of the transmitted signals over the channel is centred at the carrier frequency.
- 3. Passband Communication• Passband Analog communication – A baseband analog signal is modulated onto a carrier using AM, FM, PM techniques.• Passband Digital communication – A baseband digital signal is modulated onto a carrier using digital signalling techniques such as • ASK (Amplitude shift keying), • PSK (Phase shift keying), • FSK. (frequency shift keying),
- 4. Passband Digital Signalling• In digital signalling, the binary digital information is first encoded using a particular coding scheme, e.g. Polar NRZ, Unipolar RZ, Manchester, etc.• The code is then impressed on a carrier by using a conventional modulation technique.• For digital signalling, modulation is the switching (keying) of a carrier waveform parameter (amplitude, phase or frequency) between preset levels, whose values are discrete.• This impresses the code, and therefore the information, on the carrier.
- 5. Two types of digital signallingDigital signalling can be1. Binary (ASK, BPSK, FSK) The digital information is transmitted a bit at a time2. Multilevel (QAM, Quardrature PSK, M-ary PSK) Several bits are grouped together too form symbols, and then symbols are modulated and then transmitted as one unit.The primary performance issues in digital transmission:• Optimal transmitter and receiver design,• Minimising average probability of symbol error,• spectral properties, bandwidth occupied by a modulation scheme• An important property of any modulation scheme is its spectral characteristic. This determines the bandwidth required for transmission.
- 6. Binary Digital SignallingAmplitude-Shift Keying (On-Off Keying)• Switching the amplitude of the carrier signal between two discrete levels.• e.g. switching (keying) a carrier sinusoid "on" and "off" with a unipolar binary signal• The ASK signal is represented by s ( t ) = Ac m( t ) cos( ω c t )• m(t) is the unipolar baseband data signal.• Ac is the amplitude of carrier.• ωc is the angular frequency of the carrier.
- 7. • The spectrum of ASK signal, S(f), can be found from the Fourier transform ∞• S ( f ) = ∫ A m( t ) cos( ω t )e − j 2πft dt c c −∞ ∞ A = ∫ c m( t ) e − j 2π ( f − f c )t + e − j 2π ( f + f c )t dt −∞ 2 Ac = (M ( f − f c ) + M ( f + f c )) 2• Note: the bandwidth required to transmit the ASK s(t) is twice the bandwidth of the modulating signal m(t). i.e. BT=2B
- 8. ASK & OOK
- 9. Detection of ASK• ASK can be generated and detected in the same fashion as AM.• ASK may be detected by coherent or noncoherent approaches.
- 10. Frequency-Shift Keying (FSK)• Switching the frequency of the carrier signal between discrete levels.• In FSK, each discrete level of a code is represented by a specific frequency.• e.g. A binary code with two levels is represented by two discrete frequencies.
- 11. Frequency-Shift Keying (FSK)• In the time domain, the FSK signal is represented by ⎡ t ⎤ s (t ) = Ac cos ⎢ωc t + D f ∫ m(t )dt ⎥ ⎣ −∞ ⎦m(t) is the baseband data signal.Ac is the amplitude of the carrier.Df is the frequency deviation [rad./volt/sec].
- 12. FSK
- 13. Detection of FSK• FSK can be detected by either coherent or incoherent detection
- 14. Binary Phase-Shift Keying (BPSK)• Switching the phase of the carrier signal between two discrete levels. Usually the two levels differ by 180º.• The BPSK signal is represented by [ s ( t ) = Ac cos ω c t + D p m( t ) ] m(t) is the baseband data signal. Ac is the amplitude of the carrier. Dp = is the peak phase deviation [rad./volt].
- 15. Binary Phase-Shift Keying (BPSK) ref Couch pg 343• let m(t) a polar signal with peak values of ± 1 and rectangular pulse shape. Then ( ) ( ) s ( t ) = Ac cos D p m( t ) cos( ω c t ) − Ac sin D p m( t ) sin( ω c t ) = Ac cos( D ) cos( ω t ) − A p c c ( ) sin D p m( t ) sin( ω c t )Since cosine is an even function. – The first term is called the pilot carrier term which does not carry any useful information --- power wasted! – The second term is the data term carrying the coded message. – The peak phase deviation Dp determines the ratio of the data to the pilot term.
- 16. Optimal BPSK• To maximise the signalling power efficiency (large proportion of power is to be carried by the data term), the pilot term must be minimised, i.e. Dp = ∆θ = 90º = π/2• Hence for the optimal BPSK, the signal becomes s ( t ) = − Ac m( t ) sin( ω c t )• The transmitted signal phase is switched between two values, 0, 1800
- 17. BPSK
- 18. Detection of BPSK• Coherent (Synchronous) detection must be used multiplier BPSK signal baseband digital signal LPF cos ( ω c t)
- 19. Detection of BPSK• If a low-level pilot carrier component is transmitted a PLL can be used to recover carrier• Otherwise a Costas loop may be used to synthesise the carrier reference from this BPSK-SC signal – 180º phase ambiguity must be resolved • Can be accomplished by sending a known test signal • or using differential coding
- 20. Figure 3-57 Ziemer Costas phase-locked loop. Note: m2(t) ambiguity for ±1
- 21. Costas phase-locked loop• Note: m2(t) thus ambiguity for ±1• Hence loop is just as likely to phase lock such that the demodulated output is proportional to – m(t) as it is to m(t)• Thus we cannot be sure of the polarity of the output and binary 1’s could be read as binary 0’s
- 22. Differential Phase-Shift Keying (DPSK)• Motivation of DPSK: – Remove the 180º phase ambiguity in BPSK by using differential coding at the Tx and differential decoding at the Rx. – When serial data is passed through many circuits along a communications channel the waveform is often unintentionally inverted • Often occurs during switching between several data paths
- 23. Differential Phase-Shift Keying (DPSK)• Differential Coding – In differential coding, an input binary data sequence {dn} is encoded into differential data {en} determined by en = d n ⊕ en −1 where ⊕ is a modulo 2 adder or exclusive OR gate (XOR) operation
- 24. Differential Phase-Shift Keying (DPSK)• Each digit in the encoded sequence is obtained by comparing the present input bit with the past encoded bit• A binary 1 is encoded if the present input bit and the past encoded bit are of opposite state• A binary 0 is encoded if the states are the same
- 25. Differential Phase-Shift Keying (DPSK)• At the receiver side, the encoded data {en} is ~ decoded to produce the data {d n } by ~ ~ ~ d n = en ⊕ en −1 The tilde denotes data at the receiver.• Hence the encoded signal is decoded by comparing the state of adjacent bits – If the present received encoded bit has the same state as the past encoded bit a binary 0 is decoded – A binary 1 is decoded for opposite states
- 26. General Differential Coding System
- 27. Example of Differential Coding
- 28. Generation of DPSK logic network phase-shift keying DPSK signalbaseband binary data carrier one bit delay
- 29. Detection of DPSK multiplier DPSK signal baseband data LPF one bit delayWhen the baseband data is represented in polar code:e.g. 1 --- -1v and 0 --- +1v,the modulo 2 adder logic at the receiver side can be easilyrealised using a one-bit delay and a multiplier. 0 = 1 ⊕ 1 +1 = -1 × -1 1 = 1 ⊕ 0 -1 = -1 × +1 1 = 0 ⊕ 1 -1 = +1 × -1 0 = 0 ⊕ 0 +1 = +1 × +1

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