Chapter Four Bandpass Modulation and Demodulation Dept of
Chapter Four Bandpass Modulation and Demodulation Dept. of EE, NDHU 1
Bandpass Signaling Dept. of EE, NDHU 2
Why Modulate? • The transmission of EM fields through space is accomplished with the antenna • The size of the antenna depends on the wavelength l – Telephone industry benchmark of l/4 as the antenna dimension – Example : 3 k. Hz baseband signal needs about 15 miles for the antenna diameter – Example: 900 MHz signal needs about 8 cm for the antenna diameter • Bandpass modulation is an essential step for all systems involving radio transmission • Modulation can separate the different signals (Ex. FDMA) • Modulation can also be used to place a signal in a frequency band where design requirement can be easily met Dept. of EE, NDHU 3
Digital Bandpass Modulation Techniques • Bandpass modulation is the process by which an information signal is converted to a sinusoidal waveform (carrier waveform) • Three features can be used to distinguish the sinusoidal waveform – Amplitude, frequency, phase • Coherent detection – The receiver exploits knowledge of the carrier’s phase to detect the signa – PSK, FSK, ASK, CPM, and Hybrid forms • Non-coherent detection – The receiver does not utilize the carrier’s phase reference information – DPSK, FSK, ASK, CPM, and Hybrid forms Dept. of EE, NDHU 4
Digital Modulations Dept. of EE, NDHU 5
Detection of Signals in Gaussian Noise • Bandpass model of the detection process is virtually identical to the baseband model • Decision regions – Minimum error decision rule is to choose the signal class that the distance d(r, si) is minimized, where r is the received signal • Correlation receiver – Transform the received waveform into a point in the decision space – Determine in which decision region the point is located Choose the si(t) whose index corresponds to max zi(T) Dept. of EE, NDHU 6
Decision Regions Dept. of EE, NDHU 7
Correlator Receiver with Reference Signals Dept. of EE, NDHU 8
Binary Correlator Receiver Dept. of EE, NDHU 9
Coherent Detection of PSK • BPSK signal • Decision stage chooses the signal with largest output value of matched filter Dept. of EE, NDHU 10
Sampled Matched Filter Dept. of EE, NDHU 11
Coherent Detection of MPSK • MPSK signal • Signal space and decision regions for a QPSK (M=4) system – As shown in Fig. 4. 11 – Make a decision by the phase information Dept. of EE, NDHU 12
Demodulator for MPSK Signals Dept. of EE, NDHU 13
Coherent Detection of FSK • FSK signal • The distance between any two signal vectors is • Choose the largest output of matched filter Dept. of EE, NDHU 14
Signal Space for a 3 -ary FSK Signal Dept. of EE, NDHU 15
Signal Space for DPSK Dept. of EE, NDHU 16
Detection of Differential PSK • Differential encoding for the PSK signal • Signaling characteristics • Non-coherent detection • Compare with PSK and DPSK – PSK detection is with only one noise signal – DPSK detection is with two noise signal (differentially decoding) Dept. of EE, NDHU 17
Binary Differential PSK Example Suboptimum detection Optimum detection Dept. of EE, NDHU 18
Non-coherent Detection of FSK Dept. of EE, NDHU Quadrature Receiver 19
Non-coherent Detection of FSK Non-coherent detection of FSK with envelop detector Dept. of EE, NDHU 20
Tone Spacing for Non-coherent Orthogonal FSK Signaling • Two tones f 1 and f 2 are orthogonal – For a transmitted tone f 1, the sampled envelop of the receiver output filter tuned to f 2 is zero • Minimum tone spacing for orthogonal FSK signaling – Non-coherently detected FSK – Coherent FSK signaling is 2/T Dept. of EE, NDHU 21
Minimum Tone Spacing for Noncoherent Orthogonal FSK • • For binary FSK, bandwidth is two times the tone spacing For M-ary FSK, bandwidth is M/T Dept. of EE, NDHU 22
D 8 PSK Modulator Dept. of EE, NDHU 23
D 8 PSK Demodulator Dept. of EE, NDHU 24
Error Performance for Binary Systems • Bit error probability for BPSK signaling • Probability of bit error for coherent detected, differential encoded binary PSK • Probability of bit error for coherently detected binary orthogonal FSK • Probability of bit error for non-coherently detected binary orthogonal FSK Dept. of EE, NDHU 25
Binary DPSK • DPSK signaling • Pairs of DPSK signals, S 1(t) and S 2(t) are orthogonal • DPSK detection can be implemented by matching signal envelopes • Bit error probability is similar to the one for non-coherently detected binary FSK Dept. of EE, NDHU 26
DPSK Detection Dept. of EE, NDHU 27
Bit Error Probability of Binary Systems Dept. of EE, NDHU 28
M-ary Signals and Performance Dept. of EE, NDHU 29
Ideal Probability of Bit Error Performance Dept. of EE, NDHU 30
Bit Error Performance for M-ary Orthogonal Signaling Dept. of EE, NDHU 31
Bit Error Performance for Multiple Phase Signaling Dept. of EE, NDHU 32
M-ary Signaling • M-ary signaling instructs the modulator to produce one of M=2 k waveforms • M-ary multiple phase signaling – The BER curve moves in the direction of degraded error performance as k increases – A larger bit rate can be transmitted within the same bandwidth as k increases • M-ary orthogonal signaling – The BER curve moves in the direction of improved error performance as k increases – The required system bandwidth increases as k increases Dept. of EE, NDHU 33
Vectorial View of MPSK Signaling Dept. of EE, NDHU 34
Relation Between Eb/N 0 and S/N • General relationship between Eb/N 0 and S/N • For the QPSK signaling – QPSK bit stream is usually partitioned into an even and odd stream; each new stream is at half the bit rate of the original stream – Each of the quadrature BPSK signals half of the average power of the original QPSK signal (as shown in Fig. 4. 31) Dept. of EE, NDHU 35
Vectorial View of MFSK Signaling Dept. of EE, NDHU 36
Symbol Error Performance for Coherent FSK Signaling Dept. of EE, NDHU 37
Eb/N 0 and SNR in the MFSK Dept. of EE, NDHU 38
Symbol Error Versus Bit Error for FSK Signaling Dept. of EE, NDHU 39
Symbol Error Performance for Mary Systems • Symbol error performance for coherently detected M-ary PSK • Symbol error performance for differentially coherent detection of MPSK signal • Probability of symbol error for coherently detected MFSK signal • Probability of symbol error for non-coherently detected MFSK signal Dept. of EE, NDHU 40
Symbol Error Performance for Coherently Detected MPSK Dept. of EE, NDHU 41
Symbol Error Performance for Coherently Detected MFSK Dept. of EE, NDHU 42
Symbol Error Performance for Non -coherently Detected MFSK Dept. of EE, NDHU 43
Bit Error Versus Symbol Error Probability • Orthogonal signal Dept. of EE, NDHU 44
Bit Error Versus Symbol Error Probability • Multiple Phase signals with Gray coded • For BPSK and QPSK signaling Dept. of EE, NDHU 45
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