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ECE 4331, Fall, 2009

ECE 4331, Fall, 2009. Zhu Han Department of Electrical and Computer Engineering Class 20 Nov. 3 rd , 2009. Outline. Digital Carrier Systems Carrier band vs. baseband Baud rate, bit rate, bandwidth efficiency Spectrum Gray coding Coherent, noncoherent receiver BER Comparison

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ECE 4331, Fall, 2009

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  1. ECE 4331, Fall, 2009 Zhu Han Department of Electrical and Computer Engineering Class 20 Nov. 3rd, 2009

  2. Outline • Digital Carrier Systems • Carrier band vs. baseband • Baud rate, bit rate, bandwidth efficiency • Spectrum • Gray coding • Coherent, noncoherent receiver • BER • Comparison • Practical implementation example

  3. Digital Carrier System Baseband analysis Signal in baseband: mean symbol energy: signal in carrier band: mean symbol energy: Conclusion: analysis of carrier band = base band. Fc=0 in project

  4. Baud Rate, Bit Rate, Bandwidth Efficiency • Remember channel capacity C=Wlog2 (1+ SNR)> fb

  5. Power Spectrum, ASK • Baseband • Sy(W)=Sx(W) P(W) • ASK: Sy(t)=b Acoswct, Square wave convolute with sinusoid.

  6. FSK Spectrum • FSK: two sinc added together

  7. BPSK Spectrum • BPSK: Sx(W): NRZ. P(t): raised cosine function. Sy(W)= P(W) • Rb baud rate

  8. QPSK Spectrum • Same Rb Narrow BW

  9. Pulse Shaped M-PSK • Different 

  10. Bandwidth vs. Power Efficiency • Bandwidth efficiency high, required SNR is high and low power efficiency

  11. QAM efficiencies • For l =1  PSD for BPSK • For l =2  PSD for QPSK, OQPSK … • PSD for complex envelope of the bandpass multilevel signal is same as the PSD of baseband multilevel signals • Same baud rate, higher bit rate. • Same bit rate, less bandwidth. But higher power

  12. Minimum Shift Keying spectra • Continuous phase and constant envelop. So narrow spectrum

  13. GMSK spectral shaping

  14. Gray coding • It is very unlikely that switches will change states exactly in synchrony. So there might be misunderstanding. E.g. 011->100 • In a digital modulation scheme such as QAM where data is typically transmitted in symbols of 4 bits or more, the signal's constellation diagram is arranged so that the bit patterns conveyed by adjacent constellation points differ by only one bit. By combining this with forward error correction capable of correcting single-bit errors, it is possible for a receiver to correct any transmission errors that cause a constellation point to deviate into the area of an adjacent point. This makes the transmission system less susceptible to noise. • Graduate student for 16-QAM

  15. Coherent Reception • An estimate of the channel phase and attenuation is recovered. It is then possible to reproduce the transmitted signal, and demodulate. It is necessary to have an accurate version of the carrier, otherwise errors are introduced. Carrier recovery methods include:

  16. Coherent BER • PSK • BPSK QPSK • MPSK

  17. Coherent BER performance • ASK • FSK • MSK: less bandwidth but the same BER • MQAM

  18. Non-coherent detection • Non-coherent detection • does not require carrier phase recovery (uses differentially encoded mod. or energy detectors) and hence, has less complexity at the price of higher error rate. • No need in a reference in phase with the received carrier • Differentially coherent detection • Differential PSK (DPSK) • The information bits and previous symbol, determine the phase of the current symbol. • Energy detection • Non-coherent detection for orthogonal signals (e.g. M-FSK) • Carrier-phase offset causes partial correlation between I and Q braches for each candidate signal. • The received energy corresponding to each candidate signal is used for detection.

  19. Differential Reception

  20. Differential Coherent • DBPSK • 3dB loss

  21. Non-coherent detection of BFSK Decision stage: + -

  22. Rician pdf Non-coherent detection BER • Non-coherent detection of BFSK • Similarly, non-coherent detection of DBPSK Rayleigh pdf

  23. BER Example

  24. Example of samples of matched filter output for some bandpass modulation schemes

  25. Comparison of Digital Modulation

  26. Comparison of Digital Modulation

  27. Figure 6.45 Comparison of the noise performance of different PSK and FSK schemes.

  28. Bandwidth vs. Power Efficiency • Bandwidth efficiency high, required SNR is high and low power efficiency

  29. Spectral Efficiencies in practical radios • GSM- Digital Cellular • Data Rate = 270kb/s, bandwidth = 200kHz • Bandwidth Efficiency = 270/200 =1.35bits/sec/Hz • Modulation: Gaussian Minimum Shift Keying (FSK with orthogonal frequencies). • “Gaussian” refers to filter response. • IS-54 North American Digital Cellular • Data Rate = 48kb/s, bandwidth = 30kHz • Bandwidth Efficiency = 48/30 =1.6bits/sec/Hz • Modulation: pi/4 DPSK

  30. Modulation Summary • Phase Shift Keying is often used, as it provides a highly bandwidth efficient modulation scheme. • QPSK, modulation is very robust, but requires some form of linear amplification. OQPSK and p/4-QPSK can be implemented, and reduce the envelope variations of the signal. • High level M-ary schemes (such as 64-QAM) are very bandwidth efficient, but more susceptible to noise and require linear amplification. • Constant envelope schemes (such as GMSK) can be employed since an efficient, non-linear amplifier can be used. • Coherent reception provides better performance than differential, but requires a more complex receiver.

  31. Homework 5 • 6.2, 6.5, 6.7, 6.8, 6.11, 6.15, 6.21, 6.32, • Due on 11/17 • Make up class 11/6, 11/13, 11/20 4:00pm-5:30pm

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