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Wireless Communication Engineering (Fall 2004)

Learn about digital modulation in wireless communication engineering, covering modulation types, demodulation processes, detection methods, metrics such as power and bandwidth efficiency, and considerations for robust communication. Explore coherent and noncoherent detection techniques, modulation schemes, efficiency metrics, and practical implementations for reliable communication systems. ####

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Wireless Communication Engineering (Fall 2004)

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  1. Wireless Communication Engineering(Fall 2004) Lecture 5 Professor Mingbo Xiao Oct. 28, 2004

  2. Diagram of Digital Comm. System

  3. Basic Mixing Process • In most communications systems the frequency content of the baseband data stream does not match the frequency transmission property of the transmission channel. • For example a radio channel will have a bandpass response, only passing frequency components many times higher than those making up the input data stream

  4. Basic Mixing • To translate our baseband data stream to the frequency of our transmission channel we multiply or mix the baseband with a carrier frequency (wct) Carrier Modulation. • Spectrum of the modulated signal: wc

  5. Modulation • The process involves modulating the amplitude, frequency and/or phase of a carrier sinewave. • Carrier is commonly written cos(wct) • Choice of modulation affects: • ease of implementation • noise tolerance • occupied channel bandwidth • Two well-known analog modulation, AM and FM

  6. Analog Modulation: AM Radio

  7. Noise has a greater effect on amplitude than frequency Sufficient to detect zero crossings to reconstruct the signal Easy to eliminate amplitude distortion Constant envelope, i.e., envelope of carrier wave does not change with changes in modulated signal This means that more efficient amplifiers can be used, reducing power demands Analog Modulation: FM Radio

  8. Detection of FM Signal • Noise translates into amplitude changes, and sometimes frequency changes • Detection based on zero crossings: the limiter • Alternative schemes to translate limited signal into bit streams

  9. Digital Modulation • Serve as interface to the channel. • Map the binary information sequence into signal waveforms. • Alternate certain property of the carrier. • Carrier wave has the frequency of the wireless channel used for communication. • Two well-known analog modulation, AM and FM; FM is more resistant to noise.

  10. Digital Modulation Techniques

  11. Digital Demodulation • Demodulation – Process of removing the carrier signal • Detection – Process of symbol decision – Coherent detection » Receiver uses the carrier phase to detect signal » Cross correlate with replica signals at receiver » Match within threshold to make decision – Noncoherent detection » Does not exploit phase reference information » Less complex receiver, but worse performance

  12. Coherent Versus Noncoherent • Coherent (aka synchronous) detection: process received signal with a local carrier of same frequency and phase • Noncoherent (aka envelope) detection: requires no reference wave • Coherent examples: Phase shift keying (PSK), Frequency shift keying (FSK), Amplitude shift keying (ASK), Continuous phase modulation (CPM), Hybrids • Noncoherent examples : FSK, ASK, Differential PSK (DPSK), CPM, Hybrids

  13. Metrics for Digital Modulation • Power Efficiency – Ability of a modulation technique to preserve the fidelity of the digital message at low power levels – Designer can increase noise immunity by increasing signal power – Power efficiency is a measure of how much signal power should be increased to achieve a particular BER for a given modulation scheme – Signal energy per bit / noise power spectral density: Eb / N0 • Bandwidth Efficiency – Ability to accomodate data within a limited bandwidth – Tradeoff between data rate and pulse width – Throuput data rate per hertz: R/B bps per Hz

  14. Other Considerations • Robust to multipath effects • Low cost and ease of implementation • Low carrier-to-cochannel interference ratio • Low out-of-band radiation • Constant or near constant envelope – Constant: only phase is modulated – Non-constant: phase and amplitude modulated

  15. Illustration of Modulation

  16. Amplitude Shift Keying (ASK) • Simplest form of bandpass data modulation, symbols are represented as discrete amplitudes of a fixed frequency carrier oscillator. • Binary ASK, two symbol states, carrier is simply turned on or off. Also known as ON–OFF Keying (OOK).

  17. Symmetry in ASK • Spectrum of an ASK signal can be determined from its baseband data stream if the ASK modulation process if seen as a multiplication or mixing of the baseband symbol stream with the carrier wave. • Consider a single frequency cos(wmt) from within the baseband spectrum and perform the mathematical multiplication with the carrier cos(wct) ...

  18. ASK Modulation • The modulated signal becomes: cos(wct) . cos(wmt) = 0.5 cos (wc - wm)t + 0.5 cos (wc + wm)t • two identical components symmetric about the carrier frequency.

  19. ASK Modulation …2 • If we include all the components in the baseband stream, the resulting spectrum is again symmetrical about the carrier, a positive and reversed image of the ‘sinc’ spectrum for unfiltered binary data.

  20. Generation of ASK Signals • Simplest method for binary ASK is to use a switch to gate the carrier on and off, driven by the data signal. • Butpulse shaping at RF is difficult / expensive

  21. Baseband Filtering • Using a mixer based approach, the baseband data stream can be pre–filtered using a low pass (RRC) filter and this pulse shaping will be imposed as the envelope variation of the carrier.

  22. Non–Coherent ASK Detection • With ASK information is conveyed in the amplitude or envelope of the modulated carrier. • Simplest form of envelope detector is a diode rectifier / smoothing filter, known as non-coherent detection.

  23. Coherent ASK Detection • A coherent detector operates by mixing the incoming data signal with a locally generated carrier reference and selecting the difference component from the mixer output.

  24. Coherent ASK Detection (Cont’d) • If the modulated data signal is a(t).cos(wct) and the reference carrier cos(wct + q) where q is the phase error between the source and reference carriers, the mixer output becomes: a(t).cos(wct).cos(wct + q) = 0.5a(t).cos(q) + 0.5a(t).cos (2wct + q) • If q = 0 (reference carrier phase coherent) output is proportional to a(t) • Coherent detection has better noise performance

  25. Frequency Shift Keying (FSK) • Frequency shift keying has been the most widely used form of digital modulation until recently. • simple to generate and detect • insensitive to amplitude fluctuations on the channel • distinct carrier frequencies represent symbol states

  26. Frequency Shift Keying (Cont’d) • Can be viewed as two, separate ASK symbol streams summed prior to transmission:

  27. FSK Generation • switch –> phase jumps • VCO (Bateman p118) • Continuous Phase Frequency Shift Keying (CPFSK)

  28. Filtered FSK • we can control the spectral shape of CPFSK by a pulse shaping filter prior to the modulator as in ASK; although the mapping is not precise since the VCO is non–linear. • Commonly use a Gaussian filter as in GSM which uses Gaussian Minimum Shift Keying (GMSK)

  29. Non-coherent FSK Detection • Simplest approach is to pass the signal through two bandpass filters tuned to the two signalling frequencies and detect which has the larger output averaged over the symbol period. Equates to two ASK detectors plus a comparator. • coherent detection also used, more complex but better performance.

  30. Comparison of Modulation Types • assuming equal average symbol energy • coherent > • non–coherent. • phase > • frequency > • amplitude.

  31. Phase Shift Keying (PSK) • With PSK, the information is contained in the instantaneous phase of the modulated carrier. • It is usually imposed and measured with respect to a fixed carrier of known phase – coherent PSK. • For binary PSK (BPSK), phase states of 0o and 180o are used.

  32. Differentially Coherent PSK • It is also possible to transmit data encoded as the phase change between consecutive symbols; this is known as Differentially Coherent PSK.

  33. Binary Phase Shift Keying (BPSK) • Use alternative sine wave phase to encode bits • Simple to implement, inefficient use of bandwidth • Very robust, used extensively in satellite communications

  34. Quadrature Phase Shift Keying • – Multilevel modulation technique: 2 bits per symbol • – More spectrally efficient, more complex receiver • Output waveform is sum of modulated ±Cosine and ±Sine wave • 2x bandwidth efficiency of BPSK

  35. QPSK Symbols

  36. Minimum Shift Keying • Special form of (continuous phase) frequency shift keying • Minimum spacing that allows two frequencies states to be orthogonal • Spectrally efficient, easily generated

  37. Generating Minimum Shift Keying

  38. Gaussian Minimum ShiftKeying(GMSK) • MSK + premodulation Gaussian low pass filter • Increases spectral efficiency with sharper cutoff, excellent power efficiency due to constant envelope • Used extensively in second generation digital cellular and cordless telephone applications • GSM digital cellular: 1.35 bps/Hz • DECT cordless telephone: 0.67 bps/Hz • RAM Mobile Data

  39. π/4-Shifted QPSK • Variation on QPSK • Restricted carrier phase transition to +/-π/4 and +/- 3π/4 • Signaling elements selected in turn from two QPSK constellations, each shifted by π/4 • Maximum phase change is ±135° vs. 180° for QPSK, thus maintaining constant envelope (i.e., amplitude of QPSK signal not constant for short interval during 180° phase changes) • Popular in Second Generation Systems • North American Digital Cellular (IS-54): 1.62 bps/Hz • Japanese Digital Cellular System: 1.68 bps/Hz • European TETRA System: 1.44 bps/Hz • Japanese Personal Handy Phone (PHP)

  40. π/4-Shifted QPSK (Cont’d) • Advantages: • Two bits per symbol, twice as efficient as GMSK • Phase transitions avoid center of diagram, remove some design constraints on amplifier • Always a phase change between symbols, leading to self clocking

  41. Quadrature AmplitudeModulation (QAM) • Quadrature Amplitude Modulation (QAM) • Amplitude modulation on both quadrature carriers • 2n discrete levels, n = 2 same as QPSK • Extensive use in digital microwave radio links

  42. Differential PSK • simple receiver – no carrier recovery mechanism and still good performance. • logic ‘1’–> change of logic state from previous coded bit • logic ‘0”–> no change of state from the previous coded bit

  43. QUIZzzzzzzzzzzzzzzzzzzzzz • Why do we also need retransmission mechanism in the transport layer, while it is provided in the data link layer? • What are the expected data rates of 3G systems invehicular, pedestrian, and indoor environment? • Give two examples and explain what is Companding? • What are the effects of packet size in a packet switching network? • If you cut a watermelon N times in horizon and M times in vertical, how many chunks are with skin?

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