communications (elec3505) summary notes
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ELEC3505 – Communications
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Communications
Table of Contents General ................................................................................................................................................................................... 2
Introduction ............................................................................................................................................................................ 2
Continuous-Wave Modulation ............................................................................................................................................... 4
Pulse Modulation .................................................................................................................................................................. 13
Pulse-amplitude modulation (PAM) ................................................................................................................................. 13
Other forms of PAM .......................................................................................................................................................... 15
Quantisation process ............................................................................................................................................................ 15
Pulse code modulation (PCM) .......................................................................................................................................... 16
Baseband Pulse Transmission ............................................................................................................................................... 19
Matched LTI filter .............................................................................................................................................................. 19
Error rate due to noise ...................................................................................................................................................... 20
Inter-symbol interference ................................................................................................................................................. 22
Signal-Space Analysis ............................................................................................................................................................ 25
Summary of digital communications systems .................................................................................................................. 25
Geometric representation of signals ................................................................................................................................ 25
Signal space ....................................................................................................................................................................... 26
Passband Data Transmission ................................................................................................................................................ 27
Passband transmission model .......................................................................................................................................... 28
Coherent BPSK .................................................................................................................................................................. 28
Quadriphase-shift keying (QPSK) ...................................................................................................................................... 29
ELEC3505 – Communications
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General Lecturer
o Prof. Abbas Jamalipour
o EE 732
o Consultation time: Wednesdays 1300h-1400h
Assessment
o 10% Group lab reports and individual lab assessments
o 5% Individual tutorial attendance (short report submission)
o 15% Mid-semester exam Monday 5 May 08
o 70% Final Exam (open book)
Introduction Communications – transmission of information with acceptable quality
Elements of communication system:
o Transmitter, channel, receiver
o Received signal is a corrupted version of transmitted signal: noise, channel imperfections
o Basic modes – broadcast, point-to-point
Primary communication resources – use as efficiently as possible
o Transmitted power
Power limited system: satellite
o Channel bandwidth
Bandwidth limited system: telephone channel
Noise – unwanted disturbances to signal
o SNR (dB) – quantitative way to account for noise
Sources of information
o Speech (300-3100Hz), music (15kHz), picture (4.2MHz), computer data
Communication networks
o An interconnection of routers and links to route data
o Network in terms of layered architecture
Each layer performs a specific function
Layers communicates with each other through a well-defined interface
E.g. OSI Model
Communication channel – transmission of information across physical layer
o Guided channels – lines and cables
o Unguided channels – wireless radio
o Linear (wireless), nonlinear (satellite)
o Time varying (mobile), time invariant (optical fibre)
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Modulation process
o Definition: modifying the message signal into a form suitable for transmission
o Recreate the signal at the receiver by demodulation
o Classification
Continuous wave modulation
Sinusoidal wave used as carrier
Vary amplitude (AM), or phase (angle modulation) by frequency (FM) or phase (PM)
Pulse modulation
Period sequence of rectangular pulse
Analog – varying amplitude (PAM), duration (PDM), position of pulse (PPM)
Digital – quantisation of pulse of PAM into PCM
o Allows transmission of information
o Allows multiplexing – FDM, TDM, CDM, WDM
Pulse Code Modulation (PCM)
o PAM quantisation convert to binary symbols
o Preferred method for transmitting
Robust, flexible, common format
Digital communication systems
o Source encoder-decoder – remove redundant information, generates source code word
o Channel encoder-decoder – produce new sequence of symbols, called channel code word
o Modulator-demodulator – represent code word by analog waveform
Block diagrams
o Transmitter:
o Channel:
o Receiver:
Analog and digital communication system
o Digital – complex but easy to build, robustness against physical effects
Provides efficient and reliable information transmission
o Analog – conceptually simple, difficult to implement stringent requirements
Natural affinity for operating at high speed while consuming little power
Study of analog modulation may motivate other digital modulation schemes
Shannon’s Information Capacity Theorem
o Goal: deliver message signal efficiently, reliably given:
Allowable transmit power
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Available channel bandwidth
Affordable cost of system
o Expression of reliability – bit error rate (BER) or probability measured at Rx output
In ideal case, it is possible to operate with BER = 0 in noisy channel
o SNRBC 1log2
C – Information capacity of channel (bits)
B – channel bandwidth
SNR – received signal to noise ratio
o If actual signal rate is less than C, then it is possible to transmit without error in noisy environment
o System efficiency C
R provides a bound for attainable transmission rate
o Therefore can reduce SNR by trading off bandwidth B
o Equation provides for comparing noise performance of different modulation schemes
o Design encoder and decoder to change symbols close to Shannon’s limit
Continuous-Wave Modulation CW Modulation
o Baseband signals – information at frequencies of original source; unsuitable for transmission
o Modulation – vary some characteristics of a carrier according to a modulating wave
o Continuous-Wave – sinusoidal carrier, using baseband signal as modulating wave
Components
o Transmitter
o Receiver
Amplitude modulation (AM) – vary amplitude of carrier
o Amplitude of carrier wave is linearly varied about a mean value with the baseband signal
o Carrier wave: tfAtc CC 2cos
o Baseband signal: tm
o Modulated wave: tftmKAts CaC 2cos1
Ka – amplitude sensitivity (constant)
o Envelope of s(t) has essentially the same shape as m(t) if
Carrier frequency WfC message bandwidth
1tmKa for all t, otherwise carrier becomes “over-modulated”
Envelope distortion, phase reversal
Percentage modulation – 100max tmKa
ELEC3505 – Communications
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o AM Process
Baseband signal m(t) AM wave Over-modulation
Fourier transform
o tftmKAts caC 2cos1
o CCCa
CCC ffMffM
AKffff
AfS
22)(
o Negative frequencies now become visible (measurable)
o We can distinguish 2 parts: upper and lower sidebands
o Problems with amplitude modulation
Waste of power – no information in carrier c(t)
Waste of bandwidth – symmetry of upper and lower sidebands
Linear modulation schemes
o Definition: tftstftsts CQCI 2sin2cos
sI(t) – in-phase component of s(t)
sQ(t) – quadrature component of s(t), used to modify the power in one of the sidebands
In-phase component is solely linearly dependent on m(t)
Quadrature component is linearly dependent on a filtered version of m(t)
o DSB-SC: 0 , tstmts QI
o Upper SSB: tmtstmts QIˆ ,
21
21 tm̂ is Hilbert transform of m(t)
o Lower SSB: tmtstmts QIˆ , 2
121
o Vestigial Sideband (VSB):
Vestige (trace) of LSB: tmtstmts QI ' , 21
21 m’(t) is HQ(f) filtered output
Vestige (trace) of USB: tmtstmts QI ' , 21
21
DSB-SC
o Product modulator: tftmAts CC 2cos
o Fourier transform: CCC ffMffM
AfS
2)(