Unformatted text preview: of
9600bps
which have two amplitudes
which Quadrature PSK (QPSK)
Quadrature QPSK and OQPSK
Modulators
Modulators Performance of Digital to
Analog Modulation Schemes
Analog bandwidth ASK/PSK bandwidth directly relates to bit rate
multilevel PSK gives significant improvements
FSK bandwidth related to data rate for lower
FSK
frequencies, but to offset of modulated frequency
from carrier at high frequencies
from in presence of noise: bit error rate of PSK and QPSK are about 3dB
bit
superior to ASK and FSK
superior
for MFSK & MPSK have tradeoff between
for
bandwidth efficiency and error performance Quadrature Amplitude
Modulation
Modulation QAM used on asymmetric digital subscriber line
QAM
(ADSL) and some wireless
(ADSL)
combination of ASK and PSK
logical extension of QPSK
send two different signals simultaneously on
send
same carrier frequency
same use two copies of carrier, one shifted 90°
each carrier is ASK modulated
two independent signals over same medium
demodulate and combine for original binary output QAM Modulator
QAM QAM Variants
QAM two level ASK each of two streams in one of two states
four state system
essentially QPSK four level ASK combined stream in one of 16 states have 64 and 256 state systems
have improved data rate for given bandwidth but increased potential error rate Analog Data, Digital Signal
Analog digitization is conversion of analog data
digitization into digital data which can then:
into be transmitted using NRZL
be transmitted using code other than NRZL
be converted to analog signal analog to digital conversion done using a
analog codec
codec pulse code modulation
delta modulation Digitizing Analog Data
Digitizing Pulse Code Modulation (PCM)
Pulse sampling theorem: “If a signal is sampled at regular intervals at a
If
rate higher than twice the highest signal
frequency, the samples contain all information
in original signal”
in
eg. 4000Hz voice data, requires 8000 sample
eg.
per sec
per strictly have analog samples
strictly Pulse Amplitude Modulation (PAM) so assign each a digital value Pulse Code Modulation(PCM) 4 bit system gives 16 levels Quantized Quantizing error or noise
Approximations mean it is impossible to recover
Approximations
original exactly
original 8 bit sample gives 256 levels Quality comparable with analog transmission 8000 samples per second of 8 bits each
8000 gives 64kbps
gives SNR = 6.02n + 1.76dB PCM Example
PCM PCM Block Diagram
PCM NonLinear Coding
NonLinear Companding
Companding Delta Modulation
Delta analog input is approximated by a
analog staircase function
staircase can move up or down one level (δ) at each
at
sample interval
sample has binary behavior since function only moves up or dow...
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 Fall '11
 DRyinyan

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