Download - Digital CW Techniques
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Desirable features Low bit error rate at low received S/N Performs well in multipath conditions
Occupies a minimum of BW
Easy and cost-effective to implement
None of existing techniques satisfy all of these
requirements
Depending upon application some of the abovefeatures are prioritized over other and accordingly
modulation technique is selected
Digital CW Modulation Techniques
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Bandwidth efficiency (R/B; max=log2(1+S/N)) Ability to accommodate data within a limited bandwidth. (may be
greater than 1)
In general increasing data rate implies decreasing pulse width i.e.increase in BW of signal.
Power efficiency (Eb/N0) Ability to keep the minimum BER at low power levels (may be greater
than 1)
With decrease in power, the BER increases but rate of increasedepends upon modulation tech. (The distance between valid symbols in
constellation diagram governs BER)
Phase characteristics If phase change is smooth then side lobe level is low thus ICI is
reduced.
Nonlinear amplifiers can be used which reduces implementationcomplexity, the cost and the power consumption (enhances battery life).
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Phase Shift Keying (PSK)
Binary Phase Shift Keying (BPSK) demonstrates better
performance than ASK and FSK.
PSK can be expanded to a M-ary scheme, employing multiple
phases and amplitudes as different states.
Filtering can be employed to avoid spectral spreading.
Baseband
Data
Binary PSK modulatedsignal
s1 s1s0 s0
where s0 = -A cos wct and s1 = A cos wct
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The Constellation Diagram
1 2
1
2 2cos 2 , cos 2 ; ; 0
energy per bit; bit period
For this signal set, there is a single basic signal
2cos 2 ; 0
b bBPSK c c b
b b
b b
c b
b
BPSK
E ES s t f t s t f t t T
T T
E T
t f t t T T
S E
1 1
,b b
t E t
-Eb Eb
Q
I
Constellation diagram
Distance = 2Eb= 2Es
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Types of QPSK
Conventional QPSK has transitions through zero (ie. 180ophase transition). Highly linear amplifier required.
In Offset QPSK, the transitions on the I and Q channels are
staggered. Phase transitions are therefore limited to 90o
.In/4-QPSK the set of constellation points are toggled eachsymbol, so transitions through zero cannot occur. Thisscheme produces the lowest envelope variations.
All QPSK schemes require linear power amplifiers.
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Amplitude Shift Keying (ASK)
Pulse shaping can be employed to remove spectral spreading.
ASK demonstrates poor performance, as it is heavily affectedby noise and interference.
Long string of zeros causes synchronization loss.
Baseband
Data
ASK modulated
signal
A cos wct A cos wct0 0
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M-ary Phase and Amplitude Modulation
Amplitude and phase shift keying can be combined to transmit several bits persymbol (in this case M=4). These modulation schemes are often referred to aslinear, as they require linear amplification.
16QAM has the largest distance between points, but requires very linearamplification. 16PSK has less stringent linearity requirements, but has lessspacing between constellation points, and is therefore more affected by noise.
M-ary schemes are more bandwidth efficient, but more susceptible to noise.
16 PSK 16 APSK16 QAM
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Frequency Shift Keying (FSK)
Bandwidth occupancy of FSK is dependant on the spacing of the two
symbols.
If either f0or f1are chosen such that there is no-integer number of
periods in symbol period Tb then discontinuities in the phase will result.
FSK can be expanded to a M-ary scheme, employing multiplefrequencies as different states (which should be orthogonal).
BasebandData
FSK modulated
signal
f1 f1f0 f0
where f0 = A cos(wc-Dw)t and f1 = A cos(wc+Dw)t
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GMSK Signals
1-2
0T 2TT T
Time
GMSK Pulse Shapes and ISI
MSKGMSK,
BT=0.5
GMSK
BT=0.3
The Gaussian pre-modulation filtersmoothes the phase trajectory of the MSK
signal thus limiting the instantaneous
frequency variations. The result is an FM
modulated signal with a much narrower
bandwidth.
This bandwidth reduction does not comefor free since the pre-modulation filter
smears the individual pulses in pulse
train. As a consequence of this smearing
in time, adjacent pulses interfere with
each other generating what is commonlycalled inter-symbol interference or ISI.
In the applications where GMSK is used,the trade-off between power efficiency
and bandwidth efficiency is well worth thecost.
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Coherent Detection An estimate of the channel phase and attenuation
is recovered. It is then possible to reproduce thetransmitted signal and demodulate.
Requires a replica carrier wave of the samefrequency and phase at the receiver.
The received signal and replica carrier are cross-
correlated using information contained in theiramplitudes and phases.
Also known as synchronous detection
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Optimum binary detection (a) parallel matched filters (b) correlation detector
Figure 14.2-3
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Non-Coherent Detection
Requires no reference wave; does notexploit phase reference information(envelope detection)
Differential Phase Shift Keying (DPSK) Frequency Shift Keying (FSK)Amplitude Shift Keying (ASK) Non coherent detection is less complex than
coherent detection (easier to implement), buthas worse performance.
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Noncoherent detection of bianry FSK
Figure 14.3-5
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Optimum Signal selectionThe simplex signal set, which form a M-
dimensional pyramid having centre of gravity atorigin, is the optimum signal set as it maximizes
the distance thus minimizes error
Orthogonal signal set is easy to generate and
decode and approximates the simplex set forlarge value of M
The transmission BW for orthogonal
signals>=M/2D
The threshold effect can be observed for M>>1
i.e. above it a slight increase in S/N causes huge
reduction in Pe.
The errorless transmission at any data rate is
possible for M=infinity for very small S/N0.
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Hard Vs. Soft decision decoding
In error coded transmission
Bitwise evaluation of all received bits of a vector thenhamming distance with valid vectors. (hard decision taken)
Euclidean distance of received vector as a whole (basedon the soft decision)
The representation ofdemodulator o/p (m) >1 bit
corresponds to soft
decision decoding.
2dB gain with 3 bit and 2.2
dB with infinite bitrepresentation.
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Trellis coded modulation
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8-ary PSK encoder for TCM (a) 4-state,
(b) 8 state,
Figure 14.5-3
2, 1m m
2, 2m m
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Partitioning of an 8-PSK signal set,Ungerbroeck, 1982Figure 14.5-4
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Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Trellis diagram for the encoder of Fig. 14.5-3a,
Ungerbroeck, 1982
Figure 14.5-4
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Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Two possible error events for encoder of Fig. 14.5-3a.
Figure 14.5-6transmitted sequence
most likely error event
error event
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Spread Spectrum Technology
Low power high bandwidth transmission.
Better performance in noisy condition.
Anti jamming feature
Security, etc.
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Types of SS techniques
Direct Sequence Spread Spectrum (DS-SS)e.g. CDMA
Frequency Hopping Spread Spectrum (FH-SS)
Time hopping Spread Spectrum (TH-SS)
Hybrid (DS/FH, DS/TH, FH/TH,DS/FH/TH)
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Direct Sequence Spread Spectrum (DS-SS)
This figure shows BPSK-DS
transmitter and receiver (multiplicationcan be realized by RF-mixers)
DS-CDMA is used in WCDMA, cdma2000 and IS-95 systems
2
22
av av
AP A P
spreading
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Effect of narrowband noise signal
Effect of wideband Interference
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Basic principle of CDMA
Assumptions
Polar line coding is used. All the users are synchronized All the users produce same power level at the base
station. Base station has a copy of all chipcodes of its
network
All the codes should be either PN-sequence or
orthogonal codes (preferable) i.e.Cx*Cy = +1 if x= y
= -1 if x= complement (y)
= 0 otherwise.
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Algorithm
A unique codeword of same length
{Cx=(c1,c2,c3)} is assigned to each user. A user transmits its codeword for transmittinglogic 1 and its compliment for 0.
The base station receives the algebraic sumof chips {D=(d1,d2,)} transmitted by all theactive users.
The BS calculates Sx = Cx(i)*D(i) for allusers.
If it is above +ve (-ve) threshold user x hastransmitted logic 1 (0)
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Problem:
There are three users A,B,C in a CDMA networkwith corresponding chip sequences Ca = {1,-1,-
1,1,-1,1}, Cb = {1,1,-1,-1,1,1}, Cc = {1,1,-1,1,1,-1}.
If both the thresholds are set at 0 volts, find the
data decoded by BS when(i) A transmits logic 1.
(ii) A and B transmit logic 1
(iii) A and C transmit logic 1.
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PN - sequences
Uniform distribution and independence.
Period = 2n1 bits; n= length of shift register
To identify a PN both algorithm (interconnection oftaps) and seed (starting point) must be known.
Seed can change the starting and ending point ofa PN sequence but the contents remains thesame. (this property is used for cell identificationin CDMA mobile systems.)
Walsh codes are known to be orthogonal codes.
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Frequency Hopping Transmitter and Receiver
May be slow (two or more
symbols are tx at same fc) or
fast (two or more frequencies
are used to transmit single
symbol)
d
BW W
d
BW W
s
BW W
sBW W
2 frequenciesk
2 level modulationL
Hopping frequencies aredetermined by the code.
This method is applied inBlueTooth
Frequency Hopping Spread Spectrum (FH SS) (ex: tx
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Frequency Hopping Spread Spectrum (FH-SS) (ex: tx
of two symbols/chip)
2 levelsL
2 slotsk
:chip duration
: bit duration
: symbol duration
c
b
s
T
T
T
2 ( data modulator BW)
2 ( total FH spectral width)
L
d d
k
s d
W f
W W
b
T
2L
4-level FSK modulation
Hopped frequency
slot determined by
hopping code
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B th th d fi t d i ilit
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Both methods were first used in military comm, FH can be advantageous because the
hopping spancan be very large (makeseavesdroppingdifficult)
DS can be advantageous because spectraldensitycan be much smaller than
background noise density (transmission isunnoticed)
By using hybrid systemssome benefits can becombined: The system can have a lowprobability of interception and negligible near-far effect at the same time. (Differentially
coherent modulationis applicable)
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FDMA, TDMA and CDMA compared
TDMA and FDMA principle: TDMA allocates a time instantfor a user
FDMA allocates a frequency bandfor a user
CDMA allocates a codefor user
CDMA-system can be synchronousorasynchronous:
Synchronous CDMA difficult to apply in multipath
channels that destroy code orthogonality
Therefore, in wireless CDMA-systems as in IS-95,cdma2000, WCDMA and IEEE 802.11 users
are asynchronous
d
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FDMA, TDMA and CDMA
yield conceptually the same
capacity
However, in wireless
communications CDMA has
improved capacity due to
statistical multiplexinggraceful degradation
Performance can still beimproved by adaptive antennas,
multiuser detection, FEC, and
l i di