Spread Spectrum Modulation

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Digital Modulation
 Two
general classifications
modulation methods:
of
digital
1) Linear : amplitude of Tx signal varies linearly
with modulating information signal, m(t).
2) Constant Envelope : non-linear methods where
amplitude of Tx signal held constant regardless of
the variation in the modulating information signal,
m(t).
ECE 4730: Lecture #15
1
Constant Envelope Modulation
 BFSK
 Binary Frequency Shift Keying
 Frequency
of constant amplitude carrier shifted
between two possible states
» fH = “1” and fL = “0”
0
Df
fL
0
1
0
Df
fc
t
fH
1
f
Tb
Continuous Phase Transitions
ECE 4730: Lecture #15
2
Constant Envelope Modulation

BFSK  Binary Frequency Shift Keying
 Df = frequency offset from fc
 Note that phase between bits can be continuous
» No discontinuity  constant envelope retained !!
 Discontinuous
phase  can be allowed but leads to
envelope variations in bandlimited system and spectral
broadening when non-linear amplifiers are used
 BFSK
Rx’s
signals can be demodulated with non-coherent
» Simple and cheap
» Unlike BPSK which requires coherent detection
ECE 4730: Lecture #15
3
Constant Envelope Modulation
 BFSK
 Binary Frequency Shift Keying
 Coherent
detection
implemented
of
BFSK
can
also
be
» Better BER for same Eb / No as non-coherent Rx
» Never done in practice b/c coherent detection of BPSK
has best possible BER vs. Eb / No
 Bandwidth
» RF BW = BT = 2 Df + 2 B (Carson’s rule like FM!)
where B = baseband BW (single null)
ECE 4730: Lecture #15
4
Constant Envelope Modulation

MSK  Minimum Shift Keying
 Specific type of continuous phase (CP) FSK
 Choose minimum allowed frequency spacing
such that
high & low FSK tones are orthogonal
» Orthogonal  no ISI due to demodulation (other ISI still present)
» Modulation index = 0.5 = 2 Df / Rb  Df = 0.25 Rb
 Fig. 6.38, pg. 317
 MSK RF signal BW
» MSK has lower sidelobes than QPSK  –23 dB vs. –10 dB
» Larger null-to-null BW than QPSK  1.5 Rb vs. 1.0 Rb
» 99% RF BW much better than QPSK (1.2 Rb vs. 8.0 Rb !!)
 Very low ACI!!
ECE 4730: Lecture #15
5
MSK vs. QPSK PSD
Sidelobe
Levels
FNBWs
ECE 4730: Lecture #15
6
Constant Envelope Modulation
 MSK
 Minimum Shift Keying
 Constant
envelope achieved due to continuous
phase
» DC/RF efficient non-linear Tx amplifiers (Class C)
» Long battery life for mobile units
 Non-coherent detection
» Simple & inexpensive Rx’s
 Very popular modulation scheme for mobile radio
ECE 4730: Lecture #15
7
Constant Envelope Modulation
 GMSK
 Gaussian MSK
 Spectral efficiency of MSK further enhanced using
baseband Gaussian pulse-shaping filter
» Reduce signal BW
 Gaussian filter will introduce some ISI
» Does NOT satisfy Nyquist criterion
 ISI not severe if BG Tb > 0.5
 BWbit duration product  BG = 3 dB filter BW
ECE 4730: Lecture #15
8
GMSK Bandwidth
ECE 4730: Lecture #15
9
Constant Envelope Modulation
 GMSK
RF BW
 BW  as BG Tb  but ISI 
 GMSK with BG Tb < 0.5
used with no adverse
effects if BER < irreducible MRC BER
 MRC BER caused by multipath delay + mobile
velocity
 BER floor inherent in MRC
 T-Mobile, Cingular, and AT&T Wireless all used
GSM standard  0.3 GMSK (BG Tb = 0.3)
ECE 4730: Lecture #15
10
Spread Spectrum Modulation

Spread Spectrum Modulation (SSM)  read pgs.
329-334
 Tx

expands (spreads) signal BW many times and the
signal is then collapsed (despread) in Rx
Trade BW for signal power like in FM
Tx PSD
Both have
same Pav
Tx PSD
f
fc
f
fc
ECE 4730: Lecture #15
11
Spread Spectrum Modulation

SSM signal spreading done by multiplying baseband
data signal by pseudo-noise (PN) code or sequence
Ts
0
1
0
1
0
1
to RF Mod
0
Data Signal Ts = 100 msec
Data Rate = 10 kbps
“chip”
bw  1 / Ts
Tc
0
1
0
1
0
1
0
Spreading Sequence Tc = 1 msec
Chip Rate = 1 Mcps
BW  1 / Tc
f
fc
ECE 4730: Lecture #15
12
Spread Spectrum Modulation

SSM Advantages
1) Combats multipath fading  no equalization needed
2) Resistant to narrowband interference
3) Allows multiple users with different codes to share
same MRC
 No frequency reuse!!
4) As # simultaneous users  the bandwidth efficiency 
ECE 4730: Lecture #15
13
Spread Spectrum Modulation

Pseudo-random Noise (PN) Codes
 Code Division Multiple Access (CDMA)
» Mobiles users share spectrum using codes
 Binary sequence with random properties  noise-like
  equal #’s of 1’s and 0’s
 Very low correlation between time-shifted versions of

same sequence (high-correlation at exact time overlap)
Very low cross-correlation between different codes
» Each user assigned unique code
» Other user’s signal appears (approximately) like random noise!
 White noise properties
ECE 4730: Lecture #15
14
Spread Spectrum Modulation

White noise properties
Autocorrelation
in Time
d(t)

t
Frequency
PSD
f
 Delta Function  Autocorrelation in Time
 Flat PSD in Frequency  equal amount of energy at all
frequencies
ECE 4730: Lecture #15
15
Spread Spectrum Modulation

PN Spreading Codes
Example: 0 0 0 1 1 0 1
   + +  +
 let “0” =  & “1” = +
Matched
   + +  +
   + +  +
Time shifted by 1 step
   + +  +
   + +  +
1
0
1
1
1
1
1
1
 S=7
1
1
-1
1
-1
-1 0
 S=0!
Uncorrelated !!
ECE 4730: Lecture #15
16
Spread Spectrum Modulation

Auto-correlation of PN code  noise-like!

Time
d(t)
t
Frequency
PSD
f
 Cross-correlation
between different users’ codes has
similar noise-like properties
 Spread Spectrum Modulation (SSM) must be used with
PSK or FSK to encode data bits
ECE 4730: Lecture #15
17
Spread Spectrum Modulation

Two types of SSM
1) Direct Sequence (DS)  used with PSK
» Multiply baseband data by PN code (same as diagram above)
2) Frequency Hopping (FH)  used with FSK
» Randomly change fc with time

Processing Gain = PG
 SSM resistant to narrowband interfering signals
 Narrowband interfering signal converted to
wideband
energy in SS Rx after despreading + LPF

Fig. 6.50, pg. 333
ECE 4730: Lecture #15
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Spread Spectrum Modulation
PG 
Ts
Tc

Rs
Rc

Wss
B
where Wss : SS BW and B : signal BW
ECE 4730: Lecture #15
19
Spread Spectrum Modulation

Sprint PCS and Verizon Wireless
 Both used 2G DS-SSM (CDMA) technology
 Sprint PCS  first nationwide deployment
of 2G CDMA
system in the world in 1998-99

Main disadvantage of DS-SSM is that perfect power
control of mobiles is required to maximize capacity
 Near/far
problem where one mobile unit can dominate
base station Rx thereby wiping out other users!!
ECE 4730: Lecture #15
20
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