Transcript Spread Spectrum Modulation
Digital Modulation
Two
general
classifications modulation methods: of digital 1)
Linear
: amplitude of Tx signal varies with modulating information signal,
m
(
t
) .
linearly
2)
Constant Envelope
:
non-linear
amplitude of Tx signal held methods where
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 »
f H
= “1” and
f L
= “0” 0 1 0
f L
0 D
f f c
D
f f f H
1
T b
Continuous Phase Transitions ECE 4730: Lecture #15
t
2
Constant Envelope Modulation
BFSK D
f
Binary Frequency Shift Keying = frequency offset from Note that phase between bits
f c
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 signals can be demodulated with Rx’s » Simple and cheap » Unlike BPSK which requires coherent detection
non-coherent
ECE 4730: Lecture #15 3
Constant Envelope Modulation
BFSK Binary Frequency Shift Keying Coherent detection of BFSK can also implemented be » Better BER for same
E b
/
N o
as non-coherent Rx » Never done in practice b/c coherent detection of BPSK has best possible BER vs.
E b
/
N o
Bandwidth » RF BW = where
B B T
= 2 D
f
+ 2
B
(Carson’s rule like FM!) = 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 Fig. 6.38, pg. 317 D
f
/
R b
D
f
= 0.25
MSK RF signal BW » MSK has lower sidelobes than QPSK
R b
–23 dB vs. –10 dB » Larger null-to-null BW than QPSK 1.5
R b
» 99% RF BW
much
better than QPSK (1.2
R b
vs. 1.0
vs. 8.0
R b R b
!!) Very low ACI!!
ECE 4730: Lecture #15 5
FNBWs
MSK vs. QPSK PSD
Sidelobe Levels 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
B G T b
> 0.5
BW bit duration product
B G
= 3 dB filter BW ECE 4730: Lecture #15 8
GMSK Bandwidth
ECE 4730: Lecture #15 9
Constant Envelope Modulation
GMSK RF BW BW as
B G T b
GMSK with effects
if
B
G
but ISI
T b
< 0.5 used with no adverse 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 (
B G T b
= 0.3) ECE 4730: Lecture #15 10
Spread Spectrum Modulation
Spread Spectrum Modulation (SSM) 329-334 Tx expands (spreads) signal BW
many
signal is then collapsed (despread) in Rx Trade BW for signal power like in FM read pgs. times and the Tx PSD
f c
Both have same P
av f
Tx PSD
f f c
ECE 4730: Lecture #15 11
Spread Spectrum Modulation
SSM signal spreading done by multiplying baseband data signal by pseudo-noise (PN) code or sequence
T s
0 1 0 1 0 1 Data Signal
T s =
100 m Data Rate = 10 kbps sec 0 to RF Mod
bw
1 / T s BW
1 / T c
“chip”
T c
0 1 0 1 0 1 0 Spreading Sequence
T c =
Chip Rate = 1 Mcps 1 m sec
f f c
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 equal #’s of 1’s and 0’s noise-like 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 Frequency PSD
t f
d
(t)
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 + + + + + + 1 1 1 1 1 1 1 S = 7 Time shifted by 1 step + + + + + + 0 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 Frequency PSD d
(t) t
Cross-correlation between different similar noise-like properties
f
users’ codes has
S
pread
S
pectrum
M
odulation (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
f c
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 18
Spread Spectrum Modulation
PG
T s T c
R s R c
W ss
where
W ss B
: 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