dr. moshe ran- spread spectrum 1 טכניקות בתקשורת מרחיבת סרט (spread spectrum)...
Post on 19-Dec-2015
224 views
TRANSCRIPT
Dr. Moshe Ran- Spread Spectrum1
טכניקות בתקשורת מרחיבת סרט
(Spread Spectrum) Chapter 2b
"ר משה רןד
. MostlyTek Ltdכל הזכויות שמורות לחברת
אין לצלם, לשכפל או להעתיק בכל צורה שהיא ללא קבלת אישור בכתב מד"ר משה רן
Dr. Moshe Ran / Spread Spectrum2
נושאי לימוד
הרחבת ספקטרום – Spread Spectrumמבוא הסטורי לטכניקות לשם מה? חזרה- מושגי יסוד ועקרונות של מערכות תקשורת ספרתיות; רעשים והפרעות במערכות תקשורת, דרישות מערכתיות על התקשורת,
השוואת שיטות אפנון ספרתיות, יעילות ספקטרלית.
1פרק
: קונספט ( -Spread Spectrum)מבוא למערכות מרחיבות סרט מרחיבות סרט; שיטות הרחבת סרט המבוססות על ומודלים למערכות
(FH)( דילוגים בתדר ( THדילוגים בזמן ((DSהרחבה ישירה
2פרק
3פרק LFSR, Gold Sequence, Walsh - סדרות קוד למערכות מרחיבות סרט
; ביצועים של מערכות עם (DS) ביצועים של מערכות עם הרחבת סרט ישירה Spread ; שיטות גילוי, עקיבה וסנכרון של אותות (FH)דילוגי תדר
Spectrum
4פרק
עם קודים Spread Spectrum קודים לתיקון שגיאות, ביצועים של מערכות Viterbi לתיקון שגיאות, אלגוריתם
5פרק
6פרק בתקשורת תאית CDMAעקרונות
7פרק Spread Spectrumשימושים ואפליקציות של מערכות
שו"ת 8
שו"ת8
שו"ת8
שו"ת8
שו"ת8
שו"ת8
שו"ת4
Dr. Moshe Ran / Spread Spectrum3
Chapter 2b – Frequency- Hop Spread Spectrum
Dr. Moshe Ran / Spread Spectrum4
1. Basics of Frequency Hopping
Method to change the carrier frequency periodically
0 1 2 2, kf f f f
Typically, modulation code is k-bits that select 1-out-2k
frequencies )or frequency bands( which are spaced approximately
1/ hf T
Rx removes the FH by mixing )down-converting( with LO signal which is hopping synchronously with the received signal.
Usually coherent FH is not implemented – since carrier phase estimation is required per each hop.
Dr. Moshe Ran / Spread Spectrum5
1.1 Classification of Frequency Hopping Systems
Academic Classification
Slow Frequency Hopping
In every hop there are more than one symbol
Fast Frequency Hopping
In every hop there is one symbol or less
Industrial Classification
Slow Frequency Hopping: Less then 50hps
Medium Frequency Hopping: 50hps to 500hps
Fast Frequency Hopping: More then 500hps
h ST T
h ST T
1/h hR T
Dr. Moshe Ran / Spread Spectrum6
2.1 Coherent slow FH Spread Spectrum
Tx implementation for Coherent slow FH Spread Spectrum
( )d t
Code
GENERATOR
Data
Modulator
( )ds t
Frequency
synthesizer
nf
Bandpass
filter( )Th t
1 2 3 k
FH code clock
( )ts t
02 cos(2 )P f t
NRZ data
Data modulated carrier
Dr. Moshe Ran / Spread Spectrum7
Coherent slow FH Spread Spectrum Receiver
nd
Code
GENERATOR
Image reject
filter
( )ds t
Frequency
synthesizer
nf
Bandpass
filter( )Rh t
1 2 3 k
FH code clock
Data demod
( )y t
Dr. Moshe Ran / Spread Spectrum8
Analysis of Coherent slow FH Spread Spectrum
Output of freq. synthesizer at Tx is the “hop carrier” hT(t): bandpass signal with random sequence of tones fn, of duration Th
( ) 2 ( )cos(2 )T h n nn
h t p t nT f t
1 0,
0 otherwiseh
hT
t Tp t
, 1, 2, , 2kn m m
Coherent assumption: the same phase is used each time m
( )Th t returns to the same frequency mf
)1(
Dr. Moshe Ran / Spread Spectrum9
PSD of transmitted slow FH
Analysis of Coherent slow FH Spread Spectrum – cont.
( ) ( ) ( )t d hS f S f S f
is the convolution of the two signals
The transmitted signal is the data-modulated carrier up-converted to a new frequency f0+fn for each FH hop
( ) ( ) ( )t d Ts t s t h t
22
21
2 2 22
' '1 1 ' 1
''
1( ) ( )
1 2(1 ) ( ) ( ) ( )
k
k k k
h m mn m h hh
m m m m m m mm m mh h
m mm m
n nS f p G f
T TT
p p G f p p e G f G fT T
)2(
)3(
)4(
Dr. Moshe Ran / Spread Spectrum10
Notations:
Analysis of Coherent slow FH Spread Spectrum – cont.
2 ( )cos( ) 00 elsewhere
= Pr
( ) exp [ ( ) ] [( ) ]
exp [ ( ) ] [( ) ]
( )
m m
m h m h m m h
h m h m m h
m m hp t t t Tm
p f f
G f T j f f T Sinc f f T
T j f f T Sinc f f T
g t
PSD Sh(f) can be simplified by assuming
m'
'
( ) and G ( ) are nonoverlapping for m m'
( ) ( ) 0
m
m m
G f f
G f G f
This assumption is valid whenever 1/Th is small compared to the minimum frequency spacing i.e. Slow FH
)5(
)6(
)7(
Dr. Moshe Ran / Spread Spectrum11
Simplified PSD – further assume
Analysis of Coherent slow FH Spread Spectrum – cont.
22
21
22
1
1( )
( 2 )
1 1 11 ( )
2 2
k
k
h mkn mh h h
mk kmh
n nS f G f
T T T
G fT
1,
2m kp m
Note: PSD has discrete components, due to the “coherent FH assumption”. The same phase is used each time hT(t) returns to frequency fm . If
2 1khT
Then - the discrete components are negligible.
)8(
Dr. Moshe Ran / Spread Spectrum12
Example:
Calculate the PSD of a transmitted coherent slow FH signal with the following parameters.Modulation: BPSK, Rb=1Mbps, Rh=100Khops/sec, 4 frequencies are employed, minimum spacing equals data rate.
Solution:
Since minimum spacing >> hop rate
we can use simplified PSD formula.510 , 2 4, ( ) given by (7)kh mT G f
22 2
21
22 2
1
1( ) { [( )] [( )]} ( )
2
11 [( ) ] [( ) ]}
2 2
k
k
h m h m hkn m h
hm h m hk k
m
nS f Sinc n f T Sinc n f T f
T
TSinc f f T Sinc f f T
Dr. Moshe Ran / Spread Spectrum13
Receiver for slow FH
The received signal
0 0
( )
2 ( ) cos[( ) ( ) ( ) ]
t d
d h n n d d n dn
s t T
P p t T nT t t T T
This signal is down converted using hR)t(
ˆ ˆ( ) 2 ( )cos(2 )R d h n n n dn
h t p t T nT f t T
)9(
)10(
Assuming d̂ dT T No tracking errors
( ) ( ) ( )t d R bpfy t s t T h t
Dr. Moshe Ran / Spread Spectrum14
Receiver for slow FH
0
0 0
( ) ( ) ( )
2 ( )cos[ ( ) ( )]
2 cos[ ( )]
t d R bpf
d h d d dn
d d d
y t s t T h t
P p t T nT t T t T
P t T t T
d̂ dT TUsually- there are tracking errors and
Thus – the recovered carrier is phase modulated by terms with the form:
ˆ( )d d nnT T
)11(
Recovered data-modulated carrier
)12(
Dr. Moshe Ran / Spread Spectrum15
Dealing with tracking errors in slow FH:
Need means for coherent carrier tracking, independent of the FH tracking loop.
Possible way: estimating Rx phase every hop – “feed forward carrier phase estimation” using
known sync. Word– Need to protect sync. word to avoid hostile jammer
)I.e., by changing periodically the sync word(
Dr. Moshe Ran / Spread Spectrum16
Non coherent slow FH
Coherent FH are complex to implement– Non-coherent – Differentially coherent
When freq. synthesizer phase is random for each successive time interval, PSD of hT(t)
2
2 2
1
( ) [( ) ] [( )2
k
hh m h m hk
m
TS f Sinc f f T Sinc f f T
)13(
Dr. Moshe Ran / Spread Spectrum17
Typical Example of Slow non coherent FH:
Data modulation: 2L-ary FSK Each Ts=LTb sec. the modulator outputs 1-out-2L
tones
Spacing between the tones 1/LTb
Bd, Bandwidth of modulated signal: Bd = 2L/LTb
Each Th the data modulated signal is translated to a new frequency by a FH modulator, 2k
frequency bands of wide Bd
Total BW of system: Bss=2kBd
Th > LTb slow FH condition
Plot this for k=3 and L=2 .
That is 4-FSK and 8 bands for the FH signals
Dr. Moshe Ran / Spread Spectrum18
Non Coherent fast FH
Here , sh s b
h
TT T LT K T
o Hop-bands can change many times per symbol
o Data modulator can operate in different modes with different complexities. Can use the K hops per symbol based on majority vote or Maximum Likelihood sequence estimation.
Benefits: frequency diversity gain on each transmitted symbol
o Partial-band jammer
o improving performance in “fast fading” multipath channel
Dr. Moshe Ran / Spread Spectrum19
Typical Example of Fast non coherent FH:
Data modulation: 2L-ary FSK Each Th=(1/K) * Ts sec. the modulator outputs 1-out-2L
tones. That is – the tones are subdivided into K “chips”
Spacing between the tones 1/Th = K / LTb =K / Ts
Bd, Bandwidth of modulated signal: Bd = 2L / Th= K2L / LTb
Each Th the data modulated signal is translated to a new frequency by a FH modulator, 2k
frequency bands of wide Bd
Total BW of system: Bss=2kBd
Th < LTb fast FH condition
Plot this for L=K=k = 2 .
That is 4-FSK and 4 bands for the FH signals
Dr. Moshe Ran / Spread Spectrum20
Combined DS-FH spread spectrum transmitter + DPSK data modulator
Code
GENERATOR
( )dss t
Frequency
synthesizer
nf
Bandpass
filter( )Th t
1 2 3 k
FH code clock
( )ts t
( )d tDifferential
encoder
02 cos(2 )P f t
NRZ data
Data modulated carrier
( )c t
'( )d t
Dr. Moshe Ran / Spread Spectrum21
Combined DS-FH spread spectrum receiver + DPSK data demodulator
Code
GENERATOR
BP
filter
( )ds t
Frequency
synthesizer
nf
Bandpass
filter( )Rh t
1 2 3 k
FH code clock
nd
DPSK demod
( )y t
Bandpass
filter
ˆ( )dc t T
02cos[( ) ]IF t