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1. Spread Spectrum & CDMA
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n a app ca on: m ary
Benefitsn - amm ng
Robust to multipath fading
Multi le user access CDMA
High-resolution ranging (DS)
3
DATAd(t) r(t) r'(t) d'(t) Ts
Source
0
)( fSd
1/Ts
c c
d(t)T
s
)( fSc
)( fSr
1/Tcc(t)
)(' fSd
=r'(t)
d'(t)
4
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DS/SS makes noise-like waveforms
Maximal-length shift register makes the binary sequence thathave noise like properties
PN sequence
PN sequence is mapped to a chip spreading sequence of1s
The spreading comprises of chips of duration Tc
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Receiver: when the receiver knows the correct spreading sequence
Po
wer
De
nsity
received signalTIME
spreading sequence(spreading code)
10110100
0100101110110100 10110100
RadioFrequency
you can find thespreading timingwhich ives themaximum detectedpower, and
Accumulate for
gathering energy !
1011010010110100 10110100
one ura on
Demodulated data
1111111100000000 00000000
Base-bandFrequency
0 01
7
Receiver: when the receiver does not know the correct s readin se uence
received signalPower
Density
TIME
spreading sequence(spreading code) 01010101 01010101 01010101
0100101110110100 10110100
you cannot findthe spreadingtimin
a oFrequency
10101010 10101010 10101010
without correctspreading code,and
1011010010110100 10110100
Accumulate for
one bit duration
No data can be detected
- --
Base-bandFrequency
Demodulated data
8
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9
-
form of frequency diversity)
- If the maximal delay spread (due to multi-path) is Tm and if the
chip rate 1/Tc = W>> 1/Tm, then individual multi-path signal
components can be isolated
Amplitudes and phases of the multi-path components are
found by correlating the received waveform with delayed
versions of the signal Multi- ath with dela s less than 1/T cannot be resolved
10
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The carrier frequency is pseudo randomly hopped over
Slow hopping: Tsymbol < Thopping
Fast ho in : T > T
11
d t
DATA Mod Filter BPF BPF
Data
demod
Freq. Synthesizer Freq. Synthesizer
....Code Generator
....Code Generator
* Noncoherent detection is common
12
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Allows multi le users to share same bandwidth at the same
time
process
Matched filter pulls out desired users waveform, while
DS-SS is one popular way to make the noise-like waveforms
13
Freq.Freq.Freq.Freq.
BPFDespreader
Code A
Data A
Code A
BPF
MS-A
Data A
Freq.Freq.Freq.Freq.
BPFDespreader
Code B
Data B BPF
MS-B
Data B
BS
14
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Advanta es
Universal frequency reuse
Soft capacity
Soft handoff
Robust to multipath fading
o us o amm ng
Disadvantages
ear- ar pro em
Difficult synchronization
15
Objectives of a Wireless communication system
Deliver desired signal to a designated receiver
Minimize the interference that it receives
One way is to use disjoint slots in frequency or time in the same
cell as well as adjacent cells limited frequency reuse
In CDMA, universal frequency reuse (frequency reuse factor = 1)
app es no on y o users n e same ce u a so n a o er ce s
No frequency plan revision as more cells are added
esource a ocat on o eac user s c anne s energy nstea o
time and frequency)
16
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Handoffs between cells are supported while the mobile is in
traffic or idle MS continuously keeps searching for new cells as it moves across
the network
Target cell search
MS maintains active set, neighbor set, and remaining set as well
as candidate set
Two types of handoffs Hard handoff
Soft/softer handoff
17
margin exceeds
Base A
T_ADDBase B
T_DROP
B_Active
candidate list
starts Drop timerresets
18
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o an o
Mobile commences communication with a new BS without interrupting
communication with old BS (make-before-break)
Same frequency assignment between old and new BSs
Provides different site selection diversity, called macro diversity
Neither the mobile nor the base station is required to change frequency
Handoff between sectors in a cell
19
-
P
CDMADATA A
Lp-a
CDMAReceiverCODE A
Demodulated DATA
P Lp-b
CDMADATA B
CODE A
Desired signal power = P/Lp-a
Transmitter
When user B is close to the receiver and user A is
Interference power = P/Lp-b/PG
far from the receiver, Lp-a could be much bigger
than Lp-b. In this case, desired signal power is
smaller than the interfered ower.
20
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Overcomes near-far problem
CDMA would not work without it
Copes with path loss and fading
tedPower
from A
from B
Time
Dete
21
Power control is capable of compensating different path loss and
fading fluctuation
ac c anges transm t power ynam ca y so t at t e rece ved power at the BS from all MSs is controlled to be equal
dPower
from MS Bfrom MS A
Time
Detecte
22
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pen-Loop Power ontro ose -Loop Power ontro
transmitmeasuringreceived power decidetransmission
power
power controlcommand
est mat ng patloss about 1000 times
per second
transmission
power
transm t
received power
transmit receive
23
2. OFDM
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Multicarrier modulation/multi lexin techni ue
Available bandwidth is divided into several subcarriers
The subcarriers are overla in but ortho onal
Parallel data transmission
A high rate stream is partitioned into several low rate streams
25
Each subcarrier has exactly an integer number of cycles
Adjacent subcarriers have exactly one cycle difference
Transmit signal1
2( ) exp , 0
sN
k
ks t d j t t T
= 0
where : number of subcarriers
: transmit symbol for the th subcarrier
k
s
k
N
d k
=
Note:
: sym o urat on
0
2 2exp exp ( )
T kj t j t dt k
T T
=
ll
Subcarriers in time
( )01 2
exp
T
s t j t dt d T T
= l
l
1
Subcarriers in frequency
u carr er spac ng:T
26
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ransm tter an rece ver arc tecture
27
gna transm ss on an recept on
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2 2
2
1
( ) , 0
Ns
j it
TNs
Nsi
S t d e t T
++=
2
21
0
[ ] ( )s j in
N
N
i
i
S n S nT N d e
=
+
=
= = T
0
s
2 2 2
2 2 2 2
0 1 2
0 1 2
2
[0]
[1]j j
N N
j j
N N
S d d d
S d d e d e
= + +
= + + + L
L0 1 2
0
d
( )0S
1sND
IDFT P/S
( 1sS N
1d (1S
29
30
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Guard time
The guard time is chosen larger than the expected delay spread, so that
the delayed symbol cannot interfere with the next symbol (RemoveISI
Zero insertion or cyclic prefix
31
Preserve orthogonality between subcarriers, once the largest
multipath delay is less than the duration of the cyclic prefix
No nter-carr er nter erence (I I) Even with long enough cyclic prefix, ICI can occur due to
0
2 2 ( ) (1 exp( 2 ))exp exp
T k k m T jj t j t dt
+ + = where denotes the normalized frequency offset
32
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FCC manages spectrum
Specifies power spectral density mask
Adjacent channel interference
Roll-off requirements
Implications to OFDM
Zero tones on edge of the band
Time domain windowing smoothes adjacent symbols
33
Shar si nal transitions at the OFDM s mbol boundariescan cause significant out-of-band emission
Windowing smoothens the transitions, making the power
spectrum decay faster Raised cosine window
0.5 0.5cos( ( ) ), 0 1
[ ] 1.0, (1 ) 1
n N N n N
w n N n N
+ +
= + . . ,
SYM sampleN T T
TT T
(1 )SYM SYM T T = +
time
34
SYM SYM
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Data (48) Pilots (4) Nulls (12)
Subcarrier Index (-32 ~ 31)
-25 -20 -15 -10 -5 -1-30 0 5 10 15 20 25 261-32 317 21-7-21-26 30
0 1 2 3 61 62 63NU
25 26 27 28NU
NU
36 37 38 39NU
NU
IFFT Block
LL
61 62 630 1 2 3
LL
LL
LL
LL
25 26 27 28 36 37 38 39
r0
r1
r2
r3
r4
r5
r62
r63
r0
r1
r56
r62
r63
Prefix Postfix
r2
r3
r54
r55
OFDM Symbol
Windowing Function
21 7 7 211, 1, 1, 1p p p p = = = = Pilot Symbols:
(the same as 802.11a)
35
36
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Advanta es
Spectrally efficient
Conveniently implemented using IFFT and FFT
Easy to handle frequency-selective fading channel (wideband
transmission)
sa van ages
More complex than single carrier transmission
Long symbol duration vulnerable to frequency offset and fast fading
37
Complexity
Single-carrier systems need equalizer when delay spread over
FFT does not need full multiplication but rather phase rotation
The complexity in OFDM grows slightly faster than linear
Robustness
Single-carrier system performance degrades abruptly when
delay spread exceeds the value for which the equalizer is
designed
OFDM s stems are robust a ainst dela s read
38
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Number of subcarriers
Guard time > delay spread
39
Carrier frequency: 900 MHz
Signal bandwidth 5 MHz
Channel characterization Frequency selectivity
RMS delay spread < 100 msec
90% coherence bandwidth (CB) = 1/(50m) = 209.2 kHz Time selectivity
Assumption for user mobility: 0-3 km/hr (maximum Doppler frequency
(fm) = 2.5 Hz at the carrier frequency of 900 MHz)
50% coherence time C = / 16 = 71.62 msec m
40
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su carr er spac ng s ou mee e o ow ng
relationship:1/CT= 13.9 Hz < f< CB = 209.2 kHz,
which is required for an OFDM system not to be vulnerable to both
frequency-selective and time-selective fading
We select a subcarrier spacing of 72.27 kHz
Set the number of subcarriers to 64
Total bandwidth = 72.27 kHz 64 = 4.625 MHz
41
Parameter Value
Total bandwidth (W) 4.625 MHz
Total number of subcarriers (NT) 64
Number data subcarriers (ND
) 48
Number pilot subcarriers (NP) 4
Number of guard or null subcarriers (NG) 12
Subcarrier frequency spacing (F) 72.27 kHz
IFFT/FFT period (TFFT
) 13.838 sec (64 samples)
uar n erva ura onGI
. sec samp es
OFDM symbol duration (TSYM
) 16.0 sec (TFFT+ TGI)
42
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Given sk, k pilot tone indices, solve for hl Find
2
0
kL jN
k k ky h e s n
=
= +l
l
l
2 kLj N
kH h e
= l
l
Interpolation for all subcarriers
More pilot tones give
0=l
Better noise resilience
Lower throughput
43
-
Frequency domain equalizer (FDE)
-
k k k k
Noise enhancement factor
Use MMSE to reduce noise enhancement
44
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Different channel fadin for different subcarriers
Bad subcarriers will cause many errors (channel-selective
errors)
Two approaches
Error correction coding across subcarriers
Adaptive modulation & coding and/or unequal power allocation
Coding across subcarriers realizes frequency diversity gain as
well as the coding gain
45
Two types of errors
Random errors: primarily caused by noise
Channel-selective errors: caused by magnitude distortion in channel
frequency response
Error correcting codes are effective for random errors
Interleaving is often used to scramble data bits so that standard
error correcting codes can be applied Interleaving and coding provide frequency diversity as well as the
46
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Transmitter Receiver
47
Multiple transmit antennas are separated geographically
Enables same radio/TV channel frequency throughout a country
Creates artificially large delay spread No problem in OFDM
48
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-
Effective to handle large bandwidth for high data rate
Takes advantage of multipaths through simple equalization
Resource allocation (AMC, power, etc)
Easy combination with MIMO
49
-
50
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. ransm vers y
MIMO
Fading
Fluctuation of received SNR
Diversity Reduce the fluctuation of received SNR
, , ,
Antenna diversity
, ,
Transmit diversity
52
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Space-Time Transmit Diversity (STTD)
Ortho onal Transmit Diversit (OTD
Time-Switched Transmit Diversity (TSTD)
Transmit Antenna Array (TxAA)
Classification
Open-loop
STTD, OTD, TSTD
Closed-loop Need feedback information
TxAA
53
-
P
2 h Alamouti STC
STTDEncoder
STTDDecoderh
1x 2x
0 T 2T
1x*
2x
0 T 2T
2x*
1x
0 T 2T
P
2
*
1 2*
x x Ant. 1Time 1 Time 2
( )1 1 1 2 2 12* *
Pr h x h x n= + +
2 1x xAnt. 2
STTD Decoder:2 2* *
2 1 2 2 1 22r x x n= + +
( )
1 1 1 2 2 1 2 1 12
2 2* *
2 1 2 2 1 1 2 2 22 P
x r r x
x r h r h h h x
= + = + +
= = + +
Diversity without BW
expansion!
54
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.
Ph1
2 h1
Source ReceiverEncoder Decoderh
2
2
2 P Tx. Power = P Tx. Power = P
1 : +h h2
1 2 SNR:
P 02
Avg. SNR: P 02
Avg. SNR:
z Average SNR gain = 0
z Diversity order = 2
z Average SNR gain = 0
z Diversit order = 1
55
P
2 h1
OTD
EncoderOTD
1x 2x
1x
0 T 2T
1x
20 T 2T
2x0 T 2T
P
2x
2
( )1 1 1 2 2 12Pr h x h x n= + +
Received Signal: OTD Encoder:
1 1x x Ant. 1Time 1 Time 2
( )2 1 1 2 2 22Pr h x h x n= +
OTD Decoder:
2 2x x Ant. 2
Different s mbol different fadin
( )( )
2*
1 1 1 2 1 1 1
2*
2 2 1 2 2 2 2
2
2
x h r r P h x
x h r r P h x
= + = += = +
Interleaving effects
Interleaving depth (diversity order) = 2
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-
P h1TSTD
TSTD1x 2x
1x
0 T 2T
0
20 T 2T
2x
0 T 2T
0
( )1 1 1 1r P h x n= + Received Signal: TSTD Encoder:
1 0x Ant. 1Time 1 Time 2
( )2 2 2 2r P h x n= +
TSTD Decoder: Different s mbol different fadin
20 x Ant. 2
2*
1 1 1 1 1 1
2*
2 2 2 2 2 2
x h r P h x
x h r P h x
= = +
= = +
Interleaving effects
Interleaving depth (diversity order) = 2
57
h1
Coded
w1 hm
Weight
Channelization &Scrambling Code
wm
Mi
wM Feedback
wh
mm=*
r w h x nm m
M
=F I
+
= hmm
M2
1
hii=
2
1
x h n
m
m
M
= +
=
1
2z Diversity order Mz Avera e SNR ain M
m=1
58
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10-1
No. of Tx antennas=1
No. of Tx antennas=2
No. of Tx antennas=4No. of Tx antennas=8
-3
10-2
edBER
10-4
Unco
-6 -4 -2 0 2 4 6 8 10 12 14 16 1810
-5
Eb/No (dB)
z 4 tx antennas: 5.2 dB gain at BER 10-2 compared with 2 antennas
z 8 tx antennas: 9.2dB gain
59
multiple receive antennas
Single I nput Single Output
Multiple I nput Single Output
Multiple I nput Multiple Output
s gna process ng n e space oma n can r ng
enormous capacity enhancement without bandwidth expansion,
60
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Mantennas Nantennas
RxTx
1x
1y
11 12 1
21 22 2
M
M
H H H
H H H
=H
L
L
Mx Ny
Received signal for flat fading channels
1 2N N NMH H H
M M O M
L
Total transmit powerE[xHx] = PT Noise covariance matrix at the receiver EnnH = 2I
= +y Hx n
Assumptions Receiver perfectly knows the channel H
-
61
General ca acit formula
2 2log det HT
PC
M
= +
I HH
SISO:M=N= 12
2 112log 1 T
SISO
PC H
= +
SIMO:M= 1,N 2
2NP
MISO:M 2,N= 1
2 121
ogSIMO nn =
= +
Logarithmical increase withM
SNRMISO = SNRSIMO/M
2
2 121
log 1
MT
MISO m
m
P
C HM =
= +
62
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MIMO:M 2,N 2
2 2
rank( )
log det HTMIMO
K
PC
M
P P
= +
H
I HH
where K= min MN and
2 22 21 1
og ogk k
k kM M = == + = +
ks are eigenvalues ofHHH that is ordered such that1 2 rank(H) rank(H)+1 = = K= 0
Interpretation:
12
rank(H)
Tx Rx
SISO channels
Capacity increases linearly with rank(H) at high SNR63
-
,
transformed into parallel SISO channels2 H
( : eigenvalues of )H=H UDV D HH
Maximize the capacity( )H H= + = +y U HVx n Y Dx U n
1s
HTn Rn
1
~S
min( , )
2
1min( , )
log (1 ),T R
T R
n n
k k
kn n
C p =
= +
VDecouplingTransform
kp
ksUH
DecouplingTransform k
S~1
kk
p P=
1 1
Tnp
Tns
TnS~,
0 elsewhere
k o
o kkp =
64
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i.i.d. Rayleigh channel: full rank
65
Linear detectors
Zero-Forcing (ZF)
Minimum Mean Square Error (MMSE)
Nonlinear detectors
ZF-OSIC, MMSE-OSIC (V-BLAST)
ML
Reduced-complexity ML (sphere decoding, QRM-MLD)
66
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ece ve s gna mo e
1 1 1N N M M N = +y H x n ZF
1 1 ( ) ( )H H H+ = = = +x H y H H H y x HH H n
MMSE12
arg min ( )H HMMSE M
E M = = +W x Wy H H I H
ML
( )1
( )H HMMSE M
M
= = +W
x W y H H I H y
2 arg min
c
c=
x
x y Hx
67
- -
:
1
Initialization
i
1
12
1
arg mink
+
+
==
= < >
y y
G H
H
:
i ik i k
Recursion
=< >=
w G
w
1
( )
[ ]
i i
i i
i i
k k
i i k k
x D z
x++
== y y H
1 2
1
2
1{ , , , }
arg min
i
i
i
i k
i jkj k k k
k
+
+
=
= < >H
+
68
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V-BLAST Exam le (3x2 s stem)
1 1
12 2
1 0.5
: 0.2 0.3
y n
xy n
= + = + y Hx n
First layer
2
3 30.2 0.7y n
1
. . .
0.2976 0.4762 1.0119
+ = = G H
2
= = =1 . , .jj j
[ ]1 1
0.8433 0.3175 0.4663k k=< > = w G
1 1 1 1 1
1
1 1
2
k k k k k
xz x
x= = + = +
w y w H w n w n
= =
69
1 11 1 1k k
Second layer
0.5
1 1 1 1
2 2. ,
0.7
0.5
k k k k
+
[ ]1
2[ ] 0.3 0.6024 0.3614 0.8434
0.7
k
+ = = =
G H
[ ]2
2
2
2
0.6024 0.3614 0.8434k
k =
= = w G
2 2 2 2
2 2 2 2
2
( ) ( )
k k k k
k k k k x D z D x
= =
= = + w n
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71
wireless systems
channel linear increase in capacity in contrast to
ogar m ca ncrease or an
Greatest capacity improvements are obtained under rich
scattering channels with full rank