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Dip. Ingegneria dell’Informazione, Univ. Pisa, Pisa, Italy
Basics of 3G Communications
Giacomo Bacci, Marco Luise marco.luise@unipi.it
Computer Engineering
Electronics and Communications Systems
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
3G systems
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
3G systems: The need for more throughput
3G systems
o When GSM was designed (late 1980s), voice services were the main
(only?) focus for mobile networks
o GSM extensions (GPRS and EDGE) improved the bitrates, but the
demand (late 1990s) was growing larger and larger
o The only way to meet the rate demands was to increase the
bandwidth:
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Giacomo Bacci, Marco Luise
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Baseband Data (PAM) Signal
s(t) ak p(t kT )k
+A
-A
a0 a1 a2 a3 a4
T
Binary Pulse-Amplitude Modulation (PAM)
Symbol Interval: T = Bit Interval: Tb
Symbol Rate: R=1/T = Bit Rate: Rb=1/Tb
{ak}=+1 Binary Data
Basic Pulse p(t):
T
A
p(t)
t
s(t)
Bipolar Format
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Power Spectral Density (PSD) of the PAM Signal
Ss ( f ) A2
TP( f )
2
T
A
p(t)
t
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Normalized Frequency, fT
A=1
-60
-50
-40
-30
-20
-10
0
10
6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Normalized Frequency, fT
-13.3 dB
No
rm.P
SD
Ss(f
)/T
(d
B)
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Band-Limited Signaling
Pulse-Shaping Filter p(t)
Energy of Square-root Raised Cosine (SRC) pulse
ak s(t)
Average power of PAM signal
Ep | p(t) |2
dt
| P( f ) |2
df
T
Ps Ep /T 1
T cos2 ( f (1 ) / 2T)
4 / T
=roll-off factor
f
P(f)
T
Nyquist
Frequency
1/2T
Roll-off
Region
/T
Bandwidth
(1+ )/2T
(1-)/2T
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Baseband Data Detection with Noise
p(t)
AWGN w(t)
Clock Sync
Matched Filter
ak ak ^
s(t) r(t) rk
kT
h(t)
h(t) 1
Tp(t)
Matched Fiter Received Signal
r(t) s(t) w(t) h(t) ai
i
g(t iT) n(t )
g( t) p(t) h(t) is the basic received pulse
White, noise,
PSD N0/2
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Bandpass (modulated) Signal
0
0 0
0 0
( )cos( ( ))
( ) ( )cos(2 ( ))
c
( )
os(2 ) sin(2 )
c (
(
os(2 ) sin(2 )
)sin(
)
( ))
BP
QI
A t
x t A t f t t
f t f t
f t f t
A t t
x t x t
t
f
XBP(f)
f0f0 0
BB
XBP(f)
XBB(f)
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
The I/Q Modulator
0 0( ) cos(2 ) sin(2 )( ( ))IBP Qx x t x tt f t f t
xI(t)
I/Q
Carrier
Generator
xQ(t)
xBP(t)
-sin(2f0t)
cos(2f0t)
Two signals are “multiplexed” on the same carrier
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
X(f)
f0 f-f0
Recovering the Baseband Signals 1/2
X(f+f )
f0 f-f0
0
2X(f+f )H(f)
f0 f-f0
0
B
H(f)
02( ) 2 ( ) ( )
j f t
BPx t x t e h t
( )BPx t
02( )
j f t
BPx t e
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Recovering the Baseband Signals 2/2
02( ) 2 ( ) ( ) ( ) ( ) ( )exp( ( ))
j f t
BP I Qx t x t e h t x t jx t A t j t
The complex “baseband equivalent”
02
00cos(2 si( ) ( ) ( ) n() ( )) 2
j f t
BP I Qx t x t e x ft t fx tt
I-carrier Q-carrier
xI
xQ
A
xBB
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
The I-Q Demodulator
• Allows to recover I/Q components from bandpass modulated signal • B is the modulation bandwidth
02( ) 2 ( ) ( ) ( ) ( )
j f t
I Qx t x t e h t x t jx t
xI(t)
I/Q
Carrier
Generator
xQ(t)
xBP(t)
-2sin(2f0t)
2cos(2f0t)
hLP(t)
hLP(t)
Bandwidth B
Bandwidth B
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
BPSK and QPSK Modulations
k-th signaling interval kT≤t<(k+1)T
xQPSK(t) a2k cos(2f0t) a2k1 sin(2f0t)
˜ x QPSK(t) a2k ja2k1
T=Tb
T=2Tb
x(t)cos( 2šf t )0
-sin( 2šf t)0
S/Pa
k
a2k
a2k+1
x(t)
cos( 2šf t )0
ak
xBPSK(t) ak cos(2f0t)
˜ x BPSK (t) ak
2f0t
2f0t
2f0t
0cos(2 )f t
0sin(2 )f t
0cos(2 )f t
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
“Constellations” of signals
+1-1
1
{ ̃x }
{˜ x }
BPSK QPSK
8PSK 16-QAM
1-1
I
Q
1010
1011
1001
1000
1110
1111
1101
1100
0110
0111
0101
0100
0010
0011
0001
0000
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Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Band-Limited I/Q Modulation...
x(t)cos( 2šf t )0
-sin( 2šf t)0
Map
ak
Shaping Filter p(t)
Shaping Filter p(t)
sI,k
sQ,k
˜ x (t) ˜ s k p(t kT )k
(sI ,k j sQ ,k )p(t kT)k
T=Tb·log2M
M=# of points in constellation
1
Tb
1
T
1
T
0cos(2 )f t
0sin(2 )f t
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Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
...and I/Q Demodulation
ak ^
The receiver may produce errors when the noise is large
Ik
Qk
2f0t
2f0t
x(t) 2cos( 2f t ) 0
-2sin( 2f t ) 0
Matched Filter p(-t)/T
Matched Filter p(-t)/T
Complex Decision
De- Map
kT
kT
w(t)
r(t)
0sin(2 )f t
0cos(2 )f t
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
How an Error is Made
Pe Q2Eb
N0
BPSK, QPSK
Transmitted
symbol
Correct decision zone
I k
Q k
ERRORS!
Received Complex Samples
0( / )b bE C N T
Energy-per-bit
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Baseband Data (PAM) Signal
s(t) ak p(t kT )k
+A
-A
a0 a1 a2 a3 a4
T
Binary Pulse-Amplitude Modulation (PAM)
Symbol Interval: T = Bit Interval: Tb
Symbol Rate: R=1/T = Bit Rate: Rb=1/Tb
{ak}=+1 Binary Data
T
A
p(t)
t
s(t)
Bipolar Format
Basic Pulse p(t):
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Spreading the Spectrum
M=8
+1
-1
s(t)
T t
+1
-1
c(t)
T t T c
s(t)
c(t)
x (t)SS
SPREADING CODE
The Spread-Spectrum signals
runs at the same clock (the
chip-rate) as the spreading code
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Spread Spectrum !! 1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2 12 11 10 9 8 7 6 5 4 3 2 1 0
Normalized Frequency, fT
Spreading Code PSD
Data PSD
M=8
10 -6
10 -5
10 -4
10 -3
10 -2
10 -1
10 0
10 1
24 22 20 18 16 14 12 10 8 6 4 2 0
Normalized Frequency, fT
M=16
Linear Scale
Log (dB) Scale
GPS: chip rate 1.023 Mchip/s
bit rate 50 bit/s M=20460
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
The DS/SS Receiver
z(t) s(t)c(t) w(t) c(t) s(t)c2(t) w' (t)
=1 !
r(t) x (t) SS
w(t) c(t) Code
Sync
De-spreader Correlator
1 T
kT
(k+1)T
( · ) dt z(t) âk
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Narrowband Interference Rejection
In-band Interfering Power
Pi P
1/Tc
1
T P
Tc
T
P
M
BEFORE Despreading
Frequency
Narrowband Interference Power P
SS signal
1/Tc
AFTER Despreading
1/Tc 1/T
De-spread SS signal
Spread Interference
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
BER of the DS/SS Receiver
NO difference w.r.t a narrowband modulation!
Spectrum spreading/despreading
is a transparent operation
P e Q
T b C
N 0
/ 2
÷ Q
2 E b
N 0
÷
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Frequency-Hopping SS Transmitter...
ak
x (t)FH-SS
1/T
1/Tc
p(t)
NCO
S/P
1/MTc
M-bit
PN Code Gen.
cm
s(t)
MTc 2MTc
FSK Modulation is most often used
The overall spectrum is partitioned in 2M bins
Time
HOP Pattern
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
...and Receiver
The receiver just demodulates the SS
signal via a hopping oscillator with the
same hop pattern
Not used in multiple-access communications or positioning
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Who’s invented Spread-Spectrum ?
Just to know..
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Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Hedy Lamarr, actress
Can you believe ?
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Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Generation of PN Codes
Linear Feedback Shift-Register LFSR
The CODE POLYNOMIAL describes the outputs that are to be XORed:
G(x)=1+x+x2+x4+x6
D Q D Q D Q D Q D Q D Q
Chip Clock
Parallel InputLSB MSB
with P delay elements
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Maximal-Length Sequences
Def: G(x) has no prime factors and divides 1+xL in GF(2P)
Properties:
• M-sequences have the maximum possible periodicity: L=2P-1
• They contain 2P/2 1s and 2P/2-1 0s (BALANCED sequences)
• XORing an M-sequences with a delayed replica of itself gives the same M-sequence
with a third phase
• If you slide a P-bit window on the sequence, you get all of the numbers between 1
and 2P
• There is a limited number of M-sequences for a given P
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Gold Codes
Generator of an L=63 Balanced Gold Code • Family of L+2 codes with period L=2P-1
• (Logical) sum of two M-sequences of length
L
• May be done balanced (preferentially
phased)
• Used in GPS, UMTS
• Good spectrum
• The average cross-correlation is low (quasi-
orthogonal codes)
1
N 1
1
Lcm
(1)cm
(k )
m0
L1
2
k 2
N
1
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Autocorrelation Functions of a PN sequence
Rp[k]1
Lcmcm k
m 0
L1
1
k
L-1=7
-1/L
R [k] p
Linear (Aperiodic) Autocorrelation
for an M-sequence
Rl[k ]1
L kcmcm k
m0
L k1
Periodic Autocorrelation
Determines the spectral properties of a
DS/SS signal with short codes and data
modulation
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
The True Spectrum of SS Signals
The code is only PSEUDO random (actually, it is periodic !)
C( f ) cm
m0
L1
e j2fTc
The “Code Spectrum” C(f) depends on the
autocorrelation properties of c(t)
Sss( f )| C( f ) |2
TQ( f )
2
-30
-20
-10
0
10
20
30
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Normalized Frequency, fT c
Gold Code, L=63
Co
de
PS
D (
dB
)
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Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Multiplexing: Time- and Frequency-Division
. . .
FREQUENCY
Carrier #1 Carrier #2 Carrier #N
. . .
TIME
Slot #1 Slot #2 Slot #N
TDMA: The user signals are separated in TIME(overlapped in frequency)
FDMA: The user signals are separated in FREQUENCY (overlap in time)
User data signals to be multiplexed
User Data #1
User Data #2
User Data #N
. .
.
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Giacomo Bacci, Marco Luise
Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Code Division Multiplexing
FREQUENCY
TIME
UserData #1
UserData #2
UserData #N
..
.
SpreadingCode #1
SpreadingCode #2
SpreadingCode #N
Each user is assigned a signature code
used to spread his/herown data signal
The users are overlapped
both in time and frequency
- They can be separated
only via de-spreading with
the appropriate code
The users are overlapped
both in time and frequency
- They can be separated
only via de-spreading with
the appropriate code
Spread Spectrums
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Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Orthogonal Codes
c ( i ) ( t ) c ( k ) ( t ) dt 0 , i k 0
T
H2 1 1
1 1
HH HH / 2 H H / 2
HH / 2 HH / 2
WALSH-HADAMARD Matrices & Functions
t
t
t
t
1 1 1 1
1 -1 1 -1
1 1 -1 -1
1 -1 -1 1
T=LTc
L=2 =4 m
Chip Time
c (t) (0)
c (t) (1)
c (t) (2)
c (t) (3)
H4
1 1 1 1
1 1 1 1
1 1 1 1
1 1 1 1
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Giacomo Bacci, Marco Luise
Basics of 3G communications
36
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Synchronous Orthogonal CDMA
w (t)0
w (t)1
w (t)2
w (t)3
w (t)4
w (t)5
w (t)6
w (t)7
Table of Walsh-Hadamard Functions L=8
wi(t)wk(t)dt=0, ik
Frequency
Time
CODE
The user signals are ORTHOGONAL
(but synchronicity is needed)
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Giacomo Bacci, Marco Luise
Basics of 3G communications
37
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Code-Division Multiplexing and Multiple Access
Asynchronous MULTIPLE ACCESS: no need of user
synchronization (sort of random access)
Synchronous multiplexing: all users share the same data and
chip clock
• Co-located user signals
• Broadcast Pilot Signal
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Giacomo Bacci, Marco Luise
Basics of 3G communications
38
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
The UMTS standard (1/2)
The universal mobile telecommunications system (UMTS), designed in the
late 1990s, and operational in Europe, Asia and Australia since 2002, offers:
o throughput:
• 2 Mb/s for fixed users (very low mobility)
• 384 kb/s for pedestrian users (low mobility)
• 144 kb/s for vehicular users (up to 500 km/h)
o latency: 100 ÷ 200 ms
o multiplexing of multiple services with different quality of service (QoS)
requirements
o asymmetric traffic for uplink and downlink channels
3G systems
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Giacomo Bacci, Marco Luise
Basics of 3G communications
39
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
The UMTS standard (2/2)
Two modes are supported by the UMTS standard:
o UMTS terrestrial radio access FDD (UTRA-FDD)
o UMTS terrestrial radio access TDD (UTRA-TDD) (not covered)
3G systems
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Giacomo Bacci, Marco Luise
Basics of 3G communications
40
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD (1/3)
The UTRA-FDD mode adopts CDMA as the multiple-access technique
Since B = 5 MHz, this technology is called wideband CDMA (WCDMA)
In both the uplink (UL) and the downlink (DL), the input signal is processed as
follows:
3G systems
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Giacomo Bacci, Marco Luise
Basics of 3G communications
41
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD (2/3)
Uplink:
3G systems
spreading codes: used to separate the streams at the transmit side (MS)
scrambling codes: used to separate the users in the nertwork
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Giacomo Bacci, Marco Luise
Basics of 3G communications
42
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD (3/3)
Downlink:
3G systems
spreading codes: used to separate the messages for each receiver at the
transmit side (BTS)
scrambling codes: used to separate a BTS from each other in the network
message for
message for
message for
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Giacomo Bacci, Marco Luise
Basics of 3G communications
43
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD: Spreading codes (1/2)
The spreading codes used in UTRA-FDD belong to the family of orthogonal
variable spreading factor (OVSF) codes, built upon Walsh-Hadamard codes:
3G systems
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Giacomo Bacci, Marco Luise
Basics of 3G communications
44
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD: Spreading codes (2/2)
In the OVSF codes used by UTRA-FDD, the chiprate is fixed:
3G systems
o slow streams: large M’s
o fast streams: small M’s
Distinctive features of the OVSF codes:
o orthogonal codes:
if is not a parent of
o careful management of the codes to avoid code shortage
o need for synchronization across the codes: all codes are generated by the
same transmitter
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Giacomo Bacci, Marco Luise
Basics of 3G communications
45
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD: Scrambling codes (1/2)
o Orthogonal codes such as the OVSF codes cannot be used for aggregate
signals (e.g., the signals from all MSs in the UL)
o To separate users/cells in the UL/DL, the UTRA-FDD makes use of different
PN codes with chiprate Rc = 3.84 Mc/s (no bandwidth spreading):
• autocorrelation
• crosscorrelation
3G systems
UTRA-FDD uses non-orthogonal codes as scrambling codes
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Giacomo Bacci, Marco Luise
Basics of 3G communications
46
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Frequency Reuse Patterns
F/TDMA Seven-cell cluster CDMA Universal Frequency Reuse
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Giacomo Bacci, Marco Luise
Basics of 3G communications
47
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Channelization (spreading) and Scrambling Codes
CDMA cellular network with hexagonal cells
w1 w2
w3
wL
. .
.
S1 w1
w2
w3
wL
. .
.
S2 w1
w2
w3
wL
. .
.
S3
Intra-cell Channelization Codes:
wi
Inter-cell Scrambling Codes:
Sk
Composite Codes:
Ck,i= Skwi
Code Reuse !!
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Giacomo Bacci, Marco Luise
Basics of 3G communications
48
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Back to Basic CDMA
Gaussian approximation for the
Multiple-Access Interference
Pe Q2Eb
N0 I0
I0 is the MAI PSD
U
Downlink
Uplink
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Giacomo Bacci, Marco Luise
Basics of 3G communications
49
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
The Capacity of Cellular F/TDMA...
F/TDMA with RE-USE factor Q (number of cells/cluster)
• Each cell is allocated 1/Q of the total spectrum
• Total bit-rate in a cell NRb
• Total Bandwidth =QNRb
S 1/ Q
The re-use factor is determined by the
level of the co-channel interference
coming from the nearest cell using the
same frequency
GSM: Q=9, S=0.11
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Giacomo Bacci, Marco Luise
Basics of 3G communications
50
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
...and Cellular CDMA
CDMA with UNIVERSAL RE-USE
Worst-Case user location
With M=63, b=0.4, BER=10-3
S=0.26
N
N N-1
FWD LINK
Similar computation for the return link
b2/1
0IE
S b
bE
MNNI 0
0
bb
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Giacomo Bacci, Marco Luise
Basics of 3G communications
51
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Spreading and scrambling codes: A summary
3G systems
spreading
codes
scrambling
codes
groups UL: streams
DL: MSs
UL: MSs
DL: BTSs
length UL: M=4÷256
DL: M=4÷512 38,400
cardinality M UL: 224
DL: 512
family OVSF Gold
codes
spreading
factor M 1
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Giacomo Bacci, Marco Luise
Basics of 3G communications
52
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD: Frame structure
Each frame has a duration
3G systems
power control rate:
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Giacomo Bacci, Marco Luise
Basics of 3G communications
53
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD: Downlink structure (1/2)
The DL makes use of a quaternary phase shift keying (QPSK) constellation,
using a root-raised cosine (RRC) with rolloff as the shaping filter
3G systems
Considering the guard bands, the
dedicated physical channel (DPCH)
occupies 5 MHz
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Giacomo Bacci, Marco Luise
Basics of 3G communications
54
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD: Downlink structure (2/2)
Time slot structure:
3G systems
QPSK
M=4 1,248 bits/slot Since each user can be
assigned up to 3 parallel DPCHs,
we get data rates around 2 Mb/s channel
coding rate
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Giacomo Bacci, Marco Luise
Basics of 3G communications
55
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD: Uplink structure (1/2)
The UL makes use of a dual-code binary phase shift keying (BPSK)
constellation, using a root-raised cosine (RRC) with rolloff
3G systems
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Giacomo Bacci, Marco Luise
Basics of 3G communications
56
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UTRA-FDD: Uplink structure (2/2)
Time slot structure:
3G systems
M=4 640 bits/slot Since each user can be assigned up
to 6 parallel DPCHs, we get again
data rates around 2 Mb/s channel
coding rate
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Giacomo Bacci, Marco Luise
Basics of 3G communications
57
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Discontinuous transmission
The UL channel uses a dual-code BPSK in order to:
o improve the efficiency of the power amplifier, by reducing the peak-
to-average power ratio (PAPR)
o avoid a 1,500 Hz oscillation during discontinuous transmission (DTX)
3G systems
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Giacomo Bacci, Marco Luise
Basics of 3G communications
58
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Handover management
The UMTS standard supports three different handover (HO) procedures:
o intersystem HO: backward compatibility with 2G systems ensures HOs
when the 3G coverage is not available
o hard HO: the connectivity when moving across cells is ensured by the
BTS of the cell just entered, and is available for both FDD and TDD
modes
o soft HO: in the FDD mode, the MS can be connected simultaneously to
both the BTS of the exiting cell, and the BTS of the entering cell, by
exploiting the frequency diversity recovered by the rake receivers
3G systems
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Giacomo Bacci, Marco Luise
Basics of 3G communications
59
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Power control (1/3)
Let’s consider code spreading when the near-far effect takes place:
3G systems
despreading
If the MSs are received at the BTS with very
unbalanced powers, the despreading is
not successful due to a non-negligible
multiple-access interference (MAI)
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Giacomo Bacci, Marco Luise
Basics of 3G communications
60
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Power control (2/3)
The received signal-to-interference (SIR) is:
o measured by the receivers using pilot symbols
o fed back to the transmitter to regulate the transmit power level (closed-loop
power control)
3G systems
Powers are increased/decreased on a logarithmic scale every 2/3 ms (1500 Hz)
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Giacomo Bacci, Marco Luise
Basics of 3G communications
61
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Power control (3/3)
Due to power control, UMTS cells have variable coverage areas:
3G systems
As the number of users increases, so does the level of MAI: to keep an acceptable
level of , the radius of the cell is reduced
cell
breathing
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Giacomo Bacci, Marco Luise
Basics of 3G communications
62
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
UMTS evolution: The HSPA/HSPA+ standards
The high-speed packet access (HSPA) improves data rates (DL: 14.4 Mb/s,
UL: 5.76 Mb/s) and latencies (<100 ms), by introducing:
o 16-QAM (quadrature
amplitude modulation)
o hybrid automatic
repeat-request (HARQ)
3G systems
The HSPA+ has eventually increased the UMTS performance, achieving 56
Mb/s for the DL, and 22 Mb/s for the UL:
o 64-QAM constellation
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Giacomo Bacci, Marco Luise
Basics of 3G communications
63
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Limits of 3G systems
Due to the use of WCDMA (B=5 MHz), the receiver needs to estimate the
channels taps, with parameters , , and
4G systems
This can be done by using rake receivers:
This significantly increases the receiver complexity,
thus limiting the bandwidth of the CDMA signal
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Basics of 3G communications
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Bibliography
[01] A.J. Goldsmith, Wireless Communications. Cambridge, UK: Cambridge Univ.
Press, 2005.
[02] J.G. Proakis and M. Salehi, Digital Communications, 5th ed. New York, NY:
McGraw-Hill, 2007.
[03] A.F. Molisch, Wireless Communications. West Sussex, UK: J. Wiley & Sons,
2005.
[04] H. Holma and A. Toskala, WCDMA for UMTS, 2nd ed. West Sussex, UK: J. Wiley
& Sons, 2002.
[05] H. Holma and A. Toskala, HSDPA/HSUPA for UMTS. West Sussex, UK: J. Wiley &
Sons, 2006.
[06] H. Holma and A. Toskala, WCDMA for UMTS: HSPA Evolution and LTE. West
Sussex, UK: J. Wiley & Sons, 2010.
[07] A. Jamalipour, T. Wada, and T. Yamazato, “A tutorial on multiple access
technologies for beyond 3G mobile networks,” IEEE Commun. Mag., vol. 43,
no. 2, pp. 110-117, Feb. 2005.
[08] L. Hanzo, M. Münster, B.J. Choi, and T. Keller, OFDM and MC-CDMA for
Broadband Multi-User Communications, WLANs and Broadcasting.
Chichester, UK: J. Wiley & Sons, 2003.
Bibliography
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