Computer Engineering Electronics and … of 3G Communications ... Computer Engineering Electronics...

Dip. Ingegneria dell’Informazione, Univ. Pisa, Pisa, Italy

Basics of 3G Communications

Giacomo Bacci, Marco Luise [email protected]

Computer Engineering

Electronics and Communications Systems

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g

Giacomo Bacci, Marco Luise

Basics of 3G communications

2

Dip. Ingegneria dell’Informazione

University of Pisa, Pisa, Italy

3G systems

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



3



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:

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



4



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



5



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


-13.3 dB

No

rm.P

SD

Ss(f

)/T

(d

B)

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



6



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



7



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



8



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)

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



9



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



10



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



11



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



12



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



13



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



14



“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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



15



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



16



...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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



17



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



18



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):

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



19



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



20



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


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


M=16

Linear Scale

Log (dB) Scale

GPS: chip rate 1.023 Mchip/s

bit rate 50 bit/s M=20460

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



21



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



22



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



23



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

÷

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



24



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



25



...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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



26



Who’s invented Spread-Spectrum ?

Just to know..

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



27



Hedy Lamarr, actress

Can you believe ?

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



28



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



29



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



30



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



31



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



32



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

)

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



33



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

. .

.

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



34



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



35



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



36



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)

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



37



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



38



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



39



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



40



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



41



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



42



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



43



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



44



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



45



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



46



Frequency Reuse Patterns

F/TDMA Seven-cell cluster CDMA Universal Frequency Reuse

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



47



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 !!

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



48



Back to Basic CDMA

Gaussian approximation for the

Multiple-Access Interference

Pe Q2Eb

N0 I0

I0 is the MAI PSD

U

Downlink

Uplink

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



49



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



50



...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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



51



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



52



UTRA-FDD: Frame structure

Each frame has a duration

3G systems

power control rate:

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



53



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



54



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



55



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



56



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



57



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



58



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



59



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)

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



60



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)

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



61



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



62



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



63



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

Ele

ctr

on

ics

an

d C

om

mu

nic

atio

ns

Sy

ste

ms

Co

mp

ute

r En

gin

ee

rin

g



64



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

Computer Engineering Electronics and … of 3G Communications ... Computer Engineering Electronics...

Documents

Transcript of Computer Engineering Electronics and … of 3G Communications ... Computer Engineering Electronics...