computer engineering electronics and … engineering electronics and communications systems. ... o...

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

Basics of 4G Communications (and Beyond)

Giacomo Bacci, Marco Luise

[email protected]

Computer Engineering Electronics and Communications Systems

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Basics of 4G communications and beyond

2

Dip. Ingegneria dell’Informazione

University of Pisa, Pisa, Italy

4G systems

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3



Evolution of Cellular Standards

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4



IMT-advanced requirements

4G systems

o peak data rates of 100 Mb/s for high-mobility users, and 1 Gb/s for low-mobility users

o larger bandwidths (up to 40 MHz)

o lower latencies (< 15 ms)

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5



4G deployment status

4G systems

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6



4G standards

4G systems

There were two competing systems labeled as 4G technologies:

o LTE-advanced (LTE-A), standardized by the

3rd generation partnership project (3GPP)

o IEEE 802.16m, standardized by the Institute

of Electrical and Electronic Engineers (IEEE)

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7



LTE-advanced

4G systems

o The long-term evolution – advanced (LTE-A)

has been standardized by the 3GPP in

March 2011, as 3GPP Release 10

(current version: Release 13)

o LTE-A adopts OFDMA for the DL, and SC-FDMA for the UL, achieving peak

rates of 3 Gb/s (DL) and 1.5 Gb/s (UL), and maximum latency 10 ms

o Carrier frequencies: 700 MHz, 900 MHz, 1800 MHz, 2100 MHz, 2600 MHz

o Carrier spacing: 15 kHz

o Bandwidths: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, 20 MHz

o Constellations: QPSK, 16-QAM, 64-QAM

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8



Two-ray channel Amplitude response

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

|H(f

)|

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

Frequenza (MHz)

A=1

A=0.1

A=0.5

t =1 ms

fN = 0.5 MHz

See the notch

frequencies!

2( ) 1 j j fH f Ae e t

2 ( )( ) 1 Nj f f

H f Ae t

or

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9



Time-Invariant Channel of DVB-T

Rs=6 Mbit/s

Ts=0.16 ms

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10



Amplitude Response of the DVB-T Channel

-25

-20

-15

-10

-5

0

5

|H

(f)|

(d

B)

1.0 0.8 0.6 0.4 0.2 0.0

Normalized Frequency, fT

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11



Too many notches !

The equalizer is too complicated !

How to cope with severely selective channels ? 1/2

-25

-20

-15

-10

-5

0

5

|H

(f)|

(d

B)

1.0 0.8 0.6 0.4 0.2 0.0


DVB-T 20-ray Channel Model

Modulated Signal Spectrum

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12



-25

-20

-15

-10

-5

0

5

|H

(f)|

(d

B)

0.60.50.40.30.2


Split your (single-carrier) high-rate stream into many

“parallel” low-rate streams on

different subcarriers that

“see” each a FLAT channel response

!!

How to cope with severely selective channels ? 2/2

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13



exp{ 2 0 }scj f t

exp{ 2 1 }scj f t

exp{ 2 ( 1) }scj N f t

.

.

.

cmN+k S/P

(DeMUX)

symbol time: NT=Ts

S b(t)

N=# of subcarriers

fsc=subcarrier spacing

m=block index

k=intra-block subcarrier index,

0k N-1

+1 -1

1

( )

0

mc

( )

1

mc

( )

1

m

Nc

symbol time: T

Multi-Carrier Modulation (DVB-T, ADSL,WLAN)

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14



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)

Equivalent to

xI(t)+jxQ(t) x(t)

exp(j2f0t)=cos(2f0t)+jsin(2f0t)

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15



Orthogonal FDM

To convey the stream of infomation symbols on multiple subcarriers without

any INTERFERENCE, we use a set of orthogonal subcarriers:

4G systems

normalized frequency

no

rma

lize

d P

SD

classical frequency division multiplexing


no

rma

lize

d P

SD

OFDM

This solution requires the minimum bandwidth occupancy

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16



The OFDM Signal Format

k-th data stream

12 /( )

0

( ) ( ) s

Nj kt Tm

k s

k m

x t c p t mT e

m is the time index of an OFDM symbol

k-th subcarrier

exp{ 2 0 }scj f t

exp{ 2 1 }scj f t


..

..

..

ccmNmN+k+k

S/P SSbb((tt))

( )

1

mc

( )

1

m

Nc

exp{ 2 0 }scj f t

exp{ 2 1 }scj f t


..

..

..

ccmNmN+k+k

S/P SSbb((tt))

( )

1

mc

( )

1

m

Nc

12 /

0

( ) ( ) s

Nj kt T

k

k

x t x t e

1

0

( ) ( / )k

N

x x s

k

S f S f k T

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17



Power Spectrum of OFDM

-30

-25

-20

-15

-10

-5

0

5

10

Sx(f

) (d

B)

1.21.00.80.60.40.20.0-0.2


N=2048

N=64

N=64

IEEE 802.11 Wireless LAN

N=2048

DVB-T Terrestrial Digital Video

Broadcasting

1

0

( ) ( / )k

N

x x s

k

S f S f k T

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18



Orthogonal FDM again

To convey the stream of infomation symbols on multiple subcarriers without

any INTERFERENCE, we use a set of orthogonal subcarriers:

4G systems


no

rma

lize

d P

SD

classical frequency division multiplexing


no

rma

lize

d P

SD

OFDM

This solution requires the minimum bandwidth occupancy

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19



Digital Implementation

How can we implement OFDM?

o Using L local oscillators to sinthesize at the transmitter

and the receiver is a highly inefficient architecture

o Let us try to sample our signal at intervals mTb:

4G systems

inverse discrete Fourier

transform (IDFT)

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20




4G systems

exp{ 2 0 }scj f t

exp{ 2 1 }scj f t


.

.

.

cmN+k S/P

(DeMUX) S b(t)

( )

0

mc

( )

1

mc

( )

1

m

Nc

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21




4G systems

.

.

.

cmN+k S/P

(DeMUX)

b(t) IFFT P/S

(MUX) DAC

.

.

.

.

.

.

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22




4G systems

.

.

.

cmN+k S/P

(DeMUX)

b(t) IFFT P/S

(MUX) DAC

.

.

.

.

.

.

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23



…and in the Receiver

4G systems

.

.

.

cmN+k S/P

(DeMUX)

b(t) FFT P/S

(MUX) ADC

.

.

.

.

.

.

OFDM is very efficient, as it can exploit both at the TX and at the RX (fast) FFT

processing with L=2D (e.g., L=2048 for LTE)

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24



Virtual Carriers

Carrier # 0 Carrier # N-1

Signal BW

Carrier # Nv/2 Carrier # N- Nv/2-1

Reduced Signal BW

Symbols # 0,.., Nv/2-1 and N- Nv/2 ,..,N-1 are set to 0 to control signal bandwidth

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25



Channel equalization in OFDM (1/3)

What happens when introducing the additive white Gaussian noise (AWGN)?

4G systems

filtered

noise term

Considering channel selectivity

In this case, multipath propagation can lead to inter-carrier

interference (ICI), and orthogonality is lost

maxt

( ) ( ) ( ) k k k

m m mr c nm st mT

m st mT

MF

MF

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26




To mitigate the ICI, we can add a special guard interval, called the cyclic

prefix (CP), with length :

4G systems

Using the CP, we have “artificially” introduced a cyclic inter-symbol

interference (ISI), that can now be controlled

maxtpT

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27




Adopting the same receiver technique,

4G systems

In this case, channel equalization is extremely simple:

( ) ( ) ( ) ( ) ( )

k k k k k

m m m k m m

s

kr H c n H c n

T

sfT

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( ) ( ) ( ) k k k

m k m mr H c n

FFT P/S

…

Channel

Estimator

ˆkH

( )-1

ˆ1/ kH

( ) ( ) ( ) ( )1

ˆ ˆ k k k kk

m m m m

k k

Hc n c n

H H

Channel Estimation with known PILOT SYMBOLS (1 every a few)

Channel distortion is canceled !

Equalization in the Frequency Domain

( )k

mr

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How can the receiver estimate the coefficients Hk in practice?

4G systems

OFDM symbols contain sparse pilot subcarriers, with known symbols

ck=1, to let the receiver get an accurate estimation of the channel

response:

( ) ( ) ( )ˆ1 k k k

m k m k mr H n H r

Of course there are etimation errors caused by the presence of the noise

term nm(k)

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30




How can the receiver estimate the coefficients Hk in practice?

4G systems

OFDM symbols contain sparse pilot subcarriers, with known symbols, to

let the receiver get an accurate estimation of the channel response:

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31



LTE DL Pilots

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32



OFDM architecture

4G systems

Features:

o optimal implementation via (I)FFT

o no ICI due to carrier orthogonality

o controlled OOB emissions thanks to the virtual carriers

o frequency-domain equalization thanks to the CP

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33



Orthogonal frequency division multiple access (OFDMA)

How can we adapt the OFDM technology to the multiuser case?

4G systems

Each user can be assigned a subset of subcarriers, by zeroing the inactive subcarriers

o Subcarrier allocation is critical to exploit

the frequency diversity

o This allocation introduces some overhead

in the network

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34



Fequency Reuse with Factor K

Introduction to modern wireless communications systems

End of 1950s/beginning of 1960s: introducing cells to provide seamless coverage

available channels

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35



Universal Fequency Reuse of CDMA (K=1)


Typical of 3G systems

available channels

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36



Fractional frequency reuse (FFR)


available bandwidth

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Limits of OFDM(A)

o sensitivity to synchronization errors:

• single-carrier systems: the residual frequency offset must be

• multicarrier systems:

o high peak-to-average power ratio (PAPR):

• the superposition of L sinusoidal signals yields a large PAPR, thus

calling for linear radio-frequency (RF) amplifiers

• to improve the efficiency of the RF stage at the MSs, the uplink can

adopt a modified version of OFDMA, called single-carrier FDMA (SC-

FDMA)

4G systems

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38



Single-carrier FDMA (SC-FDMA)

o FFT precoding significantly reduces the PAPR

o the IFFT at the transmitter operates on the Fourier coefficients rather than

on information symbols (as in OFDMA)

o the distinctive feature with respect to a traditional FDMA is the presence

of the CP, that allows for frequency-domain equalization

4G systems

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39



Multiple-input multiple-output (MIMO) systems

Another technology that is widely adopted in 4G standards to meet the ITU-

advanced requirements is MIMO:

4G systems

SISO:

MISO:

SIMO:

MIMO:

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40



Benefits of MIMO (1/2)

o array gain: the signal-to-noise ratio (SNR) can be increased by

beamforming at the transmitter and/or coherent combining at

the receiver

o diversity gain: channel fading can be mitigated by combining

independent copies of the transmitted signal in space,

frequency, or time

o spatial multiplexing: the throughput can be increased by

transmitting multiple, independent (at most, ) data

streams, thus increasing the capacity of the network

4G systems

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41



Limits of MIMO

4G systems

o it is not possible to exploit all the degrees of freedom to simultaneously

obtain array gain, space diversity, and spatial multiplexing: some

fundamental tradeoffs in terms of system performance need to be taken

o in most cases, full CSI at both the transmitter and the receiver cannot be

guaranteed in a realistic environment: the actual performance is poorer

than the maximum achievable one

o some tradeoffs between system complexity and performance are needed

to reduce the intensive computational load of MIMO: suboptimal

algorithms are necessary

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4G deployment status

4G systems

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43



4G standards

4G systems

There used to be two competing systems labeled as 4G technologies:

o LTE-advanced (LTE-A), standardized by the

3rd generation partnership project (3GPP)

o IEEE 802.16m, standardized by the Institute

of Electrical and Electronic Engineers (IEEE)

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44



LTE-advanced

4G systems

o The long-term evolution – advanced (LTE-A)

has been standardized by the 3GPP in

March 2011, as 3GPP Release 10

(current version: Release 13)

o LTE-A adopts OFDMA for the DL, and SC-FDMA for the UL, achieving peak

rates of 3 Gb/s (DL) and 1.5 Gb/s (UL), and maximum latency 10 ms

o Carrier frequencies: 700 MHz, 900 MHz, 1800 MHz, 2100 MHz, 2600 MHz

o Carrier spacing: 15 kHz

o Bandwidths: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, 20 MHz

o Constellations: QPSK, 16-QAM, 64-QAM

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45



Structure of the LTE-A frame

4G systems

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46



Enabling technologies for 4G standards

4G systems

o carrier aggregation

o network MIMO

o relaying

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47



Carrier aggregation

4G systems

With carrier aggregation (CA), we can increase the signal bandwidth by

grouping physical channels, in both TDD and FDD configurations

intraband, contiguous CA:

intraband, non-contiguous CA:

interband CA:

By grouping up to 5 carriers, we

can obtain a 100-MHz bandwidth

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48



Network MIMO

4G systems

Network MIMO, known as coordinated multipoint (CoMP) in LTE-A, and

coordinated MIMO (CO-MIMO) in IEEEE 802.16m, consists in coordinating (at the

transmit side) or combining (at the receive side) signals using multiple antennas

o this form of distributed MIMO achieves significant performance improvements,

especially for cell-edge users (improving coverage and cell-edge rates)

o it requires a significant feedback overhead to exchange CSI across BTSs

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49



Relaying

4G systems

Relay nodes can be introduced in the network as low-power BTSs, to

provide enhanced system performance

o improved network coverage

o increased energy efficiency

o increased spectral efficiency

o some form of coordination between the relays and the network is required

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50



Basics of beyond-4G

technologies

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51



Beyond-4G technologies (1/3)

Basics of beyond-4G technologies

Do we really need 5G?

o over 50%/year growth in data traffic

o at least, a 1000× increase every decade

o in 2018, global traffic will reach more

than 1/100 of a zettabyte (1 ZB=1021 B)

1

0×

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According to CISCO, mobile data traffic will reach the following milestones within

the next five years:

o monthly global mobile data traffic will

surpass 15 exabytes by 2018

o the number of mobile-connected devices

will exceed the world’s population by 2014

o the average mobile connection speed will surpass 2 Mb/s by 2016

o due to increased usage on smartphones, smartphones will reach 66% of

mobile data traffic by 2018

o tablets will exceed 15% of global mobile data traffic by 2016

o 4G traffic will be more than half of the total mobile traffic by 2018

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source: IMT-2020 Promotion Group

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Technology drivers


o network densitification

o massive MIMO

o mm-wave technology

and many more: full-duplex antennas, spectrum sharing, advanced PHY and

interference management, device-to-device (D2D) communications, etc.

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Network densification (1/3)


Cooper’s “law”: the wireless capacity has doubled every

30 months over the last century in the last fifty years,

capacity has increased about a million times!

5× 5× 25×

1600×

The right path to pursue is

network densification: very

dense deployment of BTSs

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Shannon’s law:

number of antennas

bandwidth

adaptive coding and modulation, interference

management

With extreme network densification, we can reuse Shannon’s law

everywhere, thus increasing the area spectral efficiency (in b/s/Hz/m2)

heterogeneous, multi-tier

dense networks: small cells,

relays, etc.

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Open challenges include:

o self-organization, exacerbated by

random/unplanned deployment of small cells

o coverage and performance predicition:

stochastic geometry and random matrix

theory could serve as powerful tools

o interference management and resource

allocation, also considering the presence of

multiple tiers

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Massive MIMO (1/2)


Massive MIMO (a.k.a. as multiuser MIMO) technology:

antennas

(hundreds)

single-

antenna terminals

The massive MIMO concept relies on the law of large numbers, to average

out frequency selectivity and thermal noise:

o spatial multiplexing gain

o array gain

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Massive MIMO (2/2)


In the uplink, the BTS:

o acquires CSI from pilot symbols

o detects the information symbols

o since , adopts linear

processing techniques (MRC, ZF,

MMSE), which are nearly optimal

In the downlink, the BTS:

o uses CSI obtained in the uplink

o applies multiuser MIMO precoding,

using low-complexity precoders

o Thanks to the unused degrees of freedom, massive MIMO can obtain:

• hardware-friendly waveform shaping

• PAPR reduction due to multiuser precoding

o The major challenge is getting accurate CSI estimation

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mm-wave technology (1/2)


Increasing the carrier frequency increases the path loss

However, network densification includes small cells with coverage radius

path loss becomes acceptable

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mm-wave technology (2/2)


o mm-waves can provide a brand-new, very wide frequency band, with

very high-gain steerable antennas at both the MS and the BTS sides

o we can augment the currently saturated 700 MHz – 2.5 GHz radio

spectrum, moving to 60 GHz

o due to very low wavelengths, we can

accommodate the antennas required

by massive MIMO

Wireless channel modeling is not valid anymore: research challenges

include new propagation models for mm-wave communications

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Perspectives and open issues


o We need to face the tremendous increase in capacity demand

foreseen for the near future

o Current challenges include multiple hierarchical network layers,

functioning seamlessly across different radio technologies, also

considering energy efficiency, spectral efficiency, and cost efficiency

o The combination of many state-of-the-art technologies shows

potential for low-power electronics, integrated antennas, space-time

processing, and many other features

o Cellular networks are undergoing many fundamental changes, thus

calling for many long-standing models to be significantly re-thought

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Sussex, UK: J. Wiley & Sons, 2010.

[04] L. Hanzo, M. Münster, B.J. Choi, and T. Keller, OFDM and MC-CDMA for

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[05] H.G. Myung and D.J. Goodman, Single Carrier FDMA: A New Air Interface

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[06] IEEE 802.16 Broadband Wireless Access Working Group, “IEEE 802.16m

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