Dip. Ingegneria dell’Informazione, Univ. Pisa, Pisa, Italy
Basics of 4G Communications (and Beyond)
Giacomo Bacci, Marco Luise
Computer Engineering Electronics and Communications Systems
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
2
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
4G systems
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
3
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Evolution of Cellular Standards
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
4
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
5
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
4G deployment status
4G systems
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
6
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
7
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
8
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
9
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Time-Invariant Channel of DVB-T
Rs=6 Mbit/s
Ts=0.16 ms
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
10
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
11
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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
Normalized Frequency, fT
DVB-T 20-ray Channel Model
Modulated Signal Spectrum
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
12
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
-25
-20
-15
-10
-5
0
5
|H
(f)|
(d
B)
0.60.50.40.30.2
Normalized Frequency, fT
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
13
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
14
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)
Equivalent to
xI(t)+jxQ(t) x(t)
exp(j2f0t)=cos(2f0t)+jsin(2f0t)
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
15
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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
normalized frequency
no
rma
lize
d P
SD
OFDM
This solution requires the minimum bandwidth occupancy
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
16
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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
exp{ 2 ( 1) }scj N 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
exp{ 2 ( 1) }scj N 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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
17
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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
Normalized Frequency, fT
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
18
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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
normalized frequency
no
rma
lize
d P
SD
classical frequency division multiplexing
normalized frequency
no
rma
lize
d P
SD
OFDM
This solution requires the minimum bandwidth occupancy
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
19
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
20
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Digital Implementation
4G systems
exp{ 2 0 }scj f t
exp{ 2 1 }scj f t
exp{ 2 ( 1) }scj N f t
.
.
.
cmN+k S/P
(DeMUX) S b(t)
( )
0
mc
( )
1
mc
( )
1
m
Nc
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
21
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Digital Implementation
4G systems
.
.
.
cmN+k S/P
(DeMUX)
b(t) IFFT P/S
(MUX) DAC
.
.
.
.
.
.
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
22
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Digital Implementation
4G systems
.
.
.
cmN+k S/P
(DeMUX)
b(t) IFFT P/S
(MUX) DAC
.
.
.
.
.
.
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
23
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
…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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
24
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
25
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
26
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Channel equalization in OFDM (2/3)
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
27
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Channel equalization in OFDM (3/3)
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
28
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
( ) ( ) ( ) 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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
29
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Channel equalization in OFDM (5/5)
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
30
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Channel equalization in OFDM (5/5)
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
31
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
LTE DL Pilots
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Basics of 4G communications and beyond
32
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
33
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
34
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Basics of 4G communications and beyond
35
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Universal Fequency Reuse of CDMA (K=1)
Introduction to modern wireless communications systems
Typical of 3G systems
available channels
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
36
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Fractional frequency reuse (FFR)
Introduction to modern wireless communications systems
available bandwidth
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
37
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
38
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
39
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
40
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Basics of 4G communications and beyond
41
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
42
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
4G deployment status
4G systems
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Basics of 4G communications and beyond
43
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
44
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Basics of 4G communications and beyond
45
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Structure of the LTE-A frame
4G systems
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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Enabling technologies for 4G standards
4G systems
o carrier aggregation
o network MIMO
o relaying
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Basics of 4G communications and beyond
47
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
48
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
49
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Basics of beyond-4G
technologies
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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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×
in
cre
ase
in
fiv
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ea
rs
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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Beyond-4G technologies (2/3)
Basics of beyond-4G technologies
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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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Beyond-4G technologies (3/3)
Basics of beyond-4G technologies
source: IMT-2020 Promotion Group
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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Technology drivers
Basics of beyond-4G technologies
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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Giacomo Bacci, Marco Luise
Basics of 4G communications and beyond
55
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Network densification (1/3)
Basics of beyond-4G technologies
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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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Network densification (2/3)
Basics of beyond-4G technologies
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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Basics of 4G communications and beyond
57
Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Network densification (3/3)
Basics of beyond-4G technologies
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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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Massive MIMO (1/2)
Basics of beyond-4G technologies
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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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Massive MIMO (2/2)
Basics of beyond-4G technologies
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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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
mm-wave technology (1/2)
Basics of beyond-4G technologies
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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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
mm-wave technology (2/2)
Basics of beyond-4G technologies
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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Basics of 4G communications and beyond
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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
Perspectives and open issues
Basics of beyond-4G technologies
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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Dip. Ingegneria dell’Informazione
University of Pisa, Pisa, Italy
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[02] A.F. Molisch, Wireless Communications. West Sussex, UK: J. Wiley & Sons,
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[03] H. Holma and A. Toskala, WCDMA for UMTS: HSPA Evolution and LTE. West
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[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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[08] J.G. Andrews, S. Buzzi, W. Choi, S.V. Hanly, A. Lozano, A.C.K. Soong, and J.C.
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