Lte course

OUTLINE
 NEED FOR MULTI-CARRIER
 OFDM ENTERS INTO THE PICTURE
 FFT/ IFFT
 GUARD TIME INSERTION
 OFDM DRAWBACKS
 CHANNEL ESTIMATION
 OFDM BLOCK DIAGRAM
 SIMULATION RESULTS

NEED FOR MULTI-
CARRIER
Time Domain Analysis

NEED FOR MULTI-
CARRIER cont.

Pulse completely distorted. Pulse extended but the extension
ISI is significant in this case. are much smaller than T the output
behaves like the transmitted
rectangular pulse.

NEED FOR MULTI-
CARRIER cont.
Frequency Domain Analysis

NEED FOR MULTI-
CARRIER cont.
Conclusion
Wide pulses is needed for simple
equalization,
But
Narrow pulses is needed for high data rate

Solution
Multiplexing

NEED FOR MULTI-
CARRIER cont.
Problem

Solution
 Orthogonality

OFDM ENTERS INTO THE
PICTURE
Interference  Orthogonality
B.W efficiency  Min Separation

OFDM ENTERS INTO THE
PICTURE cont.
 Min Separation 
 Problem
◦ Difficult Implementation with traditional
oscillators
 Solution
◦ DFT
But
◦ DFT needs high processing
Solution
◦ Easy implementation using FFT/IFFT

FFT / IFFT

IFFT DAC
Channel
FFT ADC

GUARD TIME
INSERTION
Channel Filtering

X1 X2 …. …. Xn

hv …. h1 h0

Y1 Y2 …. …. Yn

GUARD TIME INSERTION
cont.
Problem

X’1 X’2 …. …. X’n X1 X2 …. …. Xn

hv …. h1 h0

Y’’1 Y’’2 …. Y’’v Yv+1 Yv+2 …. Yn

ISI occurs

GUARD TIME INSERTION
cont.
Solution  Cyclic Prefix

Xn-
X’1 …. X’n v+1
…. Xn X1 X2 …. Xn

hv …. h1 h0

Y1 Y2 …. Yv Yv+1 Yv+2 …. Yn

 No ISI
Circular Convolution achieved.

Cyclic prefix
 The CP allows the receiver to absorb much more
efficiently the delay spread due to the multipath
and to maintain frequency Orthogonality.

 The CP that occupies a duration called the Guard
Time (GT), often denoted TG, is a temporal
redundancy that must be taken into account in data
rate computations.

OFDM DRAWBACKS cont.
Peak to Average Power Ratio (PAPR)

OFDM DRAWBACKS cont.
Sensitivity to frequency offset 

CHANNEL ESTIMATION
Pilot Based Channel Estimation 
Estimated
Received Channel
Signal after Response
FFT
Lowpass
Pilot Pilot
FIR
Signal Signal
Filter
Extraction Estimation

Known Pilots

CHANNEL ESTIMATION
cont.
 Pilot Arrangement Types
 Block Pilot Patterns Comb Pilot Patterns
Frequency( sub carriers)

Frequency( sub carriers)
Pilot
symbols
Data symbols

Time (OFDM Symbols) Time (OFDM Symbols)
High channel frequency rapid changing channels
selectivity

OFDMA
 OFDMA is a multiple access method based on
OFDM
signaling that allows simultaneous
transmissions to and from several users along
with the other advantages of OFDM.

OFDM versus OFDMA

IEEE802.16d IEEE802.16e
Fixed WiMAX,256-OFDM Mobile WiMAX

What is diversity?
Is a technique that combats the fading
by ensuring that there will be many
copies of the transmitted signal
effected with different fading over
time, frequency or space.

Diversity
types

Time Frequency Spatial
diversity diversity diversity

1- Time diversity:

We averaging the fading of the
channel over time by using :
1-The channel coding and
interleaving.
2-Or sending the data at different
times.
to explain this we will see an
example:

1-time diversity:
|H(t)|

t

No interleaving x1 x2 x3 x4 y1 y2 y3 y4 z1 z2 z3 z4 h1 h2 h3
h4
interleaving x1 y1 z1 h1 x2 y2 z2 h2 x3 y3 z3 h3 x4 y4 z4
h4

So we can see that only the 3rd symbol from each codeword lost and
we can recover them from the rest symbols in each codeword.

2- frequency diversity:
This type of diversity used for the
frequency selective channels as we
will averaging the fading over the
frequency by using:
1-Multi-carrier technique like OFDM.
2-FHSS (frequency hope spread
spectrum).
3-DSSS (direct sequence spread
spectrum).

2- frequency diversity:

We can see that each sub-band will
effecting with different fading over the
frequency.

3-spatial diversity:
we will have many copies of the transmitted signal effects
with different fading over the space .
we use multi-antenna systems at the transmitter or the
receiver or at both of them.

Spatial
diversity

MIMO-
MISO SIMO MIMO
MU

Receive diversity:

1-The receiver will has many antennas .
2-Each one has signal effecting with different
fading.
3-number of different paths =Mr.
Diversity order=Mr

MIMO:

In this type we use multi antennas at both
the transmitter and receiver as shown.

Diversity order=Mt x Mr

Notes:
The higher diversity order we have the
better we combat the fading

Notes:
1-The diversity
reduces the BER
of the
communication
system.
2-Diversity order
BER .

Notes:

The distance between the antennas
must be larger than the coherent
distance to ensure that data streams
are not correlated .

Question?
How the receiver get the signal from
the many copies reached ?
Answer
Diversity
combining
techniques

Maximal ratio Equal gain
Selective
combining combining
combining SC
MRC EGC

Diversity combining technique
1-Combines the independent fading paths
signals to obtain a signal that passed
through a standard demodulator.
2-The techniques can be applied to any
type of diversity.
3-combining techniques are linear as the
output of is a weighted sum of the
different fading signals of branches.
4-It needs co-phasing.

Fading of the path
The signal output from
the combiner is the
transmitted signal
s(t) multiplied by a
random complex Type of
technique
amplitude term

Diversity order

Random SNR
from the
combiner


Types of combining techniques

Selection Threshold Maximal ratio Equal gain
combining combining combining combining

selection combining technique
1-the combiner
outputs the
signal on the
branch with the
highest SNR .
2-no need here for
0 0 1 0
the co-phasing.

Threshold combining technique
As in SC since only one branch output is used
at a time and outputting the first signal with
SNR above a given threshold so that co-
phasing is not required.

Does not take the
largest SNR so that
its performance less
than the SC
technique.

Special case at diversity order
=2 (SSC)

Maximal ratio combining
In maximal ratio
combining
(MRC) the
output is a
weighted sum h1* h2* h3* hi*
of all branches
due to its SNR

Equal gain combining
technique
A simpler technique is equal-gain
combining, which co-phases the signals
on each branch and then combines them
with equal weighting

MIMO
 Traditional diversity is based on multiple receiver
antennas
 Multiple-In Multiple-Out (MIMO) is based on both
transmit and receive diversity
 Also known as Space Time Coding (STC)
 With Mt transmission antennas and Mr receiver
antennas we have Mt Mr branches
 Tx and Rx processing is performed over space
(antennas) and time (successive symbols)

47

MIMO or STC
 In Mobile communication systems it may be difficult
to put many antennas in the mobile unit
 Diversity in the downlink (from base station to
mobile station) can be achieved by Multiple-In
Single-Out (MISO) (i.e., Mr=1)
 In the uplink (from mobile station to base station)
diversity is achieved my conventional diversity
(SIMO)
 Hence, all diversity cost is moved to the base
station
 All 3G and 4G mobile communication system
employ MIMO in their standard
48

Type of MIMO
 Two major types of space time coding
◦ Space time block coding (STBC)
◦ Space time trellis coding (STTC)
 STBC is simpler by STTC can provide
better performance
 STBC is used in mobile
communications. STTC is not used in
any systems yet
 We will talk only about STBC

49

Space Time Block Codes
 There are few major types
◦ Transmit diversity: main goal is diversity gain
◦ Spatial multiplexing: main goal is increase
data rate
◦ Eigen steering: main goal is both. Requires
knowledge of the channel at the transmitter
side
◦ Mix of the above: Lots of research
 Transmit diversity, spatial multiplexing
and simplified version of Eigen steering
are used in 3G and 4G standards
 While in 3G standards MIMO was an
enhancement, in 4G MIMO is a main part 50

Transmit Diversity
 Take Mt=2 and Mr=1
 Two symbols so and s1 are transmitted over
two transmission periods
 No change in data rate (denoted as rate 1
STBC)
 Channel is known at receiver only

51

Transmit Diversity
Ant o Ant1
 Transmission matrix:
 s o s1   Timeo
S   * *
 s1 s o   Time1
 Transmission matrix columns are orthogonal to
guarantee simple linear processing at the receiver
 Other transmission matrices are defined in
literature
 Received signal  r   s
is: s 1   g o   no 
R    * *    
o o

 r1   s1 s o   g 1   n1 
 Performance is same as MRC with M=2
 However, if Tx Power is the same, then transmit
diversity (2x1) is 3 dB worse than (1x2)
52

Transmit Diversity
 Take Mt=2 and Mr=2
 Performance is the same as MRC with M=4
 However, if Tx Power is the same, then transmit
diversity (2x2) is 3 dB worse than (1x4)

53

Performance
 MRRC=Maximal Ratio Receiver Combining
 Note 3 dB difference in favor of Rx MRC diversity
Reference: S. Alamouti, a simple transmit diversity
technique for wireless communications,
IEEE JSAC, October 98

No diversity

Order 2

Order
4

54

Spatial Multiplexing
ro  s o g o  s 1 g 1

r1  s o g 2  s 1 g 3

 Purpose is to increase data rate (2x2 gives twice
data rate)
 The 4 gains must be known at receiver
 Simplest way zero forcing algorithm:
 ro   g o g 1  s o  ˆ
s o  1 H  o
r
r   g  s   G G  G  r 
H

 1  2 g 3   s1 
   ˆ1   
 1
G
55

Spatial Multiplexing
ro  s o g o  s 1 g 1

r1  s o g 2  s 1 g 3

 Optimum method: Maximum Likelihood
◦ Try all combinations of s1 and s2
◦ Find the combination that minimizes the squared error:
2 2
e  e  ro  s o g o  s1 g 1  r1  s o g 2  s1 g 3
2
o
2
1
ˆ ˆ ˆ ˆ
◦ Complexity increases with high order modulation

56

Performance
 Equal rate
comparison
 Reference: David
Gesbert, Mansoor Zero forcing
Shafi, Da-shan
Shiu, Peter J.
Smith, and Ayman
Naguib, From ML
theory to practice:
an overview of
MIMO space–time Alamouti
coded wireless
systems, IEEE
JSAC, April 2003

57

Eigenvalue Steering
 Assume a MIMO system

58

Eigenvalue Steering
 Example with Mt = 2 and Mr=4
 y 1   h11 h12 
 y  h
 n1   y   H x   n 
 2    21 h22   x 1   n 2 
  
x    n 
 y 3   h31 h32   2  3
     
 y 4   h41 h42  n 4 
 Any matrix H can be represented using
H

Singular Value Decomposition as
H U V H
 U is Mr by Mr and V is Mt by Mt unitary
matrices
  is Mr by Mt diagonal matrix, elements σi

59

Eigenvalue Steering
Using transmit pre-coding and receiver shaping 

y U H H x  n 
 U H U V H
x n
 U H U V H V x  n 
 U H U V H V x U H n
  x n
60

Eigenvalue Steering
 This way we created r paths between the Tx and
specific Rx without any cross interference
 The channel (i.e., Channel State Information) must
be known to both transmitter and receiver
 The value of r = rank of matrix H, r min(Mt, Mr)
 Not all r paths have good SNR
 Data rate can increase by factor r
 See Appendix C for Singular Value Decomposition
 See Matlab function [U,S,V] = svd(X)

61

Example
 Reference: Sanjiv Nanda, Rod Walton, John Ketchum, Mark
Wallace, and Steven Howard, A high-performance MIMO OFDM
wireless LAN, IEEE Communication Magazine, February 2005

62

INTRODUCTION TO LTE AND ITS
UNIQUE TECHNOLOGIES.

What is LTE??
 The 3GPP LTE is acronym for “long term evolution of
UMTS “.
 In order to ensure the competitiveness of UMTS for the
next 10 years and beyond, concepts for UMTS Long
Term Evolution (LTE) have been introduced in 3GPP
release 8.
 LTE is also referred to as EUTRA (Evolved UMTS
Terrestrial Radio Access) or E-UTRAN (Evolved
UMTS Terrestrial Radio Access Network)

What is LTE(cont.)?
 The architecture that will result from this work is called
EPS (Evolved Packet System) and comprehends E-
UTRAN (Evolved UTRAN) on the access side and EPC
(Evolved Packet Core) on the core side.
 Can be considered the real 3.9G & invited to join the
4G family.
 Also considered a competitive system to mobile
WiMAX as we will show

(a) capabilities:-
 Scalable BW: 1.25, 2.5, 5.0, 10.0 and 20.0 MHz.
 Peak data rate:
 Downlink (2 Ch MIMO) peak rate of 100 Mbps in 20
MHz channel
 Uplink (single Ch Tx) peak rate of 50 Mbps in 20 MHz
channel
 Supported antenna configurations:
 Downlink: 4x4,4x2, 2x2, 1x2, 1x1
 Uplink: 1x2, 1x1
 Duplexing modes: FDD and TDD
 Number of active mobile terminals:
 LTE should support at least 200 mobile terminals in the
active state when operating in 5 MHz.
 In wider allocations than 5 MHz, at least 400 terminals
should be supported

 Spectrum efficiency
 Downlink: 3 to 4 x HSDPA Rel. 65bits/s/Hz
 Uplink: 2 to 3 x HSUPA Rel. 62.5bits/s/hz
 Latency
 C-plane: <50 – 100 msec to establish U-plane
 U-plane: <10 msec from UE to server
 Mobility
 Optimized for low speeds (<15 km/hr)
 High performance at speeds up to 120 km/hr
 Maintain link at speeds up to 350 km/hr
 Coverage
 Full performance up to 5 km
 Slight degradation 5 km – 30 km
 Operation up to 100 km should not be precluded by
standard

INTRODUCTION TO LTE
KEY TECHNOLOGIES

(1)OFDM and OFDMA:-
 One of the key technologies used in LTE and WiMAX
systems.
 The problem ???
 Due to the multipath the signal is received from many paths with
different phases that will result in
 DELAY SPREAD :symbol received along a delayed path to
“bleed” into a subsequent symbol (ISI)

 FREQUENCY SELECTIVE FADING: : some frequencies
within the signal passband undergo constructive interference
while others encounter destructive interference.The composite
received signal is distorted

 Old solutions of multipath fading include direct channel
equalization or spread spectrum techniques(complex receiver is
needed).

 OFDM:
 OFDM systems break the available bandwidth into
many narrower sub-carriers and transmit the data in
parallel streams
 each OFDM symbol is preceded by a cyclic prefix
(CP), which is used to effectively eliminate ISI.

 In practice, the OFDM signal can be generated using IFFT
 with a CP of sufficient duration, preceding symbols do not spill
over into the FFT period and also this satisfy that the output
convolution with channel is complex gain multiplication.
 Also, Once the channel impulse response is determined (by periodic
transmission of known reference signals), distortion can be
corrected by applying an amplitude and phase shift on a subcarrier-
by-subcarrier basis.
 Problems of OFDM are: susceptibility to carrier frequency errors
(due either to local oscillator offset or Doppler shifts) and a large
signal peak-to-average power ratio (PAPR).

OFDMA
 OFDMA is a multiple access method based on OFDM
signaling that allows simultaneous transmissions to
and from several users along with the other
advantages of OFDM.

(2) Multi antenna transmission
 LTE and WiMAX targets extreme
performance in terms of
◦ Capacity
◦ Coverage
◦ Peak data rates
Advanced multi-antenna solutions is
the key tool to achieve this
 Multi antenna systems are integral part of
those systems
 Different antenna solutions needed for
different scenarios/targets
◦ High peak data rates  spatial multiplexing
◦ Good coverage Beam-forming
◦ High performanceDiversity

(3)Hybrid ARQ with soft
combining
 used in LTE and WiMAX to allow the terminal to rapidly
request retransmissions of erroneously received transport
blocks.
 The underlying protocol multiple parallel stop-and-wait hybrid
ARQ processes
 Incremental redundancy is used as the soft combining
strategy and the receiver buffers the soft bits to be able to do
soft combining between transmission attempts.

(1)Spectrum flexibility:

 A high degree of spectrum flexibility is
one of the main characteristics of the
LTE radio access.
 The aim of this spectrum flexibility is to
allow for the deployment of the LTE
radio access in diverse spectrum.
 The flexibility includes:
◦ Different duplex arrangements.
◦ Different frequency-bands-of-operation.
◦ Different sizes of the available spectrum.

(a) 3G LTE – Duplex arrangement

(b) 3G LTE – Bandwidth flexibility

LTE physical layer supports any bandwidth from 1.25
MHz to well beyond 20 MHz in steps of 200 kHz (one
”Resource Block”)

(2) Channel-dependent scheduling and rate
adaptation
 LTE use of shared-channel transmission,
in which the time-frequency resource is
dynamically shared between users.

(3)Interference coordination(soft reuse)
 Adaptive reuse
◦ Cell-center users: Reuse = 1
◦ Cell-edge users: Reuse > 1
 Relies on access to frequency domain
◦ Applicable for both downlink OFDM and
uplink SC-FDMA

(4)SC-FDMA:-
 LTE uplink requirements differ from downlink
requirements.
 power consumption is a key consideration for UE
terminals.
 The high PAPR and related loss of efficiency associated
with OFDM signaling are major concerns.
 As a result, an alternative to OFDM was sought for use
in the LTE uplink.
 Single Carrier – Frequency Domain Multiple Access
(SC-FDMA) is well suited to the LTE uplink
requirements.
 The basic transmitter and receiver architecture is very
similar (nearly identical) to OFDMA,
 and it offers the same degree of multipath protection.
 because the underlying waveform is essentially single-

 Basic block diagram:

 transmitter :a QAM modulator coupled with the addition
of the cyclic prefix. This will eliminate ISI as OFDMA
 Reciever: by using FFT & CP simple equalizer are
used (as OFDM).
 Multipath distortion is handled in the same manner as in
OFDM(removal of CP, conversion to the frequency
domain, then apply the channel correction on a subcarrier-
by subcarrier basis).

LTE practical SC-FDMA :-

 The practical transmitter is likely to take advantage of FFT/IFFT blocks
as well to place the transmission in the correct position of the
transmit spectrum in case of variable transmission bandwidth.

SC-FDMA receiver

Frequency domain equalization (FDE) using
DFT/IDFT is more practical for such channels.

 The fact of transmitting only a single symbol at a time
ensures a low transmitter waveform, compared with the
OFDMA case.
 The resulting PAR/CM impact on the amplifier is thus directly
dependent on the modulation, whereas with the OFDMA case
it is the amount of subcarriers.
 SC-FDMA subcarriers can be mapped in one of two ways:
localized or distributed
 However, the current working assumption is that LTE will use
localized subcarrier mapping.
 This decision was motivated by the fact that with localized
mapping, it is possible to exploit frequency selective gain
via channel dependent scheduling (assigning uplink
frequencies to UE based on favorable propagation
conditions).

(5) LTE Multicast/Broadcast
 MBMS – Multimedia Broadcast/Multicast Service
 OFDM allows for high-efficient MBSFN operation
◦ Multicast/Broadcast Single-Frequency Networking
◦ Identical transmissions from set of tightly synchronized
cells
◦ Increased received power and reduced interference
Substantial boost of MBMS system throughput
 LTE allows for multicast/broadcast and unicast on
the same carrier as well as dedicated
multicast/broadcast carrier

LTE RADIO INTERFACE
ARCHITECTURE

Introduction
 Similar to WCDMA/HSPA, as well as to most other modern
communication systems, the processing specified for LTE
is structured into different protocol layers.
 note that the LTE radio-access architecture consists of a
single node –the eNodeB. The eNodeB communicates with
one or several mobile terminals, also known as UEs

Packet Data Convergence
Protocol (PDCP)
 performs IP header compression
 to reduce the number of bits to transmit over the radio
interface.
 The header compression mechanism is based on Robust
Header Compression (ROHC)a standardized header-
compression algorithm also used in WCDMA
 PDCP is also responsible for ciphering and integrity
protection of the transmitted data. At the receiver side, the
PDCP protocol performs the corresponding deciphering and
decompression operations.
 There is one PDCP entity per SAE bearer configured for a
mobile terminal

Radio Link Control (RLC)
 is responsible for segmentation/concatenation, retransmission
handling, and in-sequence delivery to higher layers.
 Unlike WCDMA, the RLC protocol is located in the eNodeB since
there is only a single type of node in the LTE radio-access-network
architecture.
 The RLC offers services to the PDCP in the form of radio bearers .
 There is one RLC entity per radio bearer configured for a terminal.

Medium Access Control
(MAC)
 handles hybrid-ARQ retransmissions and uplink
and downlink scheduling.
 The scheduling functionality is located in the
eNodeB, which has one MAC entity per cell, for
both uplink and downlink.
 The hybrid-ARQ protocol part is present in both
the transmitting and receiving end of the MAC
protocol.
 The MAC offers services to the RLC in the form of
logical channels .

MAC scheduling

The basic operation of the scheduler is so-called dynamic scheduling, where the
eNodeB in each 1 ms TTI makes a scheduling decision and sends scheduling
information to the selected set of terminal.

Downlink
UL scheduling
scheduling
 dynamically controlling  dynamically control
the terminal(s) to which mobile terminals
transmit to are to transmit on their
 the set of resource UL-SCH
blocks upon which the
terminal’s DL-SCH  and on which uplink
should be transmitted. time/frequency resources
 Transport-format  uplink scheduling
selection(selection of decision is taken per
transport-block size, mobile terminal and not
modulation scheme, and per radio bearer.
antenna mapping)
 And logical-channel
multiplexing for downlink
transmissions

Physical Layer (PHY)
 handles coding/decoding, modulation/demodulation,
multi-antenna mapping, and other typical physical
layer functions.
 The physical layer offers services to the MAC layer
in the form of transport channels

LTE Generic Frame Structure
 The generic frame structure is used with FDD.(TDD is also
supported but not the trend).
 LTE frames are 10 msec in duration.
 They are divided into 10 subframes, each subframe being 1.0 msec
long.
 Each subframe is further divided into two slots, each of 0.5 msec
duration.
 Slots consist of either 6 or 7 ODFM symbols, depending on whether
the normal or extended cyclic prefix is employed.

 Different time intervals within the LTE radio-access
specification are defined as multiples of a basic time
unit Ts = 1/30 720 000.
 The time intervals can thus also be expressed as
Tframe = 307 200 Ts and Tsubframe = 30 720 Ts

OFDMA For LTE Downlink:-
 OFDMA is an excellent choice of multiplexing scheme for the 3GPP LTE
downlink
 allows the access of multiple users on the available bandwidth.
 Each user is assigned a specific time-frequency resource.
 Allocation of PRBs is handled by a scheduling function at the 3GPP base
station (eNodeB).
 The total number of available subcarriers depends on the overall transmission
bandwidth of the system. The LTE specifications define parameters for system
bandwidths from 1.25 MHz to 20 MHz as shown in Table.

 A PRB is defined as
consisting of 12 consecutive
subcarriers for one slot (0.5
msec) in duration.
 A PRB is the smallest
element of resource allocation
assigned by the base station
scheduler.
 LTE does not employ a PHY
preamble to facilitate carrier offset
estimate, channel estimation, timing
synchronization etc. Instead, special
reference signals are embedded in
the PRBs

Downlink resource block
 the OFDM subcarrier spacing has been chosen to Δf = 15 kHz.
 Sampling rate fs =15 000NFFT , where NFFT is the FFT size
 the sampling rate Δf NFFT will be a multiple or submultiple of the
WCDMA/HSPA chip rate (3.84 Mcps)
 in the frequency domain the downlink subcarriers are grouped into
resource blocks
 where each resource block consists of 12 consecutive subcarriers.
In addition, there is an unused DC-subcarrier in the center of the
downlink band. it may be subject to un-proportionally high
interference, for example, due to local-oscillator leakage.

Downlink reference signal
 To carry out coherent demodulation of different
downlink physical channels,
 a mobile terminal needs estimates of the downlink
channel
◦ Cell-specific downlink reference signals.
◦ UE-specific reference signal.
◦ MBSFN reference signals

Cell-specific downlink reference
signals
 consists of known reference symbols inserted within the first and third last
OFDM symbol of each slot and with a frequency-domain spacing of six
subcarriers
 the mobile terminal should carry out interpolation/averaging over multiple
reference symbols
 There are 504 different reference-signal sequences defined for LTE, where
each sequence corresponds to one out of 504 different physical-layer cell
identities

 In case of downlink multi-antenna transmission the mobile
terminal should be able to estimate the downlink channel
corresponding to each transmit antenna
 reference-signal structure for each antenna port in case of multiple
antenna ports within a cell:
◦ In case of two antenna the reference symbols of the second antenna
port are frequency multiplexed with the reference symbols of the first
antenna port, with a frequency-domain offset of three subcarriers.
◦ In case of four antenna ports ,the reference symbols for the third and
fourth antenna ports are frequency multiplexed within the second OFDM
symbol of each slot. Note that the reference symbols for antenna port
three and four are only transmitted within one OFDM symbol

UE-specific reference signals
 LTE also allows for more general beam-forming. In
order to allow for channel estimation also for such
transmissions, additional reference signals are
needed.
 As such a reference signal can only be used by the
specific terminal to which the beam-formed
transmission is intended, it is referred to as a UE-
specific reference signal .

LTE block diagram (DL transport
channel processing)

(1)CRC insertion:

 In the first step of the transport-channel processing, a
24-bit CRC is calculated for and appended to each
transport block.
 The CRC allows for receiver side detection of errors in
the decoded transport block.
 The corresponding error indication is then, for
example, used by the downlink hybrid-ARQ protocol as
a trigger for requesting retransmissions.

(2)Code-block segmentation and
per-code-block CRC insertion:
 The LTE Turbo-coder internal interleaver is only defined for a
limited number of code-block sizes with a maximum block size
of 6144 bits.
 In case the transport block, including the transport-block CRC,
exceeds this maximum code-block size, code-block
segmentation is applied before Turbo coding.
 Code-block segmentation implies that the transport block is
segmented into smaller code blocks that match the set of code-
block sizes defined for the Turbo coder.

 In order to ensure that the size of
each code block is matched to the
set of available code-block sizes,
filler bits may have to be inserted at
the head of the first code
 An additional (24 bits) CRC is
calculated for and appended to each
code block.
 Having a CRC per code block allows
for early detection of correctly
decoded code blocks. This can be
used to reduce the terminal
processing effort and power
consumption.

(3) FEC(forward error
correction):-
 The UL-SCH uses the same rate 1/3 turbo encoding scheme (two 8-
state constituent encoders and one internal interleaver) as the DL-
SCH.

•The older interleaver used in HSPA been replaced by QPP
based interleaving .
•the QPP interleaver provides a mapping from the input (non-
interleaved) bits to the output (interleaved) bits according to the
function:

(4) Rate-matching and physical-
layer hybrid-ARQ functionality
 The task of the rate-matching and physical-layer hybrid-ARQ
functionality is to extract, from the blocks of code bits delivered by the
channel encoder, the exact set of bits to be transmitted within a given
TTI.
 The outputs of the Turbo encoder (systematic bits, first parity bits, and
second parity bits) are first separately interleaved.
 The interleaved bits are then inserted into what can be described as a
circular buffer with the systematic bits inserted first, followed by
alternating insertion of the first and second parity bits.
 The bit selection then extracts consecutive bits from the circular buffer

(5) Bit-level scrambling
 LTE downlink scrambling implies that the block of code bits delivered by the
hybrid-ARQ functionality is multiplied (exclusive-or operation) by a bit-
level scrambling sequence (usually a gold code).
 In general, scrambling of the coded data helps to ensure that the receiver-
side decoding can fully utilize the processing gain provided by the channel
code

(6) Modulation
 The set of modulation schemes supported for the LTE downlink
includes QPSK, 16QAM, and 64QAM.
 All these modulation schemes are applicable to the DL-SCH, PCH, and
MCH transport channels.
 only QPSK modulation can be applied to the BCH transport channel.

(7) Multi antenna transmission
 LTE supports the following multi-antenna
transmission schemes or transmission modes , in
addition to single-antenna transmission:
◦ Transmit diversity
◦ Closed-loop spatial multiplexing including codebook-
based beam-forming
◦ Open-loop spatial multiplexing

Transmit diversity
 LTE transmit diversity is based on Space Frequency
Block Coding (SFBC)
 SFBC implies that consecutive modulation symbols Si
and Si+1 are mapped directly on adjacent subcarriers on
the first antenna port.
 On the second antenna port, the swapped and
transformed symbols - S*i+1 and Si*are transmitted on
the corresponding subcarriers

SFBC/FSTD(combined SFBC
Frequency Shift Transmit )and
Diversity

Closed loop Spatial multiplexing
 spatial multiplexing implies that multiple streams or ‘ layers ’
are transmitted in parallel, thereby allowing for higher data rates
 The LTE spatial multiplexing may operate in two different
modes: closed-loop spatial multiplexing and open-loop spatial
multiplexing
 where closed-loop spatial multiplexing relies on more extensive
feedback from the mobile terminal.

General beam-forming
 closed-loop spatial multiplexing includes beam-forming as a special
case when the number of layers equals one.
 This kind of beamforming can be referred to as codebook-based
beam-forming , indicating that
◦ the network selects one pre-coding vector (the beam-forming vector)
from a set of pre-defined pre-coding vectors (the ‘ codebook ’ ) with the
selection, for example, based on the terminal reporting a recommended
pre-coding vector.
◦ if not following the terminal recommendation, the network must
explicitly inform the terminal about what pre-coding vector, from the set
of predefined vectors, is actually used for transmission to the terminal.

Uplink transmission scheme
 LTE uplink transmission is based on so-called DFTS-
OFDM transmission
 Which is a‘ single-carrier ’ transmission scheme that
allows for
◦ flexible bandwidth assignment
◦ orthogonal multiple access not only in the time domain but
also in the frequency domain.
◦ the use of a cyclic prefix allows low-complexity frequency-
domain equalization at the receiver side.

Transmission According to
method
OFDM mod.
“M” position of signal
determines is determined
the BW

Mapping is
applied to
consecutive
carriers
localized

DFT implementation
 The DFT size should preferably be constrained to a power of two.
 However, such a constraint is in direct conflict with a desire to
have a high degree of flexibility of the bandwidth that can be
dynamically assigned to a mobile terminal for uplink
transmission all possible DFT sizes should rather be allowed.
 For LTE, a middle way has been adopted where the DFT size is
limited to products of the integers two, three, and five.
 For example, DFT sizes of 60, 72, and 96 are allowed but a DFT
size of 84 is not allowed.
 In this way, the DFT can be implemented as a combination of
relatively low-complex radix-2, radix-3, and radix-5 FFT
processing

Uplink physical resource
parameters
 Chosen to be aligned, as much as possible, with the
corresponding parameters of the OFDM-based LTE
downlink
◦ spacing equals 15 kHz
◦ resource blocks, consisting of 12 subcarriers
◦ Any number of uplink resource blocks ranging from a minimum of
6-110 resource blocks.
◦ time-domain structure, the LTE uplink is very similar to the
downlink
 However, in contrast to the downlink, no unused DC-
subcarrier is defined for the LTE uplink

Uplink reference signals
 Demodulation reference signals (DRS )
◦ reference signals for channel estimation are also needed
for the LTE uplink to enable coherent demodulation of
different uplink physical channels
 Sounding reference signals (SRS)
◦ are transmitted on the uplink to allow for the network to
estimate the uplink channel quality at different
frequencies.

Basic principles of uplink DRS
transmission
 Due to the importance of low power variations for
uplink transmissions
 The principles for uplink reference-signal
transmission are different from those of the
downlink
 certain DFTS-OFDM symbols are exclusively used
for reference-signal transmission,
 a reference signal is transmitted within the fourth
symbol of each uplink slot

Uplink sequences
 Limited power variations in the frequency domain to
allow for similar channel-estimation quality for all
frequencies.
 Limited power variations in the time domain to allow
for high power-amplifier efficiency.
 Furthermore, sufficiently many reference-signal
sequences of the same length, should be available to
easily assigning reference-signal sequences to cells

Zadoff–Chu sequences
 have the property of constant power in both the frequency
and the time domain.

 Zadoff–Chu sequences are not suitable for direct usage as
uplink:
◦ to maximize the number of Zadoff–Chu sequences and to maximize
the number of available uplink reference signals, prime-length
Zadoff–Chu sequences would be preferred. At the same time, the
length of the uplink reference-signal sequences should be a
multiple of 12
◦ For short sequence lengths, corresponding to narrow uplink
transmission bandwidths, relatively few reference-signal
sequences would be available

Phase-rotated reference-signal
sequences
 by cyclically extending different prime-length Zadoff – Chu
sequences .
 Additional reference-signal sequences can be derived by
applying different linear phase rotations to the same basic
reference-signal sequences

sounding reference signals
(SRS)
 estimate the uplink channel quality at different frequencies
 A terminal can be configured to transmit SRS at regular
intervals ranging from as often as once in every 2 ms (every
second subframe) to as infrequently as once in every 160 ms
(every 16th frame
 the frequency-domain scheduling:
◦ entire frequency band of interest with a single SRS OR
◦ narrowband SRS that is hopping in the frequency domain in such a way
that a sequence of SRS transmissions jointly covers the frequency band of
interest.

Uplink transport-channel
processing
 uplink transport-channel
processing are similar to the
corresponding steps of the
downlink transport-channel
processing
 no spatial multiplexing or
transmit diversity currently
defined for the LTE uplink
 As a consequence, there is also
only a single transport block, of
dynamic size, transmitted for each
TTI.

LTE cell search
 Aim
◦ Acquire frequency and symbol synchronization to a cell.
◦ Acquire frame timing of the cell, that is, determine the start of the
downlink frame.
◦ Determine the physical-layer cell identity of the cell.
 two special signals are transmitted on the LTE downlink,
◦ the Primary Synchronization Signal (PSS)
◦ Secondary Synchronization Signal (SSS)

a terminal synchronizes to a cell.
Once it knew PSS 5ms delay

acquires the physical-layer identity
(but not the identity group) of the cell
using PSS .

Acquires physical layer identity group
using SSS signal

detects the cell frame timing using SSS
signal

Once this has been achieved, the
terminal has to acquire the cell system
information

System information
 In LTE, system information is delivered by two
different mechanisms relying on two different transport
channels
◦ A limited amount of system information, corresponding to the
so-called Master Information Block (MIB), is transmitted
using the BCH.
◦ The main part of the system information, corresponding to
different so-called System Information Blocks (SIBs), is
transmitted using the downlink shared channel (DL-SCH).

Random access
 A fundamental requirement for any cellular system is the possibility
for the terminal to request a connection setup, commonly referred
to as random access .
 In LTE, random access is used for several purposes, including:
◦ for initial access when establishing a radio link (moving from
RRC_IDLE to RRC_CONNECTED;
◦ to re-establish a radio link after radio link failure;
◦ for handover when uplink synchronization needs to be established to the
new cell;
◦ to establish uplink synchronization if uplink or downlink data arrives
when the terminal is in RRC_CONNECTED and the uplink is not
synchronized;
◦ as a scheduling request if no dedicated scheduling-request resources have
been configured on PUCCH.

The first step consists of transmission of a random-access
preamble, allowing the eNodeB to estimate the transmission
timing of the terminal. Uplink synchronization is necessary as the
terminal otherwise cannot transmit any uplink data.

The second step consists of the network transmitting a timing
advance command to adjust the terminal transmit timing, based on
the timing estimate in the first step. In addition to establishing
uplink synchronization, the second step also assigns uplink
resources to the terminal to be used in the third step in the
random-access procedure.

The third step consists of transmission of the mobile-terminal
identity to the network using the UL-SCH similar to normal
scheduled data. The exact content of this signaling depends on
the state of the terminal, in particular whether it is previously
known to the network or not.

The fourth and final step consists of transmission of a
contention-resolution message from the network to the
terminal on the DL-SCH. This step also resolves any
contention due to multiple terminals trying to access the
system using the same random-access resource.

paging
 Paging is used for network-initiated connection
setup.
 An efficient paging procedure should allow the
terminal to sleep with no receiver processing most
of the time and to briefly wake up at predefined
time intervals to monitor paging information from
the network.
 In LTE, no separate paging-indicator channel is
used

LTE System Architecture cont.
 Evolved Radio Access Network (RAN)
UE: User Equipment
eNB: enhanced Node B

-Contains PHY, MAC, RLC (Radio Link Control)
, PDCP (Packet Data Control Protocol).
 eNBs are connected together through the SGW.

Functions of eNodeB:
 Radio Resources management.
 Admission control.
 Enforcement of negotiated UL QoS.
 Cell information broadcast.
 Ciphering/deciphering of user and
control plane data
 Compression/decompression of
DL/UL user plane packet headers.

 Serving Gateway (SGW)
-Routes and forwards user Data Packets.
-Mobility anchor for eNB handovers and LTE to other
3GPP systems.
(relaying the traffic between 2G/3G systems and PDN
GW).

 Packet Data Network Gateway (PDN GW)
-Connects UE to external packet data networks (serve IP
functions)
-Anchor for mobility between 3GPP and non-3GPP
technologies such as WiMAX and 3GPP2 (CDMA 1X and
EvDO).

- Performs policy enforcement , charging
support.

LTE System Architecture
cont.
 Mobility Management Entity (MME)
-Manage the UE’s mobility.
-Idle-mode UE tracking and reachability .
-Paging procedure.
-Authentication and authorization.
- choosing the SGW for a UE at
the initial attach
-Security negotiations.

Fundamental requirements for
LTE-Advanced
 complete fulfillment of all the requirements for IMT-
Advanced defined by ITU
 LTE-Advanced has to fulfill a set of basic backward
compatibility requirements
◦ Spectrum coexistence, implying that it should be possible to
deploy LTE-Advanced in spectrum already occupied by LTE
with no impact on existing LTE terminals
◦ infrastructure, in practice implying that it should be
possible to upgrade already installed LTE infrastructure
equipment to LTE-Advanced capability
◦ terminal implementation

Extended requirements beyond
ITU requirements
 Support for peak-data up to 1 Gbps in the downlink and 500
Mbps in the uplink.
 Substantial improvements in system performance such as
cell and user throughput with target values significantly
exceeding those of IMT-Advanced.
 Possibility for low-cost infrastructure deployment and
terminals.
 High power efficiency, that is, low power consumption for
both terminals and infrastructure.
 Efficient spectrum utilization, including efficient utilization
of fragmented spectrum

Technical components of LTE-
Advanced
 Wider bandwidth and carrier aggregation
 Extended multi-antenna solutions
 Advanced repeaters and relaying functionality
 Coordinated multi-point transmission

Wider bandwidth and carrier
aggregation
 LTE-Advanced will be an increase of the maximum transmission
bandwidth beyond 20 MHz, perhaps up to as high as 100 MHz or
even beyond
 In case of carrier aggregation, the extension to wider bandwidth is
accomplished by the aggregation of basic component carriers of a
more narrow bandwidth

Extended multi-antenna solutions
 support for spatial multiplexing on the uplink is
anticipated to be part of LTE-Advanced
 extension of downlink spatial multiplexing to more
four layers
 benefits of eight-layer spatial multiplexing are only
present in special scenarios where high SINR can
be achieved

Coordinated multi-point
transmission
 Coordinating the transmission from the multiple antennas can be used to
increase the signal-to-noise ratio for users far from the antenna
 for example by transmitting the same signal from multiple sites.
 Such strategies can also improve the power-amplifier utilization in the
network, especially in a lightly loaded network where otherwise some power
amplifiers would be idle

Advanced repeaters and relaying
functionality
 Repeaters simply amplify and forward the received analog signals and are
used already today for handling coverage holes.
 “L1 relays”schemes where the network can control the transmission power
of the repeater and, for example, activate the repeater only when users are
present in the area handled by the repeater
 intermediate node may also decode and re-encode any received data prior to
forwarding it to the served users. This is often referred to as decode-and-
forward relaying

The proposals could roughly be
categorized into:
 Various concepts for Relay Nodes
 UE Dual TX antenna solutions for SU-MIMO and diversity MIMO
 Scalable system bandwidth exceeding 20 MHz, Potentially up to 100MHz
 Local area optimization of air interface
 Nomadic / Local Area network and mobility solutions
 Flexible Spectrum Usage
 Cognitive Radio
 Automatic and autonomous network configuration and operation
 Enhanced precoding and forward error correction
 Interference management and suppression
 Asymmetric bandwidth assignment for FDD
 Hybrid OFDMA and SC-FDMA in uplink
 UL/DL inter eNodeB coordinated MIMO

Timeframe
 Standardization is expected to be included in 3GPP
Release 10 timeframe.
 The importance and timeframe of LTE Advanced
will of course largely depend on the success of LTE
itself.
 If possible LTE-Advanced will be a software
upgrade for LTE networks.

Technology
Demonstrations
 In February 2007 NTT DoCoMo announced the
completion of a 4G trial
 where they achieved a maximum packet
transmission rate of approximately 5 Gbit/s in the
downlink using 100MHz frequency bandwidth to a
mobile station moving at 10 km/h

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