OFDM AND OFDMA TECHNOLOGIES
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.
ISI is significant in this case.
Pulse extended but the extension
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.
NEED FOR MULTI-
CARRIER cont.
Problem
Solution Orthogonality
NEED FOR MULTI-CARRIER cont.
NEED FOR MULTI-CARRIER cont.
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
ADC FFT
FFT/IFFT
GUARD TIME
INSERTION
X1 X2 …. Xn ….
Y1 Y2 …. Yn ….
Channel Filtering
hv …. h1 h0
GUARD TIME INSERTION
cont.
X1 X2 …. Xn ….
Y’’1 Y’’2 Yv+1 Yn Yv+2 …. Y’’v
X’1 X’2 …. X’n ….
….
Problem
ISI occurs
hv …. h1 h0
GUARD TIME INSERTION
cont.
X1 X2 …. Xn Xn-v+1
Xn ….
Y1 Y2 Yv+1 Yn Yv+2 …. Yv
X’1 X’n ….
….
Solution Cyclic Prefix
No ISI
Circular Convolution achieved.
hv …. h1 h0
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
Pilot
Signal
Extraction
Lowpass
FIR
Filter
Pilot
Signal
Estimation
CHANNEL ESTIMATION
Pilot Based Channel Estimation
Received
Signal after
FFT
Estimated
Channel
Response
Known Pilots
CHANNEL ESTIMATION
cont.
Fre
qu
en
cy(
su
b c
arr
iers
)
Data symbols
Pilot
symbols
Time (OFDM Symbols) Time (OFDM Symbols) F
req
ue
ncy(
su
b c
arr
iers
)
Pilot Arrangement Types
Block Pilot Patterns Comb Pilot Patterns
High channel frequency
selectivity rapid changing channels
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
DIVERSITY AND MIMO PRINCIPLES
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 diversity
Frequency diversity
Spatial 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:
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.
|H(t)|
t
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
MISO SIMO MIMO 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
Selective combining SC
Maximal ratio combining
MRC
Equal gain combining
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.
Diversity combining technique
The signal output from
the combiner is the
transmitted signal
s(t) multiplied by a
random complex
amplitude term
Random SNR
from the
combiner
Fading of the path
Type of
technique
Diversity order
Diversity combining technique
Types of combining techniques
Selection combining
Threshold combining
Maximal ratio combining
Equal gain combining
selection combining technique
1-the combiner
outputs the
signal on the
branch with the
highest SNR .
2-no need here for
the co-phasing. 0 0 0 1
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.
Special case at diversity order
=2 (SSC)
Does not take the
largest SNR so that
its performance less
than the SC
technique.
Maximal ratio combining
In maximal ratio combining (MRC) the output is a weighted sum of all branches due to its SNR
h1* h2* h3* hi*
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
Transmission matrix:
Transmission matrix columns are orthogonal to guarantee simple linear processing at the receiver
Other transmission matrices are defined in literature
Received signal is:
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
1
* *
11 1 1
oo o o
o
s sr g nR
s sr g n
1
1
* *
1 1
oAnt Ant
o o
o
s s TimeS
s s Time
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
54
Order 2
Order
4
No diversity
Spatial Multiplexing
Purpose is to increase data rate (2x2 gives twice
data rate)
The 4 gains must be known at receiver
Simplest way zero forcing algorithm:
55
1 1o o or s g s g
1 2 1 3or s g s g
1
2 31 1
oo o
G
g gr s
g gr s
1
1 1
ˆ
ˆ
o oH Hs r
G G Gs r
Spatial Multiplexing
Optimum method: Maximum Likelihood
◦ Try all combinations of s1 and s2
◦ Find the combination that minimizes the squared error:
◦ Complexity increases with high order modulation
56
2 22 2
1 1 1 1 2 1 3ˆ ˆ ˆ ˆ
o o o o oe e r s g s g r s g s g
1 1o o or s g s g
1 2 1 3or s g s g
Performance Equal rate
comparison
Reference: David
Gesbert, Mansoor
Shafi, Da-shan
Shiu, Peter J.
Smith, and Ayman
Naguib, From
theory to practice:
an overview of
MIMO space–time
coded wireless
systems, IEEE
JSAC, April 2003
57
Zero forcing
ML
Alamouti
Eigenvalue Steering
Assume a MIMO system
58
Eigenvalue Steering Example with Mt = 2 and Mr=4
Any matrix H can be represented using Singular Value Decomposition as
U is Mr by Mr and V is Mt by Mt unitary matrices
is Mr by Mt diagonal matrix, elements σi
59
y H x n 1 11 12 1
2 21 22 1 2
3 31 32 2 3
4 41 42 4
H
y h h n
y h h x n
y h h x n
y h h n
HH U V
Eigenvalue Steering
Using transmit pre-coding and receiver shaping
60
H
H H
H H
H H H
y U H x n
U U V x n
U U V V x n
U U V V x U n
x n
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
What is LTE (cont.)?
LTE DESIGN TARGETS
-(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. 65bits/s/Hz
Uplink: 2 to 3 x HSUPA Rel. 62.5bits/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
-)OFDM and OFDMA:1( 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.
OFDM versus OFDMA
IEEE802.16d IEEE802.16e
Fixed WiMAX,256-OFDM Mobile WiMAX
) Multi antenna transmission2(
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 performanceDiversity
)Hybrid ARQ with soft 3(
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.
)Spectrum flexibility:1(
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.
Duplex arrangement –G LTE 3(a)
Bandwidth flexibility –G LTE 3(b)
LTE physical layer supports any bandwidth from 1.25
MHz to well beyond 20 MHz in steps of 200 kHz (one
”Resource Block”)
dependent scheduling and rate -) Channel2(
adaptation LTE use of shared-channel transmission,
in which the time-frequency resource is dynamically shared between users.
)Interference coordination(soft reuse)3(
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-
carrier, the PAPR is lower.
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).
-FDMA :-LTE practical SC
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
scheduling UL scheduling
dynamically controlling
the terminal(s) to
transmit to
the set of resource
blocks upon which the
terminal’s DL-SCH
should be transmitted.
Transport-format
selection(selection of
transport-block size,
modulation scheme, and
antenna mapping)
And logical-channel
multiplexing for downlink
transmissions
dynamically control
which mobile terminals
are to transmit on their
UL-SCH
and on which uplink
time/frequency resources
uplink scheduling
decision is taken per
mobile terminal and not
per radio bearer.
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
DOWNLINK PHY LAYER OF (LTE)
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
and )Frequency Shift Transmit
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 PHY LAYER OF (LTE)
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 method
“M”
determines
the BW
According to
OFDM mod.
position of signal
is determined
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 ACCESS PROCEDURE
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 ARCHITECTURE AND SAE
LTE System Architecture
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.
cont.LTE System Architecture
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.
cont.LTE System Architecture
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.
OVERVIEW OF LTE ADVANCED
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