Networks: from GSM to LTE

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12 Δεκ 2013 (πριν από 4 χρόνια και 19 μέρες)

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Maciej
Stasiak, Mariusz
Głąbowski

Arkadiusz
Wiśniewski, Piotr Zwierzykowski

LTE

Modeling and Dimensioning of Mobile
Networks:

from
GSM to LTE

LTE


Introduction

1/
1


The International Telecommunications Union has defined the
requirements for a fourth generation system


One of the systems for which the standardization process is already
much advanced

in line with the requirements set up by ITU
-
R for
4G systems

is the Third Generation Partnership Project Long
-
Term
Evolution (3GPP LTE)


The most important requirements identified for the LTE systems can
be itemized in the following way:

o
cost reduction in network data transmission (per bit):


improvement of spectrum efficiency


cost reduction in backhaul data transmission

o
reduction in setup time and round trip delay

o
improvement in functioning of quality of service (
QoS
) mechanisms for various services

o
focus on services utilizing the IP protocol

o
broadening of multimedia multicasting services for selected groups of users (enhanced
MBMS)

o
increase in the transmission rate to over 100 mbps in the downlink and 50 mbps in the uplink
direction

o
flexibility in the use of existing and new spectral resources

o
possibility of carrier allocation with different bandwidth, ranging from 1.25 to 20 MHz


System
Architecture

1/
1


The following elements can be distinguished in LTE architecture:

o
eNB

E
-
UTRAN Node B

o
eGW

access gateway

o
MME

mobility management entity

o
UPE

user plane entity


The tasks to be assigned tome include management and context
storage for the control of users’ terminals, authorization procedures
for terminals, and mobility management schemes for terminals


The tasks of the UPE unit encompass management and subscriber
terminal context storage. The unit also performs functions such as
encoding, securing integrity of data blocks, and automatic repeat
request (ARQ) to reduce data block discard



Individual data blocks located in network nodes (eGW)
communicate with one another and with external networks


The blocks cooperate with base stations (eNB), which carry on all
functions related to radio transmission


Transmission

Techniques

in

the

LTE
System 1/
1



One of the key elements of LTE is the use of orthogonal
frequency division multiplexing (OFDM) as the signal bearer
and the associated access schemes, orthogonal frequency
division multiple access (OFDMA) and single carrier
frequency division multiple access (SC
-
FDMA)


The actual implementation of the technology will be
different in the downlink and the uplink direction as a result
of the different requirements between the two directions
and the equipment at either end


However OFDM was chosen as the signal
-
bearer format
because it is very resilient to interference. It is also a
modulation format that is very suitable for carrying high data
rates

one of the key requirements for LTE


Long
-
term Evolution OFDMA in the
Downlink Direction 1/
6


The principle of the OFDMA is based on the use of narrow,
orthogonal subcarriers


In LTE the sub
-
carriers spacing is 15KHzregardless of the total
transmission bandwidth


The transmission is done after a fast
fourier

transform (FFT) block,
which is used to change between the time and frequency domain
representation of the signal


Within the OFDM signal it is possible to choose between three types
of modulation:

o
QPSK

o

16QAM

o

64QAM


The duration of the LTE system frame is 10 ms and the system
consists of ten sub
-
frames with two slots each. Within one slot,
seven OFDM symbols are transmitted, with a short cyclic prefix, and
six OFDM symbols, when a longer cyclic prefix is used

Long
-
term Evolution OFDMA in the
Downlink Direction
2
/
6


Long
-
term Evolution OFDMA in the
Downlink Direction
3
/
6


The OFDM signal includes NBW subcarriers. The signal on a single subcarrier in
one OFDM symbol is thus of fundamental importance.


There are therefore 6N
BW

or 7N
BW

such elements depending on the length of
the cyclic prefix


The total resources of a single slot are divided into the so
-
called physical
resource blocks, each being composed of 12 subsequent subcarriers
allocated in a given time slot


The physical resource block is the basic unit for radio resource allocation


Figure on the next slide shows a block diagram of the transmitter and the
receiver of the signal in the downlink direction in the case where single
antennas are used


A data block from the n
-
th interval of the modulation can result from
multiplexing several streams generated by users


Then it is arranged in the radio resource allocation block and the data is
mapped in symbols from the constellation of elementary symbols (QPSK, 16
-
QAM, 64
-
QAM), which are then attributed to appropriate subcarriers


Signal samples in the time domain are performed with the help of M
-
point IFFT
transformation

Long
-
term Evolution OFDMA in the
Downlink Direction
4
/
6


A cyclic prefix is added at the head of block of samples thus generated. This
prefix is a sub
-
block of the sequence of samples copied from the rear of the
block (trailer)


The signal constructed in this way, with the inphase and quadrature
components (the real and the imaginary part of the complex block of IFFT
samples respectively), is then converted into analog form, converted to radio
band and enhanced to be emitted through the antenna



On the receiver side the dual processes take place. So, after signal conversion
to base band and conversion to a block of digital samples, the cyclic prefix is
removed


Then, the subcarrier signals are correlated with reference signals using the FFT
method


The sample block from the FFT output, corresponding to the frequency
domain, is correlated on the basis of the estimated characteristics of the
transmission channel


The samples obtained in this way from individual subcarriers are then mapped
in the constellation points that indicate the positions of the symbols mapped in
the constellation


Eventually, the binary block is determined on the basis of the symbols. The
block includes the final decision on the properly received signal block

Long
-
term Evolution OFDMA in the
Downlink Direction
5
/
6


Long
-
term Evolution OFDMA in the
Downlink Direction
6
/
6


Long
-
term Evolution SC
-
FDMA in the
Uplink 1/
2


For the LTE uplink, a new concept is used for the access technique



Although still using a form of OFDMA technology, the implementation is called single
carrier frequency division multiple access


A fundamental problem for the OFDM transmission, from the mobile station perspective,
is limited power resources derived from battery energy. Hence, efficiency of radio blocks
is of fundamental significance, in particular that of the power enhancer


The OFDM signal is characterized by a high value of peak power ratio to average power,
which implies the need for a high degree of linearity in the power enhancer and a
decrease in the average power of emitted signal as compared to signals for which the
peak power ratio to the average ratio is insignificant. In the uplink SC
-
FDMA transmission is
therefore used


The binary data are first mapped into constellation points. The constellation is chosen
according to the quality of the modulation channel (QPSK, 16
-
lub 64
-
QAM)


The symbols are then arranged in blocks with the length N. Such a block is treated as a
sequence of samples in the time domain and undergoes a frequency transformation
according to the DFT algorithm (FFT)


The obtained frequency samples are then mapped in selected subcarriers of SC
-
FDMA
modulator


The block of samples thus obtained within the frequency domain is then transformed into
the time form by the block that performs the IFFT algorithm. The block of samples in time is
preceded with a cyclic prefix and the whole of the block is filtered so that spectral
properties of the signal are shaped. Dual operations are performed in the receiver


Long
-
term Evolution SC
-
FDMA in the
Uplink
2
/
2


Long
-
term Evolution MIMO 1/
2


Multiple input multiple output (MIMO) is another major LTE technology innovations used to
improve the performance of the system



It uses multiple antennas at both the transmitter and receiver to improve
communication performance


It offers significant increases in data throughput and link range without using additional
bandwidth or transmission power. This is achieved by higher spectral efficiency (more bits
per second per hertz of bandwidth) and link reliability or diversity (reduced fading)


Although MIMO adds complexity to the system in terms of processing and the number of
antennas required, it enables high data rates to be achieved along with much improved
spectral efficiency


As a result, MIMO has been included as an integral part of LTE. The basic concept of
MIMO uses the multipath signal propagation that is present in all terrestrial
communications



For the downlink direction, a configuration of two transmit antennas at the base station
and two receive antennas on the mobile terminal is used as baseline, although
configurations with four antennas are also being considered


Figure on the next slide presents a configuration of reference symbols within two
subsequent physical resource blocks included in a
subframe

for a single antenna
transmission (SISO) and for two antenna transmission


A similar configuration is also expected with the case of the application of four
transmitting antennas

Long
-
term Evolution MIMO
2
/
2


Channels in the Radio Interface of the
LTE System 1/
1


Like UMTS, channels in the radio interface of the LTE
system can be divided into three types:

o
logical channels

o
transport channels

o
physical channels


Radio Resource Management in LTE
1/
4


Radio resource management (RRM) aims at efficient usage of all available
algorithms and techniques offered by the system and supports appropriate
level of quality of service (
QoS
). Some examples of mechanisms for radio
resource management in LTE are presented below


Admission Control
-

AC algorithm in
eNB

decides whether or not a new call in
the cell can be serviced. Admission control takes into account available
resources in a cell, call priorities, quality requirements for a new call as well as
current quality parameters in the cell. A new call can only be serviced when
the expected quality of currently serviced calls with the same, or higher,
priorities will be retained and the assumed
QoS

parameters for the new call will
be satisfied . Admission control algorithms have not yet been specified by
3GPP. Possible solutions with regard to the issue are to be decided by
hardware suppliers


Frequency Domain Packet Scheduling
-

FDPS uses frequency selective power
variations on either the desired signal (frequency selective fading) or
interference (fading or due to fractional other cell load) by only scheduling
users on the physical resource blocks with high channel quality. Thanks to this
solution, transmission in the radio channel is executed only when it is most
efficient. The working principle for the frequency domain packet scheduling is
presented on the next slide

Radio Resource Management in LTE
2
/
4


Radio Resource Management in LTE
3
/
4


Interference Management and Power Settings
-

the LTE system
offers a number of mechanisms that underlie control of
interference between neighboring cells. Version 8 of the system
(Release 8) uses the distribution of frequencies between
neighboring cells and the possibility of power control in the radio
channel. The X2 interface supports information transmission about
inter cell interference between eNB base stations. In LTE two
methods of interference level management in the radio interface
can be distinguished:

o
The reactive method, based on monitoring the network quality. If interference is detected
that is too high the system decreases the level of transmitted power in the radio channel or
modifies the packet transmission process

o
The proactive method, based on an exchange of information between eNBs regarding
how they plan to schedule packets to its users in the future. This allows neighboring eNBs
involved in packet transmission to take into account the current load of the radio interface
effected by neighboring cells. This communication is performed by means of the X2
interface


Interference management in 3GPP Release 8 aims primarily at a
constant improvement of the quality of shared data channels in
the uplink and in the downlink direction (PDSCH and PUSCH)

Radio Resource Management in LTE
4
/
4


Discontinuous Transmission and Reception (DTX/DRX)
-
radio
resources management mechanism in the LTE system offers
discontinuous signal transmission and reception (DTX/DRX).
Discontinuous transmission/reception in LTE allows a mobile
station (MS) to enter the power
-
saving mode and to cease
monitoring the PDCCH channel in a given sub frame. This
operation can be effected based on activity requirements
for a given mobile station in the uplink and in the downlink
direction. For traffic in which a cycle pattern can be
predicted with high probability, discontinuous reception can
last 20 or 40
ms.

His functionality lowers energy consumption
in the mobile station and the level of interference in the
radio channel. However, it should not be forgotten that too
aggressive
parametrization

of the function, such as
maximization of the periods of discontinuous transmission
and reception, can eventually reduce the quality of service
offered