Digital network lecturer7

DIT
Dar es Salaam institute of Technology (DIT)
ETU 08102
Digital Networks
Ally, J
jumannea@gmail.com

 Peak data rate of 50/100 Mbps (uplink / downlink)
 Reduced latency enabling RTT (round trip time) <10 ms
 Packet-optimized
 Improved spectrum efficiency between 2- 4 times higher
than Release 6 HSPA
 Bandwidth scalability with allocations of 1.4, 3, 5, 10, 15
and 20 MHz
 Operation in FDD and TDD modes
 Support for inter-working with WCDMA and non-3GPP
systems (i.e. WiMAX)
 Good level of mobility: optimized for low mobile speeds (up
to 15km/h) but support also high mobile speeds (up to
350km/h)
 Improved terminal power efficiency
DIT
LTE Requirements

Drivers for LTE
 There are at least three major key drivers for
LTE mobile broadband networks:
 Demand for higher data-rates
 increasing device capabilities, growing
mobile data consumption
 New spectrum allocation
 Maintaining operator profitability while
continued cost reduction and competitiveness.
DIT

LTE and LTE Advanced Comparison
DIT

LTE Overview
 The multiple access schemes in LTE:
 Orthogonal Frequency Division Multiple Access
(OFDMA) in downlink
 Single Carrier Frequency Division Multiple Access
(SC-FDMA) in uplink
 LTE user transmissions can be divided in frequency and
time
 Better orthogonality between users
 Interference is less or can be cancelled more easily
 Better network capacity can be achieved
 The resource allocation in the frequency domain takes
place with a resolution of 180 kHz resource blocks both
in uplink and in downlink.
DIT

Basic LTE system architecture
Architecture is divided into four main domains:
User Equipment (UE),
Evolved UTRAN (E-UTRAN),
Evolved Packet Core Network (EPC),
Services domain.
DIT

Domains
 Three domains: UE, E-UTRAN and EPC form the so-
called Internet Protocol (IP) Connectivity Layer. This part
of the system is also called as Evolved Packet System
(EPS).
 The main function of EPS is to provide IP based connectivity
 All services will be offered on top of IP
 The biggest architectural change is that EPC does not
contain a circuit switched domain.
 Main functionalities of the EPC are equivalent to the
packet switched domain of the existing 3GPP networks.
 As a logical element the SAE GW is a combination of
the two gateways, Serving Gateway (S-GW) and Packet
Data Network Gateway (P-GW)
DIT

Domains
 Since interfaces between S-GW and P-GW are defined in
standards, it is possible that S-GW and P-GW are
implemented either separately or together.
 E-UTRAN contains only one element type: Evolved Node
B (eNodeB).
 All radio functionalities are controlled by eNodeB. All radio
related protocols are terminated in eNodeB.
 E-UTRAN network is just a mesh of eNodeBs connected
to neighbouring eNodeBs through the X2 interface.
 Functionally eNodeB acts as a layer 2 bridge between UE
and the EPC, by being the termination point of all the
radio protocols towards the UE.
 From functionality point of view the UE is similar like in
3G.
DIT

UE and eNodeB
 UE
 Access device for user.
 Provides measurements that indicate channel conditions to the
network.
 eNode B performs
 Ciphering/deciphering of the User Plane data
 Radio Resource Management (resource allocation, prioritizing,
scheduling, resource usage monitoring
 eNodeB is also involved with Mobility Management (MM)
 The eNodeB controls and analyses radio signal measurements
carried out by the UE.
 eNodeB makes signal measurements itself
 Based on measurement information eNodeB makes decisions to
handover UEs between cells.
DIT

Mobility Management Entity (MME)
 MME is the main control element in the EPC. It is
typically a server in a secure location in the operator’s
premises.
 MME operates only in the control plane and is not
involved with the user plane data.
 MME also has a direct logical control plane connection
to the UE. Connection is a primary control channel
between the UE and the network.
Main functions of MME:
 Authentication and Security
 Mobility Management
 Managing Subscription Profile and Service Connectivity
DIT

Serving Gateway (S-GW)
 S-GW takes care of user plane tunnel
management and switching, and relays data
between eNodeB and P-GW.
 The S-GW has a small role in control functions.
 When bearers for UEs are set up, cleared or
modified the S-GW allocates its resources
based on requests from MME, P-GW or PCRF.
DIT

Packet Data Network Gateway (P-GW)
 P-GW is the edge router between the EPS and external
packet data networks.
 P-GW is the highest level mobility anchor in the system,
and usually it acts as the IP point of attachment for the UE.
 Thus, typically the P-GW allocates the IP address to the
UE, and the UE uses that to communicate with other IP
hosts in external networks, e.g. the internet.
 During mobility between eNodeBs, the S-GW acts as the
local mobility anchor. The MME commands the S-GW to
switch the tunnel from one eNodeB to another.
 P-GW performs traffic gating and filtering functions as
required by the service in question.
 Both S-GW and P-GW are part of the network infrastructure
maintained centrally in operator premises.
DIT

Policy and Charging Resource Function
(PCRF), Home Subscription Server (HSS)
 PCRF is the network element that is responsible for Policy and
Charging Control (PCC).
 HSS is the data repository for all permanent subscription data.
Hence, HSS has the master copy of the subscriber profile
Main interfaces
 X2 interface: This interface is used in mobility between the
eNodeBs, and it includes functions for handover preparation,
and overall maintenance of the relation between neighbouring
eNodeBs.
 S1-MME interface: Reference point for the control plane
protocol between E-UTRAN and MME.
 S1-U interface: Reference point between E-UTRAN and
Serving GW for the user plane tunnelling and inter eNodeB
path switching during handover.
DIT

Existing and Future 3GPP Bands
DIT

UE Categories and Capabilities
DIT
UE category
Maximum Throughput Support for 64QAM in
Uplink
Downlink Uplink
1 10.3 Mbit/s 5.2 Mbit/s No
5 300.0 Mbit/s 75.4 Mbit/s Yes
8 300.0 Mbit/s 149.8 Mbit/s Yes
10 452.3 Mbit/s 102.2 Mbit/s No

OFDMA
 OFDMA is an extension of the OFDM transmission scheme
by allowing multiple users.
 That is, allowing for simultaneous frequency-separated
transmissions to / from multiple mobile terminals.
 In OFDM the user data is transmitted in parallel across
multiple orthogonal narrowband subcarriers.
 Each subcarrier only transports a part of the whole
transmission.
 The orthogonal subcarriers are generated with IFFT
(Inverse Fast Fourier Transform) processing.
 The number of subcarriers depends on the available
bandwidth.
 In LTE, they range from less than one hundred to more
than one thousand.
DIT

Cyclic Prefix (CP) Principle
 Cyclic prefixes are used by OFDM systems to fight against
the Inter Symbol Interference (ISI) due to multipath
environments.
 CP consists of a copy of the last part of a symbol shape for
the duration of a guard time and adding it to the beginning
of the symbol.
 This guard time needs to be long enough to capture all the
delayed multipath signals and avoid ISI at the receiver.
 LTE’s typical symbol duration including the CP is around
71.64 µsec.
DIT

Types of Cyclic Prefix for LTE
There are two cyclic prefix options for LTE:
Normal cyclic prefix: For use in small cells or cells with
short multipath delay spread.
Its length depends on the symbol position within the slot
being 5.21 µsec for the CP in symbol 0 and 4.6 µsec for the
rest of symbols.
The reason for these two different lengths is so that the slot
duration is 0.5ms, facilitating at the same time, that the
terminal finds the starting point of the slot.
Extended cyclic prefix: For user with large cells or those
with long delay profiles.
Its length is 16.67µs and it is constant for all symbols in the
slot.
DIT

OFDMA Benefits and Drawbacks
Benefits
 High spectral efficiency for
wideband channels
 OFDM is almost completely
resistant to multi-path
interference due to its very
long symbol duration
 Flexible spectrum utilization
 Relative simple
implementation using
FFT/IFFT
 Easy MIMO techniques
implementation
Drawbacks
 Some OFDM Systems can
suffer from high PAPR
(Peak Average Power
Ratio)
 Loss of orthogonality due
to frequency errors
 Doppler shifts impacts
subcarrier orthogonaliy
due to ISI
 Accurate frequency and
time synchronization
DIT

Comparison of OFDMA and SC-FDMA

LTE Frame Structures
 Ts is the basic time unit for LTE.
 Ts = 1/(15000 x 2048) seconds or about 32.6 ns.
 Downlink and uplink transmissions are organized into frames of
duration Tf = 307200 Ts.
 The 10 ms frames divide into 10 subframes.
 Each subframe divides into 2 slots of 0.5 ms.
 Two frame types are defined for LTE: Type 1, used in
Frequency Division Duplexing (FDD) and Type 2, used in Time
Division Duplexing (TDD).
 Type 1 frames consist of 20 slots with slot duration of 0.5 ms.
 Type 2 frames contain two half frames. Depending on the
switch period, at least one of the half frames contains a special
subframe carrying three fields of switch information: Downlink
Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot
Time Slot (UpPTS). DIT

Uplink-downlink Configurations for
the LTE TDD Mode
DIT
D is Downlink subframe, U is Uplink subframe, and S is special
subframe

The Resource Block
 Mapping of channels takes place in the time and
frequency domains in LTE.
 The primary element that support the mapping
process is the Resource Block (RB).
 The RB has a fixed size and is common to all
channel bandwidths/FFT sizes.
 In the time domain the RB is one slot ( 7 x 66.67µS
symbols).
 In the frequency domain there are 12 x 15KHz sub-
carriers.
 1 symbol and 1 sub-carrier is known as a resource
element.
DIT

Theoretical Data Rates
 LTE does not officially meet the 4G requirements issued by
ITU in the definition for IMT-Advanced.
 The data rates available in LTE (up to 300 Mbps) are
substantially higher than previous generations of cellular
standards.
 It is worth noting that the maximum theoretical data rates
of LTE Advanced (up to 3.08 Gbps) are compliant with the
‘4G’ definition of the IMT-Advanced requirements.
 Throughput of digital wireless communications channels is
defined by several factors, including:
symbol period utilization, symbol rate, modulation scheme,
code rate, number of resource blocks, and number of
spatial streams.
DIT

Throughput calculation for LTE SISO
Link
Throughput = Data Subcarriers X Slots per second X
Symbols per Slot X Bits per Symbol X Code
Rate X Spatial Streams
With LTE, the maximum throughput in a 1x1 SISO channel
occurs when the eNodeB allocates all resource blocks (1200
subcarriers) for a 20 MHz signal bandwidth using the 64-QAM
modulation scheme. In this case, the estimated theoretical
throughput is 76.9 Mbps.
Throughput = 1200 data subcarriers X 2000 slots X 7 symbols
X 6 bits X (4/5) code rate x 1 spatial stream
= 76.9 Mbps
DIT

Throughput calculation for LTE
Advanced 8x8 MIMO Link
 For MIMO schemes, the addition of carrier aggregation
increases the theoretical data rates of LTE Advanced
further.
 20 MHz channel bandwidth allows for 1,200 data
subcarriers, the use of five aggregated carriers would
increase the number of data subcarriers to 6,000. the
maximum data rate can be calculated as follows:
Throughput = 6000 data subcarriers X 2000 slots X 7 symbols
X 6 bits X (4/5) code rate x 8 spatial streams
= 3.08 Gbps
LTE Advanced is the first commercial wireless standard that
exceeds the IMT-Advanced requirements for 4G cellular
systems.
DIT

Maximum Downlink Capacity per
Radio Channel
DIT

Maximum Uplink Capacity per
Radio Channel
DIT

LTE Logical Channels
 Logical Control Channels
 Broadcast Control Channel (BCCH)
 Paging Control Channel (PCCH)
 Common Control Channel (CCCH)
 Multicast Control Channel (MCCH)
 Dedicated Control Channel (DCCH)
 Logical Traffic Channels
 Dedicated Traffic Channel (DTCH)
 Multicast Traffic Channel (MTCH)
DIT

LTE Transport Channel
 Downlink Transport Channel
 Broadcast Channel (BCH)
 Downlink Shared Channel (DL-SCH)
 Paging Channel (PCH)
 Multicast Channel (MCH)
 Uplink Transport Channels
 Uplink Shared Channel (UL-SCH)
 Random Access Channel (RACH)
DIT

LTE Physical Channel
 Downlink Physical Channel
 Physical Broadcast Channel (PBCH)
 Physical Control Format Indicator Channel (PCFICH)
 Physical Downlink Control Channel (PDCCH)
 Physical Hybrid ARQ Indicator Channel (PHICH)
 Physical Downlink Shared Channel (PDSCH)
 Physical Multicast Channel (PMCH)
 Multicast Channel (MCH)
 Uplink Physical Channel
 Physical Uplink Control Channel (PUCCH)
 Physical Uplink Shared Channel (PUSCH)
 Physical Random Access Channel (PRACH)
DIT

3GPP Evolution: From LTE to
LTE-A/B/C
DIT

Digital network lecturer7

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