3. 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
4. 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
6. 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
7. 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
9. 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
10. 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
11. 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
12. 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
13. 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
14. 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
15. 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
19. 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
2 51.0 Mbit/s 25.5 Mbit/s No
3 102.0 Mbit/s 51.0 Mbit/s No
4 150.8 Mbit/s 51.0 Mbit/s No
5 300.0 Mbit/s 75.4 Mbit/s Yes
6 301.5 Mbit/s 51.0 Mbit/s No
7 301.5 Mbit/s 102.0 Mbit/s No
8 300.0 Mbit/s 149.8 Mbit/s Yes
9 452.3 Mbit/s 51.0 Mbit/s No
10 452.3 Mbit/s 102.2 Mbit/s No
20. 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
22. 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
23. 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
24. 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
27. 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
31. 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
34. 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
35. 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
36. 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
OFDM/OFDMA has several benefits over other transmission schemes:
-High spectral efficiency: due to the orthogonality between subcarriers it is possible to pack them closely together (15kHz subcarrier spacing)
-Little interference between subcarriers due to the IFFT/FFT processing (Interference can be introduced by frequency offsets generated by either Doppler or Local Oscillator frequency innacuracies).
-Robustness in multi-path environments thanks to the cyclic prefix as mentioned before.
-Straightforward support of the operation in different spectrum allocations with different bandwidths just by varying the number of OFDM subcarriers used for transmission.
-Simpler receiver design to support high data rate communications. Detecting a rectangular pulse with cyclic prefix requires less hardware. Free capacity can be used then to implement other performance optimization techniques.
-Easy MIMO techniques implementation
Drawbacks of OFDMA
-The main disadvantage of OFDM/OFDMA is that the signal has a relatively large peak-to-average power ratio (PAPR). This is due to the nature of OFMA where modulated symbols are transmitted in parallel, each one containing a part of the transmission. The power at a certain point in time is the sum of the powers of all the transmitted symbols for a certain connection, which explains that the differences between peak and average powers can be high.
-This issue reduces the power efficiency of the RF amplifier. Expensive transmission amplifiers are needed, especially on the mobile side, in order to work on a wide range of powers; otherwise the non-linear amplification reduces the orthogonality of the OFDM signal. This is a reason why OFDMA is not optimal for use with mobile or battery-power devices.
-Other issue of OFDM/OFDMA systems is that tight spacing of subcarriers may lead to loss of orthogonality due to frequency errors. Doppler may cause inter carrier interference (ICI) and the consequent lost of orthogonality. To cope with the problems caused by close subcarrier spacing, LTE has adopted 15 kHz spacing (mobile WiMAX uses 10KHz spacing).
A graphical comparison of OFDMA and SC-FDMA as shown in the slide is helpful in understanding the differences between these two modulation schemes. For clarity this example uses only four (M) subcarriers over two symbol periods with the payload data represented by quadrature phase shift keying (QPSK) modulation. As described earlier, real LTE signals are allocated in units of 12 adjacent subcarriers.
Visually, the OFDMA signal is clearly multi-carrier with one data symbol per subcarrier, but the SC-FDMA signal appears to be more like a single-carrier (hence the “SC” in the SC-FDMA name) with each data symbol being represented by one wide signal. Note that OFDMA and SC-FDMA symbol lengths are the same at 66.7 μs; however, the SC-FDMA symbol contains M “sub-symbols” that represent the modulating data. It is the parallel transmission of multiple symbols that creates the undesirable high PAR of OFDMA. By transmitting the M data symbols in series at M times the rate, the SC-FDMA occupied bandwidth is the same as multi-carrier OFDMA. But, crucially, the PAR is the same as that used for the original data symbols. Adding together many narrow-band QPSK waveforms in OFDMA will always create higher peaks than would be seen in the wider-bandwidth, single-carrier QPSK waveform of SC-FDMA. As the number of subcarriers M increases, the PAR of OFDMA with random modulating data approaches Gaussian noise statistics but, regardless of the value of M, the SC-FDMA PAR remains the same as that used for the original data symbols.