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ENGINEERINGKnowledge object
Topic: Guidelines and Basics of LTE
Prepared by: Syed Azam Shah
Date: 19-04-2012
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LTE Architechture
LTE encompasses the evolution of: the radio access through the E-UTRAN the non-radio aspects under the term System Architecture Evolution
(SAE) Entire system composed of both LTE and SAE is called the Evolved
Packet System (EPS)
At a high-level, the network is comprised of: Core Network (CN), called Evolved Packet Core (EPC) in SAE access network (E-UTRAN) A bearer is an IP packet flow with a defined QoS between the
gateway and the User Terminal (UE)
CN is responsible for overall control of UE and establishment of the bearers
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LTE Architechture
Main logical nodes in EPC are: PDN Gateway (P-GW) Serving Gateway (S-GW) Mobility Management Entity (MME)
EPC also includes other nodes and functions, such: Home Subscriber Server (HSS) Policy Control and Charging Rules Function (PCRF) EPS only provides a bearer path of a certain QoS, control of multimedia
applications is provided by the IP Multimedia Subsystem (IMS), which considered outside of EPS
E-UTRAN solely contains the evolved base stations, called eNodeB or eNB
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LTE RADIO INTERFACE
The LTE radio interface is based on OFDM (Orthogonal Frequency Division Multiplex) and OFDMA (Orthogonal Frequency Division Multiple Access) in DL and SC-FDMA (Single Carrier Frequency Division Multiple Access) in UL. These techniques are well suited for flexible bandwidth operation. This enables operators to deploy LTE in different regions with different frequency bands and bandwidths available.
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LTE Radio Access – DownlinkOFDM - Orthogonal Frequency Division Multiplexing
Large number of 15 kHz sub carriersOrthogonal: Other carriers zero at sampling pointImproved spectral efficiencyReduce ISI effect by multipathAgainst frequency selective fading
f = 15 kHz
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LTE Radio Access – UplinkSC-FDMA – Single Carrier FDMA (DFTS-OFDM)Low Peak-to-Average Power RatioSimilar to OFDM15 kHz tones BUT consecutive
Same time-domain structure
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LTE Frame and Subframe Structure
The frame structures for LTE differ between the Time Division Duplex, TDD and the Frequency Division Duplex, FDD modes as there are different requirements on segregating the transmitted data.
There are two types of LTE frame structure:
Type 1: used for the LTE FDD mode systems.
Type 2: used for the LTE TDD systems.
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Resource Grid
One frame is 10ms 10 subframesOne subframe is 1ms 2 slotsOne slot is 0.5ms N resource blocks[ 6 < N < 110]One resource block is 0.5ms and contains 12 subcarriers from each OFDM symbol
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Type 1 LTE Frame Structure
The basic type 1 LTE frame has an overall length of 10 ms. This is then divided into a total of 20 individual slots. LTE Subframes then consist of two slots - in other words there are ten LTE subframes within a frame.
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Type 2 LTE Frame StructureThe frame structure for the type 2 frames used on LTE TDD is somewhat different. The 10
ms frame comprises two half frames, each 5 ms long. The LTE half-frames are further split into five subframes, each 1mslong.
The subframes may be divided into standard subframes of special subframes. The special subframes consist of three fields;
DwPTS - Downlink Pilot Time Slot
GP - Guard Period
UpPTS - Uplink Pilot Time Stot.
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LTE Channels
There are three categories into which the various data channels may be grouped.
Physical channels: These are transmission channels that carry user data and control messages.
Transport channels: The physical layer transport channels offer information transfer to Medium Access Control (MAC) and higher layers.
Logical channels: Provide services for the Medium Access Control (MAC) layer within the LTE protocol structure.
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LTE Channels
UL-SCHDL-SCH
PCCH
Logical Channels “type of information” (traffic/control)
Downlink Uplink
PDSCH
Physical Channels“bits, symbols, modulation, radio frames etc”
MTCH MCCH BCCH DTCH DCCH DTCH DCCH CCCH
PRACH
RACH
CCCH
MCH BCH
PUSCHPBCH PCFICH PUCCH
-CQI -ACK/NACK-Sched req.
-Sched TF DL-Sched grant UL-Pwr Ctrl cmd-HARQ info
MIB SIB
PMCH PHICHPDCCH
ACK/NACKPDCCH
info
Mapped onto Downlink physical resources Mapped onto Uplink physical resources
Transport Channels“how and with what characteristics” (common/shared)
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LTE physical channels
Downlink:
Physical Broadcast Channel (PBCH):
It carries only the MIB.
It is using QPSK.
Mapped to 6 Resource Blocks (72 subcarriers), centered around DC subcarrier in sub frame 0.
Mapped to Resource Elements which is not reserved for transmission of reference signals, PDCCH or PCHICH
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Physical Downlink Control Channel (PDCCH) :
Mapped to the first L OFDM symbols in each of the downlink sub-frame.
Number of the symbols (L) for PDCCH can be 1,2, or 3.
Number of the symbols for PDCCH is specified by PCFICH
PDCCH carries DCIs and the DCI carries Transport format, resource allocation, H-ARQ information related to DL-SCH, UL-SCH and PCH.
PDCCH also carries DCI 0 which is for UL Scheduling assignment (e.g, UL Grants).
Multiple PDCCH are supported and a UE monitors a set of control channels.
Modulation Scheme is QPSK.
PDCCH is like HS-SCCH for HSDPA and PDCCH for R99, E-AGCH/E-RGCH for HSUPA
Even though PDCCH has a lot of functions, not all of them are used at the same time so PDCCH configuration should be done flexibly.
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Physical Control Format Indicator Channel (PCFICH) :
It carries the size of PDCCH
Mapped to the first OFDM symbol in each of the downlink sub-frameThis contains the information on number of OFDM symbols for PDCCH and PHICH symbol duration received from the PBCHUE decode this channel to figure out how many OFDM symbols are assigned for PDCCH
It is 16 data subcarriers of the first OFDM symbol of the subframe.
The exact position of PCFICH is determined by cell ID and bandwidth.
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Physical Hybrid ARQ Indicator Channel (PHICH) :
Carries H-ARQ Feedback
After UE trasmitted the data in UL, it is waiting for PHICH for the ACK.
It is like E-HICH in HSPA
Sometimes several PHICH constitutes a PHICH group using the same resource elements.
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Physical Downlink Shared Channel (PDSCH):
Carries user specific data (DL Payload).
Carries Random Access Response Message.
It is using AMC with QPSK, 16 QAM and 64 QAM
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Downlink Physical Signals: The specification defines the DL SSs in two types: primary
synchronization signal (PSS) and secondary synchronization signal (SSS). DL SSs are needed to acquire frequency and time synchronization to a cell and determine the physical layer identity of the cell.
Characteristics of the PSS and SSS:
PSS and SSS are transmitted in the central 6 RBs for all possible system bandwidth (6 - 110 RBs): PSS and SSS are each mapped to the central 62 subcarriers around the unused D.C. subcarrier.
2. DL SSs are transmitted twice per 10 ms radio frame (the two PSSs within a radio frame are identical in a given cell). For the FDD case, the PSS is located in the last symbol of the first slot of subframe of 0 and 5 and the SSS is located just prior to the PSS. For the TDD case, the PSS is located in the third symbol of subframe 1 and 6 and the SSS is located three symbols ahead of the PSS.
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RS (Reference Signal):
Most of the channels (e.g, DPSCH, DPCCH, PBCH etc) is for carrying a special information (a sequence of bits) and they have some higher layer channel connected to them, but Reference Signal is a special signal that exists only at PHY layer. This is not for delivering any specific information. The purpose of this Reference Signal is to deliver the reference point for the downlink power.
When UE try to figure out DL power (i.e, the power of the signal from a eNode B), it measure the power of this reference signal and take it as downlink cell power.
These reference signal are carried by multiples of specific Resource Elements in each slots and the location of the resource elements are specifically determined by antenna configuration.
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Uplink:
Physical Uplink Control Channel (PUCCH) :
This channel can carries a lot of information (UCI), but depending on the configuration it can carry only a few of the following information.
ACK/NACK for the recieved PDSCH data
CQI
RI
PMI
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Physical Uplink Synchronization channel (PUSCH):
Carries Uplink data that UE tries to send. and it can also carries ACK/NACK for the PDSCH the UE recieved in addition to uplink data.
Physical Random Access Channel (PRACH) :
This is the only non-synchronised transmission that the UE can make within LTE. The downlink and uplink propagation delays are unknown when PRACH is used and therefore it cannot be synchronised.
The PRACH instance is made up from two sequences: a cyclic prefix and a guard period. The preamble sequence may be repeated to enable the eNodeB to decode the preamble when link conditions are poor.
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LTE transport channels
Downlink:
Broadcast Channel (BCH) :
fixed, pre-defined transport format;
requirement to be broadcast in the entire coverage area of the cell.
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Downlink Shared Channel (DL-SCH) :
This transport channel is the main channel for downlink data transfer. It is used by many logical channels.
support for HARQ;
support for dynamic link adaptation by varying the modulation, coding and transmit power;
possibility to be broadcast in the entire cell;
possibility to use beam forming;
support for both dynamic and semi-static resource allocation;
support for UE discontinuous reception (DRX) to enable UE power saving;
support for MBMS transmission.
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Paging Channel (PCH) :
support for UE discontinuous reception (DRX) to enable UE power saving (DRX cycle is indicated by the network to the UE);
requirement to be broadcast in the entire coverage area of the cell;
mapped to physical resources which can be used dynamically also for traffic/other control channels.
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Multicast Channel (MCH) :
This transport channel is used to transmit MCCH information to set up multicast transmissions.
requirement to be broadcast in the entire coverage area of the cell;
support for MBSFN combining of MBMS transmission on multiple cells;
support for semi-static resource allocation e.g. with a time frame of a long cyclic prefix.
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Uplink:
Uplink Shared Channel (UL-SCH) :
possibility to use beamforming; (likely no impact on specifications)
support for dynamic link adaptation by varying the transmit power and potentially modulation and coding;
support for HARQ;
support for both dynamic and semi-static resource allocation.
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Random Access Channel (RACH) :
limited control information;
collision risk;
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LTE logical channels
The logical channels cover the data carried over the radio interface. The Service Access Point, SAP between MAC sublayer and the RLC sublayer provides the logical channel.
Control channels:
These LTE control channels carry the control plane information
Broadcast Control Channel (BCCH) :
This control channel provides system information to all mobile terminals connected to the eNodeB.
Paging Control Channel (PCCH) :
This control channel is used for paging information when searching a unit on a network.
Common Control Channel (CCCH) :
This channel is used for random access information, e.g. for actions including setting up a connection.
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Multicast Control Channel (MCCH) :
This control channel is used for Information needed for multicast reception.
Dedicated Control Channel (DCCH) :
This control channel is used for carrying user-specific control information, e.g. for controlling actions including power control, handover, etc..
Traffic channels:
These LTE traffic channels carry the user-plane data:
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Dedicated Traffic Channel (DTCH) :
This traffic channel is used for the transmission of user data.
Multicast Traffic Channel (MTCH) :
This channel is used for the transmission of multicast data.