LTE basic.pptx

BASICS

Contents

What is LTE

Wireless Evolution

Key features of LTE

LTE system overview

Overview of LTE standard

LTE Architecture

LTE frame Structure

Generic Frame Structure

OFDMA

SC-FDMA

Downlink and uplink Signal and channels

Physical layer UL and DL Procedures.

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What is LTE

LTE (Long Term Evolution) is the project name of a new high performance air interface for cellular mobile communication systems. It is the last step toward the 4th generation (4G) of radio technologies designed to increase the capacity and speed of mobile telephone networks. Where the current generation of mobile telecommunication networks are collectively known as 3G, LTE is marketed as 4G.


Wireless Evolution


Key Features of LTE

Multiple access scheme• Downlink: OFDMA• Uplink: Single Carrier FDMA (SC-FDMA)

Adaptive modulation and coding• DL modulations: QPSK, 16QAM, and 64QAM• UL modulations: QPSK and 16QAM

Bandwidth scalability for efficient operation in differently sized allocated spectrum bands

Possible support for operating as single frequency network (SFN) to support MBMS


Key Features of LTE

Multiple Antenna (MIMO) technology for enhanced data rate and performance.

ARQ within RLC sub layer and Hybrid ARQ within MAC sub layer.

Power control and link adaptation

Implicit support for interference coordination

Support for both FDD and TDD

Channel dependent scheduling & link adaptation for enhanced performance.

Reduced radio-access-network nodes to reduce cost, protocol-related processing time & call set-up time


LTE System Overview

LTE is the latest step in moving forward from the cellular 3G services( e.g. GSM to UMTS to HSPA to LTE or CDMA to LTE). LTE is based on standards developed by the 3rd Generation Partnership Project(3GPP).

The following are the main objectives for LTE:• Increased downlink and uplink peak data rates.• Scalable bandwidth• Improved spectral efficiency• All IP network• A standard’s based interface that can support a multitude of user types.

LTE networks are intended to bridge the functional data exchange gap between very high data rate fixed wireless LAN and very high mobility cellular networks.


Overview of the LTE Standard

The original study item on Long Term Evolution (LTE) of 3GPP Radio Access Technology was initiated with the aim to ensure that 3GPP RAT(Radio Access Technology) is competitive in the future (next 10 years). Focus of the study was on enhancement of the radio-access technology (UTRA) and optimization & simplification of radio access network (UTRAN). The key driving factors for LTE are:

• Efficient spectrum utilization• Flexible spectrum allocation• Reduced cost for the operator• Improved system capacity and coverage• Higher data rate with reduced latency


UE :User EquipmenteNB : E-UTRAN Node BS-GW : Serving gatewayPDN-GW : Packet data network gatewayPCRF :Policy and charging rule functionHSS :Home subscriber serverSGSN: Serving GPRS Support NodeePDG: Evolved packet data gateway

LTE Architecture


LTE Architecture

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.


LTE Architecture

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


LTE Frame Structure

One element that is shared by the LTE Downlink and Uplink is the generic frame structure. The LTE specifications define both FDD and TDD modes of operation. This generic frame structure is used with FDD. Alternative frame structures are defined for use with TDD.

LTE frames are 10 msec in duration. They are divided into 10 sub frames, each sub frame being 1.0 msec long. Each sub frame 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


Generic Frame structure


OFDM(Orthogonal Frequency Division Multiplexing)

OFDM is based on the idea of dividing a given high-bit-rate data stream into several parallel lower bit-rate streams and modulating each stream on separate carriers-often called subcarriers.

The subcarriers are selected such that they are all orthogonal to one another over the symbol duration, thereby avoiding the need to have non-overlapping subcarrier channels to eliminate inter-carrier interference

No band gaps is required between subcarriers to prevent interference needed.

Currently OFDMA is used in 3GPP-LTE, WiMax (802.16 d/e) and Wi-Fi(802.11a/g)

The relation between FDM and OFDM is shown


How OFDM Works In OFDM many 15-kHz subcarriers are defined within the radio band. The bit stream is used to modulate the

subcarriers individually; in the most complex implementation, each of hundreds of subcarriers is used to transmit 6 bits at a time with QAM-64. These are all added together to produce a transmittable waveform… and this is calculated in one step with highly-complex digital signal processing called an Inverse Discrete Fourier Transform

LTE uses OFDM for the downlink – that is, from the base station to the terminal. OFDM meets the LTE requirement for spectrum flexibility and enables cost-efficient solutions for very wide carriers with high peak rates. OFDM uses a large number of narrow sub-carriers for multi-carrier transmission.

The basic LTE downlink physical resource can be seen as a time-frequency grid. In the frequency domain, the spacing between the subcarriers, Δf, is 15kHz. In addition, the OFDM symbol duration time is 1/Δf + cyclic prefix. The cyclic prefix is used to maintain orthogonality between the sub-carriers even for a time-dispersive radio channel.

One resource element carries QPSK, 16QAM or 64QAM. With 64QAM, each resource element carries six bits.

The OFDM symbols are grouped into resource blocks. The resource blocks have a total size of 180kHz in the frequency domain and 0.5ms in the time domain. Each 1ms Transmission Time Interval (TTI) consists of two slots (Tslot).

In E-UTRA, downlink modulation schemes QPSK, 16QAM, and 64QAM are available.


LTE OFDM - Transmitter


Why OFDM for the downlink?

OFDM already widely used in non-cellular technologies and was considered by ETSI for UMTS in 1998

CDMA was favored since OFDM requires large amounts of baseband processing which was not commercially viable ten years ago

OFDM advantages

• Wide channels are more resistant to fading and OFDM equalizers are much simpler to implement than CDMA

• Almost completely resistant to multi-path due to very long symbols

• Ideally suited to MIMO due to easy matching of transmit signals to the uncorrelated RF channels

OFDM disadvantages

• Sensitive to frequency errors and phase noise due to close subcarrier spacing

• Sensitive to Doppler shift which creates interference between subcarriers

• Pure OFDM creates high PAR which is why SC-FDMA is used on UL

• More complex than CDMA for handling inter-cell interference at cell edge


SC-FDMA(Single carrier-FDMA)

The LTE uplink transmission scheme for FDD and TDD mode is based on SC-FDMA (Single Carrier Frequency Division Multiple Access).SC-FDMA is sometimes referred to as Discrete Fourier Transform

Spread OFDM = DFT-SOFDM


Why Single Carrier FDMA (SC-FDMA)?

This is to compensate for a drawback with normal OFDM, which has a very high Peak to Average Power Ratio (PAPR). High PAPR requires expensive and inefficient power amplifiers with high requirements on linearity, which increases the cost of the terminal and also drains the battery faster.

SC-FDMA solves this problem by grouping together the resource blocks in such a way that reduces the need for linearity, and so power consumption, in the power amplifier. A low PAPR also improves coverage and the cell-edge performance.

Still, SC-FDMA signal processing has some similarities with OFDMA signal processing, so parameterization of downlink and uplink can be harmonized


Comparing OFDM and SC-FDMA


Downlink Physical Signals and Channels

Downlink Physical Signals• Reference Signals• Synchronization Signals

Downlink Physical Channels• Physical Broadcast Channel (PBCH)• Physical Downlink Shared Channel (PDSCH)• Physical Downlink Control Channel (PDCCH)• Physical Control Format Indicator Channel (PCFICH)• Physical Hybrid-ARQ Indicator Channel (PHICH)• Physical Multicast Channel (PMCH)


Uplink Physical Signals and Channels

Uplink Reference Signals• Demodulation Signals• Sounding Reference Signals

Uplink Physical Channels• Physical Uplink Shared Channel (PUSCH)• Physical Uplink Control Channel (PUCCH)• Physical Random Access Channel (PRACH)


Physical Layer UL and DL Procedures

Downlink Physical Layer Procedures

For E-UTRA, the following downlink physical layer procedures are especially important:

•Cell search and synchronization:

•Scheduling:

•Link Adaptation:

•Hybrid ARQ (Automatic Repeat Request)

Uplink Physical Layer Procedures

For E-UTRA, the following uplink physical layer procedures are especially important:

•Random access

•Uplink scheduling

•Uplink link adaptation

•Uplink timing control

•Hybrid ARQ

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LTE basic.pptx

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