Evolved UTRAN (E-UTRAN) eNodeB : Directly connected to the Core via S1 interface No RNC as in WCDMA eNodeBs interconnected via X2 interface Handovers are handled by eNodeBs it self, communicating via X2 interface This is an intelligent NodeEvolved Packet Core (EPC)Supports only packet switched domain only Mobility Management Entity (MME) : Control Plane Node of the EPC handling connection/release of bearers to a terminal handling of IDLE to ACTIVE Transition handling of security keys
Serving Gateway(S-GW) : User plane node which connects EPC to E-UTRAN Acts as a mobility anchor when Terminals move between eNodeBs Mobility Anchor for other 3GPP technologies (GSM,HSPA) Collecting information for charging purposes Packet Data Network Gateway (P-GW) : Connects EPC to the Internet Allocation of the IP address for a specific terminal QoS handling Home Subscriber Service (HSS) : A database containing subscriber information
What is Orthogonal FrequencyDivision Multiplexing (OFDM) ?
ISI – Inter Symbol InterferenceTime domain : Data Rate ISI
Time Spreading (Freq. SelectiveFading)• When an impulse is transmitted , howdoes the average power received by Power Delay ProfileMobile vary as a function of time delay ζ ? Freq. Selective Fading : Ts < ζ0 Non Freq. Selective Fading : Ts > ζ0
Power Delay Profile Spaced Freq. Correlation function FTInside Coherence BW channel passes all freq. components withequal gain and linear phase Freq. Selective Fading : W > f0 Non Freq. Selective Fading : W < f0
• Symbol rate not increased in order to achieve high data rates.• Instead of that Available BW breaks in to many narrower subcarriers and modulate generated symbols to these subcarriers.• These subcarriers then combine linearly and transmit (OFDM symbol). OFDM Modulation OFDM demodulation
Single carrier transmission Vs OFDMTransmission : Single Carrier 1 0 1 Transmission 1 : OFDM Transmission 0 1 t
Sub carrier Pulse shape and Spectrum Subcarrier BW < Coherance BW
Why “Orthogonal” ? Subcarriers “Orthogonal” in the time domainIn OFDM, Subcarriers are overlapped in Frequencydomain while maintaining orthogonality in time domain
Overlapping subcarriers in Freq.domain Overlapping Subcarriers Spectral Efficiency
OFDM Symbol • Generated by Multiplexing several overlapping subcarriers and a Cyclic Prefix (CP). CP Modulated Subcarriers • Cyclic Prefix added to the beginning of the OFDM symbol in order to eliminate IBI • At the Receiver CP is removed and only the information bearing part is further processed .
OFDM as a Multiple Access Scheme(OFDMA) OFDMA : In each OFDM symbol interval, Different subsets of the overall set of available subcarriers are used for transmission to different terminals.
What is Multiple-Input Multiple-Output (MIMO) ?
Main Transmission TechniquesSpatial Diversity : Signal copies are transmitted at multiple antennas or received at more than one antenna. Spatial Multiplexing : Transmit independent and separately encoded data streams over different antennas
Why MIMO? Significant increase in Spectral efficiency and data rates - Spatial Multiplexing High QoS - Spatial diversity Wide Coverage - Spatial diversity
Received signal y at the receiver when signal x is transmitted,
What is Single Carrier FDMA(SC – FDMA)?
SC – FDMA (DFTS-OFDM)Why not Multi Carrier OFDM in Uplink ? One of the main drawbacks in OFDM : Large instantaneous power variations in the Transmitting signal This leads to High Peak-to-Average-Power Ratio (PAPR) in the Power Amplifier. Power Amplifier Efficiency Power Amplifier Cost Hence Multicarrier OFDM is not a Viable solution for Low power Mobiles
In OFDM, each subcarrier carries information relating to one specific Symbol In SC-FDMA, each subcarrier contains information of All Transmitted symbols. Hence no need of transmitting with High Power. Signal energy is distributed among sub carriers.
User Multiplexing in SC-FDMA Localized Transmission : Distributed Transmission :User 1 User 2 User 3 User 1 User 2 User 3
LTE Physical LayerOverall RAN Protocol Architecture
LTE Physical Layer Processing
Available DL BW and Physical Resource Blocks(PRBs)Bandwidth (MHz) 1.25 2.5 5.0 10.0 15.0 20.0Subcarrier BW (kHz) 15PRB BW (kHz) 180No. of available RBs 6 12 25 50 75 100
Resource Grid 7 OFDM symbols TimeFREq R E S O U R C R E E S B O L U O R C C K E G R I DResource Element
Physical Resource Block (PRB) allocation is done by the scheduling function in eNodeB PRB is the smallest element of resource allocation assigned by the base station scheduler.
LTE Radio Access : An Overview
Channel dependent Scheduling and Rate adaptation : Depending on the channel conditions, time – frequency resources are allocated to users by the scheduler Scheduling decisions taken once every 1ms with frequency domain granularity of 180 kHz. Scheduler allocates resources depending on the Channel State Information(CSI) provided by the UE
Inter Cell interference Coordination (ICIC) : In LTE, Frequency Reuse Factor equals to one (full spectrum availability at each Cell) This leads to high performance degradation specially the Users in cell edge. ICIC reduce ICI at cell edge applying certain restrictions on resource assignment.Adaptive Fractional Frequency ReuseCoordination: 3 1 2 Inner Region Outer Region
Multicast / Broadcast Single frequency Network (MBSFN) As Identical information is transmitted from transmitters (time aligned), UEs in Cell edge can utilize received power of several surrounding cells to detect / decode broadcasted data.
Special Features in LTE – A (Rel.10)Carrier Aggregation : Relaying:
Extended Multi Antenna Transmission : DL Spatial Multiplexing has been expanded to support up to 8 transmission Layers.Heterogeneous Deployments :Ex : Pico Cell placed inside a Macro Cell
References : . “4G LTE/LTE-Advanced for Mobile Broadband” by Erik Dhalman, Stefan Parkvall, Johan Skold “Overview of the 3GPP Long Term Evolution Physical Layer ” by Jim Zyren, Dr.Wes McCoy “Wireless Communication” by Andrea Goldsmith