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Overview

The study item on Long Term Evolution (LTE) of 3GPP Radio Access Technology was initiated with the aim to ensure that 3GPP RAT 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).

Driving factors for LTE:

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

Targets

Some specific targets set for LTE are listed below [3GPP TR 25.913]

Increased peak data rate:100Mbps for DL with 20MHz (2 Rx Antenna at UE), 50Mbps for UL with 20MHz
Improved spectral efficiency: 5bps/Hz for DL and 2.5bps/Hz for UL
Improved cell edge performance (in terms of bit rate)
Reduced latency: <10ms UE-RAN edge for U-plane, <100ms for C-plane (dormant to active transition time <50ms)
Scalable bandwidth allocation: in chunks of 1.25, 2.5, 5, 10, 20 MHz)
Support for wide range of Mobile speeds: upto 350Km/h (or even up to 500 km/h depending on the frequency band)

3GPP LTE Summary

The LTE is included in 3GPP release 8 specifications. The first set of LTE specifications were released in Sept’07. The specifications mostly address the physical layer for LTE. Additionally the Service Architecture Evolution (SAE) based on All-IP Packet Core is addressed in the specifications. This document mainly discusses the E-UTRAN.

Overall Architecture

The E-UTRAN uses a simplified single node architecture consisting of the eNBs (E-UTRAN Node B). The eNB communicates with the Evolved Packet Core (EPC) using the S1 interface; specifically with the MME (Mobility Management Entity) and the UPE (User Plane Entity) identified as S-GW (Serving Gateway) using S1-C and S1-U for control plane and user plane respectively. The MME and the UPE are preferably implemented as separate network nodes so as to facilitate independent scaling of the control and user plane.
Also the eNB communicates with other eNB using the X2 interface (X2-C and X2-U for control and user plane respectively).

Overall Architecture [3GPP TS 36.300]
LTE supports an option of Multicast/Broadcast over a Single Frequency Network (MBSFN), where a common signal is transmitted from multiple cells with appropriate time synchronization.
The eNB being the only entity of the E-UTRAN supports all the functions in a typical radio network such as Radio Bearer control, Mobility management, Admission control and scheduling. The Access Stratum resides completely at the eNB.

Functional Split between E-UTRAN and EPC[ 3GPP TS 36.300]

Physical layer

Main article, LTE::PHY

The LTE physical layer is based on Orthogonal Frequency Division Multiplexing scheme OFDM to meet the targets of high data rate and improved spectral efficiency. The spectral resources are allocated/used as a combination of both time (aka slot) and frequency units (aka subcarrier). MIMO options with 2 or 4 Antennas is supported. Multi-user MIMO is supported in both UL and DL.The modulation schemes supported in the downlink and uplink are QPSK, 16QAM and 64QAM.

Downlink Physical Channel

The downlink transmission uses the OFDM with cyclic prefix . Some of the reasons for using OFDM are given below:

Multiple carrier modulation (MCM) helps in countering the frequency selective fading as the channel appears to have nearly flat frequency response for the narrow band subcarrier.
The frequency range of the resource block and the number of resource blocks can be changed (or adapted to the channel condition) allowing flexible spectrum allocation.
Higher peak data rates can be achieved by using multiple resource blocks and not by reducing the symbol duration or using still higher order modulation thereby reducing the receiver complexity.
The multiple orthogonal subcarriers inherently provides higher spectral efficiency.
The cyclic prefix (CP) is the partial repetition of the bit/symbol sequence from the end to the beginning. This makes the time domain input sequence to appear periodic over a duration so that the DFT representation is possible for any frequency domain processing. Also the duration if chosen larger than the channel delay spread, will help in reducing the inter-symbol interference.

Following pilot signals are defined for the DL physical layer:

Reference signal: The reference signal consists of known symbols transmitted at a well defined OFDM symbol position in the slot. This assists the receiver at the user terminal in estimating the channel impulse response so that channel distortion in the received signal can be compensated for. There is one reference signal transmitted per downlink antenna port and an exclusive symbol position is assigned for an antenna port (when one antenna port transmits a reference signal other ports are silent).

Synchronization signal: Primary and secondary synchronization signals are transmitted at a fixed subframes (first and sixth) position in a frame and assists in the cell search and synchronization process at the user terminal. Each cell is assigned unique Primary sync signal.

Uplink physical channel

The uplink transmission uses the SC-FDMA (Single Carrrier FDMA) scheme. The SC-FDMA scheme is realized as a two stage process where the first stage transforms the input signal to frequency domain (represented by DFT coefficients) and the second stage converts these DFT coefficients to an OFDM signal using the OFDM scheme. Because of this association with OFDM, the SC-FDMA is also called as DFT-Spread OFDM. The reasons (in addition to those applicable for OFDM for downlink) for this choice are given below:

The two stage process allows selection of appropriate frequency range for the subcarriers while mapping the set of DFT coefficients to the Resource Blocks. Unique frequency can be allocated to different users at any given time so that there is no co-channel interference between users in the same cell. Also channels with significant co-channel interference can be avoided.
The transformation is equivalent to shift in the center frequency of the single carrier input signal. The subcarriers do not combine in random phases to cause large variation in the instantaneous power of the modulated signal. This means lower PAPR (Peak to Average Power Ratio).
The PAPR (Peak to Average Power Ratio) of SC-FDMA is lesser than that of the conventional OFDMA, so the RF power amplifier (PA) can be operated at a point nearer to recommended operating point. This increases the efficiency of a PA thereby reducing the power consumption at the user terminal.

Following pilot signals are defined for the UL channel:

Demodulation reference signal: This signal send by the user terminal along with the uplink transmission, assists the network in estimating the channel impulse response for the uplink bursts so as to effectively demodulate the uplink channel.
Sounding reference signals : This signals send by the user terminal assists the network in estimating the overall channel conditions and to allocate appropriate frequency resources for uplink transmission.

RLC & MAC Layer

Initial draft of the RLC [3GPP TS 36.322] and MAC [3GPP TS 36.321] specifications are available and all the procedures are not yet fully specified.
The Hybrid-ARQ is suggested at the MAC layer in addition to the ARQ at the RLC layer.

Radio Resource Management

All the following functions are assigned to eNodeB(s) in the E-UTRAN

Radio bearer control
Radio admission control
Connection mobility management
Dynamic resource allocation
Inter cell interference coordination
Load balancing
Inter RAT RRM functions

S1 Interface

S1 interface uses SCTP/IP and GTP-U/UDP/IP for the control and user plane respectively. The signaling protocol between eNB and MME is identified by S1-AP.

References

3GPP TS 36.300: “E-UTRA and E-UTRAN Overall Description; Stage 2”
3GPP TR 25.913: "Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN)"
3GPP TS 36.201: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer; General description".
3GPP TS 36.211:"Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation "
3GPP TS 36.212: "Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding"
3GPP TS 36.213: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures"
3GPP TS 36.214: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer; Measurements"