Long Term Evolution(LTE) is first advancement towards stronger, faster and more efficient 4G data networks. The technology provide downlink peak rates of 100Mbps and uplink speeds of 50Mbit/s.
The LTE technology is a scaleable bandwidth technology for carriers operating anywhere from 20Mhz town to 1.4Mhz.
Long Term Evolution offers some excellent advantages over current 3G systems including higher throughput, plug and play compatibility, FDD (Frequency Division Duplexing) and TDD (Time Division Duplexing), low latency and lower operating expenditures. It also offers legacy modes to support devices operating on GPRS systems, while supporting seamless passthrough of technologies operating on other older cellular towers.
LTE is designed with a full Internet Protocol (IP) network infrastructure. This means it can support full voice in packet domains, while also offering advanced radio techniques for achieving higher performance levels beyond 3G data packets can currently achieve.
LTE Requirements
- Reduced cost per bit
- Increased service provisioning – more services at lower cost with better user experience
- Flexibility of use of existing and new frequency bands
- Simplified architecture, Open interfaces
- Allow for reasonable terminal power consumption and
- Provide the subscriber with a "ready-to-use" IP connectivity and an "always-on" experience.
Technologies
- OFDM (Orthogonal Frequency Division Mulplex)
- MIMO (Multiple Input Multiple Output)
- SAE (System Architecture Evolution)
- OFDM (Orthogonal Frequency Division Multiplex): OFDM technology has been incorporated into LTE because it enables high data bandwidths to be transmitted efficiently while still providing a high degree of resilience to reflections and interference. The access schemes differ between the uplink and downlink: OFDMA (Orthogonal Frequency Division Multiple Access is used in the downlink; while SC-FDMA(Single Carrier - Frequency Division Multiple Access) is used in the uplink. SC-FDMA is used in view of the fact that its peak to average power ratio is small and the more constant power enables high RF power amplifier efficiency in the mobile handsets - an important factor for battery power equipment.
- MIMO (Multiple Input Multiple Output): One of the main problems that previous telecommunications systems has encountered is that of multiple signals arising from the many reflections that are encountered. By using MIMO, these additional signal paths can be used to advantage and are able to be used to increase the throughput.
When using MIMO, it is necessary to use multiple antennas to enable the different paths to be distinguished. Accordingly schemes using 2 x 2, 4 x 2, or 4 x 4 antenna matrices can be used. While it is relatively easy to add further antennas to a base station, the same is not true of mobile handsets, where the dimensions of the user equipment limit the number of antennas which should be place at least a half wavelength apart. - SAE (System Architecture Evolution): With the very high data rate and low latency requirements for 3G LTE, it is necessary to evolve the system architecture to enable the improved performance to be achieved. One change is that a number of the functions previously handled by the core network have been transferred out to the periphery. Essentially this provides a much "flatter" form of network architecture. In this way latency times can be reduced and data can be routed more directly to its destination.
LTE Network Architecture
Source: From 3pp Specifications
Describe about Interface and Node Functionality at high level
- eNodeB: The eNodeB(eNB) is reposbile for Radio Resource managment, IP header compression, encrtyption of user data stream. Scheduling and tranmission of paging mesages. It will do measurement and measurement reporting configuration for mobility and scheduling.
- MME (Mobility Management Entity): The MME node responsible for idle mode UE (User Equipment) tracking and paging procedure including retransmissions.It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.
- SGW (Serving Gateway): The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PGW). For idle state UEs, the SGW terminates the DL data path and triggers paging when DL data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
- PGW (PDN Gateway): The PDN Gateway provides connectivity from the UE to external packet data networks by being the point of exit and entry of traffic for the UE. A UE may have simultaneous connectivity with more than one PGW for accessing multiple PDNs. The PGW performs policy enforcement, packet filtering for each user, charging support, lawful Interception and packet screening. Another key role of the PGW is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and EvDO).
Downlink
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. It is a well-established technology, for example in standards such as IEEE 802.11a/g, 802.16, HIPERLAN-2, DVB and DAB.
In the time domain there is a radio frame that is 10 ms long and consists of 10 sub frames of 1 ms each. Every sub frame consists of 2 slots where each slot is 0.5 ms. The subcarrier spacing in the frequency domain is 15 kHz. Twelve of these subcarriers together (per slot) is called a resource block so one resource block is 180 kHz. 6 Resource blocks fit in a carrier of 1.4 MHz and 100 resource blocks fit in a carrier of 20 MHz.
In the downlink there are three different physical channels. The Physical Downlink Shared Channel (PDSCH) is used for all the data transmission, the Physical Multicast Channel (PMCH) is used for broadcast transmission using a Single Frequency Network, and the Physical Broadcast Channel (PBCH) is used to send most important system information within the cell. Supported modulation formats on the PDSCH are QPSK, 16QAM and 64QAM.
For MIMO operation, a distinction is made between single user MIMO, for enhancing one user's data throughput, and multi user MIMO for enhancing the cell throughput.
Uplink
In the uplink, LTE uses a pre-coded version of OFDM called SC-FDMA. This is to compensate for a drawback with normal OFDM, which has a very high peak to average power ration(PAPR). High PAPR requires expensive and inefficient power amplifiers with high requirements on linearity, which increases the cost of the terminal and drains the battery faster. SC-FDMA solves this problem by grouping together the resource blocks in 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.
In the uplink there are two physical channels. While the Physical Random Access Channel (PRACH) is only used for initial access and when the UE is not uplink synchronized, all the data is being sent on the Physical Uplink Shared Channel (PUSCH). Supported modulation formats on the uplink data channel are QPSK, 16QAM and 64QAM.
If virtual MIMO / Spatial division multiple access (SDMA) is introduced the data rate in the uplink direction can be increased depending on the number of antennas at the base station. With this technology more than one mobile can reuse the same resources.
LTE Protocols:
The objective of EPS to provide the subscriber with a "ready-to-use" IP connectivity and an "always-on" experience.
User plane protocols:
These are the protocols implementing the actual E-RAB service, i.e. carrying user data through the access stratum.
Control plane protocols:
These are the protocols for controlling the E-RABs and the connection between the UE and the network from different aspects (including requesting the service, controlling different transmission resources, handover etc.). Also a mechanism for transparent transfer of NAS messages is included.
Brief details about interfaces:
• X2: The X2 interface connects neighboring eNodeBs to each other and is used for forwarding contexts and user data packets at inter-eNodeB handover.
• S1-MME: Reference point for the control plane protocol between E-UTRAN and MME
• S1-U: Serves as a reference point between E-UTRAN and Serving GW for the per bearer user plane tunneling and inter eNodeB path switching during handover
• S3: Enables user and bearer information exchange for inter 3GPP access network mobility in idle and/or active state. It is based on Gn reference point as defined between SGSNs.
• S4: Provides related control and mobility support between GPRS Core and the 3GPP Anchor function of Serving GW and is based on Gn reference point as defined between SGSN and GGSN. In addition, if Direct Tunnel is not established, it provides the user plane tunneling.
• S5: Provides user plane tunneling and tunnel management between Serving GW and PDN GW. It is used for Serving GW relocation due to UE mobility and if the Serving GW needs to connect to a non-collocated PDN GW for the required PDN connectivity.
• S6a: Enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between MME and HSS
• S6d: Enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between S4-SGSN and HSS
• Gx: Provides transfer of (QoS) policy and charging rules from PCRF to Policy and Charging Enforcement Function (PCEF) in the PDN GW
• S8: Inter-PLMN reference point providing user and control plane between the Serving GW in the VPLMN and the PDN GW in the HPLMN. It is based on Gp reference point as defined between SGSN and GGSN. S8 is the inter PLMN variant of S5.
• S9: Provides transfer of (QoS) policy and charging control information between the Home PCRF and the Visited PCRF in order to support local breakout function.
Brief functionality about each protocol:
S1AP:
S1AP protocol has the following functions
a)E-RAB management function
b) Initial Context Transfer function
c) UE Capability Info Indication
d) Mobiliry Functions
e) Paging
f) S1 Interface managment
g) S1 UE Context Release function
h) UE Context modification
i) Status Transfer
j)Location Reporting
k) NAS Signaling transport
X2 AP:
a) Mobility Management: This function allows the eNB to move the responsibility of a certain UE to
another eNB. Forwarding of user plane data, Status Transfer and UE Context Release function are parts of the mobility management.
b) Load Management: This function is used by eNBs to indicate resource status, overload and traffic load to each other.
c) Managing the X2 interface.
d) eNB Configuration Update. This function allows updating of application level data needed for two
eNBs to interoperate correctly over the X2 interface.
NAS(eMM/eSm):
Main functions of the protocols that are part of the NAS are:
- the support of mobility of the user equipment (UE)
- the support of session management procedures to establish and maintain IP connectivity
between the UE and a packet data network gateway (PDN GW).
- NAS security is an additional function of the NAS providing services to the NAS protocols,
e.g. integrity protection and ciphering of NAS signalling messages.
RRC:
a)Broadcast of system information
Including NAS common information.
Information applicable for UEs(cell selection parameters, neighbouring cell information and
information and common channel configuration information)
b)RRC connection control
Paging
Establishment/ modification/ release of RRC connection(SRB1 and SRB2)
Initial security activation
Establishment/ modification/ release of RBs carrying user data (DRBs)
Recovery from radio link failure
c)Measurement configuration and reporting
Establishment/ modification/ release of measurements
Setup and release of measurement gaps
Measurement reporting
PDCP:
The Packet Data Convergence Protocol do the following functions
a) header compression and decompression of IP data flows using the ROHC protocol;
b) transfer of data (user plane or control plane);
c)in-sequence delivery of upper layer PDUs at re-establishment of lower layers;
d)ciphering and deciphering of user plane data and control plane data;
e)integrity protection and integrity verification of control plane data;
f)timer based discard;
RLC:
a)Transfer of upper layer PDUs;
b) error correction through ARQ (only for AM data transfer);
c) concatenation, segmentation and reassembly of RLC SDUs (only for UM and AM data transfer);
d) re-segmentation of RLC data PDUs (only for AM data transfer);
e) reordering of RLC data PDUs (only for UM and AM data transfer);
f) duplicate detection (only for UM and AM data transfer);
g)RLC SDU discard (only for UM and AM data transfer);
h)protocol error detection (only for AM data transfer).
MAC:
a) mapping between logical channels and transport channels;
b) multiplexing of MAC SDUs from one or different logical channels onto transport blocks (TB) to be
delivered to the physical layer on transport channels;
c) demultiplexing of MAC SDUs from one or different logical channels from transport blocks (TB)
delivered from the physical layer on transport channels;
d) scheduling information reporting;
e) error correction through HARQ;
f) priority handling between UEs by means of dynamic scheduling;
g)priority handling between logical channels of one UE;
h) Logical Channel prioritisation;
i) transport format selection
PHY:
Physical channels and modulation
- Definition of the uplink and downlink physical channels;
- The structure of the physical channels, frame format, physical resource elements, etc.;
- Modulation mapping (BPSK, QPSK, etc);
- Physical shared channel in uplink and downlink;
- Reference signal in uplink and downlink;
- Random access channel;
- Primary and secondary synchronization signals;
- OFDM signal generation in downlink;
- SC-FDMA signal generation in uplink;
- Scrambling, modulation and up conversion;
- Uplink-downlink timing relation
- Layer mapping and precoding in downlink.
Multiplexing and channel coding
- Channel coding schemes;
- Coding of Layer 1 / Layer 2 control information;
- Interleaving;
- Rate matching;
Physical layer procedures
- Synchronisation procedures, including cell search procedure and timing synchronisation;
- Power control procedure;
- Random access procedure;
- CQI reporting and MIMO feedback;
- UE sounding and HARQ ACK/NACK detection;
Physical layer – Measurements
- Measurements to be performed by Layer 1 in UE and E-UTRAN;
- Reporting of measurement results to higher layers and the network;
- Handover measurements, idle-mode measurements, etc.
LTE Bearers:
Source: 3gpp Specifications
LTE Signaling procedures:
Note: Most of the information have been driven from 3gpp specifications. If any one feels/detects/finds any copy right violation please bring to me notice, So I will keep update with required information if required.
