14-08-2012, 04:19 PM
LTE Systems
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LTE Systems Overview
Long Term Evolution (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). LTE may also be referred more formally as Evolved UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS Terrestrial Radio Access Network(E-UTRAN). Even though 3GPP created standards for GSM/UMTS family, the LTE standards are completely new, with exceptions where it made sense. 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 betweenvery highdata rate fixed wireless Local Area Networks (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 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
Targets for LTE
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
Overall Network Architecture
The E-UTRAN uses a simplified single node architecture consisting of the eNBs (E-UTRAN Node B). The eNBcommunicates 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 facilitateindependent 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). Please Refer to LTE Network Infrastructure and Elements for a detailed overview of individual network elements.
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.