E-UTRAN (eNB)

Evolve - UTRAN reponsible for RAN part of EPS architecture. It only consists of one node which well known as eNode B (eNB). Its main role is  for radio transmission to and reception from UEs in one or more cells.

The eNode B is connected to EPC nodes by S1 interface.The S1 interface is devided into two part,  S1 MME interface connects eNB to MME and S1-U interface connects eNB to S-PGW. The eNB may also be connected to its neighbour by X2 interface.

Since the eNB connect to the EPC directly, thus RNC functionality have been moved to eNB. This configuration also known as flat architecture in LTE.

3GPP introduce around 36 series of standarisation as a guidance for all vendor to develop their eNB HW solution. Most of eNB HW solution consist of two main part which are :

• Digital Unit or System Module
• Radio Unit or Radio Module

Some big vendors such as Ericsson has RBS 6000 Ericsson  family solution where NSN has Flexi BTS family solution.

 

Below follows a description of the functionality provided by eNode B.

• Cell control and MME pool support
eNode B owns and controls the radio resources of its own cells. Cell resources are requested by and granted to MMEs in an ordered fashion. This arrangement supports the MME pooling concept. S-GW pooling is managed by the MMEs and is not really seen in the eNode B.

• Mobility control
The eNode B is responsible for controlling the mobility for terminals in active state. This is done by ordering the UE to perform measurement and then performing handover when necessary.

• Control and User Plane security
The ciphering of user plane data over the radio interface is terminated in the eNode B.
Also the ciphering and integrity protection of RRC signalling is terminated in the eNode B.

• Shared Channel handling
Since eNode B owns the cell resources, eNode B also handles the shared and random access channels used for signalling and initial access.

• Segmentation/Concatenation
Radio Link Control (RLC) Service Data Units (SDUs) received from the Packet Data convergence

Protocol (PDCP) layer in the AGW consist of whole IP packets may be larger than the transport block size provided by the physical layer. Thus, the RLC layer must support segmentation and concatenation to adapt the payload to the transport block size.

• HARQ
A Medium Access Control (MAC) Hybrid Automatic Repeat reQuest (HARQ) layer with fast feedback provides a means for quickly correcting most errors from the radio channel. To achieve low delay and efficient use of radio resources, the HARQ operates with a native error rate which is sufficient only for services with moderate error rate requirements such as for instance VoIP. Lower error rates are achieved by letting an outer Automatic Repeat reQuest (ARQ) layer in the eNode B handle the HARQ errors.

• Scheduling
A scheduler with support for the QoS model provides efficient scheduling of UP and CP data.

• Multiplexing and Mapping
The eNode B performs mapping of logical channels onto transport channels.

• Physical layer functionality
The eNode B handles the physical layer such as scrambling, Tx diversity, beamforming processing, and OFDM modulation. The eNode B also handles L1 functions like link adaptation and power control.

• Measurements and reporting
eNode B provides functions for configuring and making measurements on the radio environment and eNode B-internal variables and conditions. The collected data is used internally for RRM but can be reported for the purpose of multi-cell RRM.