eNB Functionalities

eNodeB

The eNB element of LTE is responsible for radio transmission and reception with UE. eNB provides the required functionality for the radio resource management (RRM), including admission control, radio bearer control, scheduling of user data, and control signaling over the air interface. In addition, eNB takes care of the ciphering and header compression over the air interface.

The clearest difference between UTRAN and E-UTRAN can be seen in the role of the base station. The eNodeB of LTE now includes basically all the functionalities that were previously concentrated on the RNC of the UTRANsystem. In addition, the traditional tasks of the NodeB are still included in the new eNodeB (eNB) element. eNB works thus as the counterpart of the UE in the radio interface but includes procedures for decision making related to the connections.

This solution results in the term “flat architecture” of LTE, meaning that there are less interfaces and only one element in the hierarchy of the architecture. As control has been moved closer to the radio interface, the respective signaling time has also been reduced. This is one of the key issues for the reduction of the latency of LTE compared to the previous solutions of the 3G [2]–[4].

More specifically, the eNB element handles the following tasks:

. Radio Resource Management (RRM);
. Radio Bearer Control;
. Radio Admission Control;

. Connection Mobility Control;
. UE scheduling (DL and UL);
. security in Access Stratum (AS);
. measurements as a basis for the scheduling and mobility management;
. IP header compression;
. encryption of the user data;
. routing of the user data between eNB and S-GW;
. handling of the paging that originates from MME;
. handling of the broadcast messaging that is originated from MME and Operations and Management System (OMS);
. selection of the MME element in case UE does not provide this information;
. handling of PWS messages, including ETWS and CMAS.

• Functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling)
• IP header compression and encryption of user data stream
• Selection of an MME at UE attachment when no routing to an MME can be determined from the information provided by the UE
• Routing of User Plane data towards Serving Gateway
• Scheduling and transmission of paging messages (originated from the MME)
• Scheduling and transmission of broadcast information (originated from the MME or O&M)
• Measurement and measurement reporting configuration for mobility and scheduling
• Scheduling and transmission of PWS (which includes ETWS and CMAS) messages (originated from the MME)

RRM functions

Radio Bearer Control (RBC)

The establishment, maintenance and release of Radio Bearers involve the configuration of radio resources associated with them. When setting up a radio bearer for a service, radio bearer control (RBC) takes into account the overall resource situation in E-UTRAN, the QoS requirements of in-progress sessions and the QoS requirement for the new service. RBC is also concerned with the maintenance of radio bearers of in-progress sessions at the change of the radio resource situation due to mobility or other reasons. RBC is involved in the release of radio resources associated with radio bearers at session termination, handover or at other occasions. RBC is located in the eNB.

Radio Admission Control (RAC)

The task of radio admission control (RAC) is to admit or reject the establishment requests for new radio bearers. In order to do this, RAC takes into account the overall resource situation in E-UTRAN, the QoS requirements, the priority levels and the provided QoS of in-progress sessions and the QoS requirement of the new radio bearer request. The goal of RAC is to ensure high radio resource utilization (by accepting radio bearer requests as long as radio resources available) and at the same time to ensure proper QoS for in-progress sessions (by rejecting radio bearer requests when they cannot be accommodated). RAC is located in the eNB.

Connection Mobility Control (CMC)

Connection mobility control (CMC) is concerned with the management of radio resources in connection with idle or connected mode mobility. In idle mode, the cell reselection algorithms are controlled by setting of parameters (thresholds and hysteresis values) that define the best cell and/or determine when the UE should select a new cell.

Also, E-UTRAN broadcasts parameters that configure the UE measurement and reporting procedures. In connected mode, the mobility of radio connections has to be supported. Handover decisions may be based on UE and eNB measurements. In addition, handover decisions may take other inputs, such as neighbour cell load, traffic distribution, transport and hardware resources and Operator defined policies into account. CMC is located in the eNB.

Dynamic Resource Allocation (DRA) - Packet Scheduling (PS)

The task of dynamic resource allocation (DRA) or packet scheduling (PS) is to allocate and de-allocate resources (including buffer and processing resources and resource blocks (i.e. chunks)) to user and control plane packets. DRA involves several sub-tasks, including the selection of radio bearers whose packets are to be scheduled and managing the necessary resources (e.g. the power levels or the specific resource blocks used). PS typically takes into account the QoS requirements associated with the radio bearers, the channel quality information for UEs, buffer status, interference situation, etc. DRA may also take into account restrictions or preferences on some of the available resource blocks or resource block sets due to inter-cell interference coordination considerations. DRA is located in the eNB.

Inter-cell Interference Coordination (ICIC)

Inter-cell interference coordination has the task to manage radio resources such that inter-cell interference is kept under control. ICIC mechanism includes a frequency domain component and time domain component. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells. The preferred ICIC method may be different in the uplink and downlink. The frequency domain ICIC manages radio resource, notably the radio resource blocks.

For the time domain ICIC, Almost Blank Subframes (ABSs) are used to protect resources receiving strong inter-cell interference. MBSFN subframes can be used for time domain ICIC when they are also included in ABS patterns. The eNB cannot configure MBSFN subframes [4] as ABSs when these MBSFN subframes are used for other usages (e.g., MBMS, LCS). ICIC is located in the eNB.