The eNB is the network entity that is responsible for radio interface transmission and reception. This includes radio channel modulation/demodulation as well as channel coding/decoding and multiplexing/demultiplexing.
System information is broadcast in each cell on the radio interface DL to provide basic information to UEs as a prerequisite to access the network.
The LTE base station hosts all RRC functions such as broadcast of system information and RRC connection control including:
- Paging of subscribers.
- Establishment, modification, and release of RRC connection including the allocation of temporary UE identities (Radio Network Temporary Identifier, RNTI).
- Initial security activation, which means the initial configuration of the Access Stratum (AS) integrity protection for the control plane and AS ciphering for both control plane and user plane traffic.
- RRC connection mobility that includes all types of intra-LTE handover (intra-frequency and inter-frequency). In the case of handover, the source eNB will take care of the associated security handling and provide the necessary key and algorithm information to the handover target cell by sending specific RRC context information embedded in a transparent container to the handover target eNB.
- Establishment, modification, and release of DRBs (Dedicated Radio Bearers) carrying user data.
- Radio configuration control, especially the assignment and modification of ARQ and Hybrid Automatic Repeat Request (HARQ) parameters as well as Discontinuous Reception (DRX) configuration parameters.
- QoS control to ensure that, for example, user plane packets of different connections are scheduled with the required priority for DL transmission and that mobiles receive the scheduling grants for UL data transmission according to the QoS parameters of the radio bearers.
- Recovery functions that allow re-establishment of radio connections after physical channel failure or Radio Link Control Acknowledged Mode (RLC AM) retransmission errors.
The most crucial part for measuring the eNB performance is the UL/DL resource management and packet scheduling performed by the eNB. This is probably the most difficult function which requires the eNB to cope with many different constraints like radio link quality, user priority, requested QoS, and UE capabilities. It is the task of the eNB to make use of the available resources in the most efficient way.
Furthermore, the RRC entity of the eNB covers all types of intra-LTE and inter-RAT measurements, in particular:
- Setup, modification, and release of measurements for intra-LTE intra-frequency, intra-LTE inter-frequency, inter-RAT mobility, transport channel quality, UE internal measurement reports to indicate, for example, current power consumption and GPS positioning reports sent by the handset.
- For compressed mode measurements it is necessary to configure, activate, and deactivate the required measurement gaps.
- The evaluation of reported measurement results and start of necessary handover procedures are also eNB functions (while in 3G UMTS all measurement evaluation and handover control functions have been embedded in the RNC). The many different parameters used in RRC measurement control functions like hysteresis values, time to trigger timer values, and event level threshold of RSRP and RSRQ (Received Signal Reference Power and Received Signal Reference Quality) are the focus of radio network optimization activities.
Other functions of the eNB comprise the transfer of dedicated NAS information and non-3GPP dedicated information, the transfer of UE radio access capability information, support for E-UTRAN sharing (multiple Public Land Mobile Network (PLMN) identities), and management of multicast/ broadcast services.
The support of self-configuration and self-optimization is seen as one of the key features of the E-UTRAN. Among these functions we find, for example, intelligent learning functions for automatic updates of neighbor cell lists (handover candidates) as they are used for RRC measurement tasks and handover decisions.
The eNB is a critical part of the user plane connections. Here the data is routed, multiplexed, ciphered/deciphered, segmented, and reassembled. It is correct to say that on the E-UTRAN transport layer level, the eNB acts as an IP router and switch. The eNB is also responsible for optional IP header compression. On the control plane level, the eNB selects the MME to which NAS signaling messages are routed.