The exemplary and non-limiting embodiments of this invention relate generally to wireless communication systems, methods, devices and computer programs and, more specifically, relate to cellular system mobility procedures, such as those performed in UTRAN and E-UTRAN, and relate even more specifically to mobility procedures used with small cells such as pico cell or home enhanced NodeBs (HeNBs), also referred to as femtocells, and to techniques adapted to facilitating user equipment (UE) discovery of such cells.
This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
ANR automatic neighbour relation
ASF autonomous search function
BS base station
CSG closed subscriber group
DL downlink (eNB towards UE)
DRx discontinuous reception
eNB E-UTRAN Node B (evolved Node B)
EPC evolved packet core
E-UTRAN evolved UTRAN (LTE)
HeNB home eNB
IMTA international mobile telecommunications association
ITU-R international telecommunication union-radiocommunication sector
LTE long term evolution of UTRAN (E-UTRAN)
LTE-A LTE advanced
MAC medium access control (layer 2, L2)
MM/MME mobility management/mobility management entity
OFDMA orthogonal frequency division multiple access
O&M operations and maintenance
PCI physical cell identifier
PDCP packet data convergence protocol
PHY physical (layer 1, L1)
PSC packet scheduling
RAT radio access technology
RLC radio link control
RRC radio resource control
RRM radio resource management
SGW serving gateway
SI system information
SC-FDMA single carrier, frequency division multiple access
UE user equipment, such as a mobile station, mobile node or mobile terminal
UL uplink (UE towards eNB)
UPE user plane entity
UTRAN universal terrestrial radio access network
One modern communication system is known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA),In this system the DL access technique is OFDMA, and the UL access technique is SC-FDMA.
One specification of interest is 3GPP TS 36.300, V10.4.0 (2011-06), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (EUTRAN); Overall description; Stage 2 (Release 10), referred to for simplicity hereafter as 3GPP TS 36.300. Another specification of interest is 3GPP TS 36.331 V10.2.0 (2011-06) Technical Specification 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification (Release 10).
FIG. 1A reproduces Figure 4.1 of 3GPP TS 36.300 and shows the overall architecture of the EUTRAN system (Rel-8). The E-UTRAN system includes eNBs, providing the E-UTRAN user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UEs. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME by means of a S1 MME interface and to a S-GW by means of a S1 interface (MME/S-GW 4). The S1 interface supports a many-to-many relationship between MMEs/S-GWs/UPEs and eNBs.
The eNB hosts the following functions:
functions for RRM: RRC, Radio Admission Control, Connection Mobility Control,
Dynamic allocation of resources to UEs in both UL and DL (scheduling);
IP header compression and encryption of the user data stream;
selection of a MME at UE attachment;
routing of User Plane data towards the EPC (MME/S-GW);
scheduling and transmission of paging messages (originated from the MME);
scheduling and transmission of broadcast information (originated from the MME or O&M); and
a measurement and measurement reporting configuration for mobility and scheduling.
Also of particular interest herein are further releases of 3GPP LTE targeted towards future IMT-A systems, referred to herein for convenience simply as LTE-Advanced (LTE-A). A goal of LTE-A is to provide significantly enhanced services by means of higher data rates and lower latency with reduced cost. LTE-A is directed toward extending and optimizing the 3GPP LTE Rel-8 radio access technologies to provide higher data rates at lower cost. LTE-A will be a more optimized radio system fulfilling the ITU-R requirements for IMT-Advanced while maintaining backwards compatibility with LTE Rel-8.
A problem relates to small cell discovery and measurement triggering, where by small cells what is generally meant are picocells or HeNB cells. Under current 3GPP assumptions the UE should be able to detect (discover) allowed and earlier visited CSG/hybrid cells using an implementation-specific Autonomous Search Function (ASF). This function is currently specified in a vague manner and the details of this function are left to individual implementations. However, 3GPP is now working towards improving small cell discovery and proximity indications (thus also improving the ASF). Reference in this regard can be made to, for example, 3GPP TSG-RAN Meeting #51, RP-110438, Kansas City, US, Mar. 15-18, 2011, Source: Nokia Siemens Networks, Nokia Corporation, Alcatel-Lucent; Title: New work item proposal for Hetnet Mobility Improvements for LTE, Agenda Item: 14.1.
The challenge that is presented is how should a UE, in the (RRC) connected mode, find a previously discovered and allowed CSG/hybrid cell. Ideally this additional UE task should not lead to a noticeable increase in the signaling load, UE battery power consumption, nor should it have a negative impact on any potentially ongoing data transmission such as, for example, requiring a lower data rate to be used. Under current assumptions the UE is to perform the needed measurements in an implementation specific way, and once a cell has been discovered the network should be notified via a proximity indication message. Based on this message the network configures the UE with an appropriate measurement configuration (on the indicated carrier) and from that time forward then standardized measurement rules are used.
By way of background, reference can be made to Section 10.5.1.2, RRC_CONNECTED, of 3GPP TS 36.300. As is currently specified, while the UE is in the RRC_CONNECTED state the UE performs normal measurement and mobility procedures based on configuration provided by the network. The UE is not required to support manual selection of CSG IDs while in RRC_CONNECTED state.
Handover to a HNB/HeNB follows the framework of UE assisted network controlled handover as described in Section 10.1.2.1. Handover to a HNB/HeNB is different from the normal handover procedure in three aspects:
1. Proximity Estimation: in case the UE is able to determine, using autonomous search procedures, that it is near a CSG or hybrid cell whose CSG ID is in the UE\'s CSG white list, the UE may provide to the source eNB an indication of proximity. The proximity indication may be used as follows:
If a measurement configuration is not present for the concerned frequency/RAT, the source eNB may configure the UE to perform measurements and reporting for the concerned frequency/RAT.
The source eNB may determine whether to perform other actions related to handover to HNB/HeNBs based on having received a proximity indication (for example, the source eNB may not configure the UE to acquire system information of the HNB/HeNB unless it has received a proximity indication).
2. PSC/PCI Confusion: due to the typical cell size of HNB/HeNBs being much smaller than macro cells, there can be multiple HNBs/HeNBs within the coverage of the source eNB that have the same PSC/PCI. This leads to a condition referred to as PSC/PCI confusion, wherein the source eNB is unable to determine the correct target cell for handover from the PSC/PCI included in the measurement reports from the UE. PSC/PCI confusion is solved by the UE reporting the global cell identity of the target HNB/HeNB.
3. Access Control: if the target cell is a hybrid cell, prioritization of allocated resources may be performed based on the UE\'s membership status. Access control is done by a two step process, where first the UE reports the membership status based on the CSG ID received from the target cell and the UE\'s CSG white list, and then the network verifies the reported status.
Mobility from eNB/HeNB to a HeNB CSG/hybrid cell takes place with the S1 Handover procedure. In the following call flow the source cell can be an eNB or a HeNB.
The procedure applies to any scenario where the CSG ID is provided by the UE or provided by the source eNB.
Reference can be made to FIG. 1B, which reproduces Figure 10.5.1.2-1 “Mobility to HeNBs CSG and hybrid cells” from 3GPP TS 36.300. The enumerated steps and procedures in FIG. 1B are described as follows.
1) The source eNB configures the UE with proximity indication control.
2) The UE sends an “entering” proximity indication when it determines it may be near a cell (based on autonomous search procedures) whose CSG ID is in the UE\'s CSG white list. The proximity indication includes the RAT and frequency of the cell.
3) If a measurement configuration is not present for the concerned frequency/RAT the source eNB configures the UE with relevant measurement configuration including measurement gaps as needed, so that the UE can perform measurements on the reported RAT and frequency. The network may also use the proximity indication to minimize the requesting of handover preparation information of CSG/hybrid cells by avoiding requesting such information when the UE is not in the geographical area where cells whose CSG IDs are in the UEs CSG White-list are located.
4) The UE sends a measurement report including the PCI (e.g., due to triggered event A3).
5) The source eNB configures the UE to perform SI acquisition and reporting of a particular PCI.
6) The UE performs SI acquisition using autonomous gaps, i.e., the UE may suspend reception and transmission with the source eNB within the limits defined in 3GPP TS 36.133 to acquire the relevant system information from the target HeNB.