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 or pursued. 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        eNB base station in a LTE network        GPS global positioning system        LTE long term evolution (evolved UTRAN or EUTRAN)        RAT radio access technology        RF radio frequency        UE user equipment        UTRAN universal terrestrial radio access network        WiFi wireless fidelity (IEEE 802.11 family of standards)        
Recent progression in the wireless arts include the deployment of smaller cells within the coverage areas (or extending the coverage area) of conventional cells. In at least the LTE system these smaller cells are generally termed pico or femto cells while the conventional cells are distinguished by the term macro cell, with the aggregated deployment referred to as a heterogeneous network or HetNet for short. An exemplary HetNet is shown at FIG. 1, in which there are pico eNBs 16, 18 with control over relatively small cells B, C are within the coverage area of a macro cell A controlled by a macro eNB 12. It is conventional to use the terms cell and eNB or access node interchangeably, so for example the UE 10 with the macro cell 12 as its serving cell will as shown in FIG. 1 see cells 16 and 18 as neighbor cells.
HetNet deployments also provide the opportunity to offload traffic from crowded cellular frequency bands to other non-cellular frequency bands such as for example license-exempt WiFi bands and pico cells operating on license-exempt secondary component carriers. Such interworking in HetNets is not yet fully developed and there are many open issues yet to be worked out. The 3GPP standardization of LTE-Advanced is undergoing some of this development, but one open issue is how to arrange for UEs to scan inter-frequency small neighbor cells in a HetNet that is power efficient for the UE's limited power supply.
Current LTE specifications provide for a UE mobility state estimation (MSE) which is based on the number of cell reselections (for a UE in the idle mode) or handovers (for a UE in the connected mode) the UE has undergone over a predefined period of time. This ratio serves as a proxy for the UE's relative speed. For idle mode UEs having medium and high mobility states, the UE will add an offset to the signaled cell reselection margin (the parameter Qhyst which is a hysteresis value) and scales the value for Treselection by a factor. Treselection is a time value signaled also in system information against which a UE will test the received signal strength (or power) of a best neighbor cell against that of its serving cell; if the received signal strength/power from the neighbor cell exceeds that from the serving cell for the duration Treselection the UE is to reselect to that neighbor cell. The connected mode UE will scale its time-to-trigger by a factor. Time to trigger is a delay period during which a UE having received new parameters is not allowed to change its operational state; introducing this hysteresis prevents the UE from transitioning too quickly from connected to idle and back to connected states again by imposing a minimum time for its transition out of the connected state. These scaling factors are parameters signaled by the network and are used at least in part to optimize mobility for fast moving UEs.
These factors were not designed with HetNets in mind, but rather for the case where the UE in FIG. 1 would see only macro neighbor cells 20, 22; at that time pico neighbor cells 16, 18 were not a part of the radio environment. The addition of all those other pico cells to the radio environment would mean higher power consumption and a faster drawdown of the UE's power supply if the UE were to scan all those small cells, or measure all the small cells it detects, according to the conventional procedure. There is a need for new network scanning schemes for different user velocities to overcome the draining of battery power, as well as to support high-speed mobility as envisioned in LTE-Advanced systems.
One relevant document is U.S. Pat. No. 7,408,506 which proposes a scanning rate control mechanism that increases the scanning rate proportional to the velocity of the mobile terminal. The velocity is measured based on the signal strength data received from a WiFi receiver and a cellular receiver as well as position and velocity data obtained from a GPS receiver. This solution proposes to control the scanning rates of the GPS receiver, the cellular receiver and the WiFi receiver. In the inventors' view, increasing the scanning rate in conjunction with the mobile's velocity as this document sets forth will serve to increase power consumption.
Another relevant document is R2-114005 by Vodafone and entitled IMPROVING MOBILITY TOWARDS SMALL CELLS (3GPP TSG RAN WG2 Meeting #75; Athens, Greece; 22-26 Aug. 2011). This document targets intra-frequency cells, those operating on a same frequency band as the mobile's serving cell. The problem with intra-frequency cells is quite different than that with inter-frequency cells, in that delays in finding small intra-frequency cells can cause severe interference problems since the intra-frequency small cells will act as interferers. That problem is not present in the inter-frequency case in which the goal is to find offloading opportunities in a power efficient manner. Additionally, while document R2-114005 may reduce how often cell changes are triggered via manipulating the Treselection parameter, the UE is still expending large amounts of power in taking those measurements it never sends.
What is needed in the art is a way for the UE to scan for offload opportunities in a power efficient manner, where the offload opportunities are identified by scanning for inter-frequency neighbor cells whether they are operating on the same RAT as the UE's serving cell or on a different RAT.