Multi-band handover via an adaptive handover threshold

BACKGROUND OF THE INVENTION

The present invention relates to a handover operation in a multi-band

communication arrangement.

As more and more spectrums are being made available for MBB due to e.g. 2G reforming or the 2.6G technology, there is increasing possibility that a single MBB technology such as e.g. HSPA+ or LTE (HSPA: High Speed Packet Access, LTE: Long Term Evolution) may operate on different frequency bands. From standard point of view, different frequency bands may be treated as different carriers.

However, the existing handover mechanisms do not differentiate from different frequency bands so that a coverage nature of individual frequency bands under different radio conditions may not be recognized. SUMMARY OF THE INVENTION

A goal to be achieved by the present invention is to provide an efficient handover mechanism in a multi-band communication arrangement which supports a prioritized use of frequency bands according to different radio conditions.

The invention is based on the finding that an efficient handover mechanism can be provided by introducing an offset to a handover threshold in particular with respect to a shared coverage area between e.g. high and low frequency bands within a communication cell, in order to create a means of prioritizing over frequency bands when e.g. making a handover decision. Thus, the handover threshold may adaptively be adjusted, wherein a feedback loop may be provided in order to take the current, resulting handover threshold and the current communication conditions into account. By way of example, the feedback loop may be used in the shared coverage area between high and low bands in order to ensure adjustments of the offsets may lead to an increase in the overall throughputs of the multi- frequency band arrangement, i.e. cluster, supported by a communication cell. By way of example, e.g. communication resources of e.g. the UMTS 900 MHz technology, which may be considered as having a better building penetration than the UMTS 2.1 GHz technology, may be saved for indoor users or for users which are located at a cell edge (UMTS: Universal Mobile Telecommunications System).

According to a first aspect, the invention relates to a method for adapting a handover threshold for a handover between a first frequency band and a second frequency band of a multi-band communication arrangement to communication conditions such as different radio conditions in a communication cell. The method comprises determining a cell throughput of the communication cell, and amending a handover offset of the handover threshold depending on the cell throughput.

By way of example, the handover may be threshold-based, wherein the threshold enables taking e.g. a cell capacity at the respective frequency band or different service scenarios into account. According to the invention, the handover threshold is provided with an additional offset which allows for a quick adaptation of the handover threshold to e.g. current communication conditions when e.g. deciding whether to handover to e.g. the first frequency band or not. By way of example, the first frequency band may be a lower frequency band than the second frequency band. Furthermore, the first frequency band may be a frequency band of the UMTS 900 MHz technology, wherein the second frequency band may be a frequency band of the UMTS 2.1 GHz technology.

According to an implementation form of the first aspect, the step of amending comprises increasing or reducing the handover offset by an offset amount. The offset amount may be static or non-static in order to take different communication conditions into account.

According to an implementation form of the first aspect, a feedback loop is used to regulate the adaptation of handover threshold.

According to an implementation form of the first aspect, the step of amending comprises amending the handover offset by a predetermined offset amount. The offset amount may be pre-stored in a lookup-table, by way of example.

According to an implementation form of the first aspect, the step of amending comprises reducing the handover offset if the cell throughput is greater than or equal to a cell throughput at a previous time instant. The cell throughput at the previous time instant may correspond to the cell throughput which was calculated at a previous iteration step when iteratively executing the method for adapting the handover threshold.

According to an implementation from of the first aspect, the step of amending comprises reducing the handover offset if the cell throughput is greater than or equal to a maximum allowable throughput. The maximum allowable throughput may be either subject to standard constraints or may be OMC configurable.

According to an implementation form of the first aspect, the step of amending comprises increasing the handover offset if the cell throughput is smaller than or equal to a cell throughput at a previous time instant, e.g. at a previous iteration step.

According to an implementation form of the first aspect, the step of amending comprises increasing the handover offset if the cell throughput is smaller than or equal to a maximum allowable cell throughput. The maximum allowable cell throughput may correspond to that maximum allowable cell throughput mentioned above.

According to an implementation form of the first aspect, the cell throughput comprises a cell throughput in the first frequency band and a cell throughput in the second frequency band. Alternatively, the cell throughput comprises only the cell throughput in the first frequency band. Alternatively, the cell throughput comprises only the cell throughput in the second frequency band. According to an implementation form of the first aspect, the step of determining the cell throughput comprises calculating the cell throughput, e.g. calculating the respective cell throughput as mentioned above. In order to calculate the cell throughput, e.g. a data rate may be calculated or determined. According to an implementation form of the first aspect, the method comprises providing the handover threshold with the handover offset. The handover offset may depend on the respective communication technology being used for communicating in the respective first frequency band or second frequency band. Preferably, the handover offset is added to or subtracted from the handover threshold.

According to an implementation form of the first aspect, the method comprises detecting a new handover trigger. The new handover trigger may be detected upon e.g. a basis of a handover request requesting to handover from e.g. the second frequency band towards the first frequency band or vice versa by user equipment forming an embodiment of a user entity.

According to an implementation form of the first aspect, the method comprises determining whether the communication cell supports communications in the first frequency band and in the second frequency band. In this regard, the method may comprise determining whether the user entity is within a coverage area supporting communications e.g. in the second frequency band according to e.g. the 2.1G technology. Correspondingly, the method may comprise determining whether the user entity is in the coverage area according to e.g. the UMTS 900 MHz

technology.

According to a second aspect, the invention relates to a method for performing a handover operation between a first frequency band and a second frequency band of a multi-band communication arrangement in the communication cell. The method comprises adapting the handover threshold according to the first aspect of the invention, and deciding whether to perform the handover upon the basis of the adopted handover threshold.

According to a third aspect, the invention relates to a network node which is configured to perform at least one of the methods of the first aspect or of the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the invention will be described with respect to the following figures, in which:

Fig. 1 shows a diagram of a method for adapting a handover threshold; and

Fig. 2 shows a diagram of a method for performing a handover.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a diagram of a method for adapting a handover threshold for handover between a first frequency band and a second frequency band. The method comprises determining 101 a cell throughput of a communication cell, and amending 103 a handover offset of the handover threshold in dependency on the cell throughput.

As depicted in Fig. 1 , an intelligent handover scheme is provided in a shared coverage area between e.g. high frequency bands and low frequency bands such that the power budget for the lower band, e.g. the first frequency band, may be saved for the user equipment which may be located outside of the shared coverage area between the high frequency band and the low frequency band. Preferably, an inter-frequency band handover is distinguished from an inter-carrier handover since carriers in different frequency bands may be treated a different way as those in the same frequency band. Furthermore, the actual downlink power consumption for the user equipment with respect to different frequency bands may be taken into account when varying the resulting handover threshold which comprises a traditional handover threshold and the variable handover offset. In addition, according to some implementation forms, prioritization of use of frequency bands according to different radio conditions may be supported where communication is taking place. Thus, the best use of the UMTS 900 MHz spectrum or the UMTS 800 MHz spectrum by encouraging users near a cell centre to operate according to e.g. the 2.1G technology may yield saving the lower band for users indoor or at the cell edge. Furthermore, according to some

implementations, the overall throughput of a multi-band arrangement may be improved by intelligent prioritizing a frequency band usage according to radio conditions. According to some embodiments, one or more inputs to making a handover (HO) decision may be provided. By way of example, an offset to the HO threshold for the lower band in the shared coverage area with the higher band may be changed which may make possible to assert HO preference for different bands for the same technology. Furthermore, service types and loading at each band can also be taken into account. According to some embodiments, an intelligent HO decision may be made. In this regard, the HO mechanism may remain the same as before for all frequency bands. The intelligent HO decision may be realised by adding an offset to the HO threshold when operating in the shared coverage area between the high and low bands.

According to some embodiments, the handover offset may be controlled via a feedback loop as shown in Fig. 2 which takes into account the effect on overall throughput and loading situation. As to the feedback loop, the following inputs and control may be used to constitute the feedback loop in the shared coverage area between high and low frequency bands:

Input 1 : Outcome of HO threshold evaluation for each band, Input 2: Overall throughput of the mullti-layer cell site (e.g. for 900/2.1 G or

800/2.6G technologies),

Input 3: Loading situation at each frequency band, Control: Regulating an offset to each HO threshold such that the overall multi-layer cell throughput is maximized.

The feedback loop may enable an iterative system adaptation of the threshold to communication conditions. The stability and performance of the resulting feedback system may be affected by the incremental steps applied to the handover threshold and/or by a filter length of a moving average filter which may be applied to the overall cell throughput.

Fig. 2 shows a diagram of a method for performing a handover between a first frequency band and a second frequency band. The method comprises determining 201 a cell throughput of a communication cell which determines a coverage area by e.g. calculating a moving average, D _T , of the first frequency band and of the second frequency band of the multilayer cluster forming an embodiment of a multi- band communication arrangement. The first frequency band may be determined by the UMTS 900 MHz technology, whereas the second frequency band may correspond to the frequency band according to the 2.1G technology.

In step 203, the cell throughput is compared with a previously calculated cell throughput, D _T - i . If the cell throughput is greater than or equal to the previous cell throughput, then, in step 205, the handover offset T _h goo is reduced by a

predetermined offset amount Δ _τ . Otherwise, in step 207, a cell throughput at e.g. the second frequency band may be determined by calculating e.g. a moving average, L _T , of the second frequency band cell throughput at the time instant T. In step 209, the cell throughput calculated in step 207 is compared to a maximum allowable cell throughput L _Ma x which may be a 2.1 G cell throughput being

configurable from the OMC. If L _T is greater than or equal to L _Max then the method proceeds with the step of reducing 205 the handover offset. Otherwise, in step 211 , the handover offset is increased by Δ _τ . Thereafter, the method proceeds with the step of determining 213 as to whether a new handover is triggered. If a new handover is triggered then the method may proceed with the step of determining 215 whether a user entity (UE) is e.g. in the coverage area of the second

frequency band, e.g. in the 2.1 G coverage area. Otherwise, the method may remain in step 213.

If the communication cell supports communications in the second frequency band, then, starting from the step 215, a next iteration step is triggered in step 217.

Otherwise, a normal handover for the first frequency band, e.g. for the lower 900 MHz band is performed in step 219.

According to some embodiments, T _h goo may denote an offset to the HO threshold for 900M band in the shared coverage area (900/2.1 G) with may default value configurable from the OMC. According to some embodiments, D _T may denote a moving average of total

900/2.1G multi-layer cell throughput at T. According to some embodiments, L _T may denote moving average of the 2.1G cell throughput at T.

According to some embodiments, L _Ma x may denote a maximum 2.1G cell throughput configurable from the OMC.

According to some embodiments, Δ _τ may denote an incremental step for T _h goo adjustment configurable from the OMC.

According to some embodiments, an optimum use of spectrum pairs like 900/2.1G, 800/2.6G and 900/2.6G may be enabled in the sense that maximum throughput can be achieved for the overall multi-band cell structure in a cluster.

According to some embodiments, more indoor traffic may be absorbed by saving the lower band for users deep indoor or at the cell edge. According to some embodiments, the invention may also be applicable to both macro and micro layers deployed with 900/2.6G dual bands.

According to some embodiments, an open loop solution without the feedback loop as shown in Fig. 2 may be implemented.

According to some embodiments, the best use of lower frequency band of paired band configurations for either UMTS or LTE (900MHz/2.1GHz, 900MHz/2.6GHz, 800MHz/2.6GHz) may be achieved by encouraging users near cell centre to operate on the higher band saving the lower band for users indoor or at the cell edge. According to some embodiments, prioritization over different frequency bands based on radio environment condition and relative cell position of the UE may be enabled. According to some embodiments, frequency bands differentiation during an HO operation for either UMTS or LTE via the introduction of an offset to the HO threshold for the lower frequency band in the shared coverage area between high and low frequency bands may be facilitated. According to some embodiments, a feedback loop may be introduced to control the adjustment of the HO offset in the shared coverage area between high and low frequency bands such that the overall through of the multi-band cluster is improved. According to some embodiments, service types and loading condition of each frequency bands may taken into account in the algorithms implementing the inventive methods.