Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/0005—Control or signalling for completing the hand-off
  • H04W36/0083—Determination of parameters used for hand-off, e.g. generation or modification of neighbour cell lists
  • H04W36/00837—Determination of triggering parameters for hand-off
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/0005—Control or signalling for completing the hand-off
  • H04W36/0083—Determination of parameters used for hand-off, e.g. generation or modification of neighbour cell lists
  • H04W36/0085—Hand-off measurements
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/24—Reselection being triggered by specific parameters
  • H04W36/30—Reselection being triggered by specific parameters by measured or perceived connection quality data
  • H04W36/302—Reselection being triggered by specific parameters by measured or perceived connection quality data due to low signal strength
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04Q—SELECTING
  • H04Q2213/00—Indexing scheme relating to selecting arrangements in general and for multiplex systems
  • H04Q2213/13098—Mobile subscriber
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W36/00—Hand-off or reselection arrangements
  • H04W36/14—Reselecting a network or an air interface
  • H04W36/144—Reselecting a network or an air interface over a different radio air interface technology
  • H04W36/1446—Reselecting a network or an air interface over a different radio air interface technology wherein at least one of the networks is unlicensed

Definitions

• This invention relates to handover control for multi-mode wireless devices: that is to say, devices which are able to support two or more wireless interfaces. Some of these devices are able to hand over a data connection or voice call from 5 one wireless interface to another without the session or call being dropped. Such arrangements are particularly useful for handsets capable of operating on both localised high bandwidth systems such as WLANs (Wireless local area networks) and public cellular networks which have greater geographical penetration but more limited facilities for example in bandwidth.
• WLANs Wireless local area networks
• public cellular networks which have greater geographical penetration but more limited facilities for example in bandwidth.
• the decision as to when to hand over is typically taken by the device, by monitoring one or more parameters such as signal strength on both the current interface and the candidate for handover, so that if a predetermined threshold is reached indicating the availability of a better interface than the one currently in use, a handover from the current wireless interface to the other, better, one is
• a single parameter is measured and a handover threshold selected, though in some cases multiple parameters are monitored and an algorithm based upon the parameters and one or more thresholds used to determine whether to hand over or not.
• a wireless communications device having a handover processor for controlling the handover of connection of the device from a first communications network to a second communications network, having a signal quality assessment system for determining values for one or more measure of signal quality currently available from at least one of the communications networks, and determining whether the measure or measures of signal quality meet threshold values set for that network, a handover processor for initiating a handover attempt if the threshold values are met, and a threshold setting unit for adjusting one or more of the threshold values according to the success or failure of the handover attempt.
• the invention also provides a process for controlling handover of a connection between two co-operating devices between a first communications network and a second communications network, in which values for one or more measure of signal quality are determined in at least one of the communications networks, the measure or measures of signal quality are compared with threshold values set for that network.
• a handover attempt is initiated if the threshold values are met, and one or more of the threshold values are adjusted according to the success or failure of the handover attempt.
• This invention differs from existing systems by introducing an iterative process to learn the optimum settings for handover thresholds from experience.
• the advantage of this is that it enables the handover thresholds to be optimised for the actual equipment in use and the way it is used, rather than making use of a conservative threshold for all devices and environments.
• This invention would significantly reduce the existing costs expended by network operators or handset manufacturers in determining the optimum handover thresholds for installing in the network or handsets. Instead, the handset or network automatically optimises coverage in use, for many different usage cases.
• Figure 1 illustrates the prior art handover process already discussed
• Figure 2 illustrates a modified handover process according to the invention.
• Figure 3 illustrates a variant of the modified handover process according to the invention.
• Figure 4 is a schematic diagram depicting the elements of a wireless handset device to which the invention may be applied.
• the handset 7 comprises a user interface 41 for processing inputs such as audio or keyboard entries and generating outputs such as audio or text displays. These inputs and outputs are encoded/decoded and modulated/demodulated by a wireless (RF) interface 42 to allow communication with two different networks 8, 9. Handover between the networks is handled by a handover processor 43. This includes a signal quality measuring system 44, and a comparison processor 45 which compares the measured signal qualities with a stored threshold value 46. The output of the comparison processor 45 causes the handover processor 43 to initiate a handover attempt if the threshold values are met.
• a network interface unit 47 handles interactions with the network, such as location update, and also detects loss of contact with the network. The success or failure of a handover is reported by the network interface 47 to a threshold setting unit 48 which generates adjustments to the stored threshold values 46 according to the success or failure of individual handover attempts.
• the flow-charts of Figures 2 and 3 illustrate an embodiment running on a handset (7, Figure 4) which supports calls over both 802.11 b WLAN (WiFi) and GSM (cellular) wireless systems (8, 9).
• the invention may also be run on the fixed infrastructure of a mobility network, to control network- initiated handovers.
• the flow-chart of Figure 2 shows the scenario for this invention whereby a call (1 ) is started over the WLAN (8) and then the WLAN signal strength is periodically monitored (21 ).
• Properties other than signal strength such as signal quality, signal to noise ratio, bit-error-rate or packet error rate, or combinations of such properties, may also be used as thresholds. If it falls below a value X(off) the handset 40 determines whether a GSM connection 9 is available
• the handover attempt (3) may fail (step 4), either because the quality of the connection 9 to which the handset is attempting handover is not adequate (branch 40x), or more usually because the quality of the existing connection 8 deteriorates further, to the point where connection is lost before the handover is completed (branch 4Oy). If this happens, the relevant threshold value X(off) and Y(on) is then raised (40) so that future handoffs are not attempted in those same conditions. Raising the threshold value Y(on) of the destination network 9 will prevent future handoffs being attempted to a network exhibiting such a low signal strength. Conversely, raising the threshold value X(off) of the originating network 8 will have the effect of causing any handover which is to be initiated to be started sooner, before the signal strength has deteriorated too far to complete the handover.
• step 5 If the handover is successful, (step 5) then one or both of the handover thresholds (X(off), Y(on)) are lowered (step 51 ). This will cause future handovers to take place less promptly, allolwing unnecessary handovers to be avoided. Because the consequences of setting the thresholds too low (failure of call) are more severe than the consequences of setting them too high (reduced efficiency or extra cost), more caution is required in reducing the threshold, so the conditions for reduction of the threshold should be more stringent than for raising it. This may be achieved by having reductions in threshold operating in smaller increments than those used for increases, (as shown in Figure 2, increments are in steps of 1 , but reductions in steps of 0.5).
• the thresholds may also be incremented alternately on successive successful handovers, rather than both being incremented together. Alternatively, it may be required that a number of successful handovers are to be completed (step 50, Figure 3) before the threshold is updated. This is in contrast to the behaviour in the event of a call failure (4), which will result in immediate modification of at least one of one of the thresholds X(off), Y(on) (step 40).
• the choice of which to modify preferably depends on the nature of the handover failure.
• the WLAN connection 8 is preferred over the GSM network 9, so handover 3 to the WLAN may be attempted whenever the signal strength of that network exceeds a threshold X(on) (test 22) regardless of the GSM strength.
• a threshold X(on) test 22
• the signal quality of the current (GSM) connection is not an issue in determining whether a handover should take place and the step (21 , Figure 2) of assessing quality of the current network is unnecessary.
• the threshold is modified as before (40, 51 ) according to the outcome of the handover attempt (3).
• Initial values of thresholds X(on) Y(on) X(off) Y(off) for different networks will typically differ, to take account of preferences for using one network or the other for reasons of cost, available facilities, etc, and maximum and minimum values for these thresholds may be specified to prevent updates disrupting these preferences.
• the invention may also be employed in a network-initiated handover process.
• the appropriate handover threshold for a given base station may be dictated by circumstances such as the presence of physical obstructions, range of the transmitter, differing RF interference environments, the speed at which users move in the locality, and other factors. Some of these factors may vary over time, for example if the base station is near a road designed for fast traffic but which is subject to frequent congestion, changes in topography such as tides, or longer term factors such as trees in leaf or temporary structures which cause changes in wireless reception characteristics.
• the invention may determine the threshold values at which handover is to be initiated for each base station (of the same or different network technologies).

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Mobile Radio Communication Systems (AREA)

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• 2008
  • 2008-01-25 EP EP08250313A patent/EP2083587A1/en not_active Ceased
  • 2008-12-19 JP JP2010543553A patent/JP2011510580A/ja active Pending
  • 2008-12-19 WO PCT/GB2008/004237 patent/WO2009092990A1/en not_active Ceased
  • 2008-12-19 US US12/863,766 patent/US8437310B2/en active Active

Patent Citations (5)

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