GB2358550A

GB2358550A - Battery saving strategy in cellular systems based on a mobile assisted handover process

Info

Publication number: GB2358550A
Application number: GB0001073A
Authority: GB
Inventors: Mark Shahaf; Rafy Carmon; Joseph Ziv
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 2000-01-18
Filing date: 2000-01-18
Publication date: 2001-07-25
Also published as: GB0001073D0

Abstract

A portable cellular device measures a quality parameter such as signal strength or interference for signals associated with nearby cells. The cell signals are arranged into at least two groups of relatively high and low quality signals. High quality signals are monitored at a relatively high sampling rate, lower quality signals are unlikely selection candidates for handoff so may be monitored at a lower rate. The sampling rate for neighbouring cells may also be reduced if received signals do not vary substantially with time. There is little benefit in measuring signals at a high rate for a stationary device. Reducing the sampling rate of the portable device reduces power consumption.

Description

2358550 BATTERY SAVING STRATEGY IN CELLULAR SYSTEMS BASED ON A MOBILE

ASSISTED HANDOVER PROCESS

FIELD OF THE INVENTION

The present invention is in the general field of cellular telephone networks and in particular concerns cell reselection techniques.

BACKGROUND OF THE INVENTION:

The problem of subscribers moving throughout the coverage area of cellular system has been addressed intensively during the last decade. The process of switching from one cell to another is called handover, handoff or cell reselection. For modem digital cellular systems, this process requires the subscriber to perform certain actions and thus to be involved effectively in the overall decision when and to which cell subscriber shall move. Such a handover process which requires a subscriber to be an active partner affecting the decision when and to which cell to switch is known by the name of a Mobile Assisted HandOver (MAHO). Generally, subscribers are required, at least, to perform certain measurements of the received signal from the current cell they are operating on (referred to as serving cell) and the signals received from the other near cells (referred to as neighboring cells). Figure I illustrates a typical cellular network (10) that includes serving cell and surrounding neighbor cells. The subscriber, being by this particular embodiment, e.g. a cellular telephone fitted in car (11). The car moves within area (12) Covered by the serving cell (13). As the car approaches area (14) the subscriber should switch to the neighboring cell (15). For advanced systems, the subscribers are making the reselection decision based on certain criteria and infonn the network about the decision.

These measurements of the signal received from neighbor cells result in an intensive procedure which requires noticeable subscriber power resource, i.e.

battery current drain. Maintaining neighbor cells signal constantly will resul shortened battery life due to the extensive operation of RF circuitry and d1i processing device incorporated in the subscriber.

There are known in the art techniques which purport to improvel 5 reselection decision and thereby reduce power consumption.

A typical example is the TETRA (Terrestrial Trunked Radio) syster in which the process of measuring neighbor cell signals was recommended t be triggered conditionally. Thus, as long as the signal level or quality of the se i Ig cell are above a predefined threshold, the process of sampling nei hb i c io signals is disabled and is triggered only if certain conditions are fulfilled (e.g., t le serving cell signals level drops below a given threshold). In case the measure (6t process of neighboring cells is active and the conditions are no longer fulfilled, serving cell signal parameters (strength and/or quality) have improved and above the specified threshold, the process of sampling the signals of neighbo 9 cells shall be disable to save the subscriber resources.

Although the specified approach improves the perfomiance in the sens f extended battery life it is not optimized, at least, with respect to the follow r g conditions:

1. Since, in practice, only part of the neighbor cells can provide'a 1 20 acceptable signal to the subscriber due to the geometrical s cture ( f the cellular system, when the level of the serving cell signal J i decreased, it is only reasonable that not all the neighbor cells will b measured at the same rate. This leveling of the measurement rate & the neighbor cells with respect to their signal quality was adopted ii t: 25 the DEN (trademark of Motorola, Inc.) trunked radio product ran C however it was not optimized in the sense of battery saving. differently, in accordance with the DEN (trademark) technique, t I mapping of he cells into foreground and background groups does n have any effect on the total mean weight, and accordingly, substantially no battery power saving is accomplished.

2. When the subscriber has triggered the measurement process but is kept at the same location (i.e. static conditions), without any change in propagation conditions, the measurement process will not produce any effective update to the information available from previous measurement. The simplest example is a subscriber kept in the office during a meeting. In this case, making the measurements at the same rate as while moving through the coverage area will use power resource without any added valued. When the subscriber is in static conditions switching between the serving cell to any of the neighboring cells is not likely to improve the quality of the received signal.

There is accordingly a need in the art to improve the cell reselection decision algorithm and consequently accomplish an improved battery power consumption.

SUMNLARY OF THE INVENTION:

Accordingly, by one aspect of the invention, there is provided for in a cellular radio device operable in a radio system having a plurality of frequency channels, a method of monitoring signals from proximate cells, comprising:

(a) identifying a set of proximate cell signals for monitoring, where the set is a subset of the plurality of frequency channels; (b) measuring a quality parameter for each proximate cell signal; (c) logically arranging the set of proximate cell signals into first group of 25 at least one relatively high quality parameter signal and a second group of at least one relatively low quality parameter signal; (d) monitoring the signals in the first group at a relatively high rate; and (e) monitoring the signals in second group at relatively low rate.

There is flirther provided in accordance with the invention, in a ce 1 flar radio device operable in a radio system having a plurality of frequency chann i els, a method of monitoring signals from proximate cells, comprising:

(a) identifying a set of proximate cell signals for monitoring, where the set is a subset of the plurality of frequency channels; (b) measuring a quality parameter for each proximate cell signal; (c) logically arranging the set of proximate cell signals into at least 1 o groups according to the quality parameter signals; and (d) monitoring the signals in the respective groups in different tes according to the respective quality parameter signals.

Still further, the invention provides for a cellular radio device operabl j n a radio system having a plurality of frequency channels; the device inclu c; a processor for monitoring signals from proximate cells; the processor being ope. a 51e 15 for:

(a) identifying a set of proximate cell signals for monitoring, where the set is a subset of the plurality of frequency channels; (b) measuring a quality parameter for each proximate cell signal; (c) logically arranging the set of proximate cell signals into first of i ro f 1 g i at least one relatively high quality parameter signal and a sec i of ond g p of at least one relatively low quality parameter signal; (d) monitoring the signals in the first group at a relatively high rate; an (e) monitoring the signals in second group at relatively low rate.

The invention further provides for a cellular radio device operable in a radio system having a plurality of frequency channels, the device includes a processor for monitoring signals from proximate cells; the processor being operable for:

(a) identifying a set of proximate cell signals for monitoring, where the set is a subset of the plurality of frequency channels; (b) measuring a quality parameter for each proximate cell signal; (c) logically arranging the set of proximate cell signals into at least two groups according to the quality parameter signals; and (d) monitoring the signals in the respective groups in different rates according to the respective quality parameter signals.

Still further, the invention provides for in a cellular radio device operable in a radio system, a method for reducing current consumption, comprising:

(a) identifying a set of proximate cell signals for monitoring; (b) measuring a quality parameter for each proximate cell signal is (c) measuring a time derivative for at least some of the quality parameters of the proximate cell signals so identified; and (d) reducing a rate of sampling for at least some of the proximate cell signals when the time derivative or a function thereof falls below a threshold.

The invention further provides for a cellular radio device operable in a radio system, the device includes a processor for monitoring signals from proximate cells, the processor being operable for:

(a) identifying a set of proximate cell signals for monitoring; (b) measuring a quality parameter for each proximate cell signal (c) measuring a time derivative for at least some of the quality parane ters I of the proximate cell signals so identified; and (d) reducing a rate of sampling for at least some of the proximat ell signals when the time derivative or a function thereof falls bel a threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried u in practice, a preferred embodiment will now be described, by way of non- li il ing i o example only, with reference to the accompanying drawings, in which:

Fig. 1 illustrates a typical cellular network that includes serving cellij d surrounding neighbor cells; Fig. 2 illustrates a data flow diagram (DFD) in accordance with one embodiment of the invention; Fig. 3 illustrates a block diagram of a typical subscriber unit; Fig. 4 illustrates, graphically, received signal quality vs. time; and Fig. 5 illustrates a flow diagram in accordance with one embodiment of he invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In accordance with a first aspect of the invention, there is proposed an optimization system and method to monitor signals from proximdte c els, i I depending upon a measurement parameter of the cells. It should be noted tha he terms "measurement" and "monitoring" are used interchangeably.

This aspect is described with reference to a specific embodiment below and with reference to Fig. 2. The invention is by no means bound by this example.

The set of proximate cells is determined, e.g. empirically, and includes by this embodiment the neighboring cells, e.g. those surrounding serving cell (12) It is proposed to organize neighbor cells into, preferably (but not necessarily), two groups, named foreground and background. The foreground group will in include only these neighbor cells with high quality parameter signal, for example, those cells having signal quality and/or strength (depending on a criteria being used) above a certain threshold. By another non-limiting example, a quality parameter signal being the signal-to-noise ratio. An exemplary threshold value being -9OdBm.

The members of the foreground group will be monitored at a relatively high rate.

The background group will include all other neighbors which have low quality parameter signal and which by this embodiment include the cells whose signal quality and/or strength drops below said threshold. The members of the background group will be monitored at a relatively low rate.

In a preferred embodiment, the foreground group size can be limited, say empirically. Thus only optimal candidates will be included in this group.

By one embodiment, described below, the measurement rate of the foreground group members (designated Rf)will be dictated by the requirements for optimal estimation of the received signal criteria, see W.C. Lee "Estimate of Local Average Power of a Mobile Radio Signal", in IEEE Trans. On Vech. Techn., VOL VT-34, NO 1, Feb., 1985. The invention is, of course, not bound by this example.

The cells from the background list will be measured at a lower monitoring rate, Rb which, according to one embodiment, varies with respect to the number of members in the foreground group. For example, the lower monitoring rate corrip les with the following algorithmic expression (1), as follows:

Rb Rf N > 0 aN f f Rf Nf 0 where aNf are calibrating coefficient and the effective numbe of foreground group members, respectively. The units of Rf and Rh are measuren. e its per seconds (mps). Effectively, Eq. (1) defines the measurement rate of he background group members to be in inverse proportion to the numbe of foreground group members, while a calibration factor allows to adjust the a to r, io desired trade-off between battery saving and system performance and can be ed for a fine tuning during field tests. As an example, for a= 10, Rf Z Nf 1, it IN ill result in Rb =0.2 which is I measurement in 5 seconds.

By this specific embodiment, as along as no foreground cells have e.-,n i i identified (i.e. Nf = 0) all the neighbor cells are measured at the optimal rate Rf.

The operation of the signal monitoring system in accordance with the a o ie referred to embodiment will now be described with reference also to Fig. 2.. I -s' I I The foreground and background cell signals are monitored in monitoring rate I'f and Rb (20) and entering wait state (21) in the interim intervals.

I I I I, I Depending upon the quality parameter signal (e.g. exceeding or drop 1i] IS! below a specified threshold in the manner specified above), signals are moved C.'A foreground group to background group (22) or vise versa (23) and consequently t ke monitoring rate Rb (which, as recalled, depends, inter alia, upon the numbe)f members in the foreground group) is updated (24). Whenever desired, sa), 11 accordance in pre-defined setting, signals originating ftom new cells are scan. d (25) and in the case that new cells are found they are arranged in either background group (26) or foreground group (27). The latter, obviously, necessitates the recalculation of Rb in the manner specified (24).

Those versed in the art will readily appreciate that by virtue of the low monitoring rate for the background cell signals, a power consumption is reduced since, during idle time (i.e. when the signal is not monitored), various current consuming modules in the subscriber are disabled. Thus, the battery life span is extended.

Fig. 3 illustrates a block diagram of a typical subscriber unit. The structure and operation of the unit is generally known per se and therefore will not be expounded upon herein. This notwithstanding it should be noted that during the specified idle time the following modules (receiver R,, 31; Synthesizer module 32; Signal Parameter monitor 33; Frequency selector 34 and Memory module 35) are disabled to therefore bring about power consumption saving. It has been determined empirically that about 10% to 30% improvement in energy saving is accomplished.

There follows now a description in connection with a second aspect of the invention which is based on the understanding that when a cellular subscriber is communicating with the serving cell under static conditions (i.e. when the signal received from the serving cell is substantially invariant), there is no point in monitoring proximate (e.g. neighboring) cell signals at high rate, since, due to the invariance of the subscriber's self location there is no chance that the neighboring cells will produce signals whose quality will exceed that of the serving cell.

Reducing the monitoring rate of the proximate cell signals under the specified conditions will reduce the pertinent battery power consumption as explained in detail with reference to Fig. 3.

In a more generalized observation of this aspect, the signals of at least one proximate cell (e.g. the serving and possibly one or more neighboring cells) signal is monitored, a quality parameter thereof is identified (e.g. intensity or i er quality) and the time derivative thereof is determined. A function (e.g. the abs ute function) is then applied to the specified time derivative value and in the cas hat the value of said function drops below a given threshold, the monitoring rate o, aid 5 proximate cell signals is reduced.

For a better understanding of the foregoing, attention is drawn to Fq. 4 illustrating graphically received signal quality vs. time.

In accordance with a preferred embodiment, there is provided a st of applying a logical condition which, if affirmative, obviates the need to apply te io power consumption procedure described below.

By a non -limiting example, the specified logical condition step ascer i s whether the quality parameter of the signal (e.g. signal intensity) of the self seiv bg cell is high. As long as the result is affirmative, there is no point is monitorinj at high rate the signals of proximate cells since the currently serving cell provids adequately high quality signal.

Reverting now to the former logical condition, by a specific embodiin -nt Threshold (41) discerns between sufficiently good serving cell signal and un(ye lower quality serving cell signal. In the case that the signal is of sufficient g( od! quality, the monitoring rate of neighboring cell signals may be relatively I)w considering that there is no need to switch to the neighboring cells. If, on the ot ier hand, the quality drops below threshold (41), a seeking procedure shoul be invoked in order to identify a more qualitative cell signal.

However, in accordance with the finding of the second aspect of he invention, the monitoring rate among proximal cell signals should be adjum-d, depending upon the change over time in the quality parameter of the proximite signals.

More specifically, As shown in Fig. 4, there are time periods (such as 42 and 43) where the received signal from the serving cell does not vary substantially (by this specific example the quality parameter signal is the signal intensity and no change is monitored). During these periods, monitoring of proximate cell signals at high rate will not add any additional information and thus will not affect the subscriber behavior insofar as reselection of neighboring cells is concerned. Put differently, maintaining high monitoring rate under invariable conditions appears to be an obsolete activity unduly consuming power resources.

However, when change in the signal is encountered (exceeding a given threshold), e.g. in areas 44 or 45, a higher monitoring rate of the proximate signal should be employed, since due to the motion of the subscriber it may receive a higher qualitative signal from a proximate cell (e.g. from one of the neighboring cells illustrated in Fig. 1).

The change in the monitoring rate may be applied by one embodiment as follows: in the case that the time derivative of the signal intensity (of the serving cell, or if desired of each of a set of proximate cells) drops below a given threshold [t (say 20), the monitoring rate of the proximate cells (except for the serving cell) is reduced to, say close to zero. The monitoring rate is resumed if the time derivative of the signal intensity of the serving cell exceeds say, the same threshold t.

Accordingly by this embodiment the adjustment of monitoring rate complies with the following algorithmic expressions:

Sqi W] max F2[ < p => Deactivate measurements i=I,Nf at 1=10 I F, [ a'5qs (0] > p => Activate measurements at 1=10 where sq, (t) is the serving cell received signal intensity as function of Iii ae, Sqi is the "any cell i " (serving or neighbor) signal intensity as function of I ii rie, 0/0 is time derivative, F, () and F2 () are functions deten-nining dynamics of response, to is the current measurement time, p is variation threshold, Nf iI he number of proximate cells including serving cell. By a specific embodiment f signifies the members in the foreground group, defined above.

The subscriber s operation in accordance with algorithmic expression is as follows: when the measurement process is enabled (i.e. the serving cell sig jal intensity drops below the threshold (41)), then for the serving cell q, and, for b, 6h monitored signal that belongs to the foreground cell (i=l..Nf), the subscriber,;f all calculate the time derivative of the respective signals for t-- to (the current time) Next, function F2 being effectively a filtering operation - e.g. e absolute operator, is applied to each derivative.

The maximum of the function values is compared to the threshold P and i -, ,:it falls below the threshold, the measurement process of the neighbor cells is disabled.

The meaning of the latter result (i.e. each of the resulting function valus of the serving cell and neighboring cells drop below the specified threshold y) is t] at the subscriber is in static or slow moving state and therefore the signal from e serving cell and the proximate cells hardly varies. Accordingly, there is no point in monitoring the proximate cell signals (e.g. neighboring signal at a high rate), a id accordingly as indicated in algorithmic expression 2A. the monitoring fr(M proximate cells is reduced, giving rise to substantial power consumption reduco n (as explained in detail with reference to Fig. 3).

Thus, having reduced the monitoring rate from proximate cells, only t le serving cell is monitored.

In the case that change is triggered (above the threshold po, the measurement process of the neighbor cells is resumed. The process is resumed since a change in the signal that exceeds the threshold M, means that the subscriber is moving relatively fast and therefore the signal changes over time as illustrated 5 e.g. in areas 44 or 45.

By this particular embodiment, the response time of operator F, () shall be higher than the response time of the operator F2 0 to avoid degradation in grade of service. This is of course not obligatory.

In accordance with a modified embodiment the proposed apparatus can be activated unconditionally, Le, as long as the subscriber is operational, or only during the time the subscriber is in idle mode, when there is an active service in progress. Since typical value for idle mode of the cellular subscriber is about'80%, a noticeable effect of extended battery life will be achieved.

In accordance with another modified embodiment, measurements of the neighbor cells may not be affected when Eq. (2) is true, but rather performed rarely for extended confidence that there was no significant change of neighbor cells signal criteria.

In accordance with still yet another embodiment, the functions F, () and F2 () can be adaptive with respect to the subscriber operation mode, compromising between the percentage of battery saving vs. degradation to the grade of service.

The operation of the device and method in accordance with one embodiment of the invention is described briefly with reference to Fig. 5. Thus, a set of proximate cell signals is identified (5 1) and confined to the foreground group (52), using to this end the procedure described in detail with reference to Fig.'2. Then, algorithmic expression 2A is applied in the manner specified above (53) and in the case that the MAX value does not drop below the threshold (54), the monitoring rate is not affected. Otherwise, the monitoring rate of the neighboring cells is. reduced (55) and algorithmic expression 2B is applied to the signal of the service cell (56). If the resulting value exceeds the threshold no change is affected i the sampling rate (57), otherwise, the monitoring rate of the foreground group of s' nal is increased (58).The net effect in saving battery power was described,v,ith reference to Fig. 3, above.

In the method claims that follow, alphabetic characters used to desi. ge claim steps are provided for convenience only and do not imply any partit lar order of performing the steps.

i i 1 i 1

Claims

CLAIMS:

1. In a cellular radio device operable in a radio system having a plurality of frequency channels, a method of monitoring signals from proximate cells, comprising:

(a)identifying a set of proximate cell signals for monitoring, where the set is a subset of the plurality of frequency channels; (b) measuring a quality parameter for each proximate cell signal; (c) logically arranging the set of proximate cell signals into first group of at least one relatively high quality parameter signal and a second group of at least one relatively low quality parameter signal; (d) monitoring the signals in the first group at a relatively high rate; and (e) monitoring the signals in second group at relatively low rate.

2. The method of claim 1, wherein the step of measuring comprises comparing the signal parameter with a threshold, and the first group comprises those signals 15 for which the respective parameter exceeds the threshold.

3. The method of claim 2, wherein the second group comprises all identified proximate cell signals not included in the first group.

4. The method according to claim 1, wherein the quality parameter signal is signal quality and/or strength.

5. The method of claim 1, wherein the relatively high rate is selected for optimal estimation of received signal parameters.

6. The method of claim 1, wherein the lower rate varies with respect to the number of proximate cell signals in the first group.

7. The method of claim 6, wherein the lower rate decreases with increasing 25 number of proximate cell signals in the first group.

8. The method of claim 6, wherein the lower rate decreases in inVe rse proportion to the number of proximate cell signals in the first group.

9. The method of claim 1, wherein at least one of the relatively high rate (1d, the relatively low rate is decreased when a time derivative of a parameter of at JC Ast: 5 one proximate signal falls below a threshold.

10. The method of claim 8, wherein the relatively low rate is decreased.

11. In a cellular radio device operable in a radio system having a plurali f frequency channels, a method of monitoring signals from proximate c comprising:

(a) identifying a set of proximate cell signals for monitoring, where th,. ; -.t is a subset of the plurality of frequency channels; (b) measuring a quality parameter for each proximate cell signal; (c) logically arranging the set of proximate cell signals into at least t- "o groups according to the quality parameter signals; and is (d) monitoring the signals in the respective groups in different r it( according to the respective quality parameter signals.

12. A cellular radio device operable in a radio system having a plurality f frequency channels; the device includes a processor for monitoring signals fri i proximate cells; the processor being operable for:

(a) identifying a set of proximate cell signals for monitoring, where the c is a subset of the plurality of frequency channels; (b) measuring a quality parameter for each proximate cell signal; (c) logically arranging the set of proximate cell signals into first group at least one relatively high quality parameter signal and a second gro, of at least one relatively low quality parameter signal; (d) monitoring the signals in the first group at a relatively high rate; and (e) monitoring the signals in second group at relatively low rate.

13. A cellular radio device operable in a radio system having a plurality of frequency channels, the device includes a processor for monitoring signals from 5 proximate cells; the processor being operable for:

(a) identifying a set of proximate cell signals for monitoring, where the set is a subset of the plurality of frequency channels; (b) measuring a quality parameter for each proximate cell signal; (c) logically arranging the set of proximate cell signals into at least two 10 groups according to the quality parameter signals; and (d) monitoring the signals in the respective groups in different rates according to the respective quality parameter signals.

14. In a cellular radio device operable in a radio system, a method for reducing current consumption, comprising:

(e) identifying a set of proximate cell signals for monitoring; (f) measuring a quality parameter for each proximate cell signal (g) measuring a time derivative for at least some of the quality parameters of the proximate cell signals so identified; and (h) reducing a rate of sampling for at least some of the proximate cell signals when the time derivative or a function thereof falls below a threshold.

15. The method according to Claim 14, wherein said sampling rate, stipulated in step (d), is reduced close to zero.

16. The method according to Claim 14, further comprising the step of.

(e) Increasing a rate of sampling for at least some of the proximat ell signals when the time derivative or a function thereof exceel'd, a threshold.

17. The method according to Claim 14, further comprising the step f applying a logical condition which, dependent on received signal conditios is capable of being false but which, as long as holding true, obviates at least said,,t,,ps (b)-(d).

18. The method according to Claim 17, wherein said logical cond t on comprises ascertaining whether the quality parameter of the signal of the elf io serving cell is high.

19. The method according to Claim 14, wherein said proximate cell si ls include foreground group of signals; said foreground group of signals are identf ed each as having relatively high quality parameter signal.

20. A cellular radio device operable in a radio system, the device includ a processor for monitoring signals from proximate cells, the processor being oper11e for:

(a) identifying a set of proximate cell signals for monitoring; (b) measuring a quality parameter for each proximate cell signal (c) measuring a time derivative for at least some of the quality paramtl" of the proximate cell signals so identified; and (d) reducing a rate of sampling for at least some of the proximate c 11 signals when the time derivative or a function thereof falls beIffi,A a threshold.