Enhanced Sleep Mode in Radiocommuniation Systems

DESCRIPTION OF THE DRAWINGS

The foregoing, and other, objects, features and advantages of the present invention will be more readily understood upon reading the following detailed description in conjunction with the drawings in which:

Figure 1 depicts an exemplary forward digital control channel;

Figure 2 shows an example of a hierarchical cell structure;

Figure 3 is a flowchart illustrating control channel measurement according to an exemplary embodiment of the present invention;

Figure 4 is an illustration of different paging frame classes;

Figure 5 depicts an exemplary SPACH header; and

Figure 6 represents a block diagram of an exemplary cellular mobile radiotelephone system,

DETAILED DESCRIPTION

According to exemplary embodiments of the present invention, the measurement periodicity for evaluating control channels for potential reselection of the serving control channel can be optimized to increase sleep time of the remote unit and extend battery life. Briefly, control channels are used for setting up calls, informing the base stations about locations and parameters associated with mobile stations, and informing the mobile stations about locations and parameters associated with the base stations. The base stations listen for call access requests by mobile stations and the mobile stations in turn listen for paging messages.

Future systems will employ additional cells. For example, new systems may include any combination of macrocells, indoor microcells, outdoor microcells, public

CA 02152943 1999-06-25 microcells and restricted or private microcells. New systems therefore will likely be designated to incorporate an increasing number of control channels. Currently, there are approximately twenty-one analog control channels available for a cluster in a typical system employed, for ex ample, in the United States.

FIG. 2 is an exemplary hierarchical, or multi-layered, cellular system. An umbrella macrocell 10 represented by a hexagonal shape makes up an overlying cellular structure. Each umbrella, cell may contain an underlying microcell structure. The umbrella cell 10 includes microcell 20 represented by the area enclosed within the dotted line and microcell 30 represented by the area enclosed within the dashed line corresponding to areas along city streets, and picocells 40, 50, and 60, which cover individual floors of a building. The intersection of the two city streets covered by the microcells 20 and 30 may be an area of dense traffic concentration, and thus might represent a hot spot.

Each of the cells illustrated in Figure 2 will include a base station transmitting on at least one control channel. Consider the remote station, e.g., a portable unit, which is travelling down the city streets and up into the building including picocells 40, 50, and 60. During various portions of this transit, potentially all of the control channels associated with the umbrella cell 10, the microcells 20 and 30 and the picocells 40, 50, and 60 may be transmitted on the neighbor list of control channel candidates for reselection. Evaluating those control channels to determine if reselection is desirable is performed by having the remote station scan those channels (e.g., measure the received signal strength (RSS) or decode portions of the signal).

The periodicity with which a mobile or remote station scans the control channels in the neighbor list during the idle mode can be controlled according to exemplary embodiments of the present invention as follows. Two information elements can be transmitted on the broadcast control channel (BCCH) for the mobile or remote unit to receive that are related to this scanning process. A SCANFREQ information element can be sent in the control channel selection message on the BCCH to inform the mobile or remote station about a default minimum number of signal strength measurements to be made per time period, for example, per superframe. Alternatively, some default value could be provided without transmitting this information element. For a complete discussion of superframes and hyperffames, the interested reader is referred to the aforementioned US Patent 5,603,081. Briefly, each superframe includes a complete set of

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F-BCCH information (i.e., a set of Layer 3 messages), using as many slots as are necessary, and that each, superframe begins with a F-BCCH slot. After the F-BCCH slot(s), the remaining slots in each superframe include one or more (or no) slots E-BCCH, S-BCCH and SPACH logical channels. A hyperframe consists of two superframes. Thus, the mobile station will perform a total of SCANFREQ signal strength measurements per superframe regardless of the size of the neighbor list. Alternatively, SCANFREQ can indicate measurements per time interval, e.g., superframe, for each entry in the neighbor list.

The number of measurements made by the mobile station on each control channel in the neighbor list per time interval raises an interesting tradeoff. On the one hand, the more measurements which are made, the more accurate the measurement information will be. On the other hand, the greater the number of measurements, the greater the drain on the mobile or remote unit's battery. Since it may be desirable to allow the mobile to take a greater or lesser number of measurements on particular control channels in the neighbor list to finely balance these competing factors, the default SCANFREQ frequency or rate can be modified by a second parameter, denoted HL_FREQ, as described below.

The HL_FREQ information element is transmitted in the neighbor list. For each entry in the neighboring list there is an associated HLFREQ information element. If the HLFREQ is set to HIGH, this particular control channel is to be measured using the default frequency or rate defined by the SCANFREQ parameter. If, on the other hand, the HL FREQ information element is set to LOW, this particular control channel can be measured, for example, at half the frequency or rate required by the default SCANFREQ parameter. Of course those skilled in the art will recognize that the particular assignment of HIGH and LOW values of HL_FREQ to the attributes described above is arbitrary and could be reversed.

For example, consider that the neighbor list contains 16 entries, eight of which have HLFREQ set to HIGH and eight of which have HLFREQ set to LOW. If the SCANFREQ parameter is set to be 12 measurements per superframe, then the number of measurements for entries marked as HIGH can be measured at a minimum rate of 12/16 per superframe. For entries marked as LOW, the mobile or remote shall measure these control channels at a minimum rate of (12/16)/2 per superframe. Thus, the total number of measurements in this example per superframe would then be:

CA 02152943 1999-06-25 (12/16)*8 + ((12/16)/2)*8 = 9

To facilitate sleep mode efficiency for the mobile or remote stations, the basic procedure outlined above can be adjusted by the mobile or remote station to reduce the frequency or rate of measurements required. The following discussion describes three exemplary techniques which may be used by the mobile or remote station to reduce the measurement frequency and thereby minimize battery drain.

The flowchart in Figure 3 illustrates three exemplary tests for reducing the measurement frequency of control channels in the neighboring list as a cumulative procedure such that the measurement frequency can be cumulatively reduced by some factor for each test that is validated. However, those skilled in the art will readily appreciate that these three tests could be used on their own or in various combinations rather than being cumulative as set forth in this exemplary embodiment.

At decision block 600, the first test is implemented by determining if the current control channel has been serving the mobile or remote station for more than a predetermined period of time, for example, one hour. This test can be used, for example, to reduce measurements when the remote station is not moving. If so, then the flow moves to block 610 where the measurement frequency on control channels identified by the neighbor list can be reduced by some predetermined factor, for example, a factor of two.

If not, then no reduction in the frequency measurement is indicated by this test and the flow moves to 620.

The second exemplary test is based on an average received signal strength (RSS) for the serving control channel and the control channels in the neighbor list. An average received signal strength can be used to provide a statistically accurate representation, as opposed to any one instantaneous measurement which could be skewed due to, for example, Rayleigh fading. For the purposes of this example, assume that the mobile or remote station keeps a running average of the last five signal strength measurements for each measured frequency. Then, the compound test expressed by decision block 620 and 630 is as follows. If the rate of change of the average received signal strength on the serving channel is not less than some predetermined threshold rate, for example, 7 dB over a previous five minutes, then no reduction in the measurement frequency is warranted and the flow moves down to block 650. Otherwise, the flow moves to decision

CA 02152943 1999-06-25 block 630 and it is deteimined whether the change in the average received signal strength on all of the control channels in the neighbor list is less than some predetermined threshold rate, for example, 7 dB over a previous five minutes. If so, then the measurement frequency or rate of the control channels in the neighbor list can be reduced by some predetermined factor, e g., two. Otherwise, no reduction in the rate of frequency is warranted. In either case, the flow moves on to the third test at block 650.

Therein, it is determined whether or not the rate of change of a difference between the average received signal strength of the serving control channel and of some specific entry in the neighbor list is less than some predetermined threshold rate, e.g., 10 dB over the last five minutes. If so, then the measurement frequency or rate of the control channels in the neighbor list can then be reduced by a factor, for example, two at block 660. Otherwise, no reduction is warranted and in any event the procedure is then completed.

The aforedescribed process illustrates exemplary ways in which the number of measurements taken by the mobile or remote station on channels in the neighboring list may be reduced to conserve battery power. As the conditions which triggered the reduction in measurement frequency change so that those conditions no longer hold, then the corresponding reduction in measurement frequency can be revoked. For example, the process illustrated y in Figure 3 can be performed periodically, e.g., every minute. Thus, if a mobile had previous ly reduced its measurement frequency of the control channels on the neighbor list based on satisfaction of the criteria in block 600 and, in a subsequent iteration the mobile had changed its serving control channel, the reduction of block 610 would be revoked.

In addition to being awake to measure control channels, a mobile or remote station is also awake periodically so that it can receive a page. Specifically, the mobile may be paged at any time, therefore the mobile must be locked to a particular cell in a location area so that the mobile may receive the page. For example, if the mobile has moved out of the location area of a. first cell to which the mobile was locked to a second cell in a different location area, a. paging request for the mobile will not be heard or received because the mobile switching center, or MSC, will page the mobile over a paging channel available to the location area in which the mobile is registered. Thus, a paging request would not be received by the mobile in the distant location area if it is not

CA 02152943 1999-06-25 registered in that location area. Therefore, the mobile should register with a new base station when entering a new location area.

According to exemplary embodiments of the present invention, pages are repeated periodically and the mobile station can be assigned to a paging frame class at registration to take advantage of knowledge of this periodicity to remain asleep longer. Exemplary paging frame classes are illustrated in Figure 4. Therein note that a first paging frame class is defined as being; every hyperffame, a second paging frame class spans two hyperframes, a tliird paging frame class spans three hyperframes, and a fourth paging frame class spans four hyperframes. When the mobile station registers, it can temporarily change its paging frame class to PF], until it receives a registration response. If the registration response contains a new paging frame class assignment, then that shall be the assigned paging frame class for the mobile. Otherwise, the mobile can retain the default paging frame class of PFj.

Having been assigned a paging frame class, either by default or through the registration response, the mobile station shall then awaken to determine if it is being paged more or less frequently based upon its paging frame class. According to exemplary embodiments of the present invention, the system can adjust the frequency with which the mobile station awakens to monitor the paging channel without actually changing the mobile station's assigned paging frame class. This provides a solution to another dilemma facing system designers, i.e., the tension between paging frequency and call-setup delay. Consider that the more frequently a mobile or remote unit is paged, the less call-setup delay there will be. However, the more frequently a mobile can be paged, the more often a mobile must awaken to potentially receive a page, resulting in greater battery drain. Thus, during periods such as nighttime hours or other times when communication with the mobile or remote station is expected to be less frequent, exemplary embodiments of the present invention provide a mechanism for allowing the mobile or remote station to sleep longer and take advantage of greater tolerance for call-setup delay.

This is accomplished, for example, by transmitting a paging frame modifier (PFM) from the system to the mobile station in a routinely transmitted overhead message via the digital control channel. One exemplary mechanism for transmitting the PFM is in a header of the SPACH channel, e.g., in every PCH subchannel. The SPACH channel, as described in more detail in the above-identified U.S. Patent 5,603,081, carries layer 2

CA 02152943 1999-06-25 messages which are used to carry point-to-point SMS, paging or ARCH information. An exemplary SPACH Header including the PFM bit is illustrated as Figure 5. By adjusting the paging frame class of a mobile or remote unit in this way, a cell specific adjustment can be made (rather than changing paging classes upon registration which requires an entire registration area, i.e., potentially many cells, change) and synchronization can be maintained. Thus, the PFM field is unaddressed in the sense that it is transmitted to every mobile or remote in a cell as opposed to a page which is directed to a particular remote or mobile station.

According to exemplary embodiments of the present invention, the paging frame modifier can comprise a, single bit which, if set to binary 0, indicates that the mobile unit should continue to use its assigned paging frame class. If, however, the PFM bit is set to binary 1 then the mobile station will adjust its paging frame class, for example, to the next higher paging frame class As an example, assume that a mobile station has been assigned to paging frame class 2 (PF2) such that it monitors its paging channel for a page every other hyperframe. Then, assume that the paging frame modifier bit transmitted on the SPACH channel is subsequently set to binary ’’ 1 At this point, the mobile will act as if its assigned paging frame class is PF3 and will sleep longer until the PFM reverts to 0. Mobiles or remote stations which are asleep when the PFM is changed will nonetheless become quickly aware in their change of paging frame class since, according to this example, the PFM is transmitted as a bit on the SPACH header.

The foregoing has described mobile and base station operation in terms of functional qualities. While specific hardware implementations of such stations per se are known to those skilled in the art, a brief example will now be described. FIG. 6 represents a block diagram of an exemplary cellular mobile radiotelephone system, including an exemplary base station 110 and mobile station 120. The base station includes a control and processing unit 130 which is connected to the MSC 140 which in turn is connected to the PSTN (not shown). General aspects of such cellular radiotelephone systems are known in the art, as described by the above-cited U.S. patents and by the additional U.S. Patents 5,175,867 entitled Neighbor-Assisted Handoff in a Cellular Communication System which issued December 29, 1992, and U.S. Patent 5,745,523 entitled Multi-Mode Signal Processing, which issued April 28, 1998.

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The base station 110 handles a plurality of voice channels through a voice channel transceiver 150, which is controlled by the control and processing unit 130. Also, each base station includes a control channel transceiver 160, which may be capable of handling more than one control channel. The control channel transceiver 160 is controlled by the control and processing unit 130. The control channel transceiver 160 broadcasts control information over the control channel of the base station or cell to mobiles locked to that control channel. It will be understood that the transceivers 150 and 160 can be implemented as a single device, like the voice and control transceiver 170, for use with DCCs and DTCs that share the same radio carrier frequency.

The mobile station 120 receives the information broadcast on a control channel at its voice and control channel transceiver 170. Then, the processing unit 180 evaluates the received control channel information, which includes the characteristics of cells that are candidates for the mobile station to lock on to, and determines on which cell the mobile should lock. Advantageously, the received control channel information not only includes absolute information concerning the cell with which it is associated, but also contains relative information concerning other cells proximate to the cell with which the control channel is associated, as described in U.S. Patent No. 5,353,332.

The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims.

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THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE

PROPERTY OR PRIVILEGE IS