NZ268683A

NZ268683A - Selecting base station as server according to signal strength and preference category in cellular telephony system

Info

Publication number: NZ268683A
Application number: NZ268683A
Authority: NZ
Inventors: Bror Ake Karlsson
Original assignee: Ericsson Telefon Ab L M
Priority date: 1993-07-09
Filing date: 1994-06-23
Publication date: 1998-03-25

Description

<div class="application article clearfix" id="description"> New Zealand No. 268683 International No. PCT/SE94/00630 TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates: 09.07.1993; Complete Specification Filed: 23.06.1994 Classification:^) H04B7/26 Publication date: 25 March 1998 Journal No.: 1426 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Best server selection in layered cellular radio systems Name, address and nationality of applicant(s) as in international application form: TELEFONAKTIEBOLAGET LM ERICSSON (PUBL), a Swedish company of S-1 26 25 Stockholm, Sweden * ¥ New Zealand No. 268683 International No. PCT/SE94/00630 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Best server selection in layered cellular radio systems Name, address and nationality of applicant(s) as in international application form: TELEFONAKTIEBOLAGET LM ERICSSON, a Swedish company of S-126 25 Stockholm, Sweden BEST SERVER SELECTION IN LAYERED CELLULAR RADIO SYSTEMS BACKGROUND OF THE INVENTION Field of the Invention The invention relates to handoff of mobile stations within cellular radio systems and, more particularly, to handoff within layered cell structures of such systems. History of the Related Art In cellular radio communication systems a geographic area is divided into a plurality of individual regions referred to as cells, each of which is provided with radio service on a plurality of separate RF channels the frequencies of which are reused among different cells sufficiently separated from one another that there is a sufficiently low level of interference between them. When a mobile station receiving radio service from a particular base station serving a particular cell moves from that cell into another adjacent cell, communication with the mobile station is "handed off" from the first base station to the second base station serving the adjacent cell. Such handoff is accomplished by a mobile switching center (MSC) to which all of the base stations are connected and which controls the allocation of communication channels to the individual mobiles moving throughout the region served by the cells. As the demand for cellular radio service increases over time, the capacity of existing systems has been severely stressed to serve all the subscribers who would like to have access to the system/ particularly in major metropolitan areas. To address this demand, cellular radio technology is currently moving from analog based systems, in which each subscriber communication channel is allocated to a single radio channel, to digital based systems in which a plurality WO 95/02309 KTT/SE94/00630 -2- ch fadbv of subscriber channels can be assigned to each channel through time division multiple access (TDMA) radio technology. In TDMA radio, each radio channel is divided into a plurality of time slots and a digitized 5 portion of .each subscriber's conversation is broadcast in an assigned time slot. However, even with such improvements in channel capacity, there exist certain areas within major metropolitan areas in which the demands on the system are so great thiat it cannot be 10 successfully satisfied by existing cellular radio architectures. For example, in the area in and around a convention center located in a major metropolitan area, the channel usage by portable cellular radio transceivers may be so great that the demands for 15 service cannot be satisfied by the entire channel capacity of the base station serving the cell within which the convention center is located. In such situations, it has been proposed to provide additional "layers" of cellular radio coverage provided by 20 additional lower powered base stations located within an existing, so-called "umbrella" cell and referred to as "microcells". Such microcells may have a coverage or service area on the order of a few hundred meters in contrast to a few kilometers of coverage by the base 25 station of the overlying umbrella cell. A plurality of such microcells may be located adjacent to one another and form a contiguous coverage area of substantial width all of which is within the overall coverage area of the umbrella cell. 30 When a layered cell structure, as described above in conjunction with umbrella cells and microcells, is used there is provided an enhanced level of radio capacity which can be configured for individual circumstances and which provides an assurance that 35 users can receive service despite an extremely high iJoftufi Office 1 6 FES 1998 of New Zealand WO 95/02309 PCT/SE94/00630 og * demand within a very small geographic area. Moreover, additional layers of radio coverage may be added, for example, by a plurality of either contiguous or separated Hpico cells" positioned within the coverage 5 or service area of the individual microcells, each of which are in turn within the overall umbrella cell. The base stations providing the radio coverage within the pico cells would be of even still lower power than the base stations serving the microcells and have a 10 coverage or service area of, for example, a hundred meters to provide coverage within a single building or a single floor within a large convention center. Thus, in layered cell architectures, the issue of server selection and handoff of each mobile radio 15 transceiver moving within a geographic area involves many more options. That is, it is possible for the mobile station to receive radio service at any given moment from either a pico cell base station, a microcell base station, or an umbrella cell base 20 station. When conventional handoff criteria used in single layered cellular architectures are applied to this situation, problems arise and the solution is less than ideal. The ability to configure the handoff arrangement for maximum efficiency with respect to the 25 utilization of channel availability and consistent with high quality radio service to each mobile subscriber is highly desirable. When handoff is effected between adjacent cells in a single layer cellular radio architecture, the 30 principle criterion used is the quality of the signal received from the mobile station by the respective base stations capable of providing radio service. That is, the quality of the signal received from the mobile station by the base station currently 35 serving the mobile is compared with the quality of 'rJecLS l'VJ » p*>Peity Office 1 6 FEB M8 of New Zealand WO 95/02309 PCT/SE94/00630 -4- signal received by a base station serving an adjacent cell and when the quality of signal in the latter exceeds the former, the mobile is handed off to the base station serving the adjacent cell. In addition, 5 a signal quality increment, known as an offset or hysteresis, is also applied to the signal quality difference value so that unless the signal quality in the adjacent base station is at least " x" amount greater than the presently serving base station, 10 handoff does not occur. This prevents oscillating handoffs due to signal quality perturbations in which the mobile is repeatedly handed back and forth between two adjacent base stations. When conventional handoff techniques, based purely 15 upon signal quality, are applied to the multiple layer cellular architecture, the resulting service quality is often not optimal and in many cases totally inappropriate. This is because it is generally preferable to serve a mobile station with the lowest 20 possible level of base station as a preferred service provider primarily because of capacity reasons. That is, if sufficient signal quality exists from the microcell, it is preferred to serve the mobile from that cell rather than handing the mobile off to the 25 umbrella cell which has fewer total channels available for service than a plurality of adjacent microcells would have. Because of the foregoing problems associated with the use of existing handoff algorithms within layered 30 cellular architectures, it is desirable to introduce a system of handoff algorithms which maximize the efficient utilization of channel availability within a multi-level cellular radio architecture. The system of the present invention provides such a technique. 35 WO 95/02309 PCT/SE94/00630 SUMMARY OF THE INVENTION In one aspect of the invention, a mobile station is served by the base station of the cell level which is the preferred service provider for that mobile as 5 long as the signal quality is above a selected minimum value. Should the signal quality deteriorate below that selected minimum value, best server selection possibilities are evaluated on the same level and such possibilities are first exhausted before handoff is 10 made to a cell on a lower preference level. In another aspect of the present invention, handoff is performed of a mobile station operating within a cellular radio system in a region in which radio channels are provided by multiple levels of cells 15 having different service areas. Categories of preference are assigned within each cell with respect to each other associated cell within the system as to which said cell's service area is either adjacent, contiguous, or overlapping. The signal strength of the 20 radio signals providing communication between the mobile station and the base station serving each of the associated cells within the system is measured and compared to a preselected threshold value. Whether or not to handoff the mobile to a base station serving an 25 associated cell is determined based upon whether the signal strength of the radio signal therefrom is greater than the threshold value on the preference assigned to the associated cell. In a still further aspect of the invention, best • 30 server selection is performed for a mobile station served on a first radio channel by a first base station serving a first cell and operating within a cellular radio system in a region in which radio channels are provided by multiple levels of cells having different 35 service areas. Categories of preference are assigned 'wJlaLS I' V 6 D ""(•atual "wuarty Otfiee * 6 FEB 1998 of New Zealand WO 95/02309 % PCT/SE94/00630 -6- withln each cell with respect to each other associated cell within the system as to which said cell's service area is either coextensive adjacent, contiguous, or overlapping. Each associated cell is assigned a 5 category of preferred, equal preference or non- preferred based upon the size of its service area relative to that of the cell within which the assignment is made. Information on a second radio channel broadcast by a second base station serving a 10 second associated cell is received and stored. The signal strength of the radio signals broadcast on the first and second radio channels, respectively, are measured. The preference category of the cell from which said second radio channel signal is received is 15 determined. The second base station broadcasting the second radio channel is selected as the best server in response to any one of the following conditions, (a) the second cell having a preference category of preferred and the signal strength of the radio signal 20 of said second radio channel being greater than a preselected value, (b) the second cell having a preference category of equal preference and the signal strength of the radio signal of the second radio channel being greater than the signal strength of the 25 radio signal of the first radio channel, or (c) the second cell having a preference category of non-preferred and the signal strength of the first radio channel being less than a preselected value and the signal strength of the radio signal of the second radio 30 channel being greater than that of the first radio channel. In yet another aspect of the invention, server selection is performed for a mobile station operating within a cellular radio system in a region in which 35 radio channels are provided by multiple levels of WO 95/02309 PCT/SE94/00630 6 6 a cells. Each of the cells is provided with radf service by a base station the broadcast power of which and the resultant service area of which varies from larger to smaller. Each cell is provided with a list 5 of neighboring cells the service areas of which are proximately associated with the service area of the cell. A minimum acceptable radio channel signal strength value is assigned for each cell. The strength of the radio signals providing communication between 10 the mobile station and the base station is measured for the cell currently serving the mobile station as well for each of the neighboring cells thereof. A server is selected for the* mobile station from among the currently serving cell and its neighboring cells by 15 choosing the cell having both the smallest service area and a radio signal strength which is at least equal to the minimum acceptable value. BRIEF DESCRIPTION OF THE DRAWINGS 20 For a more complete understanding of the present invention and for further objeers and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawings, in which: 25 FIG. 1 is a pictorial representation of a single level cellular radio communication system including a mobile switching center, a plurality of base stations a.nd a plurality of mobile stations; FIG. 2 is a pictorial representation of a multi-30 level cellular radio communication system illustrating a plurality of microcells and a plurality of overlying umbrella cells; FIG. 3 is a graph illustrating received signal strength by a mobile station moving within a i J,01§ 11 v 6 e ' weMrty Office f S FEB 1998 °f New Zeslonri 10 WO 95/02309 PCT/^4jW630 • 8 6. -8- microcell and an overlying umbrella cell within which it is included; FIG. 4 is a diagram of nominal minimum and maximum microcells sizes; FIG. 5 is a graph illustrating received signal strength 5 by a mobile station moving within a microcell and an overlying umbrella cell with different criteria than in FIG. 3 FIG. 6 is a diagram illustrating three different handoff borders for a microcell and an overlying umbrella cell; FIG. 7 is a block diagram illustrating a plurality of base stations located respectively within adjacent cells of a cellular radio system; FIG. 8 is a block diagram of the infrastructure 15 equipment within a cellular radio system; FIG. 9 is a block diagram of a mobile radio transceiver for use within a cellular radio system; and FIGS. 10-12b are flow charts illustrating procedures used in the present invention. 20 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, there is illustrated a conventional single layer cellular radio communication system of the type to which the present invention has 25 a relationship. In FIG. 1, an arbitrary geographic area may be divided into a plurality of contiguous radio coverage areas, or cells C1-C10. While the system of FIG. 1 is illustratively shown to include only 10 cells, it should be clearly understood that in 30 practice, the number of cells will be much larger. Associated vith and located within each of the cells CI -CI 0 is a base station designated as a corresponding one of a plurality of base stations Bl-B10. Each of the base stations B1-B10 includes a 35 transmitter, receiver, and base station controller as , R s e e i v s e Intellectual Property Office 1 6 FEB 1998 of New Zealand WO 95/02309 PCT/SE94/00630 -9- is well known in the art. In FIG. 1, the base stations B1-B10 are illustratively located at the center of each of the cells C1-C10, respectively, and are equipped with omni-directional antennas. However, in other 5 configurations of a cellular radio system, the base stations B1-B10 may be located near the periphery, or otherwise away from the centers of the cells C1-C10 and may illuminate the cells C1-C10 with radio signals either omni-directionally or directionally. Therefore, 10 the representation of the cellular radio system of FIG. 1 is for purposes of illustration of a single level cellular radio system only and is not intended as a limitation on the possible implementations of the cellular radio system within which the system of the 15 present invention may find utility. With continuing reference to FIG. 1, a plurality of mobile stations M1-M10 may be found within the cells C1-C10. Again, only 10 mobile stations are shown in FIG. 1 but it should be understood that the actual 20 number of mobile stations will be much larger in practice and will invariably greatly exceed ■'-.he number of base stations. Moreover, while none of the mobile stations M1-M10 will be found in some the cells C1-C10, the presence or absence of the mobile stations M1-M10 25 in any particular one of the cells C1-C10 should be understood to depend, in practice on the individual desires of the mobile stations M1-M10 who may roam from one location in the cell to another or from one cell to an adjacent cell or neighboring cell, and even from one 30 cellular radio system served by an MSC to another system. Each of the mobile stations M1-M10 is capable of initiating or receiving a telephone call to one or more of the base stations B1-B10 and a mobile switching 35 center (MSC). A mobile switching center (MSC) is WO 95/02309 PCT/SE94/00630 ft -10- r> connected by communication links, e.g., cables, via each of the illustrative base stations B1-B10 and to the fixed public switch telephone network (PSTN), not shown, or a similar fixed network which may include an 5 integrated system digital network (ISDN) facility. The relevant connections between the mobile switching center (MSC) and the base stations B1-B10, or between the mobile switching center (MSC) and the PSTN or ISDN, are not completely shown in FIG. 1 but are well known 10 to those of ordinary skill in the art. Similarly, it is also known to include more than one mobile switching center (MSC) in the cellular radio system and to connect each additional mobile switching center (MSC) to a different group of base stations and to other 15 mobile switching centers (MSCs) via cable or radio links. Each of the cells C1-C10 is allocated a plurality of voice or speech channels and at least one access or control channel, such as a forward control channel 20 (FOCC). The control channel is used to control or supervise the operation of mobile stations by means of information transmitted to and received from those units. Such information may include incoming call signals, outgoing call signals, page signals, page 25 response signals, location registration signals, voice channel assignments, and maintenance instructions as the mobile station travels out of the radio coverage of one cell and into the radio coverage of another cell. The control or voice channels may operate either in an 30 analog or digital mode or a combination thereof. FIG. 1 illustrates a relatively conventional single level cellular structure in which mobile stations are handed off from one cell to another by it being served with radio channels in communication with 35 first one base station and then an adjacent base , received Ihtafleetual Property Office i 6 FEB 1^98 of New Zealand WO 95/02309 PCT/SE94/00630 -11- station based upon the signal level of the mobile station received from each of the respective base stations. Referring next to FIG. 2, there is shown an 5 illustrative multi-level layered cellular architecture in which a plurality of umbrella macrocells A-G, analogous to the cells C1-C10 of FIG. 1, overlie and include a plurality of microcells a-e located within the same service area of one or more of the umbrella 10 cells. In FIG. 2, each of the umbrella cells A-G would be served respectively by a base station, similar to the base stations B1-B10 of FIG. 1, and it would include a service area within a cell on the order of, for example, several kilometers in diameter. 15 Similarly, each of the microcells a-e would also be served by a base station (not shown) located within the cell and providing a respective service area on the order of a few hundred meters for each. Just as the service areas of the umbrella cells A-G are adjacent, 20 contiguous or overlapping to one another, so also are the service areas of the respective microcells a-d contiguous to one another, with the service area of microcell e being isolated from the other microcells but coextensive within the service area of the umbrella 2 5 cell A. Each of the base stations providing radio coverage for both the umbrella cells A-G and the microcells a-e could be operated under the control of a single MSC (not shown). As shown in FIG. 2, a mobile station 21 moving 30 along the path identified by arrow 22 would move from the region served by umbrella cell A through the regions served by microcell d and microcell c and into the coverage area of umbrella cell B. Providing radio service to the mobile station 21 as it moves along the 35 path 22 could involve a number of different handoffs WO 95/02309 PCT/SE94/00630 -12- from different serving base stations at different points along the way depending upon the handoff criteria or handoff algorithm used to implement such handoffs. For example, at the beginning of its path, 5 the mobile station 21 could only be provided with service by the base station serving the umbrella cell A, while at point 23 along the path 22 it could be served either by the base station of umbrella cell A or the base station of microcell d. In addition, when it 10 reached point 24 on the path 22, it could be served either by the base station of umbrella cell A or the base station of microcell c. Further, at point 25, service could be provided by the base station of microcell c or the base station of umbrella cell B. 15 Finally, at point 26 on path 22, the mobile station 21 could only be provided with service by the base station serving umbrella cell B. Thus, the decision criteria with respect to which cell the mobile station 21 should be handed off to and the criteria which should be used 20 to determine both the timing and the result of that handoff are important considerations in configuring the efficiency and capacity of radio service to be provided to the mobile stations within the multi-layered cellular structure of FIG. 2. 25 As discussed above, in implementing the handoff process within a single layer cellular structure, a mobile station is only located in one cell at a time and leaves the old cell behind when it enters and begins receiving service by virtue of handoff to the 30 new cell. In such case, the signal strength received from the mobile station decreases for the old cell and increases for the new cell when the handoff border is crossed by the mobile station. However, when a microcell is placed within an umbrella cell and is 3 5 totally surrounded by the umbrella cell, there is an WO 95/02309 PCT/SE94/00630 * entirely different situation with respect to th^ ^ variation in signal strength as the mobile moves through the microcell located within the umbrella cell. For example, if the handoff border for a microcell on 5 the reverse side from the umbrella cell is crossed, both the serving cell signal strength and the candidate cell signal strength are increasing or decreasing, depending upon the direction of movement of the mobile station. This variation in signal strength becomes 10 more and more of a problem the closer to the antenna site serving the umbrella cell site the microcell is located. These problems may be illustrated by two examples set forth below, both for digital radio channels. 15 In the first example, assume that an umbrella cell has 50 watts erp (47dbm) and an enclosed microcell 0.1 watt erp (20dbm). The microcell is placed 200 meters from the base station site of the umbrella cell. 0.1 watts is an appropriate power level for a 200 meter 20 diameter microcell. As shown in FIG. 3, there is illustrated the calculated signal strength received by a mobile station using the formula L=30+351og d as the path loss formula, with d being given in meters. In addition, in an attempt to have an appropriate cell 25 size, a signal offset value is set.to 10db, i.e., lOdb is substracted from the measured umbrella cell signal strength before comparison to candidate cell signal strength in accordance with standard single level cell architecture handoff algorithms. As illustrated, the signal 30 strength of the signal received by a mobile station from the umbrella cell moving from the base station location of the umbrella cell toward and through the base station site of the microcell, begins quite high and decreases exponentially as a function of the 35 distance between the mobile station and the umbrella 'nteJlolS prLV 5 D Pr°Perty Office 1 6 FEB 1998 of New Zealand WO 95/02309 PCT/SE94/00630 -14- cell base station. The signal strength received by the mobile station from the microcell increases exponentially until it passes through the base station serving the microcell at 200 meters distance from the 5 base station of the umbrella cell and then decreases exponentially thereafter as a similar function of the distance from the microcell base station. Referring next to PIG. 4, there is shown the nominal handoff border offset value of lOdb being 10 applied when the signals are compared, meaning that the nominal handoff border is the line where the umbrella cell is lOdb stronger than the microcell. Adding a plus or minus 5db inaccuracy in the mobile assisted handoff (MAHO) measurement by the mobile station and a 15 +/- 3db hysteresis, the maximum and minimum cell size is found where the umbrella cell is 18db and 2db stronger, which are shown by the inner and outer line of FIG. 4, respectively. The lowest signal strength in the microcell is found to be around -97dbm. Such a 20 case may work and might be acceptable in some cases, however, the uncertainty of the handoff borders is relatively high. Moreover, if some fading is introduced into the consideration of signal level, such a technique is likely to become unacceptable from a 25 practical standpoint. These uncertainties in handoff borders will be even worse if fading is added to the signal strengths. Without any offset at all, the cell is much too small considering the ability to provide coverage with 20dbm output power. 30 A second example of the problems associated with using the conventional single layer handoff algorithm for a multi-layer cellular structure, may be illustrated with the case of 200 meters between two separate cell sites but with 30dbm for the micro cell. 35 In this case, the difference was that the microcell had WO 95/02309 PCT/SE94/00630 • *tis »sg. 30dbm output power. FIG. 5 is a graph showing the signal strength from the two cells at the different distances from the respective base station of the umbrella cell and the microcell as the mobile travels 5 from the base station of the umbrella cell through the base station of the microcell and then away from both. While the overall structure of the respective signal level curves is similar to that shown in FIG. 3, the overlapping relationship is somewhat different. 10 Moreover, if a nominal offset value is selected to be 17db, the nominal handoff border, i. e, the line where the umbrella signal strength is 17db higher than that of the microcell signal strength, is represented by the straight line in FIG. £. As discussed above, 15 inaccuracies in the mobile and hysteresis values are added giving borders at 9db and 25db offsets, respectively. In this case, the umbrella cell and the microcell are equivalent. In the worst case, the umbrella cell is the small one and in the best care the 20 microcell is the small one. This example illustrates what can easily happen if the offset is set too high in an attempt to get a reasonable coverage. The above two examples illustrate that there are cases where the existing locating function and handoff 25 algorithm gives highly inappropriate results in the case of multi-level cellular structures. Attempts to address these inadequacies may include: (1) providing design rules to be followed by cellular system operators to avoid dangerous situations; and (2) 30 modification of the locating algorithms to better handle the umbrella/microcell situation. The use of design rules is not an acceptable approach since rules must define restrictions on microcell power levels and distances from the umbrella cell site. Such rules have 35 to include a considerable margin of error in order to . f R % C; * I V E D intellaotual Property Office 1 G FEB 1998 of New Zealand WO 95/02309 PCT/SE94/00630 8 *6°6( cover nominal situations and will be perceived as a limitation on the operation of the system. In addition, if the rules are not followed, either deliberately or accidentally, the system will be 5 perceived to have a lower quality with degraded voice quality and more dropped calls leading to system complaints. As pointed out above, the present algorithm used in the case of single level cellular structures 10 functions adequately to use relative signal strength between current cell and neighboring cell in order to locate handoff borders when the cells are of equal preference and located side by side. Another criterion is needed for handoff between umbrella cells and 15 microcells. The system of the present invention provides handoff criterions based on absolute signal strength. That is, when the serving cell is the umbrella cell and the signal strength for a microcell neighbor, measured 20 by the mobile station in digital cellular systems and by the signal strength receiver in analog cellular systems, is above a "sufficient" signal level, so as to ensure a good speech quality in the microcell, a handoff can be made to the microcell. This criterion 25 ensures that a handoff to the microcell is performed whenever it can give service of sufficient quality. A general approach inherent in the system of the present invention is to handle each of the cells as belonging to distinctly different layers, i.e., 30 macrocells, microcells, pico cells, etc., without defining different layer types. In order to achieve, for example, three categories of neighboring cells: (1) neighbors beside, (2) neighbors above, and (3) neighbors below, such a classification is useful. In 35 such a definition, a "neighbor beside" will be an 'wJfcolS frl„V 6 0 ' ^QPerty Office 1 S FEB 1998 °f New Zealand WO 95/02309 PCT/SE94/00630 -17- existing neighbor on the same level as the currently serving cell. Neighbors above for a microcell would be an umbrella cell and neighbors below for an umbrella cell would be microcells. 5 Under the present system, handoff should be accomplished to a neighbor below whenever the signal strength in that neighbor is above a new "sufficient" threshold value to be defined as a cell parameter. On the other hand, handoff to a neighbor above, for 10 example, the umbrella cell, should only be done when the signal strength is below the same threshold provided that it is a better candidate. A hysteresis value will be applied to the threshold in order to avoid oscillating handoffs. 15 The modification of the handoff criteria employed in the application of the present invention to one exemplary implementation of a cellular radio system, the CMS 88 system of Ericsson Radio System, has an impact on both the exchange and the base station 20 software in that in the exchange software handling of new cell parameters and neighbor type information must be added. Such new cell parameters and neighbor type information are sent by the exchange to the base station and, in the base station, these parameters must 25 be added to the digital channel software as new criteria for sending handoff requests. In the case of analog channels, such corresponding software criteria must be located in the exchange itself. To be more general, there are two different 30 methods implied in best server selection in accordance with the present invention: (a) Analog, in which the signal strengths for each call are measured in the neighbor cells, and these together with the signal strength for uplink in the serving channel are 35 evaluated; and (b) Digital in which mobile assisted WO 95/02309 PCT/SE94/00630 18- handoff (MAHO) is employed and similar measurements are made by the mobile and reported to the base station and evaluated. A basic concept inherent in the present invention 5 is that it is desirable to serve a mobile station by means of a cell level within a multi-cell structure which is at the lowest possible level provided the signal quality is sufficient to provide good service. In the situation shown in FIG. 2, including an array of 10 umbrella cells and microcells positioned therein, and in which it possible to provide coverage from either the umbrella cell or the microcell, it is normally optimal to allow the microcell to handle the subscriber traffic whenever it is appropriate from the standpoint 15 of quality transmission. In other words, in such situations the microcell, i.e., the lowest level of cells in the multi-cell array, is preferred. In the case where multi-cell levels are implemented, one layer is generally preferred over the other primarily for 20 capacity reasons. The system and method of the present invention is implemented based upon the processing of a list of neighbor cells and upon the differentiation between preferred cells, non-preferred cells and cells with 25 equal preference in order to ascertain the most appropriate serving cell both at time of call setup as well as during the call itself. For example, as shown in FIG. 2, the microcells a, c, d, e are preferred to the umbrella cell A, microcells a, b, c are preferred 30 to the umbrella cell B, while umbrella cells A-G have equal preference to each other and microcells a-e have equal preference to each other. Within the present system, one goal is to ensure that the mobile subscriber radio transceiver is tuned 35 to the most appropriate cell both during the idle mode WO 95/(12309 PCT/SE94/00630 ft ?6 20 25 19- " V ~ ; s* as well as during the call process. Preferably, algorithm used in the present invention is employed to select a cell whit-h is the same in both idle mode and during the process of a call in order to make sure that the 5 cell as selected at call setup is also accepted immediately after the call setup. In the present invention, data are assigned to each cell as set forth below in the chart. Parmnfltar RflSCrlptlon 10 1 A t» elected signal strength threshold which defines a sufficiently "acceptable" signal strength for the cell 15 2 A hysteresis value to be used together with the assigned threshold 3 A list of neighboring cells 4 For each neighbor, an assigned type, i.e., preferred, non-preferred, equal preference 5 For each neighbor, an assigned hysteresis value 30 In case there is no qualified preferred neighbor and the signal strengths in the currently serving cell is below the threshold, then the "best" cell should be selected and handed off to. The hysteresis value of parameter 5 is used in this particular situation. 35 Referring briefly to FIG. 7, there is shown a plurality of neighboring cells 41-43 which may be at any level within a multi-level cellular structure. Within each cell is a base station 44-46 each of which comprises a control channel transceiver 44a-46a as well 40 as a plurality of voice channel transceivers 44b-46b. The control channel of the base station is used for communication with mobile stations during idle period "•ta&luS gmce f S FEB {998 Of New Zealand WO 95/02309 PCT/SE94/00630 6 « fi fs when no call has been established. The signalling v q placed on the control channel includes the information to be broadcast to the mobile stations relating to neighboring cells. The voice channels are used after 5 call setup to a mobile station and carry not only voice data but also certain signalling, for example, measurement channel numbers for the neighboring cells to the cell serving the mobile station, as well as a handoff order to the mobile and measured signal 10 strength from the neighboring cells to the cell serving the mobile station in the case of mobile assisted handoff (MAHO) under the IS-54 digital standard. Referring now to FIG. 8, there is shown an overall block diagram of the equipment within the cellular 15 network within which the present invention is incorporated, including the public switched telecommunication network (PSTN) 51, a mobile switching center (MSC) 52, and a pair of illustrative base stations 53-54. Within the MSC 52 is a switch 55 20 controlled by a central processor 56 both of which are connected to an interface 57. The interface 57 of the MSC 52 is coupled by means of data lines 61 and 62 to each of the respective base stations 53 and 54. Each bare station includes a multiplexer/demultiplexer 60 25 connected to a plurality of transceivers 63-65 connected to an antenna 66. Each transceiver includes a control unit 67 and a transmit unit 68 along with a receive unit 69. Calls to or from subscribers in the PSTN 51 are connected to the switch 55 within the MSC 30 52 where they are routed to the correct cell and voice channel controlled by the central processor 56. The central processor 56 also administers and processes data regarding the location of the mobile, the neighboring cells to the cell within which the mobile 3 5 is currently located, and controls both call setup and ,„ P se if v s e Intellaotu#! Property Office 1 6 FEB 1998 of New Zealand WO 95/02309 ' PCT/SE94/00630 *'ee On. handoff. The links between the MSC 52 and the baseO* y stations 53 and 54 (links 61 and 62) carries voice data, as well as control information to and from the control unit 67 within each of the different 5 transceivers 63-65 of the base stations 53 and 54. The base station equipment 53 and 54 consists of the multiplexer 60 which administrates distribution of voice data and control information to and from the different transceivers 63-65. The transceivers include 10 both control channel transceivers and voice channel transceivers. Referring next to FIG. 9, there is shown a mobile subscriber unit 71 including a receiver 72, a transmitter 73 connected to an antenna 74 through a 15 duplexer 75. A control unit 76 is coupled to a synthesizer 77 which is also coupled to the transmitter 73 and receiver 72. A microphone 78 is connected to the transmitter 73 while a speaker 79 is connected to the receiver 72. The synthesizer 77 is used to tune 20 the receiver 72 to the control channel frequencies before call setup and to different voice channel frequencies and measurement channel frequencies during the call and under control of the control unit 76. The control unit 76 measures by means of the receiver 72 and 25 the synthesiser 77 the signal strength of the signal being received by the receiver 72, receives neighbor list information from the base station via the receiver 72 and processes all of the information according to the algorithm and technique of the present invention. 30 In the present system a mobile unit in idle mode searches the assigned frequency spectrum being broadcast for an appropriate control channel for communication with the cellular network. All control channels are continuously transmitting and are 35 cyclically broadcasting neighbor cell information which , JIC8IV8D Intellectual Property Office 1S FEB 1998 of New Zealand WO 95/02309 PCT/SE94/00630 -22- is valid for the cell within which the mobile is currently located. The neighbor cell information consists of the following data: 1) Control channel frequencies for the neighbor 5 cells; 2) The type of neighbor, i.e. (a) Preferred neighbor, (b) Non-preferred neighbor, or (c) Neighbor with equal preference; 10 3) For preferred neighbors, a signal strength threshold value for each. This threshold value is the threshold of sufficient signal strength in the neighbor cell plus a corresponding hysteresis or offset value, which may be calculated in the central processor in the 15 MSC or in the control unit of the base Btation before it is sent; 4) For each control channel (cell), a signal strength threshold which is the threshold for sufficient signal strength when served by the cell, 20 i. e. , preferred, cell but this time with the hysteresis value subtracted from that value; and 5) For all neighbors (with equal preference and for non-preferred neighbors) a hysteresis value. The mobile station measures the signal strength 25 periodically for the current channel and at the frequencies defined for the current cell's neighbors. The mobile then tunes to a preferred neighbor as soon as the measured signal strength for this neighbor is above the threshold set for this neighbor cell. If 30 more than one preferred neighbor is qualified, the mobile tunes to the neighbor with the best margin with respect to the threshold value. If, however, there is no Qualified preferred neighbor cell and a neighbor ceil with equal preference has a signal strength which 35 is hi.gher than the signal strength to the current WO 95/02309 PCT/SE94/00630 -23- channel with a margin which is defined by the hysteresis value for that particular neighbor, the mobile then retunes to this neighbor cell. If the signal strength for the current channel is 5 below its threshold, defining sufficient signal strength and there is no qualified preferred neighbor and there is also no qualified neighbor with equal preference, and a non-preferred neighbor has a signal strength which is higher than the signal strength for the 10 current channel with a margin also added which is defined by the hysteresis value for that particular neighbor cell, the mobile tunes to this neighbor cell's control channel frequency. When a call setup from the mobile is attempted, 15 the mobile is already on an appropriate control channel according to the procedure set forth above and the call can be set up in the corresponding cell according to the well-established procedures. Another way to provide the mobile with the 20 frequencies, the neighbor types, two signal strength levels, i.e., the threshold and the hysteresis, and another hysteresis would be to broadcast the threshold and the hysteresis separately and let the mobile calculate the two levels. 25 Another way to describe the algorithm is: (1) tune to a preferred neighbor when the measured signal strength is "9'ood enough"; (2) do not tune to a non-preferred neighbor when the measured signal strength for the currently serving cell is "good enough"; and 30 (3) tune to the best neighbor when the signal strength for the serving cell is not "good enough". As an illustration of the above-referenced procedure, an example may be set forth with reference to PIG. 2. A mobile subscriber terminal moving from 35 umbrella cell A to umbrella cell B will first be tuned WO 95/02309 PCT/SE94/00630 •24- to cell A's control channel frequency where it will receive broadcast information regarding control channel frequencies for neighboring cells B-G with equal preference as well as their hysteresis values together 5 with the frequencies for the preferred neighbor microcells a, b, d, e and their corresponding threshold values. When the signal strength from microcell d is above the threshold value set for that cell, the mobile will retune to microcell d's control channel and 10 receive information about microcell d's neighbor cells which are now microcell a and microcell c with equal preference, and umbrella cell A as a non-preferred neighbor cell. After having retuned to microcell c, the neighbors with equal preference will be microcell 15 a, microcell b, and microcell d and the non-preferred neighbors will be umbrella cell A and umbrella cell B. Finally, when the mobile station has reached a location within umbrella cell B, it will have umbrella cells A, C and G as neighbors with equal preference and 20 microcells a, b, c as preferred neighbors. When a call is in progress within the system of the present invention and just after call setup and after handoff to a new cell, information about the control channel frequency or any other continuously 25 transmitting frequency in each neighbor cell is sent to the mobile station. The mobile measures signal strength for the serving channel and the neighboring cells periodically and reports those values on the current voice channel to the base station. This 30 information is processed in the control unit of the base station or in the central processor of the MSC according to precisely the same rules as was done in the mobile station during idle condition. As soon as a better serving base station is identified according 35 to rules set forth above, a handoff is performed to WO 95/02309 PCT/SE94/00630 " - '»oe 8 > this new serving base station. If the base station the mobile station uses power regulation, the signal strength or the threshold must be adjusted in order to get a comparison result, as if the radio transmitter 5 was using the maximum allowed power level in the cell. The system of the present invention provides an easier and more natural way to control selection of a cell at call setup and at handoff. The system also provides a system for optimal call selection within 10 such a multi-layered cell structure. Referring next to FIG. 10# there is shown a flow chart illustrating certain functional performance in the system of the present invention. FIG. 10 illustrates a process performed within the mobile 15 during idle period as follows: the routine begins at 101 and moves to 102 at which it selects a first channel. Thereafter, the mobile moves to 103 where it receives and stores information broadcast on the selected channel. Next, at 104 the mobile station 20 measures the signal strength for the neighboring cells and for the serving channel. Thereafter, at 105 the mobile selects a first channel for evaluation and moves to 106 where it is determined whether or not that channel comes from a preferred neighbor cell. If so, 25 the system moves to 10.7 at which it is determined whether or not the signal strength for the neighbor cell is greater than a preselected threshold value plus the hysteresis, if any. If not, the system moves to 108 where it is determined whether or not the last 30 neighbor has been evaluated. If, however, at 107, the signal strength for the neighbor cell is greater than the threshold value plus the hysteresis, if any, the system moves to 109 and the mobile station retunes to this neighbor cell value and returns to 103 to receive A g fi £ f V E D intelleotufll Property Office 1 G FEB 1998 of New Zealand WO 95/02309 PCT/SE94/00630 fm 2$ 8 6 and store information broadcast on this newly selected channel. If, however, at 106 the channel being evaluated is determined to not be a preferred neighbor cell, the 5 system moves to 110 where it is determined whether or not the neighbor cell is one with equal preference. If not, i.e., the cell is a non-preferred neighbor cell, the system moves to 111 at which it determines whether or not the serving channel signal strength is less than 10 a preselected threshold value plus the hysteresis, if any. If not, it moves to 108 to determine if the last neighbor has been evaluated. If, at 111, it is determined that the serving channel signal strength is lower than the threshold value minus the hysteresis 15 value, if any, the system moves to 112 where it is determined whether or not the neighbor cell's signal strength is greater than the serving channel's signal strength and, if not, moves to 108 and determines whether or not the last neighbor has been evaluated. 20 If, at 112 the neighbor cell's signal strength is determined to be greater than the serving channel's signal strength, then the mobile station retunes to this neighbor cell at 109 and returns to 103 to receive and store informat-on broadcast on this channel. 25 Similarly, if it is determined at 110 that this neighbor cell is a cell of equal preference, the system moves directly to 112 to evaluate the signal strength of that cell. If, at 108, the system determines that the last neighbor has been evaluated, the system returns to 30 104 and measures the signal strength for the neighboring cells in the serving channel as set forth above. If, however, the last neighbor cell is determined to have not been evaluated at 108, the system moves to 113 and selects the next neighbor for RECEIVER Intellectual Property Office 1 6 FEB 1998 of New Zealand WO 95/02309 PCT/SE94/00630 -27- evaluation returning to 106 to determine the preference of that particular neighbor. It can thus be seen in FIG. 10 that the mobile station during idle period is continually cycling 5 through the reception of channel information and determining whether or not it is appropriate to be handed off to a cell and applying a preference to those potential cells to which it may be haiided off in accordance with the most efficient utilization of 10 channel availability. It should be understood that the process described above relates to the implementation of the present invention within a digital cellular system employing mobile assisted handoff (MAHO) such as that specified 15 by the IS-54 standard promulgated by EIA/TIA. It should be clear that the invention could also be implemented within other digital or analog cellular systems in which radio signal strength measurements are made by the base stations of the various associated 20 cells. Referring next to FIG. 11, there is also shown an alternate procedure for the evaluation of handoff likelihood in the mobile during an idle period. The routine begins at 121 where the mobile selects a first 25 channel at 122 and moves to 123 where it receives and stores information broadcast on that selected channel. Thereafter, the system moves to 124 and measures signal strength for the neighboring cells and. serving channels thereof. Next, at 125 the system selects the first 30 channel for evaluation and clears a candidate channel list. Thereafter, at 126 the system determines whether or not the selected channel is that of a preferred neighbor and, if so, moves to 127 where the signal strength for the neighboring cell is evaluated to 35 determine whether or not it is above a preselected -28- threshold value plus the hysteresis, if any. If not, the system moves to 128 and it is determined whether or not the last neighbor has been evaluated. If, however, at 127, it has been determined that the signal strength for the neighboring cell was greater than the threshold value, the system moves to add this neighbor's cell to the candidate list at 129 and thereafter to decision 128 to evaluate whether or not the last neighbor has been evaluated. If, however, at 126 it is determined that the neighbor was not preferred, the system moves to 131 to determine whether or not the neighboring cell * is one with equal preference. If not, i.e., it is a non-preferred neighbor, the system moves to 132 and determines whether or not the serving channel strength is less than a preselected threshold value minus the hysteresis, if any, and, if not, to decision 128 to determine if the last neighbor has been evaluated. If, however, at 132 the serving channel signal strength is determined to be le'j than a threshold value minus the hysteresis, if any, the system moves to 133 at which it . » determined whether or not the neighbor cell signal strength is greater than the serving channel signal strength and, if not, to a decision as to whether or not the last neighbor has been evaluated at 128. If the neighbor cell signal strength is greater than the serving channel signal strength, the system moves to 129 and adds this neighbor to the candidate list. If, at 131 it is determined that the cell is a neighbor with equal preference, the system moves directly to decision 133 to determine if the neighboring cell signal strength is greater than the serving channel signal strength. If, at decision 128 it is determined that the last neighbor has not been evaluated, the system moves to 134 to select the next neighbor for evaluation and J 6 FEB 1998 of New Zealand WO 95/02309 PCT/SE94/00630 -29- thereafter to decision 126 to begin that evaluation. If the last neighbor has been evaluated at 12 8, the system moves to 135 and determines whether or not any candidates have been found. If not, the system returns 5 to 124 and, if so, moves to 136 to sort the candidate list. Thereafter, the system moves to 137 and retunes to the first candidate on the list followed by moving to 123 where it receives and stores the information broadcast on that selected channel. 10 As shown in FIG. 11, again the system engages in an orderly evaluation of possible channels and selects a channel based upon criteria determined to provide maximum efficiency and channel utilization in a multi-layered cell structure. 15 Referring next to FIGS. 12a-12b, there is shown a routine which may be executable in the system by either the mobile station and/or base station during an established call within an exemplary digital cellular system. The system begins at 151 by moving to 152 at 20 which it transfers neighbor cell information from the base station to the mobile. Thereafter, at 153 the system measures the signal strength for the neighboring cells and serving channels. Next, at 154 the system selects a first channel for evaluation and clears a 2 5 candidate list. Thereafter, at 155 the system determines if the selected channel is from a preferred neighbor cell and, if so, moves to 156 where it determines whether or not the signal strength for that neighbor cell is greater than the preselected threshold 30 value plus the hysteresis, if any. If not, the system moves to 157 and determines if thfi last neighbor has been evaluated. If, at 156 the sicnal strength of the neighboring cell is greater than the threshold value plus the hysteresis, if any, the system moves to 158 3 5 and adds this neighbor to the candidate list. If, at WO 95/02309 PCT/SE94/00630 155 the cell is determined not to be a prexewreA neighbor, the system moves to 159 to determine if tkk ^ neighboring cell is one with equal preference and, if not, to 161 to determine if the serving channel signal 5 strength is less than the preselected value minus the hysteresis, if any. If not, the system moves to 157 to determine if the last neighbor has been evaluated. However, if the signal strength is less than the preselected value at 161, the system moves to 162 where it 10 is determined whether or not the neighboring cell signal strength is greater than a serving channel signal strength plus the hysteresis, if any, and, if so, to 158 to add this neighbor to a candidate list or, if not, to 157 to determine if the last neighbor has been evaluated. If, at 15 159 it is determined that the neighboring cell is one with equal preference, the system moves directly to 162 to determine if that neighboring cell signal strength is greater than the serving channel signal strength plus the hysteresis, if any. 20 If it is determined at 157 that the last neighbor has not been evaluated, the system moves to 163 to select the next neighbor for evaluation and thereafter evaluates it beginning at step 155. If, at 157 it is determined that the last neighbor has been evaluated, 25 the system moves to 164 to determine if candidates are found and, if not, back to 153 to measure the signal strength for neighbors and the serving channels. If candidates are found at 164, the system moves to 165 to sort the candidate list and, thereafter to 166 to 30 select a first candidate cell for a handoff attempt. At 167, the system determines whether or not an idle channel exists in the candidate cell and, if not, moves to 168 to determine if this is the last candidate and, if not, to 169 wherein it selects the next candidate 35 cell for a handoff attempt. If the cell was the last , „ M 0 R | V B D 'ntelleotual Property Office I 6 FEB 1998 of New Zealand WO 95/02309 PCT/SE94/00630 -31- candidate at 168, the system moves back to 153 to measure the signal strength for neighbors and serving channels. If the idle channel in the candidate cell exists, the system moves to 171 and hands off to this 5 candidate cell and, thereafter, returns to 152 to transfer neighboring cell information to the mobile. As can be seen, the system of the present invention utilizes algorithms which maximize the channel utilization from various cells within a multi-10 layered cell structure in a highly efficient manner. As can be seen from the above description, the present invention allows a multi-layered cellular communication system to monitor channel signal strength in a plurality of cells on multiple levels and perform 15 an evaluation in accordance with an algorithm which maximizes the channel availability within the multi-layered cellular structure. It is believed that the operation and construction of the present invention will be apparent from the 20 foregoing description and, while the method and apparatus shown and described has been characterized as being preferred, obvious changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the following 2 5 claims. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> ?6e WHAT WE CLAIM IS: C7 8 S

1. A method of performing server selection for a mobile station operating within a cellular radio system comprised of 5 multiple levels of cells having different service areas, said method comprising the steps of: assigning to each cell according to cell level a category of preference for selection with respect to each other associated cell within the system having either a coextensive, 10 adjacent, contiguous, or overlapping service area; assigning to each associated cell a preselected signal strength threshold; measuring the signal strength of communications between the mobile station and each base station serving the associated 15 cells; comparing the measured signal strengths to the preselected thresholds for each of the associated cells; and selecting for the mobile station as server a base station serving a candidate one of the associated cells based 20 upon whether the measured signal strength to the candidate associated cell is greater than the preselected signal strength threshold assigned to said cell and whether the candidate associated cell has a higher category of preference than a currently serving cell. 25 30

2. A method of performing server selection as set forth in claim 1 in which the categories of preference assigned to each associated cell includes the categories of preferred, equal preference and non-preferred for selection.

3. A method of performing server selection as set forth in claim 2 in which an associated cell is assigned a preferred category of preference if its service area is substantially smaller than that of the currently serving cell, an equal 35 category of preference if its service area is substantially the same size as that of the currently serving ce,AtiJ00Etl|^^p^|gn- 1 6 FEB 1998 of New Zealand • - 33 - '?6j> 6 preferred category of preference if its service area isT substantially larger than that of the currently serving cell.

4. A method of performing server selection as set forth 5 in claim 2 in which said multiple levels of cells include macrocells, microcells and picocells.

5. A method of performing best server selection for a mobile station operating within a cellular radio system comprised 10 of multiple levels of cells each having a specific service area, said method comprising the steps of: assigning to each cell a category of preference with respect to each of the other cells within the system having either a coextensive, adjacent, contiguous, or overlapping 15 service area, said categories of preference comprising a category of preferred, a category of equal preference or a category of non-preferred, said category assignment based upon the size of the cell's service area relative to that of other cells; measuring the signal strength of communications between 20 the mobile station and a first base station for a first cell currently selected as best server, and between the mobile station and a second base station for a second cell in the system; identifying the assigned category of preference for the second cell; 25 selecting the second base station as the best server in response to satisfaction of any one of the following conditions, (a) the second cell having a category of preference of preferred and the measured signal strength with the second base station being greater than a preselected signal strength 30 threshold, or (b) the second cell having a category of preference of equal preference and the measured signal strength with the second base station being greater than the measured signal strength with the first base station, or (c) the second cell having a category of preference of non-preferred and the measured 35 signal strength with the first base station being less than a preselected signal strength threshold and the measu^-gc^ Intellectual Property Office 1 6 FEB 1998 - 34 - 2<ig r. s8s strength with the second base station being greater than that with the first base station.

6. A method of performing best server selection as set 5 forth in claim 5 in which a cell is assigned a preferred category of preference with respect to another cell if its service area' is substantially smaller than that of the other cell, an equal category of preference with respect to the other cell if its service area is substantially the same size as that of the other 10 cell, and a non-preferred category of preference with respect to the other cell if its service area is substantially larger than that of the other cell.

7. A method of performing best server selection as set 15 forth in claim 7 further comprising the steps of: measuring the signal strength of communications between the mobile station and each of the base stations serving associated ones of the cells in the system having either a coextensive, adjacent, contiguous, or overlapping service area: 20 identifying the assigned category of preference for each of the associated cells; selecting as the base station best serving the mobile station that particular base station for One Of the associated cells satisfying any one of the following conditions, (a) the 25 particular base station being in a cell having a category of preference of preferred and having a measured signal strength with the mobile station greater than a preselected signal strength threshold, or (b) the particular base station being in a cell having a category of preference of equal preference and 30 naving a measured signal strength with the mobile station greater than the measured signal strength between the mobile station and the first base station, or (c) the particular base station being in a cell having a category of preference of non-preferred and having a measured signal strength with the mobile station greater 35 than the measured signal strength between the mobile station and the first base station and wherein the measured signal strength . , R E 6 6 I V E D 'ntsllaoiu^l Property Office, J 6 FEB iyy8 ?68 R 6Qs between the mobile station and the first base station is less than a preselected signal strength threshold.

8. A method of performing best server selection as set forth in claim 5 in which said multiple layers of cells include macrocells, microcells and picocells.

9. A system for performing server selection for a mobile station operating within a cellular radio system comprised of 10 multiple levels of cells having different services areas, each cell assigned according to cell level a category of preference for selection with respect to each other associated cell within the system having either a coextensive, adjacent, contiguous, or overlapping service area, and for assigning to each associated 15 cell a preselected signal strength threshold, said system comprising: means for measuring the signal strength of communications between the mobile station and each base station serving the associated cells; 20 means connected to the means for measuring for comparing the measured signal strengths to the preselected signal strength thresholds for the associated cells; and means connected to the means for comparing for selecting for the mobile station as server a base station serving 25 a candidate one of the associated cells based upon whether the measured signal strength to the candidate associated cell is greater than the preselected signal strength threshold for said cell and whether the candidate associated cell has a higher category of preference than a currently serving cell. 30

10. A system for performing server selection as set forth in claim 9 in which the categories of preference assigned to each associated cell includes the categories of preferred, equal preference and non-preferred for selection. 35 S 6 fe I V E & 'ntel/Gotua, Property Office 1 G FEB 1998 of New Zealand - 36 -

11. A syr.tern for performing server selection as set forth in claim 10 in which an associated cell is assigned a preferred category of preference if its service area is substantially smaller than that of the currently serving cell, an equal 5 category of preference if its service area is substantially the same size as that of the currently serving cell, and a non-preferred category of preference if its service area is substantially larger than that of the currently serving cell. 10

12. A system for performing server selection as set forth in claim 10 in which said multiple levels of cells include macrocells, microcells and picocells.

13. A system for performing best server selection for a 15 mobile station operating within a cellular radio system comprised of multiple levels of cells each having a different service area, each cell assigned a category of preference with respect to each of the other cells within the system having either a coextensive, adjacent, contiguous, or overlapping service area, said 20 categories of preference comprising a category of preferred, a category of equal preference or a category of non-preferred, said category assignment based upon the size of its service area relative to that of other cells, said system comprising: means for measuring the signal strength of 25 communications between the mobile station ana a first base station for a first cell currently selected as best server, and between the mobile station and a second base station for a second cell in the system; means connected to the means for measuring for 30 identifying the assigned category of preference for the second cell; means connected to the means for identifying for selecting the second base station as the best server in response to satisfaction of any one of the following conditions, (a) the 35 second cell having a category of preference of preferred ard the measured signal strength with the second base sta1ftiE>G 'intellectual Property Office 1 6 FEB IM8 . _ _ _ °f New Zealand • ?686„f uo $ greater than a preselected signal strength threshold, or (b) the second cell having a category of preference of equal preference and the measured signal strength with the second base station being greater than the measured signal strength with the first 5 base station, or (c) the second cell having a category of preference of non-preferred and the measured signal strength with the first base station being less than a preselected signal strength threshold and the measured signal strength with the second base station being greater than that with the first base 10 station.

14. A system for performing best server selection as set forth in claim 13 in which a cell is assigned: a preferred category of preference with respect to another cell if its 15 service area is substantially smaller than that of the other cell, an equal category of preference with respect to the other cell if its service area is substantially the same as that of the other cell, and a non-preferred category of preference with respect to the other cell if its service area is substantially 20 larger than that of the other cell.

15. A system for performing best server selection as set forth in claim 13 further comprising: means for measuring the signal strength of 25 communications between the mobile station and each of the base stations serving associated ones of the cells in the system that are either coextensive, adjacent, contiguous, or overlapping with the first cell; means for identifying the assigned category of 30 preference for each of the associated cells; and means for selecting as the base station best serving the mobile station that particular base station for one of the associated cells satisfying any one of the following condition, (a) the particular base station being in a cell having a category 35 of preference of preferred and having a measured signal strength with the mobile station greater than a preselected signal ' o rCfl 1958 - 38 - 26Sf 6Sl strength threshold, or (b) the particular base station being in a cell having a category of preference of equal preference and having a measured signal strength with the mobile station greater than the measured signal strength between the mobile station and 5 the first base station, or (c) the particular base station being • in a cell having a category or preference of non-preferred and having a measured signal strength with the mobile station greater than the measured signal strength between the mobile station and the first base station and wherein the measured signal strength 10 between the mobile station and the first base station is less than a preselected signal strength threshold.

16. A system for performing best server selection as set forth in claim 13 in which said multiple levels of cells include 15 macrocells, microcells and picocells.

17. A cellular communications system including a plurality of cells arranged in multiple layers, comprising: a base station for each of the plurality of cells, each 20 base station assigned a category of preference for server selection with respect to each neighboring cell in the plurality of cells having either a coextensive, adjacent, contiguous, or overlapping service area, each base station further assigned a signal strength threshold for mobile station communications 25 within the cell; and a plurality of mobile stations moving through the plurality of cells and communicating with the plurality of base stations- each mobile station comprising: means for measuring the signal strength of 30 communications between the mobile station and each of the base stations within communications range; and processing means connected to the means for measuring for making a server selection determination for the mobile station based on a comparison of the measured signal 35 strength of communications with each candidate base station to the assigned signal strength threshold for those canfligjQ.^ vjo&fte 'ntellsotual Property office 1 6 FES 1998 of Klnw 39 stations in conjunction with a comparison of the category of preference assigned to each candidate base station.

18. The cellular communications system as in claim 17 5 wherein the base station further includes means for broadcasting in a communication to the mobile stations the assigned category of preference for server selection and assigned signal strength threshold for communications to the base station. 10

19. The cellular communications system as in claim 17 wherein the processing means for making the server selection determination operates to select a server when the mobile station is both camping-on to a base station in an idle operating mode and handing-off between base stations in a call processing mode.

20. The cellular communications system as in claim 17 wherein the processing means of the mobile station further includes means for determining a server selection change from a currently serving base station to the candidate base station if 20 the measured signal strength exceeds the assigned signal strength threshold for the candidate base station and further if the candidate base station has a higher category of preference than the currently serving base station. 25

21. The cellular communications system as in claim 20 wherein the determination by the processing means of the server selection change is further based on a comparison of the measured signal strength of communications with the candidate base station to the measured signal strength of communications with the 30 currently serving base station.

22. The cellular communications system as in claim 21 wherein the processing means of the mobile station further includes means for determining the server selection change from 35 the currently serving base station to the candidate base station if the measured signal strength for the candidate ba^p 15 totelieshj f 6 FEB 1998 in( Mai.. 40 ?6g exceeds the measured signal strength for the currently serving base station and further if the candidate base station has at least an equal category of preference to the currently serving base station.

23. The cellular communications system as in claim 21 wherein the processing means of the mobile station further includes means for determining the server selection change from the currently serving base station to the candidate base station, 10 even if the candidate base station has a lower category of preference than the currently serving base station, if the measured signal strength for the currently serving base station is below the assigned signal strength threshold for the currently serving bas? station, and further if the measured signal strength 15 for the candidate base station exceeds the measured signal strength for the currently serving base station.

24. In a cellular communications system including a plurality of cells arranged in multiple layers, wherein each cell 20 includes a base station, and further including a plurality of mobile stations moving through the plurality of cells and communicating with the plurality of base stations, wherein each base station is assigned a category of preference for server selection with respect to each neighboring cell in the plurality 25 of cells having either a coextensive, adjacent, contiguous, or overlapping service area, and each base station is assigned a signal strength threshold for mobile station communications within the cell, a method for determining mobile station server selection of a base station, comprising the steps of: 30 measuring the signal strength of communications between each mobile station and each of the base stations within communications range thereof; communications with each candidate base station to the assigned 35 signal strength threshold for that candidate base station; 5 comparing the measured signal strength of 1 e FEB 1998 of New Zealand comparing the category of preference fo: 41 base station to the category of preference for the base stations currently serving each of the mobile stations; and 5 mobile station serve-- selection determination for each mobile station.

25. The method as in claim 24 wherein the step of making is performed to select a server when the mobile station is both 10 camping-on to a base station in an idle operating mode and handing-off to a base station in a call processing mode.

26. The method as in claim 24 wherein the step of making including the step of selecting as a server a candidate base 15 station having a measured signal strength with the mobile station that exceeds its signal strength threshold, and also having a category of preference that is higher than the category of preference for the currently serving base station. 20

27. The method as in claim 2 6 further including the step of comparing the measured signal strength of communications with the candidate base station to the measured signal strength of communications with the currently serving base station. 25

28. The method as in claim 27 wherein the step of making includes the step of selecting as a server the candidate base station having a measured signal strength with the mobile station that exceeds the measured signal strength for the currently serving base strtion, and also having a category of preference 30 that is at least equal to the category of preference for the currently serving base station.

29. The method as in claim 27 wherein the step of making includes the step of selecting as a server the candidate base 35 station having a measured signal strength with the mobile station that exceeds the measured signal strength for th<^ responsive to the foregoing comparisons making the Intallflfltus' P^arty offlce t 6 FEB (998 » nn<II VABlonH serving base station, even if the category of preference forQh^^ candidate base station is lower than the category of preference for the currently serving base station, and also provided that the measured signal strength for the currently serving base 5 station is below its signal strength threshold.

30. A cellular communications system including a plurality of cells, comprising: a base station for each of the plurality of cells, 10 wherein each base station is assigned a category of preference for server selection with respect to each neighboring cell in the plurality of cells having either a coextensive, adjacent, contiguous, or overlapping service area, and wherein each base station is further assigned a signal strength threshold for 15 mobile station communications within the cell; a plurality of mobile stations moving through the plurality of cells and communicating with the plurality of base stations; means for measuring the signal strength of 20 communications between the mobile stations and base stations; processing means connected to the means for measuring, said processing means executing a predetermined procedure for selecting a base station to serve each of the mobile rtations, the selected base station having both the highest assigned 25 category of preference and a measured signal strength of mobile station com-aunications that exceeds the assigned base station signal strength threshold, said predetermined procedure followed for each mobile station in selecting the base station both for camping-on to a server during an idle operating mode and for 30 handing-off between servers during a call processing mode. 33..

A method of performing server selection for a mobile station operating within a cellular radio system in a region in which radio channels are provided by cells having different 35 service areas, said method comprising: Bg811 V I ft InteilsottJal Property oftlne 1 S FEB 1998 of New Zealand ee 8 ft providing for the mobile station a list of neighboring cells; assigning a threshold radio channel signal strength level which is acceptable for service of a mobile station in each 5 of said neighboring cells; assigning to each of said neighboring cells on said list a category of preference with respect to a currently serving cell; measuring the strength of the radio signals providing 10 communication between the mobile station and the base station of each of the neighboring cells; and selecting as a server for the mobile station the base station of a neighboring cell on said list whose preference category is preferred with respect to the currently serving cell 15 and whose measured strength of the radio signal providing communication therewith is greater than the assigned threshold radio channel signal strength level by a largest margin in comparison to other preferred neighboring cells. 20

32. A method of performing server selection as set forth in claim 31 in which the categories of preference assigned within each cell to each neighboring cell includes preferred, equal preference and non-preferred. 25

33. A method of performing server selection for a mobile station operating within a cellular radio system in a region in which radio channels are provided by multiple levels of cells, each of said cells being provided with radio service by a base station the broadcast power of which and the resultant service 30 area which varies in size from larger to higher level cells to smaller for lower level cells, said method comprising: providing for the mobile station a list of neighboring cells; assigning a threshold radio channel signal strength 35 value which is acceptable for service to a mobile station in each of said neighboring cells; fnt#!uiotu9i Iflperiy Office 1 fi FEB 1938 of New Zealand 8 e - 44 - 0 Q f measuring the strength of the radio signals providing communication between the mobile station and the base station of each of the neighboring cells; and selecting as a server for the mobile station from among 5 the neighboring cells the cell having both the smallest relative' service area size and a measured radio signal strength which is greater than the threshold radio channel signal strength by a largest margin in comparison to other smallest neighboring cells. 10

34. A method for control channel reselection in a cellular telephone system including a plurality of cells, the plurality of cells including, with respect to a given cell associated with a currently selected control channel, an identified plurality of neighboring cells each being assigned a signal strength threshold 15 and a category of preference with respect to the given cell, comprising the steps of: filtering the identified plurality of neighboring cells to determine candidate cells for control channel reselection which have a measured signal strength exceeding their assigned 2 0 signal strength threshold and have a categorv of preference of preferred over the given cell; and choosing for control channel reselection the candidate cell whose measured signal strength exceeds its assigned signal strength threshold by a best margin in comparison to other 25 preferred neighboring cells.

35. The method of claim 34 wherein the plurality of cells form a multi-level layered cell structure. 3 0

36. The method of claim 35 wherein the multi-level layered cell structure includes both umbrella cells and microcells, and: wherein a microcell has a category of preference of preferred with respect to an umbrella cell; wherein an umbrella cell has a category of preference 35 of non-preferred with respect to a microcell; nereit/en f 6 FEB 1998 of New Zealand • ^ 8 ft 68J wherein a microcell has a category of preference of equally preferred with respect to another microcell; and wherein an umbrella cell has a category of preference of equally preferred with respect to another umbrella cell. 5

37. The method of claim 34 wherein the step of filtering comprises the steps of: making signal strength measurements on a control channel of each of the identified neighboring cells; 10 comparing the signal strength measurements to the assigned signal strength thresholds; determining the category of preference for each of the identified neighboring cells; and selecting as the candidate cells those neighboring 15 cells having measured signal strengths exceeding the signal strength thresholds and having preferred categories of preference.

38. The method of claim 37 wherein the signal strength 20 measurements are made by mobile stations operating within the cellular telephone system.

39. The method of claim 37 wherein the signal strength measurements are made by base stations operating within the 25 cellular telephone system.

40. The method of claim 34 wherein the step of choosing comprises the steps of: sorting the candidate cells by calculated margin 30 between the measured signal strengths and associated signal strength thresholds; and selecting for control channel reselection the candidate cell with the best calculated margin. RECEIVE D Intel! amea I 6 m 1396 of New Zealand - 46 - 6

41. The method of claim 34 wherein the signal stren threshold comprises the sum of a sufficient signal strength for communication in the neighboring cell plus an offset. 5

42. The method of claim 34 wherein if no neighbor cell with a preferred category of preference is capable of being chosen in control channel reselection by the steps of filtering and choosing, further including the steps of: further filtering the identified plurality of 10 neighboring cells to determine alternate candidate cells for control channel reselection which have a measured signal strength exceeding a measured signal strength for the given cell by a certain hysteresis and have a category of preference of other than non-preferred with respect to the given cell; and 15 further choosing for control channel reselection the alternate candidate cell whose measured signal strength exceeds the sum of the given cell measured signal strength and the certain hysteresis by a best margin in comparison to other neighboring cells. 20

43. The method of claim 42 wherein the step of further filtering comprises the steps of: making signal strength measurements on a control channel of the given cell and each of the identified neighboring 25 cells; comparing the signal strength measurements for the neighboring cells to the sum of the signal strength measurement for the given cell plus the certain hysteresis; determining the category of preference for each of the 30 identified neighboring cells; and selecting as the alternate candidate cells those neighboring cells having measured signal strengths exceeding the sum of the given cell measured signal strength and certain hysteresis and having a category of preference other than non- 35 preferred. RES6IVIB Irt.n.SuS PrioirtV 0«l»» 18 h£8 iaae of New Zealand - 47 -

44. The method of claim 43 wherein the signal str! measurements are made by mobile stations operating within the cellular telephone system. 5

45. The method of claim 43 wherein the signal strength measurements are made by base stations operating within the cellular telephone system.

46. The method of claim 42 wherein the step of further 10 choosing comprises the steps of: sorting the candidate cells by calculated margin between the candidate cell measured signal strengths and the given cell measured signal strength plus the certain hysteresis; and 15 selecting for control channel reselection the candidate cell with the best calculated margin.

47. The method of claim 34 wherein the steps of filtering and choosing are performed in connection with server selection 20 during mobile station camp-on.

48. The method of claim 34 wherein the steps of filtering and choosing are performed in connection with server selection during mobile station handoff. 25

49. A method for filtering a plurality of neighboring cells to determine candidate cells for control channel reselection in a cellular telephone system, the plurality of neighboring cells esch being assigned a signal strength threshold and a category 30 of preference with respect to a given cell associated with a currently selected control channel, comprising the steps of: measuring received signal strength with respect to each of the identified plurality of neighboring cells; determining the category of preference with respect to 35 each of the identified plurality of neighboring cells; and RECEIVE D, Intaliaotud Property Offlo* 1 6 FEB 1998 of New Zealand <?<? ^°a6o identifying as candidate cells for control ctffifnnat reselection those ones of the identified plurality of neighboring cells which have a measured received signal strength exceeding their assigned signal strength threshold and have a category of 5 preference of preferred over the given cell.

50. The method as in claim 49 further including the steps of: measuring received signal strength with respect to the 10 given cell; and identifying as alternate candidate cells for control channel reselection those ones of the identified plurality of neighboring cells which have a measured received signal strength exceeding the sum of the measured signal strength for the given 15 cell plus a certain hysteresis and having a category of preference other than non-preferred with respect to the given cell.

51. A method of performing server selection for a mobile 20 station operating within a cellular radio system substantially as hereinbefore described with reference to the accompanying drawings.

52. A system for performing server selection for a mobile 25 station operating within a cellular radio system substantially as hereinbefore described with reference to the accompanying drawings.

53. A cellular communications system including a plurality 30 of cells arranged in multiple layers substantially xs hereinbefore described with reference to the accompanying drawings.

54. A method for filtering a plurality of neighboring cells 35 to determine candidate cells for control channel reselection in RECEIVED Intellectual property Office I 6 FtB ba8 of New Zetland 49 ?eB a cellular telephone system substantially as hereinbefo described with reference to the accompanying drawings. TELEFONAKTIEBOLAGET L M ERICSSON i " ~ END OF CLAIMS </div>