GB2353438A

GB2353438A - Cell selection in cellular mobile communications networks

Info

Publication number: GB2353438A
Application number: GB9919493A
Authority: GB
Inventors: Behzad Mohebbi
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1999-08-17
Filing date: 1999-08-17
Publication date: 2001-02-21
Anticipated expiration: 2019-08-17
Also published as: GB9919493D0; GB2353438B

Abstract

In a cellular mobile communications network, each cell has an actual coverage area CA' within which a predetermined signal, transmitted by the base station BTS of the cell for use by mobile stations located in that cell for cell selection purposes, is receivable. At least one of the cells is a restricted cell whose actual coverage area is sufficiently large that a desired capacity level in the cell would be exceeded were every mobile station located within that actual coverage area to select that cell. The restricted cell also has a smaller intended coverage area IA, within which it is expected that every mobile station can select the cell without the desired capacity level being exceeded. The restricted cell is assigned a correction factor for cell selection purposes, which serves to define the intended coverage area of the restricted cell. When located within the actual coverage area CA' of the restricted cell, each mobile station MS1-3 employs the assigned correction factor for that cell in its cell selection such that the mobile station does not select the restricted cell if it is outside the intended coverage area IA of that cell. The base station of the restricted cell can then transmit at maximum power, so as to achieve satisfactory indoor penetration (e.g. to MS2), without exceeding its desired capacity level.

Description

1 2353438 1_ CELL SELECTION IN CELLULAR MOBILE COMMUNICATIONS NETWORKS The

present invention relates to cell selection in cellular mobile communications networks.

In a cellular mobile communications network, each base station has with it associated a cell covering a certain area ("footprint"). A mobile station within the coverage area of-the cell communicates with the network by transmitting radio signals to, and receiving radio signals from, the base station associated with the cell. The shapes and sizes of different cells can be different and may vary over time. The respective coverage areas of adjacent cells generally overlap with is one another so that at any given time, a mobile station may be capable of communicating with more than one base station.

The ability of a particular mobile station to communicate with a base station depends on a number of factors including transmission power, distance from the base station and interference. By increasing transmission power it may be possible to increase the maximum separation between the mobile station and the base station at which signals transmitted from one can still be received by the other (since received power is dependent on the transmission power and the distance) This means that in theory the cell coverage area can be increased if the base station transmission power is increased.

The level of interference (sometimes called "multiple- access interference" or "multi-user interference") however, experienced by a particular mobile station depends largely on factors such as the number of other mobile stations operating in the cell, their proximity to the subject mobile station, their transmission powers and data rates.

Accordingly, it is of ten the case that the size and footprint of a cell are decided taking into account the expected number of mobile stations (expected subscriber density) in that cell. If the subscriber density is expected to be very low (f or example, in a rural environment), such that the maximum capacity of a cell is not utilised, the cell radius can be as large as possible. In such a cell as this, where interference from other mobile stations is very low, the communications link between the mobile station and the base station is "noise" limited. If the subscriber density is expected to be high (for example, in an urban area), the cell must be designed primarily on the basis of subscriber density, which often means a very small footprint is required. In such a cell as this, the network is interference limited, with received power at the cell boundaries exceeding the minimum required signal level for common control channels CCCH 1 (e.g. synchronisation control channel (SCH), broadcast control channel (BCCH) and pilot control channel (PCH)). In fact, the capacity of an urban cell may not just be limited by interference. Other capacity limitations include code resource allocation limitations, fixed"fletwork bandwidth limitations in the fixed-network paths connecting the base station to the remainder of the network, and processing power limitations in the base station.

At various times (for example, during call setup or network acquisition) it is necessary for a mobile station to perform a cell selection operation, i.e. to select a base station to begin communicating with. A cell selection operation is also required for hand-off between adjacent cells.

It is possible to make adjacent cells have different sizes by making a static power setting of common control channel signals (CCCH signals) broadcast by the cell I s base station dif f erent amongst the adjacent cells. The higher the CCCH static power setting the larger the cell. Accordingly, the static power setting allows the design of different cell sizes, even if they are adjacent and have to participate in soft handoff.

In practice, however, the reduction in CCCH static power setting needed to achieve a small footprint causes a problem of reduced indoor penetration. Mobile phones are of course used indoors as well as outdoors.

Radio signals suffer much more attenuation in passing through solid materials such as walls than in passing through air and the attenuation may be, say, 10-20dD per wall. Accordingly, when the CCCH static power is setting is reduced to provide a cell of small radius, although in the outdoor environment signal strength is high enough, in the indoor environment signal strength may be too low.

Since system acquisition (also known as network acquisition) and call setup are based on CCCH signals, even buildings near the base station can have much reduced indoor coverage, which means loss of revenue for the network operator and. customer dissatisfaction.

Figure 1 shows '..a base station BTS of one cell, for example a small urban cell. Such a cell typically has a radius of 100-500m. (A large cell, typically of radius 1-5km, is more often used in rural areas) A desired coverage area CA of the cell is indicated by hatching. Within the coverage area CA three mobile stations (MS1, MS2 and MS3) are in use and there is also a building B. A line Li represents schematically a received signal strength of the CCCH across the coverage area CA and line L2 represents schematically an interference level across the coverage area CA.

For the sake of simplicity, the received signal I strength is shown in reducing linearly with distance from the base station BTS, but in practice the attenuation would increase with distance in a complex non-linear way due to multipath propagation etc.

Similarly, the interference level, although shown as being constant across the cell, will vary with position in practice. A spectral density of the interference in the cell is, for example, designed to be kept to no more than 6dB higher-than a noise spectral density of the cell, where the noise spectral density represents the noise power per Hertz in the cell.

The difference in height between lines Ll and L2 at a given position represents schematically a SINR of received CCCH signals at the position concerned. The is effective boundary of the coverage area CA is where the height difference (SINR) falls below a minimum value necessary for reliable reception of the CCCH signals.

Beyond this boundary a mobile station, for example MS3, is not able to start communicating with the network through the base station shown, since the SINR of the CCCH signals is too low here. In the Figure I cell, therefore, the CCCH static power is set to a level sufficient to make the effective boundary of the coverage area CA coincide with the desired coverage area.

As also illustrated, the received signal strength (line L2) is severely reduced inside the building. MSI, situated at the side of the building farthest from the base station, where the SINR again falls below the minimum value, cannot receive reliably the CCCH signals from the base station.

MS2 which is further away from the base station from MS1 but which is outside the building, however, receives the CCCH signals much more strongly and is therefore able to start communicating with the base station shown.

Another solution to the problem of achieving adjacent differing cell sizes is to vary the antenna tilt from base station to base station. However, although this solution is partially effective with small degrees of tilt (e.g. 3-4<'), higher degrees of tilt give rise to significant problems with far penetrating side lobes.

According to a first aspect of the present invention there is provided a cellular mobile communications network including a plurality of base stations, each having an associated cell, and a plurality of mobile stations, wherein: each cell has an actual coverage area within which a predetermined signal, transmitted by the base station of the cell for is use by mobile stations located in that cell for cell selection purposes, is receivable; at least one of the said cells is a restricted cell whose said actual coverage area is sufficiently large that a desired capacity level in the cell would be exceeded were every mobile station located within that actual coverage area to select that cell, and the or each such restricted cell is assigned a correction factor for cell selection purposes; and each mobile station is operable, when located within the: said actual coverage area of such a restricted cell, to employ the assigned correction factor for that cell to influence its cell selection such that at least one mobile station located within that area, which in the absence of the assigned correction factor would select that cell, does not select that cell.

In such a network, large variations in the sizes of adjacent cells can be achieved, whilst maximising indoor coverage.

The desired capacity level can be a limit, such as a subscriber density limit, chosen with interference levels in. mind. In this case reception conditions in 1 the cell can be prevented from deteriorating undesirably as a result of interference between the mobile stations. Alternatively, the desired capacity level can be some other limit or desired level for the restricted cell, such as a code resource allocation limit, a fixed-network bandwidth limit, or a processing power limit.

According to a second aspect of the present invention there is provided a base station, for use in a cellular communications network including at least one further base station and a plurality of mobile stations, the base station embodying the second aspect of the invention and the or each said further base station each having associated therewith a cell, each is cell having an actual coverage area within which a predetermined signal, transmitted by the base station of the cell for use by mobile stations located in that cell for cell selection purposes, is receivable, the base station embodying the second aspect of the invention including: correction factor information transmitting means operable to receive a correction factor for cell selection purposes, assigned by the network to one of the said cells that is a restricted cell whose said actual coverage area is sufficiently large that a desired capacity level in the cell would be exceeded were every mobile station located within that actual coverage area to select that cell, and further operable to transmit information relating to the said assigned correction factor to mobile stations located in its associated cell for use thereby in cell selection.

According to a third aspect of the present invention there is provided a mobile station, for use in a cellular communications network including a plurality of base stations, each having associated therewith a cell, each cell having an actual coverage area within which a predetermined signal, transmitted by the base station of the cell for use by mobile stations located in that cell for cell selection purposes, is receivable, and at least one of the cells being a restricted cell whose said actual coverage area is sufficiently large that a desired capacity level in the cell would be exceeded were every mobile station located within that actual coverage area to select that cell, the mobile station embodying the third aspect of the invention including: correction factor information receiving means for receiving from the network information relating to a correction factor for cell selection purposes, assigned by the network to such a restricted cell, and operable to derive the said is correction factor from the received information; and cell selection means operable, when the mobile station is located within the said actual coverage area of that restricted cell, to employ the assigned correction factor for that cell to influence cell selection.

Thus the assigned correction factor for that cell can be employed in the cell selection process such that in at least one cell selection situation in which, in the absence of the assigned correction factor, the restricted cell would be selected, that cell is not selected.

According to a fourth aspect of the present invention there is provided a cell selection method, for use in a cellular communications network in which each cell of the network has an actual coverage area within which a predetermined signal, transmitted by a base station of the cell concerned for use for cell selection purposes by mobile stations of the network located in the cell, is receivable, in which method:

at least one of the said cells is designated a restricted cell whose said actual coverage area is sufficiently large that a desired capacity level in the 1 cell would be exceeded were every mobile station located within that actual coverage area to select that cell, and the or each such restricted cell is assigned a correction factor for cell selection purposes; and each mobile station, when located within the said actual coverage area of such a restricted cell, employs the assigned correction factor for that cell to influence its cell selection such that at least one mobile station located within that actual coverage area, which in the absence of the correction factor would select the restricted cell, does not select that cell.

Reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 illustrates a previous ly- considered cell in which a cell size is set by controlling a base station transmission power; Figure 2 illustrates a cell in a network embodying the present invention; and Figure 3 is a block diagram illustrating parts of a base station and mobile station in an embodiment of the present invention.

In Figure 2, which illustrates a cell in a network embodying the present invention, a base station BTS transmits its CCCH signals at maximum power (e.g. 3W) so that indoor penetration and coverage is maximiseA.

Lines L11 and L2 represent schematically the received signal strength of the CCCH signals and a level of interference, respectively, across the cell.

The maximum-power transmission of the CCCH signals naturally results in an extended (outdoor) actual coverage area CA, as compared for example to the Figure 1 cell discussed in the introduction. This actual coverage area CA' is larger than an intended coverage area IA of the cell, i.e. the coverage area (corresponding to the area CA in Figure 1) that would have been achieved had the CCCH signal transmission power been set taking into account cell mobile density and capacity in the cell. Thus, the actual coverage area CA' is sufficiently large that, were every mobile station in that area to select the cell concerned, a desired capacity level in the cell would be exceeded.

The intended coverage area IA, on the other hand, is small enough that it is expected that every mobile station in that area can select the cell concerned without exceeding the desired capacity level.

In this embodiment, the cell of the Figure 2 13TS is designated a "restricted cell" by the network, and the CCCH signals transmitted by the Figure 2 BTS include a correction message defining a "transmit power is correction factor" assigned by the network to the cell (referred to hereinafter as the correction factor (DO The correction f actor a is then registered by each mobile station in accordance with the received correction message. A mobile station wishing to set up a call or to "acquire" the network or to perform hand off (either hard or soft hand-off) can then use the correction factor a, in conjunction with a locally determined measure of received CCCH signal power, to identify a best-serVing baseptation.

For example, each mobile station calculates a modified or corrected measure of received CCCH power, representing what the received power would have been if the CCCH signals had been transmitted with reduced power commensurate with the intended coverage area IA, using the equation:

R-'CCCCH modii - fied _," RXCCCH_actual The modified measures RXCCCH-Modifi.d f or a set of candidate base stations within receiving range of the mobile station are then compared and the best-serving base station is identified. In this way,' the intended coverage area IA of each cell, as perceived by the mobile station, is restricted to comply with the defined capacity requirement of the cell.

As illustrated in Figure 2, a mobile station MS1 within the intended coverage area IA receives the CCCH signal with greater power than at the same position in the Figure 1 cell, but after subtracting the correction factor a from the actual received power measure arrives at substantially the same modified power measure as the actual power measure in the Figure 1 case. Thus, after comparing the modified measures RXCCCH modified for its set of candidate base stations, MS1 selects the base station of the illustrated cell as in the Figure 1 case. MS2, as discussed above, receives the CCCH signals reliably from the base station. After comparing the modified measures Rxcc,,, modified for its set of candidate base stations it too selects that base station and starts communicating with the network through that base station. MS3, outside the intended coverage area IA but within the actual coverage area CA', now receives the CCCH signals from the base station but its modified measure RxcccH modified for the Figure 2 BTS (produced by subtracting the correction factor a from the actual received power measure for that BTS) is low enough that, after the comparison with the modified measures for the other candidate base stations of MS3, MS3 decides not to select the cell of the Figure 2 BTS. Effectively, MS3 determines, by using the correction factor a, that it lies outside the intended coverage area IA. Accordingly, as before, MS3 will select another base station, different from the one shown in Figure 2, to start communicating with the network.

Incidentally, it will be appreciated that not all cells will need to be designated as restricted cells.

For example, rural cells may not need to be designated as restricted cells. For such non-restricted cells there is no need to assign any correction factor at all to the cell. When comparing the received power measures for its set of candidate base stations, the mobile station can simply use the actual received power measure RX=Hactuai (rather than any modified power measure RxcccHmodified) for any base station whose cell has no assigned correction factor, and the modified received power measure RXCCCHmodified for any base station whose cell does have such an assigned correction factor. Alternatively, all cells may have an assigned correction factor but the factor can be simply set to zero for non-restricted cells.

Figure 3 is a block diagram showing parts of a base station 1 and a mobile station 2 according to one embodiment of the present invention. Only the parts which are different from a conventional base station or mobile station will be described in detail. In this embodiment, the other parts can be typical of a base station or a mobile station which operates in a W-CDMA (wideband code-division multiple access) cellular network.

In this embodiMent, the base station 1 comprises a correction factor setting unit 10, a correction me ssage generating unit 11, and transmitter circuitry 12. The correction factor setting unit 10 has an input operatively connected to the network (e.g. to a base station controller BSC of the network) for receiving therefrom a control signal UPDATE and also has an output connected to an input of the correction message generating unit 11 for applying thereto a correction factor a. The correction message generating unit 11 is connected to the transmitter circuitry 12 for applying a correction message CM thereto.

The mobile station 2 comprises receiver circuitry 20, a correction message receiving unit 22, a plurality of received power modifying units 24, to 24,, and a cell selection unit 26. Here, n is a predetermined integer equal to the maximum number of candidate base stations from which the mobile station can select in a cell selection operation. For example, n may be 6 or 12.

The correction message receiving unit 22 has an input connected to an output of the receiver circuitry for receiving therefrom a correction message CM.

The correction message receiving unit 22 has an output connected a first input of each received power modifying unit 24 for applying thereto a correction factor a. Each received power modifying unit 24 also has a further input connected to an output of the receiver circuitry 20 for receiving therefrom a measure RXCCCHActual. An output of each received power modifying unit is connected to an input of the cell selection unit 26 for applying thereto a measure RXCCCHmod1fied operation of the Figure 3 apparatus will now be described.

In this embodiment, as described previously with reference to Figure 2, an intended coverage area IA of the cell associated with the base station 1 is effectively defined'by assigning an appropriate transmit power correction factor a to that base station. The cell has an actual coverage area (within which the CCCH signals used for cell selection can be received reliably) greater than the intended coverage area IA. The mobile station 2 employs the respective correction factors a assigned to the different candidate base stations, as well as the actual received power levels from those base stations, when selecting a best-server base station from amongst the candidate base stations. This makes it possible for each base station to transmit at maximum power whilst preventing a mobile station from selecting as its best-server base station a base station for which the mobile station is outside the intended coverage area IA.

The correction factor a could have a static value for each base station and therefore the correction factor setting unit 10 could be, for example, a register holding a constant value. However, in the Figure 3 embodiment, the correction factor a is variable so that cells can grow and shrink with time.

In this embodiment, the correction factor setting unit 10 receives from time to ti me the control signal UPDATE from the network, indicating that the currently held value of the correction factor a is to be updated.

Updating the correction factor a could be triggered by human intervention, for example an operator deciding that the correction factor a should be increased or decreased. Alternatively, the value of the correction factor a might be updated automatically and depend on one of or'a combination of factors such as the time of day, the number of users currently being handled by the cell or the quality of service currently being offered by the base station. The time of day is a potentially relevant factor because expected traffic patterns in a cell may vary in a predictable manner over the course of a day, for example.on.commuter.routes 2S The correction factor setting unit 10 outputs its currently-held value of the correction factor a to the correction message generating unit 11. The correction message generating unit 11 generates a correction message CM, providing information relating to the assigned correction factor a, for transmission and then outputs it to the transmitter circuitry 12. The transmitter circuitry 12 broadcasts the correction message CM via one or more of its common control channels CCCH, for example via the so-called "broadcast" channel which, in a wideband CDMA network, is a primary common control physical channel PCCPCH.

Signals of this common control channel are transmitted at the maximum transmit power that the base station can transmit, for example 3W.

The correction message CM transmitted can take any number of forms. For example, it could be an a increment or decrement message for instructing each receiving mobile station to increment or decrement by a fixed step size its current a value. Alternatively, the correction measure CM could embody an absolute value of a, or a codeword representing a, or a difference from a nominal a value preset in the mobile station. Such a nominal value of the correction factor a might correspond to a standard cell size of a cell in the network, especially if most of the cells are the is same size. In this case, the correction message would only need be transmitted by the base station if the a value assigned by the network to the base station becomes different from this nominal value.

Signals, including the common control channel signals, transmitted from any base station within receiving range are received by the receiver circuitry in the mobile station 2. Up to n such base stations are taken as a set of candidate base stations by the mobile station 2 at'any one time, and each candidate base station is allocated a corresponding one of the n received power modifying units 241 to 24,. As well as performing the normal receiver circuitry functions, the receiver circuitry 20 outputs to the correction message receiving unit 22 any correction message CM identified in the received CCCH signals. The correction message receiving unit 22 decodes the correction message CM and determines a new value of the correction factor a for the candidate base station from which the CCCH signals were received. The new a value is output to that one of the received power modifying units 241 to 24, that corresponds to the candidate base station concerned.

The receiver circuitry 20 also determines, for each candidate base station, the power of the received CCCH signals and outputs this as the measure Rxcccv_A,1111 to the corresponding received power modifying unit 24, to 24,.

Each received power modifying unit 24 calculates the modified power value measure RXCCCHmodified for its corresponding candidate base station by subtracting the latest value of a for that base station, supplied by the correction message receiving unit, from the measure RXCCCH Actual for that base station provided by the receiver circuitry 20.

The received power modifying units 24, to 24, output their respective modified received power measures RXCCCH modified to the cell selection unit 26.

Incidentally, if a candidate base station does not have an assigned correction factor a (e.g. because its associated cell is a rural cell), instead of outputting a modified received power measure Rxcccsmodified for the cell, the received power modifying unit outputs the actual received power measure RXCCCH actual for that cell.

When the mobile station is required to make a cell selection (selection of best-server base station), the cell selection unit26 compares the modified received power measures RXCCCH modified (or, in the case of any candidate base station having no assigned correction factor, the actual power measure RXCCCH actual) and selects the cell of the candidate base station that has the highest measure amongst the set of candidate base stations. It then outputs the SELECT signal identifying the selected cell (or base station) Such a cell selection operation is required, for example, as part of a call setup process, network acquisition process, cell roaming or cell camping. Such a cell selection operation is also required for hand-off (soft or hard hand-off). In the case of soft hand-off two or I more best-server base stations may be selected for use simultaneously in the cell selection operation.

In another embodiment of the present invention, different correction factors a, a. correspondinj respectively to different types of services or traffic (e.g. voice and data) can be assigned to each base station. Accordingly, the cell can have different intended coverage areas for different types of service.

It will also be.appreciated that the correction f actor a does not have to be a transmit power correction factor a In another embodiment, it is simply a correction factor which influences the cell selection process in the mobile station. For example, a base station assigned a higher correction factor is less likely to be selected by a mobile station than one transmitting a lower value of the correction factor.

In one embodiment, the correction factor a may be increased each time a user (mobile station) sets up a call with that base station and decreased when a call is terminated, in which case the cell selection is still influenced in such a way as to tend to inhibit selection of the cell by mobile stations in the actual coverage area of the cell. In this case,however, it can be seen that the correction factor does not correspond to an intended coverage area; instead it corresponds to some extent to an intended number of users and sets the desired capacity level in terms of such a number.

In another embodiment, the correction factor could be set in dependence upon the number of spare codes remaining to be allocated to mobile stations in the cell in a CDMA network. This would be useful as it would enable the desired capacity level of the cell to be a code resource allocation capacity level of the cell. In a TDMA network, the correction factor could be set in. dependence upon the number of spare timeslots remaining to be allocated to mobile stations in the cell. The correction factor could also be set in dependence upon congestion levels in fixed-network communication paths used to limit the base station to the remaining parts of the cellular network, for example a base station controller (BSC). The correction factor could also be set in dependence upon a level of spare processing capacity in the base station, for example the spare processing power of a digital signal processor (DSP) of the base station.

Embodiments of the invention can be used in any situation in which it is desired to make a cell a "restricted" cell for cell selection purposes so that, even though the cell has a sufficiently large actual coverage area that a desired capacity level in the cell would be exceeded were every mobile station in that actual coverage area to select the cell, the selection of the cell is restricted in some way to avoid the level being exceeded. For example, the consequence of exceeding the level could be that the cell becomes interf erence- limited, i.e. increasing the transmission powers does not lead to improved signal reception. By influencing at least one mobile station in the actual coverage area not to select the restricted cell, when in the absence of the correction factor it would have selected that cell, the correction factor- can enable the transmission power to be set high enough to ensure proper indoor penetration without the breach of the desired capacity level that would otherwise accompany the enhanced actual coverage area, especially in small urban cells.

The present invention is not limited to the details of the cell selection method provided in the embodiments described above. For example, the cell selection made by the mobile station is not limited to being based on the received power or received signal strength of the CCCH signals transmitted by each base station for cell selection purposes. Instead of, or in addition to, using the received power, other properties of the received signal could be measured, for example a bit or frame error rate. Alternatively, if the signals transmitted by the base stations for cell selection purposes include further information (e.g. a BCCH message) specifying a transmission power of the signals concerned, the mobile station can use this information, together with the received power of those signals, to calculate a path loss for the signals concerned. Such a path loss measure is a useful measure to use for cell selection purposes, as it enables selection of cells f or which the path loss is high to be avoided, potentially reducing multi-user-interference in the network as a whole. Thus, any suitable cell selection criteria can be used in embodiments of the present invention.

It will also be understood that it is not necessary that each base station transmits the assigned correction factor of its own cell alone. For example, each member of a group of adjacent base stations could transmit all of the assigned correction factors for the cells of that group'of. base stations. Alternatively, a 2S base station having an assigned correction factor may not transmit that factor so long as another adjacent base station has a sufficient actual coverage area to ensure that the assigned correction factor concerned is available throughout the other base station's cell.

It will also be understood that it is not necessary for the transmission power of the CCCH or other signals used for channel selection purposes to be set to a maximum power level or even to a fixed (constant) power level. The transmission power could be adjustable in use of the network.

Also,, although the foregoing description has referred to "cells", with each base station having an associated cell, it will be understood that in a network in which a base station at a particular site has plural "sectors" (whether or not those sectors are said to belong to the same cell), an individual sector can be designated a restricted sector by assigning it a correction factor. Thus, references to "cell selection" and "cell" should be interpreted as covering sector selection and sector as well, and wherever hand off is mentioned, this can be sector hand-off (sometimes called "softer hand-off").

Although the present invention has been described above in relation to a wideband CDMA network, it will be appreciated that embodiments of the invention are applicable to other networks as well. These networks could be other CDMA networks such as an IS95 network.

These networks could also be, or be adapted from, other cellular mobile communications networks not using CDMA, for example networks using one or more of the following multiple-access techniques: time-division multiple access (TDMA), wave length- division multiple access (WDMA), f requencydivision multiple access (FDMA) and space-division multiple access (SDMA).

Although embodiments of the invention have been discussed as having distinct units, such as the received power modifying unit, those skilled in the art will appreciate that a microprocessor or digital signal processor (DSP) may be used in practice to implement some or all of the functions of the base station and/or the mobile station in embodiments of the present invention.

Claims

CLAIMS:

1. A cellular mobile communications network including a plurality of base stations, each having an associated cell, and a plurality of mobile stations, wherein:

each cell has an actual coverage area within which a predetermined signal, transmitted by the base station of the cell for use by mobile stations located in that cell for cell selection purposes, is receivable; at least one of the said cells is a restricted cell whose said actual coverage area is sufficiently large that a desired capacity level in the cell would be exceeded were every mobile station located within is that actual coverage area to select that cell, and the or each such restricted cell is assigned a correction factor for cell selection purposes; and each mobile station is operable, when located within the said actual coverage area of such a restricted cell, to employ the assigned correction factor for that cell to influence its cell selection such that at least one mobile station located within that area, which in the absence of the assigned correction factor Would select that cell, does not select that cell.

2. A network as claimed in claim 1, wherein the or each restricted cell also has an intended coverage area, smaller than the said actual coverage cell of the cell, within which it is expected that every mobile station can select the cell without the said desired capacity level being exceeded, and the correction factor assigned to the or each restricted cell serves to define the said intended coverage area of the restricted cell; and each mobile station is operable, when located within the said actual coverage area of such a 1 restricted cell, to employ the assigned correction factor for that cell in its said cell selection such that the mobile station does not select said restricted cell if it is outside the said intended coverage area of that cell.

3. A network as claimed in claim 1 or 2, wherein:

each said mobile station is operable to take one or more said cells, in whose respective actual coverage areas it is located, as respective candidate cells for its said cell selection, and to produce, for the or each said candidate cell, a measure of a predetermined property of the said predetermined signal of that cell, and to employ the said measure for the or each said candidate cell, together with its assigned correction factor, if any, to select one of the said candidate cells.

4. A network as claimed in claim 3, wherein the said measure is an actual power measure representing a power at which the said predetermined signal is received by the mobile station.

5. A network as claimed in claim 4, wherein the said correction factor is a transmit power correction factor and each said mobile station is operable to produce, for each said candidate cell that is such a restricted cell, a modified power measure based on the actual power measure and on the transmit power correction factor for the cell concerned, and for each such restricted cell the modified power measure is used in place of the actual power measure in the said cell selection.

6. A network as claimed in any preceding claim, wherein the base station of the or each restricted cell transmits its said predetermined signal at a preselected fixed power.

7. A network as claimed in claim 6, when read as 1 appended to claim 2, wherein the said preselected power is set high enough to facilitate reception of said predetermined signal throughout indoor environments within the said intended coverage area.

8. A network as claimed in claim 6 or 7, wherein the said preselected power is a maximum power at which the base station concerned is capable of transmitting.

9. A network as claimed in any preceding claim, wherein information relating to the or each assigned correction factor is transmitted to the mobile stations from one or more base stations.

10. A network as claimed in any one of claims 1 to 8, wherein each base station transmits information relating to the correction factor assigned to its own cell only.

11. A network as claimed in claim 9 or 10, wherein the or each said base station that transmits the said information transmits that information via a common control channel whose signals are receivable by any mobile station located in the said actual coverage area of the cell of that base station.

12. A network as claimed in any preceding claim, wherein the said predetermined signal is transmitted by each base station via a common control channel whose signals are receivable by any mobile station located in the said actual coverage area of the cell of that base station.

13. A network as claimed in any preceding claim, further including correction factor control means for enabling an operator of the network, during use of the network, to selectively designate one of the said cells as being such a restricted cell and/or to selectively adjust the correction factor previously assigned to such a restricted cell.

14. A network as claimed in any one of claims 1 to 12, further including correction factor control' means operable, during use of the network, to automatically designate one of the said cells as being such a restricted cell and/or to automatically adjust the correction factor previously assigned to such a restricted cell.

15. A network as claimed in claim 14, wherein the correction factor control means carry out the said designation and/or adjustment taking into account different expected traffic conditions at different respective times.

16. A network as claimed in claim 14 or 15, wherein the correction factor control means carry out the said designation and/or adjustment taking into account one or more of the following parameters:

quality of service provided by the base station of the cell concerned to mobile stations communicating therewith; the number of such mobile stations; respective transmission rates of services between the base station and such mobile stations; a level of congestion in a fixed-network communication path used to connect the base station to remaining parts of the said network; a level of spate processing capacity in the base station; the number of spare codes remaining to be allocated by the base station to mobile stations of the cell in the case in which the network is a code division multiple access network; and the number of spare timeslots remaining to be allocated by the base station to mobile stations in the case in which the network is a time-division multiple access network.

17. A network as claimed in any preceding claim, wherein the said cell selection is used in a call set up process of the network and/or a network acquisition process and/or a cell camping process and/or a cell roaming process and/or a hand-off process.

18. A network as claimed in any preceding claim, wherein at least one said restricted cell is assigned a plurality of such correction factors corresponding respectively to different service types, and each mobile station is operable, when a service of one of the said service types is to be used in the restricted cell concerned, to employ in its said cell selection the assigned correction factor corresponding to the said one service type.

19. A base station, for use in a cellular communications network including at least one further base station and a plurality of mobile stations, the claimed base station and the or each said further base station each having associated therewith a cell, each cell having. an actual coverage area within which a predetermined signal, transmitted by the base station of the cell for use by mobile stations located in that cell for cell selection purposes, is receivable, the claimed base station including:

correction factor information transmitting means operable to receive a correction factor for cell selection purposes, assigned by the network to one of the said cells that is a restricted cell whose said actual coverage area is sufficiently large that a desired capacity level in the cell would be exceeded were every mobile station located within that actual coverage area to select that cell, and further operable to transmit information relating to the said assigned correction factor to mobile stations located in its associated cell for use thereby in cell selection.

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20. A base station as claimed in claim 19, wherein the said information transmitted by the correction factor information transmitting means relates to the correction factor assigned to the cell of the claimed base station only.

21. A mobile station, for use in a cellular communications network including a plurality of base stations, each having associated therewith a cell, each cell having an actual coverage area within which a predetermined signal, transmitted by the base station of the cell for use by mobile stations located in that cell for cell selection purposes, is receivable, and at least one of the cells being a restricted cell whose said actual coverage area is sufficiently large that a is desired capacity level in the cell would be exceeded were every mobile station located within that actual coverage area to select that cell, the claimed mobile station including:

correction factor information receiving means for receiving from the network information relating to a correction factor for cell selection purposes, assigned by the network to such a restricted cell, and operable to derive the said correction factor from the received information; and cell selection means operable, when the mobile station is located within the said actual coverage area of that restricted cell, to employ the assigned correction factor for that cell to influence cell selection.

22. A cell selection method, for use in a cellular communications network in which each cell of the network has an actual coverage area within which a predetermined signal, transmitted by a base station of the cell concerned for use for cell selection purposes by mobile stations of the network located in the cell, is receivable, in which method:

-26 at least one of the said cells is designated a restricted cell whose said actual coverage area is sufficiently large that a desired capacity level in the cell would be exceeded were every mobile station located within that actual coverage area to select that cell, and the or each such restricted cell is assigned a correction factor for cell selection purposes; and each mobile station, when located within the said actual coverage area of such a restricted cell, employs the assigned correction factor for that cell to influence its cell selection such that at least one mobile station located within that actual coverage area, which in the absence of the correction factor would select the restricted cell, does not select that is cell.

23. A cellular mobile communications network substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

24. A base station of a cellular mobile communications network substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

25. A mobile station for use in a cellular mobile communications netw6rk substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

26. A communications method for use in a cellular mobile communications network substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

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