GB2339994A

GB2339994A - Collision avoidance by restricting channel access requests according to RSSI

Info

Publication number: GB2339994A
Application number: GB9816160A
Authority: GB
Inventors: Timothy Simon Klein; Anthony John Wray
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1998-07-25
Filing date: 1998-07-25
Publication date: 2000-02-09
Anticipated expiration: 2018-07-25
Also published as: GB9816160D0; GB2339994B

Description

File ref. CM00463P/Klein/GB 2339994 Radio communication system

Technical Field

The invention relates to the field of radio frequency (RF) communication systems.

Backgroun Radio frequency communication systems are often designed to have a base station, and one or more subscriber stations. The subscriber stations are usually either'mobile'orportable'radios. Henceforth, the termmobile'will be used for both of these types of subscriber. Examples of systems which have base stations and mobile radios are public mobile radio (PMR) systems, one example being TETRA trunked radio systems. Notably, radios whose location is fixed can also use these systems. Cellular telephone systems such as GSM also consist of base stations and mobile subscribers, these being mobile telephones.

In a trunked mode of operation, a mobile radio normally sets up a communication link with one of the system!s base stations in order to communicate with another mobile radio. In order to facilitate this, the communication system permits the mobile radio to transmit a request for access to the base station. The base station may make predetermined time periods available for such access requests. The base station may make other, different time periods available for actual communication between mobiles.

A mobile radio transmits its request for access to the base station in one of the predetermined time intervals which the base station has made available.

However, other mobile radios have the same predetermined time intervals available to them in which to request access to the base station. Clearly, two or more mobile radios may transmit a request for access in the same predetermined time interval, which results in a 'claslY.

2 File ref: CM00463PAKlein/GB Clashing requests are two or more simultaneous radio signals which are both received by the same base station, and which both request access. They may in fact simply manifest themselves as a combined signal of high intensity which cannot be deciphered by the base station. The usual result of this situation is that neither request, or none of the requests, is accepted. These mobile radios must then submit a request at some later time. Importantly, the clash has effectively resulted in a waste of an opportunity for a communication channel to be assigned to a mobile. This is a waste of one of the predetermined time intervals which was reserved for mobiles to request access.

When the communication system is in a steady state, mobile radios request access as and when they need it. With a sufficiently large number of mobile radios, the rate at which access requests are received by a given base station will occur at a certain mean rate. There will be some statistical variation in this rate. It may also be somewhat higher at certain times for predictable reasons, e.g. when a group of workers equipped with mobiles needs to communicate particularly often, such as at the start or finish of a work shift. The system needs to have enough predetermined time slots available for this pattern of access requests. In particular, clashes between two or more requests must be relatively rare.

However, unusual circumstances may arise which cause mobile radios to make an enormous number of requests for access to a particular base station in a very short time interval. A typical example of this might be when the system undergoes a 'soft re-stare. A soft re-start occurs when the communication system once more becomes available immediately after having been shut down. This shut down might be because of a software error, power interruption or some other unforeseen event.

An enormous number of requests for access will lead to an overload situation where there is a clash of two or more requests in a large proportion of the predetermined time periods reserved for access requests. These clashes lead to a situation where, with far more requests arriving per unit time than in 3 File ref: CM00463P/Klein/GB normal operation, virtually none of the mobiles is actually granted access. Clearly this results in very few of the mobiles being assigned communi cation channels and actually being able to communicate at exactly the time when the largest proportion need to. Almost all the individual mobiles will experience a very long wait for connection when this situation occurs. This 'saturation of the slots available for access requests in the communication system is highly undesirable.

The number of successful attempts to establish a connection per unit time under different conditions in a radio system can be visualised easily by plotting this variable on a graph. Figure 1 shows such a plot, with the rate of successful attempts at connection on the y-axis being plotted against the rate of access attempts on the x-axis. The higher the number on the x-a-,ds of figure 1, the higher the demand on the system to set up connections.

The graph of figure 1 shows a substantially linear plot towards the left of the figure. This indicates that almost all requests from mobiles to gain access to the base station are successful, because there are virtually no clashes. However, as the number of requests per unit time increases along the x-axis, the number of clashing requests per unit time increases. This causes the plot to fall away from further linear progression. As the number of clashes rises to the point where clashes occur in a significant proportion of all the time slots available for access requests, the plot falls away with a negative gradient. See the portion of the plot towards the right of figure 1.

When a radio system undergoes a soft re-start, there may be a very large number of mobiles attempting to gain access to a base station. The radio system will therefore find itself operating in a manner corresponding to the portion of the curve at the extreme right of figure 1. Most attempts by mobiles to access the base station will result in clashes with attempts by other mobiles. Very few mobiles will be successful in accessing the base station.

A prior art solution for the problem outlined above is known. This solution involves increasing the number of time slots which are available for mobiles to

4 File ref. CM00463P/Klein/GB make requests for access. This will reduce the likelihood of dashes between requests for access, for any given number of mobiles. However, the disadvantage of this arrangement is that the number of time-slots available for actual communication between mobiles is reduced. This is because the extra time slots which the system makes available for access requests from mobiles are no longer available for ongoing communication between mobiles which are in communication with one another via that base station.

A further approach to solving this problem could be to limit access to certain groups of mobile users, each group containing mobiles with some form of assigned'identity'. This technique for limiting access would involve temporarily excluding certain groups of users from making access requests. This approach has limitations. It is administratively complex, particularly because it involves dividing the users into groups. As users would need to register and re- register after periods of absence, this represents a significant overhead. There may also be users who have a very good or urgent reason to seek a connection, who might then be prevented from doing so simply because of the group to which they belong. In a TETRA system, this sub-division technique requires additional signalling beyond that specified in the standard, and thus uses up down-link capacity.

A need exists to alleviate the problems of the prior art.

Summary f the]Invention

A radio communication system in accordance with the invention, for use with a plurality of portable and/or mobile radio units, can comprise at least one base station for broadcasting a signal to a plurality of portable and/or mobile radio units, the signal defining a minimum value of a received signal strength at the location of a radio unit for the radio unit to be permitted to make a request for access to the base station, and means for varying the defined minimum value of the received signal strength in order to maximise the channel efficiency of the radio communication system.

File ref. CM00463P/Klein/GB Further features which may be employed together with such a radio commurdcation system are the subject of appended dependent claims 2-5.

A portable or a mobile radio unit in accordance with the invention, for use in a cellular radio communi cation system, can comprise means for measuring the currently received signal strength at the location of the radio unit from a base station, means for decoding a dynamically varying minimum value of the received signal strength in a communication from the base station, means for comparing the currently received signal strength with the minimum value of the received signal strength, and means for maldng an access request for assignment of a communication channel from the base station only if the currently received signal strength equals or exceeds the minimum value of the received signal strength.

A radio communication system in accordance with the present invention can comprise a plurality of portable and/or mobile radio units, at least one base station for broadcasting a signal to the radio units, the signal defining a threshold minimum value of a received signal strength at the location of a radio unit, means for varying the defined threshold minimum value of the received signal strength in order to maximise the channel efficiency of the radio communication system, the radio units comprising means for measuring the currently received signal strength at the location of the radio unit from the base station, means for comparing the currently received signal strength with the defined threshold minimum value of the received signal strength, and means for selecting one of two sub-sets of access request time slots in dependence on a comparison of the currently received signal strength and the defined threshold minimum value of the received signal strength.

Further features which may be employed together with such a radio communication system are the subject of appended dependent claims 8 and 9.

A portable or a mobile radio unit in accordance with the invention can comprise means for measuring the currently received signal strength at the 6 File ref. CM00463P/Klein/GB location of the radio unit from a base station, means for decoding a dynamically varying threshold minimum value of the received signal strength in a broadcast communication from the base station, means for comparing the currently received signal strength with the threshold minimum value of the received signal strength, means for selecting a particular sub-set of time slots reserved for access requests in dependence on a comparison of the currently received signal strength with the threshold minimum value of the received signal strength, and means for making an access request for assignment of a communication channel from the base station in an access request time slot from the particular sub-set of time slots selected by the means for selecting.

The invention has the advantage of reducing the number of clashes between competing requests for access from mobile users. Reducing this rate of clashes leads to successful assignment of communication channels to a greater proportion of the mobiles than is the case with prior art systems under the same circumstances. Therefore the communication system operates more efficiently, with lower mean delay in assignment of communication channels to mobiles.

Brief description of the drawings

Figure 1 shows a graph illustrating the number of successful requests for access per unit time, plotted against the number of requests for access per unit time, for a prior art radio system.

Figure 2 illustrates an embodiment of a radio communication system in accordance with the present invention.

Figure 3 is a simphfied illustration of a signal broadcast by a TETRA (Terrestrial Trunked Radio) base station.

Figure 4 illustrates an embodiment of a radio in accordance with the present invention.

7 File ref. CM00463P/lGein/GB Detailed description of the preferred embodiment

The invention relies on a technique which provides a means of restricting the number of clashes between competing requests for access, and therefore of increasing the number of successful requests for access per unit time within the system. This is done in a preferred embodiment of the invention by controlling the number of mobile users of a radio system who are permitted to make requests for access. This technique can be applied at times of particularly high demand, such as at times of soft re-start of the radio system.

The invention can best be understood from consideration of the illustrative embodiment outlined below in connection with figures 2-4.

Figure 2 illustrates a radio communication system. A base station 1 is shown which forms part of the radio communication system. Base station 1 may be one of a number of such base stations forming part of the radio communication system. The base station may be controlled by the switching and management infrastructure of the radio communication system. This infrastructure has been shown as element 2 on figure 2. The switching and management infrastructure is likely to be located away from base station 1. Base station 1 is shown on figure 2 as being dose to element 2 only for the purposes of simplicity of illustration.

Also shown on figure 2 are a number of users 10, 12, 14, 16, 18 and 20. These users may be either mobile or portable radios (PMR) or a mixture of both. The users 10-20 are located at different distances from the base station 1. Although not shown in this embodiment, the invention is equally applicable to a radio communication system comprising mobile telephones as users. 30 Figure 2 also shows two boundaries 30 and 32 surrounding the base station 1. These boundaries indicate the limits of regions within which the signal strength received by a user from the base station 1 is above a particular level. The signal strength within boundary 30 can be considered to be above signal 8 File ref. CM00463P/Klein/GB strength S1. The signal strength within boundary 32 can be considered to be above signal strength S2. Beyond each of these boundaries, the received signal strength falls below each of these respective levels. The boundaries 30 and 32 are not necessarily circular, due to differences in the propagation of the signal from base station 1 in various directions. These differences are caused by topographical variations over the propagation paths, such as variations in the height of the land, and/or the presence of obstacles such as buildings.

In accordance with the standards set for radio communication systems, base station 1 broadcasts a signal which all mobiles may receive. An arrangement of the signal broadcast by a TETRA (Terrestrial Trunked Radio) base station is shown in figure 3. TETRA is the ETSI standard for trunked terrestrial radio service.

The signal shown in figure 3 is divided into pre-defined periods of time. Blocks such as Block 1 and Block 2 are time periods reserved for mobile units to communicate with each other. The signal also comprises time periods for an access assignment channel (AACH), located between the blocks.

The base station broadcasts information in the access assignment channel about channel availability. In particular, the information tells mobile users which time slots are available for the mobile users to request a communication link. These slots take the form shown by slots 1,2 and 3 on figure 3. These access requests by mobile users are typically random access within the available time slots. Using this information, a mobile user knows when to transmit a request for access to the base station.

The arrangement of the invention goes beyond the basic broadcast in accordance with the TETRA standard explained above. In accordance with the invention, the base station 1 can include variable information in its broadcast signal about the minimum signal strength which a mobile must be receiving in order for it to be allowed to transmit an access request to the base station. Specifically, this information sets a minimum value for the received 9 File ref. CM00463P/Klein/GB strength of the signal which the mobile is receiving from the base station. The minimum which is set can be varied.

In operation of the inventive arrangement, the minimum value of the received signal strength which is specified sets a threshold of received signal strength at the mobile's location. The communication system can raise or lower this threshold in order to respectively decrease or increase the number of mobile units which are permitted to make access requests at that time. This acts therefore as a variable 'throttle' on the number of mobiles which are entitled to make access requests to the base station in the time slots reserved for these requests. This throttling of the number of requests for access to the communication system can be applied at any time, not merely when a failure within the system leads to a peak in demand. The base station broadcasts the defined minimum value for the received signal strength in part of the AACH portion of the broadcast signal, such as in slots 1,2 or 3 shown in figure 3.

The switching and management infrastructure section 2 of the communication system can be responsible for setting and varying the defined minimum value of the received signal strength which is incorporated into the signal broadcast from the base station. Alternatively, this can be done by the base station I or by a combination of both. The actual means for defining the minimum value can be an algorithm incorporated in software. This algorithm raises the defined minimum value if the rate of dashing of access requests exceeds the rate where a certain proportion of the available access request slots suffer clashing. MThen the rate of clashing subsides again, the algorithm lowers the defined minimum value. An alternative arrangement would be for an algorithm to simply raise the defined minimum value of signal strength for a predetermined time period after the communication system has once more become available after a shut-down or soft re-start.

Clearly, the technique described above constitutes dynamic control of the cell size around the base station. The effective cell size is a direct:ftmction of the variable value of the minimum signal strength parameter which the communication system sets.

File ref. CM00463P/lUein/GB Although a software arrangement within the switching and management infrastructure section 2 of the communi cation system is the preferred arrangement for setting the defined minimum value of signal strength, other means could perform this ftmction. Clearly for example, an application specific integrated circuit located in each base station could perform this function.

Each mobile unit is equipped with a received signal strength indicator (RSSI).

Such circuits are known to the person skilled in the art, and will not be described in detail here. The received signal strength indicator provides a measure of the strength of the signal which the mobile is receiving from the base station. Clearly therefore, each mobile unit is in possession of a measurement of the signal strength which it is receiving at any time and can compare this actual measurement with the minimum signal strength specified at that particular time in the broadcast signal from the base station.

If a particular mobile's currently received signal strength exceeds the currently specified minimum, then that mobile may make a request for access to the base station. If the currently received signal strength is less than the currently specified minimum, then that mobile is barred from making an access request at that time.

Figure 4 illustrates an embodiment of a radio 100 in accordance with the present invention. Such a radio operates as part of the radio communication system whose structure and function are described above. Radios 10-20 of figure 2 may correspond to a radio 100 of the form shown in figure 4.

Radio 100 comprises an antenna 114. The antenna is connected to receiver, or IrRX), circuitry 104. Transmission, orTx" circuitry 102 is likewise connected to antenna 114. Both the transmission stage 102 and the receiver stage 104 of the radio are connected to microprocessor 108. Microprocessor 108 is responsible for signal generation and decoding within the radio.

Also shown in radio 100 are keypad input means 112 and display 110. The keypad and display allow the user to operate and control the radio.

File ref. CM00463P/Klein/GB Within receiver circuitry 104 is a signal strength detection circuit 106, Commonly known as anRSSI', circuit 106 measures and provides a value for the strength of the signal which the radio is currently receiving. The measurement output of signal strength detection circuit 106 is supplied to microprocessor 108. Microprocessor 108 also controls the operation of the signal strength detection circuit 106.

In a preferred embodiment of the invention, the minimum received signal strength is varied dynamically by the communication system in response to the prevailing rate of receipt of requests from mobiles for access. However, it is clearly possible to operate other schemes for varying this minimum. For example, the minimum value of received signal strength could be set to a high level for a predetermined time after start up or soft restart of the communication system or the base station. After this predetermined time, the base station could then broadcast a lower specified value of minimum received signal strength at the mobile unit. This scheme would help alleviate any initial surge of requests for access after the base station first became available, and would be simple to implement.

Considering figure 1 once more, it is clear that the inventive arrangement can control the operating point of the entire radio communication system within the operating characteristic shown in figure 1. This means for example that in response to the system operating at the state depicted at the extreme right of the characteristic, i.e. being swamped with requests for access, the system can move itself back to operate at a point on the upward slope of the curve on the left of figure 1. The part of the characteristic of figure 1 with a positive gradient is the part of the characteristic where the comm 11 ni cation system is functioning most efficiently. Thus the inventive arrangement alleviates the problem of saturation of the time slots reserved for access requests.

The modification required to allow the communication system to include the feature of determining and broadcasting a variable required minimum received signal strength is possible without requiring major changes to the TETRA 12 File ref. CM00463P/Klein/GB standard. This is because the signal broadcast by the base station in accordance with the TETRA standard already contains bits for specifying a fixed value of minimum received signal strength for mobile units. However, it is emphasised here that the standard does not indicate a variable value for this minimum received signal strength. Furthermore, the standard does not envisage a variable minimum received signal strength acting as a parameter to control access requests at times of excessive numbers of such requests.

Example

The effect of the invention can be illustrated by considering the fimetion of the system of figure 2 when equipped with the arrangement of the invention. Specifically, the example explained here incorporates dynamic variation of the minimum received signal strength specified in the broadcast transmission from the base station.

In normal operation of the system of figure 2, the system is in a steady state. Some of the mobile units such as mobile units 10-20 will be in communication through the base station 1, typically with one another. Others will be switched off or simply not in use. A small number may be in the act of requesting access to base station 1. Mobile units 10-20 are illustrative of a large number of units in the system. The total number of mobile units in a real system may for example be in the range of 10-1000.

The system will be designed to handle the number of mobile units requesting access with ease in this steady state, so clashes between two or more mobile units requesting access in the same time slot will be very rare. They Will even be rare at times when the natural statistical variation in the number of users wanting to make a radio call at any one time causes a temporary peak in the number of mobile units requesting access.

In this steady state, the minimum received signal strength specified in the broadcast signal from the base station will be very low. This means that any 13 File ref. CM00463P/Klein/GB mobile unit may request access, even those such as mobile unit 20 which are situated in the regions of lowest received signal strength.

Consider now a failure of the base station 1. This will cause failure of the ongoing communications between all currently active mobile units which were using the base station 1. Assume further that the base station becomes available once more after some time has elapsed.

When base station 1 first becomes available once more, it is likely to receive a very large number of requests for access.

Some of requests for access will be from mobile units trying to reestablish their lost on-going communication. There will also be 'new' requests from mobile units which are trying to request access, but which were not in the act of communicating at the time of shut-down of the base station. In the case of an emergency network such as that operated by police or fire services, this number may be way above the number requesting a new connection in the steady state. This is because the very event which caused the base station to fail, such as a lightening strike or an earthquake, may also require activity by the emergency services. This very large number of requests for access has the consequence that a large number of these requests will clash with one another. Comparatively few of the requests, if any, are successful.

The situation of a clash between requests for access is readily detectable.

Typically, the arrival of two or more requests for access in the same request time slot from different mobile users will manifest itself as there being received signal strength at the base station, but with the base station unable to decode any received signal. The received signal strength may in fact be of a comparatively high level, due to the simultaneous arrival of the two or more clashing signals. So the base station can assume that there has been a clash of two or more requests for access when the base station detects that signal strength is present in an access time slot, but a request cannot be decoded. Thus the base station can relatively easily determine the number of time slots in which access requests are clashing.

14 File ref. CM00463P/Mein/GB In this situation, the base station will increase substantially the minimum received signal strength specified in its broadcast signal. This minimum win be set to a level which exceeds the actual signal strength likely to be received by a substantial number of the mobile stations. This can only be done to a 'likely' level because the communication system and base station 1 do not have precise details of the locations of the mobile units. They therefore know only to an approximation how many mobiles will be excluded from maldng access requests as a result of any particular value of minimum received signal strength which they set. This is not however a great disadvantage, since dynamic variation of the minimum value which is set allows an iterative approach to setting the optimum value.

In the example of figure 2, the base station could specify that a signal strength S I be received by a mobile unit before it is allowed to make a request for access to the base station. Boundary 30 indicates the regionwithin which mobiles receive signal strength S1. Mobiles units such as 16, 18 and 20 would be disqualified from making requests for access. This action would significantly reduce the number of requests for access, and thereby reduce the rate of dashes between competing mobiles. This would therefore increase the likelihood of a successful request for access from each one of those mobile units such as 10, 12 and 14 which are still permitted to make requests for access.

When a certain number of mobiles within boundary 30 have been successful in their requests to establish a communication link, they no longer need to make requests for access. Consequentially, the number of requests for access from mobile units within boundary 30 win fall. At this point in time, the base station can lower the threshold minimum received signal strength specified in the broadcast signal to level S2. Now units such as 16 and 18 can make requests for access to the communication system for the first time since the system has become available once more.

File ref- CM00463P/Klein/GB Finally, when the number of requests for access has decreased further, the base station can lower the minimum received signal strength specified in the broadcast signal to a level whereby mobile units such as 20 in the very weakest signal reception areas can make access requests. When all these mobile units which want to make requests for access have been successful, the system once more returns to the steady state. In particular, the communication system is back in the steady state when the minimum signal strength specified in the broadcast signal from the base station has reached the lowest value used by the system and the rate of clashes has reached its long-term steady state value. There is thus no longer a backlog of requests for access.

The invention described above has clear advantages. It is notable that at times when the system chooses to exclude any mobile units, the system excludes those mobile units in the weakest signal reception areas. These are the units whose signal strength received by the base station is also likely to be weakest, and most prone to interruption. Due to both of these facts, these are the mobile units which are least efficient at transmitting decipherable requests for access to the system. For the communication system as a whole, there is an advantage in excluding these mobile units. This is because these are the units which may require most attempts to get their request for access accepted, and therefore which are least efficient in their use of the available time slots for requesting access, even when no dash occurs. Because they are the mobile units whose signal is most prone to interruption and thus call drop- out during an on-going communication, they also require re-connection more often than mobile units which are in areas of higher signal strength reception. They therefore also tend to submit more requests for access, and there is clearly a further advantage to preferentially excluding them. Finally, these are the mobiles which may transmit at or close to their maximum transmission power. They therefore provide more interference in the system, e.g. to nearby mobile units, than do mobile units operating at lower power levels.

The invention has been described with a minimum received signal strength as the determining parameter for which of the mobiles may make access 16 File ref.- CM00463P/Klein/GB requests at times when some limitation is to be imposed. However, the invention could be adapted to permit access requests from mobiles which receive a signal level from the base station which is either: (i) below a given threshold maximum; or 5 (ii) within a certain range of values. This might prove advantageous when, for example, mobiles are equipped to call one another directly, without using the base station. In this case, at times of limiting access requests to the base station, the mobiles closest to the base station may be close enough to one another to be able to employ direct mode communication with one another. They can therefore be preferentially excluded from communicating through the base station.

Furthermore, the above invention has been described in terms of interaction of mobiles with a single base station. However, the invention can be extended to a further embodiment whereby the cell sizes around more than one base station are changed in response to an excessive number of requests for access to a particular base station 1. As the effective cell size is reduced around base station 1 using the principle of the invention, neighbouring base stations could increase the size of the cells from which they can accept access requests. This would temporarily re-direct demand from mobiles which are at the peripheries of the cell around base station 1 to base stations which neighbour base station 1.

In another alternative arrangement, the base station could limit access requests by varying the specified maximum power with which a mobile may transmit. Applying this technique would also serve to reduce the number of dashing access requests. The TETRA standard does allow the base station to specify the maximum transmit power of mobiles. This is however envisaged as being a constant setting, and foresees neither doing this dynamically, nor in a manner which mitigates temporary saturation of a base station with access requests.

17 File ref. CM00463P/Mein/GB Second embodiment An alternative embodiment of the invention is possible. In the arrangements described above, the base station transmits a signal which indicates the signal strength which a mobile must receive before it is- permitted to make an access request to the base station. Instead of this however, the mobile units can be sub-divided into two sub-groups using only the received signal strength information and the threshold minimum signal strength value defined in the broadcast signal. One sub-group would consist of all those mobiles which are receiving a signal at a level higher than the threshold minimum value of the received signal strength set in the broadcast signal. The other sub- group would be tho se mobiles which receive a signal below that threshold minimum value. The available access request slots would be divided into two sub-sets. One sub-group of the mobiles would be constrained to making their access requests in one of these sub-sets. The other sub-group of mobiles would be constrained to making their access requests in the other sub-set of access request timeslots. This arrangement would reduce the number of clashes within the available access request time slots, compared to prior art systems without such sub-division. It would therefore achieve a higher rate of channel assignments than prior art systems achieve.

A radio which forms part of a communication system as outlined in the previous paragraph takes the general form shown in figure 4. The microprocessor 108 compares the currently received signal strength and the threshold minimum value of the received signal strength which has been set in the broadcast from the base station. The microprocessor uses the result of this comparison for selecting a particular sub-set of time slots reserved for access requests. If the signal strength currently received is above the threshold minimum value set by the base station, then the radio uses one of a first set of access request time sub-slots when it wants the communication system to assign a communication channel to it. If the signal strength is below the threshold minimum value, then the radio uses one of a second subset of access request time slots when it wants a communication channel. The microprocessor 108 and transmit circuitry 102 make the access request.

18 File ref. CM00463P/Klein/GB In a communication system which uses the division of mobiles into two subgroups as outlined above, variation of the defined threshold minimum value of the received signal strength will vary the number of mobiles in each of the two subgroups of mobiles. Thus by the base station varying the threshold minimum value of the received signal strength which it sets in its broadcast signal, the communication system can control the number of mobile units which make their access requests in each of the two sub-sets of access request slots. To overcome the fact that the communication system does not know precisely where the mobile units are located, and therefore the signal strength which each mobile unit receives, the system can change the threshold minimum value of the received signal strength set in its broadcast signal mi order to adapt to the geographical distribution of the units. Preferably, this threshold minimum value can be adjusted iteratively. This can be done to ensure allocation of the mobile units into the two sub- groups at all times in appro)dmately the correct numbers to equalise the rates of clashing in the two sub-sets of access slots. This will cause the system to operate as efficiently as possible. This state of operation, with the same proportion of both sub-sets of access request slots suffering clashes, widl result in the operation of the system for each of the two sub-sets of access request slots being represented by substantially the same point on the graph of figure 1.

The number of mobile units requesting access to the base station will vary with time. Mobile units will also move around within the cell which is centred on base station 1, moving to regions of higher or lower received signal strength. Variation of the threshold minimum value of the received signal strength set by the base station in its broadcast signal can adjust for these changes. Thus the rates of clashing in the two sub-sets of access request slots can be kept equal.

19 File ref.- CM00463P/Klein/GB

Claims

1. A radio communication system, for use with a plurality of portable and/or mobile radio units (10-20, 100), comprising:

at least one base station (1) for broadcasting a signal (AACH) to a plurality of portable and/or mobile radio units (10-20, 100), the signal defining a minimum value of a received signal strength (SI, S2) at the location of a radio unit (10 20, 100), for the radio unit (10-20, 100) to be permitted to make a request for access to the base station (1); and means for varying the defined minimum value of the received signal strength (S1, S2) in order to maximise the channel efficiency of the radio communication system.

2. A radio communication system in accordance with claim 1, further comprising the said means for varying the defined minimum value of the received signal strength (S1, S2) forming part of the base station (1) and/or the switching and management infrastructure (2) of the radio communication system.

3. A radio communication system in accordance with claim 1 or claim 2, further comprising the said means for varying the defined minimum value of the received signal strength (SI, S2) being adapted to vary the defined value dynamically in dependence on the number of clashes between requests from radio units (10-20, 100) for access to the base station (1), thereby regulating the number of radio units permitted to make a request for access to the base station (1) at any particular time.

4. A radio communication system in accordance with claim 3, further comprising the means for dynamically varying the defined minimum value of the received signal strength (S1, S2) being adapted to set a minimum value of the received signal strength (S1, S2) which corresponds to a ma3dmum. cell File ref. CM00463P/lGein/GB size (30, 32) appropriate to the number of clashes between requests from radio units (10-20, 100) for access to the base station (1) at any particular time.

5. A radio communication system in accordance with claim 1 or claim 2, further comprising the said means for varying the defined minimum value of the received signal strength (SI, S2) being adapted to set a high value (Sl) for the said minimum value of the received signal strength (S1, S2) during a predetermined time after start up or soft restart of the radio communication system, and to set a lower value (S2) for the said minimum value of the received signal strength (S1, S2) from the end of this predetermined time.

6. A portable or a mobile radio unit (10-20; 100) for use in a cellular radio communication system, comprising:

means (106) for measuring the currently received signal strength at the location of the said radio unit from a base station (1); means (104, 108) for decoding a dynamically varying minimum value (S1, S2) of the received signal strength in a communication (AACH) from the base station (1); means (108) for comparing the said currently received signal strength with the said minimum value (S1, S2) of the received signal strength; means (102, 108) for making an access request for assignment of a communication channel from the base station (1) only if the currently received signal strength equals or exceeds the said minimum value of the received signal strength (S1, S2).

21 File ref. CM00463P/Klein/GB

7. A radio communication system, comprising: a plurality of portable and/or mobile radio units (10-20, 100); at least one base station (1) for broadcasting a signal (AACH) to the said radio units (10-20, 100), the signal defining a threshold minimum value of a received signal strength (S1, S2) at the location of a radio unit (10-20, 100); means for varying the defined threshold minimum value of the received signal strength (S1, S2) in order to maximise the channel efficiency of the radio communication system; the said radio units (10-20, 100) comprising:

means (106) for measuring the currently received signal strength at the location of the said radio unit from the base station (1); means (106, 108) for comparing the currently received signal strength with the defined threshold minimum value of the received signal strength (S1, S2); and means (108) for selecting one of two sub-sets of access request time slots in dependence on a comparison of the currently received signal strength and the said defined threshold minimum value of the received signal strength (SI, S2).

8. A radio communication system in accordance with claim 7, further comprising the said means for varying the defined threshold minimum value of the received signal strength (S1, S2) forming part of the base station (1) and/or the switching and management infrastructure (2) of the radio communication system.

9. A radio communication system in accordance with claim 7 or claim 8, further comprising the said means for varying the defined threshold minimum value of the received signal strength (S1, S2) being adapted to vary the defined 22 File ref. CM00463PAKlein/GB threshold minimum value dynamically in dependence on the number of clashes between requests from radio units (10-20, 100) for access to the radio communication system in each of the two sub-sets of access request time slots, so as to substantially equahse the proportion of access request time 5 slots in each of the two sub-sets in which clashes occur.

10. A portable or a mobile radio unit (10-20; 100) for use in a cellular radio communication system, comprising:

means (106) for measuring the currently received signal strength at the location of the said radio unit (10-20; 100) from a base station (1); means (104, 108) for decoding a dynamically varying threshold minimum value of the received signal strength (S1, S2) in a broadcast communication (AACH) from the base station (1); means (108) for comparing the said currently received signal strength with the said threshold minimum value of the received signal strength (S1, S2); means (108) for selecting a particular sub-set of time slots reserved for access requests in dependence on a comparison of the said currently received signal strength with the said threshold minimum value of the received signal strength (S1, S2); and means (102, 108) for making an access request for assignment of a communication channel from the base station (1) in an access request time slot from the particular sub-set of time slots selected by the said means (108) for selecting.