GB2359453A

GB2359453A - Method of allocating slots in a TDMA cellular communication system having picocells

Info

Publication number: GB2359453A
Application number: GB0003564A
Authority: GB
Inventors: Davood Molkdar; Thomas Quirke
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2000-02-17
Filing date: 2000-02-17
Publication date: 2001-08-22
Also published as: GB0003564D0

Abstract

System control logic (132) operates such that stationary (or slow moving) subscriber units (110, 111, 113, 114) are assigned to time-slots 0, 1 and 7 in a time division multiplex pico cellular-type communication system (100). Relatively fast moving mobile units (112) will be placed in time-slots 2-6. Moreover, time-slots 0, 1 and 7 are preferably left vacant, system loading permitting, with intra-cell handoff between time-slot channel resources actioned to ensure (preferably) non-utilisation of time-slots 0, 1 and 7. In the event that deemed stationary subscriber units are allocated to time-slots 0, 1 and 7 (or any time-slot for that matter), then uplink measurements therefrom are reduced. In this way, non-continuous downlink systems avoid excessive re-tuning of transceiver equipment required to monitor uplink traffic in adjacent cells, especially when such re-tuning is detrimental to monitoring of broadcast control channel (BCCH) information allocated, for example, to time-slot zero of a dedicated BCCH carrier.

Description

2359453 Apparatus and Method of Reducing Uplink Measurements in a Cellular

Communication System

Background to the Invention

This invention relates, in general, to a cellular communication system and is particularly, but not exclusively, applicable to a multi-cellular environment in which a plurality of relatively small radius cells are overlaid by a larger cell that effectively administers call control through the use of a single dedicated control channel (i.e. the so-called pico, cellular capsule concept). More especially, the present invention relates to a method of reducing uplink measurements from a mobile transceiver within a pico-cellular environment.

Summary of the Prior Art

With increasing demand for cellular services, communication system (such as the Global System for Mobile communication, GSM) are now employing multilayer cellular techniques in which an umbrella (or macro) cell overlays a plurality of smaller cells. Generally, the aim of this particular configuration is to enhance capacity, to increase spectral efficiency and to reduce administrative overhead. In the first respect, capacity is increased through the ability of the smaller cells to use different traffic (and control) channel frequencies that additional (and generally) benefit from a tighter re-use pattern than that used in the umbrella cell. Multi-layer techniques therefore directly address the limited frequency spectrum available in the wireless domain by employing frequency re-use in ever increasingly tight frequency re-use patterns.

The deployment of such multi-layer cellular techniques is likely to remain prevalent in resolving capacity issues for present and proposed cellular systems, including third generation systems such as UMTS (Universal Mobile Telecommunication System).

As will now be appreciated, a subscriber terminal may therefore be located within both a macro-cell and at least one of a micro-cell of pico- cell.

2 From an administrative perspective, however, subscriber management (both in terms of handoff and call set-up and control) is significant, with it being preferably to avoid repeated handovers; during on-going calls. Indeed, in a multilayer cellular system, characteristics of (mobile) subscriber unit operation are determined to assess whether the subscriber unit is better served by the umbrella cell or an underlying micro- (or pico-) cell. For example, to ensure call set-up, it is generally better to communicate directly with a base station and associated control channel serving a macro-cell since boundary transitions and potential loss of the control channel are less likely. Once call set-up has been established, then handoff to a micro- (or pico- ) cell can occur to avoid traffic handling congestion within the macro- cell. Of course, if the subscriber terminal is actually mobile, then its rate of micro- (or pico-) cell boundary transitions may be such that administrative overhead (through the use of dedicated control channels and the actual time required for handover to take place) is significant within the system as a whole. In fact, increasing the number of handovers in a short amount of time decreases the call reliability and increases the number of breaks in communication, thus reducing the quality of communication and the perceived quality of the service. Indeed, in extreme cases of excessive handover, calls can be lost through the inability of the system to produce a stabile communication channel environment. Fast moving mobile subscriber units therefore generally warrant a more stable call environment offered by the larger cell radius of the serving cell.

With respect to pico-cellular systems, these are employed in relatively small but densely populated serviceable areas, such as within buildings, between floors of buildings and in indoor environments generally. Picocells therefore have cell radii of between about ten and fifty metres and hence utilise a frequency re-use pattern for associated traffic channels that repeats over a significantly smaller distance than a corresponding re-use pattern for traffic channels in either a micro-cellular or macro-cellular scheme. In other words, successively smaller cells generally benefit from statistical frequency multiplexing to obtain tighter frequency re-use schemes.

3 Infrastructure cost and deployment are also issues of significant concern in a multi-layer cellular communication system. Indeed, when one considers pico- cellular systems, it is clearly undesirable for each pico-cell to contain a fullyfledged base station having all base station signal processing functionality. Consequently, systems have been developed that deploy transceiver heads, with limited processing capabilities, in each pico- cell. Clearly, by reducing base station complexity, both base station size and overall cost are reduced and so deployment becomes less obtrusive and more commercially viable.

In one particular cellular system (the so-called capsule concept), a plurality of adjacent serviceable cells are assigned traffic channel carriers according to a regimented (or sequenced) frequency re-use pattern but no dedicated individual control channel carriers for individual cells. In other words, a single broadcast control channel (BCCH) overlays a multitude of pico-cellular type traffic channels, with individual pico-cellular base station heads operating in a simulcast mode to transmit the BCCH control information. Effectively, therefore, a single wide-area BCCH services a plurality of cellular capsules, with the BCCH transmitted in the downlink from, effectively, a global base station. On the other hand, TCHs are distributed over a serviceable area by providing localised coverage in small cells referred to as capsules. This form of system design is relatively easy to implement, especially in an indoor environment, and is not wasteful of frequency resources that would otherwise be required in the provision of individual BCCHs in individual cells according to a BCCH frequency re-use pattern. However, with a unitary but generally simulcast BCCH, the system as a whole is unable to benefit from being able to continuously monitor uplink transmissions, since BCCH transmissions are curtailed and are therefore effectively noncontinuous across the cellular service area.

As will be understood, the BCCH control information is actually associated with many aspects of a cellular call, including channel allocation and handover. Moreover, a conventional BCCH channel is actually part of a control channel (CC) multiframe, with each CC frame being time division multiplexed (TDM) into 4 eight time-slots (in the specific instance of GSM) and wherein the BCCH is assigned to time-slot zero (TS-0s). The BCCH is therefore realised by a combination of bursts in successive CC frames made up from various channels 5 having specific and dedicated uses.

Having regard, in general, to the architecture of the BCCH carrier, although TS-0 is only ever used for control purposes, the remaining timeslots on the BCCH carrier (i.e. TSABCCH to TS-713CCH) may be either: i) assigned to support traffic (which is the predominant default operation), ii) dynamically assigned to provide additional control channel capacity in a fully-loaded system, or iii) to transmit dummy bursts (preferred option).

In certain environments, it will be appreciated that a single carrier cell anticipates low usage (i.e. small Erlang or traffic unit quantities) and therefore may simply make use of the BCCH carrier for both control and traffic purposes. A multi carrier cell will, in contrast, also contain specific traffic carrier frequencies assigned to provide reasonable system access and service for a multiplicity of user terminals.

With an overlaid BCCH (although generally realised by local simulcast broadcasts from base station heads) and a plurality of pico-cells, noncontinuous down link transmissions are advantageous since the environment supports a higher (i.e. better) carrier to interference (ClI) ratio. In other words, downlink interference is only caused periodically by intermittent data traffic. With improved C/1, the system is generally better able to support data transfer, such as required in the Group Packet Radio System (GPRS). Moreover, in data services, it is preferable to have a good C/I since this reduces the requirement for overhead associated with forward error correction coding and hence results in a system that has a corresponding increase in data traffic throughput. However, the improved C/1 environment is obtained at a cost offset related to overall system knowledge. In conventional cellular communication systems, handover decisions make use of data gathered by mobile units in determining a distribution of control and traffic responsibilities; this is known as mobile-assisted handover.

The necessity for the transceiver head or BTS to monitor adjacent cells generally requires the transceiver head or BTS to tune away from the BBCH carrier frequency (and its constant duplex-spaced uplink companion for RACH) during time-slot zero JS-0) periods. More specifically, as previously described, with the BCCH carrier being generally a multiframe artefact, the transceiver head or BTS inherently must locate and lock to traffic channels in neighbouring cells, i.e. there is a re-tuning requirement for the head transceiver/BTS.

Generally, the BSS or MSC structures its handover decisions on a combination of serving cell information and adjacent neighbour-cell information acquired by transceiver heads or base transceivers (BTSs). Typically, measurements of traffic in neighbouring cells are converted to equivalent downlink measurements from which the mobiles are allocated the best serving cell. To date, BCCH head architectures use a single slow tuning receiver whose use generally results in the loss of at least two (2) time-slots during the tuning and de-tuning process, with the effected time-slots contiguous to TS-0. Alternatively, a single wide band receiver could be used within the transceiver head/BTS, although this has an undesirable cost implication since such wideband receivers are considerable more expensive than narrowband receivers and processing complexity (and hence the cost of signalling processing components) increases significantly. Effectively, therefore, the implication for a fully loaded system is that BCCH TS-0 will be deleted either due to a tuning/de-tuning process or when it is deleted for measuring neighbouring mobiles in different cells in TS-0. Indeed, as will now be appreciated, the latter situation applies even if a fast tuning receiver is employed.

There are ways to more efficiently manage time-slot zero deletion over time, but these still ultimately result in the general loss of some TS-0 periods to the transceiver head/BTS. In essence, the frequency of loosing time-slot 0 depends on the method employed to cycle through the time-slots of the BCCH. For example, rather than having to look to traffic channels in each and every successive BCCH frame, it is conceivable that one could simply monitor the traffic environment over a multiplicity of contiguous frames, but it can be shown 6 than this is statistically less effective based on averaging. Additionally, and more importantly however, for a combined multi-frame control channel (i.e. BCCH/CCCH/DCCH), the deletion (or inability to access) TS-0 of the BCCH will result in reduced capacity as RACH, SACCH and FACCH frames will generally be lost. The slow associated control channel (SACCH) conveys power control and timing information in the downlink.

It would therefore be desirable to develop a system that could more effectively 10 manage time-slot zero deletion in the BCCH, and such that a continuous downlink capsule-type transmission scheme can nevertheless be operated to provide meaningful handover decision information to the transceiver head/BTS.

Summary of the Invention

According to a first aspect of the present invention there is provided a cellular communication system having a plurality of cells administered by a broadcast control channel, the cellular communication system comprising: a cell controller for administering transmission of the broadcast control channel; and a plurality of transceiver heads coupled to the cell controller, the transceiver heads supporting transmission of the broadcast control channel in a dedicated communication channel resource and wherein each of the plurality of cells includes a transceiver head arranged to monitor uplink traffic originating from different cells and supported on traffic channel resources; the cellular communication system characterised by: means for perceiving a level of movement of a subscriber unit within the cellular communication system; and means for implementing an intra-cell handoff of a call to a traffic channel resource offset in time to the dedicated communication channel resource based on the level of movement perceived associated with the subscriber unit.

The means for perceiving the level of movement of the subscriber unit is, preferably, operationally responsive to at least one of: a mobility report generated from a subscriber unit; an assessment of subscriber unit movement 7 based on uplink measurements; and a projected movement characteristic based on at least one of historical data and an identify of a subscriber unit.

The cellular communication system preferably further comprises: means for deeming a subscriber unit to be substantially stationary; and means for reducing a regularity of assessing uplink measurements from those subscriber units deemed to be substantially stationary.

In one embodiment, the cellular communication system of claim further comprises: means, operational in a partially loaded system, for assigning subscriber units to traffic channel resources offset in time to the dedicated communication channel resource; and means, operational in a fully loaded system, for assigning subscriber units deemed to be stationary to traffic channel resources proximate to the dedicated communication channel resource.

There may be provided means for re-allocating traffic channel resources based on the perceived level of subscriber unit movement, wherein traffic channel resources aligned with and adjacent to said dedicated communication channel resource are substantially used to support subscriber units perceived to be substantially stationary.

In a second aspect of the present invention there is provided a method of structuring transmission to subscriber units of a cellular communication system having a plurality of cells administered by a broadcast control channel, the cellular communication system further comprising a cell controller for administering transmission of the broadcast control channel and a plurality of transceiver heads coupled to the cell controller and each arranged to support transmission of the broadcast control channel in a dedicated communication channel resource, wherein each of the plurality of cells includes a transceiver head, the method comprising: at a transceiver head in a first cell, monitoring uplink traffic originating from different cells and supported on traffic channel resources between subscriber units and other ones of the plurality of transceiver heads; and characterised by: perceiving a level of movement of a subscriber unit 8 within the cellular communication system; and implementing an intra-cell handoff of a call to a traffic channel resource offset in time to the dedicated communication channel resource based on the level of movement perceived associated with the subscriber unit.

Preferably, the method includes the step of reducing a regularity of assessing uplink measurements from those subscriber units deemed to be substantially stationary. In another embodiment, the method further comprises: assigning subscriber units to traffic channel resources offset in time to the dedicated communication channel resource in a partially loaded system; and assigning subscriber units deemed to be stationary to traffic channel resources proximate to the dedicated communication channel resource. in a fully loaded system.

It is preferred that there is a step of re-allocating traffic channel resources based on the perceived level of subscriber unit movement, wherein traffic channel resources aligned with and adjacent to said dedicated communication channel resource are substantially used to support subscriber units perceived to be substantially stationary.

In a further aspect of the present invention there is provided a method of intracell handoff in a cellular communication system employing a dedicated broadcast control channel time-aligned with a time division multiplexed traffic carrier resource, the method comprising: assessing a mobility of subscriber terminals within the cellular communication system; in a full-loaded traffic environment, assigning those subscriber units deemed to be stationary to time division multiplexed channels of the traffic carrier resource adjacent to or corresponding with the dedicated broadcast control channel; and in a partiallyloaded traffic environment, assigning subscriber units to time division multiplexed channels remote from the dedicated broadcast control channel.

The method of intra-cell handoff may also comprise increasing a time between taking uplink measurements from those subscriber units deemed to be stationary.

9 In another aspect of the present invention there is provided a cellular communication system employing a dedicated broadcast control channel timealigned with a time division multiplexed traffic carrier resource, the cellular communication system comprising: means for assessing a mobility of subscriber terminals within the cellular communication system; means for assigning those subscriber units deemed to be stationary to time division multiplexed channels of the traffic carrier resource adjacent to or corresponding with the dedicated broadcast control channel; and means for assigning subscriber units to time division multiplexed channels remote from the dedicated broadcast control channel in a partially-loaded traffic environment.

In yet another aspect of the present invention there is provided a method of servicing a subscriber unit in a cellular communication system having a plurality of cells administered by a broadcast control channel carrier and a plurality of traffic channel resources, the method comprising:

undertaking uplink traffic measurements from subscriber units within the cellular communication system for system management purposes; estimating a level of movement of at least some the subscriber units within the cellular communication system; and reducing the frequency of undertaking uplink measurements for those subscriber units deemed to be substantially stationary.

In still yet another aspect of the present invention there is provided a cellular communication system having a plurality of cells administered by a broadcast control channel carrier and a plurality of traffic channel resources, the cellular communication system comprising: means for undertaking uplink traffic measurements from subscriber units within the cellular communication system for system management purposes; means for estimating a level of movement of at least some the subscriber units within the cellular communication system; and means for reducing the frequency of undertaking uplink measurements for those subscriber units deemed to be substantially stationary.

The preferred embodiment of the present invention therefore advantageously reduces the number of uplink measurements in a cellular communication system. Beneficially, the set-up of the operating methodology of the preferred embodiment of the present invention increases the measurement accuracy for those mobiles at risk of loosing a radio resource connection (perhaps as a result of the mobile unit in question being fast moving or overlapping adjacent cells). Additionally, the number of deletion of timeslot 0 in the BCCH is reduced, and hence the capacity of the BCCH improved. Furthermore, the speed of call set-up is increased as the number of RACH and SDCCH opportunities increases through the more frequency accessibility to TS-0 in the BCCH.

Generally, there is a placing of deemed stationary mobile units in timeslots 0, 1 and 7, while fast moving mobiles will be placed in time-slots 2-6. Time-slots 0, 1 and 7 are preferably left vacant, system loading permitting, with intra-cell handoff between time-slot channel resources actioned to ensure (preferably) non-utilisation of time-slots 0, 1 and 7. In the event that deemed stationary mobile units are allocated to timeslots 0, 1 and 7, then uplink measurements therefrom are reduced. Of course, stationary subscriber units allocated to any time-slot can also undergo reduced uplink measurements. In fact, based on system knowledge, a subscriber terminal (i.e. a unit address known to have a fixed location) could preferably always be assigned to time-slots 0, 1 or 7 when system loading meant that no other time-lost channel resources were available; this may then result in improved system operation by minimising intra-cell handoff (and hence control overhead) during call set-up or in-call situations.

Brief Description of the Drawings

Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is representative of a multi-layer cellular communication system employing an overlaid control channel (BCCH) and a plurality of traffic serving cells; FIG. 2 is a block diagram of a cellular communication system of a preferred embodiment of the present invention as typically required to assess support for the underlying inventive concept; 11 FIG. 3 is a diagrammatic representation of fluctuations in confidence in received signal strength with mobile speed having regard to a varying number of uplink measurements; and FIG. 4 is a flow diagram of an operating methodology employed by a preferred embodiment of the present invention.

12 Detailed Description of a Preferred Embodiment

As will be appreciated by the skilled addressee, a cellular radio channel resource is narrowband in nature and is characterised by short and long term fading. For a channel serving a stationary communication device, such as a terminal, long term variations of the channel can be assumed quasi-static, i.e. there are only small variations in the mean signal level (whether this is reviewed on a received signal strength basis of bit error rate (BER) basis). For a moving mobile unit, however, the mean signal level can change over short distances, with short-term fading (and particularly extreme cases thereof) generally characterised by Rayleigh distribution.

Turning to FIG. 1, there is shown a multi-layer cellular communication system 10 employing an overlaid control channel (BCCH) 12 and a plurality of traffic serving cells 14-60; this system may be adapted to support the concepts of the present invention. The cellular system includes at least one base transceiver station (BTS) (head) controller 62-64 coupled to provide operational control and synchronisation to transceiver heads (such as head 66 in pico-cell 36) in each of the cells 14-60. If multiple BTS (head) controllers 62-64 are implemented in the cellular communication system, then interconnection of the heads through, typically, a wireline or optical interface ensures that consistent system operation is maintained, as will be readily appreciated. Each transceiver head 66, which typically has limited signal processing functionality to reduce head size and complexity, is coupled to a serving BTS (head) controller 62-64 through a suitable communication resource (again typically realised by a wireline connection, such as a coax, optical fibre, twisted pair or local area network). The BTS (head) controller(s) 62-64 is/are coupled, ultimately, into an MSC or the like and then into a comprehensive telecommunications network, such as an extended cellular network, a broadband network (e.g. an asynchronous transmission mode (ATM) domain) or a trunked network.

In deployment, the pico-cellular configuration of cells could, for example, be arranged to service a particular floor or floors of a shopping complex or building, 13 or a commercial group within an office suite. As regards operation, the transceiver heads 66 of each cell generally operate in a simulcast mode of operation to establish the single control channel coverage indicated in the diagram. Generally, as will be appreciated, each transceiver head has a low power output both in terms of the control channel and associated traffic channels, with the network of pico-cells 14-60 co- operating to produce a merging of service areas (both traffic and control) at adjacent cell boundaries to provide area specific coverage. Interactions, such as call set-up procedures and handover, between subscriber units within the cellular system 10 and the transceiver heads 66 (as well as functional partitioning between the transceiver heads 66 and the BTS (head) controller for signal processing requirements) will be readily appreciated by the skilled addressee.

FIG. 2 is a block diagram of a cellular communication system 100 of a preferred embodiment of the present invention as typically required to assess support for implementing the underlying inventive concept (expressed in detail later in relation to the flow diagram of FIG. 4). For the sake of clarity, FIG. 2 shows a restricted view of a typical cellular system, such as that shown in FIG. 1.

Basically, each pico-cell 102-108 will include a varying number of subscriber units 110-114, some of which may be mobile (such as mobile phones 110, 113 114) and some of which may be fixed equipment, such as an infrared-cou pled computer modem 111. Clearly, on an aperiodic basis, some cells may contain no mobiles (which is generally unusual), whereas other cells may contain mobiles which are simply not active, i.e. that are in an idle/sleep mode (such as mobile 114) and hence are affiliated to the system but generally do not generally contribute to traffic or interference. Generally, however, each pico-cell will regularly service a plurality of subscriber units on a TIDIV1 basis (in the specific instance of GSM), with the subscriber units operationally responsive to the BCCH and providing uplink traffic 116 on dedicated traffic channel resources (e.g. specific time slots on allocated frequency carriers). For the sake of completeness, each pico-cell 102-108 is shown having a transceiver head 120 126 coupled to a BTS (head) controller 130 administering operational control of the system, in general. In a capsule-type environment, each cell will normally 14 contain two transceiver heads; one for continuous downlink BCCH transmissions and the other for supporting traffic.

It has been identified that, in a pico-cellular environment (e.g. a system having a high density of cells and a relatively high number of users), the majority of mobile units appear to remain stationary during a call that necessarily requires use of a traffic channel. Strictly speaking, whilst a mobile unit is stationary, the channel is not entirely static and fluctuation in signal level occur because of multi-path and a dynamically varying local physical environment. As such, a mobile unit can generally be considered as having some form of residual speed associated with it. Nominally, a residual speed of the order of about 0.2 metres per second (0.2ms-1) qualifies the mobile unit as a slow moving mobile, and such a slow moving mobile will have a mean signal level that varies insignificantly with time. In contrast, moving mobile unit (having a speed of say 1.Oms-1 generally encounter a more rapid short term fading variation with either a graceful reduction or sudden change in the mean level of the signal.

An assessment of the perceived relative speed of mobile units is important in relation to the operating methodology of the preferred embodiment of the present invention. The speed of a mobile unit can either be implied by the BTS (head) controller (or the like) based on received signal measurements and, possibly, cell identity information or, alternatively,it can be reported to the BTS (head) controller 130 directly by the mobile unit.

With the fall and rise times (required for frequency lock and operational stabilisation) resulting in the loss of time-slots in the BCCH carrier on either side of a time-slot allocated for control channel purposes, the present invention contemplates the use of an intra-cell handoff (to more beneficial radio frequency channel resources) to support reduced uplink measurements.

To crystallise the problem that is now being addressed, it is beneficial to briefly flesh-out the scenario in which tuning and re-tuning of the BCCH for uplink measurements occurs. Assuming a fully-loaded system in which a cell of interest is surrounded by six (6) neighbouring cells each having one traffic channel. Potentially, therefore, the six traffic channels can support forty-eight (48) subscriber units within the forty-eight (48) available time-slots. This means that, in a fully-loaded system, six subscriber units are allocated to TS-0 in the various channel frequencies of the six traffic channels. In relation to transceiver head/BTS uplink measurements, re-tuning of the BCCH could be based on a simple non-dynamic and sequential approach in which, in contiguous frames, uplink measurements of traffic calls in adjacent cells is targeted to BCCH time- slots 2 to 6 in a first BCCH frame (thus using time-slots 0, 1 and 7 for re-tuning and stabilisation) and then time-slots 5, 6, 7, 0 and 1 in the next frame. Effectively, therefore, TS-0 is lost/deleted on a regular basis for BCCH control purposes. Consequently, in a GSM SACCH frame, for example, there will be approximately eleven samples for subscriber units on time-slots 0, 1, 2, 3, 4 and 7, and twenty-two samples for subscriber units on time-slots 5 and 6. This, however, means that time-slot 0 is not available for half of the time, and it is this that is generally unacceptable to the system operator. By employing a dynamic (i.e. "on the fly") approach based on the number of active mobiles in the surrounding cells and the number of tolerable TS-0 deletions, the situation may possibly be improved. However, such a dynamic approach would still cause problems with BCCH TS-0 access and availability, even in a lightly loaded system, since there are still only four available dedicated control channels (over the entire simulcast system when considering GSM as an example) every fiftyone multiframes.

Of course, the tuning of the BCCH and uplink measurements may be carried out in various ways having regard to the sequencing of measurements of mobiles in neighbour cells.

It has been identified that it is advantageous to look to time-slot allocation to avoid loss of TS-0 in the BCCH, with the preferred embodiment of the present invention working on two approaches.

16 First, the BTS/head controller 130 of the preferred embodiment of the present invention operates to allocate traffic calls to time-slots 0, 1 and 7 only after traffic is fully packaged within the remaining timeslots 2 to 6; this process may therefore involve intra-cell handoff of calls between channel resources. Indeed, if is preferable that only those subscriber units deemed to be stationary (or having a relatively low speed) are assigned to time-slots 0, 1 and 7. In fact, it is preferable never to assign time-slot zero to traffic and only occasionally assign subscriber units to time-slots 1 and 7, although efficient system operation often precludes this.

Second, and either separately or in combination with the allocation of calls principally to time-slots 2 to 6, the number of measurements made in time-slots 0, 1 and 7 is minimised. In this second regard, characteristics of the pico-cell radio channel are determined to identify if a subscriber unit (such as a portable cellular telephone) is perceived/deemed stationary or moving (as typically perceived by control intelligence 132 within the BTS/head controller 130). If the subscriber unit is stationary, then a frequency (i.e. period icity/reg u la rity) of its uplink measurements is reduced to an extent that the stationary subscriber terminal can be serviced in TS-0. In other words, subscriber units believed by the system to be stationary are generally unlikely to suffer significant changes in tra ns miss ion/rece ptio n characteristics since their immediately physical environments remain generally unchanged. Consequently, for system operation and servicing (such as handoff determination), such deemed-stationary subscriber units do not require intensive uplink monitoring/measurements. The system (and especially the BTS/head controller can now assign these deemedstationary subscriber units to but preferably around TS-0 (in the traffic channel carrier domain), with uplink measurements of these deemed stationary subscriber units, if desired, systematically reduced over time to avoid BCCH TS- 0 deletion issues. Putting this another way, the servicing overhead of a deemedstationary subscriber unit is relatively low compared to that of a moving subscriber unit, and so uplink measurements (associated with subscriber terminals in adjacent cells and requiring the re-tuning of a transceiver head away from the BCCH frequency and TS-0 channel resource) do not need to be 17 so regularlfrequent. Hence, there is a saving of BCCH TS-0 deletions (within the system as a whole). A control algorithm based, for example, on received signal strength indication (as being indicative of subscriber unit movement) can therefore be used to increase or decrease the number of measurements per SACCH frame (in a GSM context, for example) per subscriber unit.

FIG. 3 is a diagrammatic representation of fluctuations in confidence in received signal strength with mobile speed having regard to a varying number of uplink measurements. As can be seen, and as will be now appreciated, increasing subscriber unit speed and decreasing measurement numbers results in a greater deviation in confidence (i.e. a larger standard deviation in error). However, with decreased measurements on a slow moving subscriber unit the deviation in error is generally small and, in fact, practically insignificant, yielding an ability to intra-cell handoff and/or back-off uplink measurements. Effectively, therefore, FIG. 3 shows the relation between the number of measurements required and the speed of the subscriber unit against the root mean square (rms) measurement error. On balance, results generally indicate that a tolerable window of error is obtained at a standard deviation of between about 0.7 and 1.0 (and preferably less than 0.7), implying that at least about six measurements are required for a mobile with an assessed speed of about 1. Oms-1. Clearly, the number of measurements increases or decreases in proportion with mobile speed, with the frequency of measurements generally determined using a number of input criteria such as (but not limited to):

i) the absolute signal strength of the mobile; ii) the rolling average of the received signal level (either strength of BER); and iii) time-slot allocations in neighbour cells.

The input criteria therefore effectively determine the number of measurements required for mobiles on time-slots 0, 1 and 7.

18 One preferred mechanism for determining whether a subscriber terminal is moving or stationary is realised by periodically estimating the rolling mean of received signal strength and comparing it with N previous samples therefor. Should the rolling mean of received signal strength from a particular subscriber unit drop significantly and/or fluctuate meaningfully from a substantially constant level, then that subscriber unit is likely to have moved. Furthermore, if the rolling mean dips below a predetermined threshold then this may also be indicative of movement. In more detail, a received signal level profile of a low-speed or stationary subscriber terminal is nominally a straight-line function, although there are generally slight signal levels excursions thereabout. Should the received signal level plummet in a single time sample, then this may simply be due to blocking (or loss of direct line of sight), and so it would be necessary to view a succeeding received signal strength level to meaningfully interpret such a large single change as being indicative of movement.

In addition, as indicated, it may be preferable also to utilise an absolute received signal strength as another parameter in assessing subscriber unit mobility, since absolute received signal strength may act to stabilise the number of measurements for those subscriber units that remain in the close proximity of a capsule cell.

On an assumption that, at any one time, about 40% of the in-call subscriber (mobile) units remain stationary within a pico cellular-type environment having fully-loaded neighbour cells, then implementation of the present invention yields a system in which time-slot 0 (for BCCH use) will be available almost all the time.

FIG. 4 is a flow diagram of an operating methodology employed by a preferred embodiment of the present invention. Basically, as can be seen, the underlying algorithm continually acts to move active calls away from traffic channel resources that are aligned or in close proximity to dedicated control channel resources, which may therefore require intra-cell handoff. In the event that calls are aligned or adjacent to dedicated control channel resources then upiink 19 monitoring goes to successively more restrictive levels (in an analogous way to that of a battery-powered pager or the like goes into deeper and deeper sleep modes to conserve energy). The algorithm, which may be provided to a system operator by way of a software program on a CD-ROM or the like, further operates to ensure deemed stationary subscriber units are assigned, if necessary to obtain optimum system capacity, to traffic channel resources aligned or proximate (e.g. adjacent) to the dedicated control channel resource. Calls that are on time offset traffic channel resource can undergo uplink measurements on a standard/regular basis, since the re-tuning of the transceiver head does not therefore affect receipt of control channel information of the dedicated control channel resource.

In summary, an underlying inventive concept behind the preferred embodiment of the present invention requires a determination of a mobility/movement condition of a subscriber unit, which mobility condition is assessed, for example, by monitoring a mean variation of the received signal on a per mobile basis. This mobility/movement condition, together with other parameters (such as historical data) is used to form a matrix from which a pre-loaded flow-chart based algorithm may be driven at the BTS, thus enabling the head transceiver/BTS controller to increase or decrease the number of uplink measurements particular to a mobile. The method of the present invention then acts to adjust, dynamically, the width of an observation time window, its frequency of occurrence and a number of measurements within the window on a per mobile basis. In combination, these variations in time of the transceiver head/BTS monitoring up-link traffic from adjacent cells culminate in a sub-fully loaded run system achieving a statistically significant performance improvement at no cost increase. In an independently implementable solution, a statistically determined subscriber unit (such as a mobile) goes through a intra-cell handoff to time-slot channel resources isolated in time away from TS-0, such that re-tuning of a BTS/head controller is preferably limited to time-slots that do not conflict with BCCH TS-0 monitoring.

In an ostensibly fuily-loaded system, the arrival of a new subscriber unit wishing to access the network through a RACH mechanism could immediately cause the BTS/Head controller 130 to assign the new subscriber unit to a particular traffic time slot (traffic channel resource) based on the subscriber unit's identity (e.g.

the subscriber is effectively a fixed-site terminal) or on historical data suggestive of subscriber unit operation. Generally, the historical data would be stored in memory 131 associated with the BTS/head controller 130, and would be updated regularly. In this way, intra-cell handoff could be reduced by effectively ensuring that subscriber units likely to move around are allocated to traffic 10 channel resources remote in time from any dedicated control channel resources.

It will, of course, be appreciated that the above description has been given by way of example only and that modifications in detail may be made within the scope of the present invention.

21

Claims

1. A cellular communication system (100) having a plurality of cells (1460) administered by a broadcast control channel (12), the cellular communication system (100) comprising:

a cell controller (130) for administering transmission of the broadcast control channel (12); and a plurality of transceiver heads (120-126) coupled to the cell controller (130), the transceiver heads supporting transmission of the broadcast control channel (12) in a dedicated communication channel resource and wherein each of the plurality of cells (14-60) includes a transceiver head (120-126) arranged to monitor uplink traffic originating from different cells and supported on traffic channel resources; the cellular communication system (100) characterised by:

means (132) for perceiving a level of movement of a subscriber unit (110 114) within the cellular communication system (100); and means (132) for implementing an intra-cell handoff of a call to a traffic channel resource (116) offset in time to the dedicated communication channel resource (12) based on the level of movement perceived associated with the subscriber unit (110-114).

2. The cellular communication system of claim 1, wherein the means for perceiving the level of movement of the subscriber unit is operationally responsive to at least one of: a mobility report generated from a subscriber unit; an assessment of subscriber unit movement based on uplink measurements; and a projected movement characteristic based on at least one of historical data and an identify of a subscriber unit.

3. The cellular communication system of claim 1 or 2, further comprising: means for deeming a subscriber unit to be substantially stationary; and 22 means for reducing a regularity of assessing uplink measurements from those subscriber units deemed to be substantially stationary.

4. The cellular communication system of claim 1, 2 or 3, further comprising:

means, operational in a partially loaded system, for assigning subscriber units to traffic channel resources (116) offset in time to the dedicated communication channel resource; and means, operational in a fully loaded system, for assigning subscriber units deemed to be stationary to traffic channel resources proximate to the dedicated communication channel resource.

5. The cellular communication system of claim 4, further comprising means for re-allocating traffic channel resources based on the perceived level of subscriber unit movement, wherein traffic channel resources aligned with and adjacent to said dedicated communication channel resource are substantially used to support subscriber units perceived to be substantially stationary.

6. A method of structuring transmission to subscriber units of a cellular 20 communication system (100) having a plurality of cells (14-60) administered by a broadcast control channel (12), the cellular communication system (100) further comprising a cell controller (130) for administering transmission of the broadcast control channel (82) and a plurality of transceiver heads (120-126) coupled to the cell controller (130) and each arranged to support transmission of the broadcast control channel (12) in a dedicated communication channel resource, wherein each of the plurality of cells (14-60) includes a transceiver head (120126), the method comprising: at a transceiver head in a first cell, monitoring uplink traffic originating from different cells and supported on traffic channel resources between 30 subscriber units and other ones of the plurality of transceiver heads; and characterised by: perceiving a level of movement of a subscriber unit (110-114) within the cellular communication system (100); and 23 implementing an intra-cell handoff of a call to a traffic channel resource (116) offset in time to the dedicated communication channel resource (12) based on the level of movement perceived associated with the subscriber unit (110-114).

7. The method of structuring transmission to subscriber units of a cellular communication system (100) according to claim 6, wherein perceiving the level of movement of the subscriber unit further includes: generated a mobility report at a subscriber unit and transmitting the report to cell controller; assessing movement of a subscriber unit based on uplink measurements; and projected a movement characteristic of a subscriber unit based on at least one of historical data and an identify of the subscriber unit.

8. The method of structuring transmission to subscriber units of a cellular communication system (100) according to claim 6 or 7, further comprising:

reducing a regularity of assessing uplink measurements from those subscriber units deemed to be substantially stationary.

9. The method of structuring transmission to subscriber units of a cellular communication system (100) according to claim 6, 7 or 8, further comprising:

assigning subscriber units to traffic channel resources (116) offset in time to the dedicated communication channel resource in a partially loaded system; and assigning subscriber units deemed to be stationary to traffic channel resources proximate to the dedicated communication channel resource. in a fully loaded system.

10. The method of structuring transmission to subscriber units of a cellular communication system (100) according to claim 9, further comprising:

re-allocating traffic channel resources based on the perceived level of subscriber unit movement, wherein traffic channel resources aligned with and 24 adjacent to said dedicated communication channel resource are substantially used to support subscriber units perceived to be substantially stationary.

11. A method of intra-cell handoff in a cellular communication system employing a dedicated broadcast control channel time-aligned with a time division multiplexed traffic carrier resource, the method comprising: assessing a mobility of subscriber terminals within the cellular 10 communication system; in a full-loaded traffic environment, assigning those subscriber units deemed to be stationary to time division multiplexed channels of the traffic carrier resource adjacent to or corresponding with the dedicated broadcast control channel; and in a partially-loaded traffic environment, assigning subscriber units to time division multiplexed channels remote from the dedicated broadcast control channel.

12. The method of intra-cell handoff according to claim 11, wherein the 20 broadcast control channel is discontinuous in nature.

13. The method of intra-cell handoff according to claim 11 or 12, further comprising: increasing a time between taking uplink measurements from those 25 subscriber units deemed to be stationary.

14. A cellular communication system employing a dedicated broadcast control channel time-aligned with a time division multiplexed traffic carrier resource, the cellular communication system comprising: means for assessing a mobility of subscriber terminals within the cellular communication system; means for assigning those subscriber units deemed to be stationary to time division multiplexed channels of the traffic carrier resource adjacent to or corresponding with the dedicated broadcast control channel; and means for assigning subscriber units to time division multiplexed channels remote from the dedicated broadcast control channel in a partiallyloaded traffic environment,

15. The cellular communication system according to claim 14, wherein the broadcast control channel is discontinuous in nature.

16. The method of intra-cell handoff according to claim 14 or 15, further comprising: means for increasing a time between taking uplink measurements from those subscriber units deemed to be stationary.

17. A method of servicing a subscriber unit in a cellular communication system having a plurality of cells administered by a broadcast control channel carrier and a plurality of traffic channel resources, the method comprising: undertaking uplink traffic measurements from subscriber units within the cellular communication system for system management purposes; estimating a level of movement of at least some the subscriber units within the cellular communication system; and reducing the frequency of undertaking uplink measurements for those subscriber units deemed to be substantially stationary.

18. The method of servicing a subscriber unit according to claim 17, wherein at least one of said plurality of traffic channels is substantially timealigned with the broadcast control channel.

19. A cellular communication system having a plurality of cells administered by a broadcast control channel carrier and a plurality of traffic channel resources, the cellular communication system comprising: means for undertaking uplink traffic measurements from subscriber units within the cellular communication system for system management purposes; means for estimating a level of movement of at least some the subscriber units within the cellular communication system; and 26 means for reducing the frequency of undertaking uplink measurements for those subscriber units deemed to be substantially stationary.

20. A cellular communication system substantially as hereinbefore described with reference to FIGs. 2 to 4 of the accompanying drawings.

21. A method of servicing a subscriber unit of a cellular communication system substantially as hereinbefore described with reference to FiGs. 2 to 4 of 10 the accompanying drawings.

22. A method of intra-cell handoff in a cellular communication system substantially as hereinbefore, described with reference to FIGs. 2 to 4 of the accompanying drawings.

23. A method of structuring transmission to subscriber units of a cellular communication system substantially as hereinbefore described with reference to FIGs. 2 to 4 of the accompanying drawings.