GB2357399A

GB2357399A - Control channnel utilisation within a cellular communication system

Info

Publication number: GB2357399A
Application number: GB9929857A
Authority: GB
Inventors: Thomas Michael Quirke; Howard John Thomas
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1999-12-18
Filing date: 1999-12-18
Publication date: 2001-06-20
Anticipated expiration: 2019-12-18
Also published as: GB9929857D0; GB2357399B

Abstract

A cellular telecommunication system such as GSM, has a plurality of pico-cells administered by a single broadcast control channel. Transceiver heads within each cell generally transmit simulcast control channel information and support cell-specific traffic channels. A cell controller estimates a co-channel interference environment within the system by assessing up-link subscriber unit transmissions and then partitions logically distinct control channels, such as SDDCH and SACCH, into a first simulcast service group and a second dedicated control channel information group. Subsequent re-use of bursts in the second group associated with the SDCCH and SACCH results in different control information associated with different subscriber units being transmitted on exactly the same time slot and BCCH carrier frequency in physically separate and interference isolated pico-cells serviced by different transceiver heads. Generally, therefore, all transceiver heads transmit simulcast except for SDDCH (and SACCH) periods in which some transceiver heads continue to transmit identical simulcast data whereas others change control data transmissions to address limited call-origination capabilities and control channel overload problems otherwise associated with using a single overlaid BCCH.

Description

2357399 CELLULAR COMMUNICATION SYSTEM AND METHOD OF CONTROL CHANNEL

UTILISATION THEREIN Backqround to the Invention This invention relates, in general, to a cellular communication system and method of control channel utilisation therein, 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.

Summarv of the Prior Art

With increasing demand for cellular services, communication system (such as the Global System for Mobile communication, GSM) are now employing multi layer 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).

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 multi- layer 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 arid 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.

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.

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. Pico-cells 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.

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 fully fledged 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, 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. 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 downlink transmissions, since BCCH transmission are curtailed and are therefore effectively non- continuous across the cellular service area.

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 (CII) ratio. In other words, downlink interference is only caused periodically by intermittent data traffic. With improved CII, 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 preferably 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.

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 eight time slots (in the specific instance of GSM) and wherein the BCCH is assigned to time slot zero (TS-0s). Turning to the BCCH in more detail, a BCCH multiframe is constructed from a cyclic transmission of fifty-one (or sometimes one hundred and two) TS-0 information bursts in contiguous CC frames. Within the BCCH multiframe, various TS-0 bursts (each of 0.577 millisecond duration) are assigned to specific management and control tasks. These bursts may be packaged so that several successive bursts each relate to a particular management or control function. The BCCH is therefore realised by a combination of bursts in successive CC frames made up from: the frequency correction channel (FCCH) that addresses the correction of a receiving terminal's internal time base; the synchronisation (SCH) channel for time slot synchronisation; the common control channel (CCCH) for controlling call origination and call paging functions; the standalone dedicated control channel (SIDCCH) that supports the transfer of data to and from a subscriber terminal during call set-up; and the slow associated control channel (SACCH) that conveys power control and timing information in the downlink. The SIDCCH and SACCH are therefore assigned both to the same time slot within each frame and to the same BCCH carrier frequency, although the SDCCH and SACCH are distinguishable in time.

As will be understood, the SDCCH is clearly not as robust as the certain other control channels, such as the paging channel of the BCCH, because there is no re-transmission associated with the SDCCH. Consequently, any bursts lost due to C/I have a significant impact on system performance.

As will be understood, a large proportion of call set-up procedures take place whilst a subscriber unit, such as a mobile, is on a SDCCH. It is therefore imperative that the dimensioning of SDCCHs in a cell is carefully considered during any planning process. The three principal factors that effect SDCCH dimensioning are:

i) to support call set-up in a cell; ii) the requirement to support the GSM defined short message service (SMS) feature; and iii) to enable location update.

For an in-building cellular system, adequate provisioning for the first two factors is vital in view of location update being less important as mobiles (and subscribers generally) within a building are associated with a single (and known) base station controller (BSC). However, mobile units may still perform periodic location updates regardless of mobile movement, with such uplink data transmission used for management purposes.

One drawback with the use of a single BCCH overlaying a plurality of traffic cells is that there is clearly a limitation in the number of available SDCCHs. As previously described, it is usual for the logical SDCCH channel to be subsumed/combined within the BCCH, although it is feasible to consider the multiplexing of eight SDCCHs onto a single dedicated time slot; the latter respect is, however, considered wasteful of resources and would significantly reduce the capacity of a system in its entirety. As will now be appreciated, the combination of the logical SDCCH into a simulcast BCCH limits the availability of the logical SDCCH to four groups of four time slot bursts in every fifty- one BCCH multiframes, representing a time occupancy of the four (4) SDCCHs in the BCCH multiframe (in GSM) of 9.23ms in every 235.365ms (or a utilisation of 3.92%). A similar access issue arises with SACCHs, although there is access to only four distinct SACCHs in every one hundred and two frames, or two SACCHs every fifty-one BCCH frames.

Unfortunately, in high capacity applications, overload of the control channel can occur, and this is certainly the case in multi-layer cellular environments where there is a periodic migration of subscribers into a particular service area.

Although this capacity issue can be addressed by using more frequencies or by assigning individual control channels to small underlaid cells, the associated costs and increase in design complexity seldom warrant the provision of a permanent solution that, in any event, may run in an inefficient manner for a majority of the operating time. Consequently, system operators are left to resolve the viability of providing additional channel resources that are seldom used (rendering the system less commercially attractive), or in operating a system that can stall at peak times when overload of the control channel causes either blocking of call set-up requests or refusal of a call set-up process in its entirety. In both latter events, a subscriber therefore perceives a lower standard of service.

It is therefore preferably for a capsule-based cellular system in which traffic cells are administered by a single BCCH to have the MCCH sent on a simulcast carder, although this unfortunately leads to an arrangement in an in- building cellular system, for example, having an insufficient MCCH capacity. It would therefore be desirable to increase the availability to SDCCHs and SACCHs within frame structures of existing (and future) cellular communications standards, such as GSM and U1VITS, without adversely affecting system capacity.

Summarv 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 incorporating a plurality of logical control channels, 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 and providing a simulcast transmission service in relation to the broadcast control channel, wherein each of the plurality of cells includes a transceiver head; the cellular communication system characterised by: means for assessing an interference environment of the broadcast control channel in at least some of the plurality of cells; means for identifying cells having a co-channel interference environment capable of supporting cell-specific logical control channel transmissions complementary to simulcast logical control channel transmissions across at least some of the plurality of cells; and wherein the cell controller is operable to cause selected ones of said transceiver heads associated with identified cells having co- channel interference environments capable of supporting cell-specific logical control channel transmissions simultaneously to transmit different control messages in selected ones of said logical control channels.

in another aspect of the present invention there is provided a cellular communication system having a plurality of cells administered by a broadcast control channel incorporating a plurality of logical control channels, 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 and providing a simulcast transmission service in relation to the broadcast control channel, wherein each of the plurality of cells includes a transceiver head; the cellular communication system characterised by: means for determining a cell interference environment; means, responsive to the determination of the cell interference environment, for differentiating cells into a first group in which logical control channels can be broadcast on a re-use basis and a second group in which logical control channels must be simulcast; and means for causing, when desired, different control channel messages to be sent in selected logical control channels from transceiver heads associated with those cells in the first group.

In a further aspect of the present invention there is provided a method of providing control information to subscriber units in a cellular communication system having a plurality of cells administered by a broadcast control channel incorporating a plurality of logical control channels, 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 and providing a simulcast transmission service in relation to the broadcast control channel, wherein each of the plurality of cells includes a transceiver head; the method comprising the steps of: determining a cell interference environment; responsive to the determination of the cell interference environment, differentiating cells into a first group in which logical control channels can be broadcast on a re-use basis and a second group in which logical control channels must be simulcast; and causing, when desired, different control channel messages to be sent in selected logical control channels from transceiver heads associated with those cells in the first group.

Preferably, the method further includes transmitting a simulcast logical control channel message from the plurality of transceiver heads in the plurality of cells in at least one of the logical control channels; and in at least one cell of the first group, periodically transmitting differing control channel messages in at least one of the logical control channels.

The step of periodically transmitting differing control channel messages may also further include substantially simultaneously transmitting a simulcast logical control channel message in cells of the second group.

The step of determining the cell interference environment may be performed periodically during cellular system operation, or at another time (such as at system initiation).

The cellular communication system may be a multi-service area in-building pico cellular communication system, and the method of a preferred embodiment may include the step of allocating said first group and said second to different service areas within the multi-service area. In fact, a preferred method re-uses said first group and said second group within different service areas within said multi service area.

In one embodiment, the method causes subscriber units within the plurality of cells to transmit up-link information substantially simultaneously to assess the co-channel interference environment.

Another aspect of the present invention provides a cell controller arranged to administer control of a plurality of cells through generation of a broadcast control channel incorporating a plurality of logical control channels, the cell controller operationally coupled to a plurality of transceiver heads arranged to provide a simulcast transmission service in relation to the broadcast control channel and wherein each of the plurality of cells includes a transceiver head, the cell controller comprising: means for determining a cell interference environment; means, responsive to the determination of the cell interference environment, for differentiating cells into a first group in which logical control channels can be broadcast on a re-use basis and a second group in which logical control channels must be simulcast; and means for causing, when desired, different control channel messages to be sent in selected logical control channels from transceiver heads associated with those cells in the first group.

Preferably, the cell controller further comprises: means for causing transmission of a simulcast logical control channel message from the plurality of transceiver heads in the plurality of cells in at least one of the logical control channels; and means for assessing a system demand in relation to said logical control channels and, in response to a requirement for additional control channel resources, for periodically transmitting differing control channel messages in at least one of the logical control channels in at least one cell of the first group.

Advantageously, the present invention therefore provides a system that can vary its capacity in handling call control down-link messages without resorting to additional time slot utilisation. More specifically, the system of the preferred embodiment has an ability to increase logical control channel capacity within a broadcast multiframe control channel, which is especially beneficial in relation to channel associated with call set-up and control, e.g. SIDiCCH and SACCH in GSM. The preferred embodiment of the present invention can be easily integrated into existing pico-cellular type systems, since the invention can be controlled by a software upgrade to install the requisite system functionality; although hardware remains essentially unaffected although operational capabilities are altered. Once the present invention has been introduced into a system, any new cells subsequently introduced into the coverage area serviced by the single BCCH can be automatically assessed in terms of their cochannel interference problems and suitability for supporting the present invention.

Indeed, a cellular communication system supporting the preferred embodiment of the present invention can be considered as self-configuring and essentially independent of planning overhead.

Brief Description of the Drawims

An exemplary embodiment of the present invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 illustrates inter-relationships between time slots, frames and a multi frame in a conventional time division multiplex (TDM) communication system, such as GSM; FIG. 2 is representative of a multi-layer cellular communication system employing an overlaid control channel (BCCH) and a plurality of traffic serving cells; FIG. 3 illustrates an underlying inventive concept of MCCH and SACCH re-use within a mixed simulcast and dedicated control channel system according to the present invention; -]IFIG. 4 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 of FIG. 3; and FIG. 5 is a flow diagram of an operating methodology employed by a preferred embodiment of the present invention in assessing an interference environment and cell suitability for supporting a cell-specific MCCH and SACCH.

Detailed Description of a Preferred Embodiment

Referring briefly to FIG.1, this diagram illustrates inter-relationships between time slots 10-24, frames 26-30 and a multi-frame 32 in a conventional time division multiplex (TDM) communication system, such as GSIVI. The specific structure has already been discussed above, with the diagram more clearly demonstrating the construction of a BCCH multi-frame. It is worthy to note the groupings of time slot bursts to construct SDCCHs (DO-D3) and SACCHs (A0-Al) in the exemplary fifty-one time-slot multiframe 32. A contiguous multi frame generally follows an identical structure, although optionally third (A2) and fourth (A3) logically independent SACCHs take the place of the SACCHs AO and Al.

Turning to FIG. 2, there is shown a multi-layer cellular communication system 80 employing an overlaid control channel (BCCH) 82 and a plurality of traffic serving cells 84-128; 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 130-132 coupled to provide operational control and synchronisation to transceiver heads (such as head 136 in pico-cell 106) in each of the cells 84-128. If multiple BTS (head) controllers 120-132 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 136, which typically has limited signal processing functionality to reduce head size and complexity, is coupled to a serving BTS (head) controller 130-132 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) 130-132 islare 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, or a commercial group within an office suite. As regards operation, the transceiver heads 136 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 84-128 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 handover, between subscriber units within the cellular system 80 and the transceiver heads 136 (as well as functional partitioning between the transceiver heads 130 and the BTS (head) controller for signal processing requirements) will be readily appreciated by the skilled addressee.

The principal problem of how to increase SDCCH and SACCH capacity without using additional dedicated timeslot bursts in the control channel is met by differentiating pico-cells heads into those which must operate in a simulcast control channel mode (to serve subscribers) and those which can operate a re-use pattern for the SDCCH and SACCH. Differentiation between these two groups is based on an assessment of the interference environment within pico cells of the communication system 80 as a whole. Re-use of the bursts associated with the SDCCH and SACCH therefore results in different information associated with different subscriber units being transmitted on exactly the same time slot and BCCH carrier frequency in sufficiently physically separate pico-cells serviced by different transceiver heads, i.e. some heads continue to transmit simulcast whereas others transmit different control information during identical bursts. In other words, there is a re-use of the same MCCH and SACCH on the same frequency and same channel where it is known that there is no significant interference between pico (or capsule. ) cells having head transceivers that could otherwise co-operate in simulcast to provide a solitary control channel within the combined coverage area of the pico- cells.

As will be appreciated, the interference of concern is essentially cochannel interference generated from the various transceiver heads using the same BCCH multiframe and BCCH carrier frequency.

Effectively, therefore, if a prevailing interference environment within specific pico-cells is deemed sufficiently good (i.e. low and of little concern) and therefore capable of supporting re-use of the MCCH andlor the SACCH, then a control channel re-use pattern, as exemplified in FIG. 3, emerges. Looking to FIG. 3, one can see that the majority of transceiver heads 130 (not shown for the sake of clarity) continue to operate in a simulcast mode (as noted by the use of the reference W' to indicate identical control channel transmissions). Other cells, such as pico-cells 92, 100 and 106, can benefit from transmitting information (denoted by the letters "B", "C" and V) directed to the control of different subscriber units within such other cells. The overall effect is to increase servicing capacity in the entire system by re-using the same SDCCH and SACCH logical channels for both simulcast and non-interfering discrete cells.

FIG. 4 is a block diagram of a cellular communication system 180 of a preferred embodiment of the present invention as typically required toassess support for the underlying inventive concept of FIG. 3. For the sake of clarity, FIG. 4 shows a restricted view of a typical cellular system, such as that shown in FIG. 2.

Basically, each pico-cell will include a varying number of subscriber units 182 186, some of which may be mobile (such as mobile phones 184 and 186) and some of which may be fixed equipment, such as an infrared-cou pled computer modem 182. Clearly, on an aperiodic basis, some cells may contain no mobiles, whereas other cells may contain mobiles which are simply not active, i.e. that are in an idle/sleep mode (such as mobile 186) 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 TDM basis (in the specific instance of GSM). For the sake of completeness, each pico-cell is shown having a transceiver head 136, 190- 194 coupled to a BTS (head) controller administering operational control of the system, in general.

The present invention is related to co-pending UK patent application number 99xxxxx.x (Applicant Docket Number CE30582P-Thomas et al), filed simultaneously with the present application. The co-pending applicationdescribes a method of increasing paging capacity by periodically re-using the paging channel within a building, thereby departing from simulcast control channel transmissions from the transceiver heads. The question of interference in the co-pending application can be tolerated through a combination of re transmission of information and routinely switching paging back to simulcast operation. A consequence of the technique employed in CE30582P is that, in a preferred embodiment of the present invention, it has been identified that it is possible to determine which pico-cells suffer from co-channel interference when operating in a re-use type of mode. More specifically, the present invention makes use of the fact that subscriber units that are able to receive and decode Access Grant Channel (AGCH) or Paging Channel (PCH) information during re use modes of operation are sufficiently distant from re-use boundaries, implying that associated pico-cells are somewhat isolated and generally exhibit a good C/l. The present invention therefore makes use of any suitable measurement technique that can identify co-channel interference, and then makes use of this interference information to set up dynamic re-use of the SDCCH and associated SACCH.

In a preferred embodiment, the hardware of FIG. 4 operates to force idle subscriber units 186 to transmit 200 (in an uplink) in the same time slot; this can be accomplished by having the transceiver heads use a generic page command or the like. The forcing of up-link transmissions can be on a systemlarea wide basis, or by a mechanism of selective pairing or grouping of pico-cells of interest. In view of the simulcast nature of the overlaid BCCH, substantially all subscriber terminals receive the page instruction and respond. The BTS (head) controller 130, responsive to paging acknowledgements (if any) from the addressed subscriber units, can then assess pico-cell capability in supporting group partitioning of SIDCCH and SACCH. More specifically, with all subscriber units transmitting in the same burst, a level of co-channel interference will result in head transceivers either: i) failing to receive an acknowledgement to the global paging request implying high interference or no serviceable subscriber units; ii) receiving a poor quality response from a subscriber unit consequential on a poor C/1; or iii) receiving of a subscriber unit's identity, implying good C/L Although up-link transmission from idle subscriber terminals possibly presents a (limited) drain on battery capacity, many subscribers in a pico-cellular environment are associated with some form of battery charging station.

Therefore, whilst in an idle mode, subscriber units often reside in a battery charging cradle or the like which mitigates any downside with up-link transmission required ta assess the interference and hence re-use environment.

The technique of assessing the interference environment can be made at any time from system deployment and periodically thereafter, or alternatively on a regular, perhaps hourly or daily, basis.

Once the BTS (head) controller 130 has determined which cells are tolerant to re-use (from, for example, use of the paging channel) by specifically observing pico-cellular statistics during any re-use mode, the BTS (head) controller logically divides/al locates the WCCH logical channels into different service groups. A first group of SIDiCCHISACCH is for simulcast operation (denoted in FIG. 4 by the label "A"), while a second group (denoted in FIG. 4 by the!abels "B", "C" and "D") is dedicated to a re-use mode of operation. The cells determined as being prone to poor W conditions during re-use are therefore assigned the MCCHs from the first (simulcast) group. The cells that are robust during re-use can therefore make use of the second group of SDCCHs, if desired, which second group is also subject to re-use. As will be appreciated, re-use from the second group effectively means that different transceiver heads in different cells will simultaneously transmit different MCCH information to different subscriber units. Therefore, one can use BCCH TS-Os for additional call control and set-up and thereby provide an improved call originating capacity.

Furthermore, in one embodiment, the groups of MCCH and SACCH channels within the BCCH multi-frame are actually separated in time so that all cells occasionally receive a simulcast broadcast in relation to one particular MCCH (e.g. DO), whereas a succeeding MCCH transmission (e.g. D2) is split between both simulcast and dedicated re-use group transmissions or only re-use group transmissions. A mixed transmission environment is preferably since time separation of the groups ensures that simulcast transmission is maintained for at least some proportion of all transmissions. In other words, generally, all transceiver heads 136, 190-194 transmit simulcast except for SDDCH (and SACCH) periods in which some transceiver heads will continue to transmit simulcast whereas others will change data transmission for improved system efficiency. Furthermore, if it can be ascertained that pico-cell boundary separation is sufficiently spatially separated, then simulcast logical channels (specifically MCCH and SACCH) can be re-used in different locations, e.g.

distinct floors in a building can be assigned either the simulcast or reuse SWCH/SACCH groups.

FIG. 5 is a flow diagram of an operating methodology employed by a preferred embodiment of the present invention in assessing an interference environment and cell suitability for supporting a cell-specific MCCH and SACCH. At some point, whether this is periodic to provide dynamic allocation of control channel resources or otherwise, the BTS (head) controller 130-132 instructs 250 head transceivers 136, 190-194 in a plurality of cells to assess a cell co- channel interference environment. The head transceivers broadcast 252 a control channel message requiring individual subscriber terminals (at least those which are idle, and preferably all serviceable units) to transmit an acknowledgement in a specified up-link slot. Each subscriber terminal responds to this "page"-type requirement by sending 254 an identifiable acknowledgement (such as the subscriber unit's identity or address) in the designated up-link time slot. The various head transceivers within the cells of interest are then able potentially to resolve 256 the individual subscriber unit responses if a prevailing co- channel interference environment fails to degrade an ability to resolve the subscriber unit's identity or address. The BTS (head) controller 130 is therefore able to identify those cells in which favourable (good) C/I conditions exists, by interrogating (or receiving reports from) th e various head transceivers. Thus, the BTS (head) controller is able to identify or mark (at least until the next test is ordered) which pico-cells, if any, covered by a unitary overlaid BCCH are able to support logical control channel re-use (step 258) on the allocated BCCH carrier frequency or whether the cell is limited to support simulcast transmissions (step 260). As will be appreciated, the system (and more usually the BTS controller 130 in a pico-cellular system) will include a system-wide intelligence device, such as a dedicated microprocessor (210 of FIG. 4) and associated memory.

More usually, the dedicated microprocessor will interact with its memory to contrast signal quality determination obtained from the head transceivers against predetermined acceptable threshold levels, and then to store the results for later use. Clearly, once a cell's ability to re-use the BCCH frequency and time slot has been assessed, then control channel information (such as SDCCH and SACCH) can be distributed 260 across the system, as a whole, to increase capacity.

Of course, the forcing of subscriber terminal transmissions required by the preferred embodiment can, in any event, be used to enhance the location update facility of the cellular system, in general, which in turn can be used for generating co-ordinated up-link transmissions which can aid in automatic frequency allocation.

The present invention therefore advantageously supports an increase in control channel handling capacity, especially in relation to SDCCH and SACCH (although other forms of multi-frame assembled control channels can benefit from a corresponding re-use assessment and separation, if possible). Indeed, in the preferred embodiment, the increase in capacity has been obtained without requiring an additional, dedicated channel (although an effective noise floor for the entire system may have been increased, but without affecting a perceived quality of service). Clearly, if all members of the simulcast group can be re-used in a dedicated mode, then a significant increase in system control is acquired.

The underlying inventive concept could be adapted to operate in a principally downlink orientated environment, such as a paging system, provided that an assessment of the interference environment can be determined.

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. For example, the principals of the present invention can be employed within any cellular communication system employing a relatively wide area dedicated control channel frequency that serves a plurality of underlying traffic cells. Consequently, a pico-cellular environment is an exemplary and not limiting environment in which the present invention can find application. Additionally, the fact that the system of the preferred embodiments chooses to use transceiver heads for simplification should not be consideired as limiting, since the transceiver head could equally be replaced by a fully functioning base transceiver station. The term "transceiver head" should therefore be construed in a broad sense to encompass a device that has a considerable range to functionality, including that of a fully configured base station.

1

Claims

1. A cellular communication system (180) having a plurality of cells (84128) administered by a broadcast control channel (82) incorporating a plurality of logical control channels (DO-D3, AO-Al), the cellular communication system (180) comprising:

a cell controller (130-132) for administering transmission of the broadcast control channel (82); and a plurality of transceiver heads (136, 190-194) coupled to th e cell controller (130-132) and providing a simulcast transmission service in relation to the broadcast control channel (82), wherein each of the plurality of cells (84-128) includes a transceiver head (136,190-192); the cellular communication system (180) characterised by:

means (130-132, 136, 190-194) for assessing an interference environment of the broadcast control channel in at least some of the plurality of cells; means (130-132, 210) for identifying cells having a co-channel interference environment capable of supporting cell-specific logical control channel transmissions complementary to simulcast logical control channel transmissions across at least some of the plurality of cells; and wherein the cell controller (130-132) is operable to cause selected ones of said transceiver heads (136, 190-194) associated with identified cells (84- 128) having co-channel interference environments capable of supporting cell- specific logical control channel transmissions simultaneously to transmit different control messages in selected ones of said logical control channels.

2. The cellular communication system of claim 1, wherein the cell controller is arranged to cause subscriber units (182-186) within thepturality of cells to transmit up-link (200) information substantially simultaneously to assess the co-channel interference environment.

3. The cellular communication system of claim 2, wherein the up-link transmission is one of a subscriber unit identity and a subscriber unit address.

4. A cellular communication system (180) having a plurality of cells (84128) administered by a broadcast control channel (82) incorporating a plurality of logical control channels (DO-D3, AO-Al), the cellular communication system (180) comprising:

a cell controller (130-132) for administering transmission of the broadcast control channel (82); and a plurality of transceiver heads (136, 190194) coupled to the cell controller (130-132) and providing a simulcast transmission service in relation to the broadcast control channel (82), wherein each of the plurality of cells (84-128) includes a transceiver head (136,190-192); the cellular communication system (180) characterised by:

means for determining a cell interference environment; means, responsive to the determination of the cell interference environment, for differentiating cells into a first group in which logical control channels can be broadcast on a re-use basis and a second group in which logical control channels must be simulcast; and means for causing, when desired, different control channel messages to be sent in selected logical control channels from transceiver heads associated with those cells in the first group.

5. The cellular communication system claim 4 or 5, wherein the system is a multi-service area in-building pico-cellular communication system and wherein the cell controller is arranged to allocate said first group and said second to different service areas within the multi-service area.

6. The cellular communication system claim 5, wherein said first group and said second group are subject to a re-used scheme within said multiservice area.

7. A method of providing control information to subscriber units in a cellular communication system (180) having a plurality of cells (84-128) administered by a broadcast control channel (82) incorporating a plurality of logical control channels (DO-D3, AO-Al), the cellular communication system (180) comprising:

a cell controller (130-132) for administering transmission of the broadcast control channel (82); and a plurality of transceiver heads (136, 190-194) coupled to the cell controller (130-132) and providing a simulcast transmission service in relation to the broadcast control channel (82), wherein each of the plurality of cells (84-128) includes a transceiver head (136,190-192); the method comprising the steps of.

determining a cell interference environment; responsive to the determination of the cell interference environment, differentiating cells into a first group in which logical control channels can be broadcast on a re-use basis and a second group in which logical control channels must be simulcast; and causing, when desired, different control channel messages to be sent in selected logical control channels from transceiver heads associated with those cells in the first group.

8. The method of claim 7, further comprising:

transmitting a simulcast logical control channel message from the plurality of transceiver heads in the plurality of cells in at least one of the logical control channels; and in at least one cell of the first group, periodically transmitting differing control channel messages in at least one of the logical control channels.

9. The method of claim 8, wherein the step of periodically transmitting differing control channel messages further includes:

substantially simultaneously transmitting a simulcast logical control channel message in cells of the second group.

10. The method of claim 8 or 9, wherein the step of determining the cell interference environment is performed periodically during cellular system operation.

11. The method of claim 8, 9 or 10, wherein the cellular communication system is a multi-service area in-building pico-cellular communication system, and the method further comprises:

allocating said first group and said second to different service areas within the multi-service area.

12. The method of claim 11, further comprising re-using said first group and said second group within different service areas within said multiservice area.

13. The method of any one of claims 8 to 12, further comprising.

causing subscriber units within the plurality of cells to transmit uplink information substantially simultaneously to assess the co-channel interference environment.

14. The cellular communication system of any one of claim 1 to 6 or the method of any one of claims 7 to 13, wherein the logical control channels are at least one of an SIDDCH and an SACCH.

15. A computer program product containing program modules for executing the method steps of any one of claims 7 to 13.

16. A cell controller (130-132) arranged to administer control of a plurality of cells (84-128) through generation of a broadcast control channel (82) incorporating a plurality of logical control channels (DO-D3, AO-Al), the cell controller operationally coupled to a plurality of transceiver heads (136, 190-194) arranged to provide a simulcast transmission service in relation to the broadcast control channel (82) and wherein each of the plurality of cells (84-128) includes a transceiver head (136,190-192), the cell controller comprising:

means for determining a cell interference environment; means, responsive to the determination of the cell interference environment, for differentiating cells into a first group in which logical control channels can be broadcast on a re-use basis and a second group in which logical control channels must be simulcast; and means for causing, when desired, different control channel messages to be sent in selected logical control channels from transceiver heads associated with those cells in the first group..

17. The cell controller of claim 16, further comprising:

means for causing transmission of a simulcast logical control channel message from the plurality of transceiver heads in the plurality of cells in at least one of the logical control channels; and means for assessing a system demand in relation to said logical control channels and, in response to a requirement for additional control channel resources, for periodically transmitting differing control channel messages in at least one of the logical control channels in at least one cell of the first group.

18. A cellular communication system substantially as hereinbefore described with reference to FIG. 3 to 5 of the accompanying drawings.

19. A method of providing control information to subscriber units in a cellular communication system substantially as hereinbefore described with reference to FIG. 3 to 5 of the accompanying drawings.

20. A cell controller substantially as hereinbefore described with reference to FIG. 3 to 5 of the accompanying drawings.