System and method for slot allocation with reduced need for measurement

Abstract: In a communication system in which mobile terminals may roam throughout an area with a plurality of base stations with overlapping coverages and wherein the base stations employ multiplexed slots all in the same frequency range, a system and method for allocating slots based on the quality classes of the transmitted traffic wherein certain slots are assigned as owned by some base stations and certain other slots are assigned as shared between base stations. Slots assigned as owned by a base station are assigned as avoided by adjacent base stations. When assigning a slot for communication from a mobile terminal, the number of slot quality measurements that must be made in order to find a slot with interference below an acceptable level is reduced, compared to conventional dynamic slot allocation schemes.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multiplexed radio system with a plurality of base stations having overlapping coverage areas, and particularly to the dynamic apportionment of channels in a multiplexed radio system. 2. Description of the Related Art In many communication applications, a user with a mobile terminal communicates via a relatively short range radio link with a base station, which may pass his communication to other proximate mobile terminals via the short range link, or which may pass his communication to other base stations or to networks such as the PSTN. In many such applications a user may roam throughout an area larger than the reliable coverage area of a single base station, yet the user may wish to maintain his communication capability as he roams. Typically, multiple base stations with overlapping coverage areas are provided for such applications. Since a great many communication services are in place or contemplated, space in the radio spectrum is quite scarce. Thus, while it might be desirable for each base station to have frequencies allocated for its own exclusive use, multiple base stations must operate within lower spectrum bandwidth than ideal, and in many instances must share frequencies. Yet, this must be accomplished while minimizing interference between communication channels, i.e., timeslots in a time-multiplexed system, such as a UMTS/TDD radio system. Typically, each base station has a number of slots on which it is capable of communicating with mobile stations. In typical conventional systems, when a mobile station wishes to initiate communication the associated base station makes path-loss and interference measurements on each of the slots that it is not presently using, in order to find a slot not being seriously interfered with by another base station or a mobile station. An existing example of a radio communication system with dynamic channel apportionment is the DECT (Digital European Cordless Telephone) system in which the mobile terminal chooses the slot to be used for communication, without any consideration of the type or quality class (QoS class) of service. There is a need for faster and more reliable operation with more optimal radio resource usage, especially when the requirements of the traffic become more diverse with data and multi-media transmissions. US Patent No. 5,926,763 describes a telecommunication system where channels are assigned to cells together with a figure predicting potential voice quality for each channel. When a caller arises in a cell, a channel is selected for assignment to the call. The channel is selected from channels assigned during a channel-to-cell assignment phase. A channel assignment algorithm ensures that potentially lower quality channels are the least used channels at the cell. When potentially lower quality channels are assigned to calls, the calls are handed off to potentially higher quality channels as soon as potentially high quality channels become available. The document titled "Channel Segregation for Slot Assignment in Integrated Voice and Data Systems" by Al Agha K et al published in "Personal, Indoor and Mobile Radio Communications", 8 September 1998, pages 831 - 835, XP 010314581, ISBN 0-7803-4872-9, is an article describing an integrated voice and data system using a channel segregation protocol. To achieve each service target carrier to interference ratio, and meet the associated bit error rate requirement, a learning process is described in relation to channel segregation, which is extended by weighting of slot priorities, based on previous successful attempts, according to each service carrier to interference requirement. The document titled "Channel Assignment Schemes for Cellular Mobile Telecommunications Systems: A Comprehensive Survey" by Katzela I. et al, IEEE Personal Communications, IEEE Communications Society, US, Vol 3, No. 3, 1 June 1996, page 10 to 31, XP 000533925 ISSN 1070-9916, provides a discussion of wireless resource and channel allocation schemes. Fixed, dynamic and hybrid channel allocation schemes are discussed and compared in terms of complexity and performance. SUMMARY OF THE INVENTION An aim of embodiments of the present invention is to reduce the number of slots measured by a base station or a mobile station in order to find a slot not interfered with by another base station or mobile station, and to direct measurements to particular slots determined according to the needed quality class of the communication. According to one aspect of the present invention there is provided a method as defined in claim 1. According to another aspect of the present invention, there is provided an apparatus as defined in claim 12. According to one embodiment, a centralized set of information is assembled indicative of the interference patterns among the transmitters (be they base stations or mobile stations) of a particular installation. A slot on which it is found that base stations interfere with each other is assigned as "owned" by one of the base stations and as "avoided" by the other base stations interfered with. A slot on which two adjacent base stations would interfere with each other but is not assigned as owned by either of them is assigned as "shared" by those two base stations. When a slot is to be allocated for communication between a base station and a mobile station, the base station's owned slots that are not presently in use are measured (path-loss measurements are taken) first in an attempt to find one not being interfered with by any other transmitter. If such a slot is found, the communication is assigned to it. If no such owned slot is available, shared slots are measured to find one not interfered with by another transmitter; if one is found, it is used. In one embodiment, if no owned or shared slot is available, avoided slots will be measured to determine if one is not in use by the owning base station; if one such is found, it is used. In one embodiment, communications which do not have a high quality class requirement are assigned only to shared or avoided slots while owned slots are reserved for communications with high quality class requirement. Other objects and features of embodiments of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, wherein like reference numerals denote similar elements: Fig. 1 is a block diagram of an environment in which embodiment of the present invention is deployed; and Fig. 2 is a flow chart of slot allocation embodying to the present invention. DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS Fig. 1 depicts a typical environment in which embodiments of the present invention are useful. A system 10 is deployed in a building which might be an office building, a factory building, a department store, etc. The exemplary embodiment of Fig. 1 depicts a 5-story building, although nothing implicit in the invention constrains it to any particular number of stories or even to deployment in a building; it might, for example, be deployed on terrain, in a plurality of buildings, or in one or more buildings in conjunction with adjacent terrain. It is desired that users equipped with mobile terminals 4 (of which there typically are many, only one of which is shown in Fig. 1) can always be in communication through the system 10 no matter where they may roam within the system 10. Communication from a mobile terminal 4 is through a base station (BS) 2. Since the system 10 occupies an area larger than the reliable coverage area of a single BS 2, a plurality of BSs 2 are provided. In the exemplary embodiment of Fig. 1, there is a BS 2 near each end of each story of the five-story building, for a total of ten BS 2s denominated 2-1 through 2-10. In accordance with the present invention, a controller 6 is located somewhere in system 10 and has a connection to each of BSs 2. In a present embodiment, the connection is by means of wiring. A present embodiment of the invention is in a time-division duplex (TDD) system conforming to the Universal Mobile Telecommunication System (UMTS) specification, thus known as a UMTS/TDD system, but the invention is suitable to other systems in which a plurality of transmitters might interfere with one another. All BSs 2 can transmit on the same frequency band, and each BS 2 employs fifteen time-division slots, connoted slots 1 through 15. Taking BS 2-3 as exemplary, an initial assessment might be arbitrarily made that slots of BSs 2-2, 2-4, and 2-8 interfere with BS 2-3. That assessment can be refined over time based on empirical observation. Empirical observation may also indicate that some slots of some BSs other than 2-2, 2-4, and 2-8 interfere with BS 2-3. Based on the current assessment, slots of each BS are assigned as one of owned, shared, or avoided. For example, slots 1, 2, and 3 might be assigned as owned by BS 2-3. Slots 4, 5, and 6 might be assigned as owned by BS 2-2; they would thus be assigned as avoided by BS 2-3. Slots 7, 8, and 9 might be assigned as owned by BS 2-4; they would thus be assigned as avoided by BS 2-3. Slots 10, 11, and 12 might be assigned as owned by BS 2-8; they would thus be assigned as avoided by BS 2-3. Slots 13, 14, and 15 would then be assigned as shared by BS 2-3. When a user requests to initiate communication via mobile terminal 4 (which is in BS 2-3's coverage area), BS 2-3 must allocate a slot for the communication. BS 2-3 determines what slot to use by making path loss measurements between itself and mobile terminal 4 and interference measurements at various slots. The first slots to be tried are those assigned as owned by BS 2-3 (slots 1, 2, and 3 in the present example) and not already in use. If at least one of slots 1, 2, and 3 is not presently in use, the probability is very high that it will be found to be free of interference, since it was assigned as avoided by adjacent base stations. It is thus likely that a usable slot will be found after far fewer measurements than under the prior-art method in which all slots are measured until a usable one is found. However, if slots 1, 2, and 3 are all in use by BS 2-3, or are found to have excessive interference (which might be because of a peculiarity of the present position of mobile terminal 4 or because an adjacent transmitter is using the slot under special permission from controller 6 as will be described below), the slots assigned as shared by BS 2-3 are measured (slots 13, 14, and 15 in the present example). If one of them is found to be sufficiently free of interference, it is used for the communication. In another embodiment of the invention, the quality class required for the communication affects the order of slot measurement. Real-time transmission, for example, requires a high quality class, since lost data are not automatically recovered. Packet data, on the other hand, is transmissible under a lower quality class, since typical protocols automatically retransmit packets which have undergone transmission errors. The term "quality class" is being used herein to connote what is generally known as QoS (quality of service) class. In this embodiment, communications requiring a high quality class start measuring in the slots assigned as owned by a BS, but communications not requiring a high quality class start measuring in the slots assigned as shared by a BS, thus permitting the owned slots (with their higher probability of low interference) to remain available for subsequent high quality class transmissions. In another embodiment of the invention, if neither a BSs owned slots nor its shared slots are available and suitably interference free, controller 6 may grant special permission to a BS to use a slot marked as avoided by that BS and owned by another BS, if controller 6 determines that the other BS is not presently using the slot. Fig. 2 is a flowchart of initiating a communication according to the present invention. A request indicated to a BS from a user at a mobile terminal 4 (such as by putting the mobile terminal into an off-hook condition) initiates the flow. The flow centers on a particular one of the BSs, namely the particular BS in whose coverage area a mobile terminal 4 is located when requesting to initiate communication. In the present example, a mobile terminal 4 requesting to initiate communication is in the coverage area of BS 2-3, so BS 2-3 will be treated as central in the present example. The flow of Fig. 2 is entered at block 202. In block 204 it is determined whether a high quality class path is needed. (In some embodiments the check of block 204 may be omitted; control may dispatch directly from block 202 to block 206, and all communications regardless of quality class requirement will be tried on owned slots first.) Blocks 206 and 208 may be repetitively looped through (as indicated by Note 1 on Fig. 2). For each slot owned by BS 2-3 but not currently in use by it, path loss measurements are made in block 206 and the results are evaluated at block 208. As soon as a slot is found with acceptably low interference (indicated by a path loss measurement meeting predetermined criteria) control dispatches block 210 where the communication is established on that slot. If no owned slots are found with suitably low interference, shared slots are tried next by dispatching to block 212. Blocks 212 and 214 may execute repetitively in the manner of blocks 206 and 208, but for BS 2-3's shared slots. If one is found with suitably low interference, block 210 is reached (through connector "A") and communication is initiated on the found slot. If control dispatches to block 216, none of BS 2-3's owned slots or shared slots are available for use. Controller 6 (Fig. 1) is interrogated (the interrogation is not shown in Fig. 2) to determine whether any of BS 2-3's avoided slots (which would be slots assigned as owned by other BSs) are in use by their owners, and whether it is permitted for BS 2-3 to use such slots at this time. If there are any such slots, blocks 216 and 218 may execute repetitively to find one with suitably low interference. Finding one dispatches to block 210 to initiate communication on that slot. Failure to find one indicates that there are no slots at all available to BS 2-3 at this time. Control dispatches to block 220, and after some predetermined delay time control dispatches back to the beginning of the flow (through connector "B") to try again, on the premise that a communication on some slot may have terminated thus making the slot available, or the interference on some slot may have abated. Not shown in Fig. 2 is that if the user terminates his request to establish communication (as by putting mobile terminal 4 back in an on-hook condition) the flow of Fig. 2 is abandoned. Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art.

Claims: A method of allocating communication channels in a communication system (10) comprising a plurality of base stations (2-1, 2-2, 2-3) each for communicating with at least one mobile station (4), the base stations (2-1, 2-2, 2-3) capable of communicating via any of a predetermined group of channels, and some of the base stations being susceptible of being interfered with by other of the base stations in some of the channels of said group of channels, the method comprising the step of predetermining, for each base station, a classification for each channel according to the probability of interference at the channel, characterized in that said step of predetermining a classification comprises assigning each channel to one of a plurality of classification categories according to the probability of interference, and said method further comprises the step of allocating on request a channel for transmission according to said predetermined classification and a desired quality class of transmission. The method of claim 1, wherein each said channel is a time slot The method of claim 1 or 2, wherein the classification categories for each channel are owned, avoided or shared, and said step of predetermining comprises: assigning as owned by said each base station and as avoided by said other base stations a channel in which said other base stations interfere with said each base station; assigning as owned by said other base stations and as avoided by said each base station remaining channels in which said other base stations interfere with said each base station; and assigning as shared by said each base station and said other base station channels in which said other base stations interfere with said each base station if used simultaneously with said each base station and which are not assigned as owned by either. The method of claim 3, wherein the step of allocating a channel comprises: measuring at each channel in turn a characteristic of the channel until the characteristic of the channel satisfies a predetermined criterion; then selecting said channel for allocating to said transmission. The method of claim 4, wherein each channel is measured in an order dependent on the desired quality class. The method of claim 5, wherein: if the desired quality class is high, then the step of measuring starts with owned channels first, then shared channels, and finally avoided channels until a channel satisfies the predetermined criteria; and if the desired quality class is low, then the step of measuring starts with shared channels first, and then avoided channels until a channel satisfies the predetermined criteria. The method of claim 5 or 6, wherein the characteristic is a path loss. The method of claim 5, 6 or 7, wherein the predetermined criterion is a predetermined level of interference. The method of any preceding claim wherein: the communication system further includes a controller (6) connected to each base station; said predetermination for each base station is reported to the controller; and said allocating is performed in the controller. The method of claim 9, wherein the controller maintains an indication of which channels are currently allocated for each base station. The method of claim 1 wherein the step of allocating is further according to a measured level of interference. Apparatus for allocating communication channels in a communication system (10) comprising a plurality of base stations (2-1, 2-2, 2-3) each for communicating with at least one mobile station (4), the base stations (2-1-, 2-2-, 2-3) capable of communicating via any of a predetermined group of channels, and some of the base stations being susceptible of being interfered with by other of the base stations in some of the channels of said group of channels, the apparatus comprising a logic unit configured to predetermine, for each base station, a classification for each channel according to the probability of interference at the channel, characterized in that said logic unit is further configured to assign each channel to one of a plurality of classification categories according to probability of interference, and allocate on request a channel for transmission according to said predetermined classification and a desired quality class of transmission. The apparatus of claim 12, wherein each said channel is a time slot. The apparatus of claim 12 or 13, wherein the classification categories for each channel are owned, avoided or shared, and said logic unit is configured to perform said predetermination by: assigning as owned by said each base station and as avoided by said other base stations a channel in which said other base stations interfere with said each base station; assigning as owned by said other base stations and as avoided by said each base station remaining channels in which said other base stations interfere with said each base station; and assigning as shared by said each base station and said other base station channels in which said other base stations interfere with said each base station if used simultaneously with said each base station and which are not assigned as owned by either. The apparatus of claim 12, 13 or 14 further comprising a controller (6) connected to each base station and configured to: receive said predetermination for each base station is reported to the controller; and to be a portion of said logic unit for performing said allocating. The apparatus of claim 15, wherein the controller maintains an indication of which channels are currently allocated for each base station. The apparatus of claim 16, wherein: if neither an owned channel nor a shared channel of a first base station is available for a requested communication, the controller is configured to determine whether any avoided channel of the first base station is not in use by a second base station owning that channel, and if so, to allocate that channel for the requested communication. The apparatus of claim 12, wherein the logic unit is configured to allocate a channel further according to a measured level of interference.

Inventors: JOLMA PETRI, LEHTINEN OTTO, PASSOJA KALLE, PATRONEN PETRI, JOLMA, PETRI, LEHTINEN, OTTO, PASSOJA, KALLE, PATRONEN, PETRI
Applicants: NOKIA CORP, NOKIA CORPORATION
Application year: 2001
Main classification: H04Q7
IPC codes: H04W72, H04W16