CONTROL HANDOFF METHOD IN A CELLULAR TELECOMMUNICATIONS SYSTEM

Abstract: A cellular telecommunications system, comprising mobile exchanges (MSC), base stations (BS) and user mobile stations (MS) roaming in the system. The mobile exchanges comprise means for inter-exchange soft handoff with diversity combining. The mobile exchanges further comprise means for control handoff, wherein user communications control and signal diversity combining functions involved with said user communications are handed off from a first mobile exchange to a second mobile exchange.

Description: Control handoff method in a cellular telecommunications system Field of the invention The present invention relates to cellular telecom- munications systems. More specifically, the present in¬ vention relates to a novel and improved system for cont¬ rolling soft handoff between a mobile station and base stations which are connected to different mobile swit¬ ching centers within a cellular telecommunications sys- tern. Background of the Invention The use of code division multiple access (CDMA) modulation is but one of several techniques enabling di- gital communications among a number of mobile users uti¬ lizing a common part of the radio spectrum, as is the case for cellular telecommunications system. Other well- known radio access techniques are time division multiple access (TDMA) and frequency division access (FDMA) . The concept of control handoff is indeed applicable to all three of the mentioned multiple access techniques. The background of the present invention will be illustrated by means of a CDMA cellular telecommunications system, however, it should be understood that the present inven- tion is not limited to CDMA..An exemplary application of CDMA to cellular telecommunications systems has been substantially described in "On the System Design Aspects of Code Division Multiple Access (CDMA) Applied to Digi¬ tal Cellular and Personal Communications Networks", Allen Salmasi and Klein S. Gilhousen, presented at the 41st IEEE Vehicular Technology Conference on May 19-22, 1991 in St. Louis, MO. In the above mentioned publication a direct-se¬ quence CDMA (DS-CDMA, or shortly, CDMA in the following) technique is described in which a number of user mobile stations (MSs) communicate via CDMA radio spread spectrum signals with base stations (BSs, also referred to as cell sites) in the uplink (mobile station to base station) and downlink (base station to mobile station) direction. The base stations convert these CDMA radio signals ori¬ ginating from, respectively terminating at, the user's MS into a form appropriate for use in conjunction with ter¬ restrial telecommunications transmission equipment such as the commonly deployed Pulse Code Modulation (PCM) cir- cuit facilities. The base stations further relay these user signals in the uplink and downlink direction to the mobile switching center (MSC, also referred to as mobile exchanges or mobile telephone switching office (MTSO) ) for further processing. The above mentioned user communication signals comprise digitized voice signals and control information (also referred to as signalling) . The MSC performs multi¬ plexing and conversion operations on the another men¬ tioned tributaries and relays the voice signal to another user, e.g. within the Public Switched Telephone Network (PSTN) . The MSC also interprets, reacts upon and genera¬ tes sig-nalling information, thus controlling the overall communication link between the users. These communica¬ tions link control functions comprise the management of general call related events such as call setup or tear down as well as CDMA radio link related events such as the deterioration of the CDMA radio link quality and sub¬ sequent handoff initiation. If CDMA is deployed within the typical medium to large sized cells of land mobile telecommunications sys¬ tems then the average time delay spread of the multipath radio propagation environment is usually larger than the chip duration of the DS-CDMA signal. This forces CDMA to operate in an asynchronous mode with the consequence that the orthogonality of the spread spectrum multiple access user signals cannot be achieved by means of orthogonal spreading codes alone. Therefore, the communications suffer from system self-induced interference not only among signals originating from different cells but in ad- dition to that also considerably within a single cell (referred to as CDMA intra-cell interference) . For such CDMA cellular systems it is therefore an important ove¬ rall system design objective to minimize any excessive CDMA interference among the communicating, users and comp- lementary, to capture and utilize as much energy from a desired CDMA user signal as possible. This system design requirement, although a generic requirement applicable to any multiple access method within cellular telecommunica¬ tions systems, is less stringent for FDMA and TDMA based systems in which intra-cell interference is avoided by the intrinsic properties of the respective multiple ac¬ cess method and inter-cell interference is limited by means of pre-planned cellular frequency re-use schemes. Thus, CDMA unlike FDMA or TDMA operates in a strictly interference limited manner. In the following, the back¬ ground of the invention is examplified for the case of a CDMA cellular telecommunications system. Several methods implementing the above mentioned CDMA system design objective can be readily identified for the above referenced exemplary embodiment of a CDMA cellular telecommunications system. For example, the described closed loop MS transmit power control method has the objective to continuously equalize the received qualities of all uplink CDMA signals within a single BS against the background of rapidly changing radio propaga¬ tion channels undergoing fast and slow fading processes. For this purpose, the BS measures periodically the re¬ ceived Eb/No value, indicative of the signal quality, from each MS CDMA uplink communication and subsequently transmits an appropriate power control command on the downlink communication channel to the MS which in turn sets the CDMA transmitter power accordingly. Ideally, all MS CDMA uplink signals are received at the BS with the same quality and in addition to that, minimum strength necessary in order to maintain the communication link subject to a predetermined quality threshold. Another embodiment of the previously mentioned system design objective is the method of mobile assisted soft handoff in conjunction with signal diversity com- bining during an active CDMA communication which will be summarized in the following. Mobile assisted soft handoff in conjunction with signal diversity combining comprises the method of re¬ laying user communication signals on the transmission segment between MS and MSC concurrently via a first and a second BS in the uplink and downlink direction and per¬ forming signal diversity reception at the MS and MSC in order to enhance the user signal quality. This method is invoked by the MSC when a MS communicating initially with a BS has moved into the overlapping coverage areas of this first BS and a second BS and has reported the avai¬ lability of a sufficiently strong signal from this second BS to the MSC. At no time instant during soft handoff in conjunction with signal diversity combining does the MS interrupt its communications .with the MSC. The MSC typi¬ cally deploys post-detection/decoding, selective com¬ bining of the digitally encoded speech frames. In order to enable the reporting assistance of the MS during soft handoff initiation all BSs may transmit a CDMA downlink reference signal, referred to as pilot sig¬ nal . MSs when roaming throughout the service area of the CDMA cellular telecommunications system periodically de¬ modulate the pilot signals " of the various neighboring BSs during an ongoing communication with a first BS and deri- ve a corresponding pilot signal quality indication. This indication determines a ranked list of candidate BSs for handoff and is transmitted in form of signalling informa¬ tion to the MSC. It should be understood that also the first BS may perform continuously CDMA uplink signal qua- lity strength and measurements and based upon these ob¬ servations may give a soft handoff request indication to the MSC. Usually, soft handoff in conjunction with signal diversity combining is initiated by the MSC if the MS re- ports that the pilot signal quality of a second BS in ad¬ dition to that of the first BS is sufficiently good ac¬ cording to the predetermined thresholds made available to the MS and the MSC as well as the second BS can obtain the required resources for the soft handoff transition. Subsequently, the MS will be instructed by the MSC via the first BS by means of signalling to initiate a soft handoff and to commence signal diversity combining on the downlink. Moreover, MSC initiates the additional relay of user signals via the second BS and commences diversity combining of the user signal in the uplink direction. Both participating BSs invoke autonomously the previously mentioned closed loop power control method. The MS sets its CDMA transmit power to the minimum of the two comman- ded power levels in order to reduce excessive CDMA inter¬ ference with the other communication links. Finally, when the MS is firmly established within the area of the second BS and the pilot signal received from the first BS has weakened sufficiently according to the predetermined thresholds made available to the MS it will report this condition to the MSC which in turn deci¬ des to terminate the soft handoff with signal diversity combining and will use subsequently only the second BS for maintaining the CDMA communications. This process of soft handoff with signal diversity combining may be repeated as the MS moves within the ser¬ vice area of the CDMA cellular telecommunications system and as the measured CDMA signal quality indications suggest . Some of the contemporary TDMA based cellular tele¬ communications systems also utilize the assistance of the MS in form of corresponding MS downlink signal quality measurements as trigger for requesting a handoff from a first BS to a second BS, much in the same way as summari- zed above. However, these systems usually use a scheme referred to as hard handoff in which the MS in response to instructions from the MSC disrupts the communication with the first BS, tunes into the indicated TDMA radio channel of the second BS, and then resumes the uplink and downlink communications. At no time instant does the MS communicate with more than one BS simultaneously and hen¬ ce, no corresponding signal diversity combining takes place in the MS nor in the MSC as is the case for the soft handoff method described above. This hard handoff scheme is applicable to CDMA in a similar manner as well, but should be avoided whenever possible, for reasons of CDMA system capacity as explained in the following. Within the context of soft and hard handoff the previously mentioned predetermined thresholds used in conjunction with the MS downlink signal quality measure¬ ments for the determination of handoff candidate BSs are also referred to as handoff margins. The use of these handoff margins in conjunction with time averaging pro¬ cesses is necessary in order to avoid frequent handoffs (also referred to as handoff Ping-Pong effect) when the MS moves within the usually fuzzy border between the ra¬ dio signal coverage areas of two neighboring BSs. Such frequent handoffs would overload the processing capacity of the MSCs . For the purpose of controlling the CDMA soft handoff with signal diversity combining the handoff mar- gin can be chosen as small as 1 - 3 dB in contrast to the hard hand¬ off case where usually 6 - 10 dB are required in order to avoid the deleterious handoff Ping-Pong effect. Referring to the previously mentioned interference limited operation of CDMA, small handoff margins are in¬ deed an essential requirement for an efficient operation of CDMA. The use of CDMA hard handoff in conjunction with the necessary large hard handoff margins would substanti- ally decrease the CDMA system capacity. In a CDMA cellu¬ lar telecommunications system hard handoff can be tolera¬ ted only in exceptional situations but not as a normal mode of system operation. Soft handoff with signal diver¬ sity combining therefore is to be provided on a seamless basis throughout the CDMA system service area. Prior art CDMA cellular telecommunications systems provide soft handoff with signal diversity combining only between BSs which are connected to one and the same MSC (referred to as intra-MSC soft handoff with signal diver- sity combining) . If the MS is to be handed off between BSs connected to different MSCs prior art CDMA cellular telecommunications systems use CDMA hard handoff instead. In the copending patent application of the present applicant is presented a system and a method for soft handoff with signal diversity combining among base sta¬ tions connected to different mobile exchanges of a cellu¬ lar telecommunications system, in the following referred to as inter-exchange soft handoff with signal diversity combining. The novel approach thus enables a seamless soft handoff throughout the whole system service area. One disadvantage is that when the control of the user communications and diversity combining functions remain at the first exchange involved in a sequence of subse¬ quent inter-exchange soft handoffs many inter-exchange connections remain reserved and many exchanges are invol- C munications system, comprising mobile exchanges, base stations, and user mobile stations roaming in the system, each of the mobile exchanges comprising means for inter- exchange soft handoff with diversity combining, wherein said mobile exchanges further comprise means for control handoff, wherein user communications control and signal diversity combining functions involved with said user communications are handed off from a first mobile ex¬ change to a second mobile exchange. Brief description of the drawings The features and advantages of the present inven¬ tion will become more apparent from the detailed descrip¬ tion set forth below when taken in conjunction with the drawings: Figure 1 is a schematic overview of an exemplary CDMA cellular telecommunications system in accordance with the present invention; Figure 2 is a block diagram showing a preferred embodiment of a mobile switching center for use within a CDMA cellular telecommunications system; Figure 3 is a block diagram showing a preferred embodiment of a base station for use within a CDMA cellu¬ lar telecommunications system. Detailed Description of the Preferred Embodiments Figure 1 shows an exemplary embodiment of a CDMA cellular telecommunications system. Figure 2 shows an exemplary embodiment of a MSC used in a CDMA cellular telecommunications system to which the present invention relates . Digital links (120,122,124,126) connect the mobile exchange MSC with the Public Switched Telephone Network PSTN, other mobile exchanges MSC and base stations BS, respectively. These digital links carry the user informa- ling information related to this connection, are carried in a digital, framed format suitable for the terrestrial transmission links 124,126 between the BSs and the MSC. These frames are subsequently referred to as transcoder & combiner frames. In addition to this user information, transcoder & combiner frames may also contain information supplied by the BSs which is relevant to the signal qua¬ lity as used for the signal diversity combining within the MSC in the uplink direction. These transcoder & combiner frames arriving and leaving on the circuits 130,132,134 are buffered in the digital memory 104 for the uplink and downlink direction, respectively. Digital processor 102 reads and writes cy¬ clically the transcoder & combiner frames from and to di- gital memory 104. In the uplink direction, a signal qua¬ lity indication attached to the transcoder & combiner frames arriving from circuits 130,132 into the memory 104 is inspected and processor 102 performs the diversity se¬ lection based on these indications. In the downlink di- rec-tion, voice samples arriving from circuit 134 into the memory 104 are transcoded and packed into transcoder & combiner frames by processor 102. Transcoder & combiner equipment 100 by means of the digital processor 102 also extracts, respectively inserts, the user signalling information from, respecti¬ vely into, the transcoder & combiner frames and offers, respectively receives, this signalling information to MSC control processor 110 via a circuit 140. By these means, MSC control processor 110 receives MS signalling informa- tion such as pilot signal quality measurement reports. Thus, MSC control processor 110 possesses the necessary information to initiate and terminate inter-MSC or intra- MSC soft handoffs, as well as control handoffs. Further¬ more, by these means MSC control processor 110 can issue the appropriate handoff commands to the MS via circuits 140,130,132 and links 124,126 as well as via the digital packet switch 114 and link 122 to other MSCs, should this be required. Figure 3 shows an exemplary embodiment of a BS used in a CDMA cellular telecommunications system to which the present invention relates. Block 200 shows the apparatus required to support a single CDMA communication within a BS, referred to as CDMA channel equipment (only one is shown) . In the uplink direction the CDMA user communicati- on signals are received from the digital CDMA radio link 230, demodulated by the CDMA demodulator 202, de-inter¬ leaved and channel decoded by the de-interleaver & deco¬ der 206, converted into transcoder & combiner frames and buffered for the terrestrial transmission within digital memory 210 and finally transmitted on the digital link 232 towards the MSC. In the downlink direction, the transcoder & com¬ biner frames are received from the MSC via digital link 232, buffered and converted into a presentation appro- priate for the BS within digital memory 210, channel en¬ coded and interleaved by encoder & interleaver 208, CDMA modulated by the CDMA modulator 204 and finally transmit¬ ted on the digital radio link 230. In the preferred embodiment of the present inven- tion the BS possesses a network independent timing source 220 which provides a reference signal of high accuracy as required for efficient CDMA operation and utilized by the CDMA channel equipment 200. Such a timing source may be derived e.g. from the GPS satellite signal and can be provided globally to each BS thus enabling a network of mutually synchronized BSs. The BS further comprises a BS control processor 222. The BS control processor 222 receives and transmits signalling information from and to digital link 232 con- nected to the MSC. The BS control processor 222 performs the resource management of the BS, such as the allocation and releasing CDMA channel equipment for user connections (calls) . The BS control processor 222 thus responds to CDMA channel assignment requests related to a call setup as well as to CDMA channel assignment requests related to soft handoff requests from the MSC. Digital processor 212 in conjunction with the buf¬ fer memory 210 performs the packing and unpacking of the BS internal representation of the CDMA user communica¬ tions signal to and from transcoder & combiner frames in the uplink, respective downlink direction. In the prefer¬ red embodiment of the present invention, the previously mentioned transcoder & combiner frames also contain in¬ formation supplied by the de-interleaver & channel deco¬ der 206 and provided to the digital processor 212 which is indicative of the signal quality of the CDMA radio frames as received from the uplink CDMA radio link 230 and which is used for the signal diversity combining wit¬ hin the MSC in the uplink direction. To support the description of the present inventi- on, control handoff, mobile assisted inter-exchange (or inter-MSC) soft handoff and macrodiversity signal combi¬ ning techniques in a CDMA cellular telecommunications system is described in detail in the following. Initiation of inter-MSC soft handoff with signal diversity combining (Figure 1) In the following it is assumed that a MS 30 com¬ municates via a first BS 24 connected to a first MSC 14 which provides access to the PSTN and other MSCs 10,12. The inter-MSC soft handoff with signal diversity combining is initiated when the MS 30 moves from the coverage area of the serving BS 24 connected to the first MSC 14 to the coverage area of a second BS 22 connected to a second MSC 12 and the MS pilot signal quality measu¬ rements indicate that a soft handoff to the second BS 22 is appropriate. The MS signals this measurement indicati- on via the first BS 24 to the first MSC 14 including identification information of the second BS 22. The first MSC 14 in turn detects from cellular configuration data that BS 22 is connected to another MSC 12 and passes subsequently an inter-MSC soft handoff re¬ quest to this second MSC 12. This handoff request identifies the CDMA code channel and frequency which MS 30 currently uses and additionally, the identity of the inter-MSC circuit 50 reserved by MSC 14 for the this transaction. MSC 12 passes this handoff request further on to BS 22 after reserving and switching through an appro¬ priate circuit 72. BS 22 analyzes the handoff request and if the requested resources are available allocates a further CDMA code channel to be used by MS 30 for the downlink connection 82 which is signaled back to MSC 12 and MSC 14. BS 22 will also activate the downlink direc¬ tion of connection 82 using the newly assigned CDMA code channel. BS 22 will further start demodulating the CDMA uplink connection 82 using the CDMA context information related to MS 30 and subsequently relaying the user com¬ munication signals via MSC 12 back to MSC 14 for diversi¬ ty combining. BS 22 may signal the successful acquisition and reception of the CDMA uplink connection 82 to MSC 14 via MSC 12. MSC 14 will send a handoff request via BS 24 to MS 30 including the identity of the newly allocated CDMA code channel. MSC 14 will also commence diversity signal combing of the user communication signals on the uplink once the user communications signals following the legs 84 - 74, respectively 82 - 72 - 50, have been received in synchronism. MS 30 after having received the handoff request com¬ mences signal diversity combining of the first CDMA down- link connection 84 and the second, newly allocated down- link connection 82. The successful initiation of the in¬ ter-MSC soft handoff with signal diversity combining is then signaled from MS 30 to MSC 14. Termination of inter-MSC soft handoff with signal diversity combining (Figure 1) The inter-MSC soft handoff with signal diversity combining is terminated if the MS leaves completely the coverage area of one of the participating BSs and pene- trates deeply into the coverage area of the other BS. In the following, it is assumed that MS 30 has penetrated deeply into the cell covered by BS 22 and therefore the pilot signal coming from BS 24 has weakened below a predetermined threshold in the previously descri- bed inter-MSC soft handoff configuration. Thus, the leg 84 - 74 shall be removed from the inter-MSC soft handoff with signal diversity combining. MS 30 will inform MSC 14 via a pilot signal quali¬ ty measurement report that the signal coming from BS 24 has weakened below a predetermined threshold. MSC 14 de¬ cides to drop the leg 84 - 74 and thus to terminate the inter-MSC soft handoff with signal diversity combining. To this end MSC 14 sends a handoff termination signal via BS 24 and MSC 12 - BS 22 to MS 30. MS 30 will stop the downlink demodulation diversity combing of the signal co¬ ming from BS 24 and will communicate from now on with BS 22 only. MS 30 signals via BS 22 and MSC 12 the suc¬ cessful termination of the inter-MSC soft handoff with signal diversity combining back to MSC 14 which in turn will request BS 24 to terminate the CDMA radio link 84 and to release the corresponding resources. MSC 14 will also free the terrestrial link 74 and terminate the up¬ link diversity combining. This completes the termination of the inter-MSC soft handoff with signal diversity com- bining. It should be understood that MSC 14 is still in versity combining, the additional leg 80 - 70 - 50 will be backhauled to MSC 14 in addition to the already exis¬ ting leg 82 - 72 - 50. However, this results in two in¬ ter-MSC circuits on the link 50 to be used for this co - munication. It would be advantageous in this situation, if the system could handoff the control and signal diver¬ sity combining functions from MSC 14 to MSC 12, so that all control and diversity combining functions take place closer to BSs 20,22 and thus only one inter-MSC circuit on the link 50 would be necessary. This novel type of handoff, in the following re¬ ferred to as control handoff, is the object of the pre¬ sent invention. The control handoff does not affect radio links directly, in fact the network control handoff is executed by the MSCs only, without direct involvement of the MS, and may be involved much later than the last in¬ ter-exchange soft handoff. However, as there are switching and circuit re¬ arrangements taking place within the network it may hap- pen that signalling information between MS and the cont¬ rolling MSC is lost. Therefore, a reset operation of the signalling link layer (Layer 2 within the OSI model) of the MS - MSC links will be necessary in order to restore the consistency of the signalling connections. However, such a reset operation does not affect the physical radio link layer (Layer 1) nor the call processing (Layer 3) . As a consequence, the usual CDMA closed loop power cont¬ rol operation can be performed between MS and the suppor¬ ting BSs without any disruption by the control handoff. The control handoff method is described in detail for the following exemplary configuration (Figure 1) : MS 30 communicates during an inter-MSC soft handoff via BSs 20,22 and the relaying MSC 12, and is controlled by the MSC 14 which also performs the diversity combining. Thus, two circuits are currently in use for this transaction on figure 1, would be embodied as TDMA radio links in which several timeslots are used to provide communications channels to the systems users. During soft handoff, in particular, during inter-exchange soft handoff with di- versity combining, two (or more) timeslots could be used to provide the concurrent radio channels used by the MS and BSs involved in the handoff. All the other mentioned characteristics of the present invention remain the same for TDMA cellular telecommunications system. The previous description of the preferred embodi¬ ments are provided to enable any person skilled in the art to make or use the present invention. Various modifi¬ cations to these embodiments will be readily apparent to those skilled in the art, and the generic principles de- fined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present in¬ vention is not intended to be limited to the embodiment herein, but is to be accorded the widest scope consistent with the principles as novel features disclosed herein.

Claims: Claims 1. A control method in a cellular telecommunica¬ tions system, in which a user mobile station relays user communications signals via at least one of a plurality of base stations and in which said base stations further relay said user communications signals via at least one of a plurality of mobile exchanges to and from another system user, said method comprising steps of executing an inter-exchange soft handoff between a first and a second mobile exchange and having user communications control and signal diversity combining functions involved with the handoff at the first mobile exchange, c h a r a c t e r i z e d by the step of executing a handoff of said user communications control and signal diversity com¬ bining functions from said first mobile exchange to said second mobile exchange. 2. Method according to claim 1, c h a r a c t e - r i z e d in that the control handoff comprises the steps of requesting a control handoff from said second mo¬ bile exchange by said first mobile excange , allocating and activating user communication cont¬ rol and diversity combining resources required by said second mobile exchange for processing and controlling said ongoing user communications, taking control of said user communications by means of said allocated resources, informing user's mobile station about the control handoff, acknowledging the successful completion of the control handoff to said first mobile exchange, releasing said resources which were involved with said user communications at said first mobile exchange. 3. Method according to claim 1 or 2 , c h a r a c t e r i z e d in that the control handoff is utilized in a CDMA system. 4. Method according to claim l or 2 c h a r a c t e r i z e d in that the control handoff is utilized in a TDMA system. 5. A cellular telecommunications system, comp¬ rising mobile exchanges, base stations, and user mobile stations roaming in the system, each of the mobile ex¬ changes comprising means for inter-exchange soft handoff with diversity combining, c h a r a c t e r i z e d in that said mobile exchanges further comprise means for control handoff, wherein user communications control and signal diversity combining functions involved with said user communications are handed off from a first mobile exchange to a second mobile exchange. 6. System according to claim 5, c h a r a c t e r i z e d in that said means for the control handoff comp¬ rises means at said first mobile exchange for requesting a control handoff from said second mobile exchange, means at said second mobile exchange for alloca¬ ting and activating user communications control and di¬ versity combining resources required by said second mobi¬ le exchange for processing and controlling said ongoing user communications, means at said second mobile exchange for taking control of said user communications by means of said al¬ located resources, means at said second mobile exchange for informing said user's mobile station about the control handoff, means at said second mobile exchange for acknow¬ ledging the successful completion of the control handoff to said first mobile exchange, means at said first mobile station for releasing said resources which were involved with said user com- munications at the said first mobile exchange. 7. System according to claim 5 or 6, c h a r a c t e r i z e d in that the system is a CDMA system. 8. System according to claim 5 or 6, c h a r a c¬ t e r i z e d in that the system is a TDMA system.

Inventors: MUSZYNSKI PETER, MUSZYNSKI, PETER
Applicants: MUSZYNSKI PETER, NOKIA TELECOMMUNICATIONS OY, MUSZYNSKI, PETER
Application year: 1993
Main classification: H04Q7
IPC codes: H04B7, H04W36