Abstract:
A communication system supports multiple source coding and/or channel coding schemes. When a handoff is needed that transfers radiocommunication service support responsibility from a base station controlled by one MSC to a base station controlled by another MSC, signaling procedures are employed to handle the transfer as it pertains to the source coding and/or channel coding, i.e., ensuring that the new MSC links in an appropriate codec to support the handed off connection. Various techniques are described to provide such signaling, both in the direction of the new (target) MSC and in the direction of the user equipment being handed off.

Description:
RELATED APPLICATION 
     This application is related to, and claims priority from, U.S. Provisional Patent Application Ser. No. 60/174,165, entitled “Inter-MSC Handover,” filed on Jan. 3, 2000, the disclosure of which is incorporated here by reference. 
    
    
     BACKGROUND 
     The present invention generally relates to coding in the field of communication systems and, more particularly, to handling the signaling for controlling transcoding when a connection is handed off between mobile switching centers (MSCs) in radiocommunication systems. 
     The growth of commercial communication systems and, in particular, the explosive growth of cellular radiotelephone systems, have compelled system designers to search for ways to increase system capacity without reducing communication quality beyond consumer tolerance thresholds. One technique to achieve these objectives involved changing from systems wherein analog modulation was used to impress data onto a carrier wave, to systems wherein digital modulation was used to impress the data on carrier waves. 
     In wireless digital communication systems, standardized air interfaces specify most of the system parameters, including speech coding type(s), burst format, communication protocol, etc. For example, the European Telecommunication Standard Institute (ETSI) has specified a Global System for Mobile Communications (GSM) standard that uses time division multiple access (TDMA) to communicate control, voice and data information over radio frequency (RF) physical channels or links using a Gaussian Minimum Shift Keying (GMSK) modulation scheme at a symbol rate of 271 ksps. In the U.S., the Telecommunication Industry Association (TIA) has published a number of Interim Standards, such as IS-54 and IS-136, that define various versions of digital advanced mobile phone service (D-AMPS), a TDMA system that uses a differential quadrature phase shift keying (DQPSK) modulation scheme for communicating data over RF links. 
     TDMA systems subdivide the available frequency into one or more RF channels. The RF channels are further divided into a number of physical channels corresponding to time slots in TDMA frames. Logical channels are formed of one or several physical channels where modulation and coding is specified. In these systems, the mobile stations communicate with a plurality of scattered base stations by transmitting and receiving bursts of digital information over uplink and downlink RF channels. 
     The growing number of mobile stations in use today has generated the need for more voice and data channels within cellular telecommunication systems. As a result, base stations have become more closely spaced, with an increase in interference between mobile stations operating on the same frequency in neighboring or closely spaced cells. In fact, some systems now employ code division multiple access (CDMA), using a form of spread spectrum modulation wherein signals intentionally share the same time and frequency. Although digital techniques provide a greater number of useful channels from a given frequency spectrum, there still remains a need to maintain interference at acceptable levels, or more specifically to monitor and control the ratio of the carrier signal strength to interference, (i.e., carrier-to-interference (C/I) ratio). 
     Another factor which is increasingly important in providing various communication services is the desired/required user bit rate for data to be transmitted over a particular connection. For example, for voice and/or data services, user bit rate corresponds to voice quality and/or data throughput, with a higher user bit rate producing better voice quality and/or higher data throughput. The total user bit rate is determined by a selected combination of techniques for speech coding, channel coding, modulation, and resource allocation, e.g., for a TDMA system, this latter technique may refer to the number of assignable time slots per connection, for a CDMA system, this latter parameter may refer to the number of assignable codes per connection. 
     Speech coding (or more generally “source coding”) techniques are used to compress the input information into a format which uses an acceptable amount of bandwidth but from which an intelligible output signal can be reproduced. Many different types of speech coding algorithms exist, e.g., residual excited linear predictive (RELP), regular-pulse excitation (RPE), etc., the details of which are not particularly relevant to this invention. More significant in this context is the fact that various speech coders have various output bit rates and that, as one would expect, speech coders having a higher output bit rate tend to provide greater consumer acceptance of their reproduced voice quality than those having a lower output bit rate. As an example, consider that more traditional, wire-based telephone systems use PCM speech coding at 64 kbps, while GSM systems employ an RPE speech coding scheme operating at 13 kbps. 
     In addition to speech coding, digital communication systems also employ various techniques to handle erroneously received information. Generally speaking, these techniques include those which aid a receiver to correct the erroneously received information, e.g., forward error correction (FEC) techniques, and those which enable the erroneously received information to be retransmitted to the receiver, e.g., automatic retransmission request (ARQ) techniques. FEC techniques include, for example, convolutional or block coding (collectively referred to herein as “channel coding”) of the data prior to modulation. Channel coding involves representing a certain number of data bits using a certain number of code bits. Thus, for example, it is common to refer to convolutional codes by their code rates, e.g., ½ and ⅓, wherein the lower code rates provide greater error protection but lower user bit rates for a given channel bit rate. 
     Conventionally, each of the techniques which impacted the user bit rate were fixed for any given radiocommunication system, or at least for the duration of a connection established by a radiocommunication system. That is, each system established connections that operated with one type of speech coding, one type of channel coding, one type of modulation and one resource allocation. More recently, however, dynamic adaptation of these techniques has become a popular method for optimizing system performance in the face of the numerous parameters which may vary rapidly over time, e.g., the radio propagation characteristics of radiocommunication channels, the loading of the system, the user&#39;s bit rate requirements, etc. This type of dynamic adaptation of coding techniques has been referred to in the GSM standard as adaptive multirate (AMR) communications. AMR techniques and the like are likely to be used in next generation radiocommunication systems, e.g., Universal Mobile Telecommunication Systems (UMTS). 
     In GSM systems, AMR techniques have been traditionally been coordinated by the base station controller (BSC). For context, consider FIG. 1, which depicts various nodes in a hybrid UMTS/GSM communication system  10 . This Figure will also be referred to below in describing aspects of the present invention. The system  10  is designed as a hierarchical network with multiple levels for managing calls. Using a set of uplink and downlink frequencies, mobile stations  12  operating within the system  10  participate in calls using time slots allocated to them on these frequencies. At an upper hierarchical level, a group of Mobile Switching Centers (MSCs)  14   a - 14   c  are responsible for the routing of calls from an originator to a destination. In particular, these entities are responsible for setup, control and termination of calls. One of the MSCs  14   a , known as the gateway MSC, handles communication with a Public Switched Telephone Network (PSTN)  18 , or other public and private networks. 
     At a lower hierarchical level, each of the MSCs  14   a - 14   c  are connected to a group of BSCs  16   a-b  (although the BSCs are only depicted for MSC  14   b  to simplify the figure) using, for example, PCM (pulse code modulated) links. Under the GSM standard, the BSCs  16   a - 16   b  communicate with MSCs  14   a - 14   c  under a standard interface known as the A-interface, which is based on the Mobile Application Part (MAP) of CCITT Signaling System No. 7. Under the UMTS standard, the BSCs are referred to as radio network controllers (RNCs) and communicate with the MSCs  14   a - 14   c  over a standard interface known as the Iu interface. 
     At a still lower hierarchical level, each of the BSCs (RNCs)  16  controls a group of base transceiver stations (BTSS)  20 . Each BTS  20  includes a number of TRXs (not shown) that use the uplink and downlink RF channels to serve a particular common geographical area, such as one or more communication cells  21 . The BTSs  20  primarily provide the RF links for the transmission and reception of data bursts to and from the mobile stations  12  within their designated cell. As mentioned earlier, the information transmitted on these RF links may be speech coded in various ways to enhance various aspects of communication over the air interface, which coding is controlled in conventional GSM systems by the responsible BSC. 
     However, in next generation systems (e.g., UMTS) it has been proposed that the coding of this information be controlled by the MSC rather than the radio network controller (RNC), which is the nomenclature for the node corresponding to the BSC in third generation systems. This shifting of responsibility leads to a number of problems regarding management of coding, particularly during handoffs of connections between MSCs. For example, it will be necessary to know which MSC, i.e., the serving (anchor) MSC or the target MSC, will control the transcoder. Additionally, it will be useful to define where the radio access network protocol is terminated and the format for signaling between the MSCs involved in the handoff to control coding parameters. 
     Accordingly, it will be desirable to provide methods and systems for handling the transcoding of connections being handed off between MSCs which take into account these various aspects of emerging, next generation communication systems. 
     SUMMARY 
     These and other drawbacks, problems and limitations of conventional radiocommunication systems are overcome according to the present invention, wherein various signaling techniques and protocols are described for handling codec issues during inter-MSC handoff. For example, exemplary embodiments of the present invention handle codec issues associated with inter-MSC handoff by determining at least one codec that is available in a target MSC, selecting, by an anchor MSC, one of the available codecs for handling the connection, signaling the target MSC with an indication of the selected codec, and informing the user equipment of the selected codec. 
     Various exemplary embodiments perform these signaling steps in different ways. For example, the determination of available codecs in the target MSC may (or may not) require signaling to ascertain the target MSC&#39;s codec capabilities. If such signaling is performed, mobile application part (MAP) protocol messages may be used. 
     According to another aspect of the present invention, the signaling used to inform the target MSC of the selected codec may take any of a plurality of different forms. For example, the selected codec information may be forwarded to the target MSC as a MAP message which is independent of another message wherein the radio access bearer (RAB) assignment is forwarded. Alternatively, the selected codec information can be passed to the target MSC using the same message by which the RAB is assigned. Different protocols, e.g., radio access network application protocol (RANAP) or base station system management application part (BSSMAP) can be used to as the signaling protocol for these messages. 
     Likewise, signals informing the user&#39;s equipment of the selected codec can be provided in different forms. For example, a call control message can be sent to the user&#39;s equipment identifying the selected codec so that it is prepared to properly process messages for transmission and reception after the inter-MSC handoff. Alternatively, an RANAP message identifying the RAB can also be used to convey selected codec information to the user equipment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will become more apparent upon reading from the following detailed description, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram of an exemplary GSM communication system which advantageously uses the present invention; 
     FIG. 2 is a flow chart depicting a general set of steps for handling inter-MSC handoff according to exemplary embodiments of the present invention; 
     FIG. 3 is a signaling/block diagram which depicts inter-MSC handoff signaling according to an exemplary embodiment of the present invention; 
     FIG. 4 is a signaling/block diagram which depicts inter-MSC handoff signaling according to another exemplary embodiment of the present invention; 
     FIG. 5 is a signaling/block diagram which depicts inter-MSC handoff signaling according to yet another exemplary embodiment of the present invention; 
     FIGS.  6 ( a ) and  6 ( b ) depict protocol stacks for the control plane and user plane, respectively, for the exemplary embodiments of FIG. 5; 
     FIG. 7 is a signaling/block diagram which depicts inter-MSC handoff signaling according to still another exemplary embodiment of the present invention; 
     FIGS.  8 ( a ) and  8 ( b ) depict protocol stacks for the control plane and user plane, respectively, for the exemplary embodiment of FIG. 7; and 
     FIG. 9 is a signaling/block diagram which depicts inter-MSC handoff signaling according to still another exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The various features of the invention will now be described with reference to the figures, in which like parts are identified with the same reference characters. In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, and circuits are omitted so as not to obscure the description of the present invention. 
     The following exemplary embodiments are provided in the context of TDMA radiocommunication systems. However, those skilled in the art will appreciate that this access methodology is merely used for the purposes of illustration and that the present invention is readily applicable to all types of access methodologies including frequency division multiple access (FDMA), TDMA, code division multiple access (CDMA) and hybrids thereof. 
     Moreover, operation in accordance with GSM communication systems is described in, for example, European Telecommunication Standard Institute (ETSI) documents ETS 300 573, ETS 300 574 and ETS 300 578, which are hereby incorporated by reference. Therefore, the operation of the GSM system is only described herein to the extent necessary for understanding the present invention. Similar standards related documents are also available for UMTS. Although, the present invention is described in terms of exemplary embodiments in a UMTS/GSM system, those skilled in the art will appreciate that the present invention may be applicable to digital communication systems. 
     When user equipment (UE), e.g., a mobile phone or personal digital assistant, moves to a location wherein its wireless connection with a fixed network has deteriorated to a point where a new connection is desirable, a handoff procedure is initiated to provide the UF with continued service via a different portion of the fixed network. In some cases, the best candidate base station for providing this continued service is controlled by a different (target) MSC than that which is currently controlling the UE&#39;s service, i.e., the anchor MSC, in which case the handoff can be referred to as an “inter-MSC” handoff. Various control signaling is performed in order to accomplish inter-MS C handoffs. 
     In general, the handling of codec control during inter-MSC handoff according to the present invention follows a number of steps as illustrated in FIG.  2 . Therein, at step  100 , the codecs available in the target MSC are determined. Those skilled in the art will appreciate that not all MSCs will contain the same type and number of codecs and, therefore, it is useful to determine the target MSCs codec capabilities before making a codec selection. This determination may be rather straightforward if the codecs supported by the target MSC are administered by the anchor MSC. Alternatively, a message can be transmitted from the anchor MSC to the target MSC to request a supported codec list therefrom. These messages may be formatted as, for example, MAP messages since MAP messages will terminate in the target MSC. 
     In any event, once the anchor MSC has determined the codecs available in the target MSC, a codec selection is made to determine which codec in the target MSC shall be linked in to handle the connection once it has been handed off (step  102 ). This selection can be based on a comparison between the codec list associated with the target MSC and the codecs supported by the UL, the latter of which can be identified to the anchor MSC during call set-up. For example, the call setup message (or call confirmation message) transmitted from the UE to the anchor MSC may include a GSM codec list and a UMTS codec list. Separate lists may be provided for each system in order to ensure unique codec identification, e.g., since AMR for GSM and AMR for UMTS may differ in rate adaptation and discontinuous transmission (DTX) modes. Additionally, the UE may transmit a separate intersystem code handoff list which may include preferences for the codec selected during inter-MSC handoff described below. If the target MSC and the UE support more than one of the same codecs, then a selection from among the common codecs can be made based on preferences, e.g., either network preferences, UE preferences or some combination thereof. The particular mechanism by which a particular codec is selected for handling the connection after the handoff is beyond the scope of this discussion. 
     After a codec is selected for use in the target MSC, the target MSC needs to be informed of that selection in step  104  so that it can link in the selected codec to handle the connection once it is handed off. Likewise, at step  106 , the UE needs to be informed of the codec selection so that it can employ the appropriate codec post-handoff. Those skilled in the art will appreciate that steps  104  and  106  can be performed in any temporal order including simultaneously. 
     Those skilled in the art will also appreciate that steps  100 - 106  illustrated in FIG. 2 can be implemented in many different ways depending, for example, upon network topologies and which systems, e.g., GSM and/or UMTS, are involved in the inter-MSC handoff process. Several examples will now be provided to illustrate these variations. 
     Consider the example of FIG. 3 for a handoff between two MSCs which operate in accordance with the UMTS standard. Therein, a UE  300  needs a handoff (relocation required by RNC) due to, for example, a change in position relative to a serving RNC  302  or base station (not shown) associated therewith. A relocation request is communicated through to the anchor MSC  304  which then begins, among other tasks, to coordinate the codec transition to the new (target) MSC  306 . In this particular example, the anchor MSC  304  administers the codecs in the target MSC  306 , such that no signaling needs to be performed between the two MSCs to establish the codec capability of the target MSC  306  prior to a selection being made. The anchor MSC  304  can, therefore, make its codec selection based upon its a priori knowledge of the codecs available in the target MSC  306 , UE  300  and preferences. 
     Once a codec selection is made, the anchor MSC  304  informs the target MSC  306  and UE  300  of the selection using different mechanisms. According to this exemplary embodiment, the anchor MSC  304  sends a BSSMAP Prepare HandoverRequest Message which includes, as a parameter, the preferred speech version over a synchronous transfer mode (STM) link  308 . The preferred speech version indicates which type of codec is to be seized by the target MSC  306  for this handoff. Those skilled in the art will recognize that BSSMAP is the acronym for Base Station System Management Application Part which identifies a protocol which is used over the A-interface. Those interested readers desiring more information regarding BSSMAP protocols are directed to the published standard GSM 08.08: “Digital cellular telecommunications system (Phase 2+); Mobile Switching Centre—Base Station System (MSC-BSS) interface Layer 3 specification,” the disclosure of which is incorporated here by reference, for more information. The BSSMAP message may also inform the target MSC  306  which radio access bearer (RAB) is requested for the new connection. The target MSC  306 , in turn, provides the relevant RNC  310  with a Relocation Request over the Iu interface to alert it to its responsibilities for the handoff of UE  300  thereto. The target MSC  306  also acknowledges the PrepareHandover Request message to the anchor MSC  304 . In the UE direction, the anchor MS  304  sends a call control (CC) message informing the UE  300  of the selected codec. 
     According to another embodiment of the present invention illustrated in FIG. 4, the inter-MSC handoff occurs from a UMTS network to a GSM network. Therein, similar reference numerals are used to refer to similar functional units as set forth above with respect to FIG.  3 . Moreover, to avoid redundancy, those similarities between the two embodiments are not repeated here and reference is made to the foregoing for the description of similar signaling. Unlike the embodiment of FIG. 3, however, the target MSC  306  coordinates with a target BSC  400  which contains the transcoder unit (TRAU) to be used to support the UE  300  after the handoff. Thus, in this embodiment, the selected codec information is passed from the target MSC  306  to the BSC  400  over the A-interface via a Handover Request message. Also note that the link  308  between the (GSM) target MSC  306  and the (UMTS) anchor MSC  304  can either be an STM link or an asynchronous transfer mode (ATM) link. 
     Another variation of a UMTS MSC to UMTS MSC handoff is depicted in the exemplary embodiment of FIG.  5 . Again, similar reference numerals are used to refer to similar functional units as set forth above with respect to FIG.  3  and differences are discussed with respect thereto. Therein, the anchor MSC  304  is not responsible for administering the codecs supported by the target MSC  306 . Accordingly, step  100  of FIG. 2 is performed by sending a MAP message requesting that the supported codecs be identified over the STM link  308  and receiving a MAP response indicating those supported codecs so that the anchor MSC  304  can then select a codec, e.g., by comparison to a list supplied by UE  300  and preferences, for the handoff. 
     Once a codec is selected, the anchor MSC  304  sends a MAP message indicating the selected codec to the target MSC  306 . Unlike the embodiment of FIG. 3, the signaling associated with the selected codec in this embodiment is separate from the signaling relocation request signaling, which permits the relocation request signaling to be codec independent. This, in turn, provides for a clear separation of functionality as MAP signalling is used in both GSM and UMTS systems and terminates in the target MSC. Additionally, the embodiment of FIG. 5 employs the RANAP protocol for transmitting the relocation request message, as opposed to the BSSMAP protocol employed in the exemplary embodiment of FIG.  3 . Those skilled in the art will recognize that RANAP is the acronym for Radio Access Network Application Protocol which identifies the protocol defined for use over the Iu interface between UMTS MSCs and RNCs to maintain links there between. For more information regarding RANAP signaling in general, the interested reader is directed to the published standard 3 GPP 25.413: “Iu interface RANAP signaling,” the disclosure of which is incorporated here by reference. 
     The embodiments of FIGS. 3-5 are depicted using signaling diagrams and functional block units. However, as will be appreciated by those skilled in the art, the choice of signaling protocols used to perform codec handling during inter-MSC handoff according to the present invention can also be described using protocol stacks. The resulting protocol stacks after handoff from the anchor MSC  304  to the target MSC  306  (with an STM link there between) are illustrated in FIGS.  6 ( a ) and  6 ( b ) for the control plane and user plane, respectively. A non-access stratum (NAS) container can be used to covey the chosen codec information from the MSC to the UE. 
     If ATM links are used between the MSCs involved in the handoff, then ATM need not be terminated in the target MSC  306  and compressed speech, for example, can be passed transparently through the target MSC  306  to the anchor MSC  304 . This means, in turn, that transcoding need not necessarily be performed in the target MSC  306  and, instead, can be performed in the anchor MSC  304 . One exemplary embodiment which illustrates an inter-MSC handoff using an ATM link is provided as FIG.  7 . Therein, it will be noted that the signaling used to perform the inter-MSC handoff is similar as that described above and illustrated in FIG. 5, with the exception that there is no need to inform the target MSC  306  of the selected codec, since the same codec in the anchor MSC  304  is used both pre- and post-handoff. Thus, from the protocol stack perspective (see FIGS.  8 ( a ) and  8 ( b )), the target MSC  306  is transparent on the user plane and is used only for performing RANAP control signaling. Those skilled in the art will appreciate that although the embodiment of FIGS. 7,  8 ( a ) and  8 ( b ) provides certain advantages, e.g., compressed speech through to anchor MSC and continued use of the same codec after handoff, that the techniques described above with respect to FIGS. 3-5 can also be used in circumstances where ATM links exist between the relevant MSCs. 
     FIG. 9 depicts yet another example of signaling between various units associated with inter-MSC handoff according to the present invention. Therein, the UE  300 &#39;s codec lists are forwarded to the system during call setup. The target MSC is interrogated via MAP signaling for its list of supported/available codecs, after which the anchor selects a codec for the handoff. The UE and the target MSC are informed of the selection via various MAP and/or RANAP signaling as shown. 
     Although the invention has been described in detail with reference only to a few exemplary embodiments, those skilled in the art will appreciate that various modifications can be made without departing from the invention. Accordingly, the invention is defined only by the following claims which are intended to embrace all equivalents thereof.