Abstract:
A communications system includes a wireless telephony network and a wireless Local Area Network (LAN), both accessible by a mobile communications device ( 16 ). To facilitate transitioning of the mobile communications device to the wireless LAN from the wireless telephony network, the wireless LAN includes a beacon transmitter, which generates a synchronization channel having a pattern unique to the wireless LAN. The Wireless LAN synchronization channel is received at a first receiver in the mobile communications device together with a synchronization channel from the wireless telephony. The wireless LAN synchronization channel enables the mobile communication device to synchronize with, for transitioning to, the wireless LAN.

Description:
The applicants hereby claim the priority under 35 USC 119(a) of International Application PCT/US2003/031536, filed Oct. 3, 2003 and was published in accordance with PCT Article 21(2) on May 12, 2005 in English. 
     TECHNICAL FIELD 
     This invention relates to a technique for enabling a mobile communications device to transition from a first wireless communications network to a second network. 
     BACKGROUND ART 
     Wireless telephony technology continues to evolve over time. Most recently, the European Telecommunications Standards Institute (ETSI) has promulgated a new standard for mobile telephony service, known as “Universal Mobile Telecommunications Service” or UMTS which offers broadband, packet-based transmission of voice, text, video and multimedia information at rates as high as 2 Mbs. The proposed UMTS standard describes a wireless network that includes one or more radio access nodes, each typically referred to as a “Node B.” One or more Radio Network Controllers (RNC) exist within the UMTS network to manage the radio access nodes. Each RNC has a broadband connection, typically in the form of an Asynchronous Transport Mode (ATM) Link, to a UMTS core network that provides Authorization, Authentication and Accounting (AAA) Functions. 
     Advances in the field of wireless LAN technology have resulted in the emergence of publicly accessible wireless LANs (e.g., “hot spots”) at rest stops, cafes, libraries and similar public facilities. Presently, wireless LANs offer mobile communications device users access to a private data network, such as a Corporate Intranet, or a public data network such as the Internet. The relatively low cost to implement and operate a wireless LAN, as well as the available high bandwidth (usually in excess of 10 Megabits/second) makes the wireless LAN an ideal access mechanism through which the mobile communications device user can exchange packets with an external entity. 
     Given the lower access cost and higher bandwidth afforded by wireless LANs, users of mobile communications devices often find it advantageous to transition from a wireless telephony network, such as the UMTS network, to a wireless LAN. To that end, many present day wireless LAN operators provide coverage areas that overlap the coverage area of a UMTS network. Also, many mobile communication device manufacturers will include a dual protocol stack within the device to enable the device user to transition between networks. 
     Ideally, transitioning from the wireless telephone network to the wireless LAN (and vice versa) should occur seamlessly, i.e., without any loss of communications. To achieve a seamless transition from a UMTS network to the wireless LAN, the mobile communications device must first detect entry into the coverage area of the wireless LAN before undertaking a handoff from the wireless telephony network to the wireless LAN. Presently, the mobile communications device undertakes detection of a wireless LAN by operating a second receiver tuned to the wireless LAN while a first receiver remains tuned to the wireless telephony network. Continuously operating both the first and second receivers concurrently consumes battery resources in the mobile communications device, often a precious commodity. 
     Thus, there is a need for a technique for enabling a mobile communications device to seamlessly transition from one wireless network to another while overcoming the aforementioned disadvantage of the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly, in accordance with the present principles, there is provided a technique for enabling a mobile communications device to transition from a first wireless communications network, such as a wireless telephony network, to a second wireless network, such as a wireless LAN. To enable such a transition, a synchronization signal (hereinafter referred to as the “second network synchronization signal”) is generated in the second wireless network with a synchronization pattern that is unique to that network. In other words, the second network synchronization signal is readily distinguishable from a first network synchronization signal generated by the first network. The second wireless network broadcasts the second network synchronization signal for receipt by a common receiver in the mobile communications device that also receives the first network synchronization signal. Upon receipt of the second network synchronization signal, the mobile communications device can establish a communication session with the second wireless network to enable a transition thereto from the first wireless communications network. Transmitting the second network synchronization signal for receipt at the same receiver in the mobile communications device that receives the first network synchronization signal obviates the need to continuously operate two separate receivers, thus reducing the consumption of battery resources. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a block schematic diagram of a first wireless communications network interworked with a second wireless communications network; 
         FIG. 2  depicts a block schematic diagram of the details of the second wireless communications network modified in accordance with the present principles; 
         FIG. 3  depicts a first embodiment of a synchronizing signal for synchronizing a mobile communications device with the second wireless communications network of  FIGS. 1 and 2 ; and 
         FIG. 4  depicts a second embodiment of a synchronizing signal for synchronizing a mobile communications device with the second wireless communications network of  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a block schematic diagram of a communications system  10  that includes a first wireless communication network  12  interworked with a second wireless communication network  14  for providing communications services to one or more mobile communications devices, exemplified by device  16 . In practice, the mobile communications device  16  can comprise a wireless telephony handset, a wireless Personal Data Assistant (PDA) or a personal computer having a wireless modem. In the illustrated embodiment, the first wireless communications network  12  comprises a wireless telephony network having an architecture in conformance with the well-known Universal Mobile Telephone (i.e., 3G) standard. To that end, the wireless telephony network  12  includes at least one, and preferably, a plurality of radio access nodes  18  (each typically referred to as a “node B”), each node providing wireless telephony service within a corresponding area (cell)  20 . 
     The wireless telephony network  12  of  FIG. 1  also includes at least one Radio Node Controller (RNC)  22  for controlling one or more of the plurality of radio access nodes  18 . Depending on the number cells  20  and hence, the number of radio access nodes  18 , the wireless telephony network  12  could include a plurality of RNCs  22 . Each RNC  22  is controlled by a core network  24  that typically includes at least a Serving Gateway Service Node (SGSN) (not shown) that provides Authentication, Authorization and Accounting (AAA) functions within the core network. The second wireless network  14  may be partially or fully overlaid by a first one of the cells  20 , with  FIG. 1  depicting a fully overlaid cell  20 . 
     Referring to  FIG. 2 , the second wireless communications network  14  typically takes the form of a wireless LAN that includes one or more access points  26 . Each access point  26  within the wireless LAN  14  includes a radio transceiver (not shown) for transmitting RF signals to, and for receiving RF signals from the mobile communications device  16 . In practice, one or more of the access points  26  of the wireless LAN  14  reside within one of the cells  20  of the wireless telephony network  12  of  FIG. 1 . Thus, the mobile communications device  16 , while resident in a cell  20 , will enter the coverage area of one of the access points  26  of the wireless LAN  14  and can initiate a communications session with the wireless LAN. 
     Ideally, the transition (hand off) of the mobile communications device  16  from the wireless telephony network  12  of  FIG. 1  to the wireless LAN  14  illustrated in  FIG. 2  should occur seamlessly. In other words, the capability should exist for the mobile communications device  16  to establish a communications session with the wireless LAN  16  while the device remains in communications with the wireless telephony network  12  of  FIG. 1 . In order for the mobile communications device  16  to establish a communications session with the wireless LAN  14 , the device must have the ability to detect the presence of the wireless LAN. 
     Traditionally, the mobile communications device  16  has continuously operated two protocol stacks, and two receivers (not shown), one tuned to the wireless telephony network  12 , and the other tuned to the wireless LAN  14 . In this way, the mobile communications device can detect the presence of the wireless LAN while still in communications with the wireless telephone network. This approach to achieving seamless transitioning between networks, while effective, nonetheless requires the continuous use of two separate receivers, and the attendant consumption of precious battery power. 
     In accordance with the present principles, seamless transitioning can be effected without the need to operate two separate receivers simultaneously within the mobile communications device  16 . To enable the communications device  16  to detect the presence of the wireless LAN  14  by means of a first receiver (not shown) in the device, the wireless LAN  14  includes a basic transmitter  30  that transmits an identifying signal at the same frequency as a synchronizing signal transmitted by the wireless telephony network  12 . The identifying signal transmitted by the transmitter  30  of  FIG. 2  has a format compatible with the wireless telephony network  12  but is unique to the wireless LAN  14 , thereby avoiding interference with the synchronizing signal from the wireless telephony network  12 . In practice, the identifying signal transmitted by the transmitter  30  of  FIG. 2  has a signal strength corresponding to the coverage of the wireless LAN  14 . 
     The identifying signal transmitted by the transmitter  30  of  FIG. 2  can take one of several forms. Referring to  FIG. 3 , the signal from the transmitter  30  of  FIG. 2  can take the form of a Primary Synchronization Channel (P-SCH) corresponding to a downlink signal transmitted in the wireless telephony network  12  for effecting synchronization at the outset of a cell search. The P-SCH signal depicted in  FIG. 3  comprises a periodic synchronizing code having fifteen slots per frame, each frame being typically 10 ms in length. The synchronizing code is the same for each slot and has a format unique to the wireless LAN  14  so that the mobile communications device, upon receiving the code can specifically identify the wireless LAN  14  of  FIG. 2 . 
     In practice, the mobile communications device  16  establishes the identity of the wireless LAN  14  by matching the characteristics of the received P-SCH channel to one of a plurality of stored patterns in memory, each pattern corresponds to a particular wireless network technology. Once the mobile communications device  16  establishes the identity of the wireless LAN  14 , the device stops the search for a match. Thereafter, the mobile communications device  16  initiates operation of a second receiver dedicated to the associated wireless LAN radio technology to effect the transition from the wireless telephony network  12  of  FIG. 1 . In the illustrated embodiment, the Frequency Division Duplex (FDD) mode is assumed when the wireless telephony network  12  embodies WCDMA technology. In a wireless telephony network embodying TDMA technology, the Time Division Duplex (TDD) mode or a similar mechanism would apply. 
     As an alternative solution, the transmitter  30  could transmit a Secondary Synchronization Channel (S-SCH) as seen in  FIG. 4  which is comparable to the downlink signal transmitted by the wireless telephony network  12  of  FIG. 1  to effect frame synchronization and scrambling code detection as part of the cell search operation. In practice, the S-SCH channel corresponds to the repetition of a sequence of fifteen Secondary Synchronization codes (SSCs). Each code is chosen among sixteen such codes and is modulated within a given time slot. There exist sixty-four different possible combinations (or SSC sequences), each sequence corresponding to a separate scrambling code group and each having the same duration as the collective fifteen slots of a radio frame. Upon receipt of the SSC sequence, the first receiver in the mobile communications device  16  matches the sequence to a corresponding stored value indicative of the scrambling code group. Thereafter, the first receiver in the mobile communications device  16  finds the scrambling code group and becomes frame synchronized to the basic transmitter  30 . 
     After finding the scrambling code group, the first receiver within the mobile communications device  16  looks for a primary CPICH (Common Pilot Channel) channel that carries a well-specified bit sequence that is the same for all cells. From knowledge of the scrambling code group, the receiver will attempt a match to the CPICH channel bit sequence to the stored value (1 among 8 possible sequences). In this way, the first receiver will detect the wireless LAN  14 . The wireless LAN detection mechanism is based on the reservation of a scrambling code (the combination of a scrambling code group and a scrambling code in that group) allocated for inter-working of the wireless LAN  14  of  FIG. 2  with the wireless telephony network  12  of  FIG. 1 . 
     In case the operator of the wireless LAN  14  implements more than one wireless LAN technology, the above-described detection mechanism requires enhancement in one of the two ways: 
     First Solution:
         Each new wireless LAN inter-working technology receives its own scrambling code. Standardization of this solution could prove potentially problematic since the number of downlink primary scrambling codes is limited.       

     Second Solution:
         After recognition of this wireless LAN scrambling code, the mobile communication device  16  will go through an extra step to identify the radio technology of the wireless LAN by looking for the Primary Common Control Physical Channel (PCCPCH) that carries system information. The P-CCPCH channel has a well-specified channelization code and thus can bring a new type of system message that will allow the mobile communications device  16  to discover the type of wireless LAN cell it is entering.
 
In order to implement the second solution, the following are required:
   The mobile communications device  16  must monitor listed cells (including the wireless LAN  14 ). To identify a cell, the mobile communications device  16  must identify its primary scrambling code (one among 512). This solution works as long as the cell belongs to the monitored list. Typically, the mobile communications device  16  is constrained in terms of the maximum numbers of cells the device can measure in parallel.   Once it detects the wireless LAN  14 , the mobile communications device  16  stops searching for cells and no longer performs any measurement, nor does it signal the presence of any subsequently detected cell to its corresponding cell controller. After being detected, the wireless LAN  14  manages future communication with the mobile communications device  16 .
 
Ideally, there should be standardization of the scrambling codes dedicated to wireless LAN technology by the 3GPP standardization body or the like. In addition, there should also be standardization of the message format and content. Although standardization of the message format and content isn&#39;t mandatory, such standardization will assure that all mobile terminals will be compliant regardless of the network operator.
       

     The foregoing describes a technique for enabling a mobile communications device to seamlessly transition from a wireless telephony network to a wireless LAN without unnecessary consumption of battery resources.