Abstract:
The present invention takes the advantage of the WLAN resources in hotspots for voice and data connections and enables seamless handover between WWAN and WLAN and vice versa. This allows two users in the hotspots to communicate with each other by making a voice over IP (VoIP) or video call, thus achieving cost savings for the end users and WWAN resources savings for the wireless operators. According to one embodiment of the invention, a mobile terminal has wireless wide area network (WWAN) and wireless local area network (WLAN) interfaces. The mobile terminal via a WWAN. If a WLAN is present, the mobile terminal sends a handover request together with its network address to the called mobile terminal, using a short message service (SMS) technique or a watermarking technique, for initiating a WLAN call with the called mobile terminal via a WLAN. After the WLAN call is established with the called mobile terminal, the mobile terminal releases the WWAN call.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to wireless communication technology, and more particularly to wireless communication systems having wireless wide area network (WWAN) and wireless local area network (WLAN) interfaces. 
       FIG. 1  illustrates typical service areas in which mobile terminals such as mobile phones are used. In these areas, the WWAN service area (e.g., GSM, GPRS, CDMA, 3G) may or may not overlap with a WLAN service area (as shown in  FIG. 1 ). In the case where there is an overlap between the WWAN and WLAN service areas, both the WWAN and WLAN services are available. These overlapping areas are also referred to as hotspots, which include airports and hotels. 
     In hotspots (e.g., airports and hotels), people normally use mobile terminals for making voice calls and using low-speed data services via the WWAN and for using high-speed data services via the WLAN through an access point (AP). In situations in which two mobile terminals in the same hotspot or in different hotspots run by the same service provider are trying to communicate with each other, they still need to use the WWAN service to make voice calls. This is not cost effective even though a low cost alternative communication network, i.e., the WLAN, is available to both parties. 
     Therefore, there is a need to provide a cost effective method and system for seamlessly roaming between a WWAN and a WLAN in areas in which both services are available. 
     SUMMARY OF THE INVENTION 
     The present invention takes the advantage of the WLAN resources in hotspots for voice and data connections and enables seamless handover between WWAN and WLAN and vice versa. This allows two users in the hotspots to communicate with each other by making a voice over IP (VoIP) or video call, thus achieving cost savings for the end users and WWAN resources savings for the wireless operators. 
     According to one embodiment of the invention, there is provided a wireless communication system, e.g., a mobile terminal, having wireless wide area network (WWAN) and wireless local area network (WLAN) interfaces. The mobile terminal detects presence of a WWAN and a WLAN and establishes a WWAN call between this mobile terminal as a calling mobile terminal and a called mobile terminal via a WWAN. If the WLAN is present, the mobile terminal sends extra information including a handover request together with its network address (e.g., an IP address) to the called mobile terminal, using a short message service (SMS) technique or a watermarking technique, for initiating a WLAN call with the called mobile terminal via a WLAN. If a response received from the called mobile terminal indicates an acceptance of the handover request. The mobile terminal establishes the WLAN call with the called mobile terminal via the WLAN. Thereafter, the mobile terminal releases the WWAN call. However, if, during the WLAN call, the mobile terminal detects that it is moving away from the WLAN, the mobile terminal establishes a new WWAN call with the called mobile terminal via the WWAN. 
     In preferred embodiments of the invention, no modifications on the existing network infrastructure are required. Thus, the invention has no impact on current wireless network standards and can be easily deployed in the existing networks. 
     Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
         FIG. 1  illustrates typical service areas in which mobile terminals are used; 
         FIG. 2  shows a basic architecture of a mobile terminal with dual radio interfaces, according to one embodiment of the invention; 
         FIG. 3  illustrates a method for seamlessly roaming between a WWAN and a WLAN, according to one embodiment of the invention; 
         FIG. 4  shows a circuit for adding extra information as analog signals in a calling mobile terminal, in accordance with one embodiment of the invention; 
         FIG. 5  shows a circuit for demodulating the extra information added as analog signals in a called mobile terminal, in accordance with one embodiment of the invention; 
         FIG. 6  shows a circuit for adding the extra information as digital signals in a calling mobile terminal, in accordance with one embodiment of the invention; and 
         FIG. 7  shows a circuit for extracting the extra information inserted as digital signals in a called mobile terminal, in accordance with one embodiment of the invention. 
     
    
    
     Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows a basic architecture of a mobile terminal  10  with dual radio interfaces—WWAN and WLAN interfaces—for use in the invention. When a user tries to make a call with mobile terminal  10 , a user agent, which is responsible for profile management, service mediation, etc., will first make a WWAN call. After the call is set up, the user agent will determine which radio interface to use based on the user profile and the available radio interfaces. The user profile contains information about the radio interface preferences, handover permission during an active connection, etc. For example, if the user prefers a connection via the WLAN interface, the user agent will, after setting up the WWAN call, try to connect both parties via a WLAN. After the WLAN call (e.g., a voice or video call) is successfully set up, the WWAN call will be released without the user&#39;s direct involvement. 
       FIG. 3  illustrates a method  20  for seamlessly roaming between a WWAN and a WLAN, according to one embodiment of the invention. When a party enters into a hotspot, the user agent of this party&#39;s mobile terminal will detect the types of wireless services available. When the party tries to make a call, the user agent of this calling party&#39;s mobile terminal will make a call via a WWAN in a conventional way, using the MSISDN (Mobile Station International ISDN Number) of a called mobile terminal (steps S 22  and S 26 ). After the WWAN call is properly set up, the user agent of the calling mobile terminal will decide which radio interface to use based on the user profile of the calling party. If a WLAN is preferred, and the WLAN interface is available, the user agent will send extra information including a handover request together with information about the IP address, user profile, etc. of the calling mobile terminal to the called mobile terminal for handing over the call to the WLAN (step S 32 ). The extra information is transferred using the short message service (SMS) or the watermarking technology. The watermarking technology allows the extra information to be hidden within the voice, as will be described later. 
     Upon receiving the handover request, the user agent of the called mobile terminal will decide whether to accept the request, based on its own user profile and the available radio interface of the called mobile terminal. If the called mobile terminal agrees to hand over the call to the WLAN, its user agent will send similar extra information including a handover accept response together with information about the IP address, user profile, etc. of the called mobile terminal to the calling mobile terminal, using the SMS or the watermarking technology (step S 36 ). The calling mobile terminal will then initiate a call (e.g., a voice or video call) via a WLAN, using the received IP address of the called mobile terminal (steps S 42  and S 46 ). When the WLAN call has been successfully set up, the WWAN call will be released by both parties (steps S 52  and S 56 ). After the WLAN call is completed, it will also be released by both parties (steps S 62  and S 66 ). 
     In the above, during an active WLAN call, one of the parties may leave a WLAN service area. In such case, the user agent of this party&#39;s mobile terminal will recognize a change in the radio signals. If the calling party is leaving the area, the user agent of the calling mobile terminal will try to initiate a new WWAN call to the called mobile terminal. On the other hand, if the called party is leaving the area, the user agent of the called mobile terminal will send a handover request via the WLAN to the calling mobile terminal to initiate a new WWAN call. This new WWAN call will be automatically accepted by the called mobile terminal. After the new WWAN call is successfully set up, the WLAN call will be immediately released by both parties. Optionally, the called mobile terminal may also initiate the new WWAN call to the calling mobile terminal when the called party is leaving the area. 
     In the above, the handovers from a WWAN to a WLAN and vice versa are transparent to both users, so that the call between them is not affected. Also, the parties may communicate with each other via the Internet connecting two WLANs, within each of which one party is present. 
     The watermarking technology as used in the invention will now be described. By using this technology, the extra information (i.e., the handover request, IP address, user profile, etc.) is exchanged along with the voice in one channel, e.g., the voice channel between two mobile terminals. Such exchanges will not affect the normal voice conversation, since during a typical telephone conversation, 50% of the time is used for listening and 10% of the time is used for pausing between words and sentences. Therefore, there are sufficient resources for exchanging the extra information, without requiring modifications to the existing network infrastructure. The extra information may be added by the user agent in a mobile terminal as either analog signals or digital signals. 
       FIG. 4  shows a circuit  50  for adding the extra information (i.e., the handover request, IP address, user&#39;s profile, etc.) in the voice channel of a calling mobile terminal, in accordance with one embodiment of the invention. Circuit  50  includes a silence control circuit  54 , a modulation circuit  56 , and a time division multiplexer  62 . The user agent of the calling mobile terminal provides the extra information in the form of a digital sequence to modulation circuit  56  for modulation into analog signals. When silence control circuit  54  detects a silence in the input voice, it sends a control signal to modulation circuit  56 . Upon receiving the control signal, modulation circuit  56  outputs the extra information in an analog form to multiplexer  62 . Multiplexer  62  time multiplexes the voice output with the extra information and sends a combined output to an ADC for transmitting to a called mobile terminal in a voice channel. 
     The extra information can be modulated as some patterns of frequencies in the voice band (0˜4000 Hz). For example, the Dual Tone Multi Frequency (DTMF) technique may be used to transfer 16 digits (0˜9, A˜F). Frequencies that last certain time durations (e.g., 5 ms) stand for a symbol. As an example, the combined frequencies of 1336 Hz and 770 Hz stand for 6, and the combined frequencies of 1477 Hz and 852 Hz stand for 8. If a party wants to transmit “68”, the frequencies to be added into the voice will be |1336 Hz+700 Hz| simultaneously for 5 ms and |1477 Hz+852 Hz| simultaneously for 5 ms. 
       FIG. 5  shows a circuit  70  for demodulating the extra information received by a called mobile terminal in the voice channel, in accordance with one embodiment of the invention. Frequency detection circuit  70  includes a frequency detection circuit  72  and a demodulation circuit  76 . Frequency detection circuit  72  receives combined analog signals from a digital-to-analog converter (DAC), including both the voice and the extra information. Circuit  72  recognizes the frequencies used for representing symbols, and outputs the recognized frequencies to demodulation circuit  76  for decoding the pre-defined frequency patterns to extract the extra information. For example, if the frequency patterns |1336 Hz+700 Hz| and |477 Hz+852 Hz| described above are recognized by frequency detection circuit  72  and output to demodulation circuit  76 , demodulation circuit  76  will decode the frequency patterns and output a digital sequence of “68”. 
     To reduce decoding errors, the maximum time duration for the same frequency pattern is defined. For example, the same frequency that lasts longer than 50 ms is not allowed. If a calling mobile terminal sends more than 10 identical digits within a time duration over 50 ms (i.e., 10×5 ms), a disturbance frequency pattern (e.g., |1888 Hz+888 Hz|) will be added after the 10th digit. The called mobile terminal can locate the disturbance frequency pattern after the 10 identical digits. For instance, when the calling mobile terminal sends a digital sequence of 999,999,999,999 to the called mobile terminal, it will be received as 9,999,999,999,x99, where x represents a disturbance frequency pattern. 
       FIG. 6  shows a circuit  80  for adding the extra information as digital signals in a calling mobile terminal in accordance with one embodiment of the invention. Circuit  80  includes a silence control circuit  82  and a watermarking circuit  83 . Silence control circuit  82  detects silent periods in the input voice and upon detecting a silent period, provides a control signal to watermarking circuit  83 . Watermarking circuit  83  receives the input data, including the IP address, user profile, etc. from the user agent and upon receiving the control signal, outputs the data to a time multiplexer  84 . Watermarking circuit  83  may be implemented with a latch circuit. An ADC  86  converts the input voice into digital signals and provides the digital signals to a coding circuit  88  for coding to comply with a specific communication standard (e.g., GSM). The output from coding circuit  88  and the data from watermarking circuit  83  will be time multiplexed by multiplexer  84  to provide a combined output for baseband processing. In this way, the extra information is inserted in the silent periods of the input voice. 
       FIG. 7  shows a watermarking detection circuit  90  for extracting the extra information inserted as digital signals, in a called mobile terminal in accordance with one embodiment of the invention. Watermarking detection circuit  90  detects a pair of pre-defined digital sequences (described below in detail) from the input voice to recover the data as the extra information. The voice will be allowed to pass through watermarking detection circuit  90  and will be output to a decoding circuit  96 , which decodes the voice in accordance with a specific communication standard. A digital-to-analog (DAC)  98  converts the decoded voice into analog signals and provides them to an output device. 
     In  FIGS. 6 and 7 , a pre-defined digital sequence is used for marking the start point and end point of the extra information. For example, a digital sequence 1010101010101010 may be used for marking the start and end of the extra information. All the extra information must be enclosed within a pair of two such sequences. If the extra information or the input voice contains the same sequence pattern as the start or the end sequence, then this sequence pattern must be transformed. For example, if the extra information contains 1010101010101010, this sequence will be changed to 101010101010101110, with “11” being inserted in between the last two bits. 
     In the above, the invention has been illustrated in conjunction with a WWAN and WLAN. However, the invention may be used for roaming between any two wireless communication networks. 
     While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.