Abstract:
A terminal for transmitting a voice signal, comprising: a transmitter having a first transmission module arranged to transmit signals over a packet-switched data network and a second transmission module arranged to transmit signals over a circuit-switched telephony network; a microphone for generating a voice signal; and signal processing apparatus configured to generate a first signal and a second signal from the voice signal, each representing information from the voice signal over a same portion of time. The signal processing apparatus is further configured to supply the first signal to the first transmission module for transmission to a receiver via the packet-switched data network, and to supply the second signal to the second transmission module for transmission to the receiver via the circuit-switched telephony network. There is also provided a terminal for receiving such signals and reconstructing the voice signal, and corresponding methods and program products.

Description:
FIELD OF THE INVENTION 
     The present invention relates to enhancing the quality of calls made over a circuit-switched network. 
     BACKGROUND 
     Traditionally, phone calls have been made over circuit-switched (CS) networks in which a reliable connection for a call is created between the communication end points. The circuit-switched connection (i.e. the “circuit”) provides a channel in the form of a specified, predetermined path for routing the voice information of the call. Because the required network resources are thus reserved for the call, there is a low probability of information being lost unless there is actually a network fault. Furthermore, the delay per unit information is constant. For these reasons the connection can be considered reliable. However, circuit-switched networks typically only support a limited bandwidth, i.e. only a limited range of frequencies from the voice signal can be encoded for transport over the circuit switched network. 
     More recently, voice over internet protocol (VoIP) has become a widely used alternative to circuit-switched calling, with advantages in cost and quality, but suffering from lower reliability. To transmit using VoIP (or indeed any such packet-switched medium), the encoded voice data of the call is divided into a plurality packets and the relevant destination address is added into a header of each packet. The route for each packet is then determined “on the fly” by the routing equipment of the packet-switched network. The packet-switched channel may therefore be considered a “virtual” channel. That is, in contrast with a circuit-switched channel, the physical path for each packet is not fixed or reserved for the call, and different packets of the same call may be routed differently without any predetermined path. Instead, the channel exists in the sense that the end points have performed a handshaking procedure in order to establish a session between them. The fact that the voice encoding is not a fixed feature of the network means that packet-switched calls allow a larger frequency range to be encoded and therefore potentially better call quality. On the other hand, the packet-based routing is less reliable because a certain amount of packet loss is expected (i.e. that expectation is an intrinsic feature of the system, e.g. with overloaded routers being arranged to discard packets, and error detection algorithms at routers or endpoints being arranged to reject corrupted packets). Furthermore, different packets can be delayed by different amounts, leading to further reliability issues for real-time communications. 
     The VoLGA (“Voice over LTE via Generic Access”) forum provides specifications for implementing mobile VoIP calls using Long-Term Evolution (LTE) technology. VoLGA specifies the use of a “fall-back” mechanism which resorts to a circuit-switched channel when conditions over the packet-switched channel are poor. 
     SUMMARY 
     According to the VoLGA fall-back mechanism, the voice call is routed either over an LTE packet-switched data channel or over a circuit-switched channel, but not over both. 
     However, the inventors of the present invention have recognized that the perceived quality of voice calls can be enhanced by sending information about the speech signal simultaneously via both a circuit-switched channel and a data channel, and then combining both information streams on the receiver side. 
     According to one aspect of the present invention, there is provided a terminal for transmitting a voice signal, comprising: a transmitter having a first transmission module arranged to transmit signals over a packet-switched data network and a second transmission module arranged to transmit signals over a circuit-switched telephony network; a microphone for generating a voice signal; and signal processing apparatus coupled to the transmitter and the microphone, configured to generate a first signal and a second signal from the voice signal, each representing information from the voice signal over a same portion of time; wherein the signal processing apparatus is further configured to supply the first signal to the first transmission module for transmission to a receiver via the packet-switched data network, and to supply the second signal to the second transmission module for transmission to said receiver via the circuit-switched telephony network. 
     Preferably transmission over the circuit-switched telephony network is prioritized relative to transmission over the packet-switched data network, but the packet-switched data network supports a higher bandwidth than the circuit-switched telephony network. Thus although the circuit-switched network may support a lower frequency, it can still guarantee a certain lower limit to the call quality when channel conditions on the packet-switched network are poor. Preferably the first signal comprises a higher frequency band than the second signal. 
     There are two alternative preferred solutions. In the first solution, the first signal may comprise the higher frequency band without a lower frequency band, and the second signal may comprise the lower frequency band without the higher frequency band, such that the receiver may be enabled to reconstruct the voice signal from a combination of the first and second signals. 
     In this embodiment the receiver will be provided with two alternative signals via the two respective channels, and can switch between them in dependence on some criterion such as received quality. Although this requires transmission of some redundant information and therefore requires extra resources, the inventors have recognized that this downside is outweighed by the advantage that running the circuit-switched and packet-switched calls simultaneously can avoid a gap while switching between the two signals on the receiving side. 
     In the second preferred solution, the first signal may comprise both the higher frequency band and a lower frequency band, whilst the second signal may comprise the lower frequency band without the higher-frequency band, such that the receiver may be enabled to reconstruct the voice signal by switching between the first and second signals. 
     This embodiment avoids the redundant use of resources by only using the packet-switched network to transmit frequencies of the voice signal that cannot be accommodated by the circuit-switched network. 
     Another advantage that can be achieved by running the circuit-switched and packet-switched calls simultaneously is that it allows improved synchronization between the two voice signals at the receive side. This is because the receiver will have available two versions of the same information, one from each respective channel, which it can therefore use to align the two decoded information streams in time. The inventors have recognized that this advantage outweighs the extra resources required to transmit the two signals simultaneously over both the circuit-switched and packet-switched networks. 
     Furthermore, in embodiments the invention can be implemented before LTE deployment, and can be implemented without requiring any change of the underlying network hardware or software. 
     In particularly preferred embodiments, the circuit-switched telephony network may be a wireless cellular circuit-switched telephony network, the second transmission module being arranged to transmit over the wireless cellular circuit-switched telephony network, and the signal processing apparatus may be configured to supply the second signal to the second transmission module for transmission to said receiver over the wireless cellular circuit-switched telephony network. 
     The first signal may comprise one of a Wideband and a Super Wideband signal, and the second signal may comprise a Narrowband signal. 
     The signal processing apparatus may be further configured to generate a third signal from the voice signal and to supply the third signal to the first transmission module for transmission to said receiver via the packet-switched data network, wherein the first signal may comprise a higher frequency band without an intermediate frequency band and lower frequency band, the second signal may comprise an intermediate frequency band without the higher frequency band and lower frequency band, and the third signal may comprise the lower frequency band without the higher frequency band and intermediate frequency band. 
     In some embodiments, the receiver may not have circuit-switched capability, and the second transmission module may be arranged to transmit to the receiver via a gateway from the circuit-switched network to the packet-switched data network. 
     According to a further aspect of the present invention, there is provided a terminal for receiving a voice signal, comprising: a receiver having a first reception channel arranged to receive at least a first signal over a packet-switched data network and a second reception channel arranged to receive a second signal over a circuit-switched telephony network, the first and second signals each representing information from the voice signal over a same portion of time; a speaker for playing the voice signal; and signal processing apparatus coupled to the receiver and the speaker, configured to reconstruct said voice signal from the first and second signals, and to output the voice signal to the speaker. 
     Preferably the circuit-switched telephony network is a wireless cellular circuit-switched telephony network, the second reception channel being arranged to receive the second signal over the wireless cellular circuit-switched telephony network. 
     In further embodiments, the first signal may comprise the higher frequency band and a lower frequency band, whilst the second signal may comprise the lower frequency band without the higher frequency band, and the signal processing apparatus may be configured to reconstruct the voice signal by switching between the first and second signals. 
     The signal processing apparatus may be configured to switch between the first and second signals in dependence on a quality measure of the second signal. For example the quality measure may comprise one of a delay, loss rate, and a distortion measure. 
     According to another aspect of the present invention, there is provided a method of transmitting a voice signal, comprising: generating a voice signal from a microphone; and operating a signal processing apparatus to generate a first signal and a second signal from the voice signal, each representing information from the voice signal over a same portion of time; transmitting the first signal to a receiver via a packet-switched data network, and transmitting the second signal to said receiver via a circuit-switched telephony network. 
     According to another aspect of the present invention, there is provided a method of receiving a voice signal, comprising: receiving at least a first signal over a packet-switched data network and a second signal over a circuit-switched telephony network, the first and second signals each representing information from the voice signal over a same portion of time; operating signal processing apparatus to reconstruct said voice signal from the first and second signals, and to output the voice signal to a speaker. 
     These methods may comprise further steps in accordance with any of the above terminal features. 
     According to another aspect of the present invention, there is provided a communication application for transmitting a voice signal, the communication application comprising code embodied on a computer-readable medium and configured so as when executed on a processing apparatus of a user device to: receive a voice signal from a microphone of the user device; and generate a first signal and a second signal from the voice signal, each representing information from the voice signal over a same portion of time; supply the first signal to a first transmission module of the user device for transmission to a receiver via a packet-switched data network, and supply the second signal to a second transmission module for transmission to said receiver via a circuit-switched telephony network. 
     According to another aspect of the present invention, there is provided a communication application for receiving a voice signal, the communication application comprising code embodied on a computer-readable medium and configured so as when executed on a processing apparatus of a user device to: receive at least a first signal over a first reception channel of a packet-switched data network and a second signal over a second reception channel of a circuit-switched telephony network, the first and second signals each representing information from the voice signal over a same portion of time; and reconstruct said voice signal from the first and second signals, and output the voice signal to a speaker of the user device. 
     These communication applications may be further configured in accordance with any of the above terminal features. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention and to show how it may be put into effect, reference will be made to the accompanying drawings in which: 
         FIG. 1 a    is a schematic illustration of a communication system comprising a circuit-switched telephony network and a packet-switched data network, 
         FIG. 1 b    is a schematic block diagram of a mobile terminal, 
         FIG. 1 c    is another schematic illustration of a communication system comprising a circuit-switched telephony network and a packet-switched data network, 
         FIG. 1 d    schematically illustrates an enhanced circuit-switched call involving a gateway to a packet-switched data network, 
         FIG. 2  is a schematic block diagram of a first solution for providing enhanced circuit-switched calls, and 
         FIG. 3  is a schematic block diagram of a second solution for providing enhanced circuit-switched calls. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1 a    shows a communication system comprising a first, packet-switched network  130  such as the Internet; and a second, circuit-switched network  140  such as a mobile cellular network. The mobile cellular network comprises a plurality of base stations  104   a ,  104   b  (sometimes referred to as node Bs in 3GPP terminology). Each base station  104   a ,  104   b  serves a corresponding cell of the cellular network  140 . Further, the packet-switched network  130  comprises a plurality of wireless access points  103   a ,  103   b  for accessing the Internet such as wi-fi access points. These may be the access points of one or more wireless local area networks (WLANs). Internet access by such means is sometimes referred to as Unlicensed Mobile Access (UMA). 
     A plurality of mobile terminals such as mobile phones  102   a ,  102   b  are arranged to communicate over the circuit-switched network  140  via the base stations  104   a ,  104   b , and to communicate over the packet-switched network  130  via the wireless access points  103   a ,  103   b . For example, each mobile phone  102   a ,  102   b  may comprise a short-range wireless transceiver (e.g. wi-fi) for accessing the Internet  101  via the wireless access points  106  over an unlicensed RF band (whilst cellular wireless transceivers typically operate on licensed RF bands). 
     A plurality of dedicated packet-switched terminals such as a desktop or laptop PC  102   c  and one or more servers  105  may also be connected to the packet-switched network  130 . In addition, one or more gateway terminals  108  may be connected between the packet-switched network  130  and the circuit-switched network  140 . 
     Each mobile terminal  102   a ,  102   b  comprises a transmitter and receiver. In each mobile terminal  102   a ,  102   b , the transmitter comprises a first transmission module arranged to transmitting signals over the packet-based network  130  via the access points  103   a ,  103   b , and a second transmission module arranged to transmit signals over the circuit-switched network  140  via the base stations  104 . The receiver comprises a first reception module arranged to receive signals over the packet-based network  130  via the access points  103   a ,  103   b , and a second reception module arranged to receive signals over the circuit-switched network  140  via the base stations  104   a ,  104   b.    
     As illustrated in  FIG. 1 b   , each terminal  102   a ,  102   b  preferably comprises a processor  112 , a memory  110  coupled to the processor  112 , a first radio-frequency (RF) front-end  113  coupled to the processor  112 , a first antenna  115  coupled to the first RF front-end, a second RF front-end  114  coupled to the processor, and a second antenna coupled to the RF front-end  114 . The RF front-end  114  comprises dedicated circuitry for transmitting and receiving signals over the air interface to and from a base station  104 , via the antenna  116 . The RF front-end  113  comprises dedicated circuitry for transmitting and receiving signals over the air interface to and from the access points  103 , via the antenna  115 . The processor  112  is arranged to execute code stored in the memory  110 ; and under control of the executed code to output signals to the first RF front-end  113  for transmission to a wireless access point  103 , to take input signals from the first RF front-end received from a wireless access point  103 , to output signals to the second RF front-end  114  for transmission to a base station  104 , and to take input signals from the second RF front-end  114  received from the base station  104 . 
     The first transmission and reception modules are thus formed of the relevant hardware in the first RF front-end  113  combined with a first portion of code stored in the memory  110  and arranged for execution on the processor  112 . The first portion of code comprises a codec configured to encode voice signals for transmission over the packet-switched network  130  and to decode voice signals received over the packet-switched network  130 , and may also comprise a suitable protocol for establishing a virtual channel with another terminal  102  over the packet-switched network  130 . The second transmission and reception modules are formed of the relevant hardware in the second RF front-end  114  combined with a second portion of code stored in the memory  110  and arranged for execution on the processor  112 . The second portion of code is configured to encode voice signals for transmission over the circuit-switched network  140  and to decode voice signals received over the circuit-switched network  140 . So in embodiments the first and second modules comprise physically separate RF transceivers, albeit with a software element, for transmitting on different frequencies according to different protocols. 
     Each of the user terminals  102   a ,  102   b ,  102   c  is installed with a communication client application on the memory  110 , arranged for execution on the processor  112  and configured so as when executed to establish a VoIP call with another user terminal  102 . In embodiments this may be achieved using a de-centralized peer-to-peer call set-up procedure, whereby the calling client application looks up the IP address of the callee from a distributed database, distributed amongst other end-user terminals  102  (i.e. other peers). The caller and callee then exchange digital certificates in order to prove their identities. However non-P2P call set-up is also an option, whereby address look-up and authentication are performed via a server  105 . 
     The invention enhances the perceived call quality on voice calls by sending information about the speech signal simultaneously via a circuit-switched and a data channel, and combining both information streams on the receiver side. 
     This may provide some or all of the following advantages over the VoLGA system mentioned above:
         (a) running the circuit-switched and VoIP calls simultaneously simplifies the synchronization between the two voice signals on the receiving side;   (b) in certain embodiments, running the circuit-switched and VoIP calls simultaneously avoids a gap while switching between the two voice signals on the receiving side;   (c) it is possible to implement before LTE deployment;   (d) it need not require any change of the underlying network hard- or software; and   (e) in particularly preferred embodiments it can be used to enable wideband (WB) and/or super wideband (SWB) calls in networks where the circuit-switched channel is only narrowband (NB).       

     Cellular circuit-switched (CS) calls come with the advantage that they provide consistent QoS because they are prioritised on the cellular network  140 . However, they are limited in acoustical bandwidth to 4 kHz, commonly known as narrowband (NB) quality. In the world of VoIP on the other hand, NB speech is long deprecated and today the standard is wideband (WB; 8 kHz)—or even super wideband (SWB; 12 kHz) as used in Skype. (S)WB calls provide a far better user experience than NB calls—simply since more information content of the original speech signal is conveyed to the receiver. This not only sounds better, but also overcomes intelligibility problems inherent to NB (e.g., it is difficult to distinguish “s” and “f” over a NB connection). Cellular data channels however only guarantee a “best effort” in service. This is why sending pure VoIP over a cellular data channel is unreliable. We therefore propose to use the CS voice channel in conjunction with additional information about the speech signal on the data channel to enable the best of both worlds: the consistent QoS of CS and the enhanced user experience of (S)WB calls. 
     To implement the invention, the user equipment  102  should preferably support the following features:
         the microphone signal is processed by the VoIP application before being encoded and transmitted by the circuit-switched system,   the received and decoded circuit-switched signal is processed by the VoIP application before playback through the loudspeaker,   the circuit-switched network  140  and packet-switched data network  130  can be accessed simultaneously.       

     In some embodiments, a gateway  108  can be used if only one of the two end points  102  is connected through the circuit-switched network  140 , as shown in  FIG. 1 d   . The gateway  108  is arranged to convert a NB circuit-switched call to a NB VoIP call and vice versa (the voice signal can travel both ways, the gateway  108  having two-way functionality). In parallel to this NB call, a (S)WB all-VoIP call is performed over the packet-switched network  130 . 
     In one particularly preferred embodiment of the present invention, the input signal gets split into three frequency bands (LF+NB+HF) at the encoder side, where the NB band is the standard speech band used by the CS voice calls (typically 300-3500 Hz). The low-frequency (LF) band contains all frequencies below the NB band (typically 0-300 Hz), and the high-frequency (HF) band all frequencies above the NB band up to the desired limit, i.e., 3500-8000 Hz for WB or 3500-12000 Hz for SWB. The LF band may optionally be dropped depending on the implementation. 
     There are at least two possible solutions for reconstructing the information contents sent over the two different channels at the receive side  102   b.    
     A first solution is illustrated schematically in  FIG. 2 . The transmitting user terminal  102   a  comprises a microphone  202  and a VoIP client application  201   a . The VoIP client application  201   a  comprises an analysis filter bank  204 , and a first encoder  206  (part of the first transmission module). The transmitting terminal  102   a  also comprises a second encoder  208  (part of the second transmission module). The analysis filter bank  204  is coupled between the output of the microphone  202  and the inputs of the first encoder  206  and second encoder  208 . Further, the receiving terminal  102   b  comprises a speaker  218  and a client application  201   b . The client application  201   b  comprises a switch  216 , a synchroniser  214 , and a first decoder  210  (part of the first reception module). The receiving terminal  102   b  also comprises a second decoder  212  (part of the second reception module). The synchroniser  214  is coupled between the outputs of the first decoder  210  and second decoder  212  and respective inputs of the switch  216 . The output of the switch  216  is coupled to the input of the speaker  218 . 
     The analysis filter bank  204  receives the voice signal from the microphone  202  and filters it into different bands which are supplied to the encoders  206 ,  208 . The first encoder  206  is a VoIP encoder which is arranged to encode the full-band (S)WB signal, which is then transmitted over the packet-switched data network  130 . The second encoder  208  encodes the NB signal, which is transmitted over the circuit-switched network  140 . At the receive side, the receiver  102   b  can decode both versions of the signal simultaneously using the first decoder  210  and second decoder  212  respectively, time aligns them in the synchronization module  214 , and the switch  216  decides which of the circuit-switched NB data and the full-band VoIP data to play out based on a simple distortion measure or even a rule. For example the rule could be to play out the full-band audio from the VoIP decoder  210  whenever it is available within a certain maximum delay (a typical limit required for reasonable conversational quality would be 300-500 ms), but otherwise to play out the circuit-switched NB signal from the second decoder  212 . Other criteria could also be used. The switching between the signal received over the packet-switched and circuit-switched channel is preferably based on some measure of quality derived from the received signal, evaluated at the receiver, and preferably performed dynamically (i.e. on the fly in response to changing quality). Alternatively however, it could be based on reported channel conditions, or on a user selection based on the user&#39;s perceived experience of the call. 
     This first solution can be enhanced with some temporal alignment performed on both signals by the synchroniser  214  to avoid short segments of the speech being played out twice or getting dropped at the time of switching. A time alignment accuracy of 10 to 50 ms is good enough. Such synchronization can be done by delaying either the VoIP or the circuit-switched signal such that a correlation measure between the signals is maximized. Alternatively, the correlation measure can be computed between the envelopes of the VoIP and circuit-switched signals, where the envelope signals can be downsampled to once per frame or once per subframe to reduce complexity. The synchronization needs to be adaptive over time to respond to changes in the transmission delay of the VoIP and circuit-switched signals. 
     The above solution has the advantage of simpler implementation and high quality, at the cost of a higher data rate on the packet-switched data network  130 . 
     A second solution show in  FIG. 3  reduces the data rate on the packet-switched data network  130 , but is limited in quality for the NB part of the signal by the circuit-switched network  140 , and requires tighter synchronization between the circuit-switched and VoIP signals. 
       FIG. 3  shows an embodiment of this second solution. In this embodiment, the VoIP encoder  206  only encodes information about the LF and HF bands, not any information about the NB band. Thus only the LF and HF bands are transmitted over the packet-switched network  130 , and not the NB band, thereby avoiding unnecessary redundancy on the data channel. At the receive side, instead of a switch  216  the receiver  102   b  comprises a synthesis filter block  220  which is configured to combine the signal from the circuit-switched NB decoder  212  with the LF and HF signals from the VoIP decoder  210  and thereby form a full-band signal. Note that typically most of the available bit rate is spent in the NB part of the signal, thereby keeping the data rate on the packet-switched data channel low. 
     This second solution requires relatively accurate time alignment of the VoIP signal to the NB signal in the synchroniser  214  at the receiver side, as both signals are played out simultaneously. The time alignment accuracy should be no more than a few milliseconds to avoid loss in quality. Such synchronization can be done based on the temporal envelope of the speech waveforms or the LPC excitation signals of the VoIP and circuit-switched signals, by delaying either signal such that a correlation measure between the envelopes is maximized. The synchronization should preferably be adaptive over time to respond to changes in the transmission delay of the VoIP and circuit-switched signals. 
     The present invention may be implanted in VoIP clients for mobile devices, and at least the receive functionality may also be implemented in desktop clients  102   c  on the packet-switched data network  130  for interoperability. 
     It will be appreciated that the above embodiments have been described only by way of example. 
     For instance, the present invention is not limited to the embodiment employing wireless access points  103  such as wi-fi access points. In other embodiments, access to the packet-switched network could be achieved by other means such as GPRS (General Packet Radio Service) or a High Speed Packet Access (HSPA) service.  FIG. 1 c    shows such an embodiment. At a higher level of the cellular hierarchy, the cellular network  140  further comprises a plurality of cellular controller stations  106  each coupled to a plurality of base stations, including a first controller station  106   a  coupled to the first base station  104   a  and a second cellular controller station  106   b  coupled to the second base station  104   b . The controller stations  106  may be referred to as Base Station Controllers (BSCs) in GSM/EDGE terminology, Radio Network Controllers (RNCs) in USTM or HSPA terminology, or VoLGA Access Network Controllers (VANCs) in LTE terminology. The controller stations  105  are thus arranged to allow access to packet-based communications via the base stations  104 , including access to the Internet  101 . 
     As in the previous embodiment, in each mobile terminal  102   a ,  102   b  the transmitter comprises a first transmission module providing a first transmission mechanism for transmitting signals over the packet-based network  130 , and a second transmission module providing a second transmission mechanism for transmitting signals over the circuit-switched network  140 . The receiver comprises a first reception module providing a first reception mechanism for receiving signals over the packet-based network  103 , and a second transmission module providing a second transmission mechanism for receiving signals over the circuit-switched network  101 . However, in this case the first and second transmission and reception mechanisms may be implemented solely in the form of different software modules stored in the memory  110  and arranged for execution on the processor  112 , which both access the packet-based and circuit switched networks via the same physical front-end  114 . The first transmission mechanism of the mobile terminal  102   a  preferably comprises a VoIP encoder and a protocol for transmitting packets over a virtual channel established over the GPRS system, e.g. by transmitting packets with session information including a session identifier. Reciprocally, the first reception mechanism of the mobile terminal  102   b  comprises a VoIP decoder and a protocol for receiving packets over the virtual channel, e.g. be interpreting the received session information. The second transmission mechanism then comprises a more conventional cellular voice encoder and signaling protocol, and the second reception mechanism in the mobile terminal  102   b  comprises a conventional cellular voice decoder and signaling protocol. 
     In other alternative embodiments, the circuit-switched network need not be a mobile cellular network, but could instead be a landline network (sometimes called a “plain old telephone system”, POTS) 
     Furthermore, the present invention is not limited to use over the Internet, to VoIP or to a P2P topology. Other packet-switched networks or protocols could be used, and other call set-up techniques could be employed. 
     Other variations of the present invention may be apparent to a person skilled in the art given the disclosure herein. The present invention is not limited by the described embodiments, but only by the appendant claims.