Patent Publication Number: US-2001000137-A1

Title: Leased line optimization and voice quality improvement in bandwidth constrained communication systems

Description:
BACKGROUND OF THE INVENTION  
       1. (a) Field of the Invention  
       2. The present invention relates generally to digital communications and, more particularly to a method and apparatus for (i) backhauling compressed voice in a digital communication system without additional degradation in voice quality, thereby reducing the number of leased PCM trunks and (ii) improving voice quality on calls that involve multiple voice compressed links using similar voice compression technology such as mobile-to-mobile calls in cellular or mobile satellite systems.  
       3. (b) Description of Related Art  
       4. The most common form of digital modulation used in communications is pulse code modulation (PCM). PCM represents a message as a sequence of coded digital pulses. PCM is used in communication systems to transfer voice or data from one location to another via a communication link, which may include a wire link such as an E 1  line or a wireless link such as a satellite communication link. Typically, PCM is used to represent an analog signal in digital form. In the communications industry, PCM is used to refer to a digital communication link that operates at a 64 kilobits per second (kbps). There are eight thousand samples of an analog signal taken every second, and each sample is represented by eight bits, hence 64 kbps.  
       5. Bandwidth conservation is a concern in almost all communication applications. In a digital satellite link, for example, a 64 kbps signal is compressed and transmitted at 4 kbps to save bandwidth. For example, a subscriber unit in a communication system may sample a user&#39;s voice at a rate equivalent to 64 kbps or more, however, that signal is compressed into a signal that is 4 kbps before it is uplinked to a satellite. The satellite receives the signal from the subscriber unit and rebroadcasts it to a gateway. The gateway receives the downlinked signal from the satellite and decompresses the signal back to a 64 kbps signal. The compression and decompression of a signal in such a manner mildly compromises the integrity of the signal. However, each successive signal compression and decompression introduces more noise into the signal.  
       6. In many satellite communication systems the gateway decompresses the signals to 64 kbps and couples them to leased terrestrial E 1  transmission lines that eventually connect with a conventional PSTN network. The number of E 1  lines needed is proportional to the bandwidth that must be carried from the gateway to the PSTN network. In many applications the leased E 1  lines may span many hundreds of miles and may traverse the boarders of many countries. The lease rates for E 1  lines are based on required bandwidth and line length. Additionally, traversal of national borders may increase the cost to lease a particular E 1  line. All of these line leasing costs are typically passed on to the user placing the telephone call.  
       7. The cost associated with leasing E 1  lines for hauling traffic between a gateway (or base station) to a point of presence (POP) in a national or international network can be readily appreciated. Accordingly, it would be desirable to have a method and apparatus for reducing the costs associated with the leasing of the E 1  lines in a satellite communications.  
       8. It is possible to perform one more (possibly different) compression and decompression steps on the information to be sent across E 1  lines to achieve cost reduction. However, the overall voice quality degradation can be easily noticeable and will be worse than the worst performance of the two compression and decompression schemes. A similar problem exists in many satellite and cellular systems when voice calls are established between two satellite users (using two satellite hops) or two cellular users. The heart of the present invention is to achieve cost reduction and, at the same time, avoid voice quality degradation.  
       SUMMARY OF THE INVENTION  
       9. The present invention is embodied in a gateway for transferring data representative of voice from a first location to a second location. The gateway includes a voice decoder for decompressing a compressed voice signal received from a satellite; a voice packetizer and embedder for embedding the compressed signal within the decompressed signal; and a transcoder for removing the embedded compressed signal from the decompressed signal and delivering the compressed signal to a communication line.  
       10. The present invention is also embodied in a method for transferring data representative of voice from a first location to a second location comprising. The method includes the steps of decompressing a compressed voice signal received from a satellite to create a decompressed signal; embedding the compressed signal within the decompressed signal; removing the embedded compressed signal from the decompressed signal; and delivering the compressed signal to a communication line.  
       11. The present invention is further embodied in a transcoder for transferring data representative of voice from a first communication line to a second communication line. The transcoder includes a voice decoder for decompressing a compressed voice signal received from the first communication line and a linear to A-law format converter for converting the decompressed signal to an A-law format and for coupling the decompressed A-law format signals to the second communication line.  
       12. The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     13.FIG. 1 is an illustration of a satellite communication link that may employ the present invention;  
     14.FIGS. 2A and 2B illustrate a system employing the method and apparatus of the present invention;  
     15.FIG. 3 is a detailed block diagram of a MSC transcoder;  
     16.FIG. 4 is a detailed block diagram of a POP transcoder;  
     17.FIG. 5 is an illustration of a mobile-to-mobile communication link carried by a single satellite in accordance with the method of the present invention; and  
     18.FIG. 6 is an illustration of a mobile-to mobile communication link carried over leased lines in accordance with the present invention.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     19. Currently satellite communication systems require a large number of leased E 1  lines to backhaul communication traffic from a satellite gateway (downlink station) to PSTN networks located at points of presence (POP). In many instances, the leased E 1  lines may cover vast distances and may cross international borders, thereby increasing the cost to lease the lines. This increased cost is passed on to the communication system users.  
     20. The present invention is embodied in a method and apparatus for reducing the number of leased E 1  lines needed for backhauling communication traffic in a satellite system without voice quality degradation. The present invention receives and decompresses compressed signals from a satellite. After the signals are decompressed, the compressed information is embedded into the decompressed signals. The system may then communicate the compressed or decompressed information to landline users. The compressed information may be sent across leased E 1  lines to reduce the number of lines needed. That is, by sending compressed information across the leased lines, fewer lines are needed.  
     21. Referring now to FIG. 1, an illustration of a typical communication system  50  that may employ the present invention is shown. In the disclosed example, a system user in Spain places a call using a subscriber unit  54 . The subscriber unit  54  transmits voice information to a satellite  58 , which is located in high or medium earth orbit. The satellite receives the uplinked information, translates its frequency, and downlinks the information to a gateway station  62 , which may be located, for example, in Poland. Due to the expense associated with gateway stations, a single gateway station may support a significant geographical area. For example, as shown in FIG. 1, a gateway station  62  in Poland may support many surrounding European countries.  
     22. The gateway station  62  receives the downlinked signal and routes the voice information to a PSTN  68  or  84  at a point of presence (POP) that serves a person to whom the subscriber unit user is speaking (i.e., the person on the land line). For example, if the person on the land line is located in Poland, the gateway station  62  may couple the received satellite signal to a local PSTN  68  in Poland. The local PSTN  68  proceeds to route the call to the land line user&#39;s telephone  72  in a conventional manner.  
     23. Alternatively, the call placed at the subscriber unit  54  may be intended for a person whose telephone  76  is located in, for example, Spain. In this case, the gateway  62  located in Poland, which serves a broad geographic region, receives the downlinked signal and couples it to leased E 1  lines  80 . The E 1  lines  80  link the signal from the gateway station  80  to a PSTN  84  located in Spain. The process of linking the gateway station to a PSTN via E 1  lines is commonly referred to backhauling. The PSTN, in turn, couples the signal to the land line user&#39;s telephone  76  located in Spain. The present invention may be implemented at the gateway station  62  of the communication system  50  shown in FIG. 1. The present invention reduces the number of E 1  lines  80  needed to backhaul communication traffic from the gateway station  62  to various PSTN networks.  
     24.FIGS. 2A and 2B show a detailed illustration of a system employing the method and apparatus of the present invention. The subscriber unit  54  includes a microphone  100 , which is coupled to an analog-to-digital converter (ADC)  104 . The ADC  104  quantizes the output of the microphone  100  into 8N kbps (eight thousand samples per second, wherein each sample is represented by N bits). The quantized 8N kbps voice information signal is coupled to a voice encoder  108 , which compresses the 8N kbps signal to, for example, a 4 kbps signal. This compression is accomplished using techniques known in the art. The 4 kbps voice information signal is coupled to a transceiver  112 , which converts the 4 kbps signal to an appropriate amplitude and frequency for transmission to the satellite  58 . The satellite receives the 4 kbps voice information signal and retransmits it down to the gateway station  62 .  
     25. In the receive path, the gateway station  62  includes a transceiver  116  capable of receiving the signal downlinked from the satellite  58 . The output of the transceiver  116  is the same 4 kbps signal that was uplinked to the satellite  58  by the transceiver  112  of the subscriber unit  54 . The signal from the transceiver  116  is coupled to a voice decoder  120 . The voice decoder  120  decompresses the 4 kbps signal into a 8N kbps (eight thousand samples per second, each sample represented by N bits) PCM signal. The 8N kbps signal is coupled to a linear to an A-law converter  128 , which converts the 8N kbps signal from a linear format to a 8N kbps A-law format, using known techniques.  
     26. According to the present invention, the A-law formatted PCM signal is coupled to a voice packetizer and embedder (VPE)  124 , which steals the least significant bits (LSB) of PCM samples and replaces those bits with the compressed bits received from the satellite  58 . This encodes the compressed information into the decompressed information, and destroys the decompressed information stored in the LSBs of the samples. However, the information lost by stealing the LSBs will negligibly effect the voice quality of the communication link. Additionally, the VPE  124  appropriately adds a header on the data to indicate the presence of the compressed information in the LSBs of the samples.  
     27. After the VPE  124  appropriately processes the voice information signals, the signals are passed to a mobile switching center (MSC)  132 . The MSC  132  determines the PSTN  68  or  84  to which the voice information should be routed. If the voice information needs to be routed to a local telephone  72 , the 8N kbps voice information is routed to a local PSTN  68 , and further routed to the appropriate telephone  72 . The signal path uses the 8N kbps voice information signal to reproduce voice as sent from the subscriber unit  54 .  
     28. If the voice information needs to be routed to a international PSTN  84 , the MSC  132  passes the voice information to a MSC transcoder  136  according to the present invention. FIG. 3 is a detailed illustration of the functions included in the MSC transcoder  136 . The information from the MSC  132  is passed to a voice packet detector  137 , which examines the headers on the voice information from the MSC  132  to determine if compressed voice information has been placed in the LSBs of the decompressed voice signal. It is noted that PCM signals arriving at the MSC  132  could be voiceband data in which case the voice encoded bits will not be present in the LSBs. If the compressed voice information is present in the decompressed voice information, a voice packet extractor and transmitter  138  uses the appropriate LSBs to assemble the compressed voice information as relayed by the satellite  58 . Once assembled, the compressed voice information is sent across leased E 1  lines  80  to a point of presence (POP) transcoder  140 . Since the information transferred across the leased E 1  lines  80  is compressed, fewer leased E 1  lines  80  are needed to transfer information from the MSC  132  to the international PSTN  84 .  
     29. According to the present invention, the POP transcoder  140  is located physically at the end of the leased line near the international PSTN  84  location. The POP transcoder  140 , as shown in FIG. 4, includes a voice decoder  141  and a linear to A-law converter  142 . The voice decoder  141  decompresses the compressed voice information in a similar fashion to the voice decoder  120  of the gateway station  62  and passes the decompressed voice information to the linear to A-law converter  142 , which functions in an identical fashion to block  128  of the gateway station  62 . After the information is converted to A-law format it is coupled to the international PSTN  84 . In turn, the international PSTN  84  routes the decompressed voice information to the appropriate telephone  76 .  
     30. In addition to receiving signals from the satellite  58 , the gateway station  62  is capable of receiving signals from land line users at telephones  72 ,  76 . If the land line signals that are to be transmitted to the subscriber unit  54  originate from the local PSTN network  68  at 8N kbps, the MSC  132  receives the signals and transfers them to a voice packet detector  144 , which functions in a similar fashion to the voice packet detector  137  of the MSC transcoder  136 . The voice packet detector  144  determines if the LSB of the signal is coded with compressed voice information. If the signals originate from the local PSTN  68 , they will not be encoded with the compressed voice information. The local PSTN signal is then passed to an A-law to linear converter  148 , the function of which is known in the art. The linear formatted 8N kbps signal is passed to a voice encoder  152 , which compresses the 8N kbps signal into, for example, a 4 kbps signal in a manner similar to that of the voice encoder  108  of the subscriber unit  54 .  
     31. Alternatively, information may be received from the international PSTN  84 . That is, voice information from a user at a telephone  76  is passed to the international PSTN  84 , which passes the 8N kbps signal to the POP transcoder  140 . The POP transcoder  140 , according to the present invention, converts the information from A-law format to linear format using an A-law to linear converter  153 . After the format conversion is complete, the A-law formatted 8N kbps information is compressed into, for example, a 4 kbps signal by a voice encoder  154  for transmission across the leased E 1  lines  80 . By transmitting the compressed signal (rather than the 8N kbps signal) across the leased E 1  lines the present invention reduces the number of leased lines  80  needed as there is less information transferred per user for a given period of time.  
     32. The leased E 1  lines  80  transfer the information from the POP transcoder  140  to the MSC transcoder  136 . The MSC transcoder  136 , as shown in FIG. 3, converts the compressed signal to a 8N kbps signal that has compressed version of its information encoded in the LSBs of the signal. Specifically, a block  155  performs the function of extracting and decompressing the voice information from the leased E 1  line  80 . After the voice information is extracted and decompressed, it is passed to a voice packet embedder  156 , which performs a similar function to the VPE  124  located in the gateway  62 . This conversion is necessary because the MSC  132  is designed to interface only with 8N kbps signals.  
     33. After the information is properly processed by the MSC transcoder  136 , the information is passed to the MSC  132 . The MCS  132 , in turn, routes the information to a voice packet detector  144 . When the 8N kbps signals encoded with compressed information are received at the voice packet detector  144  via the MSC  132 , they are routed to an externally voice encoded information block  158 . The externally voice encoded information block  158  strips the LSBs containing the compressed voice information from the 8N kbps signal, which results in a compressed signal representative of voice information.  
     34. The voice information from blocks  152  and  158  is passed to an OR function  160 , which selects whether the signal from the voice encoder  152  or the signal from the externally voice encoded information block  158  us passed to the transceiver  116 . The information routed to the transceiver  116  is transmitted via satellite  58  to the subscriber unit  54 . The transceiver  112  of the subscriber unit  54  receives the compressed voice information from the satellite  58  and appropriately down-converts the signal for use by a voice decoder  164 , which converts the compressed voice information into decompressed voice information in a similar fashion to the voice decoder  120  of the gateway station  62 . The voice information is then passed from the voice decoder  164  to a speaker  200 , or other audio device, which manifests the audio to the user of the subscriber unit.  
     35. The functions of the system of the present invention may be mathematically described with respect to FIGS. 2A and 2B. Considering FIGS. 2A and 2B, let B=[b o b 1 . . . b M-1 ] be the bit-stream received by the voice decoder  120  in the gateway station  62 . Let S out =[S 0 S 1  . . . S N-1 ] represents a frame of 8 bit PCM samples at the output of the A-law converter  128 . The inputs to the VPE  124  are S out  and B. The output of the voice packetizer in the VPE  124  may be represented as shown in Equation 1.  
               B   ′     =       B                            0         0                 …0           1                 0                 0                 …                        0                 0                 0                 …                 0                          0         0                 …0           01                 0                 …           0                 0                 0                 …                 0             0         0                 …0           001                 …           0                 0                 0                 …                 0             ⋮                                                 0                      0                 …0           0                 0                 0                 …           1                 0                 0                 …                 0                                                                  M   x          N   1               M   x        N             M   x          N   2                    +           [       h   0          h   1          h   2                   …                   h       N   1     -   1          000                 …                 0        t   0          t   1          t   2          t   3          …t       N   2     -   1         ]                 Equation                 1                       
 
     36. Wherein B′ is the voice encoded packet in the VPE  124 . Additionally, h i  is a N 1  sequence of header bits of length N 1  that includes a start flag, unique word, and other voice related flags. The variable t i  denotes the tail bits, which typically is an end-of-packet flag.  
     37. The output of the VPE  124  of the gateway  62  is described by Equations 2 and 3.  
     S′ out =[s′ 0 s′ 1  . . . s′ N-1 ]  Equation 2  
     38.               s   i   ′     =     {                 s   i     &amp;                [   1111110   ]     +     b   i   ′             0   ≤   i   ≤       N   1     +   M   +     N   2     -   1                 s   i               N   1     +   M   +     N   2       ≤   i   ≤     N   -   1                       Equation                 3                         
     39. Wherein &amp; represents a bitwise logical AND operation. Furthermore, the 8 bit A-law encoded speech sample and modified speech sample are denoted as [MSB . . . LSB].  
     40. Input to the MSC transcoder  136  is the output of the VPE  124  of the gateway station  62  as defined by Equations 2 and 3. The output of the MSC transcoder  136  is represented by Equations 4 and 5.  
     B″=[b″ 0 b″ 1  . . . b″ N     1     +M+N     2     −1 ]  Equation 4  
     b″ i =LSB(s′ i &amp;[00000001])   Equation 5  
     41. The compression achieved over the leased E 1  lines for voice transfer is shown in Equation 6.  
             g   =       8        N   1           N   1     +   M   +     N   2                 Equation                 6                       
 
     42. For example, consider a mobile satellite system operating at a frame interval of 40 ms. At 8 Khz sampling rate for voice, the number of speech samples for a frame is N=320. Assume that the voice encoder used in the system is 3 kbps and, therefore, M=120. Assuming one byte for a start flag, one byte for an end flag, two bytes of unique word, and one byte of voice-related flags such as voice activity or voice quality, we have N 1 =32, N 2 =8. Therefore, compression over the leased E 1  line is 16. For this example, the voice encoded packet B′ is represented by Equation 7.  
     B′[01111110UU . . . UFFFFFFFFb 0 b 1  . . . b 119 01111110]  Equation 7  
     43. The POP transcoder  140  extracts the voice encoded bits from the received bit stream B″ before providing it to the voice decoder  141 . Extraction of the voice encoded bitstream is represented by Equation 8.  
               B   ′′′     =       B   ″                     000                 …                 0               000                 …                 0               ⋮                   N   1     ×   M               000                 …                 0               100                 …                 0               010                 …                 0               001                 …                 0               ⋮                 M   ×   M               000                 …                 1               000                 …                 0               000                 …                 0               ⋮                   N   2     ×   M               000                 …                 0           ]               Equation                 8                       
 
     44. Equations 2-5 and 8 verify that B′″=B. Therefore, the input to the voice decoder  141  in the POP transcoder  140  is identical to the input to the voice decoder  120  in the gateway station  62 . Since the inputs are identical the speech represented by the inputs is identical. The voice information is transferred efficiently over the leased E 1  lines.  
     45. FIGS.  5  and  6  illustrate two additional applications of the preferred embodiment of the present invention. The configurations shown in FIGS.  5  and  6  are relevant to mobile-to-mobile call configurations. The embodiment shown in FIG. 5 improves the voice quality that would otherwise be degraded due to two compression/decompression schemes. The embodiment shown in FIG. 5 allows the system to transfer the voice encoded bits generated at subscriber unit  54  to another subscriber unit without an intermediate compression/decompression scheme.  
     46. The configuration shown in FIG. 6, reduces the number of leased lines  80  needed between two gateway stations  62  handling satellite communications from subscriber units  54 . In this configuration, only the voice encoded bits are transferred between the transcoders  136  across leased lines  80 . This configuration is illustrative of the fact that the present invention is not only useful in mobile to terrestrial communications, but also in mobile to mobile communications.  
     47. Of course, it should be understood that a range of changes and modifications can be made to the preferred embodiment described above. For example, as shown in FIGS. 2A and 2B, the present invention may be used in voice mail applications. Specifically, a voice mail system  210  connected to the MSC  132  may be used to store voice mail having compressed information embedded in the decompressed information. In conventional voice mail applications, if a mobile user deposits voice mail to another mobile user, and if the other mobile user accesses his voice mail box using a mobile phone, then voice information is compressed and decompressed twice, thereby resulting in loss of voice quality. By using the preferred embodiment of the present invention, it is possible to steal LSB of PCM samples and embed voice packets before storing the voice information in the voice mail system. When the voice mail is retrieved, then the gateway identifies the presence of voice encoded packet according to the present invention and hence passes it straight to the user retrieving the voice mail, thereby avoiding further loss in voice quality. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of this invention.