6,138,022
`[11] Patent Number:
`[19]
`United States Patent
`
`Strawczynski et al.
`[45] Date of Patent:
`*Oct. 24, 2000
`
`IJSOO6138022/\
`
`[54] CELLULAR COMMUNICATION NETWORK
`WITH VOCODER SHARING FEATURE
`
`[75]
`
`Inventors: Leo Strawczynski, Ottawa; Bill W. A.
`Gage, Stittsville; Rafi Rabipour, Cote
`St Lac, all of Canada
`
`7/1998 DeJaco et al.
`5,784,406
`......................... 455/67.4
`
`8/1998 Han et al.
`5,793,810
`..... 455/422
`5,825,760 10/1998 Siira ........................................ 370/331
`
`FOREIGN PATENT DOCUMENTS
`96/19907
`6/1996 WIPO .
`
`[73] Assignee: Nortel Networks Corporation,
`Montreal, Canada
`
`Primary Examiner—Dwayne D. Bost
`Assistant Examiner—Raymond B. Persino
`
`[*] Notice:
`
`This patent issued on a continued pros-
`ecution application filed under 37 CFR
`1.5301), and is subject to the twenty year
`patent
`term provisions of 35 U.S.C.
`5400(2).
`
`[21] Appl. No" 08/898367
`[22]
`Filed:
`Jul. 23, 1997
`
`7
`
`[51]
`
`Int. Cl.
`
`[52] US. Cl.
`
`............................... H04Q 7/20, H04B 1/38,
`H04M 1/00; G01R 31/08
`.......................... 455/445; 455/422; 455/560;
`370/238
`[58] Field Of Search ..................................... 455/445, 422,
`455/436, 674, 11-1, 561, 560; 375/242;
`370/238
`
`[56]
`
`_
`References Clted
`U.S. PATENT DOCUMENTS
`
`4,890,327 12/1989 Bertrand et al.
`........................ 704/219
`574227935
`6/1995 Spear
`~~~~~~~~~~~~~~~~
`455/445
`
`379/88.06
`5,440,615
`8/1995 Caccuro et al.
`..
`
`5,513,172
`4/1996 Shikama et al.
`..
`370/428
`5,608,779
`3/1997 Lev et al.
`..........
`455/436
`
`5,666,348
`9/1997 Thornberg et al.
`370/230
`
`............................ 370/398
`5,729,536
`3/1998 Doshi et al.
`
`[57]
`
`ABSTRACT
`.
`.
`.
`Thepresent invention prov1des a novel system and a method
`for improvmg the v01ce quality of Wireless-to-wireless calls
`or w1reless-to-fixed terminal calls, While permitting a more
`efficient utilization of network resources. In a first aspect, the
`invention
`rovides a communication s stem includin
`a
`plurality ofJbase stations connected to ofie another throfigh
`a digital network (such as TDM, frame relay or ATM). Also,
`a plurality of vocoder channels, separate from the individual
`base stations, provide a data flow path from the base stations
`to wireline terminals first through the digital network via a
`Mobile Switching Center and finally through a landline
`network.
`In a wireless-to-wireless call,
`the compressed
`audio signal travels from one base station to another without
`undergoing any de-compression/compression. This avoids
`undesirable vocoder tandeming known to degrade voice
`quality. If the call is identified as being of a wireless-to-
`wireline terminal nature, the data is directed to a vocoder
`channel for decompression into PCM samples. The samples
`are then transmitted to the fixed terminal through the land-
`line network. The main advantage of this network architec—
`ture is that fewer vocoders are required by comparison to
`prior art systems,
`therefore fewer costs are incurred. In
`-
`-
`-
`.
`.
`.
`addlmn’ V01ce quamy 15 ImPIOVed
`
`17 Claims, 3 Drawing Sheets
`
`300
`
`Mobile
`terminal A
`
`RF cna'nnel A
`
`360
`
`Mobile
`termlllal B
`
`RF Cmirlllel B
`
`7
`
`542Base
`
`Transcelver
`
`
`Slatlun
`
`
`7
`
`31?
`Base
`
`Transceiver
`
`Sleliorl
`
`
`310
`340
`
` 3150
`
`Ease Stallari
`Base Statlnn
`Digital
`
`
`Controller 8
`Controller A
`Network
`
`
`
`
`Slgnal
`
`Slgnal
`and
`arid
`Control
`370
`
`Control
`only
`Vocoder
`
`only
`380
`
`
`
`
`Lalidllne
` 352
`372
`Network
`
`
`Moblie SWltcnlng
`Moblle Swncnlng
`
`Celller B
`
`Center A
`
`354
`374
`Echo
`Echo
`
`Canceller
`
`Cancellrer
`
`
`
`
`
`1
`
`APPLE 1043
`
`APPLE 1043
`
`1
`
`

`

`US. Patent
`
`Oct. 24,2000
`
`Sheet 1 0f3
`
`6,138,022
`
`CELL
`
`4‘
`
`T
`T
`.Lx
`
`CELL
`4~
`
`CELL
`
`NW
`Mi
`
`CELL
`
`CELL
`
`ELL
`
`T “T
`
`i
`
`LA
`
`150
`
`10
`
`i
`
`LA
`
`LA
`
`5
`
`10
`
`i5.
`
`
`
`
`
`100
`100
`
`Base Station
`Base Station
`
`Controller
`Controller
`
`
`110
`
`
`Mobile
`
`
`
`Switching
`
`identification
`Center
`
`Databases
`
`\F = Radio Channels including
`FCC, RCC, FVC and RVC
`MT = Mobile Terminal
`FWT = Fixed Wireless Terminal
`BTS = Base Transceiver Station
`LT : Land Terminal
`FCC = Forward Control Channel
`RCC = Reverse Control Channel
`FVC = Fonivard Voice Channel
`RVC = Reverse Voice Channel
`
`
`
`130
`
`Landline or
`
`
`"backbone"
`
`Network
`
`
`Figure 1
`Prior Art
`
`2
`
`

`

`US. Patent
`
`Oct. 24, 2000
`
`Sheet 2 0f3
`
`6,138,022
`
`200
`
`Mobile
`terminal A
`
`280
`
`.
`Mobile
`terminal B
`
`RF Channel A
`
`RF Channel B
`
`275
`
`
`
`215
`
`Base
`Base
`
`
`
`
`Transceiver
`Transceiver
`
`
`
`Station
`Station
`
`
`
`
`270
`
`
` 220
`
`Base Station
`
` Base Station
`
`Controller B
`Controller A
`
`
`
`
`
`
` 240
`
`
` LandHne
`Network
`
`
`265
`Vocoder
`
`230
`
`Echo Canceller
`
`260
`
`Echo Canceller
`
`
` Fixed
`
`terminal
`
`
`255
`235
`
`
`
`Mobile
`Mobile
`
`
`
` 245
`
`Switching
`
`
`Switching
`Center B
`Center A
`
`
`
`Digital
`
`Network
`
`
`Figure 2
`
`Prior Art
`
`3
`
`

`

`US. Patent
`
`Oct. 24, 2000
`
`Sheet 3 0f3
`
`6,138,022
`
`300
`
`Mobile
`terminal A
`
`360
`
`Mobile
`terminal B
`
`RF Channel A
`
`RF Channel B
`
`312
`Base
`Transceiver
`Station
`
`Base Station
`Controller A
`
`Network
`
`
`330
`
`
`
` Digital
`
`
`
`Signal
`and
`Control
`only
`
`542
`Base
`Transceiver
`Station
`
`
`
`Base Station
`
`Controller B
`
`Signal
`and
`Control
`only
`
`
`370
`350
`Vocoder
`Vocoder
`
`
`380
`
`
`
`LandHne
`
`
`372
`
`
`352
`
`Network
`
`
`
`Mobile Switching
`
`Mobile Switching
`
`Center B
`CenterA
`
`
`
`
`m-F' ..
`354
`
`
`
`
`374
`Echo
`Echo
`-\
`
`
`
`Canceller
`Canceller
`
`
`
`Fixed
`terminal
`
`Fig u re 3
`
`4
`
`

`

`6,138,022
`
`1
`CELLULAR COMMUNICATION NETWORK
`WITH VOCODER SHARING FEATURE
`
`FIELD OF THE INVENTION
`
`The invention relates to digital signal processing in a
`digital cellular network and more particularly to a commu-
`nication system featuring one or more vocoder channels
`functionally separate from the base stations. The vocoder
`channel is bypassed in the case of wireless-to-wireless calls,
`thus avoiding vocoder tandeming that can degrade the voice
`quality of the connection. In the case of a wireless-to-fixed
`terminal call the data flow is directed through the vocoder
`channel for de-compression and transported to the fixed
`terminal through a landline network, such as a PSTN, ISDN
`or Internet network.
`
`BACKGROUND OF THE INVENTION
`
`5
`
`10
`
`15
`
`the telecommunications industry has
`In recent years,
`witnessed the proliferation of a variety of digital vocoders in
`order to meet bandwidth demands of different wireline and
`
`20
`
`wireless communication systems. The name <<vocoder>>
`stems from the fact that its applications are specific to the
`encoding and decoding of voice signals primarily. Avocoder
`therefore is comprised of an encoder stage and a decoder
`stage. Vocoders are usually integrated in mobile telephones
`and the base stations of the communication network. They
`provide compression of a digitized voice signal as well as
`the reverse transformation. Typically, a voice signal
`is
`digitized through one of many quantization techniques.
`Examples of these techniques are Pulse Code Modulation
`(PCM) and Delta Modulation. For the purposes of this
`description we will refer to PCM as the input format for the
`vocoder. Thus a vocoder includes an encoder stage that will
`accept as input a digitized voice signal and output a com-
`pressed signal, the typically compression ratio being in the
`order of 8:1 to 12:1. As for the reverse transformation the
`
`vocoder is provided with a decoder stage that will accept the
`compressed speech signal and output a digitized signal, such
`as PCM samples.
`The main advantage of compressing speech is that it uses
`less of the limited channel bandwidth for transmission. The
`
`main disadvantage is loss of speech quality.
`The rapid growth in the diversity of networks and the
`number of users of such networks is increasing the number
`of instances where two vocoders are placed in tandem to
`serve a single connection. Tandem connections of low
`bit-rate codecs are known to cause additional distortions and
`
`reduce the quality of the speech signal. One example of such
`a scenario in a wireless context is a wireless-to-wireless link.
`
`In such a case, a first encoder is used to compress the
`speech of the first wireless user. The compressed speech is
`transmitted to a base station serving the local wireless
`terminal and it is then decompressed (converted to PCM
`format samples) by a vocoder. The resulting PCM samples
`arrive at
`the base station serving the second wireless
`terminal, over the digital trunk of the telephone network,
`after being compressed by a second encoder. The speech
`signal is then ready for transmission to the second wireless
`terminal. A speech decoder at the speech wireless terminal
`decompresses the received compressed speech data to syn-
`thesize the original speech signal from the first wireless
`terminal.
`
`to eliminated the condition of vocoder
`In an attempt
`tandeming, a method called <<bypass>> has been proposed
`in the past. The basic idea behind this approach is the
`provision of digital signal processors including a vocoder
`
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`
`2
`and a bypass mechanism that is invoked when the incoming
`signal is in a format compatible with the vocoder. In use, the
`digital signal processor associated with the first base station
`that receives the RF signal from a first wireless terminal
`determines, through signaling and control that a compatible
`digital signal processor exists at the second base station
`associated with the wireless terminal at which the call is
`
`directed. The digital signal processor associated with the
`first base station, rather than converting the compressed
`speech signals into PCM samples,
`invokes the bypass
`mechanism and outputs the compressed speech to the trans-
`port network. The compressed speech signal, when arriving
`at the digital signal processor associated with the second
`base station, is routed such as to bypass the local vocoder.
`Decompression of the signal occurs only at
`the second
`wireless terminal. The “bypass” approach is described in the
`international application serial number PCT/CA95/00704
`dated Dec. 13, 1995. The contents of this disclosure are
`incorporated herein by reference.
`Although this solution is effective in reducing vocoder
`tandeming,
`it still requires a dedicated vocoder per base
`station. This vocoder deployment strategy is not particularly
`effective because the vocoder units are not utilized in the
`
`most efficient manner. More specifically, when a call is
`made, the system determines whether the vocoder should be
`enabled or the bypass mechanism should be invoked. This is
`not an optimal utilization of network resources since the
`vocoder functionality, or the bypass functionality of the base
`station, are alternative elements and if one is invoked during
`a given call, the other remains idle.
`OBJECTS AND STATEMENT OF THE
`INVENTION
`
`An object of the invention is to provide a cellular com-
`munication network configured to reduce the likelihood of
`vocoder tandeming in the course of a call.
`Another object of the invention is to provide a method for
`call routing in a cellular communication network to reduce
`the likelihood of vocoder tandeming in the course of a
`wireless-to-wireless call.
`
`As embodied and broadly described herein, the invention
`provides a communication network, comprising:
`a) a group of base stations, each base station being
`capable to establish an RF communication with a
`wireless terminal over an air interface, each base station
`being capable of exchanging with a respective wireless
`terminal data packets of compressed audio information,
`each data packet including a coefficients segment and a
`excitation segment;
`b) a digital network interconnecting said base stations to
`one another, said digital network establishing a path-
`way to allow a given base station to exchange data
`packets of compressed audio information with another
`base station of said group;
`c) a branch connected to said digital network to establish
`a pathway from said digital network toward a landline
`network, said branch including a decoder capable of
`decoding a data packet of compressed audio informa-
`tion directed through said branch;
`d) a control system to control routing of a data packet of
`compressed audio information received at one of said
`base stations from an associated wireless terminal, said
`control system capable of selectively directing the data
`packet of compressed audio information toward either
`one of said one base station, a base station other than
`said one base station and said branch, in dependence
`
`5
`
`

`

`6,138,022
`
`3
`upon the destination of the data packet of compressed
`audio information.
`
`the term “wireless terminal” is
`In this specification,
`intended to include both mobile terminals and fixed wireless
`terminals.
`The term “Base station” in a wireless communication
`
`to designate a fixed station enabling,
`network is meant
`through radio communications with wireless terminals, a
`link between the wireless terminals and the “backbone”
`
`network (also called landline network) through radio com-
`munication with wireless terminals Base stations are located
`
`at the center or on the edge of a coverage region (cell) and
`may include, among others, a base transceiver station (BTS),
`a base station controller (BSC), antennas, etc.
`The expression “data packet” will refer to a group of bits
`organized in a certain structure that conveys some informa-
`tion. Typically, a data packet when representing a sample of
`compressed audio information will usually include a user
`segment (containing the speech information), a signaling
`and control segment, an address segment, a header and a
`trailer segment, etc. The expression “Compressed audio
`information” is limited to include a “Coefficients segment”
`and an “Excitation segment”.
`The term “Coefficients segment” is intended to refer to
`any set of coefficients that uniquely defines a filter function
`that models the human articulatory tract. In conventional
`vocoders, several different types of coefficients are known,
`including reflection coefficients, arcsines of the reflection
`coefficients,
`line spectrum pairs,
`log area ratios, among
`others. These different
`types of coefficients are usually
`related by mathematical transformations and have different
`properties that suit them to different application. Thus, the
`term “Coefficients segment” is intended to encompass any of
`these types of coefficients.
`The “excitation segment” can be defined as information
`that needs to be combined with the coefficients in order to
`
`in a non-
`provide a representation of the audio signal
`compressed form. Such excitation segment may include
`parametric information describing the periodicity of the
`speech signal, an excitation signal as computed by the
`pseudo-decoder, speech framing control
`information to
`ensure synchronous framing in the pseudo-encoder associ-
`ated with the remote vocoder, pitch periods, pitch lags, gains
`and relative gains, among others. The coefficients segment
`and the excitation segment can be represented in various
`ways in the signal transmitted through the network of the
`telephone company. One possibility is to transmit the infor-
`mation as such,
`in other words a sequence of bits that
`represents the values of the parameters to be communicated.
`Another possibility is to transmit a list of indices that do not
`convey by themselves the parameters of the common format
`signal, but simply constitute entries in a database or code-
`book allowing the pseudo-encoder to look-up this database
`and extract on the basis of the various indices received the
`
`pertinent information to construct the common format sig-
`nal.
`
`In a preferred embodiment, the present invention provides
`a novel cellular Communications Network including a plu-
`rality of base stations that are connected to one another
`through a digital network, such that a data packet of a
`compressed audio signal received at one base station can be
`transmitted over the digital network toward another base
`station. This mode of data exchange is suitable for wireless-
`to-wireless calls.
`
`One or more branches connect the digital network to the
`landline network to allow establishment of wireless-to-
`wireline terminal calls. Each one of these branches incor-
`
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`
`porates a vocoder whose function is to decode (de-
`compress) the data packet issued by a base station. The
`resulting signal
`is usually PCM samples that are then
`transported through the landline network toward the central
`office to which the fixed terminal is connected. At the central
`
`office the PCM samples are converted to analog format and
`conveyed to the handset of the subscriber. In the Integrated
`Services Digital Network (ISDN) though, the PCM signals
`are sent to the end user in a digital format.
`The path thus followed by the data packet is dependent
`upon its final destination. For wireless-to-wireless calls, the
`data packets remain in the digital network interconnecting
`the various base stations of the network. As such, no vocoder
`tandeming can occur since the vocoder functions are now
`separate from the base stations. As a result,
`the usual
`degradation in the quality of speech observed to arise when
`vocoders are serially connected when a call is established
`can be avoided.
`
`Another important advantage that results from this novel
`network architecture is the possibility of reducing the num-
`ber of vocoders required in the network. Prior art network
`configurations are designed such as to incorporate a vocoder
`function in every wireless-to-wireless telecommunication.
`Those vocoders are useful when wireless-to-wireline termi-
`
`nal calls are made, and serve the purpose of converting the
`data packet into PCM samples. However, during wireless-
`to-wireless calls, the vocoders serve no beneficial function,
`rather they can be detrimental in that the vocoders can
`establish tandemed connections known to degrade the qual-
`ity of the speech.
`The routing of the data packet, in other words whether the
`packet will remain in the digital network or move through
`the vocoder branch, is effected through the existing signaling
`and control mechanisms and protocols in the network. As it
`is well known to those skilled in the art, the data packets
`carry signaling and control bits that in conjunction with
`external control channels direct
`the packet
`towards the
`correct destination. Thus, on the basis of the telephone
`number dialed by the subscriber at the wireless terminal, the
`system can determine where and how to route the packet. If
`the packet is directed to a wireline terminal, then, through
`switching, it is directed to a vocoder branch. Otherwise, the
`packet remains in the digital network.
`It should be pointed out that the transport of the data
`packet through the digital network connecting the various
`base station can be effected without any transformations on
`the packet as long as the vocoders in the respective wireless
`terminals associated with the various base stations are
`
`compatible with one another.
`As embodied and broadly described herein, the invention
`further provides a communication network for exchanging
`audio information between a wireless terminal and either
`one of a wireless terminal and a wireline terminal,
`the
`wireline terminal being accessible through a landline
`network, said communication network including:
`a) a group of base stations, each base station being
`capable to establish an RF communication with a
`wireless terminal over an air interface, each base station
`being capable of exchanging with a respective wireless
`terminal data packets of compressed audio information,
`each data packet including a coefficients segment and a
`excitation segment;
`b) a digital network interconnecting said base stations to
`one another, said digital network establishing a path-
`way to allow a given base station to exchange data
`packets of compressed audio information with another
`base station of said group, said base stations and said
`
`6
`
`6
`
`

`

`6,138,022
`
`5
`digital network defining a sub-network for transporting
`primarily data packets of compressed audio informa-
`tion;
`c) a data communication channel including a first end and
`a second end, said first end being connected to said
`sub-network, and said second end being capable of
`being connected to the landline network,
`d) a decoder in said data communication channel between
`said first end and said second end, said decoder being
`capable of decompressing a data packet of compressed
`audio information that is received from a base station.
`As embodied and broadly described herein, the invention
`further provides a communication network for exchanging
`audio information between a wireless terminal and either
`one of a wireless terminal and a wireline terminal,
`the
`wireline terminal being accessible through a landline
`network, said communication network including:
`a) a first base station capable to establish an RF commu-
`nication with a first wireless terminal over an air
`
`interface, said first base station being capable of
`exchanging with said first wireless terminal data pack-
`ets of compressed audio information, each data packet
`including a coefficients segment and a excitation seg-
`ment;
`
`b) a second base station capable to establish an RF
`communication with a second wireless terminal over an
`
`air interface, said second base station being capable of
`exchanging with said second wireless terminal data
`packets of compressed audio information, each data
`packet including a coefficients segment and a excitation
`segment;
`c) a digital network connecting said first base station to
`said second base station to allow data exchange
`therebetween, said digital network and said base sta-
`tions constituting a pathway permitting transfer of a
`data packet of compressed audio information received
`from said first base station toward said second base
`
`station substantially without any decoding,
`d) a data communication channel including a first end and
`a second end, said first end being connected to said
`pathway, and said second end being capable of being
`connected to the landline network,
`e) a decoder in said data communication channel between
`said first end and said second end, said decoder being
`capable of decompressing a data packet of compressed
`audio information that is received from a base station.
`
`As embodied and broadly described herein, the invention
`further provides a method for transporting audio information
`received from a first wireless terminal, including the steps
`of:
`
`a) providing a communication network that includes:
`1) a first base station capable to establish an RF
`communication with the first wireless terminal over
`
`an air interface, said first base station being capable
`of exchanging with the first wireless terminal data
`packets of compressed audio information, each data
`packet including a coefficients segment and a exci-
`tation segment;
`2) a second base station capable to establish an RF
`communication with a second wireless terminal over
`
`interface, said second base station being
`an air
`capable of exchanging with the second wireless
`terminal data packets of compressed audio
`information, each data packet
`including a coeffi-
`cients segment and a excitation segment;
`3) a digital network connecting said first base station to
`said second base station to allow data exchange
`
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`6
`therebetween, said digital network and said base
`stations constituting a pathway permitting transfer of
`a data packet of compressed audio information
`received from said first base station toward said
`
`second base station substantially without any
`decoding,
`4) a data communication channel including a first end
`and a second end, said first end being connected to
`said pathway, and said second end being capable of
`being connected to the landline network,
`5) a decoder in said data communication channel
`between said first end and said second end, said
`decoder being capable of decompressing a data
`packet of compressed audio information that
`is
`received from a base station,
`
`b) determining a destination of a call initiated from the
`first wireless terminal;
`1) if said call is directed toward the second wireless
`terminal associated with said second base station,
`directing data packets of compressed audio signal
`received from the first wireless terminal associated
`
`with said first base station through said digital
`network,
`2) if said call is directed toward a second wireless
`terminal associated with said first base station,
`directing data packets of compressed audio signal
`received from the first wireless terminal associated
`
`with said first base station directly to second wireless
`terminal,
`3) if said call is directed toward a wireline terminal
`connected to the landline network, directing data
`packets of compressed audio signal received from
`the wireless terminal associated with said first base
`
`station through said data communication channel.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram representation of a partial
`cellular wireless telecommunications network;
`FIG. 2 is a block diagram that shows a portion of the
`wireless network illustrated in FIG. 1;
`FIG. 3 is a block diagram that shows a partial represen-
`tation of the wireless network in accordance with the inven-
`tion.
`
`DESCRIPTION OF A PREFERRED
`EMBODIMENT
`
`FIG. 1 is a block diagram representation of a portion of a
`cellular wireless telecommunications network. In this figure,
`Mobile terminals (MT) are on the move in the hexagonal
`areas defined as cells. Fixed wireless terminals (FWT) are
`also included in the areas defined as cells. Each cell covers
`
`a predetermined geographical area and has a Base Trans-
`ceiver Station (BTS) which communicates through radio
`channels with the MTs and FWTs. Typically, these channels
`are in the 900 MHZ, 1.8 GHZ or 1.9 GHZ ranges. A number
`of these BTSs (i.e. cells) may be connected by land line or
`microwave link 150 to one Base Station Controller 100
`
`(BSC), which controls handoff functions and routes the
`signal as requested. Anumber of BSCs are in turn connected
`to a Mobile Switching Center 110 (MSC). The MSC coor-
`dinates the activities of all its BSCs, verifies/acknowledges
`MT information through its identification database 120 and
`provides a connection to the landline network 130. The
`landline network may include, among others,
`the Public
`Switched Telephone Network (PSTN), the Integrated Ser-
`vices Digital Network and the Internet. Land terminals 140
`7
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`6,138,022
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`7
`
`(LT) are also known for completeness. Note that the landline
`network and LTs are not part of the cellular wireless com-
`munication network.
`
`When a call is made to or from a wireless terminal (MT
`or FWT), four radio channels are involved in each of the
`links between a wireless terminal and a BTS. The channel
`used to communicate voice data from the BSC to the
`
`wireless terminal is called Forward Voice Channel (FVC).
`The channel used to communicate voice data from the
`wireless terminal to the BSC is called the Reverse Voice
`
`10
`
`Channel (RVC). Two other channels carry the handshaking
`information required to establish communications with
`wireless terminals. They are the Forward Control Channel
`(FCC) and the Reverse Control Channel (RCC). Among
`other things, the FCC and RCC are used to broadcast the
`mobile identification number (MIN) (i.e. the wireless ter-
`minal’s phone number). Transmission power in the MTs and
`BTS is monitored and controlled, and when MTs move from
`one cell to another, a switching technique called handoff is
`initiated.
`
`In existing wireless-to-wireless or wireless-to-wireline
`terminal communications, speech data is carried on a trans-
`port network such as depicted in FIG. 2. FIG. 2 is a block
`diagram that shows a part of the wireless network illustrated
`in FIG. 1 in more detail. The drawing illustrates an example
`of a telecommunication path involving wireless links and
`embodying vocoders. Vocoders are installed in all wireless
`terminals and in the fixed part of the communication net-
`work.
`
`In wireless communications, channel bandwidth is at a
`premium. In order to use the smallest possible portion of a
`radio transmission channel, it is necessary to modulate and
`compress the voice signal of a user before it is transmitted.
`Typically, the voice signal is first digitized by means of one
`of many modulation techniques. Examples of these tech-
`niques are Pulse Code Modulation (PCM) and Delta
`Modulation, PCM being perhaps the most popular.
`Basically, in PCM, samples of an analog signal are taken at
`a specified rate (8 kHz is common) and quantized into
`discrete values for representation in digital format.
`Encoders and decoders (codecs) are then used to com-
`press (and decompress) the digital signals at the source and
`reception point, respectively, in order to optimize the use of
`transmission channels. Codecs used specifically for voice
`signals are dubbed <<voders>> (for voice coders). By
`encoding only the necessary characteristics of a speech
`signal, fewer bits need to be transmitted than what
`is
`required to reproduce the original waveform in a manner
`that will not significantly degrade speech quality. With fewer
`bits required, lower bit rate transmission can be achieved.
`The main advantage of compressing speech is that it uses
`less of the limited available channel bandwidth for trans-
`
`mission. The main disadvantage is some loss of speech
`quality.
`Returning back to FIG. 2, once the RVC and FVC have
`been established for, say,
`the telecommunication link
`between a wireless mobile terminal (MT) A 200 and a
`wireline terminal
`(WT) 250, speech is compressed
`(encoded) by a vocoder located in MTA 200 and sent via a
`wireless link (RF channel A) and a Base Transceiver Station
`(BTS) 215, to Base Station Controller (BSC) A 210. It is
`then converted into PCM samples by the decoder of a second
`vocoder 225. The signal is directed, through the mobile
`switching center A (MSC) 235, to the landline network 240
`to which WT 250 is physically connected. In the landline
`network, the digital signal is converted into analog format
`
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`8
`before it is routed to WT 250. In this example, speech is
`compressed and decompressed only once.
`For a telecommunication link between two wireless
`
`mobile terminals 200 and 280 (or any two wireless
`terminals), proper communication channels are established
`as described earlier, then speech is compressed (encoded) by
`a vocoder located in MT A200 and sent, via a wireless link
`(RF channel A) and BTS 215,
`to BSC A 220. It is then
`converted into PCM samples by the decoder of a second
`vocoder 225. The PCM samples are sent via MSC A235 and
`the Digital Network 245 to a second MSC B 255 (if the
`BSCs are served by different MSC). They are compressed
`(encoded) a second time by the encoder of another vocoder
`265. The compressed signal is then sent via BSC B 270, BTS
`275 and a wireless link (RF channel B) to MT B 280 where
`it is decoded a second time by its vocoder. Audible speech
`is then available at mobile terminal 280.
`
`this arrangement of
`In such a communications link,
`vocoders is called “tandemed vocoding”. Other examples of
`tandemed vocoding are situations where a wireless mobile
`terminal is communicating with a fixed wireless terminal
`and when any type of wireless terminal is retrieving mes-
`sages from a central voice-mail system which uses a vocoder
`to compress speech before it stores the data. In such cases,
`speech is put through the compression and decompression
`algorithms of vocoders more than once. When vocoders are
`tandemed in such a manner,
`the quality of speech is
`degraded.
`Tandemed connections are not desirable particularly
`when low bit-rate codecs (vocoders) are used because they
`produce additional distortion and reduced quality of the
`speech signal as compared to a single stage of vocoding. To
`avoid this difficulty, the prior art has proposed a method
`called bypass that eliminates the double decoding/encoding
`performed by vocoders 225 and 265. More specifically,
`when BSC A220 detects the destination of the message, and
`if the destination (BSC B 270) is bypass capable, BSC A220
`overrides the local vocoder 225 so the data stream issued by
`BSC A220 is compressed speech rather than PCM samples.
`The data stream is transported to the BSC B 270 via the
`MSCs 235 and 255 and the digital network 245 as com-
`pressed speech. Similarly, BSC B 270 observes the incom-
`ing data stream and, if it notes the presence of compressed
`speech data blocks, overrides the local vocoder 265 in such
`a manner that the compressed speech samples are transmit-
`ted without any coding to MT B 280. Note that in the bypass
`mode, the ECs 230 and 260 are also bypassed. The structure
`of this system, and the signaling and command protocol
`required to effect by bypass operations, are described in the
`international patent application PCT/95CA/00704. The con-
`tents of this document are incorporated herein by reference.
`The bypass solution, although effective,