(12) United States Patent
`DeMartin et al.
`
`USOO6421527B1
`(10) Patent No.:
`US 6,421,527 B1
`(45) Date of Patent:
`Jul. 16, 2002
`
`(54) SYSTEM FOR DYNAMIC ADAPTATION OF
`DATA/CHANNEL CODING IN WIRELESS
`COMMUNICATIONS
`
`(75) Inventors: Juan-Carlos DeMartin, Richardson;
`Alan V. McCree; Krishnasamy
`Anandakumar, both of Dallas, all of
`TX (US)
`(73) Assignee: Texas Instruments Incorporated,
`Dallas, TX (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(*) Notice:
`
`(21) Appl. No.: 09/311,008
`(22) Filed:
`May 13, 1999
`
`Related U.S. Application Data
`(60) Provisional application No. 60/086.217, filed on May 21,
`1998.
`(51) Int. Cl." .................................................. H04Q 7/20
`(52) U.S. Cl. ........................ 455/67.3; 455/63; 455/501;
`455/226.3; 375/227
`(58) Field of Search ............................... 455/67.3, 67.7,
`455/63, 501, 423, 9, 560, 226.3, 517,466,
`422,426, 67.1; 375/227, 225, 222
`References Cited
`
`(56)
`
`U.S. PATENT DOCUMENTS
`4,939,731. A * 7/1990 Reed et al. ................. 455/466
`
`5,818,826 A * 10/1998 Gfeller et al. .............. 370/342
`5.839,077 A * 11/1998 Kowaguchi ................ 455/67.3
`5,857,147 A * 1/1999 Gardener et al. .......... 455/67.3
`5,923,679 A * 7/1999 Itoh et al. .................. 455/67.3
`6,108,374. A * 8/2000 Balachandran et al. .. 455/226.3
`6,141,388 A * 10/2000 Servais et al. ............. 455/67.3
`
`* cited by examiner
`
`Primary Examiner William Trost
`Assistant Examiner Keith Ferguson
`(74) Attorney, Agent, or Firm-Robert L. Troike; Frederick
`J. Telecky, Jr.
`
`ABSTRACT
`(57)
`A System for dynamic adaptation of wireleSS communication
`between a Mobile Station (11) and a Base Station (13)
`wherein the transmitted frame from the Mobile Station
`includes a convolutionally coded portion containing a down
`link measurement bit and a repetition code identifying the
`codec mode of the frame. The transmitted frame from the
`Base Station (13) includes a codec mode command signal
`for the Mobile Station (11) in the convolutionally encoded
`portion and the repetition code identify the codec mode of
`the down-link frame. The Base Station (13) includes means
`for analyzing the quality of the up-link frame and means
`from the received down-link measurement bit for determin
`ing the down-link quality.
`
`4 Claims, 2 Drawing Sheets
`
`(
`(3 BIT
`CHANNE
`CONVOL.
`CODE) ENCODED) MEASUREMENT
`210
`21b.
`(1 BIT)
`
`CODEC MODE
`BEACON
`(1 BIT)
`21C
`
`10N
`
`ri
`
`
`
`
`
`Cld
`Y
`d
`
`11
`
`(BEACON) CODECMODE
`25C
`COMMAND
`
`(CONVOL. (3 BIT
`ENCODED) CODE)
`25b
`250
`
`

`

`U.S. Patent
`
`Jul. 16, 2002
`
`Sheet 1 of 2
`
`US 6,421,527 B1
`
`CHANNE
`(CONVOL.
`(3 BIT
`CODE) ENCODED) MEASUREMENT
`210
`21b.
`(1 BIT)
`
`CODEC MODE
`BEACON
`(1 BIT)
`21C
`
`VV
`
`- 21
`TRANS in
`
`
`
`23 N 13
`-1
`e
`
`(BEACON) CODECMODE
`23
`COMMAND
`
`(CONVOL. (3 BIT
`ENCODED) CODE)
`25b
`230
`
`
`
`11
`
`11
`Y
`
`31
`
`55
`
`UP-LINK
`
`35
`
`
`
`CHANNEL
`ENCODER
`
`MS
`
`
`
`
`
`Q
`SSS
`SSS
`(Š
`S)
`
`MODE
`(REPETITION
`CODE)
`
`CHANNEL MEASURE
`FOR DOWN-LINK
`(DELTA MOD ON 1 BIT)
`
`|
`
`- - -
`
`
`
`
`
`SPEECH
`ENCODER
`
`
`
`
`
`UPLINK
`CODEC
`MODE
`
`DOWN-LINK
`CHANNEL
`ANALYSIS
`
`METRICS FROM
`DEMOD/EQ.
`(e.g., RXLEV)
`
`DOWN-LINK
`
`FIC 2
`
`

`

`U.S. Patent
`
`Jul. 16, 2002
`
`Sheet 2 of 2
`
`US 6,421,527 B1
`
`CHANNEL MEASUREMENTS
`FOR DOWN-LINK
`(1 BIT A MOD)
`
`CHOOSE
`UP-LINK
`MODE
`SELECTOR
`
`CHOOSE
`DOWN-LINK MODE
`SELECTOR
`
`NETWORK
`MODE
`OVERRIDE
`
`
`
`A
`E3 KC 2 2 2
`
`ES f t
`
`(ASSUMING
`NOTFO)
`
`41
`
`
`
`DEMOD
`
`45
`
`CHANNEL
`DECODER
`
`
`
`
`
`
`
`
`
`
`
`
`
`METRICS FROM
`DEMOD/EQ
`---
`
`
`
`
`
`|
`-
`Sy: E
`
`
`
`SPEECH
`DECODER
`
`64 kb/s
`PCM
`
`49
`
`DOWNLINK
`MODE
`
`SPEECH
`ENCODER
`
`64 kb/s
`PCM
`
`53
`
`DOWN-LINK
`
`FIC 3
`
`UPNK DOWNLINK
`
`
`
`MOWING
`AVG FILTER
`
`MOVING
`AVG FILTER
`
`HEURISTC
`RULES FOR
`FULL FRAME
`
`HEURISTIC
`RULES FOR
`HALF RATE
`
`FIC 4
`
`MODE 1 OR MODE O
`
`

`

`1
`SYSTEM FOR DYNAMIC ADAPTATION OF
`DATA/CHANNEL CODING IN WIRELESS
`COMMUNICATIONS
`
`This application claims priority under 35 USC S119(e)
`(1) of provisional application No. 60/086,217, filed May 21,
`1998.
`
`TECHNICAL FIELD OF THE INVENTION
`
`This invention relates to wireleSS communications and
`more particularly to a System for dynamic adaptation of
`data/channel coding.
`
`BACKGROUND OF THE INVENTION
`Transmission of digitally encoded speech over wireleSS
`channels in a cellular environment usually requires the use
`of error control techniques to combat the noisy nature of
`Such channels. In cellular applications, however, the char
`acteristics of the channel are highly non-Stationary, that is,
`periods of relatively error-free Signal alternate with periods
`of Strongly deteriorated Signal. The traditional Solution to
`this problem is to allocate to error detection and correction
`enough bandwidth to deal with the “average channel”,
`Sacrificing optimality for the two extreme cases of good and
`bad channels.
`This Static approach is clearly not optimal: in good
`channel conditions most of the resources employed by error
`control are redundant, and could be better used to increase
`the Speech quality, while in bad channels, error control
`should be reinforced by using resources made available by
`a lower bit rate speech codec. Moreover, the cellular channel
`is quite bipolar, that is, oscillates in time between good and
`bad channels, passing only a fraction of the time of a call in
`the “average channel” condition for which the static solution
`was designed. Unequal Error Protection is used in most
`cellular standards. In Unequal Error Protection, speech bits
`are divided into classes of decreasing perceptual importance
`and each class is encoded with appropriate rates of protec
`tion including no protection. Although the Unequal Error
`Protection approach used in most cellular Standards Some
`what mitigates the flaw of using the “average channel”
`approach, a better Solution is desirable.
`
`SUMMARY OF THE INVENTION
`In accordance with one embodiment of the present
`invention, a System is presented that allows one Station to
`communicate with a Second Station. The Station monitors the
`quality of the channels connecting them and adapts their
`data and error control rates accordingly.
`
`DESCRIPTION OF THE DRAWINGS
`FIG. 1 is an overall block diagram of the system in
`accordance with one embodiment of the present invention;
`FIG. 2 is a block diagram of the mobile station;
`FIG. 3 is a block diagram of the base station; and
`FIG. 4 is a block diagram of the mode selector in the base
`Station.
`
`DESCRIPTION OF PREFERRED
`EMBODIMENTS OF THE PRESENT
`INVENTION
`In the present invention, a System for dynamic adaptation
`of Speech/channel coding to the varying conditions of wire
`leSS channels in a cellular environment is presented. In an
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`adaptive System, the ratio between Speech bits and error
`control bits changes as a function of the condition of the
`channel.
`In the present invention, there is more error control and
`leSS Speech bits in bad channels, and more speech bits and
`less error control in good channels, where error control is
`less needed.
`In accordance with the present invention, the up-link and
`down-link codec modes are dynamically changed to account
`for the estimated error rates on the up-link and down-link. To
`implement an adaptive System and, more specifically, one
`that is in accordance to the requirements of the new Global
`System for Mobile Communications (GSM) Adaptive
`Multi-Rate (AMR) system as specified by European Tele
`communication Standards Institute (ETSI), we also need to
`Send in-band information and a mode indicator.
`In a preferred embodiment of the present invention, the
`adaptation control is located in the Base Station (BS).
`Dynamic Switching requires the transmission of two dif
`ferent kinds of in-band information: a channel measurement
`of the quality of the down-link channel, Sent via the up-link,
`that is, from Mobile Station (MS) 11 to Base Station (BS)
`13; and a codec command Sent via the down-link, that is
`from Base Station (BS) 13 to Mobile Station (MS) 11 (see
`FIG. 1). The former describes how good the down-link
`channel is, the latter is the codec mode that the Mobile
`Station (MS) 11 encoder is asked to use.
`The objective is to Send this information accurately and
`frequently enough to make the adaptation mechanism work
`effectively, but using as few bits as possible, to minimize
`overhead.
`We have chosen to send the in-band information for both
`directions in every frame (20 ms), using a single bit. This
`in-band information bit is placed in Class 0, to achieve
`maximum protection. Class 0 is the most channel protected
`Subset of information bits.
`One Substantial problem of an adaptive System is that the
`channel decoder has to know which mode had been used to
`encode the frame before it can Successfully decode it.
`Several Solutions have been considered, the Simplest to
`understand and implement being that of attaching a header
`to the convolutionally encoded frame as presented herein.
`Prior to channel decoding, the header is decoded and the
`mode extracted, allowing the decoding of the rest of the
`frame. A specific implementation of this approach is a
`repetition code with a novel decoding Scheme.
`A relatively simple Solution to the mode indicator prob
`lem is to code the mode with a 3-bit repetition code: Mode
`0 is represented by the codeword '000, while Mode 1 by
`*111. Such codeword is sent together with the convolution
`ally encoded data and represents a header that the channel
`decoder reads in order to determine how to decode the
`convolutionally encoded part of the frame. Traditional
`majority-vote decoding of a 3-bit repetition code would not
`perform well enough in the kind of channels we are dealing
`with, where the bit error rate can be 19% or more. Using
`more bits, however, would diminish the number of bits
`available for speech or for error control. In order to mini
`mize the overhead, applicants, in accordance with one
`embodiment, decode the repetition code taking advantage of
`the characteristics of the information represented by the
`Sequence of modes, a slowly varying, highly correlated
`Sequence of just two modes. With Such decoding, which is
`referred herein as “unanimous decoding, the mode is
`changed only if a unanimous codeword is received, i.e.,
`000 or '111'. Such codewords can only be the result of no
`
`

`

`3
`errors or three errors on the channel. All the remaining
`codewords, caused by all the possible combinations of 1 and
`2 errors, are ignored, leaving the mode unchanged. Errone
`ous decoding is still possible, it will happen every time that
`we have three errors, but it can be shown that it is a relatively
`rare occurrence even in very bad channels, and anyway it
`results, because of the CRC protection (parity discussed
`later), in a frame repeat, which often goes unheard. On the
`other hand, unanimous decoding could slow down a mode
`change, Since it is based on the assumption that a mode
`change is unlikely, but it can be shown that the probability
`of making the transition to the right mode within three
`frames is very high even in bad channels. In Short, a
`traditional repetition code decoded in a novel way allows
`good performance in a time-varying channel with a mini
`mum amount of overhead (3 channel bits). If even higher
`Switching performance is needed, the System can be Straight
`forwardly extended to use 4 or more bits.
`Referring to the system 10 of FIG. 1, a cellular Mobile
`Station (MS) 11 comprising a transmitter, a receiver, an
`antenna and transmit/receive control Switch (TR) transmits
`a packet frame 21 to a Base Station (BS) 13. The packet
`frame 21 is made of three parts or subsets. The first and main
`part or Subset 21b is the Speech bits, parity bits, and in-band
`information all convolutionally encoded. A Second part or
`Subset 21a is the 3-bit codec mode header discussed above.
`The third part or subset 21c is a 1-bit codec mode beacon
`(explained later). The Base Station 13 includes a transmitter,
`a receiver, and an antenna System for transmit and receive.
`The Base Station 13 determines the best speech and channel
`coding combinations for both the up-link and the down-link.
`It then creates a frame 23 using the appropriate mode for the
`down-link. The frame 23 is made of the same three parts: the
`Subset of the convolutionally encoded section 23b, the codec
`header 23a and codec mode beacon 23c. The convolution
`ally encoded Subset 23b includes a codec mode command
`for the up-link as part of the in-band information. The Base
`Station (BS) 13 receives the frequencies of the channel used
`by the Mobile Station (MS) 11 and the Mobile Station (MS)
`receives the frequencies of the channel used by the Base
`Station (BS) 13. The system 10 can change the source and
`channel bit rates to adapt to the quality of the channels. The
`present invention does this in accordance with the con
`straints and requirements for the new Global System for
`Mobile Communications (GSM) Adaptive Multi-Rate
`(AMR) system as specified by European Telecommunica
`tion Standards Institute (ETSI).
`In the new GSM AMR, there are two channel modes, full
`rate or half rate. In the full rate, there are 456 bits per frame
`at an overall bit rate of 22.8Kbits per second. In the half rate,
`there are 228 bits per frame at an overall bit rate of 11.4K
`bits per second. Within each of the two channel modes (full
`rate or half rate) there are in the present embodiment two
`different Speech/error control combinations. These are the
`codec modes. With the full rate there are two options. For a
`bad channel, the codec mode is Mode 0 and the Source
`coding rate for Speech is 7.45 Kb/sec. For the good channel,
`the codec mode is Mode 1 and the Source coding rate for
`speech is 11.85 Kb/sec. The rest of the bits are used for
`in-band Signaling, channel coding, codec mode header and
`codec mode beacon. The channel coding adds redundancy to
`correct for bit errors. For the half rate there are two options.
`For a bad channel, the codec mode is Mode 0 and the Source
`coding rate is 5.15 Kb/sec. For the good channel, the codec
`mode is Mode 1 and the source coding rate is 7.45 Kb/sec.
`Referring to FIG. 2, there is illustrated the Mobile Station
`(MS) 11 according to one embodiment of the present inven
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`4
`tion. The up-link input speech is Sampled/digitized and
`encoded with the up-link codec mode in encoder 31. The
`Speech/data bits are multiplexed with a down-link channel
`measurement bit from down-link analyzer 39 in multiplexer
`33 and the multiplexed speech bits and channel measure
`ment bit output is applied to the channel encoder 35. The
`channel encoder is operated at the codec up-link mode
`received via the down-link from the Base Station 13.
`Speech bits are divided into classes of decreasing percep
`tual importance. Each class is then encoded with convolu
`tional codes of appropriate rate (including, possibly, rate 1,
`i.e., no protection). The first class, Class 0, includes the most
`important bits. On the up-link frame, the bits are protected
`by a Cyclic Redundancy Code (CRC) parity check. A CRC
`parity check is computed over the bits of Class 0 to detect
`any error at the receiver. At the receiver, the received CRC
`is compared to the CRC computed over the received bits: if
`they are equal, all bits in Class 0 are assumed to be correct.
`The down-link channel measurement bit (1 bit) is part of
`Class 0 and therefore has the CRC protection. With no
`convolutional encoding and no error detection, we have the
`codec mode identifier (repetition code) 21a. This identifier
`21a is Sent as header information and for the example is the
`repetition code discussed above. We also have the codec
`mode beacon (1-bit channel) 21c described later. Everything
`else is channel coded which in this case means that a
`convolution code with different levels of redundancy for
`every bit of information is used. This is what the Mobile
`Station (MS) 11 is transmitting. The channel encoded infor
`mation 21b, the codec mode header 21a and beacon bit 21c
`are sent in the frame 21. They are modulated on the RF
`carrier of the mobile transmitter.
`Referring to FIG. 3, the Base Station (BS) 13 antenna
`System picks up the radiated packet frame signal and down
`converts to the base band Signal which is detected at
`demodulator 41 and the analog signal is Sampled back to
`digital bits, for example, in maximum likelihood equalizer
`43. The receiver recognizes the header 21b 3-bit (repetition
`code for example) code and knows the codec mode to use for
`the frame. The equalizer 43 makes a decision as to whether
`a logic 1 or Zero and passes the result to the channel decoder
`45. The reliability of the received bits is reflected by their
`soft values, a number, for example, between -127 and +127
`that is directly proportional to the probability of error. If the
`bits are strongly 1, the value is close to +127. If the bits are
`strongly 0, the value is close to -127. All the intermediate
`values reflect a lesser degree of confidence. This level of
`confidence is used to choose the up-link mode at up-link
`mode select 47.
`A Suitable moving average of the Soft-values is a good
`estimator of the current Carrier to Interference (C/I) Ratio of
`the channel, a parameter which is directly connected to the
`amount of errors introduced by the channel. The aim of the
`mode selector, for both up- and down-link, is to follow the
`C/I profile faithfully and quickly enough to allow a good
`mode adaptation between the two available modes.
`Two different sets of thresholds have been chosen for the
`specific case of the GSM half- and full-rate channel modes.
`For the half-rate case, the absolute values of the Soft-bits for
`the current frame are averaged together and the resulting
`value is then fed to a moving average filter 63 of memory
`size 40 (See FIG. 4). The filter 63 averages over 40 frames.
`The output of the filter, called “average value', is then used
`to estimate the current C/I value and consequently the most
`suitable mode to be used for the up-link. The mode is chosen
`according to a number of heuristic rules:
`If average value < 120, change to mode 0 (the mode with
`greater error control protection);
`
`

`

`S
`If average value and previous one is >126, Switch to mode
`1 (the mode with less error protection);
`If the last 30 values of average values >124, Switch to
`mode 1.
`For the full-rate case, the memory of the moving average
`filter 65 is over 35 frames. The rules are:
`If average value < 104, change to mode 0 (the mode with
`greater error control protection);
`If average value >113, Switch to mode 1 (the mode with
`less error protection).
`These rules intend to induce a mode Switch between 16 and
`13 dB C/I for the half-rate, and between 10 and 7 dB C/I for
`the fill-rate in the present embodiment.
`AS Stated previously, the codec mode used for a given
`frame is Sent outside the convolutionally encoded part of the
`frame in a header. This header is decoded (by the unanimous
`decoder for example) and used to select the codec mode for
`the channel decoder 45. The compressed speech and the
`mode are sent to the speech decoder 49 for decoding the
`Speech at, for example, 64 kb/sec pulse code modulation
`(PCM). This is sent out at this rate to the user to which the
`call is addressed. The quantized down-link channel mea
`surement bit sent by the Mobile Station 11 in-band infor
`mation via the up-link frame 21 is decoded by the channel
`decoder 45 as part of Class 0 and provided to the down-link
`mode selector 51. This information is used to select the
`appropriate codec mode at the Speech encoder 53 and
`channel encoder 55. Upon reception of this information, the
`Base Station 13 performs inverse Delta Modulation
`quantization, obtaining the quantized "average value'
`described in the up-link. The same heuristic rules are applied
`for mode decision as in the up-link mode Selector. In the
`present embodiment of the invention, the Delta Modulation
`is set up slightly different between half and full rate. The
`quantization Step size is 3 in the first case and 2 in the
`Second. The mean is 80 in the first and 90 in the second. The
`codec mode may be overridden at the base Station by the
`network mode override. The down-link codec mode is also
`Sent outside the convolutionally encoded part of the frame as
`a header to the mobile station 11. The 64 kb/s PCM speech
`bit/data bits is applied to the speech encoder 53 which
`encodes the Speech at the down-link codec mode Selected at
`selector 51. The encoder 53 output is multiplexed with the
`codec mode command bit from up-link selector 47 and the
`combination is convolutionally encoded, with the codec
`mode command bit in the Class 0 protected by the CRC, as
`explained before. The channel encoder 55 is operated at the
`down-link codec mode as determined at the selector 51. The
`repetition code identifying the down-link codec mode is sent
`outside the convolutionally coded frame as a header 23a.
`The frame 23 with header 23a and codec beacon bit 23c are
`up converted to the down-link frequency of the Base Station
`(BS) 13. At the Mobile Station 11 this down-link frequency
`Signal is picked up at the mobile antenna and down con
`verted to base band. The frame 23 with convolutionally
`coded Subset 23b, header 23a and beacon bit 23c is demodul
`lated at demodulator 37 and the codec mode of the header
`23a is decoded and applied to the channel decoder 32 and
`speech decoder 34. The demodulator 37 output is applied to
`a maximum likelihood equalizer 38 which makes a decision
`as to whether a logic 1 or logic 0 and provides a reliability
`based on the Soft values between, for example, -127 and
`+127 that is directly proportional to the probability of error.
`The soft values are fed to the channel analysis 39 where their
`absolute values are averaged over the frame. This measure
`is encoded with Delta Modulation (change) in a measure
`ment bit which is applied to multiplexer 33 to be sent to the
`
`6
`Base Station 13 and used at down-link mode selector 51 as
`discussed previously. The channel decoder 32 output is
`applied to the demultiplexer 36 where the up-link codec
`mode command bit (1 bit) is removed and applied to the
`Speech encoder 31 and channel encoder 35 for encoding at
`the command codec mode rate determined at the base Station
`13. The down-link speech is decoded at decoder 34.
`The command bit from the Base Station (BS) 13 com
`manding the Mobile Station (MS) 11 to change the codec
`mode to change the rate is done by variable-length coding.
`The entire code word is not transmitted at once. If the bit is
`0 you know to use mode 0. If you get a “1” and then in the
`following frame a “0”, then you switch to mode 1. If it stays
`0, you stay in mode 0. It also allows more codes.
`A minimum amount of overhead is possible by the
`adoption of a variable-length, Suffix coding Scheme for the
`codec mode and of differential coding for the channel
`measurement.
`
`TABLE 1.
`
`CODEC MODE:
`
`AMR Mode 0 (FR)
`AMR Mode 1 (FR)
`AMR Wideband O
`AMR Wideband 1
`GSM FR (full rate)
`GSM EFR (enhanced full rate)
`GSM HR (half rate)
`
`O
`1O
`110
`1110
`11110
`111110
`1111110
`
`Extended Modes
`
`Note that this variable-length Solution has the advantage
`of being expandable at will. The delay is significant, Several
`frames, only for Extended Modes (EM), which should be
`fine, since a fairly slow Switch to EM’s should be in line with
`current GSM practice in the context of Tandem Free Opera
`tions (TFO).
`Channel Measurement
`It may be desirable to use mean-removed Delta Modula
`tion quantization for channel measurement. The 1-bit mean
`removed Delta Modulation quantizer has fixed prediction
`coefficient of for example a coefficient=0.95.
`The quantized channel measures give an indication of
`how good the channel is, like channel grade 0, channel grade
`1, channel grade 2 or channel grade 3.
`Codec Mode Beacon Bit for Extended Modes
`The requirement for extended modes poses Several prob
`lems. It seems clear that a full Solution allowing Switching
`from a mode to any other mode (say, from AMR mode 0 to
`GSM EFR) at every frame is not necessary. What seemed
`necessary is, instead, a Signaling Solution capable of han
`dling Switching to an extended mode at a reasonable Speed
`and with good robustness.
`Again, we looked for a Solution with the lowest bit rate
`impact possible. We chose to add a single Codec Mode
`Beacon (CMB) (1 bit) to the bitstream. This bit, through the
`Same variable-length code used for the in-band information,
`keeps repeating the codec mode currently in use, working
`like a color-coded lighthouse beam. Since the bit goes into
`the channel unprotected, the beam is often shadowed and
`misinterpreted, but in time the message gets through cor
`rectly.
`When working with AMR modes, the bit is generally
`disregarded. When Switching to one of the extended modes,
`instead, the CMB performs an anti-lock function.
`For an Extended Mode, the mobile station (MS) 11
`recognizes the in-band codec mode command in the coded
`
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`Subset 23b and Switches to the corresponding Extended
`Mode (EM) (see FIG. 1 and Table 1). As described above,
`the Base Station (BS) 13 periodically tells the Mobile
`Station (MS) encoder which codec mode to use, and does so
`through the codec mode in-band information (via the down
`link). When the command to switch to an EM is received,
`the mobile station (MS) 11 encoder 31 will process the next
`Speech frame with the corresponding mode, and the mobile
`station (MS) 11 decoder 32 will decode the next frame
`coming from the base station (BS) 13 with the same mode.
`With error-free channels, this scheme would work correctly,
`allowing to switch back and forth between AMR and
`Extended Modes (please remember that EMs will have
`appropriate channel coding and in-band Signaling).
`However, Since the channel is error prone, we need a way to
`make Sure that the decoders, both on the Mobile Station
`(MS) 11 and on the Base Station (BS) 13, will not enter a
`crash State of continuous decoding of frames coded with a
`mode different from the one considered correct.
`The CMB provides lock-breaking protection. After a
`Switch, the MS/BS decoder will start monitoring the CRC
`behavior; if it is decoding the wrong mode, a continuous
`string of frame repeats will be observed. After a number of
`them (Such as ten, for example), the decoder will start paying
`attention to the CMB, in order to determine the actual mode
`being used by the encoder. Since, over time, even an
`unprotected CMB will convey the right mode information,
`the decoder will be able to resynchronize itself with the
`mode used by the encoder.
`Change to a different mode, either EM or AMR, will be
`possible through the in-band information included in the bit
`stream of the EMs themselves.
`To improve robustness, the Switch to an EM can be made
`only after the Successful reception of more than one appro
`priate codec mode commands.
`What is claimed is:
`1. A wireleSS communication System comprising:
`a Mobile Station and a Base Station adapted to transmit
`and receive a communication signal frame containing
`data and error control from each other, Said frame
`including a header with a unanimous repetition code
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`identifying the codec mode of the transmitted frame
`and a convolution encoded Subset including Speech
`coding with different level coding and parity; Said
`Mobile Station and said Base Station including a unani
`mous bit decoder for identifying the codec mode of the
`header with the repetition code, Said frame includes an
`extra codec mode beacon code out of band to guarantee
`mode Switching for additional robustness.
`2. A wireleSS communication System comprising:
`a Mobile Station and a Base Station adapted to transmit
`and receive a communication signal frame containing
`data and error control from each other, Said frame
`including a header with a unanimous repetition code
`identifying the codec mode of the transmitted frame
`and a convolution encoded Subset including Speech
`coding with different level coding and parity; Said
`Mobile Station and said Base Station including a unani
`mous bit decoder for identifying the codec mode of the
`header with the repetition code; said Mobile Station
`and Said Base Station are adapted to transmit and
`receive a communication signal frame containing data
`and error control from each other; Said frame from Said
`Mobile Station to said Base Station containing down
`link channel measurement using delta modulation; and
`said frame from said Base Station to said Mobile
`Station includes a codec mode command with a vari
`able suffix code with one bit sent per frame.
`3. A wireleSS communication System comprising:
`a mobile Station and a base Station adapted to transmit and
`receive a communication signal frame containing data
`and error control from each other; Said frame from Said
`mobile Station to Said base Station containing downlink
`channel measurements using delta modulation; Said
`base Station performing inverse delta modulation quan
`tization; and Said frame from Said base Station to Said
`mobile Station includes a codec mode command with a
`variable suffix code with one bit sent per frame.
`4. The method of claim 3 wherein said frame includes an
`extra codec mode beacon code out of band to guarantee
`mode Switching for additional robustness.
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