EP1198934A1 - Procede et dispositif acceptant la modulation tdd/tty sur des voies telephoniques equipees en vocodeur - Google Patents

Procede et dispositif acceptant la modulation tdd/tty sur des voies telephoniques equipees en vocodeur

Info

Publication number
EP1198934A1
EP1198934A1 EP00952564A EP00952564A EP1198934A1 EP 1198934 A1 EP1198934 A1 EP 1198934A1 EP 00952564 A EP00952564 A EP 00952564A EP 00952564 A EP00952564 A EP 00952564A EP 1198934 A1 EP1198934 A1 EP 1198934A1
Authority
EP
European Patent Office
Prior art keywords
bits
sequence
frame
original
vocoder
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP00952564A
Other languages
German (de)
English (en)
Inventor
Nikolai K. N. Leung
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Qualcomm Inc
Original Assignee
Qualcomm Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Qualcomm Inc filed Critical Qualcomm Inc
Publication of EP1198934A1 publication Critical patent/EP1198934A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/08—Arrangements for detecting or preventing errors in the information received by repeating transmission, e.g. Verdan system
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10L—SPEECH ANALYSIS TECHNIQUES OR SPEECH SYNTHESIS; SPEECH RECOGNITION; SPEECH OR VOICE PROCESSING TECHNIQUES; SPEECH OR AUDIO CODING OR DECODING
    • G10L19/00—Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis
    • G10L19/005—Correction of errors induced by the transmission channel, if related to the coding algorithm
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04M—TELEPHONIC COMMUNICATION
    • H04M1/00—Substation equipment, e.g. for use by subscribers
    • H04M1/72—Mobile telephones; Cordless telephones, i.e. devices for establishing wireless links to base stations without route selection
    • H04M1/724—User interfaces specially adapted for cordless or mobile telephones
    • H04M1/72475—User interfaces specially adapted for cordless or mobile telephones specially adapted for disabled users
    • H04M1/72478—User interfaces specially adapted for cordless or mobile telephones specially adapted for disabled users for hearing-impaired users
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W48/00—Access restriction; Network selection; Access point selection
    • H04W48/08—Access restriction or access information delivery, e.g. discovery data delivery
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W72/00—Local resource management

Definitions

  • the present invention relates to the field of telecommunication devices for the deaf (TDDs) or text telephone yokes (TTYs). More particularly, the invention relates to modification of standard vocoder operation to enable reliable transport of TDD/TTY signals within a telecommunication system.
  • the system may include wireless links.
  • TDDs telecommunication devices for the deaf
  • TTYs Text Telephone Yokes
  • TDDs include a keyboard and a display connected to a telephone via a modem (modulator /demodulator).
  • the modem is built into the TDD and is either directly connected to a telephone line or coupled by an acoustic coupler to a normal telephone handset.
  • TDDs are capable of transmitting information over telephone lines by means of coded tones to other TDDs connected at opposite ends of the telephone line through another modem. These tones are referred to as low activity communications because the frequency and amplitude envelopes remain relatively constant.
  • the code and protocol that is in widespread conventional use for TDD communications is an idiosyncratic one.
  • the code set known as Baudot
  • TDD protocol the communication protocol
  • the TDD protocol was constructed for a set of constraints that no longer are relevant to present day devices. Those constraints work to create a code protocol and a telecommunication network of users and devices operating under that protocol, that is somewhat antiquated.
  • TDD communications are conducted at 50 Baud (45.5 Baud in some countries), representing a transfer of 6 characters per sec.
  • Other protocols now available for TDD communications incorporate higher Baud rates, such as the ASCII (American Standard Code Information Interchange) and enhanced Baudot protocols.
  • a normal TDD communication character set consists of characters that are 5 bits long. These characters are analogous to a letter in an alphabet where each letter represents a word or idea.
  • a character is grouped with overhead information bits prior to transfer, where each group of bits to be transferred has a duration or unit interval equal to 22 milliseconds.
  • a group of bits to be transferred comprises 8 bits: a start bit (one source or zero bit), five bits representing the character, and at least one and 1/2 bits marking the stop point of the transfer group.
  • TDD transmissions occur at a snail's pace.
  • a bigger problem is that TDD signals are substantially constant. These slow paced, monotone signals can create havoc in digital telecommunication systems that transmit higher activity signals at very high rates, and especially in telecommunication systems that include wireless links.
  • One example of such a telecommunication system is a code division multiple access (CDMA) system having a large number of wireless subscriber units. Each subscriber unit has a transceiver and communicates within the system through satellite repeaters or terrestrial stations referred to as cells. Each cell includes a physical plant called a base station. A cell covers a limited geographic area and routes calls carried over subscriber units to and from the telecommunication network via a mobile switching center.
  • CDMA code division multiple access
  • a subscriber unit When a subscriber moves into the geographic area of a new cell, the routing of that subscriber's call may be eventually made through the new cell by a process called a "handoff."
  • a subscriber unit generically referred to as a cell phone, transmits a signal that is received by a base station. The signal is relayed to a mobile switching center that routes the signal to a public switched telephone network (PSTN) including telephone lines or other subscriber units. Similarly, a signal may be transmitted from the PSTN to a subscriber unit via a base station and a mobile switching center
  • PSTN public switched telephone network
  • the interface between the subscriber unit and the base station is referred to as the air interface.
  • the telecommunications industry association has provided a standard for CDMA call processing on the air interface entitled "IS- 95 Mobile Station - Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System.” Addendum to IS-95 are provided as Telecommunications Service Bulletins (TSB).
  • Telecommunications Service Bulletins The standard IS-95 + TSB74 includes provisions for service negotiation on the air interface and is incorporated herein by reference. Service negotiation is critical to successfully transmit any communication, especially a low activity TDD communication, over a digital telecommunication system.
  • a vocoder - a device used in the system to encode a voice or TDD analog signal into a digital signal, and to decode a digital signal into a voice or TDD analog signal- has difficulty in handling the substantially monotone signal and slow speed dictated by the TDD protocol.
  • a low activity communication signal such as a TDD communication would probably be treated by the vocoder as background noise or signal interference and be disregarded.
  • What is needed is an invention that can easily be integrated into existing communication systems and can be capable of reducing frame error rates by invoking a protocol to be used by the vocoders during transmission of the low activity communication signal.
  • the invention should be compatible with wireless telecommunication modulation systems, such as CDMA systems, servicing large numbers of system users.
  • wireless telecommunication modulation systems such as CDMA systems
  • a more robust discussion of CDMA systems and techniques used in multiple access communication systems may be found in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” assigned to the assignee of the present invention and incorporated by reference herein.
  • the invention should also be compatible with other modulation systems and techniques used in other types of communication systems, such as time division multiple access (TDMA), frequency division multiple access (FDMA), and amplitude modulation (AMPS) schemes.
  • TDMA time division multiple access
  • FDMA frequency division multiple access
  • AMPS amplitude modulation
  • the present invention involves the modulation of a low activity communication by a telecommunication system using encoded signals and increased transmission power levels. More particularly, the invention concerns a method that uses specialized encoding, decoding, or both, on a low activity communication signal to minimize a transmitted signal's frame error rate. The invention also provides for decoding a low activity signal by looking at "soft bits" contained in erred frames, or in frames adjacent to an erred frame, in an attempt to determine the content of the original frame. Certain disclosed embodiments of the invention provide unique decoding methods for a TDD signal that was encoded using standard encoding protocol.
  • the decoder may compare a frame containing transmission errors (erred frame) with a vocoded frame from a known TDD signal and determine the most likely vocoded frame that was transmitted. In another embodiment, the decoder may review adjacent frames to determine the most likely vocoded frame that was transmitted but received in error. In yet another embodiment, the decoder can be modified to include a signal enhancer or repeater that "cleans up" corrupted bits in the transmitted frame before the decoding methods are applied upon the transmitted frames. And although a TDD communication is discussed throughout this application, it should be understood that any slow or low activity communication may be transmitted using this invention.
  • Another embodiment of the invention provides for decoding as discussed above but invokes vocoder parameters that are different from standard vocoder parameters.
  • the encoder switches to "Baudot encoding mode," notices the decoder of the protocol change, and uses channel coding redundancy to further improve the decoder's chances of determining the correct TDD signal sent even if it is contained in a bad frame.
  • This version of the invention replaces standard vocoder parameters with vocoder "signatures" that are better spaced apart, thus making it easier to distinguish between tones.
  • Another version of the invention provides for encoding a TDD signal in vocoder frames using redundancy, but doing the encoding across numerous vocoder frames.
  • the information is interleaved across "N" frames so that if a frame is lost, the decoder can extract necessary information from adjacent frames to determine the content of the lost frame.
  • Yet another version of the invention provides a cost-efficient system for combining the aforementioned encoding and decoding methods of the invention with methods for controlling transmission power levels. Modifying the design of a standard vocoder chipset within a mobile station is expensive, which could bar the implementation of the aforementioned encoding and decoding methods. However, a communication system can be implemented wherein the aforementioned methods are utilized in the system's base stations, but not in the mobile stations. Frame error rates for low activity communication signals transmitted from the base station to the unmodified vocoder in a mobile station can be minimized using methods for controlling transmission power levels.
  • the invention provides its users with numerous advantages.
  • One advantage is that a TDD message can be transmitted using a digital transmission medium having wireless links.
  • a TDD device can be connected to a mobile device or subscriber's unit, such as a digital cellular telephone, connected to the telecommunications system by a wireless link.
  • the invention also provides a number of other advantages and benefits that should become even more apparent after reviewing the following detailed descriptions of the invention.
  • FIGURE 1A is a block diagram of hardware components and interconnections of a telecommunications system incorporating wireless links in accordance with one embodiment of the invention
  • FIGURE IB is a block diagram of a vocoder capable of implementing the present inventions encoding and decoding methods coupled to a prior art noticing apparatus in accordance with one embodiment of the invention
  • FIGURE 2 illustrates a typical prior art TDD communication device used in accordance with one embodiment of the invention
  • FIGURE 3 shows a traffic channel frame format for a rate set 1 used by a variable rate vocoder
  • FIGURE 4 is a flow diagram of a method aspect in accordance with one embodiment of the invention.
  • FIGURE 5 illustrates a block diagram of a wireless telecommunication system configured according to an embodiment of the invention
  • FIGURE 6 illustrates a block diagram of a wireless telecommunication system configured according to an embodiment of the invention
  • FIGURE 7 is a flow diagram of a method for controlling transmission power levels between a base station and a mobile station.
  • FIGURES 1 through 7 illustrate examples of various method and apparatus aspects of the present invention. For ease of explanation, but without any limitation intended, these examples are described in the context of a TDD communication device attached to a digital telecommunication system incorporating wireless links, one example of which is described below.
  • FIG. 1 illustrates one type of telecommunications system 100 including wireless links and a TDD communication device (TDD) 200 as used in the present invention.
  • TDDs usually include a keyboard and a display that are connected to a telephone via a modem (modulator/demodulator).
  • the modem is built into the TDD and is either directly connected to a telephone line or coupled by an acoustic coupler to a normal telephone handset.
  • TDDs are capable of transmitting information over telephone lines by means of coded tones to other TDDs, such as TDD 102 shown in FIG. 1, connected at opposite ends of a telephone line through another modem.
  • the TDD 200 may be coupled to a subscriber unit 104 that is used in the telecommunications system 100 to transmit received signals.
  • a subscriber unit 104 are digital signal telephones, such as the Q-800 manufactured by Qualcomm Incorporated, and commonly referred to as cell phones.
  • the subscribers unit 104 as shown in FIG. 1 includes a noticing apparatus 106 communicatively coupled to circuitry of the subscribers unit 104.
  • a hardwire 108 may be used to connect the TDD 200 to the subscribers unit 104 via the noticing apparatus 106, or a device port may be used. Examples of such a noticing apparatus and device ports are disclosed in the U.S.
  • the device port may be configured to receive a low activity communication device attachment such as a plug, connector, or receiver. These items are commonly used today for connecting telephone and computer equipment, and are readily available from electronics suppliers.
  • the device port interfaces with the attachment to communicatively connect a low activity communication device (not shown) such as the TDD 200 to the subscriber unit 104 of the telecommunications systemlOO.
  • the device port allows information to be exchanged between a low activity communication device and the subscriber unit 104. Regardless of whether a device port or a hardwire is used, the noticing apparatus 106 allows for the system 100 to be noticed that a TDD signal needs to be transmitted.
  • the signal is processed by the subscriber unit 104.
  • a signal for transmission is created that includes the information contained in the low activity signal.
  • the system adapts to assure a decipherable transmission occurs.
  • an analog signal received from the analog circuitry 228 shown in FIG. 2 normally would undergo signal or "voice" processing including digitizing the signal, setting a transmit power level to protect against signal fading during transmission, compressing the signal, and filtering.
  • These functions may be performed by the circuitry (not shown) of the subscriber unit 104 that includes a vocoder.
  • a variable rate vocoder - generically referred to in this application as a vocoder - may dynamically determine and negotiate service within the telecommunications system 100 to provide successful transmission and decoding of the signal. This negotiation involves establishing the values for multiple parameters, such as the rate the vocoder should use, the transmission power, and compression technique.
  • TIA/EIA/IS-95-A entitled "Mobile Station-Based Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systems, referred to as "IS-95" and incorporated by reference herein, and other transmission standards, including standard vocoder protocol, are well known in the art.
  • a vocoder may identify the signal as either noise, a pause, or a signal not intended to be transmitted. Simply, a vocoder doesn't know what service to use because it cannot identify the low activity signal received. By noticing the system 100 that a low activity signal is being sent, the vocoder will establish the service needed to assure the best possible transmission and decoding of the signal.
  • a signal may be transmitted using an antenna 112 over a wireless link 114.
  • the digitized signal is received by another antenna 116 at a remote location, such as a base station 118, and processed by base station circuitry (not shown) including a vocoder 120.
  • base station circuitry (not shown) including a vocoder 120.
  • Various based station circuitry arrangements for telecommunications systems are well known in the art, and a further understanding may be found in TTA/EIA/IS-95-A referenced above.
  • a second noticing apparatus 106 is shown coupled to the base station 106. This provides for a low activity signal to be sent from the low activity communication device 102 back to the TDD communication device 200.
  • Communication link 120 appears bifurcated to emphasize that the base station 118 may not be connected directly to the low activity device 102.
  • the base station 118 is usually connected to a standard PSTN switching station commonly used by telephone companies for coordination of telephone calls and the low activity device 102 is connected to the PSTN.
  • a second mobile station (not shown) connected to the low activity communication device 102 may be linked to the base station 118.
  • the telecommunication system may include mobile switching stations as mentioned above.
  • FIG. 2 Shown in FIG. 2 is a schematic block diagram of the circuitry of a typical TDD device 200, either a standard or enhanced TDD, operating in accordance with the present invention.
  • a keyboard 202 is provided into which the user may input data characters.
  • the output of the keyboard 202 is connected to a processor 204 that serves to control the circuit elements contained in FIG. 2.
  • Characters that are received or transmitted by the processor 204 are also displayed on a display 206.
  • the same characters received or transmitted may be reproduced on a device such as printer 208.
  • Some TDD devices may not have a printer, although it standard for TDDs to have a visual display of some kind so that a user can see the characters being typed and received.
  • the keyboard 202 thus functions as an input source of data characters to the processor 204 while either or both the display 206 and the printer 208 serve as local destinations for the data stream characters.
  • the processor 204 may be connected by a suitable data and address bus that would typically be used for this type of application by one schooled in the art.
  • the bus 210 connects a read only memory (ROM) 212 to a non- volatile random access memory (NVRAM) 214.
  • NVRAM non- volatile random access memory
  • Appropriate control lines 216 and 218 are connected from the processor 204 to the ROM 212 and the NVRAM 214 providing interactive control of these units.
  • the ROM 212 is intended to permanently store the program that dictates the operation of the processor 204 as well as certain data used by the program. For example, special character strings for machine-to-machine communication and for synchronizing two TDDs in an enhanced operating mode may be stored.
  • the NVRAM 214 is used as a buffer, a floating storage place for data coming into or out of the TDD device 200, and for storage of standard messages as entered by the user through the keyboard 202 and intended for rapid.
  • Other circuitry configurations may be used, such as combining the microprocessor 202 with the ROM 212 and the NVRAM 214 in a single integrated circuit.
  • a telephone keypad 220 is Also connected to the processor 202 in FIG. 2 is a telephone keypad 220 that permits the entry of telephone numbers for dialing by the processor 202 through telecommunications system 100.
  • a standard telephone handset 224 rests on a cradle 226 that incorporates a switch (not shown) indicating whether the handset 224 is in use and thus removed from the cradle 226.
  • the processor 204 is communicatively connected through analog circuitry 228 to the telecommunications system 100.
  • This connection is shown as a hardwire connection 230, but may be any type of connection that can communicatively link the analog circuitry 228 with the telecommunications system 100.
  • the analog circuitry 228 provides a connection between the handset and the processor 202 allowing both Baudot tones and dialing tones to be received by the telecommunications system 100.
  • the analog circuitry 228 provides an interface of voice information to and from the handset 224.
  • the analog circuitry 228 of the TDD device 200 is connected to the telecommunication system 100 using a connector such as the device discussed above.
  • multiple subscriber unit 104 may be linked to the base station 118, or the low activity communication device 200 may be integrated with the subscriber unit 104.
  • FIG. 3 shows a typical variable rate vocoder frame format for a traffic channel using a rate set 1.
  • the variable rate vocoder produces a frame every 20 milliseconds using Code Excited Linear Prediction (CELP) techniques that are well known in the art.
  • CELP Code Excited Linear Prediction
  • the frames may be formatted in full, half , quarter, or eighth rate formats depending upon voice activity.
  • variable rate vocoder will usually detect low activity and use the eighth rate format, assuming the standard vocoder currently in use can detect that a signal is being sent. Commonly, a Baudot signal will be treated as noise and generally ignored.
  • Full rate refers to the fact that each bit contained in each frame is not repeated.
  • Half-rate refers to sending the same number of bits per frame, but each bit is repeated once in the frame; that is, each unique bit will appear twice in the frame.
  • Quarter-rate refers to each unique bit appearing four times per frame, and so on. The more repetitively a bit of information is sent, the less total information is sent per frame. At full rate the signal is sent at a higher power because a given bit is sent only once.
  • This full rate power level is referred to as the reference power for purposes of this application. Because bits are repeated at lower rates, a reduced power level is used because the power for each repeated bit is accumulated over the frame. Assuming a fixed miriimum power is used for the transmission, a full rate transmission will contain more frame errors than would a half rate transmission of the same information.
  • the power level is set based upon a selected frame error rate (FER) for the transmitted signal as received at a remote location, also referred to as the target of the transmitted signal, such as the subscriber unit.
  • FER frame error rate
  • a desired FER is selected because when a low activity signal is being sent, the actual FER increases using current methods.
  • This selected FER range is between a 0.1% and a 1.0% error rate, but may be greater or lesser if necessary for preservation of the quality of the transmitted signal.
  • an FER of 0.2% is desirable for low activity signals.
  • implementing specialized encoding and decoding techniques controls the frame error rate.
  • FER being defined as the total number of erred frames even after reconstruction of vocoder frame information
  • - methods to adjust transmission power levels can also be used along with the specialized encoding and decoding techniques.
  • the vocoders will be locked at full rate and the transmission power will be increased for transmitting low activity signals. It should be realized that any increase required would still be less than the increase required if the present encoding/decoding techniques were not implemented.
  • the system 100 when a TDD call is received, the system 100 is either noticed or detects the call type.
  • the system 100 processes the call from TDD unit 200 for transmission using standard processing techniques known in the art.
  • the frame is received at a remote point, for example base station 118, the call is decoded using the present invention. If a frame error has occurred in the physical layer, that is, if the frame does not pass the checksum as described within IS-95, the frame is still delivered to the vocoder 120 for decoding. Delivering the erred frame to the vocoder is currently not done in standard IS- 95 implementations. Bits contained in an erred frame are referred to as "soft bits" because they may not all be in error and information may be gleaned from them individually to reconstruct information contained in erred frames.
  • the vocoder decoder in the present invention processes erred frames by looking at the vocoder parameters received and comparing these parameters against "signatures" of TDD modulation signals or tones as seen in the vocoder parameter space. This compares the vocoder parameters of stored vocoded TDD tones with those received. This comparison results in a determination being made as to which TDD signal was most likely received.
  • a vocoder representation of a Baudot tone of "0" is represented as sixteen “0”s in sequence, and that the representation of a Baudot tone of "1" is represented as sixteen “l”s.
  • the present method considers these to be voice-parameter-space signatures.
  • three layers are identified as: vocoder frame boundaries: I — voc frame 'N' — I baudot tone boundaries: I -- baudot 'X' — I , and received vocoder parameter: 000000000000000 or
  • the vocoder decoder receives the following parameters:
  • the decoder recognizes the baudot tone boundaries and recognizes that the received parameters for the second baudot '0' are closer to '0' than '1.' The decoder decides on baudot tone '0' and modifies the suspected error bits before decoding. For the next baudot tone '1/ the decoder recognizes that the vocoder parameters are closer to 'V than '0' and modifies the bits accordingly.
  • the decoder now uses the following sequence to produce a corrected TDD signal: ⁇ corrected frame ⁇
  • the vocoder decoder may look to adjacent non-erred or "good" frames to reconstruct the erred frame.
  • the adjacent frame will contain a portion of the baudot tone that was lost in the erred frame. For example, suppose the following signal is received:
  • the vocoder parameters for the second baudot tone '0' are too ambiguous to make an accurate decision on the tone because the number of 'O's is almost the same as the number of 'l's in the vocoder frame parameters.
  • the vocoder looks at the next adjacent frame (voc frame 3) and determines that the tone appears to continue as a '0' into this frame. The decoder therefore decides that this is meant to be a baudot '0' tone in the latter half of vocoder frame 2.
  • the decoder continuously monitors and updates the received baudot tone boundaries in task 408. Otherwise, any non-low activity signal is processed using traditional methods. If an erred frame is received as detected in the physical layer, the frame is assigned an indicator N and the vocoder examines the erred frame in task 410. If a "reliable" decision concerning whether or not the frame is a baudot '0' or '1' can be made, such as when the frame parameters are quite distinct, then the erred frame is modified to reflect the parameters of the decision. A reliable decision is one that falls within a prescribed probability of obtaining the original frame parameters. For purposes of this invention, the desired probability would be in the range of 51% to certainty. If a modification is made, the method returns to task 402 and determines the next signal.
  • the vocoder reviews the next adjacent frame N+1 or, alternatively, N-1. If this frame is good in task 416, the decision to modify the erred frame is made in task 418 based upon the parameters contained within frame N+1, or alternatively, frame N-1. If neither next adjacent frame is good, then a next best reliable decision is made based upon the parameters contained within next adjacent frame N+1 and frame N's parameters are modified accordingly.
  • the decoder implementation in this embodiment of the invention is similar to that disclosed above. However, to further reduce the error rate and improve upon the accuracy and reliability of the signal decoded, the encoder also takes advantage of the "soft bits.”
  • the encoder When the vocoder encoder detects baudot tones are to be sent, the encoder switches to a "baudot tone encoding mode." In this mode the encoder decides whether the tone received for encoding is a '0' or a '1.' The encoder then sends this decision to the decoder using a vocoder frame, but using channel-coding redundancy to improve the decoder's chances of determining the proper baudot tone. Even if the decoder receives a tone in an erred frame, it will have a greater likelihood of determining the correct tone sent because of the forwarded decision.
  • the encoder detects a baudot '1' is to be transmitted, it sends a series of Is to the decoder.
  • the series may be any length, but must be sufficient so that the decoder can operate as discussed above in section A if necessary.
  • This version of the invention replaces the standard vocoder parameters with vocoder "signatures" that are better spaced apart (i.e., easier to differentiate), thus making it easier to decide between two tones even when frames are in error.
  • This embodiment of the invention is another version of the methods described in sections A and B, but the decoder is not given the soft bits from any erred vocoder frames to process.
  • the vocoder encoder detects a ⁇ or '0' baudot tone
  • the vocoder also encodes the tone in a vocoder frame using redundancy, but the encoding may be done across many vocoder frames.
  • the 'l's and 'O's are interleaved across a number of frames M so that if one frame is lost, the decoder can extract the necessary information from adjacent frames.
  • the following example shows interleaving taking place across four frames, but any number of frames could be used. Assume the encoder detects the following baudot tones for transmission:
  • the encoder encodes the frames as follows for transmission to the decoder:
  • the vocoder frame parameters for each frame are segmented where four bits represents the detected baudot tone in a particular vocoder frame. The entire sixteen bits represents the detected baudot tones from the last four vocoder frames:
  • the invention uses the following four-bit sequence where XXYY indicates that the code in the current vocoder frame reflects a baudot code of 'X' followed by a baudot code of 'Y': I —voc frame 1 — I —voc frame 2— I —voc frame 3 — I —voc frame 4 — I - voc
  • sections A, B and C can be advantageously accomplished through the use of a standard vocoder connectively communicating with a signal enhancer, such as an estimator or a repeater or any other device capable of performing a signal enhancing function.
  • a signal enhancer such as an estimator or a repeater or any other device capable of performing a signal enhancing function.
  • the methods of sections A, B, or C can be accomplished through the use of a modified vocoder. It would be apparent to one skilled in the art that a vocoder can be modified to further incorporate the functions of a signal enhancer, i.e., to make an estimation of whether corrupted bits received by the communication unit were originally transmitted as 'O's or 'l's.
  • the system of FIG. 5 illustrates one embodiment of the invention.
  • the transmission from base station 550 to mobile station 510 is referred to as the forward link and the transmission from the mobile station 510 to the base station 550 is referred to as the reverse link.
  • unmodified vocoder 520 in mobile station 510 encodes the Baudot signal into standard vocoder parameters and transmits the vocoder parameters to base station 550.
  • Modified vocoder 560 receives the encoded Baudot signal and enhances the Baudot signal to restore corrupted bits. A clean version of the Baudot signal is then generated.
  • unmodified vocoder 580 in base station 550 encodes the Baudot signal using standard vocoder parameters and transmits the vocoder parameters to mobile station 510.
  • Modified vocoder 530 receives the encoded Baudot signal and enhances the Baudot signal to restore corrupted bits. A clean version of the signal is then generated.
  • the frame error rate of the system can be reduced by using a signal enhancer in a base station on the reverse link, and by using power control techniques to adjust signal transmission power levels from the base station to the mobile station on the forward link.
  • base station In the reverse link of the communication system of FIG. 6, base station
  • base station 655 reconstructs the original Baudot signal contained within a vocoder encoded frame according to the methods discussed in sections A, B and C. However, in the forward link, base station 655 transmits the encoded Baudot signal using power control techniques as discussed in U.S. Patent Application No. 09/114,344, entitled “METHOD AND APPARATUS FOR ESTABLISHING TDD/TTY SERVICE OVER VOCODED CHANNELS,” assigned to the assignee of the present invention and incorporated by reference herein.
  • FIG. 7 is a flow diagram of a method for controlling transmission power between the base station and the mobile station.
  • the method begins in task 702, where a Baudot signal is received by the mobile station 605. After the signal is received, vocoders used by mobile station 605 during processing of the signal are locked into a full rate in task 704.
  • the transmission power does not decrease from the transmission power used by the telecommunications system for full rate transmissions in task 710.
  • the power level is typically set based upon a selected FER for the transmitted signal as received at the mobile station 605. A desired FER is selected because when a Baudot signal is being sent, the actual character error rate of the Baudot signal is about 9 to 10 times that of the FER.
  • This selected FER range is between a 0.1% and a 1.0% error rate, but may be less if necessary for preservation of the quality of the transmitted signal. Preferably, an FER of 0.2% is desirable for transmitting Baudot signals.
  • mobile station 605 notifies base station 655 in conventional fashion during task 706 that a system adjustment to reduce the FER is needed. Accordingly, an adjustment is made in task 708.
  • An adjustment typically includes increasing the transmission power for the full rate transmission, but may also include adjusting other parameters known to reduce FER. If the FER is acceptable in task 712, the signal transmission may continue in task 714 and dynamic adjustments to the telecommunications system continue throughout the transmission of the entire transmitted signal 710. Otherwise, when the transmission of the Baudot signal ends, the vocoders are unlocked, and the telecommunications system returns to normal operation.

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  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Human Computer Interaction (AREA)
  • Acoustics & Sound (AREA)
  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Audiology, Speech & Language Pathology (AREA)
  • Computational Linguistics (AREA)
  • Multimedia (AREA)
  • Computer Security & Cryptography (AREA)
  • Mobile Radio Communication Systems (AREA)
  • Detection And Prevention Of Errors In Transmission (AREA)
  • Telephonic Communication Services (AREA)
  • Dc Digital Transmission (AREA)
  • Time-Division Multiplex Systems (AREA)

Abstract

L'invention concerne un procédé permettant de coder/décoder un signal de communication à faible débit tel qu'un signal sonore Baudot devant être transmis sur un système (100) de communication téléphonique. Ce système (100) peut comprendre un nombre quelconque de liaisons sans fil. Une fois que le système (100) est informé qu'un signal à faible débit doit être transmis, chaque vocodeur utilisé dans le système (100) pour coder/décoder le signal exécute un processus unique de codage/décodage. Dans un mode de réalisation, des trames contenant des erreurs nuisant au signal sont dirigées vers le vocodeur et les « bits souples » qu'il contient sont utilisés pour déterminer le signal d'origine transmis. Dans un autre mode de réalisation, le codage du signal peut consister à coder le signal à l'aide de la redondance avec le signal codé diffusé à travers diverses trames de vocodeurs.
EP00952564A 1999-08-04 2000-08-03 Procede et dispositif acceptant la modulation tdd/tty sur des voies telephoniques equipees en vocodeur Withdrawn EP1198934A1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US36671499A 1999-08-04 1999-08-04
US366714 1999-08-04
PCT/US2000/021441 WO2001011840A1 (fr) 1999-08-04 2000-08-03 Procede et dispositif acceptant la modulation tdd/tty sur des voies telephoniques equipees en vocodeur

Publications (1)

Publication Number Publication Date
EP1198934A1 true EP1198934A1 (fr) 2002-04-24

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EP00952564A Withdrawn EP1198934A1 (fr) 1999-08-04 2000-08-03 Procede et dispositif acceptant la modulation tdd/tty sur des voies telephoniques equipees en vocodeur

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Country Link
EP (1) EP1198934A1 (fr)
JP (1) JP2003506968A (fr)
KR (1) KR20020037342A (fr)
CN (1) CN1178438C (fr)
AU (1) AU773419B2 (fr)
BR (1) BR0012932A (fr)
CA (1) CA2379973A1 (fr)
HK (1) HK1047836B (fr)
IL (1) IL147717A0 (fr)
MX (1) MXPA02001199A (fr)
NO (1) NO20020517D0 (fr)
RU (1) RU2002105522A (fr)
UA (1) UA70375C2 (fr)
WO (1) WO2001011840A1 (fr)

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BR0313259A (pt) 2002-08-06 2005-06-14 Siemens Ag Processamento de erros de informações úteis recebidas através de uma rede de comunicação
US9226123B2 (en) * 2013-02-21 2015-12-29 Telefonaktiebolaget L M Ericsson (Publ) Limiting processing of calls as text telephony calls

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WO1993023947A1 (fr) * 1992-05-20 1993-11-25 Ultratec, Inc. Dispositif de telecommunications exploite selon un protocole pour dispositifs de telecommunications pour sourds
FI100932B (fi) * 1995-04-12 1998-03-13 Nokia Telecommunications Oy Äänitaajuussignaalien lähetys radiopuhelinjärjestelmässä
AU5957798A (en) * 1996-12-19 1998-07-15 Diva Communications, Inc. Methods and apparatus of providing transparency of dtmf signaling tones across a digital cellular voice channel
US6434198B1 (en) * 1998-08-28 2002-08-13 Lucent Technologies Inc. Method for conveying TTY signals over wireless communication systems

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Title
See references of WO0111840A1 *

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NO20020517L (no) 2002-02-01
NO20020517D0 (no) 2002-02-01
AU773419B2 (en) 2004-05-27
BR0012932A (pt) 2003-04-29
CA2379973A1 (fr) 2001-02-15
JP2003506968A (ja) 2003-02-18
WO2001011840A1 (fr) 2001-02-15
KR20020037342A (ko) 2002-05-18
CN1178438C (zh) 2004-12-01
AU6523800A (en) 2001-03-05
RU2002105522A (ru) 2003-09-10
IL147717A0 (en) 2002-08-14
HK1047836A1 (en) 2003-03-07
HK1047836B (zh) 2005-07-22
CN1375146A (zh) 2002-10-16
MXPA02001199A (es) 2002-07-30
UA70375C2 (uk) 2004-10-15

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