AU773419B2 - Method and apparatus supporting TDD/TTY modulation over vocoded channels - Google Patents

Description

WO 01/11840 PCT/US00/21441

I

METHOD AND APPARATUS SUPPORTING TDD/TTY MODULATION OVER VOCODED CHANNELS BACKGROUND OF THE INVENTION I. Field of the Invention Generally, 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.

II. Description of the Related Art Many deaf or hearing-impaired people use communication terminals specifically constructed and designed to enable them to communicate over standard telephone lines. Such devices, referred to as telecommunication devices for the deaf (TDDs) or Text Telephone Yokes (TTYs), are collectively referred to as TDDs in this application. Typically, 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, and the communication protocol (TDD protocol) evolved historically at a time when many telecommunication devices for the deaf were based on mechanical or electromechanical devices rather than electronic devices. Accordingly, 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.

Traditionally, 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.

Regardless, a normal TDD communication character set consists of characters that are 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. For example, under conventional TDD protocol, 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.

Compared to modem telecommunication systems, TDD transmissions occur at a S snail's pace. A bigger problem is that TDD signals are substantially constant. These slow S•2.q paced, monotone signals can create havoc in digital telecommunication systems that S 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.

S. 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 S 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 centre. 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 centre 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 centre.

The interface between the subscriber unit and the base station is referred to as the air interface. The telecommunications industry association (TIA) has provided a standard for CDMA call processing on the air interface entitled "IS95 Mobile Station- Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System". Addendum to IS-95 is provided as Telecommunications Service Bulletins (TSB).

Service negotiation is critical to successfully transmit any communication, especially a low activity TDD communication, over a digital telecommunication system.

One problem with modern systems, including the one described above, is that 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. In current systems, 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 o• communication signal.

The invention should be compatible with wireless telecommunication modulation systems, such as CDMA systems, servicing large numbers of system users.

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. Further, 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.

SUMMARY OF THE INVENTION In a first aspect the present invention accordingly provides a method for communicating a noticed low activity Baudot signal from a first vocoder to a second vocoder in a telecommunication system, the method including: transmitting a vocoder frame conveying at least one section of a Baudot tone representation from the first vocoder to the second vocoder; receiving the transmitted vocoder frame at the second vocoder; modifying the transmitted vocoder frame if a frame error is present rather than discarding the transmitted vocoder frame; and processing the transmitted vocoder frame to obtain additional information that will be used to identify at least one section of a Baudot tone representation carried by another vocoder frame.

0 In a second aspect the present invention accordingly provides a method for communicating a noticed low activity Baudot signal from a first vocoder to a second vocoder in a telecommunication system, the method including: transmitting a vocoder frame from a mobile station, wherein the vocoder frame conveys a portion of a Baudot tone; receiving the vocoder frame at a base station; modifying the received vocoder frame by replacing the contents of the received vocoder frame with a known Baudot bit S sequence; delivering the modified vocoder frame to obtain additional information that will be used to identify at least one section of a Baudot tone carried by another vocoder frame; and transmitting all vocoder frames that carry Baudot tone information from the 30 base station at an increased transmission power level.

00 In a third aspect the present invention accordingly provides apparatus for communicating a low activity Baudot signal in a telecommunication system, the apparatus including: means for encoding a frame of the low activity Baudot signal into at least one vocoder frame; means for transmitting the at least one vocoder frame; means for receiving the at least one vocoder frame; and means for enhancing the contents of the at least one vocoder frame to reproduce a Baudot tone.

In a fourth aspect the present invention accordingly provides apparatus for communicating a noticed low activity Baudot signal in a telecommunication system, the apparatus including: a vocoder for encoding a frame of the low activity Baudot signal into at least one vocoder frame; a transmitter for transmitting the at least one vocoder frame; a receiver for receiving the at least one vocoder frame; and a signal modifier for enhancing the contents of the at least one vocoder frame to reproduce a Baudot tone.

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. Yet another advantage is that a TDD device can be connected to a mobile device or subscriber's unit, such as a digital cellular telephone, connected to the 20 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.

S

6 BRIEF DESCRIPTION OF THE DRAWINGS The nature, objects, and advantages of the invention will become more apparent to those skilled in the art after considering the following detailed description in connection with the accompanying drawings, in which like reference numerals designate like parts throughout, and wherein: FIGURE 1 illustrates one type of telecommunications system including wireless links and a TDD communication device; 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; o .20 FIGURE 5 illustrates a block diagram of a wireless telecommunication system o 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; and FIGURE 7 is a flow diagram of a method for controlling transmission power Sgo.

levels between a base station and a mobile station.

00 "DETAILED DESCRIPTION OF SELECTED EMBODIMENTS 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.

HARDWARE COMPONENTS AND INTERCONNECTIONS 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. As shown in detail in FIG. 2, 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.

In digital telecommunications systems using wireless links, the TDD 200 may be coupled to a subscriber unit 104 that is used in the telecommunications system 100 to transmit received signals. Exemplary embodiments of 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 *9 subscribers unit 104. A hardwire 108 may be used to connedct 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. Patent Application No. 09/114,344 entitled "METHOD AND APPARATUS FOR ESTABLISHING TDD/TTY SERVICE OVER VOCODED CHANNELS", now U.S.

Patent No. 6,205, 339 to Nikolai K.N. Leung et al, assigned to the assignee of the present invention and incorporated by reference herein.

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 8 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 system100. 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.

Returning to FIG. 1, after the noticing apparatus 106 receives the low activity communication signal, the signal is processed by the subscriber unit 104. Very basically, a signal for transmission is created that includes the information contained in the low activity signal. Because the telecommunications system 100 has been noticed that a low activity signal is being transmitted, the system adapts to assure a decipherable transmission occurs. For example, an analog signal received from the analog circuitry 228 shown in FIG. 2 normally would undergo a signal or "voice" processing including digitising 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 110. Depending upon the signal received, 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, S and compression technique. A fuller discussion concerning the processing of signals for transmission in telecommunication system may be found in the Electronic Industry Association standard 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.

0 However, when a low activity signal is received, 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.

After the low activity communication signal has been processed and the service determined, a signal may be transmitted using an antenna 112 over a wireless link 114.

The digitised 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. Various based station circuitry arrangements for telecommunications systems are well known in the art, and a further understanding may be found in referenced above. By processing the signal after receipt, a low activity signal reflecting the information contained in the transmitted low activity signal may be delivered to the low activity device 102 via communication link 122. A second noticing apparatus 106 is shown coupled to the base station 118. 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 122 appears bifurcated to emphasise that the base station 2 118 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. In another embodiment, a second mobile station (not shown) connected to the low activity communication device 102 may be linked to the base V. 25: station 118. Further, the telecommunication system may include mobile switching S stations as mentioned above.

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. In the TDD device 200 of FIG. 2, 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. Optionally, 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 is 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. In FIG. 2, the bus 210 connects a read only memory (ROM) 212 to a non-volatile random access memory (NVRAM) 214. 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 synchronising 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 S device 200, and for storage of standard messages as entered by the user through the keyboard 202 and intended for rapid access. 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.

Also connected to the processor 202 in FIG. 2 is a telephone keypad 220 that permits the entry of telephone numbers for dialling by the processor 204 through 25: telecommunications system 100. A standard telephone handset 224 rests on a cradle 226 ooo, Sthat 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 dialling 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.

Despite the specific foregoing descriptions, ordinarily skilled artisans having the benefit of this disclosure will recognise that the apparatus discussed above may be implemented in a telecommunications system of different construction without departing from the scope of the present invention. As a specific example, 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.

OPERATION

After a TDD signal is received, vocoders used by the system 100 during processing of the signal is noticed or detect that a low activity signal has been received for transmission and may use an eighth rate traffic channel frame format to transmit the signal. However, adaptation of the following methods for quarter-to-full rate traffic channel transmissions may be accomplished, as discussed below.

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 S using Code Excited Linear Prediction (CELP) techniques that are well known in the art.

The frames may be formatted in full, half, quarter or eighth rate formats depending upon voice activity. If a Baudot tone is received, the 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.

.*oo Full rate refers to the fact that each bit contained in each frame is not repeated.

30 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 minimum 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.

Typically, 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. 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.

Preferably, an FER of 0.2% is desirable for low activity signals.

In the present invention, implementing specialised encoding and decoding techniques controls the frame error rate. In the circumstance that the o O .o go *o *ooo WO 01/11840 PCT/US00/21441 13 disclosed techniques fall short of a desired FER in this case 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. Typically, 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.

A. Decoder Using Soft Bits In one embodiment, 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. When 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 ISimplementations. 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.

However, detecting or being noticed that a TDD call has been received, 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.

For example, suppose 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 is represented as sixteen The present method WO 01/11840 PCT/US00/21441 14 considers these to be voice-parameter-space signatures. For the following examples, three layers are identified as: vocoder frame boundaries: I voc frame I baudot tone boundaries: I baudot I ,and received vocoder parameter: 000000000000000 or 11111111111111111.

Assume that the vocoder decoder receives the following parameters: (erred frame) I voc frame 1 I -voc frame I -voc frame 3- I -voc frame 4 voc I baudot'0'- I baudot'0'- I baudot I baudot I 1111111100000000000000000000000000000110000111111111110000000000000000

AAAAA

[frame errors] The decoder recognizes the baudot tone boundaries and recognizes that the received parameters for the second baudot are closer to than The decoder decides on baudot tone and modifies the suspected error bits before decoding. For the next baudot tone the decoder recognizes that the vocoder parameters are closer to than and modifies the bits accordingly. The decoder now uses the following sequence to produce a corrected TDD signal: (corrected frame) I -voc frame 1 I voc frame 2- I -voc frame I -voc frame 4 I voc t I baudot I baudot I baudot I baudot I 1111111100000000000000000000000000000011111111111111110000000000000000 This example shows the error transitions to occur at a frame boundary, which isn't always the case. If these transitions commonly fall within a frame, another version of the invention can be used as follows.

If the vocoder decoder receives an erred frame where the erred bits are contained within the frame, the vocoder may look to adjacent non-erred or "good" frames to reconstruct the erred frame. The adjacent frame will contain a S I, portion of the baudot tone that was lost in the erred frame. For example, suppose the following signal is received: {erred frame} I voc frame I voc frame voc frame I -voc frame 4- 1 -voc I -baudot'O'-- I baudot'O'-- I baudot'l'-- baudot'O'- I 111111110000000000111111111111000000000111111111111111000000000000

AAAAAAAAAA

[frame errors] The vocoder parameters for the second baudot tone'O'are too ambiguous to make an accurate decision on the tone because the number of'0's is almost the same as the number of'l's in the vocoder frame parameters. To make a better determination, 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.

As shown in the flow chart of FIG. 4, after it is determined if a low activity signal is being received in tasks 402 and 404, the decoder continuously monitors and updates the received baudot tone boundaries in task 408. Otherwise as in task 406, any non-low S 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'l' 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 S to task 402 and determines the next signal.

If a reliable decision cannot be made as shown in task 412, the vocoder reviews the next adjacent frame N+1 in task 414 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 in task 420 based upon 1 1 16 the parameters contained within next adjacent frame N+1 and frame N's parameters are modified accordingly.

B. Encoder and Decoder Using Soft Bits 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".

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'l.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.

In a simplified example, if the encoder detects a baudot'l'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 9 9 version of the invention replaces the standard vocoder parameters with vocoder :24' "signatures" that are better spaced apart easier to differentiate), thus making it easier to decide between two tones even when frames are in error.

C. Encoder and Decoder Not Using Soft Bits 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.

o WO 01/11840 PCT/US00/21441 17 In this case, when the vocoder encoder detects a or 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: 11001 The encoder encodes the frames as follows for transmission to the decoder: I -voc frame 1- I -voc frame 2- voc frame 3 -1 voc frame 4 I voc frame I baudot I baudot I baudot I baudot I baudot xxxxxxxxxxxlllxxxxxxxx11llllllxxxx1llll 110000111111110000000011110 00000001111.

In this example, 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: I baudot for frame N-3 I baudot for frame N-2 I baudot for frame N-1 I baudot for frame N I.

To account for baudot tones not corresponding to vocoder frame boundaries, the invention uses the following four-bit sequence where XXYY indicates that the code in the current vocoder frame reflects a baudot code of followed by a baudot code of WO 01/11840 PCTIUS00/21441 18 I voc frame 1 -voc frame I voc frame 3 I -voc frame 4 I voc t I baudot I baudot I baudot I baudot I xxxxxxxxxxxxx0011xxxxxxxx00111111xxxx00llll11110000111111110000001111 D. Modified Vocoder with Increased Transmission Power In another embodiment, the prohibitive cost of modifying the chipset in a mobile station to accomplish the encoding and decoding processes discussed above in sections A, B and C is advantageously minimized.

The encoding and decoding methods of 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.

In addition, 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, to make an estimation of whether corrupted bits received by the communication unit were originally transmitted as 'O's or '1'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. In 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. In the forward link, 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 WO 01/11840 PCT/US00/21441 19 the encoded Baudot signal and enhances the Baudot signal to restore corrupted bits. A clean version of the signal is then generated.

However, using signal enhancers or modified vocoders on both the forward link and the reverse link to accomplish the decoding methods discussed in sections A, B, and C can be prohibitively expensive to produce. In yet another embodiment of the invention, 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.

In the reverse link of the communication system of FIG. 6, 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/TY 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. In this embodiment, 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. If the FER exceeds the selected range in task 712, 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.

OTHER EMBODIMENTS While there have been shown what are presently considered to be preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the scope of the invention as defined by the appended claims.

It will be understood that the term "comprise" and any of its derivatives (eg comprises, comprising) as used in this specification is to be taken to be inclusive of S features to which it refers, and is not meant to exclude the presence of any additional features unless otherwise stated or implied.

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Claims (19)

1. A method for communicating a noticed low activity Baudot signal from a first vocoder to a second vocoder in a telecommunication system, the method including: transmitting a vocoder frame conveying at least one section of a Baudot tone representation from the first vocoder to the second vocoder; receiving the transmitted vocoder frame at the second vocoder; modifying the transmitted vocoder frame if a frame error is present rather than discarding the transmitted vocoder frame; and processing the transmitted vocoder frame to obtain additional information that will be used to identify at least one section of a Baudot tone representation carried by another vocoder frame.

2. The method recited in Claim 1, wherein the step of modifying the transmitted vocoder frame further includes: comparing a portion of the transmitted vocoder frame with a known low activity Baudot bit sequence; determining whether the comparison result is statistically reliable; and if the comparison result is statistically reliable, then substituting the portion of the transmitted vocoder frame with the known low activity Baudot bit sequence.

3. The method recited in Claim 2, further including: .if the comparison result from the step of processing the transmitted vocoder frame is not statistically reliable, then processing a neighbouring vocoder frame to determine if the neighbouring vocoder frame contains any errors; if the neighbouring vocoder frame is errorless: then determining a low activity Baudot tone representation defined by a bit sequence contained in the neighbouring vocoder frame; and modifying the transmitted vocoder frame based upon the bit sequence 0 contained in the neighbouring vocoder frame. 0 *go

4. The method recited in Claim 3, wherein said comparison results indicate statistical reliability if the probability that the portion of the transmitted vocoder frame is the known low activity Baudot bit sequence is 51% or greater.

5. The method recited in any one of claims 1 to 4, wherein the first vocoder is located at a mobile station and the second vocoder is located at a base station.

6. The method recited in any one of claims 1 to 5, wherein the method further includes the step of replacing standard vocoder parameters with vocoder signatures, wherein sequenced bits contained within the vocoder signatures are spaced further apart than sequenced bits contained within standard vocoder parameters, said replacement step preceding the transmitting step.

7. The method recited in Claim 6, wherein the step of replacing standard vocoder parameters further includes: communicating to the second vocoder that a low activity Baudot signal is being transmitted; and encoding the low activity Baudot signal using channel coding redundancy within an information bit field of the vocoder frame, the encoding also noticing the second vocoder that a low activity signal is being transmitted.

8. The method recited in Claim 6, wherein the step of modifying the transmitted vocoder frame further includes the steps of: o comparing a portion of the transmitted vocoder frame with a known low activity Baudot bit sequence; determining whether the comparison result is statistically reliable; and iif the comparison result is statistically reliable, then substituting the portion of S the transmitted vocoder frame with the known low activity Baudot bit sequence.

9. A method for communicating a noticed low activity Baudot signal from a first vocoder to a second vocoder in a telecommunication system, the method including: transmitting a vocoder frame from a mobile station, wherein the vocoder frame conveys a portion of a Baudot tone; receiving the vocoder frame at a base station; modifying the received vocoder frame by replacing the contents of the received vocoder frame with a known Baudot bit sequence; delivering the modified vocoder frame to a base station vocoder; processing the modified vocoder frame to a base station vocoder; processing the modified vocoder frame to obtain additional information that will be used to identify at least one section of a Baudot tone carried by another vocoder frame; and transmitting all vocoder frames that carry Baudot tone information from the base station at an increased transmission power level. The method recifed in Claim 9, wherein the step of transmitting all vocoder frames that carry Baudot tone information from the base station at an increased transmission power level includes the steps of: locking all vocoders into a full rate; and transmitting all vocoder frames that carry Baudot tone information at a transmission power level higher than a normal full rate transmission power level, 20 wherein the transmission power level is set to maintain a minimum target frame error 00 rate for the vocoder frames carrying Baudot tone information.

11. The method recited in Claim 9 or 10, wherein enhancing the received vocoder S* frame by replacing the contents of the received vocoder frame with the known Baudot bit sequence includes: comparing the contents of the received vocoder frame with the known low 2 activity Baudot bit sequence to produce a comparison result; and if the comparison result indicates statistical reliability, then replacing the contents of the vocoder frame with the known low activity Baudot bit sequence; but if the comparison result is not statistically reliable, then processing a neighbouring frame to determine if the neighbouring frame contains any errors; if the neighbouring frame is errorless: determining a low activity Baudot tone defined by a bit sequence contained in the neighbouring frame is errorless: modifying the received vocoder frame based upon the bit sequence contained in the neighbouring frame.

12. The method recited in Claim 11, wherein said comparison results indicate statistical reliability if the probability that the contents of the transmitted vocoder frame is the known Baudot bit sequence is 51% or greater.

13. The method recited in Claims 11 or 12, wherein the step of transmitting all vocoder frames that carry Baudot tone information from the base station at an increased transmission power level includes the steps of: locking all vocoders into a full rate; and transmitting all vocoder frames that carry Baudot tone information at a transmission power level higher than a normal full rate transmission power level, wherein the transmission power level is set to maintain a minimum target frame error rate for the vocoder frames carrying Baudot tone information.

14. The method recited in Claim 13, wherein the minimum target frame error rate is S less than

15. Apparatus for communicating a low activity Baudot signal in a telecommunication system, the apparatus including: •means for encoding a frame of the low activity Baudot signal into at least one oo0 vocoder frame; means for transmitting the at least one vocoder frame; 30 means for receiving the at least one vocoder frame; and means for enhancing the contents of the at least one vocoder frame to reproduce a Baudot tone.

16. The apparatus recited in Claim 15, wherein the apparatus further includes: means for comparing the contents of the at least one vocoder frame with a known low activity Baudot bit sequence; means for determining whether the result of comparing the contents of the at least one vocoder frame with the known low activity Baudot bit sequence is statistically reliable; and means for replacing the contents of the at least one vocoder frame with the known low activity Baudot bit sequence.

17. Apparatus for communicating a noticed low activity Baudot signal in a telecommunication system, the apparatus including: a vocoder for encoding a frame of the low activity Baudot signal into at least one vocoder frame; a transmitter for transmitting the at least one vocoder frame; a receiver for receiving the at least one vocoder frame; and a signal modifier for enhancing the contents of the at least one vocoder frame to reproduce a Baudot tone. 20 18. A method as claimed in claim 1, substantially as herein described with reference to the accompanying drawings. to the accompanying drawings. An apparatus as claimed in claim 15, substantially as herein described with reference to the accompanying drawings.

21. An apparatus as claimed in claim 17, substantially as herein described with 30 reference to the accompanying drawings.

22. A method substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings.

23. An apparatus substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings. DATED this 27t day of February, 2004. Qualcomm Incorporated By its Patent Attorneys MADDERNS 09 o 0 *at 0* OS S**