TTY/TDD INTEROPERABLE SOLUTION IN DIGITAL WIRELESS SYSTEM

Abstract: A method and apparatus are disclosed for transmitting a communication through a communications channel when the communication has been encoded with any number of signals. The invention comprises detecting a presence of one of the encoding signals and then generating another predetermined signal, and then combining the encoding signal with the predetermined signal to form a combined signal. The combined signal is then transmitted through a communications channel and received with a terminal, after which the combined signal is separated into the encoding and predetermined signals. The receipt of the encoding signal is verified by the presence of the predetermined signal. Also disclosed is an application of the method and apparatus to the transmission of Baudot tones through a telecommunications network.

Description: TTY/TDD INTEROPERABLE SOLUTION IN DIGITAL WIRELESS SYSTEM FIELD OF THE INVENTION This invention relates generally to transmitting communications through a communication channel and, in particular, to improving the transmission of encoded communications. BACKGROUND OF THE INVENTION Communications transmitted through a communication channel are typically encoded, especially when the communication is between electronic devices. For example, in order for a computer to communicate with another computer using a modem, the bits being exchanged are changed, or encoded, to signals so that they may be sent over telephone lines. As encoded communications are transmitted through communication channels, they may become distorted due to disturbances in the communication channel and the ability of the communications channel to transmit the particular type of communication. A specific example of these types of problems concerns transmitting Teletype®/Text Telephone Device (TTY/TDD) communications through wireless communications channels. The use of Teletype®/Text Telephone Devices (TTY/TDD devices) is well known. When using a text telephone device, a user types characters with a keyboard, rather than speaking into a microphone. The text characters are encoded as tones and then transmitted over a telephone network. At a receiving station, the transmitted tones are received and then converted back to text to be read by the receiving user. Text based systems can be either stand-alone or computer based, and users are required to have the ability to type and read. There are many standards for TTY/TDD communication with various modulation and character encoding schemes, but virtually all TTY/TDD devices support the Baudot code communications protocol. The Baudot code is a digital text encoding method which uses a half-duplex communications channel with no channel turnaround, and shift-keyed modulation (i.e. the carrier is present only when a character is being transmitted). Binary states are represented by one of two tones, binary l's being represented by 1400 Hz (+/- 5%) and binary 0's by 1800 Hz (+/- 5%). The tone on-time, or duration, is either 20 or 22 ms (+/- 0.4 ms), depending on the international region, resulting in signal rates of 50 bits/sec or 45.45 bits/sec respectively. Each character is represented by 5 bits preceded by a start bit of 1800 Hz and followed by at least 1 stop bit of 1400 Hz. There are two special 5-bit characters that signify which type of alphanumeric characters will follow, letters or figures. Letters are designated by the character 11111, and figures are designated by the character 11011. Figures include the symbol keyboard characters, i.e.: ~ ! @ # $ % A etc. When operating to transmit and receive Baudot tones, TTY/TDD devices typically employ a band pass filter to minimize frequencies outside the specified tone ranges. It is desirable to use TTY/TDD devices in conjunction with radiotelephones and with radiotelephone terminals, fixed stations, or mobile stations capable of operating with a digital wireless network. TTY/TDD devices that are capable of operating with wireless networks provide their users with the same mobility, flexibility, and access to emergency services available to non-TTY/TDD users. Wireless networks include fixed or mobile stations having vocoders, optimized to transmit and receive speech. At the Baudot tone frequencies the vocoder introduces variations in frequency, amplitude, and tone duration that are not well suited for reception by conventional TTY/TDD terminals. In particular, digital fixed or mobile stations typically include a voice encoder (vocoder) which converts sound into a digital data stream by accurately modeling the way the human body produces speech, particularly in the 300-500 Hz range. Passing Baudot tones, which are single frequency sine waves, through a vocoder in the same way as speech results in distortions, such as frequency, amplitude and tone duration (i.e.: the 22ms/20ms duration mentioned above) distortions that, while undistinguishable by the human ear, cause errors in conventional TTY/TDD terminals. Present techniques where Baudot tones are fed directly into radiotelephone systems yield unacceptable results. In fact, experiments using a commercial text telephone in a digital wireless system demonstrated a character error rate (CER) of about 1.5% when using a clean transmission channel in a laboratory environment. Real world transmission conditions cause even higher CER figures, especially where there are frame erasures and bit errors in the received data stream. Generally when a communication channel performs a particular function that distorts or corrupts a particular type of communication, the distortion may be avoided by substituting that function with one more suited to the particular communication, but still operable in the communication channel. In the case of TTY/TDD communications through a wireless communication channel, one solution for improving transmission of TTY/TDD Baudot tones is to convert the tones independently of the vocoder. In this case a Baudot tone detector is implemented before the vocoder. Upon recognizing Baudot tones, the detector converts them into a binary format, bypasses the vocoder, and then embeds those binary strings into the output data stream where the encoded speech data is normally found. Special measures (i.e. a no-gain method) are taken to ensure that various components of the wireless system are able to process the modified transmission stream. While errors may still be introduced in the wireless transmission channel, the tones are not being processed as speech by the vocoder, providing far better performance than simply feeding the tones directly into the vocoder. This solution provides interoperability with presently available TTY/TDD devices, meaning that presently available devices do not need to be modified to receive the benefits of this technique. However, this technique requires hardware and software changes to the wireless infrastructure, especially to the functions surrounding the speech vocoder. Another solution to the general case of distorted encoded communications in a communication channel is to change the encoded communication to a form that the communication channel transmits more reliably. In the case of TTY/TDD communication through a wireless network, the Baudot tones may be converted to other, more easily transmitted signals. Before the tones are introduced into the fixed or mobile station vocoder, a detector is used to detect the 1400/1800 Hz tones and then convert them to different signals that are processed more reliably by the wireless network. The converted signals are then introduced into the vocoder and transported through the wireless network system and eventually to the receiving TTY/TDD equipment. The receiving TTY/TDD equipment is then equipped with a device that detects the new signals and regenerates the original Baudot tones. This solution offers lower CER's, and incorporation of this type of device into currently available radiotelephone equipment has much less of an impact than bypassing the speech vocoder. This type of device may also be made available as a separate accessory to conventional TTY/TDD equipment. However, this technique provides no interoperability with conventional TTY/TDD equipment that is not equipped to receive the converted signals. SUMMARY OF THE INVENTION The forgoing and other problems are overcome by methods and apparatus in accordance with embodiments disclosed herein. A method and apparatus are directed toward transmitting an encoded communication through a communication channel by converting the signals comprising the communication to new signals that the communication channel is optimized to deliver, or is known to deliver reliably, and then combining the original communication signals with these new signals. The disclosed embodiments are further directed toward maintaining compatibility with receiving devices that are not equipped to receive the new signals. The disclosed embodiments are further directed toward improving the error rate in TTY/TDD devices when used in conjunction with radiotelephones and with radiotelephone terminals, fixed stations or mobile stations capable of operating with a wireless network, while maintaining compatibility with presently available devices. This is achieved by converting the Baudot TTY/TDD tones to signals that the wireless system is currently optimized to deliver, or is known to deliver reliably, and then combining the original Baudot tones from the TTY/TDD device with these signals. The combined signals are then transmitted through the wireless network and eventually received by another TTY/TDD device. Before being presented to the TTY/TDD device the combined signals are re-converted to the originally generated Baudot tones. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows an illustration of a conventional Text Telephone Device. Figure 2 shows a block diagram of a communications system that is constructed and operated in accordance with an embodiment of this disclosure. Figure 3 is an illustration of the relationship among the signals utilized in a preferred embodiment. Figure 4 shows the disclosed embodiments as incorporated into a wireless station. Figure 5 shows the disclosed embodiments as incorporated into a wireless telecommunication systems infrastructure. DETAILED DESCRIPTION OF THE INVENTION A conventional text telephone device or terminal 10 is shown in Figure 1. The terminal 10 may be conventional in construction and operation, and is not discussed in any detail herein. The components include a keyboard 20 for typing messages and a display 30 for displaying typed and received messages. Optionally, a handset 40 for voice communication may also be included. The terminal 10 is connected to a communications network 50, usually a telephone network. A block diagram of a communication system incorporating an embodiment of the invention is shown in Figure 2. A device for generating combined signals 55 is shown comprised of a TTY/TDD device or terminal 10, capable of producing Baudot tones 20 which is connected to a device 60 responsive to the Baudot tones 20. A tone detector 70 detects the presence of the Baudot tones 20. Because the Baudot tones are ideally pure sine wave tones, they are easily distinguished from speech. The tone detector 70 triggers over signal line 78 a signal generator 80 to generate converted signals 90, corresponding to the Baudot tones. Ideally, the converted signals 90 are composed of frequencies that the digital wireless system is currently optimized to deliver, or is known to deliver reliably, that is, without appreciably distorting the frequency, amplitude, or duration of the converted signals. The frequencies utilized to compose the converted signals 90 are preferably lower than the 1400 and 1800 Hz. Baudot tones. The presently preferred embodiment most preferably utilizes frequencies in the range of about 300-1200 Hz. Those skilled in the art may recognize that the terminal 10 need not be limited to encoding text using frequency bursts (Baudot tones) but may be capable of using other encoding schemes. In this case it should also be noted that tone detector 70 is capable of detecting the encoding scheme generated by the terminal 10. That is, while Baudot tones are a universally accepted standard for text encoding, there are other encoding techniques for text communications. In response to the presence of the Baudot tones 20, the Baudot tone detector 70 also triggers over signal line 72 a high pass filter 75 and a low pass filter 85. In a preferred embodiment, the high pass filter 75 and the low pass filter 85 are linear phase filters, selected in order to avoid introducing amplitude distortion and non-uniform frequency sensitive delays into the Baudot tones 20 or the converted signals 90. The original Baudot tones 20 pass through the high pass filter 75 and the converted signals 90 pass through the low pass filter 85. The converted signals 90 are then combined in summing junction 100 with the original Baudot tone output of the TTY/TDD terminal 10. The high pass linear phase filter 75 and low pass linear phase filter 85 serve to reduce interference between the converted signals 90 and the Baudot tones 20. The combined signals 110 are next introduced into a wireless station 120 which delivers the combined signals 110 to a wireless network 130 through link 125. It should be noted that wireless station 120 may be fixed or mobile and typically includes a vocoder 122. The combined signals therefore provide the Baudot tone information in two forms, the original Baudot tones themselves, and, in the presently preferred embodiment, the lower frequency converted signals. The converted signals serve to convey the Baudot tone information, but at frequencies that are different from the Baudot frequencies and in a frequency range that can be transmitted more reliably by a wireless network. The converted signals 90 generated by the signal generator 80 can be chosen from a wide range of frequencies, so long as they are not significantly attenuated by the band pass filter 152 found on the input of the TTY/TDD terminals 150A, 150B. Ideally, the converted signals 90 are composed of frequencies that the digital wireless system is currently optimized to deliver, or is known to deliver reliably through vocoder 122, and then through the wireless communications channel. As was stated previously, the present invention preferably utilizes tones in the range of about 300-1200 Hz. The converted signals 90 may be composed of several different frequencies, and may be transmitted at a rate faster than the 50 bits/sec or 45.45 bits/sec typical of Baudot tones. For example, a Baudot representation for a character may be 01011011, including start and stop bits, and may have a transmission time of 176ms (8 * 22ms). Stripping off the start and stop bits leaves the actual character, 101 10. By channel coding or simply repeating the character 14 times, the original 5 bits may be encoded into 70 bits. Every two bits in the resulting 70 bit stream may then be mapped into one of a plurality of frequencies, each frequency having an on-time duration of about 5ms. For example, the first 2 bits, "1" and "0", could be mapped to a frequency of 400Hz, the second 2 bits, "1" and "1", could be mapped to a frequency of 600Hz, etc. Because the mapped frequencies have an on-time duration of about 5ms, the 70 bit encoded stream has a transmission time of 175 ms (70/2*5), which is less time than the transmission time for the original Baudot representation of the character. In the example described herein, the 70 bit encoded stream has a high level of redundancy and greatly enhances the robustness of the transmission. It is important to note that the converted signals may be of any frequency and need not be audible, but only need to be those that can be transmitted reliably through a vocoder and then through a communications channel. In the event that the TTY/TDD terminal 10 is also capable of transmitting speech, and speech is being produced rather than Baudot tones, the tone detector 70 fails to detect Baudot tones and inhibits the signal generator 80 from producing the converted signals 90. The output of summing junction 100 is then simply the speech signals produced by the TTY/TDD terminal lOas filtered by the high pass filter 75. In the presently preferred embodiment the wireless network 130 delivers the combined signals 110 to a receiving station 140 through link 135. If receiving station 140 is a wireless receiving station, it may contain a vocoder 142. Link 135 may be a wireless link or a wired link such as the Public Switched Telephone Network (PSTN). The receiving system 140 in turn may convey the received combined signals 110A to a conventional TTY/TDD terminal 150A which, as stated above, typically employs the band pass filter 152 in the input circuitry. The band pass characteristics of such filters allow the Baudot frequencies of 1400 and 1800 Hz to pass substantially unattenuated, while frequencies outside the range of the Baudot tones are attenuated. Figure 3 is illustrative of the relationship among the various signals. Figure 3 shows the 1400 and 1800 Hz. Baudot tones 160 and the frequency response of a typical TTY/TDD band pass filter 170. The at least two converted signals 90, corresponding to the 1400 and 1800 Hz. Baudot tones 160, are composed of frequencies in a range of about 300-1200 Hz. indicated generally by the designation 180, in accordance with an aspect of this disclosure. Returning to Fig. 2, in accordance with the presently preferred embodiment, the converted signals 90 are filtered out of the combined signals 110 by the input band pas filter 152 of the TTY/TDD terminal 150A, while the remaining Baudot tones are received and processed in a conventional manner. In accordance with the presently preferred embodiment, the wireless network 130 also delivers the combined signals 1 10 to a device 190 for utilizing the combined signals 110. A tone detection and generation device 200 includes a signal separator 210 which splits the received combined signals 110A into their received Baudot tone 20A and received converted signal 90A components. A receiving tone detector 220 detects the presence of the Baudot tones 20A and triggers, over signal line 230, a Baudot tone generator 240 to generate Baudot tones in response to the presence of the received Baudot tones 20A and the received converted signals 90A. Thus, in the tone detection and generation device 200 the presence of the received Baudot tones 20 A and the received converted signals 90A is used to verify the receipt of the Baudot tones 20A. The received Baudot tones 20A in the received combined signals 110A are not used directly by the receiving TTY/TDD terminal 250, but is used instead as a trigger over signal line 230 for the Baudot tone generator 240. The trigger function is designed so as to be less sensitive than a TTY/TDD device to the variations in frequency, amplitude, and duration of the Baudot tones that are introduced by the passage through the wireless network. The generated Baudot tones 245 are then delivered to the TTY/TDD terminal 250. Thus, in the presently preferred embodiment, the converted signals 90 convey information that is used to generate the Baudot tones 245 to be delivered to the receiving TTY/TDD device 250. Ideally, the device 250 receives and processes the generated Baudot tones 245 in a conventional fashion. In an alternate embodiment, the presence of the received Baudot tones 20A may be used to trigger a decoding of the received converted signals 90A to reconstruct the original Baudot tones. By example, the Baudot tone detector 220 may be used to trigger the tone generator 240 over signal line 230 to decode the received converted signals 90A, and utilizing the information conveyed by the received converted signals 90A, to generate the specified Baudot tones for use by the receiving TTY/TDD device 250. For example, the received converted signal 90A may be a channel coded bit stream mapped into at least two frequencies, as used as an example above. In this case the tone generator 240 would decode the frequencies, construct and recognize the character represented by the 70 bit stream and generate the corresponding Baudot tones. In the event that the received combined signal is speech, or some other signal, the receiving tone detector 220 fails to detect Baudot tones from the combined signals 110 and inhibits the tone generator 240 from generating Baudot tones. The tone generator 240 may then pass through the received signals to the TTY/TDD device 150B. Although described in the context of preferred embodiments, it should be realized that a number of modifications to these teachings may occur to one skilled in the art. By example, the disclosed embodiments may be incorporated into currently available TTY/TDD equipment, or it may be incorporated into currently available radiotelephone equipment. The disclosed embodiments may also be made available as a separate accessory, or a system in accordance with the disclosed embodiments could incorporate any combination of the components described above. For example, the transmitting TTY/TDD device 10, the device 60 responsive to the transmitting TTY/TDD device, and the wireless station 120 can be an integral part of the device 55, and need not be separate, external components. As another example, the device 60 responsive to the transmitting TTY/TDD device may be an integral part of the transmitting TTY/TDD device 10, or could be an attachment thereto. As a further example, device 55 for generating combined signals may be incorporated into a computer either as an integral component or as a plug in module. Correspondingly, the receiving TTY/TDD device 250 and the device 200 for utilizing the combined signals can be an integral part of the device 190, and need not be separate, external components. The device 190 for utilizing the combined signals may also be incorporated into a computer either as an integral component or as a plug in module, and as a further example, the device 200 could be an integral part of the receiving TTY/TDD device 150A or 150B, or could be an attachment therefor. As shown in Figure 4, in an alternate embodiment, the device 55 for generating combined signals 110 and the device 190 for utilizing received combined signals 110A may be incorporated into the hardware circuitry of a wireless station 300, permitting wireless to wireless, or wireless to wired communication of Baudot encoded text. A key pad 305 integral to the wireless station may serve as an input device, or a keyboard may optionally be attached to the wireless station. The vocoder 122 then encodes the combined signals 110 and conveys them to an antenna 350 by way of a transceiver 340. The vocoder receives signals from the antenna 350 by way of the transceiver 340 and conveys the received combined signals 110 A to device 190 where text may be displayed by the display integral to the wireless station 320 or optionally by a display connected to the wireless station 300. The functions of the device 55 for generating combined signals and the device 190 for utilizing the combined signals may be incorporated into the program code of the wireless station vocoder 122, eliminating the need for changes to the hardware circuitry in the wireless stations. In this case, the radiotelephone may be considered a wireless TTY/TDD device. Turning to Fig. 5, the disclosed embodiments may also be incorporated into a wireless telecommunication systems infrastructure 355, either into the base station transceiver 360, or the base station controller 370, or the mobile switching center 380, and would operate in accordance with the functions and embodiments described above. It is preferred to incorporate the disclosed embodiments into the mobile switching center 380 because it serves as an interface among many base stations and the PSTN. Thus, while the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from its scope and spirit.

Claims: CLAIMS What is claimed is: 1 . A method for transmitting a communication through a communications channel, the communication being encoded with a plurality of first signals, comprising steps of: detecting a presence of one of said first signals; in response to detecting a presence of one of said first signals, generating a second signal; combining said first signal with said second signal to form a combined signal; and, transmitting said combined signal through said communications channel. 2. The method of claim 1 , further comprising steps of: receiving said combined signal with a first terminal; separating said combined signal into said first signal and said second signal; and, responsive to a presence of said first and second signal, regenerating said first signal. 3. The method of claim 2, further comprising the step of decoding the communication in response to the regeneration of said first signal. 4. The method of claim 2, wherein said first terminal has an input band pass characteristic, wherein the frequency of said first signal lies within said band pass characteristic, and wherein the frequency of said second signal lies outside said band pass characteristic. 5. The method of claim 1 , wherein said plurality of first signals have a frequency selected from the group consisting of about 1400 Hz and about 1800 Hz. 6. The method of claim 1 , wherein said plurality of first signals are Baudot tones. 7. The method of claim 1 , wherein said second signal has a frequency in the range of about 300 Hz. to about 1200 Hz. 8. The method of claim 1 , wherein said communications channel is a telecommunications channel. 9. The method of claim 1 , wherein said communications channel is part of a wireless network. 10. The method of claim 1 , wherein said communication comprises alphanumeric information. 1 1. The method of claim 1 , further comprising steps of: receiving said combined signal with a second terminal; and, filtering out said second signal from said combined signal so as to respond only to said first signal. 12. An apparatus for transmitting a communication through a communications channel, the communication being encoded with a plurality of first signals, comprising: a detector for detecting a presence of one of said first signals; a signal generator responsive to said detector for generating a second signal; a junction for combining said one of said first signals with said second signal to form a combined signal; and, a transmitter for transmitting said combined signal through said communications channel. 13. The apparatus of claim 12, further comprising; a first terminal for receiving said combined signal; a separator for separating said combined signal into said first signal and said second signal; a receiving detector for detecting the receipt of said first signal and said second signal; and, a generator, responsive to said receiving detector for regenerating said first signal. 14. The apparatus of claim 13, further comprising a decoder, responsive to said generator for decoding the communication in response to the regeneration of said first signal. 15. The apparatus of claim 13, wherein said first terminal has an input band pass characteristic, wherein the frequency of said first signal lies within said band pass characteristic, and wherein the frequency of said second signal lies outside said band pass characteristic. 16. The apparatus of claim 12, wherein said plurality of first signals have a frequency selected from the group consisting of about 1400 Hz and about 1800 Hz. 17. The apparatus of claim 12, wherein said plurality of first signals are Baudot tones. 18. The apparatus of claim 12, wherein said second signal has a frequency in the range of about 300 Hz. to about 1200 Hz. 19. The apparatus of claim 12, wherein said communications channel is a telecommunications channel. 20. The apparatus of claim 12, wherein said communications channel is part of a wireless network. 21. The apparatus of claim 12, wherein said communication comprises alphanumeric information. 22. The apparatus of claim 12, further comprising: a second terminal for receiving said combined signal comprising: a filter, receptive of said combined signal, for filtering out said second signal from said combined signal, so that only said first signal remains; and, a detector, receptive of said remaining first signal for detecting said first signal. 23. The apparatus of claim 12, wherein said apparatus comprises a portion of a wireless telecommunications system. 24. The apparatus of claim 12, wherein said apparatus comprises a portion of a wireless mobile station. 25. The apparatus of claim 12, wherein said apparatus comprises a portion of a wireless telecommunication systems infrastructure. 26. An apparatus for receiving a communication through a communications channel, the communication being encoded with a plurality of first signals, comprising: a first terminal for receiving said combined signal; a separator for separating said combined signal into said first signal and said second signal; a receiving detector for detecting the receipt of said first signal and said second signal; and, a generator, responsive to said receiving detector for regenerating said first signal. 27. The apparatus of claim 26, further comprising a decoder, responsive to said generator for decoding the communication in response to the regeneration of said first signal. 28. The apparatus of claim 26, further comprising a device for forming said combined signal, said device comprising: a detector for detecting a presence of one of said first signals; a signal generator responsive to said detector for generating a second signal; a junction for combining said one of said first signal with said second signal to form said combined signal; and, a transmitter for transmitting said combined signal through said communications channel. 29. The apparatus of claim 26, wherein said first terminal has an input band pass characteristic, wherein the frequency of said first signal lies within said band pass characteristic, and wherein the frequency of said second signal lies outside said band pass characteristic. 30. The apparatus of claim 26, wherein said plurality of first signals have a frequency selected from the group consisting of about 1400 Hz and about 1800 Hz. 31. The apparatus of claim 26, wherein said plurality of first signals are Baudot tones. 32. The apparatus of claim 26, wherein said second signal has a frequency in the range of about 300 Hz. to about 1200 Hz. 33. The apparatus of claim 26, wherein said communications channel is a telecommunications channel. 34. The apparatus of claim 26, wherein said communications channel is part of a wireless network. 35. The apparatus of claim 26, wherein said communication comprises alphanumeric information. 36. The apparatus of claim 26, further comprising: a second terminal for receiving said combined signal comprising: a filter, receptive of said combined signal, for filtering out said second signal from said combined signal, so that only said first signal remains; and, a detector, receptive of said remaining first signal for detecting said first signal. 37. The apparatus of claim 26, wherein said apparatus comprises a portion of a wireless telecommunications system. 38. The apparatus of claim 26, wherein said apparatus comprises a portion of a wireless mobile station. 39. The apparatus of claim 26, wherein said apparatus comprises a portion of a wireless telecommunication systems infrastructure. 40. The apparatus of claim 27, wherein said apparatus comprises a portion of a wireless telecommunication systems infrastructure.

Inventors: FATINI MASOUD, WEI CHAO, FATINI, MASOUD, WEI, CHAO
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LIMITED
Application year: 2000
Main classification: H04M11
IPC codes: H04B1, H04L12