Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1998-09-12
2003-09-16
DeCady, Albert (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
Reexamination Certificate
active
06622275
ABSTRACT:
BACKGROUND OF THE INVENTION
1. 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.
2. 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 TTDs in this application. Typically, TTDs 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 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 20 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 ½ bits marking the stop point of the transfer group. This 20 milliseconds interval is also the frame length produced by a vocoder, discussed below, for transmitting one frame of information in a wireless telecommunications system.
Compared to modem telecommunication systems, 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. 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.
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 “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). 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. 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 that does not require an increase in transmission power to reduce frame error rates. The invention should be able to sense when a low activity communication is received, or notice should be sent to the system, and be able to reconstruct frames containing errors by reviewing adjacent frames. Alternatively, the invention should 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. 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
Broadly, the present invention involves the modulation of a low activity communication by a telecommunication system using encoded signals. More particularly, the invention concerns a method, article of manufacture, and apparatus that uses specialized encoding, decoding, or both, on a low activity communication signal to minimize a transmitted signal's frame erasure rate without increasing transmission power. 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. In one embodiment, the decoder may compare a frame containing transmission errors (erred frame) with a vocoded frame from a known TDD signal
Baker Kent D.
DeCady Albert
Macek Kyong H.
Qualcomm Incorporated
Torres Joseph D.
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