Pulse or digital communications – Systems using alternating or pulsating current
Reexamination Certificate
1998-08-28
2002-08-13
Pham, Chi (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Reexamination Certificate
active
06434198
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention provides a method for conveying teletype information over a wireless communication system and more particularly to a method in which the teletype information is encoded so that it can be reliably conveyed over the wireless telecommunication system.
2. Description of the Related Art
Teletype terminals are part of telecommunication technology developed more than 30 years ago. The teletype terminals are equipment that were designed to transmit and receive teletype signals where such teletype signals contain information arranged in accordance with a well known signaling protocol such as Baudot signaling. A signaling protocol is a set of rules followed by a provider of a communication system that defines how information is to be encoded, transmitted, received and decoded by equipment of the communication system. The Baudot signaling protocol has remained virtually unchanged for the past 30 years. Baudot signaling is an analog signaling protocol in which analog signals are used to digitally encode information that are conveyed over a telephony system such as the Public Switched Telephone Network (PSTN).
There are still many active teletype terminals in the United States and throughout the world. Many of these active terminals still transmit and receive information in accordance with the 45.45 baud Baudot analog signaling protocol or other similar protocols. In the 45.45 baud Baudot signaling protocol, each character is coded as an 8 bit block of information comprising 1 start bit, 5 information bits and 2 stop bits where each bit is 22 milliseconds in length. Thus, 45.45 baud (1/22 msec.) represents the speed at which the information is conveyed and can be generally referred to as the information rate. The actual information is represented by the 5 information bits. The start bit, which is usually defined as a “0” bit and the stop bits, which are usually defined as “1” bits, indicate the beginning and end of each 8 bit block of information respectively. For many signaling protocols, including the Baudot signaling protocol, the 8 bit block of information usually represents an alphanumeric character. An 8 bit block of information is commonly referred to as an 8 bit word. The start and stop bits are typically referred to as synchronization bits and are part of signaling information. Signaling information is information used by the system that enable the system to properly transmit, receive and process the user information. The Baudot signaling protocol being an analog signal protocol, each bit is represented by a tone (i.e., a single frequency sinusoid). Thus, a “0” bit is represented by an 1800 Hz tone and a “1” bit is represented by a 1400 Hz tone.
With the advent of wireless telecommunication systems, owners of teletype terminals have attempted to transmit and receive teletype signals over such systems. Moreover, the Federal Communications Commission (FCC) has adopted rules requiring providers of wireless telecommunication systems to ensure that teletype users can access Emergency 911 services over wireless telecommunication systems. The Emergency 911 services are the well known types of services typically provided via the PSTN (or other communication network) in which a subscriber of the PSTN simply dials 911 to obtain emergency assistance from local authorities (e.g., police dept., fire dept., hospital/ambulance services). However, system providers have discovered that the teletype terminals have performed poorly over wireless telecommunication systems. One indication of poor performance is the ratio of the number of erroneous characters received to the total number of characters transmitted; this ratio is known as the Character Error Rate (CER). Several studies and trials conducted by various wireless equipment manufacturers confirm the poor performance (e.g., unacceptably high CER) of teletype communications over wireless telecommunication systems. The following are some of the more recent studies: E911 TDD Compatibility Testing With IS-136, Jan. 22, 1998, Ericsson Corp.; E911 TDD Compatibility to GSM, 1998, Ericsson Corp.; TTY Study using Vocoder direct link Gallaudet University Technology Assessment Program; TTY/TDD Compatibility Measurements Preliminary Results”, May 1998, Nokia Corp.; TDD Communications over CDMA September 1997, Qualcomm Inc.; “Initial TTY Report” May 1998, Sprint PCS.
It is widely thought that the poor performance of teletype communications over wireless telecommunication systems is due to the use of vocoders in such systems. Vocoders are speech coding systems designed to represent human voice as mathematical models comprising various parameters. Also, vocoders comprise speech coding systems that simply digitize analog voice signals into Pulse Coded Modulation (PCM) samples (i.e., bit streams) without any mathematical parameter modeling. The parameters are transmitted and received over wireless telecommunication channels. At the transmit end, a vocoder produces a set of parameters for every basic timing interval of the wireless telecommunication system. The basic timing interval is part of a format being followed by a wireless telecommunication system. The format is a particular method of arranging and structuring information to be conveyed over a wireless telecommunication system. The basic timing interval is a period of time during which a block of information representing, for example, voice, data, video and other communication signals, is conveyed. For certain wireless telecommunication systems such as Code Division Multiple Access (CDMA) wireless telecommunication systems, the basic timing interval is commonly referred to as a frame. Each frame contains a plurality of digital bits representing digitally encoded information. At the receive end, another vocoder uses the received parameters to reproduce human voice. In current CDMA wireless telecommunication systems the frame is 20 milliseconds long and some of the vocoders used are the QUALCOMM Code Excited Linear Predictive Coder (QCELP 13 k or 8 k) and the Enhanced Variable Rate Coder (EVRC).
Signals that do not originate from human speech such as tones are sounds that can adversely impact the quality and accuracy of the mathematical model parameters produced by the vocoders, thus, the ability of a vocoder at the receive end to accurately reproduce such signals would also be adversely affected. Many have reasoned that since teletype signals (i.e., tones) are not sounds typically produced by human voice, some vocoders have difficulty in reproducing such signals resulting in the poor performance of teletype signals when conveyed over wireless telecommunication systems. However, tests and performance studies have shown that the poor performance of teletype signals conveyed over wireless telecommunication systems is mainly due to the quality of the communication channels of these systems and not to the use of vocoders. The quality of these communication channels may be such that the conveyed teletype signals are damaged to the extent that an unacceptably high information error rate (i.e., high CER) results.
Referring to
FIG. 1
, there is shown a test setup designed to evaluate the performance of teletype signals over a communication channel of a CDMA system using QCELP 13k vocoders. At the transmit end teletype signals from a teletype terminal (not shown) are fed to vocoder which produces the mathematical model parameters. The parameters produced by vocoder
100
are transmitted as frames over communication channel
102
. In this setup, communication channel
102
is designed to be ideal in the sense that the transmitted frames experience no adverse effects. Adverse effects are any existing conditions in the wireless telecommunication system which cause errors to occur in the information being conveyed through the system. A non-zero information error rate, which is usually defined as the ratio of erroneous information to total information, directly results from the adverse effects.
In an actual CDMA wireless telecommunication sys
Lucent Technologies - Inc.
Narcisse Claude R.
Pham Chi
Tran Khai
LandOfFree
Method for conveying TTY signals over wireless communication... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for conveying TTY signals over wireless communication..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for conveying TTY signals over wireless communication... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2920787