Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-06-12
2001-02-06
Hunter, Daniel S. (Department: 2749)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S450000
Reexamination Certificate
active
06185424
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to signal processing in a digital cellular system and more particularly, to a method of providing an automatic switching capability from a tandem to a single Vector Sum Excited Linear Prediction (VSELP) speech encoder/decoder configuration in a TDMA mobile-to-mobile connection.
BACKGROUND OF THE INVENTION
Digital signal processor (DSPs) are used in digital mobile cellular radio systems to enable echo cancellation, speech encoder/decoder (codec) functions and channel encoder/decoder functions for TDMA channels. These digital signal processors perform in some installations echo cancellation on the switch side of the cellular infrastructure. Some equipment manufacturers provide for these functions on the cell site end of the cellular system. In both cases, the speech and channel encoder/decoder (codec) designs and implementations are made to comply with Communications Technology Industry Association's (CTIA) Digital Cellular Interim Standard (IS-54) specification.
In a TDMA mobile-to-land connection, audio signals arriving at the digital signal processor from the land source is first subject to echo cancellation. The echo cancelled signal is then compressed into IS-54 VSELP signal format with a 8:1 ratio and the compressed signal is channel encoded and sent over the air to the mobile. VSELP signal arriving at the mobile is channel decoded and then used to reconstruct the original audio signal by the speech decoder. The entire speech encoding and decoding process introduces perceptible coding noise to the reconstructed speech signal. Audio signal travelling in the reverse direction (mobile to land) is subject to the same encoding and decoding processes but with no echo cancellation by the digital signal processors.
A TDMA mobile-to-mobile connection is realized basically by connecting two TDMA mobile-to-land calls back to back between two digital signal processors. Thus, an audio signal travelling in either direction is subject to two speech encoding and decoding processes operating in tandem. Thus, the coding noise introduced at each end degrades the quality of voice signal received by each subscriber.
Accordingly, a requirement exists to reduce the amount of coding noise which is generated during mobile-to-mobile communication.
SUMMARY OF THE INVENTION
With the design of the present invention, the digital signal processor is provided with the capability to automatically switch configuration such that two digital signal processors can automatically identify a TDMA mobile-to-mobile connection and bypass the speech encoding and decoding processes within the digital signal processors. The two digital signal processors are virtually connected at the channel codecs. Thus enhancing the end-to-end audio signal quality as well as improving system performance.
According to an aspect of the present invention, there is provided a digital cellular radio system for enabling digital mobile radios to communicate with each other and the public switched telephone network via radio cell sites and a mobile switching office, said digital cellular radio system having digital signal processor means for providing echo cancellation, speech encoding/decoding and channel encoding/decoding of audio signals, comprising:
a first digital signal processor for enabling echo cancellation, speech encoding/decoding and channel encoding/decoding of audio signals from a local digital mobile telephone;
a second digital signal processor for enabling echo cancellation, speech encoding/decoding and channel encoding/decoding of audio signals from a remote digital mobile telephone;
a message transmitter for transmitting in-band signalling information to a remote digital signal processor;
a message receiver for detecting in-band signalling information at a local digital signal processor; and
a controller for monitoring the transmission of signalling information from a remote message transmitter to a local message receiver, wherein when the local message receiver detects inband signalling information from the remote message transmitter indicating that a digital mobile-to-mobile connection is established, the first and second digital signal processors can switch to a by-pass mode to by-pass the speech encoding/decoding and echo cancellation functions such that each digital mobile radio communicates audio signals directly with each other in a VSELP format via the channel encoding/decoding function.
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Chu Chung-Cheung
Pon Hermon
Rabipour Rafi
Haszko Dennis R.
Hunter Daniel S.
Nortel Networks Limited
Wyche Myron K.
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