Method and apparatus for error correction

#The present invention relates generally to communication systems and in particular, to a method and apparatus for error correction within such communication systems.

#Within a Code Division Multiple Access (CDMA), and other communication system types, communicated information, either voice or data, is carried between communication resources, e.g., a radio telephone and a base station, on a communication channel. Within broadband, spread spectrum communication systems, such as CDMA based communication systems in accordance with Interim Standard IS-95B, a spreading code is used to define the communication channel.

#CDMA systems have the capability of transmitting user information at variable rates. For example in voice calls the data rate of each speech frame is varied based on the speech activity. When a user is speaking, compressed speech information is typically sent at full rate. Between words and sentences the data rate is typically reduced to eighth rate. Half and quarter rates are also used for speech to quiet transitions and when data rate reductions are required, such as to allow for multiplexing of signaling information or to increase system capacity. In data services calls, full, half, quarter and eighth rate frames can be selected based on the data rate of the user requested information.

#FIG. 1 is a block diagram of a transmitter in accordance with the preferred embodiment of the present invention.

#FIG. 2 is a block diagram of the convolutional encoder of FIG. 1 in accordance with the preferred embodiment of the present invention.

#FIG. 3 is a block diagram of a decoder in accordance with the preferred embodiment of the present invention.

#To address the need for forward error correction within a communication system, a convolutional encoder/decoder is provided herein. The convolutional encoder comprises controller, having a transmission rate (e.g. frame rate) as an input. The controller initializes the encoder to an initial state based on a transmission rate currently being utilized. When decoding, a decoder utilizes a Trellis diagram having an initial and final state based upon the frame rate.

#Since the initial state of the encoder is different for all possible data rates, the probability of falsing is greatly reduced. More particularly, the hamming distances are significantly increased between valid encoded sequences for one transmission rate and the valid encoded sequences for another transmission rate. This increase in the hamming distance improves the likelihood of correctly decoding the frame.

#The present invention encompasses a method for encoding data within a communication system. The method comprises the steps of receiving a data rate and setting an initial state of a convolutional encoder based on the data rate.