GB2131253A

GB2131253A - Error-correcting decoder

Info

Publication number: GB2131253A
Application number: GB08233471A
Authority: GB
Inventors: Graham Edgar Beesley; Ian Michael Dewey
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1982-11-24
Filing date: 1982-11-24
Publication date: 1984-06-13

Abstract

An error-correcting decoder (10) for decoding a remotely transmitted coded signal comprises a random-error-correcting decoder (14) for decoding a received coded signal in accordance with a random-error-correcting strategy, a burst-error-correcting decoder (16) for decoding the received coded signal in accordance with a burst- error-correcting strategy and an error detector (16, 18) for detecting the presence in the received signal for random errors and of burst errors and for causing either the random-error- correcting decoder or the burst-error- correcting decoder to be used dependent on whether random errors or burst errors are detected. <IMAGE>

Description

SPECIFICATION Error-correcting decoder This invention relates to decoders for decoding remotelytransmitted coded signals which employ error-correcting to correct for errors introduced into the coded signals during transmission.

Error-correcting decoders are used, for example, in paging system receivers and in data communications system receivers where communication may take place over a fading channel, such as mobile radio systems and portable signalling systems. Atypical, known, error-correcting decoder applies a predetermined decoding strategy to a coded signal so that errors which may be present in the signal are corrected if the error level is below a predetermined threshold and the signal is rejected if the error level is above the predetermined threshold.

However, errors present in a received signal may be of more than one type. For example, in a car radio transmitter or receiver the signals may be subject to burst type or errors caused by the car's ignition system and may also be subjectto random type or errors as the vehicle passes beneath bridges or behind other sorts of signal attenuators.

Known error-correcting decoders compromise their error-correcting performance by adopting either an error-decoding strategywh ich is designed to correct for random errors but is very intolerant of burst errors, or an error-correcting strategy which is designed to correctfor burst errors but is very intolerant of random errors.

It is an object of the present invention to provide an error-correcting decoder in which the above disadvantage may be overcome oratleastalleviated.

In accordance with the invention a decoder for decoding a remotelytransmitted coded signal comprises first decoding means for decoding a received coded signal in accordance with first predetermined decoding strategy so as to correctfor a firsttype of error in the received signal; second decoding means fordecodingthe received signal in accordance with a second predetermined strategy so as to correct for a second type of error different from said first type of error in the received signal; and error detecting means for detecting the presence in the received signal of said firsttype of error, for detecting the presence in the received signal of said second type of error and for causing the decoderto use a selected one of said first decoding means and said second decoding means in dependence on whether said first type of error or said second type of error is detected.

A radio paging system receiver incorporating a decoder in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawing,which shows a blockschematic view of the receiver.

A paging system receiver 2 includes an antenna 4 for receiving coded radio signals and a tuner 6for passing only those signals in a predetermined fre- quency channel. The coded information is recovered from the signals by a de-modulatorand amplifier 8 and is passed to a decoder 10. As will be further explained below, decoder 10 detectswhetherthe signal information is encoded with a predetermined code, and if so produces an output. The decoder output activates a warning device 12 such as an audible our visual indicator.

The decoder 10 comprises a random-error-correcting decoder 14, a burst-error-correcting decoder 16, an errordetector18and a switch 20 controlled bythe error detector 18.

The random-error-correcting decoder 14, known per se, decodes the coded signal in accordance with a known, random-error-tolerant decoding strategy. The decoder 14 produces an output if the signal information is encoded with a selected, predetermined code and continues to produce its output when the detected code with which the signal information is encoded varies from the selected code in a random way, provided that the code error level is within acceptable limits.

Similarly,the burst-error-correcting decoder 16, also known peruse, decodes the coded signal in accordance with a known, burst-error-tolerant decoding strategy. The decoder 16 produces an output ifthe signal information is encoded with the selected code and continues to produce its output when the detected code with which the signal information is encoded varies from the selected code in a burst way, provided thatthe code error level is within acceptable limits.

The error detector 18 detects which ofthe two types of error, random or burst, is present in the encoded signal and operates switch 20 accordinglyto connect the output of the appropriate error-correcting decoder 14 or 1 6to the vyarning indicator 12. If neither burst nor random errors are detected by the detector 18, or if random errors alone are detected, then the output of the random-error-correcting detector 14 is connected; if burst errors alone are detected then the output of the burst-error-correcting detector 16 is connected. If both types of error are present then the predominanttype of error is detected and the output ofthe decoder 14 or 16 which detects for the predominanttype of error is connected to the warning indicator 12.It will, however, be appreciated that the burst-error-correcting detector 16 may be connected instead of the randomerror-correcting detector 14 when neither type of error is detected.

Where synchronous codes are used to encodethe transmitted signal, the error detector 18may detect the presence of random or burst errors in the received signal by comparing the synchronising information in a number of consecutive codes in known manner.

Alternatively, where synchronous codes are used to encode the transmitted signal the error detector 18 may detectthe presence of random or burst errors in the received signal by comparing the pre-amble and post-amble portions of the codes, again in known manner.

It will be appreciated that the paging system receiver described above exhibits improved sensitivity because by using a decoding strategy relevant to whichever type of error is detected the receiver is tolerant of either type of error and so is able to receive its predetermined code over a wide error range.

It will also be appreciated that although in the above description simple switch 20 is used for selecting between the outputs of the error correcting decoders 14 and 16, the switch may alternatively take the form of a data squelch circuit or a signal level detector circuit. It will also be appreciated that the switching may alternatively be embodied, possibly together with the decoding strategies and error detection, in a control program for a microprocessor.

It will also be appreciated that although the invention has been described above with reference to a radio paging system receiver, the invention is not limited to such an application and is applicable to any error-correcting decoder in which a plurality of differenttypes of error may be encontered.

Claims

1. Adecoderfordecoding a remotelytransmitted coded signal comprising: first decoding means for decoding a received coded signal in accordance with a first predetermined decoding strategy so as to correct for a firsttype of error in the received signal; second decoding means for decoding the received signal in accordance with a second predetermined strategy so as to correct for a second type of error different from said first type of error in the received signal; and error detecting means for detecting the presence in the received signal of said firsttype of error,for detecting the presence in the received signal of said second type of error and for causing the decoder to use a selected one of said first decoding means and said second decoding means in dependence on whether said firsttype of error or said second type or error is detected.

2. A decoder according to claim 1 wherein said error detecting means is arranged to detect, when said first and said second types of error are present in the received signal, the predominant one of said first and second types of error.

3. A decoder according to claim 1,or2wherein said first type of error is a random error and said second type of error is a burst error.

4. A decoder according to claim 1,2 or 3 wherein said first decoding means and said second means are arranged to receive at their inputs the received signal and said error detector is arranged to select between the output of said first decoding means and the output of said second decoding means.

5. A decoder substantially as hereinbefore described with reference to the accompanying drawing.

6. A radio paging system receiver incorporating a decoder according to any one of claims 1 to 5.

7. A radio paging system receiver substantially as hereinbefore described with reference to the accompanying drawing.