WO1990014735A1

WO1990014735A1 - Apparatus and method for enhancing the reliability of digital messages

Info

Publication number: WO1990014735A1
Application number: PCT/US1990/002446
Authority: WO
Inventors: Kazimierz Siwiak
Original assignee: Motorola, Inc.
Priority date: 1989-05-24
Filing date: 1990-05-04
Publication date: 1990-11-29

Abstract

A transmitter (100 or 300) and receiver (205 or 305) for use in a data transmission system are disclosed. In order to provide reliable communication, data to be transmitted is both encoded and encrypted. An EXCLUSIVE OR operation is utilized for encryption and decryption of the data.

Description

APPARATUS AND METHOD FOR ENHANCING THE RELIABILITY OF DIGITAL MESSAGES

BACKGROUND OF THE INVENTION

This invention relates in general to data paging systems and, more particularly, to an apparatus and method for enhancing the reliability of data messages in such a system.

In paging systems, typically a single data transmitting system serves as the originator of messages which have unique addresses associated with them. A multiplicity of paging data receivers served by such a transmitting system, each being responsive to at least one such paging address, simultaneously receive the uniquely addressed messages. A pager which is assigned a unique address that was transmitted then decodes the data message associated with the unique paging address and alerts the pager user that a message was received. The user may then retrieve and read the message on the display in the pager.

One such data paging system encodes binary data using the Post Office Code Standardization Advisory Group (POCSAG) signalling pattern. Although the POCSAG signalling pattern in more fully described in, "PMR 2000 Series POCSAG Alphanumeric Display Personal Message Receiver," Instruction Manual 68P81048C80-0, published by Motorola, Incorporated, it is important to note here that one portion of a POCSAG transmission pattern consists of address code words followed by data code words. The POCSAG address and data code words are 32 binary bits encoded in a BCH (31 ,21 with parity bit) code, that is, a BCH (32,21) code having one additional parity bit. Each code word consist of 21 information bits (including a data/address flag bit), 10 redundancy bits and a parity bit. The flag bit is a binary ^H0" for address code words and a binary "1 " for data information code words. The 10 redundancy bits are generated by the 21 information bits from BCH construction rules at the encoder.

When the POCSAG signals are passed through a noisy communication channel, bit errors may occur. According to BCH theory, the BCH (31 ,21 with parity bit) allows the detection of up to 4 bit errors and the correction of up to two error bits in the 21 information bits.

A paging receiver searches for address code words, and attempts to match (to within 2 bits) its assigned code with one that is transmitted. When a match (to within 2 bits) between sent address and a paging receiver address occurs, the paging receiver begins decoding the information data words which follow the address word. An end of message is marked by the reception of the next address word sent on the communication channel. It will be appreciated that information data messages can be misdirected in at least two ways. First, an address word which was corrupted by noise in the noisy communication channel can be misread as a valid address in a paging receiver. Second, during the reception of a valid page, the end of message (which is an address word for another paging receiver) can be misread as a continuing information word, thereby subsequent information words are misdirected information unintended for that particular paging receiver.

When such a paging receiver responds falsely to a paging address which was not intended for that pager or mistakes an end of message mark for another data word, the data which is associated with that falsely read address is displayed on the pager as if it were a message intended for that pager. Since messages in paging systems, typically do not have user identification labels within the message, a pager user receiving such an unintended and misdirected message may mistake it for one that is intended. Paging systems are designed to minimize such false responses to a very small number.

However, when such a false response does occur, the misdirected message information becomes a serious degradation of message reliability in a data paging system. In the event of a false paging response it is therefore, desirable to obscure or suppress the data message so as not to confuse the pager user with erroneous and unintended information.

A prior art paging system where a falsely responding message can occur is shown in FIG. 1 as a simplified block diagram 10 of a POCSAG data transmission system. Simplified block diagram 10 shows only those components which affect the processing of the address and information data word portions of the POCSAG signaling pattern.

As shown in FIG. 1 , a data transmitter system 20 includes an address supplier 30 and a word generator 35 for data messages. Generator 35 organizes the address words supplied by address supplier 30 with the associated message data into a format that is defined by the POCSAG signaling pattern. The address and data words are passed through a channel encoder 40 which operates on the address and data words and appends the 10 redundancy bits and one parity bit according to BCH (31,21 , with parity bit) encoding construction rules, as further defined by the POCSAG signaling pattern. Such encoded address and information data words are supplied by channel encoder 40 to transmitter system 20 output terminal 45 which further supplies the POCSAG encoded signal through noisy communication channel 50 to an input terminal 55 of a data receiver 60.

The noise corrupted POCSAG signal pattern at input terminal 55 of data receiver 60 is supplied to address detector 65 which is also supplied with a POCSAG encoded data receiver address internally from receiver address supplier 70. Address detector 65 attempts to match POCSAG encoded noise corrupted signals supplied at input 55 with the internally generated address from address supplier 70. When a match (within an error of two bits) occurs, subsequent POCSAG signals are supplied by address detector 65 to channel decoder 75 for validation and error correction according to specific BCH decoding rules. BCH construction rules specific to this BCH coding pattern allow for the detection of up to four error bits and the correction of up to two error bits. Typically, however, only one erroneous bit is corrected in the interest of pager receiver simplicity. Decoded information data words are supplied by channel decoder 75 to input 80 of display apparatus 85. In the event that channel decoder 75 detects more errors than can be corrected by channel decoder 75, decoded words containing such errors are marked by channel decoder 75. Correctly decoded words and error marked words are supplied to display apparatus 85 which formats and displays the message words as on an alphanumeric display.

Often, pager receivers are designed to suppress such error marked data words. One pager which suppresses the display of error marked data words when excessive errors are detected is the Model PMR-2000 POCSAG Alphanumeric Display Personal Message Receiver manufactured by Motorola, Inc. In that pager receiver, error marked symbols are displayed on the alphanumeric display as asterisk (^*) symbols. When channel decoder 75 validates a supplied POCSAG encoded signal, channel decoder 75 also checks the flag bit to determine whether the word is an information data word or another address word, hence an end of message marker for the current message. Occasionally, due to noise corruption by communication channel 50, an initial address may be misread by address detector as a valid address or an end of message marker may be read as a data word. Subsequent data words continue to be read as information data words until another end of message is found. Spurious misdirected and unintended message data are consequently displayed to an unintended data receiver user. Although paging systems are designed to minimize such false responses, in the event of such a false paging response, it is desirable to obscure or suppress the data message so as not to confuse the pager user with erroneous and unintended information.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method for transmission or reception of data in which a receiver responding to a wrong address will decode jumbled data.

A transmitter for use in such system includes means for supplying an address signal and word generator means for supplying data. Both a channel encoder means and a data encryption means are provided so that the data can be both encoded and encrypted. The encryption is performed based upon the particular address. A receiver includes an input, for receiving the address signals and the data that has been both encrypted and encoded. The address detector detects the address signal received by the receiver input. Both data encryption means and channel decoder means are provided for decrypting and decoding the data. In one aspect of the invention, the decryption is based upon the received address. In another aspect of the invention, data that is recognized as containing errors is not displayed. In still another aspect of the invention, the data is encoded as POCSAG data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a prior art data transmission system.

FIG. 2 is a simplified block diagram of a data transmission system in accordance with the present invention. FIG. 3 is a simplified block diagram of an alternative embodiment of a data transmission system in accordance with the present invention.

FIG. 4 is a block diagram of a data encryption apparatus.

FIG. 5 is a block diagram of a data decryption apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 illustrates a simplified block diagram of a data transmission system in accordance with the present invention. A data transmitter system 100 includes a channel encoder 110 for encoding data words supplied at its input terminal 115 into BCH (31 ,21 with parity bit) words of the POCSAG signaling pattern according to BCH construction rules. Such BCH construction rules are well known to those skilled in the art. The data signal at output 135 of channel encoder 110 consists of 32 bit address and data words of the POCSAG signaling code.

Channel encoder 110 is supplied with address words and information data words by word generator 125 which is supplied address words by address supplier 130. Address supplier 130, word generator 125 and channel encoder 110 of FIG. 2 perform substantially the same functions as corresponding address supplier 30, word generator 35 and channel encoder 40 shown in FIG. 1. Returning again to FIG. 2, output 135 of channel encoder 110 is coupled to an input 140 of a data encryption apparatus 145. Data encryption apparatus 145 is also supplied an encryption code word at input 150 by encryption code word supplier 155. Data encryption apparatus 145 and encryption code word supplier 155 collectively constituting data encryption means. Encryption code word supplier 155, constituting means for supplying an encryption code word, is coupled at its input 160 to address supplier 130. A unique encryption code word is selected by code word supplier 155 for each address supplied to word generator 125 by address supplier 130. Output 165 of encryption apparatus 145 is coupled to output terminal 170 of data transmitter system 100. Data encryption apparatus 145 operates on the 21 bit information portion of the 32 bit POCSAG encoded data words using a 21 bit encryption code word supplied to input 150 of data encryption apparatus 145 by encryption code word supplier 155. The encryption code word is uniquely linked to the address word supplied by address supplier 130. The encryption operation provided by data encryption apparatus 145 is the logical EXCLUSIVE OR operation between the encryption code word supplied by encryption code word supplier 155 and the data word provided by channel encoder 110. Only data words are encrypted by data encryption apparatus 155 as will be explained later. The encrypted POCSAG signaling pattern at output terminal 140 of transmitter system 100 is passed through noisy communication channel 190 as a channel signal to input terminal 200 of data receiver 205. Noise is introduced by communication channel 190 and the channel signal can be corrupted by such noise.

Input 210 of address detector 220 is coupled to input terminal 200 of data receiver 205. A pager address supplier 230 supplies a pager address to address input 235 of address detector 220. When a valid address is detected (within an error of two bits) by address detector 220, subsequent data words are passed from input terminal 200 of data receiver 205 to data decryption apparatus 240. Apparatus 240 is supplied at its input 255 by decryption code word supplier 250 with a receiver code word. Data decryption apparatus 240 and decryption code word supplier 250 collectively constituting data decryption means.

Decryption code word supplier 250, constituting means for supplying a decryption code word, supplies the same code word to data decryption apparatus 240 within data receiver 205 as the encryption code word supplier 155 supplied to data encryption apparatus 145 of transmitter system 100.

Since pager data receiver 205 may be assigned several pager addresses, a unique decryption code word supplied by decryption code word supplier 250 is associated with each such pager address by the connection from pager address supplier 230 to input 265 of decryption code word supplier 250. The unique decryption code word supplied by decryption code word supplier 250 which is associated with each pager address is the same unique encryption code word associated with that address by encryption code word supplier 155 of data transmitter system 100. As will be described later, apparatus 240 decrypts the 21 bit information portion of the data words by applying the logical EXCLUSIVE OR operation between the supplied data words and the supplied decryption code word, and then supplies channel decoder 260 with 32 bit POCSAG encoded data words.

Channel decoder 260 validates and, if needed, error corrects at least one error bit of the POCSAG encoded words according to specific BCH decoding rules. Such validation and error correction of data words using the POCSAG signaling words are well known to those skilled in the art. Decrypted and decoded data words, including words having uncorrectable errors, are supplied by channel decoder 260 to decision means 270 at decision means data input 275. Data words having more bit errors than can be corrected by channel decoder 260 cause an error output signal at output 280 of channel decoder 260 which is supplied to error signal input 285 of decision means 270.

Decision means 270 normally passes information data words which are supplied to input 275 through to display means

290 for display of the message information to the user of data receiver 205. In the event that uncorrectable errors are detected by decision means 270, decision means 270 suppresses data supplied on input 275, thereby suppressing the display of such information on alert and display means 290. That is, when an error signal signifying that a data word has uncorrectable errors is present at error signal input 285 of decision means 270, such words containing uncorrectable errors are suppressed by decision means 270. Address detector 220, pager address supplier 230 and channel decoder 260 shown in FIG. 2 perform substantially the same functions respectively as corresponding address detector 65, receiver address supplier 70 and channel decoder 75 shown in FIG. 1. The channel decoder 260 decodes, error corrects, and validates data words supplied by data decryption means 240.

An end of message is marked by the detection of an address flag in a decoded word. If that address word is misread as a data word, then the subsequently sent data words continue to be processed by apparatus 240 and channel decoder 260. Since, however, the encryption code word supplier 155 in transmitter system 100 changes during a subsequent message, that subsequently used encryption code word no longer matches the decryption code word supplied by decryption code word supplier 250. In data receiver 205 the ten redundancy bits of the data words no longer relate to the 21 data information bits according to the BCH construction rules, causing channel decoder 260 to detect bit errors in such data words. An error detect signal is generated at output 280 of channel decoder 260 and supplied to error signal input 285 of decision means 270. Decision means 270 suppresses such error marked words by reading the presence of the error signal at input 285, and potentially misdirected information data is thereby suppressed from display in data receiver 205.

It will be appreciated that as a consequence of the encryption operation by data encryption apparatus 145, transmitter system 100 of FIG. 2 sends encoded encrypted data words through communication channel 190 that are not always

BCH (31 , 21 with parity bit) words. That is, after encryption by apparatus 145, the resulting encoded encrypted data words are likely to be words that are outside the code space of BCH (31 ,21 with parity bit) code words. Since it is known from the construction rules for this particular code that the minimum distance between code words is 4 bits, there is a high likelihood that the encrypted words will resemble code words having two or more errors. Fewer than 20 percent of all possible 32 bit word combinations are within the code space BCH (31,21 with parity bit) code words.

Only two error bits can be corrected according to BCH construction rules, but paging receivers are typically restricted to correcting only one such error bit in the interest of simplicity. Data words decoded by channel decoder 260 and found having two or more bit errors are thereby marked as invalid data words which are suppressed by decision means 270.

Where it is desired to send only valid POCSAG code patterns through communication channel 190, all data words sent though channel 190 must be valid code words within the code space of the BCH (31 ,21 with parity bit) code. The code word supplier 155 of transmitter 100 must, in that case operate on the entire 32 bit word using a valid BCH (31 ,21 with parity bit) encryption code word. Furthermore, the encryption process is the EXCLUSIVE OR logical operation between the 32 bit encryption code word and the 32 bit data word. It will be apparent to those skilled in the art that since the BCH (31,21) code is of the cyclic code type, an EXCLUSIVE OR logical operation between any two BCH (21,31) code words is another BCH (31,21) code word.

Correctly coded POCSAG signal patterns are thereby sent through communication channel 190. A falsely responding data receiver would decode and decrypt such a signal with no errors (except for errors due to channel noise), but the resulting displayed message will appear garbled because message words supplied by word generator 125 are mapped into other words by data encryption apparatus 145. At data receiver 205, those mapped words are again mapped into still different words. Although the resulting message is displayed to the user, such an unintended message is rendered unreadable by the encryption process.

An alternative embodiment of the invention which is equivalent to encrypting with 32 bit BCH (31 ,21 with parity bit) encryption words is shown in FIG. 3. Comparing the embodiments of FIG. 2 with the embodiment of FIG.3, like numbers are used for like elements. In FIG. 3, encryption of data by data encryption apparatus 145 precedes the BCH encoding operation of channel encoder channel 110. That is, the relative positions of the data encryption and the data encoding have been changed compared with the embodiment of FIG. 2. In FIG. 3, data words are first encrypted, then encoded, so that only valid BCH (31 ,21 with parity bit) POCSAG signal patterns appear at output 170 of transmitter system 300, and thereby in communication channel 190. A corresponding change in the relative positions of channel decoder 260 and data decryption apparatus 240 is seen in data receiver 305 of FIG. 3 as compared to the like elements of FIG. 2. In FIG. 3, channel decoder 260 of data receiver 305 precedes data decryption apparatus 240. Data words supplied by communication channel 190 are thereby first decoded by channel decoder 260, then decrypted by data decryption apparatus 240. Encryption apparatus 155 of data transmitter system 300 maps data words supplied by message generator into other words by the EXCLUSIVE OR logic operation between the encryption code word supplied by encryption code word supplier 155 and the data word supplied by message word generator 125. Channel encoder 110 then encodes the encrypted data words and supplies a POCSAG signal pattern to output terminal 170 of data transmitter system 300 for transmission through the communication channel 190 to input 200 of data receiver 305 and subsequently to channel decoder 260. Channel decoder 260 decodes the POCSAG signal pattern, checks bit errors, and corrects at least one bit error if required. Uncorrectable error words cause the generation of an error detect signal at output 280 of channel decoder 260. The error signal is supplied to input 285 of decision means 270 so that such uncorrectable error words may be suppressed from display on display apparatus 290. Such uncorrectable error words are associated with channel noise in communication channel 190. Words with no errors or with corrected errors are decrypted in decryption apparatus 240 by the EXCLUSIVE OR logic operation between the 21 bit data word supplied by channel decode 260 and the 21 bit decryption code word supplied by decryption code word supplier 250. Decoded decrypted words are then supplied to display apparatus 290 for display to the user of data receiver 305. The encryption operation provided by data encryption apparatus 145 and encryption code word supplier 155 of data transmitter systems 100 and 300 are best understood by reference to FIG. 4. Data encryption apparatus 145 is supplied with binary data words at its input 140. The binary data words have a bit pattern represented by bit mask 350. A single leading binary bit 365 is a binary "0" when the binary data word is an address word or is a binary "1" when the binary data word is a message data word as required by the POCSAG signaling pattern. In the event that bit 365 is a "0" signifying an address word, data switch 370 routes that address word directly to output 165 of data encryption apparatus 145. When binary bit 365 is "1" signifying a message word, data switch 370 routes the message word to input 375 of EXCLUSIVE OR gate 380. An encryption code word uniquely associated with this message word is supplied on input 150 of apparatus 145. The encryption code word conforms to encryption mask 390 having a leading ^H0" bit in leading bit position 395. The encryption code word in encryption mask 390 is supplied to second input 385 of EXCLUSIVE OR gate 380. Gate 380 provides means for performing a bit by bit EXCLUSIVE OR operation between the corresponding bits in mask 350 and mask 390 and supplies output bit mask 400 with the encrypted result. Output bit mask 400 provides an output to output 165 of data encryption apparatus 145. The leading bit in encryption bit mask 390 is always a "0" so that the EXCLUSIVE OR operation by apparatus 380 always results in a ^M1" leading bit 405 in output bit mask 400.

Encryption code word supplier 155 supplies a unique encryption code word for every new address generated by address supplier 130 (shown in FIG. 2). One method of supplying an encryption code word is from a look up table 410 which associates each address supplied at input 160 of encryption code word supplier 155 with a unique code word 415 which is then supplied to input 150 of encryption apparatus 145. Those skilled in the art will recognize that alternative methods of choosing an encryption code word will work equally well.

The decryption operation provided by data decryption apparatus 240 of data receivers 205 and 305 are best understood by reference to FIG. 5. Data words supplied to input 450 of data decryption code words supplied by decryption code word supplier 250 at input 255 conform to code word mask 460. An

EXCLUSIVE OR gate 465 constitutes means for performing the EXCLUSIVE OR operation on the corresponding bits in masks 455 and 460, and supplies the result as output 470 of data decryption apparatus 240. The decryption code word is supplied from code word table entry 475 of table 480 in code word supplier 250. A unique decryption code word is associated with each address supplied by address supplier 230 (shown in FIG. 2 and FIG. 3). A data receiver having four addresses has correspondingly four unique code words in table 480.

I claim as my invention:

Claims

1. A receiver for use in a data transmission system in which address signals and BCH encrypted encoded data are transmitted comprising: a receiver input terminal for receiving the address signal and BCH encrypted encoded data, an address detector for detecting the address signal received by said receiver input terminal, a data decryption means operatively coupled to said receiver input terminal, and including means for supplying a BCH decryption code word, said decryption means performing an EXCLUSIVE OR operation between the BCH encrypted encoded data supplied to said receiver input terminal and the BCH decryption code word to provide decrypted encoded data, and a channel decoder means for decoding said encoded decrypted data to produce decoded decrypted data.

2. A receiver as defined in claim 1 , in which: said encoded data is encoded as POCSAG data.

3. A receiver as defined in claim 1 , in which: the address detector detects a plurality of addresses, and the means for supplying a decryption code word provides one of a plurality of decryption code words based upon the detected address.

4. (amended) A receiver for use in a data transmission system in which address signal and BCH encoded encrypted data are transmitted comprising: a receiver input terminal for receiving the address signals and BCH encoded encrypted data, an address detector for detecting the address signal received by said receiver input terminal, a channel decoder means operatively coupled to said receiver input terminal for decoding said BCH encoded encrypted data to produce decoded encrypted data, and a data decryption means including means for supplying a decryption code word, said decryption means utilizing the decryption code word to decrypt said decoded encrypted data and provide decoded decrypted data.

5. A receiver as defined in claim 4, in which: said encoded data is encoded as POCSAG data.

6. A receiver as defined in claim 4, in which: the address detector detects a plurality of addresses, and the means for supplying a decryption code word provides one of a plurality of decryption code words based upon the detected address.

7. A receiver as defined in claim 4, in which: said data decryption means includes means for performing an EXCLUSIVE OR operation between the decoded encrypted data and the decryption code word.

8. A transmitter comprising: means for supplying an address signal, a word generator means for supplying data words, a channel encoder means for encoding said data words and providing BCH encoded data words, a data encryption means including encryption code word supplying means responsive to said address signal supplying a BCH encryption code word, and utilizing a supplied BCH encryption code word to encrypt said encoded data words, performing an EXCLUSIVE OR operation between said encoded data and said encryption code word, and supplying the BCH encoded encrypted data for transmission.

9. A transmitter as defined in claim 8, in which: said channel encoder means encodes said data as POCSAG data.

10. A transmitter comprising: means for supplying an address signal, a word generator means for supplying data, a data encryption means including encryption code word supplying means responsive to said address signal supplying means for suppling an encryption code word, and utilizing a supplied encryption code word to encrypt said data, and a channel encoder means for encoding said encrypted data and providing BCH encoded encrypted data, and supplying the BCH encrypted encoded data for transmission.

11. A transmitter as defined in claim 10 in which: said data encryption means includes means for performing an EXCLUSIVE OR operation between said data and said encryption code word.

12. A transmitter as defined in claim 11 , in which: said channel encoder means encodes said encrypted data as POCSAG data.

13. A transmitter as defined in claim 10, in which: said channel encoder means encodes said encrypted data as POCSAG data.

14. A method of transmitting a data signal comprising the steps of: providing an address, providing data, both encrypting and encoding the data, and transmitting the address and the encrypted and encoded data, the encrypting of the data comprising performing an EXCLUSIVE OR operation between the data and the address.