RADIO PAGER WITH ERROR DETECTION

Abstract: A transmission for a radio pager includes ''label'', ''address'' and ''message'' signals, preferably in that order. The pager stores the label and message signals in buffers (16, 18) and if the appropriate address is detected by a comparator (12), the stored label signal is compared with previously-received label signals stored in a label memory (22). If the label comparison indicates that the message (and associated label) have already been received, and error checking procedures show that errors are present in the received signals, a message combiner (28) pieces together a corrected message signal from appropriate parts of the currently-received and stored message signals, which corrected signal is then stored at the appropriate location in the message memory (24). If the label comparison indicates that the message (and associated label) have not previously been received, the message signal is stored in the message memory (24) and the label signal is stored in a corresponding portion of the label memory (22).

Description: RADIO PAGER WITH ERROR DETECTION .This invention relates to a radio pager, and more particularly to a radio pager having the capability of detecting errors in its received messages. A number of radio paging systems include some error detecting feature whereby small errors arising from such causes as weak transmissions or interference can be either corrected or at least detected as errors. For example, in the format devised by the British Post Office Code Standardization Advisory Group (POCSAG) and adopted by CCIR as Radiopaging Code No. 1, a series of address signals follows a preamble signal, with regular synchronisation signals being interspersed. An address signal for a particular pager or group of pagers may then be followed by a message signal which can be displayed by the pager. Fuller details are given in "A Standard Code for Radiopaging - a Report of the Studies of the British Post Office Code Standardisation Advisory Group (POCSAG)" published in 1978 by British Telecom (then part of the British Post Office). In Appendix 1 (page 11) of this publication, details are given of codeword generation which is of the form 31, 21 BCH + Parity. It is common for pagers to be able to identify and correct a single bit error on the basis of the check bit information. However, correction of further bits can only be done with a decreasing probability that the process has been successful, although it is often possible to predict that an error is present. In so-called wide area paging systems, it is known for a number of transmitters to transmit the same message sequentially, so as to increase the probability of the correct message being received. If the pager is situated in an overlap zone, the same message may be received (for example) three times but, each of the three transmissions may be subject to some degradation, since such an overlap zone may be on the fringes of transmission range. Pagers capable of storing more than one message tend to fill up with such repeat messages, unless some message comparison process is provided. Even then, the comparison will only successfully reject further messages if no errors are present, otherwise the messages will not be found to be identical. The effect of this is that a user will read two near-identical messages and may have no indication (for example, in the case of telephone numbers) which is correct. Furthermore, particularly in a fringe overlap zone, it is possible that none of the messages is correct, although statistically it will be unlikely that errors will have been introduced at the same positions within each message. According to the present invention there is provided a radio pager for receiving a transmission that comprises a plurality of selective address signals, a plurality of respective message signals which include error checking information capable of providing an indication of the position of errors within the message signals, and label signals each providing an indication of a message code associated with the respective message signal, the radio pager comprising: address detection means operative to detect a predetermined said address signal, a memory operative to store at least one message signal and label 'signal associated with respective detected predetermined address signals, comparing means operative to compare the or each label signal stored in the memory with the currently-received label signal and to provide an indication when the message codes of the compared label signals are the same, and , Jl error correcting means responsive to an indication from the comparing means that the compared message codes are the same to provide a corrected message signal derived from the stored and currently-received message signals having the same message codes, by using parts of each of the message signals for which the error checking information indicates that there is no error. According to the present invention there is also provided a radio pager for receiving a transmission that comprises a plurality of selective address signals, a plurality of respective message signals, and label signals each providing an indication of a message code associated with the respective message signal, the radio pager comprising: address detection means operative to detect a predetermined said address signal, a memory for storing at least one message signal and label signal associated with respective detected predetermined address signals, and comparing means operative to compare the or each label signal stored in the memory with the currently-received label signal and to provide an indication when the message codes of the compared label signals are the same, the memory being operative to store the message signal only if the comparing means does not indicate that the message codes of the compared label signals are the same. It will therefore be apparent that the error correcting means is capable of deriving the corrected message signal by judicious combination of parts of message signals identified by their labels as being the same, the error checking part of each message signal indicating where in the signal errors are ' present, so that those errors are not included in the composite corrected signal. In certain formats, such as the POCSAG format, each message signal comprises a number of message codewords, each having its own. error correction capability. This capability can be utilised (as . previously described) to provide a degree of error correction, but if insufficient, the error correcting means of the present invention will provide a further correction mechanism. It is necessary for the transmitting system to assign a message code, which can be a simple message number, to each message when it is received from a caller so that when the message is transmitted at different times by different transmitters, the same message number will be transmitted each time, and this will provide a positive result in the comparison process. In certain systems, this message label could be related to a global variable such as time of day. The label can conveniently be provided as a "header" portion to the message signal, which allows easy processing since the label comparison can be performed as soon as the address signal has been identified as relating to the particular pager. Alternatively, the label may instead be included at some other location within the received signal, for example it may be interleaved with the message signal or even the address signal. However, the presently-preferred arrangement is for the label to be transmitted as a codeword before the address codeword. Where a multiple message storage capability is provided, the comparison step ensures that only messages having different message numbers are stored each time, whereas if the message numbers are the same, the error correcting means will check for message integrity and, if necessary, derive an improved or corrected message and then store that improved or corrected version. Thus the performance of the pager is considerably enhanced. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 shows a schematic block diagram of one embodiment of the invention having a multiple message storage capability; and Figure 2 shows a schematic block diagram of a second embodiment. Referring to Figure 1, a radio pager includes an address comparator 12 receiving a signal from an RF receiver and preamble detector section (not shown) of the pager. In some formats including the POCSAG format, synchronisation signals are provided at regular intervals, and the circuit should be capable of detecting these signals so as to attain synchronisation with the received signal. The circuit will be described with reference to an adaptation of the aforementioned POCSAG signal format, but any other format having an error detecting facility and some form of address code discriminating ability could be adapted instead. In the case of the U.K. POCSAG format, the label signal needs to be included somewhere in the code, and a convenient position is as a "header" portion to the address and message signals, in other words it is transmitted just before the address signal and the message signal, as shown in Figure 1. Accordingly, the label signal is the first to be received and this signal is fed from the RF receiver section to a label buffer 16 for temporary storage. The next signal in the transmission is the address code. The address comparator 12 compares the received address code from the RF receiver section with its own stored address or addresses 14 under the control of a timing controller 10 which ensures that all operations are correctly synchronised. When the comparison made by the address comparator 12 is positive, the timing controller 10 receives an enabling signal as a result of which the timing controller allows the message signal (which follows the address) to be stored in a message buffer store 18. Conventional error detection and/or error correction techniques are used at this stage so that uncorrcctable message errors can be determined. Typically, if one or more uncorrectable errors have been detected, error data will be added to the message data in the message buffer store 18, the error data indicating where uncorrectable errors are present in the message signal. If the comparison made by the address comparator 12 indicates that the message is not intended for the particular receiver, the next label signal is admitted to the label buffer store 16 to replace the previous stored label signal. Also the message signal is not accepted into the message buffer store 18. A label comparator 20 compares the label signal in the label buffer store 16 with previously received and stored label signals in a label memory 22 having sufficient capacity for a particular number of label signals to be stored (four as shown, at locations A-D). Associated with the label memory 22 is a message memory 24 having the capacity for an equivalent number of message signals to be stored. If the label comparison shows that the message associated with the particular label signal has not yet been received and stored, the label and message signals are respectively loaded into suitable locations in the label and message memories 22, 24. ' If however the comparison shows that the message has already been received, a switching means 26 causes the message signal to be fed to a message combiner 28 instead. Also, the equivalent stored message signal is read out of the appropriate location, say C, of the message memory 24, again to the message combiner 28. The message combiner 28 analyses the two message signals and, if differences are present, and the check bit information in the code is sufficient to indicate the position of the or each erroneous codeword, a process of judicious combination of parts of each message signal can provide a corrected composite message signal. This signal can then be loaded back into the same location in the message memory 24. The contents of the message memory 24 can be displayed on a display 30, or sent to a printer 32 or to another output device (not shown). Although it was previously stated that such operation is particularly advantageous in overlapping zones of a wide area paging system, where the repeat messages come from different transmitters, it will be seen that the system is equally applicable to a situation where messages are repeatedly transmitted from a single transmitter. Operation will be identical and equally effective, as long as the same message number is assigned in the label signal of each transmission, so that the message correction can be effected. A significant advantage of the described system is that, although an amount of correction capability is already inherent in certain codes, such as the POCSAG format, even if repeat messages are received, it is not possible to ascertain whether uncorrectable messages are the same or not. The present system provides this capability. It is statistically unlikely that the same portions of repeat messages will be degraded, since such degradation in transmission is a random process. In the POCSAG format or similar formats, to allow effective error correction of the above type, the system must be able to determine whether or not the incoming message data is a repeat of a previously received message, and this information is provided by message numbering in the label signal. In addition, it must be able to determine the total message length, so that interference cannot cause premature termination of the decoding process. This information may be part of the transmitted label or may be programmed into the decoder where fixed, length messages only are involved. In the 31, 21 BCH + parity code (which forms the basis of the POCSAG code) the information for both of the above requirements may be transmitted in a single code word. In any given system where surplus capacity exists in this codeword, fixed data patterns should be transmitted in order to reduce the probability of errors. Equally if the address codeword capacity is not fully utilised, then the information regarding message length and repetition could be included here as the label signal. The label signal will not generally be displayed by the paging receiver. The transmission system will generate the labels. If the receiver decoder determines from the labels that the same message has been received more than once, then it will construct the best common message as described above. This technique also overcomes the problem of wasteful data storage in the receiver due to reception of the same message emanating from multiple transmitters operating sequentially when the receiver is in an overlap zone. The preferred signal format of "label", "address" and "message" has the advantage that in existing formats, such as POCSAG, it is possible to have compatible transmissions to receivers whether provided with the label detection facility or not. No change to the basic format is necessary. A receiver not having the label detection facility will simply ignore any transmitted label code and check the address code to see whether the receiver's address is present or not, in conventional manner. Where such compatibility is not required, a simpler receiver can be provided, for example as shown in Figure 2. The receiver of Figure 2 is capable of receiving a signal format of the type "address", "label" and "message". The circuit is simpler since label and message storage need not take place unless the address comparison is positive. Similar reference numerals designate similar parts and the basic operation of the circuit is similar to that of Figure 1, once a positive address comparison has been made and accordingly label and message data have been read into the respective buffers 16, 18. In both Figure 1 and Figure 2 embodiments (as well as any others), the label signal may additionally offer a means of system control of the receiver, for example transmitting a certain label signal may switch particular receiver facilities on or off. The following features may, for example, be included. 1. Switching, the pager into and out of different operational modes. These modes could, for example, be different battery economy cycles or data rates. 2. Adding or removing the ability of the receiver to respond to certain predetermined addresses. 3. Enabling an address of the receiver to be programmed via the radio link. In this case the new address would be part of the received message data. 4. The label may also be used for updating a particular message previously transmitted. In addition, the label may include check bits calculated from the total message so that the probability of an incorrect decoding of the message is further reduced.

Claims: CLAIMS 1. A radio pager for receiving a transmission that comprises a plurality of selective address signals, a plurality of respective message signals v/hich include error checking information capable of providing an indication of the position of errors within the message signals, and label signals each providing an indication of a message code associated with the respective message signal, the radio pager comprising: address detection means operative to detect a predetermined said address signal, a memory operative to store at least one message signal and label signal associated with respective detected predetermined address signals, comparing means ' operative to compare the or each label signal stored in the memory with the currently-received label signal and to provide an indication -when the message codes of the compared label signals are the same, and error correcting means responsive to an indication from the comparing means that the compared message codes are the same to provide a corrected message signal derived from the stored and currently-received message signals having the same message codes, by using parts of each of the message signals for which the error checking information indicates that there is no error. 2. A radio pager according to claim 1, wherein buffer stores are provided for temporarily storing the currently-received message and label signals. 3. A radio pager according to claim 1, wherein the transmissions to be received include the label signal, the address signal and the message signal in that order, the address detection means being operative to enable further processing of the label and message signals when the predetermined address signal is detected. 4. A radio pager according to claim 3, wherein buffer stores are provided for temporarily storing the currently-received message and label signals, the label signal being sent to the comparing means upon detection by the address, detection means of the predetermined address signal, the message and label signals stored in the buffer stores being updated upon receipt of new message and label signals. 5. A radio pager according to claim 1, wherein the label signal is arranged to provide a control function in the receiver in addition to the error correcting function. 6. A radio pager for receiving a transmission that comprises a plurality of selective address signals, a plurality of respective message signals, and label signals each providing an indication of a message code associated with the respective message signal, the radio pager comprising: address detection means operative to detect a predetermined said address signal, a memory for storing at least one message signal and label signal associated with respective detected predetermined address signals, and comparing means operative to compare the or each label signal stored in the memory with the currently-received label signal and to provide an indication when the message codes of the compared label signals * are the same, the memory being operative to store the message signal only if the comparing means does not indicate that the message codes of the compared label signals are the same.

Inventors: LAX ALEXANDER PETER, SPIERS JOHN, LAX, ALEXANDER, PETER, SPIERS, JOHN
Applicants: MULTITONE ELECTRONICS PLC
Application year: 1986
Main classification: H04Q7
IPC codes: G08B5, H04L1, H04W88