GB2136179A - Pager Decoding System - Google Patents

Abstract

A pager decoding system which is suitable for use with signal formats, such as POCSAG, in which code words are sent in batches, each batch containing a synchronisation code word and (n-1) address/message code words, n being the number of code words in a batch, each code word comprising m bits. The pager is able to detect or regain a synchronisation code word when the carrier signal is lost due to fading. In a fade recovery mode, (m-1)th, mth, and (m+1)th bit positions are searched to see if a preamble or synchronisation code word is present. If none is detected after a predetermined number of batches has been searched then the pager assumes a loss of synchronisation mode. In this latter mode input data received by the pager is combed through by switching-on the pager for a duration of m bits at intervals corresponding to (n+1) code words. The data received is stored in a shift register 108 having m stages and in which a synchronisation code word can be assembled within two batch periods. Once the synchronisation code word has been detected the pager can be switched to a normal data receive mode. <IMAGE>

Description

SPECIFICATION Pager Decoding System The present invention relates to a pager decoding system for use in a paging system employing a standard code such as the CCIR Radiopaging Code No. 1 otherwise known as POCSAG (British Post Office Code Standardisation Advisory Group).

Although the POCSAG code is becoming widely known, in order to understand the present invention it is worth mentioning the signal and code word formats of POCSAG and in this respect reference is made to Figures 1 and 2 of the accompanying drawings. Figure 1 shows the signal format which comprises a preamble 10 of at least 576 bits, which comprises alternate ones and zeroes and a series of batches 1 2, 1 4 each of 544 bits. The preamble 10 at its shortest has a duration of a number of bits corresponding to one of the batches plus a thirty-two bit code word. A batch 12 or 1 4 comprises a thirty-two bit synchronisation code word 16 and eight frames 1 8, each of which frames comprises two code words 20 each of thirty-two bits length. Thus each batch 1 2, 1 4 is formed by seventeen code words 20 each of 32 bits.

There are two types of code words 20, there are address code words 22 and message code words 24. The first bit of a code word determines whether it is an address code word because its value is zero or whether it is a message code word because its value is one. In the case of an address code word, bits 2 to 1 9 are address bits corresponding to the eighteen most significant bits of a twenty-one bit identity assigned to the paging receiver. The three least significant bits are not transmitted but serve to define the frame within a batch in which the address code word must be transmitted. Four discrete addresses are assigned to each paging receiver having a given twenty-one bit identity, selection of a particular one of the four addresses is according to the values assigned to the bits 20 and 21.Bits twenty-two to thirty-one are cyclic redundancy check bits and the final bit, bit 32, is chosen to give even parity on the complete code word.

In the case of a message code word 24 the bits 2 to 21 are assigned as message bits which do not follow the allocations of the address code word 22, however bits 22 to 32 do.

A batch is formed by a synchronisation code word which precedes in time sixteen other code words. Since the identity of a paging receiver is defined by an address code word transmitted in a given time frame 1 8 within a batch 12, 14, then it is unnecessary for the paging receiver to receive any address code words other than those in its allocated frame. Thus the paging receiver may switch off when other frames are being transmitted thus providing a battery saving capability. In any transmission of a batch, an idle (unallocated) address code word is transmitted in the event that a particular code word location within that batch is not required for the transmission of a paging call.

A paging call requiring transmission of message code words 24 is formatted such that an appropriate number of message code words 24 related to the length of the message are concatenated onto one of the address code words 22 assigned to the particular paging receiver.

Although message code words 24 (Figure 2) may continue into a subsequent batch due to the length of the messages, the normal batch structure, that is sixteen code words 20 preceded by a synchronisation code word 1 6 is maintained.

With the POCSAG signalling structure, a paging decoder has to synchronize itself first with the preamble 10 and second with the synchronisation code word 16. Unless the paging decoder is synchronised to the synchronisation code word, it will be unable to decode successfully address code words in their assigned frame.

In operation, a paging receiver in its carrier-off mode when there are no transmissions from its base station is usually switched on once every seventeen code words for a duration of a thirtytwo bit code word in order to detect the preamble bit pattern which may be transmitted. As the preamble for POCSAG is at least eighteen code words in duration it will quickly be detected.

Thereafter the paging receiver is continuously energised for a duration of eighteen code words in order to detect the synchronisation code word 1 6 which is concatenated onto the preamble 10.

Thereafter the paging receiver assumes its data receive mode and switches its receiver section off until its assigned time frame and then switches it on for that time frame in order to decode address code words. Then the decoder will be switched off until the time slot allocated to the synchronisation code word in a subsequent concatenated batch, the decoder is then switched on in order to decode that synchronisation code word and subsequently the address code word in the following allocated frame. If the synchronisation code word is not detected then the paging decoder may not decode address code words in the allocated subsequent time frame. Thus it is essential to achieve and maintain word synchronisation. It is important that synchronisation and address code words are decoded acceptably in order to keep a sufficiently lowfalsing rate.

British Patent Specification 2,086,106A discloses a pager decoding circuit with an intelligent synchronisation circuit. This known circuit employs a synchronisation strategy which tolerates at least some degree of error in an attempt to achieve batch synchronisation. The decoding circuit includes means for examining the received bit pattern in order to search initially for the presence of'preamble. When a match or near match to the preamble bit pattern is detected, the decoding circuit examines the received bit pattern for the synchronisation code word. When a match or a near match to the synchronisation code word s achieved the decoding circuit is deemed to be n batch synchronisation in which case it is then 3ble to examine the address code words in its assigned frame in order to detect the receipt of a raging call.

The decoding circuit then examines each synchronisation time slot in subsequent batches in order to detect the synchronisation code word in those batches and thereafter detect address code words in the allocated time frame within those batches. If a match between the received bit pattern and the stored reference synchronisation code word is not achieved nor a near match to a certain number of bits in error is obtained then the address frame is not examined for address code words. If the synchronisation code word is again not detected in the time slot allocated for the synchronisation code word in the next following batch, assuming that a batch has been transmitted, then the decoding circuit deems that batch synchronisation has been lost and thence reverts to its carrier-off mode in which it examines the received bit pattern for the preamble bit pattern or a near match to it.

When examining the received bit pattern for preamble the known circuit switches on for one code word slot in each batch, as before, thus guaranteeing detection of preamble if it is being transmitted. Once preamble has been detected the receiver then examines the bit pattern for the synchronisation code word. When this has been detected the known circuit assumes a data receive mode as described previously.

This known decoding circuit has two drawbacks. Firstly it cannot resume correct batch synchronisation should a long fade, greater than eighteen code words (worst case) occur, causing irrecoverable errors in the received bit pattern because the circuit will have reverted to preamble detection operation in a carrier-off mode whilst coded data is still being transmitted.

Consequently the probability of detecting preamble in coded data is very low causing batches of data (e.g. addresses) to be overlooked.

The second drawback in the operation of this known decoding circuit may occur if the paging receiver is used in a heavily loaded, zoned, transmission system. A zoned transmission system as specified within the POCSAG description would allow for the transmission of a preamble immediately concatenated to the end of a batch of code words should the paging receiver be in an overlap region of two transmission zones and the data transmission period in the former zone was continuous for the complete zone time period.Under such circumstances the known circuit would not detect the transmitted preamble (assuming that the received bit pattern is decodable as preambie) since immediately following failure to detect a synchronisation code word, as would occur at the end of transmission on one zone or in an unzoned system, the known decoder will be only examining the received bit pattern for the following synchronisation code word which will not be present because preamble is being transmitted.

It is the object of this present invention to recover and maintain batch synchronisation more effectively than is possible in the prior art system.

According to the present invention there is provided a pager decoding system suitable for use with a signal format in which code words of m bits are sent in batches of n code words, each batch containing a synchronisation code word and (n-1) address/message code words, the system comprising timing means for controlling the switching-on of a receiver section of the pager, shift register means coupled to the receiver section, preamble and synchronisation code word detecting means coupled to the shift register means, means for detecting the absence of a synchronisation code word when the pager is operating in a data receive mode, said lastmentioned detecting means producing an output signal in response to detecting the absence of a synchronisation code word such that the pager decoding system operates in a fade recovery mode in which the output signal is applied to the timing means, said timing means enabling data input to the shift register means for at least (m+2) bit periods and activating the preamble and synchronisation code word detecting means for the (rn-i )th, mth, and (m+1)th bit periods in every nth code word, whereby in response to detecting a synchronisation code word, the timing means is reset so that the pager operates in a data receive mode but if no synchronisation code word is detected after a predetermined number of batches then the pager adopts a carrier-off mode.

By being able to examine the input signal in the fade recovery mode for a predetermined number of batches, the pager decoding system is capable of recovering synchronisation in the event of a fade lasting several, up to thirty, batches whereas the cited known system reverts to a carrier-off mode very quickly which means that batch synchronisation is not achieved until a new transmission is made, i.e. preamble transmitted.

In the pager decoding system in accordance with the present invention the shift register means may have m stages, wherein in the carrieroff mode the timing means switches on the receiver section for a duration corresponding to m bits at intervals corresponding to (n+ 1) code words, the received data is concatenated in the shift register means onto the data received in the previous interval, the preamble and synchronisation code word detecting means produces output signals in response to a preamble bit pattern being detected or a synchronisation code word being detected, one of said output signals, produced in response to the detection of preamble, is used to set the timing means to wait for synchronisation code word detection, and another of said output signals produced in response to the detection of the synchronisation code word is used to reset the timing means so that the pager operates in the data receive mode.

In such a carrier-off mode the transmitted signal can be detected and properly synchronised if there has been a loss of signal due to a deep fade as well as due to the termination of a previous transmission and the commencement of a new one. Additionally the system is inherently capable of greater battery economy in the carrieroff mode because it is turned-on every (n+ 1) code words rather than every n code words as is done in known POCSAG pagers.

The present invention will now be described, by way of example, with reference to Figures 3 to 5 of the accompanying drawings, wherein: Figure 3 is a block schematic circuit diagram of a paging receiver and shows those parts which are necessary for the understanding of the pager decoding system in accordance with the invention, Figure 4 comprises waveform diagrams 4A to 4D which illustrate the change from the data receive mode to the face recovery mode, and Figure 5 comprises waveform diagrams 5A to 5E which illustrate the operation of the decoding system in its carrier-off mode.

The paging receiver 100 comprises a receiver section 102 which is turned-on and -off by a receiver power control circuit 104 which is controlled by a timer control circuit 1 06. A thirtytwo stage shift register 108 is coupled to the receiver section 102. Outputs of each of the stages of the shift register 108 are coupled to an address detector 110 and to a preamble and synchronisation detector 11 2. In the interests of clarity not all thirty-two outputs have been shown. The detector 11 2 has two outputs 11 4, 11 6 on which appear respectively output signals indicating that the preamble bit pattern and synchronisation bit pattern have been detected.

These outputs 114, 11 6 are connected to a preamble and synchronisation pulse generator 118. In response to an output signal on the output 11 4, the generator 11 8 produces a pulse on a line 120 and in a similar manner an output signal on the output 11 6 causes a pulse to be produced on the line 122. The lines 120 and 122 are coupled to the timer control circuit 106. A frame number store 124 which holds the number of the frame in which the address code word is transmitted is also coupled to the timer control circuit 106.

An address store 1 28 which stores the addresses allocated to the paging receiver 100 is coupled to the address detector 110. An output of the detector 110 is connected to an alert control circuit 1 30 which controls the energisation of an acoustic transducer 1 32. The timing control circuit has an output connected to the address detector 110.

Assuming that the paging receiver 100 is already in bit and batch synchronisation then the timing control circuit 106 causes the power control circuit 104 to energise the receiver section 102 at the synchronisation code word interval and the allocated frame interval in each batch. If an address allocated to the paging receiver 100 is detected then the alert control circuit 1 30 causes the transducer 1 32 to be energised.

The operation of the decoding system in the fade recovery and carrier-off modes will now be described with reference to Figures 4 and 5 of the accompanying drawings.

In Figure 4 the left-hand part refers to the data receive mode and the right-hand part refers to the fade recovery mode wherein the data is lost to the receiver, this is shown in broken lines in waveform diagram 4A.

In the data receive mode the preamble 10 (not shown in Figure 4) and the synchronisation code word 1 6 have been detected, the receiver is both bit synchronised and batch synchronised. This is shown in waveform diagram 4B wherein the receiver section 102 (Figure 3) is switched-on or powered-up at the intervals 40 to receive the synchronising code word 1 6. The pulse generator 118 produces synchronising pulses 42, waveform diagram 4C, at times corresponding to the end of the parity bit, bit 32, of the synchronisation code words 1 6. These pulses, when generated, are used by the timing control circuit 106 to control the switching of the receiver 102 by means of signals to the receiver power control circuit 104 for the detection of address code words and to predict the time occurrence of the synchronisation code word at the start of the subsequent batch by means of a counter within the timing control circuit 106 which produces an output every seventeenth code word in anticipation of the receipt of the subsequent synchronisation code word.

Once the synchronisation code word has been detected, the timing control circuit 106 switches off or powers down the receiver section 102 by means of the receiver power control circuit 104 until the frame 18 (Figure 1) in which its address code words are assigned, at which time the timing control circuit 106 switches on the receiver section 102 for the duration of that particular frame, this is denoted by the pulses 44 in diagram 4B. At the end of the address frame, the timing control circuit 106 switches off the receiver section 1 02 again until the start of the subsequent synchronisation code word time slot, at which time the receiver section 102 is again switched on in order to repeat the process.

When data is lost or corrupted beyond detection, say due to a fade in the received signal, as shown in broken lines in waveform 4A, the synchronisation code word cannot be detected when the receiver section 102 is switched on for the duration of the synchronisation code word time slot and no synchronising pulse is produced by the pulse generator 118. On failing to detect -the first of these synchronisation pulses, denoted by an arrow 46 in timing diagram 4D, the timing control circuit 1 06 switches the pager from the data receive mode 48 to the fade recovery mode 50. As no synchronising pulse 42 is generated, the timing control circuit 106 inhibits the receiver power control circuit 104 so that the receiver section 102 is not switched on at the assigned address frame time slot.The timing control circuit continues running and causes the receiver section 102 to be switched on every seventeenth code word time slot in order to attempt to retain synchronisation should the pager be in a deep fade such that data reception would be maintained when the fade ends. In the fade recovery mode it is not possible to determine whether true data is being received and hence whether bit synchronisation is being maintained.

Thus in order to account for any timing errors introduced, the decoding system inspects the incoming data for preamble as well as for synchronisation code word just in case the loss of data is due to a cessation of transmission of data, at the 31st, 32nd and 33rd bit positions every seventeenth code word. The fade recovery mode is maintained for a predetermined number of batches, for example thirty batches, whereupon the timer control circuit 106 switches the decoding system to the carrier-off mode. The exact number of batches during which the fade recovery mode is maintained is a function of the stabilities of the transmitter and receiver data frequency crystal oscillators and of the characteristics of the bit synchronisation technique employed.In order to detect a synchronisation code word at the 31st bit position the receiver section 102 is turned-on one bit earlier than is customary. In addition, to detect the synchronisation code word at the 33rd bit position the receiver section 102 is turned-off one bit later than is customary, In the carrier-off mode the paging receiver section 102 is switched-on and the input signal is clocked in to the 32-stage shift register 108 for 32 bit periods and then the receiver section 102 is switched-off. Outputs of the shift register stages are coupled in parallel to the preamble and synchronisation signal detector 12.Assuming that the preamble bit pattern or the synchronisation code word is not stored in the shift register 108 then precisely eighteen code words later the receiver section 102 is switchedon and another thirty-two bits are stored by being concatenated onto the already stored data. As each bit is clocked in a check is made by the synchronisation and preamble detector 11 2 to determine whether thirth-two bits of preamble or the synchronisation code word is present in the shift register 108.If preamble is detected which is indicative that a new transmission is being made then the synchronisation pulse generator 118 produces an output on the line 122 coupled to the timing control circuit 106 which maintains the receiver section 102 on until synchronisation code word is detected up to a maximum of eighteen code words duration should a synchronisation code word not be detected. If the synchronisation code word is detected, the pulse generator 11 8 produces a synchronising pulse which causes the timer control circuit 106 to switch the decoding circuit into the data receive mode.

If the loss of signal is due to a long fade then the first thing detected will be the synchronisation word. Thus by switching-on the receiver section 102 every eighteen code words, the decoder is able to comb through all the relative bit positions in a batch in seventeen batch durations. By using the shift register 118 in the manner described it is possible to pick-up the synchronisation code word in spite of the fact that the switching-on and switching-off of the receiver section 102 is not synchronised to the incoming data. How this is done will be described with reference to Figure 5.

Waveform diagram 5A illustrates the incoming data with a synchronisation code word 1 6 every seventeen code words as is usual with the POCSAG code structure. Diagram 5B illustrates the switching-on of the receiver 102 every eighteen code words. Diagram 5C illustrates the blocks of thirty-two bits of data being read into the shift register 114. The left-hand block received first comprises thirty-two bits of which some are the first part X of a synchronisation code word. The next time the receiver section 102 is turned on the first data to be read-in is the second part Y of the subsequent synchronisation code word.At the instant that the concatenated parts X and Y are configured in the shift register to form a complete synchronisation code word, see diagram 5D, the synchronisation signal detector 112 recognises that the shift register 108 contains the synchronisation code word and causes the synchronising pulse generator 11 8 to produce a synchronisation pulse, diagram 5E, which is used by the timer control circuit 106 to reset the receiver power control circuit 104 so that the pager operates in a data receive mode as before.

Although the decoding system in accordance with the present invention has been described with particular reference to the POCSAG format, it is capable with suitable adaptions to other formats having a fixed length regular batch structure, each batch comprising a synchronisation code word whose position in a batch is fixed relative to address and/or message code words, and multiples of such batches being concatenated with the preceding preamble to form a transmitted signal.

Claims (4)

1. A pager decoding system suitable for use with a signal format in which code words of m bits are sent in batches of n code words, each batch containing a synchronisation code word and (n-1) address/message code words, the system comprising timing means for controlling the switching-on of a receiver section of the pager, shift register means coupled to the receiver section, preamble and synchronisation code word detecting means coupled to the shift register means, means for detecting the absence of a synchronisation code word when the pager is operating in a data receive mode, said lastmentioned detecting means producing an output signal in response to detecting the absence of a synchronisation code word such that the pager decoding system operates in a fade recovery mode in which the output signal is applied to the timing means, said timing means enabling data input to the shift register means for at least (m+2) bit periods and activating the preamble and synchronisation code word detecting means for the (rn-i )th, mth, and (m+1)th bit periods in every nth code word, whereby in response to detecting a synchronisation code word, the timing means is reset so that the pager operates in a data receive mode but if no synchronisation code word is detected after a predetermined number of batches then the pager adopts a carrier-off mode.

2. A paging system as claimed in Claim 1, wherein the shift register means has m stages and wherein in the carrier-off mode the timing means switches on the receiver section for a duration corresponding to m bits at intervals corresponding to (n+ 1) code words, the received data is concatenated in the shift register means onto the data received in the previous interval, the preamble and synchronisation code word detecting means produces output signals in response to a preamble bit pattern being detected or a synchronisation code word being detected, one of said output signals, produced in response to the detection of preamble is used to set the timing means to wait for synchronisation code word detection, and another of said output signals produced in response to the detection of the synchronisation code word is used to reset the timing means so that the pager operates in the data receive mode.

3. A pager decoding system constructed and arranged to operate substantially as hereinbefore described with reference to the accompanying drawings.

4. A paging receiver including a decoding system as claimed in any one of Claims 1 to 3.