GB2100486A - Radio receiver (pager) decoding system - Google Patents

Abstract

A decoding system is provided as part of a portable radio paging unit which is capable of detecting and storing up to four different alerting signals per user. Each user's pager is uniquely addressable. The decoding system is responsive to a transmission protocol in which the transmission signal includes a preamble to enable bit-synchronization, a recurring synchronization codeword, and one or more address codewords within a batch of codewords transmitted between each synchronization codeword a recurring synchronization codeword separating first, second and subsequent batches of codewords in a plurality of frames, characterised in that the systeme has means for decoding after the receipt of an erroneous synchronization codeword or the loss thereof at its immediate specific decoding frame by virtue of the known time in the transmission between an obtained synchronization codeword immediately before the erroneous or lost synchronization codeword and said specific decoding frame immediately following said erroneous or lost synchronization codeword.

Description

SPECIFICATION Improved radio receiver (pager) decoding system and pager for use therewith This invention relates to a radio receiver (pager) decoding system and a pager arranged to deploy said system. One system producing a code format for use in wide area radio paging is outlined in Appendix A ofA Standard Code forRadiopaging (A report of the studies of the British Post Office Code Standardisation Advisory Group (POCSAG) 1978/1979.

The specification for a standard code format for use in the POCSAG system is now well known the signal format being a transmission comprising inter alia a preamble followed by batches of complete codewords, each batch beginning with a synchronization codeword (SC).

It is known that the transmission of such a system when in use is vulnerable to fading and loss of definition from impulsive noise. It is highly desirable to accept fading and even the loss of a synchronization codeword without the loss of the ability to decode.

This desideratum is achieved according to the present invention by the provision of a radio receiver (pager) decoding system arranged to receive a transmission signal that comprises a periodically recurring synchronization codeword separating first, second and subsequent batches of codewords in a plurality of frames, characterised in that the system has means for decoding afterthe receipt of an erroneous synchronization codeword or the loss thereof at its immediate specific decoding frame by virtue of the known time in the transmission between an obtained synchronization codeword immediately before the erroneous or lost synchronization codeword and said specific decoding frame immediately following said erroneous or lost synchronization codeword.

In a preferred system decode operations are terminated after two consecutive erroneous synch ron- ization codewords are received or lost and are not taken-up again until a synchronization codeword is correctly received.

The invention will be more fully understood from the following description given by way of example only with reference to the figures of the accompanying drawings in which: Figure lisa diagram of perfect signal format.

Figure 2 is a diagram similar to that of Figure 1 but showing a signal format with the receipt of an erroneous or the loss of a synchronization codeword (SC) and the ability of the system of the invention to decode.

Figure 3 is a block diagram showing an embodiment of a pager for incorporation of the system of the present invention.

Figure 4 is a flow diagram of operations to be performed by one decoding circuit.

Figure 5 is a detail flow diagram relating to a part of Figure 4.

Figure 6 is a schematic.

In Figure 1 is shown the well known POCSAG signal format in which a transmission comprises a preamble followed by batches of complete codewords, each batch beginning with a synch ron- isation codeword (SC).

Each transmission starts with a preamble to permit a pager to attain bit synchronization and to prepare it to acquire word synchronization. The preamble is a pattern of reversals, 101010... repeated for a period of at least 576 bits, i.e. the duration of a batch plus a codeword.

In any batch codewords are transmitted comprising a synchronization codeword followed by 8 frames each containing 2 codewords. The frames are numbered 0 to 7 and the pager population is similarly divided into 8 groups. Each pager is allocated to one of the 8 frames according to the 3 least significant bits (Isb) of its 21 bit identity and will only examine address codewords in that frame. Therefore each pager's address codewords must only be transmitted in the frame that is allocated to those codewords.

Message codewords for any receiver may be transmitted in any frame but will follow, directly, the associated address codeword. A message may consist of any number of codewords transmitted consecutively and may embrace one or more batches but the synchronization codeword must not be displaced by message codewords. Message termination is indicated by the next address codeword or idle codeword. In any frame an idle codeword will be transmitted whenever there is no address codeword or message codeword to be transmitted.

In Figure 2 the signal format is extended and shown to a reduced scale. Preamble (P) commences and is followed by synchronization codeword SC, and Sic,, SC4, SC5, SC6; SC3 and SC7 are erroneous or lost owing to impulsive noise, but decoding is still achieved in the batch following SC3 and SC, since both the respective decode frames are at predetermined time distance Z from SC2 and SC6.

In Figure 3 a block diagram of a pager unit, includes a decoding circuit The radio pager comprises a receiver unit 10 having a binary detector 12, which is responsive to receiver 10 and is also responsive to a data enable signal from a microprocessor unit 20 over line 11 and which transmits received pulses to unit 20 over line 13. Unit 20 in one embodiment of the invention is an MC146805 8-bit microprocessor by Motorola.

Also coupled to unit 20 are an identity ROM (read only memory) 22, a program ROM 24, and a support I.C. 26. The identity ROM 22 is programmed, using fusable links, for example, with the unique identity number of the specific pager. The identity number has two separate parts (1) a base identity code and (2) the frame number which defines the relative position within a batch at which the frame assigned to the specific pager appears. Each pager in the pager population is assigned one of eight possible frame numbers within a batch. The identity ROM 22 is plug-interchangeable.

Each pager can respond to four different paging addresses and accordingly generates four distinguishable alerting tones. The four different paging addresses are calculated by the unit 20 utilising the base identity code contained in the identity ROM 22, as will be described in greater detail below.

The program ROM 24 contains the computer prog ram which controls the operation of the unit 20 to carry out the desired decoding functions of the decoding circuit. The portion of the computer program which specifically pertains to the decoding system of the present invention is attached hereto in source code form as given in Appendix A to be understood with Figure 5.

The support l.C. 26 is a general purpose support circuit which, in the context of the present invention, performs three functions. First, it generates and regulates the secondary supplyforthe unit 20, the identity ROM 22, and the program ROM 24. Secondly, it contains a timer circuit known as the "deadman timer" (DMT), whose function is to reset the unit 20 after a predetermined interval in the event that a specific instruction either is not received or remains in the instruction register for an excessive period.

The function of the "deadman timer" is to prevent the decoder circuit from being held up in a lengthy software loop. Thirdly, the support I.C. 26 tests the primary power supply level on command and returns an indication of its condition.

Unit 20 generates the paging alert signals over line 27 to audio frequency amplifier 28 and ultimately transducer 30. A power supply switch 36 is responsive to an enable signal from unit 20 over line 35.

When the power supply switch 36 is enabled, the receiver power supply is connected to receiver 10 and to binary detector 12 over line 37. Battery 40 provides the primary power supply. Switch 38 is a user-operated onloff switch. Switch 32 is a "cancel" switch, the closure of which cuts short a sounding paging alert. Switch 34 is a switch used to make a selection between "normal mode" and "memory mode". In "memory mode" upon receipt of a paging request the pager does not generate the paging alert, but it stores the page request in a memory allocated for such purpose in the unit 20. When switch 34 is switched over to the "normal mode", or when the "cancel" switch 32 is depressed at any time, any page requests stored in memory are sequentially read out and their corresponding audible alerts (each uniquely distinguishable) are generated to the pager user.

In Figure 4 is given by means of an orthodox flow sheet, understood by one skilled in the art of wide area radio paging, a logical sequence for use by the unit 20 of Figure 3 which is as given below: 101 is turn on 102 is battery test 103 is valid battery? 104 is initialize 105 is audio bleep on completion of initialize 104 106 is bleep cancelled? 107 is bleep complete? 108 is bit synchronization 109 is page received? 110 is bit synchronization achieved? 111 is word synchronization 1 112 is word synchronization 1 achieved? 113 is battery save 114 is decode 1 115 is decode 2 116 is battery save 117 is page re 0lived? 118 ismemor ,node? 119 is delay 120 is word synchronization 2 I21 is word synchronization 2 achieved? 122 is clear flag 123 is flag set 124 is set flag 125 is decrement count 126 iscount equals zero? 127 is delay 128 is memory mode? 129 is set asynchronous mode 130 is memory interogate 131 is set asynchronous mode 132 is page received? 633 is test for page function 1,2,3 or 4 134 is requisite audio tone 135 is page cancelled? 136 is asynchronous mode? 137 is bit synchronization and decode 138 is bit synchronization and decode 139 is eight second tone complete? 140 is clear tone flag 141 is memory mode? After switch-on the battery is checked and the pager initialized, (A) (101-107). This consists of setting up registers and obtaining identity and operational information from the identity ROM 22 (Figure 3).The pagerthen proceeds to lookforthetransmis- sion characteristics and attempt synchronization to bit stream 108. After obtaining bit synchronization initial word synchronization is sought (111). When this has been successfully achieved decode is attempted in the appropriate frame ofthetransmis- sion B (113-116). Following this it is necessaryto re-establish word synchronization in a routine called Word Synchronization 2 (120). This routine also optimizes bit synchronization.

Word synchronization 2 checks the number of errors in the received synchronization codeword and if less than or equal to the error criterion clears the excess error flag and returns (122) (F) to decode. If the number of errors is greater than the prescribed error criterion then the flag is checked to see if it is already set (123). If clear the flag isthen set (124) and the routine returns to decode. If the flag is already set (YT(123), indicating that the previous synchronization word was received with excessive errors, the routine returns to the bit synchronization and transmission squelch routine (108)withoutdecoding.

Thus decode operations are terminated after two consecutive erroneous synchronization codewords have been received.

The essential steps of the invention reside pne- limarily between points Q1 and Q2 generally of the flow sheets as shown in Figures4 and 5.

In Figure 5 the sequence is asfollows: 201 is JSR WD SYN2 202 is subroutine WD SYN2 203 is 4, BFLAG clear? 204 is GNOWD2 205 is wait 206 is reset timer, DMT reset receiver off 207 is wait 208 is zero, GFLAG 1 clear? 209 isGNWHOP 210 is set zero GFLAG 1 211 is BRA to GWDOK 1 212 is clear zero, GFLAG 1 213 is reset timer 214 isJMP GBATOK 215 is clear zero GFLAG 1 216 is BRA, GWDOK1 The advantage of the system of the invention is shown by the following table in which the performance in continuous noise for three bit error rates for a system of the flow sheet minus that part between 02 and O3 and a system with the refinement of the present invention are compared respectively in columns A and B.

A B Bit Error Rate Success Rate Success Rate .05 .618 .697 .1 .134 .222 .15 .015 .043 It is to be understood that the system of the invention gives decoding after the erroneous receipt or loss of one synchronization codeword (SC) such as SC, or SC7 for example (Figure 2) but will not give decoding if the noise and the loss extends to a second consecutive synchronization codeword and will then revert to the acceptance of the next synchronization codeword (SC) that is obtained correctly, this is shown schematically in Figure 6.

SC1 is correct and decode occurs at d1.

SC3 is lost, but decode occurs at d in harmony with the present invention.

SC, is lost and there are too many erronious synchronization codewords to give a decode atX.

SC4 is correct and decode occurs at d,.

SC5 is lost, but decode occurs at d4 in harmony with the present invention.

Note SC1 to d: and SC4 to d4 are at a predetermined time period Z as shown more fully in Figure 2. Appendix A

1AOB CD 1BC1 A JSR WDSYN2 1AOE 09 28 16 1A27 BRCLR 4,BFLAG, 1A11112B A BCLR 0,GFLAG1 1A13 20 DO 19E5 BRA GWDOK1 1A158F GNOWD1 WAIT 1A16 DMTOFF 1A1C8F WAIT 1A1 D 8D NOP 1A1E9D NOP 1 A1 F 9D NOP 1A20 9D NOP 1A21 9D NOP 1A221C09 A BSET 8,TIMCON 1A24CC19BD A JMP GBATOK 1A27 8F GNOWD2 WAIT 1A28 DMTOFF 1A2E8F WAIT 1A2F 01 2B 0C 1A3E BRCLR 0, GFLAG1,GNWHOP 1A32112B A BCLR 0,GFLAG1 1A349D NOP 1A359D NOP OA369D NOP 1A379D NOP 1A389D NOP 1A39 1C 09 A BSET 8, TIMCON 1A3BCC19BD A JMP GBATOK 1A3E 10 2B AGNWHOP BSET 0,GFLAG1 1A40 20 A3 19E5 BRA GWDOK1

Claims (3)

1. A radio receiver (pager) decoding system arranged to receive a transmission signal that comprises a periodically recurring synchronization codeword separating first, second and subsequent batches of codewords in a plurality of frames, characterised in that the system has means for decoding after the receipt of an erroneous synchronization codeword or the loss thereof at its immediate specific decoding frame by virtue of the known time in the transmission between an obtained synchronization codeword immediately before the erroneous or lost synchronization codeword and said specific decoding frame immediately following said erroneous or lost synchronization codeword.

2. A radio receiver (pager) decoding system substantially as hereinbefore described.

3. A radio receiver (pager) decoding system substantially as hereinbefore described and as shown in the figures of the accompanying drawings and the appendix.