Abstract:
A transmission signal frame format includes an address field and a message field. The address field includes a plurality of addresses associated with the subscriber&#39;s receivers to be called, respectively. The message field includes a plurality of messages corresponding to the addresses, respectively. A message includes a transmitting sequence number which is coincident with the transmitting order of the address corresponding to this message. A selective calling receiver, receiving a frame, sequentially searches the address field for the ID address of its own while incrementing a receiving sequence count at each the address. When the ID address is found, the receiver stores the receiving sequence count at that time. The receiver selects its own message data from the message field when the transmitting sequence number of a message is coincident with the receiving sequence count.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a selective calling system, and more specifically to a format of a transmission signal, a method associated with transmitting and receiving the transmission signal and a receiver for receiving the transmission signal. 
     2. Related Art 
     As to a selective calling receiver which is usually taken on the road, the life of a battery as a power supply is a fundamental performance category. Especially, with the miniaturization of the receiver includes in recent years, a smaller battery is increasingly demanded. Therefore, how to make the battery life long becomes a more important technical problem. 
     The battery life is determined by two main parameters: the amount of the power consumption of the receiver and the intermittent receiving interval. Since a considerable portion of the power consumption is made in a radio frequency system, so far as a receiving operation is performed, one can not hope for a drastic improvement of the power consumption. Then, a method of making the intermittent receiving interval longer is proposed to reduce an average power consumption. In order to make the intermittent receiving interval longer, a synchronous system is under consideration in place of an asynchronous system (for example, POCSAG system) which is used widely at present. One example of the synchronous system is ERMES (European Radio Message System). 
     ERMES employs the following method. A frame number is given to each time division frame in advance and a base station transmits a calling signal with the frame number which a certain receiver should receive. This method allows the intermittent receiving interval to be lengthened dramatically as compared with the asynchronous system. 
     A transmitting signal of ERMES is comprised of a synchronization field, an address field and a message field. The message field is further comprised of a message and a message header indicating attributes of the message. Since the message header indicates which address the message belongs to, the transmission order of the message can be determined arbitrarily regardless of its address. For example, a paging system which is disclosed in Japanese Patent Unexamined Publication No. 63-158924 (publication date: Jul. 1st, 1988) employs a method for lengthening a receiving internal by designating the frame number which should be received. 
     However, the conventional selective calling system mentioned above needs a complicated system configuration. Further, a lot of additional bits are required in a transmission message signal, resulting in increased overhead of the message signal, which causes the message transmission efficiency to be reduced. Therefore, even though a high-speed transmission system is adopted, the expected increase of the subscriber capacity cannot be achieved. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a data communication method which achieves an improved transmission efficiency, a larger capacity of subscribers and lower power consumption of a subscriber receiver. 
     Another object of the present invention is to provide a receiving method which achieves high reliability of receiving message signals with simple steps. 
     Still another object of the present invention is to provide a selective calling data which achieves receiving of message signals with high reliability, simple structure and lower power consumption. 
     A transmission signal format according to the present invention is as follows. A specific time period is divided into a plurality of frames. Each the frame is comprised of a synchronization field, an address field, and a message field. The address field is comprised of a plurality of addresses associated with the subscriber&#39;s receivers, respectively. The message field is comprised of a plurality of messages corresponding to the addresses, respectively. A message is comprised of a message and a message header which includes a transmitting sequence number of the address corresponding to that message. 
     A base station transmits the transmission signals to the receivers. The receiver receives a frame of each transmission signal intermittently. When receiving the frame, the receiver sequentially searches the address field for an ID address of the receiver itself while incrementing a receiving sequence count at each address. When the ID address is found, the receiver stores the receiving sequence count at that time. Subsequently, the receiver searches the message field for the message corresponding to the ID address by comparing the receiving sequence count stored with the transmitting sequence number included in the message. When the transmitting sequence number is coincident with the receiving sequence count, the message data addressed to the receiver is detected from the message field. 
     The address field is preferably terminated by a predetermined word. The receiver stops searching the address field when the predetermined word is found, and then back to the receiving step. 
     More specifically, the message header is further comprised of a first field for containing a message header indicator, a second field for containing the transmitting sequence number, a third field for containing a message type, and a fourth field for containing a message length. In this case, the receiver detects a message header from the message field when comparing the receiving sequence count stored with the transmitting sequence number included in the message header. And, the message addressed to the receiver itself is input using the message type and the message length when the receiving sequence count stored is coincident with the transmitting sequence number. 
     As described above, the communication method according to the present invention provides the relationship between a transmission order of each address in an address field and an address transmission number of each message header in a message field in each message frame. Therefore, a message is defined by a short message header even when a synchronous system is employed. 
     In addition, the selective calling receiver according to the present invention stores the transmission order of its own address in the address field of the message frame received. When the transmission order stored is coincident with the transmission sequence number of the message header, the receiver selects the message data following the message header as a message addressed to the receiver itself. Therefore, high reliability can be realized with a simple structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating a frame structure of a message signal according to an embodiment of the present invention; 
     FIG. 2 is a diagram illustrating an example of a message header MH in the message signal format of FIG. 1; 
     FIG. 3 is a diagram illustrating a word format of BCH (31,21) code employed in the present embodiment; 
     FIG. 4 is a block diagram showing a base station in a selective calling system of an embodiment according to the present invention; and 
     FIG. 5 is a block diagram showing a selective calling receiver in the embodiment; and 
     FIG. 6 is a flow chart showing a receiving operation of the selective calling receiver of the present embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A frame of a message transmission signal is determined in such a manner that a specific time period is equally divided into a plurality of frames. For example, if one minute is equally divided by 120, a single frame has a time period of 0.5 seconds. When employing a transmission rate of 9600 bps and a word format of the BCH (31,21) code, a frame of 0.5 second consists of 150 words. The word format will be shown in FIG.  4 . In this case, a selective calling receiver performs a receiving operation during only one frame of 120 frames. Which frame is received is decided depending on the purpose of use. 
     Signal Frame Format 
     As illustrated in FIG. 1, a frame of 150 words is comprised of a synchronization (SYNC) field, an address field and a message field. The SYNC field is further comprised of a bit synchronization signal, a frame synchronization signal and a frame information signal, each signal having a length of one word. The selective calling receiver corrects a bit error by using the bit synchronization signal of the synchronization field and establishes word synchronization by using the frame synchronization signal. The frame information signal of the synchronization field includes a frame number signal (8 bits), a time information signal (5 bits) and another frame information signal (8 bits), which are not shown. If a frame is previously set at a predetermined time period and the frame #0 is coincident with a certain time instant, the receiver obtains time information by receiving the frames. In addition, the receiver can also realize some functions such as displaying the time of day and the time of receiving the message by monitoring the time information signal (5 bits) of the frame information signal. 
     The address field, as shown in FIG. 1, is a set of addresses (here five addresses A 1 -A 5 ) each having a length of one word. In the case of the BCH (31,21) code, since one word has an information field of 21 bits, it is possible to address up to about 2 millions users. The address field is terminated by the address end word P. When detecting the address end word P, the receiver judges that there is no address of its own within the frame received, and terminates the address retrieval. Note that the address transmission order in the address field is previously determined in the base station. 
     The message field is a set of submessages (here five submessages M 1 -M 5 ) each having a variable length and comprising a message header MH and a message. The message header MH is further comprised of a message header indicating signal F (1 bit), an address transmission number signal A (5 bits), a message type signal T (2 bits), and a message length signal N (7 bits), totaling to 15 bits. The remaining bits except for the message header MH of one word are managed as a message. Therefore, this embodiment is capable of defining a message using the overhead of 15 bits, resulting in drastically improved efficiency of message transmission as compared with the conventional system. 
     The message header indicating signal F is a binary bit which discriminates between the message header MH and code words of the message according to the value 0 or 1. For example, the value 1 indicates the message header MH and the value 0 indicates the message code words. Therefore, the receiver may search for a message header MH by only checking the 1st bit of each word. 
     The address transmission number signal A indicates the transmission order of each address (A 1 -A 5 ) within the address field. For example, the message header MH corresponding to the first address A 1  which is transmitted at first has the address transmission number A=‘1’. The message header MH corresponding to the fifth address A 5  has the address transmission number A=‘5’. As described later, the receiver searches the address field for its own address. When detecting the address of its own, the receiver stores the number of addresses checked so far, that is, the address transmission order. Subsequently, while retrieving the message field, the receiver finds the message having an address transmission number A coincident with the number of addresses stored. 
     The message type signal T shows the type of a message which is transmitted following the message header MH. As an example, the message type signal T of 2 bits is defined as shown in table 1. 
     
       
         
               
               
             
           
               
                  TABLE 1 
               
               
                   
               
               
                  2-bit Message Type signal 
                 Message Type 
               
               
                   
               
             
             
               
                 00 
                 Tone only 
               
               
                 01 
                 Numeric 
               
               
                   
                 (4 bits/character) 
               
               
                 10 
                 Alphanumeric 
               
               
                   
                 (7 bits/character) 
               
               
                 11 
                 Reserved 
               
               
                   
               
             
          
         
       
     
     In the embodiment, up to 4 types of a message can be identified and the fourth message type of T=‘11’ is reserved. By defining it in another way, for example, binary data may be transmitted. 
     The message length signal N indicates the length of a message or the number of code words in a message. Since the message length signal N is 7 bits in length, it can specify a message length of up to 128 words. When receiving a message, the receiver counts the number of words of the message while comparing the count with the message length signal N. When the count reaches the message length N, the receiver judges the end of the message addressed to itself. Since the length of a message is indicated by the message length signal N, a more accurate message display can be achieved, resulting in higher reliability as compared with the case where a message display range is decided by only the message header indicating signal F. 
     FIG. 2 shows one example of a message header MH. Since the bit string ‘100101100001110’ has the 1st bit of ‘1’, this word includes a message header. Since the address transmission number A is ‘00101’, this message is the 5th submessage M 5  corresponding to the address A 5 . Since the message type T is ‘10’, the message type is an alphanumeric. Finally, since the message length signal N is ‘0001110’, this message consists of 40 characters. 
     It should be noted that a word used in this embodiment is a BCH code, for example, the BCH (31,21) code of 32 bits which is used in POCSAG system. A word format of the BCH (31,21) code is shown in FIG.  3 . One word is comprised of an information bit field of 21 bits, a parity bit field of 10 bits, and an even-number parity bit. 
     Base Station 
     As shown in FIG. 4, a plurality of telephone lines are connected to a switching system  1  of a central base station. A calling number and a message received from the telephone set of a calling party are transferred to a message processing unit  2  through the switching system  1 . The message processing unit  2  generates necessary message data for selective calling and transfers it to a protocol encoding processor  3 . The protocol encoding processor  3  generates a transmission signal having the frame format as shown in FIGS. 1-3 using an address transmission count memory  4 . In this way, the transmission signal is transmitted from the transmitter  5  to the selective calling receivers. 
     Selective Calling Receiver 
     FIG. 5 is a block diagram showing an embodiment of a selective calling receiver according to the invention. A battery-powered receiver has a receiver circuit which is comprised of a built-in antenna  10 , a RF receiver  11  and a demodulator  12 . Receiving demodulated signal from the demodulator  12 , a decoder  13  decodes it into receiving data. 
     When inputting the receiving data from the decoder  13 , a processor  14  performs a receiving control by using an ID address memory  15 , an address count memory  16  and a message memory  17  in accordance with instructions input by a user through a key pad  18 , which will be described in detail later. The ID address memory  15  stores the address of its own or the predetermined ID (identification) number of this receiver. 
     Under the control of the processor  14 , the received message and other necessary data are displayed on a display  19  through a display driver, and a calling indicator  20  is operated through a driver upon receipt of an incoming call. The receiver has a battery  21  therein, the power of which is supplied to the receiver circuits such as the RF receiver  11  and the demodulator  12  by a power supply controller  22  under control of the processor  14 . For example, the power is supplied to those circuits intermittently at the time of intermittent receiving. 
     Receiving Operation 
     FIG. 6 shows a receiving operation in the selective calling receiver of FIG.  5 . The demodulator  12  establishes synchronization using the synchronization field of a signal frame as shown in FIG.  1 . After the synchronization of the demodulator  12  has been established, the demodulated signal is transferred from the demodulator  12  to the decoder  13  which decodes the demodulated signal into the receiving data. 
     The processor  14  searches the address field (A 1 -A 5 ) of the frame for its own address using the following procedure. First of all, when receiving the 1st address A 1  (S 101 ), the processor  14  increments an address counter (S 102 ) and then determines whether the 1st address A 1  coincides with the ID address stored in the ID address memory  15  (S 103 ). 
     If the 1st address Al does not coincide with the ID address (No of S 103 ), it is further determined whether it coincides with the address end word P (S 104 ). 
     If it is not the address end word P, the processor  14  inputs the subsequent 2nd address A 2  (S 101 ) and increments the address counter (S 102 ). Such a n address retrieval step is repeated until the ID address is detected. 
     Assuming that the ID address is located at the 3rd address A 3 . Each time inputting the 1st address A 1  and then the 2nd address A 2 , the processor  14  increments the address counter but skips these addresses. 
     When inputting the third address A 3 , the processor  14  increments the address count to the count ‘ 3 ’, (S 102 ), and then detects the ID address (Yes of S 103 ). When the ID address is detected, the address count ‘ 3 ’, at that time is stored onto the address count memory  16  (S 105 ). 
     Subsequently, the message retrieval is started (S 106 ). The processor  14  discriminates the message header MH from the bit string received from the decoder  13  by finding the message header indicating signal F. After extracting the address transmission number A of 5 bits from the message header MH (S 107 ), 
     the processor  14  determines whether this address transmission number A is coincident with the address count stored in the address count memory  16  (S 108 ). 
     If not coincident, the subsequent message header MH is input (S 106 ) and its address transmission number A is checked similarly (S 107  and S 108 ). If the address transmission number A is coincident with the address count stored in the address count memory  16  (Yes of S 108 ), the message which follows the message header MH is stored onto the message memory  17  as the message addressed to itself (S 109 ). And, the receiving operation is terminated (S 110 ). 
     Assuming that the ID address is located at the 3rd address A 3 , the address transmission numbers of the messages M 1  and M 2  are not coincident with the address count stored in the address count memory  16  but the address transmission number of the message M 3  is coincident with it. Therefore, the message of the message M 3  is stored onto the message memory  17  in accordance with the message type T and the message length signal N of the message header MH of the message M 3 . The processor  14  informs the user of an incoming call by the calling indicator  20  and reads out the message from the message memory  17  to display the message and other necessary data on the display  19 . 
     Comparison 
     The selective calling system as mentioned above will be compared with other systems in message efficiency. In order to transmit an alphanumeric message of 40 characters, the embodiment according to the present invention requires a total of 16 words comprising an address of 1 word and a message of 15 words. On the other hand, the POCSAG system requires 15 words for transmitting the same message. Therefore, the overhead of the message signal increases by about 7% in comparison with the POCSAG system. 
     Since the embodiment is different in word structure from the ERMES system, a simple comparison is not useful. However, the ERMES system requires 20 words for transmission of the similar message. Therefore, the embodiment causes the transmission efficiency to be improved by about 20%. Especially, in the case of a message of a relatively short length such as a numeric message, a remarkable improvement in transmission efficiency is achieved. For example, when a message of 10 characters is transmitted, the ERMES system needs 7 words but the embodiment only 4 words.