Abstract:
A method of transmitting messages in an answer-back paging system in which a primary station ( 10,14,16 ) transmits a series of messages addressed to respective secondary stations ( 20 ) on a down-link. The primary station then invites the secondary stations to transmit their responses, for example as CDMA signals, on an up-link. On receipt the primary station analyses those responses which are deemed to be intelligible, matches them to the originally transmitted messages and transmits acknowledgements to the relevant secondary stations. The primary station then issues another invitation to those stations whose responses had not been detected requesting them to reply on the up-link. Those responses which are deemed intelligible are processed as before and acknowledgements are transmitted together with another invitation on the down-link. The cycle of operations is repeated until either a predetermined number of cycles have elapsed or it is considered that all the intelligible responses have been received. The method can be adapted to include handling requests for service as well as handling responses to messages sent on the down-link.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of, and system for, transmitting messages and also to a primary station and a secondary station for use in the system. An example of such a system is an answer back paging system and for convenience reference will be made to an answer back paging system but it is to be understood that the teachings of the present invention can be applied to other 2-way messaging systems. 
     Answer back paging systems have been disclosed for example in PCT Patent Specification WO96/14716, which corresponds to U.S. Pat. No. 5,872,522. In an elementary form such a system requires a paging network controller (PNC) to arrange for a message to be transmitted to a predetermined addressee. The addressee on receiving a call is able to transmit a simple reply by way of a low power transmitter incorporated into the pager. The above mentioned Patent Specification discloses a system in which a series of messages are transmitted to respective addressees and the PNC then transmits invitations for the addressees to transmit their replies substantially simultaneously as spread spectrum signals which are de-spread at the PNC and the replies forwarded to the respective party requiring the reply. In order to avoid having to apply power control techniques in the pagers to ensure that replies are received by the PNC at comparable power levels, the PNC transmits its invitations at stepwise increasing (or decreasing) power levels and only those pagers just able to receive a respective one of the invitations transmit their replies. A feature of this known method is that relatively strict control of the received power is necessary. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the present invention is to introduce more flexibility into the operation of answer back messaging systems. 
     According to one aspect of the present invention there is provided a method of operating a message transmission system comprising at least one primary station making transmissions on a down-link and a plurality of secondary stations making transmissions on an up-link, characterised by the primary station transmitting a control signal on the down-link inviting secondary stations wishing to make an up-link transmission to proceed, those of said secondary stations wishing to respond to said invitation making their transmissions, the primary station receiving at least some of the transmissions and analysing those of the received transmissions which are intelligible and determining in the case of those of the analysed, intelligible transmissions which of them comprise responses to messages sent previously on the down-link, acknowledging at least the responses and repeating the transmission of the control signal on the down-link. 
     The transmissions on the up-link may comprise responses to messages sent on the down-link and/or requests for a service. These transmissions may be in the form of CDMA signals transmitted substantially simultaneously. 
     According to a second aspect of the present invention there is provided a method of operating a message transmission system, comprising a primary station transmitting messages to a plurality of secondary stations on a down-link, the primary station inviting the secondary stations to transmit their responses on an up-link, the primary station analysing the responses received from the up-link and acknowledging receipt of those responses which were analysed successfully, the primary station determining which messages have been responded to and repeating the invitation to those secondary stations whose responses have not been acknowledged to transmit/re-transmit their responses on the up-link. 
     According to a third aspect of the present invention there is provided a message transmission system comprising at least one primary station and a plurality of secondary stations, the primary station having means for compiling messages to be sent on a down-link to the secondary stations, means for transmitting a signal on the down-link inviting the secondary stations to transmit responses to the messages on an up-link, means for receiving and analysing the responses, means for acknowledging receipt of responses which were analysed successfully, means for determining which of the transmitted messages have been responded to and repeating the invitation to the secondary stations whose responses have not been acknowledged to transmit/re-transmit their responses, and each of the secondary stations having means for receiving transmissions on the down-link, means for determining if any one of the messages is addressed to it, means for receiving the invitation message from the down-link, means for transmitting a response to a message addressed to the secondary station on an up-link, and means responsive to not receiving an acknowledgement of its response for repeating the transmission of the response when invited to do so. 
     By means of the method of, and system for, transmitting messages in accordance with the present invention it has been found that the cycle of analysing responses, acknowledging and re-inviting secondary stations not receiving acknowledgements to send or re-send their responses or other up-link transmissions increases the rate of processing of the responses compared to known systems which in turn means that the capacity of the system is increased without the need to increase the amount of infrastructure equipment. 
     The secondary stations may transmit their responses or other up-link transmissions substantially simultaneously as CDMA signals in which case the stronger response signals will mask the weaker signals. This is known as the near-far problem. In the method in accordance with the present invention the stronger signals on the up-link are detected first and acknowledged and then the secondary stations whose responses or other up-link transmissions have been masked are invited to send or re-send their response signals or other up-link transmissions. After several iterations of the cycle of analysing the responses or other up-link transmissions, acknowledging the successful responses or other up-link transmissions and issuing another invitation or control signal, it has been found that a very high percentage of the responses or other up-link transmissions have been processed. Those which are not processed typically are too weak to be capable of processing and/or the secondary station is either switched-off or out of range. 
     The time allocated to the above-mentioned cycle may be variable in that a shorter time is allocated to processing a relatively small number of strong responses or other up-link transmissions and a longer time is allocated to processing a relatively large number of weak responses or up-link transmissions and if desired the allocation of time can be made adaptive to the numbers of the response or other up-link transmission signals. This will save processing time compared to the option of allocating intervals of constant duration to processing each batch of responses or other up-link transmissions. 
     The acknowledgement signal and the invitation signal may be transmitted simultaneously. One method of doing this is for the primary station to transmit messages in an ordered sequence and for the next following invitation message to contain a field comprising elements allocated in an ordered sequence corresponding to the message sequence and in which the elements are adapted to distinguish those responses successfully analysed from those which are unsuccessful, for example by changing a binary ‘0’ to a ‘1’ when successful. 
     In order to increase message throughput, the population of the secondary stations may be sub-divided into at least 2 groups and while responses or other up-link transmissions from one group ire being analysed the otherwise idle transmitter can be transmitting messages to the other group, and vice versa. The division of the population may be done on the basis of odd numbered and even numbered addresses or on signal strengths (RSSIs). 
     According to a fourth aspect of the present invention there is provided a primary station for use in a message transmission system in which messages are transmitted on a down-link to a plurality of secondary stations which are capable of transmitting responses on an up-link, the primary station comprising means for compiling messages to be sent on the down-link to the secondary stations, means for transmitting a signal on the down-link inviting the secondary stations to transmit responses to the messages on the up-link, means for receiving and analysing the responses, means for acknowledging receipt of responses which were analysed successfully, means for determining which of the transmitted messages have been responded to and for repeating the invitation to the secondary stations whose responses have not been acknowledged to transmit/re-transmit their responses. 
     According to a fifth aspect of the present invention there is provided a secondary station for use in a message transmission system in which messages are transmitted on a down-link to a plurality of secondary stations which are capable of transmitting responses on an up-link, the secondary station comprising means for receiving transmissions on the down-link, means for determining if any one of the messages is addressed to it, means for receiving an invitation to reply message from the down-link, means for transmitting a response to a message on the up-link, and means responsive to not receiving an acknowledgement of its response for repeating the transmission of the response when invited to do so. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a block schematic diagram of a message transmission system, 
     FIG. 2 is a diagram showing the transmission of messages and acknowledgements by a primary station and reception of TDMA responses, 
     FIG. 3 is a diagram showing the transmission of invitation signals, reception of CDMA responses and the analyses of the responses 
     FIG. 4 is a diagram showing the interlaced operation of a CDMA response type of system, 
     FIGS. 5A and 5B illustrate how acknowledgements are combined with an invitation signal transmitted on the down-link, 
     FIG. 6 is a block schematic diagram of the PNC  10 , 
     FIG. 7 is a flow chart showing the sequence of operations, and 
     FIG. 8 is a block schematic diagram of a pager. 
    
    
     In the drawings the same reference numerals have been used to indicate corresponding features. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the message transmission system comprises a paging network controller (PNC)  10  having a message entry port  12  which receives pager addresses and associated messages from an operator equipped with a personal computer (PC) or directly from a subscriber having a PC and a modem. The PNC  10 , which comprises directories containing information such as pager receiver identity codes (RICs), areas to be paged, frequencies, pager types, prevailing protocols, for example POCSAG (or CCIR Radiopaging Code No 1) and ERMES, and status of the pagers, assembles the messages and their associated RICs together with other relevant information into data packets which are forwarded to a paging area controller (PAC)  14  which formats the RICs and associated messages into a format which can be transmitted by base station transmitters (or transmitter section of a base station transceiver)  16  to one- and two-way pagers  18 , 20  respectively, by way of a down-link. 
     If a two-way pager  20  identifies that a message is being transmitted having its RIC, it receives the message and decodes it. If the user wishes to send a brief response then, by means of an integral key pad, he selects a pre-stored response and when invited by the PAC  16  it transmits its response by way of an up-link. The response signals may be sent simultaneously as CDMA signals or in a frame of time slots, one time slot being usable by a response. 
     One or more receivers (or receiver sections of a transceiver)  22  are provided for receiving the responses and for relaying them to the PAC  14  in which they are decoded in the case of CDMA signals or demultiplexed and sent as data packets to the PNC  10 . The PNC  10  comprises means for analysing the signals and for matching the responses with the messages transmitted on the down-link. 
     Those responses which are matched are relayed to the respective users in any suitable form, for example by e-mail or by transmission as one-way paging messages. Alternatively the responses are sent to a message answering service operated by the paging network. In either case an acknowledgement is sent to the respective 2-way pager  20 . However, not all the responses are matched because for example in a time division multiplex arrangement two or more responses clash and are unintelligible and in a CDMA system, strong responses smother the weak responses as a result of the near-far effect. If the PNC  10  determines that only a small proportion of the messages transmitted on the down-link have received responses then it issues a general invitation to those pagers which have not responded to the messages or have not received an acknowledgement, to transmit or re-transmit their responses on the up-link. The newly received responses are analysed, matched where possible and acknowledgements are transmitted. If it is determined that the total number of successful responses is still below a statistically determined threshold level, the cycle is repeated by transmitting another invitation signal on the down-link. The process is repeated until either a predetermined number of cycles have elapsed or the threshold level has been exceeded and it is evident that no more intelligible responses to the batch of messages are recoverable. 
     In the case of substantially simultaneously transmitted CDMA response signals, each time a batch of responses is received, those responses which are analysed successfully will generally have the greatest power levels at the antenna of the receiver  22 . Thus as they will be eliminated from the pool of response signals, then when the next invitation signal is responded to signals from weaker sources, that is, the more distant pagers, will be decoded and matched. 
     In a TDMA type of response system one method of operating the system will be described with reference to FIG. 2 which assumes that the protocol has a batch structure in which a signal channel is divided in the time domain into a succession of batches B. Each batch commences with a synchronisation code word S and has a plurality of concatenated frames F. One of the batches B(Tx) is dedicated to transmitting paging messages M 1  to M 8  in accordance with a known paging protocol, such as POCSAG, on the down-link. The last code word in the batch is an invitation to the addressed pagers to transmit their responses in the corresponding frames of the next following batch B(Rx). As the responses are received they are analysed and match to the outgoing messages wherever possible. Once a match has been made, an acknowledgement ACK is transmitted to the pager on the down-link. When the analysis is complete, the PNC  10  decides if responses to all the messages have been received and if not it transmits another invitation on the down-link and the cycle of operations is repeated one or more times. 
     FIG. 3 illustrates an example of a system in which responses comprise CDMA signals which are transmitted simultaneously in response to an invitation signal on the down-link. In FIG. 3 messages (not shown) already have been transmitted on the down-link. A first invitation INV 1  is transmitted on the down-link. The pagers which have detected a message addressed to them respond to the invitation signal INV 1  by transmitting a code sequence within a defined time slot RES 1 . A search routine SCH is initiated following expiry of the time slot. In the search routine, codes stored in the PNC  10  (FIG. 1) are successively compared with the response code sequences and one by one the responses to particular ones of the messages are identified. However, due to the near/far problem only the strongest of the response signals are detected and these are eliminated from the next search by acknowledgement signals being transmitted on a down-link to inform those pagers which have been successful not to the respond to the subsequent invitation signals INV 2  and INV 3  in the sequence. 
     It is anticipated that in a practical system the majority of the pagers  20  (FIG. 1) will be some distance from the antenna of the receiver(s)  22  which means that they will have a low power at the antenna. Accordingly, although the durations of the time slots RES 1 , RES 2  and RES 3  may be equal, as shown, it is preferable that variable slot lengths be allocated according to the anticipated number of responses, for example a low number of relatively high powered responses and a high number of relatively low powered responses. Short slots are allocated initially so that the few, strong powers contending against low noise and interference can be eliminated efficiently. Longer slots are then allocated to accommodate the weak received powers contending against significant levels of noise and interference. 
     If desired the pagers  20  may have power control on their transmitters in order to vary the strength of their response signals and in so doing reduce the number of invitation/response cycles. 
     In a refinement of the embodiment described with reference to FIG. 3, FIG. 4 shows dividing the population of the pagers into two groups and interleaving the transmission of messages and invitations for one group Gp 1  on the down-link with analysing the responses on the up-link from the other group Gp 2 . 
     One method of dividing the pager population is to assign the odd numbered pagers to one group, say Gp 1 , and the even numbered pagers to a second group, say Gp 2 . An alternative method is for the pagers to measure the strength (RSSI) of the received down-link signal and by using a pre-set threshold determine to which one of say two groups they belong. 
     One method of issuing an invitation message whilst simultaneously informing those pagers whose responses have been analysed successfully is to send the messages M 1  to M 14  in an ordered sequence as shown in FIG.  5 A and in the invitation signal, FIG. 5B, providing a field FD with a corresponding number of time slots on a 1 to 1 basis, thus slot  3  corresponds to message M 3 . When the first invitation signal INV 1  is transmitted, say all the bits in the field FD are zero indicating that no responses have been received. However, after the first round of analyses, acknowledgements are transmitted to say the pagers to which the messages M 1 , M 3 , M 4 , M 9 , M 11  and M 13  were addressed by changing the bits in slots  1 ,  3 ,  4 ,  9 ,  11  and  13  of the field FD from “0” to “1”. Further bits are changed as more of the messages are acknowledged. 
     The number of cycles in which invitations are transmitted may be fixed. However, if it is determined that the number of successfully decoded responses exceeds a statistically determined threshold value, then further iterations are stopped. 
     FIG. 6 shows in block schematic form a PNC  10  coupled to a PAC  14  and a base station transmitter  16  and a receiver  22 . The entry port  12  is coupled to a microcontroller  24  to which are connected directories  26  to  36  relating respectively to RICs, paging areas, frequencies, pager types, prevailing protocols and status. A message store  38  is coupled to the microcontroller  24  for storing messages as they are received at the entry port  12 . The store  38  has an area  40  for storing indications confirming that a response to a respective message has been received and acknowledged. An output  42  from the microcontroller  24  is coupled to the PAC  14  to supply data packets to be formatted prior to being transmitted by the transmitter  16 . 
     Responses received by the receiver  22  are relayed to a signal analyser  44  by way of the PAC  14 . As each response is analysed successfully, it is forwarded to the microcontroller  24  for matching with the messages in the store  38 . Once matched, the microcontroller arranges for an acknowledgement to be transmitted when sending the next invitation signal on the down-link. At the appropriate moment the recipients of the responses are informed, for example by e-mail or a one-way paging message, or the responses are stored together with the respective pager number so that a subscriber can interrogate the store at his or her convenience. Once the microcontroller  24  has decided that for all practical purposes all the responses have been received, it erases the messages in message store  38  in readiness for receiving more messages by way of the entry port  12 . More conveniently the store  38  can comprise two halves with one half handling the acknowledgements of the messages already sent on the down-link and the other half storing messages to be sent. 
     FIG. 7 is a flow chart showing the sequence of operations involved in transmitting messages, receiving responses and acknowledging responses. Block  50  represents start. Block  52  relates to the transmission of a sequence of messages and this is followed in block  54  by the transmission of an invitation signal. Block  56  relates to the reception of the CDMA responses which are then analysed and matched with their respective messages, block  58 . Block  60  relates to the transmission of the acknowledgements. In block  62  a check is made to see if the number of successful responses exceeds a threshold value indicating that as many as possible responses have been received. If the answer is No(N) the flow chart proceeds to block  64  in which a check is made to see if the predetermined maximum number of invitations has been exceeded. If the answer is No(N) the flow chart reverts to the block  54 . A Yes(Y) answer from the blocks  62  and  64  causes the flow chart to revert to the block  52  and the cycle is repeated. 
     Referring to FIG. 8, the pager  20  comprises a receiver  68  connected to a decoder  70  which in turn is coupled to a control processor  72 . The processor  72  operates in accordance with a program stored in a read-only memory  74 . The processor also includes an address store (not shown) which contains the unique addresses of that pager. In the event of the pager receiving a message then this is stored in a random access memory  76 . The messages can be displayed subsequently on a LCD panel  80  which has its associated driver  78  coupled to the control processor  72 . Annunciating devices which may comprise an acoustic transducer  82 , a light emitting transducer  84  and a vibrator  86  are coupled to the control processor  72 . A keypad  88  provides a man machine interface whereby a user can instruct the processor to carry out various functions, for example to display a stored message on the panel  80 . A transmitter  90  is coupled to an output of the processor  72  and to an antenna  92 . A receiver power control stage  94  is coupled between the processor  72  and the receiver  68  in order to practice battery conservation in accordance with the provisions of the paging protocol being followed. In the event of the pager sending response signals as spread spectrum signals then the control processor  72  stores a number of pseudo random codes, each one being representative of a particular reply which is selected by the user actuating the keypad  88 . The selected code is then relayed to the transmitter  90  for onward transmission. If as an option power control is to be applied to the transmitter  90  then a power control signal is supplied by the processor  72  through a control line  96 . 
     Although the present invention has been described with reference to a digital paging system the present invention may be used in a cellular or cordless telephone system having provision for two-way paging. 
     Further the signals transmitted on the up-link may also comprise requests for services, such as registration, and accordingly the present invention is equally applicable to processing such requests for services in the same way as a response with the exception there will be no match with an outgoing message. 
     From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of message transmission systems or component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.