Abstract:
Receivers synchronize to signals sequentially transmitted from several transmitters prior to receiving signals simulcast by the transmitters. The relative delay differential of the received sequential signals is measured and used to determine the optimal sampling positions for the simulcast signals.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to simulcast communication systems, and more specifically to a method of synchronizing one or more receivers prior to receiving a simulcast transmission. 
     BACKGROUND OF THE INVENTION 
     In contemporary communication systems, transmitter simulcasting is frequently utilized to expand the geographic area in which receivers may receive an information signal. Regrettably, however, by focusing upon simultaneous transmission of the information signals, existing simulcast systems have not fully addressed the effects of differential delay of the received simulcast signals. Differential delay arises when a receiver receives simulcast signals from multiple transmitters that are substantially identical in composition, but are shifted (in time) with respect to one another. As communication systems employ higher data rates to maximize information throughput, the acceptable differential delay between signals received from multiple transmitters is reduced and tolerance to timing error minimized. Accordingly, a need exists to compensate for differential delay effects in a simulcast communication system without unduly impeding information throughput. 
     SUMMARY OF THE INVENTION 
     Briefly, according to the invention, receivers synchronize to signals sequentially transmitted from several transmitters prior to receiving a signal simulcast by the transmitters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a simulcast communication system suitable for use with the present invention. 
     FIG. 2 is a transmitter coverage map in accordance with the present invention. 
     FIG. 3 is a timing diagram illustrating the signal transmission process in accordance with the present invention. 
     FIG. 4 is a timing diagram illustrating the bit-sampling process in accordance with the present invention. 
     FIG. 5 is a flow diagram in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a block diagram of a simulcast communication system comprises at least two transmitter sites, each transmitter site comprising a transmitter and an antenna such as a transmitter A 103 and an antenna 104 and a transmitter B 113 and an antenna 114. Radio frequency signals are sent from the transmitter sites over the air to at least one selective call receiver (SCR) such as a SCR 112. The radio frequency signals are typically modulated by a digital signal comprised of: (a) a synchronization signal portion for the purpose of synchronizing the SCR&#39;s in the system and (b) an address and message data portion. 
     A typical SCR 112 receives the modulated radio frequency signals from the air through an antenna 105. The signals are conducted to a receiver 106, where the address and data portions are recovered and provided to a microprocessor 107. The recovered address is compared to one or more addresses stored in a memory 108. If there is a match, the message data is stored in the memory 108, and an alert 109 is activated to inform the SCR user of a received message. The stored message may then be displayed on a display 111 either automatically or by actuation of one or more controls 110. 
     Referring to FIG. 2, a transmitter coverage map in accordance with the present invention comprises an arrangement of a plurality of transmitter coverage areas, each represented in FIG. 2 by a circle and a number at the center. In accordance with the present invention, a transmitter such as the transmitter A 103 or transmitter B 113 is located at the center of each of the circular areas to provide radio frequency coverage for each particular area. The transmitters and their coverage areas comprise a predetermined number of groups; in FIG. 2, the predetermined number of groups is four, although it may be a different number. Typical coverage areas 201, 202, 203, and 204 are shown for transmitters within group one (1), group two (2), group three (3), and group four (4), respectively. An overlapping coverage area 205 is shown. Within the geographical area 205, transmissions from the transmitter for the coverage area 201 and the transmitter for the coverage area 202 may be received. The groups of transmitters are arranged in a manner that prevents simultaneous receipt of more than one signal from any particular group. In other words, in FIG. 2, like circles never intersect. 
     Referring to FIG. 3, a timing diagram illustrating the signal transmission process comprises signals from each of the four groups of transmitters. The signals include a sequential calibration 301 and a simulcast transmission 302. During the sequential calibration 301, the four transmitter groups sequentially broadcast a calibration signal over the air to the SCR&#39;s in the system. The calibration signal may comprise a synchronization signal portion for time-synchronizing the SCR&#39;s. The calibration signal may also be used to determine which signals are of sufficient strength to be further considered by a particular SCR. If, for example, a particular SCR is located within the geographical area 205, (FIG. 2) the SCR may, following the sequential calibration 301, determine that the received calibration signals from transmitter groups three and four are weak relative to those from transmitter groups one and two. The SCR may, then, decide to consider calibration signals only from transmitter groups one and two. 
     The simulcast transmission 302 may comprise, for all of the transmitter groups in the system, an identical address and message data portion that may include addresses of and data messages for particular ones of the SCR&#39;s in the system. 
     Referring to FIG. 4, a timing diagram illustrating the bit-sampling process comprises typical sequential calibration signals received by a selective call receiver from transmitter groups one (1) 401 and two (2) 402. As shown in FIG. 4, a bit 420 of the signal 401 is received by the SCR 112 at time t=0+x, where t=0 is the time of broadcast from the transmitter and x is a time delay due to factors such as propagation delay. If the delay in travel from transmitter to SCR were equal for signals 401 and 402, the SCR would begin receiving the bit 420 and a bit 421 of the signal 402 at time t=a+x. However, as shown in FIG. 4, the bit 421 is delayed by an additional amount of time y due to the signal 402 travelling a longer distance to the SCR because the SCR is located closer to a group one transmitter than a group two transmitter or because the signal 402 is reflected off an object (e.g., a building) prior to reaching the SCR. It is assumed in the example of FIG. 4 that the signals from transmitter groups three and four are very weak relative to signals 401 and 402 and are, therefore, rejected by the SCR. In accordance with the present invention, the sampling times for the simulcast transmission portion of the received signal are adjusted within the selective call receiver based on the overlapping portions of the time-shifted bits 420 and 421. Specifically, as shown at signal 403, the bit-sampling process comprises sampling each bit within the overlapping portions 422 of the bits 420 of the signal 401 and the bits 421 of the signal 402. The adjusted sampling times may occur relative to the midpoint of the common overlapping portions, 425. In the example shown in FIG. 4, the first overlapping portion 422 begins at time t=d+x+y. The arrows shown indicate sampling time 425 as determined by the received signals 401 and 402. 
     Referring to FIG. 5, a flow diagram in accordance with the present invention comprises setting a counter N=1, step 501. The process then proceeds to step 502 in which the sequential calibration signal is received from transmitter group N. In step 503, the signal strength and bit timing for the group N transmitter are determined. Next, the signal strength and bit timing information for the group N transmitter is stored in the memory 108 of the SCR 112, step 504, and the counter N is checked to see if it is equal to four (4), step 505. 
     If the counter N is not equal to four in step 505, the counter N is incremented by one (N=N+1), step 509, and the process returns to step 502. If the counter N is equal to four in step 505, the signal strength of all transmitter groups is compared and the groups that are below a predetermined level are rejected, step 506. The predetermined level in step 506 may comprise a level relative to the strongest received signal or, alternatively, an absolute signal strength level. Next, the bit timing of the remaining groups is compared and the sampling reference for the following simulcast transmission is determined, by selecting the center 425 of the overlapping bits 422 created by the bits 420 of the signal 401 and the bits 421 of the signal 402 (FIG. 4) step 507. The simulcast transmission is then received and sampled, step 508, and the process returns to step 501.