System and method for dynamic overlap compensation in a simulcast network

[0002] The present invention relates generally to telecommunications systems and methods for simultaneously broadcasting a signal from multiple transmitters in a mobile radio network, and specifically to compensating for overlap between simulcast signals in a simulcast radio network.

[0003] Simulcast, or Simultaneous Broadcast, is a mobile radio system architecture that uses two or more transmitters operating on a single radio frequency, in which the same information is carried by all of the transmitters, and the coverage area of two or more of the transmitters overlap. Simulcast provides some significant advantages including wide area communications with a limited number of channels without the use of a multisite switch. In addition, a simulcast system provides more efficient use of channels in situations where groups operate in multiple locations. Furthermore, simulcast systems offer seamless roaming within the total simulcast coverage area, provide efficient coverage in areas with difficult terrain and provide improved in-building coverage in some cases due to the multiple transmitter concept.

[0004] However, there can be performance issues in simulcast systems, especially when digital voice or digital data is being transmitted. With reference now to FIG. 1 of the drawings, a simulcast system 10 having only two transmitters 24aand 24bis shown. Both transmitters 24aand 24bare connected to a central control point 23, or master base station, which utilizes special circuitry to transmit a signal to each transmitter 24aand 24bfor simultaneous broadcast of the signal in a cell 22aand 22bassociated with each transmitter 24aand 24b,respectively, using the same radio frequency (RF). Each transmitter 24aand 24bis connected to the control point 23 via a dedicated, phasestable microwave or optic fiber backbone system.

[0005] Typically, there is a delay introduced by the control point 23 in the sending of the signal to the various transmitters 24aand 24bdepending upon the distance between the control point 23 and the transmitters 24aand 24b.For example, if a first transmitter 24ais 10 kilometers away from the control point 23, while a second transmitter 24bis 20 kilometers away from the control point 23, the control point 23 will delay sending the signal to the first transmitter 24a,so that the signal will arrive at both transmitters at the exact same time. This difference in transmission times is generally referred to as the timing differential.

[0006] Theoretically, if a Mobile Station (MS) 20, which is the physical equipment, e.g., a car-mounted mobile radio or other portable radio, used by mobile subscribers to communicate with the mobile radio network 10, each other, and users outside the subscribed network, both wireline and wireless, is located exactly between the two transmitters 24aand 24b,the signal transmitted from each of the transmitters 24aand 24bwould be received by the MS 20 at exactly the same time, e.g., digital bits received by both transmitters 24aand 24bwould line up exactly. As the MS 20 moves towards the edge of the overlap zone 25, the MS 20 captures the transmission from one of the transmitters, such as transmitter 24b.This “capture zone” can be defined as the area in which the carrier signal (signal strength) of the closer transmitter 24bexceeds the signal strength of the farther transmitter 24aby at least approximately 10 decibels (dB).

[0007] However, as the MS 20 moves through the overlap zone 25 from one of the transmitters 24atowards the other of the transmitters 24b,the interference increases. This is due to the fact that a bit transmitted from the closer transmitter 24bwould be received by the MS 20 at an earlier time than the same bit would be received by the MS 20 from the farther transmitter 24a.If this time difference (hereinafter referred to as the delay spread) becomes too large, the bits begin to interfere with each other, and the MS 20 may demodulate the bit in error. The bit errors caused by this self-imposed interference manifest themselves as problems such as no access to the system, retransmissions of the signal, loss of audio and/or loss of data.

[0008] For example, in the Enhanced Digital Access Communication System (EDACS) system, the two transmitter overlap design parameters allows approximately 30-40 microseconds (usec) of delay spread with capture ratio ranges of 8-12 dB. The system is theoretically designed so that the MS 20 can always receive the signal without significant error. Unfortunately, in reality, most simulcast systems have some overlap regions where the rules are exceeded and the system coverage is severely degraded or unusable. In addition, in some simulcast systems, more than two transmitters overlap, which can amplify this problem. Furthermore, this overlap problem is more severe for higher data rates because the ratio of the size of the overlap zone to the capture zone increases. Thus, for digital radio transmissions such as control channel, digital voice, or data, a simulcast system almost always provides non-uniform coverage.

[0009] It is, therefore, an object of the present invention to dynamically shift the delay spread in the overlap zone to reduce interference between simulcast signals.

[0010] The present invention is directed to telecommunications systems and methods for deliberately producing synchronous timing jitters in order to dynamically shift the delay spread in the overlap zone in a simulcast system. At a synchronous periodic rate, the timing differential between transmission of the signal from the control point to each of the transmitters can be continuously adjusted to continuously change the amount of delay spread in the overlap zone. Alternatively, the timing adjustments can be performed only upon reception of a retransmission request. In either case, the timing adjustments are performed so as to not be noticed by the mobile subscriber. In addition, in preferred embodiments of the continuously adjusting embodiment, the timing jitters cycle above and below the optimal timing values determined from the system coverage analysis. Advantageously, the dynamic shifting of the delay spread in the overlap zone can improve signaling, voice quality perception and data transmissions in an otherwise unusable overlap area.

[0011] The disclosed invention will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein:

[0012] FIG. 1 is a block diagram of a conventional simulcast system;

[0013] FIG. 2 illustrates the effects of introducing a timing jitter in accordance with preferred embodiments of the present invention;

[0014] FIG. 3 illustrates a sample implementation of the introduction of a timing jitter in accordance with preferred embodiments of the present invention; and

[0015] FIG. 4 illustrates an exemplary twelve frame jitter cycle produced by implementing a timing jitter in accordance with embodiments of the present invention.

[0016] The numerous innovative teachings of the present application will be described with particular reference to the presently preferred exemplary embodiments. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others.

[0017] In simulcast systems, the timing differential, e.g., the difference between the time that the control point transmits the signal to two different transmitters, is already precisely controlled, typically using high stability oscillators with synchronization to control the data timing. However, most simulcast systems have some overlap regions where the system coverage is severely degraded or unusable. These overlap regions create areas of non-uniform coverage for digital radio transmissions, such as control information, voice, or data transmissions.

[0018] With reference now to FIG. 2 of the drawings, in order to prevent system 10 coverage from becoming severely degraded or unusable in areas of non-uniform coverage within the overlap zone 25, a synchronous timing jitter can be purposefully introduced by the control point 23 to continuously dynamically shift the delay spread in the overlap zone 25. At a synchronous periodic rate, the difference between the time that the control point 23 transmits the signal 30ato the first transmitter 24aand the time that the control point 23 transmits the signal 30bto the second transmitter 24bcan be adjusted to change the delay spread in the overlap zone 25. This new time difference between the times that the control point 23 transmits the signals 30aand 30bto the first and second transmitters 24aand 24b, respectively, is defined as the timing jitter. For example, if the timing between the two transmitters 24aand 24bis shifted by 1 usec, the relative position of a point in the overlap zone 25 where the delay spread limit is exceeded can be moved by approximately 1000 feet.

[0019] As shown in FIG. 2 of the drawings, the control point 23 has purposefully adjusted the transmission time so that the transmission of the signal 30bto the second transmitter 24ais delayed by 1 usec as compared to the normal transmission time to the second transmitter 24a. Thus, the relative position of the receiving MS 20 with respect to overlap zone 25 timing parameters has shifted approximately 1000 feet closer to the second transmitter 24aand approximately 1000 feet further from the first transmitter 24b.As stated hereinbefore, normally, the control point 23 synchronizes the transmission of the signals 30aand 30bto the two transmitters 24aand 24b, respectively so that if an MS 20 is located at a point (not shown) exactly between the two transmitters 24aand 24b,e.g., the distance from the MS 20 to the first transmitter 24ais the same as the distance from the MS 20 to the second transmitter 24b,the signals 30aand 30btransmitted from each of the transmitters 24aand 24bwould be received by the MS 20 at exactly the same time. However, by introducing the timing jitter, the location at which the MS 20 receives the signals 30aand 30bat exactly the same time has shifted.

[0020] As an example, if the MS 20 is originally located at an edge 27 of the overlap zone 25, which is typically the area where the MS 20 would experience the highest interference levels between the two signals 30, and thus, signal degradation, when the timing jitter is introduced, the MS 20 is now located in an area of improved performance 26. Thus, the MS 20 receives the signals 30aand 30bshifted in time, resulting in a reduced delay spread, and thus a higher probability of decoding the signal 30 correctly. It should be understood that by introducing a timing jitter in a simulcast system may improve performance for some users, but may also degrade the performance for other users who were not originally located in an area of excessive timing delay spread 28. Thus, if the MS 20 was originally located in an area where the delay spread was acceptable, when the timing jitter is introduced, the MS 20 may now be in an area where the timing rules are exceeded 28.

[0021] However, the timing jitters should improve the overall performance of all of the MS's 20 located within the simulcast system 10 by allowing an acceptable degradation in previously good areas, while providing some coverage in areas previously unserviceable. For example, in a trunked mobile radio system, access through the control channel typically uses an algorithm, such as slotted-Aloha, where multiple retries are performed when no response is received to a service request. In this case, no response would be caused by the mobile radio incorrectly decoding the control channel response due to interference caused by excessive delay spread in the received composite signals. The synchronous timing jitter discussed herein can allow an MS 20 to receive the response correctly after a retry request.

[0022] With reference now to FIG. 3 of the drawings, a sample implementation of the introduction of the timing jitter within the control point 23 is shown. When the control point 23 receives data 32 to be transmitted to the MS 20, a digital voter circuit 35 within the control point 23 selects the best received data 32. Thereafter, the digitized data 32 is assigned to a channel, which can be either a control channel 40 for control information or a working channel 45, e.g., traffic channel, for speech or other type of data. The data 32 is also segmented into separate frames, in which one of these frames forms the signal 30 that is transmitted on the assigned channel 40 or 45 to the MS 20 via each of the transmitters 24aand 24b.Each signal 30 (whether included within the control channel 40 or within one of the working channels 45) is passed onto a separate programmable delay generator 50aand 50bfor each of the transmitters 24aand 24b,respectively. A control processor 60 within the control point 23 coordinates the data 32 and signal 30 transmission process.

[0023] The transmission of the signals 30aand 30bfrom the control point 23 to each of the transmitters 24aand 24b, respectively, is controlled by a timing generator 55 and separate programmable delay generators 50aand 50bfor each of the transmitters 24aand 24b,respectively. The timing generator 55 synchronizes to a master clock signal 56, and creates synchronization signals 58aor 58bto each of the programmable delay generators 50aand 50b. It should be understood that the timing generator 55 transmits two separate synchronization signals 58aand 58b,one for control channel 40 transmission and one for working channel 45 transmissions, respectively. This is due to the fact that the control channel 40 frame size, e.g., the number of bits transmitted over the control channel 40 per frame, is different than the working channel 45 frame size. For example, in the EDACS system, one control channel 40 frame is thirty milliseconds (msec) long and contains 288 bits, while the working channel 45 frame is eighty msec long.

[0024] The programmable delay generators 50aand 50btransmit the signals 30aand 30bto each of their respective transmitters 24aand 24bbased upon the delay associated with each of the transmitters 24aand 24b. For example, if the first transmitter 24ais closer to the control point 23 than the second transmitter 24b,the second programmable delay generator 50btypically transmits the signal 30bto the second transmitter 24bimmediately upon reception of the synchronization signal 58aor 58b.In other words, the delay time for the second programmable delay generator 50bis zero. In order to ensure that the signals 30aand 30bare received by all of the transmitters 24aand 24bat the same time, the first programmable delay generator 50acan delay the transmission of the signal 30ato it's respective transmitter 24afor a predefined time period.

[0025] However, if a timing jitter is introduced into the system 10, the programmable delay generators 50aand 50bcan adjust the delay time for the transmission of the signals 30aand 30bto each of the transmitters 24aand 24b.In preferred embodiments of the present invention, each of the programmable delay generators 50aand 50bcan continuously cycle through a respective set of values 75aand 75bstored in a respective memory 70aand 70b, changing values every time a synchronization signal 58aor 58bis received. The set of values 75aand 75bfor each programmable delay generator 50aand 50b, respectively, could be stored in, for example, a respective look-up table 70aand 70b.Each programmable delay generator 50aand 50bcan access its respective table 70aand 70band increment a respective counter 80aand 80bassociated with the respective table 70aand 70bevery time a synchronization signal 58aor 58bis received.

[0026] For example, considering only the first programmable generator 50a,when a first synchronization signal 58aor 58bis received by the first programmable delay generator 50a,the counter 80aassociated with that first programmable delay generator 50ais initialized, and the first programmable delay generator 50aretrieves the first delay value 75a₁in the table 70a.The first programmable delay generator 50auses this delay value 75a₁to determine the delay time for transmission of the signal 30ato the first transmitter 24aassociated with the first programmable delay generator 50a.When the first programmable delay generator 50aretrieves the delay value 75a₁, the counter 80ais incremented by one to indicate to the first programmable delay generator 50athat a second delay value 75a₂in the table 70ashould be retrieved next. Therefore, when the next synchronization signal 58aor 58bis received by the first programmable delay generator 50a,the first programmable delay generator 50aretrieves the second delay value 75a₂from the table 70a,and uses this delay value 75a₂to delay the transmission of the next signal 30ato the transmitter 24a.This process continues until the first programmable delay generator 50aretrieves the last delay value 75a_nin the table 70a.Thereafter, the counter 80 is reinitialized, and the first programmable delay generator 50aretrieves the first delay value 75a₁from the table 70awhen the next synchronization signal 58aor 58bis received.

[0027] In an alternative embodiment, these timing jitters can be implemented such that a separate delay time can be utilized for each channel 40 and 45. In this case, a separate look-up table 70aand 70band counter 80aand 80bwould be required for each channel 40 and 45. Thus, each programmable delay generators 50aand 50bwould have multiple look-up tables 70aand 70band counters 80aand 80bfor each channel 40 and 45 within the system 10. Advantageously, by implementing the timing jitters on a channel by channel basis, the timing jitters can be selectively applied only to those channels experiencing difficulty. For example, if an MS 20 receives a corrupted signal 30 (due to interference), the MS 20 normally requests retransmission of the corrupted signal 30. Upon receipt of this retransmission request, the control processor 60 within the control point 23 can request the programmable delay generators 50aand 50bto access their respective look-up tables 70aand 70bfor the channel 40 or 45 that the MS 20 is using and retrieve respective delay values 75aand 75bin order to shift the delay spread in the overlap zone 25 for the MS 20, which should put the MS 20 in a better position to correctly receive that signal 30.

[0028] Each time the delay spread in the overlap zone 25 is shifted by varying the transmission delay of the signal 30aand 30bfrom the control point 23 to the transmitters 24aand 24b,there is a possibility that the performance for some MS's 20 that are not located in an area of excessive delay spread in the overlap zone 25 may degrade. Therefore, these timing adjustments must be performed fast enough so as to not be noticed by MS subscribers. In addition, these timing adjustments preferably cycle above and below the optimal timing values determined from the system coverage analysis.

[0029] A sample twelve frame jitter cycle for one programmable delay generator 50ais shown in FIG. 4 of the drawings. The control point timing reference and normal transmit times are compared with the transmit times of twelve separate frames, each containing eight bits. As can be seen in FIG. 4, the timing jitters cause frames 1-5 to be transmitted at a later time than the normal transmit time, while frames 7-11 are transmitted at an earlier time than the normal transmit times. Frames 6 and 12 correspond to the normal transmit times. By cycling the timing adjustments above and below the normal transmit time, the position in the overlap zone 25 where the delay spread is zero can be forced to constantly change relative to the transmitters 24aand 24b.In one cycle, the delay spread in the overlap zone 25 shifts away from the transmitter 24aand then back to the normal position, while in the next cycle, the delay spread in the overlap zone 25 shifts towards the transmitter 24aand then back to the normal position.

[0030] Ideally, these timing adjustments occur at specific boundaries, such as between control channel frames, or between working channel frames. For example, a control channel frame 30 typically contains 288 bits. When the MS 20 receives this control channel frame 30, most of the bits within the frame 30 supply a message to the MS 20, while other bits aid in bit error detection. If the programmable delay generator 50atransmitted the first half of one control channel frame 30, and delayed the transmission of the second half of the control channel frame 30, an MS 20 in an area with good reception at the beginning of the transmission may receive the first half, but lose the second half if the shift causes the MS 20 to be located near the edge of the overlap zone 25. Thus, the entire frame 30 would be considered corrupt by the MS 20, and the MS 20 would need to send a request for retransmission of the entire frame 30 to the transmitter(s) 24a.If the MS 20 is stationary, this situation could potentially keep occurring, which results in the MS 20 never receiving the entire frame 30 correctly.

[0031] However, if the timing jitter is initiated between the last bit of a first frame 30 and the first bit of a second frame 30, decoding errors can be minimized. For example, if the MS 20 is located in a poor reception area when the control channel frame 30 is first received, when the MS 20 requests retransmission of the corrupted frame 30, the timing differential will have shifted, which can potentially put the MS 20 in a location with improved reception capabilities. Thus, upon retry, if the MS's 20 location with respect to the delay spread in the overlap zone 25 has changed, the MS 20 should be able to receive the entire frame 30 correctly.

[0032] As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a wide range of applications. Accordingly, the scope of patented subject matter should not be limited to any of the specific exemplary teachings discussed, but is instead defined by the following claims.