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Estimating time delays in a simulcast communication systemRelated Patent Categories: Telecommunications, Transmitter And Receiver At Separate Stations, Plural Transmitters Or Receivers (i.e., More Than Two Stations), Noise, Distortion, Or Singing Reduction, Synchronized Stations, Simulcast SystemEstimating time delays in a simulcast communication system description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070184862, Estimating time delays in a simulcast communication system. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF INVENTION [0001] The present invention relates generally to telecommunications systems and methods for simultaneously broadcasting a signal from multiple transmitters in a mobile radio network, and, more specifically, to compensating for overlap between simulcast signals in a simulcast radio network. BACKGROUND AND OBJECTS OF THE PRESENT INVENTION [0002] A simulcast, or simultaneous broadcast, network is a well known wireless communication system. Such systems are described, for example, in U.S. Pat. No. 6,266,536, herein incorporated by reference. Briefly, a simulcast system is a mobile radio system architecture in which two or more transmitters operate on a single radio frequency over a common area and transmit the same information. 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. [0003] Nevertheless, simulcast systems are faced with a number of performance issues, especially when digital voice or digital data are transmitted. These issues are addressed with reference to FIG. 5, which shows a simulcast system 510 having only two transmitters 524a and 524b. Both transmitters 524a and 524b are connected to a central control point 523, or master base station, which utilizes special circuitry to transmit a signal to each transmitter 524a and 524b for simultaneous broadcast of the signal in a cell 522a and 522b associated with each transmitter 524a and 524b, respectively, using the same radio frequency (RF). Each transmitter 524a and 524b is connected to the control point 523 via a dedicated, phasestable microwave or optic fiber backbone system. [0004] Typically, there is a delay introduced by the control point 523 in the sending of the signal to the various transmitters 524a and 524b depending upon the distance between the control point 523 and the transmitters 524a and 524b. For example, if a first transmitter 524a is 10 kilometers away from the control point 523, while a second transmitter 524b is 520 kilometers away from the control point 523, the control point 523 will delay sending the signal to the first transmitter 524a, 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. [0005] Between the transmitters 524a, 524b is a mobile unit 520 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 510, each other, and users outside the subscribed network, both wireline and wireless. Theoretically, if the mobile unit is located exactly between the two transmitters 524a and 524b, the signal transmitted from each of the transmitters 524a and 524b would be received by the mobile unit 520 at exactly the same time--i.e., digital bits received by both transmitters 524a and 524b would line up exactly. As the mobile unit 520 moves towards the edge of the overlap zone 525, the mobile unit 520 captures the transmission from one of the transmitters, such as transmitter 524b. This "capture zone" can be defined as the area in which the carrier signal (signal strength) of the closer transmitter 524b exceeds the signal strength of the farther transmitter 524a by at least approximately 10 decibels (dB). [0006] However, as the mobile unit 520 moves through the overlap zone 525 from one of the transmitters 524a towards the other of the transmitters 524b, the interference increases. This is due to the fact that a bit transmitted from the closer transmitter 524b would be received by the mobile unit 520 at an earlier time than the same bit would be received by the mobile unit 520 from the farther transmitter 524a. If this time difference (hereinafter referred to as the delay spread) becomes too large, the symbols begin to interfere with each other, and the mobile unit 520 may demodulate a symbol in error. The symbol errors caused by this self-imposed interference manifest themselves as problems such as limited access to the system, retransmissions of the signal, loss of audio and/or loss of data. [0007] Some systems are tolerant of time delay. For example, in the Enhanced Digital Access Communication System (EDACS.RTM.) (M/A Com, Lowell Mass.), transmitter site overlap design parameters allow approximately 30-40 .mu.sec of delay spread with capture ratio ranges of 8-12 dB. The system is theoretically designed so that the mobile unit 520 can receive the signal without significant error. In practice, however, most simulcast systems have some overlap regions in which the overlap design parameters are exceeded and the system coverage is severely degraded or unusable. Additionally, in some simulcast systems, more than two transmitters overlap, which can exacerbate this problem. [0008] 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. In other words, the probability that a delayed symbol will be demodulated causing a symbol error increases as the clock speed increases. For example, as the bit rate increases from 9.6 kb/s and to 16 kb/s, the theoretical time delay tolerance drops from 50 .mu.sec to 30 .mu.sec, respectively. Furthermore, in practice, the real time delay tolerance is likely to be lower for the reasons mentioned above. Thus, for digital radio transmissions such as control channel, digital voice, or data, a simulcast system almost always provides non-uniform coverage. [0009] Therefore, some kind of equalization of the signals is required to compensate for this time delay. Equalization is a known concept in simulcast networks and involves adjusting the magnitude and phase of received signals using complex channel coefficients to make the signals from different sites essentially equal in magnitude and phase. This concept is considered in greater detail in Equalization--Digital Communications Digital Communications 4th edition 2001 Chapter 11 by John G. Proakis. One well-known approach for equalization is covered by GSM. In GSM, equalization is performed at the receiver with the help of the training sequences transmitted as part of the midamble in every time slot. The type of equalizer used for GSM is not specified and is left up to the manufacturer as to the method of implementation. Although this standard relates to a high-speed communication system (270 kb/s), it is limited in the time delay for which it can compensate--about 15 .mu.sec. Applicant has determined, however, that such a limitation renders this approach inadequate for typical simulcast networks in which time delays of up to 100 .mu.sec can be expected. [0010] Another possible equalization approach for managing long time delays in a high speed simulcast network involves determining the actual distance between the transmitting sites and using this information to calculate the expected time delay. To this end, the transmitting sites would be outfitted with global positioning devices to provide location information, which would be encoded in the transmitted signal. This approach, although viable, would add complexity and expense to the system. [0011] Therefore, there is a need for equalizing signals in a high speed simulcast network to compensate for large time delay range, while avoiding complex and costly location-determining devices. The present invention fulfills this need among others. SUMMARY OF INVENTION [0012] The present invention facilitates signal equalization in a high-speed simulcast network having significant time delays while avoiding complicated GPS devices and high computational overhead. Specifically, the system of the present invention determines the time delay and complex channel coefficients for equalization using a pre-established set of time delay filters. The pre-established set of time delay filters is determined ahead of time based on presumed time delays and is stored in memory. The concept of calculating time delay filters ahead of time is based on the recognition that there are only a limited number of dominate sites in a simulcast system--usually two. Furthermore, the expected time delay of the sites which are likely to have potentially interfering signals is also limited--typically to about 100 .mu.sec or so. [0013] Using this set of time delay filters, predicted simulcast signals are generated which are then compared to the actual simulcast signal. The actual time delay data may be derived from the predicted simulcast signal which is closest to the actual simulcast signal. Thus, rather than determining time delay based on the received simulcast signal which is difficult and time consuming, the present invention approximates the time delay using a predetermined, stored set of time delay filters. This saves computation time and overhead. [0014] Accordingly, one aspect of the present invention is a method for determining channel coefficients for a simulcast signal based on a predetermined set of time delay filters. In a preferred embodiment, the method comprises: (a) receiving a simulcast signal comprising known data simulcast from at least two different sites; (b) generating a number of simulated simulcast signals, each simulated simulcast signal being based on at least two sites transmitting the known data with a presumed time delay, each simulated simulcast signal having a different presumed time delay; (c) determining which of the simulated simulcast signals is an optimum simulated simulcast signal that correlates most closely to the received simulcast signal; and (d) providing time delay data of the optimum simulated simulcast signal for use in equalization of the simulcast signal. [0015] Another aspect of the invention is a mobile unit suitable for use in a simulcast network which equalizes received simulcast signals using a predetermined time day function. In a preferred embodiment, the mobile unit comprises: (1) a receiver for receiving a simulcast signal; (2) a transmitter for transmitting a signal to a simulcast network; (3) a processor operatively connected to the receiver and the transmitter; and (4) memory operatively connector to the processor and being configured to instruct the process to perform the following steps: (a) receiving a simulcast signal comprising known data simulcast from at least two different sites; (b) generating a number of simulated simulcast signals, each simulated simulcast signal being based on at least two sites transmitting the known data with a presumed time delay, each simulated simulcast signal having a different presumed time delay; (c) determining which of the simulated simulcast signals is an optimum simulated simulcast signal that correlates most closely to the simulcast signal; and (d) providing time delay data of the optimum simulated simulcast signal for use in equalization of the simulcast signal. [0016] Still another aspect of the invention is software for having a processor of the mobile unit described above perform the method described above. In a preferred embodiment, the software comprises a computer-readable medium, such as a disk, having instructions for having a mobile unit processor perform the following steps: (a) receiving a simulcast signal comprising known data simulcast from at least two different sites; (b) generating a number of simulated simulcast signals, each simulated simulcast signal being based on at least two sites transmitting the known data with a presumed time delay, each simulated simulcast signal having a different presumed time delay; (c) determining which of the simulated simulcast signals is an optimum simulated simulcast signal that correlates most closely to the simulcast signal; and (d) providing time delay data of the optimum simulated simulcast signal for use in equalization of the simulcast signal. [0017] Yet another aspect of the invention is a simulcast network comprising a mobile unit which equalizes received simulcast signals by determining channel coefficients for the signal based on presumed time delays. In a preferred embodiment, the network comprises: (1) at least two sites for simulcasting a signal having known data; (2) a mobile unit positionable between the two sites such that a simulcast signal received from the two sites has a time delay, and comprising: (i) a receiver for receiving a simulcast signal; (ii) a transmitter for transmitting a signal to a simulcast network; (iii) a processor operatively connected to the receiver and the transmitter; and (iv) memory operatively connector to the processor and being configured to instruct the process to perform the following steps: (a) receiving a simulcast signal comprising known data simulcast from at least two different sites; (b) generating a number of simulated simulcast signals, each simulated simulcast signal being based on at least two sites transmitting the known data with a presumed time delay, each simulated simulcast signal having a different presumed time delay; (c) determining which of the simulated simulcast signals is an optimum simulated simulcast signal that correlates most closely to the simulcast signal; and (d) providing time delay data of the optimum simulated simulcast signal for use an equalization of the simulcast signal. BRIEF DESCRIPTION OF DRAWINGS [0018] FIG. 1 shows a flow chart of a preferred embodiment of the method of the present invention. [0019] FIG. 2 shows an error surface created using coarsely-incremented filters. [0020] FIG. 3 shows an error surface created using finely-incremented filters. Continue reading about Estimating time delays in a simulcast communication system... Full patent description for Estimating time delays in a simulcast communication system Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Estimating time delays in a simulcast communication system patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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