Abstract:
Mobile radio receivers in a broadcasting system having more than one transmission channel sending the same programs use time diversity to prevent service outages during periods when none of the channels can be received.

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 08/426,713, filed Apr. 20, 1995, entitled &#34;MOBILE RADIO RECEIVERS USING TIME DIVERSITY TO AVOID SERVICE OUTAGES IN MULTICHANNEL BROADCAST TRANSMISSION SYSTEMS&#34; by Robert D. Briskman, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     Broadcasting of television, radio, data and other forms of communications to receivers on mobile platforms using radio frequencies in the range from approximately 1-40,000 MHz suffer from transmission service outages. These transmission outages prevent the mobile user from receiving the broadcast and, if enough occur, create an unacceptable service. These outages are generally caused by multipath fading and reflection of the transmission path, physical blockage between the transmitter and receiver and interference to the mobile receiver from co-frequency transmitters other than the intended one. 
     Radio frequency transmission systems are subject to multipath fading which is caused by reflections of the desired signal from objects far from the mobile receiver such as mountains and from objects close to the mobile receiver such as a passing truck or large buildings. These reflections can either be specular or diffuse but are received along with the desired signal. Since the reflections are mostly out-of-phase with the desired signal, they either tend to cancel the desired signal or add to the receiver noise or both. When the cancellation and/or noise become sufficient with respect to the desired signal, a service outage occurs. See Reference Data for Engineers, 8th Edition, Prentice Hall, 1993, pp 46-10 for more details. Physical blockage between the transmitter and receiver is typically caused by the mobile receiver passing through tunnels, under overpasses or by close buildings and trees. During these blockage periods, whose durations are a function of the obstruction size/orientation and of the mobile receiver platform&#39;s speed, the desired signal is not available and an outage results. 
     Radio frequency broadcasting to mobile receivers is currently being implemented for satellite digital audio radio systems (see Report and Order, Federal Communications Commission, GEN Docket No. 90-357, Released Jan. 18, 1995 for more details) and has been used for many years for terrestrial radio including recently the inclusion of auxiliary digital data along with the audio program. It is possible in designing a radio broadcasting system to incorporate two or more transmission channels which transmit the same program information. Such multi-transmission channels may be provided for other purposes. Examples are transmission systems using spatial or frequency diversity for mitigating multipath. Alternatively, a multichannel transmission system can be purposely built to utilize time diversity. Hereafter the specification refers only to a two channel broadcast transmission system for simplicity. However, the same methods and apparatus described subsequently for two channel broadcast transmission systems are effective for three or more channel broadcast transmission systems. 
     The invention uses two transmission channels to provide time diversity for mitigating service outages from multipath, physical blockages and interference in mobile broadcast receivers. Each channel contains exactly the same data (e.g., program material). The program material in one of the channels is delayed at its origination source by a predetermined amount with respect to the second channel. The length of the delay is determined by the duration of the service outage which is to be avoided. At the mobile receiver, the transmission channel arriving first (i.e., with earlier data being the channel whose data were not delayed at its origination) is delayed using a storage type delay whose duration is the same as the duration of the delay in the data introduced at the origination point. The two received program channels are then combined or the program material in the two channels progressively selected by the receiver circuitry. The output of the combiner or selector in the mobile receiver always provides a time-ordered stream of all the program data even if there was a service outage in both transmission channels for a period equal to or less than the predetermined amount of delay introduced at the program origination point(s). 
     This invention is, in part, related to inventions disclosed and claimed in U.S. Pat. No. 5,278,863 which issued on Jan. 11, 1994 and U.S. Pat. No. 5,319,673 which issued on Jun. 7, 1994. Both patents are entitled &#34;RADIO FREQUENCY BROADCASTING SYSTEMS AND METHODS USING TWO LOW-COST GEOSYNCHRONOUS SATELLITES&#34;. The contents of these two patents are incorporated herein by reference as though fully set forth here. 
     SUMMARY OF THE INVENTION 
     The invention relates to mobile radio receivers for broadcasting systems in which two or more transmission channels are used to send the same information. The apparatus and methods of the invention use time diversity to avoid service outages for a predetermined time duration. Examples of such outages are those caused by physical blockage of the transmissions between the broadcast center(s) and the mobile receivers, or by lack of sufficient received signal strength at the mobile receivers due to multipath fading, or by external interference preventing reception of the desired transmissions. The apparatus and methods of this invention delay one of the transmissions at its point of origin with relation to the second transmission by a desired predetermined amount of time to avoid or minimize service outage. In preferred embodiments, this time delay is in the range of about 1 second to about 5 minutes, or less. 
     The mobile receivers are designed to receive both transmission channels. The receivers delay the earlier arriving transmission (determined from a priori knowledge of which transmission was not delayed at origination, or from time header data in the transmission, or from other transmission characteristics specifically inserted for this purpose) by the predetermined amount of time using some form of storage delay circuitry or device. The transmission internally delayed in the receiver, and the other received transmission, which had been delayed at its origination point, are applied in the mobile receiver to a comparison circuit, typically one that either selects the transmission which is available, or combines the two transmissions when one or both are above an acceptable service threshold. The combining method results in twice the signal being available when no service outage conditions exist. The resultant signal out of the selector or combiner is an unbroken, continuous stream of the original broadcast data (e.g., program material) for use by the mobile receiver operators, even though both transmissions had not been receivable during the predetermined dual outage avoidance period. If the transmissions contain several different programs multiplexed together, and if it is desired to avoid dual outage on only the program being played by the receiver, the mobile receiver can be configured to apply the internal delay after the demultiplexer for the selected program. 
     The recovered signal at the mobile receiver output is an exact replica of the original broadcast transmission, despite the dual outage occurrence, but is delayed in real time from the time of origination to the time available at the output of the selector/combiner by the predetermined dual outage avoidance period. For one-way broadcast services, this real time delay poses no technical, operational or service problems. The duration of the predetermined dual outage avoidance period is preferably chosen to be longer than the expected multipath fades and the blockage outages caused by obstructions. For blockage outages, this avoidance period preferably exceeds the average lengths of time such outages are expected to persist at the mobile receiver platforms. The duration of the blockage outages is a function of platform speed. 
     The delay storage device in the mobile receiver can take many forms. For digital transmissions, a buffer shift register is preferred. Since the storage function is sequential, random access to the stored data is not required. As an alternative, the delay storage device could be a relatively inaccurate bulk storage element followed by a delay synchronizer. The synchronizer accurately adjusts the transmission to be delayed so that its time alignment is identical to the other received transmission. 
     A mobile receiver using time diversity can operate within a broadcast system where the origin for the two transmissions is the same point or different points, and where the transmitter(s) are collocated or remote. The simplest configuration is a single origination and single transmitting facility. For example, two transmissions may be sent from the same transmitting facility at different radio frequencies, such as by terrestrial broadcast transmitters or satellite broadcast transmitters as shown in FIGS. 1 and 2, or at the same frequency using intertransmission isolation techniques such as code division multiple access (CDMA) or cross polarization. Alternatively, a broadcast channel operating at a single radio frequency can be divided in half to create two transmission channels from a single transmission channel. One way to accomplish this digitally is to allocate half the transmitted bits to one radio broadcast signal and half to the other radio broadcast signal, alternating between the two on a bit basis or block basis, for example, by time division multiplexing. Two ways to accomplish this in an analog scheme are: (a) use of two subcarriers or (b) use of frequency division multiplexing. In any channel division scheme, the resulting broadcast capacity in terms of broadcast power, or number of programs broadcast, is half what the capacity would be without division. 
     In broadcast systems employing spatial diversity, as shown in FIG. 3, two transmission paths already exist. It is only necessary to delay one transmission with respect to the other to achieve time diversity. Where two non-collocated transmitters are used, the mobile receivers also have a differential delay between the two received signals, except where a receiver is equidistant from the two transmitters. The receiver easily compensates for this differential delay since the correct amount of time diversity delay is predetermined and the differential delay is normally small. Differential delays also arise where the origin points for the two transmissions are physically different. For such delays, further mechanization is required at the transmitters to delay accurately one transmission with respect to the other, and to maintain this delay precisely. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The apparatus and methods of this invention can better be understood by reference to the drawings, in which: 
     FIG. 1 shows a mobile radio receiver in a terrestrial broadcast system using a central origination and transmitting facility configured for time diversity. The program output signal is divided into two identical parts by the splitter. This program output signal could be audio, such as voice or music, or data, or both in multiprogram broadcasts using multiplexers. The program material could be in analog or digital form. One of these two parts goes directly to a modulator/transmitter, and is broadcast from the antenna at frequency f 1 . The second part goes through a device which delays it with respect to the first part by, for example, 3 minutes. The second part then goes to a modulator/transmitter, and is broadcast from the antenna at frequency f 2 . The two frequencies f 1  and f 2  are received by the mobile radio receivers everywhere within the coverage area, and the operation of these receivers is described below with reference to FIG. 7. 
     FIG. 2 shows a mobile radio receiver in a single satellite broadcast system using a central origination and transmitting facility configured for time diversity. The technical operation at the central origination facility is the same as described for FIG. 1 except the delay has been chosen for illustration to be 0.5 seconds, and the antenna transmitting frequencies f 1  and f 2  is an uplink satellite type. The two transmissions are received by a satellite, which is assumed to be carrying a frequency translation repeater. The repeater receives and amplifies f 1  and f 2 , translates them to the desired downlink frequencies, herein designated f 3  and f 4 , amplifies f 3  and f 4 , and transmits them to the earth through a satellite antenna whose beamwidth encompasses the desired geographical service area. The mobile radio receivers in the service area receive f 3  and f 4 . The operation of these receivers is described below with reference to FIG. 7. 
     FIG. 3 shows a mobile radio receiver in a multi-satellite broadcast system using a central origination facility configured to provide both spatial and time diversity. For simplicity, only two satellites are shown. The technical operation at the central origination facility is similar to FIG. 2 with three differences. One difference is that a 2-second delay is used for illustration. A second difference is that two uplink satellite antennas are used. Third, the two program signals are modulated using spread spectrum with two specific pseudo-noise codes which are orthogonal to each other. This allows both program signals to be transmitted at the same radio frequency, and shows that time diversity operation is feasible with a wide range of modulation types. The two satellites receive the transmissions at frequency f 1  and, identically to the satellite in FIG. 2, retransmit f 1  at translated frequency f 2  to the desired geographical service area. The mobile radio receivers in the service area receive the f 2  transmissions with the orthogonal pseudo-noise codes. Their technical operation is described below with reference to FIG. 8. Since both satellites carry the same program, spatial diversity is also available in this illustration. Spatial diversity mitigates service outages from blockage and multipath which includes situations where the blockage of one satellite is longer than the service outage avoidance period provided in the mobile receiver, where the mobile receiver is slowly moving or stopped, etc. 
     FIG. 4 illustrates service blockage outage of mobile radio receivers in a broadcast system. As shown, service outages from blockage affect both terrestrial and satellite broadcast systems. 
     FIG. 5 illustrates the existing causes of multipath fading in mobile radio receivers which affects both terrestrial and satellite broadcast systems. As shown, the exiisting mobile receiver has the signal in a desired transmission path corrupted by the same signal arriving later through specularly or diffusely reflected transmission paths. 
     FIG. 6 provides an illustrative timing diagram of the two transmissions as originated in the broadcast system and after delay processing in a mobile radio receiver configured for time diversity as shown below in FIG. 7. 
     FIG. 7 shows a generalized block diagram of a dual frequency mobile radio receiver configured for time diversity operation. The configuration is compatible with the broadcast systems described in FIGS. 1 and 2. The mobile receiver contains an antenna whose bandwidth and beamwidth are designed to receive both f 1  and f 2 . The received transmissions are amplified in a preamplifier and downconverted to a convenient frequency for demodulation. The downconverted transmissions f 1  &#39; and f 2  &#39; are filtered and demodulated. The demodulated f 1  &#39; is delayed by the same time period that f 2  was delayed at the origination facility (see FIGS. 1 or 2). The f 1  &#39; delay is accomplished using an adjustable delay whose adjustment is dynamically controlled by a comparator which is designed to keep f 1  &#39; after delay in synchronism with f 2  &#39;. After this has been accomplished, f 1  &#39; and f 2  &#39; are fed to a selector or combiner whose timing diagram is illustrated in FIG. 6 and whose operation is described below. The output of the selector or combiner is used by the program presentation device(s) in the mobile receiver such as a digital to analog converter followed by stereo audio amplifiers and speakers. Although the adjustable delay is shown at baseband following the demodulator, the delay could also be inserted at intermediate or radio frequency portions of the receiver. 
     FIG. 8 shows a generalized block diagram of a single frequency mobile radio receiver configured for time diversity operation. The configuration is compatible with the broadcast system described in FIG. 3. It operates identically with that described in FIG. 7, except that the two demodulators are designed for spread spectrum demodulation with isolation between the two transmissions obtained by code division multiple access (CDMA) using two orthogonal pseudo-noise codes. 
     FIG. 8A shows one specific configuration in a mobile radio receiver where the broadcast system is assumed to transmit standard FM analog modulation for the program, accompanied by a subcarrier carrying a timing signal. The timing signals permit the comparator in the mobile radio receiver to align the received f 1  &#39; and f 2  &#39; transmissions for achieving time diversity operation as shown in the block diagram. 
     FIG. 8B shows a configuration similar to FIG. 7, but here the broadcast system transmits 4-phase offset quadrature phase shift keyed (OQPSK) digital modulation with several programs multiplexed using time division multiplexing. FIG. 8B shows how the time diversity delay in the mobile radio receiver can be applied only to the specific program selected by the mobile user. 
     FIG. 8C shows a mobile radio receiver wherein the radio frequency portion contains a first antenna, amplifier, downconverter and filter for frequency f 1  and a second antenna, amplifier, downcoverter and filter for f 2 . This configuration allows each of the previously mentioned receiver elements to be narrowband (i.e., just wide enough to pass their respective transmission), and is particularly effective when f 1  and f 2  are far apart in the radio frequency spectrum. The configuration in FIG. 8C is also useful if one radio frequency is used for broadcasting both programs, and isolation between the two transmissions is achieved using cross polarization. In such a case, one of the receiving antennas operates either right hand circular or vertically polarized, and the other, left hand circular or horizontally polarized. 
     FIG. 8D shows a mobile radio receiver design using digital signal processing (DSP). The DSP device employs a combination of digital hardware and software algorithms to filter, demodulate, delay and process the received transmissions including, depending on the transmission radio frequency and economics, down conversion as shown in the diagram by the dotted lines. 
     FIG. 8E shows an embodiment of the adjustable delay and comparator circuits shown in other figures. The design assumes digital transmission of the broadcast programs with the data in blocks containing identifying headers. FIG. 8E shows that the comparator uses these headers to achieve precise alignment of the two program digital streams. If the headers are not aligned at the start of each data block, a clock is started for the length of the time offset, and the clock output is converted to a correction signal for the adjustable delay. FIG. 8E also shows that, for long service outage avoidance periods, the majority of the delay need not be adjustable. This allows more economical delay devices to be used because bulk, fixed delay storage works satisfactorily for most of the required delay period followed by a smaller adjustable delay for precise time alignment. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Two preferred embodiments are described. The first is a local area terrestrial broadcast system, and the second is a national coverage satellite broadcast system. 
     The configuration of the local area terrestrial broadcast system, shown in FIG. 1, includes a central program origination facility in proximity to a radio frequency transmitter. To employ time diversity, the transmitter has been constructed so that the program material can be broadcast at two radio frequencies rather than one as is currently done. It is assumed for purposes of illustration that the transmission is standard frequency modulated (FM) program material with one of the FM subcarriers modulated with a time code, and that a three minute service outage avoidance period has been chosen. The service outages to the mobile radio receivers can be caused by blockages as illustrated in FIG. 4, or by multipath fading as illustrated in FIG. 5, or by radio interferers, or by combinations thereof. The program information sent on frequency f 2  is delayed by three minutes with respect to that on frequency f 1  at the transmitter before transmission. 
     Mobile radio receivers in various automobiles and trucks throughout the local area served by the previously described FM radio station are designed for time diversity operation. They receive both transmitted frequencies f 1  and f 2 , demodulate the FM modulated signals using known techniques, and receive the program material and the time code signals from the subcarriers. The program material received on frequency f 1  is delayed in the receiver by three minutes, using a comparison of the time codes to make the final delay adjustment precise. The program material from the now-delayed f 1  received signal, and the program material from the f 2  received signal are applied to a selector or combiner circuit. The selector circuit chooses the program from f 1  or f 2  depending on availability, and in a continuous sequence, based on the time code. If a combiner is used, the two program signals are continuously summed, provided one or both are above a predetermined threshold level. In any case, the output from the selector/combiner is the continuous program material which has no discontinuities, even during times that the mobile radio receivers were unable to receive both transmission for up to three minutes. The generalized block diagram of the radio receiver is shown in FIG. 7, and a specific configuration is illustrated in FIG. 8A. 
     The second embodiment is a satellite radio broadcast system providing national coverage. For illustration purposes, a two-satellite broadcast system is described as shown in FIG. 3 where the two satellites are used to provide spatial diversity. Spatial diversity is also useful for mitigating multipath and blockage service outage, and is particularly effective for slow or stopped mobile receivers. In this embodiment, program materials are assumed to be transmitted in digital form, divided into data blocks each having a header which includes, among other information, a sequential block number. This is a normal digital data transmission format based on efficient communications coding and information handling. Since spatial diversity was already employed in the illustrated system, no additional transmission facilities are required to implement time diversity in the origination, transmission or radio reception. The mobile radio receivers are already capable of two channel operation and, in this case, require primarily the addition of the delay storage device. 
     For this second embodiment, the program material is sent from the originating center through the uplink facilities to the two satellites. The program material sent to the satellite on the right side of FIG. 3 is delayed at the program origination facility by the desired service outage avoidance time period, here assumed to be 2 seconds, with the digital data block previously described assumed to be 1 second in length. For this embodiment, the two transmissions are assumed to be sent at the same radio frequency using code division multiple access (CDMA), a form of spread spectrum modulation which is one method of providing the isolation needed between the two transmissions in spatial diversity systems. However, time diversity also works well if the isolation in a spatial diversity system was achieved by using two different radio frequencies, or by using cross-polarized radio transmissions from the satellites. 
     The mobile radio receivers receive and demodulate both transmissions. The generalized block diagram of the radio receiver is shown in FIG. 8. The receiver delays the transmission received from the satellite on the left side of FIG. 3 by the predetermined service outage period, in this case 2 seconds. The receiver determines which transmission is from which satellite by the particular pseudo-noise code used in the CDMA, and can precisely align the two transmissions using synchronization data in the data block headers. The two received transmissions are then sent to the selector/combiner circuit. 
     The operation of the mobile radio receiver using time diversity is illustrated by the timing diagram of FIG. 6. The diagram shows the two signals received from the satellites at a particular mobile receiver versus real time. The broadcast signal has been digitized (e.g., analog music to digital), and placed in data blocks at the source for processing and handling since, preferably, signal compression, multi-source multiplexing, coding and interleaving would be performed before transmission to the satellites. Each data block would have a digital block sequential identification number in its header which is shown in the diagram as an Arabic number ascending with time. 
     It is assumed that a dual transmission outage of two seconds duration occurs as shown in the diagram. In real time, this means that data blocks 7 and 8 are not received from satellite one (the undelayed satellite transmission), and data blocks 5 and 6 are not received from satellite two (the delayed satellite transmission). In this example, all data blocks are 1 second long. The third line from the top of the diagram shows the satellite one transmission which has now been delayed two seconds by the mobile receiver internal delay and storage circuitry. This third line and the second line are used by the receiver selector or combiner subsystem which picks the best quality data block to output to the program user. These two lines are marked with stars and, depending on initial conditions, the output from a selector is either: 
     (1) Satellite two, data blocks 1, 2, 3 and 4 followed by satellite one, data blocks 5 and 6 followed by satellite two, data blocks 7, 8, 9 . . . or 
     (2) Satellite one, data blocks 1, 2, 3, 4, 5 and 6 followed by satellite two, data blocks 7, 8, 9 . . . 
     In both cases, all data blocks are recovered despite the dual transmission outage in correct real time sequence, but with a 2-second delay from real time. Alternately, the output from a combiner would be data blocks 1 through 10 with data blocks 1, 2, 3, 4, 9 and 10 at twice the amplitude of data blocks 5, 6, 7 and 8. 
     In this embodiment, there is an additional delay time between receiving the transmission from one satellite and the other unless the mobile receiver is equidistant from both satellites. This additional differential delay is preferably compensated by the precise alignment adjuster prior to the selector/combiner. 
     In all embodiments, the output of the selector/combiner feeds the normal mobile radio receiver program presentation device such as an audio detector, amplifier and loudspeakers and/or a visual display such as a Liquid Crystal Display (LCD) or facsimile and/or a data terminal such as a computer.