Abstract:
Method and apparatus for facilitating the simultaneous bi-directional transmission of data on a single AM carrier is described. In particular, the process is implemented using two or more transceivers, each comprising a modulator, a demodulator, and an oscillator operating at the same frequency and in locked phase. One of the oscillators functions as a master, to which all other oscillators (&#34;slaves&#34;) are locked. Each of the transceivers also includes means for recovering the appropriate signal to be demodulated by the particular transceiver.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of commonly assigned U.S. patent application Ser. No. 08/873,619 filed on Jun. 12, 1997, and claims the benefit of U.S. Provisional Patent Application Ser. No. 60/029,033, filed on Oct. 28, 1996, both hereby incorporated by reference in their entireties. 
     This application is related to copending U.S. patent application Ser. No. 08/955,482, U.S. patent application Ser. No. 08/955,533, U.S. patent application Ser. No. 08/956,244, U.S. patent application Ser. No. 08/955,480 (now U.S. Pat. No. 5,994,952 issued Nov. 30, 1999), all filed on even date herewith, assigned to the assignee of the present invention, and hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to radio frequency communications and, more particularly, to method and apparatus for enabling the simultaneous bi-directional transmission of information on a single AM carrier via a wired or wireless transmission medium. 
     BACKGROUND OF THE INVENTION 
     Transmission of data using a transmission system is typically conceptualized as a one-way transfer of data or information from a transmitting end, or transmitter, to a receiving end, or receiver. In radio frequency (&#34;RF&#34;) transmission systems that utilize amplitude modulation (&#34;AM&#34;) techniques to transmit data or information, an audio signal containing the data is used to modulate the amplitude of an RF carrier signal, wherein the data is contained in the side-band(s) of the signal created by the modulation technique. As is well known in the art, the side-band(s) comprises one or both the sum and difference of the carrier and data energy. A transmission system such as that described above is illustrated in FIG. 1. 
     As illustrated in FIG. 2, two-way, or bi-directional, simultaneous transmission is typically accomplished through the use of two carriers at different frequencies. Current technology enables either direction of transmission on a single carrier frequency, but not both directions at the same time. 
     Because individual carriers must be used for the transmission of amplitude modulated data, two carriers and their associated side-bands must be employed for the simultaneous transmission of two channels of data. The primary disadvantage associated with this technique is that it requires two transmissions, each occupying a specific amount of bandwidth based on the bandwidth of the data being processed to accomplish simultaneous bi-directional data transmission. 
     Therefore, what is needed is a technique for facilitating the simultaneous bi-directional transmission data using a single carrier. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment, the invention comprises method and apparatus for facilitating the simultaneous bi-directional transmission of information on a single AM carrier. In particular, the process is implemented using two or more transceivers, each comprising a modulator, a demodulator, and an oscillator operating at the same frequency and in locked phase. One of the oscillators functions as a master, to which all other oscillators (&#34;slaves&#34;) are locked. Each of the transceivers also includes means for recovering the appropriate signal to be demodulated by the particular transceiver. 
     A technical advantage achieved with the invention is that it significantly decreases the bandwidth needed to transmit two simplex or full duplex signals. 
     Another technical advantage achieved with the invention is that it improves the performance of signal transmission through the use of truly synchronous detection of the data and lower noise due to the common modality of noise and signal aberrations in both the carrier and data, resulting in their mutual cancellation. 
     Another technical advantage achieved with the invention is the operation of the modulation process down to DC signals. 
     Yet another technical advantage achieved with the invention is the use of one master clock for the entire transmission system. 
     Still another technical advantage achieved with the invention is the use of one or more high transient response narrow-band filters for carrier recovery. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a unidirectional transmission process using a single carrier. 
     FIG. 2 illustrates a bi-directional transmission process using two carriers. 
     FIG. 3 is a system block diagram of an RF transceiver system embodying features of the present invention. 
     FIG. 4 illustrates an alternative technique for separating data signals simultaneously transmitted on a single AM carrier. 
     FIG. 5 illustrates another alternative technique for separating data signals simultaneously transmitted on a single AM carrier. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 and 2 respectively illustrate techniques for enabling uni- and bi-directional transmission of data via one and two carriers, respectively. 
     FIG. 3 is a system block diagram an RF transceiver system 300 embodying features of the present invention. As shown in FIG. 3, the system 300 comprises a &#34;master&#34; transceiver 302, and one or more &#34;slave&#34; transceivers, represented in FIG. 3 by a slave transceiver 304, interconnected via a transmission medium 305. It should be noted that, although represented in FIG. 3 as a cable, the transmission medium may comprise any available type of medium, including both wired and wireless media. 
     The master transceiver 302 comprises a local oscillator 306M, a modulator 308M, a demodulator 310M, and a carrier recovery device 312M Similarly, the slave transceiver 304 comprises a modulator 308S, a demodulator 310S, and a carrier recovery device 312S. Each of the transceivers 302, 304, also optionally includes a bi-directional coupler 314M, 314S, respectively, for purposes that will be described in detail below. 
     In operation, an original master carrier (&#34;MCARRIER&#34;) generated by the local oscillator 306M is input to the modulator 308M, which modulates MCARRIER using a first input data stream, designated DATA1, resulting in a modulated data signal MDATA1, comprising a carrier and one or more side-bands, being output from the modulator 308M onto the transmission medium 305. 
     At the slave transceiver 304, MCARRIER is extracted from the signal received via the transmission medium 305 by the carrier recovery device 312S, which, in the preferred embodiment, is a fast response, low transient characteristic narrow-band filter, such as that described in the above-referenced U.S. patent application Ser. No. 08/955,480 (now U.S. Pat. No. 5,994,952 issued Nov. 30, 1999). The recovered carrier, hereinafter referred to as CARRIER --  A, as well as the signal received via the transmission medium 305 (i.e., MDATA1), are provided to the demodulator 310S for effecting the synchronous demodulation and recovery of DATA1 from the demodulator 310S, as will be described in greater detail below. It should be noted that, at this point, it is likely that the phases of MCARRIER and CARRIER --  A will be different, but the difference will remain fixed over time. MCARRIER and CARRIER --  A are otherwise of the same frequency. 
     Due to the characteristics of the carrier recovery device 312S, CARRIER --  A contains all of the same instantaneous phase and amplitude errors as the modulated side-band(s), due to mutually induced errors from both the cable 316 and the transmission process. Accordingly, a characteristic of the balanced demodulation process known as common-mode rejection reduces the effects of the errors on the modulated data, thus improving throughput, bandwidth, and error rate of the system 300. In addition, this exacting relationship between the carrier and the modulated data permits operation down to DC for DSB, SSB, AM and QAM. 
     CARRIER --  A is also input to the modulator 308S, which modulates CARRIER --  A using a second input data stream, designated DATA2, resulting in a modulated data signal MDATA2, comprising a carrier and one or more side-bands, being output from the modulator 308S onto the transmission medium 305. It will be recognized that the side-band(s) of MDATA2 exists at the same frequencies as the side-band(s) of MDATA1. 
     At this point, MDATA1 and MDATA2 both exist on the transmission medium 305 and therefore, unless corrective measures are taken, MDATA1+MDATA2 will be presented to both demodulators 310M and 310S, such that the signal output each of the transceivers 302, 304, will be DATA1+DATA2. There are at least three options for enabling the recovery of DATA1 and DATA2 separately by the slave transceiver 304 and master transceiver 302, respectively. The first, which is illustrated in FIG. 3, is to use bi-directional couplers 314M, 314S, to sort out MDATA1 from MDATA2 and send each along its respective path. The second, which is illustrated in FIG. 4 with reference to the slave transceiver 304, is to subtract MDATA2 from the MDATA1+MDATA2 signal, resulting in the recovery of DATA1 alone. Finally, as illustrated in FIG. 5, again with reference to the slave transceiver 304, DATA2 as it is input could be subtracted the output of the demodulator 310S and the remainder of MCARRIER could be filtered off. 
     This same event occurs back at the master transceiver 302. The carrier recovered by the carrier recovery device 312M (CARRIER --  B) is used to drive the demodulator 310M to recover DATA2. As indicated above, it is likely that the phases of CARRIER --  A and CARRIER --  B will be different, but the difference will remain fixed over time. CARRIER --  A and CARRIER --  B are otherwise of the same frequency. The issue of separation of combined signals is handled as described above with reference to the slave transceiver 304. 
     Alternatively, it will be recognized that the carrier recovery device 312M could be eliminated and MCARRIER provided directly to the demodulator 310M by the local oscillator 306M. The disadvantages to this embodiment are that MCARRIER, unlike CARRIER --  B, will not contain any noise or error information induced in the signal during transmission from the slave transceiver 304 to the master transceiver 302 and therefore such noise and error information will not cancel out. Accordingly, the preferred embodiment is to include in the master transceiver 302 the carrier recovery device 312M. 
     It is important to note that the receive portions of both transceivers 302, 304, can operate as true synchronous detectors. As the side-band(s) and the recovered carrier possess the same phase and amplitude anomalies acquired in the transmission transfer process, the induced signal aberrations are reduced or eliminated in the detection process through their common modality when demodulated with a balanced modulator-type detector. Advantages of this are the reduction of the effects of outside noise or other transmission errors, as well as the ability to demodulate single and double side-band, quadrature amplitude modulation, and variants of those, including FSK with higher fidelity, i.e., wider bandwidth, lower noise, lower distortion, and extended low frequency response, than conventional techniques. 
     In addition to these aspects of the process, the stable relationship between the two carriers and their respective side-bands makes it easy to use the two (upper and lower) side-bands to carry independent data while retaining the simultaneous bi-directional characteristics. In this variation, the data density of the single carrier is again doubled, providing dual-channel bi-directional data flow on a single carrier. 
     Although an illustrative embodiment of the invention has been shown and described, other modifications, changes, and substitutions are intended in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.