MXPA98000841A - Cancellation of autointerference for communication bipartita retransmit - Google Patents

Abstract

The present invention relates to a signal transmitted from a source scanned at the receiver associated with the transmitter, so that the desired received signal can be extracted from a received composite signal, the received composite signal consisting of the transmitted signal of retransmitted origin of the Retransmitter station (Retransmitter Device) together with the other user's desired received signal in the pair, plus additive noise. The invention takes advantage of the fact that each of the users (User 1, User 2) knows a priori the exact structure of their transmitted signal of origin and can estimate the characteristics of the channel between the relay station and the same.

Description

CANCELLATION OF AUTO-INTERFERENCE FOR REPRESENTATIVE BIPARTITE COMMUNICATION BACKGROUND OF THE INVENTION The present invention relates to a radiofrequency or optical communication system in which a relay station is used to aid communication between two parts and more particularly to an improvement allowing the most efficient use of the available channel resource. In Fig. 1, the radiofrequency or optical communication between two physically separated devices is illustrated using a relay station being the third part. The relay station receives a signal from each of the two user devices and retransmits the composite signal received back to both users, generally in a frequency channel shifted from the reception frequency channel. Two examples are: 1. Satellite communication between two ground terminals (ground, on board aircraft, or on board ship) using a repeater without "elbow pipe" processing in a linear mode. 2. Earth optical path radio communication between two terminals, using a relay radio to connect them (as in an airplane "in captivity" or an antenna tower). The relay station can retransmit many more signals simultaneously than the two shown in Fig. 1. The retransmitter resource is usually shared using one of the many well-known access techniques such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Multiple Code Division Access (CDMA) or differential couplers which use one or more of these techniques in combination. In each of these techniques, the retransmitter resource is efficiently channeled, so that the receiver can, by tuning in at a specific frequency, at a specific time, with a specific code, or a combination thereof, isolate a particular received signal. of all the others. For simplicity, the following discussion will be based on the FDMA technique, although the apparatus can be used with TDMA, CDMA or differential access multiple access techniques. Fig. 2 shows a pair of separate signals in the frequency domain between User 1 and User 2. The signal transmitted by User 1 has a central frequency of f1 Hz and a bandwidth of Ñ1 Hz. The signal transmitted by User 2 has a central frequency of f2 Hz and has a bandwidth of 2 Hz. With the FDMA procedure, the total bandwidth required to support simultaneous transmissions of the user pair is (W -, + W2) Hz. The other signals that are retransmitted are assigned to other non-superimposed bandwidths. In this way the user number is limited by the bandwidth of the relay. Although it is a direct partition of the retransmitter resource among a number of users, it is not necessarily the most efficient use. In telephone communications, due to the limited bandwidth of the twisted pair circuits, there is a need for a more efficient use of the available bandwidth, in which the transmission and reception signals share the same bandwidth. However, this type of communication is not retransmitted, in the sense of the word used in the present, rather it is between fixed points, although the signal can be routed through several intermediate nodes on its way from the origin to the destination. Because the transmit and receive signals occupy the same bandwidth, devices known as differential couplers are used to separate the combined signals on the two wires at the transmit and receive portions. However, these devices, which are located in the two central offices of the telephone company, do not perfectly separate the signals, therefore, as an undesired consequence, echoes occur. In this way the received signal will contain two echoes of the retransmitted signal, a close echo from the local central office and a distant echo from the remote central office. For data communications, these echoes are eliminated by an echo suppressor, which subtracts a duly graduated and delayed replica of the retransmitted signal of the received signal. Because the signal is not retransmitted and there is no frequency shift between the transmit and receive signals, the echo suppressor can be implemented using an adaptive filter method. The filter coefficients can be estimated using any of the well-known algorithms such as the LMS algorithm (minimum quadratic mean). However, this method can not be extended to the case of a retransmitted communication system in which the transmission and reception frequencies are considerably different. Therefore, a different cancellation technique must be employed in order that the retransmitted signals are able to share the same bandwidth. What is needed is a mechanism to provide a more efficient use of the available channel resource to support a bilateral retransmitted communication for a pair connection between a group of radiofrequency or optical terminals. More specifically, what is needed is an improvement that is independent of the multiple access technique in use, independent of the modulation and coding of the system and that does not have the benefit of data compression techniques or needs special equipment in the station relay station SUMMARY OF THE INVENTION According to the invention, an originating transmitted signal is canceled at the receiver associated with the transmitter, so that the desired received signal can be extracted from a received composite signal, the received composite signal consisting of the transmitted signal of retransmitted origin of the relay station together with the desired received signal of the other user in the pair, plus additive noise. The invention takes advantage of the fact that each of the users knows a priori the exact structure of their transmitted signal of origin and can estimate the characteristics of the channel between the relay station and itself. The device at the beginning of the origin signal uses this knowledge to help estimate: • The round trip propagation delay to and from the relay station. • Relative signal amplitude, carrier frequency and phase of the origin signal retransmitted against the origin signal known a priori. Once the device in the sending station has determined these parameters, it can subtract a version with adjustment of time, frequency, phase and amplitude of the transmitted signal known a priori from the retransmitted composite signal (downlink) of the relay station. The remaining signal consists of the desired signal of the other user, plus additive noise due to the communication channel, plus an error signal resulting from the residual mismatch of time, phase, frequency and amplitude. The invention will be better understood by analyzing the drawings and technical description attached.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram illustrating the operation of a retransmitted communication system. FIG. 2 is a diagram illustrating the bandwidth required for conventional integral duplex communication. FIG. 3 is a diagram illustrating the bandwidth required for integral duplex communication with the present invention. FIG. 4 is a waveform diagram illustrating the self-interference cancellation technique. FIG. 5 is a block diagram showing the relationship of an interference suppressor according to the present invention with the other components of the transmitter / receiver, for applications in which the source information signal is digital in nature. FIG. 6 is a block diagram showing the relationship of an interference suppressor according to the present invention with the other components of the transmitter / receiver, for applications in which the source information signal is analog in nature. FIG. 7 is a block diagram of a specific embodiment of the present invention.

FIG. 8 is a waveform schedule illustrating a possible instruction sequence for use with the present invention. FIG. 9 is a block diagram illustrating an embodiment of the delay parameter estimator. DESCRIPTION OF SPECIFIC EMBODIMENTS Fig. 3 shows a benefit of the invention, mainly, the greater use of the bandwidth available through the superposition of signals. Using this invention, User 1 and User 2 can be transmitted simultaneously using the same central frequency, f 1 Hz. The resulting total bandwidth occupied by the pair of signals is only Y 1 or 2 Hz, whichever is greater, a saving of up to 50% compared to the (W1 + W2) Hz required without the invention, leaving the bandwidth free for other users. In a TDMA environment, User 1 and User 2 can share the same slot, leaving other slots free for more users. In a CDMA system, users can also share the same expansion code. The invention works because the device that creates a signal has knowledge of the source signal and is able to estimate the characteristics of the channel.

The concept of combined signals is illustrated in Fig. 4. In trace 1, a combined signal S1 + S2 is represented. Trace 2 shows the first signal of origin SI. The trace 3 shows the second origin signal S2. The relay station always transmits combined signals. The first source signal can be subtracted from the combined signal to recover the second source signal, that is, the signal from the other device. While the displayed signal employs amplitude shift manipulation, other modulations can also be used. In this example, it is observed that at various times the two origin signals, SI and S2, cancel each other, producing a level of zero in the combined signal S1 + S2. It is important to note that although sometimes the combined signal can be zero, subtract the first source signal IF it still recovers the second source signal S2 and therefore this phenomenon will not affect the operation of the current invention. The invention can be implemented through hardware, firmware, or digital signal processing software. It can also be implemented largely in analog (linear) hardware. Recognizing that the invention can serve as an improvement to the existing communication equipment as illustrated in Figs. 5 and 6, the choice of implementation for the invention will depend on the particular application. Fig. 5 illustrates the configuration of a transmitter / receiver employing the current invention for an application in which the source information signal is digital in nature. In this case it is more efficient to delay, or store, the source information signal instead of the modulator output. In this way, the representation of the origin signal SI that is used by the interference suppressor is the origin information signal. On the other hand, if the source information signal is analog by nature, the configuration of Fig. 6 may be preferable, in which the output of the modulator is the representation of the origin signal used by the interference suppressor. If the round-trip transmission time to and from the retransmitting device is sufficiently small, the choice of configuration becomes arbitrary. If the configuration of Fig. 5 or Fig. 6 is employed, it should be evident that both communicating parties should use a transmitter / receiver employing the current invention. For simplicity, only the operation of the equipment of the User 1 will be described. Referring to Fig. 5, each device 100 has a modulator 102, a transmitter 104, a transmission antenna 106 or equivalent signal radiator, a receiving antenna 108. , a receiver 110, an interference suppressor 112 and a demodulator 114. Alternatively, the user device could be configured with a single antenna and a diplexer in place of the transmission antenna 106 and the reception antenna 108. The modulator 102 converts the Source information signal in a format suitable for transmission. The transmitter 104, through the transmission antenna 106, directs a signal of origin SI to the relay station. Through the reception antenna 108, the receiver 110 receives the composite signal Sl '+ S2' and the interference suppressor compares a signal representing the composite signal Sl '+ S2' and subtracts a signal representing SI 'on the basis of an estimate of the transmission characteristics of the channel and prior knowledge of the signal representing SI, contained in the source information signal, to obtain a signal representing S2 '. The demodulator 114 recovers the desired information signal from the signal representing the recovered signal S2 '. The configuration of Fig. 6 differs from the configuration of Fig. 5 just described only in that the interference suppressor obtains prior knowledge of the signal representing SI of the output of the modulator 102 and not of the origin information signal. Fig. 7 illustrates an embodiment of the interference suppressor 112. It has five elements: a variable delay element 20 for storing a representation of the source signal, a modulator / mixer element 22, a programmable gain element 24, a subtracter 26 and a parameter estimation element 28. The origin signal which is transmitted as SI, or the origin information signal used to produce SI, is provided through the variable delay element 20, after the delay, through the modulator element / mixer 22 and in the programmable gain element 24 to the subtractor 26. The signal representing the received composite signal Sl '+ S2' is supplied to both the parameter estimation element 28, and the subtractor 26. The element of parameter estimation 28 is dynamically programmed to provide time, frequency, phase and gain correction to the variable delay element 20, the modulator / mixer 22 and the programmable gain element 24, respectively. The output of subtractor 26 is the desired component of the received signal. More specifically, the interference suppressor 112 consists of the following main elements: • Variable delay element 20: It is used to delay the signal of known origin by a quantity of time t, so that it is correctly aligned in time with the component of the origin signal of the received composite signal. • Modulator / mixer element 22: Used to adjust the frequency and phase of the signal of known origin so that it has the same frequency and phase of the component of the origin signal of the received composite signal. The component of the signal of known delayed origin is multiplied by eJ <; 2pfot + 0 > where f0 is the estimated relative frequency difference between the known and received signals and 0 is the estimated relative phase difference. If the signal of known origin is stored in the delay element 20 in the form of the origin information signal, then this element must also repeat the function of the modulator 106. • Programmable gain element 24: Used to adjust the amplitude relative to the signal of known origin so that it is the same as the component of the origin signal of the received composite signal. • Subtractor 26: Used to linearly subtract the adjusted source signal of the received composite signal. • Parameter estimation element 28: Used to derive the adjustment parameters, t, f0, 0 and G, each of which may be of variable time, are used to correctly align the source signal with the component of the origin signal of the received composite signal. Each of these components has one or more well-known embodiments. There are many possible implementations for the parameter estimation function and the inven does not depend on a particular one. An example method for initially estimating the desired parameters is described below. It is assumed that the retransmitter channel is linear, therefore the presence or absence of the other user's signal, S2, will not affect the reliability of the estimation of any of the four parameters (t, fof 0 and G) required to modify the stored representation of SI to produce the cancellation signal. The parameters can be estimated using convenal open or closed cycle estimation algorithms. For example, the correlation techniques can be used if the other signal is present or not, the only difference is that if the other signal is present it seems as a considerable noise component towards the correlation medium and in this way the duration of the correlation it must be increased. If a rapid initial estimation of the four parameters is required, a simple training protocol can be used, as illustrated in Fig. 8. In this embodiment, by prior agreement, User 2 suppresses the transmission of his signal in and by a predetermined time period. During that time, the User 1 device estimates its parameters. Then the User 1 device suppresses its signal for a predetermined period of time while the User 2 device estimates its parameters. Then both users can start the communication, using the estimated parameters to initialize the cancellation devices. With the present inven, many different training protocols are possible. After the parameters of the interference cancellation device have been initialized, they must be updated (tracked) often to decrease the component of the error signal over the course of the bilateral communication. In Fig. 9 a technique for generating an error signal to track the delay parameter is shown. It uses an "early-late" activation technique to regularly determine whether a slightly smaller or slightly larger delay improves the correlation of the a priori known signal with the composite signal received from the relay. The error signal is used as the input to a locked classical delay cycle to maintain the best estimate of the delay parameter t. The carrier and phase frequency parameters can be tracked using a similar technique.

By way of summary, the inven comprises a method for bilateral retransmitted electromagnetic communication, such as radio or optical, between a first device and a second device through a relay station, such as a satellite repeater or a terrestrial repeater, comprising the following steps: generate on each device a source signal; storing a representation of the origin signal, which may be the origin signal itself or the origin information signal carried by the origin signal; estimating the characteristics of the channel of a round trip path to the retransmitting station of the device to obtain the compensation parameters; transmit the origin signal to the relay station where all the signals are retransmitted; receiving on each device a composite signal from the relay station, where the composite signal contains a copy of the representation of the origin signal of each pair transmission device; modify the stored representation using the compensation parameters to obtain a cancellation signal and subtract the cancellation signal of the composite signal to recover a desired signal, the desired signal being the origin signal of the other device. The method could be limited for use with transmission in a first common frequency transmission channel and reception in a second common reception frequency channel to optimize bandwidth utilization. The method could also be used in time division multiple access systems or code division multiple access systems, sharing time slots and expansion codes, respectively. In addition, the method could be used in a differential combination of these multiple access techniques (FDMA, TDMA and CDMA). An apparatus according to the invention is used in devices in a system for bilateral communication retransmitted through a relay station, wherein each device includes a modulator, a transmitter, a receiver and a demodulator. The apparatus may comprise the combination of means for capturing an origin signal addressed to the transmitter of the modulator or the origin information signal, means for storing a representation of the origin signal or origin information signal, means for estimating the characteristics of the channel of a round trip path to the relay station to obtain compensation parameters, means for extracting a composite signal from the receiver where the composite signal contains a copy of the source signal, means coupled to the estimator to modify the stored signal using the compensation parameters to obtain a cancellation signal and means coupled to the modification means and to the extraction means to subtract the cancellation signal of the composite signal to recover a desired signal, the desired signal being the origin signal of the other device. The invention has now been explained with reference to specific embodiments. Other embodiments will be apparent to those skilled in the art. Therefore, it is not intended that the invention be limited except as indicated by the appended claims.

Claims (8)

1. CLAIMS 1. A method for bilateral electromagnetic communication retransmitted between a first device and a second device through a relay station comprising the steps of: generating on each first and second device a source signal; store a representation of the source signal as a signal stored in each device; estimate the characteristics of the channel of a round trip path to the relay station to obtain the compensation parameters of each device; transmit the source signal of each device to the relay station; receiving on each device a composite signal from the relay station, the composite signal containing a copy of the source signal of each device; modify the signal stored in each device using the compensation parameters to obtain a respective cancellation signal for each device; and subtracting in each device the respective cancellation signal of the composite signal to recover a desired signal, the desired signal being the source signal of the other device. The method according to claim 1 wherein each device transmits on a common first common frequency channel and receives on a second common reception frequency channel. The method according to claim 1 wherein each device transmits and receives in a common transmission time slot. The method according to claim 1 wherein each device transmits and receives an extended spectrum signal using a common expansion code. The method according to claim 1 wherein each device transmits and receives using a common channel allocation of a multiple access scheme of differential couplers. The method according to claim 1 wherein each first device and second device transmits an instruction sequence during periods where the other respective device is not transmitting to allow rapid analysis of the channel. The method according to claim 1 wherein each first device and second device performs the analysis of the channel without the benefit of an instruction sequence. 8. In a system for bilateral communication retransmitted between a first device and a second device through a relay station, wherein each first device and second device includes a modulator, a transmitter, a receiver and a demodulator, an apparatus in each first device and second device for allowing the system to share common communication channels, the apparatus comprising: means for capturing an origin signal addressed to the modulator transmitter or an origin information signal addressed to the modulator; means for storing a representation of the source signal; means for estimating the channel characteristics of a round trip path to the relay station to obtain compensation parameters; means for extracting a composite signal from the receiver, the composite signal containing a copy of the source signal; means coupled to the estimation means for modifying the stored signal using the compensation parameters to obtain a cancellation signal; and means coupled to the modification means and to the extraction means for subtracting the cancellation signal of the composite signal to recover a desired signal, the desired signal being the origin signal of the other device.