CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims benefit under 35 USC 119(e) of US Application Serial Number 62/699,208, filed July 17, 2018, the content of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to a communication system, and more particularly to a scalable phased array that includes isolated transmitter and receiver units.

BACKGROUND

[0003] Phased arrays are used in communications, arbitrary field manipulation, ranging, and sensing applications like radar. A phased array system capable of beamforming using only electronic components— and without any mechanical parts— has an enhanced flexibility and robustness, a relatively higher resolution scan, faster scanning rate, and an improved overall performance.

[0004] A phased array includes an array of antenna elements each with an independent phase controller. By tuning these phases, an array can utilize the coherent addition and subtraction of propagating waves to shape the reception or transmission pattern. A phased array transmitter often drives its antennas with the same signal and different phases to generate a steerable propagating beam. A phased array receiver adds the signals it receives from its elements in order to increase sensitivity to a certain direction while attenuating signals received from other directions. Such beamforming allows for wireless transmission and reception along a given direction without the need to physically move a large antenna.

[0005] Systems such as radars and telecommunication equipment need to rapidly transmit and receive data. Such systems may use two antennas, one for transmission and one for reception. Alternatively, such systems can use one antenna to receive and transmit data but at different times. Systems that use one antenna for signal transmission and another antenna for signal reception occupy twice the amount of space or have half the bandwidth of system using a single antenna. Therefore, it is highly desirable to have a full duplex system capable of transmitting and receiving data continuously with the same antenna. Such a system would require a high degree of isolation between its transmitter and receiver units.

[0006] In order to achieve full transmission from the transmitter to antenna and antenna to receiver, conventional systems require multiple components such as magnetic elements, active circuits, or time-varying circuits. Such elements are relatively large and expensive. A need continues to exist for a full duplex phased array that used a single non-reciprocal element to drive its antennas.

SUMMARY OF THE INVENTION

[0007] A communicate device, in accordance with one embodiment of the present invention, includes in part, a first transmitter coupled to a first terminal of a first 90° phase shifter having a second terminal coupled to a first antenna, and to a first terminal of a second 90° phase shifter having a second terminal coupled to a first node; a second transmitter coupled to a first terminal of a third 90° phase shifter having a second terminal coupled to a second antenna, and to a first terminal of a fourth 90° phase shifter having a second terminal coupled to the first node; a receiver coupled to a first terminal of a fifth 90° phase shifter having a second terminal coupled to the first antenna, and to a first terminal of a sixth 90° phase shifter having a second terminal coupled to the second antenna; and a non reciprocal element coupled between the receiver and the first node. The non reciprocal element provides a 90° phase shift from the receiver to the first node and a -90° phase shift from the first node to the receiver.

[0008] In one embodiment, the communicate device further includes, in part, a third transmitter coupled to a first terminal of a seventh 90° phase shifter having a second terminal coupled to a third antenna, and to a first terminal of an eighth 90° phase shifter having a second terminal coupled to the first node. The third antenna is coupled to the receiver via a ninth 90° phase shifter. [0009] In one embodiment, the communicate device further includes, in part, a fourth transmitter coupled to a first terminal of a tenth 90° phase shifter having a second terminal coupled to a fourth antenna, and to a first terminal of an eleventh 90° phase shifter having a second terminal coupled to the first node. The fourth antenna coupled to the receiver via a twelfth 90° phase shifter.

[0010] In one embodiment, the communication device further includes, in part, a first phase shifter adapted to shift a phase of a signal generated by the first transmitter, and a second phase shifter adapted to shift a phase of a signal generated by the second transmitter.

[0011] In one embodiment, the communication device further includes, in part, a first phase shifter adapted to shift a phase of a signal generated by the first transmitter, a second phase shifter adapted to shift a phase of a signal generated by the second transmitter, and a third phase shifter adapted to shift a phase of a signal generated by the third transmitter.

[0012] In one embodiment, the communication device further includes, in part, a third phase shifter adapted to shift a phase of a signal received by the first antenna; and a fourth phase shifter adapted to shift a phase of a signal received by the second antenna. In one embodiment, the communication device further includes, in part, a fourth phase shifter adapted to shift a phase of a signal received by the first antenna, a fifth phase shifter adapted to shift a phase of a signal received by the second antenna, and a sixth phase shifter adapted to shift a phase of a signal received by the third antenna.

[0013] A communicate device, in accordance with one embodiment of the present invention, includes, in part, a first receiver coupled to a first terminal of a first 90° phase shifter having a second terminal coupled to a first antenna, and to a first terminal of a second 90° phase shifter having a second terminal coupled to a first node; a second receiver coupled to a first terminal of a third 90° phase shifter having a second terminal coupled to a second antenna, and to a first terminal of a fourth 90° phase shifter having a second terminal coupled to the first node; a transmitter coupled to a first terminal of a fifth 90° phase shifter having a second terminal coupled to the first antenna, and to a first terminal of a sixth 90° phase shifter having a second terminal coupled to the second antenna; and a non reciprocal element coupled between the transmitter and the first node. The non- reciprocal element provides a -90° phase shift from the transmitter to the first node and a 90° phase shift from the first node to the transmitter.

[0014] In one embodiment, the communicate device further includes, in part, a third receiver coupled to a first terminal of a seventh 90° phase shifter having a second terminal coupled to a third antenna, and to a first terminal of an eighth 90° phase shifter having a second terminal coupled to the first node. The third antenna is coupled to the third receiver via a ninth 90° phase shifter.

[0015] In one embodiment, the communicate device further includes, in part, a fourth receiver coupled to a first terminal of a tenth 90° phase shifter having a second terminal coupled to a fourth antenna, and to a first terminal of an eleventh 90° phase shifter having a second terminal coupled to the first node. The fourth antenna is coupled to the fourth receiver via a twelfth 90° phase shifter.

[0016] In one embodiment, the communicate device further includes, in part, a first phase shifter adapted to shift a phase of a signal received by the first receiver, and a second phase shifter adapted to shift a phase of a signal received by the second receiver.

[0017] In one embodiment, the communicate device further includes, in part, a first phase shifter adapted to shift a phase of a signal received by the first receiver, a second phase shifter adapted to shift a phase of a signal received by the second receiver, and a third phase shifter adapted to shift a phase of a signal received by the third receiver.

[0018] In one embodiment, the communicate device further includes, in part, a third phase shifter adapted to shift a phase of a signal delivered for transmission by the first antenna, and a fourth phase shifter adapted to shift a phase of a signal delivered for transmission by the second antenna.

[0019] In one embodiment, the communicate device further includes, in part, a fourth phase shifter adapted to shift a phase of a signal delivered for transmission by the first antenna, a fifth phase shifter adapted to shift a phase of a signal delivered for transmission by the second antenna, and a sixth phase shifter adapted to shift a phase of a signal delivered for transmission by the third antenna.

[0020] A method of communication, in accordance with one embodiment of the present invention, includes, in part, coupling a first transmitter to a first terminal of a first 90° phase shifter having a second terminal coupled to a first antenna, and to a first terminal of a second 90° phase shifter having a second terminal coupled to a first node. The method further includes, in part, coupling a second transmitter to a first terminal of a third 90° phase shifter having a second terminal coupled to a second antenna, and to a first terminal of a fourth 90° phase shifter having a second terminal coupled to the first node. The method further includes, in part, coupling a receiver to a first terminal of a fifth 90° phase shifter having a second terminal coupled to the first antenna, and to a first terminal of a sixth 90° phase shifter having a second terminal coupled to the second antenna. The method further includes, in part, coupling a non-reciprocal element between the receiver and the first node. The non-reciprocal element provides a 90° phase shift from the receiver to the first node and a -90° phase shift from the first node to the receiver

[0021] In one embodiment, the method further includes, in part, coupling a third transmitter to a first terminal of a seventh 90° phase shifter having a second terminal coupled to a third antenna, and to a first terminal of an eighth 90° phase shifter having a second terminal coupled to the first node. The third antenna is coupled to the receiver via a ninth 90° phase shifter.

[0022] In one embodiment, the method further includes, in part, coupling a fourth transmitter to a first terminal of a tenth 90° phase shifter having a second terminal coupled to a fourth antenna, and to a first terminal of an eleventh 90° phase shifter having a second terminal coupled to the first node. The fourth antenna is coupled to the receiver via a twelfth 90° phase shifter

[0023] In one embodiment, the method further includes, in part, shifting a phase of a signal generated by the first transmitter, and shifting a phase of a signal generated by the second transmitter. In one embodiment, the method further includes, in part, shifting a phase of a signal generated by the first transmitter, shifting a phase of a signal generated by the second transmitter, and shifting a phase of a signal generated by the third transmitter.

[0024] In one embodiment, the method further includes, in part, shifting a phase of a signal received by the first antenna, and shifting a phase of a signal received by the second antenna. In one embodiment, the method further includes, in part, shifting a phase of a signal received by the first antenna, shifting a phase of a signal received by the second antenna, and shifting a phase of a signal received by the third antenna.

[0025] A method of communication, in accordance with one embodiment of the present invention, includes, in part, coupling a first receiver to a first terminal of a first 90° phase shifter having a second terminal coupled to a first antenna, and to a first terminal of a second 90° phase shifter having a second terminal coupled to a first node. The method further includes, in part, coupling a second receiver to a first terminal of a third 90° phase shifter having a second terminal coupled to a second antenna, and to a first terminal of a fourth 90° phase shifter having a second terminal coupled to the first node. The method further includes, in part, coupling a transmitter to a first terminal of a fifth 90° phase shifter having a second terminal coupled to the first antenna, and to a first terminal of a sixth 90° phase shifter having a second terminal coupled to the second antenna. The method further includes, in part, coupling a non-reciprocal element between the transmitter and the first node. The non-reciprocal element provides a -90° phase shift from the transmitter to the first node and a 90° phase shift from the first node to the transmitter

[0026] In one embodiment, the method further includes, in part, coupling a third receiver to a first terminal of a seventh 90° phase shifter having a second terminal coupled to a third antenna, and to a first terminal of an eighth 90° phase shifter having a second terminal coupled to the first node. The third antenna is coupled to the third receiver via a ninth 90° phase shifter.

[0027] In one embodiment, the method further includes, in part, coupling a fourth receiver to a first terminal of a tenth 90° phase shifter having a second terminal coupled to a fourth antenna, and to a first terminal of an eleventh 90° phase shifter having a second terminal coupled to the first node. The fourth antenna is coupled to the fourth receiver via a twelfth 90° phase shifter

[0028] In one embodiment, the method further includes, in part, shifting a phase of a signal received by the first receiver, and shifting a phase of a signal received by the second receiver. In one embodiment, the method further includes, in part, shifting a phase of a signal received by the first receiver, shifting a phase of a signal received by the second receiver, and shifting a phase of a signal received by the third receiver. [0029] In one embodiment, the method further includes, in part, shifting a phase of a signal delivered for transmission by the first antenna, and shifting a phase of a signal delivered for transmission by the second antenna,

[0030] In one embodiment, the method further includes, in part, shifting a phase of a signal delivered for transmission by the first antenna, shifting a phase of a signal delivered for transmission by the second antenna, and shifting a phase of a signal delivered for transmission by the third antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a simplified high-level block diagram of a communication transceiver, in accordance with one embodiment of the present invention.

[0032] Figure 2 is a simplified high-level block diagram of a communication transceiver, in accordance with one embodiment of the present invention.

[0033] Figure 3 is a simplified high-level block diagram of a communication transceiver, in accordance with one embodiment of the present invention.

[0034] Figure 4 is a simplified high-level block diagram of a communication transceiver, in accordance with one embodiment of the present invention.

[0035] Figure 5 is a simplified high-level block diagram of a phased array transmitter/receiver in accordance with one embodiment of the present invention.

[0036] Figure 6 is a simplified high-level block diagram of a phased array transmitter/receiver in accordance with one embodiment of the present invention.

[0037] Figures 7A, 7B, 7C, 7D and 7E are computer simulations showing the impedances at various ports of the transceiver of Figure 1, in accordance with one embodiment of the present invention.

[0038] Figures 8A, 8B, 8C, and 8D are computer simulations showing the isolations between the transmitters, antennas and receiver of the transceiver of Figure 1, in accordance with one embodiment of the present invention.

[0039] Figure 9 is a computer simulation showing the isolation between the two antennas of the transceiver of Figure 1, in accordance with one embodiment of the present invention. [0040] Figures 10A and 10C are computer simulation respectively showing the amount of power delivered to the receiver by antenna 1 and antenna 2 of the transceiver Figure 1.

[0041] Figures 10B and 10D are computer simulation respectively showing the phases of the signal delivered from antennas 1 and 2 to the receiver disposed in the transceiver of Figure 1.

[0042] Figure 11 A and 11C are computer simulation showing that a quarter of the powers received by antenna 1 and antenna 2 cancel each other at transmitter 1.

[0043] Figure 11B and 11D are computer simulation showing respectively the phase of the signal travelling from antennas 1 and 2 to the transmitter 1 disposed in the transceiver of Figure 1.

DETAILED DESCRIPTION OF THE INVENTION

[0044] A scalable communication system, in accordance with one embodiment of the present invention, includes transmission and reception units that are isolated from one another. In one embodiment, the communication system includes N independent phased array elements and a single non-reciprocal element. Accordingly, the system enables full-duplex beam steering that is critical to many radars and next generation of cellular communication devices.

[0045] In one embodiment, the scalable system includes one non-reciprocal element that may be passive or active, such as a gyrator, a ferrite based element or a time-varying circuit, adapted to provide two isolated beams for transmitter and receiver while using the same antenna. The scalable system may be a non reciprocal phased array system adapted to generate any receiver and/or

transmitter beam pattern.

[0046] Figure 1 is a simplified high-level block diagram of a transceiver 100, in accordance with one embodiment of the present invention. Transceiver 100 is shown as including, in part, first and second transmitters 10, 15, receiver 40, first and second antennas 60, 65, and phase shifters 22, 24, 26, 28, 30, 32 each of which is adapted to generate a 90° phase shift. Transceiver 100 is also shown as including, in part, a non-reciprocal element 50 adapted to provide a 90° phase shift from node d to node b, and a -90° phase shift from node b to node d, as shown. Transmitters 10 and 15 have the same terminal impedance. The receiver has an impedance that is ½ of the impedance of the transmitters.

[0047] The signal generated by transmitter 10 reaches both antennas 60 and 65; however, such a signal arrives at antenna 65 through different paths, one from node d to node f, and another one from node c to node f. Due to the presence of the 90° phase shifters, and the non-reciprocal element, the signal generated by transmitter 10 and travelling from node d to node f is 90° out-of-phase with respect to the signal travelling from node c to node f; accordingly, these two signals cancel each other at node f. In other words, the signals generated by transmitted 10 and reaching antenna 65 are added together destructively and thus cancel each other. Therefore, the signal generated by transmitter 10 is not received by antenna 65 for transmission. For similar reasons, the signal generated by transmitter 15 is not received by antenna 60 for transmission. Accordingly, the signal generated by transmitter 10 is only transmitted by antenna 60, and the signal generated by transmitter 15 is only transmitted by antenna 65.

[0048] Due to the symmetry of the paths from the antennas to receiver 40, the in-phase signal received by antenna 60 and delivered to receiver 40 will remain in-phase with the signal received by antenna 65 and delivered to receiver 40 and thus is added constructively to this signal.

[0049] Furthermore, the signal received by antenna 60 and reaching transmitter 10 from node e has a phase shift of 90° relative to the signal received by antenna 60 and reaching transmitter 10 from node b. Therefore, the signal received by antenna 60 and reaching transmitter 10 from node cancels the signal received by antenna 60 and reaching transmitter 10 from node b. In other words, the signal received by antenna 60 is not received at transmitter 10. For similar reasons, the signal received by antenna 60 is not received at transmitter 15; the signal received by antenna 65 is not received at transmitter 10; and the signal received by antenna 65 is not received at transmitter 15. Accordingly, because the signals delivered by the antennas to receiver 40 is not received by the transmitters, receiver 40 is isolated from transmitters 10 and 15.

[0050] Figure 2 is a simplified high-level block diagram of a transceiver 150, in accordance with one embodiment of the present invention. Transceiver 150 is shown as including, in part, first and second receivers 40, 42, transmitter 10, first and second antennas 60, 65, and phase shifters 22, 24, 26, 28, 30, and 32 each adapted to generate a 90° phase shift. Transceiver 150 is also shown as including, in part, a non-reciprocal element 50 adapted to provide a 90° phase shift from node b to node d, and a -90° phase shift from node d to node b. Receivers 40 and 42 have the same terminal impedance. Transmitter 10 has an impedance that is twice the impedance of the receivers.

[0051] For the same reasons as those described above with reference to transceiver 100 of Figure 1, the signal generated by transmitter 10 is transmitted in-phase by antennas 60 and 65; the signal generated by transmitter 10 is not received at receivers 40 and 42; the signal received by antenna 60 is only received by receiver 40; and the signal received by antenna 65 is only received by receiver 42.

[0052] Figure 3 is a simplified high-level block diagram of a transceiver 200, in accordance with one embodiment of the present invention. Transceiver 200 includes N transmitters 10i, 10 _{2. . .} 10N , N antennas 60i, 60 ₂ . . . 60N and a receiver 40, where N is an integer greater 2. Each transmitter lOi is associated with one antenna 60i, where i is an integer ranging from 1 to N. Therefore, for the reasons described above with reference to Figure 1, the signal generated by transmitter lOi is only transmitted by its associated antenna 60i. For example, the signal generated by transmitter lOi is only transmitted by its associated antenna 60i _, and the signal generated by transmitter l0 ₃ is only transmitted by its associated antenna 6O _3. Each transmitter lOi is controlled independently. The transmitter ports are mutually isolated. The transmitter pots are also isolated from the antennas and the receiver.

[0053] Transceiver 200 is also shown as including, in part, a non-reciprocal element 50 adapted to provide a 90° phase shift from node d to node b, and a -90° phase shift from node b to node d. All transmitters lOi have the same terminal impedance. Each transmitter has an impedance that is N time the impedance of the receiver.

[0054] Transceiver 200 is also shown as including, in part, a 90° phase shifter

15 ₁ disposed between transmitter lOi and node b. For example, 90° phase shifter

15 ₂ is disposed between transmitter l0 ₂ and node b. Transceiver 200 is also shown as including, in part, a 90° phase shifter 20i disposed between transmitter 40i and its associated antenna 60i. For example, 90° phase shifter 20 ₂ is disposed between transmitter l0 ₂ and antenna 60 ₂ . Transceiver 200 is also shown as including, in part, a 90° phase shifter 25i disposed between each antenna and node d. For example, 90° phase shifter 25 ₂ is disposed between antenna 60 ₂ and node d. For the same reasons as described above with respect to Figure 1, the signal received by the receiver is not received at the transmitters. Therefore, the receivers and the transmitter of transceiver 200 are isolated form one another. The signals received by the antennas adds constructively at the receiver.

[0055] Figure 4 is a simplified high-level block diagram of a transceiver 300, in accordance with one embodiment of the present invention. Transceiver 300 includes N receivers 40i, 40 ₂ ...40N , N antennas 60i, 60 ₂ ...60N and a transmitter 40, where N is an integer greater 2. Each receiver 40i is associated with one antenna 60i, where i is an integer ranging from 1 to N. Therefore, for the reasons described above with reference to the transceivers shown in Figures 1 and 2, the signal generated by transmitter 10 is transmitted in-phase by all antennas 60i; the signal generated by transmitter 10 is not received at receivers 40i; and the signal received by antenna 60i is only received by receiver 40i. Each receiver 40i is controlled independently. The receiver ports are mutually isolated. The receiver ports are also isolated from the antennas and the transmitter port.

[0056] Transceiver 300 is also shown as including, in part, a non-reciprocal element 50 adapted to provide a 90° phase shift from node b to node d, and a -90° phase shift from node d to node b, as shown. All receivers 40i have the same terminal impedance which is l/N the impedance of transmitter 10.

[0057] Transceiver 300 is also shown as including, in part, a 90° phase shifter 15i disposed between receiver 40i and node b. For example, 90° phase shifter l5 ₂ is disposed between receiver 40 ₂ and node b. Transceiver 300 is also shown as including, in part, a 90° phase shifter 20i disposed between receiver 40i and its associated antenna 60i. For example, 90° phase shifter 20 ₂ is disposed between receiver 40 ₂ and antenna 60 ₂ . Transceiver 300 is also shown as including, in part, a 90° phase shifter 25i disposed between each antenna and node d. For example, 90° phase shifter 25 ₂ is disposed between antenna 60 ₂ and node d. For the same reasons as described above with respect to Figure 1, the signal received by the receivers is not received at the transmitter. Therefore, the receivers and the transmitter of transceiver 300 are isolated form one another. The signals received by the antennas adds constructively at the receivers.

[0058] Figure 5 is a simplified high-level block diagram of a phased array transmitter/receiver 250 in accordance with one embodiment of the present invention. Phased array 250 includes, in part, a transceiver 200 the details of which are shown in in Figure 3 and described above. Phased array 250 is also shown as including, in part, a phase shifter block 80 having N phase shifters each adapted to shift a phase of the signal generated by a different one of the N transmitters l0i (see Figure 3) in response to signal Ctrl tx. Phased array 250 is also shown as including, in part, a second phase shifter block 85 having N phase shifters each adapted to shift the phase of the signal received by a different one of the N antennas in response to signal Ctrl rx. It is understood that antennas 60 _I . . . 60 _N are the same antennas as shown in Figure 3 and are used to transmit a field pattern generated by the transmitters and phase shifter block 80. The signal received by antennas 60 _I . . . 60 _N is phase shifted by phase shifter block 85 to generate a desired received field pattern. It is also understood that the phase shifters in each of phase shifter blocks 80 and 85 are independently controlled.

[0059] Figure 6 is a simplified high-level block diagram of a phased array transmitter/receiver 350 in accordance with one embodiment of the present invention. Phased array 350 includes, in part, a transceiver 300 the details of which as shown in Figure 4 and described above. Phased array 350 is also shown as including, in part, a phase shifter block 90 having N phase shifters each adapted to shift a phase of the signals received by a different one of the N receivers 40i (see Figure 3) in response to signal Ctrl_rx. Phased array 250 is also shown as including, in part, a second phase shifter block 95 having N phase shifters each adapted to shift the phase of the signal generated by the transmitter 10 by a different amount in response to signal Ctrl tx. It is understood that antennas 60 _I . . .60 _N are the same antennas as shown in Figure 3 and are used to transmit a field pattern generated by the transmitter in response to phase shifter block 95. The signals received by antennas 60 _I . . .60 _N are phase shifted by phase shifter block 90 to generate a desired received field pattern. It is also understood that the phase shifters in each of phase shifter blocks 90 and 95 are independently controlled. [0060] Figures 7 A, 7B, 7C, 7D and 7E are computer simulations showing the impedances at ports of transmitter 10, antenna 60, transmitter 15, antenna 65 and receiver 40, respectively of transceiver 100 of Figure 1. The x-axis show the frequency in GHZ and the y-axis show the power in dB. As is seen from these Figures, the impedances are matched at lGHz, which is the frequency selected for the simulation. The impedances are also matched at 3GHz and 5GHz.

[0061] Figures 8 A, 8B, 8C, and 8D are computer simulations showing respectively the isolation between transmitter 10 and antenna 60, transmitter 10 and receiver 40, transmitter 10 and antenna 65, and transmitter 15 and receiver 40 of transceiver 100 of Figure 1. As is seen from Figure 8 A, at the simulation frequency of lGHz, the path from transmitter 10 to antenna 60 has the least amount of power loss. Figures 8B, 8C and 8D respectively show that the path between transmitter 10 and receiver 40, the path between transmitter 10 and antenna 65, and the path between transmitter 15 and receiver 40 are fully isolated at the simulation frequency of lGHz. Figure 9 is a computer simulation showing the isolation between the first and second antennas 60 and 65 of transceiver 100 of Figure 1.

[0062] Figures 10A and 10C respectively show power loss in the path between antenna 60 and receiver 40, and in the path between antenna 65 and receiver 40 of transceiver 100 of Figure 1. As is seen from these two figures, the signals from the antennas to the receiver are in phase. Figure 10A shows that half of the power delivered to the receiver is supplied by antenna 1 and Figure 10C shows that half of the power delivered to the receiver is supplied by antenna 2. Figure 10B shows the phase of the signal travelling from antenna 1 to the receiver and Figure 10D shows the phase of the signal travelling from antenna 2 to receiver.

[0063] Figure 11 A shows that a quarter of the power received by antenna 1 is present at transmitter 1, and Figure 11C shows that quarter of the power received by antenna 2 is present at transmitter 1 but because these two power signal are out of phase at the transmitter, they cancel each other out. Therefore, no power is delivered by the antenna to the transmitters. Figure 11B shows the phase of the signal travelling from antenna 1 to transmitter 1, and Figure 11D shows the phase of the signal travelling from antenna 1 to transmitter 1. [0064] The above embodiments of the present invention are illustrative and not limitative. Other additions, subtractions or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.