The growing demand for higher data rates to enable new broadband media applications and services has spurred increased research into full-duplex system architectures, which can effectively double the information capacity of wireless communication systems.
The present teaching, in accordance with preferred and exemplary embodiments, together with further advantages thereof, is more particularly described in the following detailed description, taken in conjunction with the accompanying drawings. The skilled person in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating principles of the teaching. The drawings are not intended to limit the scope of the Applicant's teaching in any way.
The present teaching will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings. While the present teaching is described in conjunction with various embodiments and examples, it is not intended that the present teaching be limited to such embodiments. On the contrary, the present teaching encompasses various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. Those of ordinary skill in the art having access to the teaching herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present disclosure as described herein.
Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the teaching. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
It should be understood that the individual steps of the method of the present teaching can be performed in any order and/or simultaneously as long as the teaching remains operable. Furthermore, it should be understood that the apparatus and method of the present teaching can include any number or all of the described embodiments as long as the teaching remains operable.
The RF front end of a broadband full-duplex communication architecture is described, in which a single antenna simultaneously transmits and receives signals that can overlap spectrally to any degree. This architecture not only minimizes the power of the transmitted signal at the receiver in the ideal and purely hypothetical case in which the antenna has infinite return loss, but also successfully suppresses the portion of the transmitted signal reflected off of an actual antenna while successfully recovering received signals at the same frequency. The hardware that enables the transmit signal that is reflected from the antenna to be separated from a same-frequency signal received by that antenna performs this separation purely in the analog domain without the need for a control circuit or any other digital-domain hardware. Known antenna interfaces do not distinguish the reflected transmit signal from a same-frequency receive signal in the analog domain. Using this antenna interface, it has been demonstrated that single-antenna full-duplex capability with better than −30 dB of analog transmit/receive isolation nearly everywhere in the 1-18 GHz band can be achieved. In any sub-band within this 17 GHz-wide instantaneous bandwidth, additional analog and digital techniques can be applied to further improve the transmit/receive isolation. See, for example, K. Kolodziej, B. Perry, and J. Herd, “In-Band Full-Duplex Technology: Techniques and Systems Survey,” IEEE Trans. Microwave Theory Tech., vol. 67, pp. 3025-3041, July 2019).
To enable single-antenna full-duplex, a commonly employed method of achieving the necessary bi-directional interface to the antenna is to use a three-port device known as a circulator. This is typically a passive component with three ports arranged in a waveguide ring around a ferrite disc that induces a direction-dependent phase shift, causing the two counter-circulating halves of an input signal to add up constructively at the next port in one circumferential direction along the ring but destructively at the next port in the opposite direction. The fact that they suppress the undesired signal in this way, that is using two paths designed to differ in electrical length by 180° at a certain frequency, inherently limits the operational bandwidth over which ferrite circulators can isolate the receive-signal port from the transmit-signal port. This is true even when the bi-directional signal port is connected to an “ideal” 50-Ω load, such as one of the ports of a calibrated network analyzer.
The ferrite circulator's unimpressive combination of transmit/receive (Tx/Rx) isolation and bandwidth, which typically is no better than −20 dB over bandwidths of only one octave or less, has motivated research into bi-directional interfaces with improved isolation over broader bandwidths. For example, one analog approach that leverages the inherently broadband (from a radio-frequency perspective) of photonics technology has demonstrated Tx/Rx isolation of better than −40 dB across three decades of bandwidth from 20 MHz to 20 GHz when the interface's bi-directional port is terminated in 50Ω. See, for example, C. Cox and E. Ackerman, “Maximizing RF Spectrum Utilization with Simultaneous Transmit and Receive,” Microwave J., pp. 114-126, September 2014. When an actual antenna replaces the 50-Ω load at the bi-directional port in any such interface, such impressive Tx/Rx isolation has not been possible across such bandwidths, at least in part, because no antenna exists with similarly impressive return loss across such bandwidths. In fact, it is rare to find an antenna with a combination of return loss and bandwidth that is any better than the combination of Tx/Rx isolation and bandwidth mentioned above for ferrite circulators, that is, no better than −20 dB across no more than one octave.
One method that has been employed by some researchers to mitigate the effect of reflections off of the antenna in single-antenna full-duplex systems is to tap off a portion of the transmit signal prior to the interface and to adjust, for example, using a vector modulator, the amplitude and phase of this “reference copy” of the transmit signal prior to a point just before the receiver front-end where it can be combined with the signal in the receive path. When this amplitude and phase adjustment is performed perfectly, the vector-modulated reference copy of the transmit signal subtracted from the antenna-reflected copy of the transmit signal results in perfect transmit signal cancellation at the receiver. In practice, however, an attractive degree of cancellation can only be achieved over a limited bandwidth, and therefore architectures that employ this method include control circuits that modify the settings to the vector modulator as a function of the transmit signal frequency to enable a large degree of cancellation over just the instantaneous bandwidth of that transmit signal. A recent example of such an architecture achieved Tx/Rx isolation of better than −40 dB across a tunable band 30 MHz in width. See, for example, C. Campbell, J. Lovseth, S. Warren, A. Weeks, and P. Schmid, “A BST Varactor Based Circulator Self-Interference Canceller for Full Duplex Transmit Receive Systems,” Proc. IEEE Int. Microwave Symp. Dig., pp. 1195-1198, August 2020.
The present teaching relates to an analog-domain bi-directional interface that, when connected to an actual transmitting and receiving antenna, separates the transmit signal that reflects off of the antenna from the receive signal rather than attempting to perform cancellation by subtracting a reference copy of the transmit signal.
The approach as described herein involves the use of an antenna that has two waveguide ports which enable it to transmit signals with either linear or circular polarization depending on the difference in phase between the components of a signal supplied to these two ports.
A communication link between two such antennas 102, 102′, separated by approximately 10 feet, was assembled within an anechoic chamber by connecting 90° hybrid couplers 104, 104′ as shown in
Communication between the two antennas 102, 102′ that are both transmitting and receiving results in a four-port network that can be characterized using S-parameters. With the ports numbered as shown in
The two-polarization antennas used in the antenna chamber set-up were selected for their low cross-port isolation of −35 dB, but the advertised worst-case return losses of their individual waveguide ports were in the neighborhood of only −14 dB, and the isolation of the 90° hybrid coupler was quoted at only −17 dB.
In operation, light from a laser 212 at the optical frequency fopt is supplied to this modulator 208 from the end of the device that causes the light to co-propagate with RF signals received by the antenna 206 so that these signals modulate the light with maximum efficiency. The RF signals to be transmitted by the antenna 206 propagate on the modulator electrodes of the modulator 208 in the opposite direction, causing them to modulate the light much less efficiently. Moreover, a 90° difference in RF phase between the receive-signal waves on the MZ modulator's two drive electrodes causes the receive signal to impose only one modulation sideband on the optical carrier at frequencies fopt−fRX, and a 90° difference of the opposite sign between the transmit-signal waves on the two drive electrodes imposes the transmit signal on the other modulation sideband at frequencies fopt+fTX, as shown by the spectral diagram just below the modulator in
One feature of the architecture shown in
Another feature of the present teaching that can be illustrated in the architecture shown in
A laser 212, such as a semiconductor diode laser, the dual-drive MZ modulator 208 with electrode access from both ends, optical bandpass filter 210, and p-i-n photodiode detector 214 were connected in the anechoic chamber to the antenna ports 204, 204′ and 90° hybrid coupler ports 202 as shown in
Also included as shown in
For the bi-directional interface described in connection with
Furthermore, for the interface described in connection with
For the bi-directional interface of
The benefit imparted by the bi-directional interface of
By contrast, the plot shown in
While the Applicant's teaching is described in conjunction with various embodiments, it is not intended that the Applicant's teaching be limited to such embodiments. On the contrary, the Applicant's teaching encompasses various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art, which may be made therein without departing from the spirit and scope of the teaching.
The present application is a non-provisional application of U.S. Provisional Patent Application No. 63/108,304, entitled “Bi-Directional Signal Interface with Suppression of Reflected Signals” filed on Oct. 31, 2020. The entire contents of U.S. Provisional Patent Application No. 63/108,304 are herein incorporated by reference.
Number | Date | Country | |
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63108304 | Oct 2020 | US |