Patent Publication Number: US-2022231711-A1

Title: Frequency-agnostic wireless radio-frequency front end

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a non-provisional application of U.S. Provisional Patent Application No. 62/850,502, filed on May 20, 2019, entitled “Frequency-Agnostic Wireless Radio-Frequency Front End” and a non-provisional application of U.S. Provisional Patent Application No. 62/850,574, filed on May 21, 2019, entitled “Frequency-Agnostic Wireless Radio-Frequency Front End”. The entire contents of U.S. Provisional Patent Application Nos. 62/850,502 and 62/850,574 are herein incorporated by reference. 
    
    
     The section headings used herein are for organizational purposes only and should not to be construed as limiting the subject matter described in the present application in any way. 
     INTRODUCTION 
     Widespread and increasing demand for wirelessly-connected systems has created a tremendous need for more efficient use and sharing of electromagnetic spectrum bandwidth. Compact, flexible and cost-effective wireless front ends are key subsystems for sharing that electromagnetic spectrum bandwidth. These wireless front ends should ideally support flexible and efficient transmit and receive operation using popular conventional wireless frequency bands. Such front ends need to support communications across numerous wireless systems, including cellular systems, WiFi and other Wireless LAN systems, satellite and various military RF communications systems. 
     More flexible RF front ends are needed to efficiently use the RF spectrum for cellular telephony and other applications. A specific example of the need for more flexible RF front ends is seen with the dominant spectral sharing method used in mobile cellular systems, Frequency Division Duplexing (FDD). In these systems, a different frequency is used for uplink and downlink communications between a cellular device of a mobile user and the cell tower base station. To support more user demand, access is needed to more frequency bands used for uplink and downlink connections. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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&#39;s teaching in any way. 
         FIG. 1  illustrates a schematic of a multi-antenna cell phone. 
         FIG. 2  illustrates a plot of the frequency bands for the International Mobile Telecommunications (IMT) as a function of year. 
         FIG. 3A  illustrates the upper left quadrant of a schematic of example Long Term Evolution (LTE) Frequency Division Multiplexing (FDD) bands. 
         FIG. 3B  illustrates the upper right quadrant of the schematic of example Long Term Evolution (LTE) Frequency Division Multiplexing (FDD) bands described in connection with  FIG. 3A . 
         FIG. 3C  illustrates the lower left quadrant of the schematic of example Long Term Evolution (LTE) Frequency Division Multiplexing (FDD) bands described in connection with  FIG. 3A . 
         FIG. 3D  illustrates the lower right quadrant of the schematic of example Long Term Evolution (LTE) Frequency Division Multiplexing (FDD) bands described in connection with  FIG. 3A . 
         FIG. 4  illustrates a known RF system that includes a front end comprising multiple duplexers. 
         FIG. 5  illustrates an embodiment of an RF system that includes a frequency-agnostic front end according to the present teaching. 
         FIG. 6  illustrates an embodiment of an RF system that includes a frequency-agnostic front end and includes common amplifiers according to the present teaching. 
         FIG. 7A  illustrates an embodiment of an RF system that includes a frequency-agnostic receive front end for a diversity antenna according to the present teaching. 
         FIG. 7B  illustrates an embodiment of an RF system that includes a frequency-agnostic transmit and receive front end for a primary antenna according to the present teaching. 
         FIG. 8  illustrates an embodiment of a frequency-agnostic interference reducer processor according to the present teaching. 
         FIG. 9  illustrates an embodiment of an RF antenna system comprising a frequency-agnostic transmit and receive front end that utilizes travelling wave structures to form a connector according to the present teaching. 
         FIG. 10  illustrates an embodiment of an antenna system comprising a frequency-agnostic transmit and receive front end that utilizes subtraction according to the present teaching. 
         FIG. 11  illustrates another embodiment of an antenna system comprising a frequency-agnostic receive front end that utilizes a subtractor according to the present teaching. 
         FIG. 12  illustrates an embodiment of an antenna system comprising a frequency-agnostic transmit and receive front end that utilizes a fast switch according to the present teaching. 
         FIG. 13  illustrates an embodiment of an antenna system with a frequency-agnostic receive front end that utilizes a fast switch according to the present teaching. 
     
    
    
     DESCRIPTION OF VARIOUS EMBODIMENTS 
     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 methods of the present teachings may be performed in any order and/or simultaneously as long as the teaching remains operable. Furthermore, it should be understood that the apparatus and methods of the present teachings can include any number or all of the described embodiments as long as the teaching remains operable. 
     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 teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass 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. 
     Marconi&#39;s original “wireless”, circa 1894, used a spark-gap transmitter, which produced a waveform that covered a wide, poorly-defined, portion of the RF spectrum. Since Marconi was the only one broadcasting, there was no issue of interference with other users. However, with the rapid increase in both the popularity of wireless and the sophistication of the technology to support it in the early 1900s, interference among wireless users quickly became an issue. The first solution to manage interference was via regulation that was first formally implemented in the Radio Act of 1912. Then the Federal Radio Commission was established in 1926, which became the Federal Communications Commission (FCC) in 1934. The FCC established regulations that defined user frequency bands and regulated transmit powers levels for those bands as well as the residual power levels that were allowed to spill over into adjacent bands. 
     The early regulations were essentially frequency-specific, but signal-agnostic, regulations. That is, any signal that was at an unwanted regulated frequency needed to be suppressed by the user regardless of its signal format. To meet these requirements, RF filters were developed throughout the remainder of the 20 th  century using many different designs that were implemented using a myriad of technologies to filter out specific frequencies. 
     In the first two decades of the 21 st  century, there has been an exponential expansion in the use of wireless technology with the major application of wireless technology being mobile, cellular phones. There are currently on the order of about 4.6 billion users of such devices, which is roughly two thirds of the world&#39;s population. The increased demand for RF spectrum, which is a fixed resource, has resulted in skyrocketing costs for RF spectrum. A recent U.S. Government spectrum auction sold RF spectrum for $3.3 billion dollars/MHz. 
     The increasing demand for, and cost of, RF spectrum has greatly increased the incentive for developing new technologies to more efficiently utilize existing RF spectrum. One approach to more efficiently utilize existing RF spectrum is to reduce the frequency separation between frequency bands. A finite separation between any two adjacent bands, called a guard band, is presently required because any realizable RF filter needs a minimum frequency band over which to transition between passing and blocking signals. For example, for cellular telephony, there is presently greater than 100 MHz of spectrum tied up in the guard bands between up- and down-link bands. However, using sharper filters that can more rapidly transition between passing and blocking is technically challenging and expensive. Thus, there are conflicting constraints of the economic necessity for narrower guard bands vs. the cost and complexity of deploying such sharper filters. A conventional filter-based approach is not likely to yield a practical solution that will result in more economical and efficient use of the RF spectrum. 
     One aspect of the present teaching is the realization that the interfering signals are not random, unknown signals. Rather, many of the characteristics of the interfering signals that need to be suppressed are actually known. The methods and apparatus of the present teaching use this known information about the characteristics of the interfering signals to suppress these signals. The suppression can occur regardless of the frequency of the interfering signals. 
     The methods and apparatus of the present teaching result in a signal-specific, frequency-agnostic method of suppressing signals. Using the methods of the present teaching, there is no need for conventional pass and stop bands, and hence no need for a transition or guard band between them. Therefore, the very valuable spectrum that is presently used for the guard bands can be freed up and made available to convey additional signals. The signal-specific suppression technique described herein is a technology-based solution to mitigating interference, in contrast to the current regulation-based solutions. The result of using these signal-specific suppression techniques is efficient and flexible sharing of wireless spectrum. 
     Some aspects of the present teaching are described more specifically in connection with cellular telephony. The dominant format used to share spectrum in mobile cellular phone systems is Frequency Division Duplexing (FDD), which uses one frequency band for the downlink from the base station to the mobile user and another frequency for the uplink from the user to the base station. 
     In modern cellular FDD telephony systems, these uplink and downlink frequencies are separated in an electronic front end that connects transmitters and receivers to wireless antennas in the base stations and in the mobile devices. In order to support efficient high-capacity communications, the frequency bands are narrow and tightly spaced so the front end device that separates and combines the uplink and downlink signals must have high frequency selectivity and must be capable of operating over a wide range of frequency bands. 
     However, the components used in these front end devices that allow them to perform well separating out signals in noisy RF cellular environments do not scale well as the number of bands increases and the spacing between bands decreases. Making suitable components is technically challenging as these devices need to operate with narrow, closely-spaced bands. Consequently, current state-of-the-art cellular devices are limited in the number of frequency bands they can accommodate. 
     More flexible RF front ends are needed to efficiently use the RF spectrum for cellular telephony and other applications. At the wireless front end, frequency duplexers convey the uplink and downlink transmit and receive signals using filters with fixed central frequencies and bandwidths, or frequency spans, to pass the desired signals. In general, a separate duplexer is used for each frequency band. As the number of frequency bands increases so does the required number of duplexers needed to support the many bands. Using a large number of duplexers significantly adds to the physical size and cost of the wireless front end. Because space is at a premium in mobile devices, this often means that the number of bands accessible by a particular device is restricted, greatly limiting the connectivity options for that device. As such, new approaches are needed for front ends that allow a single wireless device, such as a cellular device, to transmit and receive over an arbitrary number of frequency bands with performance that exceeds the performance of a device using prior art front end components. 
     One feature of the present teaching is the recognition that recently developed technologies that were designed to enable single-channel, full-time full duplex communications can be applied to enable a single wireless device, such as a cellular device, to transmit and receive over an arbitrary number of frequency bands with performance that exceeds the performance of a device using prior art front end components. The key insight to eliminating all the filtering components—duplexers and channel filters—is to appreciate that there are actually two sources of interference that need to be suppressed. One class of interference is self-interference. This type of interference is internally generated within a particular system; it arises from the leakage of a particular system&#39;s own high power transmit signal(s) into the sensitive receiver of the particular system. The second class of interference is externally generated; it arises from transmit signals of other systems that are co-located with a particular system entering the receive path of a particular system. 
     The reason it is important to distinguish these two classes of interference is that different methods are needed to remove each class of undesired interference signals from desired receive signals. For internally-generated interference, a reference copy of the transmit signal is available to assist in the cancellation, whereas for the externally-generated interference generally no reference copy is available. There are two categories of externally-generated interference signals. For the first category, the system can generate a reference copy of the externally-generated interference signal. For the second category, the system cannot generate a reference copy of the externally-generated signal. Thus, the first category of externally-generated interference signal can be treated in the same way as internally-generated interference signals. That is, if for some reason a reference copy of an externally-generated interference is available to a particular system, then suppression of such a signal can use the same methods as used for internally-generated interference. 
     One feature of the present teaching is that the cancellation architectures for interference signals for which reference copies are available are inherently broad bandwidth, thus supporting a wide range of frequency bands. Similarly, the cancellation architectures for interference signals for which reference copies are not available are inherently broad bandwidth and support a wide range of frequency bands. Both architectures support the separation of receive signals from transmit signals with high fidelity regardless of whether these signals are in the same or disjoint frequency bands, even when those transmit and receive signals are operating simultaneously. Both architectures also support the separation of receive signals from interfering signals with high fidelity regardless of whether these signals are in the same or disjoint frequency bands, even when those interfering and receive signals are operating simultaneously. 
     Many cell phones utilize multiple antennas. In various configurations, these multiple antennas can have various transmit and receive functions. For example, separate antennas can be used to transmit and receive in particular separate bands. Some antennas may transmit and receive and some antennas may receive only or transmit only. One challenge for cell phone design is mitigating interferences that arise amongst the various antennas. This is particularly true given the small footprint of the cell phone, which means antennas must be closely spaced leading to opportunities for high levels of interference both from transmitter to receiver, as well as reception of desired over-the-air signals as well as undesirable signals incident on the antennas. 
     Another feature of the frequency-agnostic wireless radio-frequency front end of the present teaching is that it mitigates interference between antennas, especially antennas that are co-located in a cell phone in very close proximity. Another feature of the present teaching is that it can reduce the number of components needed in a cell phone while still supporting a desired number of operating bands. 
     To provide specificity to the material to be presented below, we will use a mobile telephone system as a specific example. The present teaching is applicable to any number of known wireless systems. For cellular applications, the present teaching is applicable to both the cell phone as well as the base station end of a mobile cell phone system; we will focus our description on the mobile cell phone end.  FIG. 1  illustrates a schematic of a multi-antenna cell phone  100 . A cellular telephone case  102  that contains antenna elements  104 ,  106 ,  108 ,  110  is shown. The schematic in  FIG. 1  shows a physical layout with the approximate relative positions of the antennas  104 ,  106 ,  108 ,  110  in the case  102 . In some modern cell phone designs, the primary antenna is antenna  104  that is located on the bottom of the case  102 . In various embodiments, the primary antenna  104  both transmits and receives on various cell phone bands. As is well known to those skilled in the art, the particular operating bands of a cellular phone can depend, for example, on the particular service provider providing the phone service, whether or not the phone is configured for international service, and/or the intended types of cellular service (3G, 4G, 5G, LTE, CDMA, GSM, UMTS, etc.) being offered for the particular phone  100 . Different cellular phones will provide different service capabilities that are based in part on what cellular frequency bands are supported by the various antennas  104 ,  106 ,  108 ,  110 , associated front end components and transmit and receive electronics. 
     In the configuration shown in  FIG. 1 , a diversity antenna  106  is located near the top of the case  102 . A diversity antenna provides a sample of the various signals that are incident on the cell phone to the cell phone electronics. Typically, the diversity antenna operates in receive-only mode. The independent received signal generated at an output of the diversity antenna is used in a number of ways depending on the desired configuration. For example, this signal can be used as a separate receive signal. In this configuration, the signal from the diversity antenna is switched into the receiver and data recovery system to provide a primary receive signal if the diversity signal is stronger than the primary receive antenna&#39;s signal. The signal can also be summed with the primary receive antenna&#39;s signal. 
     In the configuration shown in  FIG. 1 , a Global Positioning System (GPS) antenna  108  is located near the top of the case across from the diversity antenna  106 . Also, a WiFi antenna  110  is located across from the diversity antenna near the top of the case  102 . The antenna configuration shown in  FIG. 1  is, of course, just an example. It should be appreciated that numerous other configurations of various antennas are possible. However, it should be noted that the physical antenna configuration is restricted to a footprint of less than 10-20 cm on a side and therefore the antennas  104 ,  106 ,  108  are closely spaced. Typically, the diversity antenna  106  is positioned far enough away from the primary antenna  104  such that known diversity receiving techniques can be used to improve signal reception. 
     Improving the ability to separate and suppress signals within the front end electronics associated with the antennas allows for closer spacing of the various antennas while still supporting required reception and transmission performance. One challenge with managing the configuration of antennas and transmit receive functions on a small device such as a cell phone  100  is the incredible growth in the number of frequency bands that are desirable to be supported in a single device. In addition to the growth, there is also uncertainty in the number and size of the bands that should be included. One example of this significant and uncertain growth in the numbers and frequency allocations of frequency bands for cellular systems is seen in international cellular standards evolution. 
       FIG. 2  illustrates a plot  200  of the frequency bands for the International Mobile Telecommunications (IMT) as a function of year. This plot  200  is taken from an International Telecommunication Union (ITU) presentation entitled “Spectrum Management for 4G-CTE”, www.itu.int. Clearly, the plot  200  shows very significant growth in the number of frequency bands used as a function of recent time. New bands added over 3-5 year time frames are allocated in both higher frequency bands and lower frequency bands as compared to the previous years. As of 2015, there was still a lack of clarity within the regulatory community as to exactly which bands and which frequency allocations would be utilized in 2019. This is illustrated by the question mark over the 2019 date. 
     It is highly desirable to provide a front-end system that can easily accommodate the increasing numbers of frequency bands. However, it is also important to be flexible enough to support frequencies that may be unknown at the time of design. That is, it is highly desirable to have a design that can accommodate a wide variety of frequencies, even any frequency, in the RF spectrum. 
     In FDD, to achieve bi-directional communication, a pair of channels is used, one for transmit and one for receive, that occupy disjoint parts of the electromagnetic spectrum. Thus, FDD up-link and down-link signals are communicated simultaneously using different frequency channels. FDD is widely used because it has a number of well-known practical advantages. Currently, approximately 80% of commercial wireless cellular communications systems use this form of duplexing. State-of-the-art cell phones are designed to support the known FDD pairs of uplink and downlink frequency bands. It is also desirable looking forward to be able to flexibly pair an uplink frequency band and a downlink frequency band, even if those two frequency bands are not contiguous or closely spaced in the spectrum. Thus, it is desirable that the front ends provide both wide bandwidth operation and the flexibility to select narrow operating bands anywhere within that wide bandwidth of operation. 
       FIG. 3A  illustrates the upper left quadrant of a schematic  300  of example Long Term Evolution (LTE) Frequency Division Multiplexing (FDD) bands.  FIG. 3B  illustrates the upper right quadrant of the schematic  300  of example Long Term Evolution (LTE) Frequency Division Multiplexing (FDD) bands of  FIG. 3A .  FIG. 3C  illustrates the lower left quadrant of the schematic  300  of example Long Term Evolution (LTE) Frequency Division Multiplexing (FDD) bands of  FIG. 3A .  FIG. 3D  illustrates the lower right quadrant of the schematic  300  of example Long Term Evolution (LTE) Frequency Division Multiplexing (FDD) bands of  FIG. 3A . The different frequency allocations  302  for LTE are provided for each column across the top row of the schematic  300 . Band numbers  304  for each allocation are provided for each row in the first column. Specific uplink bands in various frequency allocations are shown in shaded rectangles  306 , while down link bands in various frequency allocations are shown in the other type of shaded rectangles  308 . 
     In various service provider systems and/or generations of cellular technology, numerous different schemes are used to aggregate bands so that a single information signal, sometimes referred to as a data channel, uses two or more different bands. This is referred to as Carrier Aggregation (CA). For example the uplink and downlink bands in the two frequency allocations that are outlined in the dashed lines  310  can carry a single data channel or information signal. These are passed through a single antenna simultaneously. The signals are in disjoint frequency bands. There are multiple types of CA depending on the frequency difference between the aggregated bands. For example, the bands may be contiguous and in the same frequency allocation. The bands may also be non-contiguous, but in the same frequency allocation. The most challenging case is where the bands are in separate frequency allocations, which is the case of aggregation  310  shown in  FIG. 3A . 
     Some services also utilize duplexer band aggregation (DBA) that enables two or more bands to be accessed via the same duplexer. In DBA, frequency bands for multiple uplink and downlink bands overlap. The dashed line  312  in  FIG. 3C  illustrates three different uplink/downlink bands with a common frequency allocation that can share a duplexer. 
     In a typical mode of operation, an uplink frequency band and a downlink frequency band are sent and received in pairs via a common antenna. The front end that connects the common antenna to the transmitter and receiver in, for example, a cellular device or base station, includes a frequency duplexer to separate signals from, and combine signals to, the antenna. A frequency duplexer is a three-port device that interfaces to the common antenna and routes the downlink signal to the input of the receive path while simultaneously conveying the output of the transmit path to the antenna. 
     Because of the tight spacing between the frequency bands—for example, uplink  306  and downlink  308  in  FIG. 3A —the duplexer needs to have high selectivity. That is, the duplexer filter must have sharp cutoff at the edges of the passband. This high selectivity demands specialized filters, such as surface acoustic wave (SAW)-type filters. One of the constraints associated with using a SAW-type implementation is that the frequency response e.g., center frequency and frequency span, are fixed at the time of fabrication. Consequently, each frequency band pair needs its own duplexer. This was not an issue with early cellular telephony since only a few bands were available, and hence only a few SAW-type filters were required in cell phones. 
     However, the dramatic growth in the number of cell phone users and the amount of data each user needs has led to a demand for more spectrum to support these needs. Unfortunately large, contiguous spans of spectrum are simply not available. The result is that there are now more than forty paired frequency bands that are scattered throughout the spectrum, as illustrated by the sample of the spectrum shown in the frequency band schematic  300 . Although it would appear that more than forty duplexers would be needed, a careful examination of the bands reveals that in some cases it is possible to have one duplexer cover more than one band. See, for example, box  312  on  FIG. 3C . Hence it is possible to cover all forty bands with about twenty-one duplexers. However, even with duplexer band aggregation, a large number of duplexers is still required to cover all bands. The reduction in the number of duplexers to twenty-one is not sufficient to allow a practical cell phone configuration utilizing all bands given the extremely tight physical space constraint within a cell phone. As a result, it is currently impractical for manufacturers to offer a truly global phone, that is, one phone that can access all bands. 
     One solution is to construct a tunable duplexer that uses tunable filter technology instead of fixed filters. However, currently there is no such tunable filter technology suitable for this purpose that would provide the required tunability to maintain the present required frequency selectivity and be physically small enough to be suitable for integration into a cell phone. 
     One aspect of the present teaching is to provide a wireless front-end solution that can operate over all the present and future frequency bands in a configuration that eliminates all duplexers. These front end solutions utilize methods and apparatus according to the present teaching that suppress both internal and external interference in both primary, transmit/receive, antennas as well as diversity, receive-only antennas to allow a single wireless device (such as a cellular device) to transmit and receive over an arbitrary number of frequency bands, including those frequency bands illustrated in the schematic  300  as well as other frequency bands that are not shown in  FIG. 3 . The resulting wireless front-end provides a performance that not only meets, but also exceeds, the performance of a device that utilizes multiple frequency duplexers. 
       FIG. 4  illustrates a known RF system  400  that includes a front end  402  comprising multiple duplexers  404 ,  404 ′,  404 ″,  404 ′″,  404 ″″.  404 ′″″. The front end  402  connects antennas  406 ,  408  to a transceiver  410 . The transceiver  410  connects to the front end  402  and to transmit paths  412  that provide an input transmit signal to the transceiver  410  and to receive paths  414  that provide the receive signal from the transceiver  410 . The transmit section of the transceiver  410  includes transmit power amplifiers  416 , a transmit modulator  418  and digital-to-analog converters (DAC)  420 . A transmit/receive digital interface  422  provides formatting and timing needed to interface the typically serial data on  412  and  414  with the parallel data typically needed by ADC  430  and DAC  420 . The receive section of the transceiver  410  includes low-noise receive amplifiers  424  that connect to receive outputs from the front-end  402 . The receive amplifiers  424  are connected to mixers  426 , filters  428  and analog-to-digital converters (ADC)  430 . The ADCs  430  provide a digital receive signal to the transmit/receive digital interface  422  that in turn provides a receive signal at the receive path outputs  414 . The front end  402  includes a number of fixed band pass filters  432 ,  432 ′,  432 ″,  432 ′″,  432 ″″,  432 ′″″,  432 ″″″,  432 ′″″″ that each pass a particular band of the FDD frequency bands. The primary antenna  406  connects to the duplexers  404 ,  404 ′,  404 ″,  404 ′″,  404 ″″.  404 ′″″ and the transceiver  410  using switch  434 . The diversity antenna  408  connects to the fixed band pass filters  432 ,  432 ′,  432 ″,  432 ′,  432 ′″,  432 ′″″,  432 ′″″,  432 ′″″″ and the transceiver  410  using switch  436 . 
     In operation, the primary antenna  406  produces a transmit signal  438  and receives receive signals  440 . The diversity antenna receives receive signals  440 ′. In general, receive signals  440 ,  440 ′ carry the same information on the same RF frequency channel. However, the receive signals  440 ,  440 ′ follow different physical channels because the two antennas  406 ,  408  are physically distinct and located at different positions. Both antennas receive interference signals  442 ,  442 ′. In addition, the diversity antenna  408  receives an interference signal  444  that is generated by the primary antenna transmit signal  438 . The switches  434 ,  436  may be active coupling devices, such as active switches or couplers, or can be passive couplers. 
     The RF front end  402  is implemented with one duplexer  404 ,  404 ′,  404 ″,  404 ′″,  404 ″″,  404 ′″″ per paired-bands of frequency. Also, the RF front end  402  includes fixed band pass filters  432 ,  432 ′,  432 ″,  432 ′″,  432 ″″,  432 ′″″,  432 ″″″,  432 ′″″″ for most of the paired-bands of frequency and some other bands that are needed to process receive signals. Clearly using duplexers and fixed band pass filters for each band is not a scalable solution as the number of bands grows. Presently there is no technology that will enable a filtering-based approach to meet the needs of present and future systems. Methods and apparatus of the present teaching utilize a new approach to frequency-agnostic wireless radio-frequency front ends that does not involve filtering. 
       FIG. 5  illustrates an embodiment of an RF system  500  that includes a frequency-agnostic front end  502  according to the present teaching. This RF system  500  configuration includes a primary antenna  504  and a diversity antenna  506 . First, it is important to appreciate that there are actually two sources of interference at the antennas  504 ,  506 . One source of interference is generated internally in the RF system  500 . The other source of interference is generated externally.  FIG. 5  illustrates a desired receive signal  508 , a desired transmit signal  510  and an external interference signal  512  all in electromagnetic communication with the primary antenna  504 . Also illustrated are a desired receive signal  516  and an external interference signal  518  in electromagnetic communication with the diversity antenna  506 . In addition, there is an internal interference signal  520  at the diversity antenna  506  that is generated by radiation from the primary antenna  504 . 
     As described herein, internal interference arises when the relatively high-power, internal transmit signal enters the internal high sensitivity receive path. This internal interference can enter via several paths, such as coupling from the internal transmit antenna, or leakage through the internal interface (from, for example, a ferrite circulator) that interfaces between the antenna and the transmit/receive paths. Internal interference is illustrated in  FIG. 5  as the coupling from the internal transmit antenna, which is the primary antenna  504 , into diversity antenna  506  by interference signals designated by arrow  520 . 
     Thus, for internal interference, there are two topologically distinct cases to consider. The first is interference coming from the primary antenna  504  that is both transmitting and receiving and the second is interference coming from the diversity antenna  506  that is only receiving. The external interferences  512 ,  518  shown enter both of these antennas  504 ,  506 . The main difference is that since the diversity antenna  506  does not radiate a transmit signal like the primary antenna  504 , the diversity antenna  506  needs only a uni-directional path from the antenna. 
     To illustrate the benefits of the front end  502  described in connection with  FIG. 5 , the front end is shown integrated into a system that would operate similar to the system  400  of  FIG. 4 . In particular, the front end  502  is coupled to a transceiver  522  using three switches  524 ,  526 ,  528 . Transceiver  522  is similar to transceiver  410  of  FIG. 4 . The transceiver  522  includes ports that support the input transmit paths  530  and output receive paths  532 . The transmit section of the transceiver  522  includes transmit amplifiers  534 , a transmit modulator  536  and digital-to-analog converters (DAC)  538 . A transmit/receive digital interface  540  provides formatting and timing needed to interface the typically serial data on  530  and  532  with the parallel data typically needed by ADC  550  and DAC  538 . In the receive section of transceiver  522  receive amplifiers  542  connect to the front end  502  via switches  524 ,  526 . Mixers  544 , filters  546 ,  548  and ADCs  550  are connected to a transmit/receive digital interface  540 . The transmit/receive digital interface  540  includes external interference reducer processors  552 ,  552 ′ that connect to the ADCs  550 . The external interference reducer processors  552 ,  552 ′ are described in more detail in connection with  FIG. 8 . Some embodiments of external interference reducer processor  552 ,  552 ′ execute a self-isolating reference (SIR) algorithm, as described herein. The external interference reducer processors  552 ,  552 ′ are used to improve the rejection of interference from both known and unknown, or partially known interference sources. By partially known, we mean that, for example, the modulation signal or data information is not known, but the modulation format and/or multiplexing format is known. 
     This configuration includes two frequency-agnostic front end devices, one for each of the two types of antennas: one that transmits and receives  504 , and one that is receive-only  506 . A receive front end of the present teaching  554  connects the diversity antenna  506  to the transceiver  522 . A transmit-receive front end  556  connects the primary antenna  504  to the transceiver  522 . The transmit and receive front end  556  may also be referred to as a connector-based front end in the sense defined in U.S. Pat. No. 9,935,680, entitled “Same-Aperture Any-Frequency Simultaneous Transmit and Receive Communication System”. The front ends  554 ,  556  connect to respective antenna  506 ,  504  via respective antenna matching circuit  558 ,  560 , which are optional. Each front end  556 ,  554  has a first port  562 ,  564  for connecting to the antenna  504 ,  506 , a second port  566 ,  568  for receiving a transmit signal or a reference transmit signal, and a third port  570 ,  572  for providing a receive signal. A transmit signal can be received directly from the transceiver  522 , as in the case of transmit and receive front end  556 . The transmit signal can be a transmit reference signal that can be input from a processor (not shown), as in the case of receive front end  554 . The third port  570 ,  572  supplies the receive signal to the transceiver  522 , via switches  524 ,  526 . Receive front end  554  includes a canceller  574  that has individual connections to each of the first port  564 , second port  568  and third port  572 ; details of its operation will be explained in conjunction with  FIGS. 7A, 11 and 13 . Transmit and receive front end  556  includes a connector  580  that has individual connections to each of the first port  564 , second port  566  and third port  570 ; details of transmit and receive front end  556  operation will be explained in conjunction with  FIGS. 7B, 9, 10 and 12 . 
     As described herein, external interference arises when the high-power transmit signal of external systems, which may be co-located with a particular system, enters the particular system&#39;s sensitive receive path. Specifically, the coupling from the external systems into diversity antenna  506  is illustrated by interference signal designated by arrow  518 . The coupling from the external systems into primary antenna  504  is illustrated by the interference signal designated by arrow  512 . The transmit signal from primary antenna  504  that causes internal interference in diversity antenna  506  is illustrated by arrow  520 . The receive signal, which is sometimes referred to as the desired receive signal or desired signal, enters the primary antenna  504  as illustrated by arrow  508 , and enters the diversity antenna  506  as illustrated by arrow  516 . 
     As compared the RF system  400  described in connection with  FIG. 4 , the RF system  500  of  FIG. 5  provides clear advantages in that it eliminates the multiple duplexers  404 ,  404 ′,  404 ″,  404 ′″,  404 ″″.  404 ′″″ and fixed bandpass filters  432 ,  432 ′,  432 ″,  432 ′″,  432 ″″,  432 ′″″,  432 ″″″,  432 ′″″″. The embodiment of the system  500  of  FIG. 5  includes multiple power amplifiers  534  and multiple low-noise receive amplifiers  542  that are also used in the known system  400  described in connection with  FIG. 4 . 
     Thus, the RF system  500  suppress both external interference sources  512 ,  518  and internal interference  520  from the transmit signal. The RF system  500  benefits from an innovative combination of two technologies. For the internal interference, which is filtered out by a frequency-selective duplexer in prior art systems, the techniques of the present teaching use the transmit or uplink signal, a reference copy of the internal interference, parameters derived from the transmit or uplink signal, or parameters derived from the reference copy. Since interference suppression of the present teaching uses the signal causing the internal interference, or parameters derived from it, to suppress the interference—rather than filtering as is used in prior art suppression systems—the amount of suppression is independent of the frequency at which the interference occurs. Hence using the suppression techniques of the present teaching, there is no need for a minimum frequency separation between the interference and desired signals as there was when using prior art suppression techniques. The minimum frequency separation results in guard bands, which are regions of the spectrum between signals that allow for separation of signals using frequency filters. Guard bands are presently technically necessary for FDD systems, but economically unproductive. Systems and methods of the present teaching result in guard bands being significantly reduced, or even eliminated. This advantageously makes the present guard band frequencies available for re-assignment to revenue generating transmit and/or receive bands. 
     Some embodiments of the frequency-agnostic wireless radio-frequency front end of the present teaching even further reduce the number of components needed to provide a multi-band RF system by, for example, utilizing common amplifiers.  FIG. 6  illustrates an embodiment of an RF system  600  that includes a frequency-agnostic front end  602  comprising common amplifiers according to the present teaching. The embodiment of the RF system  600  of  FIG. 6  is similar to the RF system  500  described in connection with  FIG. 5 , except that it uses fewer components. The frequency-agnostic front end  602  is coupled to a transceiver  604  via a primary common low-noise amplifier  606 , a diversity common low-noise amplifier  608 , and a common transmit power amplifier  610 . The common amplifiers  606 ,  608 ,  610  amplify their respective signals in all bands simultaneously, rather than separate amplifiers being used for each band as described in connection with the systems shown in  FIGS. 4 and 5 . 
     The transceiver  604  includes ports that support input transmit paths  614  and output receive paths  612 . The transmit section of the transceiver  604  also includes a transmit modulator  616  and digital-to-analog converters (DAC)  618 . A transmit/receive digital interface  620  provides formatting and timing needed to interface the typically serial data on  612  and  614  with the parallel data typically needed by the ADC  628  and DAC  618 . There is only one transmit power amplifier  610  connected to the modulator  616 . The receive section of transceiver  604  includes mixers  622 , filters  624 , and ADCs  628 . The transmit/receive digital interface  620  includes interference reducer processors  630 ,  632  that connect to the ADCs  628 . The interference reducer processors  630 ,  632  are described in more detail in connection with  FIG. 8 . The interference reducer processors  630 ,  632  are optional, and are used to improve the rejection of interference from both known and unknown, or partially known interference sources. By partially known we mean that some information is known while other information is not known. For example, the modulation signal or data information can be unknown, while the modulation format and/or multiplexing format is known. 
     A receive-only front end  634  connects a diversity antenna  636  to the transceiver  604 . A transmit and receive front end  638  connects a primary antenna  640  to the transceiver  604 . The front ends  634 ,  638  connect to respective antennas  636 ,  640  via antenna matching circuits  642 ,  644 , that are optional. Each front end  634 ,  638  has a first port  646 ,  648  for connecting to the respective antenna  636 ,  640 , and a second port  650 ,  652 , for receiving a transmit signal or a reference transmit signal, and a third port  654 ,  656  for providing a receive signal. A transmit signal can be received directly from the transceiver  604 , through a second port  652 , as in the case of transmits-receive front end  638 . In the case of receive front end  634 , the input to the second port  650  can be a transmit reference signal that can be provided by a processor (not shown) or by an internal transmit source reference, depending on the nature of the transmit signal interference being cancelled. The third ports that supply receive signals include third ports  654 ,  656  that supply the receive signal to their respective common receive low power amplifiers  608 ,  606 . 
     Operation of the receive front end  634  will be described in detail in conjunction with  FIGS. 7A, 11 and 13 . Operation of transmit and receive front end  638  will be described in detail in conjunction with  FIGS. 7B, 9, 10 and 12 . 
     In some embodiments, at least some of the undesired external interference signals collected by antenna  640  that are passed to the third port  656  are cancelled in the interference reducer processor  630 . The interference reducer processors  630 ,  632 , receive front end  634 , and transmit and receive front end  638  are all configured to be inherently broad band enough to be capable of providing their full function over any or all of the frequency bands used in cellular communications. In some embodiments, the frequency bands are the FDD LTE frequency bands. In other embodiments, the frequency bands are all the international standard frequency bands for cellular communications. 
       FIG. 7A  illustrates an embodiment of an RF system  700  that includes a frequency-agnostic receive front end  702  for a diversity antenna  704  according to the present teaching. The diversity antenna  704  receives receive signals  706  and interference signals  708 . The internal interference is suppressed by frequency-agnostic front end  702 ; the external interference is assumed to be of sufficiently low enough power that it passes through to the output  714  and eventually to the interference reducer processors  552 ,  552 ′,  630 ,  632  in  FIGS. 5 and 6  respectively where it is suppressed. The diversity antenna  704  is electrically connected to the first port  710  of the frequency-agnostic receive front end  702 . The first port  710  connects to an input of an electronic differencing device  712 , which in this embodiment comprises a low noise amplifier. The output of differencing device  712  is connected to the third port  714 , which provides a receive output signal along a receive signal path  716 . Part of the output signal from differencing device  712  is split at splitter  718  and sent to a down converter  720  that translates the frequency spectrum of the signal down to a lower frequency, which can be an intermediate frequency (IF) or baseband. The down-converted signal is sent to an analog-to-digital converter  722  and adaptive signal processor  724 . 
     A transmit reference input signal  728  is provided along a transmit signal path  728  through the second port  730  of the frequency-agnostic receive front end  702 . Part of this transmit reference signal  726  is split off at splitter  732  and provided to a down converter  734 , analog-to-digital converter  736 , and to the adaptive signal processor  724 . The other part of the transmit reference input signal  726  is sent to a vector modulator  738 . The output of the vector modulator  738  is connected to the negative input of differencing device  712 . The adaptive signal processor  724  is used to correlate the reference transmit signal with the output of differencing device  712  to isolate interfering transmit components in the receive signal at the differencing device  712  output. The adaptive signal processor  724  then forms an estimate of the optimum complex value of the reference transmitter signal that needs to be injected into the differencing device  712  so as to minimize the residual interfering transmitter signal that is present at the differencing device  712  output. 
     The adaptive signal processor  724  generates two signals at a first and second output that contain the desired settings on the transmit signal adjuster. The first and second signals generated at the output of the adaptive signal processor  724  are provided to respective digital-to-analog converters  740 ,  742 . The digital-to-analog converters  740 ,  742  generate analog outputs that are sent to vector modulator  738 . Some vector modulators include these digital-to-analog converters. Depending on the implementation, the vector modulator  738  may include complex settings for the in-phase (I) and quadrature (Q), or for the magnitude and phase, portions of the transmit reference signal. 
     In this embodiment, the frequency-agnostic receive front end  702  is a cancelling front end. That is, the first port  710  is a unidirectional port, and only receives signals from antenna  704 . The second port  730  takes in a transmit reference signal that is used to remove residual transmit interfering signals that are collected by antenna  704 . There is no transmit signal that is passed from the second port  730  to first port  710  to be sent by antenna  704  because antenna  704  is a receive-only antenna. 
       FIG. 7B  illustrates an embodiment of an RF system  750  that includes a frequency-agnostic transmit and receive front end  752  for a primary antenna  754  according to the present teaching. The frequency-agnostic transmit and receive front end  752  for the primary antenna  754  has many of the same components as the front end  702  for the diversity antenna  704  described in connection with  FIG. 7A . However, transmit and receive front end  752  for the primary antenna  754  supports the simultaneous transmission of a transmit signal with receipt of a receive signal at antenna  754 . As such, the antenna  754  collects signals that include a desired receive signal  756  as well as received interference signals  758 . Antenna  754  also propagates transmit signal  760 . Thus, the first port  762  is a bidirectional port. The second port  764  connects to a transmitter (not shown) and accepts an input transmit signal  766  on a transmit signal path  768  that connects to an input of an isolating power amplifier  767 . In this embodiment, the isolator is an isolating power amplifier with a voltage source isolator. The third port  769  connects to a receiver (not shown) and provides a receive signal  770  on a receive signal path  772 . 
     A connector  774  includes a low noise amplifier  773  with two input ports connected in parallel to a 50 ohm resistor  775 . The first port  762  is connected to an input of the low noise amplifier  773  and the 50 ohm resistor  775 . The lower impedance, ideally zero, output of isolating power amplifier  767  is connected to an input of the low noise amplifier  773  and the 50 ohm resistor  775 . The input resistance, R in  can be greater than 50 Ohms in some configurations. The voltage divider on the negative input is shown as being ½, which is the ideal value assuming that the antenna impedance equals the 50 Ohms of resistor  775 . In practice, the actual antenna impedance may differ from 50 Ohms. As is well known by those skilled in the electrical circuit design art, the voltage divider on the negative input would need to be changed to give the same voltage divider ratio as the voltage divider formed by the 50 Ohm resistor  775  and the antenna impedance. Connector  774  is inherently broadband and capable of connecting the respective signals from first port  762  to the differencing device  776  and from the isolating power amplifier  767  at multiple frequency bands, such as any of the frequency bands used in cellular communications. 
     The output of differencing device  776  is connected to the third port  769 , which provides the output receive signal path  772 . Part of the output signal from differencing device  776  is split at splitter  778  and sent to a down converter  780  that translates the frequency spectrum of the signal down to a lower frequency, which can be an intermediate frequency (IF) or baseband. The down-converted signal is sent to an analog-to-digital converter  782  and then to the adaptive signal processor  784 . 
     Transmit input signal  766  is provided to the second port  764  of the transmit and receive front end  752 . Part of this signal is split off at splitter  786  and provided to a down converter  788 , analog-to-digital converter  790 , and to the adaptive signal processor  784 . The other part of the transmit input signal  766  is sent to an isolating power amplifier  767 . The higher impedance output, which is 50 Ohms in this example, of the isolating power amplifier  767  is connected to a vector modulator  792 . The adaptive signal processor  784  is used to correlate the transmit signal  766  with the output of differencing device  776  to isolate the residual transmitter component in the differencing device  776  output. The adaptive signal processor  784  then forms an estimate of the optimum complex value of the transmitter signal that needs to be injected into the differencing device  776  so as to minimize the residual or at least partially block the transmitter signal that is present at the output of the differencing device  776 . 
     The output of the adaptive signal processor  784  includes two signals that contain the desired settings on the transmit signal adjuster. Since many vector modulators require analog inputs, outputs of the adaptive signal processor  784  comprising these two signals are provided to respective digital-to-analog converters (DACs)  794 ,  796 , but some configurations of vector modulators include the digital-to-analog converters. The analog outputs from the DACs are sent to vector modulator  792 . Depending on the implementation of the vector modulator  792 , the complex settings from the adaptive signal processor  784  may be represented as the in-phase (I) and quadrature (Q), or for the magnitude and phase, portions of the transmit reference signal. The output of vector modulator  792  is provided to the negative input port of differencing device  776 . 
     One feature of RF systems comprising apparatus and methods of the present teaching is that they can substantially reduce or eliminate interference from sources in which the format, but not the data, of the transmitted signal is known. That is the situation, for example, when no actual reference copy is available for the transmit signal, such is often the case for externally-generated interference signals. In current state-of-the art systems, these externally-generated interfering transmit signals are filtered out by the out-of-band rejection of the frequency-selective duplexer. To eliminate the duplexer, an alternative method is needed to eliminate or reduce substantially the undesired, externally-generated signals. In embodiments for which no reference copies of externally generated interfering signals are available, we need to use an interference cancellation technique that does not require a reference signal. One such technique is described in U.S. Pat. No. 10,158,432, entitled “RF Signal Separation and Suppression System and Method”, which is incorporated herein by reference. These techniques utilize an algorithm referred to as a Self-Isolating Reference (SIR) algorithm. 
       FIG. 8  illustrates an embodiment of a frequency-agnostic interference reducer processor  800  of the present teaching. The interference reducer processor  800  can be used, for example, as the interference reducer processors  552 ,  552 ′,  630 ,  632  described in connection with  FIGS. 5 and 6 . The interference reducer processor  800  can use the reproduction-based RF signal separation and suppression described in U.S. Pat. No. 10,158,432, entitled “RF Signal Separation and Suppression System and Method”. An input  802  of the interference reducer processor  800  couples in signals that comprise desired and undesired signals. The desired signals are also referred to herein as receive signals. The undesired signals are referred to herein as externally-generated interference signals. The input  802  is electrically connected to an input  804  of a reproduction generator  806 . The reproduction generator  806  includes a signal conditioner  807  having an input that is connected to the input  804 . The signal conditioner  807  includes a first output connected to a first input  808  and a second output that is connected to a second input  810  of a correlator  812 . The correlator  812  performs correlation of signals provided to the first  808  and second  810  inputs. The correlation is generated at an output  814  of the correlator  812 . 
     The reproduction generator  806  includes a parameter generator  816  that provides basis function parameters to a basis function generator  818 . One skilled in art will appreciate that a basis function is an element of a particular basis for a function space. Every continuous function in the function space can be represented as a linear combination, with proper weighting, of basis functions. In some methods according to the present teaching, the function of interest is the signal to be separated. The parameter generator  816  also provides parameters to the correlator  812 . The parameters provided by the parameter generator  816  include the signal parameters of the signal to be separated. In some embodiments, if the desired signals are to be separated, then their signal parameters are generated by the parameter generator  816 . In some embodiments, if the undesired signals are to be separated, then their signal parameters are generated by the parameter generator  816 . 
     Using the parameters provided by the parameter generator  816 , a basis function generator  818  produces the desired functional representation of the aggregate of the signals to be separated as a linear combination of weighted basis functions. In some embodiments, using the parameters provided by the parameter generator  816 , a basis function generator  818  produces the carrier(s) and subcarrier(s) of the signal to be separated. The output of the basis function generator  818  is connected to one input of a basis function adjuster  820 . A second input of the basis function adjuster  820  is connected to the output  814  of the correlator  812 . The basis function adjuster  820  output is connected to a second input of the signal conditioner  807 . The basis function adjuster  820  adjusts the weighting of the basis functions, which in some methods according to the present teaching comprises the amplitudes and the phases of the carrier(s) and subcarrier(s) provided by the basis function generator  818 , in order to maximize the correlation between the output of the reproduction generator and the sum of the signal(s) to be separated and the signal(s) to be kept. 
     A third output of the signal conditioner  807  is the output of the reproduction generator  806 . The output of the reproduction generator  806  is electrically connected to a first input  822  of a subtractor  824 . The input  802  of the interference reducer processor  800  is electrically connected to the second input  826  of the subtractor  824 . The subtractor  824  subtracts the reproduction of the signal to be separated from the sum of the signal(s) to be kept and the signal(s) to be separated. The output of the subtractor  824  provides an output signal that includes the signal(s) to be kept with the signal(s) to be separated suppressed. Thus, the output signal from the interference reducer processor  800  includes the desired signal(s) and a suppressed aggregate of the undesired signal(s). 
     In some embodiments, the reproduction generator  806  performs a self-isolating reference (SIR) algorithm using three steps. In the first step, the basis function generator  818  generates the basis function(s) of the signal to be separated. In the second step, the basis function adjuster  820  adjusts the signals provided by the basis function generator  818 . In the third step, the correlator  812  performs the correlation. The adjuster  820  operates in conjunction with the correlator  812  to maximize the correlation between the output of the reproduction generator  806  and the sum of the signal to be separated and the signal to be kept. For use in the RF system that includes the frequency-agnostic front end embodiments described in connection with  FIGS. 5-6 , the frequency-agnostic interference reducer processor  800  operates with signals in digital form. In various embodiments, the functions and/or steps associated with the interference reducer processor  800  can be performed in analog and/or digital form. 
     One important feature of the interference reducer processor  800  of the present teaching is that it only requires information about the signal format including, for example, the multiplexing and physical layer transmission format (e.g. carriers), of the interference signals to render a portion of the interference signal(s) distinguishable and thus, separable and/or suppressible. Thus, frequency-agnostic the interference reducer processor  800  of the present teaching is effective at suppressing the undesired signals irrespective of the data that are being conveyed by that format. In cellular telephony systems according to the present teaching, the format of the multiplexing and physical layer transmission is standardized and well known. As a particular example, consider the case where the up-link signals being transmitted by mobile users who are co-located with a particular user constitute the externally-generated interference that is incident on the antennas of a particular user&#39;s cell phone. In such a case the format of the externally-generated interference is well known, since it follows the known standards for cell phone up-links. 
     The frequency-agnostic interference reducer processor  800  can be implemented in RF systems that include a frequency-agnostic front end  500 ,  600  that are described in connection with  FIGS. 5-6 . The input  802  can be electrically connected to the output of the analog-to-digital converters  550 ,  628 . The output of the subtractor  824  is connected to output receive path  532 ,  614 . 
     One feature of the present teaching is the frequency-agnostic interference reducer processor  800  has a broad bandwidth, such that any or all of the frequency bands of a particular cellular system, such as an FDD-based cellular system, can be appropriately processed by the interference reducer processor  800 . Therefore, the input  802  of the interference reducer processor  800  couples in signals that include desired and undesired signals that occupy a variety of signals in one or more of multiple frequency bands of cellular systems. The desired and undesired signals can occupy the same frequency bands and/or different frequency bands. This reduces component count and size of a front end as compared to known front ends that rely on filtering or filtering frequency duplexers to reduce the content of undesired signals in a signal that contains a desired signal. In some embodiments, these include signals in FDD frequency bands for cellular systems. 
       FIG. 9  illustrates an embodiment of an RF antenna system  900  comprising a frequency-agnostic transmit and receive front end  902  that utilizes travelling wave structures  904 ,  906  to form a connector according to the present teaching. For example, the connector can be one of the connectors described in U.S. Pat. No. 7,555,219, which is assigned to the present assignee, and which is incorporated herein by reference. The first port  908  connects to an antenna  910 , which is a primary antenna that produces a transmit signal  912  and that collects a desired receive signal  914  and undesired interference signals  916 . In some embodiments, the primary antenna  910  is a cellular system primary antenna. The connection to the antenna  910  may pass through an antenna matching circuit  918 , which causes a part of the transmit signal to be reflected back on bi-directional path where it enters the connector thereby producing an internally-generated interference that needs to be suppressed. 
     The transmit and receive front end  902  has a first port  908  for connecting to the antenna  910  that is bidirectional, and a second port  920  for taking in a transmit signal  922 , and a third port  924  for providing a receive signal and the externally-generated interference  926  out. The transmit and receive front end  902  includes a non-reciprocal waveguide device  928  having a first  904  and a second  906  traveling-wave waveguide that are positioned such that electromagnetic fields couple in an interaction region  930  between the first traveling-wave waveguide  904  and the second traveling-wave waveguide  906  in a non-reciprocal manner. The term “non-reciprocal manner” is as used herein to mean non-reciprocal coupling of electromagnetic fields where electromagnetic fields strongly coupled in one direction and are substantially prevented from coupling in another direction, such as in a circulator device. In some embodiments, the non-reciprocal waveguide device  928  is an optical modulator. In other embodiments, the non-reciprocal waveguide device  928  is an electronic distributed amplifier. 
     The non-reciprocal waveguide device  928  passes a transmit signal  922  from the second port  920  to the first port  908 . Simultaneously, the non-reciprocal waveguide device  928  passes a receive signal and externally-generated interference signal(s) from the first port  908  to the third port  924  while not passing the transmit signal from second port  920  to the third port  924 . The non-reciprocal waveguide device  928  also prevents the passage of the receive signal and externally-generated interference signal(s) from the first port  908  to the second port  920 . 
     One feature of the present teaching is the use a non-reciprocal waveguide device  928  with broad RF spectral bandwidth, such that any or all of the frequency bands of a particular cellular system, such as an FDD-based cellular system, are appropriately passed amongst the first port  908 , second port  920  and the third port  924  as described. Therefore, the second port  920  can pass transmit signals on one or more of multiple frequency bands for cellular transmit signal bands to the first port  908  while the third port  924  passes receive signals on one or more of multiple frequency bands for cellular receive signal bands from the first port  908 . 
     The one or more frequency bands for transmit and receive can be spectrally disjoint, and/or can have overlapping spectra. In some embodiments, the transmit signal and the receive signal occupy the same cellular frequency band at the same time. In other embodiments, the transmit signal and the receive signal occupy different cellular frequency bands at the same time. For example, the transmit signal  922  can be a cellular uplink channel for an FDD system while the receive signal  926  is a cellular downlink channel for an FDD system, or vice versa. Even in the case where the transmit signal  922  and the receive signal  926  occupy disjoint cellular frequency bands, the signal separation occurs without the use of filtering, or filtering frequency duplexers to separate the transmit and the receive signals. This reduces component count and size of the front end  902  as compared to known front ends that rely on filtering or filtering frequency duplexers. In some embodiments, the front end  902  is used as the primary antenna front end  556 ,  638  for the system described in connection with  FIGS. 5-6 . 
       FIG. 10  illustrates an embodiment of an antenna system  1000  with a frequency-agnostic transmit and receive front end  1002  that utilizes subtraction according to the present teaching. The first port  1004  connects to an antenna  1006  via bi-directional path  1024 . The antenna  1006  is a primary antenna producing a transmit signal  1008  and collecting a desired receive signal  1010  and undesired interference signals  1012 . In some embodiments the primary antenna  1006  is a cellular system primary antenna. The connection from the first port  1004  to the antenna  1006  may pass through an antenna matching circuit  1014 , which causes a part of the transmit signal  1018  to be reflected back on bi-directional path  1024  where it enters the connector  1030  thereby producing an internally-generated interference that needs to be suppressed. 
     The front end  1002  also has a second port  1016  for inputting a transmit signal  1018 , and a third port  1020  for providing a receive signal  1022 . Thus, the transmit and receive front end  1002  connects three signal paths via the first port  1004 , the second port  1016  and the third port  1020 . These paths are a bidirectional signal path  1024  from and to the antenna  1006 , a transmit path  1026 , and a receive path  1028 . The first port  1004  connects the antenna  1006  to a first port of a signal connector  1030 . An isolator  1032  is connected between the second port  1016  and a second port of the signal connector  1030 . 
     The combined tap and isolator  1032  also provides a portion of the transmit signal  1026  to a transmit signal adjuster  1034  and to a signal processor  1036 . Alternatively, as is well known in the RF art, the tap and isolator can be implemented using separate components. The signal processor  1036  also has an input  1038  that is connected to the third port  1020 , and also provides an output to the transmit signal adjuster  1034 . A third port of the signal connector  1030  is input to a subtractor  1040 . In various embodiments, the signal connector  1030  is one of the connectors described in U.S. patent application Ser. No. 14/417,122, entitled “Same-Aperture Any-Frequency Simultaneous Transmit and Receive Communication System.” For example, the signal connector  1030  can be configured so that the impedance at each port of the signal connector  1030  is designed to match the impedance of the component that is connected to that respective port, which can be the antenna  1006 , the isolator  1032 , and the subtractor  1040 . 
     The output of the subtractor  1040  is also connected to the signal processor  1036  via path  1038 . In operation, the subtractor  1040  removes the large transmit signal  1018  from the receive signal passed by the connector  1030 . The signal-processed and adjusted transmit signal is derived from the input transmit signal  1018 . The signal processor  1036  determines the precise complex value of the transmit signal that should be fed to the second terminal of the subtractor  1040  so as to minimize the residual transmit signal that is present in the receive path. The transmit signal adjustor  1034  is used to set the complex value of the transmit signal that is fed to the subtractor  1040 . 
     Thus, only the receive signal and the externally-generated interference signal(s) are present at the third port  1020 , with the transmit signal  1018  removed. This is true regardless of whether the frequency bands of the transmit and receive signals are in the same or disjoint portions of the spectrum. The separation occurs without the need for any filtering or frequency duplexer between the first port  1004  and the second port  1016  and/or the third port  1020 . 
     One feature of the present teaching is that the transmit and receive front end  1002  and subtractor  1040  has a broad bandwidth, such that any or all of the frequency bands of a particular cellular system, such as an FDD-based cellular system, can be appropriately passed amongst the first port  1004 , second port  1016 , and the third port  1020  as described. Therefore, the second port  1016  can pass transmit signals on one or more of multiple frequency bands for cellular transmit signal bands to the first port  1004  while the third port  1020  can pass receive signals from first port  1004  on one or more of multiple frequency bands for cellular receive signal bands. The one or more frequency bands for transmit and receive can be spectrally disjoint, and/or can have overlapping spectra. 
     In one method of operation, the transmit signal and the receive signal occupy the same cellular frequency band at the same time. In another method of operation, the transmit signal and the receive signal occupy different cellular frequency bands at the same time. For example, the transmit signal may be a cellular uplink channel for an FDD system, while the receive signal can be a cellular downlink channel for an FDD system, or vice versa. In the case where the transmit signal and the receive signal occupy disjoint cellular frequency bands, the signal separation occurs without the use of a filter, or a filtering frequency duplexer, that separates transmit and receive signals. This reduces component count and size of the transmit and receive front end  1002  as compared to known front ends that rely on filtering or filtering frequency duplexers. In some embodiments, the front end  1002  is used as the primary antenna front end  556 ,  638  for the RF system described in connection with  FIGS. 5-6 . 
       FIG. 11  illustrates another embodiment of an antenna system  1100  comprising a frequency-agnostic receive front end  1102  that utilizes a subtractor according to the present teaching. The first port  1104  connects to a diversity antenna  1106 . The diversity antenna  1106  does not transmit. The diversity antenna  1106  receives both a desired receive signal  1108  and undesired, interference signals  1110 . For example, the diversity antenna can be a cellular system diversity antenna. The connection to the diversity antenna  1106  can pass through an antenna matching circuit  1112 . The front end  1102  has a first port  1104  for connecting to the antenna  1106 , a second port  1114  for inputting a transmit reference signal  1116 , and a third port  1118  for providing a receive signal  1120 . The front end  1102  provides a receive signal and externally-generated interference signal(s)  1120  at the third port  1118  with substantially reduced undesired internal interference signal  1110  content. 
     A transmit reference signal  1116  is provided to the second port  1114 . A portion of the transmit reference signal  1116  is provided to a transmit signal adjuster  1122  and to a signal processor  1124 . The first port  1104  connects the antenna  1106  to an input port of a subtractor  1126 . An output of the subtractor  1126  provides a receive signal to the third port  1118 . A portion of the output receive signal from the subtractor  1126  is provided to the signal processor  1124 . The signal processor  1124  provides an output to the transmit signal adjuster  1122  based on the input from the output of subtractor  1126  and the input transmit reference signal  1116 . The output of the transmit signal adjuster  1122  is connected to a second input of the subtractor  1126 . 
     In operation, the subtractor  1126  removes the internal interfering transmit signal(s) of undesired receive signals  1110  from the desired receive signal  1108  by using the signal-processed and adjusted transmit signal that is derived from processing the input transmit reference signal from the second port  1116 . The signal processor  1124  determines the precise complex value of the transmit reference signal that should be fed to the second input of the subtractor  1026  so as to minimize the residual internal interfering transmit signal that is present in the receive path. The transmit signal adjustor  1122  is used to set the complex value of the transmit reference signal that is fed to the subtractor  1126 . 
     Thus, after subtraction, only the receive signal and the externally-generated interference signal(s) are substantially present at the third port  1118 , with internal interfering signals removed. This is true regardless of whether the frequency bands of the internal interfering transmit and receive signals are in the same or disjoint parts of the spectrum. The removal of the internal interfering signals occurs without the need for any filtering or frequency duplexer between the first port  1104  and the second ports  1114  and/or the third port  1118 . 
     One feature of the frequency-agnostic wireless radio-frequency front end of the present teaching is that the receive front end  1102  and subtractor  1126  have a broad bandwidth, such that any or all of the frequency bands of a particular cellular system, such as an FDD-based cellular system, can be appropriately passed amongst the first port  1104 , the second port  1114  and the third port  1118  as described. Therefore, the second port  1114  can pass transmit reference signals  1116  on one or more of multiple frequency bands used for cellular transmit signal bands to the signal processor  1124  and transmit signal adjuster  1122  while the third port  1118  can pass receive signals and the externally-generated interfering signal(s) from the first port  1104  on one or more of multiple frequency bands used for cellular receive signal bands. 
     The one or more frequency bands for transmit and receive can be spectrally disjoint, and/or can have overlapping spectra. In some embodiments, the transmit signal and the receive signal occupy the same cellular frequency band at the same time. In other embodiments, the transmit signal and the receive signal occupy different cellular frequency bands at the same time. For example, the transmit signal may be a cellular uplink channel for an FDD system while the receive signal is a cellular downlink channel for an FDD system, or vice versa. In the case where the transmit signal and the receive signal occupy disjoint cellular frequency bands, the signal separation occurs without the use of a filter or a filtering frequency duplexer to separate transmit and receive. This reduces component count and size of the receive front end  1102  as compared to known front ends that rely on filters or filtering frequency duplexers. In some embodiments, the receive front end  1102  is used as a diversity antenna receive front end  554 ,  634  for the system described in connection with  FIGS. 5-6 . 
     Another feature of the frequency-agnostic wireless radio-frequency front end of present teaching is that a fast switch can be used in a front end for a diversity antenna to pass one or more of multiple frequency bands in a cellular system.  FIG. 12  illustrates an embodiment of an antenna system  1200  with a frequency-agnostic transmit and receive front end  1202  that utilizes a fast switch according to the present teaching. The first port  1204  connects to a primary antenna  1206  that radiates a transmit signal  1208 . The primary antenna  1208  also collects a desired receive signal  1210  and undesired, externally-generated interference signals  1212 . In some embodiments, the primary antenna  1206  is a cellular system primary antenna. The bidirectional connection from the first port  1204  to the antenna  1206  may pass through an antenna matching circuit  1214 . 
     The transmit and receive front end  1202  has a first port  1204  for connecting to the antenna  1206 , a second port  1216  for inputting a transmit signal  1218 , and a third port  1220  for providing a receive signal  1222 . Thus, the front end  1202  connects three signal paths, a bidirectional signal path from and to the antenna  1206 , a transmit signal path, and a receive signal path via the first port  1204 , second port  1216 , and the third port  1220 . The first port  1204  connects to a common port of a single-pole double-throw switch  1224 . The switched ports of the switch  1224  connect to the second port  1216  and to the third port  1220 . The switch  1224  samples the receive signal by connecting the first port  1204  to the third port  1220  for a short sampling time, and connects the first port  1204  to the second port  1216  for the remainder of a period between samples. In some embodiments, the sampling time is from 1-10% of the period and the sampling time is synchronized to occur when the transmit signal passes through zero. In one specific embodiment, the period is a Nyquist sampling period. See, for example, U.S. Pat. No. 9,209,840, entitled “Same-Aperture Any-Frequency Simultaneous Transmit and Receive Communication System”, which is assigned to the present assignee and which is incorporated herein by reference. 
     Yet another feature of the present teaching is that the transmit and receive front end  1202  and fast switch  1224  can operate with a broad bandwidth, such that any or all of the frequency bands of a particular cellular system, such as an FDD-based cellular system, can be appropriately passed amongst the first port  1204 , second port  1216 , and the third port  1220  as described. Switch devices that operate at the Nyquist rate of typical receive signals in a cellular system and a sample interval that is less than 10% of that sample period are available. Therefore, the second port  1216  can pass transmit signals  1218  on one or more of multiple frequency bands for cellular transmit signal bands to the first port  1204  while the third port  1220  can pass receive signals from the first port  1204  on one or more of multiple frequency bands for cellular receive signal bands. 
     The one or more frequency bands for transmit and receive can be spectrally disjoint, and/or can have overlapping spectra. In some embodiments, the transmit signal and the receive signal occupy the same cellular frequency band at the same time. In other embodiments, the transmit signal and the receive signal occupy different cellular frequency bands at the same time. For example, the transmit signals can be a cellular uplink channel for an FDD system while the receive signal can be a cellular downlink channel for an FDD system, or vice versa. In the case where the transmit signal and the receive signal occupy disjoint cellular frequency bands, the signal separation occurs without the use of a filter or a filtering frequency duplexer to separate transmit and receive. This reduces component count and size of the front end  1202  as compared to known front ends that rely on filtering or filtering frequency duplexers. In some embodiments, the transmit and receive front end  1202  is used as the primary antenna transmit and receive front end  556 ,  638  for the system described in connection with  FIGS. 5-6 . 
       FIG. 13  illustrates an embodiment of an antenna system  1300  with a frequency-agnostic receive front end  1302  that utilizes a fast switch according to the present teaching. The first port  1304  connects to a diversity antenna  1306  that collects a desired receive signal  1308  and undesired, externally-generated interference signals  1310 . In some embodiments, the diversity antenna  1306  is a cellular system diversity antenna. 
     The receive front end  1302  has a first port  1304  for connecting to the antenna  1306 , a second port  1312  for inputting a transmit reference  1314 , and a third port  1316  for providing a receive signal  1318 . The first port  1304  connects to a common port of a single-pole single-throw switch  1320 . The switched port of the switch  1320  connects to the second port  1312  and the switch control connects to the third port  1316 . The switch  1320  samples the receive signal by connecting the first port  1304  to the third port  1316  for a short sampling time. The sampling time is from 1-10% of the period and the sampling time is synchronized to occur when the transmit signal passes through zero. In some embodiments the sampling time is synchronized to at least some of the transmit signal zero crossings. 
     EQUIVALENTS 
     While the Applicant&#39;s teaching is described in conjunction with various embodiments, it is not intended that the Applicant&#39;s teaching be limited to such embodiments. On the contrary, the Applicant&#39;s teaching encompass 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.