# Patent

## Title
Digital routing switch matrix for digitized radio-frequency signals

## Abstract

Routing and distribution of radio-frequency (RF) signals is commonly achieved in the analog domain. However, improved performance and simplified circuit architectures may be obtained by first digitizing the RF signal, and then carrying out all routing in the digital domain. A new generation of scalable digital switches has been developed, which routes both the data and clock signals together, this being necessary to maintain the integrity of the digitized RF signal. Given the extremely high switching speeds necessary for these applications (tens of GHz), this is implemented using Rapid-Single-Flux-Quantum (RSFQ) logic with superconducting integrated circuits. Such a digital switch matrix may be applied to either the receiver or transmitter components of an advanced multi-band, multi-channel digital transceiver system, and is compatible with routing of signals with different clock frequencies simultaneously within the same switch matrix.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to and claims priority to the following applications: (1) U.S. application Ser. No. 11/243,019, filed Oct. 5, 2005, by inventors Alexander Kirichenko, Deepnarayan Gupta, and Saad Sarwana entitled Superconducting Digital Mixer; (2) U.S. application Ser. No. 11/243,022, filed Oct. 5, 2005, by inventor Alexander Kirichenko entitled A Digital Programmable Frequency Divider; (3) U.S. application Ser. No. 11/243,020, filed Oct. 5, 2005, by inventor Alexander Kirichenko entitled A Digital Programmable Phase Generator; and (4) U.S. application Ser. No. 11/360,749, filed Feb. 23, 2006, by inventors Alexander Kirichenko, Timur Filippov, and Deepnarayan Gupta entitled Superconducting Circuit for High-Speed Lookup Table. Each of these applications is hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION 
1. Field of the Invention
This invention relates to the field of switching circuits and, more particularly, for switching circuits for advanced digital radio receivers and transmitters.
2. Description of the Prior Art
A simple classic radio receiver consists of a single antenna coupled to a downconverter that extracts a single “baseband” channel. In a modem receiver, the baseband signal is digitized with an analog-to-digital converter (ADC) and further processed in the digital domain. A classic transmitter contains essentially the same components working in the reverse direction; a transceiver contains both a transmitter and a receiver packaged together.
A multi-band, multi-channel RF communications system (see FIG. 1a) can include multiple antennas, and can extract multiple baseband channels or groups of channels simultaneously. This requires a switch matrix, which permits distribution of various signals between each antenna (corresponding to each band) and one or more appropriate radio receiver channels. If the system is to be flexible and reconfigurable, it should be possible to redirect input signals to selected output channels.
The signal at the antenna is an analog waveform, even if it may be encoding a digital signal. In a conventional receiver system of the prior art, as illustrated in FIG. 1a, both the switch matrix and the channelizing receivers are analog components, and similarly for the transmitter. However, these band-specific precision analog components are often expensive and limited in their flexibility and reconfigurability. Furthermore, these analog switching systems have severe deficiencies in terms of losses, isolation, crosstalk, and ability to multicast.
For these reasons, the communications industry would like to move toward an approach known as “software-defined radio” (SDR) or “software radio”, where all data processing is carried out in the digital domain, except right at the antenna itself. This requires ultrafast data converters, with sampling rates of tens of GHz and excellent linearity. ADCs with the requisite properties have recently been demonstrated, based on superconductor electronics using Josephson junctions, with circuit designs based on rapid-single-flux-quantum logic (RSFQ). It is natural that this data conversion be carried out right at the antenna, as illustrated in FIG. 1b. But in this case, the switching must also be carried out directly on the digital-RF signals. Furthermore, the precision and linearity of these signals can be maintained in the distribution network only if the sampling clock is distributed along with the data bits. This requires a new type of digital-RF switch matrix, which has not been reported before, and which is the subject of the present invention.
Furthermore, the digital-RF transceiver architecture allows natural partitioning between band-specific (analog) and band-independent (digital) components. Analog components, such as antennas and amplfiers are optimized for performance within a particular frequency band. Even data converters between analog and digital formats, ADCs and DACs, work best with designs that target specific frequency bands. Furthermore, an ADC or DAC optimized for a particular frequency band will typically have a particular sampling frequency (clock frequency f[clock]) that is preferred for best performance. For example, a radio-frequency bandpass ADC designed for a center frequency f) may exhibit the greatest dynamic range for a sampling frequency that is four times the center frequency (f[clock]=4*f)). On the contrary, digital signal processing units, operating on numbers, are independent of the signal characteristics. This partitioning enables the true software radio paradigm by allowing full software programmability of the RF distribution network. Superconductor electronics are fast enough to digitize at multi-GHz RF and perform subsequent processing completely in the digital domain.
Switch matrices based on superconducting electronic circuits have been recently reported by several inventors. For example, see (1) U.S. Pat. No. 6,960,929, issued Nov. 1, 2005 by inventor Fernand D. Bedard, entitled Superconductive Crossbar Switch, (2) U.S. Pat. No. 6,917,537, issued Jul. 12, 2005 by inventor Paul I. Bunyk entitled RSFQ Batcher-Banyan Switching Network, (3) U.S. Pat. No. 6,865,639, issued Mar. 8, 2005 by inventor Quentin P. Herr entitled Scalable Self-Routing Superconductor Switch, and (4) Hashimoto et al., Implementation of a 4×4 Switch With Passive Interconnects, IEEE Trans. Appl. Supercon., vol 15, no. 2, June 2005, pp. 356-359.
However, none of these patents was designed for an application in RF communications, and none of these include switches which route the clock signal together with the data signal, which are two of several reasons this invention distinguishes over the prior art. See also the article by D. K. Brock, O. A. Mukhanov, and J. Rosa, “Superconductor Digital Development for Software Radio,” IEEE Commun. Mag., pp. 174-179, February 2001, and K. K. Likharev and V. K. Semenov, “RSFQ Logic/Memory Family: A new Josephson junction technology for sub-THz digital systems”, IEEE Trans. Appl. Supercond., vol. 1, pp. 3-28, 1991.
PROBLEMS OF THE PRIOR ART 
The prior art switches have been expensive and limited in their flexibility and ability to reconfigure. In addition, they have severe deficiencies in terms of losses, isolation, cross talk and ability to multicast.
It is natural and desirable that data conversion be carried out right at the antenna, but, in such a case, the switching must also be carried out directly on the digital-RF signals. Further, the precision and linearity of these signals can be maintained in the distribution network only if the sampling clock is distributed along with the data bits. This requires and new type of digital-RF switch matrix which is the subject of the present invention.
SUMMARY OF THE INVENTION 
A multi-carrier, multi-channel RF communication system requires a switch matrix to route various signals between a set of antennas and a set of radio transceivers. This can be carried out most efficiently in the digital domain, but requires the use of ultrafast circuits that can accurately process multi-GHz RF signals.
One aspect of the invention is directed to a switch matrix which is suitable for routing various signals between a set of antennas and a set of radio transceivers. The transceivers can be multi-carrier, multi-channel RF communication devices. The routing is carried out in the digital domain and uses ultra fast superconductive circuits that can accurately process multi-gigahertz RF signals. For best performance the basic switching cell must carry both the data bits and the sampling clock, where the sampling clock may be at different frequencies for signals from different RF bands distributed within the same switch matrix.
Preferred exemplary embodiments of the invention are implemented using ultra fast RSFQ superconducting logic elements.



BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1A is a block diagram of a multi-band, multi-channel RF transceiver system incorporating an analog switch matrix as known in the prior art.
FIG. 1B is a block diagram of a multi-band, multi-channel RF transceiver system incorporating a new digital switch matrix for use with digital RF architectures in accordance with one aspect of the invention.
FIG. 2 shows a basic digital switch using Josephson junctions.
FIG. 3A is a block diagram of a digital switch based on a non-destructive readout (NDRO) cell.
FIG. 3B is a schematic diagram of the digital switch based on a non-destructive readout (NDRO) cell.
FIG. 4 is a novel switch cell for routing clock and data together.
FIG. 5 is a block diagram of a 2×2 building-block for a 1:1 router matrix.
FIG. 6 is a block diagram of a 4×4 switch matrix consisting of four 2×2 building-blocks in a Banyan network.
FIG. 7 shows a block diagram of a multicasting switch matrix in accordance with one aspect of the invention.
FIG. 8 shows an optional delay compensation network for a multicasting switch matrix of the type shown in FIG. 7.
FIG. 9A and 9B show implementations of a multicasting switch using RSFQ cells, respectively with and without clock accompanying a data line.
FIG. 10 is a block diagram of a digital-RF switch matrix for receiver systems showing size of the switching array.
FIG. 11 shows a block diagram of a digital switch matrix included in both receiver and transmitter of a complete digital-RF transceiver system.



DETAILED DESCRIPTION OF THE INVENTION 
I. Basic Switching Cells
RSFQ logic, first developed by Likharev, Semenov, and Mukhanov, is built around Josephson junctions (JJs), with lossless propagation of single-flux-quantum (SFQ) voltage pulses, each corresponding to an integrated pulse of 2 mV-ps. Two different types of RSFQ switch cells are described, both of which are well known in the literature: the DC switch and the NDRO switch. In either case, the general principle of operation is the same. A JJ is biased such that an SFQ pulse will cause it to temporarily exceed its critical current and then return to its previous state, thus emitting another SFQ pulse. The switching time, which depends on the critical current density (Jc) of the JJ, is about 2 ps for the Jc=4.5 kA/cm2. This ultrafast switching allows a very high rate (40 Gbps and higher) of SFQ digital signals. Whether or not a switch responds to an input SFQ pulse depends on its designed control conditions (DC bias, control current, or other concurrent digital events). For example, the state of the switch—whether it will pass or block a digital input—may be controlled by the state of a coupled flip-flop, where control information could be stored.
FIG. 2 shows a schematic diagram of a simple DC-switch with only 2 JJs, which is controlled by a applying DC current to the control line.
FIGS. 3A and 3B show another digital switch element, called a non-destructive readout (NDRO) switch, where the control information is stored in a reset-set flip-flop (RSFF). (The NDRO switch with 8 JJs is more complex for a single switch, but is more easily scalable to large switch matrices.) An SFQ pulse applied to the Set input sets the RSFF to the ‘1’ state. While it is in the ‘1’ state, any pulse applied to the input will be read out as an SFQ pulse at the output synchronized to the clock input. This corresponds to the ‘ON’ (closed) state of the switch.
If a pulse is applied to the Reset input, the RSFF is reset to the ‘0’ state. While it is in the ‘0’ state, any pulse applied to the input will escape through J6 and no SFQ pulse will be produced at the output. This corresponds to the ‘OFF’ (open) state of the switch. One advantage of this type of switch is that only one external control line is required for each cell to write the desired switch states for the entire matrix. Thus the control for the entire matric can be stored as a serial digital word. This is achieved by simply connecting the RSFFs of different switch cells as a shift register by applying the FF Out to the Set input of the next RSFF.
One needs to route both data and clock from each input source through the switch to desired output ports, for fully asynchronous operation. Of course, this can be accomplished by two switch cells which are manually set (reset) together. However a more elegant solution is part of one aspect of the invention by routing the clock signal through an NDRO switch and controlling propagation of the data stream through an RSFF (see FIG. 4 where the clock line is shown as a dashed line), thus reducing the JJ count necessary.
FIG. 4 can be extended to accommodate a n-bit parallel data stream by replicating instance of the Data RSFF for each bit stream of the n-bit parallel data stream, with each RSFF being reset by the clock output signal.
II. Routing Switch Architecture
Consider a switch architecture where each input is routed only to a single output. (This constraint will be relaxed later.) To see how to scale up from a single switch, consider first a 2×2 building block (see FIG. 5). This is done with 4 switch cells. The data and clock signals from the first input are applied to Switch 11 and Switch 12 respectively, and the data and clock signals from the second input are applied to Switch 21 and Switch 22 respectively. The data and clock outputs from Switch 11 and Switch 21 are merged (combined) at the first output port, and the data and clock outputs from Switch 12 and Switch 22 are merged (combined) at the second output port. We have shown both clock and data paths, as well as the control (set) line, explicitly to illustrate the complexity of the design. We did not show the reset line, which will add to the routing complexity, but could be laid out similarly to the set lines in a way that should be obvious to one skilled in the art. Throughout the layout, maintaining accurate relative timing between paired clock and data signals is essential for correct operation.
Once a 2×2 switching network is complete, standard network approaches can be used to generate larger networks. For example, FIG. 6 shows the design of a 4×4 switch using the well-known, scalable Banyan network, with the double lines indicating routing of both data and clock signals together.
III. Architecture of Multicasting, Cross-Point Switch Matrix
For some applications, it is necessary to copy one input to multiple outputs, or vice versa. The relevant switch architecture here is an M×N cross-point switch matrix that connects M inputs to N outputs (FIG. 7). Here each input propagates horizontally through a set of switch cells. If a switch is turned on, a copy of the input data stream is routed down the corresponding column towards an output port; there is no degradation of signal quality since digital copying is lossless. Any number of switches may be turned on in each row to produce copies of the input at multiple output ports. There is a potential problem of latency in this structure, since the signal propagation paths from an input port to different output ports is different. For most communications applications, such small differences in propagation delays (a nanosecond or less) do not matter. Delay compensation networks (shown as triangles in FIG. 8) can be added for applications, such as a network switch for multiprocessor supercomputers, that are sensitive to delay mismatches. Delay compensation can be achieved using passive lines of matched physical lengths or active transmission structures with matching delay times (such as tunable Josephson transmission lines).
We can use the same DC and NDRO switches (FIGS. 2 and 3 or the switch cell of FIG. 4) to build this multicasting switch matrix depending on wheather the clock line accompanies the data line. The building block of this matrix is a row (FIGS. 9A and 9B).
The multicasting switch matrix architecture in FIG. 7 is composed of M×N switches as in FIG. 3. Each switch has a set and reset line that permits the routing to be dynamically modified. For many implementations, as suggested in FIG. 9B, the “set” lines may be connected in series, and the entire M×N array addressed by a single digital word. Given the very fast clock speed, the entire array can be reconfigured in a short time. For very large arrays, partial parallel addressing (of separate rows, for example) may be used to speed up the reprogramming rate. In this way, one may direct any input to any and all of the outputs, in a way that is rapidly reconfigurable.
IV. Integration of Switch Matrix into Transceiver System
To exemplify the advantages of digital routing of RF signals, let us examine the receive side in greater detail (FIG. 10). The analog RF input signals are digitized directly at RF using an ADC behind each antenna. Once the RF signals are in the digital domain, multiple copies can be generated without compromising signal power and quality. Therefore, the digital-RF signal can be simultaneously applied to a bank of digital channelizing units, each operating independently to extract a subband from a wide input band. For a multi-band system, digitized data streams from multiple ADC front-ends can be distributed to a bank of channelizers through a digital non-blocking, multicasting switch matrix. This architecture is scalable to an arbitrary number of channelizers (or more general digital processors) and banded antenna-ADC pairs. Furthermore, the digital switch matrix can be programmed in real time to dynamically reconfigure the communication system: changing band-to-channel allocation, cross-banding, etc.
In general, signal processing involves multiple steps, including multiple levels of channelization. For simplicity, here we have considered only the first level channelization function: extraction of a sub-band through mixing and filtering. This step requires digital-RF processing at clock speeds of 40 Gbps and beyond, and therefore, may be accomplished using superconductor electronics.
One special requirement for the switch matrix is its ability to support multiple input data rates. The ADCs may not share the same clock frequency. For example, the choice of clock frequency may depends on the center frequency of the band for convenient digital in-phase and quadrature (I&Q) mixing, which requires the clock to be 4 times higher or multiples thereof. An asynchronous or better said multi-synchronous switch, routing both the clock and digitized data together, is essential to address this requirement. This ability to distribute digital signals with several different clock frequencies simultaneously within the same general-purpose switch matrix is a unique feature of one aspect of the invention.
A general block diagram of a complete direct digital-RF communication system is shown in FIG. 11. A multi-band RF communication system consists of an antenna subsystem to capture electromagnetic energy in different RF bands and a transceiver subsystem to transmit and receive information from each RF band through a variety of signal processing steps (e.g., up/down-conversion, filtering, modulation/demodulation, coding/decoding, etc.). The goal is to dynamically assign the available signal processing resources to the input bands to meet the communication needs. This requires dynamic RF distribution and routing. The top half of FIG. 11 shows the receive side, where direct digitization of RF signals by analog-to-digital converters (ADCs) is followed by programmable digital routing to a bank of digital processing units. The bottom half shows the reciprocal transmit side. Here, multiple transmit signals are digitally synthesized and connected to digital-to-analog converters (DACs), coupled to digital-RF predistorters that linearize the RF transfer function of high power amplifiers (HPAs) directly.
A similar switch matrix may also be incorporated within a digital-RF transmitter system. In one embodiment of the invention, as indicated in the Multicasting Switch Matrix in FIG. 7, it may be desirable to direct more than one digital-RF input to be combined in the same output. Functionally, the multiple inputs may be added in a Combiner circuit, where this must be carried out in the Digital Domain in the digital-RF signals. Let us assume here that the clock frequencies of these signals to be added are the same. That is generally the case for signals designed for transmission using the same output antenna, which are within the same output band. Then the Combiner circuit can be implemented as simply a fast clocked Binary Adder, as has already been demonstrated in RSFQ technology. The outputs of the Binary Adder circuit may be passed along to a multi-bit Digital-to-Analog converter (DAC), the output of which can then be passed to an RF Power Amplifier, and then to a Transmission Antenna.
While various embodiments of the present invention have been illustrated herein in detail, it should be apparent that modifications and adaptations to those embodiments may occur to those skilled in the art without departing from the scope of the present invention as set forth in the following claims.

## Claims

1. A switching element comprising a data signal input and a clock signal input for selectively directing data from said data signal input to a data signal output depending upon the state of a reset/set flip flop and for directing a clock signal from said clock signal input to a clock signal output, in which the data signal moves in parallel with, and is resynchronized by, the clock signal.

2. A switching matrix comprising switching elements of claim 1, comprising:
a. a first set of input signals, comprising a data signal and a clock signal, connected to respective inputs of both a first switching element and to a second switching element;
b. a second set of input signals, comprising a data signal and a clock signal, connected to respective inputs of both a third switching element and to a fourth switching element; wherein
c. outputs from the first and third switching elements are connected to a first set of outputs, comprising a data line and a clock line; and
d. outputs from the second and fourth switching elements are connected to a second set of ouputs, comprising a data line and a clock line;
e. wherein input signals from said first set of input signals and said second set of input signals can be selectively routed to either said first set of outputs or said second set of outputs depending on the state of a control signal.

3. A non-blocking switch comprising at least one switching matrix of claim 2, in which at least four switching matricies are interconnected to selectively interconnect four sets of input signals to four sets of output signals in a non-blocking manner.

4. The switching element of claim 1 in which a data input comes from a low pass ADC modulator.

5. The switching element of claim 4 in which the low pass ADC modulator is a Delta modulator.

6. The switching element of claim 4 in which the low pass ADC modulator is a Delta-Sigma modulator.

7. The switching element of claim 1 in which a data input comes from a band pass ADC modulator.

8. The switching element of claim 7 in which the band pass ADC modulator is a Delta-Sigma modulator.

9. The switching matrix of claim 2, in which a data output goes to a digital signal processor.

10. The switching matrix of claim 9 in which the digital signal processor is configured as a cross correlator.

11. The switching matrix of claim 10 in which the cross correlator is a digital multiplier.

12. The switching matrix of claim 10 in which the cross correlator utilizes a digital waveform generator.

13. The switching matrix of claim 10 in which the cross correlator comprises a digital decimation filter.

14. The switching matrix of claim 9 in which the digital signal processor is configured as an autocorrelator.

15. The switching matrix of claim 2, in which a data input to the switching matrix comes from a digital transmit waveform generator.

16. The switching matrix of claim 2 in which the transmit waveform generator comprises a digital interpolation filter.

17. The switching matrix of claim 2 in which the transmit waveform generator comprises a digital multiplier.

18. The switching matrix of claim 2 configured to connect at least one output to a combiner.

19. The switching matrix of claim 18 in which the combiner is connected to a Direct Digital Synthesizer or a Digital to Analog Converter.

20. The switching matrix of claim 18 in which the Direct Digital Synthesizer or a Digital to Analog Converter is connected to an amplifier.

21. The switching matrix of claim 18 in which the combiner is connected to a Direct Digital Synthesizer or a Digital to Analog Converter through a pre-distortion circuit.

22. The switching matrix of claim 21 in which the predistortion circuit compensates for non-linearities in at least one of a power amplifier and a transmission medium.

23. The switching matrix of claim 21 in which output of the Direct Digital Synthesizer or a Digital to Analog Converter is applied to an amplifier.

24. A superconducting switching element comprising a data signal input and a clock signal input for selectively directing data from said data signal input to a data signal output depending upon the state of an RSFQ reset/set flip flop and for directing a clock signal from said clock signal input to a clock signal output, in which the data signal moves in parallel with, and is resynchronized by, the clock signal.

25. A switching matrix comprising switching elements of claim 24, comprising:
a. a first set of input signals, comprising a data signal and a clock signal, connected to respective inputs of both a first switching element and to a second switching element;
b. a second set of input signals, comprising a data signal and a clock signal, connected to respective inputs of both a third switching element and to a fourth switching element; wherein c. outputs from the first and third switching elements are connected to a first set of outputs, comprising a data line and a clock line; and
d. outputs from the second and fourth switching elements are connected to a second set of ouputs, comprising a data line and a clock line;
e. wherein input signals from said first set of input signals and said second set of input signals can be selectively routed to either said first set of outputs or said second set of outputs depending on the state of a control signal.

26. A non-blocking switch comprising at least one switching matrix of claim 25, in which at least four switching matricies are interconnected to selectively interconnect four sets of input signals to four sets of output signals in a non-blocking manner.

27. The switching element of claim 24 in which a data input comes from a low pass ADC modulator.

28. The switching element of claim 27 in which the low pass ADC modulator is a Delta modulator.

29. The switching element of claim 27 in which the low pass ADC modulator is a Delta-Sigma modulator.

30. The switching element of claim 24 in which a data input comes from a band pass ADC modulator.

31. The switching element of claim 30 in which the band pass ADC modulator is a Delta-Sigma modulator.

32. The switching matrix of claim 25, in which a data output goes to a digital signal processor.

33. The switching matrix of claim 32 in which the digital signal processor is configured as a cross correlator.

34. The switching matrix of claim 33 in which the cross correlator is a digital multiplier.

35. The switching matrix of claim 33 in which the cross correlator utilizes a digital waveform generator.

36. The switching matrix of claim 33 in which the cross correlator comprises a digital decimation filter.

37. The switching matrix of claim 32 in which the digital signal processor is configured as an autocorrelator.

38. The switching matrix of claim 25, in which a data input to the switching matrix comes from a digital transmit waveform generator.

39. The switching matrix of claim 25 in which the transmit waveform generator comprises a digital interpolation filter.

40. The switching matrix of claim 25 in which the transmit waveform generator comprises a digital multiplier.

41. The switching matrix of claim 25 configured to connect at least one output to a combiner.

42. The switching matrix of claim 41 in which the combiner is connected to a Direct Digital Synthesizer or a Digital to Analog Converter.

43. The switching matrix of claim 41 in which the Direct Digital Synthesizer or a Digital to Analog Converter is connected to an amplifier.

44. The switching matrix of claim 41 in which the combiner is connected to a Direct Digital Synthesizer or a Digital to Analog Converter through a pre-distortion circuit.

45. The switching matrix of claim 44 in which the predistortion circuit compensates for non-linearities in at least one of a power amplifier and a transmission medium.

46. The switching matrix of claim 44 in which output of the Direct Digital Synthesizer or a Digital to Analog Converter is applied to an amplifier.

47. The switching element of claim 1 in which the data signal input and the data signal output each comprise a respective plurality of binary data lines, with signals from each input binary data line being selectively directed to a corresponding output binary data line depending upon the state of said set/reset flip flop.

48. The switching element of claim 24 in which the data signal input and the data signal output each comprise a respective plurality of binary data lines, with signals from each input binary data line being selectively directed to a corresponding output binary data line depending upon the state of said set/reset flip flop.