MIMO passive channel emulator

Embodiments of methods and means for passively emulating channels in a multiple-input multiple output (MIMO) system are provided. Such embodiments include passively splitting a plurality of radio frequency signals into a greater plurality of such signals. Each of the greater plurality of radio frequency signals can then be selectively and passively attenuated, delayed and/or phase shifted. The resulting modified radio frequency signals are then recombined crossed over channels and coupled to a plurality of output nodes. Economical and versatile device and system testing is thus facilitated in a low-noise radio frequency environment without the need for complex up/down frequency or analog/digital conversions.

BACKGROUND

Multiple-input multiple-output (MIMO) communication techniques exploit performance gains achieved by using multiple transmit and receive antennas within a system to provide un-correlated propagation channels between a transmitter and receiver. Typically, it is the correlation between different antenna elements (e.g., propagation paths) that enables multiple-input multiple-output techniques to realize advantageous performance in a realistic usage environment. Such performance advantages include increased throughput and operating range at the same bandwidth and same overall transmit power as other prior communications techniques.

During testing and development of multiple-input multiple-output communications equipment, channel emulators are sometimes employed to simulate usage conditions. It is desirable that a channel emulator be able to simulate realistic multiple-input multiple-output scenarios with accuracy, repeatability and performance that does not limit the performance (or apparent performance) of a device under test (DUT). At least one known multiple-input multiple-output channel emulator is based on: down-conversion of a signal from radio frequency to baseband; conversion of baseband signal from analog to digital; application of a predetermined baseband channel model (i.e., simulation scenario); conversion of the model signal from digital back to analog; and up-conversion of the analog signal from baseband back to radio frequency.

Under the known operational sequence outlined above, the channel model is applied digitally at a baseband sample rate, thus permitting such a channel emulator to apply virtually any sophisticated, dynamically varying channel model. However, this known approach also introduces noise and distortion at each step in the sequence described above, resulting in a noise floor on the signal that may limit the ultimate performance of the device under test and/or provide misleading indications as to one or more aspects the devices overall behavior. In some situations—including almost all high data rate scenarios—this noise floor problem is such that a device or system under scrutiny cannot be fully validated. These known channel emulators also tend to be relatively expensive, with some units exceeding $500,000 in cost, while being limited to a four-transmit/four-receive channel operation.

DETAILED DESCRIPTION

Introduction

Embodiments contemplated herein provide multiple-input multiple-output passive channel emulators that can repeatably emulate user-defined or measured static channel models. Such passive channel emulators are test tools that can be utilized to evaluate the effects of channel attenuation, frequency selectivity, and channel correlation. These embodiments utilize integrated design aspects including passive signal splitting and re-combining, and provide a stable test bench for the emulation of multi-channel environments. Embodiments of apparatus and techniques herein can be used for the development of multiple-input multiple-output algorithms, as well as for the integration and verification or comparison of multiple-input multiple-output-capable wireless products and systems. In accordance with these embodiments, dynamic path delay models are constant, and losses (i.e., signal attenuations) are selectively variable. The corresponding attenuation, delay and/or phase shifting parameters can be respectively set manually (requiring no software interface), or with a simple graphical user interface, depending on the particular embodiment under consideration.

Also, the embodiments presented herein are relatively economical to provide and use, and utilize an overall architecture (topology) that achieves increased noise floor performance and does not limit the performance of the device under test. Unlike known emulators, the passive channel emulator embodiments herein emulate a radio frequency channel, input to output, exclusively at radio frequency and without the need for analog-to-digital or digital-to-analog conversion, or down-conversion or up-conversion to/from baseband. Because the foregoing complexities are eliminated, the embodiments herein do not require relatively expensive components such as, for example, radio frequency and/or digital field programmable gate arrays.

Embodiments of passive channel emulators provide static channel emulation that is sufficient for most development and testing. Also, these embodiments allow efficient testing, making emulation possible for a high signal-to-noise ratio wireless fading channel, while further permitting more effective scheduling of more costly channel emulator resources.

As used herein, the terms “802.11”, “802.16”, “WiMAX” and “WiFi” refer to respective signaling standards defined by the Institute of Electrical and Electronics Engineers, Inc. (IEEE), Piscataway, N.J., USA. In particular, “WiMAX” refers to standards 802.16e-2005, 802.16e-2004/Cor 1-2005, and 802.16-2004, respectively, as defined by the IEEE. In regard to “WiFi”, that term refers to standards 802.11g-2003, 802.11a-1999, 802.11b-1999, and 802.11-1997, respectively, as defined by the IEEE. As also used herein, “3GPP” refers to standards defined by the 3rdGeneration Partnership Project, a collaborative agreement established December, 1998 via international cooperation between ETSI (Europe), ARIB/TTC (Japan), CCSA (China), ATIS (North America) and TTA (South Korea).

Exemplary Embodiments

Attention is now turned toFIG. 1, which depicts a device100topology in accordance with one embodiment. The device100illustrates a four input/four output (i.e., 4×4) passive channel emulator. It is to be understood that the device100is exemplary of general topological aspects that can be readily employed in other N×M (e.g., 4×3, 2×2, etc.) passive channel emulator embodiments consistent with the subject matter herein. Therefore, the device100ofFIG. 1is exemplary and non-limiting in its overall teachings.

The device100includes a circuit102. The circuit102is also designated as “CIRCUIT A” inFIG. 1. Circuit102includes four inputs104respectively configured to receive a corresponding radio frequency signal. The four inputs104are electrically coupled to four respective circuits (blocks, or sub-circuits)106. Each circuit106is also designated as “B” inFIG. 1. In turn, each circuit106includes four circuits (blocks, or sub-circuits)108. Each circuit108is also designated as “C” as illustrated byFIG. 1. Thus, circuit102reflects a hierarchical, building-block type structure. In any case, it is to be understood that the circuit102ofFIG. 1is inclusive of four circuits106and sixteen circuits108.

The circuit102(i.e., CIRCUIT A) also includes four outputs110. Each output110is configured to provide, or facilitate electrical coupling to, a radio frequency signal that has been derived (processed) by the overall constituency of circuit102. Each of the inputs104and outputs110is also referred to as a node for purposes herein. The circuit102also include a plurality of signal splitters (hereinafter, splitters)112. Radio frequency signals are coupled from each of the circuits108, via the corresponding circuits106, to one or more of the outputs110by way of the splitters112. As depicted inFIG. 1, each splitter112is configured to passively re-combine a pair of radio frequency signals. Thus, the splitters112depicted inFIG. 1function essentially as signal “re-combiners”.

The circuit102ofFIG. 1also includes a plurality of connectors114. Each connector114can be defined by any suitable known means for coupling wiring or cabling to the outputs (i.e., nodes)110. In one or more embodiments, the connectors114are respectively defined by circuit board-mounted coaxial cable connectors. Other suitable connectors114can also be used. In this way, the outputs110of circuit102can be conveniently coupled to other electronic entities generally external to the circuit102. Non-limiting examples of such generally external entities include radio frequency transceivers, oscilloscopes, frequency counters, signal analyzers, signal acquisition and detection devices, etc.

The circuit102ofFIG. 1is inclusive of overall circuitry such that four discrete radio frequency signals can be received, and then the four signals passively split into sixteen distinct radio frequency signals (e.g., by way of circuits106), and then these sixteen radio frequency signals passively split into a total of sixty-four distinct radio frequency signals (e.g., by way of circuits108). Each of the sixty-four, passively derived radio frequency signals can also be considered a portion of one of the original four received radio frequency signals. The particular means for this passive splitting operation shall be discussed in greater detail below. Greater detail of each of circuits106and108(B and C, respectively) of the overall circuit102is provided below.

The circuit102ofFIG. 1is intended to represent an overall passive channel emulator as a unitary whole in accordance with the present subject matter. The circuit102can be used as a module in an overall testing system. In one system-level embodiment, two like circuits102are coupled to a corresponding number of transceivers, signal circulators, and/or other devices as a part of a development and testing strategy. Other usage configurations incorporating the circuit102, or other embodiments consistent with this subject matter, can also be defined and used.

FIG. 2is now considered, which illustrates a circuit (or sub-circuit)106as introduced above in greater detail. The circuit106(i.e., CIRCUIT B) includes a plurality of passive signal splitters112. Each of the splitters112ofFIG. 2is configured to passively derive a pair of radio frequency signals or signal portions. As illustrated, the splitters112ofFIG. 2are configured to derive a total of four radio frequency signals (or portions). The circuit106further includes a connector114configured to facilitate coupling the circuit106to a radio frequency input signal. The connector114ofFIG. 2corresponds to an input (node)104of circuit102ofFIG. 1. The connector114can be defined by any suitable connector as discussed above in regard toFIG. 1. The circuit106also includes four circuits108as introduced above.

In turn,FIG. 3illustrates a circuit (or sub-circuit)108in greater detail. The circuit108(i.e., CIRCUIT C) includes a plurality of passive splitters120. Each of the splitters120is configured to derive a respective pair of radio frequency signals (or portions) from an input radio frequency signal. In one embodiment, the splitters120ofFIG. 3are essentially equivalent to the splitters112ofFIG. 2.

The circuit108ofFIG. 3further includes a plurality of connectors114. Each connector114can be suitably defined by any means such as, for example, a coaxial cable connector, etc. Other connectors114can also be used. In any case, each connector114is configured to facilitate radio frequency signal inter-connections within the circuit108by way of corresponding cables122.

Each cable122is of a length corresponding to a predetermined radio frequency path delay. Thus, each cable122can be individually selected with respect to length (and/or other salient parameters) so as to establish respective signal delays within the circuit108. In one embodiment, each cable122is selected so as to establish a twenty nanosecond delay. Other delays can also be used. In this way, discrete radio frequency signals (or portions thereof) can be selectively and passively delayed so as to establish an overall static path delay model for use in device and/or system testing and validation. In another embodiment (not shown), each cable122is represented by a plurality of cables of differing respective lengths and suitable switching means are employed such that varying delay characteristics can be selected during use of the circuit108, without the need to shut down and/or manually swap out different cables122.

As depicted inFIG. 3, four discrete radio frequency signal pathways, respectively designated as130,132,134and136, are ultimately defined by way of cooperation of the signal splitters120, connectors114and/or cables122. In this way, radio frequency signal pathways132,134and136can include user-selected static delays. Each signal pathway130-136ofFIG. 3includes a fixed attenuation element138coupled to a corresponding one of the splitters120. In one embodiment, the respective attenuation elements138are defined by fixed resistors of predetermined value (i.e., Ohms). Other attenuation elements138can also be used. Each signal pathway130-136also includes a pair of interlocked switches140, a pair of predetermined resistive loads142, a variable radio frequency attenuator (also herein, variable attenuator)144, and a variable phase shifter150. In another embodiment, the variable phase shifter150is omitted from one or more of the signal pathways130-136.

A first position of each pair of the interlocked switches140electrically couples the resistive loads142(e.g., fifty Ohms each, etc.) into the corresponding pathway (130-136), while isolating the corresponding variable attenuator144and variable phase shifter150. Such a first position can be used, for example, during calibration of the circuit108, the circuit102ofFIG. 1that is host thereto, and/or some other aspect of a system-level testing arrangement. A second position of each pair of the interlock switches140electrically couples the variable attenuator144and variable phase shifter150into the corresponding pathway (130-136), while isolating the corresponding resistive loads142. Such a second position is typically used during actual radio frequency device testing at a system level.

Each variable attenuator144ofFIG. 3can be defined numerous ways as contemplated herein. In one embodiment, each variable attenuator144is defined by a dual in-line package (DIP) switch (not specifically shown) coupled to a respective plurality of fixed resistors (not specifically shown). Manual actuation of the individual switches (i.e., bits) permits discrete attenuation values to be selected, either alone or in selective combination with one another. In one such embodiment, a five-bit, dual in-line package switch and corresponding resistors are selected so as to permit attenuation values to be selected in accordance with Table 1 below:

In another embodiment, the variable attenuator144is defined by a digital attenuator with attenuation values to be selected in accordance with Table 1 above and is used in conjunction with an input/output (I/O) port expander or CPLD (not shown) so that discrete attenuation levels may be provided under remote computer control (e.g., using an SPI or I2C interface, etc.). One such digital attenuator144is defined by a model AT90-0001 Digital Attenuator available from M/A-COM, Lowell, Mass., USA.

Each variable phase shifter150ofFIG. 3can be defined by any suitable such device for use in the radio frequency range of interest. In one embodiment, each variable phase shifter150is defined by a voltage-variable phase shifter configured to operate in the range of about 3.5 GHz to about 6.0 GHz. One such phase shifter150is defined by a model MAPCGM0002 6-bit Phase Shifter available from M/A-COM, Lowell, Mass., USA. In one embodiment, such a variable phase shifter150is configured and operable via the same interface used for the digital attenuator144in accordance with Table 2 below:

In such a computer controlled embodiment, corresponding software enables various automated test sequences to be defined and used, and eliminates the tedium (and potential for manual setting error) that can occur under the manual bit-setting procedures discussed immediately above. Furthermore, such a computer controlled value-setting embodiment can facilitate automated emulation of specific IEEE I-METRA channel model propagation scenarios. Additional information regarding I-METRA testing protocols is provided in Jean Phillipe Kermoal et al,A Stochastic MIMO Radio Channel Model With Experimental Validation, IEEE Journal On Selected Areas in Communications, Vol. 20, No. 6, pp. 1211-1226, August, 2002. Other means for providing variable radio frequency signal attenuation by way of corresponding elements144can also be used.

Other suitable variable phase shifters150can also be used, such as are available from Agile Materials & Technologies, Inc., Goleta, Calif., USA. In any event, each phase shifter150is configured to permit independent, selective phase shifting of the radio frequency signal (or portion) corresponding to each signal pathway130-136.

The circuit108also includes a plurality of passive splitters146. Each splitter146is configured to re-combine a pair of radio frequency signals into a single radio frequency signal output. Thus, each of the splitters146is essentially being operated in “reverse”, so as to unite a pair of radio frequency signals (or portions) at a single node. In one embodiment, the splitters146ofFIG. 3are essentially equivalent to the splitters112ofFIG. 1. As also illustrated inFIG. 3, the splitters146combine pairs of radio frequency signals in a cascading fashion such that four radio frequency signals—as respectively attenuated, delayed and/or phase shifted (i.e., modified) within the signal pathways130-136—are recombined and provided at a single output148of the circuit108.

Exemplary Methods

FIG. 4is a flowchart200that describes a method in accordance with one embodiment. While the flowchart200describes particular methodical acts and order of execution, it is to be understood that the method of flowchart200is contemplated to be suitably varied, broadly applicable, and is not limited as specifically presented. Thus, other embodiments contemplated herein can be configured and/or performed wherein selected acts represented by the flowchart200are modified and/or omitted, and/or other acts not specifically depicted therein are executed.

At202ofFIG. 4, a plurality of radio frequency signals is received. Such reception is understood to take place using a multiple-input multiple-output passive channel emulator in accordance with the present subject matter. For purposes of example, it is assumed that four discrete radio frequency signals are received by an emulator consistent with circuit102ofFIG. 1(i.e., N=4). The received radio frequency signals can correspond, for example, to wireless local area network signals, WiMAX signals, IEEE 802.11 signals, WiFi signals, etc. Other suitable radio frequency signal formats can also be used and received.

At204ofFIG. 4, the plurality of received radio frequency signals is passively split into a greater plurality of radio frequency signals. For purposes of ongoing example, it is assumed that corresponding elements of circuits106and108of the exemplary circuit102(seeFIGS. 1-3) function to passively split the four received radio frequency signals into sixty-four discrete radio frequency signals or portions of the original radio frequency signals (i.e., K=64). It is further understood that each of these sixty-four radio frequency signals corresponds to a respective signal pathway (e.g.,130-136, etc.) of a corresponding one of the circuits (i.e., blocks, or sub-circuits)108of the overall circuit102.

At206ofFIG. 4, each of the greater plurality of radio frequency signals is selectively delayed, attenuated and/or phase shifted so as to derive a like plurality of individually modified radio frequency signals. In some embodiments, phase shifting is not performed. In the ongoing example, it is assumed that sixteen signals are attenuated (only), forty signals are attenuated and delayed and phase shifted, and the remaining eight signals are delayed and phase shifted, such that sixty-four modified radio frequency signals are derived. This is but one of numerous operational scenarios in accordance with the present subject matter. Thus, each of the sixty-four exemplary radio frequency signals is individually and selectively modified by way of corresponding elements of the circuits108of the circuit102(FIGS. 1-3).

At208ofFIG. 4, each of the greater plurality of modified radio frequency signals is coupled to at least one of a plurality of outputs. In the example, each of the sixty-four modified radio frequency signals is coupled to at least one of four outputs110(nodes) of the circuit102ofFIG. 1(i.e., M=4). This exemplary coupling or routing is also referred to as cross-over channel routing as the radio frequency signals are delivered to the output nodes110.

Exemplary System

FIG. 5depicts an exemplary system300according to another embodiment. System300is intended to exemplify but one of any number of possible systems inclusive of means and/or methods provided herein. An M×N emulator can be configured as an Q×R emulator with Q<=M and R<=N by appropriately terminating inputs/outputs—thus, a single M×N emulator can cover a wide range of multiple-input multiple-output channel topologies. Thus, the exemplary system300is understood to be illustrative and non-limiting in its overall teachings.

The system300includes a pair of circuits102as defined and described above in regard toFIGS. 1-3. Thus, each of the circuits102is further defined to be a four-input, four-output (i.e., multiple-input multiple-output) passive channel emulator in accordance with the present subject matter. As depicted inFIG. 5, each of the circuits102is configured to receive four radio frequency signals by way of corresponding inputs (i.e., nodes)104, and to provide four modified radio frequency signals by way of corresponding outputs110.

The system300also includes a total of eight radio frequency transceivers (transceivers)302(four transceivers for two multiple-input multiple-output systems). Each of the transceivers302can be respectively defined by a standard such device of known calibration and performance, or by a transceiver device under test, in any suitable combination. In any case, each transceiver is configured to transmit and receive radio frequency signals of a corresponding format (WiFi, WiMAX, etc.). As depicted inFIG. 5, one (or more) of the transceivers302may be coupled to an antenna314.

The system300also includes a total of eight signal circulators304. Each signal circulator is configured to couple one of the transceivers302in radio frequency signal communication with an input104of a particular circuit102, and with an output110of the other circuit102. As depicted inFIG. 5, for example, the transceiver designated306transmits radio frequency signals to the circuit designated160, and receives radio frequency signals from the circuit designated162, by way of action of the circulator designated308.

In one non-limiting operational example, the four transceivers302within grouping310are assumed to be standard devices of known calibration and performance criteria. These four transceivers are coupled so as to transmit radio frequency signals of predetermined characteristics to the passive channel emulator represented by circuit160.

The circuit160then passively modifies the four radio frequency signals received from the transceiver group310in accordance with desired delay, attenuation and/or phase shifting criteria. In one scenario, such criteria are defined by a selected IEEE I-METRA protocol. In any case four modified, cross-over channel radio frequency signals are provided at the outputs110of the circuit160.

The transceivers302within the grouping312are presumed to be respective transceiver devices under development or other testing. The transceivers302of group312receive the modified radio frequency signals from the circuit160in accordance with the assumed exemplary test scenario and perform respectively in accordance therewith.

Each transceiver302in the test grouping312then transmits a respective radio frequency signal that is coupled to the inputs104of passive channel emulator represented by circuit162. Therein, the received radio frequency signals are modified in accordance with a selected protocol, and coupled to the transceivers302of the standard grouping310. In this way, two similar or different testing protocols can be used simultaneously to evaluate the four transceivers302of the test group312. In accordance with the description above, this testing procedure can be manually controlled, automated, or performed under some select combination of manual and automatic means.

CONCLUSION

Embodiments and methods presented herein can provide versatile and economical multiple-input multiple-output passive channel emulators for testing and evaluating radio frequency equipment and system under WiFi, WiMAX 802.16, 802.11, 3GPP and/or other wireless protocols. These embodiments facilitate selective radio frequency signal attenuation, delaying and/or phase shifting such that numerous known as well as novel testing protocols can be performed.

Furthermore, the present subject matter performs without the need for frequency up-conversion or down-conversion, nor the need for conversion between analog and digital domains. The simplistic elegance of the present subject matter makes possible a low noise floor, exclusively radio frequency testing environment that is readily scalable to N×M (input×output) system configurations.