Simultaneous transmission of multiple signals through a common shared aperture

Abstract

In phased array antennas, multiple signals that perform different functions, such as radar, electronic warfare (EW) and telecommunications, can each be simultaneously transmitted only through a different sub-aperture of the array. For maximum power and efficiency, the power amplifiers operate on one signal at a time. This patent provides the technique of forming a common waveform from multiple signals for transmission through a common aperture. To practically implement this technique in wideband operations, processes performing the required waveform-shaping and amplitude-to-phase-modulation are devised that can transmit the high-power diverse waveforms without serious intermodulations and spectral distortion through every element of the array.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

In phased array antennas, multiple signals (such as radar, electronic warfare (EW), and communications waveforms) can each be simultaneously transmitted only through a different sub-aperture of the array. For maximum power and efficiency, the power amplifiers operate on one signal at a time. This invention is to form multiple signals as a common waveform and transmit them through a common shared array aperture. The user signals are thus transmitted simultaneously and independently with full antenna gain in any direction.

2. Description of the Related Prior Art

The number of electronic equipments and associated antennas carried on military platforms continues to grow rapidly. In many instances, the platforms can no longer properly carry, nor operate, all the desired electronic equipment. To alleviate this problem, a significant initiative called the Advanced Multi-Function Radio-Frequency Aperture Concept (AMRFC)¹ was undertaken by the Office of Naval Research (ONR) and Naval Research Laboratory (NRL). The AMRFC objective is to provide many military electronic services for communications, radar, and electronic warfare (EW) by means of shared electronic equipment and through the use of a common antenna. ¹ P. K. Hughes and J. Y. Choe, “Overview of Advanced Multifunction RF System (AMRFS),” Proceedings of the 2000 IEEE International Conference on Phased Array Systems and Technology, May 2000.

Currently, if multiple signals, such as radar and EW, are to be simultaneously transmitted in time, each individual signal is transmitted through a different sub-aperture of the antenna such that any power amplifier in the array only operates on one signal at a time. The sub-aperture that transmits each signal is dynamically allocated. It is necessary to transmit one signal at a time through a power amplifier since the power amplifiers operate at saturation for maximum power and efficiency. If two signals are simultaneously present at the power amplifier input, the resulting output signal will generally be highly distorted and contain extremely high intermodulations, thus causing serious problems for the nearby receive antenna. Alternately, if the amplifier is operated in a linear mode, significant power and efficiency are lost. Consequently, at the present time only one type of signal at a time is distributed to each sub-aperture. As a result, the full gain of the antenna cannot be realized on any of the user signals that are required for simultaneous transmission.

Clearly, to form and transmit a noise-like combined waveform without mutual interaction, the signals' spectra must be well confined and located anywhere within the available frequency band, as long as they do not overlap. However, sharply confining the spectra requires signal shaping² in the time domain that causes amplitude modulation of the waveforms. It is desirable to pass these combined spectrally clean, amplitude-varying waveforms through conventional power amplifiers that are operated near their full-rated RF power levels for greater efficiency. Since the power amplifiers exhibit nonlinear operation, the combined amplitude-varying input signals need to be converted into signals of constant amplitude before being transformed back to a diverse high power signal. ² W. M. Waters and B. R. Jarrett, “Bandpass Signal Sampling and Coherent Detection,” NRL Report 8520, December 1981.

Here a new technique along with its practical implementation is invented, which provides a transformation on the sum of multiple signals through the Chirex outphasing³ prior to power amplification, and an inverse transformation of the combined signal through power amplification. This technique allows simultaneous transmission of multiple signals through every saturated power amplifier in the array antenna without serious intermodulations and spectral distortion. In the case that these multiple signals are radar and EW waveforms, the radar waveform cannot degrade the effectiveness of the Electronic Attack (EA) technique, and similarly the EA technique cannot interfere with the radar's target-detection function. Since the individual signals of this combined waveform are spectrally confined and can be hopped about the common portion of the available frequency bands, all Navy platforms such as ships and decoys could use this waveform to prevent Electronics Surveillance (ES) systems from distinguishing between them. Transmission of this diverse waveform can deny antiradiation missiles (ARMs) from acquiring and targeting Navy platforms. On the other hand, Navy ships can target hostile platforms without being targeted themselves. ³ Chirex, H., “High Power Outphasing Modulation,” Proc. IRE, Vol. 23, No. 11, November 1935.

SUMMARY OF THE INVENTION

In this invention, a diverse waveform is formed from multiple signals and transmitted through a common array aperture. The invention provides a new technique that provides a transformation that shapes and combines multiple signals prior to power amplification and an inverse transformation that converts the transformed phase-modulated (PM) signals back into a high-power, low-distortion, amplitude-varying waveform. This technique allows simultaneous transmission of multiple signals through every saturated power amplifier in the array antenna without serious intermodulations and spectral distortion. To implement this technique in wideband operations, innovative amplitude-to-phase-modulation (AM-PM) processes in associated with the inverse transformations capable of transmitting combined waveforms are devised.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Transmission of multiple signals with the signals combining before power amplifiers

FIG. 2 Generation of clean-spectrum signals

FIG. 3 Implementation for simultaneous transmission of multiple signals (Double sideband circuitry)

FIG. 4 Formation of a single sideband diverse waveform

FIG. 5 Shaped and spectrally confined Signal No. 1, r₁(t)

FIG. 6 Shaped and spectrally confined Signal No. 2, r₂(t)

FIG. 7 A spectrally diverse waveform r(t) by combining r₁(t) and r₂(t)

FIG. 8 Up-converted signal after AM-PM conversion for the bottom channel at Blk 335

FIG. 9 Output signal in time after Chireix outphasing for the double sideband signal at Blk 338

FIG. 10 Output signal in frequency after Chriex outphasing for the double sideband signal at Blk 338

FIG. 11 The upper sideband high-power low-distortion combined signal out of the difference channel of the combiner Blk 401

FIG. 12 The lower sideband high-power low-distortion combined signal out of the sum channel of the combiner Blk 401

DETAILED DESCRIPTION OF THE INVENTION

The detailed description of the invention closely follows the block diagrams and figures shown in FIGS. 1 through 12. FIG. 1 is the overall block diagram describing the new technique in transmitting a high power diverse waveform without serious intermodulations and spectral distortion. FIGS. 2 through 4, which are sub-blocks of FIG. 1, detail the practical implementation. FIGS. 5 through 11 are the signal outputs, appearing in time or frequency domains, for some critical blocks shown in FIG. 3

Transmission of Multiple Signals With the Signals Combining Before Power Amplifiers—FIG. 1

FIG. 1 is the overall block diagram describing the new technique in transmitting a high power diverse waveform without serious intermodulations and spectral distortion.

In Blks 101 and 102, clean-spectrum signals r₁(t) and r₂(t) are generated through the waveform generators in that desired waveforms are generated and passed through the correlators and Digital to Analog converters (DAC). These signals r₁(t) and r₂(t), which are time-continuous and band-limited, can be summed to form a spectrally confined waveform r(t), i.e., r(t)=r₁(t)+r₂(t). It has to be pointed out that the above two signals r₁(t) and r₂(t) can be transmitted coincidently and independently. The signals need not be aligned with each other for combining and transmission. We next up-convert this diverse signal r(t) through power amplifiers that would operate in saturation at their full-rated RF power. Clearly r(t) varies widely in magnitude and cannot be transmitted through power amplifiers without distortion since the power amplifiers operate in saturation at their full-rated RF power. We thus use the Chireix outphasing principle to transform r(t) into PM signals of opposite sense in two separate channels as processed in Blks 110 through 111. Here the signals out of Blks 110 and 111 are: s₁(t)=cos (ω₀t+cos⁻¹ r(t)) and s₂(t)=cos (ω₀t−cos⁻¹ r(t)), respectively, where ω₀ is the carrier frequency. It can be seen that constant amplitudes are maintained in the above modulation processes.

In Blk 114, we sum the encoded signals s₁(t) and s₂(t) through power amplifiers to form a high-power, low-distortion, amplitude-varying waveform, u(t)≡G·(s₁(t)+s₂(t))=2·G·r(t)·cos ω₀t, where G is the power amplifier gain, r(t) is the modulating signal, and ω₀ is the up-converted frequency. Here, the up-converted high-power signal u(t) is double sidebanded. Since both sidebands contain identical information, it is sufficed to transmit either sideband of the combined signal. To obtain either the upper or the lower sideband of this signal, we have to combine u(t) with an analogous signal v(t) through a combiner Blk 120. Here Blk 104 and Blks 115 through 119 are processed to generate v(t) in that r(t) is 90° phase-shifted and transformed through a modified Chriex outphasing process. Blks 115 and 116 actually perform the following functions: ŝ₁(t)=sin (ω₀t+cos⁻¹ r_h(t)) and ŝ₂(t)=sin (ω₀t=cos⁻¹ r_h(t)). In the above, r_h(t) is the quadrature function of r(t). Clearly r_h(t) is the Hilbert transform of r(t) in that the phase of r(t) is shifted by −π/2 for positive frequencies and π/2 for negative frequencies. The signal v(t), corresponding to u(t), is obtained by summing ŝ₁(t) and ŝ₂(t) after passing through the power amplifiers Blks 117 and 118, v(t)═G·(ŝ₁(t)+ŝ₂(t))=2·G·r_h(t)·sin (ω₀t). Since the upper sideband high-power signal is defined and given by y_U═Re{r(t)+jr_h(t)}e^jω0t, We obtain y_U=r(t) cos ω₀t−r_h(t) sin ω₀t, or, y_U=K·(u(t)−v(t)) (where K is a constant) since u(t)=2·G·r(t) cos ω₀t and v(t)=2·G·r_h(t) sin ω₀t as obtained in Blks 114 and 119. Similarly we can show that the lower sideband signal y_L is the sum of u(t) and v(t) and y_L=K·(u(t)+v(t)).

The configuration shown in FIG. 1 provides a transformation that shapes and combines multiple signals prior to power amplification and an inverse transformation that converts the transformed PM signals back into a high-power, low-distortion, amplitude-varying waveform. Either the double sideband signal u(t) or the single sideband signal y_U or y_L can be transmitted out of every element of a common shared aperture. This technique allows simultaneous transmission of multiple signals through every saturated power amplifier in the array antenna without serious intermodulations and spectral distortion.

Generation of Clean-spectrum—FIG. 2

It is our objective to transmit a noise-like, combined waveform through a common shared aperture with little distortion. Therefore, the individual signals to be combined and transmitted must be shaped such that their spectra are bandpass limited. These so-called clean-spectrum signals can be obtained through interpolation (the correlation process performed by Blks 203 and 204) and conversion of the sampled signals to analog by means of DACs.

From the basic theory involving sampling of a band-limited signal, the following interpolation function can be derived if the signal is to be limited to one-side bandwidth W: h(t)=[ sin (2πmWt)−sin (2π(m−1)Wt)]/2πWt, where m is an integer. Clearly, at baseband m=1, h(t)=sin (2πWt)/(2πWt). Let r_1,2(t_n) be the time series samples of the user waveforms. Then the shaped signals become $r_{1,2}\left(t\right)=\sum _{n=-\infty }^{\infty }{r}_{1,2}\left(t_n\right)·h\left(t-t_n\right)·w\left(t-t_n\right),$ where w(t) is a weight function. Indeed the signals r_1,2(t_n) are spectrally confined from −mW to −(m−1)W and from (m−1)W to mW. The interpolation or sampling rate is 1/(2W). Generally, the signal bandwidth B is less than or equal to W. For summation in finite length, the clean spectrum or smoothed signal may still be properly band-limited if a weighting factor is included in the interpolation function h(t) described above. Here h(t) is simply weighted with a Gaussian function, $w\left(t\right)=\left(\sin \mathrm{kt}\text{/}\mathrm{kt}\right)·e^{-\frac{t^2}{2{\sigma }^2}}.$

In Blks 201 and 202, desired waveforms r₁(t_n) and r₂(t_n) are generated through the pattern generators. These waveforms are then passed through the correlators (Blks 203 and 204) to generate digital clean-spectrum data and store in the computer memory.

Implementation for Simultaneous Transmission of Multiple Signals (Double Sideband Circuitry)—FIG. 3

In FIG. 1, we configure the transformation that encodes the combine signals r(t) into signals of constant amplitudes prior to power amplification and an inverse transformation that converts the transformed PM signals back into a high-power amplitude-varying waveform u(t) or v(t). All these function blocks are realized in FIG. 3 with the circuitry implemented for wideband array operation.

It may not be practical to form the analog PM signals from a carrier modulated by ±cos⁻¹ r(t), as configured in Blks 110, 111, 115 and 116 in FIG. 1, when wide bandwidth signals are required. For practical implementation, we consider the PM signal output from Blk 110s₁(t)=cos (ω₀t+cos⁻¹ r(t)) in a different form. Since cos⁻¹ r(t)≡ tan⁻¹ ({square root}{square root over (1−r(t)²)}/r(t)), this PM signal becomes s₁(t)=r(t)·cos ω₀t−{square root}{square root over (1−r(t)²)}·sin ω₀t. By a simple series expansion or a polynomial fit to the above square-root term, the signal can be practically approximated as s₁(t)≅r(t)·cos ω₀t−(a+b·r(t)²)·sin ω₀t, where a and b are constants. The circuitry composed of Blks 331, 332, 333 and 334 accomplishes the above function. Similarly, the PM signal for the process of cos(w₀t−cos⁻¹ r(t)) can be obtained through the Blks 331, 332, 333 and 335.

In FIG. 3, practical 14-bit DACs are used to convert the output data from FIG. 2 into analog (Blks 301 and 302). These signals are up-converted to IF (Blks 303 through 308) and then combined as r(t) in Blk 310. Next, the combined signal r(t) is encoded into four signals s₁(t), s₂(t), ŝ₁(t) and ŝ₂(t) through Blks 334, 335, 324 and 325, respectively. These encoded signals are all nearly constant amplitudes. The Chireix outphasing process for the upper two channels (Blks 321 through 325) is modified such that the corresponding functions sin (ω₀t±cos⁻¹ r_h(t)), instead of cos (ω₀t±cos⁻¹ r(t)), are performed. Here r_h(t) is the input with r(t) being 90° phase shifted (Blk 311). The above four signals are next passed through four saturated power amplifiers. The sum of these signal outputs from the lower two channels and from the upper two channels are then formed, indicated here as u(t) and v(t), respectively. It is pointed out that both u(t) and v(t) generated in FIG. 3 are high-power, low-distortion, amplitude-varying waveforms. Both signals are double sidebanded.

Formation of a Single-sideband Diverse Waveform—FIG. 4

The double-sidebanded signals u(t) and v(t), obtained by combining the lower two channels and upper two channels in FIG. 3, are then summed or subtracted through a combiner (Blk 401) to obtain either the lower sideband y_L(t) or the upper sideband y_U(t) of the up-converted high-power signal.

This implementation is based on the algorithms that the upper sideband of the combined signal r(t) is given by y_U≡Re({r(t)+jr_h(t)}e^jω01), or equivalently y_U=r(t) cos ω₀t−r_h(t) sin ω₀t, where r(t) and r_h(t) are defined in FIG. 3. Consequently y_U=K·(u(t)−v(t)), since u(t)=2·G·r(t)·cos ω₀t and v(t)=2·G·r_h(t)·sin ω₀t. Similarly, y_L=r(t) cos ω₀t+r_h(t) sin ω₀t=K·(u+v). Here we consider ω₁=25 MHz, ω₂=60 Mhz and ω₀=730 MHz in the frequency up-conversion process.

Shaped and Spectrally Confined Signal No. 1, r₁(t)—FIG. 5

This is the spectrally confined signal r₁(t) output from Blk 101 in FIG. 1 or from Blk 307 in FIG. 3. The pattern generator Blk 201 generates the 63-element maximum-length shift-register pseudorandom code r₁(t_n.), which is smoothed up through interpolation and converted to analog. The signal is centered at 25 MHz and spectrally confined from 15 to 35 MHz.

Shaped and Spectrally Confined Signal No. 2, r₂(t)—FIG. 6

This is the spectrally confined signal r₂(t) out of Blk 102 in FIG. 1 or out of Blk 308 in FIG. 3. The waveform generator Blk 202 generates the 13-element Barker code r₂(t_n), which is then smoothed up through interpolation and converted to analog. The signal is centered at 60 MHz and spectrally confined from 40 to 80 MHz.

A Diverse Combined Signal r(t) by combining r₁(t) and r₂(t)—FIG. 7

The spectrally confined waveforms r₁(t) and r₂(t), which are time-continuous and band-limited, are summed in Blk 310 to form a diverse waveform r(t). It is pointed out that the above two signals can be transmitted coincidently and independently. The signals need not be aligned with each other for combining and transmission.

Up-Converted Signal After AM-PM Conversion for the Bottom Channel at Blk 335—FIG. 8

This is one of the decoded signals transformed from the amplitude-varying combined signal r(t) through the processors Blks 331, 332, 333 and 335. This signal s₂(t) output from Blk 335 is up-converted to 730 MHz and maintains nearly constant amplitude.

Output Signal In Time After Chireix Outphasing for the Double Sideband Signal at Blk 338—FIG. 9

This is the signal out of the reconfigured Chriex outhpasing by summing two encoded signals input to Blk 338. This signal showing in time domain is double sidebanded with the carrier ω₀ set at 730 MHz. The summation process converts the transformed PM signals back into a high-power, low-distortion, amplitude-varying waveform.

Output Signal In Frequency After Chriex Outphasing for the Double Sideband Signal at Blk 338—FIG. 10

The same signal in FIG. 9 is shown here in frequency domain. This is the signal out of the reconfigured Chriex outhpasing by summing two encoded signals input to Blk 338. This signal showing in frequency domain is double sidebanded with the carrier ω₀ set at 730 MHz.

The Upper Sideband High-Power Low-Distortion Combined Signal Out of the Difference Channel of the Combiner Blk 401—FIG. 11

Since both sidebands of the transformed back high power signals, u(t) and v(t), contain identical informatin, it is sfficed to transmit either sideband of the combined signal. The upper sideband of this high-power, low-distortion, amplitude-varying signal is attained through the difference channel of the combiner Blk 401.

The Lower Sideband High-Power Low-Distortion Combined Signal Out of the Sum Channel of the Combiner Blk 401—FIG. 12

The lower sideband of the high-power, low-distortion, amplitude-varying waveform is attained through the sum channel of the combiner Blk 401. This converted common signal combined from multiple signals can thus transmitted out of every element of the array.

Summary

In this patent, multiple signals are combined to form a common waveform and are then transmitted through a common aperture composed of array modules. Specifically, a combined EW/radar waveform that applies an electronic attack (EA) technique and performs a radar function can be applied here. As a result, the full aperture gain can be realized on both EW and radar signals when they are simultaneously transmitted. If the beam-steering time delay is applied to each signal before combining, the signal can be coincidently and independently transmitted at different angles. Since the spectrally interlaced combined signal can be potentially noise-like, Navy platforms that use these common waveforms would be difficult to discriminate. Both EA and radar functions can be performed simultaneously without degradation and interference.

A technique that provides a transformation that shapes and combines signals prior to power amplification and an inverse transformation that converts the phased-modulated signals back into a high-power, low-distortion, amplitude-varying waveform is described. This technique allows simultaneous transmission of multiple signals through every saturated power amplifier in the array antenna without serious intermodulations and spectral distortion. To implement this technique in wideband operations, an innovative amplitude-to-phase-modulation process in associated with the inverse transformation capable of generating and transmitting either the double-sidebanded or single-sidebanded combined signal is devised in this patent.

Claims

1. An integrated radar antenna system comprising: a radar comprising a plurality of planar arrays of radiating elements; a structure for receiving signal information associated with said radar; a structure for transmitting signal information associated with said radar; a signal generator for transmitting multiple signals through a common shared array aperture.

2. The system as in claim 1, further including transmission of multiple signals through every saturated power amplifier in said arrays without distortions.

3. The system as in claim 1, wherein the transmission of diverse waveform can deny antiradiation missles from targeting the structures for transmitting signal information.

4. The system of claim 1, wherein the structure for transmitting signal information combines multiple signals for transmission through a common shared aperture.

5. The system of claim 1, wherein the system transforms combined radar and EA signals into phase modulated signals, passes said phase modulated signals through high power amplifiers with low distortion and transforms said phase modulated signals into their original form for transmission.

6. The system of claim 1, where in the structure for transmitting signal information includes waveform generators that transmit signals coincidently and independently.

7. The system of claim 6, wherein the signals from the waveform generators are combined into a single signal.

8. The system of claim 7, wherein said single signal is approximated by a series expansion of a square root function.

9. The system of claim 7, wherein the signals from the waveform generators are combined to form a single power, low-distortion, and amplitude varying signal.

10. The system of claim 1, wherein either a double sideband signal or a single sideband signal can be transmitted out of every element of a common shared aperture.

11. The system of claim 1, wherein a double sideband signal is generated from four separate signals which are combined into an upper channel and lower channel to form two signals for transmission through a single aperture.