System and method for fast acquisition of ultra wideband signals   

The present application is a continuation of application titled “System and Method for Fast Acquisition of Ultra Wideband Signals”, Ser. No. 10/955,118 118 filed Sep. 30, 2004 by Fisher et al., which is a continuation in part of application titled: “Methods and Systems Acquiring Impulse Signals,” Ser. No. 10/713,615, filed Nov. 14, 2003 by Brethour et al., which claims the benefit of provisional application titled: “Methods and Systems for Acquiring Impulse Signals,” Ser. No. 60/426,949 filed Nov. 15, 2002 by Brethour et al., and provisional application titled “Method And Apparatus For Ultra Wideband Signaling And Modulation,” Ser. No. 60/426,857 filed Nov. 15, 2002 by Brethour et al.; the present application is also a continuation in part of application titled: “System And Method For Processing Signals In UWB Communications”, Ser. No. 10/712,269, filed on 14 Nov. 2003 by Brethour et al., which claims the benefit of provisional application titled: “Method And Apparatus For Ultra Wideband Signaling And Modulation,” Ser. No. 60/426,857 filed Nov. 15, 2002 by Brethour et al.; the present application further claims benefit of provisional application titled: “Selective Auto Gain Control with Auto Threshold,” Ser. No. 60/506,761 filed Sep. 30, 2003 by Fisher et al.; all of which are incorporated herein by reference.

The US Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract DAAB07-03-D-213 awarded by the U.S. Army CECOM Night Visions Lab, Fort Belvoir, Va.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of communications, and more particularly, the present invention relates to a mechanism for acquiring an ultra wideband signal.

2. Background Art

An impulse radio system typically includes an impulse radio transmitter for transmitting an impulse signal and an impulse radio receiver for receiving the impulse signal. An exemplary impulse signal includes a train of impulse signal frames each including one or more impulses. The transmitter can pulse position modulate the impulses within the impulse signal frames based on a modulating signal, and then transmit the impulse signal frames to the receiver.

The impulse radio receiver receives the impulse signal frames and associated received impulses transmitted by the impulse radio transmitter. In one known application, the impulse receiver coherently samples the received impulses to produce impulse samples. The receiver can use such impulse samples for subsequent signal processing relating to radar, position-locating, and communication applications, for example. However, before the impulse receiver can coherently sample the received impulses, it is necessary for the impulse receiver to determine a frame timing associated with the received impulses. That is, the impulse radio receiver must achieve frame synchronization (also referred to as proper frame alignment). For example, it is useful for the impulse receiver to determine when each received impulse signal frame begins and/or ends. Therefore, in an impulse radio capable of receiving an impulse signal including a train of impulse signal frames, there is a need to determine received impulse signal frame timing, such as a time when each impulse signal frame begins and/or ends.

Additionally, if the received impulse signal is coded (e.g., pulse position modulated based on a pseudo-random (PN) code), then the impulse radio receive must achieve code synchronization before it can coherently sample the received signal. Thus, there is also a need to provide code synchronization in an impulse radio capable of receiving an impulse signal including a train of code modulated impulse signals.

The impulse radio transmitter can transmit source information (i.e., digital data) to the impulse radio receiver. For example, the impulse transmitter uses the source information to pulse position modulate the impulses within the impulse signal frames, thereby producing information bearing symbols. The transmitter transmits the symbols (i.e., the impulse signal frames including the pulse position modulated impulses) to the impulse receiver. It is likely that the transmitter also codes the symbols as mentioned above prior to transmitting the impulse signal frames.

The impulse radio receiver receives the symbols transmitted by the impulse transmitter. Before the impulse receiver can demodulate the received symbols to recover the source information therein, the impulse receiver needs to recover a symbol timing associated with the received symbols. For example, the receiver needs to determine when each received symbol begins and/or ends. Once the receiver recovers such symbol timing, then the receiver can demodulate the received symbols to recover the data therein. Therefore, in an impulse radio capable of receiving symbols, there is a need to determine (or recover) received symbol timing, thereby enabling the impulse radio receiver to demodulate the symbols.

The impulse radio transmitter can pulse position modulate the impulses within the impulse signal frames based on different types of code sequences (codes), to produce a coded impulse signal. One type of code is a PN code used to channelize the impulse signal and/or combat relatively narrowband interference signals. These codes are relatively long (e.g., a code length of 1024, 2048 or 4096) for at least two reasons: first, so energy is spread across the frequency spectrum; and second, so a relatively large number of independent communication channels are provided.

In order to demodulate the coded impulse signal, the impulse radio receiver must be code synchronized with the impulse signal transmitter. Accordingly, there is a need to code synchronize the impulse radio receiver with the impulse radio transmitter.

It is typically beneficial to accomplish necessary requirements in fast and efficient manners that utilize reduced amounts of hardware to thereby increase throughput and/or reduce hardware costs. More specifically, it would be beneficial to satisfy each of the above discussed needs in fast and efficient manners that utilize reduced amounts of hardware. For example, it would be beneficial to achieve proper frame alignment in a fast and an efficient manner that utilizes reduced amounts of hardware. Further, it would be beneficial to recover received symbol timing in a fast and efficient manner that utilizes reduced amounts of hardware. Additionally, it would be beneficial to code synchronize an impulse radio receiver with the code of an impulse radio transmitter in a fast and efficient manner that utilizes reduced amounts of hardware. Still further, it would be beneficial if the same hardware could be used (or reused) to satisfy as many of the above needs as possible.

SUMMARY

Briefly, the present invention is an ultra wideband system employing a threshold to detect signal quality during acquisition wherein the threshold is adjusted based on signal characteristics such as packet traffic rate. In one embodiment, the threshold is adjusted by adjusting the gain of a variable gain stage ahead of the threshold. In another embodiment, gain and threshold are adjusted in a coordinated manner wherein gain is adjusted for low signal levels and threshold is adjusted for high signal levels.

In one embodiment, packet traffic rate is evaluated over an interval based on maximum packet length, number of monitor packets, and inter-packet delay.

In a further embodiment, multiple ramp builders are operated in parallel at multiple code offset values to generate signal statistics to compare with the threshold. The signal statistics may include:

Number Threshold Equation 1 N * MaxR/MA_meanV > C 2 MaxR/NextR > C 3 Logic AND or OR of equation 1 and 2 (via the Master Sequencer) 4 N * MaxR/MaxV > C 5 N * (MaxR/MA_meanV) * (MinV/MaxV) > C 6 N * (MaxR/MA_meanV) * [FORCE_ZERO(MinV − MA_MinV)/FORCE_ONE(MaxV − MA_MinV)] > C 7 N * (MaxR − NextR)/MA_meanV > C 8 N * [(MaxR − NextR)/MA_meanV] * (MinV/MaxV) > C

In a further embodiment, frame offset space is scanned to determine the maximum signal strength and thresholds are set below the maximum signal strength.

In a further embodiment, frame offset space is scanned to determine a noise floor and a maximum signal strength wherein the threshold value is established between the noise floor and the maximum signal strength.

In a further embodiment having multiple stages of acquisition, the thresholds for each stage are adaptively adjusted according to performance criteria wherein the multiple stages include at least one of the following:

Initial Detection—First detection of a possible signal.

Initial Tracking—Initial closed loop tacking of the signal

Sig-Opt—Signal Optimization, refinement of tracking loop lock point

Quick Check—Verification of the first detection.

Lock Check—verification of tracking loop lock

Long Code (Ratchet Code) Detection—Detection of long length code

Detect Delimiter—Detect end of acquisition header

In a further embodiment having multiple stages of acquisition, the thresholds for each stage are adaptively adjusted according to performance criteria, the performance criteria include at least one of the following:

false alarm rate,

packet acquire rate, or

packet loss rate

In a further embodiment, packet sent information is encoded in transmitted packets and the packet sent information is used to derive the packet loss rate for use in setting threshold levels.

In a further embodiment, wherein a system may receive a given transmitter multiple times on a known schedule, the threshold parameters associated with a given transmitter may be stored in memory to be retrieved from memory to search for the given transmitter.

In a further embodiment, thresholds may be optimized by a first transceiver and the optimization information sent to a second transceiver to aid in the reception of the first transceiver by the second transceiver.

The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.

FIG. 1A illustrates a representative Gaussian Monocycle waveform in the time domain, which is the first derivative of a Gaussian pulse;

FIG. 1B illustrates the frequency domain amplitude of the Gaussian Monocycle of FIG. 1A;

FIG. 1C represents the second derivative of a Gaussian pulse;

FIG. 1D represents the third derivative of the Gaussian pulse;

FIG. 1E represents the Correlator Output vs. the Relative Delay of a measured pulse signal;

FIG. 1F depicts the frequency domain amplitude of the Gaussian family of the Gaussian Pulse and the first, second, and third derivative;

FIG. 2A illustrates a pulse train comprising pulses as in FIG. 1A;

FIG. 2B illustrates the frequency domain amplitude of the waveform of FIG. 2A;

FIG. 2C illustrates the pulse train spectrum;

FIG. 2D is a plot of the Frequency vs. Energy;

FIG. 3 illustrates the cross-correlation of two codes graphically as Coincidences vs. Time Offset;

FIGS. 4A-4E illustrate five modulation techniques to include: Early-Late Modulation; One of Many Modulation; Flip Modulation; Quad Flip Modulation; and Vector Modulation;

FIG. 5A illustrates representative signals of an interfering signal, a coded received pulse train and a coded reference pulse train;

FIG. 5B depicts a typical geometrical configuration giving rise to multipath received signals;

FIG. 5C illustrates exemplary multipath signals in the time domain;

FIG. 5D represents a signal plot of an idealized UWB received pulse with no multipath;

FIG. 5E represents a signal plot of an idealized UWB received pulse in moderate multipath;

FIG. 5F represents a signal plot of an idealized UWB received pulse in severe multipath;

FIG. 5G illustrates the Rayleigh fading curve associated with non-impulse radio transmissions in a multipath environment;

FIG. 5H illustrates a plurality of multipaths with a plurality of reflectors from a transmitter to a receiver;

FIG. 5I graphically represents signal strength as volts vs. time in a direct path and multipath environment;

FIG. 6 is an illustration of an example general purpose architecture for an impulse radio;

FIG. 7 is a more detailed block diagram of the impulse radio of FIG. 6;

FIG. 8A is an illustration of a transmitted impulse transmitted by a remote impulse radio and received by an impulse radio antenna;

FIG. 8B is an illustration of an example impulse response of an impulse radio receiver front-end;

FIG. 9 is a block diagram of an example (IJ) correlator pair arrangement corresponding to a sampling channel in the impulse radio of FIG. 7;

FIG. 10A is an example timing waveform representing a correlator sampling control signal in the impulse radio of FIG. 7, and in the (IJ) correlator pair arrangement of FIG. 9;

FIG. 10B is an example timing waveform representing a first sampling signal derived by a sampling pulse generator of FIG. 9;

FIG. 10C is an example timing waveform representing a second sampling signal produced by a delay of FIG. 9;

FIG. 11 is a block diagram of an exemplary lock loop used for tracking a receive impulse signal;

FIGS. 12A and 12B illustrate exemplary packets 1202 transmitted by an impulse radio transmitter, according to embodiment of the present invention;

FIG. 13 illustrates an example timing relationship between a portion of a received impulse signal including received header frames and a receiver sample timeline initially established by the impulse radio to sample the received impulse signal;

FIG. 14 shows an exemplary acquisition code sequence and its corresponding delimiter;

FIG. 15 shows a high level flow diagram illustrating a method for acquiring a pulse position modulated (ppm) impulse signal, according to an embodiment of the present invention;

FIG. 16a illustrates additional details of one of the steps shown in FIG. 15, according to an embodiment of the present invention;

FIG. 16b depicts discriminants;

FIG. 16c depicts a quantity threshold comparison flowchart;

FIGS. 17 and 18 illustrate time-lines that are useful for explaining specific parallel steps of discussed in FIG. 16;

FIG. 19 shows an exemplary packet that is useful for explaining the ambiguity that results from the integration length used by an impulse radio receiver being greater than the number of ramp builders in the impulse radio receiver;

FIG. 20 illustrates an exemplary packet and relative times over which ramp values can be generated, is useful for explaining how a ratchet codes of the present invention can be used to resolve a four (4) way ambiguity that results when a receiver uses four (4) ramp builders during signal acquisition, and the integration length is sixteen (16), and is useful for explaining the steps of FIG. 21;

FIGS. 21 and 22 illustrate additional details of one of the steps of the method shown in FIG. 15, according to an embodiment of the present invention;

FIG. 23 illustrates a table that is useful for showing how ratchet codes can be generated based on a length four (4) short acquisition code, according to an embodiment of the present invention;

FIG. 24 shows an exemplary portion of an impulse radio receiver according to an embodiment of the present invention;

FIG. 25 illustrates an exemplary portion of an impulse radio receiver that is used to produce back ramp values, according to an embodiment of the present invention;

FIG. 26 illustrates an exemplary frame, which is useful for showing how multiple states can be represented according to various embodiments of the present invention;

FIG. 27 shows an exemplary portion of an impulse radio receiver according to an embodiment of the present invention;

FIG. 28 illustrates a table that is useful for showing how ratchet codes can be generated based on a length sixteen (16) short acquisition code, according to an embodiment of the present invention;

FIG. 29 illustrates a portion of a frame, which is useful for showing how impulses are deliberately jittering in an embodiment of the present invention;

FIG. 30 shows an exemplary portion of an impulse radio receiver according to an embodiment of the present invention; and

FIG. 31 is an example computer system environment in which the present invention can operate.

FIG. 32 shows a gain plot for a variable gain system wherein both gain and threshold are varied in a coordinated manner, in accordance with the present invention.

FIG. 33 is a functional flow diagram for a receiver employing a variable gain and threshold system in accordance with the present invention.

FIG. 34 is a functional flow diagram illustrating an exemplary multi-stage acquisition process in accordance with the present invention.

DETAILED DESCRIPTION

A. Overview

B. Waveforms

C. Pulse Trains

D. Coding

E. Modulation

F. Reception and Demodulation

G. Interference Resistance

H. Processing Gain

I. Capacity

J. Multipath and Propagation

K. Distance Measurement and Positioning

L. Power Control

A. Overview

B. RF Sampling Subsystem

C. Timing Subsystem

D. Control Subsystem

E. Baseband Processor

F. Paired Correlators

G. Lock Loop

A. Terminology

B. Exemplary Packets

C. Problem Description

D. Overview of Solution to Problem

E. Packet Protocol

F. Frame Alignment (First Stage Acquisition)

G. Threshold Determination

H. Tracking

I. Back Ramps

J. Detect Beginning of Data Payload (Second Stage Acquisition)

K. Ratchet Codes

L. Generating Ratchet Codes

M. Exemplary Impulse Radio Receiver Subsystem 1. Operation During First Stage Acquisition 2. Tracking and Back Ramps 3. Operation During Second Stage Acquisition 4. Frame Format and Additional Receiver Embodiments for Use with Longer Short Acquisition Codes and Ratchet Codes 5. IJ or IQ Correlator Pairs and Ramp Builder Pairs

N. Radio Command Channel

O. Jittering Impulse Positions

P. Acquisition Times 1. First Stage Acquisition Time 2. Tracking Time 3. Second Stage Acquisition Time 4. Time Required for Complete Acquisition 5. Time Required in a Typical Conventional System 6. Comparison

Q. Hardware and Software Implementations

R. Automatic Acquisition Threshold

S. Gain Control

T. Automatic Multi-Stage Threshold Adjustment

The present invention builds upon existing impulse radio techniques. Accordingly, an overview of impulse radio basics is provided prior to a discussion of the specific embodiments of the present invention. This section is directed to technology basics and provides the reader with an introduction to impulse radio concepts, as well as other relevant aspects of communications theory. This section includes subsections relating to waveforms, pulse trains, coding for energy smoothing and channelization, modulation, reception and demodulation, interference resistance, processing gain, capacity, multipath and propagation, distance measurement, and qualitative and quantitative characteristics of these concepts. It should be understood that this section is provided to assist the reader with understanding the present invention, and should not be used to limit the scope of the present invention.

A. Overview

Ultra Wideband is an emerging RF technology with significant benefits in communications, radar, positioning and sensing applications. In 2002, the Federal Communications Commission (FCC) recognized these potential benefits to the consumer and issued the first rulemaking enabling the commercial sale and use of products based on Ultra Wideband technology in the United States of America. The FCC adopted a definition of Ultra Wideband to be a signal that occupies a fractional bandwidth of at least 0.25, or 0.5 GHz bandwidth at any center frequency. The 0.25 fractional bandwidth is more precisely defined as:

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