Spread spectrum localizers

Details Spread spectrum localizers Spread spectrum localizers

1999-07-22

2002-05-07

Le, Amanda T. (Department: 2634)

Pulse or digital communications

Miscellaneous

C343S905000, C455S269000

Reexamination Certificate

active

06385268

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related generally to spread spectrum radio communication systems, and more particularly to nonsinusoidal spread spectrum radio communication systems used to determine the locations of transceivers.
2. Prior Art
In traditional radio communication systems the transmitted electromagnetic power is concentrated in a narrow frequency band, whereas in spread spectrum communication systems the power is distributed over a relatively large bandwidth. Spread spectrum radio communications are used in place of traditional systems to circumvent communications jamming by interference signals, prevent detection and interception by unwanted receivers so as to provide privacy, provide tolerance to multipath transmissions, send multiple independent signals over a frequency band, and/or provide accurate ranging information.
The standard categories of spread spectrum radio communication techniques include direct sequence, frequency modulation, chirp modulation (or linear frequency modulation), and time hopping. In chirp modulation each transmitted pulse has a carrier frequency which changes linearly with time. The reception circuitry is designed such that the propagation of the signal through the circuitry is inversely related to the carrier frequency, so that the length of chirped pulses are shortened and there is increased signal power during the pulses in comparison to unchirped signals. In frequency hopping transmissions the frequency of the carrier changes according to a pseudorandom sequence. The receiver must know the pseudorandom sequence to be able to tune to the correct carrier frequencies at the proper times. Time hopping transmissions consist of a sequence of frames, each frame having a single impulse or monocycle (see R. A. Scholtz, “Multiple Access with Time-Hopping Impulse Modulation,” MILCOM'93, Bedford, Mass. 1993). The location of the pulses in the frames is determined by a pseudorandom sequence, and without knowledge of the sequence the signal cannot be detected.
In direct-sequence spread-spectrum transmissions, the transmitted signal a(t) is encoded with a data string d(t) of positive and negative unity according to
a
(
t
)=&PSgr;(
t
)
d
(
t
) cos(&ohgr;
t
+&thgr;),
where t is time, &ohgr; is the angular frequency of the carrier, &thgr; is a phase angle, and the modulating signal &PSgr;(t) is a string of pseudorandom sequences &PHgr;(t), the pseudorandom sequences &PHgr;(t) being repeated M times, i.e.,
&PSgr;(
t
)=&PHgr;(
t
mod
t
s
),
where “mod” represents the modulo operation, t
s
is the length of the pseudorandom sequence, &PHgr;(t) is zero outside the range 0≦t≦t
s
, and &PSgr;(t) is equal to zero outside the range 0≦t≦Mt
s
. The signal d(t) is a data stream of positive and negative unity values, the values of d(t) changing at the beginning of each cycle of the pseudorandom signal &PHgr;(t). Each pseudorandom sequence &PHgr;(t) consists of a string of basic units called “chips.”Each chip consists of a chip function &ggr;(t)—in the simplest cases the chip function &ggr;(t) has a constant value—multiplied by a code sequence &sgr;(i), where i is an integer and &sgr; takes on the phase factor values of plus and minus one. Each chip has a length of &dgr;t, and if there are L chips per pseudorandom sequence &PHgr;(t) then &PHgr;(t) has a period t
s
of (L*&dgr;t). The pseudorandom sequence &PHgr;(t) therefore has the functional form
&PHgr;(
t
)=&ggr;(
t
mod &dgr;
t
)*&sgr;[
t/&dgr;t],
where the square brackets indicate the largest integer less than or equal to the argument within the brackets, and &ggr;(t) is zero outside the range 0≦t≦&dgr;t. When 1/&ohgr; is much smaller than the chip length &dgr;t, and the value of &PHgr;(t) changes much faster than the value of d(t) (for instance when L is much greater than unity) the transmitted spectrum has a width on the order of the width of the spectrum of the chip function &ggr;(t). This relation allows the transmitted spectrum to be “spread” by imposing a chip function &ggr;(t) with a wide spectrum. To receive the direct sequence transmission it is necessary to know the pseudorandom sequence &PHgr;(t) and the phase of the carrier and the chip function &ggr;(t). It should be noted that all the above-mentioned spread spectrum techniques rely on a sinusoidal carrier.
Historically, localization systems have been used for surveying and military applications, such as troop positioning and aircraft and missile guidance. The most prevalent military localization systems, Loran, Omega, and Global Positioning System (GPS), determine position based on the propagation time of the electromagnetic signals from beacons at known locations. Loran and Omega beacons are ground-based, and GPS transmitters are satellites. In these localization technologies, pulses are modulated onto a sinusoidal carrier, and traditional resonant circuits are used for transmission and reception.
The GPS system utilizes 21 active satellites in 12-hour orbits above the Earth to allow determination of three-dimensional position to an accuracy to about 10 meters, and velocity to an accuracy of 0.03 meters/second. At any location on Earth at least four GPS satellites are above the horizon at all times. Each satellite transmits the position of the satellite in space, and highly accurate time information derived from an onboard atomic clock. The user's position and the bias of the user's clock are determined by measuring the time for propagation of the signals from the satellites to the user.
Because refraction effects, deviations in velocity and variations in amplitude of radio waves propagating through the ionosphere increase with decreasing frequency, the accuracy of a satellite-based localization system is increased by using high frequency radio transmissions. However, the frequency is limited by practical circuit-design considerations and the fact that the absorption of radio transmissions by water molecules, even in fair weather, increases sharply near 10 GHz. The satellites transmit at two frequencies 1.227 GHz and 1.575 GHz, to allow corrections to be made for frequency-dependent time delays in the propagation of the signals through the atmosphere. Because gigahertz radio transmissions are line-of-sight, a GPS receiver must have a clear view of the sky and at least four satellites to function. This limits the applications to which this technology can be applied.
Since it is difficult to construct gigahertz resonant systems using inexpensive integrated circuitry, such localization systems must be constructed from discrete components, and powered accordingly. To provide reasonable efficiency, antennas for gigahertz reception must be on the order of at least ten centimeters. These limitations in the miniaturization of sinusoid-based localizers, while within bounds acceptable for military and industrial applications, provide limits as to the applicability of sinusoid-based localization technologies, such as GPS, to many aspects of everyday life.
OVERVIEW OF THE PRESENT INVENTION
The present invention is directed to a network of nonsinusoidal spread spectrum (NSS) transceivers capable of precisely locating objects in three-dimensional space. In the preferred embodiment these “localizers” are about the size of a coin, can operate without a clear view of the sky, have a low enough power consumption that they can operate for long periods of time powered by a small battery, do not require a system of satellites, and are very inexpensive.
Such localizers can be utilized in a myriad of applications, serving as extensions of the senses of people and machines. It is envisioned that these localizers will someday be commonly used in almost every aspect of everyday life. For instance, a localizer could be attached to the clothing of a young child, and if that child were to enter an area that presented a danger to her an alarm could sound to alert her or her guardians. Such localizers might also be used to help members of a group, such as

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