Pulse or digital communications – Systems using alternating or pulsating current
Reexamination Certificate
1999-12-21
2001-04-17
Vo, Don (Department: 2734)
Pulse or digital communications
Systems using alternating or pulsating current
C455S456500, C342S463000, C701S213000
Reexamination Certificate
active
06219385
ABSTRACT:
BACKGROUND OF THE INVENTION
Recent years have witnessed ever-increasing interest in radio positioning and navigation systems, with interest spanning a very broad range of government and commercial applications. Probably the best known, and most applied system to date is the U.S. Government Global Positioning System (GPS), which relies on many simultaneous, and globally distributed satellite broadcasts. All satellite transmissions rely on highly stable, and mutually synchronized clocks, and apply well-established spread-spectrum signaling techniques. In the absence of blockage (e.g., from buildings in urban areas) or multipath, suitable equipped radio receivers can provide highly accurate position (e.g., 100 meter to even centimeter accuracy). GPS performance, however, degrades rapidly when blockage and/or multipath is present, thereby dramatically reducing its effectiveness for a broad range of personal services in urban and suburban areas—including traveler services and FCC-mandated, wireless 911.
In recognition of the importance of enabling positioning in urban/suburban regions, considerable effort has been expended in recent years in defining and developing terrestrial-based approaches that can overcome or mitigate propagation phenomena in such difficult regions. These include: passive or active positioning via the evolving digital cellular and PCS networks; positioning via existing terrestrial AM radio transmission; positioning via existing FM radio transmission; positioning via existing TV transmissions; hybrids of GPS and terrestrial signaling. Each of these approaches offers benefits but also limitations. For example, the IS-95 spread-spectrum signal is ideally suited for ranging and positioning, but the power control inherent in the system limits the ability to simultaneously receive from multiple, geographically distributed cell-sites/base stations; multipath degradation is also a factor if very accurate positioning is desired. On the other hand, FM and TV transmissions offer very high power, but multipath, range resolution and/or geometry may limit positioning accuracy. Tracking of AM carriers offer extremely attractive resolution, and the long wavelength of AM makes it fairly immune to multipath, but AM is very sensitive to burst noise and nighttime propagation phenomena. In addition, several of these approaches require additional terrestrial infrastructure and interfaces to enable calibration of inherent error sources (e.g., oscillator clock biases).
What is clearly needed is a system approach and implementation that simultaneously draws upon the strengths of several of the above, and does so in a manner that simultaneously mitigates inherent weaknesses of any one of the approaches. In other words, two or more approaches are applied in a complementary fashion, and leverages inherent capabilities not exploited to date. Furthermore, the system approach must minimize infrastructure/interface augmentation.
The GPS system is an example of prior art in regards to positioning systems. It is a spread spectrum system where each satellite transmits a unique Pseudo Noise (PN) sequence. A GPS receiver correlates to these different PN sequences generating an impulse signal that mark the arrival time from each satellite signal. The time differences between these signals are then used to calculate a position. To maximize position accuracy, these PN Sequences are selected for their good auto-correlation properties (i.e., close to impulse). It can be shown that an OFDM symbol also has an excellent auto-correlation properties, and can be used in a similar but unique way.
It is easiest to understand the OFDM's excellent auto-correlation property by examining the signal in the frequency domain. An OFDM symbol is typically constructed by placing a symbol in each carrier (i.e., frequency bins), performing an Inverse Fast Fourier Transforms (IFFT), and transmitting the time domain signal. Assuming PSK modulation, the OFDM symbols have a constant magnitude across its bandwidth and “information” on the phases on each carrier. If the phase “information” is removed, then the resulting signal has a constant magnitude and phase. This resulting signal is the frequency response of a discrete impulse. Note that removal of phase information can be performed by bin-wise multiple across the OFDM symbol with a complex conjugate of itself. This operation is the equivalent to a circular auto-correlation in the time domain.
It should be noted that most OFDM systems have a guard interval after (and/or before) each OFDM symbol and that this guard interval contains a repeated portion (i.e., circular shift) of the OFDM symbols. This makes the system more tolerant to intersymbol caused by timing offset or multipath. It also is beneficial to positioning techniques using the circular correlation operation.
OVERVIEW OF THE INVENTION
The object of this invention is to provide a terrestrial navigation system, characterized by the following general features.
1. A distributed population of vehicles or people that are either stationary or mobile—i.e., a population of mutually independent “users”.
2. The users may be indoors or outdoors.
3. User navigation must be passive—i.e., the user performs navigation via passive reception of radio signals that are always present.
4. The navigation capability is embedded in a host device that can also be used to actively or automatically transmit the user's position at periodic intervals (e.g., every 5 seconds). The host device can, for example, be a cellular or PCS phone.
5. Applications of interest include, but are not necessarily limited to: E911; traveler services (e.g., vehicle routing); surveying.
6. The navigation system should be capable of operating without GPS, although augmentation with GPS is certainly permissible.
Toward this end, a novel concept and functional implementation is disclosed that uniquely and efficiently applies the signals transmitted by planned Digital-AM and Digital-FM terrestrial radio stations to position determination and relevant services, such as E911 and traveler information. The concept presented in this invention is an extension of Schuchman et al U.S. Pat. Nos. 5,422,813 and 5,365,450 incorporated herein by reference, on a system concept to passively monitor and track the carriers of local AM and FM radio stations. The enhanced system presented herein can operate totally independently of GPS, and is termed Digital AM/FM Positioning System (DAFPS). The present DAFPS leverages current activities to transition the AM and FM broadcast stations to all-digital broadcasts, following a multi-year transition period wherein analog and digital will coexist. The unique features of this invention include the following:
1. National (and potentially an international) system; this reflects the expected standardization of the Digital AM and FM signals.
2. Passive reception and processing of Digital AM and FM radio signals, without any interaction or coordination with the AM and FM systems.
3. Availability of a multitude of AM and FM signals in a single user/customer radio receiver that is (or may be) an augmentation of standard Digital AM/FM radio; the associated quantity and geometrical distribution of the AM and FM transmitters directly leads to robust and accurate position determination.
4. Within the urban canyon environment, the propagation characteristics of AM and FM signals complement each other, by providing robustness and diversity; i.e., AM signal propagation reflects a degree of robustness against multipath, relative to FM radio signals, due to the long wavelength of AM signals, while FM offers robustness in certain indoor environments, as well as during nighttime and thunderstorms (i.e., impulsive noise).
5. Radio receiver signal processing uniquely takes advantage of the Orthogonal Frequency Division Multiplex (OFDM) signal structure, used for the Digital AM/FM transmissions. The multi-coherent-carrier nature of OFDM directly permits implementation of tone ranging, with inherent ambiguity resolution, while the desired high resolutio
ITT Manufacturing Enterprises Inc.
Vo Don
Zegeer Jim
LandOfFree
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