Multiplex communications – Wide area network – Packet switching
Patent
1988-08-04
1991-11-05
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
375 1, H04J 1300, H04L 2730
Patent
active
050635603
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates generally to improvements in spread-spectrum transmission systems and in a preferred embodiment the invention is applied to a vehicle location and tracking system.
A number of information bearing channels can share the same medium and approximately the same frequency band and yet be separated at the receiving end with satisfactory interchannel isolation if suitable pseudo-noise (PN) codes are used asynchronously to direct-sequence modulate the channel carriers at a high rate relative to the data rate. This has the effect of spreading the spectrum of the transmitted energy.
At the receiver, the information in each channel is extracted by cross-correlating the incoming composite stream with the code associated with the desired channel. When the clock rate and the epochs of the in-coming and locally-generated codes match, the spread-spectrum energy is collapsed to the relatively narrow, data bandwidth for that channel whilst all the other channel spectra remain spread.
This method enables a particular medium (e.g. a coaxial-cable transmission line) to carry a virtually unlimited number of channels, separation being achieved at the receiving end by code-division multiple access (CDMA). The performance of the scheme in terms of signal-to-noise ratio depends on the relative orthogonality of the codes; that is, on their cross-correlation properties. A unique feature is the smooth degradation of signal-to-noise ratio as more users come into the system compared to the sudden loss of performance which occurs in a conventional frequency division multiple access (FDMA) system once the channel capacity is exceeded.
The capability of a spread-spectrum channel to reject interference from other signals in other channels and from noise is called the process gain. Mathematically, process gain is given as: enough for the spectra to be considered continuous.
Consider now the case of one transmitter, one receiver and no data. According to equation (1) the process gain is infinite because b.fwdarw.0. The zero-data example might be a ranging system where it is necessary only to locate the code epoch and, knowing the propagation delay, the range to the transmitter may be calculated; range ambiguity can be avoided by making the code repetition period much greater than the propagation delay. In practice the process gain can be very large, but not infinite, and is limited mainly by the extent of the loss of coherence of the carrier at the receiver relative to the receiver local oscillator. If the `coherence time` of the received carrier is .tau. then b.about..tau..sup.-1 and process gain can be increased only by spreading the spectrum of the transmitted signal still further. This can be done by increasing the chip-rate (code clock rate) of the PN code up to a limit set by the electronics or by the ability of the transmission medium to support the spread-spectrum bandwidth
Referring to FIG. 1 it may be seen that in a spread-spectrum location and tracking system, the vehicle 10 or object to be located emits a continual direct sequence spread-spectrum radio signal 11. This transmission is received at a number of well-spaced receiving stations 12 in the coverage area and the differences in the times of arrival of the signals at these receivers are measured. Inverse hyperbolic navigation techniques then may be used to compute the position of the transmitter at the central computer 13 which then sends this information to an operator terminal.
Direct sequence spread-spectrum modulation is employed for a number of reasons, one of which is to minimise multi-path effects. Also, since for location and tracking purposes there is no data transmission requirement, there would appear to be potential for very high process gain. Unfortunately the process gain is severely limited in practice. Firstly transmissions from a vehicle moving in an urban, or suburban, area experience Rayleigh scattering and Doppler frequency-shift. As a result, at each receiving site 12 the received signal spectrum
REFERENCES:
patent: 4494228 (1985-01-01), Gutleber
patent: 4511885 (1985-04-01), Gutleber
patent: 4601047 (1986-07-01), Horwitz et al.
patent: 4697260 (1987-09-01), Gravel et al.
Long Range Trackings of Crocodylus Porosus, M. J. Yerby, Handbook on Biotelemetry and Radio Tracking, 1979, pp. 765-776.
The Control of Foreign Microprocessors by C. R. Drane, Proceedings of Digital Equipment Users Society, Jul. 1984.
Hurst Gregory C.
Yerbury Michael J.
Advanced Systems Research Pty. Limited
Chin Wellington
Olms Douglas W.
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