Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-04-01
2001-08-28
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S339000
Reexamination Certificate
active
06282232
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to communications receiving systems designed to receive multiple burst transmissions, occurring on the same frequency and overlapping in time, using direct-sequence spread-spectrum (DSSS), code-division multiple-access (CDMA), phase-shift-keyed modulation. This type of transmission can be employed in a star-type communications network from the remote nodes to the central node, or in a mesh-type communications network between the nodes, and may or may not include relay stations such as those that would be used in satellite communications networks.
2. Description of the Related Art
Several transmission schemes have been presented for CDMA communications systems. The Telecommunications Industry Association IS-95 forward link (base-to-subscriber-terminal) scheme is an orthogonal synchronous CDMA system in which the spreading sequences used to communicate with different users are Walsh-Hademard functions. These highly orthogonal codes are used to minimize mutual interference between signals addressed to different users. However, there is a need for very precise synchronization to maintain orthogonality between signals transmitted to and by the various users. On the reverse link (subscriber-unit-to-base), each subscriber unit is assigned a unique spreading sequence. These spreading sequences are chosen to have good cross-correlation properties.
The C200 system, developed by Equatorial communications Company, uses an asynchronous scheme that provides one code per very small aperture terminal (VSAT). The spreading sequences used in this system are modified Gold codes. These codes have good asynchronous cross-correlation properties. However, since each VSAT has a unique spreading sequence and any VSAT may transmit at any time, each VSAT requires a dedicated receiver at the hub.
In a paper entitled “Fundamentals of Packet Multiple Access for Satellite Networks,” by Norman Abramson (IEEE Journal on Selected Areas in Communications, Vol. 10, No. 2, February 1992), the author suggests a transmission scheme for VSAT terminals that he refers to as “spread ALOHA.” In this scheme, all users would employ the same spreading sequence. This spreading sequence would be chosen to have a good autocorrelation peak and low autocorrelation sidelobes. The hub earth station would transmit a pilot signal to facilitate the synchronization of the transmissions from the various VSATs in the network. However, Abramson does not describe how to implement a hub earth station receiving system for multiple simultaneous inbound VSAT transmissions. Moreover, the capacity of such a system in terms of the number of simultaneous users depends heavily on the length of the spreading sequence employed (and hence the satellite transponder bandwidth required for a given inbound data rate, coding, and modulation technique) and how well the VSAT system can be synchronized.
To overcome the technical difficulties of the synchronization of various transmitters and the economic problems of requiring a dedicated receiver for each possible transmitter, a new CDMA transmission and reception scheme is needed. This scheme should operate like a conventional TDMA or FDMA scheme in that a set of receiving system resources can be shared between multiple users whose transmissions do not collide with one another. It should also not impose burdensome synchronization requirements for successful operation. It is therefore desirable to provide a system for properly routing incoming signals to a plurality of demodulators in a manner that will avoid the problems set forth above.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an effective and efficient receiving system for asynchronous, direct-sequence spread-spectrum (DSSS) code-division multiple-access (CDMA) burst transmissions wherein the transmitting stations use a common spreading sequence or choose from a small set of spreading sequences.
It is another object of this invention to provide an assignment mechanism such that as long as overlapping DSSS CDMA signals that are nearly simultaneously received are spread with different spreading sequences, or received with a spreading sequence epoch difference of more than one or two chips, they will be detected and assigned to different demodulator channels within a pool of demodulator channels for demodulation.
It is yet another object of this invention to provide an assignment mechanism that assigns one and only one demodulator channel to demodulate each received signal.
It is still another object of this invention to provide an assignment mechanism that is economical to implement.
Another object of this invention is to provide an assignment mechanism that facilitates the reception of different signals using the same spreading sequence, data rate, and frequency with minimal time-separation between their spreading sequence epochs, or symbol boundaries.
A further object of this invention is to provide a demodulator assignment mechanism that maximizes the efficiency of a pool of burst DSSS CDMA demodulators.
According to one aspect of the present invention, a method is provided for receiving and processing burst-mode code-division multiple access (CDMA) direct-sequence spread-spectrum (DSSS) signals, the method comprising the steps of receiving a data signal at first through k
th
demodulators, the first through k
th
demodulators being set in a first enumerated order and divided into a set of m ready demodulators and a set of n busy demodulators, the ready demodulators being set in a second enumerated order analogous to their order in the first enumerated order, detecting a preamble in the data signal at the ready demodulators, sending one or more request signals respectively as a grant signal from the ready demodulators to a first ready demodulator, first in the second enumerated order, processing the data signal by the first ready demodulator, and moving the first ready demodulator from the set of ready demodulators to the set of busy demodulators, wherein k is an integer greater than one, m is an integer greater than zero and less than or equal to k, n is an integer less than or equal to k, and m plus n equals k.
The step of sending a grant signal to the first ready demodulator may be carried out by sending the grant signal through all of the n busy demodulators that are higher in the first enumerated order than the first ready demodulator.
A method is also provided for receiving and processing burst-mode code-division multiple access (CDMA) direct-sequence spread-spectrum (DSSS) signals, the method comprising the steps of receiving a signal at first through k
th
demodulators, the first through k
th
demodulators being set in a first enumerated order and divided into a set of m ready demodulators and a set of n busy demodulators, the ready demodulators being set in a second enumerated order analogous to their order in the first enumerated order, detecting a preamble in the signal at the ready demodulators, sending one or more request signals respectively from the ready demodulators to an arbitrator, sending a grant signal from the arbitrator to a first ready demodulator, first in the second enumerated order, processing the signal by the first ready demodulator, and moving the first ready demodulator from the set of ready demodulators to the set of busy demodulators, wherein k is an integer greater than one, m is an integer greater than zero and less than or equal to k, n is an integer less than or equal to k, and m plus n equals k.
The step of sending a grant signal from the arbitrator to a first ready demodulator may be carried out by sending the grant signal through all of the n busy demodulators that are higher in the first enumerated order than the first ready demodulator.
A signal receiving and demodulating system is provided, comprising first through k
th
demodulators, arranged in an enumerated order, each having an IF signal input for receiving an IF signal, an i
th
request signal output, an i
th
busy signal output, an i
th
grant signa
Check William A.
Chisholm Joseph A.
Decker David G.
Fleming, III Robert F.
Glinsman Brian J.
Jones Volentine PLLC
Nguyen Dung X.
Pham Chi
Spacenet, Inc.
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