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Dual code spread spectrum communication system

Dual code spread spectrum communication system description/claims

The invention was made with Government support under contract No. F04701-00-C-0009 by the Department of the Air Force. The Government has certain rights in the invention.

The invention relates to the field of direct sequence spectrum spreading (DSSS) communications systems. More particularly, the present invention relates to DSSS communications for selectively communicating from a transmitter to a select portion of a group of receivers using switched spreading codes.

Spread spectrum communication systems use direct sequence spread spectrum (DSSS) codes for spectrum spreading channel signals communicated over respective communication channels and over a common channel bandwidth. Prior to transmission, an input signal having a narrow frequency spectrum, is spectrum spread by mixing the input signal with the DSSS code for providing a spread spectrum channel signal that has a spread spectrum extending in frequency over a wide channel bandwidth. The power spectra density of the communicated channel signal is spread over the channel bandwidth and is difficult to acquire without advance knowledge of the spreading code. As such, the communication system uses code division signaling for maintaining cochannel isolation of the multiple channel signals communicated over the channel bandwidth. Signal components of a channel signal are spread over the channel bandwidth providing low power components across the bandwidth rendering the isolation and reception of the input signal difficult without advance knowledge of the spreading code for correlation with the channel signal of interest. In order to reconstitute the spectrum spread signal upon reception, the channel signal is despread by correlating the received channel signal by a replica code generated in the receiver.

In a transmitter, spectrum spread channel signals are superimposed as an output signal that modulates a carrier for communicating by code division a plurality of channel signals, each of which is spectrum spread by the respective spreading code. Hence, each channel uses a respective spreading code, and hence, each receiver is equipped with a respective spreading code used for isolating a respective channel signal so that each receiver can acquire one and only one respective channel signal. During acquisition in the receiver, code phase is determined using the autocorrelation properties of the spreading code. When the receiver replica code is phase shifted in time to coherently match the code phase of the incoming channel signal, the signal is reconstituted and can be detected during code phase lock. When the code phase shifts, there is a drop in the level of the detection signals, and hence a lock drop preventing acquisition of the channel signal. Hence, the receivers automatically adjust the code phase of the replica code to dynamically match the code phase of the invention channel signal so as to maintain code lock during continuous reception of the channel signal. In the event the replica code has a large code phase differential, coherently code phase lock is lost, and the signal cannot be reconstituted nor detected.

The use of spreading codes have long been used to isolate channel signals so that only receivers with predetermined codes can only receive channel signals spectrum spread by the particular respective codes. In practice, each user or receiver is assigned apriori a respective unique spreading code. In some situations, a common channel signal can be broadcast to a plurality of receivers all having the same identical spreading code, such as in the broadcast of a GPS signal from a satellite to a plurality of GPS receivers each having the same spreading code for respective coherent reception. That is, the spreading code is used to broadcast a DSSS channel signal to a group of users all having a receiver generating a replica of the same spreading code for receiving the broadcasted signal. Once the grouped receivers are equipped with the same replica code, all user receivers will receive the same information at all times. In the presence of a jamming signal, all receivers will be equally jammed and communications will be equally jammed amongst all of the users. In clandestine or black out communications, it may be desirable to send a communication to a selected segment of the group and not to others of the group at various times. Such selective communications can be used in field operations where a first group is a friendly or desired group of recipients and the second group is a hostile or undesired group of recipients. Such selective communications during the broadcast of a spread spectrum signal could be used to communicate with the entire group at one time, and a preferred portion of that group at another time. Conventional broadcast spread spectrum communication systems using a single spreading code prevents time variable selective communications to a portion of a group of receivers. These and other disadvantages are solved or reduced using the invention.

An object of the invention is to provide a communication system that broadcasts upon transmission a channel signal using a plurality of spreading codes at respective different times for selective communication to a plurality of receivers.

Another object of the invention is to provide a communication system that broadcasts upon transmission a channel signal using a plurality of spreading codes at respective different times for selective communication to differing segments of the plurality of receivers at the respective different times.

Yet another object of the invention is to provide a communication system having a plurality of receivers for receiving a broadcast channel signal spectrum spread at different times by respective spreading codes.

The present invention is directed to a spread spectrum broadcast communication system that selectively communicates a spread spectrum channel signal by differing spreading codes at differing times to a group of receivers. A first portion of the receivers receive the channel signal at a first time using a first spreading code and a second portion of the receivers receives the channel signal at a second time using a second spreading code. In the preferred form, a transmitter transmits a first channel signal at a first time period using a first spreading code and transmits a second channel signal at a second time period using a second spreading code, while, a first portion of the receivers generate a replica of the first spreading code for receiving the first channel signal during the first time period and a second portion of the receivers generate replicas of the first and second spreading code for receiving the first channel signal during the first time period and for receiving the second channel signal during the second time period, respectively, using spreading code detection for determining which spreading code is currently being used to transmit the broadcast signal. In the preferred form, all of the receivers receive and correlate the first channel signal during a first time period, but only a portion of the receivers receive and fully correlate the second channel signal during the second time period. In this manner, the first message is communicated in the first channel signal to all of the receivers while a second message is communicated in the second channel signal to only a subset of all of the receivers for selective communications within a group of receivers. Hence, the broadcast communication system can be used for sending secret messages to a portion of a group of receivers at one time and for sending public messages to the entire group at another time. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.

FIG. 1 is a block diagram of a degraded spectrum spreading communication system.

FIG. 2 is a dual code autocorrelation graph.

FIG. 3 is a block diagram of spreading code detection receiver.

• Provisional Patent
• Utility Patent

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