Abstract:
A spread-spectrum communication system in which the energy output is more smoothly distributed than the length of the pseudo-random code would otherwise indicate. A spread-spectrum communication system in which the code sequence is pseudo-randomly inverted on data-bit boundaries, so that the code sequence appears longer, for energy spreading, than it otherwise would appear.

Description:
This application is a continuation of Ser. No. 08/146,499 filed on Nov. 1, 1993, now U.S. Pat. No. 5,436,941. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to spectral density techniques for use with spread-spectrum modulation. 
     2. Description of Related Art 
     In direct-sequence spread-spectrum communication, a data stream may be modulated with a pseudo-random code, so that the energy of the modulated signal is spread over a bandwidth which is larger than the bandwidth of the data stream. Present regulations, for communication in a band of electromagnetic spectrum in which spread-spectrum communication is allowed, generally require all parts of the modulated signal to be no more than +8 db power over the signal average, measured over a 3 KHZ resolution bandwidth. In a spread-spectrum system which uses relatively short pseudo-random codes, the modulated signal may at times not be sufficiently random to meet the regulatory requirement. Accordingly, it would be advantageous to generate a spread-spectrum signal using relatively short pseudo-random codes which meets the regulatory requirement. 
     SUMMARY OF THE INVENTION 
     The invention provides a spread-spectrum communication system in which the energy output is more smoothly distributed than the length of the pseudo-random code would otherwise indicate. In particular, the invention provides a spread-spectrum communication system in which the code sequence is pseudo-randomly inverted on data-bit boundaries, so that the code sequence appears longer, for energy spreading, than it otherwise would appear. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of a spread-spectrum communication transmitter and receiver. 
     FIG. 2 shows a block diagram for a pseudo-random code generator for use in a spread-spectrum communication system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a block diagram of a spread-spectrum communication transmitter and receiver. 
     A spread-spectrum transmitter 101 may comprise an input port 102 for input data 103, a chip sequence transmitter generator 104, a modulator 105, and a transmitting antenna 106 for transmitting a spread-spectrum signal 107. A spread-spectrum receiver 108 may comprise a receiver antenna 109, a chip sequence receiver generator 110, a demodulator 111, and an output port 112 for output data 113. In a preferred embodiment, a single chip sequence 114 is identically generated by both the transmitter generator 104 and the receiver generator 110, and appears essentially random to others not knowing the spreading code upon which it is based. An extensive discussion of spread-spectrum communication, spreading codes, and chip sequences, may be found in R. Dixon, SPREAD SPECTRUM SYSTEMS (1984). 
     FIG. 2 shows a block diagram for a pseudo-random code generator for use in a spread-spectrum communication system. 
     The transmitter generator 104 and the receiver generator 110 may comprise a code generator 201, having an output 202 for the chip sequence 114. In a preferred embodiment, the chip sequence 114 may comprise a 63-chip maximal-length pseudo-random chip sequence, which is pseudo-randomly inverted by XOR-ing with a second chip sequence at each data bit boundary. 
     The code generator 201 comprises a first generator 203 which generates a first code 204. In a preferred embodiment, the first code 204 may comprise a 63-chip linear maximal-length code. An output from the first generator 203 is coupled to a first input of an XOR gate 205. A second generator 206 is clocked at the same rate as the data stream, and generates a second code 207, which is coupled to a second input of the XOR gate 205. The output of the XOR gate 205 is coupled to the output 202 for the code generator 201. 
     The code generator 201 thus generates a complete sequence of the first code 204 for each data bit, but pseudo-randomly inverts the first code 204 by XOR-ing it with the second code 207 at each data bit boundary (i.e., each full data bit is modulated either with the full length of the first code 204 or with the full length of the inverse of the first code 204). 
     It will be clear to those of ordinary skill in the art, after perusal of this application, that the effect of pseudo-randomly inverting the first code 204 at each data bit boundary is to more smoothly distribute the energy of the modulated signal over the bandwidth it occupies. In a preferred embodiment, the first code 204 is 2 N  -1 chips long, e.g., 63 chips long, the second code 207 is 2 P  -1 chips long, e.g., 63 chips long, and the modulated signal has about 4 to 7 db maximum power over the signal average, measured in a 3 KHz resolution bandwidth. 
     Alternative Embodiments 
     While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention, and these variations would become clear to one of ordinary skill in the art after perusal of the specification, drawings and claims herein. 
     For example, information which is transmitted from transmitter to receiver is referred to herein as &#34;data&#34;, but it would be clear to those of ordinary skill in the art that these data could comprise both data and error-correcting codes, control information, or other signals, and that this would be within the scope and spirit of the invention.