Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1993-01-15
2003-09-16
Cangialosi, Salvatore (Department: 2661)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S130000
Reexamination Certificate
active
06621854
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to digital communication systems, and more particularly to a spectrum spreading technique for use in multi-node digital communication systems such as digital networks and digital radios.
BACKGROUND OF THE INVENTION
Spectrum spreading techniques for use in digital communication networks have been described in many books and papers. A classic publication in this field is
Spread Spectrum Communications
by M. K. Simon, J. K. Omura, R. A. Scholtz and B. K. Levift, Computer Science Press, 11 Taft Court, Rockville, Md. 20850, 1985. Particular kinds of spectrum spreading techniques that have been implemented in digital communication networks in the prior art include “direct-sequence spreading”, “frequency hopping”, “time hopping”, and various hybrid methods that involve combinations of the aforementioned techniques.
Multi-node spread-spectrum communication networks developed in the prior art were generally characterized as code-division multiple-access (CDMA) networks, which utilized “code-division multiplexing” (i.e., a technique in which signals generated by different spreading-code sequences simultaneously occupy the same frequency band). Code-division multiplexing requires that the simultaneously used spreading codes be substantially “mutually orthogonal”, so that a receiver with a filter matched to one of the spreading codes rejects signals that have been spread by any of the other spreading codes.
In a typical multi-node spread-spectrum communication network using either a conventional direct-sequence spectrum spreading technique, or a hybrid technique involving,e.g., direct-sequence and frequency-hopped spectrum spreading, only a single spreading code is employed. At regular intervals, the polarity of the spreading code is either inverted (i.e., each 0 is changed to 1, and each 1 is changed to 0) or left unchanged, depending on whether the next bit of information to be transmitted is a 1 or a 0. The resulting signal is an “information-bearing” sequence, which ordinarily would be transmitted using some type of phase-shift keyed (PSK) modulation—usually, binary phase-shift keyed (BPSK) modulation or quaternary phase-shift keyed (QPSK) modulation.
A publication entitled
Spread Spectrum Techniques Handbook
, Second Edition, March 1979, which was prepared for the National Security Agency by Radian Corporation of Austin, Tex., describes a number of spread-spectrum techniques that had been proposed in the prior art. Of particular interest is a direct-sequence technique described on page 2-21 et seq. of the
Spread Spectrum Techniques Handbook
, which involved transmitting one bit of information (either a 0 or a 1) by switching between two independent signals that are generated by different spreading codes. Ideally, the spreading codes of the two independent signals should be “almost orthogonal” with respect to each other, so that cross-correlation between the two sequences is very small. In practice, in such early spread-spectrum communication systems, the two independent signals were maximal-length linear recursive sequences (MLLRSs), often called “M-sequences”, whose cross-correlations at all possible off-sets had been computed and found to be acceptably low. However, this technique of switching between two independent signals did not achieve widespread acceptance, mainly because it required approximately twice the electronic circuitry of a polarity-inversion technique without providing any better performance.
Two recent papers, viz., “Spread-Spectrum Multiple-Access Performance of Orthogonal Codes: Linear Receivers” by P. K. Enge and D. V. Sarwate, (
IEEE Transactions on Communications
, Vol. COM-35, No. 12, December 1987, pp. 1309-1319), and “Spread-Spectrum Multiple-Access Performance of Orthogonal Codes for Indoor Radio Communications” by K. Pahlavan and M. Chase, (
IEEE Transactions on Communications
, Vol. 38, No. 5, May 1990, pp. 574-577), discuss multi-node spread-spectrum communication networks in which multiple orthogonal sequences within a relatively narrow bandwidth are assigned to each node, whereby a corresponding multiplicity of information bits can be simultaneously transmitted and/or received by each node—thereby providing a correspondingly higher data rate. A specified segment of each sequence available to a node of the network is designated as a “symbol”. In the case of a repetitive sequence, a symbol could be a complete period of the sequence. The time interval during which a node transmits or receives such a symbol is called a “symbol interval”. In a multi-node spread-spectrum network employing multiple orthogonal sequences, all the nodes can simultaneously transmit and/or receive information-bearing symbols derived from some or all of the sequences available to the nodes.
The emphasis in the aforementioned Enge et al. and Pahlavan et al. papers is on network performance, especially in certain kinds of signal environments. Neither paper recommends or suggests using any particular set of mutually orthogonal spreading codes for generating multiple orthogonal sequences; and neither paper discloses how to derive or generate suitable mutually orthogonal spreading codes. However, methods of generating families of sequences that are pairwise “almost orthogonal” by using two-register sequence generators have been known for some time.
In a paper entitled “Optimal Binary Sequences for Spread-Spectrum Multiplexing” by R. Gold, (
IEEE Transactions on Information Theory
, Vol. IT-13, October 1967, pp.119-121), so-called “Gold codes” were proposed for use as spreading codes in multi-node direct-sequence spread-spectrum communication networks of the CDMA type. A Gold code is a linear recursive sequence that is generated by a product f
1
f
2
, where f
1
and f
2
comprise the members of a so-called “preferred pair” of primitive polynomials of the same degree n over a field GF(2). A primitive polynomial of degree n is defined as a polynomial that generates a maximal-length linear recursive sequence (MLLRS), which has a period of (2
n
−1). The required relationship between f
1
and f
2
that makes them a preferred pair is described in the aforementioned paper by R. Gold.
A Gold code is a particular kind of “composite code”. Other kinds of composite codes include “symmetric codes” and “Kasami codes”. A symmetric code is similar to a Gold code in being generated by a product f
1
f
2
of a pair of primitive polynomials, except that for a symmetric code the polynomial f
2
is the “reverse” of primitive polynomial f
1
, i.e., f
2
(x)=x
n
f
1
(1/x), where n=deg f
1
=deg f
2
. The correlation properties of Gold codes and symmetric codes are discussed in a paper entitled “Cross Correlation Properties of Pseudorandom and Related Sequences” by D. V. Sarwate and M. B. Pursley (
Proceedings of the IEEE
, Vol. 68, p.5 May 1980, pp. 593-619). Kasami codes differ from Gold codes in that for Kasami codes, the polynomials f
1
and f
2
are not of the same degree. Kasami codes are also discussed in the aforementioned paper by M. B. Pursley and D. V. Sarwate. The concept of a “composite code” can be broadened to include sequences obtained from a two-register sequence generator, where the sequences generated in the two registers can be quite general.
Predominant among the reasons that have militated against using direct-sequence spreading codes for multi-node spread-spectrum communication networks of the prior art is the so-called “near-far” problem. If the nodes of a multi-node spread-spectrum communication network are widely distributed so that power levels for different nodes can differ markedly at a given receiver in the network, then at the given receiver the correlations of a reference sequence with a sequence that is transmitted by a nearby node are apt to be stronger than correlations of the reference sequence with a version of the reference sequence that has been transmitted from a greater distance. Adverse effects of the “near-far” problem can include periodic strong correlations in information-bit errors, and
Cangialosi Salvatore
Fenwick & West LLP
Lockheed Martin Corporation
Morrissey John J.
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