System for improving signal-to-noise ratio in a direct sequence spread spectrum signal receiver

• US 4,644,523 A
• Filed: 03/23/1984
• Issued: 02/17/1987
• Est. Priority Date: 03/23/1984
• Status: Expired due to Term

• Current Assignee: Itron Electricity Metering, Inc. (Itron, Inc.)
• Original Assignee: Sangamo Weston, Inc. (Atos SE)
• Inventors: Horwitz, Lawrence B.
• Primary Examiner(s): Griffin, Robert L.
• Assistant Examiner(s): Huseman, M.
• Application Number:
  US06/592,674
• Time in Patent Office: 1,061 Days
• Field of Search: 375/1, 375/115, 375/2.1, 375/96, 370/18
• US Class Current: 370/479
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04J 13/00 Code division multiplex systems for frequency hopping H04B1/713

Abstract

A plurality of transmitters synchronized to a common clock each transmit a data signal spread by a common bipolar pseudo-random code having a different assigned code sequence shift. A receiver, synchronized to the clock, discriminates the signal transmitted by a predetermined transmitter from signals transmitted by the others by generating a first pseudo-random code that is a replica of the common bipolar pseudo-random code and has a code sequence shift corresponding to that of the predetermined transmitter, and a second bipolar pseudo-random code that is a replica of the common bipolar pseudo-random code and has an unassigned code sequence shift. The difference between the first and second bipolar pseudo-random code sequences, which is a trinary code sequence, is cross-correlated with the incoming signal. The cross-correlation despreads only the signal having the predetermined code sequence shift. Each receiver includes a number of correlation detectors offset from each other by a fraction of a code chip, together with decision circuitry to identify cross-correlation peaks for optimum synchronization. The output of each sub-receiver is processed to extract data using weighting factors selected according to the particular distortion present, to improve signal-to-noise ratio.

Claims

1. In a direct sequence spread spectrum code division multiplex system, including a plurality of transmitters synchronized to a common timing signal and each transmitting a data signal spread by a bipolar pseudo-random code which is a different assigned shift of a common bipolar sequence:

a receiver synchronized to said timing signal for receiving said transmitted signal spread by a bipolar pseudo-random code having a predetermined assigned code sequence shift, said receiver including a plurality of correlation detectors and means for applying to each of the correlation detectors (1) a first reference bipolar pseudo-random sequence that is a replica of the common bipolar pseudo-random sequence and has a code shift that is within one code chip of the assigned shift of a predetermined transmitter and is displaced from the common bipolar pseudo-random code sequence applied to the other correlation detectors by a fraction of a code chip less than unity, and (2) a second reference bipolar pseudo-random sequence that is a replica of the transmitted common bipolar pseudo-random sequence and has an unassigned code sequence shift, each of said correlation detectors including first means for obtaining the product of the transmitted sequences and the first reference bipolar pseudo-random sequence;

second means for obtaining the product of the transmitted sequences and the second reference bipolar pseudo-random sequence and third means for obtaining a difference between the products obtained by the first and second means;

synchronous integrator means for integrating the difference;

means for synchronously sampling an output of the integrator means and signal processor means responsive to outputs of said correlation detectors to synchronize said receiver to said predetermined transmitter;

the improvement comprising;

means responsive to the output of each of the correlation detectors for recovering data transmitted by said preselected transmitter.

2. The receiver of claim 1, wherein said data recovering means includes means for storing weighting factors depending upon a particular form of distortion present and means for applying said weighting factors to amplify the respective outputs of said sub-receivers, to optimize signal-to-noise ratio.

3. In a direct sequence spread spectrum code division multiplex system, including a plurality of transmitters synchronized to a common timing signal and each transmitting a data signal spread by a bipolar pseudo-random code which is a different assigned shift of a common bipolar sequence;

a receiver synchronized to said timing signal for receiving said transmitted signal spread by a bipolar pseudo-random code having a predetermined assigned code sequence shift, said receiver including a plurality of correlation detectors and means for applying to each of the correlation detectors (1) a first reference bipolar pseudo-random sequence that is a replica of the common bipolar pseudo-random sequence and has a code shift that is within one code chip of the assigned shift of a predetermined transmitter and is displaced from the common bipolar pseudo-random code sequence applied to the other correlation detectors by a fraction of a code chip less than unity, and (2) a second reference bipolar pseudo-random sequence that is a replica of the transmitted common bipolar pseudo-random sequence and has an unassigned code sequence shift, each of said correlation detectors including first means for obtaining the product of the transmitted sequences and the first reference bipolar pseudo-random sequence;

second means for obtaining the product of the transmitted sequences and the second reference bipolar pseudo-random sequence and third means for obtaining a difference between the products obtained by the first and second means;

synchronous integrator means for integrating the difference;

means for synchronously sampling an output of the integrator means and signal processor means responsive to outputs of said correlation detectors to synchronize said receiver to said predetermined transmitter;

the improvement comprising;

a method of improving signal-to-noise ratio comprising the steps of storing weighting factors corresponding to the output of each of said correlation detectors, the weighting factors selected depending upon the particular form of distortion present;

amplifying the outputs from said correlation detectors as functions of said weighting factors; and

combining said amplified outputs.