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IMPROVED IMPLEMENTATION TECHNIQUE FOR DIGITALLY PHASE-CODED SYSTEMS

IP.com Disclosure Number: IPCOM000007162D
Original Publication Date: 1994-Feb-01
Included in the Prior Art Database: 2002-Feb-28
Document File: 4 page(s) / 237K

Publishing Venue

Motorola

Related People

Tom Koehler: AUTHOR [+2]

Abstract

This publication describes an implementation technique that is a key element in improving the rejection of unwanted uncorrelated signals in digitally phase-coded radars and communications systems, The principles involved will be explained using a widely used phase coded correlation radar applica- tion which uses a maximal length pseudorandom- noise (PN) code sequence! hereafter refered to as a PN code or PN code sequence. The commonly accepted limit for the rejection of these unwanted uncorrelated signals is 1010g(r/L2) B, where L is the length of the PN code sequence, which is based on the underlying autocorrelation properties of the PN code being used. However, many applications have generated requirements for the rejection ofuncorre- lated signals, hereafter called out-of-range rejection (ORR), beyond this limit.

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MOTOROLA Technical Developments Volume 21 February 1994

IMPROVED IMPLEMENTATION TECHNIQUE FOR DIGITALLY PHASE-CODED SYSTEMS

by Tom Koehler and Carl Willis

  This publication describes an implementation technique that is a key element in improving the rejection of unwanted uncorrelated signals in digitally phase-coded radars and communications systems, The principles involved will be explained using a widely used phase coded correlation radar applica- tion which uses a maximal length pseudorandom- noise (PN) code sequence! hereafter refered to as a PN code or PN code sequence. The commonly accepted limit for the rejection of these unwanted uncorrelated signals is 1010g(r/L2) B, where L is the length of the PN code sequence, which is based on the underlying autocorrelation properties of the PN code being used. However, many applications have generated requirements for the rejection ofuncorre- lated signals, hereafter called out-of-range rejection (ORR), beyond this limit.

  The authors have reduced two ORR improve- ment techniques to practice which demonstrate hard- ware performance well in excess of this lOlog(r/L2) da limit. They are: 1) a cancellation technique which implements a cancellation channel in addition to the desired signal channel and coherently subtracts the two waveforms after the correlation process, and
2) a phase bias technique which intentionally biases the relative phase difference between the two PN code states at the biphase modulator output slightly from the 180 degrees normally affected, improving the ORR in the correlation process. In both cases, the block diagram implementation was a key ele- ment in successfully reducing the technique to practice.

  The implementation technique central to the suc- cessful implementation of these two ORR improve- ment techniques consists of re-arranging the block diagram of the PN correlation radar from that seen in textbooks and as normally implemented. Specifi- callv. it is imoortant to modulate the PN code onto

the carrier in the LO path of the down convertion mixer where the range correlation process takes place. Figure 1 contains simplified block diagrams of the three approaches used to implement a PN coded radar including: a) Demodulator at Video, b) Demod- ulator in the Mixer RF Path, and c) Modulator in the Mixer LO Path. The Demodulator at Video cir- cuit, Figure la, has been used in several Target Detecting Devices for missiles and is the approach found in most of the literature on radar and com- munications. The Demodulator in the Mixer RF path is another possible radar approach which is not widely used and is shown for completeness. The Modula- tor in the Mixer LO Path circuit, Figure lc, is the approach recommended here to successmlly imple- ment the ORR improvement techniques in hardware.

  The transmit modulation path is the same in all three block diagrams in Figure 1. Typically an RF generator 11,21,31 generates an RF carrier frequency which is sent to the tra...