Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-07-30
2001-01-23
Bocure, Tesfaldet (Department: 2731)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S326000, C375S343000
Reexamination Certificate
active
06178197
ABSTRACT:
BACKGROUND
The invention relates to processing a spread spectrum signal.
In wireless systems, information typically is transmitted by modulating the information onto carrier waves having frequencies that lie within preassigned frequency bands. Radio frequency (RF) receivers demodulate the carrier waves to recover the transmitted information.
Spread spectrum communication systems spread transmitted signals over bandwidths much larger than those actually required to transmit the information. Spreading a signal over a wide spectrum has several advantages, including reducing the effects of narrow band noise on the signal and, in many situations, providing increased protection against interception by unwanted third parties. In a direct sequence spread spectrum (DSSS) system, the bandwidth of a transmitted signal is increased by modulating the signal onto a known pseudo-noise (PN) signal before modulating onto the carrier wave. The PN signal typically is a digital signal having an approximately equal number of high and low bits (or “chips”), which maximizes the spectrum over which the signal is spread. A typical implementation of a DSSS receiver recovers the transmitted information by demodulating the carrier wave and then multiplying the resulting signal with a local replica of the PN signal to eliminate the PN signal. The DSSS technique offers heightened security because the receiver must know the PN sequence used in the transmission to recover the transmitted information efficiently. Other spread spectrum techniques include frequency hopped spread spectrum (FHSS).
A DSSS receiver must be tuned to the carrier frequency of the signal to be received. In many systems (e.g., GPS and CDMA cellular telephony) the received signal is continuous or of long duration, so that it is practical for the receiver to use narrow-bandwidth phase-lock techniques to track the carrier frequency of the incoming signal. But in a data collection system, such as might be used for wireless meter reading, the incoming signal is typically a short packet whose carrier frequency is subject to considerable uncertainty due to the low cost and simplicity of the transmitter. In such a case, phase-lock techniques become difficult, and the wider bandwidths required for fast acquisition render them less advantageous due to increased noise.
Receivers in a short-packet system where phase-lock techniques are impractical and where frequency tracking is required must somehow measure the frequency of the incoming signal and gain the information needed to tune onto the carrier frequency. Typically, such receivers use a frequency discriminator to measure the carrier frequency of the incoming signal. Commonly-used analog discriminator techniques include delay lines and stagger-tune detectors, while digital techniques include dual differentiators and arctangent algorithms. However, if a system is otherwise designed only to measure and report the magnitude response to DSSS signals of a particular code phase and frequency, each of these frequency measurement techniques requires the addition of system hardware that is dedicated to the task of frequency discrimination.
SUMMARY
The invention provides a way to simplify and reduce the cost of frequency discrimination by using hardware already incorporated in a wireless system for other purposes. For example, the same correlators used to recover modulated data from a spread spectrum signal can be used to determine the signal's carrier frequency relatively precisely, e.g., to within 2 kHz, from a large interval (e.g., 30 kHz) of frequency uncertainty. Only a few correlators covering narrow frequency intervals are needed to discriminate the carrier frequency this precisely. Calculating the carrier frequency in this manner is quicker and easier than traditional frequency determination techniques, including the “dual differentiator” technique., which requires the differentiation and cross-multiplication of in-phase and quadrature signals, and the “arctangent technique,” which requires the computation and subsequent differentiation of the tangent of the phase angle implied by the in-phase and quadrature components of the spread spectrum signal. The invention can be implemented with simple, inexpensive hardware components that are realized in a variety of technologies such as application-specific integrated circuit (ASIC) technology, or with a relatively simple set of instructions for use in a microprocessor or microcontroller.
In some aspects, the invention involves determining an actual carrier frequency of a spread spectrum signal. A pair of correlators is tuned to a pair of search frequencies and then used to correlate the spread spectrum signal against one or more reference signals to produce a pair of correlation magnitude values. The ratio of the difference between the correlation magnitude values to the sum of the correlation magnitude values determines the offset between the actual carrier frequency and an estimated carrier frequency.
In some embodiments, one of the search frequencies is greater than the estimated carrier frequency and the other search frequency is less than the estimated carrier frequency. In other embodiments, both of the search frequencies are greater than the estimated carrier frequency. In still other embodiments, both of the search frequencies are less than the estimated carrier frequency.
Some embodiments involve correlating the spread spectrum signal against the reference signal over multiple correlation periods to produce multiple correlation magnitude values for each correlator. For each correlation period, the difference and the sum of correlation magnitude values for the pair of correlators is calculated. In some cases, the ratio of the difference to the sum of the correlation magnitude values is calculated for the pair of correlators during each correlation period.
Other embodiments involve tuning at least one additional correlator to another search frequency and correlating the spread spectrum signal against the reference signal to produce at least one additional correlation magnitude value. In many cases, the search frequencies are evenly spaced. The ratio is determined using the difference and the sum of correlation magnitude values for a chosen pair of the correlators tuned to adjacent search frequencies. The offset is proportional to the ratio and includes a constant determined by which pair of the correlators is chosen. In some cases, a cumulative total of correlation magnitude values is calculated for each pair of correlators tuned to adjacent search frequencies. The chosen pair of correlators often includes the correlators that produce the highest cumulative total of correlation magnitude values. The estimated carrier frequency often is equidistant from the highest and lowest search frequencies.
Other embodiments and advantages will become apparent from the following description and from the claims.
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pat
Froelich Robert K.
Fulton Forrest
Bocure Tesfaldet
CellNet Data Systems, Inc.
Fish & Richardson P.C.
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