Browse Prior Art Database

ACCELERATED COARSE ACQUISITION METHOD

IP.com Disclosure Number: IPCOM000009842D
Original Publication Date: 2000-May-01
Included in the Prior Art Database: 2002-Sep-23
Document File: 3 page(s) / 162K

Publishing Venue

Motorola

Related People

Mark J. Johnson: AUTHOR [+2]

Abstract

In many communication systems, there exists an issue with initial detection of a signal on a channel. The purpose of the initial detection is to find and characterize a signal for demodulation. This detection must be sensitive enough to characterize any signal which could be demodulated, while at the same time algorithmically simple enough to be run as a background process in a portable product without draining power.

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MOTOROLA

Technical Developments

ACCELERATED COARSE ACQUISITION METHOD

by Mark J. Johnson and Ken Conroy

ABSTRACT

In many communication systems, there exists an issue with initial detection of a signal on a channel. The purpose of the initial detection is to find and characterize a signal for demodulation. This detection must be sensitive enough to characterize any signal which could be demodulated, while at the same time algorithmically simple enough to be run as a background process in a portable product without draining power.

It is preferable to use coherent averaging to detect such a signal. At baseband (zero IF), the digitized signal can be represented as a stream of complex samples. It is important that the samples be complex since the phase of the incoming signal is unknown. Further, the phase of the incoming signal may vary continuously as a result of the frequency offset betWeen the transmitter and receiver. Often pilot signals are embedded in the transmitted signal to facilitate coherent averaging. Coherent averaging adds the samples as vectors, rather than just adding the amplitudes, in order to increase sensitivity by taking advantage of the phase-ordered nature of the desired signal.

One of the most annoying problems with coherent averaging is the frequency offset between the transmitter and receiver, which results in a gradual rotation of the signal phase over the duration of the averaging period. Since the samples are no longer perfectly phase-ordered, their vector sum is less than it would otherwise be. This results in failure to initially detect signals which could otherwise be decoded successfully. Many systems include a hardware mechanism to fine-tune the frequency offset in response to the detection algorithm, allowing it to "search" for a signal at different frequency offsets. This gains some sensitivity at the expense of product cost and search time.

Motorola. Inc. 2000

This paper proposes a simple algorithm, easily implemented in digital hardware or on a DSP with multiply capability, which circumvents the frequency offset issue and allows initial detection and frequency characterization of a signal without recourse to changing the receiver characteristics.

BACKGROUND

A complex vector can be represented as having an amplitude and a phase, and can be represented by:

X = Aeie

where A is the amplitude of the vector and 8 is the phase. This vector has a complex conjugate

X* = Ae-j9

If we assume another vector

y = Dei+

then we can get the phase difference by multiplying one vector by the complex conjugate of the other:

X*y = Ae-jeDei+

= ABe-je ei+ = ABe -j9+j+ = ABei(+-e)

The resulting vector has an amplitude equal to the product of the tWo amplitudes, and a phase equal to the difference of the two phases.

159

May 2000

MOTOROLA

Technical Developments

If the complex multiplication is perfonned without the conjugate operation, the phase will be the sum of the two phases. Although a complex sigual is generally represented in car...