Modulators – Phase shift keying modulator or quadrature amplitude modulator
Reexamination Certificate
2001-12-07
2003-09-16
Choe, Henry (Department: 2817)
Modulators
Phase shift keying modulator or quadrature amplitude modulator
C332S104000, C375S302000
Reexamination Certificate
active
06621366
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to digital modulator systems.
2. Description of the Related Art
Digital modulators find use in a variety of modern communication systems. Cable modems, for example, employ digital modulators to provide high-speed Internet service over bidirectional transmission systems (e.g., coaxial cables and optical fibers). Downstream data has typically been processed with quadrature amplitude modulation (QAM). Although quadrature phase-shift keying (QPSK) is a typical upstream modulation process, other processes (e.g., QAM) are often used to enhance the efficiency (bits/Hz) of the upstream data path. Cable modems have typically operated with an upstream data rate (e.g., 10 Mb/s) in an upstream communications band (e.g., 5-42 MHz) and with a higher downstream data rate (e.g., 36 Mb/s) in a different downstream communications band (e.g., 42-850 MHz).
Realizing digital modulators with extremely high carrier frequencies F
c
is challenging because the Nyquist criteria requires that the system sampling rate R
s
be at least twice the carrier frequency F
c
. For a carrier frequency in the region of 500 MHz, a modem's quadrature modulator must therefore operate at a system rate R
s
of 1 GHz. Design of a 1 GHz modulator generally requires extensive pipelining of the modulator's adders and multipliers with consequent increase of modulator costs.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to digital modulator structures and methods that can operate at extended system rates R
s
and thus realize high carrier frequencies F
c
.
These goals are realized with modulator structures and methods that facilitate the use of M quadrature modulators which modulate, at a reduced modulation rate R
s
/M, respective ones of M polyphase cosine elements and M polyphase sine elements with respective ones of interpolated I elements and interpolated Q elements to thereby form M polyphase modulated elements.
Modulator embodiments of the invention are thus significantly easier to realize for a high carrier frequencies F
c
and their structure can be substantially simplified.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.
REFERENCES:
patent: 5638401 (1997-06-01), Jones
patent: 5781076 (1998-07-01), Iwamatsu et al.
patent: 6240142 (2001-05-01), Kaufman et al.
patent: 6384677 (2002-05-01), Yamamoto
Analog Devices Inc.
Choe Henry
Koppel, Jacobs Patrick & Heybl
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