Pulse or digital communications – Equalizers – Automatic
Patent
1993-09-28
1996-02-27
Chin, Stephen
Pulse or digital communications
Equalizers
Automatic
375229, 375234, 375235, 3647242, 381103, H03H 730, H03H 740
Patent
active
054955017
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a communication system employing a digital roll-off filter.
BACKGROUND ART
A quadrature amplitude modulation, referred to hereinafter as QAM, has been increasingly employed in a multi-channel digital communication system due to its high efficiency capability. Concept of a system configuration of a typical QAM communication system is schematically illustrated in FIG. 1. Explanation will be given hereinafter representatively with the I channel because the circuit configurations of I and Q channels are symmetrical with each other. In a transmitting station, an internal frequency band (referred to hereinafter as IF) or a base band (both referred hereinafter to representatively as IF) of an I channel and a Q channel are input to input terminals of first digital roll-off filters 102ai and 102aq, respectively. Circuit configuration of roll-off filters will be described later in detail. Frequency characteristics of the first roll-off filters are determined by the tap rating ratios stored in a first read-only memory, referred to hereinafter as a ROM, 113.
Output of first roll-off filter 102ai is input to a digital-to-analog (referred to hereinafter as D/A) converter 131i. Unnecessary higher frequency spectrum generated in the output of D/A converter 131i is eliminated by a low-pass filter 132i. Outputs from low-pass filters 132i and 132q respectively of the I channel and Q channel are input to a QAM modulator 134, to which an carrier frequency signal is also input from an carrier generator 133. A QAM modulated radio frequency signal transmitted via a radio frequency amplifier (not shown in the figure) to a receiving station.
In the receiving station, a preamplifier (not shown in the figure) amplifies and converts the received radio frequency signal to an IF signal, which is then input to a QAM demodulator 154, to which a local frequency signal is input from a local frequency oscillator 156. I channel and Q channel signals output from QAM demodulator 154 is input via a low-pass filter 152i to an analog-to-digital (referred to hereinafter as A/D) converter 151i. Each bit line of a parallel digital signal output from A/D converter 151i is input to a second roll-off filter 102bi. Frequency transmission characteristic of second roll-off filter 102bi is determined by tap rating ratios stored in, and output from, a ROM 6-1.
Frequency characteristics of the first and second roll-off filters are chosen such that overall transmission characteristics, i.e. a total of the frequency characteristics of the two filters in each channel, allow the second roll-off filter to output signal pulses in an adequately good shape which causes no intermodulation in the QAM-modulated signals, as well as to eliminate unnecessary upper frequency spectrum generated from the circuits, such as D/A converter, etc.
Even though the overall frequency characteristics that are the sum of both the first and second roll-off filters are set so that the pulse form at the output at second roll-off filter is in a good shape, fading or some other factors in the transmission system always varies the transmission characteristics, such as frequency vs amplitude, or frequency vs phase-delay, which accordingly deteriorate the pulse forms output from the second roll-off filter.
In order to remedy this deterioration an automatic equalizer 160i is provided at the output of the second roll-off filter 102bi. Output of second roll-off filter 102bi is input via a pulse divider 146i, which returns the signal to have the symbol rate, to a first input terminal of automatic equalizer 160i. Moreover, an output of roll-off filter 102bq of Q channel is input to a second input terminal of automatic equalizer 160i of the I channel. Symmetrically the same cross-connection is done in the Q channel.
Automatic equalizers are formed of roll-off filter, the frequency characteristics of which are variably determined by tap rating ratios given from a calculating circuit 7. Calculating circuit 7 is formed of a micro computer system which monito
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IEE Journal of Solid-State Circuits, vol. 23, No. 5, Oct. 1988, Meier et al. "A2 m CMOS Digital Adaptive Equalizer Chip for QAM Digital Radio Modems".
Iwamatsu Takanori
Mitsuta Norihide
Chin Stephen
Fujitsu Limited
Luther William
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