Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-09-02
2001-05-29
Tse, Young T. (Department: 2634)
Pulse or digital communications
Spread spectrum
C370S206000
Reexamination Certificate
active
06240122
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a receiving apparatus in a code spreading system, and, more particularly, to a receiving apparatus that has as few circuit portions as possible which process a reception signal of a high frequency band.
2. Description of the Related Art
CDMA (Code Division Multiple Access), one of spread spectrum communication systems, is receiving an attention as a communication system which can produce a greater number of channels within a limited frequency in portable telephone communication, fast data communication and so forth. In this CDMA communication system, communication between two communication units is carried out with a transmission signal spread with a specific spreading code in the frequency band of the communication channel. As a result, a signal to be communicated is in the same frequency band as other signals to be communicated, and is separated from them only by the specific spreading code. That is, a transmitted signal is subjected to spread demodulation (despreading) with a spreading code on the reception side, thereby obtaining the original transmission signal from the communication channel. The transmission signal spread with the spreading code is subjected to digital modulation such as quadrature phase shift keying and is then transmitted. The digital modulation like quadrature phase shift keying can make the frequency band of the transmission wave as narrow as possible and can reduce interference with other transmission waves on the same communication channel.
Because the code spreading process, quadrature phase shift keying and the like are all carried out with digital signals, recently, those processes are generally executed by a digital signal processor.
FIG. 7
is a diagram exemplifying a conventional receiving apparatus of a code spread communication type. In this prior art, a communication signal received at a reception antenna
1
is passed through a band-pass filter
2
to yield a signal of the frequency band of the communication channel, which is in turn amplified by a low-noise amplifier
3
. A multiplier
4
multiplies the amplified signal by a signal Lo
1
of a local frequency f
L
so that the carrier frequency f
R
is dropped to an intermediate frequency f
M
. As a result, the frequency becomes f
R
−f
L
and f
R
+f
L
, and the resultant signal is then passed through a band-pass filter
5
to yield a signal
6
of the intermediate frequency f
M
=f
R
−f
L
.
Then, multipliers
7
and
8
multiply this signal by a signal whose frequency is a second local frequency Lo
2
shifted by &pgr;/2, thereby accomplishing orthogonal detection (orthogonal demodulation). Specifically, a transmission signal, which has undergone orthogonal modulation on the transmission side, is multiplied by a signal having the second local frequency Lo
2
shifted by &pgr;/2, thus yielding an I component signal and a Q component signal, both of intermediate frequencies. Those two component signals pass through the respective low-pass filters and are converted to digital signals by AD (Analog-to-Digital) converters
56
and
58
, respectively. The digital signals are then input to a digital signal processor
70
. In the digital signal processor
70
, a code spread band-band signal is subjected to spread demodulation or despreading (code correlation) with a spreading code to return to the original digital signal.
According to this prior art, however, spread demodulation using a spreading code is carried out in the digital signal processor
70
at the last stage, so that the circuit up to the digital signal processor
70
should process signals of a dynamic range which include signals of other channels of the same frequency band. Particularly, the greater the multiplicity becomes by increasing the number of channels, the wider the required dynamic range and the band width of signals become. It is thus necessary to secure a sufficiently wide dynamic range for the analog circuit extending from the reception antenna
1
to the digital signal processor
70
. This demands a large SN ratio.
Further, spread demodulation or despreading (code correlation) with a spreading code requires fast digital processing. In particularly, as the multiplicity gets higher, the demand for this fast digital processing is further intensified, which results in increased power dissipation of the digital signal processor
70
. This is contradictory to the demand for portable communications terminals or the like to reduce power consumption due to which the operation speed of the digital signal processor cannot be made sufficiently fast.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the present invention to provide a receiving system of a code spread communication type which need not widen the dynamic range of an analog circuit.
It is another object of this invention to provide a receiving system of a code spread communication type which does not employ digital processing in spread demodulation or despreading (code correlation) with a spreading code.
It is further object of this invention to provide a receiving apparatus of a code spread communication type which can directly perform code despreading and orthogonal demodulation on a high-frequency reception signal.
To achieve the above objects, a receiving apparatus of a code spread communication type according to this invention includes:
a reception unit for receiving a transmitted signal;
a demodulation signal generator for performing orthogonal modulation on a spreading code assigned to a communication channel, generating a reciprocal of a complex signal having undergone the orthogonal modulation, and multiplying the reciprocal of the complex signal by a local frequency signal to generate a demodulation signal; and
a first multiplier for multiplying the transmitted signal, received at the reception unit, by the demodulation signal to perform spread demodulation and orthogonal demodulation.
With this structure, spread demodulation and orthogonal demodulation can be performed simultaneously by multiplying a reception signal of a high frequency by the aforementioned demodulation signal. Therefore, an analog circuit at the subsequent stage need only to process a signal of a low frequency, thus making it possible to narrow the dynamic range. As the spread demodulation process is carried out by an analog circuit, a burden on a digital processing unit at the subsequent stage can be reduced.
In the above receiving apparatus, the demodulation signal generator may include:
a spreading code generating section for generating a spreading code in phase with one on a transmission side;
a serial-parallel converting section for performing serial-parallel conversion on the spreading code to convert the spreading code to a plurality of symbol points;
a reciprocal converting section for converting a complex number including an I component and a Q component of each of the converted symbol points to a reciprocal; and
a second multiplier for multiplying an I component and Q component of the reciprocal by the local frequency signal whose phase is shifted by &pgr;/2.
To achieve above objects, a receiving apparatus of a code spread communication type according to this invention includes:
a receiving unit for receiving a transmitted high-frequency signal acquired by subjecting a transmission data signal to code-spreading, orthogonal modulation and then frequency modulation;
a demodulation signal generator for performing orthogonal modulation on a spreading code assigned to a communication channel, and generating a demodulation signal obtained by multiplying a complex signal undergone the orthogonal modulation or a reciprocal signal of the complex signal by a local frequency signal; and
a demodulator for detecting a polarity of the reception signal received by the receiving unit in response to a phase timing of the demodulation signal and detecting the transmission data signal in accordance with the detected polarity.
According to this invention, the receiving apparatus
Fujitsu Limited
Staas & Halsey , LLP
Tse Young T.
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