Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-02-17
2004-09-21
Deppe, Betsy L. (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C370S342000
Reexamination Certificate
active
06795488
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spread spectrum communication apparatus in which a spread spectrum signal obtained by performing a code division multiplex with orthogonal codes for transmission data is transmitted and the spread spectrum signal is received to reproduce the transmission data.
2. Description of the Related Art
To transmit data of a plurality of channels, a division multiplex is generally performed for the data. As a method of the division multiplex, a frequency division multiplex (FDM) method, a time division multiplex (TDM) method and a code division multiplex (CDM) method are well-known.
In the CDM method, a time-frequency-power space, in which orthogonal codes are spread, is divided into a plurality of sub-spaces corresponding to code channels along the power axis, an orthogonal transformation for data of each code channel is performed by using an orthogonal code spreading within one limited power zone of the time-frequency-power space, so that orthogonally-transformed transmission data is obtained for each code channel. Because a data rate and a weighting rate can be easily set for the transmission data for each code channel in the CDM method, this CDM method is appropriate for a hierarchical transmission.
In the CDM method used for a broadcasting field, a plurality of code channels are used for broadcast data, a weighting factor applied for the broadcasting data is set for each code channel, a broadcast signal is transmitted to a receiving side through each code channel, and the number of code channels, through which the broadcast signals added to each other on the receiving side are transmitted, is adjusted according to the quality of the broadcast signals received on the receiving side. Therefore, a graceful degradation for the broadcast data can be performed. A digital video signal transmission method performed in this graceful degradation has been studied for practical use.
In a mobile communication field, IS-95 method standardized as a code division multiple access (CDMA) cellular telephone system, in which a direct sequence (DS) type spread spectrum is used, is well-known. In this IS-95 method, code channels such as a control code channel and a communication code channel are used, control information and audio information are orthogonally coded with an orthogonal code in each code channel on a transmission side, the orthogonally coded information transmitted from the transmission side through one code channel is demodulated according to a communication procedure on a receiving side in a RAKE reception using a plurality of fingers, so that a communication quality for the control information and the audio information can be improved.
The RAKE reception is briefly described. The RAKE reception is a signal receiving processing peculiar to a spread spectrum communication method, and a path diversity reception can be performed in the RAKE reception.
In a digital communication such as a spread spectrum communication method, when a transmission signal is transmitted from a transmission side, a direct incoming wave directly incoming from the transmission side and a reflected wave transmitted from the transmission side while being reflected by buildings are generally received on a receiving side. In this case, because there are many paths for the reflected wave, a plurality of reflected incoming waves passing many transmission paths are received as a receiving signal on the receiving side. Therefore, a plurality of incoming waves, which each have a propagation delay time depending on the transmission path, are received on the receiving side. As a result, the incoming waves originated in the transmission signal interfere with each other, so that a receiving disturbance occurs on the receiving side.
However, in cases where a pseudo noise (PN) code used for the spread spectrum of the transmission signal is offset in point of time, a correlation among PN codes corresponding to various offsets (or phase offsets) is not obtained. This phenomenon that the correlation among the PN codes is not obtained is utilized to avoid the receiving disturbance occurring in a receiving signal composed of a plurality of incoming waves. In detail, in cases where a despread spectrum is performed for the receiving signal with a PN code in a despread spectrum unit of the receiving side after a phase offset corresponding to the propagation delay time of a particular incoming wave is given to the PN code, the despread spectrum is successfully performed only for the particular incoming wave, and the despread spectrum is not performed for the other incoming waves. Therefore, a despread spectrum can be selectively performed for each incoming wave without the interference of the incoming waves with each other by giving a phase offset corresponding to the propagation delay time of each incoming wave to the PN code.
Accordingly, in cases where a plurality of despread spectrum units are arranged in parallel to each other, a despread spectrum can be simultaneously performed for a plurality of incoming waves having different propagation delay times in the despread spectrum units by giving a phase offset corresponding to the propagation delay time of each incoming wave to the PN code in each despread spectrum unit, and a plurality of despread spectrum signals independent of each other can be obtained from the incoming waves.
The despread spectrum signals are weighted with weighting factors and are added to each other in an addition unit, so that a superior demodulated signal corresponding to the transmission signal can be obtained.
The spread spectrum signal receiving method described above is called a RAKE reception. Because a plurality of incoming waves of a receiving signal passing through different paths are selectively inverse-spread and are added to each other, the RAKE reception can be applied for a path-diversity reception.
Also, a W-CDMA method, in which a wide frequency band is used, is useful as a third generation type radio access method. In this W-CDMA method, a communication of audio data, video data and control data has been proposed by dividing the time-frequency-power space into a plurality of code channels in the code division multiplex.
3. Problems to be Solved by the Invention
In the digital video signal transmission method and the CDMA cellular telephone system described above, the number of code channels allocated to each user is generally fixed, and the incoming waves of the allocated code channels are demodulated in a demodulating unit of each user.
However, in the mobile communication field, though a service performing a high speed data transmission is required as well as a service mainly performing a low speed data transmission such as an audio data transmission, because the high speed transmission is performed in a conventional spread spectrum communication by heightening a data transmission rate for each code channel, there is a problem that an occupied zone for each code channel is increased. Therefore, it is difficult to perform the high speed data transmission in the conventional spread spectrum communication.
Also, conditions of each propagation path change with time in the mobile communication because of a selective fading and a non-selective fading occurring in the transmission signal and an interference between transmission signals of non-synchronized code channels, so that there is a problem that a communication quality for the receiving signal is degraded.
Also, to perform a data transmission based on the CDMA without widening a transmission band of each code channel, a data transmission method, in which a plurality of code channels are allocated to each user, is known. In this case, a degree of interference between the signals of the code channels is increased. Also, in cases where the number of code channels allocated to each user and a transmission electric power per code channel are adaptively changed or in cases where a plurality of code channels are used by a plurality of users, a degree of the inte
Deppe Betsy L.
Lowe Hauptman & Gilman & Berner LLP
YRP Telecommunications Key Technology Research Laboratories Co.,
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