Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-02-12
2001-10-30
Patel, Ajit (Department: 2661)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000, C455S522000
Reexamination Certificate
active
06310868
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a signal transmission method, a mobile station and a base station in a CDMA communications system for carrying out communications in a code division multiple access scheme.
More particularly, the present invention relates to a signal transmission method by which a mobile base station transmits a signal to a base station in a CDMA communications system, and to a mobile station and a base station for implementing the signal transmission method.
BACKGROUND ART
In the mobile communications system, since a reverse link signal transmitted from a mobile station is affected by distance attenuation, shadowing, fading or the like, the level of the signal received by a base station sharply fluctuates. In the CDMA communications, however, it is of critical importance for the received signal level to be kept constant, and hence the mobile station carries out power transmission control so that the received signal level of the base station is kept constant.
As the power transmission control method, two methods are widely known: an open loop control method and a closed loop control method. The open loop control method is a technique in which a mobile station receives a forward link signal transmitted from a base station, and sets the transmission level of a reverse link signal in response to the received level of the forward link signal. Although it can be implemented with ease, large control errors will result if the correlation between the reverse link signal and the forward link signal is small. In contrast, the closed loop control is a technique in which the base station receives the reverse link signal transmitted by the mobile station, makes a decision whether its received level is higher or lower than a reference value by comparing them, and commands the mobile station to change the transmission level using the forward link signal. Although this method can reduce the control errors in the steady state, the mobile station may continue to transmit with large control error before entering the steady state.
In conventional CDMA mobile communications systems, although the closed loop control method is used in conjunction with the open loop control method, the reverse link signal undergoes only the open loop power transmission control at the beginning of the communications. This will result in large control error, which in turn can cause undue interference to message signals of other mobile stations. Thus, the IS-95 system according to the EIA/TIA standard in the United States sets the transmission power of the initial reverse link signal of the mobile station at a low level to avoid the foregoing problem. If the base station cannot receive the reverse link signal, and hence no channel can be established, the mobile station retransmits the initial signal with increasing the transmission power step by step until the channel is established. Although this method has little possibility to cause the undue interference, it takes a long time until the channel has been established.
In short, a signal transmission method in the CDMA mobile communications has not yet been conceived which can establish the channel in a short delay time without transmitting at unduly large transmission power.
DISCLOSURE OF THE INVENTION
As described above, each mobile station in the CDMA communications system carries out the power transmission control so that the signal level received by the base station is kept constant. The conventional CDMA communications system, however, has a problem in that it cannot achieve highly accurate power transmission control at the initial stage of the communications, and can cause undue interference to the message signals of other mobile stations. To avoid this, the IS-95 system sets the initial transmission power of the reverse link signal at a lower value, and retransmits the initial signal with gradually increasing the transmission power until the channel is established. Although this method has little possibility of causing undue interference, it has a problem in that it takes a long time to establish the channel and wasted signals not received by the base station cause extra interference in spite of their low levels.
The present invention is implemented in view of the foregoing problems. Therefore, an object of the present invention it to provide a signal transmission method, a mobile station and a base station in the CDMA mobile communications system which can establish the channel in a short delay time with avoiding transmission at undue power.
According to the first aspect of the present invention, a signal transmission method of a CDMA mobile communications system carrying out communications between a plurality of mobile stations and a base station by code division multiplex access, the signal transmission method comprises the step of:
setting, in a mobile station, transmission power of an initial portion of a signal to be transmitted to the base station at a value lower than transmission power of a portion following the initial portion.
Here, the signal transmission method of the CDMA mobile communications system may further comprise the step of setting a spreading factor of the initial portion at a value higher than a spreading factor of the portion following the initial portion by setting an information transmission rate of the initial portion lower than that of the following portion.
In the signal transmission method of the CDMA mobile communications system, the step of setting may set, when the plurality of mobile stations randomly access the base station through a common channel at any timings to reserve a channel particular to each mobile station to carry out communications, a signal of the random access through the common channel at a level lower than transmission power of a signal of communications through the channel particular to each mobile station.
In the signal transmission method of the CDMA mobile communications system, a signal transmitted from the base station in response to the signal of the random access through the common channel may include a signal for designating transmission power of a signal transmitted by the mobile station thereafter.
In the signal transmission method of the CDMA mobile communications system, the signal transmission method may be applied to a CDMA mobile communications system in which a mute portion of speech is not sent and a voice signal in a speech spurt portion is encoded into a burst signal to be transmitted, and wherein the step of setting sets a spreading factor, transmission power and a burst length of the burst signal which is transmitted at first after transition from the mute portion to the speech spurt portion at a higher spreading factor, lower transmission power and a longer burst length than those of the following burst signals.
According to the second aspect of the present invention, a mobile station of a CDMA mobile communications system in which a plurality of mobile stations randomly access a base station through a common channel at any timings, and subsequently reserve a channel particular to each mobile station to carry out communications, the mobile station comprises:
means for setting a spreading factor and transmission power of a signal of the random access through the common channel at a higher spreading factor and lower transmission power than those of a signal of communications through the channel particular to each mobile station.
In the mobile station of the CDMA mobile communications system, the mobile station may transmit a random access signal through the common channel, and subsequently receives from the base station a signal designating transmission power, and further comprises means for setting transmission power of a signal to be transmitted at a transmission power value designated by the signal when communicating through the channel particular to the each mobile station.
According to the third aspect of the present invention, a mobile station of the CDMA mobile communications system which suppresses transmission of a mute portion of speech and encod
Tanno Motohiro
Uebayashi Shinji
Brown & Raysman Millstein Felder & Steiner LLP
NTT Mobile Communications Network Inc.
Patel Ajit
Phunkulh Bob A.
LandOfFree
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