Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1997-11-14
2001-08-14
Urban, Edward F. (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S515000, C455S164100
Reexamination Certificate
active
06275699
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a terminal of a mobile communication system which is capable of performing automatic frequency control (AFC) for the first time when a receive signal is normally demodulated as in a code division multiple access (CDMA) system, and more particularly to a technique by which a transmit frequency of a reference oscillator is adjusted.
In recent mobile communication systems, there has been used a high frequency of 900 MHz to several GHz. For that reason, even though a high-accuracy TCXO (temperature compensated crystal oscillator) which is about 3 ppm in frequency error is employed, a frequency error of 3 kHz or more may occur. However, because such a frequency error makes it difficult to demodulate a receive signal, a higher accuracy of the TCXO is being studied. On the contrary, a demand for the higher accuracy brings an expensive mobile communication terminal. Under those circumstances, there is required a technique by which a reference frequency that is oscillated by the TCXO is adjusted in conformity with the receive signal.
A conventional example for adjusting the reference frequency in conformity the receive signal is disclosed, for example, in Japanese Patent Unexamined Publication No. Hei 6-326740.
FIG. 1
is a block diagram for explanation of this conventional example.
In
FIG. 1
, a receive wave received by an antenna
1
is converted in frequency into an intermediate frequency band signal and then inputted to a demodulator
3
. The intermediate frequency band signal which has been inputted to the demodulator
3
contains a frequency error based on the output frequency error of a reference oscillator
6
. The demodulator
3
demodulates the intermediate frequency band signal to output demodulated data as well as frequency data indicative of a frequency of a regenerative carrier wave. The frequency data is supplied to a frequency error detecting circuit
4
.
The frequency error detecting circuit
4
detects a frequency error between the frequency data and a frequency of a predetermined intermediate frequency band signal. A detection output from the frequency error detecting circuit
4
is supplied to a reference oscillator control circuit
5
. The reference oscillator control circuit
5
produces a frequency error compensation signal (a reference oscillator control signal) for compensating the frequency error and inputs the frequency error compensation signal to a reference oscillator
6
to control the transmit frequency of the reference oscillator
6
until the frequency error becomes a predetermined value or less, thus conducting stabilizing operation. The frequency error detecting circuit
4
may be formed of a frequency counter or the like. The reference oscillator
6
outputs a signal as a reference of the output frequency of a frequency synthesizer.
As mentioned above, the conventional system employs a method in which the frequency error is detected by some manner, and the detected frequency error is fed back to the output frequency of the reference oscillator, to thereby reduce the frequency error.
By the way, in a mobile communication system using a CDMA which is represented by IS95 system of U.S.A., or the like, a spread spectrum system is used as the modulation system, and a channel is determined by a spread code. Therefore, a plurality of code channels exist in the identical frequency channel so that signals different from each other are transmitted through the respective code channels. Also, since base stations also use the identical frequency channel, various base stations and code channels are mixed in an electric wave which is received by a terminal. This is largely different between the normal analog system or TDMA-digital system and the CDMA system.
Therefore, the CDMA system is required to extract a signal transmitted to an operator from a plurality of signals contained in the identical frequency channel. To achieve this, complicated processing such as base station search, synchronization or spectrum de-spreading are required.
There is proposed that the conventional frequency adjusting method is used in the CDMA system. This method suffers from a first problem that the frequency error can be detected for the first time after all of the complicated processing such as base station search, synchronization or spectrum de-spreading has been well performed. In other words, the processing such as base station search, synchronization or spectrum de-spreading must be well performed before frequency adjustment is conducted.
The case where the conventional frequency adjusting method is used in the CDMA system also suffers from a second problem that an error in the reference frequency must be sufficiently small in order to normally conduct the processing such as the base station search, synchronization or spectrum de-spreading.
As mentioned above, the first and second problems have no order of priority, and are contradictory to each other in that one of those problems cannot be solved without the solution of the other problem.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems with the conventional system, and therefore an object of the present invention is to provide a technique by which an error in frequency is well squeezed from an initial state where a power supply is switched on, and a base station is grasped.
In order to achieve the above object, according to the present invention, there is provided a mobile communication terminal, which comprises:
a receive circuit for receiving an electric wave which is transmitted from a base station;
local signal generating means for outputting a local signal to the receive circuit;
a reference oscillator for transmitting a reference signal indicative of a reference of an oscillation frequency of the local signal generating means;
reference oscillator control means for generating a control signal that controls the transmit frequency of the reference oscillator;
demodulating means for demodulating a signal received by the receive circuit;
base station search means that inputs a demodulated signal outputted from the demodulating means for searching an identifying signal indicative of the base station which is included in the demodulated signal to detect the presence/absence of the base station; and
frequency offset generating means that changes the control signal generated by the reference oscillator control means according to a search result of the base station search means for offsetting the transmit frequency of the reference signal oscillator.
Also, in the case where the identifying signal indicative of the base station can be correctly detected by the base station search means, the base station search means stops the frequency offset generating means from offsetting the transmit frequency of the reference signal oscillator.
The mobile communication terminal further includes frequency error detecting means that inputs the demodulated signal outputted from the demodulating means for detecting an error in the frequency of the reference oscillator, wherein after a function of offsetting the output frequency of the reference signal oscillator by the frequency offset generating means is stopped, the control signal generated by the reference oscillator control means is produced according to a frequency error signal outputted by the frequency error detecting means to control the transmit frequency of the reference signal oscillator.
Moreover, in one mode of the present invention, the mobile communication terminal includes, in addition to the above structural elements, storage means for storing the control signal which is outputted to the reference oscillator control means so that data stored in the storage means can be used as an initial value of the control signal at the time of turning on a power supply of the moving communication terminal.
The mobile communication terminal according to the present invention is designed such that the frequency is adjusted on a terminal of the moving communication system which is required for
Foley & Lardner
NEC Corporation
Urban Edward F.
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