Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-07-29
2001-07-17
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
C375S145000, C370S335000, C370S342000, C370S516000, C455S524000
Reexamination Certificate
active
06263010
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a spread spectrum communication apparatus used in a digital cellular radio system adopting a code division multiple access (CDMA) method as a radio channel access method.
In recent years, the spread spectrum communication method immune to interference and disturbance attracts attention as one of communication methods applied to mobile communication systems. The spread spectrum communication method is used mainly to implement a cellular radio system adopting the CDMA method.
In the cellular radio system adopting the CDMA method, a transmitting device modulates digitized audio data and image data by using a digital modulation method such as a PSK modulation method, then converts the modulated data to be transmitted to a wideband baseband signal by using a short code, up-converts the wideband baseband signal to a signal of a radio frequency band, and transmits the up-converted signal. The short codes are generated by shifting a phase of a spreading code having a periodicity. For example, a pseudo random noise code (PN code) is used as the spreading code. On the other hand, a receiving device down-converts the received radio frequency signal to a received signal of an intermediate frequency or a baseband frequency, despreads the down-converted signal using same short code used in the transmitting device, then conducts digital demodulation by using a digital demodulation method such as a PSK demodulation method, and thus reproduces received data.
In other words, in the CDMA method, different short codes are assigned to radio communications between a plurality of mobile stations and a base station to ensure channel separation among radio communications.
When a mobile station starts communication in such a system, it is necessary to establish synchronization for the short code of a pilot signal transmitted by a base station. As its synchronization acquisition method, the following method, for example, has been proposed heretofore.
A PN code has a length of, for example, 2
15
(one period=26.6 msec). Each base station generates a PN code to produce the short codes from a preset different chip phase every radio area managed by its own station. The chip phase whereat generation of the PN code is started is called offset.
When power is turned on, each mobile station first generates a short code from its own PN code generator while successively shifting the chip phase from an arbitrarily set initial reference phase. By monitoring a correlation value between this short code and the short code of the pilot signal broadcast from a base station, the mobile station searches for the offset of the PN code used by the base station. If the offset of the PN code used by the base station is acquired by this search, then a sync channel is received and a system identification (SID) for identifying the mobile communication network is checked. If this SID is one corresponding to a home system in which its own station was registered beforehand, then the mobile station proceeds to a waiting state.
However, such a short code synchronizing method heretofore proposed has the following problem to be solved. Whenever power of the mobile station is turned on in the conventional short code synchronizing method, the mobile station generates a PN code from an arbitrarily set initial phase and conducts the operation for acquiring the synchronization of the pilot signal as described above. Even in the case where, for example, the power of the mobile station is temporarily turned off and the power of the mobile station is turned on again without moving from that radio area, therefore, the mobile station necessarily generates a PN code from the initial reference phase and acquires the synchronization of the pilot signal. As a result, it takes a long time to acquire the synchronization of the short code, depending upon the positional relation between the offset of the PN code to be acquired and the initial reference phase of the mobile station.
BRIEF SUMMARY OF THE INVENTION
The present invention has been made in view of the above described situations. An object of the present invention is to provide such a spread spectrum communication apparatus that acquisition of synchronization of the pilot signal can be conducted in a short time with a high probability when a mobile station stays in the same radio area or in its vicinity.
In accordance with one aspect of the present invention, there is provided a spread spectrum communication apparatus for transmitting/receiving digital signals to/from a base station in a spread spectrum form by using a spreading code having a periodicity and having a different offset value for each radio area comprising: spreading code generation means for generating a plurality of short codes by shifting a phase of the spreading code, during a synchronization acquisition interval for a pilot signal transmitted by the base station; signal search means for conducting synchronization acquisition of the pilot signal on the basis of a correlation between a short code of the pilot signal and each of the generated short codes and for recognizing a timing information of the spreading code when the short code of the pilot signal and the generated short code are synchronized; and phase holding means for holding the recognized timing information during power off interval and controlling a phase of generation start of the spreading code conducted by the spreading code generation means on the basis of the held information when power is turned on again.
Even if power of the apparatus is turned off, therefore, the phase information of the spreading code obtained before power is turned off is held during the power off interval according to the present invention. When power is turned on again, the synchronization acquisition operation of the pilot signal is started on the basis of the held phase information. In the case where the communication apparatus stays within the same radio area or in the vicinity thereof, therefore, the synchronization acquisition can be achieved in a shorter time than the conventional technique.
When power is turned on again, the phase of generation start of the spreading code conducted by the spreading code generation means is preferably set to a position delayed from a phase derived on the basis of the phase information held during the power off interval by a predetermined phase value. By doing so, the synchronization acquisition with high certainty considering the propagation delay time difference of the pilot signal from the base station can be conducted, even when the apparatus has largely moved within the radio area during the power off interval, and consequently the distance between the base station and the apparatus at the time when power is turned on again has largely changed from that at the time when power is turned off.
Furthermore, after power is turned on again, the synchronization acquisition operation is preferably monitored. If the synchronization acquisition of the pilot signal of acquisition subject cannot be conducted while the spreading code is shifted by a predetermined value, then preferably the generation phase of the spreading code is reset to a predetermined reference phase, and the synchronization acquisition of the pilot signal is conducted from the reference phase.
By doing so, a shift to the synchronization acquisition from the reference phase is conducted in the case where the pilot signal of acquisition subject could not be acquired during a predetermined phase shift interval for some reasons. Therefore, repetition of the synchronization acquisition operation within a predetermined phase range for an indefinite time is prevented. As a result, the pilot signal can be acquired certainly.
Furthermore, after the synchronization acquisition of the pilot signal of the acquisition subject has been conducted by the signal search means when power is turned on again, the second control means causes the signal search means to conduct signal search operation for another pilot signal including anothe
Nakamura Ken
Naruse Akiko
Obayashi Arata
Bayard Emmanuel
Gray Cary Ware & Freidenrich LLP
Kabushiki Kaisha Toshiba
Pham Chi
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