Pulse or digital communications – Spread spectrum
Reexamination Certificate
1999-03-08
2002-07-16
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Reexamination Certificate
active
06421369
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a receiver and receiving method for spread spectrum signals, and more particularly, is preferably applied to a radio communication system such as a portable telephone system.
2. Description of the Related Art
In a cellular radio communication system, an area of providing a communication service is divided into cells of desired size and every base station serving as a fixed radio station is installed in a cell. A portable telephone as a mobile radio station radio-communicates with the base station in the cell in which the portable telephone itself is located.
There have been proposed various kinds of systems as a communication system between the portable telephone and the base station. One of the systems is a code division multiple access (CDMA) system.
In The CDMA system, a pseudo noise sequence (PN) code having a characteristic pattern composed of a pseudo random number series code is assigned to each of communication lines in a transmitting side. The assigned PN code is multiplied by a primary modulation signal of the same carrier frequency, so that the PN code is spread to a band wider than an original frequency band (this is called a spectrum diffusion hereinafter) and a secondary modulation signal subjected to the spectrum diffusion is transmitted.
On the other hand, a receiving side, receiving a transmitted signal sent from the transmitting side, multiplies the received signal by the PN code having the same series pattern and the same phase as those of the PN code assigned to each of the communication lines in the transmitting side, so that the received signal is subjected to an inverse diffusion process to obtain a primary modulation output. In addition, the primary modulation output is demodulated so as to restore transmitted data.
As described above, according to the CDMA system, the same PN code is previously set to be mutually generated in the transmitting side and the receiving side. In the receiving side, only when the inverse diffusion process is carried out by using the PN code having the same series pattern and the same phase as those of the PN code used in the transmitting side, the primary modulation output can be demodulated, and therefore, an excellent secrecy can be advantageously achieved.
Further, in a cellular radio communication system utilizing the CDMA system, the base station of the transmitting side repeatedly transmits pilot signals generated by diffusing data comprising all “1” or “0” with the PN code in order to get a synchronization, to track a synchronization (tracking) and to reproduce a clock in a mobile station side. The mobile station of the receiving side first receives the pilot signals constantly sent from the base stations at the time of turning power on.
The mobile station of the receiving side receives a plurality of pilot signals transmitted in a multipass through a plurality of transmission paths in a superposed state to obtain receiving timings different from each other for the respective pilot signals. Then, when an actual talking is started, the mobile station multiplies the received signals in the multipass (referred as multipass signals, hereinafter) which are received in a delayed state with a plurality of demodulators provided therein by the respective PN codes having the phases corresponding to the obtained receiving timings in order to perform an inverse diffusion process. A combiner combines the plurality of inverse-spread received signals in a state in which their timings are set to the same timing. Thus, a signal-to-noise power ratio (SN ratio) of the demodulation signal is improved. That is, the mobile station is designed to constitute a Rake receiver which prevents direct waves in a multipass and reflection waves from interfering with each other so as to lower electric power.
For instance, in the CDMA system which has been already standardized in U.S.A. as an IS-95 system, as illustrated in FIG.
1
, a mobile station
1
of the receiving side receives pilot signals P
1
to P
3
in a multipass which are sent from an object base station
2
with a time delay due to the reflection of buildings
3
and
4
. Here, the pilot signals P
1
to P
3
, which are received by the mobile station
1
, have the same series pattern, however, have different phase shifts due to the time delay.
The mobile station
1
calculates the correlation values of the pilot signals P
1
to P
3
and the respective PN codes generated therein with a circuit called a searcher (not shown) provided therein while moving phases of the PN codes, so that the phases of the pilot signals P
1
to P
3
are detected. Then, the mobile station
1
synchronizes them with a system clock cn to all the base station and mobile stations in a whole system with the pilot signal P
1
having the largest correlation value as a reference. Thus, the pilot signals P
1
to P
3
are composed of the PN codes whose cycles have the same series pattern of 32768 series (2
15
), and they have a common position of a phase “0” and the phases of the pilot signals are shifted respectively by several tens of chips.
In the pilot signals P
1
to P
3
, the direct wave is the largest correlation value. As the arrival time of the reflection wave is late, the correlation value of the reflection wave becomes smaller than that of the direct wave. This is associated with the phase shift representing the difference of arrival time among the pilot signals P
1
to P
3
which reach the mobile station. As a matter of course, the pilot signal P
1
being the direct wave, which has the largest correlation value, has the smallest phase.
When the mobile station starts an actual talking after exchanging control data including such pilot signals P
1
to P
3
with the base station, it initially detects received signals (multipass signals) S
1
, S
2
and S
3
in a multipass state by calculating the correlation values with a searcher, as shown in FIG.
2
.
Then, when the mobile station detects the multipass signals S
1
to S
3
, the multipass signal S
1
having the largest correlation value among them is used as a reference signal. The mobile station periodically detects (referred to as a steady search, hereinafter) with the searcher whether or not the multipass signals S
1
to S
3
exist within an arbitrary search window range called a “search window” having the phase position
64
of the reference signal at a center which is designated by the base station. When the mobile station can detect the multipass signals S
1
to S
3
, it demodulates transmitted data by employing the multipass signals S
1
to S
3
(phase positions
64
,
68
and
70
) which are the three highest signal strength.
Here, the “search window” is determined to range between ±20 of the multipass signal S
1
serving as the reference signal S
1
(from
44
to
84
in the phase position). The mobile station always carries out the steady search within the above-described search window range even during receiving of real data.
Since it is generally difficult to consider that there exists an extremely big time difference between the multipass signal S
1
at the phase position
64
which arrives at the mobile station first, and the multipass signals S
2
and S
3
which arrive with a delay due to the reflection of buildings, in this case, a search window range is determined so as to detect the multipass signals S
1
to S
3
, which are the three highest signal strength, by searching the phase range between ±20 of the multipass signal S
1
at the phase position
64
, which arrives first and is served as the center.
As described above, according to the CDMA system of the IS-95 system, the multipass signal S
1
at the phase position
64
, which arrives at the mobile station first, is used as the reference signal, and the time management such as the acquisition of synchronization, the tracking of synchronization, the reproduction of clocks, etc. is performed based on the reference signal.
However, in the case where the reference signal on lost due to the chang
Iwaskai Jun
Watanabe Nobuhiko
Yamamoto Katsuya
Bayard Emmanuel
Maioli Jay H.
Pham Chi
Sony Corporation
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