Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-08-10
2003-07-22
Chin, Vivian (Department: 2682)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S335000, C370S477000
Reexamination Certificate
active
06597674
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a handover method, a base station, a mobile station, and a mobile communication system according to code division multiple access (CDMA).
In a mobile communication system, an exchange station (a base-station controlling station) is connected to base stations. Further, a mobile station may move during communication from the service area of a base station to the service area of another base station. At this time, a handover technique is employed for the mobile station to switch the base station to another without interrupting the communication of the mobile station. Therefore, it is desirable to improve the efficiency of the handover technique.
FIG. 12
shows an example of a handover technique. A mobile communication system includes base stations
101
1
,
101
2
, and the like, an exchange station
102
, and a mobile station
103
. The mobile station
103
communicates with a fixed station or another mobile station through the base station
101
1
, while the mobile station moves toward the service area of the base station
101
2
as indicated with an arrow mark. At this time, the exchange station
102
and base station
101
1
,
101
2
transmit and receive control information to hand over the mobile station
103
from the base station
101
1
to the base station
101
2
so that the mobile station
103
may continue the communication.
The above described stations to carry out the handover may be part of a CDMA mobile communication system. If the operating frequencies f
1
, f
2
of the base stations
101
1
,
101
2
are equal to each other, it is possible to achieve a soft handover by switching the base stations
101
1
,
101
2
from one to another while maintaining the communication of the mobile station
103
through both the base stations
101
1
,
101
2
. If the operating frequencies f
1
, f
2
are different from each other, a hard handover is required since the reception frequency of the mobile station
103
must be changed.
When being handed over from the base station
101
1
to the base station
101
2
in a CDMA mobile communication system, the mobile station
103
finds a time difference between a traffic channel used to communicate with the base station
101
1
and a perch channel of the base station
101
2
, and then informs the base station
101
1
of the time difference. The base station
101
1
then informs the base station
101
2
of the time difference through the exchange station
102
. According to the time difference, the base station
101
2
adjusts transmission-reception timing with respect to the mobile station
103
and starts to communicate with the mobile station
103
.
In order to perform the soft handover without interrupting the communication of the mobile station
103
, the base station
101
2
must obtain a long code phase of the mobile station
103
. The mobile station
103
has a long code period of 2
9
×72 frames, and thus the base station
101
2
is unable to automatically obtain the long code period. Accordingly, the mobile station
103
measures a phase difference between the perch channel of the base station
101
2
and an outgoing traffic channel.
FIG. 13
shows an example of a signal format employed by perch channels. In
FIG. 13
, (a) shows a system frame having a period of 36,864 ms, (b) is a radio frame having a period of 10 ms, (c) is a 0.625-ms radio slot, (d) is a first perch channel signal in the radio slot having a pilot PL, data, and a long code mask symbol LM, and (e) is a second perch channel signal in the radio slot having only a long code mask signal LM. In
FIG. 13
, (f) and (g) show the signal format and long code of the first perch channel, while (h) and (i) show the signal format and group short code of the second perch channel. The time axis of (d) and (e) differs from that of (f) to (i).
One radio frame (b) consists of 16 radio slots, and one super frame (a) consists of 72 radio frames. The first and second perch channels are each made of slot periods. The first perch channel signal of (d) and (f) is spread according to the long code of (g). The long code mask symbol LM of the second perch channel signal of (e) and (h) is spread according to the group short code of (i).
FIG. 14
shows parts of a perch channel transmitter for a base station. Reference numeral
111
designates an antenna,
112
,
113
transmitters,
114
-
116
spreading units,
117
a long code generator,
118
a common short code generator,
119
a group short code generator, and Swa, SWc switches.
The spreading unit
115
spreads data for the first perch channel according to a common short code from the common short code generator
118
. The switch Swa is closed except during the period of the long code mask symbol LM (FIG.
13
). This enables the spreading unit
114
to spread the data from the spreading unit
115
according to a long code from the long code generator
117
. The transmitter
112
modulates the data from the spreading unit
114
into a signal having a transmission frequency, which is then transmitted from the antenna
111
.
The switches SWb and SWc are closed in response to the long code mask symbol LM. This enables the spreading unit
116
to spread the data for the second perch channel according to a group short code provided by the group short code generator
119
through the switch SWb. The data from the spreading unit
116
is transferred to the transmitter
113
through the switch SWc and is modulated into a signal having the transmission frequency. The signal is transmitted from the antenna
111
together with the signal of the first perch channel.
The mobile station receives signals from the perch channels of peripheral base stations, despreads the signals according to the common short code, memorizes timing having a high correlative value, uses the timing to despread the signals according to all group short codes used for the second perch channels, and selects a group short code having a high correlative value. For example, a group short code involves 16 long codes, and 16 group short codes are prepared, so that 256 long codes are divided into 16 groups. Sixteen long codes belonging to the group short code having the high correlative value are used to despread the received data, and a long code having a high correlative value is found. The found long code is used to despread the received data and carry out a reception process.
Referring back to
FIG. 12
, if adjacent base stations
101
1
,
101
2
employ the same frequency, a single frequency handover operation is carried out. In this case, the mobile station
103
may have a single receiver. The mobile station
103
is provided with a despreading unit for a traffic channel and a despreading unit for a perch channel. This enables the mobile station
103
to select a perch channel having the highest reception level during the handover process. If the adjacent base stations
101
1
,
101
2
employ different frequencies, a different frequency handover operation is carried out. In this case, the mobile station
103
is provided with a receiver for the traffic channel frequency and a receiver for the perch channel frequency.
Providing the mobile station
103
with the two receivers increases the size, power consumption, and cost of the mobile station. In order to cope with this problem, it has been proposed to provide the mobile station with a single receiver and to switch the reception frequency of the receiver in time division to receive signals from both the traffic and perch channels.
In this case, each base station transmits data through a traffic channel in a compression mode as shown in FIG.
15
. In the figure, (a) shows normal frames #
1
, #
2
, and the like transmitted in a normal mode at a rate of, for example, 8 kbps, (b) shows compressed frames that are made from the normal frames and are transmitted in a compression mode at a rate of, for example, 16 kbps. In other words, the compression mode temporally compresses two normal frames into a single frame to form a spare frame and transmits the compress
Kawabata Kazuo
Obuchi Kazuhisa
Yano Tetsuya
Chin Vivian
Fujitsu Limited
Katten Muchin Zavis & Rosenman
Moore James K
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