Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-03-12
2002-10-01
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S147000
Reexamination Certificate
active
06459724
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CDMA mobile communication system and, more particularly, to a perch channel slot timing detection method and circuit in a W-CDMA (Wide-band Code Division Multiple Access) scheme, together with a mobile station and mobile communication system in a CDMA scheme.
2. Description of the Prior Art
In a CDMA transmission/reception system, as shown in
FIGS. 1A
to
1
D, on the base station side, a spreading unit
17
spreads a logic symbol
15
with a spreading code B (see FIG.
1
A), and a spreading unit
18
spreads a logic symbol
16
with a spreading code A (see FIG.
1
C), thereby transmitting the logic symbols by using carrier waves having the same frequency. In this case, the transfer rate of spreading codes is several 10 to several 100 times the transfer rate of logic symbols. The period of such a spreading code is called a chip. On the receiver side on which the carrier waves are received, a despreading unit
19
can extract the logic symbol
16
by despreading the carrier wave with the spreading code A (see FIG.
1
B), and a despreading unit
20
can extract the logic symbol
15
by despreading the carrier wave with the spreading code B (see FIG.
1
D).
As described above, in the CDMA transmission/reception system, multiple access can be performed using the same frequency by performing spreading/despreading operation with a plurality of spreading codes.
These spreading codes include long codes (long-period spreading codes) and short codes (short-period spreading codes). In a W-CDMA scheme, logic symbols are double-spread with these long and short codes.
In this case, a long code is a code having a very long period corresponding to several 10 to several 100 symbols, where as a short code is a code having a short period corresponding to one symbol.
FIG. 2
shows the arrangement of the transmission section of a CDMA transmitter designed to perform double-spreading.
The transmission section of this CDMA transmitter includes a baseband modulator
60
, a clock signal generator
61
, a short code generator
63
, a long code generator
64
, exclusive OR circuits (EX-ORs)
65
and
66
, a multiplier
67
, a carrier wave generator
68
, and an amplifier
69
. In addition, the exclusive OR circuits
65
and
66
constitute a spreading unit
70
.
The baseband modulator
60
receives a digital signal and converts it into a baseband-modulated signal. The exclusive OR circuit
65
multiplies this baseband-modulated signal by a short code output from the short code generator
63
, thus spreading the spectrum. The exclusive OR circuit
66
multiplies the baseband-modulated signal by a long code output from the long code generator
64
, thus spreading the spectrum. In this case, the short and long codes have the same chip period, and both the short code generator
63
and the long code generator
64
are driven by clock signals generated by the clock signal generator
61
.
The multiplier
67
multiplies the spread baseband-modulated signal output from the exclusive OR circuit
66
by a carrier wave generated by the carrier wave generator
68
. The resultant signal is amplified by the amplifier
69
and transmitted as a transmission modulated wave from an antenna.
In this CDMA transmission/reception system, however, despreading cannot be properly performed on the receiver side unless the spreading timing at which the base station performs spreading is accurately obtained. Even a deviation of one chip from this spreading timing makes the receiver unable to receive any signals transmitted from the base station. In a W-CDMA scheme, which is one of the CDMA schemes, since synchronization between output signals is not established between base stations, the receiver must establish synchronization every time the base station to which the receiver is connected is changed.
In addition, since each base station uses a plurality of spreading codes, the spreading codes used by the base station to which the receiver is to be radio-connected cannot be known in advance. For example, in the W-CDMA scheme, 32 types of short codes are prepared, but the receiver cannot specify which short codes, of the 32 types of short codes, the base station to which the receiver is to be connected is using. If the spreading codes used by the base station cannot be specified, no information can be obtained from the base station. The receiver cannot therefore be radio-connected to the base station.
A perch function is designed to solve this problem. The perch function is the function of allowing a receiver to obtain base station information, e.g., the spreading codes used by a base station in spreading logic symbols and the spreading timing. The receiver uses this perch function to know various information about the base station, e.g., the spreading codes in use and the spreading timing, by performing a perch search, thereby establishing radio connection.
FIG. 3
is a block diagram showing the transmission/reception section of a mobile station having such a perch function.
As shown in
FIG. 3
, the transmission/reception section of the mobile station is comprised of an RF/IF section
21
, a transmission section
22
, and a reception section
23
.
The transmission section
22
outputs a baseband signal, which is to be transmitted from the receiver to a base station, to the RF/IF section
21
.
The RF/IF section
21
modulates a carrier wave with the baseband signal output from the transmission section
22
and transmits the carrier wave to the base station. The RF/IF section
21
also demodulates a signal transmitted from the base station and outputs the signal to the reception section
23
.
The reception section
23
includes a finger reception section
24
, a search section
25
, a perch search section
26
, and a rake reception section
27
.
The perch search section
26
obtains various information about the base station from the baseband signal demodulated by the RF/IF section
21
, and outputs the information to the subsequent circuit. The perch search section
26
has a slot timing detection circuit
28
for detecting the slot timing of the base station from which signals are currently received. The slot timing is the timing at which time slots as units of data are transmitted from the base station. Since this slot timing is identical to the spreading timing, the spreading timing can be obtained by obtaining the slot timing.
The search section
25
detects a timing deviation between direct and reflected waves due to multipath components in a baseband signal.
The finger reception section
24
delays the baseband signal in accordance with the timing deviation detected by the search section
25
to correct the timing deviation between signals.
The rake reception section
27
synthesizes the signals received by the finger reception section
24
at an optimal ratio, and outputs the resultant signal to the subsequent circuit.
FIG. 4
shows the data structure of a perch channel
30
received by the perch search section
26
.
The perch channel
30
is one of 640 msec super frames and made up of 64 radio frames
31
1
to
31
64
that are used to transfer information for each receiver.
The radio frame
31
1
consists of 16 time slots
32
1
to
32
16
. The time slot
32
1
consists of a pilot symbol
33
, BCCH (Broadcast CHannel) symbol
34
, and a long code mask symbol
35
.
A W-CDMA scheme using short codes each having a spreading code length of 256 chips will be described below. One bit of a logic symbol is therefore spread to 256 chips.
The long code mask symbol
35
is a signal obtained by spreading a given 1-bit logic symbol with only a short code without using any long code. All logic symbols other than the long code mask symbol
35
are spread with both long and short codes as spreading codes. For this reason, when the signal demodulated by the RF/IF section
21
is despread with only a short code, only the long code mask symbol
35
appears as the original symbol. The slot timing detection circuit
28
detects the slot timing by using th
Chin Stephen
Lugo David B.
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