Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2002-08-21
2004-09-07
Bayard, Emmanuel (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
Reexamination Certificate
active
06788732
ABSTRACT:
PRIORITY
This application claims priority to an application entitled “Initial Acquisition and Frame Synchronization in Spread Spectrum Communication System” filed in the Korean Industrial Property Office on Jul. 21, 1998 and assigned Ser. No. 98-29344, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a spread spectrum communication system, and, in particular, to a device and method for performing initial acquisition and frame synchronization using a spreading code for a mobile station.
2. Description of the Related Art
FIG. 1
illustrates an IS-95 forward link of a base station, for transmitting channel signals to a mobile station in a Code Division Multiple Access (CDMA) mobile communication system. As shown, in a CDMA mobile communication system, the forward link includes a pilot channel, a sync channel and a paging channel. Though not illustrated, the forward link further includes a traffic channel for transmitting the voice and data of a user.
Referring to
FIG. 1
, a pilot channel generator
110
generates a pilot signal comprised of all “1”s for a pilot channel, and a multiplier
114
multiplies the pilot signal by an orthogonal code W
0
to orthogonally spread the pilot signal. Here, a specific Walsh code is used for the orthogonal code W
0
. A multiplier
115
multiplies the pilot channel signal output from the multiplier
114
by a PN (Pseudo Noise) sequence to spread the pilot channel signal.
With regard to the structure of a sync channel generator
120
, a coding rate R=½, constraint length K=9 convolutional encoder can be used for an encoder
121
. A repeater
122
repeats sync symbols output from the encoder
121
N times (N=2). An interleaver
123
interleaves the symbols output from the repeater
122
in order to prevent burst errors. A block interleaver is typically used for the interleaver
123
. A multiplier
124
multiplies the sync channel signal by a specific orthogonal code assigned to the sync channel to orthogonally spread the sync channel signal. The sync channel outputs the positional information, standard time information and long code information of the base station, and also outputs information for system synchronization between the base station and a mobile station. As stated above, the sync channel generator
120
encodes an input sync channel signal, and multiplies the encoded sync channel signal by a specific Walsh code W
sync
assigned to the sync channel out of available Walsh codes to orthogonally spread the sync channel signal. A multiplier
125
multiplies the sync channel signal output from the multiplier
124
by the PN sequence to spread the sync channel signal.
With regard to a paging channel generator
130
, an encoder
131
encodes an input paging channel signal. An R=½, K=9 convolutional encoder can be used for the encoder
131
. A repeater
132
repeats the symbols output from the encoder
131
N times (N=1 or 2). An interleaver
133
interleaves the symbols output from the repeater
132
in order to prevent burst errors. A block interleaver is typically used for the interleaver
133
. A long code generator
141
generates a long code which is the user identification code. A decimator
142
decimates the long code so as to match the rate of the long code to the rate of the symbol output from the interleaver
133
. An exclusive OR gate
143
XORs the encoded paging signal output from the interleaver
133
and the long code output from the decimator
142
to scramble the paging signal. A multiplier
134
multiplies the scrambled paging signal output from the exclusive OR gate
143
by an orthogonal code W
p
assigned to the paging channel in order to maintain orthogonality with other channel signal. A multiplier
135
multiplies the paging channel signal output from the multiplier
134
by the PN sequence to spread the paging channel signal.
As stated above, the orthogonally spread transmission signals of the respective channels are multiplied by the PN sequence to be spread, and up-converted into an RF (Radio Frequency) signal to be transmitted. In the IS-95 standard, spreading is performed using two different PN sequences for the I and Q arms. The PN sequences used herein have a period of 32,768.
In the forward link structure of
FIG. 1
, the pilot channel does not carry data and spreads a signal of all “1”s with a PN sequence of period 32,768 to transmit. In a system having a chip rate of 1.2288 Mcps (chips per second), one PN sequence period corresponds to 26.7 msec (80/3 msec). Upon power-on, the receiver in a mobile station acquires the pilot channel signal on the forward link shown in
FIG. 1
in order to acquire synchronization with a base station.
FIG. 2
illustrates a receiver in a mobile station, which receives forward link channel signals from a base station.
Referring to
FIG. 2
, an RF receiver
212
receives an RF signal transmitted from a base station and then down-converts the received RF signal into a baseband signal. An analog-to-digital (A/D) converter
214
converts the baseband signal output from the RF receiver
212
to digital data. A searcher
222
acquires the pilot channel signal out of the forward channel signals in order to synchronize the mobile station with the base station. N fingers
231
-
23
N despread corresponding forward channel signals to detect a correlation value among the channel signals. A combiner
226
combines the output signals of the respective fingers
231
-
23
N.
As illustrated in
FIG. 2
, a receiver of a mobile station is comprised of the searcher
222
, the N fingers
231
-
23
N and the combiner
226
. Acquisition of the pilot signal is performed by the searcher
222
.
FIG. 3
is a timing diagram of forward channel signals that a base station transmits, in which the frame offset of a traffic channel is assumed to be 0.
Referring to
FIG. 3
, reference numeral
311
represents a 80 ms boundary of a base station, which is determined from a two second boundary of the Global Positioning System (GPS). Reference numeral
313
represents the pilot offset of the base station. Reference numeral
315
represents the boundaries of three spreading sequence periods within 80 ms, from which it is clear that one spreading sequence period is 26.7 ms (=80/3 ms). Herein, the spreading sequence is assumed to be a PN sequence. Each spreading sequence period is synchronized with a 26.7 ms frame boundary where a sync channel is interleaved. Here, the 80 ms frame will be referred to as the second frame and the 26.7 ms frame the first frame.
Reference numeral
317
represents an 80 ms frame boundary of the sync channel, and the 80 ms frame structure of the sync channel is illustrated in FIG.
4
. For the sync channel signal, the 80 ms frame represented by reference numeral
412
is comprised of three 26.7 ms frames each including a sync bit SOM (Start of Message) set according to a pilot sequence period. For example, in the 80 ms period, the sync bit SOM for the first 26.7 ms frame period is determined as “1” (or “0”), and the sync bits SOMs for the following 26.7 ms frames are determined as “0” (or “1”). Therefore, detecting a sync bit SOM of “1” (or “0”) in the 80 ms period means detection of an 80 ms sync channel signal.
Reference numeral
319
represents the frame boundaries of the paging channel and the traffic channel. For the traffic channel, the 80 ms frame is comprised of four 20 ms frames. Therefore, it is noted from
FIG. 3
that in the 80 ms period, the sync channel is comprised of three 26.7 ms frames and the traffic channel is comprised of four 20 ms frames.
Referring to
FIGS. 3 and 4
, a description will be made regarding the synchronizing procedure performed between a base station and a mobile station. The standard timing of the base station is derived from the 80 ms boundary
311
which is determined from the two second boundary of the GPS. The pilot signal of the base station is offset by the pilot offset
313
in the
Bayard Emmanuel
Dilworth & Barrese LLP
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