Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
2000-04-28
2004-04-27
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000, C375S142000, C375S145000, C375S149000
Reexamination Certificate
active
06728229
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for searching for a cell between an asynchronous base station (BS) and a mobile station (MS).
2. Description of the Related Art
The UMTS (Universal Mobile Telecommunication System) is an asynchronous base station system in which each constituent BS is assigned to a unique cell specific code for identification. For example, if the UMTS includes 512 cells, i.e., 512 BSs, 512 cell specific codes are assigned to the respective 512 BSs. To reduce calculations for cell search in an MS, the BSs in the UMTS are divided into a predetermined number of groups (e.g., 32 groups), each BS group being assigned to a different group specific code. The MS first detects a BS group that a serving BS belongs to by searching for the specific code of the BS group and then determines the serving BS in the BS group. This multi-step cell search algorithm includes the steps of (1) receiving a primary synchronization channel (P-SCH) signal from a serving BS and synchronizing to the slot time of a slot received with the highest power; (2) receiving a secondary synchronization channel (S-SCH) signal from the BS while the MS is synchronized with the slot time and detecting frame synchronization and the group specific code of the BS; and (3) receiving a broadcasting channel (BCH) signal from the BS based on the frame synchronization and the BS group specific code and searching for the cell specific code of the BS.
FIG. 1
illustrates the structure of UMTS channels. Referring to
FIG. 1
, one UMTS channel frame includes 16 slots and each slot has 2560 chips. Therefore, one frame has 40,960 chips (T
frame
=16 Tslots). A BS transmits a synchronization code #0, SC
0
, to an MS on a P-SCH for a tenth of the each slot period, that is, for 256 chips. Then, the MS receives the P-SCH signal C
p
and synchronizes with the BS slot time (the first cell search step). All BSs of the UMTS transmit the same synchronization code to the MS on P-SCHs. The MS synchronizes its timing with the slot time of the serving BS based on the offsets of the P-SCH signals. Then, for the “i
th
” BS group the BS maps its BS group specific code C
s
i,1
-C
s
i,16
on an S-SCH, and the slot-time synchronized MS acquires the BS group specific code and frame synchronization of the serving BS from the received S-SCH signal. The BS specific code is set in accordance with a comma free code produced by selectively combining 16 codes from the 17 synchronization codes SC
1
-SC
17
available (the second cell search step).
The second cell search step will be described in detail with reference to FIG.
2
. To do convenient explanation of decision variable calculation procedure, it is assumed that the correlation result array of one frame is s[
17
][
16
] (the correlation accumulator
217
), the decision variable array is Y[
32
][
16
] (the correlation accumulator
223
), and comma free code table array is C[
32
][
16
] (comma free code table
221
).
FIG. 2
is a block diagram of a conventional cell search apparatus. The following description is conducted on the assumption that a serving BS is in a first BS group. Referring to
FIG. 2
, the BS maps synchronization codes corresponding to its BS group specific code on an S-SCH signal. The MS, synchronized with the slot time of the BS according to a slot time synchronization index detected in the first cell search step, receives the S-SCH signal at an S-SCH correlation unit
211
in the cell search apparatus. The S-SCH correlation unit
211
is comprised of 1
st
to 17
th
S-SCH correlators
213
to
215
for computing correlation values of the received S-SCH signal by 1
st
to 17
th
synchronization code auto-correlation functions. The S-SCH correlation unit
211
has as many correlators as the synchronization codes, that is, 17 correlators for detecting correlation values of each slot in an input frame with respect to the respective synchronization codes. Upon detection of a first slot of the input frame, the S-SCH correlation unit
211
computes correlation values of the S-SCH channel in the first slot with respect to the respective synchronization codes. Since 17 correlation values are computed for one slot with respect to the 1
st
to 17
th
synchronization codes, a matrix of 16×17 correlation values are obtained for one frame. An S-SCH correlation value storage
217
stores the correlation values received from the S-SCH correlation unit
211
in the form of a 16×17 matrix, as shown in FIG.
3
A. The correlation values of the SCH in each slot of the input frame are stored column by column. That is, correlation values of the S-SCH in the first slot are arranged in a first column, s(
1
,
1
), s(
2
,
1
), . . . , s(
16
,
1
), s(
17
,
1
). Correlation values of the S-SCH in the second slot are arranged in a second column, s(
1
,
2
), s(
2
,
2
), . . . , s(
16
,
2
), s(
17
,
2
). Finally, correlation values of the S-SCH in the 16th slot are arranged in a sixteenth column s(
1
,
16
), s(
2
,
16
), . . . , s(
16
,
16
), s(
17
,
16
).
When the 16×17 correlation values are completely stored in the S-SCH correlation value storage
217
at the end of the input frame, a shift comparator
219
reads a comma free code table as shown in
FIG. 4
from a comma free code table storage
221
. The shift comparator
219
compares the S-SCH correlation values received from the S-SCH correlation value storage
217
with comma free codes in the comma free code table and feeds the resulting mapped correlation values to a correlation value accumulator
223
. The shift comparator
219
refers to the comma free code table for the initial S-SCH correlation value s(
1
,
1
) shown in FIG.
3
A.
FIG. 4
shows that a first codeword in the comma free code table has 1 as its first symbol, which implies that the BS mapped a synchronization code corresponding to symbol 1, that is, the first synchronization code SC
1
on the S-SCH prior to transmission. Hence, the correlation value accumulator
223
stores s(
1
,
1
) in Y(
1
,
1
). As the same manner, the other correlation values mapped to the other symbols of first codeword inserted in the other slot are sequentially accumulated in Y(
1
,
1
), too. The same procedures are performed to the other codeword. So, the accumulated values of 0 cyclic shift version are stored in the first row of 32×16 decision variable memory Y(correlation value accumulator
223
). While the shift comparator
219
cyclically shift S-SCH correlation values 15 times as shown in
FIG. 3B
, the procedure is described in the top is performed. The correlation accumulator
223
accumulatively stores 32×16 decision variables. If do the detailed description of calculation of the decision variable (Y[
32
][
16
]), the Calculation algorithm is carried out in the following way:
for(i=0
; i
<32
; i
++)
for(j=0
; j
<16
; j
++){
Y[i][j
]=0; for(
k=
0
; k
<16
; k
++)
Y[i][j]+=s[C[i
][(
k+j
)%16
]][k];},
These calculations are performed with the purpose of summation of correlation results in various combinations (32×16=512). Within the correlation value accumulator
223
, a maximum correlation value detector
225
searches for the maximum value (i, j) whose first index i corresponds to BS group specific code and the second index j determines frame synchronization, i.e. the beginning of next frame in (
16
−j)%16 slots. Therefore, the MS detects an offset to thereby acquire frame synchronization to the BS and find out the BS group.
For detection of frame synchronization and a BS group in the above second cell search step, the conventional cell search apparatus is required to have a memory capable of storing the 16×17 correlation va
Dilworth & Barrese LLP
Kizou Hassan
McLoughlin M. I.
Samsung Electronics Co,. Ltd.
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