Cell search procedure for time division duplex communication...

Details Cell search procedure for time division duplex communication... Cell search procedure for time division duplex communication...

2000-05-22

2004-04-06

Nguyen, Chau (Department: 2661)

Multiplex communications

Communication over free space

Having a plurality of contiguous regions served by...

C370S350000, C375S145000

Reexamination Certificate

active

06717930

ABSTRACT:

BACKGROUND
This invention generally relates to spread spectrum Time Division Duplex (TDD) communication systems using Code Division Multiple Access (CDMA). More particularly, the present invention relates to cell search procedure of User Equipment (UE) within TDD/CDMA communication systems.
FIG. 1
depicts a wireless spread spectrum TDD/CDMA communication system. The system has a plurality of base stations
30
1
to
30
7
. Each base station
30
1
has an associated cell
34
1
to
34
7
and communicates with user equipments (UEs)
32
1
to
32
3
in its cell
34
1
.
In addition to communicating over different frequency spectrums, TDD/CDMA systems carry multiple communications over the same spectrum. The multiple signals are distinguished by their respective code sequences (codes). Also, to more efficiently use the spectrum, TDD/CDMA systems as illustrated in
FIG. 2
use repeating frames
38
divided into a number of time slots
36
1
to
36
n,
, such as sixteen time slots
0
to
15
. In such systems, a communication is sent in selected time slots
36
1
to
36
n
using selected codes. Accordingly, one frame
38
is capable of carrying multiple communications distinguished by both time slot
36
1
to
36
n
and code.
For a UE
32
1
to communicate with a base station
30
1
, time and code synchronization is required.
FIG. 3
is a flow chart of the cell search and synchronization process. Initially, the UE
32
1
must determine which base station
30
1
to
30
7
and cell
34
1
to
34
7
to communicate. In a TDD/CDMA system, all the base stations
30
1
to
30
7
are time synchronized within a base station cluster. For synchronization with UEs
32
1
to
32
7
, each base station
30
1
to
30
7
sends a Primary Synchronization Code (PSC) and several Secondary Synchronization Code (SSC) signals in the time slot dedicated for synchronization. The PSC signal has an associated chip code, such as an unmodulated 256 hierarchical code, and is transmitted in the dedicated time slot, step
46
. To illustrate, a base station
30
1
may transmit in one or two time slots, such as for a system using time slots
0
to
15
in time slot K or slot K+8, where K is either 0, . . . , 7.
One technique used to generate a PSC signal is to use two 16 hierarchical sequences, such as X1 and X2 in Equations 1 and 2.
X
1=[1, 1, −1, −1, 1, −1, 1, −1, −1, −1, −1, −1, 1, 1, 1, −1]  Equation 1
X
2=[1, 1, −1, −1, −1, −1, 1, −1, 1, 1, −1, 1, 1, 1, −1, 1]  Equation 2
Equation 3 illustrates one approach to generate a 256 hierarchal code, y(i), using X1 and X2.
y
(
i
)=
X
1 (
i
mod 16)×
X
2 (
i
div 16), where
i=
0, . . . , 255  Equation 3
Using y(i), the PSC is generated such as by combining y(i) with the first row of length 256 Hadamarad matrix, h
0
, to produce C
p
(i) as in Equation 4.
C
p
(
i
)=
y
(
i
)×
h
0
(
i
), where
i=
0, . . . , 255  Equation 4
Since the first row of the Hadamarad matrix is an all one sequence, Equation 4 reduces to Equation 5.
C
p
(
i
)=
y
(
i
), where
i=
0, . . . , 255  Equation 5
The C
p
(i) is used to produce a spread spectrum PSC signal suitable for transmission.
To prevent the base stations' communications from interfering with each other, each base station
30
1
to
30
7
sends its PSC signal with a unique time offset, t
offset
, from the time slot boundary
40
. Differing time offsets are shown for time slot
42
in FIG.
4
. To illustrate, a first base station
30
1
has a first time offset
44
1
, t
offset,1
for the PSC signal, and a second base station
30
2
, has a second time offset
44
2
, t
offset,2
.
To differentiate the different base stations
30
1
to
30
7
and cells
34
1
to
34
7
, each base station
30
1
to
30
7
within the cluster is assigned a different group of codes (code group). One approach for assigning a t
offset
for a base station using an n
th
code group
44
n
, t
offset,n
is Equation 6.
 t
offset,n
=n·
71
T
c
  Equation 6
T
c
is the chip duration and each slot has a duration of 2560 chips. As a result, the offset
42
n
for each sequential code group is spaced 71 chips.
Since initially the UE
32
1
and the base stations
30
1
to
30
7
are not time synchronized, the UE
32
1
searches through every chip in the frame
38
for PSC signals. To accomplish this search, received signals are inputted to a matched filter which is matched to the PSC signal's chip code. The PSC matched filter is used to search through all the chips of a frame to identify the PSC signal of the base station
30
1
having the strongest signal. This process is referred to as step-
1
of cell search procedure.
After the UE
32
1
identifies the PSC signal of the strongest base station
30
1
, the UE
32
1
needs to determine the time slot
36
1
to
36
n
in which that PSC and SSC signals are transmitted (referred to as the Physical Synchronization Channel (PSCH) time slot) and the code group used by the identified base station
30
1
. This process is referred to as step-
2
of cell search procedure. To indicate the code group assigned to the base station
30
1
and the PSCH time slot index, the base station
30
1
transmits signals having selected secondary synchronization codes (SSCs), step
48
. The UE
32
1
receives these SSC signals, step
50
, and identifies the base station's code group and PSCH time slot index based on which SSCs were received, step
52
.
For a TDD system using 32 code groups and two possible PSCH time slots per frame, such as time slots K and K+8, one approach to identify the code group and PSCH time slot index is to send a signal having one of 64 SSCs. Each of the synchronization codes corresponds to one of the 32 code groups and two possible PSCH time slots. This approach adds complexity at the UE
32
1
requiring at least 64 matched filters and extensive processing. To identify the code group and PSCH time slot index, 17,344 real additions and 128 real multiplications are required in each PSCH time slot and 64 real additions are required for the decision.
An alternative approach for step-
2
of cell search procedure uses 17 SSCs. These 17 SSCs are used to index the 32 code groups and two possible PSCH time slots per frame. To implement this approach, at least 17 matched filters are required. To identify the code group and time slot, 1,361 real additions and 34 real multiplications are required for each PSCH time slot. Additionally, 512 real additions are required for the decision.
It would be desirable to reduce the complexity required by a UE
32
1
to perform cell search procedure.
SUMMARY
A base station sends a synchronization signal in an assigned time slot to a user equipment in a time division duplex code division multiple access communication system. The base station has an assigned code group out of a predetermined number of code groups. The base station transmits selected secondary synchronization code signals out of a set of secondary synchronization code signals. The plurality of secondary synchronization code signals numbers less than half of the predetermined number of code groups. The user equipment identifies the transmitted selected secondary code signals. Based on in part the identified secondary synchronization code signals, the assigned code group is determined.


REFERENCES:
patent: 5559789 (1996-09-01), Nakamo et al.
patent: 5715521 (1998-02-01), Fukasawa et al.
patent: 6185244 (2001-02-01), Nystrom et al.
patent: 6246673 (2001-06-01), Tiedemann, Jr. et al.
patent: 6363060 (2002-03-01), Sarkar
patent: 6526091 (2003-02-01), Nyström et al.
TSG-RAN Working Group 1 (Radio) Meeting #3, “A New Hierarchical Correlation Sequence with Good Properties in Presence of a Frequency Error”, Eskilstuna, Sweden, Mar. 22-26, 1999.
Nadir Sezgin and Fatih Ozluturk, “BPSK Modulated Secondary Synchronization Codes for Cell Search in UTRA TDD”, May 28,

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