Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-07-16
2003-07-22
Le, Thanh Cong (Department: 2684)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S329000, C370S335000
Reexamination Certificate
active
06597677
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spreading code allocation system and method, and in particular to a spreading code allocation system and method, for allocating uniquely spreading codes to base transceiver stations.
2. Description of the Related Art
In a mobile communications system and a licensed personal communication service (PCS) system, an access scheme capable of accommodating as many users as possible In a limited frequency band has been desired. As such an access scheme, there is a frequency division multiple access (heretofore referred to as FDMA) scheme, in which frequency division is conducted so as to allow common use of the frequency band by a plurality of users. There is also a time division multiple access (hereafter referred to as TDMA) scheme, in which division into periodic time slots is conducted.
In addition, the code division multiple access (hereafter referred to as CDMA) scheme capable of having more user channels than the FDMA and TDMA schemes has been standardized by TIA (Telecommunication Industry Association) and EIA (Electronic Industries Association). In the CDMA scheme, the spectrum of a transmission frequency is widened by using a spreading code uniquely allocated to a user to be identified. As a result, a plurality of users can share the same frequency band. At the time of reception, demodulation is conducted by using the same spreading code as used at the time of transmission. Since signals from other users remain to have wide bands, it is possible to easily obtain a received signal which is addressed to its own station and is improved in signal-to-interference power ratio by a spreading gain. Typically as the spreading code, a pseudo-noise (hereafter abbreviated to PN) code functioning as a cyclic code has been used.
PN codes are uniquely allocated to a plurality of users using the same frequency band. Therefore, compared with the FDMA and TDMA schemes, a larger number of users can be accommodated per a predetermined bandwidth. Furthermore, since the transmission frequency band is wide, the frequency selectivity is strong. Therefore, the CDMA scheme provides concealment of information because demodulation cannot be conducted without using the same PN code as that used at the time of transmission.
When adopting the CDMA scheme, for example, in a mobile communications system of a cellular scheme, besides a spreading code allocated to a mobile station, another spreading code Is allocated to the mobile station in some cases. In the mobile communication system of the cellular scheme, there are pluralities of base transceiver stations, and consequently the mobile station needs to identify a base transceiver station with which the mobile station is communicating. Especially near the boundary of an area in which each base transceiver station provides service, It is necessary to change the base transceiver station providing service whenever the mobile station moves from a service area to another service area. In order to identify such a base transceiver station, the different spreading code is needed.
As shown in
FIG. 1
, consider a mobile communications system including a plurality of base transceiver stations each Identified by a different spreading code. Each of the base transceiver stations (hereafter referred to as BTSs)
23
1
,
23
2
, . . . ,
23
N
provides communication services to a mobile station (hereafter referred to as MS)
24
located within its service area. The first to Nth BTSs
23
1
through
23
N
are connected to a base transceiver station controller (hereafter referred to as BSC)
26
via transmission paths
25
1
through
25
N
, respectively. The BSC
26
is connected to a mobile switching center (hereafter referred to as MSC)
27
. The MSC
27
has a data base
28
associated therewith. The MSC
27
is connected to a public network (not shown) via a public line
29
. The MS
24
can communicate with the first through Nth BTSs
23
1
through
23
N
by radio signals
30
1
through
30
N
, respectively. A radio signal may convey incoming call information to the MS
24
, outgoing call information from the MS
24
to a terminal of the opposite party, or communication information.
Each of the
1
st through Nth BTSs
23
1
through
23
N
has a unique PN code allocated thereto as a spreading code. In the DB
28
, PN codes allocated to the respective BTSs are stored in advance. The MSC
27
notifies the respective BTSs of corresponding PN codes via the BSC
26
and the transmission paths
25
1
through
25
N
.
The MSC
27
performs call processing between the terminal of the opposite party and the MS
24
managed by the MSC
27
. The BSC
26
monitors the BTSs including one communicating with the MS
24
at any time and changes the BTS communicating with the MS
24
to an adjacent BTS depending on movement of the MS
24
. When an incoming call addressed to the MS
24
has occurred, the MSC
27
conducts call processing for the BTS serving an area in which the MS
24
exists.
Each BTS periodically sends a pilot PN code of its own BTS, called “pilot channel”, to MSs existing in the service area served by the BTS, and transmits information for synchronization acquisition and holding, and clock recovery in each MS. Each MS responds to a BTS sending a pilot channel which is the largest in received intensity. As a result, each BTS can determine whether a MS under search is present in its own service area or not, and notifies the BSC of its result.
For example, it s now assumed that an incoming call to the MS
24
existing in an area served by the Nth BTS
23
N
has occurred. Since the MS
24
has been recognized to be located within the service area of the Nth BTS
23
N
by responding to a pilot channel from the Nth BTS
23
N
, the MSC
27
conducts call setup processing with the MS
24
via the BSC
26
and the Nth BTS
23
N
. At this time, the MS
24
can recognize from the PN code of the Nth BTS
23
N
received over the above pilot channel that the call setup is caused by the Nth BTS
23
N
. In other words, the MS
24
selects and receives only the radio wave from the Nth BTS
23
N
without being affected by radio waves from the first through (N−1)th BTSs
23
1
through
23
N−1
. If the MS
24
has moved into an area served by the third BTS
23
3
, then the movement is periodically monitored by the BSC
26
as a result of MS
24
responding to a pilot channel sent from each BTS. Thereafter, communicating with the MS
24
is performed via the third BTS
23
3
.
As for the synchronization between the BTSs and MSs using PN codes as described above, the time required for the synchronization becomes longer according to the code length of the PN code. To improve the error rate and shorten the time required for synchronization, there has been proposed a technique for generating a PN code such that the code length thereof varies depending on an error rate detected by the BTS in Japanese Patent Application Lad-Open. No 9-181647.
Another technique for shortening the time for synchronization acquisition and the time required for synchronization establishment has been disclosed in. Japanese Patent Application Laid-Open No. 10-56405. More specifically, the time for synchronization acquisition is shortened by reducing the length of a PN code for synchronization used in a pilot channel dedicated for initial synchronization acquisition between a MS and a BTS. The time required for synchronization establishment of the MS with the communication channel transmitted by the BTS is shortened by a phase of an identification code contained in an identification pilot channel agreeing with a phase of an identification PN code contained in a communication channel.
In the case where the FDMA or TDMA scheme is adopted in the mobile communcations system or the licensed PCS system, a limited frequency band is shared by a plurality of users and therefor the frequency allocation to the users is needed. In the system design stage, this frequency allocation is a work greatly influencing the communication qu
Chow C.
Cong Le Thanh
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