Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-01-25
2002-11-19
Yao, Kwang Bin (Department: 2664)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S350000, C455S502000
Reexamination Certificate
active
06483825
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of synchronizing time in a CDMA system and, more particularly, to a time synchronization method for synchronizing the time of a plurality of base stations and the time of a base station controller that controls these base stations in a CDMA system.
In an IS-95—based N-CDMA (Narrow-band Code Division Multiple Access) system, it is required that signals transmitted from all base stations be synchronized in time (to within ±3 &mgr;s of absolute time) for the following two reasons:
(1) In an N-CDMA system, a base station is identified by the phase offset of a pilot PN sequence, which is a code string, where PN stands for pseudo-random noise. If base stations are not synchronized in time, the phase offset cannot be stipulated and a terminal such as a cellular telephone will not be able to identify the base station.
More specifically, signals transmitted from each base station to a mobile terminal include a pilot signal and a synchronizing signal, and each base station transmits these signals upon spreading and modulating the signals by a pilot PN sequence. Though the pilot PN sequence code string used is itself the same for each base station, each base station is provided with a different phase offset (a 64-chip unit) so that the mobile terminal can identify the particular base station. (A maximum of 512 base stations can be identified.) The reference for the phase offset is CDMA system time. In regard to a pilot PN sequence for which the phase offset is 0 (zero), the standard is that 15 consecutive “0” s followed by a “1” be output at time 00:00:00 on Jan. 6, 1980 (the moment at which the changeover from 0 to 1 is made is time 00:00:00). Unless the base stations are synchronized in time, therefore, it is not possible to stipulate the phase offset of the pilot PN sequence and, as a result, it will not be possible for a mobile terminal to identify to which base station it is wirelessly connected.
(2) In order to avoid transmission line congestion and a fluctuation in transmission delay that accompanies such congestion in an N-CDMA system, phase offsets are imposed on the traffic channels (a traffic channel is a channel for voice signals between a mobile terminal and a base station) in steps of 1.25 ms on a per-call basis. If base stations are not synchronized to one another in terms of time, therefore, it will no longer be possible to implement soft handoff between base stations, soft handoff being a characterizing feature of an N-CDMA system. (Soft handoff is the ability of a mobile station to move from one base station to another without an interruption in service.)
More specifically, an example of a signal sent and received between each base station and a mobile terminal is a voice signal transmitted via a traffic channel. In order to avoid the effects of congestion, delay and a fluctuation in delay time in the transmission lines between base stations and a base station controller and between the base station controller and switching equipment or the like, phase offsets are allocated to the voice signals in steps of 1.25 ms per call on each traffic channel. Since a 20-ms frame is partitioned into units of 1.25 ms, in such case there will be 16 offsets.
FIG. 7
is a diagram useful in describing the necessity of a phase offset. Shown in
FIG. 1
are a base station controller
1
, a base station
2
and mobile terminals
5
1
,
5
2
, . . . ,
5
n
currently communicating with the base station
2
. Though data transmission from each terminal is illustrated in the form of bursts in order to make it easier to visualize operation, in actuality the data is transmitted continuously or discretely along the time axis. When voice signals from the terminals
5
1
,
5
2
, . . . ,
5
n
arrive at the base station
2
at the same timing, as shown in
FIG. 7
, the signals are queued because there is only one transmission line between the base station
2
and the base station controller
1
. As a consequence, a certain signal will be sent from the base station
2
to the base station controller
1
late in terms of the numerical order. For example, in terms of the numerical order, a voice signal n is sent late at a timing a in
FIG. 7
, and a voice signal
1
is sent late at a timing b. If the queuing time and the numerical order are always constant, no problems arise. However, since the terminals move, a slight disparity develops in the order in which the voice signals arrive at the base station
2
. When the order of signal arrival differs, the order in which signals are sent from the base station to the base station controller
1
also changes and, as a result, a large variation in transmission delay time is produced. For this reason the 1.25-ms offset is set for each call and only two to three terminals are allocated to one offset to prevent a large fluctuation in transmission time.
Hitless handoff (soft handoff) between base stations under the control of the same base station controller is possible on the condition that the radio frequency before and after handoff is the same and, moreover, that the phase offset allocated to the traffic channels is the same. If the radio frequency is different, an interruption in service will be unavoidable owing to the frequency changeover. If the phase offset is different, this will result in a long standby time at a voice decoder or the like and eventually lead to an interruption in service. It should be noted that one item of voice data should be receivable in 20 ms and that any fluctuation is less than 1.25 ms at most.
Unless the base stations are synchronized in time, therefore, specifying the same offset before and after handoff will be meaningless and hitless soft handoff will be impossible to accomplish.
In view of reasons (1) and (2) set forth above, a first CDMA system according to the prior art is such that a highly accurate GPS receiver (having a time error on the order of ±0.1 &mgr;s) is deployed at all base stations and the circuitry in each base station is actuated based upon time information and a clock signal received from GPS satellites.
With a second CDMA system according to the prior art, namely the system disclosed in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 8-265838, a GPS clock output by a highly accurate GPS receiver deployed at a base station controller is adopted as a master clock and base stations are kept in frequency and time synchronization taking into account the transmission delay time between the base station controller and each base station. According to this second CDMA system, the transmission delay time is measured immediately prior to the sending and receiving of voice signals.
The first CDMA system according to the prior art is disadvantageous owing to the high cost of the base stations and system overall. The high-precision GPS receiver is costly (several hundred thousand yen) and a redundant configuration is necessary in order to reduce base station downtime due to failure. Furthermore, since an inexpensive GPS receiver has a time error on the order of 2 to 3 &mgr;s, such a receiver cannot meet the system specifications.
The second CDMA system according to the prior art is capable of maintaining the time and phase synchronization between the base station controller and base stations but a problem that arises is that the base stations themselves cannot achieve time synchronization to absolute time (or to a specific time standard). Further, the second CDMA system is such that transmission delay time is not measured periodically but only just prior to sending/receiving of a voice signal (voice communication). This means that the system cannot deal with a situation in which the transmitting apparatus re-synchronizes for some reason or in which the apparatus recovers after the occurrence of a failure.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a time synchronization method through which the time of all base stations can be synchronized to absolute time highly accurately wit
Fujitsu Limited
Katten Muchin Zavis & Rosenman
Yao Kwang Bin
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