Pulse or digital communications – Spread spectrum – Chirp
Reexamination Certificate
1998-07-10
2002-08-27
Tse, Young T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Chirp
C375S150000, C375S343000
Reexamination Certificate
active
06442189
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a binary code phase offset calculation method of a mobile communication system.
2. Discussion of the Related Art
Among the codes with q elements which are used for transmitting information in a CDMA or CTDMA of a digital communication system, a binary code whose q is 2 is generally used. The analog signals such as voice signals or image signals are converted into a binary code by an analog to digital converter. This binary code is modulated according to a frequency translation to be transmitted by a radio or wire communication system. The receiver recovers the analog signal from the modulated binary code. Thus, the binary code is not only transmitted as information itself but also used as a spread spectrum code in the code division multiple access system or the code time division multiple access system of the mobile communication.
To increase the capacity of cellular mobile communication, the digital cellular mobile communication using the global system for mobile (GSM) communication has been commercialized in European countries and the TDMA and CDMA systems have commercially been used in the USA. The T1A/ELA/IS95CDMA system developed by Qualcomm of the USA is used as the standard communication system of Korea.
For the CDMA systems mentioned above, a code having a special autocorrelation function is used as a spread code and it is also used for synchronization of a receiver and cryptography, etc.
In the CDMA system, each base station is assigned to its own phase offset and is identified by the phase offset. Therefore, the soft handoff which matches the phase offsets between the pseudo noise (PN) signals each of which belonging to each base station is possible and a mobile station belonging to one cell uses the phase difference of identical signals received from other base stations in order to identify the base stations from which the signals are transmitted.
According to a conventional CDMA system, each of the received PN spread codes is in one to one correspondence with a reference PN spread code having a length of 2
15
bits. Since the method for acquiring the phase offset information of the PN spread code to identify each base station is used in the conventional CDMA system, the amount of calculation for calculating the phase offset is very large and the hardware complexity for the system is also high. It is practically impossible for a terminal or a mobile station to process the large amount of calculations for the phase offset information of the PN spread code. Therefore, the phase offset of the PN spread code is calculated at each base station and then transmitted to each terminal by a modulated signal.
In the CDMA system of the International Telecommunication Standardization Sector (ITSS), the Barker code with a length of 13 is used as its spread code, and many subscribers using the same frequency bandwidth in one base station is assigned to the identical Barker spread code but each code assigned to each subscriber has a different phase offset or time offset from one another. In other words, the information of the phase offset information is the same as the channel number assigned to each subscriber and therefore each subscriber is identified according the phase offset information.
However, the CDMA system of the ITSS, as the other conventional CDMA system described beforehand, uses a modulation method by which the offset information is modulated with a transmission signal in a base station and the modulated signal is transmitted from the base station to a terminal.
A conventional phase synchronization method of a binary code is described in a paper of Acquisition Time Performance of PN Spread Spectrum System, by C.C. Chen published in IEEE Transaction on Communication Vol COM-25 NO.8 PP778-784, August 1997.
In the phase synchronization method of binary code disclosed in the paper, as shown in
FIG. 1
, a received pseudo noise (PN) code is multiplied with a local PN code provided from the stored local PN code updater
4
in the multiplier
1
. The output of the multiplier
1
is integrated by the integrator
2
for the dwell time &tgr;d. The output of the integrator
2
is the correlation value. This correlation value is compared with a predetermined threshold value in the threshold comparator
3
. When the correlation value is not larger than the threshold value, the comparator
3
transmits a control signal to the local PN code updater
4
to update the local PN code so that the local PN code is generated in advance or delayed by one chip interval or a half chip interval. The output of the updated local PN code is multiplied by the received PN code in the multiplier
1
. By performing repeatedly this procedure until the correlation value generated from the integrator
2
is larger than the threshold value, the acquisition of PN code, that is a coarse synchronization, is achieved. If the PN code acquisition is achieved and therefrom the correlation value obtained from the integrator
2
is larger than the threshold value, the PN code tracking, that is a fine synchronization, is performed.
When the total number q of the cell to be searched during one period satisfies q>>1, the average synchronization time of the PN code synchronization method is
T
_
acq
=
(
2
-
P
D
)
(
1
+
KP
FA
)
2
P
D
(
q
τ
D
)
,
[
equation
1
]
where q equals the length of the PN code or a multiple of the length. For example, q equals the length of the PN code when the local PN code is updated by one chip interval and q is twice the length of the PN code when the local PN code is updated by a half chip interval. P
D
is a signal detection probability and P
FA
is a false alarm probability. K is an average penalty according to the false alarm.
The average time from the time at which the system enters the tracking loop by the false alarm and to the time at which the system escapes from the loop is K&tgr;
D
.
In the mobile communication system used in a conventional CDMA system described in detail above, the mobile station (terminal) cannot acquire the phase offset information by itself if the phase offset information concerned with its neighbor cells is not provided by the base station. Therefore, the mobile station cannot simultaneously demodulate both the signal received from the base station to which the mobile station belongs and the signal received from other base stations. Furthermore, when a mobile station is going to move out of the area covered by its present base station and enter another area covered by another base station having a different phase offset, the mobile station cannot match the phase offset of the present base station with the phase offset of the spread code of the new base station. Therefore, the mobile station cannot have a handoff function for itself and the traffic of the base station is increased. Even in the CDMA system, since a subscriber cannot acquire the phase offset being used by the other subscriber without the help of the base station and cannot even pick up a phase offset or a channel which is not occupied by other subscribers, the conventional method has a problem of an increased traffic in the base station.
Since the conventional binary code synchronization method performs repeatedly the procedure that updates the local PN code by increasing or decreasing the code by one chip or a half chip interval until the correlation value is larger than the threshold value, the conventional method has a problem of a long acquisition time.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to solve the problems of the conventional techniques described above.
The present invention provides a binary code phase offset calculation method that the binary code phase offset is easily calculated and therefore the calculation is done in the subscriber terminal.
The present invention also provides a synchronization method of the binary code that greatly reduces the acquisition time by using the binary code phase offset.
Han Young Yearl
Jeong Ok Hyun
Song Young Joon
Birch & Stewart Kolasch & Birch, LLP
LG Electronics Inc.
Tse Young T.
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