Pulse or digital communications – Spread spectrum
Reexamination Certificate
2002-09-25
2004-08-17
Tran, Khai (Department: 2631)
Pulse or digital communications
Spread spectrum
Reexamination Certificate
active
06778588
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to mobile communications technology, and more particularly to a cell initial search method of user equipment (UE) in a CDMA digital cellular mobile communications system with a training sequence (pilot).
BACKGROUND OF THE INVENTION
In a digital cellular mobile communication system, after a UE is turned on, a cell initial search is made first. The purposes of the cell initial search are to select a suitable working frequency and to obtain downlink synchronization between the UE and a base station at this working frequency. Only in this way can the UE correctly receive a message sent by the base station.
Additionally, in a real digital cellular mobile communication system, the main clock in the UE and the main clock in the base station are completely independent. Even if both are working at the same working frequency, there is a carrier deviation (also called frequency difference or frequency deviation) between them. During demodulation, if the UE cannot implement relatively accurate carrier deviation recovery (or said calibration, correction), then there will be a carrier component remaining in the baseband signal, this will affect the processing of the baseband signal, further causing code error and make the UE unable to correctly receive information sent by a base station.
Therefore, for a digital cellular mobile communication system, a UE must perform the following work during the cell initial search: lock-in a working frequency, obtain downlink synchronization with the base station at the locked working frequency, and correct carrier frequency deviation.
Naturally, during a real cell initial search, because the independent main clocks of a base station and UE drift along with time, carrier frequency deviation also must also be traced at the same time.
In a real CDMA cellular mobile communication system, a pilot channel generally performs the downlink synchronization. The conventional implementation of a downlink synchronization is: first, locking-in at a working frequency, solving the correlation between the whole data frame received and a preset pilot sequence (training sequence), and then continuously sliding the working frequency for solving the correlation until the correlation peak is greater than a preset threshold. The downlink synchronization is then performed at this working frequency. The working frequency where the correlation peak is located is the UE receiving position.
In any CDMA cellular mobile communication system, a correlation operation is needed for synchronization. Nevertheless, the conventional correlation operation has the following limitation: as the correlation operation is slid at each chip or even fractional chip level of the whole data frame, the operation volume is huge and a long calculation time is required. Besides, as the correlation operation is taken for the whole data frame, it increases the probability of an error decision, especially, in the Time Division Duplex CDMA (TDD-CMDA) system. In a TDD-CDMA system, suppose, for example, near UE “A” there is another UE “B” in conversation. As the distance between UE “A” and UE “B” is short, UE “A” receives a more powerful signal from UE “B” than the signal power sent by the base station. This leads to a mistaken decision about the correlation peak position that is not the real receiving position of the UE, and produces mistaken downlink synchronization information.
In general, carrier frequency deviation is corrected at a digital demodulator (for general situations, a certain degree of carrier deviation will not affect downlink synchronization, but will affect the demodulated information). An analog phase-locked circuit, which is mature technology and well known in the art, is conventionally used. Disadvantages of this solution are: it is difficult to take account of performance and capture bandwidth at the same time, it is sensitive to carrier jitter and it is a complex hardware circuit.
In the China Patent CN 97115151.2 named “A Method And Device For Carrier Recovery And Compensation In A Frequency Spread Communication System,” a digital correction method for carrier frequency deviation has been proposed. Nevertheless, the method makes an optimal estimation under a channel model without noise and multipath interference, and is not suitable for a cellular mobile communication system.
SUMMARY OF THE INVENTION
The purpose of the invention is to provide a cell initial search method for a CDMA digital mobile communication system. The method improves the conventional cell initial search method, i.e. proposes a solution for downlink synchronization and carrier deviation correction during cell initial search. With this solution the UE can rapidly and accurately perform downlink synchronization with a base station and has a better effect for carrier deviation correction.
An implementation of the invention can be as follows:
A cell initial search method for a digital mobile communication system with CDMA is used for user equipment so that such user equipment may correctly receive information sent by a base station. The method is characterized in that the user equipment selects a working frequency, and obtains downlink synchronization with the base station at the working frequency, comprising:
a. based on a power characteristic window value method of training sequence, a range of downlink training sequence timeslot is decided first;
b. in the range, by calculating correlation of received data and training sequence, an accurate receiving position of the user equipment is obtained.
The said “based on a power characteristic window value method of training sequence,” comprises:
a. in a base station frame structure, increasing the transmitting power of synchronization symbols in the downlink pilot sequence timeslot (DwPTS), and making no transmitting power on protected symbols located before and after the synchronization symbols in the DwPTS;
b. when receiving, user equipment first searches the power characteristic window values of the DwPTS; after a position range of the synchronization symbols has been discovered the correlation operation is only made near the position.
The said “searching the power characteristic window values of the DwPTS to discover a position range of the synchronization symbols,” includes:
UE locking-in at a working frequency, then receiving a data frame; calculating each synchronization symbols' power in the DwPTS; calculating power characteristic window values at each synchronization symbols' position; calculating an average power characteristic window value for the whole data frame; searching for a minimum value of the power characteristic window values at all synchronization symbols position of the whole data frame received; deciding whether the ratio of average power characteristic window value and minimum power characteristic window value is greater than a threshold value, and, if it is, then the position of the minimum power characteristic window value is the beginning position of the DwPTS; solving correlation near the beginning position to obtain an accurate receiving beginning point and end the downlink synchronization.
The said “calculate each synchronization symbol power,” is to assume that the receiving moment is a synchronization symbol beginning point, then adding the powers of all chips belonging to the symbol to obtain the power of each synchronization symbol.
The said “calculate power characteristic window value at each synchronization symbol position,” is for the whole data frame received, calculating each position power characteristic window value R(i) with symbol level sliding at each position with the following formula:
R
i
=
(
∑
k
=
i
i
+
N
-
1
P
(
k
)
+
∑
k
=
i
+
N
+
M
i
+
2
N
+
M
-
1
P
(
k
)
)
/
∑
k
=
i
+
N
i
+
N
+
M
-
1
P
(
k
)
wherein i represents a real receiving position, P(k) represents power value of each symbol, N and M are characteristic window parameters.
The said “calculate power characteristic window value at each synchronization
He Yusong
Li Feng
Ran Xiaolong
Xu Tiezhu
Alston & Bird LLP
China Academy of Telecommunications Technology
Tran Khai
LandOfFree
Method for cell initial search in a CDMA mobile... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for cell initial search in a CDMA mobile..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for cell initial search in a CDMA mobile... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3304429