Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2000-01-05
2003-07-15
Tran, Khai (Department: 2731)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S152000
Reexamination Certificate
active
06594324
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a correlator (matched filter) used in de-spreading method on a receiver in code division multiple access (CDMA) for use in a spread spectrum communication system, particularly to a matched filter which can reduce a circuit scale without deteriorating a path detection sensitivity.
2. Description of the Related Art
In a CDMA system, after primary modulation of information data is performed in a transmitting unit, code modulation (secondary modulation) is performed using a sequence of codes (a sequence of transmitted codes) having a faster rate than that of the data modulation (primary modulation), so that a transmitted complex signal is generated.
In CDMA data communication in which quadrature phase shift keying (QPSK) is used as a system of modulating the information data and codes, when the information data is set to S, and the transmitted code sequence is set to C, a transmitted complex signal TX can be represented as follows:
TX
=
S
·
C
=
(
Si
+
j
Sq
)
·
(
Ci
+
j
Cq
)
=
(
Si
·
Ci
-
Sq
·
Cq
)
+
j
(
Si
·
Cq
+
Sq
·
Ci
)
=
TXi
+
j
TXq
[
Equation
1
]
Here, for the information data S, when an in-phase component is represented by Si, and a quadrature component is represented by Sq, the in-phase component and the quadrature component are in an orthogonal relation, and the quadrature component Sq is multiplied by an imaginary number j and represented. Similarly, for the code sequence C, when an in-phase component is represented by Ci, and a quadrature component is represented by Cq, the in-phase component and the quadrature component are in the orthogonal relation, and the quadrature component Cq is multiplied by the imaginary number j and represented.
Furthermore, when the transmission information data is taken from the transmitted complex signal, that is, the data demodulation (de-spreading) is performed on the side of the receiving unit, the received complex signal and the transmitted code sequence used in the spreading modulation need to be subjected to the complex conjugate correlating operation.
In this case, a searcher for use on the receiver side of the CDMA system has a role of synchronization capture to select a code which is complex/conjugate with the transmitted code sequence used in the spreading modulation on the transmission side, that is, an accurate received code sequence, and further to find the transmission timing of the transmitted complex signal.
A procedure of selecting the received code sequence in the searcher comprises, in the same manner as in the data demodulation, performing the complex conjugate correlating operation of the received complex signal and the received code sequence, and performing power adding operation with respect to the operation result of the in-phase component and the quadrature component.
Here, a principle of selecting the received code sequence in the searcher will be described.
A first case will be described in which a certain code sequence C* in a complex conjugate relation with the transmitted code sequence used for generating the transmitted complex signal in the transmitter is used as a received code sequence during the correlating operation of the searcher.
Assuming that a transmitted complex signal is TX, the transmitted complex signal TX is subjected to code modulation as shown in [Equation 1] in the transmitting unit, and that the transmitted complex signal TX is received as it is to form a received complex signal, a correlating operation result R
1
of the code sequence C* having the complex conjugate relation with the transmitted code sequence C, and the transmitted (received) complex signal TX is represented by the following equation:
R1
=
TX
·
C
*
=
(
TXi
+
j
TXq
)
·
(
Ci
-
j
Cq
)
=
TXi
·
Ci
+
TXq
·
Cq
+
j
(
TXq
·
Ci
-
TXi
·
Cq
)
[
Equation
2
]
In the above [Equation 2], the multiplying (correlating) operations of the in-phase component TXi and quadrature component TXq of the received complex signal, and the in-phase component Ci and quadrature component Cq of the received code sequence used in the searcher are independently performed. This means that four correlators have to be prepared as hardware.
Moreover, for the second stage of the above [Equation 2], when TXi+jTXq is developed according to [Equation 1], the following is obtained:
R1
=
{
(
Si
·
Ci
-
Sq
·
Cq
)
+
j
(
Si
·
Cq
+
Sq
·
Ci
)
}
·
(
Ci
-
j
Cq
)
=
(
Si
·
Ci
·
Ci
-
Sq
·
Cq
·
Ci
+
Si
·
Cq
·
Cq
+
Sq
·
Ci
·
Cq
)
+
j
(
Si
·
Cq
·
Ci
+
Sq
·
Ci
·
Ci
-
Si
·
Ci
·
Cq
+
Sq
·
Cq
·
Cq
)
When the multiplication of the code sequence is represented as a correlating operation result by a correlation function Rxx, the following is obtained:
=
(
Si
·
Rii
-
Sq
·
Riq
+
Si
·
Rqq
+
Sq
·
Riq
)
+
j
(
Si
·
Riq
+
Sq
·
Rii
-
Si
·
Riq
+
Sq
·
Rqq
)
=
Si
·
(
Rii
+
Rqq
)
+
j
Sq
·
(
Rii
+
Rqq
)
[
Equation
3
]
Here, the correlation function Rxx indicates the correlating operation result of a certain code sequence and another code sequence. When two affixed letters are the same, a result (auto-correlation function) of the correlating operation of the same code sequence is indicated. When the affixed letters are different, the function is classified as a result (cross-correlation function) of the correlating operation of different code sequence.
Here, for the system of codes in the CDMA system, the auto-correlation function is highest, and the cross-correlation function has a sufficiently small value as compared with the auto-correlation function. Therefore, for the sake of simplicity, the auto-correlation function is defined as 1, and the cross-correlation function is defined as 0 in the description.
According to the above-described definition, the correlating operation result R
1
of the searcher obtained by [Equation 3] can be represented as follows:
R
1
=2
·Si+j
2
·Sq=X+jY
[Equation 4]
When power adding operation is performed on the operation result of the in-phase component and the quadrature component obtained by [Equation 4], the following results:
P
1
=|
X|
2
+|Y|
2
=4·(|
Si|
2
+|Sq|
2
)
For the information data Si, Sq, when data of ±1 is transmitted, the following result is obtained:
P
1
=4·(1+1)=8
This means that when the received code sequence comprises the code sequence C* having the complex conjugate relation with the transmitted code sequence C, the power adding operation P
1
obtains a constant value of 8 irrespective of the content of transmission information (information data Si, Sq).
A second case will next be described in which a code sequence Cn not placed in the complex conjugate relation with the transmitted code sequence C used for generating the transmitted complex signal in the transmitter is used as the received code sequence in the correlating operation of the searcher.
In the same manner as in the first case, assuming that the transmitted complex signal is TX, the transmitted complex signal TX is subjected to the code modulation as shown in [Equation 1] in the transmitting unit, and the transmitted complex signal TX is received as it is to form a received complex signal, a correlating operation result R
2
of the code sequence Cn not pla
Miyatani Tetsuhiko
Watanabe Jun
Jacobson & Holman PLLC
Kokusai Electric Co. Ltd.
Tran Khai
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