Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
1998-10-07
2001-10-16
Pham, Chi (Department: 2731)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C375S316000, C375S130000
Reexamination Certificate
active
06304624
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a coherent detecting method and, more particularly, to a coherent detecting method which performs a detection of synchronism by estimating a propagation characteristic of a propagation path by using a pilot symbol and a data symbol.
2. Description of the Related Art
In a mobile communication system, when a terminal moves under a condition in which a multipath is formed, fluctuation is generated in a receiving signal due to fading. When the received signal is decoded under such a condition, a method for performing a synchronism detection is generally used, in which method a propagation characteristic (a fading complex path) is estimated from a received pilot symbol so as to reduce influences of fading caused by a propagation characteristic of the propagation path.
The above-mentioned pilot symbol is a known symbol having a predetermined amplitude and a predetermined phase. The pilot symbol can be transmitted from a transmitter by being provided in a data frame of data symbols. Alternatively, the pilot symbol may be transmitted through a channel different from a channel through which the data symbols are transmitted.
When the pilot symbol is provided in the data string of the data symbols, the pilot symbol is inserted at a predetermined position of the data frame to be transmitted. On the receiver side, the position of the pilot symbol is detected according to synchronization with a preamble added to the data frame. The detected pilot symbol is demodulated so as to assume or estimate a characteristic of the propagation path from an amplitude and a phase of the detected pilot symbol.
A description will now be given of a conventional receiver including a coherent detecting circuit using a pilot symbol provided in a data string of data symbols.
FIG. 1
is an illustration of a data frame provided with pilot symbols. FIG.
1
-(A) shows a data frame to be transmitted, and FIG.
1
-(B) shows data symbol strings and the pilot symbols in encoded data. As shown in FIG.
1
-(A), the data frame
12
-
1
comprises a preamble
12
-
2
and the encoded data
12
-
3
. The encoded data includes, as shown in FIG.
1
-(B), a plurality of data symbol strings
12
-
4
and a plurality of pilot symbols
12
-
5
. A plurality of pilot symbols are provided between adjacent data symbol strings
12
-
4
at uniform intervals. The portion in which the pilot symbols are provided is referred to as a pilot block
12
-
6
.
The pilot symbol
12
-
5
is a previously determined, known data symbol. The pilot symbol
12
-
5
is transmitted at a predetermined time interval after the preamble
12
-
2
is transmitted by a transmitter. If synchronism is achieved between a receiver and a receiving signal transmitted by the transmitter and received via a propagation path, the receiver can assume a propagation characteristic of the propagation path from the received signal with respect to a time position.
The receiver can synchronize with the receiving signal by detecting the preamble
12
-
2
provided in the data frame
12
-
1
.
Now, a pilot symbol which is the k-th pilot symbol in the n-th pilot block transmitted by the transmitter is represented by Znk, and a propagation characteristic at the time the pilot signal is transmitted is represented by &xgr;nk. A receiving symbol received via the propagation path is represented by Znk·&xgr;nk.
Since a transmitted symbol at the time position is the pilot symbol Znk which is a known symbol, &xgr;nk·|Znk|
2
is obtained by multiplying the receiving symbol by Znk* which is a complex conjugate of the pilot symbol Znk. Since a magnitude (amplitude) of the pilot symbol is a known value (may be |Znk|≡1), the propagation characteristic &xgr;nk of the propagation path can be estimated.
The estimated value &xgr;nk{circumflex over ( )} of the propagation characteristic can be represented as follows.
&xgr;
nk{circumflex over ( )}=Znk·&xgr;nk·Znk*=&xgr;nk·|Znk|
2
(1)
However, in practice, since the receiving symbol is influenced by interference caused by noise and other signals, the propagation characteristic of the propagation path cannot be accurately estimated. In order to more accurately assume the propagation characteristic, a plurality of pilot symbols may be provided in a single pilot block
12
-
6
so as to obtain an estimated value of the propagation characteristic for each of the pilot symbols. An average value of the estimated value of the propagation characteristic is determined to be the estimated value of the propagation characteristic of the propagation path. It is estimated that an estimated value of the propagation characteristic of the n-th pilot block is represented by &xgr;n{circumflex over ( )}.
The propagation characteristic of the propagation path between two pilot blocks
12
-
6
can be obtained by averaging propagation characteristics at the positions of the two pilot blocks
12
-
6
or performing a liner interpolation on the propagation characteristics.
After the estimated value of the propagation characteristic of the propagation path is obtained, the transmitted data symbol is obtained as follows. It is estimated that the i-th transmitted data symbol between the n-th pilot block and the (n+1)-th pilot block is represented by Xni; an actual value of the propagation characteristic of the propagation path is represented by &xgr;ni; an estimated value of the propagation characteristic of the propagation path is represented by &xgr;ni{circumflex over ( )}; and a decoded data symbol is represented by Xni{circumflex over ( )}.
The received data symbol transmitted through the propagation path is represented by Xni·&xgr;ni, which is obtained by multiplying the transmission data symbol Xni by the actual value &xgr;ni of the propagation characteristic of the propagation path. The transmission data symbol Xni can be obtained as the decoded data symbol Xni{circumflex over ( )}, in which the influence of the propagation characteristic &xgr;ni is reduced, by multiplying Xni·&xgr;ni by the complex conjugate &xgr;ni{circumflex over ( )}* of the estimated value &xgr;ni{circumflex over ( )} of the propagation characteristic. The decoded data symbol Xni{circumflex over ( )} is represented by the following equation.
Xni{circumflex over ( )}=Xni·&xgr;ni·&xgr;ni{circumflex over ( )}*/|&xgr;ni{circumflex over ( )}|
2
(2)
The thus-obtained decoded data symbol is subjected to a diversity-combining and is determined as a predetermined discrete data symbol when compared with a predetermined threshold value by a determining circuit. Thereafter, the decoded data symbol is subjected to a decoding process such as deinterleaving or a bit error correction so as to be reproduced as data.
FIG. 2
shows a receiver including a conventional coherent detecting circuit using pilot symbols provided in data symbols.
In
FIG. 2
, a signal received by an antenna (ANT)
13
is input to a radio unit
14
. In the radio unit
14
, the received signal is amplified by an amplifier (LNA)
14
-
1
, and all but a component of a predetermined bandwidth is eliminated by a band-pass filter (BPF)
14
-
2
. Additionally, the received signal is converted into a base bandwidth by being multiplied by a signal LO in a mixer
14
-
3
. Then, a high-frequency component is removed by a low-pass filter (LPF)
14
-
4
, and the received signal is output to an A/D circuit
15
.
The A/D circuit
15
quantizes the received signal supplied by the radio unit
14
so as to convert the received signal into a digital signal, and outputs the digital signal to a timing synchronizing circuit
16
. The timing synchronizing circuit
16
performs a synchronization by using the digitized received signal, and outputs the digitized received signal to a coherent detecting circuit
17
.
In the coherent detecting circuit
17
, a propagation path estimation circuit
17
-
1
calculates the estimated value &xgr;ni{circumflex over ( )} of the propag
Kobayakawa Shuji
Seki Hiroyuki
Tanaka Yoshinori
Toda Takeshi
Fujitsu Limited
Helfgott & Karas P.C.
Pham Chi
Tran Khai
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