Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
2000-02-16
2003-08-19
Ghayour, Mohammad H. (Department: 2634)
Pulse or digital communications
Equalizers
Automatic
C375S325000
Reexamination Certificate
active
06608863
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a coherent demodulation system for multichannel modulation system used in digital radio communication system which uses multicarrier modulation system, for instance, OFDM (Orthogonal Frequency Division Multiplexing) as modulation/demodulation system. In particular, the present invention relates to such a system which can be used in an environment in which transfer function or attenuation depends upon each subcarrier in multipath transmission circuit.
In multicarrier modulation system, such as OFDM modulation/demodulation system, a plurality of subcarriers having orthogonal relations to each other are used for transmission of communication. In a transmit side, an information signal is modulated through DQPSK (Differential Quadrature Phase Shift Keying) or 16 QAM (Quadrature Amplitude Modulation) for each subcarriers. The modulated output is applied to IFFT (Inverse Fast Fourier Transform) for inverse transformation so that block demodulation of OFDM signal is obtained.
When a DQPSK system is used for modulation of subcarriers, a delay detection system may be used for demodulation in a receive side. However, a delay detection has disadvantage that the requested Eb/No ratio must be higher by 3 dB as compared with a coherent detection system. Therefore, a coherent detection system is preferable for providing a high quality transmission circuit.
When a coherent detection system is used in a demodulation circuit in a receive side, a transmit side may use a multi-level QAM system as a modulation system. The use of a multi-level QAM system increases transmission rate of information.
When multi-paths are generated in a radio transmission channel between a transmit side and a receive side, transfer function or attenuation for propagation in the circuit depends upon each subcarriers. Therefore, if a coherent detection system is used, a channel transfer function or attenuation must be estimated for estimating amplitude information and phase information of each subcarriers which are subject to vary in each multipath circuit. Further, in a radio LAN (Local Area Network) which transfers information by using a packet, a channel transfer function or attenuation must be estimated for each packet.
A packet has in general a preamble signal at the head of each packet for synchronization purpose. Therefore, a coherent detection circuit may initially carry out channel estimation by using the preamble signal. However, if it takes long time for initial channel estimation, a throughput in a circuit would decrease, and therefore, a high speed initial channel estimation is desired. Thus, an accurate channel estimation by using short preamble signal is essential.
In a radio LAN environment, it is possible to handle that a transmission channel is quasi-static in which a transmission channel does not change during a period of each packet. In such an environment, a coherent detection is possible only by initial channel estimation.
On the other hand, when there is large variation in an environment, a coherent detection system must carry out to estimate channel transfer function or attenuation continuously.
FIG. 16
shows a block diagram of a prior OFDM coherent detection system, in which a coherent detection is carried out for each packet by using a preamble signal. This is shown in S. K. Wilson, E. K. Lhayata and J. M. Cioffi, “16 QAM Modulation with Orthogonal Frequency Devision Multiplexing in a Rayleigh-Fading Environment”, Proc. of VTC'94, pp 1660-1664.
In
FIG. 16
, a symbol timing detection circuit
1
detects a symbol timing of a receive signal a
101
. An output a
102
of the symbol timing detection circuit
1
is applied to a S/P (serial to parallel) conversion circuit
2
.
A timing signal a
114
detected by the symbol timing detection circuit
1
is applied to a control circuit
11
, which generates a signal a
105
for switching a preamble part and a data part of a receive signal, and for deciding the duration for holding a channel estimation signal depending upon a period of each receive packet. The signal a
105
is applied to a hold circuit
8
.
A Fourier transform circuit (FFT)
3
carries out Fourier transformation of an OFDM demodulation signal a
103
. That is, a parallel input signal a
103
in time-domain is converted into a parallel signal a
104
in frequency-domain. An output a
104
of the Fourier transform circuit
3
is a receive vector signal of each subcarriers.
The signal a
104
is applied to a P/S (parallel to serial) conversion circuit
4
which converts a parallel input signal to a serial output signal a
106
. The switching circuit
5
switches the serial signal a
106
into a preamble signal a
107
and a data signal a
111
based upon the control signal a
105
supplied by the control circuit
11
.
The preamble signal a
107
is applied to a channel estimation circuit
7
, which further receives a reference preamble signal a
108
from a preamble signal storage circuit
6
which stores a reference preamble signal. The channel estimation circuit
7
carries out the channel estimation for a preamble signal of a receive vector a
107
of each subcarriers by using a reference preamble signal a
108
according to the following equation (1).
r
receive
/r
store
(1)
where;
r
receive
is a received preamble signal a
107
(complex number)
r
store
is a reference preamble signal a
108
(complex number)
The signal a
109
which shows the result of the channel estimation is applied to the hold circuit
8
which holds the signal a
109
for a packet period according to the control signal a
105
.
On the other hand, the data signal a
111
is applied to a divider circuit
9
which further receives an output a
110
of the hold circuit
8
. The divider circuit
9
carries out the division (a
111
/a
110
) so that coherent detection is obtained. The signal a
112
obtained by the coherent detection is applied to a decision circuit
10
which decides a data (0 or 1) and outputs the decided output signal a
113
.
As described above, a coherent detection circuit in
FIG. 16
carries out the complex division of a received preamble signal by a reference preamble signal stored in the storage circuit
6
for the channel estimation.
FIG. 17
shows another prior coherent detection circuit which has a carrier filter. This is described in P. Hoeher, “TCM on Frequency-Selective Land-Mobile Fading Channels”, Proc. of 5th Tirrenia International Workshop Digital Communication, Tirrenia, Italy, September 1991.
The most portions of
FIG. 17
are the same as those of
FIG. 16
, except that
FIG. 17
has a carrier filter
1011
at the output of the channel estimation circuit
107
.
In
FIG. 17
, a symbol timing detection circuit
101
detects a symbol timing in a receive signal a
1101
. An output a
1102
of the symbol timing detection circuit
101
is applied to a S/P conversion circuit
102
.
A timing signal a
1115
provided by the symbol timing detection circuit
101
is applied to a control circuit
1012
, which generates a control signal a
1105
according to a timing signal a
1115
. The control signal a
1105
functions to switch a preamble part and a data part of a receive signal, and to hold a signal according to duration of a receive packet.
A parallel signal a
1103
of an output of the S/P conversion circuit
102
is applied to a Fourier Transform circuit
103
which provides a receive vector signal a
1104
for each subcarriers.
A P/S conversion circuit
104
converts a receive vector signal a
1104
for each subcarriers into a serial signal a
1106
. A switching circuit
105
switches the receive vector in serial form a
1106
into a preamble signal a
1107
and a data signal a
1111
according to the control signal a
1105
.
The preamble signal a
1107
is applied to a channel estimation circuit
107
, which further receives a reference preamble signal from a preamble storage circuit
106
. The channel estimation circuit
107
carries out the channel estimation for a receive vector a
1107
of each subcarriers by using a reference preamble signal a
Kumagai Tomoaki
Mizoguchi Masato
Morikura Masahiro
Onizawa Takeshi
Sakata Tetsu
Ghayour Mohammad H.
Nippon Telegraph and Telephone Corporation
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