Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Reexamination Certificate
2000-03-27
2004-01-20
Ngo, Ricky (Department: 2697)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
C370S210000, C370S344000, C370S503000, C375S260000
Reexamination Certificate
active
06680901
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) demodulator of a burst signal transfer system using an OFDM modulating method and, more particularly, to an OFDM demodulator in which a processing delay of a synchronization circuit can be shortened.
An OFDM modulating method as a modulating method promising for a high-speed data transfer such as a high-speed wireless LAN has been being examined. A conventional OFDM demodulator for receiving an OFDM modulated signal will be described by referring to drawings.
FIG. 4
is a block diagram showing the configuration of a conventional OFDM demodulator.
FIG. 5
is a diagram showing a format of an OFDM burst signal.
As shown in
FIG. 5
, at the head of each burst data
23
, a preamble
21
for symbol synchronization and a preamble
22
for estimating a carrier frequency and a channel characteristic are provided.
In
FIG. 4
, an OFDM burst signal is received by an antenna
1
. An orthogonal component detector
2
performs quasi-synchronous detection (QUASI-SYNC) on the received signal by using a local signal having a frequency which is very close to a carrier frequency to thereby obtain an analog complex baseband signal.
A/D converters
3
and
4
sample and quantize an analog complex baseband signal of an orthogonal component and an in-phase component outputted from the orthogonal component detector
2
.
A synchronization circuit
10
receives a digital complex baseband signal sampled and quantized by the A/D converters
3
and
4
and performs a synchronizing operation.
FIG. 6
is a block diagram showing the synchronization circuit
10
. In
FIG. 6
, a symbol timing estimating circuit
14
which receives the digital complex baseband signal establishes symbol synchronization by the digital complex baseband signal sampled and quantized which is outputted from the A/D converters
3
and
4
at the time of receiving the preamble
21
for symbol synchronization shown in FIG.
5
.
The symbol timing estimating circuit
14
outputs a symbol timing to a carrier frequency estimating circuit
11
and a symbol synchronization processing circuit
13
.
On receipt of the preamble
22
for estimating the carrier frequency and channel characteristic having repetitive patterns, the carrier frequency estimating circuit
11
detects phase rotation in the same patterns which periodically appears, estimates a carrier frequency error, and outputs a frequency error compensating signal to a complex multiplier
12
. A specific configuration of a carrier frequency estimating circuit is described in, for example, F. Daffara and O. Adami, “A new frequency detector for orthogonal multicarrier transmission techniques” Proc. of VTC' 95, pp 804-809.
A delay circuit
15
receives the digital complex baseband signal, delays the signal by predetermined time (which is about time of the preamble
22
for estimating the carrier frequency and channel characteristic) and, supplies the resultant signal to the complex multiplier
12
.
The complex multiplier
12
complex-multiplies the frequency error compensated signal by the output of the delay circuit
15
, thereby compensating a frequency deviation of the carrier wave.
After establishing symbol synchronization with the output of the complex multiplier
12
by the symbol synchronization processing circuit
13
, the preamble
22
for estimating the carrier frequency/channel characteristic is outputted from the synchronization circuit
10
.
A fast Fourier transform (FFT) circuit
6
in
FIG. 4
Fourier transforms an output signal of a synchronization circuit
5
and divides the OFDM modulated signal into signals of subcarriers.
On receipt of the preamble
22
, a channel distortion estimating circuit
16
receives the signals of subcarriers from the FFT circuit
6
and estimates a channel characteristic H(&ohgr;).
The preamble
22
for estimating the carrier frequency/channel characteristic is used to estimate the channel characteristic H(&ohgr;) in the channel distortion estimating circuit
16
.
As a result of estimation, the channel distortion estimating circuit
16
outputs a coefficient 1/H(&ohgr;) for compensating channel distortion to a channel distortion compensating circuit
8
.
The channel distortion compensating circuit
8
receives the signals of respective subcarriers and compensates the channel distortion by complex-multiplying the signal by the coefficient 1/H(&ohgr;) for compensating the channel distortion.
FIG. 7
is a block diagram of the channel distortion estimating circuit
16
. In the diagram, the preamble signal
22
for estimating the carrier frequency/channel characteristics is supplied to a complex multiplier
161
in the channel distortion estimating circuit
16
. The inverse number of the pattern of the preamble signal
22
is stored in a reference signal storing circuit
163
and is supplied to the other input terminal of the complex multiplier
161
in the channel distortion estimating circuit
16
. Both input signals are multiplied by each other in the complex multiplier
161
and an estimation result H(&ohgr;) of the channel characteristic is obtained as an output of the complex multiplier
161
. The output H(&ohgr;) of the complex multiplier
161
is supplied to an inverse number circuit
162
and a coefficient 1/H(&ohgr;) for compensating the channel distortion is computed. The coefficient 1/H(&ohgr;) is multiplied by an output of the FFT
6
in the complex multiplier
8
, thereby compensating the channel distortion.
A subcarrier demodulating circuit
9
receives the distortion compensating signal and demodulates it every subcarrier.
As described above, the conventional OFDM demodulator has the delay circuit
15
in the synchronization circuit
10
used for compensating the carrier frequency deviation with respect to the preamble for estimating the carrier frequency and channel characteristic.
According to the orthogonal frequency division multiplexing (OFDM) modulating system, data to be transmitted is divided into a plurality of low-speed subcarriers. The period of a symbol is therefore long (generally, about 4 &mgr;sec) and delay time in the delay circuit
15
is integer times or the unit of the symbol. In this case, such long delay time in the synchronization circuit
10
denotes an increase in the synchronization processing time on the symbol unit basis.
There is consequently a problem of a low throughput of a whole OFDM communication system due to the delay time in the delay circuit in the synchronization circuit.
SUMMARY OF THE INVENTION
In order to solve the problems, according to the invention, there is provided an orthogonal frequency division multiplexing (OFDM) demodulator for demodulating an OFDM modulated signal, comprising: a quasi-synchronous detector for converting the OFDM modulated signal into a complex baseband signal of two components by quasi-synchronous detection; a synchronization circuit for receiving the complex baseband signal and establishing symbol synchronization; an FFT for Fourier transforming an output of the synchronization circuit into signals of respective subcarriers; a channel distortion estimating circuit for compensating a carrier frequency error on the basis of an output of the FFT and a frequency error compensation signal outputted from the synchronization circuit and estimating channel distortion; a channel distortion compensating circuit for compensating channel distortion in the output of the FFT on the basis of an output of the channel distortion estimating circuit; and a demodulating circuit for demodulating an output of the channel distortion compensating circuit every subcarrier.
Specifically, a frequency error compensation signal is detected by using a preamble signal for estimating carrier frequency/channel characteristic in a carrier frequency estimating circuit in a synchronization circuit having no delay circuit. The frequency error compensation signal is outputted to a complex multiplier in a propagation path distortion estimating circuit. After phase rotation caused by a
Okanoue Kazuhiro
Osawa Tomoki
Yamamoto Takeshi
Ngo Ricky
Swickhamer Christopher M
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