Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-05-27
2002-04-30
Vo, Don N. (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S260000, C370S203000, C370S210000
Reexamination Certificate
active
06381263
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a receiving method and a receiving apparatus for receiving a signal subjected to an orthogonal frequency division multiplexing (to be referred to as an OFDM hereinafter) modulation and, more particularly, to a technique for detecting a frequency error of a received signal.
2. Description of the Related Art
As one of modulation systems used when digital data having a relatively large capacity is transmitted in a wireless mode or the like, OFDM modulation is practically used. The OFDM modulation system is a system for transmitting transmission data as a multi-carrier which is dispersed into a plurality of sub-carriers. This modulation system can efficiently transmit a large-capacity data in a wireless mode.
As a wireless transmission state of a signal subjected to the OFDM modulation, there may be such a case that one unit of OFDM modulation signal is continuously transmitted in a predetermined frequency band. For example, as shown in
FIG. 1
, in a wireless telephone system, it is assumed that OFDM modulation signals transmitted in a wireless mode from a plurality of mobile stations MS
1
, MS
2
, MS
3
, and MS
4
are subjected to a receiving process by a base station. In this case, the OFDM modulation signals from the respective mobile stations MS
1
to MA
4
are transmitted as multi-carrier signals respectively using different bands F
1
to F
4
, and all the bands F
1
to F
4
are subjected to a receiving process by the base station. A system in which multi-connection between the base station and a plurality of surrounding mobile stations is made by using OFDM modulation waves as described above has been proposed.
By the way, as shown in
FIG. 1
, when signals transmitted from a plurality of mobile stations are subjected to a receiving process by a base station, a frequency offset of respective transmission signals must be detected.
FIG. 2
is a diagram showing an example of a circuit arrangement for detecting a frequency offset in a conventional base station. This example is a case wherein the frequency offsets of signals from the four mobile stations MS
1
to MS
4
shown in
FIG. 1
are detected. A receiving processor
2
connected to an antenna
1
performs the signals of the transmission bands at once, and then the received signals are supplied to four band-pass filters
3
a
,
3
b
,
3
c
, and
3
d
having different passing bands, respectively. In this case, the passing bands of the band-pass filters
3
a
to
3
d
are set for the transmission bands F
1
to F
4
shown in
FIG. 1
, respectively.
Outputs from the band-pass filters
3
a
to
3
d
are supplied to different frequency offset detection circuits
4
a
,
4
b
,
4
c
, and
4
d
, respectively. The frequency offsets of transmission signals from the four mobile stations MS
1
to MS
4
are independently subjected to a detection process by the frequency offset detection circuits
4
a
to
4
d
, respectively. On the basis of amounts of frequency offsets detected by the frequency offset detection circuits
4
a
to
4
d
, the following correction process is performed. That is, the reception frequencies of signals transmitted from the mobile stations are corrected, or data for correcting a frequency offset is transmitted to a corresponding mobile station.
As a frequency offset detection process performed in each of the frequency offset detection circuits
4
a
to
4
d
, for example, a detection process using a cycle prefix component included in an OFDM modulation signal is known. An example of a frequency offset detection arrangement using the cycle prefix component is shown in
FIG. 3. A
signal from a terminal
5
at which an output from each of the band-pass filters
3
a
to
3
d
can be obtained is supplied to a multiplier
7
through a delay circuit
6
for delaying the signal for a predetermined period, and a signal obtained at the terminal
5
is directly supplied to the terminal
5
. Both the signals are subjected to a multiplying process. In this case, an amount of delay of the delay circuit
6
is set to be an inherent amount of delay included in the cycle prefix component. The relationship between the cycle prefix component and the amount of delay will be described later. The signal delayed by the delay circuit
6
is a complex signal. A multiplying process of a complex conjugate is performed in the multiplier
7
.
An output from the multiplier
7
is supplied to an averaging circuit
8
to calculate an average in a predetermined period, and the average value is supplied to an output terminal
9
as a value which is in proportion to a frequency offset value. The period in which averaging is performed by the averaging circuit
8
is set to be a period corresponding to one length (time) of a cycle prefix component, for example. With such a process, a value which is in proportion to the frequency offset value is obtained at the output terminal
9
.
A cycle prefix component included in an OFDM modulation signal will be described below. First, a complex sine wave is defined by a function rot() shown by Equation (1) equation.
rot(s)=exp(j2&pgr;s) (1)
At this time, an OFDM modulation signal before windowing for transmission can be described as represented by the following Equation (2) equation.
x
(
t
)
=
∑
k
=
a
b
x
k
rot
(
k
f
c
t
)
(
2
)
Heare, symbol x
k
denotes a transmission symbol (transmission symbol put on a kth sub-carrier), and symbol f
c
denotes a sub-carrier interval. The OFDM modulation signal represented by Equation (2) and obtained before windowing is performed can also expressed as represented by the following Equation (3).
x
(
t
+
T
c
)
=
∑
k
=
a
b
x
k
rot
(
k
f
c
(
t
+
T
c
)
)
=
∑
k
=
a
b
x
k
rot
(
k
f
c
t
+
f
c
T
c
)
=
∑
k
=
a
b
x
k
rot
(
k
f
c
t
)
=
x
(
t
)
(
3
)
In this case, T
c
=1/f
c
is established, and the OFDM modulation signal is represented by a periodic function of a period Tc depending on the sub-carrier interval f
c
. More specifically, for example, when fc=4.1666 [kHz]=100 [kHz]/24, T
c
=240 [&mgr;s]. In this case, an OFDM signal obtained before windowing is performed has, for example, as shown in
FIG. 4
, a signal waveform having a periodicity of 240 [&mgr;s].
When the OFDM modulation signal is to be transmitted, the process for multiplying windowing data (time waveform) called a transmission window. A signal obtained by multiplying the windowing data is represented by the following equation. In this equation, w(t) denotes windowing data (window).
y
(
t
)
=
∑
k
=
a
b
x
k
rot
(
k
f
c
t
)
w
(
t
)
(
4
)
FIG. 5
is a diagram showing an example of windowing data. As described above, when T
c
=240 [&mgr;s], windowing data based on the time is generated. In this case, for simplifying the description, a window is represented by an rectangular wave, and the window is set to be a window in a period L (280 [&mgr;m]) obtained by adding a period T
g
(40 [&mgr;s]) to the above-mentioned period T
c
(240 [&mgr;s]). As an OFDM modulation wave multiplied by the window, an OFDM wave in the period T
g
at the start portion of one unit of window and an OFDM wave in the period T
g
, at the end portion, having a time width equal to that of the period T
g
at the start portion have completely an equal waveform. This modulation wave is called a cycle prefix. This cycle prefix corresponds to a cycle prefix CP shown in FIG.
5
.
The OFDM wave multiplied by the windowing data is represented by the following equation when a frequency offset of v [Hz] is put on the OFDM wave.
y
(
t
)
=
∑
k
=
a
b
x
k
rot
(
k
&e
Maioli Jay H.
Sony Corporation
Vo Don N.
LandOfFree
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