Multiplex communications – Communication over free space – Combining or distributing information via frequency channels
Reexamination Certificate
1999-11-04
2003-06-24
Kincaid, Lester G. (Department: 2685)
Multiplex communications
Communication over free space
Combining or distributing information via frequency channels
C375S326000, C375S340000, C375S348000, C375S349000, C455S065000
Reexamination Certificate
active
06584092
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method used for OFDM (Orthogonal Frequency Division Multiplexing) based mobile communications.
2. Description of the Related Art
A conventional communication apparatus used for OFDM-based mobile communications (hereinafter referred to as “OFDM communication apparatus”) is explained using FIG.
1
and FIG.
2
.
FIG. 1
is a block diagram showing a configuration of a conventional OFDM communication apparatus and
FIG. 2
is a schematic diagram showing a frame format in an OFDM-based radio communication.
First, the configuration of the conventional OFDM communication apparatus is explained using
FIG. 1. A
baseband signal is input to quasi-coherent detector
101
. This baseband signal is a signal received from an antenna which is not shown in the diagram and then subjected to normal radio reception processing by a radio reception section which is not shown in the diagram. Quasi-coherent detector
101
is controlled by a local signal output from oscillator
114
which will be described later and performs quasi-coherent detection on the input baseband signal. LPF (analog low-pass filters)
102
and
103
eliminate an unnecessary frequency component of the signal subjected to quasi-coherent processing. A/D converters
104
and
105
convert the analog signal with the unnecessary frequency component eliminated to a digital signal.
FFT (Fast Fourier Transform; hereinafter referred to as “FFT”) circuit
106
performs FFT processing on the A/D-converted signal using the output signal of timing generator
116
which will be described later as a trigger signal. Demodulation section
107
demodulates the FFT-processed signal. Determination section
108
determines the demodulated signal.
Delay circuits
109
and
110
delay the A/D-converted signal. Complex multiplier
111
performs complex multiplications using the A/D-converted signals and delayed signals. Accumulator
112
accumulates the complex multiplication result of complex multiplier
111
and outputs the accumulation result to maximum value detector
115
and frequency offset detector
113
.
Maximum value detector
115
detects a maximum value of the accumulation result of accumulator
112
. When maximum value detector
115
detects the maximum value, timing generator
116
outputs a signal to start FFT processing to FFT circuit
106
.
Frequency offset detector
113
calculates a frequency offset necessary for frequency offset compensation using the accumulation result of accumulator
112
and outputs the calculation result to oscillator
114
. Oscillator
114
outputs a local signal with frequency offset compensation to quasi-coherent detector
101
.
Then, the operation off the conventional OFDM communication apparatus is explained. A signal input via an antenna which is not shown in the diagram is subjected to normal radio reception processing by a radio reception section which is not shown in the diagram and converted to a baseband signal. This baseband signal is subjected to quasi-coherent detection processing by quasi-coherent detector
101
. The baseband signal subjected to quasi-coherent detection processing by quasi-coherent detector
101
is stripped of an unnecessary frequency component by LPF
102
and
103
, converted to a digital signal by A/D converters
104
and
105
, and becomes a digital baseband signal.
The digital baseband signal is subjected to FFT processing by FFT circuit
106
where a signal assigned to each sub-carrier is obtained. The signal processed by FFT circuit
106
is demodulated by demodulator
107
, determined by determination section
108
and becomes a demodulated signal.
On the other hand, the communication apparatus in an OFDM-based mobile communication needs to provide timing so that FFT is started with symbol synchronization established with a base station, the transmitting side.
The following is an explanation of how symbol synchronization is established.
In an OFDM-based mobile communication, symbol synchronization is generally established using a synchronization symbol inserted after an AGC (gain control) symbol of each symbol and a phase reference symbol which is identical to the synchronization symbol as shown in FIG.
2
. The phase reference symbol is followed by a guard segment and valid symbol.
First, complex multiplier
111
performs complex multiplications on signals before FFT processing and other signals before FFT processing which have been delayed by one symbol (unit symbol) by delay circuits
109
and
110
.
Then, accumulator
112
accumulates the output of complex multiplier
111
. Since the synchronization symbol and the phase reference symbol have the identical waveform as described above, the accumulation result shows a peak at the start of each guard segment. Maximum value detector
115
detects the accumulation result at this peak. Then, a signal indicating that maximum value detector
115
has detected a maximum value is sent to timing generator
116
. A signal to start FFT processing is sent to FFT circuit
106
by timing generator
116
that has received this signal. FFT circuit
106
receives the signal from timing generator
116
and starts FFT processing.
Through the operation described above, the communication apparatus in the OFDM-based mobile communication can establish symbol synchronization and provide FFT start timing.
Furthermore, since an OFDM-based mobile communication is greatly affected by deterioration of the reception characteristic due to a frequency offset, frequency offset compensation is carried out. The operation of frequency offset compensation is explained below.
In an OFDM-based mobile communication, frequency offset compensation is generally carried out using the synchronization symbol and phase reference symbol shown in FIG.
2
.
First, as described above, complex multiplier
111
performs complex multiplications on signals before FFT processing and other signals before FFT processing which have been delayed by one symbol (unit symbol) by delay circuits
109
and
110
. Accumulator
112
accumulates the complex multiplication results and sends the result to frequency offset detector
113
.
Frequency offset detector
113
calculates the amount of phase rotation using the accumulation result of accumulator
112
and calculates a frequency offset from this amount of phase rotation. This frequency offset is sent to oscillator
114
.
Using the frequency offset sent from frequency offset detector
113
, oscillator
114
generates a local signal with frequency offset compensation and sends it to quasi-coherent detector
101
. Quasi-coherent detector
101
performs quasi-coherent detection under the control of the local signal sent from oscillator
114
.
Through the operation described above, the communication apparatus in the OFDM-based mobile communication prevents deterioration of the reception characteristic due to a frequency offset.
However, the conventional apparatus has the following problems. That is, under a multi-path environment, as shown in
FIG. 3
, the OFDM communication apparatus receives n delay waves, delay wave
1
to delay wave n, in addition to a dominant wave. Thus, the synchronization symbol in the dominant wave receives interference by n AGC symbols of respective delay waves. That is, the synchronization symbol in the dominant wave receives interference because the synchronization symbol in the dominant wave has a time area overlapping with the AGC symbols of delay wave
1
to delay wave n.
Especially, if the delay time of each delay wave is short, the level of the delay wave is high, and therefore the synchronization symbol in the dominant wave, or more specifically, the first half of this synchronization symbol receives greater interference between symbols.
Thus, if the entire synchronization symbol is used for accumulation processing at the time of the aforementioned frequency offset compensation, the conventional apparatus has the problem of the accuracy of frequency offset detection deteriorating.
SUMMARY OF THE IN
Greenblum & Bernstein P.L.C.
Kincaid Lester G.
Matsushita Electric - Industrial Co., Ltd.
Ward Ronald J.
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