Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Reexamination Certificate
1999-07-08
2002-10-01
Yao, Kwang Bin (Department: 2664)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
C370S343000
Reexamination Certificate
active
06459679
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for receiving orthogonal frequency division multiplexed (OFDM) signals, and more particularly, to a method and apparatus for timing and frequency synchronization of an OFDM signal receiver to an OFDM signal.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a robust technique for efficiently transmitting data using a plurality of sub-carriers within a channel bandwidth. These sub-carriers are arranged for optimal bandwidth efficiency compared to more conventional transmission approaches, such as frequency division multiplexing (FDM). FDM separates and isolates the sub-carrier frequency spectra, and requires a frequency guard band to avoid inter-sub-carrier interference, thereby increasing overhead and degrading bandwidth efficiency.
By contrast, although optimal bandwidth efficiency is obtained by overlapping the frequency spectra of OFDM sub-carriers, the OFDM sub-carriers must remain orthogonal to one another to prevent interference between sub-carriers. Additionally, an OFDM symbol is resistant to multipath fading because it is significantly long compared to the length of the channel impulse response and inter-symbol interference can be completely prevented.
FIG. 1
is a block diagram of a typical OFDM signal transmitter. An encoder
110
encodes a stream of input data bits b
n
and outputs a stream of sub-symbols X
n
. An inverse fast Fourier transformer (IFFT)
115
performs an N-point inverse discrete Fourier transformation (IDFT) or inverse fast Fourier transformation (IFFT) on the stream of sub-symbols X
n
. Here, n denotes a frequency-domain index, and also can denote a sub-carrier index. N sub-symbols X
n
are equivalent to one frequency-domain OFDM symbol, and they are typically phase shift keyed (PSK) signals or quadrature amplitude modulated (QAM) signals.
A frequency-domain OFDM symbol is usually designated as zero at a zero-frequency DC and around the edges of a passband, as shown in FIG.
2
. Accordingly, a transmitter/receiver can easily perform analog filtering, and the influence of noise on a received signal is reduced. IFFT
115
transforms the frequency-domain OFDM symbol into a time-domain symbol according to the following Equation 1:
x
K
=
1
/
N
∑
n
=
0
N
-
1
X
n
ⅇ
j
2
nkn
/
N
,
k
=
0
,
…
N
-
1
(
1
)
wherein x
k
denotes samples of a time-domain OFDM symbol, and k is a time-domain index.
A digital signal processor (DSP)
120
adds a cyclic prefix or guard interval of G samples before N samples, i.e., a sequence of the output of IFFT
115
. Thus, one time-domain OFDM symbol is comprised of (G+N) samples, as shown in FIG.
3
. The cyclic prefix is comprised of the last G samples among the output of IFFT
115
. This cyclic prefix is typically longer than the channel impulse response and, therefore, acts to prevent inter-symbol interference between consecutive OFDM symbols.
The output of DSP
120
is divided into real and imaginary-valued digital components. The real and imaginary-valued digital components are then passed to digital-to-analog converters (DACs)
130
and
135
, respectively. DACs
130
and
135
convert the real and imaginary-valued digital components into analog signals at a sampling frequency of fs=1/Ts Hz as determined by a clock circuit
125
. The analog signals pass through low pass filters (LPFs)
140
and
145
and become in-phase and quadrature OFDM analog signals, respectively. The in-phase and quadrature OFDM analog signals are then passed to mixers
160
and
165
.
As a result of the above IFFT, D/A conversion, and low pass filtering, N sub-symbols in the OFDM symbol are transmitted by being carried on N sub-carriers. As shown in
FIG. 4
, the sub-carriers each display a sinc(x)=sin(x)/x spectrum in the frequency domain, and the peak frequencies of the sub-carriers are spaced fs/N=1/NTs Hz apart from each other. Here, when the time for N samples in one OFDM symbol is T. T is equal to NTs. Also, although the spectra of the sub-carriers overlap, a given sub-carrier remains orthogonal to neighboring sub-carriers because neighboring sub-carriers become null at the peak of the given sub-carrier.
In mixers
160
and
165
, the in-phase and quadrature OFDM analog signals from LPF
140
and
145
are mixed with an in-phase intermediate frequency (IF) signal and a 90° phase-shifted IF signal, respectively, in order to produce an in-phase IF OFDM signal and a 90° phase-shifted (quadrature) IF OFDM signal, respectively. The in-phase IF signal fed to the mixer
160
is produced directly by an IF local oscillator (Lo)
150
, while the 90° phase-shifted IF signal fed to the mixer
165
is produced by passing the in-phase IF signal produced by Lo
150
through a 90° phase-shifter
155
before feeding it to mixer
165
. These two in-phase and quadrature IF OFDM signals are then combined in a combiner
167
, and the combined IF OFDM signal is transmitted via a radio frequency (RF) signal transmitter
170
.
The RF signal transmitter
170
includes a bandpass filter (BPF)
175
, an RF mixer
183
, an RF carrier frequency local oscillator (Lo)
180
, another BPF
185
, an RF power amplifier
190
, and an antenna
195
. The combined IF OFDM signal from combiner
167
is filtered by the BPF
175
, and shifted by the frequency of the Lo
180
by the mixer
183
. The frequency-shifted signal is again filtered by the BPF
185
, amplified by the RF power amplifier
190
, and finally transmitted via the antenna
195
. When the sum of the frequencies of the Lo
150
and the Lo
180
is fc for convenience′ sake, fc becomes the central frequency of a passband signal, i.e., a carrier frequency. The frequency fs of the clock circuit
125
determines the bandwidth of a transmitted signal and the sub-carrier frequency interval.
A receiver for receiving signals transmitted through the above-described process and restoring original data bits is essentially configured such that its component units are arranged opposite to those of the transmitter.
FIG. 5
is a block diagram of the configuration of a typical OFDM signal receiver. An RF receiver
210
usually includes an antenna
212
, a low noise amplifier
215
, a bandpass filter BPF
217
, an automatic gain controller (AGC)
220
, an RF mixer
222
, an RF carrier frequency local oscillator (Lo)
225
, and an IF BPF
227
. The low noise amplifier
215
amplifies an RF signal received from the antenna
212
. BPF
217
bandpass-filters the amplified RF signal. AGC
220
automatically keeps the magnitude of the filtered signal at a predetermined magnitude. The mixer
222
converts the RF signal into an IF signal, and BPF
227
bandpass-filters the output of the mixer
222
and passes only a desired IF signal. Lo
225
determines the degree of frequency shifting when the RF signal is converted into the IF signal by mixer
222
.
The IF signal output from the BPF
227
is converted into an analog baseband in-phase signal and an analog baseband quadrature signal while passing through mixers
230
and
235
and LPFs
250
and
255
. An Lo
240
determines the degree of frequency shifting when the IF signal is converted into baseband signals. Analog-to-digital converters (ADCs)
260
and
265
convert the output signals of LPFs
250
and
255
into digital signals, respectively. The operational frequencies of the ADCs
260
and
265
are determined by the frequency of a clock circuit
270
.
A DSP
275
removes a cyclic prefix added to each OFDM symbol from a complex sample signal r
k
of the output signals of ADCs
260
and
265
, finds the FFT start position, and outputs N samples to an FFT
280
. FFT
280
performs a fast-Fourier-transformation on the cyclic prefix-removed signal, and outputs a frequency domain signal R
n
. R
n
is expressed by the following Equation 2:
R
n
=
∑
k
=
0
N
-
1
r
k
ⅇ
-
j
2
π
&emsp
Jain Raj
Yao Kwang Bin
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