Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1999-12-01
2002-04-16
Olms, Douglas (Department: 2732)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S342000, C375S354000
Reexamination Certificate
active
06373861
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a modulation/demodulation device in an orthogonal frequency division multiplexing/code division multiple access (OFDM/CDMA) system, and in particular, to a device for synchronizing a frequency in a time domain in OFDM/CDMA system.
2. Description of the Related Art
In general, an OFDM technique is frequently used in digital transmission systems such as a digital audio broadcasting (DAB) system, a digital television system, a wireless local area network (WLAN), and a wireless asynchronous transfer mode (WATM) system. The OFDM technique is a type of multi-carrier technique which modulates transmission data after dividing, and then transmits the divided modulated data in parallel. The OFDM technique was not widely used for the complex structure. However, the recent progress of various digital signal processing techniques including the fast Fourier transform (FFT) and the inverse FFT (IFFT) has made it possible to utilize the OFDM system. Though similar to the existing FDM system, the OFDM system may have an optimal transmission efficiency during high-speed data transmission by maintaining orthogonality between sub-carriers. Because of the optimal transmission efficiency, the OFDM/TDMA and OFDM/CDMA systems have been proposed for use with the WATM system since it requires high-speed data transmission.
Referring now to
FIG. 1
, there is shown a block diagram of a general OFDM/CDMA system. A description will now be made regarding a multi-carrier (MC) CDMA system using the OFDM/CDMA technique. The MC-CDMA system includes a transmitter
100
and a receiver
120
. The transmitter
100
and the receiver
120
may be equally applied to both the forward link and the reverse link.
With regard to the transmitter
100
, a plurality of spreaders
101
spread transmission data using orthogonal codes of length N and PN spreading sequences. Typically, N is 256 in the OFDM/CDMA system. If the transmitter
100
is a forward transmitter, the spreaders
101
include spreaders for user identification and spreaders for base station identification. On the other hand, if the transmitter
100
is a reverse transmitter, the spreaders
101
include spreaders for channel spreading and spreaders for user identification. Herein, the N-bit data will be referred to as chip data. The chip data spread by the spreaders
101
is input to a summer
102
after pilot signal insertion (not shown). The chip data is summed in the summer
102
on a chip unit basis and is output in series to a serial/parallel converter
103
. The serial/parallel converter
103
outputs the serial chip data provided from the summer
102
in parallel. At this point, the number of the parallel chip data output can be equal to N or not equal to N. Herein, the number of the parallel chip data is assumed to be N. Further, the parallel sample data is input to an inverse fast Fourier transform (IFFT) device
104
. The IFFT device
104
receiving N parallel data samples, performs OFDM modulation on the chip data. In other words, the IFFT device
104
performs IFFT on the chip data, and carries the processed chip data on different sub-carriers having orthogonality in a frequency domain. In the IFFT device
104
, the sub-carriers are output in the time domain. The data output from the IFFT device
104
will be defined as sample data, and N data samples will be defined as an OFDM symbol.
The parallel output sample data is input to a parallel/serial converter
105
. The parallel/serial converter
105
outputs the same data in series. Further, the parallel/serial converter
105
inserts a guard interval on an N-sample data unit basis, i.e., one-OFDM symbol unit basis. The guard interval is data obtained by copying some sample data at the rear of an OFDM symbol comprised of N data samples, and is inserted at the front of the OFDM symbol. Herein, the data in which a guard interval is inserted on an OFDM symbol unit basis, is defined as an OFDM frame. The length of the guard interval should be set longer than an impulse response length. A transmission filter
106
filters the data output from the parallel/serial converter
105
and transmits the filtered data over a radio channel
107
using an RF (Radio Frequency) module (not shown). The radio channel
107
is an additive white Gaussian channel, so that additive white Gaussian noises are added by an adder
109
.
The receiver
120
receives a carrier with the additive white Gaussian noises over the additive white Gaussian channel. The received carrier is converted to a baseband signal through an RF module (not shown). A multiplier
110
compensates for frequency error generated in channel
107
using a frequency correction signal received. An analog to digital converter
115
converts the frequency-corrected analog signal input from the multiplier
110
to digital sample data stream. A serial/parallel converter
111
receives the OFDM symbol in series and outputs N data samples constituting the OFDM symbol in parallel. Though not illustrated, the receiver
120
commonly includes a guard interval remover for removing the guard interval inserted on an OFDM frame unit basis before parallelizing the sample data stream. A fast Fourier transform (FTT) device
112
performs OFDM demodulation on the received sample data carried on the sub-carriers in parallel and converts the respective sub-carriers to the original chip data in the frequency domain. A parallel/serial converter
113
converts the parallel chip data to serial chip data. A despreader
114
despreads the serial chip data input from the parallel/serial converter
113
to restore the original data.
Typically in the OFDM transmission system, if local oscillators in the transmitter and the receiver are not tuned to each other, a frequency offset occurs and causes a loss of orthogonality between the sub-carriers. In this case, even a small frequency offset may cause performance degradation of the receiving system. Therefore, in the OFDM/CDMA WATM transmission system, it is necessary to implement frequency synchronization for maintaining orthogonality between the sub-carriers.
Generally, the frequency synchronization used for a receiver of the OFDM system is performed in two steps, namely, a coarse synchronization and a fine synchronization. The coarse synchronization step removes an initial frequency offset corresponding to multiples of the sub-carrier interval, and the fine synchronization step removes the residual frequency offset remaining after coarse synchronization.
There are two coarse frequency synchronization techniques; one proposed by Classen & Myer, and another by Nogammi & Nagashima.
FIGS. 2
to
4
show a frequency synchronization device for the receiver, using the coarse frequency synchronization technique and the fine frequency synchronization technique.
First, a description will be made regarding the coarse frequency synchronization technique proposed by Classen & Myer, with reference to FIG.
2
.
The technique proposed by Classen & Myer uses a test correction frequency, and calculates a correlation between known transmission data and received data while shifting the test correction frequency by a predetermined frequency interval, thereby estimating the frequency offset. This technique uses a property that the correlation value becomes maximum when the test correction frequency is nearest to an actual frequency offset shifted in the actual channel.
Referring to
FIG. 2
, there is shown a block diagram for detecting the test correction frequency offset. A multiplier
128
compensates for a frequency offset of a received signal using a test correction frequency received. An analog/digital converter (ADC)
129
converts the received analog data to digital data. A guard interval remover
122
removes the guard interval from the received data. A guard interval removing method sets a window having a length of two OFDM symbols and one guard interval, calculates a correlation value while shifting the window by samples, and removes the guard interval b
Dilworth & Barrese LLP.
Olms Douglas
Samsung Electronics CO, Ltd.
Vanderpuye Ken
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