1. Field of the Invention
The present invention relates generally to an OFDM (Orthogonal Frequency Division Multiplexing) scheme, and in particular, to an OFDM communication system and method for improving frequency utilization efficiency.
2. Description of the Related Art
An OFDM scheme recently used for high-speed data transmission over wired/wireless channels transmits data using multiple carriers. The OFDM scheme is a kind of an MCM (Multi-Carrier Modulation) scheme, which converts a serial input symbol stream to parallel symbol streams, and modulates the symbol streams with a plurality of orthogonal sub-carriers (or sub-channels) before transmission.
The MCM scheme was first applied to an HF (High Frequency) radio for military use in the late 1950's, and the OFDM scheme overlapping a plurality of orthogonal sub-carriers has been developed from 1970's. Since it is difficult to implement orthogonal modulation between multiple carriers, the application of the MCM and OFDM schemes to an actual system is limited. However, since Weinstein et al. announced in 1971 that OFDM modulation/demodulation could be efficiently processed using DFT (Discrete Fourier Transform), the technical development of the OFDM scheme has made rapid progress. In addition, as the use of a guard interval and a method of inserting a cyclic prefix guard interval are known, the negative effects of the system on multiple paths and delay spread have decreased further. Therefore, the OFDM scheme is widely applied to the digital transmission technologies such as digital audio broadcasting (DAB), digital television, wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM). That is, although the OFDM scheme was not widely used due to its hardware complexity, recent development of various digital signal processing technologies including fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) makes it possible to implement the OFDM scheme. Though similar to the conventional FDM (Frequency Division Multiplexing) scheme, the OFDM scheme is characterized in that it can obtain optimal transmission efficiency during high-speed data transmission by maintaining orthogonality among a plurality of sub-carriers. In addition, the OFDM scheme has excellent frequency efficiency and is resistant to multi-path fading, thus making it possible to obtain optimal transmission efficiency during high-speed data transmission. Further, since the OFDM scheme uses overlapped frequency spectrums, it has excellent frequency utilization efficiency, is resistant to frequency selective fading, is resistant to multi-path fading, can reduce the effects of ISI (Inter-Symbol Interference) using the guard interval, can simply design the hardware structure of an equalizer, and is resistant to impulse noises. Hence, the OFDM scheme tends to be actively applied to the communication system.
Now, a structure of a common OFDM system is described with reference to FIG. 1.
FIG. 1 illustrates a structure of an OFDM system according to the prior art. Referring to FIG. 1, received information data 101 is provided to an error correction encoder 102. The error correction encoder 102 codes the received information data 101 using error correction coding previously set in the OFDM system, i.e., Reed-Solomon coding, and provides its output to an interleaver 103. The interleaver 103 interleaves the output signal of the encoder 102 for preventing burst errors, and provides its output to a serial-to-parallel (S/P) converter 104. The S/P converter 104 forms a plurality of sub-channels by arranging serial data output from the interleaver 103 in the form of parallel data, and provides the sub-channels to a pilot adder 106. The pilot adder 106, under the control of a pilot controller 105, adds pilots to the sub-channels output from the S/P converter 104, and provides the pilot-added sub-channels to a sub-channel mapper 107. Here, the pilot controller 105 generates pilot data blocks by phase-shifting a plurality of pilot data blocks previously set in the OFDM system with a random code. The pilot adder 106 adds the pilot data blocks generated by the pilot controller 105 to the pilot sub-channels, and outputs K sub-channels [C(1), C(2), . . . , C(K)] along with a plurality of sub-channels.
The sub-channel mapper 107 performs signal-mapping on constellation for the K sub-channels output from the pilot adder 106, and outputs signal-mapped sub-channels [S(1), S(2), . . . , S(K)]. Here, the signal mapping may be performed according to BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation) or 64QAM modulation. The signal-mapped signals [S(1), S(2), . . . , S(K)] output from the sub-channel mapper 107 are provided to an inverse fast Fourier transformer (IFFT) 108. Here, the IFFT 108, a K-point inverse fast Fourier transformer, OFDM-multiplexes the signals output from the sub-channel mapper 107 and provides the OFDM-multiplexed signals [s(1), s(2), . . . , s(K)] to a parallel-to-serial (P/S) converter 109. The P/S converter 109 converts the OFDM-multiplexed signals [s(1), s(2), . . . , s(K)] in the form of parallel data output from the IFFT 108 into a serial signal, and outputs the serial signal as output data 110.
Compared with other systems, the OFDM system having the structure described in conjunction with FIG. 1 has excellent frequency utilization efficiency and is resistant to multi-path fading and frequency selective fading. However, there is a need for an OFDM system having more excellent frequency utilization efficiency and is more resistant to the multi-path fading and frequency selective fading.
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