In the basic principle of orthogonal frequency division multiplexing (OFDM), a data stream having a high rate is divided into a large number of data streams having a slow rate and the data streams are simultaneously transmitted using a plurality of carriers. Each of the plurality of carriers is called a sub-carrier. Since orthogonality exists among the plurality of carriers in the OFDM, the carriers can be detected by a receiving side although frequency components of the carriers overlap with each other. The data stream having the high rate is converted into a plurality of data streams having the slow rate by a serial-to-parallel converter, each of the plurality of data streams converted in parallel is multiplied by each of the sub-carriers, the data streams are added, and the added data streams are transmitted to the receiving side.
The plurality of parallel data streams generated by the serial-to-parallel converter may be transmitted by the plurality of sub-carriers using an Inverse Discrete Fourier Transform (IDFT), and the IDFT may be efficiently implemented using an Inverse Fast Fourier Transform (IFFT).
Since a symbol duration of each of the sub-carriers having the slow rate is increased, a relative signal dispersion on a time axis, which is generated by multi-path delay spread, is decreased. Inter-symbol interference can be reduced by inserting a guard interval longer than the delay spread of a channel between OFDM symbols. In addition, when a part of an OFDM signal is copied and arranged at a start part of a symbol in a guard interval, OFDM symbols are cyclically extended such that the symbols are protected.
Meanwhile, OFDMA is a multiple access method which provides a part of available sub-carriers to each user in a system using OFDM modulation method so as to realize multiple access. In the OFDM, each of frequency resources called sub-carriers is provided to each user. That is, the frequency resources are independently provided to the plurality of users so as not to overlap with each other. As a result, the frequency resources are exclusively allocated.
FIG. 1 is a view showing the structure of a transmitting side using a single carrier-frequency division multiple access (SC-FDMA) scheme.
The SC-FDMA (IFDMA or DFTs-OFDMA) scheme which is a modified example of the OFDMA may be considered for uplink wideband transmission. In the SC-FDMA scheme, discrete Fourier Transform (DFT) coding is performed with respect to symbols before OFDMA modulation and multiplexing. Accordingly, input symbols are spread over the entire transmission band in a frequency domain, a peak-to-average power ratio (PAPR) of a lastly transmitted signal decreases, and the required operation range of a transmitting amplifier of a mobile station can be reduced.
However, if transmission timing of a packet is adjusted and more particularly if packet transmission timing is advanced while a transmitting side continuously transmits packets by using the above modulation method two continuous packets may collide.