1. Field of the Invention
This invention generally relates to a system and method that transmits channel data, and more particularly, to a system and method that transmits data via a multi-carrier backhaul, from a relay of an Orthogonal Frequency Division Multiplexing (OFDM) relay communication system using multi-carriers.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a method of transmitting data via multiple carriers. OFDM is a form of Multi-Carrier Modulation (MCM) that divides a serial symbol stream into parallel symbol streams, modulates the parallel symbol streams into sub-carrier channels that are orthogonal to each other, and then transmits the modulated streams.
The first systems using MCM were military high frequency radio links developed in the late 1950s. OFDM, overlapping a number of orthogonal sub-carriers, has been developed since the 1970s. Since it was difficult to implement OFDM with orthogonal modulation between multi-carriers, OFDM could not easily be applied to a real system. In 1971, Weinstein, et al. proposed that modulation-demodulation using OFDM can be efficiently achieved via Discrete Fourier Transform (DFT), which led to the rapid development of systems employing OFDM. Since a method is reported that uses a guard interval and inserts a Cyclic Prefix (CP) in the guard interval, OFDM can reduce the negative affects to the system, such as a multipath and a delay spread.
With the development of related technologies, OFDM can now be applied to digital transmission technology, such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN), Wireless Asynchronous Transfer Mode (WATM), etc. OFDM was not previously applied to such systems due to the hardware complexity until digital signal process technology, such as Fast Fourier Transform (FFT), Inverse Fast Fourier Transform (IFFT), etc., was developed.
OFDM is similar to Frequency Division Multiplexing (FDM). In particular, OFDM can transmit data while retaining the orthogonality among a number of tones, and this can lead to optimal transmission efficiency during the transmission of data at a high speed. Since OFDM has a high efficiency of frequency use and efficiently deals with the multi-path fading, it can also achieve optimal transmission efficiency during the transmission of data at a high speed.
OFDM is also advantageous in that, since OFDM overlaps frequency spectra, OFDM has a high efficiency of frequency use, can deal with frequency selective fading and multi-path fading, can reduce Inter Symbol Interference (ISI) using guard intervals, allows for a simple design of equalizer in hardware, and can counteract impulse noise. These advantages make OFDM able to be applied to communication systems.
In wireless communication, the primary factors for the deterioration of a high speed of data transmission and a high quality of data service are from a channel environment. Channel environment in wireless communication is frequently subject to Additive White Gaussian Noise (AWGN), the variation of received signal strength by fading, Doppler's effect caused by the movement of user equipment and the change in movement speed, interference by multi-path signals or the other users' devices, etc. Therefore, the causes affecting channel environment need to be processed in order to achieve a high speed of data transmission and a high quality of data service in wireless communication.
In OFDM, modulation signals are expressed in two-dimensional resources of time and frequency. The time axis resources include different OFDM symbols that are orthogonal each other. Likewise, the frequency axis resources include different tones that are orthogonal each other. For example, if an OFDM symbol is set over a time axis and a tone is also set over a frequency axis, one minimum unit resource can be set, which is hereinafter referred to as a Resource Element (RE). Although different REs pass through a frequency selective channel, they still have orthogonality. Therefore, signals transmitted via different REs can be transmitted to the receiver, without interference.
Physical layer channels (i.e., physical channels) are used to transmit modulation symbols created as one or more encoded bit streams are modulated. An Orthogonal Frequency Division Multiple Access (OFDMA) system configures a number of physical channels according to the use of data stream that will be transmitted or the types of receivers, and then performs data transmission. Arranging one physical channel to an RE and transmitting data via the channel need to be set between a transmitter and a receiver, the process of which is called ‘mapping.’