Conventionally, the orthogonal frequency division multiplexing (OFDM) is one kind of broadband modulation scheme for telecommunication by dividing the distributed frequency band domain into a plurality of narrow frequency sub-bands wherein each of includes the sub-carriers of wireless frequency, i.e. radio frequency (RF), and each of sub-carriers in one channel is mathematically orthogonal to each of sub-carriers in another channel. The orthogonal status between sub-carriers allows the respective spectrum of the sub-carrier to be superposed with other carrier in no interference mode. Since the distributed frequency band domain is divided into a plurality of narrow frequency sub-bands, the OFDM scheme efficiently utilizes distributed frequency band domain by higher data transmission speed.
While performing the above-mentioned OFDM scheme, the cyclic prefix corresponding to the length and the maximum multipath delay is located in the front of the main OFDM symbol in order to eliminate the multipath fading which results in inter-symbol interference (hereinafter abbreviated as ISI). However, the above-identified method only eliminates the ISI when the length of the CP is smaller than the maximum multipath delay due to the multipath fading. Further, such a method disadvantageously results in system redundancy and degrades the spectral efficiency. Specifically, the CP is a signal which is inserted to the guard interval (GI) to prevent inter-channel interference and the GI is inserted to cancel the ISI resulting from the multipath fading during the OFDM transmission. However, when no signal is located in the GI, the orthogonal status between inter-carriers and the inter-channel interference occurs. A portion of signal in the rear of the inter symbol interval is copied and inserted, which is termed as cyclic prefix (CP).
The drawback of the above-identified OFDM method is that the operation efficiency of the receiver is downgraded while the inter-carrier interference (ICI) occurs when the carrier frequency offset exists and the channel variation resulting from the status change between the receiver and the transmitter is generated.
In this case, the signals in the rear of the cyclic prefix (CP) and the main OFDM are identical since no ISI exists. Therefore, the CP in the OFDM receiver should not be cancelled but add the CP to the main OFDM and average the CP in order to increase the operation performance including: the improvement of the signal-to-noise ratio (SNR) of the averaged CP in the main OFDM symbol and inter-carrier interference.
According to the above-mentioned descriptions, the cyclic prefix (CP) without ISI is utilized while performing the CP averaging method. If the signal with the ISI component is used to average the CP, the operation performance of the receiver will be reduced. As a result, there is a need to estimate the maximum channel delay so that the CP averaging method is performed by signal without ISI component.
Although the conventional CP averaging method is provided, channel delay spread is utilized. However, the estimation method of maximum channel delay is not provided but the channel delay spread is disclosed. In addition, the estimation method of maximum channel delay and the CP averaging method thereof in the OFDM receiver are not disclosed even if the channel delay spread is widely used.