Hiroki Ohta

A study of OFDM system applying frequency diversity

An OFDM system is widely known as a modulation scheme, which can reduce the effect of frequency selective fading. However, under a large frequency selective fading environment, it is difficult to demodulate correctly even with appropriate equalization. In this paper, an OFDM system applying a frequency diversity scheme is proposed. In this system, the same symbols are transmitted on several sets of carriers, and at the receiver end, these symbols are combined so as to maximize the signal to noise ratio. As the experimental result, it is shown that as the fading level becomes larger, the proposed system provides a larger gain in the bit error rate characteristics than the conventional system.

The channel estimation technique based on the propagation path model of an OFDM system

OFDM is widely known as a modulation scheme which can utilize the given frequency band very efficiently, even if the channel is distorted by multipath propagation. However, in order to improve the efficiency, a high-precision estimation technique is recommended. In this paper, we pay attention to the method which introduces a condition based on the propagation path model in order to obtain better precision. Although this method performs better than another conventional method especially under the fading environment, this method has large computational complexity. Hence, we simplify this method by using the FFT algorithm to make this method more effective. As a result, the proposed system performs better than the conventional system especially under fading environments, and the computational complexity is reduced.

A scheme to connect multiple channel transfer functions derived from independently broadcasted channels in Digital Terrestrial Television Broadcasting

In Japanese Digital Terrestrial Television Broadcasting (DTTB), the identification of multi-path is important to improve its reception characteristics. In order to enhance the resolution of the delay profile, it is effective that continuous multiple channel transfer functions are connected and the frequency bandwidth is extended. However, the guard band is existing between each channel and the transfer function is not obtained directly. The phases of each channel are not synchronized because the each channel is broadcasting independently. In this paper, the scheme to connect multiple channel transfer functions is proposed. In the proposed scheme, interpolation of guard band by using the linear prediction filter based on the RLS(Recursive Least Square) algorithm and compensation of phase between adjacent channels are performed. As the result, it was conformed that resolution enhancement of the delay profile is possible. It is possible to improve the time resolution of the delay profile that has 0.024[μs].

Coded OFDM Systems using Data Spread Scheme

In OFDM signal reception, frequency dips caused by multipath propagation generate burst errors and this leads to a severe degradation of performance. To minimize this degradation, an error control code such as an error correcting code combined with a frequency interleaver is usually used. We analyzed the characteristics of a data spreading scheme in the frequency domain combined with an error correcting code. We ascertained that the proposed method results in a performance improvement. Moreover, we demonstrated that a reduction in the delay time and calculations in the decoder is possible.