Robustness / reception flexibility

OFDM transmission technology (robustness against multi-path, SFN)

OFDM (Orthogonal Frequency Division Multiplex) transmission technology is a multi-carrier transmission system. The transmission symbol length of OFDM transmission system, is longer than that of single carrier transmission system. When transmission symbol is longer, less degradation by Inter Symbol Interference (ICI) from multi-path interference, which causes "ghost" occurs.

Conceptual diagram for relationship between modulation and symbol length
Figure 1  Conceptual diagram for relationship between modulation and symbol length
(Note) multi-carrier modulation process is done by IFFT (Inverse Fast Fourier Transform)

As shown in figure 2, Inter Symbol Interference (ICI) is inverse proportional to symbol length, therefore, under multi-path condition, longer symbol transmission system is better.

Relation of multi-path delay and ICI
Figure 2  Relation of multi-path delay and ICI

In addition, because ISDB-T system adds Guard Interval to each symbol, the robustness against multi-path interference is improved to almost 0dB D/U ratio (Desired to Undesired ratio) during the period of Guard Interval length.

Robustness against static multi-path interference (3 DTTB systems)
Figure 3  Robustness against static multi-path interference (3 DTTB systems)
   

As shown in Figure 3, ISDB-T shows the advantage of the robustness during +/- Guard Interval length. Robustness against multi-path is very important factor for digital terrestrial broadcasting by following reasons.

(1) In VHF/UHF band, multi-path interference occurs by mountain, building and so on. Multi-path exists not only mountain area but also urban area. ISDB-T shows excellent reception performance under such receiving condition.
(2) By applying the advantage of robustness against multi-path interference, SFN (Single Frequency Network) can be constructed.