Multipath Channel Equalizer for Echoes outside the OFDM Guard Interval and Co-Channel Interference Canceller (CCI)

Delivering digital terrestrial broadcasting radio waves throughout the country requires the installation of a large number of relay broadcasting stations. STRL has been working on technologies that will enable the construction of a low-cost broadcast-wave network of relay stations.

Besides its effective use of frequencies, a broadcast-wave relay has cost and maintenance advantages over other methods of signal transmission to relay stations (microwave, optical fiber network, etc.). However, it is prone to various forms of interference on the radio-wave propagation path, necessitating signal compensation at receivers. This article discusses two types of compensators.

The multi-path channel equalizer for echoes outside the OFDM guard interval eliminates (i.e., equalizes) degradation from multipath waves. As shown in Figure 1, it equalizes multipath signals from an SFN station, which share the same modulation content as the desired wave, along with the reflections from geographical formations and large vessels. It handles multipath waves with delays within the guard interval, as well as those outside the GI within a range of ±454 µs. It requires a single receiving antenna.

A co-channel interference canceller (CCI) receives signals with multiple antennas and controls the antenna directionality on basis of the adaptive array antenna principle. It eliminates interference waves by increasing the sensitivity in the desired wave's direction of arrival and reducing the sensitivity in the arrival directions of waves with different modulation content from that of the desired wave. It also reduces the sensitivity to multipath waves outside the guard interval. As Figure 2 indicates, the employment of four receiving antennas, for example, can create three directions with no sensitivity, which leads to cancellation of interference from these directions.

STRL has conducted many experiments that have proved the effectiveness of these compensators. Long-term testing at relay stations has also been done to ensure their reliability.

Since the compensators used for broadcast-wave relays vary in terms of their number of receiving antennas and conditions needed for effectiveness, thorough research on the reception environment at specific sites is an essential step in selecting an appropriate compensator.