Difference between fixed and mobile reception environments
- Antenna height difference (Clearance)
- Receiving antenna difference (Antenna gain)
- Field strength fluctuations (Multipath fading)
In the fixed reception environment, we generally use a directional antenna like a yagi antenna, and a receiving antenna height is about 10 m. And the receiving antenna is usually set up in a good position that provides stable and high field strength. On the other hand, in the mobile reception environment, we generally use an omnidirectional antenna or a less-sensitive directional antenna. The receiving antenna height is lower than fixed reception, so the radio waves are mostly blocked. It is poor reception condition. A problem associated with mobile reception is frequency selective fading interference caused by a moving vehicle. It was very difficult to receive HDTV program in a moving vehicle, such as a bus or car.
Key technologies for mobile reception
1. Time Interleaving
- Time interleaving is very effective for fluctuated signal, this feature is only in ISDB-T, not equipped in DVB-T and ATSC.
2. Space diversity reception
- The diversity technology, improves the signal quality. Here, ISDB-T adopts a very unique technology, that is "maximum ratio combining for each OFDM subcarriers", for the robustness against frequency-selective fading.