Segmented OFDM transmission (Portable service in the same channel)
Segmented OFDM transmission is the unique transmission system which enables to select different kinds of transmission parameter signals in the same bandwidth. This transmission system is also called as “Hierarchical transmission system”.
Figure 1 shows an image of “Hierarchical transmission system”.
One segment in the center of 13 segments is used for the portable reception service and other 12 segments are used for HDTV fixed reception service. For example, One segment transmission, using QPSK is suitable under the severe receiving condition, such as low receiving antenna height, low antenna gain and signal level fluctuation, on the other hand, for 12 segments transmission using 64QAM is applicable for fixed reception with high antenna position and high antenna gain.
As mentioned above, hierarchical transmission system can provide the different type of services in the same channel. This advantage contributes to saving frequency resource and transmission infrastructure cost. Only ISDB-T adopts “Hierarchical transmission” technology in 3 DTTB systems.
One-seg service in the same channel
1. Reduction of power consumption of portable receiver
Partial reception technology of One-Seg service reduces the power consumption of a receiver. The most important factor to reduce power consumption is to decrease the signal processing speed in the receiver. As shown in the figure, in the case of partial reception, the center segment in 6MHz OFDM signal is filtered by narrow bandpass filter whose band width is equal to 432 kHz. This narrow band signal is demodulated by low sample rate FFT (Fast Fourier Transform). Its sample rate is 1/8 of high sample rate FFT for full band demodulation. As a result, signal processing speed of demodulation block is decreased to 1/8, comparing to the case of full band demodulation.
2. Adaptive choosing in mobile reception
Figure 3 shows an example of transport stream (TS) of 2 layers transmission in the same channel.
Case 1 in the figure shows an example of wideband reception. Wideband reception is normally used for fixed and mobile receivers.
Case 2 in the figure shows an example of partial reception. Partial reception is normally used for portable receivers.
As shown in the figure, the transport stream includes the TS packets composed by A layer and B layer. As a result, when rhe receiving condition for mobile receiver is severe because of low field strength, etc, layer A is used. On the other hand, when receiving condition is normal, HD quality program using layer B is received.