Directional Diversity Reception for Hierarchically Modulated T-DMB System

We propose a reception improvement method for OFDM based DQPSK-QPSK hierarchical modulation system in severe multipath channels using directional diversity. In an OFDM based transmission system with DQPSK modulation, data rate can be increased with hierarchical modulation by adding new QPSK signal to existing DQPSK signal. New added QPSK signal has much smaller amplitude compared with the existing QPSK signal to reduce the influences on the existing DQPSK signal, so the reception performance of new added QPSK signal is degraded when there are severe multipaths interferences due to small amplitude. To overcome this problem, the directional diversity is suggested. By applying directional diversity, we can eliminate undesired multipaths and change the reception channel characteristics near to frequency flat fading, so improve reception performance. We demonstrate the performance of proposed method under severe multipath channel and show it can improve the reception of new added QPSK signal.