COMBINED INFLUENCE OF DOPPLER EFFECT AND PILOT DE-ORTHOGONALIZATION ON 2×2 TO 4×4 MIMO SYSTEMS AND IMPROVEMENT OF ORTHOGONAL SEQUENCES

Abstract

The Doppler effect in 2×2, 3×3, and 4×4 MIMO wireless communication systems with channel state estimation is studied. The orthogonal pilot signal approach is used for the channel estimation, where the Hadamard sequences are used for piloting along with the eight Romanuke orthogonal sets similar to the Walsh set. The Doppler effect is additionally aggravated by the pilot signal de-orthogonalization, where two negative-to-positive symbol errors are assumed to have occurred while signal is transmitted. MIMO transmissions are simulated for 10 cases of the frame length and pilot symbols per frame by no Doppler shift to 1100 Hz Doppler shift with a step of 100 Hz. By assuming that the carrier frequency is 5,9 GHz, the step corresponds to a motion speed of about 18.3 km/hr. Based on the simulations, it is ascertained that the Doppler effect negatively influences transmissions of long data packets. It is impracticable to apply MIMO transmissions of long packets at speeds exceeding 100 km/hr. To maintain an appropriate MIMO link data rate, the packet length should be shortened as the motion speed increases. On the other hand, the MIMO performance is substantially improved by increasing the number of antennas, except for the case of transmitting long packets. Besides, under the de-orthogonalization caused by two negative-to-positive symbol errors, the MIMO Walsh pilot sequences are outperformed by MIMO Romanuke pilot sequences, so the latter are considered as an improvement of MIMO orthogonal sequences. However, the performance difference between the Romanuke and Walsh pilot sequences decays as a greater number of transmit-receive antenna pairs is used and the motion speed increases