arXiv:2610.10327v1 Announce Type: new Abstract: A fast antenna coding technique based on pixel antennas is investigated. Antenna coding is a technique that dynamically controls the ON/OFF states of PIN diodes to adjust radiation patterns of pixel antennas. By switching the antenna coder at the baseband symbol period, this technique can modulate information on the radiation patterns in the beamspace. This allows the pixel antenna to transmit additional information, thereby enhancing the spectral efficiency (SE) of multiple-input multiple-output (MIMO) systems. Specifically in this work, we firstly introduce the concept of pixel antenna and the fast antenna coding. Then based on the beamspace channel representation, we provide the system model of MIMO with the fast antenna coding and derive the SE and bit error rate (BER). The SE enhancement achieved by fast antenna coding is also demonstrated. An optimization approach is also presented for obtaining the antenna coders that can generate the desired radiation patterns for fast antenna coding. Using the optimized antenna coders, we evaluate the SE, energy efficiency (EE), and BER of MIMO systems with fast antenna coding. The simulation results show that for a 4-by-4 MIMO system, SE is enhanced by 12 bits/s/Hz and EE can be improved by 33\% when compared with conventional MIMO, demonstrating the effectiveness of the fast antenna coding technique.