  In this letter, we incorporate index modulation (IM) into affine frequency
division multiplexing (AFDM), called AFDM-IM, to enhance the bit error rate
(BER) and energy efficiency (EE) performance. In this scheme, the information
bits are conveyed not only by $M$-ary constellation symbols, but also by the
activation of the chirp subcarriers (SCs) indices, which are determined based
on the incoming bit streams. Then, two power allocation strategies, namely
power reallocation (PR) strategy and power saving (PS) strategy, are proposed
to enhance BER and EE performance, respectively. Furthermore, the average bit
error probability (ABEP) is theoretically analyzed. Simulation results
demonstrate that the proposed AFDM-IM scheme achieves better BER performance
than the conventional AFDM scheme.
