Subcarrier Index Modulation is an OFDM variant that provides superior power and bandwidth efficiency. In this paper, we present a novel, double-sided pulse interval modulation (DS-PIM)-based SIM OFDM technique. The proposed technique exploits the variable symbol size of DPIM to provide a variable sub-block size and enable dynamic assignment of subcarriers rather than the fixed size of conventional SIM OFDM. In comparison with conventional Subcarrier Index-Modulated OFDM (SIM-OFDM), the proposed approach shows a 12.5% reduction in bandwidth usage for a 2-bit index word. On average, 3.5 subcarriers are employed by the proposed technique per sub-block, in comparison with 4 subcarriers for the conventional technique. The proposed technique provides a superior spectral efficiency compared with conventional SIM-OFDM, even for higher-order modulation.