On the Spectral Efficiency of Orthogonal Frequency-Division Multiplexing with Index Modulation

Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) promises superior bit error rate (BER) performance and transmit power savings compared to classical OFDM. The concept consists in dividing the available subcarriers into groups and activating a subset of subcarriers in each group for transmitting M-QAM/PSK symbols. The remaining subcarriers are kept idle. The transmitted symbols and the indexes of the active subcarriers both carry useful information. In the recent literature, OFDM-IM is conceived by activating half of the subcarriers in each group for transmission of M-QAM/PSK symbols, which results in low spectral efficiency (SE). This paper addresses the low SE problem in two ways. First, an inequality rule is developed by comparing the SE of classical OFDM and OFDM-IM. The rule is used to derive three limits on the group size and it is shown that OFDM-IM can overcome the SE limitation of classical OFDM for a group size of 2M or greater. Secondly, a new OFDM-IM scheme with a variable number of active subcarriers per group, termed OFDM-IM-VAS, is proposed. The optimal group size for a given M is obtained, and it is shown that OFDM-IM-VAS can achieve higher SE than classical OFDM with low BER and high power savings. The BER, bit level throughput, transmit power saving, and complexity of OFDM-IM and OFDM-IM-VAS are analyzed considering transmissions over Rayleigh fading channels and compared with classical OFDM. A closed-form BER expression is derived to support the simulation results.