  Orthogonal frequency-division multiplexing with index modulation (OFDM-IM)
has become a high-profile candidate for the modulation scheme in next
generation networks, and many works have been published to analyze it in recent
years. Error performance is one of the most important and interesting aspects
of OFDM-IM, which has been analyzed in most works. However, most of them employ
two key approximations to derive the closed-form expressions of block error
rate (BLER) and/or average bit error rate (BER). The first one is to utilize
the union bound assisted by the pairwise error probability (PEP) analysis, and
the second one is to employ an exponential approximation of Q-function. In this
letter, we apply Craig's formula to analyze the error performance for OFDM-IM
in place of the exponential approximation. Because Craig's formula is the exact
expression of Q-function, the accuracy of analytical results regarding error
performance for OFDM-IM can be improved. We examine the proposed error
performance analysis methodology based on Craig's formula by investigating both
average BLER and BER.
