If no subject (I) is used in "if clause", the verb will chang into v-ing or v-ed form. That depends on whether the verb action is performed actively or passively by the agent/subject( "I" ). Actively, use v-ing: "If I do homework, I will improve."--> If doing homework, I will improve. Passively, use v-ed: “If I am told to do homework, I will improve.” -->If told to do HW, I will improve. (For above cases, even the "if " can be omitted! ) In one word, the verb form depends on its relationship with the later agent and no verb infinitive can be used in subordinate sentences.

thanks. Similarly, 'if do so' or 'if doing so', which expression sounds more natural?

With only one "I" the correct form is "When arriving at the airport, I'll send you a message." (But you might prefer "As soon as I arrive at the airport, I'll send...")