  Due to the crowded spectrum, frequency hopping (FH) techniques are now very
difficult to achieve efficient antijamming and increase spectrum efficiency
(SE) for wireless communications. The emerging orbital angular momentum (OAM),
which is a property describing the helical phase fronts of electromagnetic
waves, offers the potential to improve reliability and increase SE in wireless
communications. To achieve efficient anti-jamming and increase SE of wireless
communications with slight computational complexity cost, in this paper we
propose an index-modulation embedded mode-hopping (IM-MH) scheme, which
simultaneously activates several OAM-modes for hopping along with additional
index information and signal information transmission. We analyze the average
bit error rates (ABERs) for our proposed IM-MH scheme with perfect channel
state information (CSI) and imperfect CSI, respectively. We also propose the
index-modulation embedded double-serial MH (IMDSMH) scheme, which randomly
activates one OAM-mode as the serial second hop to transmit the hopping signals
in the IM-MH scheme, to further decrease the ABER of wireless communications.
Extensive numerical results demonstrate that our proposed schemes within a
narrowband can achieve the low ABER and significantly increase the SE. Also,
the ABERs of our proposed IM-MH and IM-DSMH schemes are around 25% and 10%,
respectively, compared with that of the mode hopping scheme.
