# Research Progress on Router Devices for the OAM Optical Communication
## 5 OAM Channel Hopping  
There are various types of OAM-RD, and in addition to the numerous categories mentioned above, a device capable of channel hopping has gradually come into view [141; 142; 143; 144]. This device can be used in a reconfigurable multiple-access network to output different OAM modes at different time windows. [PERSON] and coworkers proposed the channel-hopping OAM-RD in 2015 [145]. Its working is similar to frequency hopping in conventional transmissions and can be realized by controlling the hop mode controller to output the desired OAM mode for a specific time from a particular output port (Figure 6a). In this scheme, the use of a digital micromirror instead of an SLM effectively improves the speed of channel hopping and can be used to arbitrarily route data streams based on the spatial pattern occupied by the channel in a multi-access optical network. [PERSON] and group succeeded in performing reconfigurable hopping of a QPSK data channel at a rate of 100 Gbit/s (with a switching protection time of 2 ns), with a signal carrying four OAM modes keeping the power loss below 5.3 dB. In order to achieve efficient channel hopping performance of OAM communication in 2018, [PERSON] et al. [146] combined the mode hopping (MH) scheme with the traditional frequency modulation scheme and proposed a mode frequency hopping (MFH) scheme. The system model is shown in Figure 6b (the transmitter on the left and the receiver  
Figure 6: Principle or experimental setup of OAM-RDs based on OAM channel hopping: (**a**) spatial domain channel hopping principle based on OAM mode [145]; (**b**) the schematic diagram MFH system model [146]. on the right). The MFH system is based on the MH system by adding two frequency synthesizers (as shown in the red dotted line in Figure 6b). The OAM transmitter and OAM receiver can utilize a uniform circular array (UCA) antenna configuration. In the case of the OAM transmitter, the N array elements are fed with the same input signal, with each successive element receiving a progressively delayed signal so that the phase of the wavefront is incremented by an integer multiple 1 of 2\(\pi\) after a complete turn. The input data symbol experiences U OAM mode or frequency hops. At the transmitter, a mode/frequency synthesizer, controlled by a pseudorandom noise sequence generator (PNG), selects an OAM mode or a frequency band range. To de-hop the OAM mode at the receiver, PNGs identical to those used in the transmitter are employed. The received signal is processed through an integrator and low-pass filter to recover the transmitted signal. The research group then compared the BERs of the scheme it developed with traditional wideband frequency modulation schemes. The numerical results show that the communication BER of the MFH scheme developed by the research group is much lower than that of the traditional wireless frequency modulation scheme, which can improve the anti-interference performance of wireless communication systems effectively, as illustrated in Table 5.  
In 2022, [PERSON] et al. introduced an index-modulation embedded mode-hopping (IM-MH) scheme [147]. This innovative scheme enables the simultaneous activation of multiple OAM modes for hopping, while also facilitating the transmission of additional index information and signal data. An analysis of the average bit error rates (ABERs) for the proposed IM-MH scheme under two scenarios--perfect channel state information (CSI) and imperfect CSI--was performed. Additionally, they propose the index-modulation embedded double-serial MH (IM-DSMH) scheme, which incorporates a random activation of one OAM mode as the second serial hop in transmitting the hopping signals within the IM-MH scheme. This further reduces the average bit error rates (ABER) in wireless communications. The input signal information is divided into signal and index components. By utilizing secure keys to determine the index information, index selector A selectively activates I out of N OAM modes, enabling the realization of OAM multiplexing at each hop.