Trends to Enhance Throughput of Data Link in Cube Satellites.
As a smaller size nanosatellite of limited size and power, a cube satellite is expected to have limited data link of low throughput. Such low data rate can be satisfactory to transfer limited information, such as telemetry data for housekeeping purposes and data of a humble sensor like a low-resolution digital camera. However, along with the progress and spread of using cube satellites, higher data rate communication systems are experiencing extremely fast evolution. Developers of cube satellites data links have started to come out with improvements to accommodate the data throughput required for advanced missions. One of those trends is to apply linear transponders or packet repeaters that are simple to implement and enable cube satellites to play as communication satellites. Higher resolution imager of small weight and power can be utilized for remote sensing, making benefit of high data rate provided. Instead of using a basic, low efficiency, radio-amateur based, 1200 bps AFSK link, missions started to consider more advanced modulation schemes, such as BPSK and QPSK that offer better spectrum efficiency. Moreover, channel coding or Forward Error Correction (FEC) help improving the data throughput. It can be clearly seen that channel coding can improve the link budget and it will start playing a significant role as soon as missions will need to go beyond a conventional low speed link. In order to achieve better performance and reliable link, digital signal processing is carried to increase the signal-to-noise ratio, and hence reducing the data bit error rate (BER). Regarding data link layer or network protocols, still the most common is AX.25, used over amateur bands. The main reasons for using it is the possibility to reuse radio amateur hardware (like TNCs). Since AX.25 is a quite simple and robust protocol but it does not allow FEC to be used, some alternatives are proposed such as FX.25 though their usage is still limited. For antennas, the main development trend has not been primarily in the improvement in the overall performances that is limited by physical size and spacecraft requirements for omni directionality, but rather in the reduction of the volume required to house them. Antenna arrays could also be possible, but their usage is limited to higher frequencies due to the available size of the panels. This forces the antennas to be generally omni-directional, such as monopoles or dipoles. Finally, Software Defined Radio (SDR) is proposed to increase the flexibility of radio equipment. Complex modulation and coding could be accommodated, which would not be feasible using traditional systems.