# Large Constellations of Small Satellites: A Survey of Near Future Challenges and Missions
## 3 Future Challenges
### Communication Issues  
Communication is a key point when operating a constellation, in fact:  
* On-Board Automation is unlikely to grow to the point of allowing fully autonomous fleet management: a large amount of satellites will thus need to communicate frequently with ground.
* Constellations are designed usually for real time--24/7-purposes, requiring data down/up-load at any time.  
The above two points rise concerns about Radio Frequency (RF) spectrum partitioning. An overcrowded RF spectrum may indeed cause physical interference of adjacent RF signals. At the same time, the traffic capacity of the communication infrastructure shall grow in parallel with the data volume travelling in the RF channels.  
The research community is actively working towards these two aspects. One of the most promising approach towards infrastructure optimization is the sharing and integration between space and ground communication networks, especially in view of the upcoming 5G service [29]. Work in this sense can be found since the turn of the century, see e.g., [30] where the possibility of integrating a satellite network with a terrestrial network is envisaged using an IP-based communication. Few years later, authors of [31] provided a survey of mobile satellite systems endorsing IP-based communication, discouraging however communication satellites other than GEO, mainly for cost efficiency. Despite the miniaturization trend in LEO spacecraft was already established by then, the recommendation towards GEO is not surprising, given the negative experiences of the firsts LEO constellations like Iridium and Globalstar. More recently, due to the rising interest in satellite constellations authors of [32; 33] brought the attention again onto the potential of LEO satellites as communication infrastructure. The first studies the integration of the 5G ground network with a space-based network emphasizing its potential for global connectivity. The second studies the efficient implementation of inter-satellite link through a routing algorithm. The algorithm takes into account maximum available link time and remnant bandwidth to increase the total traffic capacity of the network in the presence of handover.  
A more practical solution to the frequent communication needed by low-autonomy spacecraft is a conventional network of existing ground stations. This solution could be already feasible upon standards definition for communication (e.g., CCSDS) and hardware interfaces, in fact ground station providers like KSAT or KRATOS are going towards this direction. KRATOS for example designed a device called _quantumCMD_[34], a small computer able to operate up to 4 satellites when integrated in a ground station. The power of the device is its scalability with number of satellites and ground stations.  
Solutions against the increasingly crowded spectrum are devoted mainly towards (a) spectrum sharing and (b) enhancement of regulation. Examples can be found in [35], where the possibility of a Database-Assisted spectrum sharing is pushed forward. Using this approach, the temporarily unused spectrum could be reallocated for a more efficient use. Note that also [35] uses the keyword \"sharing\" between satellite and terrestrial networks.  
A completely different approach to avoid RF spectrum overcrowding consists of moving to the optical part of the spectrum. Optical communication promises higher data rates using smaller and lighter terminals, even though due its high sensitivity to atmospheric conditions it is more suited for free-space inter-satellite links rather than satellite-to-ground [36] (enabling communication between constellation elements is an asset by its own, though). An optical communication system conceived for LEO constellations is described in [37] and is currently at an advanced development stage.