Space Microdatacenter and Communication Co-design

One way to mitigate an ISL-bottleneck in context of SµDCs is to modify the network topology to increase the amount of data onboarded onto the SµDCs. Figure 12a shows how, by adding more receivers to a SµDC, the cluster topology can be changed from a ring, or ‘2-list’, to a ‘4-list’, or, more arbitrarily, a ‘\(k\)-list’ for even \(k\). While this may not help RF communication-based constellations due to limited available bandwidth, tremendous amounts of bandwidth is available in the optical frequencies, allowing linear growth in incoming data rate with the number of optical receivers [139]. Thus, for optical ISLs, \(k\)-lists for \(k > 2\) can be used to increase the SµDC’s incoming data rate at the cost of additional optical receivers on the SµDC and additional transmit power.

As \(k\) increases, the link distance between relay satellites grows. Optical ISL transmit power grows quadratically with distance [97], meaning a 4-list’s ISLs consume \(4\\times\) the power of a 2-list (while also transmitting \(2\\times\) the data). Also, this distance can eventually grow to such an extent that the ISL must aim through significant amounts of atmosphere. This results in atmospheric turbulence induced fading of the optical signal [161], degrading the channel capacity. If the distance is large enough, then the Earth’s landmass will directly block the signal. The specific value of \(k\) for which distance becomes a concern is dependent on the constellation’s formation: for evenly distributed — ‘orbit spaced’ — formations, maximum \(k\) is smaller than for tightly packed formations in which satellites are relatively close to one another.

Alternatively, SµDCs can be split Figure 12b — increasing the number of clusters in a ring-topology without increasing the compute power of the SµDCs in aggregate. By using smaller split SµDCs, costs associated with higher cluster counts (e.g., launch cost, booster fuel requirements, etc.) are mitigated. SµDC splitting is effectively a form of disaggregation, and thus can lead to increased total launch costs and constellation management costs. However, SµDC splitting is effective for all constellation formations, including orbit-spaced constellations which may see limited benefit from \(k\)-list topologies.

SµDC splitting and \(k\)-list topologies can be used in conjunction. Their benefits are orthogonal, and the increase in aggregate data rate into SµDCs scales multi-linearly with number of clusters (from splitting), and number of links into each SµDC (from \(k\)-lists). That is, the number of EO satellites supported by a \(k\)-list topology cluster is \(\\frac{k}{2}\) times those shown in Table 8, while SµDC splitting multiplies the number of clusters. Figure 13 shows that \(k\)-lists combined with SµDC splitting leads to significantly increased ISL communication capacity (the rate at which data can be transmitted from an EO satellite to an SµDC) in a frame-spaced constellation.