# Temporal Encoding to Reject Background Signals
## 2 Results
### Satellite Systems Demonstration  
The communications system presented here is applicable in many scenarios, but is particularly attractive for inter-satellite links. The semiconductor devices are extremely compact, have low power consumption and are readily integrated with control electronics. LED based visible light communications shows potential for use with cube satellites [26; 27]. The robustness of the signal to background noise and operation at picowatt levels of received optical power means the scheme could be implemented without the high accuracy pointing requirements, telescope optics and filters of current satellite systems. To highlight this capability, the system was tested in the nano-satellite hardware and software test-bed, NANOBED, shown in Figure 6a, which can simulate the available power systems on a cube satellite. To demonstrate that the full communication link was able to be powered by the NANOBED, a real time decoder, incorporating embedded clock signal recovery, was implemented on a FPGA platform to replace the oscilloscope and PC components in the characterisation setup. Details of this setup, shown in Figure 6b, are given in Section 4.3. Figure 5: (**a**) Required signal power to attain a BER of \(10^{-3}\) under constant background power for 50 and \(10\) kb/s. Equal signal and background power is indicated by the solid line. (**b**) Required signal power to attain a BER of \(10^{-3}\) under modulated background conditions. Required power increases by approximately 40% when background modulation rates are comparable to the signal data rate. The LED transmission system was integrated with one NANOBED system, while the SPAD receiver system was integrated with a second. In this work, the solar panel emulating power sources are used to supply the transmitter and receiver devices via the electrical power supply (EPS) and battery units, simulating an in-orbit scenario. The transmitter side of the real time link requires a single FPGA board, from which the CMOS micro-LED array is powered and controlled. On the receiver side, the commercial SPAD module requires a 6 V DC supply, and a second FPGA is used to process the received signals. A summary of typical power consumption is shown in Table 1. The SPAD consumes the most power in the system, however the commercial module has not been designed with power conservation in mind, and employs significant levels of active cooling. A custom SPAD receiver may show power requirements of 10-100 mW [6; 22]. While the lack of active cooling would result in higher levels of dark counts, the resilience to constant background signals demonstrated in Section 2.5 indicates this would not be problematic. Additionally, bespoke electronics in place of FPGA boards may also permit lower power consumption, therefore this demonstration should be thought of as an upper limit on power requirements. For the laboratory demonstration, the transmitter and receiver were placed 4 m apart with the micro-LED pixel projecting the light across a 4 cm wide square with received power controlled using a neutral density filter. A micro-LED emitter at 450 nm was used to improve the PDP to 25%. As shown in Figure 7, the live link requires 2.5 pW of received power to maintain a BER of 10\({}^{-3}\) at 20 kb/s. On a 20 um diameter SPAD, 3 pW corresponds to an intensity of 9.5 mW/m\({}^{2}\). To provide this over the projected 4 cm wide square, 15.3 mW must be collected from the micro-LED by the transmitter lens. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Component** & **Voltage (V)** & **Current (mA)** & **Power (W)** \\ \hline Transmitter LED \& electronics & 5 & 181 & 0.905 \\ Receiver electronics & 5 & 122 & 0.610 \\ Receiver SPAD & 6 & 595 & 3.570 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Typical power requirements of the communication system. Figure 6: **(a)** Photograph of the micro-LED transmitter board (left), NANOBED “Flatsat” test bed (middle) and a standard 3U CubeSat chassis (right).