# Development and Evaluation of a LiFi-Transceiver Module for TMTC Intra-Satellite Communication +
## 3 Transceiver-Design
### Implementation  
The Microcontroller Unit (MCU) MAX32660 from Maxim is used as a central controller. With the 16-pin wafer-level package, this microcontroller offers a small footprint of max. 1.6 mm \(\times\) 1.6 mm. Furthermore, apart from an applied voltage of 1.71 V to 3.63 V and three small capacitors to stabilize supply fluctuations, no other peripherals are required to operate the microcontroller. MAX32660 is therefore suitable for reducing the number of components required in the resulting circuit and, in turn, for reducing the space required. It directly controls the LED to send the data, and it receives the signal detected by the photodiodes. The threshold of the GPIO pin of the MCU is used indirectly as a comparator  
Figure 4: Measurements taken on a TIA.  
Figure 3: Simulated frequency response.  
to convert the analog output of the TIA to a binary signal. On-Off-Keying (OOK) is used as modulation, as this enables an almost optimal use of the available optical power and the required transmitter front-end is significantly simplified [24]. In addition, OOK can be used with existing microcontroller modules such as UART.  
The components must be powered by an external 3.3 V power supply. External connectors provide the ability to program and debug (SWD) the MCU and to communicate via a serial interface (UART). It is also possible to read sensors or control payloads. The photodiode VBPW34S [25] is used to receive the light signal. It covers a spectral range from 430 nm to 1100 nm, and it has a sensitive area of 7.5 mm\({}^{2}\) and an angle at half sensitivity of 65\({}^{\circ}\). The LED LW VH8G [26] was selected because it is very small and provides sufficiently good illumination in initial tests. In addition, the beam angle of 105\({}^{\circ}\) at 50% \(I_{v}\) is relatively high and should therefore enable a high light signal when the modules are aligned at different angles to each other. The OPA OPA355 [23] was selected as it performed well in the initial tests. It requires a power supply of 2.7-5.5 V and has a high gain-bandwidth product of 200 MHz and at the same time a small size of 3.05 mm \((L)\times 3\) mm \((W)\times 1.5\) mm \((H)\). In addition, the OPA can be put into power saving mode when not in use.  
A TIA, as described in Section 3.1, is used to amplify the light signal captured by the photodiode. The feedback resistor \(R_{FB}\) was set to 30 M\(\Omega\), which in initial tests enabled a sufficient signal for a distance of about 20 to 30 cm and thus appears sufficient for Cubesats up to a size of 3U. The photodiode has a maximum capacitance of 70 pF. The parasitic input capacitance of the operational amplifier includes the common mode and differential mode capacitances, which are specified as 4 pF and 5 pF. The total parasitic input capacitance is therefore \(C_{I}=(4\,\mathrm{pF}+5\,\mathrm{pF})+70\,\mathrm{pF}=79\,\mathrm{pF}\). The bandwidth is therefore calculated with Equation (1) to be 115.891 kHz. This bandwidth is well suited for TMTC communication.  
The components can be arranged on both sides of a PCB smaller than 9.2 mm \(\times\) 9.6 mm.