# A Roadmap for Gigabit to Terabit Optical Wireless Communications Receivers
## 4 Discussion
### Selecting between Available SiPMs at 405 nm  
The results in Figure 4a show that non-linearity alone begins to impact the achieved BER at 1 Gbps. This corresponds to an average count rate of 26 Gcps, which is approximately 5% of the maximum count rate. This strict definition of the end of the linear regions arises from the sensitivity of the BER to the number of detected photons per bit, shown in Figure 1; however, the sensitivity of the BER to the detected photons per bit means that the power penalty for higher count rates will be relatively small. This strict definition of the end of the linear regime therefore simply highlights when the transmitter irradiance may need to be increased slightly to achieve the required BER.  
The simulation results suggest two more important criteria that should be considered when selecting an existing SiPM for incorporation into a receiver. These are as follows:  
1. The results in Figure 6a show that the effects of the non-linearity and pulse width are independent if the count rate is less than 40% of the maximum count rate. Even when the pulse width penalty is negligible, the non-linearity would almost double  
Figure 8: The estimated performance of a J30020, a C1000, and a C10020. These results were obtained by first using the ratio of the bit time to the estimated equivalent Gaussian pulse width to determine the ISI count rate penalty using the results in Figure 7a. This was then converted into the required irradiance using the relevant SiPM parameters and Equation (6).  
the irradiance required to achieve a particular BER. It may therefore be prudent to expect a SiPM to operate with count rates less than 40% of its maximum count rate.
2. The results in Figure 6a also show that the count rate power penalty is less than two if the bit time is less than the pulse width. To avoid a significant increase in required irradiance, the transmitted data rate should ideally be less than the data rate whose bit time equals the pulse width. The results that arise from applying these two conditions, shown in Table 2, show that, if they are applied, the suggested data rates for a J30020 are less than the maximum data rate that has been reported [11]. This is because, as with a C10010 and a C30020, the data rate at which condition (i) is reached for a J30020 is much higher than that required for ISI to double the required irradiance, condition (ii). This means that these SiPMs have the capacity to tolerate an ISI power penalty significantly larger than two.  
A J30020 has more microcells, a higher PDE, and a higher maximum count rate than a C30020. The estimated performances of a J30020 and a C10010 are therefore shown in Figure 8. As expected, the results in Figure 8 show that there large maximum count rates that allow these two SIPMs to operate at data rates at which they incur a significant ISI power penalty; however, it is also clear that, despite having faster output pulses, the smaller area of a C10010 means that it is only expected to perform better than a J30020 at data rates higher than 3 Gbps. At these data rates the performance of a J30020 is limited by a combination of ISI power penalty and the saturation of its non-linear response; however, its smaller area means that a C10010 is only a better choice than a J30020 at irradiances that are not eye-safe [12].