# Transmitters and Receivers for High Capacity Indoor Optical Wireless Communication
## 2 Beam-Steering for Optical Wireless Communications Transmitter
### Experimental Results  
To characterize the beam steering capabilities of our system, we first evaluated the performance of the piezoelectric actuator throughout its full voltage range. The actuator was initially centered at its median voltage of 12.5 V and precisely aligned. We then performed comprehensive voltage sweep tests from 0 V to 25 V in both forward and reverse directions to assess movement linearity and quantify the hysteresis effect, defined as the positional difference between forward and return trajectories of the piezoelectric benders. The measurements revealed that along the horizontal X-axis, the actuator demonstrated a highly linear angular displacement of \(-24.4^{\circ}\) to \(26.9^{\circ}\), achieving a total field of view (FoV) of \(51.3^{\circ}\), as shown in Figure 5. The FoV is defined here as the angular range over which the system maintains a bit error rate (BER) below the forward error correction (FEC) threshold of \(10^{-3}\). By initiating operation at the median 12.5 V, we successfully minimized the hysteresis of the PZT transducer to only \(0.5^{\circ}\). For the vertical Y-axis characterization, we employed an identical testing methodology. Starting from the 12.5 V center position, the voltage was swept through its full range. The Y-axis exhibited linear angular displacement from \(-9.3^{\circ}\) to \(13.1^{\circ}\), resulting in an operational FoV of \(22.4^{\circ}\) with a comparable hysteresis of \(0.7^{\circ}\). This asymmetry in the FoV between the axes is attributed to the cascaded mounting arrangement, where the additional mass of the X-axis actuator impacts the Y-axis performance. However, these results represent a significant improvement over previous implementations using photoresist lenses, which achieved a FoV of only \(12.8^{\circ}\) vertically and \(27.9^{\circ}\) horizontally. To fully characterize the steering capabilities of the system, we analyzed the relationship between beam profile and steering angle by adjusting the MicroBlock actuator mount. The signal strength at the receiver, which directly impacts the BER, is determined by both the beam shape preservation and the geometric path loss that increases with steering angle. Using an idealized receiver setup with a perfectly aligned fiber collimator, we measured a maximum FoV of \(100^{\circ}\) for the silicon lens and lensed fiber combination, depending on  
Figure 5: Horizontal X and vertical Y movement of the actuators. the distance from the receiver, determined by the Error-Free area on Figure 6 (BER = 10\({}^{-9}\)). The BER limit was calculated by the theoretical BER with OOK modulation [13] to 15.56 dB, by the Equation (3), assuming only additive white Gaussian noise (AWGN). \[BER=\frac{1}{2}\textit{erfc}(\sqrt{\frac{SNR}{2}}) \tag{3}\]  
where _erfc_ is the complementary error funtion and SNR is the power signal to noise ration given by \(SNR=10 log(\frac{P_{\textit{Signal}}}{P_{\textit{Nused}}})\). By removing the limitation of an open receiver, the system meets the requirements for 10 Gbps transmission. However, we observed a noticeable sharp drop in performance after reaching 30\({}^{\circ}\), the value for which the lenses were designed, making it difficult to sustain the required high performance. For beams wider than 38.5\({}^{\circ}\), the system experiences an increasing loss, as the center of the beam is vignetted by the lens edge, while angles beyond this threshold are achieved by the marginal rays; they present a significantly degraded performance. The observed deviations from theoretical predictions can be attributed to various practical factors, including manufacturing tolerances, alignment precision, and minor imperfections in the experimental setup. Eye diagram measurements conducted across steering angles from 0\({}^{\circ}\) to 50\({}^{\circ}\) at 10 Gbps NRZ OOK modulation consistently showed clear openings, as seen in Figure 7, although with progressive reduction in amplitude at larger angles.