# Polarization Differential Visible Light Communication: Theory and Experimental Evaluation +
## 1 Introduction  
As the need for communication bandwidth increases persistently [1], Visible Light Communication (VLC) has emerged as an alternative means for wireless communication. VLC capitalizes on the omnipresence of light-emitting diodes (LEDs) and exploits their ability to modify the transmitted intensity at high data rates in order to modulate signals in the visible light spectrum [2]. Several physical layer implementations for VLC have been included in IEEE 802.15.7 [3]. VLC has shown to be a promising development for both indoor and outdoor communications. Indoor applications also include indoor positioning systems [4, 5]. In recent work, quadrature photodiodes equipped with dedicated apertures were employed to provide indoor localization based on Angle-of-Arrival (AoA) [6, 7]. The communication properties of such receiver have also been investigated in detail [8].  
Rather than intensity based modulation in VLC, an additional interesting opportunity lies in modulating the polarization state of visible light. For instance, PIXEL [9] uses a polarizer in conjunction with a Liquid Crystal (LC) to modulate the polarization axis of light at low data rates. Demodulation is realized by means of a polarizer-equipped camera receiver. The data rate is however severely limited by the response time of the LC and the camera refresh rate. POLI [10] also uses a cameraequipped with a polarizer but combines the polarizers with a dispersor at the transmitter side to alter the perceived color at the camera. Here also, the data rate is also constrained by the low camera update rate. Additionally, orthogonal polarizers have been employed as a multiplexing technique [11], referred to as polarization-division multiplexing (PDM). Researchers demonstrated an optimized pre-equalization circuit [12] and proposed an asymmetric 3 \(\times\) 2 multi-input multi-output (MIMO) system for PDM [13]. Recent studies have applied PDM in conjunction with Orthogonal Frequency Division Multiplexing (OFDM), achieving a 45% increase in data rate and spectral efficiency [14]. Related work demonstrated a setup consisting of red/green/blue laser diodes reaching 40 Gbit/s using PDM OFDM signals [15]. Furthermore, other research has used orthogonal polarizers at both the transmitter and the receiver side to implement differential signalling between polarization states on one hand and to increase robustness to interference on the other hand [16]. These systems deploy photodiode receivers, which increase the bandwidth significantly compared to camera-based receivers. While the receiver orientation was fixed in aforementioned work [16], the impact of the receiver rotation with respect to the transmitters was assessed in previous work [17]. In this work, this assessment is reiterated and extended. The presented polarization-based modulation scheme is functional alongside intensity-based approaches, meaning optical bandwidth can be shared among both approaches. An analytical expression is derived to express the impact of receiver rotation. This dependency is also assessed in a representative indoor scenario and compared with the theoretical elaboration. It is demonstrated that the signal quality can deteriorate heavily with receiver orientation when using a single differential receiver pair. A second differential receiver pair can be deployed to overcome this drawback. This method is experimentally evaluated using a custom design based on a quadrant photodiode. The contents of this work is structured as follows: Section 2 describes the general system model, modulation scheme and corresponding signal processing. An experimental setup used to verify this model is presented in Section 3, while the measurement results are given in Section 4. Finally, the main conclusions of this work can be found in Section 5.