## 1 Introduction
The positioning accuracy of a VLP system using a QADA has been designed for the former, but the latter has not been examined yet. This paper presents the first analysis and experimental demonstration of the communications performance of a VLP system using a QADA receiver. The importance of angular diversity receivers in VLP systems is due to the key advantages of AOA-based positioning systems compared with systems using other techniques such as RSS and TOA [1; 18]. VLP systems using RSS depend on the receiver having accurate knowledge of the transmission pattern and the transmit power of each luminaire, while systems using TOA depend on very accurate synchronization; in the order of nanoseconds. Systems using AOA have none of these limitations. They do however require a receiver that can determine the AOA of light from each beacon luminaire. Several different forms of AOA receivers have been described in the literature [19; 20; 21]. In some, angular diversity is achieved using a 3-D structure [20], where multiple PDs face in different directions. However, a 3-D structure of this form is not likely to be convenient, especially if the receiver is to be integrated into handheld devices. In others, angular diversity is achieved by using a camera [19]. Despite many innovative approaches, such as the use of the rolling shutter feature of many digital cameras, the rate of data that can be received using a camera in VLP is strictly limited. ADA-based receivers have the potential to overcome these disadvantages. They provide angular diversity within a compact structure and can receive high-speed data. It was shown in [22] that, theoretically, the original form of ADA receiver [14] can also be used for VLP systems. However, certain unrealistic assumptions were made in [22]; for example that the PDs were identical to each other and that the dimensions of the aperture and the PD were precise and perfectly aligned with each other. In practice, such constraints are hard to meet. The QADA relaxes some of these constraints. The optical front-end of a QADA receiver consists of a single quadrant PD, with four closely matched quadrants (fabricated in one process), placed below a single aperture [16]. To further improve the positioning accuracy of the QADA, the QADA-plus was introduced [23]. The QADA-plus is a two-stage receiving system that combines the QADA with a camera, but it still uses the QADA stage as the data receiver [24]. Communication is an essential aspect of VLP and this must be considered when designing any VLP system [25; 26]. The earlier work on VLP systems that uses QADA receiver considered only the positioning aspects [16; 17; 23; 24; 27]. In this paper, we investigate its communication characteristics (Please note that this paper is **not** about investigating the positioning performance as this has been extensively studied in previous work. In this paper, we **only** focus on the characterization of the communication properties of a VLP system using a QADA receiver). Several different types of modulation can be used in VLP systems [28]. In this paper, asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) is used [29]. ACO-OFDM is a simple and power-efficient modulation scheme. Using ACO-OFDM allows us to clearly demonstrate the performance of the QADA receiver without having to consider other parameters, such as DC-bias or clipping distortion which are required to fully specify other modulation formats [30]. We also investigate the performance gains that can be achieved by exploiting the diversity present in the signals received by each of the quadrants of the QADA. While diversity has been extensively applied in RF systems, there is relatively little work on diversity in VLC systems. This is probably because VLC systems provide little _spatial_ diversity at the receiver unless the receiving elements are spaced far apart [31]. In contrast _angular_ diversity has been shown to provide significant gain [32; 33].