# Evaluation of the Effects of Hidden Node Problems in IEEE 802.15.7 Uplink Performance
## 1 Introduction  
Optical wireless communications based on visible light [1], named Visible Light Communications (VLCs), use visible light to transmit data by modulating intensity in light emitting diodes (LED), employing faster switching rates than the persistence of the human eye to avoid flickering in data/light sources. In recent years, the increasing use of LEDs in illumination systems has been conducted due to the emergence of the VLC technologies, both in indoor and outdoor environments [2]. In 2011, the IEEE (Institute of Electrical and Electronic Engineers) published the IEEE 802.15.7 standard [3], which defines Physical (PHY) and Medium Access Control (MAC) layers for short-range wireless optical communications using visible light. The MAC protocol plays a significant role in the performance of this standard, and includes a channel access procedure based on the CSMA/CA mechanism [4]. The hidden node problem is a well known problem in wireless networks based on radio frequency using CSMA/CA channel access mechanisms [5; 6]. This problem is caused by signal coverage constraints, resulting in network nodes not being able to detect transmissions performed by other nodes. This problem can produce collisions due to concurrent transmissions, potentially degrading the performance of the communications in the network. The use of Resquest to Send/Clear to Send (RTS/CTS) define frames to reduce collisions produced by hidden nodes is a common solution adopted in wireless networks. For example, the IEEE 802.11 standard specification includes the option of using RTS/CTS frames in MAC layer protocol. Regarding the use of RTS/CST frames in IEEE 802.15.4 networks, authors in [7; 8; 9] have questioned the effectiveness of these solutions for several reasons, including the fact that, during backoff delays time, RTS/CTS frames sent by other nodes are not received because each node transceiver is disabled to reduce energy consumption, which is an important requirement in these type of networks [6]. The same restriction applies to backoff delays in the IEEE 802.15.7 standard. In order to improve efficiency in energy consumption, the use of directional antennas has been proposed for wireless sensor networks [10]. Directional antennas provide several advantages such as spatial reuse channel and increases in coverage distance. However, directional antennas present a new problem called the directional hidden node problem, which is quite similar to the problem found in VLC networks. Solutions to the the directional hidden node problem in wireless sensor networks have been proposed, see [10], based on the rotation of the directional antenna. The rotation of the antenna is combined with some type of scheduling scheme in order to receive the transmissions coming from surrounding nodes. However, these type of solutions involve significant changes in the standard specification. In VLC, the hidden node problem can be considered as a key issue because of the directional characteristics of both optical transmitters (LED) and receivers (photo detectors). Since the IEEE 802.15.7 standard was published in 2011, we are unaware of other studies or published works to evaluate the effects of the hidden node problem when using CSMA/CA mechanisms in a VLC network. Several research papers have been published to evaluate the IEEE 802.15.7 uplink (from user terminals to data/lightning infrastructure) performance based on CSMA/CA. Papers [11; 12] presented analytical and simulation models to analyze uplink performance, but their authors stated explicitly that there are not hidden nodes in their proposed models. In [13], the performance of transmissions using a simulation model is discussed, although the authors do not specify the physical channel model used, and they do not indicate if they take into account the presence of hidden nodes. In this paper, we present an evaluation of the effects of the hidden node problem in the performance of uplink communications when using the slotted CSMA/CA random access procedure defined in the IEEE 802.15.7 standard.