# WON-OCDMA System Based on MW-ZCC Codes for Applications in Optical Wireless Sensor Networks
## 1 Introduction  
Optical sensors have found their application in structural health monitoring (SHM) thanks to their small size, high accuracy and immunity to electromagnetic noise [1]. The use of optical multiplexing techniques has advanced the capacity, capabilities and performance of sensor networks in terms of the number of sensing points while lowering system complexity and cost [2]. Optical fibres that carry sensor signals have also been used as sensing elements themselves. The successful application of optical sensing in the construction industry has been reported for different monitoring purposes including gas leakage [3], temperature [4], strain [5], structure vibration [6], reinforced concrete beams [7] and building cladding systems [8]. It is essential to monitor civil structures to appraise the structural condition to be able to predict the internal damages at an early stage [9, 10]. Several fibre-optic methods and tools have been developed for vibration monitoring. In 2001, a fibre Bragg gratings (FBG) coupled with a broadband light source were exploited to detect vibrations based on the light intensity modulation produced by vibrating FBG [11]. The feasibility of a fully distributed vibration sensing was evaluated based on a fibre diversity detection sensor [12]. An intensity-modulated fibre-optic accelerometer was also developed for vibration monitoring of wind turbine blades [13]. Collecting and transporting data from optical sensors for their processing is the role of the optical sensor network (OSN). Optical multiplexing techniques including time and wavelength division multiplexing [14] and optical code division multiple access (OCDMA) have been adopted to connect distributed sensing points [15]. In wavelength division multiplexing, each sensor is assigned a single wavelength from the source optical spectrum [16]. Time division multiplexing dedicates different time slots to individual sensors. For OCDMA sensor networks both, synchronous and asynchronous architectures have been proposed [17] with focus on exploring different coding techniques. For example, a spectral amplitude coding (SAC) offers a reasonable multiple access interference (MAI) cancellation, simplicity of implementation [18] and the ability to support a differentiated quality of service (QoS) by varying code-weight [19]. Using the variable-weight SAC (VW-SAC) coding an efficient OCDMA communication system for supporting the vibration sensing of unequally distributed points has been proposed [6]. Most research has focused on equally distributed sensor points, where the received optical power (ROP) by each sensor should be similar. However, in SHM, most structures are distributed with different distances, and therefore it is not possible to set up an equidistant sensor network. This leads to a near-far problem. It should be noted that the use of optical amplifiers to compensate for losses is possible but increases the system noise as well as the cost of the network. To resolve this problem and support the vibration sensing of unequally distributed points, an SAC-OCDMA system was developed and proposed [20]. Similarly, for the VW-SAC system, different code families [18; 19; 21; 22] and detection techniques [23] have been proposed. A successful experimental proof of concept for its use in optical communication has been already reported [24]. Optical fibres have a large bandwidth. This provides a temptation to extend the already existing fibre infrastructure to support SHM. However, it might not be the most cost-effective to install optical fibre for monitoring hard-to-reach places. Wireless optical networks (WON) are an elegant solution where the implementation of fibre is impractical or not cost-effective [25]. The integration of OCDMA and WON systems can provide the desired solution [26; 27]. Wireless OCDMA systems have already been deployed to support healthcare monitoring applications [28]. However, the majority of developed OCDMA codes have too high cross-correlation thus not suitable for WON and SHM applications.