# Evaluation of the Effects of Hidden Node Problems in IEEE 802.15.7 Uplink Performance
## 7 Conclusions  
Since it was published in 2011, the IEEE 802.15.7 standard has become one of the most important initiatives to normalize and deploy the use of VLC in WPAN. Though the implantation of IEEE 802.15.7 based networks must face several technical challenges, among which are the difficulties in uplink transmissions (from devices to coordinators), including limitations in mobile device transmission power imposed by LED power consumption, and mobility restrictions caused by the necessity of keeping a proper orientation in devices.  
Additionally, uplink transmissions made by means of the CSMA/CA method in the IEEE 802.15.7 standard are affected by the hidden node problem, whose effects have been evaluated and analyzed in this research work. We have evaluated the hidden node problem by simulating a star network employing the directed LOS channel model. Under these conditions, we verified that all devices are hidden nodes in all of our modeled scenarios, since none of the devices are capable of detecting the signal transmitted by the rest of them. Our simulation results make evident a severe degradation in network performance produced as a consequence of the hidden node problem. Performance degradation is proven by obtained values for the analyzed metrics, like end-to-end goodput, message loss ratio, percentage of collisions, and energy consumption.  
Actual performance degradation may depend on specific characteristics of optical transmitters and receivers used in devices. The use of less directive transmitters and receivers could partially mitigate the effects of the hidden node problem in networks employing a diffuse link configuration. In any case, limitations in optical signal coverage--due to restrictions in transmission power in devices--demand not ignoring the effects of the hidden node problem, regardless of the link configuration employed.  
In conclusion, the hidden node problem has to be included as one of the difficulties to be solved in uplink transmissions made by using the CSMA/CA method in IEEE 802.15.7 networks. Therefore, as part of this research work, we elaborated on a solution proposal to the hidden node problem, which has the advantage of not requiring modifications in the specifications defined in the standard for PHY and MAC layers in devices and coordinators.  
Our proposed solution is based on the use of the idle patterns sent by the coordinator, during idle or receive operation. Appropriate selection of idle pattern type--in-band or out-of-band--sent by the coordinator can indicate if it is receiving transmissions from devices in the network. Simulation results obtained with our solution demonstrate its high degree of effectiveness, since it allows  
Figure 15: Comparative the Goodput (**a**) and the Message loss rate (**b**) among the simulation scenarios labeled as (1) when a signaling pattern is used to indicate that a device is transmitting and the scenarios labeled as (2) when a signaling pattern is not used. The results shown are for the scenarios with four and 16 devices.  
obtaining identical performance values to those obtained from the simulations performed without hidden nodes in the network.  
This research is supported by the Research Program of University of Las Palmas de Gran Canaria (ULPGC2013-15).  
[PERSON] and [PERSON] have developed the simulation model, organized the simulations, performed the data analysis and wrote the manuscript. [PERSON] and [PERSON] have supervised the work. All authors have read, revised, and approved the final manuscript.  
The authors declare no conflict of interest.