# On Optimal Cell Flashing for Reducing Delay and Saving Energy in Wireless Networks
## 1 Introduction  
The main objectives for next generation communications, although they may be different for various organizations, are high data rate, low latency and low energy consumption [1]. Data rate means not only average data rate but also cell edge data rate, which is the worst data rate, because of inter-cell interference. With the deployment of broadband data services, demands of mobile traffic are growing rapidly [2], and thus the 4 th generation wireless networks may reach their limits soon. This implies that the above-mentioned three objectives may not be achieved at the same time, and thus to resolve these challenges, research towards the 5 th generation of mobile cellular networks is actively being conducted. To improve user-perceived performance specifically focused on energy efficiency and cell edge users, many cell switch-off techniques have been proposed. The works in [3, 4] proposed interference management schemes by switching off the nearest cells. The authors in [5, 6] proposed inter-cell interference (ICI) management by exploiting adaptive fractional time reuse (FTR) instead of fractional frequency reuse (FFR). However, these works focused on improving edge user performance and neglected considering energy efficiency. A series of papers [7, 8, 9, 10, 11, 12, 13] studied about energy-efficient cellular systems by switching off base stations and/or transmit antennas. The works such as [14, 15] proposed energy harvesting wireless communication systems that exploit renewable energy such as solar power, wind power, etc. However, these works did not consider cell edge user performances. The work such as [16] proposed increasing energy efficiency and cell edge user performances simultaneously by cell switch-off with coordinated multi-point (CoMP). The work such as [17]proposed energy conservation and interference management in a multi-cell time division multiple access (TDMA) network. However, these works consider only the static system where the number of users are fixed, and thus cannot capture real systems where the number of users (or flows, equivalently) changes over time. The work such as [18] studied energy and delay in the downlink of a proportional-fair cellular system with inter-cell interferences and fading. However, this work focused on the trade-off between energy and delay. To the best of our knowledge, increasing energy efficiency and cell edge user performances (or even improving the average performance of all users) simultaneously by cell switch-off, specifically in a dynamic system, has not been studied in the literature. In this paper, we investigate a mechanism that improves three objectives at the same time, i.e., energy efficiency, cell edge user performances, and the average user performances by managing cell activation in a timely orthogonal fashion. We call it cell flashing because base stations are turned on and off periodically and rapidly so that, when one base station is turned on, the adjacent base stations that make interferences are always off; switching off adjacent cells significantly improves energy efficiency, and cell edge users obviously experience less inter-cell interference. Cell center users who were not experiencing much inter-cell interference, however, might be slightly penalized because the associated base station is not always on. Then, it becomes not clear, even though energy efficiency would be mostly improved with cell flashing, how overall system performance would be. In this regard, we are interested in under which circumstance cell flashing becomes beneficial for both energy efficiency as well as user-perceived performance (overall and/or in the cell edge). Note that, in general, switching off base stations to save energy leads to longer delay, i.e., there is a tradeoff between energy saving and user-perceived performance. Thus, a proper switching on-off mechanism and its theoretical analysis are essential.