Energy efficient algorithms and techniques for wireless mobile clients 7

CHAPTER CONCLUSIONS AND FUTURE WORK The main contribution of this thesis is to provide a suite of algorithms for conserving energy in smartphones. In contrast to previous works that suggest conserving energy by scheduling network packet access and manipulating duty cycle of wireless interfaces for latency tolerable applications in mobile devices, we focused on conserving energy for highly dynamic and challenging real-time applications such as, fast paced games. The traditional backlight scaling techniques for LCDs are not efficient for mobile games due to their computational overheads. The technique we developed have negligible overheads. Previous works on conserving energy in OLED displays, simply turns all contents to green colour (which is the power efficient colour). However seeing everything in green reduces the users interest in continuous content viewing. Our approaches, in contrast to previous works, generates multi-colour web pages that are power efficient and harmonious with each other. In this chapter, we briefly summarise our contributions and discuss potential directions of future research. 6.1 Conclusions In this thesis, we have introduced the importance of energy conservation in smartphones, especially, when running applications like games involving intensive 3D virtual environments. We have described and analysed the structure of game maps 227 (game worlds) in modern 3D games, HDR tone mapping techniques and response of HVS to colour and brightness. Based on these analysis, we have developed and evaluated a set of algorithms for conserving energy in modern smartphones. We have targeted display and network interface of the smartphone to conserve energy as these two components consume more than 80% of the energy in a smartphoene. 6.1.1 Conserving LCD Energy We have reported an effective approach for saving LCD display energy saving in Chapter 3. Our approach is based on the observation: LCD backlight can be dimmed to save energy by compensating it with brightening the contents. We have implemented this approach in mobile games and demonstrated that this approach can save up to 68% of energy consumed by the backlight while maintaining the visual quality levels acceptable to users. The main contribution of this work is in the development of an efficient backlight scaling algorithm dynamically controlled by contrast of the content. To make it efficient we have used non-linear tone mapping operator γ. The tone-mapping operator γ can be applied to a content in O(1) time as it is already integrated into the display hardware of modern smartphones. In addition, changing the brightness of the content using γ does not saturate the pixels which is a common problem in linear content brightening techniques. However, the challenge is to find the relationship among γ and other parameters required for backlight scaling. These include the relationships between “γ and contrast”, “γ and backlight level” and “contrast and brightness”. We have derived these relationships through measurements using high precision light sensors, industry standard metrics and controlled user studies. 228 6.1.2 Conserving OLED Display Energy Driven by the importance of having harmonic colour combinations among the texts, backgrounds and images in webpages for pleasing visual effect, we have developed a set of techniques and algorithms for recolouring the web content with energy efficient colours for OLED displays. Energy consumption of OLED displays depends on luminance and colour of the content. Our recolouring algorithms are presented in Chapter 4. The main contribution here, is a simple and innovative approach which extracts brand colours of an organisation from its logo and computes a set of harmonic colours for recolouring that matches smartphone users energy efficiency requirements. In addition, we have devised an algorithm for recolouring images in webpages with energy efficient colours without loosing the visual appearance of the image. Such a colour mapping is possible because human eyes are not sensitive to small changes in colour. In addition, the sensitivity is non-linear in both colour space and luminance space. Our algorithms exploit this non-linear response of HVS to intelligently recolour the images with minimum loss to the visual quality of the image. Our evaluations show that we can save about 60% energy of the display while browsing webpages with good legibility of texts and fidelity of the images. 6.1.3 Conserving Wireless Network Energy Wireless network interface is another high power consuming component in a smartphone. In Chapter we have discussed the design and development of three algorithms for determining when and how long to put the wireless interface in to sleep mode without affecting the game play quality. The first algorithm uses distance based approach, the second one uses visibility based approach, and the third one is a combi- 229 nation of both. We have implemented and evaluated the algorithms in commercially successful FPS and MMOG games. Our energy management schemes are based on a common observation: game state updates can be suspended temporarily to a client to save energy, with minimum or no loss to the game play quality, when the game state of the client is not-critical. The main contributions are the prediction methods designed in these algorithms to intelligently determine the future game state and control the network interface. These prediction methods exploits the structure of the game map, characteristics of player and game genre to determine the game state. Our simulation and real measurements show that, up to 57% of wireless interface energy can be saved without affecting the quality of the game play adversely. Even for fast paced FPS games such high amount of power saving is possible if the game map is highly occluded as it provides short but more chances to sleep. Our user studies show that the artefacts introduced by our algorithms are not noticeable. We recommend distance based approach for huge open maps with few occlusions, visibility based approach for small size highly occluded closed maps and a combination of both for highly occluded large size maps for efficient energy saving at wireless network interface level. 6.2 Directions for the Future Research The algorithms and techniques presented in this thesis are developed for games and web browsing applications. However, these techniques can be adapted to other applications. Also, the knowledge and experience we gained in developing these techniques can be applied to other domains for conserving energy. Our future works include, 230 1. Adapting OLED Colour Transformation Algorithm for Videos and Games: Our algorithm for transferring colours of an image to energy efficient colours for OLED displays can be adapted to video and game applications. As we have previously described in Section 4.4.3, the average change between successive frames in a video are minimum. Video storing and streaming formats such as MPEG-2 exploits this observation and stores the video frames efficiently in GOP structure [134]. We have plans to exploit the structure of video format to efficiently transfer colours to energy efficient colours for stored video content. We are interested in studying its impact on visual quality. In addition, we are interested in applying the technique for real-time live video which brings strict performance constraint. As per the studies of psychology and cognitive science, human perception is attention-based and selective. We want to exploit this for optimal energy saving in videos with minimum losses to visual quality. We also envision that the same saliency feature can be applied to games. Adapting our algorithm for games and videos will be our immediate following work. 2. Gamelets (Distributed Micro-Cloud) Infrastructure for Energy Efficient MMOG in Smartphones: Energy and latency (network and computational) are the two fundamental problems that affect the playability and quality of massively multiplayer 3D games in mobile devices. We are looking forward to develop an infrastructure and a set of protocols to offload the computation intensive rendering process from resource constrained mobile devices to next hop node (called - Gamelet node) and stream the video contents to mobile device for game play. Intuitively, this will 231 reduce both latency and energy cost of computation for the mobile client. However, it will increase the amount of data to be received by the mobile client over the network which costs addtional communication energy. Hence, the protocols should be carefully designed considering various trade-offs [152]. The Gamelets infrastructure will have a set of distributed and synchronised nodes forming a cloud system for serving the MMOG clients which are just one hop away. As the clients are served by next hop nodes the communication latency will be very low. In addition, the bandwidth induced delay of conventional cloud rendering is eliminated with Gamelet architecture. Conventinal clould games suffer from two fundamental issues which are latency [153] and server scalability (in terms of bandwidth and computation). In addtion to latency reduction, Gamelets infrastructure eliminate the server scalability issue by distrbuting both bandwidth and computation requirements to the Gamelet nodes which are one hop away from the mobile clients. Moreover, colour transformation for power conserving at OLEDs can be applied during rendering in the Gamelet node without taxing the smartphone client. However, such an infrastructure is possible only if there is a solid synchronisation between Gamelets and game server and synchronisation among Gamelets. The introduction of light weight Chromium OS [10] and HTML5 [9] may lead to ultra-thin mobile devices in future. Our Gamelets based rendering approach may potentially transfer these devices into good gaming devices. 3. Efficient Controller for 3D Renderer’s View Based Approach: Current implementation of 3D Renderer’s view based approach for conserving 232 energy at wireless network interface level uses simple AMID based controller with experimentally set parameters. A more robust controller such as PID controller [93] with statistically set parameter values can improve the efficiency of the algorithm further. We are interested in implementing and evaluating our algorithm with PID controller and other statistical controllers in future. 4. Adapting our Algorithms for Electrowetting Displays: A promising future display technology is Electrowetting displays (EWD) [154]. Electrowetting involves modifying the surface tension of liquids on a solid surface using a voltage. By applying a voltage, the wetting properties of a hydrophobic surface can be modified and the surface becomes increasingly hydrophilic (wettable). With Electrowetting displays, the modification of the surface tension is used to obtain a simple optical switch by contracting a coloured oil film electrically. Without a voltage, the coloured oil forms a continuous film and the colour is visible to the consumer. When a voltage is applied to the display pixel the oil is displaced and the pixel becomes transparent. When different pixels are independently activated, the display can show content like an photograph or a video. We can also use this transmissive pixel as the basis for reflective of transflective displays. The high switching speed (a few milliseconds) of electrowetting and its applicability to small (pixel) dimensions means that electrowetting is ideally suited for application to displays showing video content. EWD displays are intrinsically the brightest and most vivid. These displays are more power efficient than LCD displays. Because the displays are so significantly brighter, they require less supplementary illumination. We are interested in 233 investigating the properties of this upcoming display technology and adapt our LCD and OLED display algorithms for improving the power efficiency of devices that use EWD displays. 234 . combination of both for highly occluded large size maps for efficient energy saving at wireless network interface level. 6.2 Directions for the Future Research The algorithms and techniques presented. maps 2 27 (game worlds) in modern 3D games, HDR tone mapping techniques and response of HVS to colour and brightness. Based on these analysis, we have developed and evaluated a set of algorithms for. algorithms for recolouring the web content with energy efficient colours for OLED displays. Energy consumption of OLED displays depends on luminance and colour of the content. Our recolouring algorithms