An Intelligent Power Management Scheme for Wireless Embedded Systems Using Channel State Feedbacks Hyukjun Oh1 , Jiman Hong1 , and Heejune Ahn2 1
2
Kwangwoon University, Seoul, Korea {hj oh, gman}@kw.ac.kr Seoul National University of Technology, Seoul, Korea
[email protected]
Abstract. In this paper, an intelligent power management scheme for embedded systems with wireless applications is proposed to reduce the power consumption of the overall system. The proposed method is based on the feedback of the extreme channel state indicator that is designed to detect the extremely bad channel condition. The considerable power reduction is achieved by turning off modules within the embedded system related to the information transmissions under such an unreliable channel condition. A simple extreme channel state detector is also proposed.
1
Introduction
Minimizing power and energy dissipation is a key factor in wireless embedded system designs. Therefore, the system should be designed with respect to that particular application to be more power-efficient. This has led to a significant research effort in power-efficient designs in such applications [1]-[4]. It is natural to think of ways to reduce the power consumption considering and utilizing inherent properties of the wireless applications. The use of such properties requires power-efficient design from the inside of the communication module, and it will provide satisfactory power reduction performance inherently. However, most previous works to date have treated it as a given and fixed module and have focused on the power reduction from the top of it [1]-[4]. In this paper, an intelligent power management scheme for embedded systems with wireless communication applications is proposed using such inherent properties of the wireless applications. Any transmission related module is required to be on in the wireless applications only when the transmission is active and reliable. The proposed method is based on the feedback of the extreme channel state indicator from the inside of the baseband transmission module. Under the unreliable channel condition, carrying user information over the air link is completely impossible. The considerable power reduction can be achieved by
The present research has been conducted by the Research Grant of Kwangwoon University in 2004.
L. Wang and Y. Jin (Eds.): FSKD 2005, LNAI 3614, pp. 1170–1173, 2005. c Springer-Verlag Berlin Heidelberg 2005
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turning off several modules within the embedded system related to the information transmission like LCD, image encoder, voice encoder, and power amplifier under this condition. A simple software code can serve as a brain of such tasks given primitive feedback signals from the detector. A simple and efficient signal processing algorithm to detect the extreme channel condition is also proposed.
2
System Model
A usual embedded system comprises a processor core, an instruction cache, a data cache, a main memory, and several custom blocks [1]. Such a custom block has been regarded as a simple one that adds just a constant amount of power dissipation to the overall system. Most previous works have tried to minimize power consumption from the perspective of the rest components other than those ones [1]-[4]. The communication module in the system with the wireless application has been considered as one of this kind. Considerable portion of total power consumption is caused by the transmission related functionality itself in the wireless embedded system. Therefore, it is desirable to include this module in optimizing the power consumption of the overall system. Because our interest is focused on the system with the application of wireless communications, the best target system would be cellular phone. Fig. 1 shows simplified block diagram of a simple cellular phone. It shows only major components and it mainly consists of three parts: application related, basedband modem related, and RF/IF related components. It is assumed that the dedicated application or modem processor is equipped with an operating system and it serves as a brain for performing the proposed power management scheme in this paper. A real time operating system (RTOS) would be appropriate for this purpose, but we do not limit the form of possible operating system in our study. It simply provides a room for central control of the power management scheme.
3
Proposed Power Management Scheme
Applications in wireless embedded systems are inevitably closely related to the propagation channel and channel conditions. In normal channel condition, there is nothing special to consider for the purpose of reducing power dissipation from the point of the overall system. In this case, several existing system models are useful and many previous works on top of them can be applied. In other words, any power saving scheme assuming the transmission related blocks as simple constant power consuming black-boxes can be used. However, it is no longer valid in unsatisfactory channel condition. In the extremely bad channel condition, the real time transmission over the poor air link is not possible. It means that operating any transmission related code, function, module, logic, and component in the system is meaningless. This observation leads to the design of the power saving scheme considering the transmission related blocks beneficial resources for our purpose.
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H. Oh, J. Hong, and H. Ahn
Application Micro Processor
Caches
Multimedia DSP (Image Encoder, Vocder)
Main Memory
Protocol Stack/ Modem Micro Processor
Logics (FPGA, ASiC) Including Baseband Processor
Several I/O Interfaces
Modem DSP
RF/ Analog Tx Chains
RF/ Analog Rx Chains
Controllers (LCD, Speaker, Microphone, Keypad, Clocks, Voltage Regulation, and Etc.)
Several Peripherals
LCD, Speaker, Microphone, Keypad, Clocks, Power Supply, and Etc.
Fig. 1. A system model of a simple cellular phone
Application Microprocessor
Turn off, disable, or execute appropriate actions for predefined unnecessary components in application domain
Protocol Stack/ Modem Micro Processor
Turn off or disable predefined unnecessary components directly related to modem A signal-processing unit generates the indicator for the extreme channel condition Modem DSP
Baseband Processor
Fig. 2. The proposed power saving scheme using the extreme channel state indicator
Such a channel condition mentioned above can happen often in wireless environment due to the characteristic of mobile channels of wide and fast variations. In the case of channel being unrecoverable or/and a long-lasting bad channel condition, a dedicated higher layer function in the protocol stack kicks in to control radio link in most popular wireless communication system [5]. However, when the propagation channel falls in and out such an extremely bad conditions quite often or repeatedly. the dedicated function does not help. In this paper, we propose to disable or turn off the meaningless components (codes, functions, modules, logics, and blocks) to minimize the power consumption when the extreme channel condition is detected. Moreover, some components do not need to operate in full performance under such a condition for power efficiency. The operating system on the dedicated processor serves as a central control unit of managing and performing the required operations for given extreme channel condition indicator. First of all, we need to identify which components can be disabled or put on much lower performance under this situation. Those com-
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ponents are dependent on the configuration of the target embedded system. In our system model, LCD, image/video encoder, vocoder, and RF chains including power amplifier can be candidates for such components. Furthermore, the data memory can be flushed and/or the number of accessing internal or external memory can be minimized. Note that RF chains should be turned on during the pilot transmission to get recovered for the possible forthcoming normal channel condition. Several variants are also possible from the proposed method as per the architecture of the target embedded system. The proposed power-efficient scheme is summarized in Fig. 2.
4
Detection of Extreme Channel Condition
A computationally efficient signal-processing algorithm is proposed to detect the extremely unstable channel condition. The algorithm should be simple to implement so that it does not add another noticeable power dissipation to the overall system. The proposed algorithm is based on the use of channel state information (CSI) that is available in most wireless communication system for the reliable transmissions. We consider the CDMA cellular systems that are the most widely accepted third generation standards for the cellular communications in the world [5]. However, the proposed scheme is not limited to the applications with the CDMA systems. It can be generalized easily to other systems like OFDM based ones. In CDMA systems, binary CSI’s of the transmitted power control bits are available in plenty of time. A value of the bit is indicating that the current channel quality is good enough to satisfy the required quality of service. The other is representing the channel condition that is not good enough to achieve the reliable transmissions. The proposed detection scheme simply estimates the frequency of occurrences of this unsatisfactory channel state by counting the number of the second state of the power control bits during the given time period. Then, the current channel condition is regarded as the extremely bad state if the majority of the bits are the second state in the given time frame. Several simulation results showed that the proposed algorithm worked very well as expected. The details and the performance of the proposed scheme are not addressed in this paper due to the space limitation.
References 1. Li, Y., Henkel, J.: A framework for estimating and minimizing energy dissipation of embedded HW/SW systems. Proc. Int. Conf. Design Automation (1998) 188–193 2. Tiwari, V., Malik, S., Wolfe, A.: Power analysis of embedded software: A first step towards software power minimization. IEEE Trans. VLSI Systems (1994) 437–445 3. Benini, L., Micheli, G.: System-level power optimization: Techniques and tools. Proc. Int. Symp. Low Power Electronics and Design (1999) 288–293 4. Simunic, T., Micheli, G., Benini, L.: Energy efficient design of battery-powered embedded systems. Proc. Int. Symp. Low Power Electronics and Design (1999) 212–217 5. 3GPP Specifications, Release 5.
