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Abstract Power-saving technology is a key technology in mobile terminal design. Most of the current research is on power-saving technology in standby mode. Combining with the technical characteristics of TD-SCDMA system and terminal, this paper deeply studies and realizes the key technology of power saving in TD-SCDMA multimedia terminal in working mode, which has great technical and practical significance for promoting the development of TD-SCDMA industry.
1 Introduction
The talk time and standby time of mobile terminals have always been one of the focus issues of the industry. In addition to supporting basic voice and short message services similar to 2G terminals, 3G terminals also support rich multimedia applications such as high-speed data downloading and video telephony. This adds complex physical layer digital signal processing and high-level protocol stack software processing. The difficulty of power saving processing of the terminal also puts higher requirements on the power saving performance of the 3G terminal.
At present, the industry's research on power saving of TD-SCDMA terminals focuses on standby mode, that is, the purpose of power saving in standby mode through periodic sleep-wake, but little research on power saving in normal working mode. . Based on this situation, this paper combines the technical characteristics of TD-SCDMA system and terminal to deeply study and realize the key technology of power saving in TD-SCDMA multimedia terminal in working mode, which is of great significance to promote the development of TD-SCDMA industry. Technical and practical significance.
2. The overall structure of the TD-SCDMA multimedia terminal
One of the important features of TD-SCDMA is that it can provide data transmission rates of up to 384 kbit/s, which is much higher than the data transmission rate of GSM. This data rate can support not only ordinary voice but also multimedia services. For the 3G "killer" application. The overall structure of a TD-SCDMA multimedia terminal capable of supporting a 3 megapixel camera and a videophone is shown in Figure 1, where clk26M and clk32k are two external clocks used by the system.
Figure 1 Overall structure of TD-SCDMA multimedia terminal
The terminal is based on Philips' Dragonfly mobile terminal design platform, in which the baseband uses ARM9 as the processor, which is mainly responsible for the processing of the protocol stack and various applications and the management of the entire terminal; the Modem IC is responsible for the processing of the TD-SCDMA physical layer; ABB is The analog baseband and digital baseband interface completes the analog-to-digital conversion and digital-to-analog conversion of the signal; the RF transmission scheme uses MAXIM's MAX2507 (the chip has integrated PA), the receiving scheme uses MAXIM's MAX2392; the multimedia coprocessor acts as video, The dedicated hardware accelerator for image processing is the main hardware device that distinguishes the video terminal from the ordinary terminal; the LCD is a 65 kHz true color high pixel dual screen to support excellent video effects; the camera resolution is up to 3 million pixels. The external clock used by the entire system is the 26 MHz system reference clock (clk26M) generated by the VCXO and the 32 kHz low-speed clock (clk32k) generated by the Power Management Unit (PMU).
The power consumption of TD-SCDMA terminals is affected by many factors such as wireless environment, network configuration [1], protocol stack control and terminal hardware and software solutions, power management, low-power design of the chip itself and its process characteristics [2], among which The decisive factor is the power saving technology of the terminal itself.
3. Power management scheme for TD-SCDMA multimedia terminals
Reasonable power distribution based on the specific power consumption characteristics of different systems, improvement of the efficiency of the power conversion chip, and simple and effective control of each power supply are important means for the power saving technology to have an immediate effect.
LDOs (Low-Dropout Linear Regulators) commonly used in 3G terminals have the advantages of low cost, small package, low peripheral components and low noise. The disadvantage is low efficiency. The DC/DC converter can provide up to 95% efficiency, but it occupies a large PCB (circuit board) area, the output voltage ripple is also large, and electromagnetic interference (EMI) problems are more prominent. The use of a parameter measurement unit (PMU) that integrates almost all of the power required by a mobile terminal is an inevitable requirement for mobile terminal design.
Combined with the specific characteristics of each chip in the TD-SCDMA terminal, the principle of convenient control of each power supply under different states is as follows. The corresponding power management scheme is designed as follows: The 500 mA DC/DC output integrated in the PMU is 1.8 V. Powering the core of the Modem IC and the baseband processor to take full advantage of the high efficiency of the DC/DC. The memory is powered by a 1.8-voltage LDO output from the LMU in the memory. The RF transceiver is used in two ways in the PMU. Very low noise and very high power supply ripple rejection ratio (PSRR) LDO output 2.8 V voltage to supply it separately; LCD and keyboard backlight using PMU boost DC/DC output 5V voltage to LED anode, PMU An LDO with high current drive capability outputs 2.6 V to power the audio processing section, and then uses the other two LDOs in the PMU to output 1.8 V and 3.0 V respectively to power the high and low ports of each chip. The two special voltages of 1.5 V and 2.8 V are required for processing, and are supplied separately by a dual DC/DC. All voltage enable pins are connected to the baseband processing chip (ARM) sleep indication signal to simplify the control mechanism. When each power supply needs to be turned on/off depending on the respective register configuration and the indication signal, it can be based on the actual The use case is programmed to conveniently and dynamically control the switching of each power source at different times.
4. Power-saving design of TD-SCDMA multimedia terminal
4.1 Dynamic Power Management of TD-SCDMA Terminal RF Transceiver
The radio frequency transceiver is the first power consumption unit in the TD-SCDMA terminal. Effective dynamic management of the RF transceiver can not only improve the RF indicators such as spuriousness, sensitivity, and adjacent channel leakage, but also fully utilize the unique TDD mode of the TD-SCDMA system to save power from the perspective of saving terminal power consumption. purpose.
WCDMA works in FDD mode. In this system, the time-frequency plane is divided into M discrete frequency segments, which are adjacently distributed on the frequency axis. The user always occupies a certain frequency band in the frequency channel and takes 100% duty. More than the signal energy. TD-SCDMA operates in TDD mode, in which the user transmits signal energy with a small duty cycle using only a portion of the time slot. Since the CDMA system is a self-interference limited system, its system capacity mainly depends on multiple access interference from other users in the same system, so the average transmission power of the TDD system transmitting signals in a burst mode is inevitably significantly smaller than that of the FDD system. Ideally, if the UE transmits the signal using only one of the seven time slots in the TD-SCDMA frame structure [2], that is, when transmitting the same signal power (for example, 24 dBm), the average transmit power of the TDD system is FDD. 1/7 of the system, 8 dB less than in FDD mode.
In FIG. 2, each subframe of TD-SCDMA (including 7 service slots and 3 special time slots) has a length of 5 ms, TX_ON is a control signal opened by the transmitter, and RX_ON is a control signal opened by the receiver. As can be seen from the figure, the TDD system only opens the corresponding circuit in the time slot in which the RF unit needs to receive and transmit the useful signal, and turns off the corresponding RF circuit in the idle time slot, thereby greatly reducing the average power consumption of the RF unit.
Figure 2 Timing of control signals for TD-SCDMA systems
4.2 Clock Gating Technology
The TD-SCDMA terminal must complete signal processing procedures such as channel estimation, convolution, matrix multiplication, Cholesky decomposition, downlink synchronization, Turbo codec, and burst generation, and most of these processes are performed by corresponding functional modules. According to the specific working process of the TD-SCDMA terminal in the working state, the corresponding function modules and the distribution of software and hardware are reasonably divided, so that the functions of each module are relatively independent, and the number of times of copying and moving data between hardware and software is reduced. Then, using the clock gating technology to turn off the circuit unit that does not need to work and does not need to work for a certain period of time, the average power consumption of the terminal can be greatly reduced. Similarly, after the DSP completes the corresponding task, the terminal immediately enters the sleep mode, and needs to wake up again when the work is needed, so that the purpose of further power saving can be achieved.
4.3 Dynamic Power Management of Baseband Processors and Modem ICs
The digital baseband processor is the second largest power consuming unit in the TD-SCDMA terminal. To reduce the power consumption of CMOS chips, reducing the clock frequency or supply voltage is the most direct and effective method, but down-conversion and buck need a compromise between delay and power consumption. Simply reducing the frequency does not reduce the power consumption of the circuit, because it takes longer for the processor to handle the same task. However, there are still ways to use the down-converting method to save power on current processors, that is, dynamic voltage-frequency adjustment techniques. Current processors are designed with the maximum performance requirements in mind, and mobile terminals do not require such high performance during most of the working hours. Therefore, the clock frequency is reduced while the processor is lightly loaded. And the supply voltage can significantly reduce the average power consumption, while meeting the performance requirements of the system.
According to the hardware characteristics of the processor used, combined with the different requirements of different services of different services of TD-SCDMA terminals, all services are divided into several types of typical services of different levels. Under the premise of meeting the performance requirements of this level, the operating frequency and operating voltage of the baseband processor are appropriately reduced, thereby greatly reducing the average operating current. The dynamic power management results of the TD-SCDMA terminal baseband processor and Modem IC under different typical services are shown in Table 1.
Table 1 Dynamic power management results of TD-SCDMA terminal baseband processor and Modem IC under different typical services
It can be seen from Table 1 that when the TD-SCDMA multimedia terminal provides video services, the dynamic power management will save the power consumption of the digital baseband processing chip by more than 20%, while providing the traditional voice with less data processing. The business saves more than 30% of power consumption, which greatly extends the continuous working time of the terminal.
4.4 Dynamic Power Management of TD-SCDMA Terminal Amplifier
The power amplifier in the RF transceiver is one of the most power-consuming units in the TD-SCDMA terminal. The linear power amplifier of the TD-SCDMA terminal is different from the nonlinear power amplifier commonly used in the 2G system. Linear amplifiers are ideal for systems that use non-constant envelope modulation (such as QPSK) to reduce distortion and adjacent channel interference [3, 4], but their biggest drawback is low efficiency, typically around 30%. How to improve the actual average efficiency of the amplifier to extend the continuous working time of the terminal?
According to statistical analysis, in most cases, the transmit power of TD-SCDMA terminals is much lower than its maximum transmit power of 24 dBm, and the transmit power of about 90% is lower than 16 dBm, thanks to the TD-SCDMA system. Accurate and fast power control not only overcomes the "near-far effect" but also increases system capacity. Therefore, combined with the structural characteristics of the linear power amplifier, different from the traditional direct battery power supply mode, an improved control scheme for the TD-SCDMA terminal power amplifier is shown in Figure 3. The power supply voltage of the power amplifier is passed by the baseband processing chip through the digital-to-analog converter (DAC). The output is controlled. Different transmit powers and their corresponding supply voltages are used to satisfy the system ACLR (adjacent channel leakage power ratio) requirements. For TD-SCDMA systems, the ACLR at ±1.6 MHz from the center frequency is required to be -33 dBc (ACLR1). The ACLR at ±3.2 MHz from the center frequency is -43 dBc (ACLR2) [5].
Taking the successful set of the MAXIM 2507 used in the terminal scheme as an example, its ACLR1=-38.3 dBc and ACLR2=-53.5 dBc provide a margin of 5.3 dB and 10.5 dB, respectively, compared with the values ​​specified by the standard. The test results obtained by using the scheme shown in Fig. 3 and the conventional battery direct supply scheme are shown in Table 2.
Figure 3 TD-SCDMA power amplifier adjustable voltage supply mode
Table 2 Comparison of power consumption between voltage adjustable mode and battery direct power supply mode
It can be seen that the power-saving scheme has no obvious effect on the power consumption of the TD-SCDMA terminal, and the power consumption of the TD-SCDMA terminal is not obvious, but for the most commonly used low-transmit power, the power amplifier can save 60% on average. the above.
4.5 Power saving measures for software and algorithms
Software and algorithms also have a large impact on system power consumption. By optimizing the software code and algorithm, the flipping probability on the instruction bus and the access to the memory can be reduced, and the power consumption can also be reduced. High code density not only reduces power consumption, but also improves performance because it reduces the number of machine cycles required to complete the same function.
The optimization of the algorithm shortens the measurement time of the physical layer of the TD-SCDMA and the duration of the process of searching for the cell, and puts the most frequently called function in the working state into the cache of the baseband processor to improve the MIPS (instructions per second) ), the read and write mode of the external memory is changed from the normal mode to the burst mode to improve the access efficiency, etc., and these software optimization methods can achieve the purpose of power saving. In the course of practice, it is concluded that the power consumption of the TD-SCDMA multimedia terminal can be directly reduced by 40 mA by the above software optimization method.
5. Evaluation of the effectiveness of the power saving program
According to the "TD-SCDMA research and development and industrialization project - terminal power consumption performance test specification" [5] developed by the third generation mobile communication technology test expert group, by measuring the power consumption current when the terminal is continuously working, and then according to the terminal battery The nominal capacity can be calculated as the continuous working time. The Agilent mobile terminal is used to quickly respond to the DC power supply to test the power consumption of the terminal in the video service. The result is shown in Figure 4. The received signal level is -92 dBm.
As can be seen from Figure 4, the average current of the TD-SCDMA multimedia terminal in the video service is 195 mA, and the nominal capacity of the lithium battery used is 1 000 mAh, that is, the terminal can work continuously for 5 hours under the video service.
Figure 4: Power consumption test results of TD-SCDMA multimedia terminal under video service
It can also be seen from the test results that due to the working mode of TDD, the TD-SCDMA terminal only needs to open the transmission path in the transmission time slot and close the receiving path in the working mode, and open the receiving path and close the transmitting path in the receiving time slot. The other idle time slots completely close the transmit and receive paths, so the maximum current and the minimum current in the same sub-frame can be up to 160 mA, which greatly reduces the average power consumption.
6, the conclusion
The power-saving technology of TD-SCDMA multimedia terminal in working mode is a system-level problem, and it is necessary to achieve good results from both hardware characteristics and software decision management. Combined with the specific characteristics and working characteristics of the TD-SCDMA multimedia terminal, after using the various integrated power-saving technologies described in this paper, the power consumption current of the terminal in the videophone state is reduced from the initial 400 mA to the current 195 mA. The consumption is about 50%, and the work is stable. It solves the problem of excessive power consumption of the TD-SCDMA terminal that is troubled by the industry, and also makes a small contribution to the development of the TD-SCDMA industry.
references
[1] 3GPP TS 25.331. Radio resource control protocol specification, V5.13.0, 2005
[2] Li Shihe. TD-SCDMA third-generation mobile communication system standard. Beijing: People's Posts and Telecommunications Press, 2003
[3] Prokis J G. Digital communications, forth edition. Beijing: People's Posts and Telecommunications Press. 2002
[4] RITT.TD-SCDMA Research and Development and Industrialization Project - Terminal Power Consumption Performance Test Specification (V5.2), 2005
[5] 3GPP TS25.945.RF requirements for 1.28 Mcps UTRA TDD option, V5.1.0, 2004
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