Power Control Technology and Process Analysis in CDMA System

0 Introduction
Among the third generation mobile communication technologies, the most representative solutions are CDMA2000 in North America, WCDMA in Europe and Japan, and TD-SCDMA in China. CDMA2000 is a direct evolution of IS95 (2G CDMA with a bandwidth of 1.23 MHz); WCDMA is also known as wideband CDMA, with a bandwidth of 5 MHz or higher; TD-SCDMA is also known as time division synchronous CDMA, and its synchronization mainly refers to the complete synchronization of the uplink signals of all terminal users when they reach the demodulator at the receiving end of the base station. The above three standards are all based on CDMA technology.

Compared with the bandwidth-limited FDMA and TDMA systems, the CDMA system can provide a sufficiently large system capacity, which is mainly limited by the interference to the system. Reducing interference can directly increase the communication capacity of the system. Since the CDMA system uses simultaneous and co-frequency carriers, controlling the power of each mobile station is the key to achieving maximum capacity. The power control technology can be used to minimize the interference between mobile stations and achieve the maximum capacity of the channel.
Power control has two sides: from the perspective of power consumption, interference and electromagnetic radiation, the smaller the transmission power, the lower the power consumption of the mobile phone, the longer the standby and talk time, the less interference to other mobile phones in the same system, and at the same time expands the cell capacity. In addition, the smaller the transmission power of the mobile phone, the less interference to other wireless devices and the less radiation to the human body. On the other hand, in order to ensure the quality of communication, the mobile phone transmission power is expected to be larger. For example, when the mobile phone is at the far end of the cell, in order to ensure that the mobile phone signal can be correctly demodulated after reaching the base station after a long distance transmission, the transmission power needs to be large enough to overcome the attenuation of the signal after long-distance transmission; when the mobile phone is in a wireless shadow area blocked by buildings or other obstructions, its transmission power must also be large enough to overcome the attenuation of the mobile phone signal after multiple reflections, refractions and long-distance transmission; in the case of relatively large interference (adjacent channel interference, co-channel interference, blocking, etc.), the transmission power must also be large enough to overcome the interference of noise. Therefore, the unified statement is: the mobile phone must have sufficient transmission power to ensure communication, and the smaller the transmission power, the better, while ensuring the quality of communication.

1 Power control technology and classification
In the current mobile communication system, PHS (Personal Handyphone System) has risen for a while with its advantages such as low construction cost and simple protocol standards. PHS is often called Xiaolingtong in China. It uses micro-cellular technology, provides simple and low-cost protocol standards, reduces the manufacturing cost of mobile phones, and adopts the RCR-STD28 standard to stipulate that the average transmission power is less than or equal to 10 mW, the peak power is less than or equal to 80 mW, and the transmission power is uncontrollable.
In the second-generation mobile communication GSM system, it is stipulated that the transmission power of the mobile phone can be controlled by the base station. The base station detects the power level of the received signal and sends commands to control the transmission power level of the mobile phone through the downlink SACCH channel. The difference between adjacent power levels is 2 dB. The power level of the mobile station and the maximum and minimum powers are shown in Table 1.

GSM power control rate is relatively slow, and the requirements for power control are not very precise or strict. In addition, GSM's reliance on power control is much lower than that of CDMA systems. In communication systems based on CDMA technology, power control technology is completely indispensable. CDMA itself is an interference-limited system, that is, the size of interference directly affects the system capacity. Therefore, it is necessary to control the size of interference, and try to make the signal of each MS reach the minimum required SIR when it reaches the BS without affecting the communication quality (QoS), so as to improve the capacity and reliability of the system. Power control can control SIR and effectively overcome and suppress interference, and is one of the core technologies for improving and enhancing the reliability of 3G cellular mobile communication systems.
Generally, from the perspective of uplink and downlink communication, power control is divided into forward power control and reverse power control. Forward power control is based on the measurement report of the mobile station, and the base station adjusts the transmit power to the mobile station. Reverse power control is divided into open-loop power control and closed-loop power control. Among them, reverse open-loop power control is mainly for the mobile station to adjust the transmit power according to the change of received power; reverse closed-loop power control is for the mobile station to adjust the average output power according to the received power control bit.
2 Reverse and forward power control
2.1 Reverse open-loop control
Open-loop power control is that the mobile station estimates the loss of the forward transmission path based on the pilot signal strength it receives from the base station, thereby determining the size of the transmission power. It is that the mobile station adjusts the transmission power of the mobile station according to the change of the received power in the cell so that the signals sent by all mobile stations have the same power when they reach the base station. It is mainly to compensate for the effects of shadows, corners, etc., so the dynamic range is very large. According to the IS95 standard, it should reach at least ±32 dB dynamic range. The control process is shown in Figure 1.

The main feature of open-loop power control is that it does not require feedback information. It can respond quickly when the wireless channel changes suddenly, and it can adjust the power over a wide range. The open-loop power control is not accurate enough because the fading estimation accuracy of the open-loop power control is based on the consistent fading of the uplink and downlink. In the frequency duplex mode, the frequency bands of the uplink and downlink differ by 190 MHz, which is much larger than the relevant bandwidth of the signal. Therefore, the channel fading of the uplink and downlink is completely unrelated, which leads to the low accuracy of the open-loop power control and can only play a rough control role. In the WCDMA protocol, the control variance of the open-loop power control is required to be within 10 dB.
2.2 Reverse closed-loop control
Reverse power control is closed-loop power control when there is a base station involved. Its design goal is to enable the base station to quickly correct the open-loop power estimation of the mobile station so that the mobile station maintains the most ideal transmission power.
Closed-loop power control is completed with the assistance of the mobile station. The base station receives the signal from the mobile station and measures its signal-to-noise ratio, and then compares it with the threshold. If the received signal-to-noise ratio is greater than the threshold, the base station transmits a command to reduce the transmission power on the forward transmission channel; otherwise, it sends a command to increase the transmission power. The control process is shown in Figure 2.

Closed-loop power control can correct the changes in the gain of the reverse and forward transmission paths and eliminate the inaccuracy of open-loop power control. The base station makes adjustments to the received reverse open-loop power estimate of the user terminal so that the user terminal maintains the most ideal transmission power. Power control is achieved by inserting power control bits in the service channel frame. The insertion rate can reach 1.6 Kb/s, which can effectively track the impact of fast fading. However, the adjustment of closed-loop power control always lags behind the state value at the time of measurement. If the communication environment changes significantly during this period, it may cause the collapse of the closed loop. Therefore, the feedback delay of power control cannot be too long. Generally, the power control command generated by a certain time slot of the communication end should be fed back within two time slots. Closed-loop power control consists of two parts: inner-loop power control and outer-loop power control. In the inner-loop closed-loop power control, the base station compares the Eb/Io of the reverse channel with the target Eb/Io every 1.25 ms, and then instructs the mobile station to reduce or increase the transmission power so that the channel Eb/Io reaches the target value. Inner-loop power control is a fast closed-loop power control, which is mainly performed at the physical layer between the base station and the mobile station. In the outer loop closed loop power control, the base station specifies the target Eb/Io (from the user terminal to the base station) for each frame of the receiver every 20 ms. When a frame error occurs, its value is automatically reduced in units. The period of the outer loop power control is generally in the order of TTI (10 ms, 20 ms, 40 ms, 80 ms), that is, 10 to 100 Hz. The outer loop power control can indirectly affect the system capacity and communication quality through closed loop control.

3 Forward power control
Forward power control refers to the base station adjusting the control of the transmit power of each mobile station based on the measurement results of the mobile station. The base station periodically sends tests, and the mobile station detects the frame error rate of the forward transmission and reports the statistical results of the frame error rate to the base station. The base station decides to increase or decrease the forward transmission power based on the statistical results of the frame error rate reported by the mobile station. In the process of the base station system slowly reducing the forward link transmission power of the mobile station, when the mobile station detects that the frame error rate (FER) exceeds the predefined value, it requests the base station system to increase the forward link transmission power. Adjustments are made at regular intervals, and the reports from the user terminal are divided into periodic reports and threshold reports. The control process is shown in Figure 3.

在前向功率控制中，对路径衰落小的移动台分派较小的前向链路功率，而对那些远离基站的和误码率高的移动台分派较大的前向链路功率，通过在各个前向业务信道上合理的分配功率来确保各个用户的通信质量，同时使前向链路容量达到最大。

4 结语
在第三代移动通信系统中有许多关键技术，如多载波技术、智能天线技术、软件无线电技术、多用户检测技术等。功率控制技术是CDMA系统的核心技术之一，它使系统能维护高质量通信，显著提高系统通信容量，同时可以延长手机电池使用寿命，并减低建网成本。本文分析目前PHS、GSM系统中的功率要求，详细阐述了在CDMA系统中的功率控制，针对其中的前向功率控制和反向功率控制技术，分析其控制过程及优缺点，对于3G系统的设计具有一定指导意义。
功率控制的能力和性能很大程度上依赖于功率测量的精度和功率控制命令产生和传输处理时延。由于信号在移动通信传输中呈瑞利衰落，功率控制系统无法补偿由快衰落引起的信号功率的变化，特别是当移动台的运动速度很快时，功率控制技术会失效。要提高CDMA系统中的功率控制技术，最终需要多种关键技术的有机结合，才能够实现3G的高质量通信。此外，在CDMA中除了功率控制以外，还包括功率的分配，它们共同构成了功率管理。对于功率控制技术，更深入地研究是结合功率和速率控制技术进行联合控制，达到系统的最大优化。