0 Introduction In the third-generation mobile communication technology, the most representative solutions are CDMA2000 in North America, WCDMA in Europe and Japan, and TD-SCDMA in China. Among them, CDMA2000 is directly evolved on the basis of IS95 (2G CDMA with a bandwidth of 1.23 MHz); WCDMA is also called broadband CDMA, and its bandwidth is 5 MHz or higher; TD-SCDMA is also called time-division synchronous CDMA, and its synchronization is mainly Refers to the fact that the uplink signals of all end users are fully synchronized when they reach the demodulator at the receiving end of the base station. The above three standards are based on CDMA.
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 received by the system, and reducing the interference can directly increase the communication capacity of the system. Since the CDMA system uses the same frequency carrier at the same time, controlling the power of each mobile station is the key to achieving the maximum capacity. The power control technology can be used to minimize the interference between the mobile stations and achieve the maximum channel capacity.
There are two sides to power control: from the perspective of power consumption, interference and electromagnetic radiation, the smaller the transmission power, the smaller the power consumption of the mobile phone, the longer the standby and talk time, the less interference with other mobile phones in the same system, At the same time expand the community capacity. In addition, the smaller the transmission power of the mobile phone, the less interference with other wireless devices, and the less radiation to the human body. On the other hand, in order to ensure the communication quality and hope that the mobile phone transmit power is higher, for example, when the mobile phone is at the far end of the cell, in order to ensure that the mobile phone signal reaches the base station after long-distance transmission, the signal can still be demodulated correctly. Large enough to overcome the attenuation of the signal transmitted over a long distance; the transmission power of the mobile phone in a wireless shadow area blocked by a building or other should also be large enough to overcome the multiple reflections, refractions and long-distance transmission of the mobile phone signal In the case of relatively large interference (adjacent channel interference, co-channel interference, blocking, etc.), the transmit power should also be large enough to overcome noise interference. Therefore, the unified expression is: the mobile phone must have enough transmission power to ensure communication, and under the premise of ensuring communication quality, the smaller the transmission power, the better.
1 Power control technology and classification In the mobile communication systems currently in use, PHS (Personal Handyphone System) has emerged for a while with its advantages of low construction cost and simple protocol standards. PHS is often referred to as PHS in China. It uses microcell technology to provide a simple and low-cost protocol standard, which reduces the cost of mobile phone manufacturing. The RCR-STD28 standard is used to specify that the average transmission power is less than or equal to 10 mW, and the peak power is less than or equal to 80 mW. The transmit power is uncontrollable.
It is stipulated in the second-generation mobile communication GSM system 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 through the downlink SACCH channel to control the transmission power level of the mobile phone. The adjacent power levels differ by 2 dB. The mobile station power levels and the maximum and minimum powers are shown in Table 1.
The GSM power control rate is relatively slow, and the requirements for power control lifting are not very precise or strict. In addition, GSM is far less dependent on power control than CDMA systems. In a communication system based on CDMA technology, power control technology is 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. Without affecting the quality of communication (QoS), try to make the signal of each MS reach the minimum required SIR when reaching the BS, so as to improve the capacity and reliability of the system. Power control can control SIR and effectively overcome and suppress interference. It is one of the core technologies to improve and improve the reliability of 3G cellular mobile communication systems.
Generally, from the perspective of communication uplink and downlink, 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 transmission power to the mobile station. Reverse power control is divided into open-loop power control and closed-loop power control. Among them, the reverse open-loop power control is mainly that the mobile station adjusts the transmission power according to the received power change; the reverse closed-loop power control is that the mobile station adjusts the average output power according to the received power control bits.
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 according to the strength of the pilot signal it receives from the base station, thereby determining the size of the transmit power. It is that the mobile station adjusts the transmission power of the mobile station according to the change in the received power in the cell so that all signals sent by the mobile station have the same power at the base station. Its main purpose is to compensate for effects such as shadows and corners, so it has a large dynamic range. According to the IS95 standard, it should reach at least a dynamic range of ± 32 dB. The control process is shown in Figure 1.
The main feature of open loop power control is that no feedback information is required. When the wireless channel changes suddenly, it can respond quickly, in addition, it can adjust the power in a large range. Open-loop power control is not accurate enough, because the accuracy of fading estimation for open-loop power control is based on the fact that the uplink and downlink have consistent fading, while in frequency duplex mode, the uplink and downlink The frequency band difference is 190 MHz, which is much larger than the signal's related bandwidth, so the channel fading of the uplink and downlink is completely irrelevant, which leads to the accuracy of the open-loop power control is not very high, can only play a rough control effect. In the WCDMA protocol, the control variance of open loop power control is acceptable within 10 dB.
2.2 Reverse closed-loop control Reverse power control is closed-loop power control when the base station is 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 can maintain the most ideal transmission. power.
Closed-loop power control is accomplished with the assistance of a mobile station. The base station receives the signal of 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 transmit power on the forward transmission channel ; Conversely, it sends a command to increase the transmit power. The control process is shown in Figure 2.
Closed-loop power control can correct the gain changes in reverse transmission and forward transmission paths, and eliminate the inaccuracy of open-loop power control. The base station adjusts the received reverse open-loop power estimation value of the user terminal, so that the user terminal maintains the optimal transmission power. The realization of power control is to insert power control bits in the traffic channel frame, and the insertion rate can reach 1.6 Kb / s, which can effectively track the effect of fast fading. However, the adjustment of closed-loop power control will always lag behind the state value at the time of measurement. If the communication environment changes greatly during this period, it may lead to the collapse of the closed-loop, so the feedback delay of power control should not be too long. The power control command generated in a certain time slot should be fed back in two time slots. Closed loop power control is composed of two parts: inner loop power control and outer loop power control. In 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 transmit power so that the channel Eb / Io reaches the target value. Inner-loop power control is 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, the value of the unit automatically decreases gradually. The period of the outer loop power control is generally of the order of TTI (10 ms, 20 ms, 40 ms, 80 ms), that is, 10 to 100 Hz. Outer loop power control can indirectly affect 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 transmission power of each mobile station based on the measurement results of the mobile station. The base station periodically sends a test, and the mobile station detects the frame error rate of the forward transmission and reports the statistical result of the frame error rate to the base station. The base station decides to increase or decrease the forward transmission power according to 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 a predefined value, it requests the base station system to increase the forward link transmission power. Adjustments are made at regular intervals, and user terminal reports are divided into periodic reports and threshold reports. The control process is shown in Figure 3.
In forward power control, a smaller forward link power is allocated to mobile stations with small path fading, and a larger forward link power is allocated to mobile stations that are far from the base station and have a high bit error rate. Power is allocated reasonably on each forward traffic channel to ensure the communication quality of each user while maximizing forward link capacity.
4 Conclusion There are many key technologies in the third-generation mobile communication system, such as multi-carrier technology, smart antenna technology, software radio technology, and multi-user detection technology. Power control technology is one of the core technologies of the CDMA system. It enables the system to maintain high-quality communication, significantly increase the system's communication capacity, and at the same time can extend the battery life of mobile phones and reduce network construction costs. This paper analyzes the current power requirements in PHS and GSM systems, elaborate on the power control in CDMA systems, and analyzes the control process and its advantages and disadvantages for the forward power control and reverse power control technologies. For the design of 3G systems Has a certain guiding significance.
The capability and performance of power control depends largely on the accuracy of power measurement and the delay of power control command generation and transmission processing. Because the signal is Rayleigh fading in the mobile communication transmission, the power control system cannot compensate for the change in signal power caused by fast fading, especially when the mobile station moves at a fast speed, the power control technology will fail. To improve the power control technology in the CDMA system, it is ultimately necessary to combine a variety of key technologies in order to achieve high-quality 3G communications. In addition, in addition to power control, CDMA also includes power allocation, which together constitute power management. For power control technology, a more in-depth study is to combine power and rate control technology for joint control to achieve maximum optimization of the system.
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