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WCDMA Power Control

In terms of power management, the QOS (Quality of Service) of a subscriber can be improved by increasing the transmit power of the subscriber. However, such improvement may result in deteriorating other subscriber’s receiving quality due to the self-interference feature of the CDMA system. WCDMA adopts the broadband spreading technology with all signals sharing the same spectrum and the signal energy of each MS is allocated within the frequency band, thus for other MSs it is a kind of wideband noise. Therefore, the use of power in the CDMA system is conflicting.

In addition, there are such effects as shadow, multi-path fading and remote loss in the radio environment. The position of a cellular MS in the cell is random and changes frequently, so the path loss will fluctuate greatly, especially in the multi-cell DS/CDMA system, where all the cells adopt the same frequency. Theoretically, the address codes allocated by different subscribers are orthogonal, but in fact it is hard to guarantee them, thus causing mutual interference among the channels and serious “near-far effect” and “corner effect”. Near-far effect occurs in the uplink. If all the subscribers in the cell transmit signals to the BS with the same power, then the signals of the MS near the BS are strong while the signals of the MS far from the BS are weak. In such a case, the weak signals will be masked by the strong signals. Corner effect occurs in the downlink. When the MS is at the corner of the cell, the interference will be twice more than that in the vicinity of the cell. When the interference is severe, the communication quality of the MS will be lowered promptly.
Therefore, on the basis of ensuring QOS for subscribers, how to effectively control power, how to reduce the transmit power as much as possible, and how to reduce the system interference and increase the system capacity are the key to WCDMA technologies. The WCDMA system has such functions as forward power control (control of the BS transmit power) and reverse power control (control of the MS transmit power), of which the reverse power control is especially important, because with it, the system capacity and communication quality may be ensured and the fading and near-far effect may be avoided to a great extent

In the WCDMA system, power control may be divided into

  • Inner loop power control
  • Outer loop power control.
The inner loop power control is to converge the received SIR to the target SIR by controlling the transmit power of physical channels. In the WCDMA system, relevant power adjustment commands are sent out by estimating the received Eb/No (ratio of bit energy to interference power spectrum density). There is certain mapping relationship between Eb/No and SIR. For instance, for the 12.2 kbps voice service, the typical value of Eb/No is 5.0 dB. If the chip rate is 3.84 Mcps, the processing gain will be 10 log10 (3.84M/12.2k) = 25 dB. So, the SIR is -20 dB (= 5 dB-25 dB), that is, the Carrier-to-Interference Ratio (C/I) is more than –20 dB.

The outer loop control mechanism is to dynamically adjust the SIR target value of the inner loop control, so as to ensure that the communication quality always meets the requirements (i.e. the specified FER/BLER/BER value).The outer loop control is conducted in the RNC. The radio channels are complex, so the power control based only on the SIR value cannot reflect the real quality of the links. For instance, based on the same FER (Frame Error Rate), the requirements of static subscribers, low speed subscribers (3 km/H) and high speed subscribers (50 km/H) for SIR are different. The communication quality is finally measured with FER/BLER/BER, so it is necessary to dynamically adjust the SIR target value according to the actual FER/BLER value.

The inner power control may be subdivided into open loop power control and closed loop power control. The former aims to providing the estimates of the initial transmit power. It estimates the path loss and the interference level according to the measurement result, so as to calculate the process of initial transmit power. In the WCDMA system, the open loop power control is adopted in both the uplink and downlink.

In the WCDMA-FDD system, the fast fading conditions in the uplink and downlink are absolutely irrelevant  because the frequency spacing between the uplink and the downlink is large. Therefore, the path loss estimates obtained through the open loop power control according to the downlink signals are inaccurate for the uplink. The method to solve this problem is to introduce the fast closed loop power control mechanism. The closed loop power control mechanism is to rapidly adjust the power in the uplink/downlink during the communication period, thus making the link quality converged to the target SIR.

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