7.5 WCDMA Power Control Planning


7.5.1  Introduction


In the WCDMA system, radio resource management includes power management, mobility management, load management, channel allocation and reconfiguration, and AMR mode control. Of which, the power management is a link of great importance. Power is the ultimate radio resource of the WCDMA system, so the only method to make fully use of the radio resources is to strictly control the use of power.
In terms of power management, the QoS 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 (i.e. control of the BS transmit power) and reverse power control (i.e. 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.

7.5.2  Principles of Power Control Implementation

1. Features of fast power control

The mode of power control implementation in the WCDMA system is greatly different from that in the GSM system. Fast power control is a very important concept integrated in the WCDMA system.
The radio propagation environment is severe. In the typical cellular mobile communication environment, the transmitting signals between the BS and the MS usually reach respective receivers after many times of reflections, dispersions and refractions. In this way, it is easy to cause multi-path fading of the signal. Fast fading will cause great impact on the receiving quality of the slow mobile receiver. In the GSM system, the MS reports the measurement result every 480 ms, and the frequency of the power control does not exceed twice per second. Therefore, for the GSM system, the multi-path fading is counteracted through frequency hopping. For the WCDMA system, in the uplink the DPCCH will divide a 10 ms radio frame into 15 timeslots, each of which includes a power control command (TPC_cmd).As the speed of power control is higher than that of fast fading, the receiving quality of the slow MS is effectively ensured.
In other words, the fast power control brings some gain to the slow MS by avoiding fast fading. Table 7-1 gives a comparison of the required Eb/Io values and required relevant transmit power changes for the slow and fast power control in the case of three different motion conditions.
Table 7-1 Changes of slow and fast power control in the case of three different conditions
Required Eb/Io
Slow power control
Fast power control (1500 Hz)
Gain of fast power control
ITU Pedestrian A 3Km/h
11.3dB
5.5dB
5.8dB
ITU Vehicular A 3Km/h
8.5dB
6.7dB
1.8dB
ITU Vehicular A 50Km/h
6.8dB
7.3dB
-0.5dB
Another two advantages of fast power control are that it can quickly adjust the power of the MS to avoid far-near affect to a great extent and the fast adjustment of the power reduces the interference to other cells and MSs.

2. Power control implementation

In the WCDMA system, power control may be divided into inner loop power control and 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, 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. Two algorithms may be adopted for the closed loop uplink power control in 3GPP protocol. In the two algorithms, the step length of the uplink power control is 1 dB or 2 dB. In the DPCCH, the step adjustment of the power control is Ddpcch = Dtpc*TPC_cmd.TPC_cmd is the synthesized TPC command from different algorithms. The power of DPDCH is set according to the power offset between the DPDCH and the DPCCH.
Differences between the two modes are: The open loop is not closed. It estimates the downlink interference according to the uplink interference, or estimates the uplink interference according to the downlink interference. In comparison, the closed loop is a closed feedback loop. The initial transmit power of the open loop power control is set by the RNC (uplink) or the UE (downlink), while the closed loop power control is completed by Node B with RNC only giving the target SIR value of the inner loop power control.

3. SSDT (Site Selection Diversity Transmission)

In the soft handover, there are two or more BSs in the downlink transmitting signals to an UE simultaneously, which occupies additional system resources (transmit power), causing additional interference and reducing the forward capacity. Therefore, careful selection of the power control algorithm during the soft handover is important to improve the system capacity. Another algorithm of the power control in the soft handover is SSDT (site selection diversity transmission), according to which, the BS with minimum path loss will transmit signals, while other BSs will only receive signals of the uplink and transmit DPCCH. In this way, the total transmit power and additional interference may be reduced. SSDT is an optional macro diversity method in the software handover mode.
The specific SSDT implementation method is as follows: Firstly, the UE selects a cell from ACTIVE SET as the PRIMARY CELL, and all other cells fall into the NON PRIMARY CELL. SSDT is to transmit signals from the PRIMARY CELL in the downlink, so as to reduce the interference resulted from the multi-channel transmission in the soft handover mode. Secondly, it is required to implement the site address selection promptly if there is no network intervention, so as to maintain the advantages of the soft handover. To select PRIMARY CELL, a temporary identity code should be allocated to each cell. Then, the UE will notify other cells in ACTIVE SET of the identity code of PRIMARY CELL on a regular basis. NON PRIMARY CELL selected by the UE will turn off the transmit power, and the identity code of PRIMARY CELL will be transmitted via the uplink FBI domain of the uplink. The SSDT activation, stopping and ID code allocation are implemented by the upper signaling.
SSDT is initiated by the network according to ACTIVE SET of the soft handover. Once it is determined to adopt SSDT, the network will notify the cell and the UE of the message that SSDT is activated in the period of current soft handover. Otherwise, TPC will still operate in the usual mode, that is, each cell controls the transmit power according to the TPC instruction of the uplink. The allocation of temporary identity code should be implemented by the network and notified to all the cells in ACTIVE SET and the UE for site address selection.
The UE measures the Received Signal Code Power of Common Pilot Channels (RSCP of CPICHs) transmitted by the cells within ACTIVE SET on a regular basis to select the PRIMARY CELL. The cell with highest RSCP of CPICHs is the PRIMARY CELL.

7.5.3  Planning of the Power Control Parameters


In the 3G system, the design criteria of the network planning is based on the SIR optimization and the activity set management. How to set proper RSCP of CPICHs, SIR target value of various services and handover area (changes to the activity set scope), and how to determine the coverage and quality of each service area are the mandatory tasks of the network planning.
In the WCDMA system, the inner loop power control is implemented by NODE-B. The inner loop power control makes convergence to the target SIR, which is determined by the outer loop power control. Therefore, the power control parameters planning is mainly reflected by the outer loop parameters planning. With the research to and experiment on the relevant parameter settings of the outer loop power control, the outer loop power control can satisfy the requirements for the control accuracy and the control speed.
Specific parameters involving the outer loop power control are as follows:
l   Time factor of BLER report: The target BLER value divided by the time factor is the pieces N to be measured;
l   BLER measurement report parameters;
l   Maximum pieces to be observed: This parameter is used to control the upper limit of the pieces N to be measured;
l   SIR-converged lagging value: Check the SIR lagging value (one of measurement report parameters) converged by SIR;
l   Control parameters of the SIR measurement report: SIR measures the filter factor used by the SIRerr;
l   Uplink outer loop power control parameters: SIR variation range, SIR adjustment factor, SIR target value falling step length, SIR maximum falling step length;
l   Uplink soft capacity control parameters: voice quality level and corresponding BLER values;;
l   Default CPICH power downlink power balancing parameters: Trigger/Stop the threshold of DPB process, adjustment period and proportion of the downlink power balancing
l   Downlink outer loop power control parameters: Trigger and stop the thresholds of downlink outer loop power control;
l   Inner loop power control parameters: Initial SIR value, adjustment step length, algorithm mode selection;

The above parameters are provided by OM, and there is a close link among them.

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