Quality Of Service (QOS) that Radio cell network provides for each subscriber mainly depends on signal-to-interference ratio (SIR) of subscriber receiving signals. For CDMA cell system, all subscribers in same cell use same band and time slot, and subscribers are isolated with each other only by the (quasi-) orthogonalization of spreading code. Correlation characteristics between each subscriber signals are not so good and signals of other subscribers interfere signals of current subscribers, due to multipath and delay of the radio channels.
Increasing of subscribers or power of other subscribers may enhance interference on current subscriber. Therefore, CDMA system is a strong power-restricted system and strength of interference influences system capacity directly.
Power control is regarded as one of the key technologies of CDMA system. Power control adjusts transmission power of each subscriber, compensates channel attenuation, countervails near-far effect and maintains all subscribers at lowest standard of normal communication. It reduces interference on other subscribers at most, increases system capacity and prolongs holding time of mobile phones.
Power control is an important part in the WCDMA system. If all the MSs in a cell transmit signals at the same power, the signals from a near MS to the BTS are stronger, and the signals from a far MS to the BTS are weaker. As a result, the strong signals override the weak signals. This is called Near/Far Effect in the mobile communication. WCDMA is a self-interference system and all users use the same frequency. Therefore, the “near-far effect” is more serious. In addition, for the WCDMA system, the downlink of the BTS is power restricted. To achieve acceptable call quality when the TX power is small, both the BTS and the MS are required to adjust power needed by the transmitter in real time according to the communication distance and link quality. This process is called “power control”.
Power control of WCDMA includes inner loop power control and outer loop power control by effect.
Inner loop power control is used to combat channel fade and loss, so that the SIR or power of the received signals can reach the specific target value. Outer loop power control generates the SIR or power threshold for inner power control according to the QoS in the specific environment. By link, there is uplink power control and downlink power control. Since the CDMA system capacity is mainly restricted by that of the uplink, uplink power control is particularly important.
By link type, there is open loop power control and closed loop power control. Open loop power control is based on the assumption that the uplink and downlink channels are symmetric. It can counteract path loss and shadow fade. Close loop power control does not need this assumption, and it can counteract fast fade.
Open Loop Control
- 1. Uplink open loop control
In the WCDMA system, every MS is calculating the path loss from the BTS to the MS all the time. When the signal received by the MS from a BTS is very strong, it indicates that either the MS is very close to the BTS or the transmission path is excellent. In this case, the MS can lower the TX power, while the BTS can still receive signals normally. On the other way around, when the signal received by the MS is very weak, its TX power can be increased to counteract the attenuation. Open loop power control occurs only when the MS is powered on and only once.
- Downlink outer loop control
It is the process of estimating the initial TX power of a new requested service. The system can estimate the initial TX power of the downlink channel according to the signal quality of the primary common pilot channel (P-CPICH) measured by the UE. At the same time, the following factors have to be taken into account: QoS, data rate, quality factor Eb/No, real-time total TX power of the downlink, interferences on this cell by other cells, and so on.
Open loop power control is simple and direct, without needing exchange of control information between the MS and the BTS. In addition, it features a higher control speed and needs few overheads. However, in the WCDMA system, different frequencies are used in uplink/downlink transmission. The frequency difference is far greater than the coherent bandwidth of channels. Therefore, it cannot be assumed that the fade characteristic of the downlink channel is equal to that of the uplink channel. This is the limitation of the open loop power control.
Inner Loop Control
In this case, the receiver compares the signal-interference ratio of the received signal with the target value of the control channel. Then, it returns a transmission power control (TPC) command to the sender. The sender determines whether to increase or reduce the TX power based on the closed loop power control algorithm specified by the upper layer, and makes adjustments at the specified step according to the received command.
Outer Loop Control
Outer loop power control is a supplement to closed loop power control. The working principle of uplink outer loop power control is: Compare the actual block error (BLER) ratio of the transmission channel with the target block error (BLER) ratio, then, slowly adjust the target signal-interference ratio (SIRTarget) so that the service quality is not affected by the change of the radio environment, and that a relatively constant communication quality can be maintained.
Outer loop power control usually adjusts the target SIR based on BLER, to make the QoS meet the requirements. Since different services have different QoS, there are different target SIRs.
Downlink outer loop power control is similar to downlink inner loop power control.