In cellular networks it is essential to control the transmission power of the mobile stations as well as the transmission power of the base stations. Effective power control takes care that a base station, for example, receives the signals from various mobile stations with roughly the same intensity. When choosing the transmission power as small as possible, the interference of radio signals is reduced. The limitation is that the lower the received signal power, or actually the signal to noise ratio, the more errors occur. The suitable transmission power is thus in balance between causing interference to other radio transmissions and getting high enough quality for the transmission whose power is controlled.
The transmission power is controlled both in the uplink and in the downlink direction. In the uplink power control, for example, the base station may monitor the power of the received signals, and based on these measurements command the mobile stations to increase or decrease their transmission powers. There are two basic methods to control power: the open loop power control and the closed loop power control. The open loop power control is typically applied to the uplink power control only. Here the mobile station maintains the sum of the received and transmitted power on a constant level. This method can react fast to changes in the power levels, but its precision is poor.
FIG. 1 presents an example of the closed loop power control system 100. The close power control system consists of two loops. The inner loop 101 is in FIG. 1 implemented in the transmitter 110 and receiver 111 equipment. A radio signal (arrow 130) is transmitted from the transmitter to the receiver, and it is received in the receiver block 122. In the inner loop of a closed loop power control system usually the signal to interference ratio SIR, is determined and compared to a predetermined target value. This is presented in FIG. 1 with the measurement block 123 and the comparison block 124. The signal power, or other chosen quantity, is sent from the measurement block 123 to the comparison block 124, where the measured quantity is compared to the target value. The target value is chosen so that, for example, the errors in the data transmitted over the radio channel are rare enough. If the signal power is larger than the target value, command to decrease the transmission power is sent from the power control command block 125. If the signal power is too small, the command to increase the power is sent. In the transmitter equipment, the power adjustment block 121 adjusts the transmission power used in the transmission block 120 according to the commands (presented by arrow 130). Usually a predetermined adjustment step is defined, and the power control commands do not indicate how much the power should be increased or decreased.
The outer loop 102 of a closed loop power control system selects the target value for the inner loop. In FIG. 1 the loop is presented to be implemented in a separate network element 112. The outer loop monitors the quality of the received signal. This can be done, for example, by periodically sending frame error rate (FER) values to the outer loop. This is presented in FIG. 1 by arrow 132. The signal quality block 126 checks if the received signal quality is as good as needed. If needed, it adjusts the target value of a controlled variable in the target value adjustment block 127. The setting of a new target value for the controlled variable is presented in FIG. 1 with arrow 133.
In spread spectrum cellular networks, for example in cellular networks that employ CDMA (code division multiple access) radio techniques, all users use the same frequency domain all the time. In CDMA system, each user has a specific code sequence using which his narrow spectrum signal is spread into a signal which uses radio frequencies of a larger bandwidth. The code sequences of different users must be chosen so that they do not correlate and the code sequence of a specific user has to autocorrelate. Those signals that have been spread using a code sequence that correlates with the code sequence used in receiving the seemingly white noise radio transmission are separated. The receiver has to know the transmitters code sequence and the code sequences must be synchronized.
When received in an antenna, the radio signal has components that have been attenuated or delayed in various ways. The radio signals may, for example, scatter, diffract or reflect. This is called multipath propagation, and each of the multipath components has faded in a specific frequency-selective manner and has a specific time delay. In CDMA system, a rake receiver comprises of several correlators (fingers), each of which is tuned to resolve a strong multipath component from the received radio signal. The tuning is done by using a different delay estimation for each finger. By combining these separated multipath components the quality of the signal can be enhanced. The fading in the multipath components is frequency selective and different in each component. A combined signal is thus less faded and its quality is, consequently, better than the quality of the individual multipath components.
In spread spectrum cellular networks, for example, soft handovers are possible. A soft handover refers to a situation where a mobile station is sending and receiving information related to a certain connection to and from more than one base station simultaneously. An example is presented in FIG. 2, where the radio signal sent by a mobile station 201 is received by the base stations 202 (BS1) and 203 (BS2). Near a cell border, for example, the received signal powers from many base stations can be equal. Soft handover results in a seamless transition from one cell to another. Many base stations may receive the uplink signal sent by a mobile station, and the signals are combined in the cellular network. In FIG. 2, for example, the signals are combined in network element 204. In the downlink direction, the different fingers of a rake receiver in the mobile station may be assigned to different multipath components, which components in this case may arrive from various base stations.
The power control in a soft handover is similar to a situation where the mobile station communicates with only one base station. The difference is that all base stations involved in the handover send uplink power control commands to the mobile station. The mobile station adjusts the transmission power based on these power control commands, which in FIG. 2 are presented with arrows 210 and 211. Usually the logic is such that the transmission power is decreased if at least one of the base stations asks for power down and that power control command can be received reliably enough. The inner loop of the power control involves the mobile station 201 and the base stations BS1 and BS2 and, for example, a radio network controller (RNC) is responsible for setting the target value for the controlled variable.
The controlled variable for power control of radio transmission is preferably the energy per user data bit divided by the equivalent noise density Eb/N0. This factor tells the power of the received signal, and for different connection types the optimal power of the received signal is different. From the signal which carries the user data, however, it is quite difficult to obtain Eb/N0. For example in WCDMA (Wideband-CDMA, a third generation cellular network), there are a dedicated physical control channel (DPCCH) and a dedicated physical data channel (DPDCH) for each connection between a mobile station and the cellular system. The control charnel carries information about the data connection, for example about the bit rate, and the data channel carries the actual user data. A signal to interference ratio (SIR) is usually calculated using the pilot symbols in the control channel DPCCH. The pilot symbols do not change during a connection, so the power signal carrying the pilot symbols is easier to estimate than Eb/N0 from the data channel DPDCH.
The signal to interference ratio SIR can be used as an estimator for Eb/N0 in the power control, but it is a biased estimator. Because the outer loop of the power control adjusts the target value of the controlled variable SIR based on, for example, the frame error rate, it does not matter if the SIR is only proportional to Eb/N0, instead of being equal to it. Only in the beginning of a connection, when an initial target value for the SIR has to be chosen, this may complicate the choice.
A problem is that certain factors affect the size of the bias in SIR compared to Eb/N0. The number of rake fingers used in receiving the signal, for example, affects the signal power estimation. When the bias in the measured SIR value changes, the inner loop of the power control deduces that the actual quality of the received signal changed and starts to alter the transmission power.
The more rake fingers involved in the reception, the better the signal quality and, consequently, the larger the measured SIR value. The release or allocation of rake fingers may change the value of SIR even with 2 dB. For example, when more rake fingers are allocated the SIR value increases in a stepwise manner. The measured value of the controlled variable in the inner loop of the power control thus increases, and the inner loop starts to decrease the transmission power. After the decrease of transmission power the outer loop that monitors the quality of the received signal, for example a frame error rate, notices that the quality of the signal has deteriorated. It decides to increase the target SIR value. The target value is usually increased in steps, and this adjustment of the target value takes time during which the received signal has more errors than usually is acceptable.
If the number of rake fingers involved in receiving the signal is decreased, then the measured SIR value decreases. As a result, the inner loop starts to increase the transmission power. Thereafter the outer loop notices that the received signal quality is better than needed, and it starts to decrease the target value for the controlled variable. The adjustment of the target value takes again some time, and during that time the signal is transmitted with an unnecessary high transmission power. This may cause interference to the other connections thus deteriorating their quality.
Further problems occur in soft handovers, especially in the uplink power control. An optimal situation in a soft handover is that the power control commands sent by each base station involved in the handover affect the transmission power. Unfortunately the base stations involved in the soft handover may use different numbers of rake fingers in receiving the signal from the mobile station. Because the bias in the SIR value depends on the number of rake fingers, a certain target value of the controlled variable does not necessarily result in a situation where all base stations agree that the target value has been reached.