The present invention relates to wireless communications, and more specifically, to radio transmit power control in a mobile radio communications system.
In cellular communications systems, the mobile radio station communicates over an assigned radio channel with a radio base station. Several base stations are coupled to a switching node which is typically connected to a gateway that interfaces the cellular communications system with other communication systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway through one or more switching nodes to a base station which serves the called mobile station. The base station pages the called mobile station and establishes a radio communications channel. A call originated by the mobile station follows a similar path in the opposite direction.
Due to the rapid expansion of wireless mobile communications and the need for wideband multimedia services, there is a continuing need to better utilize the available frequency bandwidth. A common strategy in Frequency Division Multiple Access (FDMA)/Time Division Multiple Access (TDMA) systems is to reuse the frequencies in the network. The challenge with frequency reuse is to counteract or at least reduce the interference between transmitters in the system using the same frequency by controlling the transmit power levels of the radio signals and by separating to the extent practical the transmitters by a sufficient geographic distance. The radio transmit power levels of the mobile stations and base stations are ideally lowered so that only the minimum transmission power necessary to maintain satisfactory call quality is used. By reducing mobile and base station transmission power, the other radio communicators experience lower interference which means that the system capacity can be increased. The capacity of a transmission power regulated system can arguably be increased by approximately 70% compared to an unregulated system. Another reason to maintain lower transmit power levels, at least for battery-operated mobile stations, is to reduce the energy consumed by mobile stations during transmissions.
In a Code Division Multiple Access (CDMA) mobile communication system, spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same radio frequency band-hence the term xe2x80x9cspread spectrum.xe2x80x9d In other words, individual radio xe2x80x9cchannelsxe2x80x9d are discriminated upon the basis of these codes. Various aspects of CDMA are set forth in textbooks such as Applications of CDMA and Wireless/Personal Communications, GARG, Vijay K. et al, Prentice-Hall 1997.
Spread spectrum communications permit mobile transmitted signals to be received at two or more xe2x80x9cdiversexe2x80x9d base stations and processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a handover from one base station to another, (or from one antenna sector to another antenna sector connected to the same base station), without any perceptible disturbance in the voice or data communication. This kind of handover is typically called diversity handover and includes both soft and softer diversity handover.
Because all users of a CDMA communications system transmit information using the same frequency band at the same time, each user""s communication interferes with the communications of the other users. In addition, signals received by a base station from a mobile station close to the base station are much stronger than signals received from other mobile stations located at the base station""s cell boundary. As a result, distant mobile communications are overshadowed and dominated by close-in mobile stations which is why this condition is sometimes referred to as the xe2x80x9cnear-far effect.xe2x80x9d
Interference is therefore a particularly severe problem in CDMA systems. If one mobile station transmits at a power output that is too large, the interference it creates degrades the signal-to-interference ratio (SIR) of signals received from other mobile radios to the point that a receiving base station cannot correctly demodulate transmissions from the other mobile radios. In fact, if a mobile station transmits a signal at twice the power level needed for the signal to be accurately received at the base station receiver, that mobile""s signal occupies roughly twice the system capacity as it would if the signal were transmit at the optimum power level. Unregulated, it is not uncommon for a xe2x80x9cstrongxe2x80x9d mobile station to transmit signals that are received at the base station at many, many times the strength of other mobile transmissions. Such a loss of system capacity to excessively xe2x80x9cstrongxe2x80x9d mobile stations is unacceptable.
Additional problems are associated with excessive transmit power. One is the so-called xe2x80x9cparty effect.xe2x80x9d If a mobile transmits at too high of a power level, the other mobiles may increase their respective power levels so that they can xe2x80x9cbe heardxe2x80x9d compounding the already serious interference problem. Another problem is wasted battery power. It is very important to conserve the limited battery life in mobile radios. The major drain on a mobile""s battery occurs during transmission. Thus, a significant objective for any power control approach, therefore, is to reduce transmit power where possible. Except for battery consumption, the above-described problems with setting transmission power also apply to downlink radio transmissions from base stations to mobile stations.
In some mobile radio communications systems, power regulation is performed relatively infrequently being based on the unrealistic assumption that the disturbance level is more or less constant. The mobile""s transmission power is only coarsely controlled, with the objective being to maintain the received desired signal level over the interference level. In practice, however, radio conditions and interference levels vary considerably with both time and place so that infrequent power regulation is not optimal.
One common approach to power control is to try and balance the transmit power level of signals on each radio channel so that all mobile stations or base stations receive signals with the same Signal-to-Inference Ratio (SIR). For every traffic scenario, there is a maximum SIR that can be obtained at all radio receivers at the same time. Ideally, if all information is available at one location, a global or centralized power control scheme may be employed to determine and assign the various necessary transmission powers so this maximum is achieved. But this approach requires extensive overhead signaling to keep the centralized intelligence entity fully informed and up to date. Another approach, less onerous at least with respect to control signaling, is to perform power control in a distributed fashion using only local SIR type measurements.
In both approaches, the appropriate target SIR must first be determined. If the SIR target value is set too high, the radio transmit powers might be increased to maximum levels determined by the physical limits of the system without achieving the specified SIR target value. Should that be the case, xe2x80x9cgraceful degradationxe2x80x9d by uniformly reducing transmit power levels of all active radios can be employed to minimize the impact on the quality of service provided.
A significant goal for most if not all radio transmit power control procedures is to keep the signal strength and/or quality of the signal detected by a receiver above a threshold without using unnecessarily high transmit power. In contrast to infrequent power regulation schemes mentioned above, most CDMA-based systems employ a relatively high sampling rate for the power control algorithm, e.g., 1600 times per second. To minimize overhead control signaling, only one bit is used to communicate power control adjustments to the radio transmitter.
In the IS-95 CDMA standard and similar systems, the power is stepwise increased or decreased based upon a comparison of the received signal strength or other signal parameter like SIR with a threshold. The receiver controls the transmitter""s power by issuing power control commandsxe2x80x94power up or power downxe2x80x94at the same high sampling rate based on measurements of a signal-to-interference ratio. If the measured signal parameter value is less than a target signal parameter value, the power up command is issued; otherwise, the power down command is issued. The radio transmitter responds to the power control commands by increasing or decreasing its transmit output power level P, for example, by a certain incremental power step xcex94, i.e., P←P+xcex94 or P←Pxe2x88x92xcex94.
At the example sampling rate noted above, either a power up or a power down command issues every 0.625 msec. As a result, the transmit power level is never perfectly constant or static. Accordingly, even in an ideal radio environment, the incremental power control commands continually alternate between power up and power down so that the transmit power level and the received signal quality oscillate up and down an incremental step around a target value. In order to maintain the quality of the received signal always above a prescribed limit, the target value needs to be set slightly higher than that limit so that the received signal quality after the power down step is still above the prescribed limit.
There is a problem with this type of power control algorithm, namely, the sometimes considerable delay between the time a transmit power control command is issued by a radio receiving and evaluating the signal quality of the received signal and the time when the effect on the transmit power level of the transmitting radio is experienced. This delay is referred to generally below as xe2x80x9cpower control delayxe2x80x9d and can include a total time delay period or some portion thereof.
Indeed, if the delay between the issuance of the command and its effect is larger than the time intervals separating issuance of two consecutive power control commands, the transmit power level will be increased or decreased more than it should or otherwise needs to be. For example, if the receiver determines that a received signal is below a target SIR value, a power up command is issued to the transmitter. However, due to the power control loop delay, the power up command does not take effect until after the radio receiver again samples the received signal quality, determines that it is still below the target value, and issues another power up command. As a result, the transmit output power level is adjusted upwards twice even though, at least in some instances, a single power up adjustment would have been sufficient. When the received signal quality is finally detected as exceeding the target value, the same over-reactive power control procedure is likely repeated with power down commands.
Such over-reactive power control is undesirable for several reasons. First, the transmit power levels have higher peaks resulting in greater interference for other users of the system. Second, the higher peak oscillations may lead to instabilities in the radio network. Third, because the oscillation peaks have a higher amplitude, the target signal quality will have to be increased so that the negative oscillation peak does not dip below the minimum desired signal quality. This results in a higher average power increasing the battery drain for mobile stations and the interference to other users.
Reference is now made to FIGS. 1 and 2 which illustrate the various aspects of the power control delay characteristics described above. FIG. 1 shows two signal graphs for the transmitting station including power commands received and transmit output power level. FIG. 2 shows two signal graphs for the receiving station including detected signal quality, e.g., SIR, and power control commands issued. A power down command is first received. Shortly thereafter, the transmit output power is decremented by a predetermined xcex94 amount which, after time delay Ta results in a reduction in the detected signal quality at the receiver. There is also a delay Tb needed for the receiver to detect the current signal quality of the received signal and then issue an appropriate power control command, in this case, a power down command. Yet another delay Tc is incurred from the point when the second power down command is issued until it is received at the radio transmitter. A fourth delay Td occurs between the time when the second power control command is received and then executed.
Thus, FIG. 2 shows signal graphs similar to those shown in FIG. 1, but with larger power control delays. In general, because of the larger delay, the power control algorithm overreacts causing greater oscillations in transmit power and in the received signal quality. Consider the time during Td during which the signal-to-interference ratio is measured and found to be slightly below the target signal-to-interference ratio resulting in the issuance of a power up command shortly before the expiration of time period Td. For the next measurement time interval, the power up command has not yet taken effect (at the least, the receiver has not yet seen its effect), and thus the signal quality measurement still indicates that the detected SIR is below the target. Therefore, another power up command is issued. As a result, the transmit power is adjusted upwards twice when a single power up command would have been sufficient. When the received signal quality finally exceeds the target SIR, the same process is repeated with the power down commands.
Even in a static radio environment, the power control oscillation period for this kind of incremental method is six SIR measurement time intervals, including three power up and three power down control command intervals. The difference between the highest and lowest transmit output power will be three times that compared to the case where there is no delay between the time a power command is issued and the time its effect is experienced. Of course, the time delay associated problems are further exasperated with increasing delay. The larger the number of pending issued power controls commands that have not yet taken effect, the greater the oscillation, instability, interference, power consumption, etc.
These and other problems are solved by the present invention which compensates for a transmit power control time delay that adversely affects the power control process. Time delay associated with the power control loop is compensated for by adjusting a determined value of a received signal, and in particular, based on a previous power control command already sent but whose effect has not yet been experienced. Because the determined value of the received signal is adjusted using a delay compensation value, the transmit power level control command is appropriately determined so that it also takes into account the power control loop time delay. The delay compensation value may be determined based on the output power of a single, previously generated transmit power control command, or on plural previously generated transmit power control commands, depending upon the length of the time delay.
Initially, a signal quality parameter of a signal received from the transmitter is determined such as a signal interference ratio or a received signal strength. The determined signal quality parameter is adjusted using the compensation factor, and compared to a target signal quality parameter. In response to the comparison, a power control command is generated which takes into account the signal quality parameter compensation factor. The compensation factor is thereafter modified to take into account the generated power control command. The power control command may include an incremental power up as well as an incremental power down command. The incremental value may be a fixed constant or a variable. Alternatively, the power control command may be a designated transmit power value.
In any event, if the adjusted signal quality parameter exceeds the target signal quality parameter, the generated power control command directs a decrease in transmit power, and the compensation factor causes a decrease in the subsequently determined signal quality parameter. If the adjusted signal quality parameter is less than the target signal quality parameter, the generated power control command directs an increase in transmit power, and the compensation factor causes an increase in the subsequently determined signal quality parameter. As the time delay associated with the power control command decreases, the number of power control commands used to determine the compensation factor may decrease. Alternatively, as the time delay associated with the power control command increases, the number of power control commands used to determine the compensation factor tends to increase.
The present invention may be employed advantageously in a radio station for use in a mobile radio communications system. The radio station includes a transmitter for transmitting a power control command to another radio station and also includes a receiver for receiving a signal transmitted by the other radio station. A detector detects a signal quality parameter associated with the received signal. A processor predicts an effect the transmit power control command will have on the detected signal parameter. More particularly, the processor predicts the effect that the transmit power control command would have if the transmit power control command had already been received and implemented in the transmitter. Using the predicted effect, the processor adjusts the detected signal parameter, and generates the power control command based on the adjusted signal parameter. The prediction and the adjustment compensate for a time delay associated with controlling the transmit power of the radio transmitter.