Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
2000-03-01
2003-02-25
Le, Thanh Cong (Department: 2684)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S522000, C455S067150
Reexamination Certificate
active
06526261
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a control method of transmitting power in a cellular system with a transmitting side and a receiving side. The method compares a signal to interference power ratio (SIR) of a received signal at the receiving side with a target SIR, and then controls the transmitting power at the transmitting side depending upon the comparison result such as its difference and higher or lower.
DESCRIPTION OF THE RELATED ART
In general, a bidirectional radio link is connected between a base station and a mobile station in a cellular system. In a CDMA (Code Division Multiple Access) cellular system using a CDMA link which is one of radio links between the mobile station and the base station, all the mobile stations transmit uplink signals at the same frequency. Also, all the base stations transmit downlink signals at the same frequency. The frequency of the uplink signals differs from that of the downlink signals, and therefore FDD (Frequency Division Duplex) method is used.
Since the same frequency is used in the uplink or downlink transmission, transmitted signals from one mobile station or one base station will interfere with a signal from another mobile station or base station. The higher the transmitted power, the larger the interference of the transmitted signal. Thus, in the CDMA cellular system, both the mobile stations and the base stations control the transmitting power of them to hold down the transmitted power to necessary minimum and reducing the interference. In general, the transmitting power control at the mobile station is called as an uplink transmitting power control, and the transmitting power control at the base station is called as a downlink transmitting power control.
When performing the transmitting power control at the mobile station, the base station measures a received SIR of a received signal from this mobile station and compares the measured received SIR with a target SIR. If the received SIR is higher than the target SIR, the base station instructs the mobile station to reduce the transmitting power. If the received SIR is lower than the target SIR, the base station instructs the mobile station to increase the transmitting power.
The transmitting power control at the base station is similarly performed as the transmitting power control at the mobile station except that the mobile station and the base station change places.
According to the above-mentioned transmitting power control, the received SIR is substantially kept at constant. However, even if the received SIR is constant, a communication quality factor such as a frame error rate (FER) or a bit error rate is not always kept constant.
In order to keep the FER or the bit error rate at constant, an outer loop control method may be used. According to this outer loop control method, a target SIR is increased if a communication quality factor such as a FER or a bit error rate detected at the receiving side is worse than a desired quality factor, and the target SIR is reduced if the detected communication quality factor is better than the desired quality factor.
A first conventional outer loop control method is described for example in Usuda et al., “Effects of applying outer loop transmit power control to W-CDMA System”, 1998 IEICE (Institute of Electronics, Information and Communication Engineers) General Conference, B-5-114, March 1998. In this method, a FER is measured by exponent weighting, the measured FER is compared with a target FER at every fixed number of frames and then a target SIR corresponding to the target Eb/Io described in the literature is increased or decreased by a predetermined value (the same value in increasing and in decreasing) depending upon the result of the comparison. A modified method of this is described in Higuchi et al., “Experimental performance of adaptive transmit power control using outer loop for wideband-CDMA”, 1998 IEICE General Conference, B-5-92, March 1998. In the latter modified method, a FER is measured by counting the number of frame errors for every fixed number of frames.
In a second conventional outer loop control method, whether or not a frame error exists is detected for every frame. When there is a frame error, a constant value S
1
makes a target SIR increase, and when there is no frame error, a constant value S
2
decreases the target SIR. If the target FER is expressed by p, the long average FER will become equal to the target FER by determining S
1
and S
2
to satisfy the following equation;
S
1
×
p=S
2
×(1−
p
).
First Problem
However, according to the first conventional outer loop control method, if the FER measurement time is short, the target SIR will be decreased because the measured FER may often zero when the probability of FER is substantially equal to the target FER. Namely, when the outer loop control becomes a steady state, the actual FER probability is higher than the target FER. In other words, according to the first conventional outer loop control method, a long measurement time equal to or longer than 1/p with respect to the target FER (=p) is necessarily required and therefore the required measurement time becomes extremely long when the target FER is low. Thus, the control may not quickly respond to change in the required SIR when it fast varies due to for example rapid change of environment.
Whereas, according to the second conventional outer loop control method, the difference between the increment amount of SIR S
1
and the decrement amount of SIR S
2
becomes large when the target FER is low. If the SIR increment amount S
1
is increased, excess increasing of the target SIR will occur due to probable frame error generations under the conditions that the required SIR does not change, causing the average SIR to increase. This results that the average transmitting power at the transmitting side becomes higher than required. Contrary to this, if the SIR increment amount S
1
is decreased, the SIR decrement amount S
2
is decreased in proportional to it. Thus, the control may not quickly respond to change in the required SIR when it fast decreases due to for example rapid change of environment.
Second Problem
There may be a lower limit in the transmitting power due to for example functional restrictions of a unit. Thus, a state that the received SIR at the receiving side exceeds the target SIR and there is no frame error although the transmitting power has its lower limit may be happened. At such state, if the outer loop control is being executed, the target SIR will decrease without limits. After that, if the state changes into one with frame errors, since the target SIR was decreased to quite lower value than its correct required SIR, frame errors will occur at higher probability than the target FER until the target SIR comes back to its correct value by the above-mentioned outer loop control.
Third Problem
When control of changing on/off of transmission for every frame is performed at the transmitting side as in the DTX (Discontinuous Transmission) control, the receiving side may erroneously judge that a frame error is occurred at the off-transmission frame. In this case, if the above-mentioned outer loop control is executed, the FER is measured higher than the actual FER and thus the target SIR will become higher than required.
Fourth Problem
In CDMA cellular system, in general, handover operations such as soft handover operation and softer handover operation are adopted. At each of the handovers, a plurality of radio links are simultaneously connected. However, the required SIR who is necessary to obtain the target FER may vary depending on the number of the connected links and kinds of the handovers. In such case, if the above-mentioned outer loop control is carried out, response will be delayed due to change in the number of the connected links and kinds of the handovers causing many frame errors to occur and also causing the target SIR to increase higher than required.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to pr
Suzuki Toshinori
Takeuchi Yoshio
Arent Fox Kintner & Plotkin & Kahn, PLLC
KDD Corporation
Nguyen Huy
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