Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1999-08-11
2003-04-15
Legree, Tracy (Department: 2681)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S069000, C455S070000, C455S067150, C370S318000, C370S320000, C370S335000
Reexamination Certificate
active
06549785
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is generally concerned with mobile radiocommunication systems.
The present invention is more particularly concerned with power control techniques used in such systems to improve performances (in terms of quality of service, of capacity, . . . etc.).
The present invention is in particular applicable to mobile radiocommunication systems of CDMA (“Code Division Multiple Access”) type. In particular, the present invention is applicable to UMTS (“Universal Mobile Telecommunication System”).
As is known, CDMA systems use two types of power control techniques, a so-called open-loop power control technique, and a so-called closed loop power control technique (also called hereinafter CLPC). These power control techniques may be recalled for example for the uplink transmission direction, i.e. from MS (“Mobile Station”) to BTS (“Base Transceiver Station”). In the open-loop power control, a MS transmit power is controlled based on the power received by this MS from a BTS. In the CLPC, a MS transmit power is controlled based on the transmission quality of the link between this MS and a BTS, as estimated at this BTS.
The transmission quality of a link between a MS and a BTS depends on the ratio of the received signal power and the interference power, also called SIR (Signal-to-Interference Ratio). When the SIR of a MS is low, or equivalently when the powers of the other MSs are much higher than its power, its performances dramatically decrease. The CLPC algorithm enables to keep the SIR of each user as close as possible to a target SIR.
The principle of the CLPC algorithm is that the BTS periodically estimates the SIR of the received signal from each MS, and compares this estimated SIR to a target SIR (SIR
target
). If the estimated SIR is lower than the target SIR, the BTS sends a power control command to the MS, for the MS to increase its transmit power. Otherwise, the BTS sends a power control command to the MS, for the MS to decrease its transmit power.
The target SIR is an important parameter in such systems. Indeed, if the target SIR value is set to a value higher than necessary, there is a needless contribution to interference level in the system, and therefore a needless degradation of the performances of the system; on the other hand if the target SIR value is set to a value lower than necessary, the performances of the on-going communication are degraded.
The target SIR is generally chosen as a function of the required quality of service, and is currently adjusted by a so-called outer loop algorithm (as opposed to the preceding one also called inner loop algorithm). The principle of the outer loop algorithm is to regularly estimate the quality of service (generally represented by a bit error rate BER or a frame error rate FER for voice services, or a block error rate BLER for data packet services) and to compare the estimated quality with a required quality of service. If the estimated quality is below the required quality of service, the target SIR is increased. Otherwise, the target SIR is decreased.
As opposed to the inner loop algorithm which needs to be rapid to track the SIR variations as closely as possible, the outer loop algorithm needs to be slow, since the quality needs to be averaged over a certain period in order to have a reliable estimate. Typically, in third generation systems like for example UMTS (“Universal Mobile Telecommunications System”), the SIR of the received signal is determined and compared to the target SIR every slot in a frame, while the quality is averaged over several frames (a slot being an elementary time unit in a data unit, or frame, transmitted in such a system, the frame duration being typically equal to 10 ms, and the slot duration to {fraction (1/15)} of the frame duration).
Such a slow process may however raise serious problems, in particular when a so-called compressed mode is used.
The downlink compressed mode has been introduced in UMTS in order to make possible for a user equipment (UE) to perform measurements on a frequency different from its downlink transmission frequency. It consists basically in stopping the downlink transmission during a certain amount of time (or transmission gap). Simultaneous uplink and downlink compressed modes can also be used when the measurement frequency is close to the uplink transmission frequency.
Because the instantaneous bit rate will have to be increased during compressed frames (by increasing the coding rate or decreasing the spreading factor), the target SIR also needs to be approximately increased by the same proportion.
Additionally, since the closed-loop power control is no longer active during transmission gaps for downlink and uplink, the performance are significantly degraded, mainly during compressed frames and recovery frames (frames just following compressed frames). The degradation can reach several decibels. In order to keep the same quality of service as in normal (or non-compressed) mode, this effect would also need to be compensated by increasing the target SIR during these frames.
However, the outer-loop power control algorithm is a slow process and several frames will be probably required before changing the target SIR accordingly. Therefore, it is likely that this process is too slow to be able to increase the target SIR in compressed and recovery frames as required. Moreover, the target SIR even risks to be increased just after compressed and recovery frames where it would not be needed.
Thus, there is a need for a faster process than the classical outer-loop algorithm, in compressed mode, to avoid degrading the performances.
More generally there is a need for a faster process than the classical outer-loop algorithm, in any case of change in the transmission requirements, including:
change from a non-compressed mode to a compressed mode, or vice versa,
change in required service (in particular change in the transmission rate),
change in transmission rate, for a given required service (such as for example data packet services),
change in environment conditions (such as mobile speed, radio propagation conditions, . . . ), . . . etc.
Thus there is a general need for a more efficient power control, so as to improve performances.
SUMMARY OF THE INVENTION
An object of the present invention is therefore a method for improving performances of a mobile radiocommunication system using a power control algorithm for controlling a transmit power according to a transmission quality target value, and an adjustment algorithm for adjusting said transmission quality target value according to transmission requirements, said method including, upon the occurrence of a change in said transmission requirements, bypassing said adjustment algorithm by applying a corresponding change to said transmission quality target value, so as to adjust it in an anticipated way.
According to another object of this invention, said method further includes, upon the occurrence of a change in said transmission requirements, bypassing said power control algorithm, by applying a corresponding change to said transmit power, so as to control it in an anticipated way.
In this way, performances are still improved by enabling the transmit power to be as quickly as possible close to the new transmission quality target value.
According to another object of this invention, said change in the transmission requirements includes a change from a non-compressed mode to a compressed mode, and said corresponding change includes an increase in said transmission quality target value, to be applied before a compressed frame.
According to another object of this invention, said change in the transmission requirements includes a change from a non-compressed mode to a compressed mode, and said corresponding change includes an increase in said transmission quality target value, to be applied after a transmission gap of a compressed frame.
According to another object of this invention, said change in the transmission requirements includes a change from a compressed mode to a non-compressed mode,
Alcatel
Legree Tracy
Sughrue & Mion, PLLC
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