Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
2000-06-23
2003-05-13
Trost, William (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S525000, C455S069000, C370S318000, C370S332000, C370S320000, C370S465000, C370S468000, C370S477000
Reexamination Certificate
active
06564067
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).
The CDMA is a multiple access technique which makes it possible for several users to be simultaneously active, using different spreading codes.
All that follows is valid for both downlink (link from BTS (Base Transceiver Station) to MS (Mobile Station)) and uplink (link from MS to BTS), but in order to simplify the description, only the downlink case will first be considered.
The quality of the link from a BTS to a MS depends on the ratio of the received signal power and the interference power at the MS (SIR: signal-to-interference ratio). When the SIR of one MS is low, or equivalently when the interference power is much larger than its power, its performance dramatically decreases. Therefore, in order to optimize the performance of a CDMA system, some algorithms are usually used in order to keep the SIR of each MS as close as possible to the target SIR at the receiver, like the inner loop power control algorithm.
The principle of the inner loop power control algorithm is that the MS periodically estimates the SIR of the received signal from the BTS, and compares this SIR to the target SIR (SIR
target
). If this estimated SIR is lower than the target SIR, the MS sends a command to the BTS for the BTS to increase its transmit power. Otherwise, the MS sends a command to the BTS for the BTS to decrease its transmit power. The target SIR is chosen by the MS (or BTS) in function of the required quality of service.
Additionally, another and usually slower power control algorithm, namely outer loop power control algorithm, enables to choose the best value of the target SIR. The principle of this algorithm is to regularly evaluate the quality of the transmission (BER, BLER, . . . ) and to compare this quality with the required quality of service (for example BER of 10
−3
for speech service, BLER of 0.1 for packet service, . . . ). If this quality is below the required quality of service, the target SIR is increased. Otherwise, the target SIR is decreased. This algorithm is usually slow, since the quality needs to be averaged over several frames in order to have a reliable estimate. Of course many variants of this basic algorithm exist.
In some situations, the target SIR may change significantly during the transmission. For example, this is the case when the spreading factor of the physical data channel changes. Indeed, the lowest the spreading factor of this channel, the largest the required transmit power. The spreading factor can change frequently in variable rate services such as packet service. Indeed, if the spreading factor changes, the target SIR will vary much (in the ratio of the spreading factor variation). It is also the case if the MS requires to change of service, since each service has a different target SIR.
Another example is the compressed mode. In an inter-frequency hard handover, the mobile needs to make measurements on a frequency different from the frequency used for the downlink transmission. Thus, the base station needs to stop its transmission towards the concerned mobile, in order to allow this mobile to make measurements on this other frequency. In the UMTS standard, this is known as downlink compressed mode (i.e. the downlink transmission is temporarily stopped). Uplink compressed mode is also possible to make measurements on frequencies that are close to the uplink frequency. The periods where transmission is stopped are usually called transmission gaps, and the frames including transmission gaps are usually called compressed frames. Besides, to compensate for the transmission gaps, the transmission rate has to be correspondingly increased. Therefore, during compressed mode, since the inner loop power control is regularly stopped, and since the transmission rate is correspondingly increased, the target SIR needs to be larger to reach the same quality of service than during non-compressed, or normal, mode.
Because the outer-loop power control algorithm is usually a slow process, the target SIR will not change immediately and the transmission quality will be degraded during several frames. In extreme cases, this could cause the lost of the call.
Moreover, in the case of compressed mode, the target SIR needs to be changed only at certain fixed time to enable the mobile to perform measurements and then the target SIR needs to be changed back to the previous value. The outer-loop power control algorithm will not be able to track such quick variations of SIR.
In European patent application n° 99401766.3 filed on Jul. 13, 1999 by Applicant, a solution has been proposed to solve this problem. Briefly, the basic idea in this prior patent application is to anticipate the target SIR variation, i.e. to apply an expected variation, or offset, in an anticipated way, to the target SIR. This target SIR variation may be signaled from the transmitter to the receiver for a given transmission direction; for example, for downlink transmission, it may be signalled by the network to the MS or UE (User Equipment).
According to another idea in this prior patent application, in order to keep the signaling as low as possible, the target SIR increase due to the increased instantaneous bit rate and the target SIR increase due to degraded performances in compressed frames (i.e. due to transmission gaps) may be separated. For example, when the transmission rate increase in compressed mode is obtained by spreading factor reduction, this may be written:
&Dgr;
SIR
=10log(
R
CF
/R
)+&dgr;S
IR
where R is the instantaneous net bit rate before and after the compressed frame and R
CF
is the instantaneous net bit rate during the compressed frame (it being understood that the term “instantaneous bit rate” means that for a compressed frame, the time period used to calculate this rate is not the whole frame period but only the fraction of this frame period where data are transmitted); for example, 10log(R
cF
/R) is equal to 3 dB for UMTS, where the matching rate is the same for compressed and non compressed frames, when compressed mode by reducing the spreading factor by a factor of 2 is used.
Since the bit rate variation will be known by the UE, only the additional target SIR increase &dgr;
SIR
due to degraded performances during compressed frames may be signaled. The signaling overhead can be low if this variation is signaled with other compressed mode parameters (including transmission gap length (or period where transmission is stopped, periodicity, . . . ). For example, 2 bits could enable to signal the following values of &dgr;
SIR
:
−00: 0 dB
−01: 0.5 dB
−10: 1 dB
−11: 2 dB
Alternatively, &Dgr;
SIR
could be directly signaled, but a larger number of bits would be required.
The UE will have to increase the target SIR by &Dgr;
SIR
just before the compressed frames (or just after the transmission gap of the compressed frames) and decrease it back by the same value just after the compressed frames. This target SIR variation is done additionally to the usual downlink outer-loop algorithm that will have to take it into account. The Node B may increase simultaneously its transmit power by the same amount before the compressed frame and decrease it just after the compressed frames in order for the downlink received SIR to be as quickly as possible close to this new target SIR.
According to another idea in this prior patent application, at least when the transmission gap is at the end of the compressed frame, the performances in recovery frames (frames following the compressed frames) can also be degraded because of the power co
Alcatel
Rampuria Sharad
Sughrue & Mion, PLLC
Trost William
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