Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
2001-03-05
2004-12-07
Milord, Marceau (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S063100, C455S067110, C455S013400, C455S522000, C455S574000, C370S335000, C370S318000, C370S252000
Reexamination Certificate
active
06829468
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to telecommunications in general, and more particularly, to a method for controlling reverse-link power control overshoot in a wireless communications system.
BACKGROUND OF THE INVENTION
FIG. 1
depicts a schematic diagram of a portion of a conventional wireless communications system. As shown, the wireless communications system
1
includes a mobile switching center (MSC)
5
, operatively connected with a plurality of base stations (BS)
10
, for performing call connection control and mobility management. The base stations
10
include equipment to communicate with the MSC
5
, and with various mobile stations (MS)
20
. Each base station
10
transmits a pilot signal of constant power on the same frequency. The power level of the received pilot signal enables the mobile station
20
to estimate the path loss between the base station
10
and the mobile station
20
. Knowing the path loss, the mobile station
20
adjusts its output transmit power such that the base station
10
will receive signals at the requisite power level. The base station
10
measures the output power of the mobile station
20
and informs the mobile station
20
to make any necessary adjustments to its transmit power. Mobile stations (MS)
20
, operatively connected with the base stations
10
, have equipment for allowing communication with the base stations
10
, and for establishing forward links (base station to mobile station) and reverse links (mobile station to base station) therebetween.
CDMA is one scheme for defining forward and reverse link channels between base stations and mobile stations of a wireless communications system, such as a cellular telephone system. CDMA is an interference-limited system, whereby all mobile stations transmit at the same frequency, and the internal interference generated within the system plays a critical role in determining system capacity and voice quality. The CDMA system uses power control and voice activation to minimize mutual interference. Precise power control and voice activation avoids excessive transmitter power, which contributes to overall interference in the system. As the mobile station moves around, the wireless communications environment changes continuously due to fast and slow fading, shadowing, external interference, and other factors. One feature of a CDMA system is to transmit just enough power to meet the required performance objectives. The objective of power control is to limit the transmitted power on the forward and reverse links while maintaining link quality under all conditions. The transmit power from each mobile station must be controlled to limit interference therebetween. At the same time, the power level should be adequate for satisfactory voice quality. Otherwise, if more power is transmitted than necessary, one mobile station interferes with the operation of other mobile stations, and capacity is reduced.
In a CDMA system, all mobile stations
20
should achieve the same received power levels at the base station
10
to minimize interference within the wireless communications system
1
while achieving the required performance objectives. Reverse link power control affects the access and reverse traffic channels. Reverse link power control includes open-loop power control (also known as autonomous power control) and closed-loop power control.
Open-loop power control is based on the principle that a mobile station
20
closer to the base station
10
needs to transmit less power as compared to a mobile station
20
that is farther away from the base station
10
or is in fade. The mobile station
20
adjusts its transmit power based on total power received, including power received from all base stations
10
on the forward link channels. If the power received is high, the mobile station
20
reduces its transmit power, and vice versa. Dynamic or moving mobile stations (i.e., fading sources) in multipath require much faster power control than the open-loop power control can achieve.
Closed-loop power control provides additional power adjustments required to compensate for fading losses (e.g., when the signal interference between the mobile station and the base station increases). Closed-loop power control also provides correction to the open-loop power control. Once a traffic channel is established, the mobile station
20
and base station
10
engage in closed-loop power control. The quicker response time gives the closed-loop power control mechanism the ability to override the open-loop power control mechanism in practical applications.
FIG. 2
depicts a conventional method of reverse link closed-loop power control that is performed between the mobile station
20
and base station
10
to ensure that a proper communications link can be established therebetween. In step
100
, the mobile station
20
transmits to the base station
10
at a certain output power in accordance with a request from the base station
10
. In step
200
, the base station
10
detects and measures the output power of the mobile station
20
.
Next, in step
300
, the base station
10
determines whether the output power of the mobile station
20
needs adjustment. This is done by comparing output power level requested by the base station
10
with the actual output power received from the mobile station
10
, and considering the amount of signal interference in the current communications environment. If the mobile station's output power does not require adjustment, the reverse-link power control process proceeds to step
600
to establish a communications link between the mobile station
20
and the base station
10
using well known techniques in the conventional art. In contrast, if the mobile station's output power requires adjustment, the mobile station
20
calculates a new output power as shown in step
400
.
Step
400
will be described in more detail with respect to the IS-95 CDMA standard. Under the current IS-95 CDMA standard, the transmit power of a mobile station
20
should be tightly controlled by the base station
10
through closed-loop power control. The base station
10
informs the mobile station
20
, by sending a power control bit, to make any necessary adjustments (i.e., increase or decrease) to the mobile station transmit power. This is commonly referred to as the closed-loop power control correction. As known in the art, the mean output power of the mobile station
20
is calculated by the mobile station
20
using the following algorithm:
Output
power
=
-
mean
input
power
+
offset
+
interference
correction
from
the
last
access
probe
+
(
NOM
_
PWR
-
16
*
NOM
_
PWR
_
EXT
)
+
INIT
_
PWR
+
the
sum
of
all
access
probe
corrections
+
the
sum
of
all
closed
loop
power
control
corrections
.
(
1
)
In the above Equation (1), all terms are well known in the conventional art. To obtain the mobile station's output power, a negative value of the mobile station's mean input power is added with a predetermined offset value and the interference correction from the last access probe. Here, an access probe refers to a transmission during an access attempt performed between the mobile station
20
and the base station
10
to establish a communications link therebetween. Access probes are continuously performed at incremental power levels. This is referred to as performing access probe corrections. The next added term of equation (1) comprises a nominal power for handoff (NOM_PWR) multiplied with a nominal power for extended handoff (NOM_PWR_EXT). An in
Gandhi Asif Dawoodi
Ibanez Marc Benedict
Song Lei
Thomas Mathew
Vitebsky Stanley
Lucent Technologies - Inc.
Milord Marceau
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