Adaptive power control in a radio communications systems

Details Adaptive power control in a radio communications systems Adaptive power control in a radio communications systems

1999-03-15

2003-09-30

Bost, Dwayne (Department: 2681)

Telecommunications

Transmitter and receiver at separate stations

Plural transmitters or receivers

C455S069000, C455S063100

Reexamination Certificate

active

06628956

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to power control in radio communications, and more particularly, to more efficient and effective open loop power control. The adaptive open loop power control approach of the present invention is particularly well-suited for communications in a Code Division Multiple Access (CDMA) cellular radio system.
BACKGROUND AND SUMMARY OF THE INVENTION
Power control is very important in radio communications systems, and particularly so in third generation Wideband Code Division Multiple Access (WCDMA) cellular systems. Before transmitting over an “uplink” channel, a mobile station must set its transmission power level. Similarly, the radio access network must set base station transmit power on “downlink channels,” e.g., a paging channel (PCH), a forward access channel (FACH), in addition to traffic channels (TCH. Indeed, the actual power level set for mobile and base station radio transmission and the interference levels that result therefrom are significant concerns in all mobile radio communications systems.
The physical characteristics of a radio channel may vary significantly due to a number of reasons. For example, the signal propagation loss between a radio transmitter and receiver varies as a function of their respective locations, obstacles, weather, etc. As a result, large differences may arise in the strength of signals received at the base station from different mobiles. If the transmission power of a mobile station signal is too low, the receiving base station may not correctly decode a weak signal, and the signal will have to be corrected (if possible) or retransmitted. Accordingly, erroneous receipt of the signals adds to the delay associated with radio access procedures, increases data processing overhead, and reduces the available radio bandwidth because signals must be retransmitted. On the other hand, if the mobile transmission power is too high, the signals transmitted by the mobile station create interference for the other mobile and base stations in the system.
Interference is a particularly severe problem in CDMA systems where a large number of radios transmit and receive on the same frequency. 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 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 strong mobile station to transmit signals that are received at the base station at many, many times the strength of other mobile transmissions. The loss of system capacity to such excessively “strong” mobile stations is unacceptable.
Additional problems are associated with transmitting with too much power. One is the so-called “party effect.” If a mobile transmits at too high of a power level, the other mobiles may increase their respective power levels so that they can “be heard” 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. By far, the largest drain on a mobile's battery occurs during transmission. A significant objective for any power control approach, therefore, is to reduce transmit power where possible without increasing the number of retransmissions to an unacceptably high level as a consequence of that power reduction. Except for battery consumption, the above-described problems with setting transmission power also apply to downlink radio transmissions from base stations.
There are two basic approaches to power control: open loop and closed loop. In open loop power control, the transmit power is calculated at the transmitter based on one or more parameters, and the calculated value is used to set the transmit power level. In particular, the transmit power is adjusted in order to match an estimated path loss so that the signal is received at the base station at a predetermined power level. Closed loop power control relies on feedback from the receiver so that the transmitter knows, for example, at what power level (and sometimes also at what interference level) the transmitted signal was received. Using this feedback, the transmitter then appropriately adjusts its transmit power level. Alternatively, the receiver may simply order the transmitter to increase or decrease its transmit power. The additionally received feedback information means that closed loop power control is generally more accurate than open loop power control.
Common channels for both uplink and downlink shared by several mobile radios are typically used to transmit relatively short control signaling messages which do not justify the additional “cost” in terms of delay, signaling overhead, spreading code allocation, and bandwidth consumption associated with dedicated channels. Common channels may also be utilized to transmit short traffic data packets appended directly to the typical control messages sent on common channels. “Lower cost” open loop power control is well-suited for transmission over common channels being faster, less complicated, and occupying fewer radio resources than closed loop power control commonly used for dedicated channels.
One type of common channel shared by mobile stations is a random access channel which provides communication between plural mobile stations and a base station when those mobile stations have not been allocated a dedicated channel. Access channel messages may include for example call reservations, responses to pages, orders, registrations, and small size user data packets. However, because multiple mobile stations may be using the random access channel at the same time, each additional mobile station transmitting on that access channel contributes to the background noise and interference thereby diminishing the system's finite capacity. Consequently, it is important to set the appropriate output power of the mobile station before transmitting.
Thus, before performing a random access, the mobile station caculates an open loop transmission power P
tx
to be used on the random access channel in the uplink direction so that the mobile's signal is received at the base station at a predetermined power level. In particular, the mobile station strives to achieve a target Carrier-to-Interference Ratio (CIR) &ggr;
t
at the base station. The carrier-to-interference ratio actually received at the base station corresponds to the received uplink carrier power C
UL
minus the uplink interference I
UL
. The received carrier power C
UL
corresponds to the mobile's transmit power level P
tx
minus the path loss L. The open loop power control therefore can determine the transmit power {circumflex over (P)}
tx
as a function of the target carrier-to-interference ratio &ggr;
t
, an uplink interference estimate Î
UL
, and a path loss estimate L. The path loss estimate {circumflex over (L)} may be obtained with the mobile station measuring the received power of a known signal (e.g., a downlink pilot or other broadcasted signals) transmitted on a downlink channel by the base station. The known signal includes a message that informs the mobile of the power at which that known signal is being transmit by the base station. The uplink interference is estimated (measured) and broadcast by the base station over the cell together with a downlink pilot signal output power value. The transmit power {circumflex over (P)}
tx
may then be determined using the target CIR &ggr;
t
, the uplink interference estimate Î
UL
, and the path loss estimate {circumflex over (L)} in accordance with the following open loop power control algorithm:
{circumflex over (P)}
tx
=&ggr;
t
+Î
UL
&

Affiliated with

Also associated with

Rating

Adaptive power control in a radio communications systems does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Adaptive power control in a radio communications systems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Adaptive power control in a radio communications systems will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3016597