Power control of a multi-subchannel mobile station in a mobile communication ...

POWER CONTROL OF A MULTI-
SUBCHANNEL MOBILE STATION IN A
MOBILE COMMUNICATION SYSTEM

FIELD OF THE INVENTION 5

This invention relates generally co power control within mobile communication systems and, in particular, to a system and method of power control within a Direct Sequence (DS)-Code Division Multiple Access (CDMA) 10 system.

BACKGROUND OF THE INVENTION

In DS-CDMA communication systems, efficient power control of mobile station (MS) to basestation (BS) 15 communications, hereinafter referred to as reverse link communications, is essential. One key consideration with respect to the transmit power levels in reverse link communications is interference caused by other MSs transmitting on the same channel, hereinafter referred to as co-channel 20 MSs, in the same or neighboring cells. Hence, reduced transmit power levels result in a reduction in co-channel interference problems. Another key consideration is the limited battery power within the MSs, making it important to minimize unnecessary power consumption at the MSs. On 25 the other hand, reverse link communication channels have a particular Quality of Service (QoS) requirement that must be met with a minimum transmit power. Thus, the transmit power level at the MS is a balance between co-channel interference/battery utilization and QoS requirements. 30

If each radio channel is static and the transmit power of each MS is properly allocated based on its geographical location, the level of co-channel interference and battery power utilization can be minimized so as to not severely affect the BS receiver's performance for any particular MS. 35 However, since radio channels are time varying in a realistic environment, the signals sent from MSs and received at the BS experience power variations, due to the phenomenon known as channel fading, hereinafter referred to simply as fading. 40

To adjust for fading, previous standards for DS-CDMA communication systems have established the use of a power control command, consisting of a single power control bit, to be periodically transmitted from the BS to each MS communicating with the BS. Communications from the DS 45 to the MS, hereinafter referred to as forward link communications, are used to transmit these power control commands. Each power control bit, according to previous setups, is generated in response to a power detection process for data signals received at the BS. These power control bits 50 are systematically transmitted to the MSs in order to indicate whether the transmit power at a particular MS should be adjusted to compensate for power variations, such as fading, at the BS's receiver. If the power associated with the received data signals is found to be below a predetermined 55 acceptable power threshold level, the BS indicates this by transmitting a high value on the power control bit. On the other hand, if the power associated with the received data signals is above the threshold level, the DS indicates this by transmitting a low value. In response to these power control 60 bits, the MS increases the transmit power if a high value was sent and decreases the transmit power if a low value was sent. This allows for a dynamically controlled transmit power at the MSs that can compensate for each MS's particular situation. 65

The third generation DS-CDMA standard, referred to as the 3G DS-CDMA standard or cdma2000 in North America

has been denned with increased flexibility when compared to previous standards. In the reverse link of a cdma2000 system, an MS is capable of simultaneously transmitting one pilot channel and one data channel, the data channel capable of being formed by code-multiplexing multiple codesubchannels that include fundamental, supplemental, and dedicated control channels. The pilot channel and the codesubchannels all pass through the same radio transmission environment and thus, all the signals experience the same fading. Because of this fact and the fact that codesubchannel signals are designed to be mutually orthogonal, it has been assumed that it is appropriate to apply a single bit to power control the entire MS transmitter, and this is consistent with proposed power control systems for cdma2000.

A problem with this approach is that when the subchannels are demodulated, the residual interference seen by each code-subchannel signal may be different clue to loss of orthogonality caused by the existence of multipaths in the radio transmission environment, and the different processing gains of the code-multiplexed subchannels. Thus, the variation of each code-subchannel's signal-to-interference-plusnoise ratio (SINR) may be different for a given fading rate. Further, each of these code-subchannels has its own quality of service (QoS) requirement. Hence, different codesubchannels can have different power limitations and requirements at the BS's receiver. The simple conventional power control systems discussed above are incapable of addressing these issues and may require more transmit power from the MSs than what would be ideally required to achieve the same QoSs.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome at least one of the disadvantages of the prior art and, in particular, to provide for more efficient selection of transmit power levels at MSs within mobile communication systems.

The invention provides an improved power control method for controlling a mobile station which transmits using several physical channels. A base station makes measurements for each of the physical channels independently, and as a function of those, produces either a single flower control bit which is used at the mobile station to adjust all transmit gains, or a power control bit for each physical channel which the mobile station uses to adjust transmit gains of each physical channel independently.

The present invention is, according to a first broad aspect, a method of controlling the transmit power of a mobile station that is operable to transmit a plurality of physical channels to a basestation simultaneously. The method includes steps of making power characteristic estimates for at least two of the physical channels and controlling the transmit power at the mobile station with use of the power characteristic estimates.

The present invention, according to a second broad aspect, is a power control apparatus arranged to be implemented within a basestation that can receive a plurality of physical channels from a mobile station simultaneously. In this aspect, the power control apparatus includes a plurality of receivers, a plurality of estimation devices, a power control bit generator and a power control bit transmitter. Each of the receivers preferably receives a data signal from the mobile station and separates a respective physical channel within the data signal. Each estimation device preferably receives one of the individual physical channels and makes a power characteristic estimate for the received physical 3

channel. The power control bit generator preferably receives the power characteristic estimates and generates at least one power control bit on the basis of the estimates. The power control bit transmitter preferably transmits the power control bit to the mobile station. 5

The present invention, according to a third broad aspect, is a basestation including a receiver, a power control apparatus, and a transmitter. The receiver operates to receive a plurality of physical channels from a mobile station simultaneously, The power control apparatus operates to 1° make power characteristic estimates for at least two of said physical channels. The transmitter operates to send transmit power control information to the mobile station with use of the power characteristic estimates.

The present invention, according to a fourth broad aspect, 15 is a power control apparatus arranged to be implemented within a mobile station that is operable to transmit a plurality of physical channels to a basestation. The power control apparatus receives at least two power control bits from the basestation that correspond to different physical channels 20 and adjusts transmit powers corresponding to the physical channels on the basis of the received power control bits.

In further aspects, the present invention is a basestation that operates to generate a plurality of power control bits and 2J transfer them to a mobile station, and a mobile station that operates to receive a plurality of power control bits and adjust the transmit power for a plurality of physical channels on the basis of the received power control bits.

Other aspects and features of the present invention will 30 become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS 35

Preferred embodiments of the present invention are described with reference to the following figures, in which:

FIG. la is a simplified block diagram illustrating the functionality of a reverse link closed loop power control 40 apparatus at a basestation according to a broad aspect of the present invention;

FIG. lb is a simplified block diagram illustrating the functionality of a reverse link closed loop power control apparatus at a mobile station according to a broad aspect of 45 the present invention;

FIG. 2 is a block diagram illustrating the functionality of a reverse link closed loop power control apparatus at a basestation according to a first preferred embodiment of the present invention;

FIGS. 3a and 3b respectively illustrate a well-known simplified frame structure and a simplified frame structure according to one embodiment of the present invention;

FIG. 4 is a block diagram illustrating the functionality of 55 a reverse link closed loop power control apparatus at a mobile station according to the first preferred embodiment of FIG. 2; and

FIG. 5 is a block diagram illustrating the functionality of a reverse link closed loop power control apparatus at a base 60 station according to a second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENTS

65

The preferred embodiments of the present invention are directed to a power control apparatus that can help control

4

transmit power levels at a mobile station (MS), hence aiding in the control of reverse link transmit power. This power control apparatus is preferably implemented within a basestation (BS) though modifications may be required in the MS as well. In the preferred embodiments, the MS communicates with the BS via a plurality of physical channels in a mobile communication system. Preferably, the plurality of physical channels are code-subchannels within a cdma2000 communication system though this is not meant to limit the scope of the present invention. More specifically, physical channels are defined for some embodiments as channels which allow for the transmission of data information which are modulated with direct-sequence code-division spreadspectrum techniques and multiplexed onto one carrier frequency with other data information of the other channels. Further, it should be recognized chat the present invention could be implemented in any mobile communication system in which MSs transmit a plurality of physical channels to one or more BSs.

The key to the preferred embodiments is the ability of the power control apparatus to estimate the power of at least two of the physical channels as they are received at the BS and to perform closed loop power control of the transmit power of the MS based upon these estimates. Many methods of performing such closed loop power control can be used to achieve this end. Several preferred methods are described below.

FIG. la is a simplified block diagram illustrating the functionality of a closed loop power control apparatus at the BS, according to the preferred embodiments of the present invention, which receives a data channel signal S(t) transmitted from an MS and outputs at least one power control bit that is to be further transmitted to the MS. The data channel signal S(t), according to the preferred embodiments, is a code-multiplexed signal comprising multiple codesubchannels. As depicted in FIG. la, the received data (channel signal S(t) is input to a data channel Rake 102 that comprises a plurality of individual code-subchannel Rakes 104 each of which comprises a filtering device. Each Rake 104 receives the data channel signal S(t) and separates a particular code-subchannel s1(t),s2(t), . . . , s^t) from the data channel signal S(t). For the cdma2000 example, there would be a separate Rake 104 for each fundamental channel, supplemental channel, and dedicated control channel. There would also be receive circuitry for the pilot channel.

These separated code-subchannels s1(t),s2(t), . . . , s^t) are each input to one of M Signal-to-Interference-plus-Noise Ratio (SINR) estimation blocks 106 that estimate the SINR of the input code-subchannels using an estimation method described in detail herein below. It is noted that, although the noise floor will be the same for all of the code-subchannels, the interference level might be different. The results of the SINR estimates Et,E2, . . . ,EM are subsequently input to a power control bit generation block 108 which is used to generate at least one power control bit. Subsequently, the result(s) of the generation block 108 are input to a power control bit transmission block 110 which subsequently transmits the power control bit(s) to the MS, for example over a data channel signal R(t).

FIG. lb is a simplified block diagram illustrating the functionality of a closed loop power control apparatus at the MS, according to the preferred embodiments of the present invention, which operates in cooperation with the basestation's power control apparatus of FIG. la. The power control apparatus of FIG. lb receives the data channel signal R(t), extracts the power control bit(s) transmitted from the BS, and adjusts the data channel signal S(t) to be transmitted