Variable step size for power control bits to protect against power overshoot

The invention provides a system and method for controlling transmission power of a mobile unit in a wireless communication system which minimizes transmission power overshoot caused by counteracting the effects of deep fading. The power control bits are received by the mobile unit then examined on an individual basis and also as a block of bits. Initially, each time a request to increase power is received the mobile unit increases the transmission power by 1 dB. When the mobile unit determines that a predetermined number of bits in a block each indicates a request to increase power is received the mobile unit increases the transmission power by 2 dB. After the block of increase requests, if the mobile receives a request to decrease the transmission power it increases the transmission power by 2 dB, then decreases the transmission power for the next two power control periods by 3 dB each.

FIELD OF THE INVENTION
 This invention relates generally to the field of wireless communications
 systems and more particularly to a varying the step size of power control
 bits (PCBs) to protect against transmit power overshoot in Code Division
 Multiple Access (CDMA) communications systems.
 BACKGROUND OF THE INVENTION
 In CDMA communications systems, maximum uplink capacity is achieved when
 the power level of signals received by the Base Transceiver Subsystem
 (BTS) is the same for all mobile users. Such a power level is called the
 "nominal power level." If this nominal power level is maintained
 regardless of the distance between the base station and the mobile unit
 and regardless of the signal propagation environment then maximum uplink
 capacity is maintained.
 If the power level of a signal received from a mobile unit drops below the
 nominal level, the error probability for that user increases. If the power
 level of a signal received from a mobile unit exceeds the nominal level,
 the probability that the signal will interfere with signals from other
 mobile units increases. Thus transmission power deviations from the
 nominal power level decrease the capacity of the system.
 Rayleigh fading is a problem which introduces a fast power deviation from
 the nominal power level and thus degrades system capacity.
 In current Third Generation Partner Project (3GPP) systems, power control
 mechanisms are being employed which attempt to equalize the received power
 of the signal from the mobile unit and to compensate for fast power
 deviations from the nominal power level caused by the impact of Rayleigh
 fading.
 The overall uplink power control for 3GPP is shown in FIG. 1. The BTS
 employs a Rake receiver to receive and demodulate a desired signal. It
 then determines a signal to noise ratio (SNR) of the received signal using
 SNR estimation. The BTS then compares the determined SNR to the nominal
 power level to generate a power control bit. Typically power control is
 only concerned with a single bit wherein a 1 indicates to the mobile unit
 to increase transmission power and a 0 indicates to the mobile unit to
 decrease transmission power. The power control bit is multiplexed with
 data and transmitted to the mobile unit as a Transmit Power Control (TPC).
 The mobile unit receives the TPC signal, demodulates it, and separates the
 PCB (demultiplexes it). The mobile unit then converts the PCB to a power
 transmission gain (positive or negative) of the output power in the
 Extract Power Control Bit and Convert to Power Step Size block. This is
 considered closed loop power control.
 There are also methods in 3GPP considered outer loop power control. In
 outer loop power control systems, the BTS adjusts the nominal power level
 based upon a Frame Error Rate (FER) probability for a particular nominal
 power level. The BTS measures the FER probability and determines a SNR
 threshold. If at the output of the Viterbi decoder, the FER is high, the
 nominal power level is increased. If the FER is low, the nominal power
 level is decreased.
 Channel fading without power control leads to a standard deviation of 5.5
 dB for all fading frequencies. However, due to the deep fades of the
 desired signal, the standard deviation may decrease more than 20 dB with
 respect to the required signal level. This leads to the increase of the
 error probability for a particular mobile unit.
 Based on the current 3GPP specification, closed loop power control results
 in a considerable reduction of SNR deviation for small fading frequencies
 (e.g. in the range of 8-15 Hz). However, the efficiency is greatly
 decreased when the fading frequency is above 30 Hz. This is because of the
 conventional fixed power control steps of 0.25 dB, 0.5 dB and/or 1 dB and
 the delay (at least one slot of power control bit command) which are not
 able to track the changes of the signal power in the channel in fast
 fading environments. For the same reasons large power overshooting (i.e.
 too many increases or decreases to the transmission power in the mobile
 unit) occurs at the BTS input for all fading frequencies.
 Conventional 3GPP systems operate with a fixed power control step size
 (e.g. 1 dB) and a fixed power control command transmission delay. When a
 signal from a mobile unit experiences a deep fade, the BTS sends
 consecutive power increasing commands to the mobile unit. The mobile unit
 receives these commands and increases its transmission power to compensate
 for the deep fading. However the mobile unit continues to increase its
 power even after the deep fade period ends, due to the power control
 command transmission delay. This continued increase in power causes the
 power overshoot (see FIG. 2). Power overshooting negatively impacts the
 uplink power control performance by increasing the standard deviation of
 power control for the particular mobile unit and increasing the over all
 interference experienced by transmissions to the BTS. Power overshooting
 has been observed to be as high as 5 dB and has been known to occur at all
 fading frequencies.
 Accordingly, there exists a need for a power control system which minimizes
 power overshoot.
 There also exists a need for a power control system which maximizes uplink
 capacity.
 There exists the need for a power control system which minimizes the
 standard deviation from the nominal power level.
 Accordingly, it is an object of the present invention to provide a power
 control system which minimizes power overshoot.
 It is another object of the invention to provide a power control system
 which minimizes the standard deviation from the nominal power level.
 It is still another object of the invention to provide a power control
 system which employs a variable step size.
 These and other objects of the invention will become apparent to those
 skilled in the art from the following description thereof.
 SUMMARY OF THE INVENTION
 In accordance with the teachings of the present invention, these and other
 objects may be accomplished by the present invention, which provides
 transmission power control of a mobile unit in a wireless communication
 system.
 An embodiment of the invention includes a method of controlling the
 transmission power. The mobile unit receives a power control bits. Each of
 the power control bits is respectively received during a power control
 period. The mobile unit determines if each of the power control bits
 indicates that the transmission power should be increased. The mobile unit
 increases the transmission power by a predetermined amount each time it
 determines that one of the power control bits indicates that the
 transmission power should be increased. The mobile unit examines a block
 of power control bits to determine if a predetermined number of them
 indicates that the transmission power should be increased. The mobile unit
 determines that the predetermined number of power control bits indicates
 that the transmission power should be increased. The mobile unit examines
 a set of power control bits following the block of power control bits to
 determine if the set of power control bits indicates that the transmission
 power should be increased. After determining that the set indicates that
 the transmission power should be increased, the mobile unit increases the
 transmission power by an amount which exceeds the predetermined amount.
 The invention will next be described in connection with certain exemplary
 embodiments; however, it should be clear to those skilled in the art that
 various modifications, additions and subtractions can be made without
 departing from the spirit or scope of the claims.
 Another embodiment of the invention includes a system for controlling
 transmission power of a mobile unit in a wireless communication system.
 The system includes a module for receiving power control bits during power
 control periods. At least some of the power control bits indicate that the
 transmission power should be increased. The system also includes a module
 for increasing the transmission power by a predetermined amount each time
 it receives a power control bits indicating that the transmission power
 should be increased. The system includes a module for examining a block of
 the power control bits. It also includes a module for determining that a
 predetermined number of the power control bits in the block indicate that
 the transmission power should be increased. The system includes a module
 for determining if a set of power control bits following the block of
 power control bits indicates that the transmission power should be
 increased. Further, the system includes a module for increasing the
 transmission power by an amount which exceeds the predetermined amount for
 each set of power control bits following the block of power control bits
 which indicates that the transmission power should be increased.
 Yet another embodiment includes an apparatus for controlling transmission
 power of a mobile unit in a wireless communication system. The apparatus
 includes a receiver configured to receive transmit power control signals.
 It includes a demodulator in electrical communication with the receiver
 and a demultiplexor in electrical communication with the demodulator. The
 demultiplexor is configured to separate power control bits from the
 transmit power control signals. The apparatus further includes a power
 control bit analyzer in electrical communication with the demultiplexor
 and configured to determine if a predetermined number of the power control
 bits indicates a request to increase the transmission power.

DETAILED DESCRIPTION OF THE INVENTION
 The invention provides a system and method of power control in a CDMA
 communication system which minimizes power overshooting caused by deep
 fade compensation.
 Conventional power control relies upon a one bit memoryless system. The
 present invention takes advantage of sequential power control bit patterns
 to provide better performance. In a preferred embodiment, the mobile unit
 will detect conventional power control bits and increase or decrease the
 transmission power by 1 dB per received bit. However, the present
 invention will also examine blocks of ten bits (e.g. the system will look
 for a block of ten ones @800 Hz (i.e. CDMA 2000) or twenty ones @1600 Hz
 (i.e. 3GPP), etc.) If at least eight of these ten bits are determined to
 be ones, then the system will assume that any zero bits in the block were
 received in error. For each consecutive one received after a block of ten
 ones, the mobile unit will increase the transmission power by 2 dB as
 illustrated in the following table (table 1).
 TABLE 1
 1 2 3 4 5 6 7 8 9 10 11 12
 PCB 1 1 1 1 1 1 1 1 1 1 1 1
 dB +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +2 +2
 Thereafter, when the mobile unit receives a zero, it will increase the
 transmission power by 2 dB, then decrease the transmission power over the
 next two power control periods by 3 dB each period; regardless of the bits
 received during those periods. The following table (table 2) illustrates
 this aspect.
 TABLE 2
 1 2 3 4 5 6 7 8 9 10 11 12 13 14
 PCB 1 1 1 1 1 1 1 1 1 1 1 0 ? ?
 dB +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +2 +2 -3 -3
 While a particular embodiment of the invention has been described, the
 invention is not limited to this embodiment. Those skilled in the art will
 recognize that different block sizes can be examined and that the step
 size (change in power in dB) can be different than the step sizes
 provided. For example, instead of increasing the transmission power by 1
 dB for the first ten PCB ones, the mobile unit could change the
 transmission power by 0.25 dB or 0.5 dB, etc. Instead of increasing by 2
 dB upon receipt of the transition zero, the mobile unit could maintain the
 transmission power level then decrease the transmission power by 2 dB for
 each of the next two consecutive power control periods. The following
 table (table 3) illustrates this aspect of the invention.
 TABLE 3
 1 2 3 4 5 6 7 8 9 10 11 12 13 14
 PCB 1 1 1 1 1 1 1 1 1 1 1 0 ? ?
 dB +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +2 0 -2 -2
 Further, the mobile could examine blocks of five bits and require all of
 the bits in the block to be 1's (or at least 3 or 4 of the bits to be
 ones, etc) to initiate the increased step size. Those skilled in the art
 will recognize that the block size can be virtually any number of PCBs
 greater than 1. Then for every consecutive set of PCB ones thereafter
 (e.g. a set of 1 PCB one, 2 PCB ones, 3 PCB 1's, 4 PCB ones, etc.) the
 transmission power could be increased by 3 dB(or some other number larger
 than the standard increase for individual PCBs). Further the detection of
 the transition zero could be used to instruct the mobile unit to maintain
 the transmission power level, to decrease the power level or to increase
 it by 1 dB, 2 dB etc. Then the next two consecutive periods could be used
 to decrease the power level by 2 dB, 3 dB, etc. (e.g. some number greater
 than the standard increase for individual PCBs) The power drop in these
 two consecutive periods following the transition do not need to be the
 same. For example, the drop in the first period could be 3 dB while the
 drop in the next period is 2 dB, or the drop in the first period could be
 2 dB while the drop in the second period is 3 dB, the drop in the first
 period could be 4 dB while the second dropped is 2 dB, etc. Table 4
 provides an illustration of this type of configuration.
 TABLE 4
 1 2 3 4 5 6 7 8 9 10 11 12 13 14
 PCB 1 1 1 1 1 1 1 1 1 1 1 0 ? ?
 dB +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +2 +2 -4 -2
 FIG. 3 illustrates an embodiment of the invention. In this embodiment, the
 mobile unit 10 includes a receiver 12 which receives a signal from the
 Base Station 14. The received signal is demodulated by the demodulator 16
 then demultiplexed by demultiplexor 18 to separate the power control bits.
 The power control bits are then converted to a power step size which is
 determined in the manner described above by the PCB analyzer 20. One or
 more of the elements of FIG. 3 could be realized as the same or different
 microprocessors or as some other device such as an application-specific
 integrated circuit (ASIC), programmable logic array (PLA), or another
 suitable logic device. They can also be realized in software.
 It will thus be seen that the invention efficiently attains the objects set
 forth above, among those made apparent from the preceding description. In
 particular, the invention provides methods and apparatus for analyzing
 power control bits and adjusting the transmission power of a mobile unit
 based thereon to minimize power overshoot. Those skilled in the art will
 appreciate that the configuration depicted in FIG. 3 provides such
 features.
 It will be understood that changes may be made in the above construction
 and in the foregoing sequences of operation without departing from the
 scope of the invention. It is accordingly intended that all matter
 contained in the above description or shown in the accompanying drawings
 be interpreted as illustrative rather than in a limiting sense.
 It is also to be understood that the following claims are intended to cover
 all of the generic and specific features of the invention as described
 herein, and all statements of the scope of the invention which, as a
 matter of language, might be said to fall therebetween.