Abstract:
A method and apparatus for controlling transmission of an RPC channel during a handoff in a mobile communication system, wherein. only one AN is selected among a plurality of ANs to transmit reverse power control information to an AT. For this purpose, a Forward Link Selection message received at an AN from an RNC. Alternatively, a DRC cover received at an AN from AT can be used. The DRC cover indicates at least one of ANs excluding an AN having the worst chennel condition for the AT.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for transmitting an RPC (Reverse Power Control) channel in a CDMA (Code Division Multiple Access) mobile communication system.  
           [0003]    2. Description of the Related Art  
           [0004]    Long signal paths and shadowing worsen signal attenuation, inter-system interference, and fading in a radio environment. Thus the carrier-to-interference ratio (C/I) of a signal varies greatly according to channel condition. Standardized mobile communication systems adopt link adaptive techniques of controlling a data rate according to channel condition or C/I to increase channel throughput. The data rate is determined by a code rate and a modulation scheme. When the C/I is higher, the data rate is increased by using a higher code rate and a higher-order modulation. When the C/I is lower, the data rate is decreased by using a lower code rate and a lower-order modulation, thereby increasing channel reliability. A receiver estimates channel condition on the basis of a C/I, determines a data rate according to the estimated channel condition, and feeds back information about the data rate to a transmitter. Then the transmitter assigns the requested data rate to the receiver.  
           [0005]    The Third Generation Partnership Project 2 (3GPP2) has established 1× EV-DO (Evolution-Data Only) and HDR (High Data Rate) standards for the purpose of supporting high-speed data service based on cdma200 1× standards. According to the standards, a transmitter is known as an AN (Access Network) and a receiver is known as an AT (Access Terminal) if it is forward link. On the 1× EV-DO physical layer adopting a link adaptive technique, 12 data rates are available according to three modulation schemes including QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), and 16QAM (16-ary Quadrature Amplitude Modulation), two code rates (i.e., 1/5 and 1/3), and packet length.  
           [0006]    An AT determines a data rate at which it can receive a forward traffic channel by measuring the C/I of a forward pilot channel and feeds back data rate information to an AN so that the AN can select a forward data rate based on the feed-back information. The data rate information is DRC (Data Rate Control) information. The DRC information is represented by a 4-bit DRC symbol transmitted on a DRC channel. Aside from the DRC information, the AT transmits 3-bit information indicating a sector that will be serviced among eight effective sectors on the DRC channel. The 3-bit sector information is defined as a DRC cover indicative of the index of an orthogonal code for covering such as a Walsh code.  
           [0007]    [0007]FIG. 1 is a block diagram of channel structure in an AN of a conventional mobile communication system supporting high-speed data transmission. The channel structure is comprised of a traffic and control channel  101  to  108 , a preamble  111  and  112 , a MAC (Media Access Control) channel  121  to  123  and  131  to  134 , and a pilot channel  141 .  
           [0008]    In the traffic and control channel, an encoder  101  encodes forward traffic/control channel information. A scrambler  102  scrambles the code symbols received from the encoder  101  with a predetermined scrambling code and a modulator  103  modulates the scrambled symbols by one of QPSK, 8PSK and 16QAM according to a data rate. A puncturing and repetition unit  104  punctures and repeats the modulation symbols received from the modulator  103  in a predetermined rule to match the data rate. A symbol demultiplexer (DEMUX)  105  demultiplexes the output of the puncturing and repetition unit  104 . The outputs of the symbol DEMUX  105  are spread with orthogonal codes such as Walsh codes. A channel gain processor  107  multiplies each spread channel signal by a predetermined gain (e.g., 1/4) and a chip level summer  108  sums the outputs of the channel gain processor  107  on a chip level.  
           [0009]    In the preamble, a spreader  111  spreads a preamble signal with a Walsh code W i   32  assigned according to a MAC index and a preamble repeater  112  repeats the spread signal a predetermined number of times according to the data rate.  
           [0010]    A MAC channel is divided into an RA (Reverse Activity) channel and an RPC channel. Each channel is spread with a length  64  Walsh code. An RA bit repeater  121  repeats a 1-bit RAB (Reverse Activity Bit) according to a repetition factor set in RABLength. The RAB provides information about reverse link interference load and is broadcast to all ATs within the sector. An RA channel gain processor  122  multiplies the output of the RA bit repeater  121  by an RA channel gain and a spreader  123  spreads the output of the RA gain processor  122  with a predetermined Walsh code W 4   64 . An RPC channel gain processor  131  multiplies RPC bits by a channel gain G(i). The RPC bits indicate reverse power control information for an AT with MACIndex i. The AN measures the C/I of the reverse link from the AT with MACIndex i. If the C/I is lower than a threshold, the RPC bits are set to ‘0’ (UP) and if the C/I is higher than the threshold, they are set to ‘1’ (DOWN). A spreader  132  spreads the output of the RPC channel gain processor  131  with a predetermined Walsh code W i   64 . A chip level summer  133  sums the outputs of the spreaders  123  and  132  at a chip level. A repeater  134  repeats the sum according to a predetermined repetition factor (e.g., 4). The transmission power of the RA channel and the RPC channel is maintained to be the same as that of the traffic, control, and pilot channels.  
           [0011]    In the pilot channel, a spreader  141  spreads a pilot signal with all 0s on an In-phase channel with a predetermined Walsh code, Walsh 0.  
           [0012]    A time-division multiplexer (MUX)  151  time-division multiplexes the outputs of the traffic and control channel, the preamble, the MAC channel, and the pilot channel according to a predetermined rule. A complex spreader  152  complex-spreads the outputs of the time-division MUX  151  with a predetermined PN (Pseudo Noise) code. A baseband filter  153  baseband-filters the PN-spread signal. The resulting signal is modulated with a corresponding carrier and transmitted to an AT through an antenna. Here, the transmission power is maintained to be reference transmission power, which can be set usually to the highest transmission power level of the transmitter.  
           [0013]    [0013]FIGS. 2A and 2B illustrate the structures of slots in which the multiplexed forward traffic/control channel, MAC channel, and pilot channel output from the time-division MUX  151 . FIG. 2A illustrates an active slot in which a traffic/control channel is delivered. In the active slot, each of two 96-chip pilot bursts is located at the center of each half slot. A 64-chip symbol of the MAC channel containing the RA channel and the RPC channel occurs four times in the slot, before and after the two pilot bursts. The remaining 1600 chips of the slot are occupied by the traffic/control channel.  
           [0014]    [0014]FIG. 2B illustrates an idle slot free of the traffic/control channel. In the idle slot, only the pilot channel and the MAC channel are delivered. The AN transmits the time-division multiplexed traffic/control, MAC, and pilot channels with its highest transmission power on the forward link.  
           [0015]    The MAC channel includes one RA channel and up to 59 RPC channels that are code-division multiplexed using 64 Walsh codes. Each of the channels is transmitted with its channel gain. A MAC index is assigned to the RA channel and each RPC channel. Each channel is spread with a 64 Walsh code corresponding to its assigned MAC index. For example, Walsh code 4 is assigned to the RA channel and different Walsh codes between Walsh code 5 to Walsh code 63 are assigned to each RPC channel. Since the RA channel is broadcast to all ATs within a sector, its channel gain is determined such that the reception energy of RA channels accumulated in as many slots as RABLength with respect to an AT at a cell boundary satisfies a reference RF error performance. On the other hand, each RPC channel is destined for a specific AT within the sector and up to 59 RPC channels exist. Because of limited power available to all the RPC channels, a channel gain sufficient to satisfy a reference RPC error performance cannot be assigned to each AT within the sector. As the number of ATs increases or an AT moves farther from an AN, channel gain requirements are increased. If the sum of RPC channel gains required to ensure sufficient RPC performance for all ATs is higher than the total power assigned to all of the RPC channels, a required channel gain cannot be assigned to each RPC channel or an RPC channel cannot be assigned to some ATs. As a result, the power control performance of each AT is seriously deteriorated. As the number of ATs increases, as more ATs are located at a cell boundary, and as the reception channel condition of ATs are bad, an RPC channel power assigned to each AT becomes less.  
           [0016]    Some of ATs within the sector may be in a soft handoff situation and thus are assigned RPC channels from at least two sectors.  
           [0017]    If received RPC bits are identical, the AT detects the RPC bit by diversity-combining the RPC channels received from the sectors. On the other hand, if the RPC bits are different, the AT detects them and increases its transmission power only when both RPC bits are ‘0’ (UP). That is, an RPC channel delivers information for controlling the transmission power of an AT.  
           [0018]    As described above, one AT occupies RPC channels from at least two sectors to achieve the diversity gain of the RPC channels at a soft handoff. However, power that could be assigned to an RPC channel for another AT is consumed for the benefit of the diversity gain, which causes a potential RPC channel power shortage and decreases the power control performance of the AT. What is worse, since the AT at soft-handoff is near a cell boundary and experiences degraded channel condition, it requires a higher channel gain to satisfy corresponding RPC channel error performance. If there are many concurrent Ats at soft-handoff, the RPC channel power shortage becomes more severe. Although the number of ATs at soft-handoff varies with sector size, AT distribution within the sector, and channel condition, a soft-handoff AT consumes much more RPC channel power than an AT in a normal operating situation.  
           [0019]    Thus it can be concluded that since at soft handoff, an AT requires relatively high RPC channel power from at least two sectors, leading to a potential RPC channel power shortage, degrading the overall reverse link power control performance and simultaneously reducing the number of available RPC channels.  
         SUMMARY OF THE INVENTION  
         [0020]    It is, therefore, an object of the present invention to provide a method and apparatus for controlling transmission of an RPC channel that is used to control the transmission power of an AT at handoff in a mobile communication system.  
           [0021]    It is another object of the present invention to provide a method and apparatus for alleviating a power shortage of RPC channels that are used to control the transmission power of ATs at handoff in a mobile communication system.  
           [0022]    It is a further object of the present invention to provide a method and apparatus for preventing the decrease of reverse power control performance at handoff in a mobile communication system.  
           [0023]    It is still another object of the present invention to provide a method and apparatus for increasing the number of RPC channels that are available simultaneously at handoff in a mobile communication system.  
           [0024]    To achieve the above and other objects, there are provided a method and apparatus for controlling transmission of an RPC channel in a mobile communication system. According to one aspect of the present invention, at least one AN in an active set, excluding an AN having the worst channel condition, is selected for an AT. Only the selected AN transmits reverse power control information to the AT.  
           [0025]    According to another aspect of the present invention, an AN receives a message for selecting at least one of Ans excluding an AN having the worst channel condition for an AT from an RNC.The selected AN determines whether to transmit reverse power control information to the AT according to the received RNC message and begins transmitting the reverse power control information accordingly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.  
         [0027]    [0027]FIG. 1 is a block diagram of a conventional channel structure for an AN in a mobile communication system.  
         [0028]    [0028]FIG. 2A illustrates the structure of an active slot in which the channel structure illustrated in FIG. 1 transmits a traffic/control channel.  
         [0029]    [0029]FIG. 2B illustrates the structure of an idle slot free of the traffic/control channel.  
         [0030]    [0030]FIG. 3 is a block diagram of a channel structure for controlling RPC channel transmission in an AN according to an embodiment of the present invention.  
         [0031]    [0031]FIG. 4 is a flowchart illustrating a procedure for controlling RPC channel transmission according to the embodiment of the present invention.  
         [0032]    [0032]FIG. 5 is a block diagram of a channel structure for controlling RPC channel transmission in an AN according to another embodiment of the present invention.  
         [0033]    [0033]FIG. 6 is a flowchart illustrating a procedure for controlling RPC channel transmission according to the second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
         [0035]    [0035]FIG. 3 is a block diagram of forward channel structure for controlling RPC channel transmission in an AN according to an embodiment of the present invention. In the embodiment of the present invention, the transmitter is provided with an RPC channel transmission controller  341  that determines whether to transmit an RPC channel by a Forward Link Selection message received from an RNC (Radio Network Controller) (not shown). The embodiment of the present invention will be described mainly in the context of RPC channel transmission, with the same components as illustrated in FIG. 1 not described again.  
         [0036]    The RNC selects an AN having the best forward channel condition for an AT according to the statistics of DRC covers received from ANs in its active set and transmits information about the selection to each AN by a Forward Link Selection message. at handoff, at least two ANs are in an active set and in a normal situation, one ANs is in the active set. The selected AN transmits a traffic channel to the AT.  
         [0037]    Referring to FIG. 3, the AN receives a Forward Link Selection message from the RNC with respect to all ATs with MACindex i at its receiver (not shown). The RPC transmit controller  341  determines whether to transmit an RPC channel to each AT by the Forward Link Selection message, sets the channel gain of each AT to 0 or a predetermined value, and feeds the channel gain to an RPC channel gain processor  331 . The RPC channel gain processor  331  multiplies an RPC bit by a corresponding RPC channel gain. A spreader  332  spreads the output of the RPC channel gain processor  331  with a Walsh code W i   64 . The RPC channel signal output from the spreader  332  is code-division-multiplexed with other MAC channels in a chip level summer  333  and time-division-multiplexed with a pilot channel and a forward traffic/control channel in a time-division MUX  361 . Thus, one sector (AN) with the best channel condition, as selected by the RNC, transmits an RPC channel to a corresponding AT at handoff.  
         [0038]    [0038]FIG. 4 is a flowchart illustrating a procedure for controlling RPC channel transmission in the AN illustrated in FIG. 3 according to an embodiment of the present invention.  
         [0039]    Referring to FIG. 4, the RPC transmit controller  341  receives a Forward Link Selection message for MACIndex i in step  401  and determines whether the forward link of the AN has been selected with respect to each MACIndex in step  402 . If the forward link of the AN has been selected, the RPC transmit controller  341  sets an RPC channel gain to a predetermined value and provides the RPC channel gain to the RPC channel gain processor  331  in step  403 . Then the RPC channel gain processor  331  multiplies an RPC bit by the channel gain and transmits an RPC channel signal for MACIndex I in step  404 . On the other hand, if the forward link of the AN has not been selected, the RPC transmit controller  341  sets the RPC channel gain to 0 and provides the RPC channel gain to the RPC channel gain processor  331  in step  411 . Then the RPC channel gain processor  331  multiplies the RPC bit by the channel gain 0, thereby not transmitting an RPC channel for MACIndex i in step  412 .  
         [0040]    This embodiment of the present invention can be applied partially. That is the RPC channel transmission is controlled by the Forward Link Selection message only if the sum of power assigned to each RPC channel is higher than the total power available to the overall RPC channels.  
         [0041]    [0041]FIG. 5 is a block diagram of forward channel structure for controlling RPC channel transmission in an AN according to another embodiment of the present invention. In the second embodiment of the present invention, the transmitter is provided with an RPC channel transmission controller  541  that determines whether to transmit an RPC channel by a DRC cover from an AT. Again, the same components as illustrated in FIG. 1 are not described.  
         [0042]    Referring to FIG. 5, the AN receives DRC covers from all ATs with MACindex i at its receiver (not shown). A DRC cover indicates an AN having the best channel condition for a specific AT. The RPC transmit controller  541  determines whether to transmit an RPC channel to each AT by checking the DRC covers, sets the channel gain of each AT to 0 or a predetermined value, and feeds the channel gain to an RPC channel gain processor  531 . If the DRC cover of an AT indicates the AN, the channel gain of the AT is set to a predetermined value, otherwise, it is set to 0. The RPC channel gain processor  531  multiplies an RPC bit by a corresponding RPC channel gain. A spreader  532  spreads the output of the RPC channel gain processor  331  with a Walsh code W i   64 . The RPC channel signal output from the spreader  332  is code-division-multiplexed with other MAC channels in a chip level summer  533  and time-division-multiplexed with a pilot channel and a forward traffic/control channel in a time-division MUX  561 . Thus, only one sector having the highest C/I of its forward pilot channel selected by the AT transmits an RPC channel to the AT at handoff.  
         [0043]    [0043]FIG. 6 is a flowchart illustrating a procedure for controlling RPC channel transmission in the AN illustrated in FIG. 5 according to the second embodiment of the present invention.  
         [0044]    Referring to FIG. 6, the RPC transmit controller  541  receives a DRC cover for MACIndex i in step  601  and determines whether the index of the DRC cover is identical to the index of the AN in step  602 . If they are identical, the RPC transmit controller  541  sets an RPC channel gain to a predetermined value and provides the RPC channel gain to the RPC channel gain processor  531  in step  603 . Then the RPC channel gain processor  531  multiplies an RPC bit by the channel gain and transmits an RPC channel signal for MACIndex I in step  604 . On the other hand, if the indexes are different, the RPC transmit controller  541  sets the RPC channel gain to 0 and provides the RPC channel gain to the RPC channel gain processor  531  in step  611 . Then the RPC channel gain processor  531  multiplies the RPC bit by the channel gain 0, thereby not transmitting an RPC channel for MACIndex i in step  612 .  
         [0045]    The second embodiment of the present invention can also be applied partially. That is, the RPC channel transmission is controlled using a DRC cover only if the sum of power assigned to each RPC channel is higher than the total power assigned to the overall RPC channels.  
         [0046]    In accordance with the present invention, a control is provided using a Forward Link Selection message or a DRC cover for only one sector having the best forward channel condition to transmit an RPC channel to an AT at soft handoff. Therefore, power could be assigned to another AT is increased and an RPC channel power shortage is alleviated, thereby improving reverse power control performance and simultaneously increasing the number of RPC channels available.  
         [0047]    While the invention has been shown and described with reference to certain preferred embodiments thereof, they are merely exemplary applications. For example, while an AN having the best channel condition is selected and only the selected AN transmits RPC information to an AT for controlling reverse transmission power in the embodiments of the present invention, it can be further contemplated that at least one of ANs excluding an AN at the worst channel condition is selected and only the selected AN transmits RPC information to the AT to alleviate an RPC channel power shortage. Therefore, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.