Patent Publication Number: US-7715807-B2

Title: Wireless communication device and method for communicating in site selection diversity mode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/322,365, filed on Dec. 17, 2002, now U.S. Pat. No. 7,280,842 which is a continuation of PCT Application No. PCT/IB2002/005410, entitled: “WIRELESS COMMUNICATION DEVICE AND METHOD FOR COMMUNICATING IN SITE SELECTION DIVERSITY MODE,” filed on Dec. 16, 2002, which in turn is a Continuation-In-Part of U.S. patent application Ser. No. 10/021,541, entitled: “METHOD AND APPARATUS FOR GENERATING A QUALITY MEASURE TARGET VALUE BASED ON CHANNEL CONDITIONS,” filed on Dec. 17, 2001, which is assigned to the same assignee as the present application, and all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention pertains to wireless communications, and in some embodiments, to CDMA communications, and other embodiments, to wireless communication devices operating in site selection diversity transmit (SSDT) mode. 
     BACKGROUND 
     Code division multiple access (CDMA) is a technique for spread-spectrum digital communications used for many applications, including, for example, mobile communications. In CDMA systems, data signals are combined with a spreading waveform to form a coded signal for transmission. At the receiver, the received signal is combined with a similar spreading waveform to extract the data signals. The technique provides high data capacity by spreading signal energy over a wide bandwidth to increase bandwidth utilization and reduce the effects of narrow band interference. Multipath effects make synchronization more difficult since the wireless channel from a base station to a reception device may have several paths of different time-delays, which may vary due to the movement of the reception device. 
     Wireless communication devices (e.g., mobile stations), which communicate CDMA signals with remote base stations, often operate in a site selection diversity transmit (SSDT) mode for selection of base stations to communication with. One problem with SSDT mode is that some algorithms, such as power control and frequency estimation algorithms, are unable to quickly adapt to the quick changes in channel conditions that occur when transferring communications from one base station to another. This may result in poor performance, especially after the selection of a new primary base station for communication. 
     One specific problem area is power control. Mobile stations conventionally implement power control methods to minimize transmission power while maintaining desired performance levels. Conventional power control methods implement a nested loop structure having an outer loop and an inner loop to control transmit power. In the outer loop, the block error rate (BLER) of received data may be monitored and compared to a desired BLER. A signal to interference ratio (SIR) target is then developed for the receiver based on the comparison. In the inner loop, a measured SIR for a received signal is compared to the SIR target. A power control message may then be generated for delivery to the transmitter based on the result of the SIR comparison (e.g., indicating whether transmit power modifications are desirable). A problem with this power control technique (and other similar methods) is that the BLER measurement is relatively slow and, therefore, the SIR target is not able to adapt quickly to changes in channel conditions. As may be appreciated, this may result in poor communication quality after a new base station is selected, until the transmit power may be readjusted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The appended claims are directed to some of the various embodiments of the present invention. However, the detailed description presents a more complete understanding of the present invention when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures and: 
         FIG. 1  illustrates an operational environment in which embodiments of the present invention may be practiced; 
         FIG. 2  is a block diagram of a wireless communication device in accordance with an embodiment of the present invention; 
         FIG. 3  is a block diagram of a SIR target generator in accordance with an embodiment of the present invention; 
         FIG. 4  is a block diagram of a carrier frequency correction unit in accordance with an embodiment of the present invention; 
         FIG. 5  is a flow chart of a communication procedure in accordance with an embodiment of the present invention; and 
         FIG. 6  is an illustration of SIR target management in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description and the drawings illustrate specific embodiments of the invention sufficiently to enable those skilled in the art to practice it. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Examples merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in or substituted for those of others. The scope of the invention encompasses the full ambit of the claims and all available equivalents. 
     The present invention pertains to wireless communications, and in some embodiments, to code division multiple access (CDMA) communications, and other embodiments, to wireless communication devices operating in site selection diversity transmit (SSDT) mode. The present invention also pertains to wideband CDMA (WCDMA) communications.  FIG. 1  illustrates an operational environment in which embodiments of the present invention may be practiced. Operational environment  100  includes mobile station  102  which may communicate with one or more base stations  104  over communication channels  110 . Mobile station  102  may be implemented as any form of communication device or subsystem that may be used within a wireless communication system including, for example, a handheld communicator, a cellular base station transceiver, a satellite uplink, downlink, or crosslink transceiver, a transceiver within a terrestrial wireless link, a local multipoint distribution system (LMDS) or multipoint multichannel distribution system (MMDS) transceiver, a two-way radio, transceivers within wireless local area networks (LANs), metropolitan area networks (MANs), wide area networks (WANs), wireless local loop transceivers, and others. 
     Operational environment  100  may also include one or more reflecting objects  108 , which may cause multipath reflections and frequency selective fading within the spectrum utilized by base stations  104  and mobile station  102 . Operational environment  100  may also include one or more in-band interfering devices (ID)  106  which generate interference within the spectrum utilized by base stations  104  and mobile station  102 . Due to reflecting objects  108  and interfering devices  106 , base stations  104  and mobile station  102  may experience channel fading, multipath components, and interference conditions unique to a particular communication path between a base station and a mobile station. Communication channels  110  may be any form of wireless communication path, and may include, for example, CDMA and wide-band CDMA (WCDMA) communication channels. 
     In accordance with embodiments of the present invention, mobile station  102  receives CDMA signals and communicates with remote base stations  104  in site selection diversity transmit (SSDT) mode. During communications with a primary base station, communication state parameters, such as signal to interference ratio (SIR) targets, may be maintained for base stations of an active set of base stations. The communication state parameters may be used for transferring communications to one of the base stations of the active set. In one embodiment, the communication state parameters may be used for the selection of a base station from the active set. In another embodiment, the communication state parameters of the selected base station may be used for power control immediately after transferring communications. In yet another embodiment, the communication state parameters may include SIR targets and/or carrier frequency estimates, which may be maintained for base stations of the active set during communications with the primary base station. In this embodiment, when transferring communications to a selected base station of the active set, the SIR target and/or carrier frequency correction for the selected base station may be used immediately after transferring communications. 
     In an alternate embodiment, communication state parameters for the base stations of the active set may be calculated when this station is selected for communication. When a base station is selected for transferring communications, the communication state parameters for the selected base station may be updated for subsequent use after transferring communications. 
     The active set of base stations may include any base station that may be considered by a mobile station for transferring communications. The active set of base stations may include any base station of the system from which the mobile station may be capable of communicating with. The base stations of the active set may be determined from power level measurements of associated pilot channels. In accordance with one embodiment of the present invention, a base station of the active set may be selected for transferring communications based on channel state parameters. This is unlike conventional systems, which select base stations for transferring communications solely based on power level measurements. 
     Accordingly, improved wireless communications may be achieved. A base station having better error performance may be selected for transferring communications because maintained SIR targets and/or carrier frequency estimates may be used for base station selection. Improved communications may be achieved more quickly after transferring communications by use of the SIR target maintained for the selected base station. Furthermore, a more accurate power control message based on the maintained SIR measurements may be quickly generated after transferring communications to a selected base station. Furthermore, improved communications may be achieved more quickly after transferring communications because carrier frequency offset may be quickly corrected thereafter. 
     In one embodiment, SIR target approximations may be based on channel conditions, including a number of signal paths and/or speed of the mobile station. In one embodiment, a primary base station may be chosen based at least on signal to noise ratios (SNRs) of the base stations. In one embodiment, a plurality of SIR targets may be maintained (e.g., one target for each of the base stations of the primary set). In an alternate embodiment, a SIR target for a particular base station may be updated when the base station is the primary base station and held constant for base stations that are not the primary base station. The SIR targets may be determined by using one or more channels, such as the dedicated physical data channel (DPDCH), the dedicated physical control channel (DPCCH) and/or a common pilot channel (CPICH) of a WCDMA system. Alternatively, the SIR target for non-primary base stations may be updated using at least in part, a channel such as the DPCCH and/or CPICH channel that may be constantly transmitted by the particular base station. 
       FIG. 2  is a block diagram of a wireless communication device (WCD) in accordance with an embodiment of the present invention. WCD  200  may be suitable for use as mobile station  102  ( FIG. 1 ) although other devices are also suitable. WCD  200  may be used within a communication system implementing various communication techniques, including code division multiple access (CDMA) techniques as well as wide-band CDMA (WCDMA). WCD  200  may be implemented as either a mobile communicator or a base station transceiver within a communication system. When WCD  200  is implemented as a mobile communicator, receive antenna  202  may receive spread spectrum CDMA signals from one or more remote base stations. Despreader  206  despreads one or more of the signals using CDMA despreading techniques. Rake receiver  208  isolates various multipath components associated with a particular base station and may combine the components coherently. Decoder  212  decodes the resulting signal. Cyclic redundancy check (CRC) element  214  may use decoded signal information from decoder  212  to detect and quantify errors (e.g., as percentage of CRC errors) within WCD  200 . Channel estimator  210  may processes the despread information to estimate channel parameters for the corresponding channel. SIR estimator  224  may estimate a SIR of the received signal using channel parameters determined by channel estimator  210 . Any technique, including conventional techniques, for estimating a SIR from channel estimates may be suitable. 
     SIR target generator  216  generates a SIR target based on channel estimates from channel estimator  210  and performance estimates from CRC element  214 . Embodiments of the present invention may use any conventional SIR target generation technique. At least one embodiment of the present invention may use the SIR target generation approach described in U.S. Patent Application entitled “METHOD AND APPARATUS FOR GENERATING A QUALITY MEASURE TARGET VALUE BASED ON CHANNEL CONDITIONS”, filed Dec. 17, 2001, having application Ser. No. 10/021,541, which is assigned to the same assignee as the present application and which is incorporated herein by reference. In accordance with embodiments of the present invention, SIR target generator  216  may also generate active set SIR targets  217  from channel estimates. Active set SIR targets  217  may represent instantaneous SIR targets for base stations of the active set. This is described in more detail below. 
     The SIR target generated by SIR target generator  216  may be compared with the SIR estimate provided by SIR estimator  224  in comparison unit  226 . Message generator  228  may then generate a message, including a power control message, based upon the comparison results. The message may be transmitted to a remote base station using transmitter  230  and transmit antenna  232 . A similar approach may be used when WCD  200  is implemented as a base station, although receive antenna  202  may receive CDMA signals from one or more remote users rather than one or more remote base stations. Receive and transmit antennas  202 ,  232  may be replaced by a single antenna with the addition of duplexing functionality. Receive and transmit antennas  202 ,  232  may be almost any type of antenna suitable for receiving and transmitting frequencies used by WCD  200 . In one embodiment, receive and transmit antennas  202 ,  232  may be comprised of either a single or separate dipole antennas, although other types of antennas are also suitable. 
     In at least one embodiment of the present invention, SIR targets generated for base stations of the active set are used to support a site selection diversity transmit (SSDT) mode of operation. In this mode, a mobile station may select one of a number of different base stations to transmit to the mobile station at a particular time. For example, when the channel from one base station is fading, a mobile station operating in SSDT mode may switch to another base station whose channel may not be fading. As a result of the transition (i.e., transferring communications) from one base station to another, the channel conditions of the new base station may often be significantly different from the channel conditions of the previous base station. If the SIR target of the previous base station were used for receipt of communications from the new base station, a relatively large SIR target error could result. In accordance with embodiments of the present invention, a relatively quick adjustment of the SIR target may be made for communications with the new base station. In one embodiment, a mobile station maintains SIR targets for each candidate base station during normal operation. Thus, an approximate SIR target for a new base station may be available for use at the time the new base station is selected. 
     In the past, mobile stations operating in SSDT mode would typically select a base station based on the total power received from each candidate base. However, the performance of a receiver (e.g., BLER, etc.) may not be a function of receive power alone, but may also be a function of other channel parameters. Disregarding this fact may result in a transition to a base station that has a higher received power, but may result in a lower performance level. Therefore, in at least one embodiment of the invention, a mobile station uses SIR targets that are calculated using estimated channel parameters as part of a base station selection criterion in SSDT mode. 
     CRC unit  214  may estimate almost any type of performance information that may be useful for generating the SIR target for the primary base station by target generator  216 . This may include, for example, block error rate, bit error rate, symbol error rate, and other performance measures. Channel estimator  210  may estimate almost any form of channel information that may be used by SIR target generator  216  to determine SIR targets. This may include, for example, the number of paths in the channel, the path strengths, mobile velocity, path fading rates, symbol energy variances, variances between symbols of different blocks, variance of total block energy, and/or others. In addition to the channel parameters used by SIR target generator  216 , channel estimator  210  may also estimate almost any other channel parameters that may be required by SIR estimator  224  or other functions within WCD  200 . 
     WCD  200  may also include base station selector  218  which may select a base station for transferring communications as part of SSDT mode operations. Base station selector  218  may receive active set SIR targets  217  generated by SIR target generator  216  and/or channel estimates provided by channel estimator  210 . Base station selector  218  may provide base station selection signal  219  for use by SIR target generator  216 , an embodiment of which is described in more detail below. Base station selector  218  may also provide base station selection signal  219  to carrier frequency correction unit  204 . Base station selector  208  may also provide a signal to message generator  228  for use in indicating the selected base station to a system controller. 
     Although WCD  200  is illustrated as having several separate elements, one or more of the elements may be combined and may be implemented by combinations of software-configured elements, such as processors including digital signal processors (DSPs), and/or other hardware elements. Unless specifically stated otherwise, terms such as processing, computing, calculating, determining, displaying, or the like, may refer to an action and/or process of one or more processing or computing systems or similar devices that may manipulate and transform data represented as physical (e.g., electronic) quantities within a processing system&#39;s registers and memory into other data similarly represented as physical quantities within the processing system&#39;s registers or memories, or other such information storage, transmission or display devices. 
       FIG. 3  is a block diagram of a SIR target generator in accordance with an embodiment of the present invention. SIR target generator  300  may be suitable for use as SIR target generator  216  ( FIG. 2 ) although other devices may also be suitable. SIR target generator  300  includes primary base station SIR target generator  302  to generate SIR target value  310  during communications with a primary base station. SIR target generator  300  may also include one or more active set SIR target generators  304  to generate one of the active set SIR targets  308  for one or more base stations when the WCD is communicating with the primary base station. SIR target generators  302  and  304  may receive channel estimates from a channel estimator such as channel estimator  210  ( FIG. 2 ), and in one embodiment, the channel estimator may provide channel estimates for more than one channel. For example, the channel estimator may provide channel estimates for a current channel with the primary base station, as well as channel estimates for channels with other base stations of the active set. In this embodiment, channel estimator  210  ( FIG. 2 ) may include more than one functional channel estimator unit. 
     In one embodiment, a single one of active set SIR target generator  304  may be used. In this embodiment, the single active set SIR target generator  304  may generate SIR targets for the various base stations of the active set in a serial fashion. In an alternate embodiment, one active set SIR target generator  304  may be employed for each base station (or a predetermined number of base stations) of the active set in which SIR targets are to be generated. 
     SIR target generator  300  may include management unit  306 , which receives base station selection signal  312 . Base station selection signal  312  may be provided by base station selector  218  ( FIG. 2 ). In one embodiment, when a base station is selected for transferring communications, management unit  306  provides the SIR target for the selected base station as SIR target value  310 . 
     In one embodiment, active set SIR targets  308  generated by SIR target generators  302  and  304  may be used to select a base station for transferring communications. In this embodiment, base station selector  218  ( FIG. 2 ) may receive active set SIR targets  308  from one or more of SIR target generators  304  and may select a base station for transferring communications using a selection criteria that considers the SIR for the candidate base station. 
       FIG. 6  is an illustration of a SIR target generator in accordance with another embodiment of the present invention. SIR target generator  700  is another alternative suitable for use as SIR target generator  216  ( FIG. 2 ), although other devices may also be suitable. Primary base SIR target generator  702  may use any conventional SIR target generation method to generate SIR target  703 . Primary base SIR target generator  702  may use, for example, CRC and channel estimates  701 . Primary base SIR target generator  702  may also use memory  704  to store the instantaneous SIR target. Each time a new primary base station is selected, management unit  706  may replace the contents of the active memory with data for selected base station data from base station selector  709 . This may be done, for example, in one of two ways. In one embodiment, management unit  706  may simply take the old primary base station data (e.g., from memory  704 ) and may store it in the active base stations memory (e.g., one of memories  708 ), and may take the data that is relevant for the new selected base station (from one of memories  708 ) and store it in the primary base target generator memory (e.g., memory  704 ) so that SIR target generator  702  will now work with the new base station data. 
     In another embodiment, management unit  706  may also perform an update of the data from the active set memory (e.g. one of memories  708 ) once before putting the data in the SIR target generator memory (e.g., memory  704 ). This update may be done to reflect the passing time since the data&#39;s last update and whatever information available for the management unit at the specific time. For example, channel estimates, old primary base SIR target, etc. may be different and can be updated. 
     Accordingly, improved wireless communication may be achieved. A base station having better error performance may be selected for transferring communications because the maintained SIR measurements may be used for base station selection. Furthermore, improved communications may be achieved more quickly after transferring communications because the communication state parameters may be used after transferring communications. 
       FIG. 4  is a block diagram of a carrier frequency correction unit in accordance with an embodiment of the present invention. Carrier frequency correction unit  400  may be suitable for use as carrier frequency correction unit  204  ( FIG. 1 ) although other units may also be suitable. Carrier frequency correction unit  400  provides a correction to a carrier frequency by measuring the carrier frequency in carrier frequency measuring element  406  using an output of channel despreader  404 . In one embodiment, channel despreader  404  may despread a pilot channel, such as a CPICH channel of a CDMA system, although other channels may be suitable for carrier frequency measurements. A carrier frequency estimation is generated by element  408  and may be stored in memory  410 . In one embodiment, carrier frequencies are estimated for base stations of an active set of base stations. In this embodiment, despreader  404  may despread pilot channels for the base stations in a serial or sequential fashion allowing a carrier frequency estimation for base stations in an active set to be stored in memory  410 . In this embodiment, when a base station from the active set is selected for transferring communications, a carrier frequency correction for the selected base station is immediately available allowing for correction of any carrier frequency offset. Accordingly, improved communications may be achieved substantially immediately after transferring communications. As illustrated in  FIG. 4 , other despreaders  412  correspond with despreader  206  ( FIG. 2 ) and are not necessarily part of carrier frequency correction unit  400 . CPICH Despreader  404  may correspond with Despreader  206  ( FIG. 2 ) and is also not necessarily part of carrier frequency correction unit  400 . Antenna  416  may correspond with antenna  202  ( FIG. 2 ) and is also not necessarily part of carrier frequency correction unit  400 . In one embodiment, base station selection signal  419  may be received by unit  400  from base station selector  218  ( FIG. 2 ) to select a carrier frequency correction for a selected base station. 
       FIG. 5  is a flow chart of a communication procedure in accordance with an embodiment of the present invention. Communication procedure  500  may be performed by a wireless communication device, such as WCD  200  ( FIG. 2 ), as part of communication with base stations in an SSDT mode. Communication procedure  500  may also be performed by a wireless communication device, such as WCD  200  ( FIG. 2 ), as part of selecting a base station for transferring communications. Although the individual operations of procedure  500  are illustrated and described as separate operations, one or more of the individual operations may be performed concurrently and nothing requires that the operations be performed in the order illustrated. The various operations of procedure  500  may be implemented by elements of WCD  200  as well as by processing elements and memory not separately illustrated that may be configured with software and/or firmware. 
     In operation  502 , a WCD communicates with a primary base station. The WCD may communicate using CDMA techniques or may communicate in accordance with other conventional communication techniques. In operation  504 , a power control message is generated. In one embodiment, the power control message is based on SIR estimate and SIR target  505  for the primary base station. The power control message may be generated in accordance with techniques discussed above for generating power control messages by WCD  200  ( FIG. 2 ). 
     In operation  506 , during communications with the primary base station of the active set, the WCD may maintain signal to interference ratio (SIR) target values for at least some base stations of the active set. The SIR targets may be generated by SIR target generator  300  ( FIG. 3 ) and may correspond with active set SIR targets  308  ( FIG. 3 ). In operation  508 , carrier frequency estimates may be generated and maintained for at least some base stations of the active set during communications with a primary base station. These carrier frequency estimates may be stored in a memory and may be generated by a carrier frequency estimator such carrier frequency correction unit  400  ( FIG. 4 ). 
     In operation  510 , a base station is selected for transferring communications from the active set of base stations. Operation  510  may use power level measurements  512  of channels from the base stations, SNR measurements, and/or may use active set SIR targets  514  generated in operation  506 . Operation  510  may be performed by a base station selection element, such as base station selector  218  ( FIG. 2 ) and may be performed as part of SSDT mode operations. Operation  510  may include generating a base station selection signal, such as signal  219  ( FIG. 2 ). 
     In operation  516 , communications are transferred to the selected base station. In one embodiment, the SIR target from the selected base station may be used. After transferring communications to a new base station, the carrier frequency may be corrected in operation  518  using the carrier frequency estimations generated in operation  508 . In operation  520 , the SIR target may be corrected. After transferring communications to the new base station, the WCD may communicate with the new primary base station and operations  502  through  520  may be repeated. In one embodiment, a power control message may be based on the SIR target maintained for the selected base station allowing a more accurate power control message to be sent shortly after transferring communications without having to wait for error information to be generated. 
     The foregoing description of specific embodiments reveals the general nature of the invention sufficiently that others can, by applying current knowledge, readily modify and/or adapt it for various applications without departing from the generic concept. Therefore such adaptations and modifications are within the meaning and range of equivalents of the disclosed embodiments. The phraseology or terminology employed herein is for the purpose of description and not of limitation. Accordingly, the invention embraces all such alternatives, modifications, equivalents and variations as fall within the spirit and scope of the appended claims.