Abstract:
Methods and base stations for controlling downlink power, especially in systems employing virtual cells, are described. By providing attenuators in each transmit signal processing chain, e.g., after upconverting to intermediate frequencies, downlink power control can be more finely tuned to different antenna elements.

Description:
BACKGROUND 
     The present invention relates generally to mobile telecommunication systems, and especially to downlink power control in such systems which employ, for example, virtual cells. 
     The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry&#39;s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices. 
     FIG. 1 illustrates an example of a conventional cellular radio communication system  100 . The radio communication system  100  includes a plurality of radio base stations  170   a-n  connected to a plurality of corresponding antennas  130   a-n . The radio base stations  170   a-n  in conjunction with the antennas  130   a-n  communicate with a plurality of mobile terminals (e.g., terminals  120   a ,  120   b  and  120   m ) within a plurality of cells  110   a-n . Communication from a base station to a mobile terminal is referred to as the downlink, whereas communication from a mobile terminal to the base station is referred to as the uplink. 
     The base stations are connected to a central controller, such as a mobile switching center (MSC)  150 . Among other tasks, the MSC coordinates the activities of the base stations, such as during the handoff of a mobile terminal from one cell to another. The MSC, in turn, can be connected to a public switched telephone network  160 , which services various communication devices  180   a ,  180   b  and  180   c.    
     As more mobile stations subscribe to these types of systems, the demand for system capacity will increase rapidly, especially in highly populated areas. Conventionally, a process known as “cell splitting” was performed in order to enhance the originally developed cellular system to meet demand for increased capacity per unit area. As shown in FIG. 2, a base station B 1  originally has three sector antennas not shown), each antenna supporting communications within a sector, i.e., sectors  1 - 3 . To implement cell splitting, a new base station B 2  is added, for example, in sector  1  to split the cell which was previously defined by the transmissions of base station B 1 . The new base station B 2  also has three sector antennas forming three new sectors A, B and C. In conventional cell splits, the set of frequencies allocated for the base station B 2  is more or less equally distributed for usage in the three new sectors A-C using a fixed allocation. Thus, the central controller, e.g., the MSC, will treat sectors A-C as, effectively, three separate, new cells and re-plan the available frequency band(s) on that basis. Although cell splitting can provide additional system capacity, it requires additional base station sites with associated infrastructure costs. Furthermore, the system (e.g., the MSC) continues to handle handover signaling when a mobile station moves between the cell sectors in a conventional manner. Thus, conventional cell splitting results in a significant increase in the loading of the access network, i.e., the links between base stations and MSCs and its processors, as the addition of more sectors results in more handovers and hence more signaling between the base stations and MSCs. 
     More recently, a concept known as “virtual cells” was developed to overcome this inefficiency. In the virtual cell concept, the base station B 2  can use all of the frequencies allocated thereto arbitrarily in virtual cells A-C. One main difference with the virtual cell implementation as compared to the conventional cell split is that the base station B 2  handles the handoff situation which occurs when the mobile moves between the virtual cells A-C. For example, in a virtual cell network, if a mobile station moves from cell A to cell B, the base station alone may handle the transition of the mobile station from cell A to cell B and neither the MSC nor the mobile need to be involved in a handoff process. Thus, virtual cells reduce the loading on the access network as compared with cell splitting. Furthermore, since the mobile makes no handoff, there is no impact on speech quality. 
     Those skilled in the art will recognize that it is generally desirable to tailor the base station&#39;s transmit (downlink) power for each connection to be only that which is necessary to provide a desired quality of service (QoS) as measured by, for example, a signal-to-noise ratio (SNR) experienced by a mobile station. For instance, in TDMA (time-division, multiple access) systems, downlink power control implies varying the power associated with transmissions to different mobile stations which are receiving signals in each frame. For example, as shown in FIG. 3, it is generally desirable to transmit bits to mobile station  310  (which is relatively close to the base station B 2  positioned at the center of cells A, B and C) at a lower power level than those bits which are transmitted to mobile station  300  (which is more distant from the base station B 2 ). Many examples of specific downlink power control techniques are known to those skilled in the art. For example, International Patent Application, WO 99/01949 discloses a power control apparatus operable in a conventional TDMA communication system. The power control apparatus includes a power level controller coupled to amplifier circuitry of each of a plurality of transmitter branches to control the power levels at which the communication signal bursts are transmitted on a particular carrier frequency by the base station. Another example of downlink power control can be found in U.S. patent application Ser. No. 09/057,793, entitled “Modified Downlink Power Control During Macrodiversity”, filed on Apr. 9, 1998, the disclosure of which is incorporated here by reference. 
     These conventional downlink power control techniques, however, do not provide sufficiently selective power control to optimize downlink interference levels, particularly in systems which employ virtual cells. Accordingly, it would be desirable to provide communication techniques, and systems associated therewith, which would enable communications in systems employing a virtual cell structure and in a manner which was also conducive to enabling greater downlink power control. 
     SUMMARY 
     According to exemplary embodiments of the present invention, methods and apparatus for communicating in a telecommunications network include a processing unit for providing a first level of downlink power control (DPC 1 ) at a baseband level on each of a plurality of carrier frequencies that are selectively supplied to a plurality of transmitters. Each of the transmitters is optionally coupled to a selector for providing the carrier frequencies to the antenna elements. A second processing unit, e.g., a controllable attenuator, is coupled between the selector and each of the antenna elements for providing a second level of downlink power control (DPC 2 ) to improve the efficiency of the system. 
     Base stations and methods for transmitting in radiocommunication systems according to the present invention have a number of different advantages. For example, by implementing a base station configuration which includes attenuators after the transmux, a coarse and fine downlink power control loop combination can be implemented. Moreover, selectors can be eliminated and the attenuators can be used to perform both power control and path selection in the base station. By using only one transmux per carrier frequency, the amount of hardware is minimized. This, in turn, increases the serving capacity of base stations since transmux hardware is typically a limiting factor associated therewith. Additionally, it now becomes possible to use a minimum of power output from the MCPA in time slots where no transmissions are needed. This promotes additional power savings and interference reduction. 
     Moreover, another advantage of base stations and methods according to the present invention involves the fact that the step error associated with DPC 2  downlink power control is lower than that associated with DPC 1  power control. This result stems from the fact that regulation after the transmux is performed at a higher sample rate. In fact, downlink power control at the baseband level can be replaced by downlink power control using just the attenuators downstream of the transmuxes. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The objectives and advantages of the present invention will be understood by reading the following detailed description in conjunction with the drawings, in which: 
     FIG. 1 depicts an exemplary cellular system in which the present invention can be implemented; 
     FIG. 2 is used to describe conventional cell splitting as well as a virtual cell extension of a cellular system in which the present invention can be implemented; 
     FIG. 3 depicts three mobile stations operating in different virtual cells supported by a single base station; 
     FIG. 4 illustrates a conventional base station configuration; 
     FIG. 5 illustrates how the base station of FIG. 4 can be operated in a GSM system to support communications with the mobile stations of FIG. 3; 
     FIG. 6 illustrates a timeslot/frame format according to ANSI  136  specifications; 
     FIGS. 7 and 8 illustrate how the base station of FIG. 4 can be operated in a ANSI  136  system; 
     FIG. 9 illustrates a modified base station configuration related to the present invention; 
     FIGS. 10 and 11 illustrate how the base station of FIG. 8 can be operated in a ANSI  136  system; 
     FIG. 12 illustrates a base station in accordance with an exemplary embodiment of the present invention; and 
     FIG. 13 illustrates how the base station of FIG. 12 can be operated in a ANSI  136  system. 
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, and circuits are omitted so as not to obscure the description of the present invention. 
     The exemplary radio communication systems discussed herein are described as using the TDMA protocol, in which communication between the base station and the mobile terminals is performed over a number of time slots. However, those skilled in the art will appreciate that the concepts disclosed herein may find use in other protocols, including, but not limited to, frequency division multiple access (FDMA), code division multiple access (CDMA), or some hybrid of any of the above protocols. Likewise, some of the exemplary embodiments provide illustrative examples relating to the Global System for communications (GSM) system or the Digital Advanced Mobile Phone Service (ANSI  136 ) system, however, the techniques described herein are equally applicable to radio base stations in any system. 
     Referring now to FIGS. 3-8, a conventional architecture related to the present invention will now be described in order to better understand the problems associated therewith. FIG. 4 shows the architecture of a conventional base station and will be used to provide an example indicating how such a base station can be operated to serve a virtual cell (only the transmission part of the base station is shown) in radiocommunication systems operated in accordance with different standards. The base station  400  includes a base band processing unit  410  and a set of transmuxes  420  which are common to all sectors supported by the base station  400 , as well as one wide band radio part  430  for each sector. The radio part  430  includes digital to analog converters  440 , intermediate frequency (IF) to radio frequency (RF) stage (TX)  450 , and a multi-carrier power amplifier (MCPA)  460 . Each transmux  420  handles one carrier frequency (e.g., f 1  is handled by transmux  1 , f 2  is handled by transmux  2  . . . fn is handled by transmux n). Thus, it will be appreciated that the number of transmuxes limits the number of carriers that a base station  400  can support. Each transmux  420  is equipped with a selector  470  so that the carrier can be switched on or off to the different sectors by a control unit (not shown). Each carrier is then passed through combiners  480  and  490  to an appropriate radio part, i.e., depending upon which carrier frequency f 1 , f 2  . . . fn has been assigned to which virtual cell A-C. Combiners  480 ,  490  are used to add selected, individual channel bandwidths together for transmission. 
     FIG. 5 provides an example illustrating how base station  400  can be operated to transmit to the mobile stations of FIG. 3 in virtual cells in a GSM radio communication system. As will be appreciated by those skilled in the art, in the GSM system, there are eight time slots available on each frame of each carrier frequency (only four of which are shown in FIG. 5.) In this example, three mobiles  300  (MS 1 ),  310  (MS 2 ) and  320  (MS 3 ) are assigned time slots TS 1 , TS 2  and TS 4 , respectively, on the same carrier frequency f 1 . Base station  400  transmits to each mobile station MS 1 , MS 2  and MS 3  using a different transmit power level P 1 , P 2  and P 3 , respectively. Using the selector  470 , carrier frequency f 1  from transmux  1  can be fed through the appropriate path of combiners  480 ,  490  to the radio part associated with the appropriate cell in its assigned time slot. In this example, the base station transmission power can be set individually for each mobile station by the base band processing unit  410 , e.g., using a form of conventional, downlink power control described above, which thereby forms a first mechanism for downlink power control (DPC 1 ). 
     As illustrated in FIGS. 4 and 5, the conventional base station architecture can reuse the carrier frequency f 1  to transmit to different mobile stations located within different virtual cells at different downlink power levels as long as signals used by each mobile station are transmitted solely within an assigned time slot (e.g., the signals for MS 2  are transmitted only during time slot TS 2  and so forth). This permits the base station  400  to have sufficient time to adjust its output transmit power from one time slot to another time slot, i.e., ramping from P 1  to P 2  to P 3 . However, a problem arises when this conventional architecture is implemented, for example, in Digital Advanced Mobile Phone Service (ANSI  136 ) radiocommunication systems. 
     As will be appreciated by those skilled in the art, and as illustrated in FIG. 6, in ANSI  136  systems there are three time slot pairs available on each frame of each carrier frequency for a full rate channel assignment. For example, a mobile station can be assigned TS 1  and TS 4  or TS 2  and TS 5  or TS 3  and TS 6 . As seen in FIG. 6, within each time slot a burst includes a synchronization preamble (SYNC) and a data portion (DATA). In fact, as will be appreciated by those skilled in the art, the DATA portion in each time slot&#39;s burst is further subdivided into various fields specified by the ANIS-136 standard, however these other fields are not particularly relevant for this discussion. The mobile station, which is listening during its assigned time slots in each frame, can then synchronize to the DATA portion of each burst by, for example, performing a series of correlations of the known SYNC pattern to the received burst to locate the beginning of the DATA portion. 
     However, some mobile station manufacturers have decided, in order to improve the accuracy of a mobile station&#39;s time synchronization, to evaluate the SYNC patterns which are transmitted in the time slot after a mobile station&#39;s assigned time slot. That is, a mobile station which is assigned, for example, to time slot pair TS 2  and TS 5  on a particular carrier frequency may also attempt to synchronize to the SYNC field(s) in TS 3  as part of its decoding process for the DATA portion of its TS 2  burst. In this way, if the SYNC field in TS 2  is highly degraded and good synchronization to this field is not possible, a more accurate time synchronization will be achieved using the TS 3  SYNC field and better decoding of the payload data will be possible. Thus, mobile stations for the ANSI  136  system may use not only signals transmitted within its assigned time slot but also part of the time slots after its own assigned time slot in order to synchronize with the network. 
     This characteristic of certain ANSI  136  mobile stations should then be taken into consideration when determining the base station&#39;s downlink transmit power. For example, the base station  400  may need to transmit each carrier frequency with virtually the same power in all cells which have a mobile station allocated to one of that carrier&#39;s time slot pairs. Consider the example illustrated in FIGS. 7 and 8. Therein, three mobile stations  700  (MS 1 ),  710  (MS 2 ) and  720  (MS 3 ) have been assigned the three available time slot pairs, TS  1 / 4 , TS  2 / 5  and TS  3 / 6 , respectively, on the same carrier frequency f 1 . However, MS 3  is located closer to the base station  400  (not shown in this Figure) than MS 1  and MS 2 . Thus, the base station  400  transmits at a relatively high power level P 1  to MS 1  and MS 2 , and at a relatively low power level P 2  to MS 3 . Since MS 2  may be using the SYNC portion transmitted in TS 3 , the base station must continue to transmit at power level P 1  during the SYNC portion of the TS 3  burst so that MS 2  can accurately receive and process the synchronization patterns. Moreover, since the downlink power control mechanism DPC 1  is established for each carrier frequency, the base station  400  is emitting signal energy on carrier f 1  in cells A and B at a relatively high level. This causes additional co-channel interference which is undesirable. 
     One possible solution to provide improved downlink power control while also satisfying mobile stations which use SYNC from multiple time slots is to modify the base station to employ two transmuxes per carrier frequency. FIG. 9 shows base station  900  which includes a base band processing unit  410  and a set of transmuxes  920   a-n . Unlike base station  400 , base station  900  utilizes two transmuxes to handle one carrier frequency (e.g., both transmuxes  920   a  and  920   b  for frequency f 1 ), thus the number of carriers that base station  900  can support is less than base station  400  for the same number of transmuxes. 
     FIGS. 10 and 11 illustrate how base station  900  can provide improved downlink power control albeit at the expense of extra transmuxes. As illustrated in FIG. 10, both mobile stations  1000  (MS 1 ) and  1010  (MS 2 ) are in cell A with MS 1  being located further away from base station  900  than MS 2 . Referring now to FIG. 11, base station  900  uses time slot TS 1  and transmux  920   a  to transmit to MS 1  at transmission power level P 1 . Since MS 1  may need the SYNC portion of the burst that is transmitted in TS 2 , base station  900  continues transmitting the SYNC information in TS 2  at power level P 1  using the second transmux  920   b . After the SYNC portion of TS 2  has been transmitted, base station  900  uses the second transmux  920   b  to transmit information modulated onto carrier frequency f 1  at a lower level P 2 , which power level is more appropriate for MS 2  given its closer proximity to base station  900 . Similarly, base station  900  uses the second transmux  920   b  to transmit the SYNC portion of TS 3  in cell A at power level P 2 , thus enabling MS 2  to use that SYNC portion of TS 3 . After the SYNC portion of TS 3 , base station  900  uses the first transmux  920   a  at power level P 3  and selects radio part  430  associated with cell C. Again, base station  900  continues to transmit the SYNC portion of TS 1  in cell C at power level P 3 , thus enabling MS 3  to use the SYNC portion of TS 1 . At the same time, when it is time to transmit to MS 1  again in TS 4 , base station  900  can set the second transmux  920   b  to power level P 1  and use selector  470  to select the radio part  430  for cell A. 
     Therefore, base station  900  is able to individually control transmission power for each mobile station. However, as noted above, the configuration of base station  800  has a limited capacity due to the use of two transmuxes per carrier frequency. Thus, it is an object of the present invention to efficiently control downlink transmit power in virtual cells without reducing the capacity of the base stations. 
     Referring now to FIG. 12, a base station  1200  according to an exemplary embodiment of the present invention is illustrated. Therein, blocks performing similar functions as in FIG. 4 retain the same reference numerals. Thus, base station  1200  includes a base band processing unit  410  and a set of transmuxes  420  which are common to all sectors supported by base station  1200 , as well as one wide band radio part  430  for each sector. In this embodiment, each transmux handles one carrier frequency. An attenuator  1210   A ,  1210   B  or  1210   C  is used to couple each output of a selector  470  and a corresponding combiner  480 . Each attenuator  1210   A - 1210   C  is individually controllable to set the downlink transmission power level for each mobile station and thereby forms a second mechanism for downlink power control (DPC 2 ) on the carrier frequencies. The attenuation factors/values for each attenuator can be set by downlink power control unit  1230  based upon, for example, signal strength measurements reported by mobile stations. 
     Those skilled in the art will appreciate that, although only three attenuators are depicted in FIG. 12 for clarity of the figure, base stations implemented according to this exemplary embodiment of the present invention will typically have sets of attenuators associated with each transmux  420 . Moreover, depending upon the number of independent antenna elements associated with each base station, each transmux  420  may have more or fewer than three attenuators associated therewith. 
     FIG. 13 illustrates the downlink transmission power levels for each of the mobile stations  1000  (MS 1 ),  1010  (MS 2 ), and  1020  (MS 3 ) which were shown and described with respect to FIG. 10, that are now being supported by the base station  1200 . While the power levels P 1 , P 2  and P 3  in FIG. 13 are the same as those illustrated in FIG. 11, it will be immediately apparent that, unlike FIG. 11, all of the transmissions to mobile stations MS 1 , MS 2  and MS 3  are here performed using only a single transmux (TX 1 )  1220  to communicate with all three of the mobile stations. This is accomplished as follows. First, beginning at time slot TS 1 , base station  1200  transmits to MS 1  by using selector  470  to select the path including attenuator  1210   A  that feeds a signal through the radio part  430  associated with cell A. At the same time, the downlink power control unit  1230  adjusts the setting of attenuator  1210   A  such that the output power level of the signal coupled to the cell A antenna by MCPA  460  is power level P 1 . When it is time to transmit in time slot TS 2 , base station  1200  continues to transmit the SYNC portion of that time slot at power level P 1  by maintaining the setting of attenuator  1210   A . After the SYNC portion of time slot TS 2  has ended, the downlink power control unit  1230  can adjust the setting of attenuator  1210   A  such that a reduced power level P 2  emanates from the cell A antenna. At time slot TS 3 , selector  470  is also operated to select the path including attenuator  1210   C  such that signal energy is also coupled to the cell C antenna. Downlink power control unit  1230  sets the attenuator  1210   C  such that the output power emanating from the cell C antenna is the power level P 3 . Note that, since the base station  1200  should continue to transmit in cell A during the SYNC portion of time slot  3 , the selector  470  also continues to select the path through attenuator  1210   A  during this time period. Thus, the configuration of the base station of FIG. 12 permits reuse of the carrier frequency F 1  for each mobile station at their own tailored downlink power level using only one transmux  1220 . In addition, the base station transmits in the respective virtual cell sector of each mobile station only when needed. In this way, interference associated with the signaling in these virtual cells is decreased as compared with, for example, the operation of base station  400  as depicted in FIG. 8, where, for example, the signals are transmitted in cell B during the entire duration of TS 1  and TS 2  although MS 1  and MS 2  are in cell A. 
     Base stations and methods for transmitting in radiocommunication systems according to the present invention have a number of different advantages. For example, by implementing the configuration illustrated in FIG. 12, a coarse and fine (DPC 1  and DPC 2 ) control loop combination can be implemented. Moreover, the selectors  470  can be eliminated and the attenuators  1210   A  through  1210   C  can be used to perform both power control and path selection in the base station. By using only one transmux per carrier frequency, the amount of hardware is minimized. This, in turn, increases the serving capacity of base stations since transmux hardware is typically a limiting factor associated therewith. Additionally, it now becomes possible to use a minimum of power output from the MCPA in cell sectors and time slots where no transmissions are needed, e.g., the time slots associated with the signal processing chain for cell B in FIG.  13 . This promotes additional power savings and interference reduction. 
     Moreover, another advantage of base stations and methods according to the present invention involves the fact that the step error associated with DPC 2  downlink power control is lower than that associated with DPC 1  power control. This result stems from the fact that regulation after the transmux is performed at a higher sample rate as will be recognized by those skilled in the art. 
     It should be noted that the present invention has been described in accordance with exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person or ordinary skill in the art. All such variations are considered to be within the scope and spirit of the present invention as defined by the following claims.