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""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 170a-n connected to a plurality of corresponding antennas 130a-n. The radio base stations 170a-n in conjunction with the antennas 130a-n communicate with a plurality of mobile terminals (e.g., terminals 120a, 120b and 120m) within a plurality of cells 110a-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 180a, 180b and 180c. 
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 xe2x80x9ccell splittingxe2x80x9d 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 B1 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 B2 is added, for example, in sector 1 to split the cell which was previously defined by the transmissions of base station B1. The new base station B2 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 B2 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 xe2x80x9cvirtual cellsxe2x80x9d was developed to overcome this inefficiency. In the virtual cell concept, the base station B2 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 B2 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""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 B2 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 B2). 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 xe2x80x9cModified Downlink Power Control During Macrodiversityxe2x80x9d, 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.
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 (DPC1) 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 (DPC2) 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 DPC2 downlink power control is lower than that associated with DPC1 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.
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