Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
2000-11-09
2004-05-11
Hopkin, Denise (Department: 2685)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S127200, C455S522000
Reexamination Certificate
active
06735451
ABSTRACT:
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'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'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 advant
Jansson Johan
Järleholm Anders
Stolt Tomas
Hopkin Denise
Telefonaktlebolaget LM Ericsson (publ)
Vuong Quochien Ba
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