Patent Application
20070270151
An example implementation of this invention described below includes the recognition that proximity between a picocell base station unit and an edge of a macrocell or a macrocell base station has a direct relationship to a desired transmit power used by the picocell base station unit. In a disclosed example, the transmit power of the picocell base station unit is set dependent on the position of the picocell base station unit within the macrocell. When a picocell base station unit is closer to the macrocell base station (e.g., the center or “heart” of the macrocell), the transmit power of the picocell base station unit is set relatively high. When the picocell base station unit is close to an edge of the macrocell (e.g., relatively far from the macrocell base station), the picocell base station unit transmit power is set relatively low. Such transmit power settings avoid interference and allow for the picocell and the macrocell to each include the desired coverage in the areas where they overlap and use the same channel frequency.
Within the macrocell 24 a mobile station 26 is capable of conducting wireless communications by communicating with the base station 22 in a known manner. Within the macrocell 24 there is a picocell base station unit (PCBSU) 30 that provides wireless communication coverage within a picocell 32. As can be appreciated from the illustration, the size of the coverage area of the picocell 32 is much smaller than that of the macrocell 24. In this example, the picocell 32 is contained entirely within the macrocell 24.
Another PCBSU 34 provides coverage within a corresponding picocell 36. In this example, the picocell 36 is at least partially within the macrocell 24.
As can be appreciated from the illustration, the PCBSU 30 is relatively closer to the base station 22 or the center of the macrocell 24 compared to the PCBSU 34. The PCBSU 34 is much closer in proximity to an edge of the macrocell 24 compared to the PCBSU 30.
One example embodiment of this invention includes the realization that the position of the picocell base station units within the macrocell 24 has a direct impact on a desired transmit power used by each picocell base station unit to avoid co-channel downlink interference.
For example, if the transmit power of the PCBSU 30 is set too low, a mobile station 38 within the picocell 32 will not be able to effectively receive downlink signals from the PCBSU 30 because of interference from downlink signals from the base station 22. At the same time, the PCBSU 34 may use a transmit power that is too high such that it would interfere with downlink communications between the base station 22 and a mobile station in the vicinity of the PCBSU 34 because the relative signal strength of the transmissions from the base station 22 near the edge of the macrocell 24 is lower. It is not possible, therefore, to set the PCBSU transmit power for every PCBSU within a macrocell to be the same. Instead, the disclosed example includes setting the transmit power of the PCBSUs based upon a position of the PCBSU within the macrocell.
The flowchart diagram 40 in
At 42, the PCBSU is initialized when it is installed or powered up in the desired location. At 44, a downlink interference level is determined somewhere within the picocell. In one example, the PCBSU is equipped with a downlink receiver that can perform at least basic receiver functions of known mobile stations. At appropriate times, such as gaps in broadcast or data transmissions from the PCBSU, the PCBSU makes measurements of the downlink transmissions originating from nearby macrocell base stations. Given the relatively small size of the picocell, it is expected that the levels of interference measurements at the PCBSU will be similar to those experienced by mobile stations within the picocell.
As indicated at 46 in
As shown at 48 in
One example includes determining a threshold interference level or range of interference levels that indicate when the transmission power of the PCBSU should be adjusted. Given this description, those skilled in the art will be able to determine what an appropriate interference level is to achieve acceptable picocell performance and acceptable macrocell performance, given their particular situation. Once such a threshold level or range of levels has been set, a PCBSU can be appropriately programmed to determine when the transmit power of that PCBSU should be adjusted up or down depending on the relationship between the determined interference level and the predetermined threshold level or range of levels.
In one example, appropriate transmit levels for PCBSUs are set based on empirical data collected within a macrocell. The appropriate adjustments or settings in one example are pre-programmed into the PCBSU. In another example, a macrocell base station broadcasts information from which a PCBSU can determine an appropriate setting to correspond to a current interference level.
As schematically shown in
In one example, the PCBSU does not make its own interference level determinations. In this example, instead, the PCBSU signals at least one mobile station within the corresponding picocell to make downlink interference measurements. The measurements made by the mobile station are reported to the PCBSU, which then uses them for making an adjustment to the transmit power of the PCBSU.
In one example, the identity of a macrocell base station from which downlink interference is determined is signaled to the PCBSU by a network over a wire line backhaul. For example, the scrambling code or PN offset associated with a macrocell base station can provide an indication of the macrocell base station so that the PCBSU knows which base station the downlink interference is being measured from. This can be useful in embodiments, for example, where a picocell BSU may detect downlink signals from more than one macrocell base station.
One unique feature of the disclosed example is that a PCBSU may make its own downlink interference measurement or gather such information from a mobile station within the corresponding picocell regarding downlink interference originating from nearby macrocell base station and use that determination for automatically adapting or adjusting the PCBSU transmit power. The transmit power for the PCBSU broadcast and data transmissions may be controlled independently or together, depending on the needs of a particular situation.
One way in which the disclosed example is different from previous power control mechanisms in wireless communication systems is that with the disclosed example, the power control is based upon interference measurement at the transmitter (e.g., the PCBSU). Traditionally, power control mechanisms have included adjustments at a transmitter based upon measurement reports and control commands from a receiver located remotely from the transmitter. This has been due to the fact that interference environment at a receiver location typically can be substantially different from that at the transmitter. In the picocell type arrangement described above, however, power adjustments at the transmitter are based on measurements of interference made at the transmitter or in very close proximity to the transmitter.
The adaptive transmit power control technique of the disclosed example facilitates effective communications within picocells within a macrocell. Downlink interference within the picocell and in the macrocell can be minimized or eliminated when employing the techniques of the disclosed example embodiment.
The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this invention. The scope of legal protection given to this invention can only be determined by studying the following claims.
