Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-08-10
2002-11-26
Urban, Edward F. (Department: 2685)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S446000, C455S067150
Reexamination Certificate
active
06487414
ABSTRACT:
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BACKGROUND OF THE INVENTION
This invention relates generally to the field of frequency planning in wireless communication networks and, in particular, to a system and method for improved consideration of the impact of various factors in frequency planning.
In wireless communication systems, such as a cellular mobile radio communication system, the geographic area served by the system is divided into geographically defined cells. In the system, there is a finite number of carrier frequency channels, typically radio-frequency (RF) channels, that are available for use during communications in and between network areas cells. Typically, a frequency group, consisting of a subset of all of the available frequencies, is assigned to each cell for that cell's use during communications.
However, because the number of frequency groups is limited, it is necessary to reuse them within the area served by the system. To provide for a greater coverage by the system, and to provide for greater capacity through higher reuse of frequency groups, the network service area cells may be further divided up into sectors. Because the use of a particular frequency by two different sectors can result in interference during a call or even result in the call being completely cut-off or “dropped”, an effort must be made to assign frequency groups to sectors in a manner that minimizes the amount of interference.
In wireless communication networks, there are two primary types of interference: co-channel interference and neighbor or adjacent channel interference. Co-channel interference is the interference from communication sources tuned to the same frequency as the operating channel. Adjacent-channel interference comes from communication sources using channels near the operating channel in the frequency domain. To achieve the desired voice or data transmission quality, the ratio of the received signal over the combined co-channel and neighbor-channel interference must be above a specified threshold. Such channel interference can be up-link or down-link interference or a combination of these interferences. Down-link interference is channel interference received at regions serviced by a first base station caused from signals transmitted by other base stations. Up-link interference is interference at the base stations caused from signals transmitted by mobile units in regions of the coverage area that are not serviced by that base station.
Furthermore, other factors such as antenna patterns, power levels, scattering, and wave diffraction variations combined with buildings, various other structures, hills, mountains, foliage, and other physical objects contribute to the interference experienced during wireless communications.
In frequency planning for a wireless communication network, the primary task is to try to predict and attempt to reduce the amount of channel interference experienced by strategically assigning certain channels to certain sectors. Typically, this can be achieved by assigning frequencies so that the distance between co-channel and adjacent channel sectors is maximized. In this context, “distance” does not necessarily refer to geographic distance but connotes a distance in the RF sense. That is, although sectors far apart from each other geographically are less likely to “see” each other, they can still interfere with each other. For example, a high sector can interfere with a sector as far as hundreds of miles away. Maximizing this distance decreases the chances of the sectors conflicting with one another in the airwaves. However, a severe consequence of maximizing this distance is that it effectively reduces the amount of channel combinations possible in the network service area, thereby limiting the amount of coverage available for wireless communication. Typically, frequency planning is ordinarily accomplished by three primary techniques including channel sets, reuse patterns and pixel based interference analysis.
Channel sets are non-overlapping subsets of the available channels organized according to a periodic frequency spacing in terms of number of channels between, members of a given set. The principal disadvantage of using channel sets is that the number of channels required from sector to sector usually varies, and optimal frequency planning will require that just that number, rather than the number in an arbitrary set, be assigned to each sector.
In the reuse pattern scheme, the sectors in a network are arranged in a two-dimensional pattern, or “grid”. Channels or, more commonly, channel sets, are then assigned so that co-channel or adjacent channel assignments appear periodically in different sectors. The primary disadvantage of frequency planning based upon a reuse grid is that, within a given network, varying terrain and man-made “clutter”, such as buildings and other structures, will affect the characteristics of radio propagation and attenuation. Therefore, adhering to a fixed and rigid co-channel or adjacent channel spacing on a grid will likely provide inadequate isolation in some cases, resulting in excessive interference, and more than the required isolation in others, thereby reducing reuse efficiency. Furthermore, in addition to less than optimal interference levels, the fixed reuse approach results in much reduced capacity in many parts of the network where frequencies can be added freely due to an RF shield, such as a mountain ridge, but the grid prohibits such an assignment.
In pixel based interference analysis, the entire network service area is divided into a large number of very small “pixels” or “bins”. In one example, each pixel would be a 100 meter square, so that a network service area of 100 kilometers by 100 kilometers would contain 1 million pixels. For each pixel, a system engineer will ascertain the strongest incident signal level from the sectors nearby and then the incident signal levels from each of the other sectors in the network to determine potential interferences. From this information, the system engineer can determine the predicted levels of co-channel or adjacent channel interference that would be present in that pixel if certain sectors were assigned, respectively, the same radio channels as the serving sector or channels adjacent to those in the serving sector.
However, pixel-by-pixel interference analysis also has many significant limitations. While pixel by pixel analysis can predict interference problems that are likely to result from a proposed frequency plan, it does not provide any such plan in the first place, nor does it inherently suggest modifications to a frequency plan that would reduce interference.
Furthermore, there is an inherent limitation on the amount of data that can be presented in pixel by pixel interference analysis. At the same time, pixel by pixel analysis produces an amount of data which is not easily susceptible to human interpretation. Finally, because conventional pixel by pixel interference analysis relies solely on predicted levels, it carries over the inaccuracies in such data as described above and results in erroneous frequency assignments.
Thus, while these existing techniques can provide for some measure of protection and relief from channel interference in the network service area, they still fail to account for the many variables and factors which can affect wireless communications on a day to day basis. Accordingly, it would be desirable to have a system and method for frequency planning within a wireless communication network which accounts for the many variables and factors affecting the quality of wireless communications, reduces the interference experienced during wireless communication, and does not limit the covera
Davidor Yuval
Tanay Amos
Brown & Raysman Millstein Felder & Steiner LLP
Gary Erika A.
Schema Ltd.
Urban Edward F.
LandOfFree
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