Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1997-11-24
2001-04-17
Nguyen, Lee (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S447000, C455S449000, C455S450000, C455S453000, C455S509000
Reexamination Certificate
active
06219541
ABSTRACT:
BACKGROUND
I. Field of the Invention
This invention relates to cellular radio systems, and in particular to channel-assignment arrangements for such systems.
II. Related Art and Other Considerations
In cellular radio systems a number of base stations are provided which, between them, provide radio coverage for the area to be served by the system. A mobile radio unit within the service area can make radio contact with the base station having the strongest signal, which is usually the nearest. As the mobile unit travels through the service area, the mobile unit may get further from this base station so that the signal strength on this first radio link deteriorates. Cellular systems provide arrangements whereby communication can then be established with a second base station, the link with the first being relinquished. This process is known as “handover”. Each mobile unit requires a separate channel to be assigned in order to communicate with a base station. A channel is assigned to the link between the mobile unit and the base station when communication is to be set up, the channel being selected from chose allocated for use by the base station and available for setting up a link (i.e. not currently assigned to a link. The different channels may be frequencies, time slots within a frequency, or some other division of the spectrum.
In order to avoid co-channel interference, particularly at boundaries between cells, each cell must have available to it a channel or group of channels which is different from those allocated to its neighbours. Considerable research has gone into optimising re-use patterns of these channels, to determine how close together two cells using the same channels can be without causing significant co-channel interference, thereby allowing the most efficient use of the spectrum. However, a major problem is that different cells have different traffic levels at different times of day, not always on a predictable basis, and the capacity requirements have to be based on the busiest period of each cell, even when these peak times do not coincide. This can result in channels being idle in one cell despite there being heavy demand in nearby cells.
To take a particular example, a cell serving a transport centre such as a railway station is likely to be at its busiest during the peak travel hours. The number of channels which need to be allocated to that cell in order to achieve an acceptable call success rate is determined by the call traffic density at the peak time. None of these channels can be re-used in any other neighbouring cell (i.e. one not necessarily adjacent to the first cell, but close enough for co-channel interference to be a possibility). However, a neighbouring cell may have a peak traffic density at another time of day. This must also be given a channel allocation sufficiently great to handle its peak density. The result is an inefficient use of the available channels, as at any time of day there is spare capacity in one or other of the cells.
Dynamic reconfiguration of the basic channel allocation scheme in neighbouring cells, in order to match changes in demand, is liable to have knock-on effects on further cells unless strictly controlled. Any such reconfiguration must also take place promptly to react to sudden surges in demand. Regular shifts in demand could be handled automatically by switching channels from one cell to another at predetermined times of day but this can only cope with predictable shifts in demand. Moreover, if a block of channels is shifted at a set time, any calls operating on a channel when it is shifted could be lost.
It is known from PCT patent application number WO91/01073, in the name of Telecom Securicor Cellular Radio Ltd, to provide a cellular radio system using time division multiple access in which each sector of a cell structure is subdivided into two sub-sectors. When communication is set up between a mobile unit and the base station the required time slot is transmitted only within the sub-sector in which the mobile unit is located. This reduces the amount of power required and reduces the possibility of co-channel interference since a more highly directional antenna can be used. The receive antenna at the base station can also be made more directional with similar benefits.
This prior art arrangement reduces the likelihood of co-channel interference and reduces the transmitter power required at the base station. However, it does not increase the overall capacity of any sector.
Because it assigns individual time slots it is also limited to a situation where the base stations are co-located, since synchronisation would otherwise be a problem.
In areas in which for reasons of topography and/or radio traffic density the basic cell structure would not provide an adequate service, it has been proposed to provide microcells. A microcell covers a smaller area than that covered by a typical macrocell of the basic cell structure. Consequently microcell base stations can be of lower power than the base stations of macrocells. Nevertheless, there is a problem in selecting channels for use by the microcell which do not interfere with any channels in use in nearby cells.
SUMMARY
The present invention seeks to provide extra capacity for one cell of a network with the minimum of disturbance to the overall channel allocation scheme and without reducing the maximum capacity of the neighbouring cells. The channels to be assigned may be frequencies, time slots on a given frequency, or some other division of the available spectrum. In this system each cell retains its maximum capacity, but it can be used by another cell when not needed by the first cell.
According to a first aspect of the invention, there is provided a method of assigning channels in a mobile radio system having a plurality of base stations, each base station serving a cell of the system, and wherein a first pool of channels is allocated to a first base station serving a first cell which contains a second, smaller, cell served by a second base station, and wherein a second pool of channels is allocated to a group of base stations including the second base station and, the method comprising controlling the group of base stations such that the channels of the second pool can each be assigned to only one of the group of base stations at any one time.
According to a second aspect of the invention, there is provided a mobile radio network comprising a plurality of base stations, wherein a group of the base stations serving at least a first macrocell and a microcell contained within a macrocell each have means to establish radio communication with mobile radio units operating within their respective cells, including means of selecting for such communication any of a pool of channels, available for use by all members of the group of base stations, which is not currently in use.
According to a third aspect of the invention, there is provided a controller for controlling the plurality of base stations of a network of the kind referred to above comprising means for identifying that a first base station requires a channel for radio communication with a mobile unit, means for identifying which of a plurality of channels are currently available for use by the base station, means for allocating to the first base station one of the available channels, and means for identifying a channel currently in use by the first base station as not being currently available for use by any of the other base stations.
According to a fourth aspect of the invention, there is provided a method for assigning channels for a mobile radio network characterised in that at least one of the base stations of the network has available to it a plurality of pools of channels each shared with a different cooperating base station or group of cooperating stations, the channel assignment system being arranged such that the first base station is allocated a channel from the common pool of channels having the greatest number of available channels. This arrangement ensures no cell has no channels available when anot
British Telecommunications public limited company
Nguyen Lee
Nixon & Vanderhye P.C.
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