Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Patent
1997-01-07
1999-01-12
Kizou, Hassan
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
455447, 370337, 375202, H04B 7216, H04J 1306
Patent
active
058598413
DESCRIPTION:
BRIEF SUMMARY
This invention relates to allocation of transmission frequencies in a cellular telecommunications network. In particular, the invention relates to a method and apparatus for dynamically allocating frequencies amongst cells in a cellular network.
An important factor in considering the performance of a cellular telecommunications network relates to the idea of reusing transmission frequencies in separated areas or cells. This cellular concept permits a large subscriber capacity within a limited allocation of a frequency spectrum. For example, only a certain bandwidth within the radio frequency portion of the electromagnetic spectrum is allocated for cellular telecommunications, and each communications carrier requires some fraction of the total available bandwidth. Consequently, only a limited number of carriers can coexist in a single area due to the limited allocation of bandwidth.
An important consideration in cellular communications systems is co-channel interference, which refers to interference between signals from different cells which may be transmitting on the same carrier frequency. Co-channel interference is a function of the distance between the interfering cells and the transmission power and direction of transmission from those cells. Clearly, if every cell in a cellular network were to use all available carrier frequencies then a large amount of co-channel interference would result, particularly between signals from adjacent cells.
In order to alleviate the problem of co-channel interference, a cellular network can be arranged into clusters of adjoining cells, such that the cells in a particular cluster each transmit on different carrier frequencies. Each cell can be allocated a subset of carrier frequencies from the total number of available frequencies. The cells in a single cluster may therefore collectively utilise all of the available frequencies, but since the adjoining cells within the cluster each use different carrier frequencies they do not interfere with each other. Furthermore, taking into account the spatial relationship between adjacent clusters, the allocation of frequencies within a cluster can be arranged so that the frequencies in a cell of one cluster which is adjacent to a cell of another cluster do not interfere.
In addition to co-channel interference, another factor which influences the quality of signal transmission are the propagation conditions for the radio frequency signals. In particular, propagation conditions and phenomena which arise therefrom, such as multipath fading, are dependent upon the transmission frequency, and vary substantially from one carrier frequency to another. A technique known as frequency hopping can be used to reduce the effects of frequency dependent phenomena, by periodically changing the transmission frequency in a particular cell. For example, if a cell has an allocation of three carrier frequencies then the transmitting base station can periodically switch between frequencies during transmission of a particular signal. Therefore, if one of the carriers is subject to poor propagation conditions during transmission of the signal then the signal quality is only affected one third of the time on average. In digital cellular communication systems, frequency hopping facilitates signal processing and diminishes the error probability after error correction decoding. It has been found that the signal quality which can be achieved increases if more frequencies are available for frequency hopping. Unfortunately, the total number of frequencies available in a cluster, and therefore in the network as a whole, is limited and the number of frequencies allocated to a given cell is only a fraction of this amount.
In accordance with the present invention there is provided a cellular telecommunications method for use in a cellular telecommunications network having at least one cluster of adjoining cells, said cells being allocated different transmission frequencies, said method including swapping the different frequencies amongst the cells.
Preferably at an
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Engellenner Thomas J.
Kizou Hassan
Phunkulh Bob A.
Telstra Corporation Limited
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