Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-07-27
2001-06-12
Legree, Tracy (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S452200
Reexamination Certificate
active
06246881
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns a method of dynamic channel allocation in a mobile communications system.
BACKGROUND OF THE INVENTION
The main structural elements of a known mobile communications system are shown in FIG.
1
. The figure shows a mobile services switching centre (MSC), a base station controller (BSC), a base transceiver station (BTS), a mobile station (MS) and a network management system (NMS). The network also typically comprises several inter-connected mobile services switching centres (MSC), of which only one is shown in the figure for the sake of clarity. The mobile station system is connected to a fixed telephone network, e.g. a public switched telephone network (PSTN), or an integrated services digital network (ISDN), through a mobile services switching centre MSC. Typically, several base station controllers BSC hierarchically under the MSC are connected to each mobile services switching centre MSC. Several base transceiver stations BTS hierarchically under the BSC are typically connected to each base station controller BSC. The base transceiver stations can set up connections with the mobile stations MS by way of channels through a so-called air interface. For the sake of simplicity, the figure shows only one base station controller BSC, one base transceiver station BTS and one mobile station MS. The network management system NMS may be used for controlling the operation of network elements, and e.g. for changing the network configuration.
The air interface between base transceiver stations and mobile stations can be divided into channels in several different ways. Known methods are at least TDM (Time Division Multiplexing), FDM (Frequency Division Multiplexing) and CDM (Code Division Multiplexing). The band available in TDM systems is divided into successive time slots. A certain number of successive time slots forms a periodically repeating time frame. The channel is defined by the time slot available in the time frame. In FDM systems, the channel is defined by the available frequency, while in CDM systems it is defined by the spread code to be used. Combinations of the methods of division mentioned above are also used. E.g. the known GSM system uses FDM/TDM division, whereby frequency and time slot determine the channel.
To obtain sufficient capacity in the limited frequency band of the mobile communications system, the channels in use must be used several times. For this reason, the coverage area of the system is divided into cells. Each cell has its own geographical area or cell area. Each cell has a base transceiver station serving the mobile stations located within the cell area. If channels having the same frequency are reused in cells located too close to each other, the connections using these channels will begin interfering with one another. The channel is caused interference not only by reuse of the own channel but also by adjacent channels used nearby, because adjacent channels are always slightly overlapping to save the frequency band. To maximise the capacity the channels must be reused in cells as close to one another as possible, however, so that the carrier to interference ratio CIR will allow an adequate connection quality. The distance at which the same channel may be reused so that the CIR remains acceptable is called the interference distance, while the distance at which the same channel is reused is called the reuse distance. Since the CIR is a function of reuse distance and transmission power, the carrier to interference ratio may be reduced in a cellular system so as to improve the quality of the connection by increasing the distance between base transceiver stations or by using dynamic control of the transmission power.
In channel allocation the objective is to allocate channels for desired connections, which channels may all be used at the same time while the quality of signals remains acceptable. Methods of channel allocation are at least FCA (Fixed Channel Allocation), DCA (Dynamic Channel Allocation) and HCA (Hybrid Channel Allocation) obtained as a combination of these. The various methods are described quite thoroughly in a publication by I. Katzela, M. Naghshineh: “Channel Assignment Schemes for Cellular Mobile Telecommunication Systems: A Comprehensive Survey”, IEE Personal Communications, June 1996.
In fixed channel allocation, a set of channels is assigned for each cell according to some reuse scheme. Typically, the channels are reused in every 9
th
or in every 12
th
cell. Simplicity is an advantage of this method, but it suffers from an inability to adapt to traffic situations and to changes in the number of mobile subscribers in the cells. In addition, to obtain a good result the method necessarily requires frequency planning based on signal drop-outs which are difficult to predict.
In dynamic channel allocation, all channels are in a joint “channel pool”. Channels are taken dynamically from the pool for use in the cell for new calls or for channel exchanges in the cell as calls arrive in the system. It is ensured at the same time that a minimum CIR ratio is preserved. Thus there is no fixed relation between channels and cells, but any channel can be selected by any cell, provided that the carrier to interference ratio is acceptably low. Advantages of this method are great flexibility and an ability to adapt to changing traffic, but on the other hand it is more inefficient than fixed channel allocation, if the load is very high.
In hybrid channel allocation, the available channels are divided into fixed and dynamic channels, of which the fixed channels are assigned FCA typically for use by certain cells while the dynamic channels are assigned DCA typically for use by all users. The cells always attempt primarily to use their fixed frequencies.
The dynamic channel allocation methods may be divided into those based on measurements of channel carrier to interference ratios and those based on knowledge of the allocation situation.
In the methods based on knowledge of the allocation situation, the carrier to interference ratios caused by operating connections are estimated and such areas around base transceiver stations are defined within which reuse of a channel in use at the base transceiver station or use of channels adjacent to the channel would cause excessive interference. Allocation of the concerned channels is prevented in these areas.
Dynamic channel allocation methods based on measurements of channel carrier to interference ratios define the strength of the channel candidate's existing interfering signal. If the concerned channel were made available to the connection to be set up, this signal existing on the channel would cause interference to the connection. The channel carrier to interference ratio is thus defined by measuring, whereby direct information is obtained about the carrier to interference ratio to be optimised. As the carrier to interference ratio may vary even strongly e.g. due to DTX (Discontinuous Transmission), the signal measurements used for defining the carrier to interference ratio must be averaged to obtain sufficient reliability in practice.
Channel allocation is studied as an example in the situation shown in
FIG. 2
, where two mobile stations MSA and MSB request a channel of nearby base transceiver stations BTSA and BTSB. A channel is requested first by MSA and then a little later by MSB. The figure shows base transceiver stations and cells formed around these. The cells depict an area where mobile stations seek connection with the base transceiver station of the cell.
In the example, the channel allocation method is embodied in base transceiver stations BTS. The channel allocation algorithm used as an example is the MCIR (Maximum Signal to Noise Interference Ratio) method, which is a subclass of the dynamic channel allocation methods based on signal level measurement. In this method such channels are sought on which as good a signal to noise ratio as possible is achieved in the up-link direction from mobile station to base transceiver station. A method of this typ
Parantainen Janne
Salonaho Oscar
Altera Law Group LLC
Gelin Jean A
Legree Tracy
Nokia Telecommunications Oy
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