Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-09-23
2001-08-07
Hunter, Daniel (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S448000, C455S454000
Reexamination Certificate
active
06272348
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the allocation of channels and to equalising of interference suffered by connections in a radio network.
BACKGROUND OF THE INVENTION
In mobile communications systems, mobile stations and base transceiver stations may set up connections through channels of a so-called radio interface. Depending on the type of information to be transferred, demands are made on the connections in regard to faultlessness of transferred data and in regard to transfer lag.
A certain frequency area is always allocated for use by the mobile communications system. To have sufficient capacity in the mobile communications system on this limited frequency band, the channels which are in use must be used several times. For this reason, the coverage area of the system is divided into cells formed by the radio coverage areas of individual base transceiver stations, which is why the systems are often also called cellular radio systems.
FIG. 1
shows the main structural features of a known mobile communications system. The network comprises several inter-connected MSCs (Mobile Services Switching Centre). The mobile services switching centre MSC can set up connections with other mobile services switching centres MSC or with other telecommunication networks, e.g. ISDN (Integrated Services Digital Network), PSTN (Public Switched Telephone Network), Internet, PDN (Packet Data Network), ATM (Asynchronous Transfer Mode) or GPRS (General Packet Radio Service). Several base station controllers BSC are connected to the mobile services switching centre MSC. Base transceiver stations BTS are connected to each base station controller. The base transceiver station may set up connections with mobile stations MS. A network management system NMS may be used for collecting information from the network and for changing the programming of network elements.
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 a TDM system 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 used in the time frame. In FDM systems, the channel is defined by the used frequency, while in CDM systems it is defined by the used frequency hopping pattern or hash code. Combinations of the division methods mentioned above can also be used.
FIG. 2
shows an example of a known FDM/TDM division. In the figure, frequency is on the vertical axis while time is on the horizontal axis. The available frequency spectrum is divided into six frequencies F
1
-F
6
. In addition, the frequency channel formed by each frequency is divided into repeating time frames formed by 16 successive time slots. The channel is always defined by the couple (F, TS) of frequency F and time slot TS used in the time frame.
In order to maximise capacity, channels must be reused in cells which are as close to one another as possible, however, so that the quality of connections using the channels will remain sufficiently good. The connection quality is affected by the sensitivity of transferred information to transfer errors occurring on the radio channel and by the quality of the radio channel. The transfer error tolerance of the signal depends on the characteristics of the transferred information and it can be improved by processing the information with channel coding and interleaving before sending it to the channel and by using retransmission of faulty transfer frames. The Carrier to Interference Ratio (CIR) depicts the radio channel quality which ratio is the ratio between the strengths of the signal sent by the sender and perceived by the recipient on the one hand and of the interference caused to the channel by other connections on the other hand.
FIG. 3
shows the emergence of interference caused to each others by simultaneous connections. In the figure three mobile stations MS
1
, MS
2
and MS
3
communicate with base transceiver stations BTS
1
, BTS
2
and BTS
3
. The signal received by base transceiver station BTS
1
contains a signal S
1
, which is sent by mobile station MS
1
and which is showed by a solid line and the power of which depends on the transmission power used by mobile station MS
1
and on fading on the radio path between mobile station MS
1
and base transceiver station BTS
1
. Typically, the radio path fading is smaller with a shorter distance between base transceiver station and mobile station. In addition to signal S
1
, the signal received by the base transceiver station contains signal components I
21
and I
31
caused by signals sent by mobile stations MS
2
and MS
3
. Components I
21
and I
31
will cause interference in the reception, if they can not be filtered away from the signal received by the base transceiver station. Correspondingly, the signal sent by mobile station MS
1
causes signal components I
12
and I
13
in the signals received by base transceiver stations BTS
2
and BTS
3
and these signal components may cause interference in the receptions. Components of a similar kind also emerge in the signals received by mobile stations from base transceiver stations.
If signal components I
21
and I
31
are on the same channel as signal S
1
, they can not be removed by filtering. Interference may also be caused by signals occurring on other channels than on the same channel. E.g. in systems using FDM frequency division, channels which are adjacent to one another on the frequency level are always slightly overlapping in order to use the frequency spectrum as effectively as possible, which will result in reception interference also from signals which are on the adjacent channel. Correspondingly, when using code division CDM, connections using codes that are too much alike will cause interference to one another. However, so-called neighbour channel interference caused by signals on other channels is considerably smaller than the interference caused by equally powerful signals on the same channel.
The magnitude of interference caused by connections to each other thus depends on the channels used by the connections, on the geographical location of connections and on the transmission power used. These may be influenced through a systematic allocation of channels to different cells taking the interference into account, through transmission power control and through averaging of the interference experienced by the different connections.
It is an objective in channel allocation to allocate such channels to the desired connections which may all be used at the same time while the signal quality remains acceptable. To maximise capacity, channels should be reused as close to one another as possible. The distance at which one and the same channel can be reused so that the CIR remains acceptable, is called the interference distance while the distance at which one and the same channel is reused is called the reuse distance.
Known methods of channel allocation are Fixed Channel Allocation (FCA), Dynamic Channel Allocation (DCA) and Hybrid Channel Allocation (HCA) which is obtained as a combination of FCA and DCA. The idea in fixed channel allocation is to divide the channels used in the system between the cells through a frequency design which is made before the system is put into use. In dynamic channel allocation, all channels are in a common channel pool, from which for the connection to be set up the best channel is chosen for use according to some predetermined norm. In hybrid channel allocation, some of the channels used in the system are divided in a FCA fashion fixedly for use by different cells and the remaining channels are placed in a channel pool, from which they may be taken as required dynamically for use by all cells. The different methods are described very thoroughly in the publication I. Katzela and M. Naghshineh: “Channel Assignment Schemes for Cellular Mobile Tel
Saario Eija
Salonaho Oscar
Altera Law Group LLC
Hunter Daniel
Nguyen Thuan T.
Nokia Telecommunications Oy
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