Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-09-03
2001-07-10
Vu, Huy D. (Department: 2664)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S348000, C455S450000
Reexamination Certificate
active
06259685
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the allocation of channels of a cellular radio network in systems using channels having a time frame structure.
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. A certain frequency area is always allocated for use by the 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 maximize capacity, channels must be reused in cells which are as close to one another as possible. Reuse of channels is limited by the interference caused to one another by the connections in the network.
FIG. 3
shows the emergence of interference caused to each other 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 fades 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 are not 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 neighbor channel interference caused by signals on other channels is considerably smaller than the interference caused by equally powerful signals on the same channel. The interference may also be affected e.g. by using frequency or time slot hopping. In frequency hopping, the frequency used by the connection is frequently changed, whereby the interference caused to one another by connections will be averaged. In time slot hopping again the time slot used in the connection is frequently changed. When using frequency or time slot hopping, the individual connection will not suffer an interference which is considerably worse than for others, but all connections will suffer interference of the same level.
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 and through transmission power control taking the interference into account.
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. The invention to be presented in this application relates to but is not limited to fixed channel allocation FCA, wherein the required number of channels is allocated in advance to each cell with the aid of so-called frequency planning. In frequency planning it is ensured that the connections operating on channels allocated in different cells will not interfere excessively with each other. For interference control, the base transceiver station in each cell is given a maximum limit for the allowed transmission power. The distance at which one and the same channel can be reused so that the CIR (C/I, Carrier to Interference Ratio) remains acceptable, is called the interference distance while the distance at which one and the same channel is reused is called the reuse distance.
The same frequencies are reused according to a so-called reuse pattern. When using a channel structure with FDM/TDM division, typical reuse pattern sizes are 7, 9 and 12 cells, in other words, such patterns where the same frequencies are reused in every ninth or in every 12
th
cell.
FIG. 4
shows an example of a reuse pattern the size of which is 9 cells. In
FIG. 4
the frequencies are divided into 9 classes
1
-
9
. One frequency class shown beside the cell in the figure is allocated for use by each cell. Only those channels may be used in the cell which belong to the frequency class allocated for use by the cell.
The reuse pattern can be made denser e.g. by using directed antennas or by reducing the demand made on the CIR ratio of the FDM/TDM signal. The carrier to interference ratio CIR demanded of the network can be lowered e.g. by improving the spectrum characteristics of the signal by using frequency hopping or a hash code of the CDM type
Rinne Mika
Rinne Mikko
Salonaho Oscar
Altera Law Group LLC
Harper Kevin C.
Nokia Telecommunications Oy
Vu Huy D.
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