Telecommunications networks

Details Telecommunications networks Telecommunications networks

2000-11-16

2004-06-22

Gary, Erika (Department: 2682)

Telecommunications

Radiotelephone system

Zoned or cellular telephone system

C455S509000, C455S446000, C455S447000, C455S512000, C370S328000, C370S337000, C370S347000

Reexamination Certificate

active

06754499

ABSTRACT:

This application claims priority under 35 U.S.C. §§ 119 and/or 365 to 9927219.7 filed in Great Britain on Nov. 17, 1999; the entire content of which is hereby incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to telecommunications networks and in particular to cellular mobile telecommunications networks.
BACKGROUND OF THE INVENTION
FIG. 1
of the accompanying drawings illustrates a cellular mobile telecommunications network which defines a number of cells C
1
. . . C
8
, each of which has a basestation BS
1
. . . BS
8
. Mobile stations MS
1
. . . MS
3
are able to roam through the cells. In the simplest case, a mobile station will communicate with the basestation of the cell in which it is located. When more than one mobile station wishes to communicate with a basestation, the telecommunications system must allocate available radio frequency spectrum to the various mobile stations.
The amount of radio frequency spectrum is a finite resource and many cellular operators do not have as large an allocation as they would wish. There are two basic approaches for allocating channels to mobile stations to give high capacity. One approach is to use a high number of small cells, and the other is to provide a lower number of higher capacity cells. The former approach tends to be more costly to implement than the latter because of the fixed costs associated with the provision of a large number of cells.
The present very high (and still increasing) penetration of mobile phone usage therefore makes increasing the efficient use of radio frequency spectrum a critical and commercial factor for both cellular radio system manufacturers and operators. Higher spectral efficiency allows denser frequency reuse patterns, and thus allows a frequency limited operator to deploy larger (and thus fewer) cells.
To some extent, all radio channels in a cellular system interfere with each other, both in the frequency and time domains. It is therefore desirable to attempt to allocate channels such that interference between channels is at least predictable.
This can be realised by trying to allocate radio channels which have relatively low levels of interference from existing connection allocations and conversely which will not present unacceptable levels of interference to such existing connections that were allocated based on an interference situation predating the current allocation.
An efficient allocation scheme is one where the carrier/interference ratio is approximately equal for the radio links towards all active mobile stations, said carrier to interference ratio being as small as practicable compatible with the demand that the signal shall be carried with acceptable fidelity. Since different mobiles are located in different physical places that have correspondingly different propagation characteristics, these radio links typically have corresponding different values for signal strength.
The problem of channel allocation is to find an algorithm that gives this mapping of connections onto spectrum such that all parts of the available spectrum have as equal as possible carrier to interference ratio. Specifically this means that no part of the available spectrum shall be (globally) unused while other parts carry multiple connections that mutually interfere: rather the used channels shall be spread out as evenly as possible in the time/frequency space, in which the mean variance of carrier/interference (C/I) ratios are minimised.
A well known and understood multiple access scheme is the Time Division Multiple Access (TDMA) scheme in which mobile station channels are distributed in time. A brief description of a known TDMA system is given below, and more detailed information can be found, for example, in “The GSM system for mobile communications” by Mouly & Pautet, ISBN 2-9507190-0-7.
In this description, channel allocation is described in the context of a TDMA cellular system constructed according to the GSM recommendations, since this constitutes the most widely used cellular standard at this time. However one skilled in the art will readily appreciate that the principles described can equally be applied to cellular systems built to other TDMA standards.
In the TDMA system defined in the GSM recommendations, signals are transmitted in a sequence of frames, each of which comprises 8 timeslots. A frame is illustrated in FIG.
2
. The timeslots are conventionally numbered
0
to
7
. Each timeslot is a window in time that may be used to carry a burst of radio energy as a layer
1
carrier for one basic physical channel (BPC).
It is possible (and in fact is normally the case) that a cell will have several transceivers and thus can support several frames concurrently. Two frames are illustrated in FIG.
3
. In a GSM system, concurrent frames within one cell must be synchronised with one another. However, it is normally the case that frames in different cells are not synchronised.
When a new connection is established in a cellular radio system (and in certain other circumstances) the system must select a channel (BPC) from those which are currently available. This function is called “channel allocation”.
In an efficient system, the channel allocation method may be complex, since the objective is that connections will always have undisturbed radio links, and each radio link allocated presents interference towards other channels in other cells. The method may consider many factors in its attempt to solve for a mapping of all currently used radio links on the available spectrum that gives the best overall interference level on each individual channel.
Typically there are many candidate channels available from which one must be selected to be allocated to the requested new connection.
Many strategies are used and/or have been proposed and are known to those skilled in the art. Some deterministically select channels based on the characteristics of the mobile station involved, e.g. its propagation conditions, distance from the base station, if it is moving etc. These mechanisms typically reduce the number of “favoured” candidates, but typically still do not always result in just one remaining candidate channel.
Typically there are many candidate channels available from which one must be selected to be allocated to the new connection in question.
Current systems monitor candidate (ie currently idle) channels to determine the instantaneous amount of interference present on each, and thereby select the channel with the least interference for allocation.
This “idle channel measurement based allocation” scheme has several deficiencies. Each channel in a given cell potentially suffers interference from channels in surrounding cells which use the same frequencies at the same time.
These mutually interfering connections are established, handed over and terminated independently. The result is that over period of minutes (the duration of a typical call made with a mobile phone), the interference levels on each channel will repeatedly change as new calls are set up and old calls end.
If power control is in use, there will be further variations in the interference levels found on the different channels.
The result is that a channel which appears to be the least disturbed at the instant when it is selected and allocated typically will not remain the least disturbed channel in the cell until the end of the connection.
During busy periods in the capacity scenario, the interference distribution across the candidate channels will change almost continuously, that is to say, will be unstable. Channels allocated based on the instantaneous quality at allocation time will not maintain any systematic advantage over the mean channel quality over any useful time period.
In non-synchronous TDMA networks, it is statistically improbable that one timeslot in one cell interferes neatly with only one timeslot in surrounding cells. This is due primarily to the different cells involved not being synchronised, but can be further complicated by propagation delays in some scenarios, as is well understood.
This means th

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