METHOD FOR IMPROVING INTER CELL INTERFERENCE CANCELLATION IN AN CELLULAR RADIO COMMUNICATION SYSTEM AND CORRESPONDING BASE STATION AND SUBSCRIBER STATION

Abstract

The present invention relates to a method for improving inter cell interference cancellation in an cellular radio communication system, said cellular radio communication system sending OFDM frames on an air interface, each OFDM frame comprising a pilot tones part and a payload part comprising a plurality of bursts According to the present invention the method comprises the steps of: building a predefined number of sets of cells in said cellular radio communication network; associating a predefined OFDM symbol of said OFDM frames to each set of cells, at least two different sets of cells being associated two different OFDM symbols of said OFDM frame, sending from a station belonging to a predefined set of cells a set of pilot tones in said predefined OFDM symbol associated to said set of cells.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment given by the way of non-limiting illustrations, and from the accompanying drawings, in which:

FIG. 1 shows a OFDM frame structure able to support a method according to the present invention;

FIG. 2 shows a cellular network cell structure in which the method according to the present invention is used;

FIG. 3 shows a base station implementing the method according to the present invention;

FIG. 4 shows a subscriber station implementing the method according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a OFDM frame structure able to support a method according to the present invention. This OFDM frame structure is used in the framework of a multi channel communication network employing OFDMA as technology on each sub-channel. OFMDA allows resource allocations in the frequency domain and in the time domain, where orthogonality of the resource elements in frequency domain, the so-called sub-carriers, allows for a narrow spacing of the latter ones and thus and efficient usage of the frequency resource. A burst which is destined to an end user comprises consequently a frequency extension and a time extension. In this context, a frame comprises bursts which themselves are constituted of OFDM symbols sent on the different sub-channels.

In this embodiment of the invention, the radio communication system is a TDD (Time Division Duplex) system so that first a downlink frame is sent followed by an uplink frame. A person skilled in the art would nevertheless have no difficulty to map the present invention on other type of radio communication networks using FDD (Frequency Division Duplex).

A frame FR to be transferred in a radio communication network is comprising a guard interval GI between the down-link and the up-link part as well as in the uplink part of it, a part where no active interference cancellation NIRZ is performed on, a part for pilot tones PT and a plurality of bursts UL Burst#1, . . . , UL Burst#6 on which active interference cancellation is performed.

Each burst comprises information related to one end user. The preamble comprise pilot tones which are not dedicated to interference cancellation. It will be clear for those skilled in the art that the invention only concerns the pilot tones part of the frame dedicated for interference cancellation but not the payload part, the guard interval, or the part for which no active interference cancellation is performed, the latter part comprising possibly payload data, so that the invention may apply even if one or several of these parts are missing or containing other type of information.

The present invention will be illustrated using FIG. 2 which shows a cellular network cell structure in which the method according to the present invention is used, OFDM symbols (symbol #n, #n+1, #n+2) are provided for allocating dedicated pilot tones sent by the different stations which send on a corresponding sub channel in the up-link direction, where it will be clear for those skilled in the art that the method could be equivalently applied in the downlink direction for purpose of a basic training of the interference cancellation algorithm.

According to the present invention, the cellular communication network comprises cells which are allocated to a predefined number of cell sets: Cell set #1, . . . , Cell set #3. As shown in FIG. 2 the cells may be hexagonal and separated in 3 cell sets. It will nevertheless be clear for a person skilled in the art that any other shape of the cells and any other number of cell sets may be used to apply the present invention.

Further according to the present invention, subscriber stations which are located in a cell belonging to cell set 1 will be expected to send their set of pilot tones in OFDM frame symbol #n on the sub channels it has been allocated. For example, on sub channel SC#1 and SC#2, the pilot tones PT1 comprised in the OFDM symbol #n are pilot tones from the subscriber station transmitting UL burst#1 and located in cell set #1.

Similarly subscriber stations which are located in a cell belonging to cell set 2 will be expected to send their set of pilot tones in OFDM frame symbol #n+1 on the sub channels it has been allocated. For example, on sub channel SC#3 and SC#4, pilot tones PT1 comprised in the OFDM symbol #n1 are the pilot tones from the subscriber station transmitting UL burst#2 and located in cell set #2.

Finally subscriber stations which are located in a cell belonging to cell set 3 will be expected to send their set of pilot tones in OFDM frame symbol #n+2 on the sub channels it has been allocated.

Consequently, and to generalise the example shown on FIG. 2, each cell set is associated to an OFDM symbol of the pilot tones part PT in the OFDM frame, and at least two different cell sets are associated to two different OFDM symbols of the pilot tones part PT.

It will be clear for a person skilled in the art that the number of OFDM symbols provided for allocations of dedicated pilot tones for training of inter-cell interference cancellation can be chosen arbitrarily nevertheless it is preferable to limit this number to 3 OFDM symbols in order not to increase the overhead in the system and to comply with the requirement of IEEE 802.16e mandating a minimum length of the sounding zones of 3 OFDM symbols.

Consequently, pilot tones sent by several subscriber stations belonging to the same cell set will overlap since they are sent in the same OFDM symbol. Nevertheless, the ability of the base station receiving the overlapping pilot tones to distinguish them is increased since the reuse scheme is chosen so as to create overlap, preferably, for not contiguous cells using the fact that the interference between not contiguous cells is lower than between contiguous cells.

In a preferred embodiment of the present invention, as many OFDM symbols are foreseen in the pilot tones zone PT as cell sets. Nevertheless this is not a mandatory requirement for the present invention.

In a further preferred embodiment of the present invention, and as shown on OFDM symbol #n+2 only a subset of the set of pilot tones sent by a subscriber station is transmitted on the pilot tones symbols. This enables it to multiplex on the corresponding pilot tones symbols subsets of pilot tones belonging to different subscriber stations. This presents the advantage that the subsets of pilots tones do not interfere with each others even for subscriber stations belonging to the same cell set.

Consequently, a trade off may be found to still reduce the interference of pilot tones of subscriber stations belonging to the same cell set by preferably sending non overlapping subsets of the pilot tones in the associated OFDM symbol.

FIG. 3 shows a base station implementing the method according to the present invention. A base station according to the present invention comprises a module 31 for being aware of a predefined set of cells it belongs to. Module 31 is linked to module 32 for being aware of a predefined OFDM symbol (#n, . . . , #n+2) of the pilot tones part PT associated to the set of cells the base station belongs to. Module 32 is linked to module 33 for sending from said base station a set of pilot tones in said predefined OFDM symbol. Module 33 is itself linked to a Radio Frequency module 34.

According to the present invention, the base station knows in which OFDM symbols of the pilot tones part it is expected to send its pilot tones in the downlink direction. This knowledge is obtained at modules 31, 32 and either fixed as system parameter or received via signalling from a network central entity.

Accordingly module 33 selects the OFDM symbol on the appropriate sub channels on which the pilot tones of the base station are to be sent.

FIG. 4 shows a subscriber station implementing the method according to the present invention. A subscriber station according to the present invention comprises a module 41 for being aware of a predefined set of cells it belongs to. Module 41 is linked to module 42 for being aware of a predefined OFDM symbol (#n, . . . , #n+2) of the pilot tones part PT associated to the set of cells the base station belongs to. Module 42 is linked to module 43 for sending from said base station a set of pilot tones in said predefined OFDM symbol. Module 43 is itself linked to a Radio Frequency module 44.

According to the present invention, the subscriber station knows in which OFDM symbols of the pilot tones part it is expected to send its pilot tones in the uplink direction. This knowledge is obtained at modules 41, 42 and either fixed as system parameter or received via signalling from a network central entity.

Accordingly, module 43 selects the OFDM symbol of the appropriate sub channels on which the pilot tones of the subscriber station are to be sent.

Claims

1/ A method for improving inter cell interference cancellation in an cellular radio communication system, said cellular radio communication system sending OFDM frames on an air interface, each OFDM frame comprising a pilot tones part and a payload part comprising a plurality of bursts, said method comprising the steps of: Building a predefined number of sets of cells in said cellular radio communication network;Associating a predefined OFDM symbol of said pilot tones part to each set of cells, at least two different sets of cells being associated two different OFDM symbols of said pilot tones part;Sending from a station belonging to a predefined set of cells a set of pilot tones in said predefined OFDM symbol associated to said set of cells.

2/ The method according to claim 1, wherein said sets of cells are obtained by applying a frequency reuse scheme over the cells of said cellular radio communication network, the frequency reuse scheme depending on the number of sets of cells.

3/ The method according to claim 1, wherein stations belonging to different sets of cells and scheduled on the same sub channel send sets of pilots tones on two different OFDM symbols of said OFDM frame.

4/ The method according to claim 1, wherein only a subset of said set of pilot tones is sent on said OFDM symbol associated to said set of cells.

5/ The method according to claim 1, wherein a whole or a subset of several sets of pilot tones sent by stations scheduled on the same sub channel and belonging to the same set of cells are multiplexed in the OFDM symbol associated to said set of cells.

6/ The method according to claim 1, wherein said sets of pilot tones are comprised in a 3-OFDM-symbol-broad sounding zone on each of said sub channels, a frequency reuse 3 scheme being applied to the cells of said cellular radio communication network.

7/ The method according to claim 1, wherein said radio communication system is using Orthogonal Frequency Division Multiple Access technology on the air interface, each frame having a time and frequency extension.

8/ A base station adapted to be used in a cellular radio communication system, said cellular radio communication system exchanging OFDM frames on an air interface, each OFDM frame comprising a pilot tones part and a payload part comprising a plurality of bursts, said base station comprising: Means for being aware of a predefined set of cells it belongs to, said cellular radio communication network being split in several sets of cells;Means for being aware of a predefined OFDM symbol of said pilot tones part associated to said predefined set of cells;Means for sending from said base station a set of pilot tones in said predefined OFDM symbol.

9/ The base station according to claim 8, adapted to send bursts in the downlink direction.

10/ A subscriber station adapted to be used in a cellular radio communication system, said cellular radio communication system exchanging OFDM frames on an air interface, each OFDM frame comprising a pilot tones part and a payload part comprising a plurality of bursts, said subscriber station comprising: Means for being aware of a predefined set of cells it belongs to, said cellular radio communication network being splitted in several sets of cells;Means for being aware of a predefined OFDM symbol of said pilot tones part associated to said predefined set of cellsMeans for sending from said base station a set of pilot tones in said predefined OFDM symbol.

11/ The subscriber station according to claim 10, adapted to send bursts in the uplink direction.