WO2005018186A1

WO2005018186A1 - Sub-carrier allocation method for reducing inter-cell interference in ofdm cellular environment

Info

Publication number: WO2005018186A1
Application number: PCT/KR2004/002059
Authority: WO
Inventors: Dong-Wook Roh; Joon-Kui Ahn
Original assignee: Lg Electronics Inc.
Priority date: 2003-08-18
Filing date: 2004-08-17
Publication date: 2005-02-24
Also published as: EP1623549A1; JP2006522503A; KR20050019293A; US20050047259A1; KR100965338B1; CN1759582B; JP4553893B2; EP1623549A4; US7379741B2; CN1759582A

Abstract

A sub-carrier allocation method including selecting a number of cells having astrongest inter-cell interference among cells in a multiple-cell environment,and selectingmutually exclusive sub-carrier sets for each cell selected having the strongest interference.

Description

Description SUB-CARRIER ALLOCATION METHOD FOR REDUCING INTER-CELL INTERFERENCE IN OFDM CELLULAR ENVIRONMENT Technical Field

[1] The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) method, and more particularlyto a sub-carrier allocation method for reducing inter-cell interference in an OFDM cellular environment. Background Art

[2] Currentmobile communication standardization technologies includean AMPS (Advanced Mobile Phone system) andWCDMA (Wideband Code Division Multiple Access) system. In these communication systems, multiplexing is used to support multiple users by constructing multiple communication paths (channels) to transmit and receive an independent signal. In more detail, multiplexing dividesone line or transmissionpath in to multiple channels (fora fixed line, a pair of cables, and for awireless service, a pair of transceivers).

[3] Examples of multiplexing methods include aFDM (Frequency Division Multiplexing) method in which one line is divided into multiple frequency bands and then multiplexed, and a TDM (Time Division Multiplexing) methodin which one line is divided into very short time intervals and then multiplexed.

[4] AMPS, which is a first generation analog mobile communication standard, uses FDM. A second generation mobile communication system is calledIS-95, and uses a CDM (Code Division Multiplexing) method. A third generation mobile communication system is called WCDMA (Wideband Code Division Multiple Access) and uses a Code Division Multiplexing (CDM) method.

[5] Another type of multiplexing is called the Orthogonal Frequency Division Multiplexing (OFDM). OFDM is based on a principle of multicarrier modulation, which means dividing a data stream into several bit streams (subchannels), each of which has a much lower bit rate than the parent data stream. These substreams are then modulated using codes that are orthogonal to each other. Because of their orthogonality, the subcarriers can be very close to each other (or even partly overlapping) in the frequency spectrum without interfering with each other. Further, because the symbol times on these low bit rate channels are long, there is generally no intersymbol interference (ISI). The result is a very spectrum efficient system. [6] Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB) are based on OFDM. However, OFDM is not used in a cellular communication system including multiple cells. Disclosure of Invention Technical Problem [7] Accordingly, one orject of the present invention is to at least address the above and other noted objects. [8] Anotherorject of the present invention is to provide a novel sub-carrier allocation method that reducesinter-cell interference in an OFDM cellular environment. Technical Solution [9] To achieve at least the above objects in whole or in parts, the present invention provides a novel sub-carrier allocation method including selecting a number of cells having astrongest inter-cell interference among cells in a multiple-cell en- vironment,and selectingmutually exclusive sub-carrier sets for each cell selected having the strongest interference. [10] Additional advantages, orjects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The orjects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. Description of Drawings [11] Figure 1 is an overview illustratingsub-carriers used inOFDM;

[12] Figure 2 is an overview illustratinga multi-cell structure in a mobile communication environment; [13] Figure 3 is an overview illustrating a central target cell and neighboring cells having thestrongest interference in a cell structure using an omni-directional antenna; [14] Figure 4 is an overview illustratingsector constructions whenusing anomni-di- rectional antenna and directional antennas; [15] Figure 5 is an overview illustratinginterference among sectors in a 120 ° -sector structure; [16] Figure 6 is an overview illustratinga sector making the strongest interference to the target sector shown inFigure 5; [17] Figure 7 is an overview illustrating interference among sectors in a 60 ° -sector structure; [18] Figure 8 is an overview illustrating a sector making the strongest interference to the target sector shown inFigure 7;

[19] Figure 9 is a flow chart illustrating the allocation of sub-carriers to each cell in a system;

[20] Figure 10 is an overview illustrating the allocationof mutually exclusive sub-carrier subsets whenusing the omni-directional antenna;

[21] Figure 11 is an overview illustrating the allocationofmutually exclusive sub-carrier subsets in case of using al20 ° -directional antenna;

[22] Figure 12 is an overview illustrating the allocationofmutually exclusive sub-carrier subsets whenusing a60 ° -directional antenna; and

[23] Figure 13 is a flow chart illustrating a method of allocatingsub-carriers subsetsfor each terminal in each cell. Mode for Invention

[24] Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, the present invention will be described.

[25] Example of methodsfor reducing inter-cell interference in an OFDM based system includea FH (Frequency Hopping) method and a DCA (Dynamic Channel Allocation) method. In the FH method, the(transmission) order of sub-carriers is arbitrarily changed accordingto time in a cell. Toreduce inter-cell interference, the order of change of sub-carriersis selected not tooverlap in a cell with sub-carries in astrongest cell. To accomplish this, the FH method reserves severalsub-carriers (i.e., does not use all of the available sub-carriers). Thus, the FH method is not used in a cellular environment including multiple cells, but rather is used only in a single cell environment.

[26] In the DCA method, a strength of an Signal to Interference Noise Ratio (SINR)of each sub-carrier of each user is reported in one cell and a signal is transmitted using sub-carriers having a highestSINR, namely, a good channel state, to thereby reduce data transmissionpower and interference. However, in the DCA method,a feedback signal is required to inform the base station aboutthe SINR from each terminal (user). This feedback process is very complicated and produces more interference.

[27] Turning now to Figure 1, whichillustrates sub-carriers applied in the OFDM method. As shown, the multiple sub-carriers are in a mutually orthogonal relation and thusdo not affect each other even if frequencycomponents of the sub-carriers overlap with each other. Further, because the sub-carries can overlap with each other, more sub-carriers can be multiplexed. In addition, OFDM advantageously allowsserially/ parallelyconverted coding data to be allocated to each sub-carrier and digital- modulated. Thus, thegeneration of many sub-carriers improvesa transmission speed per bandwidth.

[28] Turning next to Figure 2, which illustrates a multi-cell environment. In this en- vironment,the OFDM transmission method is applied in each cell and the same frequency band is allocated to each cell. Further, a terminal located in the central cell is interfered by neighboring cells. In Figure 2,the six cells in contact with the central cell are called a first ring of cells, and the twelve cells surrounding the first ring iscalled a second ring of cells. Thus, in this example, a terminal operating inthe central cell is interfered by the six cells of the first ring and the twelve cells of thesecond ring.

[29] Further, the interference caused by first ring of cells is larger than the interference caused by the second ring of cells, because a strength of propagation is reduced the farther away a cell is from the central cell. The following expression defines the propagation strength:

[30] P P *t - - " d^* (1) [31] whereP and P indicate a transmission power and reception power, respectively, TX RX and 'd'isa distance betweena transmitter and a receiver. The value of 'n'differs depending on a channel model, and is usually n=3or 4. [32] Thus, with reference to the above equation, the power transmitted from acell changes based ondistance between the transmitter and receiver. Assuming thetransmission power of every cell is uniform as P, 'n'=4 and a distance between centers of cells is 'd', the strength of interference coming from the cells of the first ring can be expressed by the following equation:

[33] *W^« (2) [34] Further, the strength of interference coming from the cells of the second ring can be expressed as follows:

[35]

(3) [36] Note that with reference to equations (2) and (3),theinterference from the second ring of cellsis 1/16 of the interference from the first ring of cells. Thus, the first ring of cells produces the greatest amount of inter-cell interference.

[37] The aboveexampleonly considersthe distance between the transmitter and receiver, and does not considera long-term fading effect such as a log-normal fading effector a short term fading effectsuch as Rayleigh or Rician effect, which occursin an actual mobile communication environment. However, even if the fading effect is considered, the same result exists (i.e., the first ring of cells produce the greatest interference to the central cell). Thus, in this example, the interference cause by the second ring of cells is considered to be negligible. Note that Figure 3 illustrates the multi-cell environment when only the interference cause by the first ring of cells is considered.

[38] Further, the cell structure shown inFigures 2 and 3 assumean omni-directional antenna is used. However, in other cell arrangements, directional antennas are used. When directional antennas are used,each cell is divided into sectors. Directional antennas include, for example, a 120 ° directional antenna and a 60 ° directional antenna. Figure 4 illustrates the different cell structures for the omni-directional antenna, a 120 ° directional antenna and a 60 ° directional antenna, respectively. As shown, the omni-directional antenna produce one sector, the 120 ° directional antenna produces three sectors and the 60 ° directional antenna produces six sectors.

[39] Further, assuming the sectors do not interfere with each other,each sector in one cell can use a single frequency band. Thus, the frequency can be more effectively used. For example, the frequency efficiency isincreased three times when usingthe 120 ° directional antenna producing three sectorsand six times when using the60 ° directional antenna producing six sectors.

[40] However, sectors in another cell interfere with sectors of a target cell, as shown in Figure 5, for example (Figure 5 illustrates the concept of three sectors for a 120 ° directional antenna). The arrows in Figure 5 depictcenters of the transmissiondirection of the directional antennas. Further, as shown in Figure 5, the targetsector is assumed to be thelower right lower sector of the central cell and is shaded withchecks, and the six sectors in other adjacent cells interfering with the target sector are shaded withslant lines.

[41] Further, the interference at the cell boundary region is problematic,because thetransmission power is inversely proportionalto the distance as noted above in equation (1). Thus, the reception power (signal strength) from a targetcell is very low at the cell boundary region, and therefore a signal at the boundary is easilyaffected witheven with a small amount of interference.

[42] Thus, as shown in Figure 5, for theright lower sector of the central (target) cell,the two sectors nearest to the boundary region of thetarget sector cause the largest amount ofinterference to the target sector. Figure 6 illustrates this concept as well. Similarly, Figures7 and 8 illustrate theinterference effect for a target cell when a 60 ° directional antenna is used.

[43] Turning now the flowchart in Figure 9, which illustratea method according to the present invention to preventsub-carriers transmittingdata of neighboring cells from overlapping to thereby reduce inter-cell interference generated in a forward link whenOFDM is applied to a multi-cell environment.

[44] When OFDM is applied to a forward link of a mobile communication environment, sub-carriers are allocated to multiple terminals (users) and aremultiplexed to provide a user service. Further, the sub-carriers allocated to each terminal (user) are applied with a different transmission power according to a channel state of each sub-carrier. Namely, if a signal is to be transmitted through sub-carriers having a bad channel state, the signal istransmitted with a high power to compensate for the bad channel state. Thus, a certain BER (Bit Error Rate) or an FER (Frame Error Rate), for example,required for a system ismaintained to satisfy a QOS (Quality of Service) required for a service.

[45] As shown in Figure 9, a radio network controller (RNC)using the OFDM determines whattype of an antenna is used in themulti-cell structure (step S10) and then selectsthetarget cell and the other cells having the largestinterference effect on the target cell (step SI 1). Once thecells are selected, the RNC determinesthe different sub- carriers of the overall frequency band to be used (step S12). Further,the sub-carrier subsets are selected so they do not overlap with each other. Sub-carrier subsets are explained below.

[46] First, the omni-directional antenna case in Figure 3 is considered. In this example, the seven cellsincluding the central cell (referred to as the'target cell'hereinafter) and the adjacent six cells causinginterference to the target cell are considered. In addition, each cell is assumed to use the same frequencyband.

[47] Whenusing anomni-directional antenna, seven mutually exclusive sub-carrier subsets are selected. Figure 10 illustrates this concept. That is, as shown in Figure 10, seven mutually exclusive sub-carrier subsets are selected. Note, however, the mutually exclusive sub-carrier subsets donot have to be selected based on physically adjacent sub-carriers. For example, the sub-carrier subsets for the cell 2 and 3 in Figure 10, may be switched.

[48] Once the seven sub-carrier subsets are selected, the RNC allocates the seven mutually exclusive sub-carrier subsets to the seven selected cells in turn or arbitrarily (step S13). Namely, theRNC allocates the mutually exclusive sub-carrier sub- setsamong the entire sub-carrier setsto each cell.

[49] Next, Figure 11 illustrates the selection of sub-carrier subsets when a 120 ° - directional antenna is used. In this example and as shown in Figure 11, the RNC allocates mutually exclusive sub-carrier subsets to the three sectors such that they donot overlap with each other

[50] Figure 12 illustrates the selection of sub-carrier subsets when a 60 ° -directional antenna is used. In this example and as shown in Figure 12, the RNC allocates mutually exclusive sub-carrier subsets to the twosectors such that they donot overlap with each other.

[51] Turning next to Figure 13, which is a flowchart illustrating the allocation ofsub- carriers subset for each terminal in each cell according to another example of the present invention. In this example,when sub-carrier subsets are allocated to each cell (or sector) by the RNC, the RNCin communication with the base stations of the selected cells determinesthe total number of sub-carriers required in each cell to servethe requesting users (step S20).

[52] Then, the RNC determines foreach cell (or sector) whether or notthe total number of sub-carriers needed exceedsthe number of elements of the mutually exclusive sub- carrier subsets (step S21). If the total number of requested sub-carriers exceeds the number of the available mutuallyexclusive sub-carrier subsets (Yes in Step S21), the RNC calculates for each cell (or sector) the transmission power requested by each terminal (step S22), allocates sub-carriers within the mutually exclusive sub-carrier subsets with a preference for terminals requesting a high power, and then allocates sub-carriers other than the sub-carrier subset to other remaining terminals requesting less power(step S23).

[53] Further, in another example, the RNC calculates foreach cell (sector) the transmission power that each terminal requests and allocatesthe mutually exclusive sub-carrier subsets with a preference toterminals beginningwith terminalsrequesting a transmission power greater than a threshold value set for each cell.

[54] If, however, the total number of requested sub-carriers is smaller than the number of elements of the sub-carrier subsets allocatedto each cell (or sector) (No in step S21), the RNCallocates mutually exclusive sub-carrier subsets to every terminal within themutually exclusive sub-carrier subsets (step S24).

[55] Thus, in the example shown in Figure 13,if the total number of requested sub- carriers exceeds the number of availablemutually sub-carrier subsets, thesub-carriers areallocated in consideration of the transmission power requested by each terminal (user)to further remove inter-cell interference. Namely, if a transmission power request value of each terminal exceeds a predetermined threshold value, theterminal is allocated within the mutually exclusive sub-carrier subsets, whereas aterminal having a transmission power request value not exceeding the threshold value is allocated outside the mutually exclusive sub-carrier subsets. Thus, sub-carriers having a high transmission power exceeding the predetermined threshold value do not overlap with each other among cells (sectors). Namely, becausesub-carriers requesting a high transmission power do not overlap with each other, the influence of interference isreduced.

[56] For example, consider when a 120 ° directional antenna is used as shown in Figures 7, 8 and 12. In this example, there are two sectors to be considered (e.g., that have the strongest interference effect). In the example of Figure 9, terminals in sector #1 are assigned sub-carriers in subset #1 and terminals in sector #2 are assigned sub-carriers in subset #2 (see Figure 12). The subsets #1 and #2 are mutually exclusive. In the example shown in Figure 13, the total number of sub-carriers requested is first determined. For example, assume subset #1 corresponding to sector #1 can provide 100 sub-carriers. If, however, 200 sub-carriers are actually requested from terminals in sector #1 (which is Yes in step S13 of Figure 13), the requested transmission power of each terminal is calculated. Because there are 200 sub-carriers requested, which exceeds the total number of available sub-carriers, 100 sub-carriers within subset #1 are allocated to terminal having a highest transmission power, and the other terminals will be assigned sub-carriers from subset #2. The same method can be applied to the terminals in sector #2. Thus, the terminals transmitting from sectors # 1 and #2 with the highest power (and are thus more likely to cause interference than terminals transmitting with a lower power) will be mutually exclusive and interfere with each other less.

[57] As so far described, the sub-carrier allocation method for reducing an inter-cell interference in an OFDM cellular environment has the following advantages.

[58] Becauseneighboring cells (or sectors) in a forward link of a cellular OFDM system transmit data by preferentially carrying the dataon mutually exclusive sub-carriers, theregions of cells where data transmission sub-carriers overlap are reduced and thus inter-cell interference isreduced.

[59] In addition, mutually exclusive sub-carriers are preferentially allocated to terminals that are positioned near the cell boundaries and thus request ahighertransmission power, so thatinter-cell interference isreduced.

[60] This invention may be conveniently implemented using a conventional general purpose digital computer or microprocessor programmed according to the teachings of the present specification, as well be apparent to those skilled in the computer art. Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art. The invention may also be implemented by the preparation of application specific integrated circuits or by interconnecting an appropriate network of conventional component circuits, as will be readily apparent to those skilled in the art.

[61] The present invention includes a computer program product which is a storage medium including instructions which can be used to program a computer to perform a process of the invention. The storage medium can include, but is not limited to, any type of disk including floppy disks, optical discs, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPR Ms, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions.

[62] The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.

Claims

[1] A sub-carrier allocation method, comprising: selecting a number of cells having astrongest inter-cell interference among cells in a multiple-cell environment; and selectingmutually exclusive sub-carrier sets for each cell selected having the strongest interference.

[2] The method of claim 1, further comprising: allocating the selectedsub-carrier subsets to each cell.

[3] The method of claim 1, wherein selecting the number of cells having the strongest inter-cell interference is performed by a Radio Network Controller (RNC).

[4] The method of claim 1, further comprising: determining what type of antenna is being used in the multiple-cell environment.

[5] The method of claim 1, wherein if the type of antenna used is determined to be an omni-directional antenna, the number of cells selected having the strongest inter-cell interference isseven.

[6] The method of claim 1, wherein if the type of antenna used is determined to be a 120 ° -directional antenna, the number of cells selected having the strongest inter-cell interference is three.

[7] The method of claim 1, wherein if the type of antenna used is determined to be a 60 ° -directional antenna, the number of cells selected having thestrongest inter- cell interference istwo.

[8] The method of claim 1, further comprising: obtaining atotal number of sub-carriers requested by terminals in a cell; comparing the obtained total number of sub-carriers with atotal number of available sub-carriers in a first sub-carrier subset; preferentiallyallocating the available sub-carriersof the first sub-carrier subsetto as many of the terminalsin thecell as possible; and allocating sub-carriers of a second sub-carrier subset to any remaining terminals that were not preferentially allocatedin the first sub-carrier subset.

[9] The method of claim 8, wherein preferentially allocating the available sub- carriers comprises: allocating the available sub-carriers of the first sub-carrier subset based on how much transmission power is requested by the terminals, if the total number of requested sub-carriers is greater than the total number of available sub-carriers in the first sub-carrier subset.

[10] The method of claim 9, wherein allocating the available sub-carriers comprises: allocating the available sub-carriers of the first sub-carrier subset to terminals requesting a higher transmission power, and allocating the sub-carriers in the second sub-carrier subset toany remaining requesting terminals not allocated sub-carriers in the first sub-carrier subset.

[11] The method of claim 8, wherein preferentially allocating the available sub- carriers comprises: allocating the available sub-carriers of the first sub-carrier subset to all of the requesting terminals, if the total number of requested sub-carriers is smaller than the total number of available sub-carriers in the first sub-carrier subset.

[12] The method of claim 8, wherein preferentially allocating the available sub- carriers comprises: allocatingmutually exclusive sub-carrier subsets to terminals beginning with terminalsrequesting a transmission power that isgreater than a predetermined threshold.

[13] A sub-carrier allocation method, comprising: preferentiallyallocating the available sub-carriersof the first sub-carrier subsetto as many of the terminalsin thecell as possible.

[14] The method of claim 13 , further comprising: allocating sub-carriers of a second sub-carrier subset to any remaining terminals that were not preferentially allocatedin the first sub-carriersubset.

[15] The method of claim 13, further comprising: selecting a number of cells having astrongest inter-cell interference among cells in a multiple-cell environment; and selectingmutually exclusive sub-carrier sets for each cell selected having the strongest interference.

[16] The method of claim 13, further comprising: allocating the selectedsub-carrier subsets to each cell.

[17] The method of claim 13, wherein selecting the number of cells having the strongest inter-cell interference is performed by a Radio Network Controller (RNC).

[18] The method of claim 13, further comprising: determining what type of antenna is being used in the multiple-cell environment.

[19] The method of claim 13, wherein if the type of antenna used is determined to be an omni-directional antenna, the number of cells selected having the strongest inter-cell interference isseven.

[20] The method of claim 13, wherein if the type of antenna used is determined to be a 120 ° -directional antenna, the number of cells selected having the strongest inter-cell interference is three.

[21] The method of claim 13, wherein if the type of antenna used is determined to be a 60 ° -directional antenna, the number of cells selected having thestrongest inter-cell interference istwo.

[22] The method of claim 13, wherein preferentially allocating the available sub- carriers comprises: allocating the available sub-carriers of the first sub-carrier subset based on how much transmission power is requested by the terminals, if the total number of requested sub-carriers is greater than the total number of available sub-carriers in the first sub-carrier subset.

[23] The method of claim 22, wherein allocating the available sub-carriers comprises: allocating the available sub-carriers of the first sub-carrier subset to terminals requesting a higher transmission power, and allocating the sub-carriers in the second sub-carrier subset toany remaining requesting terminals not allocated sub-carriers in the first sub-carrier subset.

[24] The method of claim 13, wherein preferentially allocating the available sub- carriers comprises: allocating the available sub-carriers of the first sub-carrier subset to all of the requesting terminals, if the total number of requested sub-carriers is smaller than the total number of available sub-carriers in the first sub-carrier subset.

[25] The method of claim 13, wherein preferentially allocating the available sub- carriers comprises: allocatingmutually exclusive sub-carrier subsets to terminals beginning with terminalsrequesting a transmission power that isgreater than a predetermined threshold.