IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 4, NO. 5, SEPTEMBER 2005 2095

Dynamic Channel Partitioning With Flexible Channel Combinationfor TDMA-Based Cellular SystemsMing Yang and Peter H. J. Chong, Member, IEEE

Abstract—In this paper, we propose flexible dynamic channelpartitioning (FDCP) with a flexible channel-combination schemeto support multiple services. CP is based on the idea that differentservices may require different signal-to-interference ratios (SIRs),and thus, different reuse factors. In FDCP, different services areallocated to the channels depending on the reuse factors theyrequire. FDCP tries to minimize the effect of the assigned channelson the channel availability to the interfering cells and to reducethe overall reuse distances of the systems.

Index Terms—Cellular networks, channel partitioning (CP),communication networks, dynamic channel allocation, multipleservices.

I. INTRODUCTION

IN CELLULAR mobile communication systems, efficientlyallocating the limited-spectrum radio resources and, at the

same time, providing good quality of service (QoS) to varioustypes of services, such as voice, data, and video, is currently amajor research area. Various studies have been done to caterto these different QoS requirements [1]–[4]. In [1] and [2],high-rate data are split into two or more parts and transmittedindependently through different channels; while in [3], differentkinds of networks are built to support different types of ser-vices, with each network supporting certain kinds of services.In all these papers, a single reuse factor is used to support thesemultiple services.

In multiple traffic cellular systems, it has been observed thatdifferent services may require different bit error rates (BERs).For example, packetized voice can tolerate BERs of the orderof 10−2 without a noticeable degradation in service quality,whereas a BER of 10−5 is ordinarily acceptable for uncodeddata [5]. In a time-division multiple-access (TDMA)-basedcellular system, given a fixed channel coding and modulationmethod, different BER requirements depend on different signal-to-interference ratios (SIRs). This means that for data services,we need a higher SIR in order to achieve good QoS. In con-trast, for speech services, a lower SIR could satisfy theirrequirement. From the basic cellular concept, we also knowthat different reuse factors can provide different SIRs. However,if a single reuse factor is assumed to be used in the system, thelargest one among these reuse factors will normally be usedto support the multiple services in order to meet the cochannel

Manuscript received March 19, 2004; revised August 8, 2004 and October23, 2004; accepted October 23, 2004. The editor coordinating the review of thispaper and approving it for publication is P. Chong.

The authors are with the School of Electrical and Electronic En-gineering, Nanyang Technological University, Singapore 638798 (e-mail:mingyang@pmail.ntu.edu.sg; ehjchong@ntu.edu.sg).

Digital Object Identifier 10.1109/TWC.2005.853889

interference constraints. Thus, using a single (largest) reuse fac-tor to support multiple services may result in wasting of avail-able radio resources. In such a case, we should use large reusefactors to support high-SIR-required services, and use smallreuse factors to support low-SIR-required services. Thus, thecapacity of the system can be improved. And this idea can berealized by a channel partitioning (CP) scheme [4] applied toTDMA-based cellular systems.

This CP, combined with a network-based dynamic channel-assignment (DCA) scheme, the so-called dynamic channelpartitioning with interference information (DCP-WI), is firstintroduced and analyzed in [4]. Network-based DCA schemes[6] use predefined rules to control interference and allocatechannels to users by keeping the minimum reuse distances. InDCP-WI, a portion of channels will be assigned permanentlyto certain types of service. However, the optimum channelcombination depends on the traffic-load ratio among services.Therefore, it cannot flexibly cater to the traffic-load varia-tion among services and cells. In this paper, we introduce anew dynamic CP with a flexible channel-combination schemecalled flexible dynamic CP (FDCP). In this scheme, all thechannels are open to any incoming call as long as the allocationsatisfies the cochannel interference constraints based on thereuse-factor requirement. This means that we do not need thepreallocation of channels for each service as required in [4].FDCP attempts to derive a TDMA-based dynamic channel-allocation scheme that is flexible enough to share radio re-sources among different services. Our simulation results showthat the proposed FDCP scheme can provide better performanceas compared to a conventional fixed channel-assignment (FCA)scheme, a network-based DCA-WI [7] scheme, and the opti-mum channel combination of DCP-WI.

II. CELL MODEL

In the design of the cellular system, 49 hexagonal cells, asshown in Fig. 1, are assumed to be the service area. To avoidboundary effect, we used the “wrap-around” technique. In thispaper, two types of services, service type 1 and type 2, withreuse factors of 4 and 7, respectively, are considered to besupported. In practice, the choice of reuse factor for supportingmore services is based on their SIR requirements. For servicetype 1, the first two tiers of the cells, except some cells withthe same separation as the minimum reuse distance, are theinterfering cells. As shown in Fig. 1, twelve cells surroundedby the solid line, i.e., cell 12, 17, 18, . . . , 38, are the interferingcells of cell 25. For service type 2, all the 18 cells in the firsttwo tiers are the interfering cells of cell 25.

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2096 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 4, NO. 5, SEPTEMBER 2005

Fig. 1. Cell model of the system.

TABLE IIIT OF CELL 25

III. INTERFERENCE INFORMATION TABLE

In FDCP, the allocating and releasing of the channels arebased on the interference information table (IIT). Each cell hasone IIT that contains enough information about the channelstatus of its own and all the surrounding interfering cells.One example IIT for cell 25 is shown in Table I. The firstcolumn in the table lists the Own Cell, its interfering cellswith the small-reuse-factor interfering cells (SRF Int. Cell) forservice type 1, and large-reuse-factor interfering cells (LRF Int.Cell) for service type 2. The total number of system channels,as shown in the first row, is 210. All the other boxes referto the channel status corresponding to a particular channelof a cell.

U1 or U2 in the first row of the boxes indicates that thischannel is occupied by a service-type-1 or service-type-2 call,respectively, in the Own Cell. For example, in Table I, channel1 is occupied by a service-type-1 call and channel 3 is occupiedby a service-type-2 call in cell 25. U1 or U2 in the other rows ofthe boxes indicates that this channel, say channel i, is occupiedby a service-type-1 or service-type-2 call, respectively, in theinterfering cell H . We call channel i a locked channel andcell H a locking cell if channel i cannot be used in theOwn Cell. For example, in Table I, cell 11 is currently using

channel 207 for a service-type-2 call. Then, cell 11 is a lockingcell for cell 25 and channel 207 is a locked channel in cell 25.A letter L in the interfering cell row, say cell K and channel j,indicates that channel j is locked in cell K because one of theinterfering cells of cell K is using channel j. Then, channel jis a locked channel for cell K and cell K is called the lockedcell for the Own Cell. For example, in Table I, channel 1 is alocked channel in cell 17 because one of the interfering cells ofcell 17, say cell 10 (refer to Fig. 1), is currently using channel1. Thus, channel 1 cannot be used in cell 17 and cell 17 is calleda locked cell in cell 25. The symbol L is additive if more thanone interfering cells use the channel.

The allocating or releasing of a channel will cause the updat-ing of the IIT. When the base station (BS) allocates channel jto a service-type-1 (service-type-2) call:

1) it first updates its own IIT by inserting a U1 (U2) in the[Own Cell, channel j] box;

2) it informs all the SRF Int. Cells (LRF Int. Cells) that a U1(U2) should be inserted in the [Own Cell, channel j] boxin their IIT;

3) each SRF Int. Cell (LRF Int. Cell) informs its interferingcells (hereafter referred to as Other Int. Cell), which arenot the interfering cells of the Own Cell, that an L shouldbe added in the [SRF Int. Cell (LRF Int. Cell), channel j]box of the Other Int. Cell’s IIT.

The releasing of a channel will follow the same procedure,except that inserting U1 (U2) and adding L should be re-placed by deleting U1 (U2) and subtracting L, in the corre-sponding box.

IV. FDCP CHANNEL-ALLOCATION SCHEME

A. Channel Assignment and Reassignment

FDCP tries to minimize the effect of the channel assignmenton channel availability to the interfering cells of the Own Cell.The idea is to allocate a channel that has been locked by themaximum number of SRF Int. Cells/LRF Int. Cells. In otherwords, a channel with the most number of locked cells [thisrefers to the most number of interfering cells with symbolL (Ls) in Table I] is allocated to the new call. Informationabout whether a channel is locked or not in the SRF Int.Cell/LRF Int. Cell of the Own Cell is provided in the IIT. Achannel, say channel j, with the smallest value of the costfunctions, min{C(j) or C ′(j)}, as (1) and (2) shown below,will be allocated to the new-arrival call in Own Cell to use.If this new call is unable to get a channel, it is blocked. Thecost functions C(j) and C ′(j), for channel assignment and re-assignment, respectively, can be used for both service-type-1and service-type-2 calls.

The cost values of C(j) for channel assignment are obtainedfor all free channels in the Own Cell first. A free channel fora new service-type-1 call means no U1 (U2) in the SRF Int.Cell (LRF Int. Cell) whereas a free channel for a new service-type-2 call means no U1 or U2 in the LRF Int. Cell. When anew service-type-1 (service-type-2) call arrives in cell A, the

IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 4, NO. 5, SEPTEMBER 2005 2097

cost function of channel assignment in cell A for channel jis given by

C(j) = N(A) − M(A, j) (1)

where N(A) is the total number of SRF Int. Cells (LRF Int.Cells) of cell A. In our model, N(A) is 12 and 18 for service-type-1 and service-type-2 calls, respectively. M(A, j) is thenumber of locked cells [this refers to the number interferingcells with symbol L (Ls) in Table I] of cell A for channel j.We can see from the function that N(A) is a predefined constantin the system. If we get the maximum value of M(A, j), we canachieve the minimum value of C(j). For example, if channelj in all SRF Int. Cells (LRF Int. Cells) of cell A is locked,i.e., M(A, j) = N(A), then cell A can use this channel freelywithout any disturbance to its interfering cells. This is becauseall these SRF Int. Cells (LRF Int. Cells) are not allowed touse channel j anyway.

A single channel reassignment is considered here so as tofurther increase system capacity. The cost values of C ′(j) forchannel reassignment are obtained for all locked channels incell A with a single locking cell, say cell B. A channel, saychannel j, is a locked channel in cell A with a single lockingcell for the new service-type-1 call if there is only one U1 inan SRF Int. Cell or one U2 in an LRF Int. Cell in channel j’scolumn. A channel, say channel j, is a locked channel with asingle locking cell for the new service-type-2 call if there isonly one U1 or U2 in an LRF Int. Cell in channel j’s column.When a locked channel j is allocated to a new call in cell A,we reassign the call using channel j in cell B to another freechannel, say channel i in cell B in order to free channel j incell A. The cost function for the single channel reassignmentis given by

C ′(j) = [N(A) − M(A, j)] + [M(B, j) − M(B, i)] (2)

where M(B, j) is the number of locked cells in the SRF Int.Cell (LRF Int. Cell) of cell B for channel j. An example isshown in Table I. Channel 207 is a locked channel with a singlelocking cell, cell 11, in cell 25. In order to free channel 207in cell 25, we reassign the call using channel 207 in cell 11 toanother channel.

After obtaining the cost values for all free channels andlocked channels with a single locking cell, a channel with thelowest cost will be allocated to the new-arrival call.

B. The Priority for Channel Assignment

When there is more than one channel with the same smallestvalue from the cost functions, the selection order of the chan-nels is based on the following rules, in descending order.

1) A channel with a larger number of locked cells hashigher priority. For example, in Table I, channel 4 withthree locked cells has higher priority than channel 209with two locked cells.

Fig. 2. Simulation results with γ1 : γ2 = 1 : 1 (ρ1 : ρ2 = 2 : 1).

2) A free channel has higher priority, e.g., in Table I,channel 209 has higher priority than channel 210.

3) If several channels have the same number of lockedcells, a lower numbered channel has higher priority, e.g.,channel 208 has higher priority than channel 209.

C. Channel Rearrangement

After a call completes and releases its channel, the channel-rearrangement process is followed. An ongoing call switches tothe just-released channel to keep its cost to a minimum. Whena service-type-1 call releases its channel, the scanning processwill be first done in the lower column of channel i (cells 11 to39 in Table I).

1) If U1 exists in the lower column of channel i, an ongoingservice-type-1 call with the smallest number of lockedcells in the upper column (cells 12 to 38 in Table I) willswitch to channel i, provided that the number of lockedcells in channel i is larger.

2) If U1 does not exist in the lower column of channel i,an ongoing service-type-1 or service-type-2 call with thesmallest number of locked cells in the whole column(cells 12 to 39) will be switched to channel i, providedthat the number of locked cells in channel i is larger.

When a service-type-2 call releases a channel i, a service-type-1 or service-type-2 call using a channel with a smallestnumber of locked cells in the whole column (cells 12 to 39)will be switched to channel i if the number of locked cells inchannel i is larger.

V. SIMULATION RESULTS

We assume that a large number of cellular users are inthe system and they are uniformly distributed in the servicearea as shown in Fig. 1. The fractions of arrival rate forservice-type-1 and service-type-2 calls are assumed to be γ1

and γ2, respectively. The arrival rates per cell λ1 and λ2, for

2098 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 4, NO. 5, SEPTEMBER 2005

Fig. 3. The improvement of FDCP and DCP-WI (optimum channel com-bination) over FCA.

service-type-1 and service-type-2 calls, are assumed to followPoisson distributions and are given by λ1 = γ1λ and λ2 = γ2λ,where λ is the total call arrival rate per cell. The mean call-duration time 1/µ1 and 1/µ2, for service-type-1 and service-type-2 calls, are assumed to follow exponential distributionswith 100 and 50 s, respectively. The service-type-1 call can beconsidered a traditional voice call that requires lower reuse fac-tor and longer call duration. The service-type-2 call can be con-sidered a data service. The traffic loads per cell ρ1 and ρ2, forservice-type-1 and service-type-2 calls, are thus given by ρ1 =λ1/µ1 and ρ2 = λ2/µ2. The total traffic load per cell ρ is givenby ρ = ρ1 + ρ2.

We evaluated the average connection blocking probabil-ity Pb of the FDCP scheme. Fig. 2 shows the simula-tion results with the arrival-rate ratio of γ1 : γ2 = 0.5 : 0.5(corresponding to ρ1 : ρ2 = 2 : 1) for FCA, DCA-WI, DCP-WI, and FDCP. DCA-WI is a network-based dynamic channel-allocation scheme that is introduced in [7]. The conventionalFCA and DCA-WI use the largest reuse factor of 7 to supportboth service-type-1 and service-type-2 calls. The optimumchannel combination of DCP-WI [4] for this traffic ratio is(110:100) for (service type 1:service type 2). It can be seenthat the FDCP scheme provides the best performance amongthe other schemes. FDCP can provide about a 55% improve-ment as compared with FCA at Pb = 1% because the averagereuse factor of FDCP is smaller than 7. Thus, system capacityis improved. FDCP also gives 24% and 5% better improve-ments as compared with DCA-WI and DCP-WI, respectively.Although the average call arrival rates for both services areset to be about 1:1 in the simulation, there is still sometraffic variation between cells. Therefore, the preallocation ofchannels in the DCP-WI scheme cannot flexibly cater to thisvariation. However, FDCP can always provide the optimumperformance.

Fig. 3 shows the improvement of DCP-WI and FDCP overFCA at Pb = 1% under different arrival-rate ratios betweenservice types 1 and 2. It can be seen that as the traffic of ser-

vice type 1 increases, the improvement of DCP-WI and FDCPover FCA increases. Because more users use the reuse factorof 4 for channel allocation, the average reuse factor is smaller.In general, FDCP outperforms DCP-WI for all traffic ratiosfor about 5%. In addition, DCP-WI requires different channelcombinations for different traffic environments in order toprovide the best system capacity. Therefore, FDCP is a fullydynamic channel-allocation scheme that can cater to any traffic-load ratio between services without preallocation of channels.The improvement of DCA-WI, not shown in Fig. 3, alwayshas about a 27% improvement over FCA for different traffic-load ratios.

VI. CONCLUSION AND LIMITATIONS

In conclusion, FDCP can cater to the traffic-load variationbetween cells and between services without preallocation ofchannels to the system or predefined channel combinations.Thus, FDCP is a fully distributed dynamic channel-allocationscheme to support multiple services requiring different reusefactors that also uses the radio resources efficiently.

An undesired side effect of FDCP is the extra overheadbetween BSs due to the channel-information updating. Forexample, for a reuse factor of 7, a total of 162 updating mes-sages are sent for a new admission call or an ongoing rearrange-ment call. For a reuse factor of 4, 60 updating messages aresent. However, since the exchange of information could passthrough the wireline between BSs and the updating of informa-tion is very simple, it will not degrade the system performanceby much.

Another limitation is that in this study, we assume that bothservices, voice and data, are equally important. This means thatQoS is not considered. In order to provide a certain level ofQoS, two simple suggestions might help deal with this issue.If we consider that voice service is more time sensitive andshould have a higher priority in acceptance, the following twoadmission-control algorithms can be applied before channelassignment is performed.

1) We could reserve a certain amount of channels, e.g., 10%of the total channels, depending on the system require-ment, for voice calls only. If 90% of the total channelsare occupied and a new data-service call arrives at thesystem, then it will be rejected or queued by the system.However, the voice-service calls are allowed to use allfree channels in the system provided that the cochannelinterference requirements are satisfied.

2) Anytime a new call arrives at the system, it will havea certain acceptance probability Pa. For example, wecan set Pa for voice and data calls to be 0.8 and 0.2,respectively. We may also employ such a policy onlywhen the system reaches a certain congestion point, say90% of the total channel.

ACKNOWLEDGMENT

The authors would like to thank the anonymous reviewersfor their helpful comments.

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