Joint Opportunistic Spectrum Sharing and Dynamic Full Frequency Reuse in OFDMA Cellular Relay Networks

Wireless Pers Commun (2013) 71:1663–1681DOI 10.1007/s11277-012-0903-0

Joint Opportunistic Spectrum Sharing and Dynamic FullFrequency Reuse in OFDMA Cellular Relay Networks

Jian Liang · Hui Yin · Li Feng · Jian Zhang ·Shouyin Liu

Published online: 11 November 2012© Springer Science+Business Media New York 2012

Abstract This paper considers the problem of spectrum sharing in orthogonal frequencydivision multiple access cellular relay networks. Firstly, a novel dynamic full frequency reusescheme is proposed to improve the spectral efficiency. Different from the conventional fullfrequency reuse scheme which only allows the base station (BS) reusing the subcarriers inthe specific regions, an improved full frequency reuse scheme is proposed to allow the BSreusing all the subcarriers in the whole BS coverage region to exploit additional multiuserdiversity gain. In order to dynamically reuse the frequency resource among the BS and relaystations (RSs) to further improve the spectral efficiency, the adaptive subcarrier scheduling isintroduced into the improved full frequency reuse scheme to obtain more multi-user diversitygain, which forms the proposed novel dynamic full frequency reuse scheme. Secondly, inorder to further increase the system throughput, the opportunistic spectrum sharing scheme isintroduced to allow the RSs selectively reusing the subcarriers among each other, which jointwith the proposed dynamic full frequency reuse scheme to intelligently allocates the subcar-riers originally reused by the BS and a RS to another suitable RS which can best improvethe system performance after considering the additional interference. Thirdly, in order toselect The optimal reusing combination scheme of BS and RSs to exploit more potential

J. Liang (B) · L. Feng · J. ZhangWuhan Digital Engineering Institute, Wuhan 430074, Chinae-mail: [email protected]

L. Fenge-mail: [email protected]

J. Zhange-mail: [email protected]

H. YinWuhan Hongxin Telecommunication Technologies Co. Ltd, Wuhan 430074, Chinae-mail: [email protected]

S. LiuCentral China Normal University, Wuhan, Chinae-mail: [email protected]

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1664 J. Liang et al.

system performance, a heuristic approach based on genetic algorithm is proposed to searchthe optimal BS and RSs combination to opportunistically share the frequency resource. Sim-ulation results show that the proposed dynamic full frequency reuse scheme can obtain highspectral efficiency, fine fairness and low outage probability compared to the conventional fullfrequency reuse scheme. Furthermore, the system performance can be improved when con-sidering the opportunistic spectrum sharing among RSs. Finally, after adopting the geneticalgorithm, the system performance can be greatly improved by the frequency reusing amongthe optimal BS and RSs combination.

Keywords Full frequency reuse · OFDMA · Adaptive scheduling ·Opportunistic spectrum sharing · Genetic algorithm · Relay

1 Introduction

Multihop cellular networks that utilize relay stations in the conventional cellular infrastruc-ture are proposed to improve the throughput of cell edge users. On the other hand, orthogonalfrequency division multiple access (OFDMA) air-interface technique is the accepted candi-date technology for the next generation broadband wireless communication networks due toits inherent ability of combating frequency selective fading and flexibility in radio resourceallocation. Therefore, the combination of relaying and OFDMA techniques can obtain sig-nificant potential benefits of multiuser and frequency diversity gains while maintaining lowinfrastructure cost [1].

The next generation wireless communication networks are expected to provide ubiquitoushigh data rate coverage to satisfy the user requirements. To achieve this target, high spectralefficiency schemes are required to aggressively reuse the expensive and scarce spectrum.With the assistance of relay, higher spectral efficiency can be obtained to improve the systemthroughput through spatially reusing the spectrum resource between the base station (BS) andrelay station (RS) [2]. In [3], several resource management policies with different frequencyreuse patterns were analyzed and a partial frequency reuse scheme was proposed to increasethe system throughput. A conventional full frequency reuse scheme was investigated in [4],which significantly improved the system performance by fully reusing the spectrum resource.Four fixed frequency reuse schemes with different partitions and reuse factors were com-pared in [5]. Compared with the orthogonal allocation scheme, the throughput of the partialfrequency reuse scheme and the conventional full frequency reuse scheme were improvedthrough spatial reuse in despite of additional interference. Moreover, the throughput of theconventional full frequency reuse scheme was larger than the partial frequency reuse schemealthough bear more interference.

In the wireless communication environment, a subcarrier undergoes deep fading on onelink may supply a better channel gain on other links due to the effects of shadowing and multi-path fading, which is more common in the complex environment of relay networks. Therefore,intelligent radio resource management schemes are required to exploit the potential diversitygain [6]. Based on this point of view, the fixed frequency reuse schemes could be replacedby dynamic frequency reuse schemes to further improving the system performance. In [7],an adaptive frequency reuse scheme for interference reduction was proposed, which canincrease the spectral efficiency and decrease the interference effect. In [8], we proposed adynamic full frequency reuse scheme to improve the system spectral efficiency, which jointthe improved full frequency reuse scheme and the adaptive subcarrier scheduling to dynam-ically sharing the subcarrier resource. In the dynamic full frequency reuse scheme, only the

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Joint Opportunistic Spectrum Sharing 1665

BS and one RS reusing the subcarrier. To the best of our knowledge, there are rare otherliteratures researching on the adaptive full frequency reuse scheme among the BS and RSsin the OFDMA relay networks.

Furthermore, the opportunistic spectrum sharing is a promising solution in wireless com-munications to adaptively reuse the frequency resource, which can adapt to the dynamicnature of wireless environment and take advantage of the spatial diversity gain to acquirehigher system throughput [9,10]. In [9], an opportunistic reuse framework was introducedto alleviate the effect of extra resources that were required in multihop transmission, wherethe RS can reuse the frequency resource used by the BS as long as the RS did not introduceany significant interference to the normal transmission of BS on this resource. Another com-plementary framework was proposed in [10] for resource allocation in femtocells, whichopportunistically reused low-cost resources in a way that would cause minimal impact onthe service level of primary macro users. However, there are rare literatures researchingon opportunistic sharing among the RSs in the OFDMA relay networks, especially for theopportunistic sharing among BS and RSs.

In this paper, firstly, a novel dynamic full frequency reuse scheme is proposed to improvethe system performance of the OFDMA cellular relay networks. Different from the conven-tional fixed full frequency reuse scheme in [4], an improved full frequency reuse schemeis proposed, where the BS can reuse all the subcarriers in the whole BS coverage region toexploit more multiuser diversity gain. Moreover, the adaptive subcarrier scheduling can beintroduced into the improved full frequency reuse scheme to dynamically allocate the subcar-riers among the users for all the RSs and BS to improve the system performance. Therefore,we joint the improved full frequency reuse scheme and the adaptive subcarrier schedulingto form the proposed dynamic full frequency reuse scheme. In the proposed scheme, eachsubcarrier is adaptively reused by the best pair of users that can obtain the optimal systemperformance. One user of the best pair is selected from RSs regions and the other from BSregion. Furthermore, the opportunistic sharing is introduced into the proposed dynamic fullfrequency reuse scheme to allow the RSs selectively reusing the subcarriers among each otherto increase the system throughput, which intelligently allocates the subcarriers reused by theBS and a RS to the chosen RS that can best improve the system performance after consideringthe additional interference. Although the proposed opportunistic sharing scheme among theRSs can improve the system performance, it searches the reusing station separately, whichleads to the reusing stations combination result suboptimal. Finally, in order to search theoptimal reusing stations combination result to achieve the opportunistic sharing among BSand RSs, a heuristic approach based on genetic algorithm is proposed to tackle the sharingproblem, which obtains the optimal reusing combination by iterations to sharing the subcarri-er resource to get the optimal system performance. Simulation results show that the proposeddynamic full frequency reuse scheme achieves higher spectral efficiency and finer fairnesscompared with conventional fixed full frequency reuse scheme. And the system performanceof the proposed scheme can be greatly improved when considering the opportunistic sharingamong RSs. Moreover, the system performance will be further improved through searchingthe optimal reusing stations combination based on the genetic algorithm to opportunisticallyshare frequency among BS and RSs.

2 System Model

The downlink scenario in the OFDMA multi-hop cellular system is considered. Figure 1shows a representative cell of the fixed relay enhanced hexagonal cells. In each cell, the BS

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Fig. 1 The two-hop OFDMAcellular relay network

BS

RS

RS

RS

RS

RS RS

MS

MS

Fig. 2 Frame structure of theTDD relay mode

time

freq

uenc

yUL subframe

MS BS

MS RS

RS BS

MS BS

DL subframe

RS MS

BS MS

BS RS

BS MS

access zonerelay zone access zone relay zone

is located at the center and uniformly surrounded by six fixed RSs which assist the com-munication between BS and K mobile stations (MS). Each RS is located at 2/3 on the linethat connects the BS and cell vertices. All RSs are considered to be the decode-and-forwardrelay and work on the time division mode, which first decode the received signals and thenre-encode them to transmit. The MSs are uniformly distributed in the cell region, which candirectly communicate with a BS over a single-hop link, or be served by the RS through atwo-hop link. Therefore, three kinds of links exist in this network architecture: BS-RS, BS-MS and RS-MS. The BS-RS link is assumed to be in the line of sight (LOS) environmentwith only considering shadowing fading, whereas the other links are in the non-line of sight(NLOS) environments with undergoing both shadowing and multipath fading. The channelstate information can be easily obtained by invoking reciprocity in time division mode. Afterthe BS performs the subcarrier allocation algorithm, the allocation results will be informedto the RSs and MSs through the control channels.

For simplicity, the distance-based relaying selection [11] is chosen as the routing selectionscheme. Figure 2 illustrates the frame structure, where the downlink subframe is divided intothe access zone and relay zone. The RSs and direct MSs communicate with BS in the relayzone while the MSs communicate with its access station in the access zone. In the relay zone,the resource allocation schemes for different reuse schemes are almost the same. Therefore,similar to [12], the resource allocation target is confined to the access zone of downlinksubframe in this paper.

3 Frequency Reuse Schemes

In this section, we first introduce two full frequency reuse schemes, the conventional fullfrequency reuse scheme and the novel dynamic full frequency reuse scheme. For the pro-posed dynamic full frequency reuse scheme, the combination of subcarrier scheduling and

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Fig. 3 The conventional fullfrequency reuse scheme

RS3

RS4

RS5

RS6

RS1

RS2

S3

S4

S5

S6

S1

S2

BS

adaptive frequency reuse will be illustrated. Then the opportunistic sharing among RSs isintroduced based on the proposed dynamic full frequency reuse scheme. Finally, we discussthe opportunistic sharing among BS and RSs based on genetic algorithm, which searches theoptimal reusing stations combination to get the optimal system performance.

3.1 Conventional Fixed Full Frequency Reuse Scheme

The conventional full frequency reuse scheme proposed in [4] is different to the partial fre-quency reuse scheme due to its fully reusing the frequency resource, which can improve thespectral efficiency due to the higher reuse factor. In order to guarantee the quality of servicerequirements of the users, reusing the frequency resource should not bring strong inter-chan-nel interference (ICI). Hence, the two links which use the same subcarriers should be locatedas far as possible from each other, which can improve the system performance via reducingthe interference level.

The conventional full frequency reuse scheme is illustrated in Fig. 3. Each cell is dividedinto seven regions, the center region is the coverage of the BS and the other six regions arerespective the coverage of six RSs. Furthermore, the coverage of the BS is partitioned into sixsectors, noted as S1, S2, . . ., S6. All the available subcarriers are divided into six sets. Eachsubcarrier set is respectively reused by {S1, RS4}, {S2, RS5}, {S3, RS6}, {S4, RS1}, {S5,RS2} and {S6, RS3}, which can make the two links sharing the same subcarrier far enoughaway from each other to decrease the interference level. The MSs in the specific regions canonly reuse the assigned subcarrier set.

3.2 The Novel Dynamic Full Frequency Reuse Scheme

When considering the shadowing and multipath fading, the fixed resource allocation schemeswould be suboptimal. Since different users with equal distance from the same transmittermay have different received Signal to Interference plus Noise Ratio (SINR), and differentsubcarriers supply different channel gain for the same user, there maybe two near links shar-ing the same subcarriers causes smaller inter-channel interference due to the shadowing andmultipath fading. So for the conventional full reuse scheme in Fig. 3, two links located farfrom each other using the same subcarriers is not optimal. Based on this point of view, wepropose an improved full frequency reuse scheme is proposed and shown in Fig. 4. Similarly,

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Fig. 4 The improved fullfrequency reuse scheme

RS3

RS4

RS5

RS6

RS1

RS2

BS

all the available subcarriers are divided into six sets and each RS uses one set. However,different from the conventional full frequency reuse scheme, the BS reuses the whole band-width but no longer divided. Regardless of the distance to the RS using the same subcarrier,each of the MSs in the BS region can use any of the available subcarriers as long as it canobtain high system performance on the subcarriers.

In order to further exploit the multiuser and frequency diversity gains, adaptive subcar-rier scheduling is utilized, which forms the proposed dynamic full frequency reuse scheme.The subcarriers are adaptively reused by the chosen two links which have the best systemperformance, one is RS-MS link and the other is BS-MS link. For each subcarrier, we firstcompute the system performance of the RS-MS links and the corresponding BS-MS linkswith considering the interference from each other. Then for each RS, we select a RS-MS linkas candidate RS-MS link, which has the better system performance compared with the otherMSs. And the corresponding candidate BS-MS link is also selected similarly for each RS.So the total system performance of the pair of candidate BS-MS link and RS-MS link can beeasily obtained for each RS. Finally, from the six RSs, the pair of candidate BS-MS link andRS-MS link with the highest system performance is choused to really reuse the subcarrier.After all the subcarriers have been allocated, the high system performance will be achieved.

3.3 Opportunistic Sharing Scheme among RSs

Based on the subcarrier allocation results in the dynamic full frequency reuse scheme, theopportunistic sharing scheme is adopt to reuse the subcarriers among RSs. The opportunisticsharing scheme can further exploit the spatial diversity gain to improve the system perfor-mance. For each subcarrier, which has been shared by the BS and a RS in the dynamic fullfrequency reuse scheme, we implement the opportunistic sharing algorithm. Firstly, eachRS-MS link of the remaining RSs on their serving UTs is assumed to share the subcarrieradditionally. Secondly, we compute the total throughput of the three reusing links on the sub-carrier for each RS-MS link of the remaining RSs. Thirdly, the RS-MS link is selected as thecandidate RS-MS link if the total throughput is improved compared to the total throughput ofthe result of the dynamic full frequency reuse scheme on the subcarrier. Finally, we comparethe system performance of the candidate RS-MS links for all remaining RSs and select thecandidate RS-MS link with the best system performance as the selected RS-MS link to reallyreuse the subcarrier. If the candidate RS-MS link does not exist, the subcarrier will not be

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Fig. 5 Opportunistic sharingscheme

RS3

RS4

RS5

RS6

RS1

RS2

BS

Subcarrier A

Subcarrier B

Subcarrier C

B

B

B

A

A

C

C

C

reused by the remaining RSs. The opportunistic sharing algorithm terminates after all thesubcarriers have been implemented.

In order to clearly illustrate the process of opportunistic sharing scheme, we adopt Fig. 5to show the opportunistic reuse based on the dynamic full frequency reuse. In the figure, welist the reuse results of three subcarriers. The solid arrows denote the subcarrier allocationresults of the dynamic full frequency reuse scheme and the dotted arrows represent the resultsof opportunistic sharing scheme. Arrows of different color express different subcarriers. Forsubcarrier A, which is shared by BS and RS2, the candidate RS-MS link for opportunisticsharing does not exist. So the subcarrier A will be not reused. However, subcarrier B and Care respectively reused by RS4 and RS1 through opportunistic sharing, which illustrates theopportunistic sharing scheme can use the spectrum efficiently.

3.4 The Optimal Reusing Combination Scheme of BS and RSs

The proposed opportunistic sharing scheme reusing the subcarriers among RSs can greatlyimproved the system performance, but it searches the reusing stations separately which causesthe reusing stations combination result suboptimal. Moreover, the proposed opportunisticsharing scheme only allows the BS and at most two RSs to reuse the subcarrier, and let theBS reuse is not necessary for every subcarrier. In order to further improve the system per-formance, more RSs should be allowed to reuse subcarrier resource. So the optimal reusingstations combination can be searched to opportunistically share the spectral resource amongBS and RSs to get the optimal system performance and the scheme will become a com-binatorial problem. For a network with BS and RSs, the computational complexity of thetraversal method will be very high. In order to reduce the complexity, a heuristic approachbased on genetic algorithm is proposed to tackle this optimal combination searching prob-lem. At the beginning of each iteration, firstly, a group of reusing combinations is generated.Secondly, we calculate the throughout on every BS-MS or RS-MS link on the subcarrier foreach station in the reusing combination with considering the interference from other stations.Thirdly, for each station in the reusing combination, the link with the best system perfor-mance is selected to reuse the subcarrier resource. Finally, the total system performance ofthe reusing combination can be got by summing the system performance of each station.Through comparing the total system performance, the reusing combination with the besttotal system performance will be selected to reusing the subcarrier. Through a few iterations,the optimal reusing combination can be obtained to reuse the subcarrier resource.

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Fig. 6 Optimal reusingcombination based on geneticalgorithm

RS3

RS4

RS5

RS6

RS1

RS2

BS

Subcarrier A

Subcarrier B

Subcarrier C

B

B

A

A

C

C

C

C

B

C

B

B

C

C

B

Similar to the proposed opportunistic sharing scheme above, Fig. 6 is adopted to illustratethe feature of genetic algorithm clearly. In the figure, the reuse results of three subcarriers arealso list. For subcarrier A, the optimal system performance will be got only when the BS andRS2 reusing the subcarrier. However, for subcarrier B, all RSs reusing together can obtainthe optimal system performance. Moreover, for subcarrier C, all BS and RSs reusing togetherwill acquire the optimal system performance. From the comparison of Figs. 5 and 6, we canclearly know that the opportunistic sharing among BS and RSs scheme based on genetic algo-rithm can obtain better system performance than the proposed opportunistic sharing amongRSs scheme.

4 Scheduling Algorithms

For the purpose of analyzing the system performance of the mentioned four full frequencyreuse scheme, we adopt a classical scheduling algorithm, which is the proportional-fair (PF)scheduling scheme. In the proposed dynamic full frequency reuse scheme, each subcarrier isadaptively reused by the chosen pair of RS-MS and BS-MS links, which has the maximumratio of achievable instantaneous rate over average received rate. Different to half equivalentfor the RS link due to the TDD mode in [8], we consider the subcarriers will be redistributedin the access zone in this paper. So only the subcarrier allocation in the relay zone need beconsidered, and the RS link will be not half equivalent procession, which also consideredin the opportunistic sharing scheme. And the scheduling scheme for each subcarrier can beformulated as

maxm,km ,kB

{Pm,kB ,n(t) + Pm,km ,n(t)

}(1)

where m and n represent the index of RS and subcarrier respectively. km and kB denote theindex of MS served by the mthRS and BS respectively. Considering the interference causedby the subcarrier reusing, Pm,km ,n(t) and Pm,kB ,n(t) denotes the rate ratio of the mth RS forthe km th MS and BS for the kB th MS on the nth subcarrier respectively. And the rate ratioof the link at time t is calculated as follows [13]

P(t) = R(t)

RA(t)(2)

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R(t) denotes the achievable instantaneous rate, which is calculated by the received SINR onthe link. RA(t) denotes the long-term average rate at time t , which is updated as

RA(t + 1) =(

1 − 1

TC

)RA(t) + 1

TCR(t) (3)

where TC is the size of observation window in time slots.After completing the dynamic full frequency reuse scheme, the opportunistic sharing

scheme is adopted to reuse the subcarriers among RSs based on the allocation results. Forthe opportunistic sharing scheme, two scheduling algorithms are adopted to allocate the sub-carriers on the candidate links, the max-rate (MR) and PF scheduling algorithm. The PFscheduling for opportunistic sharing is similar to the allocation scheme in the dynamic fullfrequency reuse scheme, which allocates the subcarrier to the selected RS-MS link with max-imum total rate ratio from the candidate RS-MS links. For each subcarrier, the formulationof the scheme can be expressed as

maxm′′ �=m′,k′′

m

{P ′

m′,k′B ,n(t) + P ′

m′,k′m ,n(t) + P ′′

m′′,k′′m ,n(t)

}(4)

where m′, k′m and k′

B respectively represent the index of selected RS, selected MS servedby the m′th RS and selected MS served by BS for reusing the subcarrier n in the results ofdynamic full frequency reuse scheme. m′′ denotes the index of the remain RSs for opportu-nistic sharing the subcarrier n. k′′

m means MS served by the m′′th RS. P ′m′,k′

m ,n(t) is the rate

ratio of the m′th RS for the k′m th MS with considering the interference from the BS and m′′th

RS. P ′m′,k′

B ,n(t) express the rate ratio of BS for the k′B th MS with the interference from the

m′th RS and m′′th RS. P ′′m′′,k′′

m ,n(t) represents the rate ratio of the m′′th RS for the k′′m th MS

with the interference from the BS andm′th RS.However, for the MR scheduling algorithm, the subcarrier is shared by the chosen RS-MS

link with maximum total system throughput from the candidate RS-MS links.

maxm′′ �=m′,k′′

m

{R′

m′,k′B ,n(t) + R′

m′,k′m ,n(t) + R′′

m′′,k′′m ,n(t)

}(5)

where R′m′,k′

m ,n(t) and R′m′,k′

B ,n(t) represent the instantaneous rate of the m′th RS for the

k′m th MS and BS for the k′

B th MS respectively. R′′m′′,k′′

m ,n(t) denotes the instantaneous rate of

the m′′th RS for the k′′m th MS. The interference for the instantaneous rates is similar to the

PF scheduling algorithm.Finally, in the optimal reusing combination based on genetic algorithm scheme, we search

the optimal reusing stations combination to opportunistically share the frequency resourceamong the BS and RSs. And the PF scheduling scheme is adopted to allocate the subcarrierson the candidate links. For each subcarrier, the formulation of the scheme can be expressedas

maxρ,K

{ρB PkB ,n(t) + ρ1 Pk1,n(t) + ρ2 Pk2,n(t) + · · · + ρ6 Pk6,n(t)

}(6)

where ρ = [ρB , ρ1, ρ2, ρ3, ρ4, ρ5, ρ6] represents the binary assignment vector of the BS andRSs on the subcarrier n. And ρB represents the binary assignment variable of the BS andρ1, ρ2, . . . , ρ6 for the RS1, RS2,. . .,RS6. The binary assignment variable can be 0 or 1, where1 indicates the corresponding station reusing the subcarrier and 0 reverse. The target of thegenetic algorithm is to search the optimal assignment vector. K = [kB , k1, k2, k3, k4, k5, k6]denotes the index vector of MSs with the optimal system performance for the BS and RSs,

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which forms the optimal user links to reusing the subcarrier. PkB ,n(t) is the rate ratio of theBS for the kB th MS with considering the interference from the RSs reusing the subcarrier.Pk1,n(t), Pk2,n(t), . . . , Pk6,n(t) is the rate ratio of the RS1, RS2,. . .,RS6 respectively withconsidering the interference from the other stations reusing the subcarrier.

In order to simplify the understanding of the procedure of searching the optimal reus-ing combination based on genetic algorithm, we present the pseudo codes of the iterativealgorithms.

The pseudo code for genetic algorithm approach

Input: population size Q,maximum generation number G,subcarrier number N .

for n = 1 to N

Generate the initial population of size Q, gen = 0

(1) Decode the individuals to obtain the reuse combination;

(2) Calculate the fitness function for each individual;

while; gen < G

Ranking the individuals by the fitness function;

Select the parents with high fitness function;

Generate the offspring by crossover and mutation ;

Do process from (1) to (2);

Replace some of the worst individuals with new

individuals and record the fitness of all individuals;

end while

Find the individual with maximum fitness value and

calculate the system performance;

end for

Table 1 shows the computational complexity of the four frequency reuse schemes. It canbe observed that the proposed three schemes exist some increase in complexity comparedwith the conventional fixed full frequency reuse scheme, whereas they can greatly improvethe system performance. Especially, compared with the novel dynamic full frequency reusescheme, the opportunistic sharing scheme and the optimal reusing combination scheme canobtain considerable system performance gain with acceptable complexity increase.

Table 1 The complexity of thefrequency reuse schemes

Scheme Complexity

Conventional fixed full frequency reuse scheme O (N K/M)

The novel dynamic full frequency reuse scheme O (3M N K/8)

Opportunistic sharing scheme O (N K (4M − 1)/8)

The optimal reusing combination scheme O (N K G Q/2)

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5 Interfence and Capacity Analysis

5.1 Interference Analysis

The cochannel interference of the OFDMA cellular relay networks consists of both intra-celland inter-cell interference. Only the downlink scenario of a single cell is considered in thispaper, therefore, just the intra-cell interference will be analyzed. Intra-cell interference iscaused by the simultaneous use of the same frequency band on different links in the samecell. In the proposed frequency reuse schemes, the cochannel interference is caused by theBS or RS links which use the same subchannel resource.

For the novel dynamic full frequency reuse scheme, the cochannel interference on thesubchannel n for the MS in the RS region derives from the BS, which is defined as

IR(n) = PB G B,n (7)

where PB is the transmitting power of the BS, G B,n is the summary of the pathloss, shadowshading and multipath shading on the interference link. The cochannel interference on thesubchannel n for the MS in the BS region is

IB(n) = PR G Rm ,n (8)

where PR is the transmitting power of the RS, G Rm ,n is the summary of the pathloss, shadowshading and multipath shading on the interference link.

For the opportunistic sharing scheme among RSs, the cochannel interference on the sub-channel n for the MS in the RS region derives from the BS and another RS, denoted as

IR(n) = PB G B,n + PR Gm,n (9)

The cochannel interference on the subchannel n for the MS in the BS region is

IB(n) = PR Gm,n + P ′R G ′

m,n (10)

For The optimal reusing combination scheme of BS and RSs scheme, the cochannel inter-ference on the subchannel n for the MS in the RS region derives from the BS and all otherRSs, which is

IRm (n) = ρB PB G B,n +∑

m′ �=m

ρm′ PRGm′,n (11)

and the cochannel interference on the subchannel n for the MS in the BS region is

IB(n) =∑

m

ρm PRGm,n (12)

5.2 Capacity Analysis

In this paper, capacity analysis is focused on the access zone of downlink subframe. There-fore, the equivalent spectral efficiency of the access zone for the whole downlink framelength of a two-hop network should be half of the calculated value. Therefore, in the pro-posed dynamic full frequency reuse scheme, the sum capacity in the access zone for eachsubcarrier is calculated as

C ′n =

(Rm′,k′

B ,n(t) + Rm′,k′m ,n(t)

)TS

/2 (13)

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where Rm′,k′B ,n(t) and Rm′,k′

m ,n(t) represent the instantaneous rate of the BS for the k′B th MS

and the m′th RS for the k′m th MS respectively, TS is the duration of the downlink frame. In

the proposed opportunistic sharing scheme among RSs, the total capacity for each subcarrieris shown as

C ′′n =

(R′

m′,k′B ,n(t) + R′

m′,k′m ,n(t) + R′′

m′′,k′′m ,n(t)

)TS

/2 (14)

where R′m′,k′

B ,n(t), R′m′,k′

m ,n(t) and R′′m′′,k′′

m ,n(t) are defined in (5). Similarly, for the optimal

reusing combination among BS and RSs based on genetic algorithm, the equivalent totalcapacity for each subcarrier is obtained as

C ′′′n = (

ρB RkB ,n(t) + ρ1 Rk1,n(t) + · · · + ρ6 Rk6,n(t))

TS/

2 (15)

where RkB ,n(t) is the instantaneous rate of the BS for the kB th MS with considering theinterference from the RSs reusing the subcarrier, Rk1,n(t), Rk2,n(t), . . . , Rk6,n(t) is theinstantaneous rate of the RS1, RS2,…,RS6 respectively, and [ρB , ρ1, ρ2, ρ3, ρ4, ρ5, ρ6] aredefined in (6).

6 Simulation Results

Table 1 provides the simulation parameters that have been adopted in this paper. In order toevaluate the performance of the proposed reuse schemes, several assumptions are made asfollows. All MSs are assumed to be fully loaded, which means that the transmitting queueis always full and all the subcarriers assigned to a MS are always occupied by its links.Independent lognormal shadowing fading with time-frequency correlated Rayleigh fadingmode is assumed for all links. The path-loss model is expressed as [14]

P L =(

4πd0 f

c

)2 (d

d0

)n

(16)

where d0 is the reference distance and is set to 10 m, f is the carrier frequency, c is the speedof light, d is the distance between the transmitter and the receiver, and n is the path lossexponent and is set to 2.35 for the LOS link and 3.76 for the NLOS link (Table 2).

The spectral efficiency and the user fairness are considered to analyze the system perfor-mance. The Jain’s fairness index [6] is adopted to measure the fairness among users, whichassumes a value between 1/K and 1 for a network having K users with the same service classand priority. Mathematically, Jain’s index can be expressed as

x =(∑K

i=1 ri

)2

K(∑K

i=1 r2i

) (17)

where ri is the rate of user i . The larger index reflects the more fairness, and x = 1 meansabsolute fairness.

For comparing the system performance of the mentioned two full frequency reuse scheme,we consider four systems with different reuse schemes. In system-1, the conventional fixedfull frequency reuse scheme (CFFFR) is applied, which fixedly allocates the subcarrier setsamong BS and six RSs. The improved full frequency reuse scheme with fixed subcarrier allo-cation (IFFRF) is utilized in system-2. Different to system-1, system-2 allows the BS reusingthe subcarriers in the whole BS coverage region. In system-3, the joint adaptive subcarrier

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Table 2 Simulation parameters Parameter Value

Cell radius 1.2 Km

RS distance from BS 800 m

MS min. close-in distance to BS 35 m

Carrier frequency 2.5 GHz

No. of subchannels 128

BER 10−3

TC 100

TS 10 ms

Noise power density −174 dBm/Hz

Shadowing σ for NLOS links 8.9 dB

Shadowing σ for LOS links 4 dB

BS total Tx. power 46 dBm

RS total Tx. power 37 dBm

0 10 20 30 40 50 60 70 80 90 1000

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rage

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CFFFR

IFFRF

SCFFR

SNFFR

Fig. 7 Average spectral efficiency for the four systems

scheduling and conventional full frequency reuse scheme (SCFFR) is applied, where thesubcarriers are dynamically allocated among all the regions different to the fixed subcarrierallocation scheme in system-1. The joint subcarrier scheduling and novel full frequency reusescheme (SNRRF) is adopted in system-4, which is the main research object.

Figures 7 and 8 show the simulation results of the spectral efficiency and Jain’s fairnessindex respectively for the four systems. In Fig. 7, the average spectral efficiency are cal-culated by averaging the spectral efficiency values of all the subcarriers in every samplingpoint, which reflects the system throughput. With the increasing of user number, the spectralefficiency will be improved because more multiuser diversity gain can be obtained. Fromthe comparison of the CFFFR and IFFFR or the SCFFR and SNFFR, it can be known thatthe novel full frequency reuse scheme has higher spectral efficiency than the conventional

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CFFFR IFFRF SCFFR SNFFR0

0.1

0.2

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0.4

0.5

0.6

0.7

The

Jai

n's

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inde

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Fig. 8 User fairness for the four system (K = 100)

full frequency reuse scheme and the proposed SNFFR has about 24 % spectral efficiencygain than SCFFR. Meanwhile, from the comparison of the CFFFR and SCFFR or the IFFFRand SNFFR, we can get that the spectral efficiency can be further increased with the help ofsubcarrier scheduling and SNFFR has about 104 % spectral efficiency gain compared withIFFFR. The comparison also shows that the dynamic full frequency reuse scheme has a higherspectral efficiency than the fixed full frequency reuse scheme. The comparison of CFFFR andSNFFR shows that SNFFR can get 174 % spectral efficiency gain compared with CFFFR,which proves the benefit of multiuser diversity.

In Fig. 8, the user fairness is shown for the four systems when the user number is 100.In [8], we have half equivalent procession for the RS link, so the priority of the MSs in theBS region is relatively increased, which will cause unfairness between the MSs of RSs andthe MSs of BS. Different to [8], in this paper, the RS link is not half equivalent, and thiscan improve the throughput of the MSs in the RSs region to increase the system fairness,which can be known from Fig. 8. From the comparison of CFFFR and IFFFR or SCFFR andSNFFR, we can know that the novel full frequency reuse scheme has slightly lower fairnessthan the conventional full frequency reuse scheme, which is different to the result in [8]. Thereason is that the MSs near to BS will obtain more subcarrier resource than the MSs far fromthe BS in the BS region. This unfairness will be more obvious when the BS region is notdivided fixedly. On the contrary to [8], both the MSs far from the BS in the BS region andthe MSs in the RSs region have low throughput, which will obtain relatively higher fairness.On the other hand, the comparison of CFFFR and SCFFR or IFFFR and SNFFR illustratesthat the fairness can be further enhanced via subcarrier scheduling. For IFFFR, we can knowthat the SNFFR can increase the fairness index about 250, and 220 % for CFFFR. Therefore,the proposed dynamic full frequency reuse scheme has the ability to achieve fain fairnessthanks to the dynamic subcarrier allocation.

Based on the results of SNFFR, we further analyze the system performance for the twodifferent opportunistic sharing systems, which are opportunistic sharing with PF schedulingsystem (OSPFS) and opportunistic sharing with MR scheduling system (OSMRS). More-over, the system performance of the optimal reusing combination based on genetic algorithmwith PF scheduling system (GAPFS) is also discussed. Figures 9, 10 and 11 show the spectral

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SNFFR OSPFS OSMRS GAPFS0

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Fig. 9 Average spectral efficiency for opportunistic sharing scheme

0 10 20 30 40 50 600

0.1

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0.7

0.8

0.9

1

user spectral efficient (bps/Hz)

F(x

)

CDF of user spectral efficiency

SNFFR

OSPFSOSMRS

GAPFS

Fig. 10 CDF of user throughput for opportunistic sharing scheme

efficiency, CDF of user throughput and Jain’s fairness index respectively when K = 100. InFig. 9, we can know that the opportunistic sharing scheme can significantly improve thesystem throughput, which can further exploit the spatial diversity gain to utilize the spectrummore aggressively, and also the GAPFS can greatly increase the system throughput. Com-pared to SNFFR, OSPFS, OSMRS and GAPFS can obtain about 46, 97 and 152 % spectralefficiency gain respectively. Therefore, the proposed opportunistic sharing scheme amongRSs can aggressively utilize the spectrum resource by effectively exploiting multiplexinggain. Especially, through searching the optimal reusing combination by genetic algorithm,even higher system throughput can be obtained.

From the CDF of the user throughput in Fig. 10, we can know that the OSMRS can makesome users achieve very high throughput, but cause very low throughput to many users, whichdue to the scheduling scheme allocates the reusing subcarrier only to users with good channel

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SNFFR OSPFS OSMRS GAPFS0

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Fig. 11 User fairness for opportunistic sharing scheme

2 3 4 5 6 7 8 9 1029

30

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The number of gennerations

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GAPFS

Fig. 12 Average spectral efficiency for opportunistic sharing scheme

conditions. For OSPFS, most of the users can attain high throughput. Further, almost all theusers have very high throughput for GAPFS. This illustrates that the proposed schemes cangreatly improved the system performance. The user fairness is shown in Fig. 11. From thefigure, we can know that OSMRS reduces the user fairness about 76 % compared to SNFFRbecause of that the subcarriers are only allocate to the users with the best channel condition.However, OSPFS can enhance the user fairness about 59 % compared to SNFFR since it canallocate more subcarriers to the users with poor channel condition. Moreover, GAPFS canimprove the user fairness about 87 %, which can obtain fine user fairness. Therefore, theopportunistic sharing and optimal reusing combination based on genetic algorithm are feasi-ble solutions to further improve the system performance. In order to show the performance ofthe genetic algorithm, Fig. 12 shows the average spectral efficiency of the GAPFS for differentiteration number. From the figure, the genetic algorithm can get convergence quickly, which

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obtain the optimal reusing combination through a small iteration number. Therefore, we canget high system performance with the cost of a little complexity increasing, and the geneticalgorithm is very suitable for our system.

7 Conclusions

This paper first proposes a novel dynamic full frequency reuse scheme, which brings animproved full frequency reuse scheme and adopts the adaptive subcarrier scheduling to obtaina better system performance than the conventional full frequency reuse scheme. Furthermore,the opportunistic spectral sharing among RSs is introduced to the proposed dynamic fullfrequency reuse scheme, which can adaptively exploit the spatial diversity gain to utilize thespectrum more aggressively. Finally, the opportunistic sharing among BS and RSs based ongenetic algorithm is proposed to search the optimal reusing stations combination to get theoptimal system performance. Simulation results show that the system performance can besignificantly improved by the proposed schemes.

Acknowledgments The research presented in this paper was supported in part by the Defense Pre-ResearchProjects of the ‘Twelfth Five-Year-Plan’ of China (Nos.4010105010103, 62101050101 and 513150802), andShipsbuilding Science Foundation (No. 09J3.4.1).

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7. Hwang, J. Y., Oh, Y., & Han, Y. (2010). Adaptive frequency reuse scheme for interference reductionin two-hop relay networks. In IEEE VTC2010-Spring (pp. 1–5).

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Author Biographies

Jian Liang received the B.S. degree in telecommunication and theM.S. degree in communication from Central China Normal University,Wuhan, China, in 2009 and in 2012, respectively. He is presently work-ing in Wuhan Digital Engineering Institute, China. His current researchinterests include wireless communication and radio resource allocation.

Hui Yin received the B.S. degree and the M.S. degree in communi-cation from Central China Normal University, Wuhan, China, in 2009and in 2012, respectively. She is presently working in Wuhan HongxinTelecommunication Technologies Co. Ltd., China. Her research inter-ests are wireless communication, radio resource allocation and CR.

Li Feng received his B.S. and M.S. degrees in electrical engineer-ing from Xi’an University of Science and Technology, Xi’an, China,in 1997 and 2001, respectively and his Ph.D degree in Electrical Engi-neering from the Xi’an Jiaotong University in 2005. He was a visitor atMicrosoft Research Asia (MSRA) joining the Microsoft Communica-tion Protocols Project (MCPP) from June to August 2004. He is now asenior research scientist at Center for Dependable and Secure Comput-ing (CDSC) of Wuhan Digital Engineering Institute, Wuhan, China. Heis also a postdoctoral research fellow from September 2007 in Depart-ment of Computer Science, Xi’an Jiaotong University, Xi’an, China.His research interests currently focus on mobile Ad hoc network andinformation security. Li Feng is also a member of the IEEE ComputerSociety

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Jian Zhang received the Ph.D. Degree in communication fromHuaZhong University of Science and Technology, Wuhan, China, in2007. He is presently working in Wuhan Digital Engineering Institute,China. His current research interests include wireless communicationand spectrum management.

Shouyin Liu received the BS degree in physics and the MS degreein radio electronics from Central China Normal University, Wuhan,China, in 1985 and in 1988, respectively. He received the PhD degreefrom Hanyang University, Korea in 2005 in electronic communicationengineering. From 2004, he has been a professor at Central China Nor-mal University. His current research interests include digital communi-cation, WSN and location techniques.

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Documents

Joint Opportunistic Spectrum Sharing and Dynamic Full Frequency Reuse in OFDMA Cellular Relay Networks