IEICE TRANS. COMMUN., VOL.E92–B, NO.12 DECEMBER 20093753

PAPER

Decentralized Dynamic Sub-Carrier Assignment forOFDMA-Based Adhoc and Cellular Networks∗

Van-Duc NGUYEN†a), Harald HAAS††, Kyandoghere KYAMAKYA†††, Jean-Chamerlain CHEDJOU†††,Tien-Hoa NGUYEN††††, Nonmembers, Seokho YOON†††††, and Hyunseung CHOO†††††, Members

SUMMARY In this paper, a novel decentralised dynamic sub-carrierassignment (DSA) algorithm for orthogonal frequency division multipleaccess (OFDMA)-based adhoc and cellular networks operating in time di-vision duplexing (TDD) mode is proposed to solve the hidden and exposednode problem in media access control (MAC). This method reduces the co-channel interference (CCI), and thus increases the overall throughput of thenetwork. Reduced CCI and increased throughput can be achieved, if timeand frequency selectivity of the multi-path fading channel and the channelreciprocity offered by the TDD are fully exploited. The time and frequencyselectivity of the channel are usually the main problem in mobile communi-cation. However, in the context of channel assignment for OFDMA-basednetworks in TDD mode, the time and frequency selectivity of the channelare the key to reduce the interference. In the proposed channel assignmentmechanism, several clusters of sub-carriers are assigned for data transmis-sion between a transmitter and a receiver only if the corresponding channelsof those sub-carriers linking this transmitter to potential victim receivers aredeeply faded. In addition, the proposed algorithm works in a fully decen-tralised fashion and, therefore, it is able to effectively support ad hoc andmultihop communication as well as network self-organisation. Numericalresults show that the throughput obtained by the proposed approach for agiven quality of service is higher than those of the conventional methods inany precondition of adhoc geographic scenario.key words: decentralized dynamic sub-carrier assignment, MAC protocol,OFDMA/TDD, adhoc and cellular networks

1. Introduction

A major challenge in designing MAC protocol for multi-hop communications is the hidden node problem, which isaddressed in [1]. This problem occurs, when a node is in therange of the receiver, but not in the range of the transmitter.Since the hidden node is out of the range of the transmit-ter, it is not able to sense the out-going transmission. Ifthis node is not properly notified, its communication withother nodes will degrade the network performance. Simi-larly, an exposed terminal is a node which is in the range

Manuscript received September 1, 2008.Manuscript revised April 19, 2009.†The author is with the Faculty of Electronics and Telecommu-

nications, Hanoi University of Technology, Hanoi, Vietnam.††The author is with the International University Bremen,

Bremen, Germany.†††The authors are with the Institute for Smart-Systems Tech-

nologies, University of Klagenfurt, Austria.††††The author is with the Faculty of Electrical Engineering and

Computer Science, the University of Hanover, Germany.†††††The authors are with the School of Information and Commu-

nication Engineering, Sungkyunkwan University, Suwon, Korea.∗This paper is partially published in [8] and [9].

a) E-mail: ducnv-fet@mail.hut.edu.vnDOI: 10.1587/transcom.E92.B.3753

of the transmitter, but not in the range of the receiver [2].This node may transmit at the same time as the other trans-mitters do without causing a network performance degrada-tion. But its transmission will be blocked by the traditionalcarrier sense multiple acces (CSMA) mechanism to preventthe channel collision. Consequently, the network capacitywill be decreased. In IEEE 802.11 MAC scheme [3], theRTS/CTS dialogue is applied to reduce the effects of thoseproblems. Analysis and development for RTS/CTS mech-anism are carried out in many research works, e.g. [4], [5].However, most of previous works focused on single carriersystems, and MAC layer is still transparent to the physicallayer. In other words, the information from physical layeris not used in the MAC layer for a possible cross-layer opti-mization.

Recently, orthogonal frequency division multiplexing(OFDM) has been intensively investigated for wireless datatransmission in broadband cellular and adhoc networks. Themultiple access technique for those networks is OFDMA[6]. The concept of this technique is to assign different usersto different sub-carriers for avoiding interference.

In broadband OFDMA networks with 100% frequency-reuse, such as in IEEE 802.16 standard, the CCI is a majorproblem. This is because all users in all cells have to use thesame carrier frequency. Suppose that a terminal is receivingdata, while another node at the same time and using the samesub-carriers, but in the neighbor cell, is transmitting. In thatsituation, the CCI will be introduced. The transmitter caus-ing CCI is, actually, a hidden node. Thus, the hidden nodeproblem remains a challenge in designing a MAC mecha-nism for OFDMA networks with full frequency-reuse. Theexposed node problem can arise, if the channel sensing bylistening to the signal power from other transmiting nodes isapplied.

In this paper, a channel assignment mechanism forOFDMA/TDD networks is proposed. The CCI, namely thehidden node problem, can be mitigated by applying a con-cept of busy signal for sensing the channel. The busy signalis an in-band short signal, which is sent out by a receiverafter successfully receiving a data packet. The busy signalis broadcasted only from the sub-carriers, which should bepreserved to continue the current communication. Based onthe received busy tone signal, the hidden node (or an intend-ing transmitter) can make a decision whether it starts trans-mitting data or refrains from the networks. The proposedalgorithm works fully in decentralized mechanism, and thus

Copyright c© 2009 The Institute of Electronics, Information and Communication Engineers

3754IEICE TRANS. COMMUN., VOL.E92–B, NO.12 DECEMBER 2009

can be applied to both adhoc and cellular networks. The ini-tial idea to use the busy signal concept for a multi-carriersystem is described in [8]. This concept with an interfer-ence aware MAC structure is carefully considered for a cel-lular OFDMA/TDD networks in [9]. In our previous workhowever, the threshold optimization issue is not studied indepth. This paper presents a complete description for theproposed approach. Condition for selecting the thresholdis given. Application of the proposed mechanism for bothcellular and adhoc network are investigated.

The paper is organized as follows: In Sect. 2, conven-tional sub-carrier assignment methods are briefly described.Section 3 is dedicated to the description of the proposedDSA algorithm. The selection of an important parameter forthe proposed method is discussed in Sect. 4. Cellular and ad-hoc scenarios setup and simulation results are presented inSects. 5 and 6. Finally, conclusions are provided in Sect. 7.

2. Review of Several Traditional Sub-Carriers Alloca-tion Methods

2.1 OFDM-FDMA Fixed Allocation

The OFDM-FDMA fixed allocation method for multiusercommunications is proposed in [6], where different usersare assigned to different sub-carriers and the assigned sub-carriers for each user are maintained for the whole time theuser remains active. Therefore, this scheme offers neitheradaptiveness nor interference avoidance mechanism.

2.2 OFDM-FDMA Random Allocation

In the OFDM-FDMA random allocation method, a new userrandomly selects a required number of sub-carriers from theavailable resource. However, it does not pay any attention tothe current active users from the neighbor cells. After selec-tion, the user starts transmitting on the selected sub-carriers.Nevertheless, only the selected sub-carriers, which ensuresthe required QoS, are maintained for the next MAC-frameof communication. Thus, it offers an adaptive mechanismbut no interference avoidance in this approach.

3. Proposed DSA Algorithm

In an OFDMA/TDD network with 100% frequency reuse,the fixed channel assignment (FCA) algorithm providespoor throughput performance because of high CCI, espe-cially when the offered traffic load of the network is high.This is because, the CCI is not taken into account in the re-source assignment process, i.e. at the MAC protocol level.A simple scenario consisting of two base stations (BSs) andthree mobile stations (MSs) is depicted in Fig. 1 in order toillustrate the problem. As an example, let us assume thefollowing scenario. The mobile stations MSRx

1 and MSRx3

receive data from the base station BSTx1 , while at the same

time the mobile station MSTx2 transmits its data to the base

station BSRx2 . In this scenario, MSTx

2 causes CCI to MSRx1

Fig. 1 Co-channel interference in OFDM/TDD networks with 100%frequency reuse.

and MSRx3 ; the base station BSTx

1 causes CCI to the basestation BSRx

2 . To reduce the CCI of the network, a new con-cept of decentralized DSA for OFDMA/TDD is proposedas shown in Fig. 2. In this channel assignment strategy,the transmitter, before sending data, must find suitable sub-carriers which do not cause significant interference to theother network participants. As it is known, in a multi-pathtransmission environment, the channel is usually frequencyselective. This selectivity of the channel can be exploited tofind such sub-carriers mentioned above. In the scenario il-lustrated in Fig. 2, we assume that the mobile stations MSRx

1 ,MSTx

2 and MSRx3 enter the network one after another at the

time instants t1, t2 and t3, respectively. The mobile stationMSRx

1 requests the base station BSTx1 to transmit its data, but

before BSTx1 can send data to MSRx

1 , it must find a set ofsub-carriers which do not disturb the other receivers signifi-cantly.

In order to enable the base station BSTx1 to fulfill such a

requirement to connect to MSRx1 , the other receivers (exclud-

ing MSRx1 ) have to broadcast the busy tone at the time instant

t1 to reserve sub-carriers for the next data transmissions. Atthe time instant t1, the base station BSTx

1 receives the busytone transmitted from some other receivers. The receivedbusy tone power will be compared with a given threshold inorder to find which clusters of sub-carriers receive the busytone below the threshold (assume clusters #n and #m in theexample). These clusters of sub-carriers are selected for datatransmission between BSTx

1 and MSRx1 . This is because the

connections (channel path gain) on these sub-carriers fromBSTx

1 to other receivers are strongly attenuated. Similarly,the mobile station MSTx

2 has to find clusters of sub-carriersfor the data transmission with BSRx

2 , and in the time in-stant t3 the base station BSTx

1 has again to find also clustersof sub-carriers for the data transmission with MSRx

1 and soforth. The sub-carriers are therefore dynamically assignedfor each transmission based on the frequency selectivity ofthe received busy tone. Thereby, the channel reciprocity in-

NGUYEN et al.: DECENTRALIZED DYNAMIC SUB-CARRIER ASSIGNMENT FOR OFDMA-BASED ADHOC AND CELLULAR NETWORKS3755

(a) (b)

(c)

Fig. 2 Concept of the decentralised DCA for OFDM/TDD networks with 100% frequency reuse.

Fig. 3 MAC-Frame structure for the proposed algorithm.

herent to TDD is constructively exploited. In the context ofhidden node problem, the transmitter MSTx

2 could be in the‘range’ of the receiver MSRx

1 or MSRx3 , similarly to BSTx

1 inconnection with BSRx

2 . The meaning of the ‘range’ in thispaper relates not only to the spatial domain, but also to thetime and the frequency domain. The ‘range’ of the receiveris defined as the areas where the intending transmitter canhear the busy signal, which is sent out from this receiver.

In order to incorporate the busy tone in a OFDMA/TDDnetwork, we propose a MAC-frame structure depicted in

Fig. 3. The upper part of this illustration shows a MACframe of a node that is firstly active in Tx mode, then in Rxmode. In the context of a cellular network, the upper part isconsidered as the MAC-frame of a BS, the lower one is ofa MS. Each MAC-frame consists of two Sub-MAC framesfor two opposite directions of transmission (downlink anduplink). A Sub-MAC frame is composed by a header fortransmitting or receiving the busy signal, and a number ofOFDM data symbols. The header of each Sub-MAC frameis simply a time slot of an OFDM symbol. If the networkis perfectly synchronized, then there will be no interferenceraised by the transmitting busy signal to the useful data sym-bol. To simplify the network analysis, perfect synchroniza-tion is assumed in this work.

The proposed algorithm is based on two main stepswhich are illustrated in Fig. 4. This algorithm is describedin the following.

1) Link initialization: To simplify the explanation of theinitialization process, we make an example illustratedin Fig. 3. This example is valid for both cellular and ad-hoc network, even the communication node is namedin the following a BS or MS. It is assumed that the mthBS wants to set-up a link to the kth MS in the (i − 2)thMAC frame, i.e. the transmission request arrival is at arandom time position in the (i−2)th MAC frame, and itis, therefore, not guaranteed that the busy-channel canbe heard during this MAC frame period. Therefore, theBS has to delay its transmission until the next MAC-frame, i.e. the (i − 1)th MAC frame when it gets thefirst proper chance to listen to the busy-channel. Thereceived busy tone in the header of the (i−1)th MAC iscompared with a predetermined threshold as explainedin Fig. 2. It is assumed that the received busy-signalpower of some sub-carriers falls below the threshold

3756IEICE TRANS. COMMUN., VOL.E92–B, NO.12 DECEMBER 2009

Fig. 4 Flow chart of the proposed DSA algorithm.

(first, third and fourth sub-carrier in the example ofFig. 3) which are subsequently selected for data trans-mission. This means that the mth BS starts transmit-ting data to the kth MS on these sub-carriers. The kthMS determines the signal-to-interference plus noise ra-tio (SINR) on each of these sub-carriers. Based on theQoS requirement for that particular transmission it willdecide whether to reserve the respective sub-carrier,or whether to ‘release’ it. In the latter case it wouldnot transmit the busy-signal on that corresponding sub-carrier, whereas in the former case it would reserve therespective sub-carrier by ‘protecting’ it using the busy-signal. Note that the SINR at a particular sub-carriermight be low either because the channel of this sub-carrier on the desired link is deeply faded, or becausethere is high interference resulting from other transmis-sions. The requirement of QoS depends on the selec-tion of the modulation scheme in the physical layer de-sign. In Fig. 3, it is assumed that the QoS of the firstsub-carrier is not satisfied. Therefore, the busy tone isnot broadcast on this sub-carrier in the next, ith, MAC-frame, i.e. the MS transmits the busy tone only on thethird and fourth sub-carriers in the example.In order to mathematically model this behavior, let us

define ak,ml,i−1 which is the channel assignment symbol

for the sub-carrier l, at the (i − 1)th MAC frame forthe link between mth BS and kth MS. If sub-carrier lat MAC-frame (i − 1) is assigned to the link betweenmth BS and kth user, then ak,m

l,i−1 = 1, otherwise, ak,ml,i−1 =

0. The outcome of this assignment, i.e. whether thisparticular sub-carrier is used for this communicationlink, is obtained by comparing the received busy tonewith the threshold as follows:

ak,ml,i−1 =

{1 if |Bm

l,i−1|2 ≤ Ithr

0 otherwise,(1)

where Bml,i−1 is the received busy tone signal at the mth

BS on the lth sub-carrier in the (i − 1)th MAC-frame,and Ithr is a threshold which is a measure for the in-terference that this transmission would cause to otherco-existing transmissions. At the (i−1)th MAC-frame,the receiver, the kth MS, estimates the SINR and de-cides if this sub-carrier is to be reserved. The outcomeof this decision is described by bk,m

l,i−1, where bk,ml,i−1 = 1

if the estimated SINR γkl,i−1 is above the required SINR

γreq., otherwise bk,ml,i−1 = 0, as described in the following

equation:

bk,ml,i−1 =

⎧⎪⎪⎪⎪⎨⎪⎪⎪⎪⎩1 if (ak,m

l,i−1 = 1)and (γk

l,i−1 ≥ γreq.)0 otherwise,

(2)

Note that the decision for the value of ak,ml,i−1 is made by

the transmitter to mitigate CCI, whereas the decisionfor the value of bk,m

l,i−1 is made by the receiver to avoidcollision and release deeply faded sub-carriers. It is as-sumed in Eq. (2) that the receiver detects ak,m

l,i−1 withouterrors.

2) Dynamic sub-carrier adaptation: For any MACframe greater than (or equal to) i, the received busy-signal powers are composed of the signal powers of theintended user, the k-th MS, and the busy-signal pow-ers of all other entities which are potentially subjectto interference. This means that the busy-signal powerfor the sub-carriers used is different from that in the(i − 1)th MAC-frame in which the communication be-tween the mth BS and the kth MS was initiated, and inwhich the intended receiver, the kth MS, has not trans-mitted a busy-signal.The received busy signal in the downlink sub-frame ofthe ith MAC frame can be written as follows

Bml,i =

√gk,mHk,m

l,i Bkl,i bk,m

l,i−1

+∑∀k′�k

√gk′,mHk′,m

l,i Bk′l,i bk′,m

l,i−1 (3)

where Bkl,i and Bm

l,i are the transmitted and the receivedbusy tone in the lth sub-carrier and the ith MAC frameof the kth MS and the mth BS, respectively. The no-tation Hk,m

l,i represents the CTF (channel transfer func-tion) coefficient for the lth sub-carrier and the ith MAC

NGUYEN et al.: DECENTRALIZED DYNAMIC SUB-CARRIER ASSIGNMENT FOR OFDMA-BASED ADHOC AND CELLULAR NETWORKS3757

frame of the transmission between the mth BS and thekth MS, i.e. the desired link. Similarly, the symbolHk′,m

l,i is the CTF coefficient between the k′ th terminal(it could be a MS or BS of a co-existing link) and themth BS. Similarly,

√gk,m and

√gk′,m are the pathloss

coefficients.Note that this algorithm does not require channelknowledge as the decision is solely based on receivedbusy-signal levels. When the transmitter makes its se-lection of a sub-carrier that had not been used in theprevious frame, it chooses a sub-carrier with low inter-ference level in the busy slot. If the receiver detectsthe data correctly and replies with a busy tone, this willcause a surge in the busy tone level on the particularsub-carrier in the following frame(s). This will indi-cate to the transmitter that the receiver confirms thesub-carrier is usable, i.e. that bk,m

l,i−1 = 1. The condi-tion for the sub-carrier assignment on the desired linkbetween the mth BS and the kth MS for the followingith and subsequent MAC frames is given as follows:

ak,ml,i =

⎧⎪⎪⎪⎪⎨⎪⎪⎪⎪⎩1 if (ak,m

l,i−1|Bml,i|

2 ≤ Ithr)

or (bk,ml,i−1 = 1)

0 otherwise

(4)

where a is the logical complement of a. In (4), thecondition (ak,m

l,i−1 |Bml,i|

2 ≤ Ithr) means the sub-carrier lhad not been used in the previous (i − 1)th MAC frameand the received busy tone on this sub-carrier at the i-th MAC-frame is lower than the given threshold. If thiscondition is fulfilled, then this sub-carrier is selectedfor data transmission in the following data transmissionof the current MAC-frame. According to this mecha-nism, in the example in Fig. 3 the second sub-carrieris selected for downlink transmission in the i-th MACframe. The condition bk,m

l,i−1 = 1 means that the sub-carrier l has been selected in the previous MAC-frameand the required SINR is maintained. In this case, thesub-carrier l remains selected for this link (third andfourth sub-carriers in the example). Note that the firstsub-carrier is released as the required SINR γreq. at thekth MS has not been achieved.Let C denote a set of all sub-carriers in one cell. Thenotations Ak,m

i and Bk,mi are sets of the channel assign-

ment symbols ak,ml,i and bk,m

l,i , respectively. In the pro-posed protocol, we follow the assumption that eachsub-carrier can only be assigned to one user within agiven cell [10]. Thus, Ak,m

i ∩ An,mi = ∅ for k � n,

where k and n are the user indices in the cell m. It isalso clear that Bk,m

i ⊆ Ak,mi ⊆ C.

The complete proposed algorithm is depicted in Fig. 4and described as follows:

1. Initialization phase: It is assumed that there is a newarrival in the network in the MAC-frame i. A set ofsub-channels will be selected by comparing the squareamplitude of the received busy-signal |Bl,i|2 with the

threshold (in Fig. 4, the user indices k and m are ne-glected for simplicity). Depending on the comparisonresult, the corresponding value of the channel assign-ment symbol al,i will be set. This comparison is exe-cuted for ∀l ∈ C, where C is the set of all sub-channelsof the system. Subsequently, data symbols are trans-mitted on the selected sub-channels.

2. In addition to data equalization and detection, the re-ceiver estimates the SINR on the selected sub-channels.A selected sub-channel is continued to be used for thenext MAC-frame, i.e. bl,i = 1, if the estimated SINR γl,i

on this sub-channel is greater than the required valueγreq. which actually depends on the QoS constraint.

3. The MAC-frame index points to the next MAC-framei := i + 1. The busy-signal is transmitted by the re-ceiver at the selected sub-channels with bl,i = 1 usingthe busy-channel.

4. At the busy-channel, the transmitter continuouslychecks the condition of Eq. (4) to adapt the set of usedsub-channels for the respective link.

5. If the communication on the particular link should becontinued in the next MAC-frame, Steps 2-5 are re-peated.

4. Selection of Threshold

The expression of the received signal at a receiver is similarto that of the received busy signal at a transmitter which isgiven in (3). Let us observe again the kth MS, the receivedsignal Rk

l,i on the lth sub-carrier and the ith OFDM symbolis written as follows

Rkl,i =

√gk,mHk,m

l,i S ml,i +

∑∀m′�m

√gk,m′Hk,m′

l,i S m′l,i , (5)

where the first and second terms are the received useful sig-nal and the CCI distortion, respectively. The notation m′

(m′ � m) denotes an active transmismitter, which causesinterference to the receiver k. The received useful signalpower PS is obtained by taking the expectation of the squarevalue of the received useful signal, i.e.,

PS = E

[√gk,mHk,m

l,i S ml,i

(√gk,mHk,m

l,i S ml,i

)∗](6)

Since the pathloss, channel coefficient and the trans-mitted symbol are statistically independent, it can be furtherdeduced

PS = E[gk,m

]E

[Hk,m

l,i (Hk,ml,i )∗

]E

[S m

l,i(Sml,i)∗]

(7)

The expectation E[gm,k

]is the mean value of the

pathloss gmean, and ES = E[S m

l,i(Sml,i)∗]

represents the trans-mitted signal power. Moreover, it has been proven in [11]that

E[Hk,m

l,i (Hk,ml,i )∗

]=

τmax∫τ=0

ρ(τ)d τ, (8)

3758IEICE TRANS. COMMUN., VOL.E92–B, NO.12 DECEMBER 2009

where ρ(τ) is the channel delay profile. The integration ofthe channel delay profile with respect to the time delay τgives the channel variance Eh. Finally, the received usefulsignal power is given by

PS = gmean ES Eh. (9)

The interference power at a victim receiver is the sumof all interference powers caused from all other interferingtransmitters as follows

PI =

Na∑m′=1,m′�m

Pm′I , (10)

where (Na − 1) is the number of active interfering trans-mitters, and the Pm′

I is the interference power caused bythe transmitter m′ to receiver k. Due to the reciprocity ofthe channel and with the assumption that the transmitteduseful signal power and the transmitted busy signal powerare equal, the interference power caused from a interferingtransmitter is always smaller than the threshold level, i.e.,

Pm′I ≤ Ithr (11)

It results in

PI ≤ (Na − 1)Ithr (12)

Thus, the minimum SINR is

SINRmin =PS

(Na − 1)Ithr. (13)

In (13), we neglect the presence of additive noise forsimplicity. In order to ensure that the SINR on all sub-carriers must be larger than the required QoS, i.e. γreq., theminimum SINRmin must be also larger than γreq., or

Ps

(Na − 1)Ithr≥ γreq. (14)

The condition for the selection of the threshold is there-fore

Ithr ≤PS

(Na − 1)γreq.(15)

Substituting the expression of the received usefulpower PS from (9) in (15), we obtain the following condition

Ithr ≤gmean ES Eh

(Na − 1)γreq.(16)

or,

Ithr [dBm] ≤ gmean [dB] + ES[dBm] + Eh[dB]

−γreq.[dB] − 10 log10(Na − 1) (17)

If the threshold is not fulfilled the condition above, thenthe throughput of the system is low. This is due to the factthat the SINR at the receiver does not meet the required QoS.However, if the threshold is set to an arbitrary small value,then the throughput is also low, since the network is strictlynot tolerant to any amount of interference. The condition in

(17) does not provide a close form of the optimum thresholdwhich maximizes the throughput for a given QoS. However,the numerical results in Sect. 5.6.1 show that the optimumthreshold is close to the maximum threshold which meetsthe condition in (16).

5. Scenario Setup and Simulation Results for CellularNetworks

5.1 Cellular Network Model

A cellular network consisting of 7 cells with 500 m radiusdepicted in Fig. 5 is considered for the analysis of the busy-tone concept for OFDMA.

MSs are uniformly distributed in space, and are as-signed to corresponding BSs according to the minimum dis-tance. Both downlink and uplink are assessed. A basic blockstructure for the downlink communication is illustrated inFig. 6, where it is assumed that users are assigned to partic-ular subcarriers based on the busy-tone MAC as proposed.A similar procedure is carried out in the uplink transmission.

Cell independent channel asymmetry is assumed. Thismeans that at any instant of time the communication in acell can be downlink, while the communication in a neigh-bor cell can be uplink. The length of a downlink sub-frameLDL is set to be equal to the length of a uplink sub-frameLUL, which is 20 OFDM symbols for all simulations. Thus,a MAC-frame consists of (LDL + LUL) OFDM symbols, inwhich there are 2 busy tone OFDM symbols used for busytone signaling for both downlink and uplink. The spectralefficiency of the system will be reduced by:

ηp = 1 −2

LDL + LUL. (18)

The penalty factor ηp will be later taken into account in (24)for evaluation of the network throughput.

5.2 Channel Model

An outdoor multi-path channel with the maximum propa-

Fig. 5 Scenario for simulations of a cellular network.

NGUYEN et al.: DECENTRALIZED DYNAMIC SUB-CARRIER ASSIGNMENT FOR OFDMA-BASED ADHOC AND CELLULAR NETWORKS3759

Fig. 6 A simplified model for downlink communication.

gation delay of 2 μs is considered. The Doppler frequencyfd,max of each path is 5 Hz. The channel is therefore a slowlytime-variant channel. The multi-path channels of differentlinks are statistically independent, and are modeled by theMonte Carlo method [12], [13],

h(τp, t) =1√

Nh

L∑p=1

ρ[p]Nh∑

q=1

e j(2π fp,qt+θp,q)δ(τ − τp),

(19)

where fp,q = fd,max sin(2πup,q), θp,q = 2πup,q, and Nh

are called the discrete Doppler frequencies, the Dopplerphases, and the number of harmonic functions, respectively.The propagation delay τp relates to the p-th channel path.The quantities up,q are independent random variables, eachwith a uniform distribution in the range (0, 1] for all p =1, 2, . . . , L, and q = 1, 2, . . . ,Nh. They are independentlygenerated for each link. The number of harmonic functionsNh is chosen to be 40. In Eq. (19), the coefficients of thediscrete multi-path profile are modelled by ρ[p].

5.3 Pathloss Model

The pathloss model described in [14], [15] is used,

g = A + 10γ log10(d/d0) + ξ, (20)

where A = 20 · log10(4πd0/λ) with d0 = 100 m, and λ is thewavelength. The quantity γ is the path-loss exponent withγ = (a − bhb + c/hb), where hb is the height of the BS andis selected to be 80 m. The constant quantities a, b, and care selected from the terrain type A given in [14]. The log-normally distributed random variable ξ models shadowing

effects and its variance is assumed to be 10 dB. The trans-mitted power of MSs and BSs is 30 dBm. This is also thetransmitted busy signal and useful signal as well.

5.4 OFDM System Parameters

Again, it is assumed that at the moment, the WiMAX stan-dard [16] provides the best framework for this investigation,and therefore, the OFDM system parameters are selected asfollows:

• Bandwidth of the system B = 20 MHz,• Sampling interval ta = 1/B = 50 ns,• FFT-Length NFFT = 256,• OFDM symbol duration TS = 12.8 μs,• Guard interval length TG = 2 μs.• Carrier frequency fc = 1.9 GHz.

The selected modulation scheme for all sub-carriers is 16-QAM (quadrature amplitude modulation). Based on theresults in [7], the required minimum SINR, γreq., for datatransmission using 16-QAM is 16 dB.

5.5 Traffic Model

The packet arrivals are assumed to be Poisson distributed.The interarrival time Ti and the holding time Th are two in-dependent random variables with exponential distributionsgiven respectively by

p(Ti > t) = pTi (t) = e−t/ν, (21)

and

p(Th > t) = pTh (t) = e−t/μ, (22)

where p(ξ > t) is the probability that the random variableξ takes its value larger than t, and is denoted by pξ(t). Theaverage interarrival time ν is varied to evaluate the networkperformance for different level of the offered load. The av-erage holding time μ is chosen to be 0.15 s.

The offered load of the network is defined as the aver-age number of bits per second per cell which are requestedto be transmitted. According to the average arrival rate 1/νand the average holding time μ, the offered load per cell iscalculated as follows,

L = Mary Nmaxμ

NCells TS ν[bits/s/cell], (23)

where Mary is the number of bits per symbol, TS is theOFDM symbol duration, and NCells is the number of cellsin the network. The modulation scheme is 16-QAM on allsub-carriers, i.e. Mary = 4. Initially, it is assumed that thenetwork imposes a restriction on the maximum number ofsub-carriers Nmax that can be assigned to one user. Clearly,Nmax < NFFT, where NFFT is the FFT length and also thetotal number of available sub-carriers. If the number of theselected sub-carriers in the set Ak is larger than Nmax, thenNmax sub-carriers will be randomly selected for data trans-mission from the preselected sub-carriers. This constraint

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is to prevent situations when a single link uses up a largeproportion of the bandwidth and forces the network to denyservice to other users.

To evaluate the throughput of the network, let us as-sume that there are M instantaneously active mobile stationswithin a given OFDM symbol, and the kth MS can success-fully receive† on NBk sub-carriers (NBk ≤ NFFT), where NBkis the cardinality of the set Bk. The throughput, which is arandom variable, in bits per second per cell can therefore beobtained as:

T = ηp1

NCells

1TS

Mary

M∑k=1

NBk [bits/s/cell]. (24)

The symbols which are received below the required SINRγreq. are rejected by the receiver and are considered lost.Based on the values assigned in sets Ak and Bk, the num-ber of rejected bits at the receiver per second per cell can becomputed as follows,

R = ηp1

NCells

1TS

Mary

M∑k=1

(NAk − NBk )

[bits/s/cell]. (25)

where NAk is the cardinality of set Ak. In practice bits onsub-carriers with low SINR (below the given threshold) willnot be lost entirely, if link adaptation techniques such asadaptive channel coding techniques and adaptive modula-tion are employed. This is part of further study in this re-search work. The definition of data rejected at the receiverswill serve only for the purpose of network performance eval-uation. Moreover, perfect time and frequency synchroniza-tion is assumed. Thus, merely CCI is present in the system.

5.6 Simulation Results

5.6.1 Threshold Optimization

The performance of the proposed MAC heavily depends onthe selection of the threshold. The obtained throughput as afunction of the threshold for the required QoS γreq. = 16 dBare illustrated in Fig. 7. The maximum number of sub-carriers Nmax is 64. The numerical results show that if thethreshold is too large or too small, a maximum throughputcan not be achieved. Therefore, the threshold must be deter-mined for a given QoS to obtain the maximum throughput.Based on the numerical results shown in Fig. 7, the thresholdcorresponding to the maximum of throughput is −90 dBm.

Table 1 shows theoretical results obtained for the max-imum value of the threshold according to (17), where themean value of the pathloss modeled by Eq. (20) is −88 dB††.The number of simulated cells is 7. Thus, the maximumnumber of active interfering transmitters (Na − 1), which si-multaneously could interfere a victim receiver on a selectedsub-carrier, is 6. As previously mentioned, the required QoSγreq. is 16 dB. According to (17), the calculation result ofthe maximum value of the threshold is Ithr,max = −82 dBm,

Fig. 7 Throughput of the system versus threshold.

whereas the numerical optimum value of threshold is about−90 dBm. In this paper, a close solution for threshold opti-mization has not been obtained. Nevertheless, the optimizedthreshold obtained by simulation is not far from the theo-retical calculation of the maximum threshold that could beassigned.

Figure 8 shows, that the lower is the threshold selected,the smaller can be the probability of data rejection obtainedat receiver. This is due to the fact that the average inter-ference power of the network is proportional to the level ofthreshold. For a given QoS, the selection of threshold is thetrade off between the throughput and the probability of thedata rejection.

Figure 9 shows the dependence of the network through-put on the thresholds Ithr. Comparing the performanceof the network for three different values of threshold, e.g.Ithr = −120 dBm, −90 dBm, and −70 dBm, it can be seenthat the corresponding probabilities for achieving a through-put higher than 32.5 Mbits/s are roughly 0.0, 0.82, and 0.31,respectively.

The threshold level of −90 dBm can provide a maxi-mum throughput which is about 50 Mbits/s/cell. The the-oretical maximum throughput for this network is NFFT ·Mary/ts = 80 Mbit/s/cell. This means that 62% of themaximum possible data rate can be achieved by the pro-posed MAC protocol for the given minimum required SINRγreq.=16 dB.

5.6.2 Comparison with Conventional OFDM-FDMA Ran-dom Allocation Technique

The performance of the proposed MAC regarding the com-plementary CDF of the throughput is compared to that ob-tained by the conventional OFDM-FDMA random alloca-

†A data packet is considered to be successfully received on asub-carrier if the SINR corresponding to this sub-carrier is higherthan γreq during the time between two consecutive busy tones.††To obtain the mean value of the pathloss, gmean, all interfer-

ence links in the scenario depicted in Fig. 5 are realized, then thecorresponding pathloss values are averaged.

NGUYEN et al.: DECENTRALIZED DYNAMIC SUB-CARRIER ASSIGNMENT FOR OFDMA-BASED ADHOC AND CELLULAR NETWORKS3761

Table 1 Numerical results of threshold optimization for cellular networks.

ES [dBm] Eh [dB] gmean [dB] 10*log(Na − 1) γreq. [dB] Ithr,max [dBm] Iopt [dBm]30 0 −88 7.8 16 −82 −90

Fig. 8 Probability of data rejection at receiver versus threshold.

Fig. 9 Complementary CDF of throughput obtained by proposed MACfor different levels of threshould.

Fig. 10 Comparision of proposed MAC with conventional OFDM-FDMA technique.

tion is shown in Fig. 10. In the case of high offered load,the proposed algorithm significantly outperforms the con-ventional method in terms of throughput. Only for very lowoffered load, when there is a redundancy in the network ca-pacity, the conventional OFDMA random allocation is com-parable or slightly better than the proposed technique.

6. Scenario Setup and Simulation Results for AdhocNetworks

A simple adhoc scenario with two communication pairs asdepicted in Fig. 11 is set for simulations. For simplifica-tion, one terminal labeled in Fig. 11 is either a transmitteror a receiver, and one way of communication is drawn. Inthe implemented scenario however, two ways of communi-cation are considered. The path loss, and the channel aremodeled as the cellular scenario as described in Sect. 5. Allother quantities as such the OFDM system parameters, mod-ulation scheme, the required QoS are also identical to thoseof the cellular network. Certainly, the traffic model is differ-ent to that in the cellular network, since two communicationpairs are active in a whole simulation time.

In the simulated scenario, we assume that two inde-pendent communication pairs at the beginning are very farfrom each other, and are moving towards. The distances be-tween the transmitter and receiver in the two communicationpairs are kept constant, namely 100 meters. The antennaheight of each adhoc node is 1 meters. The transmit poweris also 30 dBm. Both communication pairs try to occupythe medium resource for themselves by using different sub-carriers allocation methods and cause the CCI to the other.In Fig. 11, the solid lines show the communication links (in-tended links), and the dot lines show the interference links.

The averaged throughput for each communication linkin terms of the selected sub-carriers, which satisfies the re-quired QoS, is observed for different allocation methods: theproposed technique, the OFDM-FDMA random and fixedallocation mechanism. The obtained results are plotted inFig. 12. It can be seen from the results, that if two com-munication pairs are far apart, the proposed method andthe OFDM-FDMA random allocation method demonstratethe same performance, namely both select all sub-carriers.However, if the two communication pairs are close to eachother, the throughput obtained by the OFDM-FDMA ran-dom allocation method is significantly reduced, and is evenmuch less than that obtained by the OFDM-FDMA fixedallocation method. This is due to the fact that the ran-dom allocation method does not have the interference avoid-ance mechanism, and thus results in high outage when thetwo communication pairs approach each other. On the con-trary, the proposed method offers the interference avoidancemechanism by using the busy tone signaling. As a result, it

3762IEICE TRANS. COMMUN., VOL.E92–B, NO.12 DECEMBER 2009

Table 2 Numerical results of threshold optimization for adhoc networks.

ES [dBm] Eh [dB] gmean [dB] 10*log(Na − 1) γreq. [dB] Ithr,max [dBm] Iopt [dBm]30 0 −71 0.0 16 −57 −60

Fig. 11 Simplification of the simulated adhoc scenario.

Fig. 12 Number of selected sub-channels obtained by differentallocation methods.

provides the same performance as that of the OFDM-FDMAfixed allocation when the potential interference is high. Ifthe two communication pairs move far from each other,the performance obtained by the proposed method with asuitable threshold is comparable with the random allocationmethod.

Similar to the case of the cellular scenario, the theoret-ical calculation of the maximum threshold can be obtainedfrom the Eq. (16). However, the mean value of the pathlossbetween the interfering transmitter to the victim receiver forthe adhoc scenario as described in Fig. 11, gmean, is −71 dB.Moreover, the number of interfering transmitter (Na − 1),is 1. The theoretical calculation of the maximum thresholdIthr,max following the Eq. (16) is −58 dBm, whereas the opti-

mal threshold obtained from the simulation results given inFig. 12 is −60 dBm. Table 2 shows explicitly this compari-son.

For the simple OFDM-FDMA fixed allocation method,only 50% of the total resource is assigned to one commu-nication pair. This is independent of their distance. TheOFDM-FDMA fixed allocation method does not maximizethe throughput. However it is simple and ensures an CCI-free network.

7. Conclusion

In this paper, a decentralized dynamic sub-carrier assign-ment algorithm is proposed for OFDMA/TDD cellularand adhoc networks. The time-, frequency, and spatial-selectivity, as well as the channel reciprocity in TDD havebeen exploited for dynamic sub-carrier adaptation. In otherwords, link adaptation in MAC layer has is carried out on thebasis of the interference channel information from the phys-ical layer. The proposed approach solves the hidden and ex-posed node problem. If a suitable threshold is selected, theoverall throughput of the network can be maximized. TheQoS for given modulation scheme can be ensured. Simu-lation results for both cellular and a simple adhoc scenariohave been obtained. Compared to conventional methods,it has been shown that our proposed approach offers bettersystem performance. In future works, we consider this con-cept for multi-hop networks. The combination of the busytone concept with the space-frequency scheduling promisesa more effective algorithm for MIMO-OFDMA networks.The influence of imperfect synchronization on the networkperformance should be investigated. In addition, adaptivemodulation combined with the proposed method promises ahigher spectral efficiency. The busy tone signal used in thispaper is only to sense the interference power. Clearly, if itis designed to carry the feedback information from the re-ceiver to the transmitter for many other purposes, such pre-coding information, then the network performance will beincreased.

Acknowledgment

This research was supported by the Vietnamese Na-tional Foundation for Science and Technology Develop-ment (NAFOSTED) under the project number 102.02.07.09,and the Information Technology Research Center pro-gram of the Institute for Information Technology Advance-ment under Grants IITA-2009-C1090-0902-0046 and IITA-2009-C1090-0902-0005, with funding from the Ministry ofKnowledge Economy, Korea.

NGUYEN et al.: DECENTRALIZED DYNAMIC SUB-CARRIER ASSIGNMENT FOR OFDMA-BASED ADHOC AND CELLULAR NETWORKS3763

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Van-Duc Nguyen received the Bachelorand Master of Engineering degrees in Electron-ics and Communications from the Hanoi Uni-versity of Technology, Vietnam, in 1995 and1997, respectively, and the Doctorate degree inCommunications Engineering from the Univer-sity of Hannover, Germany in 2003. From 1995to 1998, he worked for the Technical Universityof Hanoi as an Assistant Researcher. In 1996,he participated in the student exchange programbetween the Technical University of Hanoi and

the Munich University of Applied Sciences for one term. From 1998 to2003, he was with the Institute of Communications Engineering, Univer-sity of Hannover, first as a DAAD scholarship holder and then as a memberof the scientific staff. From 2003 to 2004, he was employed with AgderUniversity College in Grimstad, Norway, as a Postdoctoral Researcher. Hewas with International University of Bremen as a Postdoctoral Fellow. In2007, he spent 2 months at the Sungkyungkwan University, Korea, as aResearch Professor. His current research interests include Mobile RadioCommunications, especially MIMO-OFDM systems, and radio resourcemanagement, channel coding for wireless networks.

Harald Haas received his Ph.D. degreefrom the University of Edinburgh in 2001. Hisresearch interests are in the areas of cellular sys-tem engineering and digital signal processingfor wireless communication. In 1995, Dr. Haasspent six months in Mumbai, India, as a Heinz-Nixdorf Scholar before joining Siemens AGSemiconductor Division. From 1999 to 2001 hewas Research Associate at the University of Ed-inburgh. As part of this work he was a consul-tant to Nokia Networks OY, Finland, in the area

of UMTS. From 2001 to 2002, Dr. Haas was project manager at SiemensAG (Information and Communication Mobile/Networks — ICM N) lead-ing an international research project with Chinese and German universitieson new radio concepts and algorithms for future cellular systems. Dr. Haasjoined International University Bremen (IUB) in September 2002 wherehe is currently Associate Professor of Electrical Engineering. Dr. Haas re-ceived the best paper award at the International Symposium on Personal,Indoor and Mobile Radio Communications (PIMRC) in Osaka/Japan in1999 and holds several patents in the area of wireless communications.Dr. Haas contributed a chapter to the “Handbook of Information Security”entitled “Air Interface Requirements for Mobile Data Services” to be pub-lished by John Wiley & Sons, Inc.. In 2001 and 2005, he was awarded theHonorary Fellowship of Edinburgh University.

Kyandoghere Kyamakya obtained the M.S.in Electrical Engineering in 1990 at the Univer-sity of Kinshasa. In 1999 he received his Doc-torate in Electrical Engineering at the Universityof Hagen in Germany. He then worked threeyears as post-doctorate researcher at the LeibnizUniversity of Hannover in the field of MobilityManagement in Wireless Networks. From 2002to 2005 he was junior professor for PositioningLocation Based Services at Leibniz Universityof Hannover. Since 2005 he is full Professor for

Transportation Informatics and Director of the Institute for Smart SystemsTechnologies at the University of Klagenfurt in Austria.

3764IEICE TRANS. COMMUN., VOL.E92–B, NO.12 DECEMBER 2009

Jean-Chamerlain Chedjou received in2004 his doctorate in Electrical Engineering atthe Leibniz University of Hanover, Germany.He has been a DAAD (Germany) scholar andalso an AUF research Fellow (Postdoc.). From2000 to date he has been a Junior Associateresearcher in the Condensed Matter section ofthe ICTP (Abdus Salam International Centre forTheoretical Physics) Trieste, Italy. Currently, heis a senior researcher at the Institute for SmartSystems Technologies of the Alpen-Adria Uni-

versity of Klagenfurt in Austria. His research interests include ElectronicsCircuits Engineering, Chaos Theory, Analog Systems Simulation, CellularNeural Networks, Nonlinear Dynamics, Synchronization and related Ap-plications in Engineering. He has authored and co-authored 2 books andmore than 22 journals and conference papers.

Tien-Hoa Nguyen is currently writing hismaster thesis on the areas of mobile communi-cation system at the Faculty of Electrical Engi-neering and Computer Science, the Universityof Hanover, Germany. From Oct. 2007 to Aug.2008, he worked at the Advanced Driver In-formation Technology Company in Hildesheim,Germany, as an intern. His research interests in-clude test-bed implementation for a smart anten-nas MIMO-system, MIMO-OFDMA capacity.

Seokho Yoon received the B.S.E. (summacum laude), M.S.E., and Ph.D. degrees in elec-trical engineering from Korea Advanced Insti-tute of Science and Technology (KAIST), Dae-jeon, Korea, in 1997, 1999, and 2002, respec-tively. From April 2002 to June 2002, he waswith the Department of Electrical Engineeringand Computer Sciences, Massachusetts Insti-tute of Technology (MIT), Cambridge, MA, andfrom July 2002 to February 2003, he was withthe Department of Electrical Engineering, Har-

vard University, Cambridge, MA, as a Postdoctoral Research Fellow. InMarch 2003, he joined the School of Information and Communication En-gineering, Sungkyunkwan University, Suwon, Korea, where he is currentlyan Assistant Professor. His research interests include spread spectrum sys-tems, mobile communications, detection and estimation theory, and statis-tical signal processing. Dr. Yoon is a Member of the Institute of Electricaland Electronics Engineers (IEEE), Institute of Electronics Engineers of Ko-rea (IEEK), and Korean Institute of Communication Sciences (KICS). Hewas the recipient of a Bronze Prize at Samsung Humantech Paper Contestin 2000.

Hyunseung Choo has received B.S. inmathematics from Sungkyunkwan University,Korea in 1988, MS in computer science from theUniversity of Texas at Dallas, USA in 1990, andPh.D. in computer science from the Universityof Texas at Arlington, USA in 1996. From 1997to 1998, he was a Patent Examiner at KoreanIndustrial Property Office. Since 1998, he hasjoined the School of Information and Commu-nication Engineering at Sungkyunkwan Univer-sity, and he is Associate Professor and a director

of Convergence Research Institute. Currently Dr. Choo is a director of In-telligent HCI Convergence Research Center (8-year research program) sup-ported by the Ministry of Information and Communication (Korea) underthe Information Technology Research Center support program supervisedby the Institute of Information Technology Assessment. His research in-terests include wired/wireless/optical networking, mobile computing, andgrid computing. He has published over 100 papers in international journalsand refereed conferences. Dr. Choo is a member of IEEE.