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  1. 1. Sultan F. Meko / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue4, July-August 2012, pp.1501-1509Optimal Relay Placement Schemes In OFDMA Cellular Networks Sultan F. Meko IU-ATC, Department of Electrical Engineering Indian Institute of Technology Bombay, Mumbai 400 076, IndiaABSTRACT The Wireless services such as Skype and otherwise it is blocked. Depending on the SINRother multimedia teleconferencing require high experienced by an arriving user, the BS computes thedata rate irrespective of user’s location in the subcarriers that need to allocate to the user so as tocellular network. However, the Quality of Service provide the required rate. If the required sub-channels(QoS) of users degrades at the cell boundary. (i.e., a group of subcarriers) are available, then theImprovement in capacity and increase in coverage user is admitted. Note that the SINR decreases as thearea of cellular networks are the main benefits of distance between the BS and the user increases. Thus,Fixed Relay Nodes (FRNs). These benefits of the users at the cell boundary can cause blockingFRNs are based on the position of relays in the probability to be high. Since the number of admittedcell. Therefore, optimal placement of FRNs is a users is directly proportional to the revenue of thekey design issue. We propose new schemes service provider, it is imperative to design solutionsoptimal FRN placement in cellular network. Path- that allow accommodating a large number of users.loss, Signal to Interference and Noise Ratio This motivates us to propose a Fixed Relay based(SINR) experienced by users, and effects of cellular network architecture that is well suited toshadowing have been considered. Our analysis improve the SINR at the cell boundary, and thus cangive more emphasis on supporting users at the possibly increase the number of admitted users.cell-boundary (worst case scenario). The results Relaying is not only efficient in eliminating coverageshow that these schemes achieve higher system holes throughout the coverage region, but moreperformance in terms of spectral efficiency and importantly; it can also extend the high data ratealso increase the user data rate at the cell edge. coverage range of a single BS. Therefore bandwidth and cost effective high data rate coverage may beKeywords - FRN deployment, OFDMA, multi- possible by augmenting the conventional cellularhop, outage probability, Sectoring. networks with the relaying capability.1. INTRODUCTION Increase in capacity, coverage andthroughput are the key requirements of future cellularnetworks. To achieve these, one of the solutions is toincrease the number of Base Stations (BS) with eachcovering a small area. But, increasing the number ofBSs requires high deployment cost. Hence, a costeffective solution is needed to cover the required areawhile providing desired Signal to Interference plusNoise Ratio (SINR) to the users so as to meet thedemand of the future cellular networks. To achievethe high data rate wireless services, OrthogonalFrequency Division Multiple Access (OFDMA) isone of the most promising modulation and multiple Fig.1 Layout of FRN based Cellular systemaccess techniques for next generation wirelesscommunication networks. In OFDMA, users aredynamically allocated sub-carriers and time-slots so We consider a cellular system with six fixedthat it is possible to minimize co-channel interference relay nodes (FRNs) that are placed symmetricallyfrom neighboring cell by using different sub-carriers. around the BS as shown in Fig. 1. Mobile stationsTherefore, OFDMA based multi-hop system offers (MSs) in outer regions AR1 to AR6 can use relayingefficient reuse of the scare radio spectrum. to establish a better path than the direct link to BS. The key design issues in such systems are the We consider an OFDMA-based cellular following: (1) How sub-channels should be assignedsystem in which users arrive and depart dynamically. for (a) direct MS to BS links, (b) FRN to BS links,Each arriving user demands rate 𝑟. If the required and (c) MS to FRN links. Algorithm for this israte can be provided, only then a user is accepted, referred to as the channel partitioning scheme. (2) How sub-channels can be used across various cells. 1501 | P a g e
  2. 2. Sultan F. Meko / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue4, July-August 2012, pp.1501-1509Algorithm for this is referred to as channel reuse system can be maximized while providing each userscheme. with its required rate. Effective channel partitioning maximizesutilization of every channel in the system, and thus The paper is organized as follows. Inobtains high spectrum efficiency in cellular systems Section 2, we describe our system model. In Section[3]. For a cellular system enhanced with FRNs, the 3, we propose our relay placement schemes basedmain idea of channel partitioning is to optimally Path-loss, SNR and Shadowing. In Section 4, weassign the resources to the MS-BS, FRN-BS, and evaluate the performance of the proposed schemesMS-FRN links. Such intra-cell spectrum partitioning using numerical computations and simulations. Inalong with the channel reuse scheme not only grant Section 5, we conclude the paper.the data rate demanded by each user, but alsomanages the inter-cell interference by controlling the 2. SYSTEM MODEL AND DESCRIPTIONdistance between any pair of co-channel links. 2.1 System Configuration We consider a cellular system consisting of The concept of channel partitioning for FRN regular hexagonal cells each of edge length D. Eachbased cellular system has been discussed in [4], [5], cell has a BS and certain number of FRNs situated[2], [8], [7], [14]. In [4], full frequency reuse scheme symmetrically around the BS at a distance dr from thewas proposed. The authors divided the cell in to center and dm from the cell edge as shown in parts and allocated six sets of subcarriers to Let the total bandwidth available for the downlink beFRN link and the remaining to BS. The authors W units. Let the minimum SNR/SIR experienced byaimed at exploiting the multiuser diversity gain. the user at furthest location from BS/FRN be γ1.However, sectoring the inner region can further Similarly, let the minimum SNR/SIR experienced byreduce the co-channel interference. In [5], frequency FRN at furthest location from BS be γ2, i.e, both MSreuse scheme was proposed based on dividing the and FRN are placed at a location that theyouter region in to six and also sectoring the inner experienced minimum SNR or SIR. Therefore, theseregion. In [2], a preconfigured relay channel selection locations are the effective boundary for each link (i.ealgorithm is proposed to reuse the channels that are MS-BS, FRN-BS and FRN-BS). For a given MSalready used in other cells on the links between FRNs position, let the distance from BS be d* and nearestand MSs. This scheme may suffer from high co- FRN be d*m. If d* ≤ d, then MS communicates withchannel interference on FRN-MS links. In [7], [14], the BS directly; otherwise it communicates throughfrequency partitioning and reuse schemes for cellular the nearest relay using two hop route. Because of theWLAN systems with mobile relay nodes are specified routing scheme, a cell can be partitionedproposed. Relays are mobile as the MSs themselves into seven regions as shown in Fig.1. We define theact as a relay for other MSs. Since the relay is region covered by BS as the inner region (A1) and themobile, the channel between relay and the BS can region covered by FRNs as outer region (A2). Outerchange, which will result in a large number of inter- region is further divided into A2k, for k=1, ..., 6. Allrelay handoffs. Furthermore, MSs acting as relays the MSs in region A1 communicate directly to the BS,may not be cooperative because of the power and the MSs in kth A2 region communicate to BSconsumption and the security issues. In [8], a through relay kth FRN. Both the number of FRNs (K)coverage based frequency partitioning scheme is and the size of each region in a cell (d*m and D) areproposed. The scheme assumes that the relay nodes determined by the optimal relay placement algorithm.are placed at a distance equal to the two-third of the 2.2 Channel Partitioning and Reuse Schemecell radius, and does not consider optimal relay The channel partitioning scheme, partitionsplacement. Some other proposals for frequency the downlink bandwidth into 2K+1 orthogonalmanagement include the use unlicensed spectrum segments, viz. W1, W2,k and W3,k for k = 1, . . . ,K.[12], and the use of directional antennas [1]. The band W1 is used by the MSs in region A1, the band W2,k is used by kth FRN to communicate with In our paper [6], we proposed a channel the BS, and the band W3,k is used by the MSs in kthpartitioning and a channel reuse scheme for A2 region to communicate with kth FRN. Let 𝑊2 =increasing system capacity to support some preceding 6 6standards like Global System for Mobile 𝑊 and 𝑊3 = 𝑘 =1 2,𝑘 𝑊 . Because of the 𝑘=1 3,𝑘Communications (GSM). In this paper, we extend the channel partitioning, there is no intra-cellchannel partitioning and channel reuse scheme for interference, and the system performance is mainlyOFDMA cellular networks which results in increased determined by inter-cell co-channel interference.system capacity and spectral efficiency. We alsoconsider the optimal relay placement based on For channel reuse scheme, we assume thatdifferent parameters. We show that with the the frequency reuse distance is 1, i.e., each cell usesappropriate relay placement, the system performance the complete bandwidth W for the downlinkcan be improved significantly. As a result, the communication [11]. In eachnumber of users that can be accommodated in the 1502 | P a g e
  3. 3. Sultan F. Meko / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue4, July-August 2012, pp.1501-1509cell, inner region uses the same band W1. While kth the “BS-MS, BS-FRN and FRN-MS links”,FRN uses band W2k to communicate with BS and MS henceforth, we use both terms interchangeably. Onin kth A2 region uses W3k band to communicate with these three links, we assume that each link haskth FRN. minimum SNR requirement (threshold). When the received SNR exceeds a threshold, the message is correctly decoded. The distance of user from2.3 Propagation Model BS/FRN, where the received SNR equals the Wireless channel suffers from fading. threshold is defined as the effective radius ofFading is mainly divided into two types, slow and BS/FRN; it inturn determines the optimal FRNfast. Slow fading is due to path-loss and shadowing, location. In the next subsections, we describe ourwhile fast fading is due to multi-path. In this paper, proposed methods to deploy FRNs optimally basedwe assume that the code lengths are large enough to on path-loss and SNR.reveal the ergodic nature of fast fading. Hence, we donot explicitly consider multi-path effect. We focus onthe path-loss and shadowing in the analysis. Because 3.1 Relay Placement based on Path-lossof the path-loss, the received signal power is In this subsection, we propose a simplifiedinversely proportional to the distance between the relay placement algorithm that depends on the path-transmitter and the receiver. In general, the path-loss loss. Path-loss is signal attenuation between a sourcePL between a transmitter and a receiver is given as, (BS/FRN) and a receiver (MS) which depends on the propagation distance. 2 𝛾 𝑃𝑇 4𝜋𝑓 𝑑∗ 𝑃𝐿 = 𝐺𝑇 𝐺𝑅 = (1) 𝑃𝑅 𝑐 𝑑0where PT is the transmitted power; PR is the receivedpower, GT and GR are the antenna gain of transmitterand receiver respectively; f is the carrier frequency, cis the speed of light; d* is separation between thetransmitter and receiver; d0 is the reference distance,and γ > 0 is the path-loss exponent [11].3. RELAY PLACEMENT SCHEMES Improvement in capacity and increase incoverage area are the main benefits of FRNs. Thesebenefits of FRNs are based on the position of relaysin the cell. Deploying FRNs around the edge of the Fig.2 Layout of FRN enhanced Cellular systemcell help the edge users. However, when they are illustrating the coverage of MS-BS, FRN-BS andplaced at inappropriate locations, may cause MS-FRN regions.interference to the edge users of the neighboring cell. We consider a simplified cellularTherefore, optimal placement of FRNs is a key configuration consisting of BS and FRNs as shown indesign issue. Consider the downlink scenario where Fig.2. Initially, FRNs are placed at random positionthe BS encodes the message and transmits it in the in a cell. MS moves from BS towards the cellfirst time slot to nearby MSs and FRNs. FRNs boundary along a straight-line trajectory.transmits the message to MSs at the cell boundary in Determination of optimal position is based on thethe second time slot. FRNs are either Decode-and- received signal strength and distance measurementsForward (DF) type, which fully decodes and re- along the path. MS evaluates the received signalencodes the message, or Amplify-and-Forward (AF) strength till the signal strength becomes equal to thetype, which amplifies and forwards the Message to preset threshold value.MSs in the second hop. Note that the reverse will befor uplink scenario. In downlink scenarios, we As the MS moves away from the BS/FRN, ifconsider non-transparent type relays, i.e., MSs in the the received signal strength decreases below afirst hop communicate to BS while MSs in the second threshold value and the MS is not able tohop communicate only to FRNs [15]. communicate with the BS. At this position where the received signal strength is too weak, appropriately In this section, we propose two types of placed FRN can enhance the signal quality of theRelay placement schemes to deploy FRNs optimally the cell. We analytically determine the position ofFRNs within the cell so that the QoS requirement of Now, we describe our path-loss based FRNeach user is satisfied. In each scheme we evaluate the placement algorithm as follows: Referring to Fig.2,signal strength on the three links (BS-MS, BS-FRN, let PBS and PFRN be the power transmitted by BS andand FRN-MS links). The term “three links” denote FRN respectively. Let Pb be the power received by 1503 | P a g e
  4. 4. Sultan F. Meko / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue4, July-August 2012, pp.1501-1509MS located at the distance (d) from BS; P r be the transmitted signal is diffracted due to a tree or the toppower received by FRN located at the distance (dr) of a building along the path of propagation. Let Г 𝑏𝐵𝑀 ,from BS; Pm be the power received by MS located at Г 𝑏𝐵𝑅 and Г 𝑏𝑅𝑀 be the SNR experienced by the user onthe distance (dm) from one of FRNs. When MS BS-MS, BS-FRN and FRN-MS links respectively. Infollows straight line trajectory, then BS, MS and this paper the term SNR is used to denote the effectsFRN are collinear and the radius of the cell D is of noise and shadowing, henceforth, we use SNR tocomputed as D = d + 2dm and also dr = d + dm. represent the effect of SNR plus shadowing. ThisAssume that the threshold Pb = Pm and Pb ≠ Pr, the algorithm is based on evaluation of SNR on threepath-loss (1) is redefined for the three links as: links as follows, 𝛾𝑏 𝑑0 Г 𝑏𝐵𝑀 = 𝑃 𝐵𝑆 − 10𝛾 𝑏 log 𝑑 + 𝜉 𝑏 − 𝑁 𝑏 (9) 𝑃 𝑏 = 𝐾1 𝑃 𝐵𝑆 (2) 𝑑 𝛾𝑟 Г 𝑏𝐵𝑅 = 𝑃 𝐵𝑆 − 10𝛾 𝑟 log 𝑑 𝑟 + 𝜉 𝑟 − 𝑁 𝑟 (10) 𝑑0 Г 𝑏𝑅𝑀 = 𝑃 𝐹𝑅𝑁 − 10𝛾 𝑚 log 𝑑 𝑚 + 𝜉 𝑚 − 𝑁 𝑚 (11) 𝑃𝑟 = 𝐾2 𝑃 𝐵𝑆 (3) 𝑑𝑟 𝛾𝑚 𝑑0 Note that we use subscript b, r and m in this paper to 𝑃 𝑚 = 𝐾3 𝑃 𝐹𝑅𝑁 (4) 𝑑𝑚 refer to the three links BS-MS, BS-FRN and FRN-where K1, K2 and K3 are constants; γb, γr and γm are MS respectively. ξb, 𝜉 r and 𝜉 m are Gaussian randomthe path-loss exponent on BS-MS, BS-FRN and variable with standard deviation of σb, σr and σm onFRN-MS links respectively. Based on (2), (3), (4) the three links mentioned above. Similarly, Nb, Nrand simple algebraic manipulation, the ratio and Nm denote the thermal noise. 𝑃 𝐵𝑆 𝑑𝛾𝑏 This algorithm evaluates the SNR of each = 𝛾𝑟 (5) link. We consider SNR as a decision parameter for 𝑃 𝐹𝑅𝑁 𝑚 𝑑𝑟 𝛾 the success/failure of a transmission on a link; the 𝑃𝑏 𝑑𝑟𝑟 = 𝛾𝑏 (6) received message is correctly decoded when the SNR 𝑃𝑟 𝑑𝑚 experienced by MS on each link exceeds a threshold SNR. Hence, the effective radius of the three linksremain constant. Based on these expressions, we can be obtained from the threshold SNR on the threedefine the optimal radius for BS-MS link or direct links. Let the threshold SNR on BS-MS, BS-FRNlink (𝑑 ) and that of FRN-MS link or relaying link and FRN-MS links are denoted as Гb, Гr and Гm(𝑑 m) as, respectively. Accordingly, the probability of successful decoding with direct transmission over 𝑑 = max 𝑑 , BS-MS (Prb) is given as, 𝑑 >0 𝑠. 𝑡 𝑠𝑎𝑡𝑖𝑠𝑓𝑦𝑖𝑛𝑔 𝑡𝑕𝑒 𝑐𝑜𝑛𝑠𝑡𝑟𝑎𝑖𝑛𝑡𝑠 (7) 𝑃𝑟 𝑏 = 𝑃𝑟 Г 𝑏𝐵𝑀 > Г 𝑏 𝑖𝑛 𝐸𝑞𝑢. 5 𝑎𝑛𝑑 6 = 𝑃𝑟 𝑃 𝐵𝑆 − 10𝛾 𝑏 log 𝑑 + 𝜉 𝑏 − 𝑁 𝑏 > Г 𝑏 𝑑𝑚 = argmax 𝑑𝑚 , = 𝑃𝑟 𝜉 𝑏 > 10𝛾 𝑏 log 𝑑 + 𝑁 𝑏 + Г 𝑏 𝑑 𝑚 ∈(0, 𝑑 𝑟 −𝑑 ) − 𝑃 𝐵𝑆 (12) 𝑠. 𝑡 𝑠𝑎𝑡𝑖𝑠𝑓𝑦𝑖𝑛𝑔 𝑡𝑕𝑒 𝑐𝑜𝑛𝑠𝑡𝑟𝑎𝑖𝑛𝑡𝑠 (8) 10𝛾 𝑏 log 𝑑 + 𝑁 𝑏 + Г 𝑏 − 𝑃 𝐵𝑆 𝑖𝑛 𝐸𝑞𝑢. 5 , 6 𝑎𝑛𝑑 7 = 𝑄 𝜎𝑏 We consider that for a given user position, if We choose threshold greater than the probability that the received SNR above the thresholdminimum signal strength below which call drops. is 95%, then the user has good link with the BS.Thus all admitted calls never drop. Since the effects Therefore such users do not require relaying support.noise, shadowing and interference from neighboring The effective radius for direct link (𝑑 ) is thecells are not considered in this model, this algorithm maximum distance where the above criteria isprovides the simplified estimate of FRN placement satisfied. Hence, optimum value of effective radiusscheme. In the next subsection, we propose a scheme for direct link (𝑑 ) is given as,that considers the effect of noise and shadowing. 𝑑 = max 𝑑 , 𝑑>03.2 Relay Placement based on SNR and 10𝛾 𝑏 log 𝑑 +𝑁 𝑏 +Г 𝑏 − 𝑃 𝐵𝑆 Shadowing 𝜎𝑏 = 𝑚𝑎𝑥 10 (13) Relay placement algorithm based on SNRand shadowing considers the case of isolated cell, 𝑠. 𝑡 𝑑 > 0; 𝑃𝑟 𝑏 ≥ 0.95,where interference from neighboring cells is not Similarly, the probability of successful decoding onconsidered. In addition to signal attenuation due to BS-FRN link ( 𝑃𝑟 𝑟 ) and on FRN-MS link ( 𝑃𝑟 𝑚 ) canpath-loss, we consider the effects of noise and be expressed as,shadowing. Shadowing effects happen when a 1504 | P a g e
  5. 5. Sultan F. Meko / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue4, July-August 2012, pp.1501-1509 10𝛾 𝑟 log 𝑑 +𝑁 𝑟 +Г 𝑟 − 𝑃 𝐵𝑆 𝐷𝐹𝑃𝑟 𝑟 = 𝑄 (14) 𝑠. 𝑡 𝑃suc ≥ 0.95 ; 𝑃𝑟 𝑏 𝜎𝑟 ≥ 0.95. (19) 10𝛾 𝑚 log 𝑑 + 𝑁 𝑚 + Г 𝑚 − 𝑃 𝐹𝑅𝑁𝑃𝑟 𝑚 = 𝑄 (15) 𝜎𝑚 3.3.2. Amplify and Forward RelayingThe equivalent SNR over two-hop transmission In amplify and forward relaying scheme, thedepends on the type of relaying scheme. In this paper AF relay amplifies the analog signal received fromwe consider Decode and Forward Relaying (DF) and the BS and transmits an amplified version of it to theAmplify and Forward Relaying (AF) relaying MSs [14]. Let the experienced SIR for this schemeschemes. be Г 𝐴𝐹 , then it is computed as follows.3.2.1 Decode and Forward Relaying Г 𝑏𝐵𝑅 Г 𝑏𝑅𝑀 Consider a downlink scenario, where each Г 𝐴𝐹 = (20) Г 𝐵𝑅 + Г 𝑏𝑅𝑀 +1 𝑏FRN decodes the signal received from BS-FRN linkand re-transmits to MS on the FRN-MS link. In thisscheme, the end to end rate achieved from BS to MS Let Г be the threshold SIR. If the experienced SIRis determined by the minimum rate achieved among falls below the threshold, the user is said to be inthe rates of BS-FRN and FRN-MS links, i.e., if the 𝑜𝑢𝑡 outage and outage probability is given as 𝑃 𝐴𝐹 out forrequired rate for this scheme is RDF, then AF scheme [14]. 2𝑅 𝐷𝐹 ≤ 𝑚𝑖𝑛 log 2 1 + Г 𝑏𝐵𝑅 , log 2 1 + Г 𝑏𝑅𝑀 (16) 𝑜𝑢𝑡 Г 𝑏𝐵𝑅 Г 𝑏𝑅𝑀 𝑃 𝐴𝐹 = 𝑃𝑟 ≤Г (21) Г 𝐵𝑅 + Г 𝑏𝑅𝑀 + 𝑏 1If the required rate is not achieved on either of thelink, then user is said to be in outage. Let the outage It is difficult to obtain the exact closed-form 𝐷𝐹 solution for outage probability of (21) [10]. However,probability in DF scheme be Гout , it can be expressed several literatures give the closed-form lower boundas and upper bound. We are interested in upper bound solution to compute the effective radius of relaying 𝑃out = Pr 𝑚𝑖𝑛 log 2 1 + Г 𝑏𝐵𝑅 , log 2 1 + Г 𝑏𝑅𝑀 𝐷𝐹 link. Accordingly, the optimum radius for relaying ≤ 2𝑅 link ( 𝑑 𝑚 ) due to AF relay is given as: = 1 − Pr 𝑚𝑖𝑛 log 2 1 + Г 𝑏𝐵𝑅 , log 2 1 + Г 𝑏𝑅𝑀 ≥ 2𝑅 𝑑𝑚 = argmax 𝑑𝑚 , 𝐷𝐹 𝑑 𝑚 ∈ 0, 𝑑 𝑟 −𝑑 = 1 − 𝑃suc (17) 𝑃 𝐹𝑅𝑁 − 𝑁 𝑚 − Г 𝑚 −𝜎 𝑚 .𝑄 −1 ( 𝑃 𝑟 𝑚 )If user does not go into outage, this means there is 10𝛾 𝑚successful transmission on BS-FRN and FRN-MS = argmax 10 𝐷𝐹 𝑑 𝑚 ∈(0, 𝑑 𝑟 −𝑑 )links. Let the 𝑃suc be the probability of successful 𝐴𝐹transmission on DF scheme, then it can be computed 𝑠. 𝑡 𝑃suc ≥ 0.95 , (22)as [9], 3.3 Optimal Number of FRNs𝑃suc = 𝑃𝑟 Г 𝑏𝐵𝑅 > Г 𝑟 . 𝑃𝑟 Г 𝑏𝑅𝑀 > Г 𝑚 𝐷𝐹 Assume that users are distributed uniformly 10𝛾 𝑚 log 𝑑 𝑚 + 𝑁 𝑚 + Г 𝑚 − 𝑃 𝐹𝑅𝑁 in the cell. Hence, the approximate number of FRNs = 𝑄 . required in a cell can be obtained by computing the 𝜎𝑚 10𝛾 𝑟 log 𝑑 𝑚 + 𝑁 𝑟 + Г 𝑟 − 𝑃 𝐵𝑆 area of inner region A1 and that of outer region A2. 𝑄 After simple mathematical steps, the number of FRN 𝜎𝑟 required in a cell is also computed as 𝑁 𝐹𝑅𝑁 = = 𝑃𝑟 𝑟 𝑃𝑟 𝑚 (18) 𝑑 4( + 1) . Since the entire cell radius is defined as 𝑑 𝑚 We consider the criterion 𝐷𝐹 𝑃suc ≥ 95% for D = d + 2dm, the number of FRN that can support thedeploying FRNs in cellular networks. Therefore, the users in outer region depend on optimal value of deffective radius (𝑑 m) for relaying link (FRN-MS) will and dm. We choose optimal value of d and d m thatbe optimal distance which 5% outage probability is minimizes the number of FRN used in the cell. The 𝐷𝐹satisfied, i.e., 𝑃out ≥ 0.05. Hence, the optimum optimization equation is described as,radius for FRN-MS link is given as, 𝑁 𝐹𝑅𝑁 = arg min 𝑁 𝐹𝑅𝑁 , 𝑑𝑚 = argmax 𝑑𝑚 , 𝑠. 𝑡 𝑑 , 𝑑 𝑚 𝑎𝑛𝑑 𝑑 𝑚 𝑠𝑎𝑡𝑖𝑠𝑓𝑦𝑖𝑛𝑔 𝐸𝑞𝑢. 19 𝑎𝑛𝑑 22 , 𝑑 𝑚 ∈ 0, 𝑑 𝑟 −𝑑 𝑑 𝑎𝑛𝑑 𝑑 𝑚 𝑠𝑎𝑡𝑖𝑠𝑓𝑦𝑖𝑛𝑔 𝐸𝑞𝑢. 8 (23) 𝑃 𝐹𝑅𝑁 − 𝑁 𝑚 − Г 𝑚 −𝜎 𝑚 .𝑄 −1 ( 𝑃 𝑟 𝑚 ) 3.4 Outage performance analysis 10𝛾 𝑚 Co-channel interference and shadowing = argmax 10 𝑑 𝑚 ∈(0, 𝑑 𝑟 −𝑑 ) effects are among the major factors that limit the 1505 | P a g e
  6. 6. Sultan F. Meko / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue4, July-August 2012, pp.1501-1509capacity and link quality of a wireless consider correlation among interferers and also thecommunications system. In this section, we use correlation that may exist between interferer andGauss-Markov model to evaluate the statistical desired signalscharacteristics of SIR in Multihop communicationchannels. By modeling SIR as log-normally 4. RESULTS AND DISCUSSIONdistributed random variable, we investigate the In this section, we present the analytical andperformance of relay placement schemes discussed in simulation results to illustrate the performance of ourSubsection 3.1 and 3.2. We make a comparison proposed FRN placement algorithms. We use Matlabbetween the above models in terms of performance simulation for modeling cellular networks underevaluation where outage probability is a QoS varying channel conditions. We analyze theparameter. Further more the result is used to find the performance of relay placement schemes based onmore realistic way of channel partitioning and relay path-loss and SNR as described in previous section.FRN placement schemes. The list of simulation parameters are mentioned in Table 1. The co-channel uplink interference to theBS/FRN of interest is assumed to be from MSs or Table 1. List of the simulation parameters.FRNs that links to first tier or upper tier cells (MS i′sor FRNK′s). Including the shadowing effect on the Parameters Valuesthree links, the SIR on each link can be described as; Carrier Frequency 5GHz 𝑁 −1 System Bandwidth (W) 25.6MHz 10 𝜉𝑏 10 1 Standard Deviation 8, 5, 8 dBГ 𝑏𝐵𝑀 = 𝛾𝑏 𝑑 𝛾𝑏 𝑑 𝑖 10 𝜉𝑏𝑖 10 ( σ𝑑, 𝜎𝑟and 𝜎𝑚) 𝑖=1 Correlation Coefficient 0.5 𝛾𝑏 𝜉𝑏𝑖 −𝜉𝑏 −1 = 𝑁 𝑑 10 10 (24) Path-loss Exponent 3.5, 2.5, 3.5 𝑖=1 𝑑𝑖 (𝛾𝑑, 𝛾𝑟 and 𝛾𝑚) 𝑁 −1 BS Transmit Power (PBS) 40dBm 10 𝜉𝑟 10 1 FRN Transmit Power (PFRN) 20dBmГ 𝑏𝐵𝑅 = 𝛾𝑟 𝛾𝑟 𝑑𝑟 𝑑 𝑟𝑖 10 𝜉𝑟𝑖 10 MS transmit Power (PMS) 2dBm 𝑖=1 Threshold (Г) -10dBm 𝛾𝑟 𝜉𝑟𝑖 −𝜉𝑟 −1 𝑁 𝑑 Thermal Noise (N) -100dBm = 𝑖=1 10 10 (25) 𝑑 𝑟𝑖 𝑁 −1 The optimal FRNs placement results are summarized 10 𝜉𝑚 10 1 in the Table 2.Г 𝑏𝑅𝑀 = 𝛾𝑚 𝛾𝑟 𝑑𝑚 𝑑 𝑚𝑖 10 𝜉𝑚𝑖 10 𝑖=1 Table 2. Results of optimal FRNs placement based on 𝛾𝑚 𝜉𝑚𝑖 −𝜉𝑚 −1 = 𝑁 𝑑 10 10 (26) path-loss and SNR. 𝑖=1 𝑑 𝑚𝑖where d and dr are the location of desired MS and Parameters Based on Path-loss Based on SNRFRN from the BS0 on direct link, d m is the location of dr 2120m 2076mdesired MS from desired FRN under the second hop. d 1611m 1522mSimilarly, di and dri are the location of co-channel dm 509m 554minterferer MS and FRN from the BS0, while dmi NFRN 6 6denotes the location of co-channel interferer MSsfrom FRN that is associated to BS0. Shadowing forthe desired links are denoted as ξb, 𝜉 r and 𝜉 m For Fig. 3 illustrates the outage probability ofinterfering links shadowing is expressed as ξbi, 𝜉 ri and downlink cellular network where the optimal FRN 𝜉 mi to denote the interfering link of MS-BS, FRN-BS placement scheme is based on path-loss. This figureand MS-FRN; in all cases, i∈ {1, ..., 18}. compares the outage probability of relay enhanced cellular system of direct link and relay link along Basically, the outage probability analysis for with the scheme without FRNs. Furthermore, Fig. 3the three links (MS-BS, FRN-BS and MS-FRNs) is demonstrates that outage probability is significantlysimilar to the expression given in Section 3.2. improved in relay enhanced cellular system withHowever, interference from neighboring cells is optimal FRN placement. As it can be shown in thisconsidered here. We compute the mean and standard figure, at SIR threshold of 0 dB, the outagedeviation of both desired and interference signal probability of the scheme without FRNs is 40%. But,based on Fenton-Wilkinson′s and Schwartz-Yeh′s the outage probability of our proposed scheme formethod [13]. Shadowing is usually represented by FRN-MS link is nearly 0%. This means that due toi.i.d. log-normal model in wireless multi-hop models. optimal placement of FRNs, outage probability atHowever, shadowing paths are correlated. Hence, we cellular boundary improves by 40%. Since the users at the cell boundaries dominate the system 1506 | P a g e
  7. 7. Sultan F. Meko / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue4, July-August 2012, pp.1501-1509performance, our optimal relay placement scheme Fig. 5. Outage probability versus SIR threshold of DFsignificantly improves the outage probability of MSs relay enhanced cellular system with 1200 sectoring inof second hop link. the inner region; proposed optimal FRN placement Simulation of cellular radio system 00 Sectorization schemes are compared with FRNs location at 2/3rd of 100 cell radius. 90 without FRN With FRN (direct link MS-FRN) Fig. 4 illustrates the outage probability Outage Probability (%) based on Pathloss 80 With FRN (2nd hop link MS-FRN) versus SIR threshold for DF relay where the inner 70 region of the cell is sectored in to 600 sectoring (refer 60 Fig.1). We compare our proposed optimal FRN 50 placement schemes with a scheme that places FRNs at 2/3rd position of the cell radius. The results show 40 that our proposed schemes achieve significant 30 improvement on the performance of the cellular network. In Fig.4, at SIR threshold of -20 dB, the 20 outage probability of a system when FRNs are placed 10 at 2/3rd of cellular radius is 50% whereas, the outage 0 probability of our proposed scheme for FRN-MS link -50 -40 -30 -20 -10 0 10 20 30 Threshold SIRo (dB) is only 2%. However, there is no significant difference in performance among our proposedFig. 3. Outage probability versus SIR threshold of DF schemes. As can be shown in Fig.4, Fig.5 and Fig.6,relay enhanced cellular system with no sector in the comparing the performance of our two proposedinner region; optimal FRN placement is based on schemes, the outage probability of optimal FRNspath-loss. placement scheme which depends on path-loss and Simulation of cellular radio system 600 Sectoring (DF) 100 that of the scheme which depends on SNR are nearly equal. Hence, either of the two proposed schemes can 90 be implemented for optimal FRNs placement to 80 achieve the same QOS requirement. Simulation of cellular radio system 00 Sectorization (DF) 70 100 Outage Probability (%) FRN placement at 60 2/3 of total radius 90 Optimal FRN placement 50 based on pathloss 80 Optimal FRN placement Relay placement 70 40 based on SNR based on pathloss Outage Probability (%) Relay placement 60 based on SNR 30 Relay placement at 50 2/3 of total radius 20 40 10 30 0 20 -50 -40 -30 -20 -10 0 10 20 30 Threshold SIRo (dB) 10Fig. 4. Outage probability versus SIR threshold of DF 0 -50 -40 -30 -20 -10 0 10 20 30relay enhanced cellular system with 600 sectoring in Threshold SIRo (dB)the inner region; proposed optimal FRN placementschemes are compared with FRNs location at 2/3rd of Fig. 6. Outage probability versus SIR threshold of DFcell radius. relay enhanced cellular system with no sectoring in 100 Simulation of cellular radio system 1200 Sectoring the inner region; proposed optimal FRN placement schemes are compared with FRNs location at 2/3rd of 90 cell radius. 80 70 Fig.4, Fig.5 and Fig.6 also illustrate that sectoring the BS-MS direct link of the cell Outage Probability (%) 60 Optimal Relay placement significantly improves the outage performance of the 50 based on pathloss Optimal Relay placement cell in which FRNs are placed at 2/3rd of the cell 40 based on SNR radius while sectoring has no significant effect on the Relay placement at 2/3 of 30 cell radiusased on pathloss cellular networks that use the proposed optimally 20 FRNs placement schemes. Even though sectoring lowers the effect of co-channel interference, it also 10 degrades system capacity. 0 -50 -40 -30 -20 -10 0 10 20 30 Threshold SIRo (dB) 1507 | P a g e
  8. 8. Sultan F. Meko / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue4, July-August 2012, pp.1501-1509 Fig.7 shows the performance of path-loss effects of co-channel interference, but decreasesbased optimal FRNs placement schemes for 6 and 18 system interferers. We evaluate the performanceof the two hop cellular network with AF and DF 5. CONCLUSIONrelay. Since AF relay scheme amplifies the required This research work proposed two optimalsignal and noise, the cellular network performance FRNs placement schemes which are based on path-decreases with the increase in number of co-channel loss and SNR. The proposed schemes also computecells. We also observe that, outage probability of the optimal number of FRNs that are required to enhancesystem at -20 dB SIR threshold is improved by 80% the QoS of cellular system. Our schemes alsowhen the co-channel decreases from two tire (18 cell) investigate effects of sectoring the inner region of theto one tier (6 cells) cell. However such effect is not cell on optimal FRN placements. Sectoring the innersignificantly observed in DF relaying scheme. region for improves the outage probability. Our optimal relay node placement schemes significantly Simulation of cellular radio system 00 Sectorization reduce the outage of the cellular system and provide 100 better QoS for users at cell boundaries. 90 80 6. ACKNOWLEDGMENT Outage Probability (%) based on Pathloss 70 This research work is supported by India-UK 60 Advanced Technology Centre (IU-ATC) of Excellence in Next Generation Networks Systems 50 DF Relay 6 cochannel cells and Services. 40 AF Relay 6 cochannel cells DF Relay 18 cochannel cells 30 AF Relay 18 cochannel cells References [1] Zaher Dawy, Sami Arayssi, Ibrahim Abdel 20 Nabi, and Ahmad Husseini. Fixed relaying 10 with advanced antennas for CDMA cellular 0 networks. In IEEE GLOBECOM -50 -40 -30 -20 -10 0 10 20 30 Threshold SIRo (dB) proceedings, 2006. [2] H. Hu, H. Yanikomeroglu, and et al. RangeFig.7. Outage probability versus SIR threshold of AF extension without capacity penalty inand DF relay enhanced cellular system with one tier cellular networks with digital fixed relays.and two tier co-channel cells; Optimal FRNs IEEE Globecom, Dec 2004.placement is based on path-loss [3] I. Katzela and M. Naghshineh. Channel 100 Simulation of cellular radio system (AF Relay) assignment scheme for cellular mobile no sectoring telecommunication systems: A 90 0 120 sectoring comprehensive survey. IEEE Personal Outage Probability (%) based on Model SNR 80 0 60 sectoring Communication, pages 10–31, June 1996. [4] Jian Liang, Hui Yin, Haokai Chen, 70 Zhongnian Li, and Shouyin Liu. A novel 60 dynamic full frequency reuse scheme in 50 OFDMA cellular relay networks. In Vehicular Technology Conference (VTC 40 Fall), 2011 IEEE, pages 1 –5, sept. 2011. 30 [5] Min Liang, Fang Liu, Zhe Chen, Ya Feng 20 Wang, and Da Cheng Yang. A novel frequency reuse scheme for ofdma based 10 relay enhanced cellular networks. In 0 -50 -40 -30 -20 -10 0 10 20 30 Vehicular Technology Conference, 2009. Threshold SIRo (dB) VTC Spring 2009. IEEE 69th, pages 1 –5,Fig.8. Outage probability versus SIR threshold of AF april 2009.relay enhanced cellular system with 00, 600 and 1200 [6] Sultan F. Meko and Prasanna Chaporkar.sectors in the inner region of the cell; optimal FRNs Channel Partitioning and Relay Placementplacement is based on SNR. in Multi-hop Cellular Networks. In Proc. IEEE ISWCS, 7-10 Sept. 2009. Fig.8 shows the effect of sectoring on AF relay [7] S. Mengesha, H. Karl, and which the optimal FRNs placement is based on Capacity increase of multi-hop cellularSNR. Similar to above results, sectoring can improve WLANs exploiting data rate adaptation andthe outage probability of users by lowering the frequency recycling. In MedHocNet, June 2004. 1508 | P a g e
  9. 9. Sultan F. Meko / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue4, July-August 2012, pp.1501-1509[8] M. Rong et al P. Li. Reuse partitioning based frequency planning for relay enhanced cellular system with NLOS BS-Relay links. IEEE, 2006.[9] A. Papoulis. Probability, Random Variables, and Stochastic Processes. 3rd edition, McGraw-Hill,1991[10] V. Sreng, H. Yanikomeroglu, and D. Falconer. Coverage enhancement through two-hop relaying in cellular radio systems. IEEE WCNC, 2:881– 885, Mar 2002.[11] D. Tse and P. Viswanath. Fundamentals of Wireless Communications. Cambridge University Press, 2005.[12] D. Walsh. Two-hop relaying in CDMA networks using unlicensened bands. Master’s thesis, Carleton Univ., Jan 2004.[13] Jingxian Wu, N.B. Mehta, and Jin Zhang. Flexible lognormal sum approximation method. In IEEE GLOBECOM, pages 3413– 3417, Dec 2005.[14] H. Yanikomeroglu. Fixed and mobile relaying technologies for cellular networks. In Second Workshop on Applications and Services in Wireless Networks, July 2002.[15] Sultan F. Meko. Impact of Channel Partitioning and Relay Placement on Resource Allocation in OFDMA Cellular Networks, International Journal of Wireless & Mobile Networks (IJWMN) Vol. 4, No. 3, June 20124. 1509 | P a g e