Cost Based Local Forwarding Transmission Schemes for Two-hop Cellular Networks

Transcription

1 Cost Based Local Forwarding Transmission Schemes for Two-hop Celllar Networks Zhenggang Zhao, Xming Fang, Yan Long, Xiaopeng H, Ye Zhao Key Lab of Information Coding & Transmission Sothwest Jiaotong University, Chengd, China Abstract In crrent mlti-hop relay networks (MRNs, normally the commnication path is not optimized, especially when two mobile stations (MSs attached to the same relay station ( need to commnicate with each other. In this paper, a novel data transmission scheme: administrable and controllable local forwarding for OFDMA-TDD based two-hop relay celllar networks, is proposed. In the local forwarding mode, data transmission between two MSs attached to the same is forwarded withot necessarily being sent to the base station (BS. Thereby the spectral efficiency is improved. And then two cost based relay selection schemes (Ss are proposed for the efficient sage of local forwarding mode. The first is a modified traditional S (S-MT, which implements local forwarding after relay selection. And the second is a local forwarding-based S (S-LF, which takes the advantages of the local forwarding into accont dring relay selection. Nmerical and simlation reslts show that the local forwarding transmission schemes can improve the system capacity greatly. Moreover, the performance of S-LF is shown to be sperior to that of S- MT. Keywords-cost; local forwarding; OFDMA-TDD; two-hop networks I. INTRODUCTION One of the key problems in forth-generation (4G wireless systems is the capacity of the networks. In LTE-Advanced and IEEE m systems, the spectrm that will be released are sally above 2GHz, on which radio signals experience serios path loss and channel fading. To spply the very high system capacity, network operators have to lift the transmit power level of BSs or deploy more BSs in the service region, which leads to higher deployment costs and larger inter-cell interferences. Relay-based deployment for wireless networks is a good soltion for the above mentioned problems by exploiting spatial rese and combating the shadowing at high radio freqency [1-2]. covers smaller area than BS does, and two or more s can transmit simltaneosly with the same freqency partitions in the same cell to improve the spectral efficiency. With relaying, the low-qality radio link between BS and MS located in cell bondary can be replaced by two high-qality BS link and MS link, ths the overall system capacity is increased. Even thogh mlti-hop transmission has advantages, it experiences a penalty: additional resorces shold be allocated Yang Li, Yqin Chen, Hongyn Q, Ling X ZTE Corporation Shenzhen, China {li.yang8, chen.yqin}@zte.com.cn for relay links (BS links. In the crrent MRNs, all data commnications between two MSs in the same cell mst commnicate with each other via BS s forwarding no matter how close they are [3]. If two MSs attached to the same are commnicating with each other, the data transmitted from one MS shold be relayed by the to BS, and then the same data are forwarded by the BS to the same again for s relaying to the other MS. The data forwarding via BS s air interface consmes additional system resorces. To improve the spectral efficiency, we proposed an administrable and controllable local forwarding data transmission scheme to IEEE m WG [4], and then a transmission protocol is proposed in [5] for its implementation, which can be compatible with the standard of IEEE m. The main contribtion of this paper is to provide two Ss with administrable and controllable local forwarding mode for OFDMA-TDD based two-hop relay celllar networks. To or best knowledge, there is no yet sch local forwarding related S in MRNs. The rest of the paper is organized as follows. The concept of local forwarding is presented in section II, and its sage scenarios are also discssed in this section. In section III, two cost based Ss are proposed for MRN with administrable and controllable local forwarding mode. Nmerical and simlation reslts are discssed in section IV. Finally, we conclde in section V. II. CONCEPT AND USAGE SCENARIOS A. Concept of the Local Forwarding As shown in Fig. 1(a, in crrent MRN, if two MSs (MS1 and MS2 are within the same, and MS1 wishes to commnicate with MS2, the data transmission path is MS1 BS MS2. In this case, let s observe the relay links, the same data is relayed by to BS, and then forwarded back again. Obviosly, this kind of commnication path is very inefficient. As shown in Fig. 1(b, the local forwarding transmission scheme enables MS1 and MS2 to deliver packets directly by the forwarding of the withot sending packets to the BS. For local forwarding mode involved, the data transmission path MS1 BS MS2 is replaced by the path MS1 MS2, which redces the commnication hops This work is spported by NSFC nder the Grant and SWJTU Fondation of China nder the Grant 2008A /10/$ IEEE

2 (a (b Figre.1 Data transmission path in MRN with local forwarding between two MSs and resorces allocated to the relay links, meanwhile, mitigates the brden of the BS s air interface. B. Data and Contral Channel in Local Forwarding Mode Unlike traditional peer-to-peer (P2P network, in or proposal, the local forwarding is administrable and controllable to operators. As shown in Fig. 2, in the local forwarding mode, signaling messages are still sent to BS via the traditional commnication path, only data is forwarded locally by. Important signaling messages, sch as athentication, athorization and acconting, are still reported to BS by s forwarding. So operators can spervise and manage the network according to their own policies determined. And another advantage is that few modifications are needed by IEEE m standard for local forwarding spporting in this way. C. Usage Scenarios for Local Forwarding It is noticed that local area commnications play a more important role in or daily life. Some poplar scenarios of local forwarding are shown in Fig. 3. For hot spots sch as camps, hospitals, sightseeing places and enterprises, most of the commnications are of similar type and may be limited in the local area. For instance, stdents intend to share videos, large size photos, msic and files among classmates and friends in the camps. A grop of travelers are expected to have video commnications, exchange photos and videos taken in the sightseeing at the traveling place. In a company, most of commnication among the staffs are on the bsiness and shold be limited to a specific grop of internal people. Additionally, as shown in Fig. 4, local forwarding can be applied in vehicle commnication systems. For example, we can deploy an on the top of a train, and then the commnications happened between sers on the train can be established via local forwarding mode. In this case, only relay links shold be switched when handover occrs, the local forwarding path can be retained. Therefore the handover complexity is redced. III. RELAY SELECTION SCHEMES A. Slot Efficiency In this paper, we define a measrement, termed Slot Efficiency to evalate the radio resorce tilization of an individal link. Figre.2 Hospital Park Data and Contral channel in local forwarding mode Figre.3 BS1 Sightseeing School Let s take IEEE as an example. According to IEEE standards, a TDD frame consists of several slots, in which a slot is the minimm time-freqency resorce nit and it contains 48 data sbcarriers. Adaptive modlation and coding (AMC is tilized in the system. If M-ary (M=2 m modlation is chosen, and the coding rate is r, then m r bits can be carried by one data sbcarrier. Slot efficiency can be defined as 48 m r η ( mrt,, frame = (1 T frame P2P File Sharing Usage scenarios for local forwarding handover from BS1 to BS2 BS2 Enterprise Network Figre.4 Example scenaria for vehicle commnication

3 where T frame is the TDD frame length. Table I illstrates the reqired SINR level for the given modlation and coding scheme (MCS. Different SINR level corresponds to different MCS, hence we have the following eqation ( m r T η( SINR = η,, frame (2 where SINR represents the SINR of a link. The mapping relationship between SINR and (m,r,t frame can be fond in Table I. B. A Cost Based Modified Traditionanl S for Local Forwarding (S-MT In MRN, mltiple diverse transmission paths may exist between BS and an MS, and the data can be transmitted directly or forwarded by. Let MS denote the -th ser, it can select the BS as the access station by transmitting data on MS BS link (L bm. Or it can access to throgh MS link (L rm, and then connect with BS by data forwarding on BS link (L br. An example is given in Fig. 5(a. Let SINR bm, SINR rm and SINR br represent the SINR of link L bm, L rm and L br respectively. We consider the ser traffic rate is R. We frther define the following fnction to represent the cost of selecting as the access station. C = + η( SINRbr η( SINRrm brm where x represents the smallest integer more than or eqal to x. The first (or second part of eqation (3 means that how many slots shold be allocated to the first-hop link L br (or the second-hop link L rm for satisfying the target traffic rate R. And the relay selection cost C brm shold be the total nmber of slots allocated to the -th ser for transmitting data at the rate of R. Similarly, the cost of selecting to access to BS can be represented as C bm R = η( SINRbm Then the selection cost of the S-MT scheme is S MT bm brm (3 (4 C = min( C, C (5 The station with the minimm cost will be selected as the access station, becase the system can se the smallest OFDMA slots to transmit the same amont of data. In or scheme, BS is responsible for calclating the selecting cost of each station for an MS. BS can always obtain the vales of SINR on BS and BS MS link. The information of TABLE I. THE MODE OF AMC [6] MCS SINR(dB Slot Efficiency (kbps/slot QPSK (1/ QPSK (1/ QPSK (1/ QPSK (1/ QPSK (2/ QAM(1/ QAM(2/ QAM(2/ QAM(5/ L bm 1 L rm 1 2 br Lbr L ( 2 L rm 2 L bm MS 1 MS 2 MS link, inclding the vale of SINR, is reported to BS by. Therefore, the basic condition for BS to implement relay selection scheme is met. The timing to implement local forwarding in scheme of S-MT is shown in Fig. 6. Only the two MSs, which are determined to access to the same by sing the above algorithm individally, can commnicate with each other on local forwarding mode. In the scheme, it implements local forwarding after relay selection which can be flly compatible with the traditional MRN. Since the characteristics of local forwarding are not taken into consideration dring relay selection, the reslt is probably not the optimal. C. A Cost Based S for Local Forwarding (S-LF As shown in Fig. 5(a and Fig. 5(b, if two MSs (MS 1 and MS 2 which want to commnicate with each other are in the same sector of the same cell, they have five optional commnication paths: MS 1 BS MS 2, 1 L lf 2 L lf MS 1 MS 2 (a Paths in traditional MRN (b The local forwarding path Figre 5. Commnication paths and links in MRN Traditional transmission mode via BS Figre 6. BS Service is reqested by MS MS selects access station (BS or by sing S-MT No checks whether the ser called by MS is attached to it Yes Commnicate with each other via local forwarding mode The timing to implement local forwarding in S-MT

4 No MS 1 and MS 2 select access station(s by sing S-MT Traditional transmission mode via BS Figre 7. MS 1 BS MS 2, MS 1 BS MS 2, MS 1 BS MS 2 and MS 1 MS 2. In the first for paths, as shown in Fig. 5(a, both control and data information is transmitted on them. Bt in the last path (the local forwarding path, as shown in Fig. 5(b, the signaling information mst be transmitted on the relay links, whereas there is no data transmission on them. In or following analysis, the little amont of control information on BS link in local forwarding mode is neglected de to the week impact on the system performance. We can select an optimal path from the first for by sing S-MT firstly, and the total selection cost of MS 1 and MS 2 can be calclated as bm brm bm brm C ( 1, 2 = min( C, C + min( C, C (6 The total cost of selecting the last path can be calclated as C (1, 2 = η( SINRlf η( SINRlf = η( SINRrm η( SINRrm where SINR 1 2 lf and SINR lf represents the SINR of the L lf 1 and L lf 2 link respectively. Then the selection cost of the S-LF scheme is (7 C = min[ C(1, 2, C (1, 2] (8 S LF In S-MT, MS 1 and MS 2 select their own access station individally, whereas they select commnication path together in S-LF. So the latter wold be more efficient. The timing to implement local forwarding in S-LF is shown in Fig. 7, where local forwarding transmission scheme is taken into accont dring relay selection. IV. MS 1 wants to commnicate with MS 2 System checks whether the MS 1 and MS 2 are in the same sector of the same cell SIMULATION AND RESULTS A. Simlation model and parameters A 19 cell topology with wrap arond is considered in this paper. Each cell is divided into 3 sectors and two above rooftop Yes MS 1 and MS 2 select access station(s by sing S-LF Commnicate with each other via the selected transmission mode The timing to implement local forwarding in S-LF TABLE II. SYSTEM-LEVEL SIMULATION PAPAMETE Parameter Vale Nmber of sites 19 Nmber of sectors per site 3 Freqency rese factor 1/3/3 Channel bandwidth 10 MHz for TDD UL/DL dplexing scheme TDD (28 symbols for DL(control overhead of 6 symbols, 22 symbols for data Nmber of 2 s per sector Site-to-site distance 1.5Km BS- distance (r 3/8 of site-to-site distance Angle between location and BS antenna boresight 26 direction (φ transmit power 46dBm per antenna BS antenna height 32 m cable loss transmit power 36dBm per antenna antenna height 32 m noise figre 5dB cable loss 2dB MS antenna height 1.5m noise figre 7dB Traffic Model fll bffer Traffic rate (R 384Kbps Schedle Model Rond Robin TABLE III. CHANNEL MODELS parameters vale BS- link Recommendation ITU-R M.1225 [7] Propagation BS-MS and Model IEEE j EVM Type D [8] -MS link BS- link 3.4dB Log-Normal BS-MS and Shadowing SD 8dB -MS link BS MS TABLE IV. parameters antenna gain (boresight BS- link antenna gain (boresight -MS link antenna gain antenna gain ANTENNA MODELS vale 2 θ A( θ = min 12, A m, θ θ = 70, A = 30dB (ART s are deployed in each sector. Detailed system-level simlation parameters are shown in Table II. Detailed antenna models of each type of station and path loss models are shown in Table III and Table IV, respectively. We assme that 100 MSs are niformly distribted in the entire cell, and the sers are schedled by sing Rond-Robin algorithm. The traffic rate R is 384kbps for each ser. Then, the system capacity is given as 17dBi 2 θ A( θ = min 12, A m, θ θ = 20, A = 2 20dBi Omni 7dBi Omni 0dBi m m

5 N frame Capacity = Nschedled ( i R N frame (9 i= 1 where N frame is the total nmber of TDD frame for simlation; N schedled (i represents the nmber of MSs served in the i-th frame. B. Simlation reslts and analysis To compare the system capacity with and withot or proposed local forwarding scheme in the MRN, one problem shold be addressed is that how many MSs cold possibly commnicate with each other in local forwarding mode. We define a local forwarding probability p to represent the proportion in all network sers. It is the probability of commnication occrred between two MSs which are in the same sector of the same cell. In Fig. 8, the system capacity verss local forwarding probability is presented. From the reslts, the proposed local forwarding schemes achieve the higher system capacity in all cases de to the redced commnication hops. For example, with p=0.4, the system capacity withot local forwarding is Mbps, while the system capacity with local forwarding is Mbps and Mbps by sing scheme of S-MT and S-LF respectively. It can reach a gain of 8.22% by sing scheme of S-MT, and reach a gain of 15.3% by sing scheme of S-LF. Fig. 9 illstrates the increment of the system capacity with local forwarding over traditional MRN withot local forwarding by sing different relay selection scheme with different probability p. As shown in the figre, employing local forwarding can improve the system capacity significantly, and the scheme of S-LF is sperior to S-MT. V. CONCLUSIONS An administrable and controllable local forwarding data transmission mode and two cost based relay selection schemes are proposed in this paper to optimize the commnication path of the crrent MRNs. Since the signaling process is still same as that in traditional celllar networks, the operators do not necessarily need to worry abot the administrable and controllable isses. It is obvios that the local forwarding scheme redces the commnication hops between two MSs, the radio resorce which shold be allocated to relay links are saved on, ths the system capacity is increased. And the two cost based Ss, which are proposed for MRN with local forwarding, can satisfy the traffic needs of sers by sing minimm slot resorce. In S-MT scheme, local forwarding path is determined after relay selection. As a contrast, S-LF is proved to be more efficient since the path and access station are considered simltaneosly. REFERENCES [1] H. Yanikomerogl, Fixed and mobile relaying technologies for celllar networks, in the 2nd Workshop on Applicaiions and Services. in Wireless Networks (ASWN, Paris, France, Jly 3 5, 2002, pp System capacity (Mbps Capacity incrsment (% S-MT withot Local Forwarding S-MT with Local Forwarding S-LF with Local Forwarding Local forwarding probability (p Figre 9. Figre 8. Comparison of system capacity in different S S-MT with Local Forwarding S-LF with Local Forwarding Local forwarding probability (p Capacity increments (% by sing S-MT and S-LF scheme [2] R. Pabst, B. Walke, D. C. Schltz, and et al, Relay-based deployment concepts for wireless and mobile broadband radio, IEEE Commnications Magazine, vol. 42, pp , Sept [3] Relay Task Grop of IEEE , Draft Amendment to IEEE Standard for Local and metropolitan area networks Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems: Mltihop Relay Specification, IEEE P802.16j/D6, Jly [4] Yang Li, Yqin Chen, Hongyn Q, and et al, Comments on Data Forwarding in Project m System Description Docment, IEEE C802.16m-08/1310, [5] Xiaopeng H, Xming Fang, Zhenggang Zhao, Yan Long, Ye Zhao, A Local Forwarding Implementation Scheme in OFDMA-based Mltihop Celllar Networks, NISS2009, Beijing, China, 30 Jne-2 Jly [6] K, J.M., Kim, P.K., Lee, S.J., Shin, S. and Kang, C.G.., On the Performance of Broadband Mobile Internet Access System, IEEE WPC 2006, Jan. 2006, pp [7] ITU-R M.1225, Gidelines for evalation of radio transmission technologies for IMT-2000, [8] IEEE j-06/013r3, Mlti-hop Relay System Evalation Methodology (Channel Model and Performance Metric,