Telecommun Syst (2013) 52:1697–1704 DOI 10.1007/s11235-011-9661-7
Optimization of association procedure in WiMAX networks with relay stations Pavel Mach · Robert Bestak · Zdenek Becvar
Published online: 14 September 2011 © Springer Science+Business Media, LLC 2011
Abstract When a MS enters to the WiMAX network, a network entry procedure has to be performed. The aim of procedure is twofold. Firstly, several connections between the MS and BS are created, i.e. basic, primary and secondary management connections to control data transmissions. Secondly, the MS is admitted into the network. According to the IEEE 802.16 standard, a MS always tries to associate to BS with the highest received signal quality. This method is suitable as long as the MS is directly connected to the network via BS. However by introducing relay stations to the WiMAX architecture, the MS entry procedure needs to be modified. Mainly, the point of attachment influences the network performance. This paper proposes an optimized association procedure which takes into account the use of relays stations in the network. The obtained results show improvement of system performance. Keywords Association · Relay station · Radio resource cost · WiMAX · Path selection 1 Introduction The WiMAX technology is a worldwide wireless networking standard that addresses interoperability across IEEE
The paper was originally presented in IWSSIP2008 conference. P. Mach · R. Bestak () · Z. Becvar Department of Telecommunication, Czech Technical University in Prague, Technicka 2, 16627 Prague, Czech Republic e-mail:
[email protected] P. Mach e-mail:
[email protected] Z. Becvar e-mail:
[email protected] url: www.cvut.cz/en
802.16 standard-based products. Until now, two standards were approved: IEEE 802.16-2004 [1] intended for fixed scenarios and the IEEE 802.16e [2] implementing to the former standard mobility features such as handover or power management modes. Since requirements and demands to deliver high data transmission rates are increasing, existing technologies try to satisfy these trends. This is the main motivation why a new WiMAX working group, entitled as IEEE 802.16j, was established in 2006. The IEEE 802.16j [3] version introduces to WiMAX system network entities called Relay Stations (RSs). The RS has two main purposes, i.e. to enhance the system capacity and to increase the network coverage (see Fig. 1). The RS represents a simplified Base Station (BS). It provides media access management in its area and data transfer between BS/RS and Mobile Stations (MSs). The network efficiency and cost reduction can be significantly improved by this way. How to integrate the RSs into the IEEE 802.16 network is described in [4]. According to [5], three types of RSs are specified; fixed, nomadic and mobile RSs. This paper only considers fixed RSs, i.e. RSs installed at the same location. The RSs are in most cases build in, owned and controlled by service provider. The RS is not directly connected to the wired infrastructure and has minimum functionalities to support multi-hop communication. RSs can be further divided to transparent (T-RS) and non-transparent (NT-RS). In addition, two types of NT-RSs can be considered: (i) centrally controlled RS (CC-RS) and (ii) de-centrally (in some papers denoted as distributed) controlled RS (DC-RS). More information about RS’s classification can be found for example in [6]. When a new MS associates to the WiMAX network, several steps have to be accomplished during the initial process.
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Fig. 1 Utilization of RS in WiMAX networks
Although, the IEEE 802.16-2004 and IEEE 802.16e define the network entry procedure for point to multipoint (PMP) network topology (mandatory) and mesh network topology (optional), association in relay based network has to be modified due to implementation of RSs. This is the main focus of the paper. The following section summarizes works related to the association procedure in WiMAX networks with RSs. The next section briefly describes the network entry procedure for the PMP mode (the mesh topology is not taken into consideration) and it depicts the enhancement to the original network entry procedure and introduces a new metric for MS’s path selection. Section IV describes simulation model used in evaluation of our proposal. Additionally, obtained results are presented and discussed. The last section gives our conclusions.
2 Related work The initial point of MS’s attachment is provided via network entry procedure. By introduction of RSs, the network entry procedure has to be modified. In [7], the RS and MS network entry procedure is described. The MS performs exactly the same procedure as described in existing standard. Nevertheless, the RS is also involved in the MS’s association process. Other possible way of MS’s network entry is suggested in [8]. The RS receiving a ranging request message (RNG-REQ) with satisfying quality volunteers to serve the MS. The authors discuss single and multi RS scenarios where they assume the MS being always in the BS range. A relay assisted MS network entry procedure is introduced in [9]. Again, the authors consider the MS being always in the range of BS. The BS performs routing decision before the initialization procedure is completed. The proposal of a MS’s network entry procedure for a NT-RS case is addressed in [10] (for DC-RS) and in [11] (for CC-RS). The utilization of T-RS scenario is considered in [12]. All the above mentioned proposals have two drawbacks. Firstly, none of the proposals describe algorithm for a path selection which is necessary when using RSs. Although the issue of routing is
tentatively addressed in [13], the authors only focus on the RS’s network entry. More than that, no actual end-to-end path metric is proposed in the paper. Secondly, no proposal takes into account the fact that a MS always tries to associate to a station with the highest received quality of signal. This is due to the fact that the legacy MS (i.e. MS supporting only IEEE 802.16-2004 or IEEE 802.16e standard) is not able to distinguish between RS and BS. The RS appears to a MS as another BS in the system. To remedy these drawbacks, we propose a new optimized network entry procedure which addresses both previously mentioned disadvantages.
3 Association procedure The association procedure enables a MS to connect to the network via BS (according to IEEE 802.16e) or via RS (IEEE 802.16j). Both scenarios are described in the following subsections. 3.1 Procedure according to standards The network entry procedure can be divided into several phases. In every phase, specific MAC management messages are exchanged between BS and MS. For the reason of simplicity, Fig. 2 only shows the mandatory network association steps. At the beginning of procedure, the MS scans individual downlink (DL) channels searching for the DL-MAP and DCD (Downlink Channel Descriptor) messages broadcasted by BS. By means of these messages, the MS gets synchronize with the BS and obtains DL parameters. When successfully synchronized, the MS waits for the UL-MAP and UCD (Uplink Channel Descriptor) messages to learn UL parameters. Through the UL-MAP, the MS discovers initial ranging slots in which the RNG-REQ message is supposed to be sent to initiate the ranging process. Within the ranging process, the MS acquires transmission parameters such as time offset, transmitted power level along with basic and primary management Connection Identifiers (CID).
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vent above mentioned drawbacks is to implement a mechanism that would determine to which station a MS should associate (or re-associate). Two types of association procedure can be thought over: (i) the MS receives signal from one station or (ii) the MS receives signal from multiple stations. In the former case, the network entry procedure follows the one described in the previous subsection. The MS is either in the range of BS or RS. Thus, there is no need to decide which of the connections is more suitable since only one point of attachment is available. In the latter case, i.e. two or more signals is received by MS, the network entry procedure needs to be enhanced about path selection as described in next subsections. Fig. 2 Message exchange between the BS and MS during network entry procedure for PMP mode (based on [1])
The ranging process is followed by exchange of basic capabilities parameters supported by BS and MS. The next phase of initialization is dedicated for MS authorization and keys’ exchange (authorization and traffic encryption keys). After the authorization phase, the MS has to register itself into the network via the MS registration procedure. If the managed MS supports IP services, a secondary management CID is assigned to the MS and three optional phases of the network entry procedure are performed (these phases are described in more details in [1]). The whole process of association is terminated by establishing of the provisioned connections. 3.2 Proposed association procedure The network entry procedure for a MS in the relay based network is partially different. To satisfy backward compatibility with the legacy standards, a RS must behave regarding to a MS in the same way as a regular BS. Additionally, no changes in the network entry procedure from the MS’s point of view are allowed. Since a MS does not distinguish between the BS and RS, the MS tries to associate to station which offers the best signal quality (higher modulation and coding scheme can be applied to the transmission). However, this decision doesn’t have to be the most appropriate with regard to the system throughput. If the signal from a RS is stronger than that one from a BS, the MS initiates the network entry procedure with the RS. Nevertheless, the path to the BS through the RS involves two (or more) hops and the overall capacity can be lower comparing to the direct link. On the other hand, if the BS signal is stronger than the RS one, the MS associates to the BS. This network attachment can be optimal for a DL data transmission but not necessarily for an UL data transmission. An efficient way how to pre-
3.2.1 BS’s signal is better than the RS’s one After synchronization phase, the MS performs the initial ranging procedure with the BS (see Fig. 3). To that purpose, the RNG-REQ message is transmitted to the BS. The message is also received by RS. We assume that the ranging slot dedicated for RNG-REQ occurs simultaneously in both BS’s and RS’s MAC frame. The RS derives from the received message the channel quality between the RS and MS and immediately generates Path Selection Request (PSREQ) [14]. Consequently, the BS can decide which point of attachment is the most appropriate for the given MS since the channel quality between the RS and BS is supposed to be known by BS (metric for the path selection phase is described later on). Two scenarios can occur: (i) the direct connection has better quality (in Fig. 3 labeled as option 1) or (ii) the connection along the intermediate RS is more suitable (in Fig. 3 labeled as option 2). In both cases, the BS transmits Ranging Response (RNG-RSP) to the MS. If the BS decides that the MS shall directly associate with the BS, the remaining network entry procedure corresponds to the original one. In other words, the field of RNG-RSP message informing about the current ranging status is set either to continue (e.g. the transmit power or time offset needs to be further adjusted) or to success. If the ranging process is successful, the network entry follows phases described in Sect. 5. In the opposite case, the status of RNG-RSP message is set to abort. In such situation, the MS has to perform the ranging with another station within its surrounding (i.e. with the RS). The successful ranging procedure is then followed by individual association steps. 3.2.2 RS’s signal is better than the BS’s one If a RS signal is stronger, the MS initiates the ranging process with the RS. Similarly as in the previous case, the ranging message is also received by BS. In comparison with the
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Fig. 5 Path selection according SNR and RRC metrics Fig. 3 Proposed message exchange during the association process when BS signal is the better
point of attachment from the view of the best system performance. A decision based only on the received signal (i.e. decision in conventional IEEE 802.16 standard) is inadequate when introducing RSs into the system and leads to suboptimum MS’s attachment. Reference [15] proposes to use the path via RS only if CSD
6 and ≤ 8.5
100
MAC Frame duration FD [ms]
20 11.64
> 8.5 and ≤ 11.5
66.7
OFDM Symbol useful time tb [µs]
1/2
> 11.5 and ≤ 15
50
CP time tg [µs]
15.55
3/4
> 15 and ≤ 19
33.3
No. of OFDM subcarriers [–]
256
2/3
> 19 and ≤ 21
25
No. of OFDM data subcarriers nods [–]
192
3/4
> 21
22.2
No. of BS/RS/MS
1/1/1
MaxCap = min{RRCSD , RRCSR + RRCRD }
(4)
4 Evaluation of the proposal 4.1 Model of simulation To evaluate performance of the RRC metric, scenario shown in Fig. 5 is used. Firstly, the transition points between the direct and relay path for both metrics, SNR metric based on (2) and RRC metric based on (4) are evaluated. The transition point represents a point when the MS re-connects either from the direct path to relay path or vice versa. Secondly, a comparison of the system throughput for three cases is evaluated: (i) if the MS attaches according to received signal strength (scenario labeled as “According.16”), (ii) if the MS attaches according to the SNR metric and (iii) if the MS attaches according to the RRC metric. The SNRSD is set to 10 dB for all three cases, i.e. the RRC metric of MS-BS path corresponds to value of 66.7 (see Table 1). On the other hand, SNRSR and SNRRD vary from 1 dB to 40 dB and 14 dB to 20 dB. The MS location is fixed while the RS is positioned at different locations. To evaluate the system throughput, the OFDM modulation is considered. Table 2 summarizes parameters used in the simulation. The throughput for a MS connecting directly to the BS is calculated based on following equation, NOFDM FD Nods
∗
Value
3/4
Fig. 5). The RRCSD , RRCSR and RRCRD correspond to RRC between individual stations. The maximum system capacity is reached if the MS uses route with the minimum RRC (i.e. the minimum radio resources are spent for data transmission) as indicated in the following equation,
ThroughputSD =
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1 NobSD ∗CRSD
(5)
where NOFDM is number of OFDM symbol per MAC frame, FD corresponds to the frame duration and Nods represents number of subcarriers used for data. As already explained in previous section, NobSD is derived from the modulation that
is chosen based on the received SNR. The index SD indicates the direct path. If the MS attaches via RS, the throughput can be evaluated as, ThroughputSRD =
FD Nods
∗
NOFDM 1 1 NobSR ∗CRSR + NobRD ∗CRRD
(6)
where the first term in the parenthesis corresponds to the MS-RS path while the second term corresponds to the RSBS path. The evaluated throughput in this paper represents a rough system WiMAX capacity obtained at the MAC level. The rough capacity represents the capacity when all OFDM symbols per MAC frame together with all data subcarriers are exclusively used for data transmissions. Thus, the overhead introduced at the MAC layer (e.g., preamble or control information) and other higher layer protocols (e.g., network, transport, etc.) is not taken into account. The impact of such overhead is the same for all compared scenarios; therefore obtained results are not influenced by this assumption. 4.2 Simulation results Figures 6 and 7 show transition points for the SNR and RRC metric. As can be seen, if SNR on both paths, MS-RS and RS-BS, is low, the MS directly attaches to the BS. In such scenarios, the MS’s attachment via RS would decrease the system performance as channel quality on the relay path is not sufficient. On the other hand, for high values of SNRSR and SNRRD it is more appropriate that a MS connects to the system via RS. Nevertheless, the transition points for the SNR and RRC metric considerably differ. According to the RRC metric, the MS associates to RS if the SNR on MS-RS and RS-BS paths is equal to 16 dB or higher. When considering the SNR metric, the transition points for SNRRD of 16/18/20 dB are at SNRSR of 27/22.5/20 dB respectively. Additionally, if SNR of either the MS-RS path or the RS-BS path is lower than 15 dB, the direct path is always preferable (if SNRSD = 10 dB). This is due to the fact that RRCSR and RRCRD cannot be lower than 66.7 (see Table 1). Figure 8 illustrates the system throughput for different scenarios. In the first case (the MS attaches to the network
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Fig. 6 Transition points between direct and relay path for SNR metric, SNRSD = 10 dB
Fig. 7 Transition points between direct and relay path for RRC metric, SNRSD = 10 dB
according the conventional IEEE 802.16 standard) the system performance considerably decreases; mainly for poor radio channel quality on the RS-BS path. The sudden drop of system throughput for SNRSR higher than 10 dB is caused by transition of MS’s attachment point from the direct to relay path. The MS assumes that the connection through the RS is better as the received signal from the RS is better as well. However, the direct connection is more preferable in this case. If the SNR on the RS-BS path is considered (i.e. the path selection is done according the SNR metric), the improvement of system performance is distinguishable when compare to the first scenario. Nevertheless, Figs. 8b– 8d show the obvious weakness of SNR metric. For certain values of SNRSR , the system throughput is lower comparing to the first scenario. As already shown in Figs. 6 and 7, the SNR metric fails to recognize that the attachment via RS would be better and forces the MS to associate to the BS. The highest system throughput is obtained when imple-
menting the RRC metric. Figure 8 illustrates that the RRC metric outperforms both, the conventional strategy and SNR metric. If SNRRD equals to 14 dB (Fig. 8a), the direct attachment to the BS is always preferable as indicated by RRC metric (Fig. 7). If SNRRD is higher than 14 dB (Figs. 8b–d), again the optimal transition points between direct and relay path are those that are suggested by RRC metric, not by SNR metric.
5 Conclusion The paper proposes modifications in order to support relay stations in the WiMAX system. The aim of these modifications is to determine the best point of attachment for an entering MS in the WiMAX network. Firstly, the path selection procedure during the network entry procedure is proposed. Based on that, the BS is able to decide if the direct or relay
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Fig. 8 Comparison of system throughput for all considered scenarios
connection is more preferable. Secondly, the path is selected according to the RRC metric. The RRC metric indicates how many radio resources would be spent in order to transmit a certain amount of data. The simulation results show that by implementing the RRC metric, the optimal system performance can be reached; the RRC metric outperforms both, the conventional strategy and SNR metric. To implement proposed modifications into existing WiMAX equipments only changes at the MAC level are required. Thus, no additional hardware modifications have to be made and the new features can be implemented by upgrading the equipment’s firmware. Acknowledgements MSM6840770014.
This research work was supported by grant
References 1. IEEE Std 802.16-2004 (Oct. 2004). IEEE standard for local and metropolitan area networks, Part 16: Air interface for fixed broadband wireless access systems. 2. IEEE Std 802.16e-2005 (Feb. 2006). IEEE standard for local and metropolitan area networks, Part 16: Air interface for fixed and mobile broadband wireless access systems. 3. IEEE: IEEE baseline document 802.16j-2006 (2007). Air interface for fixed and mobile broadband wireless access systems: multihop relay specification. 4. Hoymann, C., & Klagges, K. MAC frame concepts to support multihop communication in IEEE 802.16 networks. 5. IEEE 802.16j (2007). Baseline document for draft standard for local and metropolitan area networks Part 16: Air interface for fixed and mobile broadband wireless access systems. 6. Okuda, M., Zhu, C., & Vorel, D. (2008). Multihop relay extension for WiMAX network—overview and benefits of IEEE 802.16j standard. FUJITSU Science Technology Journal, 44(3), 292–302.
1704 7. Jing, S., Shen, G., & Zang, K. (2007). MS/RS network entry and initialization. Paper no. C802.16j-06/286. 8. Ge, Y., Tham, C. K., & Kong, P. Y. (2006). A node entry process for IEEE 802.16j multihop relay networks. Paper no. C802.16j06/211r1. 9. Chindapol, A., Chui, J., & Xue, Y. (2007). Relay-assisted MS network entry. Paper no. C802.16j-07/125. 10. Okuda, M., et al. (2007). MS network entry for non-transparent relay station with distributive scheduling. Paper no. C802.16j07/024r2. 11. Okuda, M., et al. (2007). MS network entry for non-transparent relay station with centralized scheduling. Paper no. C802.16j07/008r2. 12. Okuda, M., et al. (2007). MS network entry for transparent relay station. Paper no. C802.16j-07/001r4. 13. Ramachandran, S., & Pandey, A. (2007). Routing announcements for network entry support. Paper no. C802.16j-06_158r1. 14. IST-27675 STP fireworks project deliverable 3D2 (October 2007). Draft specification for the enhanced WMAN and WLAN MAC protocols. 15. IST-27675 STP Fireworks Project Deliverable 3D1 (April 2007) Design and specification of MAC in relaybased cooperative networks. Pavel Mach received his M.Sc. and Ph.D. degree in Telecommunication Engineering from Czech Technical University, Prague, Czech Republic in 2006 and 2010 respectively. During his study he joined research groups at Sintronics and R&D centers focusing on wireless mobile technologies. He has been actively involved in several national and international projects. He participated in EU FP projects FIREWORKS, ROCKET and he currently participates in EU FP7 project FREEDOM. His research interests include MAN/LAN networks based on mesh and relay architectures. He is dealing with aspects relating to radio resource management in
P. Mach et al. emerging wireless technologies (WiFi, WiMAX, 4G) and focuses on cross layer optimization processes. Robert Bestak is Assistant Professor at the Dept. of Telecommunications Engineering. He received his engineering degree from CTU, Faculty of Electrical Engineering, in 1999. In 1999/2000, he attended a one-year engineering program in telecommunications and computer networks at the Institute EERIE de l’Ecole des Mines d’Alès, Nimes, France. In 2003, he received his Ph.D. degree in computer science from ENST Paris, France. In 2004, he joined the Dept. of Telecommunications Engineering where he leads the Wireless Research Group. His research interests include RRM techniques in HSPA/LTE, cognitive networks and femtocells. He participated in several national and EC funded projects (ROCKET, FREEDOM, etc.). Zdenek Becvar received M.Sc. in telecommunication at the Czech Technical University in Prague, Faculty of Electrical Engineering in 2005 and Ph.D. degree in 2010. In 2008, he became a researcher at the Department of Telecommunication Engineering. His current research interests include MAC procedures in wireless networks (LTE, LTE-A, WiMAX) with focus on radio resource management and mobility support. He participated in several ICT FP6 and FP7 projects. Furthermore, he was involved in research activities of Vodafone R&D centre and Sitronics R&D centre at CTU in Prague and in several national research projects.
