Seamless High-Velocity Handover Support in Mobile WiMAX Networks ∗
Zhiwei Yan∗ , Lei Huang† and C.-C. Jay Kuo‡
Xi’an Jiaotong University, Xi’an 710049, P. R. China Loyola Marymount University, Los Angeles, CA 90045-2659, USA ‡ University of Southern California, Los Angeles, CA 90089-2564, USA E-mails:
[email protected],
[email protected] and
[email protected] †
Abstract—The IEEE 802.16e standard (i.e., mobile WiMAX) has been proposed to provide connectivity in wireless networks for mobile users (including users at a vehicular speed). It is shown in our analysis that the probability of a successful handover decreases significantly when the user moves at a higher speed. Then, we propose a scheme that combines adaptive forward error correction (FEC) with retransmission to offer extra protection for handover signaling messages to enhance the probability of a successful handover, especially at a higher velocity. It is demonstrated by computer simulation that the proposed scheme provides a higher successful handover probability at various velocities.
I. I NTRODUCTION The rapid growth of high data rate services in wireless communication networks demands new technologies for broadband wireless access. The Worldwide Interoperability for Microwave Access (WiMAX) is an emerging technology that enables the delivery of the last mile wireless broadband access as an alternative to the wired broadband access such as cable and DSL. It provides a convenient way to build a wireless metropolitan area network (WMN), which offers a broad range of high data rate applications, such as broadband Internet access, Voice over Internet Protocol (VoIP), Internet Protocol Television (IPTV), etc. to wireless users anytime and anywhere. For more technical details, we refer to the newly developed IEEE 802.16 standard [1], [2]. Handover is one of essential issues in mobile wireless communications since it is needed to maintain uninterrupted services during user’s movement from one location to another [3], [4]. It manages mobility between subnets in the same network domain (micro-mobility) and between two different network domains. The current mobile WiMAX standard defines handover operations to support micro-mobility for the point-to-multipoint (PMP) mode communication. That is, it handles the mobile subscriber station (MS) switching from one base station to another. In general, handover techniques can be divided into soft handover (SHO) and hard handover (HHO). SHO employs a make-before-break approach where a connection to the next base station (BS) is established before an MS releases an ongoing connection to the original BS. It guarantees zero break-time during handover at the cost This work is supported in part by the China Scholarship Council (CSC).
1-4244-2424-5/08/$20.00 ©2008 IEEE
of lower spectral efficiency. This technique is suitable for latency-sensitive services such as voice communications, video conferencing, and multi-player gaming. In contrast, HHO uses a break-before-make approach where a connection with the serving BS is terminated before a mobile station switches to another BS. HHO is more bandwidth-efficient than SHO. However, it has a nonzero breaktime, thus leading to longer delay in service delivery. A typical mobile WiMAX network uses packet-switching with mostly bursty delay-tolerant data traffic. Therefore, HHO is also used in mobile WiMAX. However, for certain delay-sensitive applications, HHO must be optimized to meet the required quality of service (QoS). Recently, a large amount of research has been conducted to address the QoS of HHO in mobile WiMAX. They focused their attention on the choice of the best target BS through tracking the MS movement, analyzing the QoS information of neighbor BSs or detecting the signal level from BSs [6], [7]. In [8], data packet loss during handover was reduced via caching in the upper layer devices such as routers. To reduce delay during the handover process, Chen et al. [9] proposed a pre-coordination mechanism, which can reduce the handover latency by measuring the distance between the BS and the MS, predicting the time that handover occurs, and pre-allocating available resource for handover usage. Besides, it was shown in [9] that the handover delay could be further reduced through transmitting MAC layer messages via the IP layer. Jiao et al. [11] proposed a connection identification (CID) assignment strategy to avoid conflicts of the CID assignment during the handover procedure. Hu et al. [12] applied a different handover mechanism to classified data. Both of them can decrease data delay in handover. Most previous work considered the handover performance from the viewpoint of the MAC or IP layer, but paid little attention to the physical layer. That is, physical channel quality and/or user’s mobility are not taken into account. Usually, handover occurs in the boundary of BS’s coverage area, where signals transmitted on the radio channel are weak and unstable. When channel quality degrades, both signalling messages and data packets are at a higher risk of being lost or corrupted. Moreover, user’s mobility plays an important role in the handover process. The radio channel quality for mobile users at a higher velocity often suffers from severe
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degradation due to the Dopploar frequency shift. Since the overlap area is limited between adjacent BSs, the requirement on handover latency is more stringent for users with higher mobility. In order to support full mobility (which means seamingless handover for users moving at a speed of 120 kilometer per hour or higher), these issues become severer and have to be addressed. In this work, we analyze the handover performance affected by the radio channel quality, measured in terms of the bit error rate (BER), and user’s mobility in terms of the velocity by means of modeling the handover procedure. The primary handover performance in this work is considered as the probability of a successful handover, i.e., the probability that all necessary handover signaling messages are completed successfully within the required time limit imposed by user’s mobility. The probability of a successful handover does not only indicate the connection-level QoS, but also play an important role in application-level QoS such as delay and/or dropping of application data packets. Based on our analysis, a simple yet effective solution is proposed to improve the successful handover probability for users with higher mobility. The rest of this paper is organized as follows. We discuss the relationship between the successful handover probability and the BER and analyze the probability at different velocities in Sec. II. Then, we propose a scheme to support high mobility using forward error correction (FEC) codes to protect the handover messages in Sec. III. Simulation results are presented in Sec. IV for performance evaluation and comparison. Finally, concluding remarks and future research directions are given in Sec. V.
MS
Serving BS
MOB_SCN−REQ MOB_SCN−RSQ
Scanning Process MOB_MSHO−REQ HO Notifaction messages
MOB_BSHO−RSP MOB_HO−IND
Connect to the target BS
Figure 1.
Illustration of the handover procedure.
probability of event Ai , which indicates that the i-th signaling message is received successfully by MS or BS. By considering a scheme with no-transmission, we have P (Ai ) = pi , P (A¯i ) = qi = 1 − pi ,
P (Ai ) =
Ni j=1
A. Handover Procedure
B. Successful Handover Probability versus BER Without loss of generality, we assume that there are M (M ≥ 2) signaling messages transmitted between an MS and two BSs during the handover procedure. Let pi be the
(1)
where i = 0, · · · , M − 1. In the WiMAX handover procedure, the retransmission mechanism is used to guarantee the transmission of these signalling messages. We assume that a message is always retransmitted until it is successfully received by its peer because these signaling messages are hand shaking messages. By taking the retransmission into account, Eq.(1) can be rewritten as
II. H ANDOVER AND M OBILITY A NALYSIS In the IEEE 802.16e standard, a BS periodically broadcasts the Neighbor Advertisement Message (MOB_NBR-ADV) for the identification of the network and the channel characteristics of neighbor BSs. MSs always listen to this message and collect the information about neighbor BSs. If an MS detects a received signal that becomes weaker, it will send a request message MOB_SCAN-REQ to its serving BS. Then, the BS allocates some radio resource and informs the MS to perform the scanning operation by sending message MOB_SCAN-RSP. After the scanning operation, the MS initiates the handover process by sending message MOB_MSHO-REQ to the serving BS. The BS replies to this request with message MOB_BSHORSP, and then the MS sends message MOB_HO-IND to inform the BS to cut all communications. In the final phase of handover, the MS continues to exchange ranging messages and re-entry messages with the target BS. The handover procedure described above is illustrated in Fig. 1. Thus, a successful handover is based on several handshaking messages transmitted successfully.
Neighoring BS1 Neighoring BS2
MOB_NBR−ADV
qij−1 pi =
Ni
(1 − pi )j−1 pi ,
(2)
j=1
where Ni = 1, 2, 3, · · · is the total number of the i-th message to be transmitted untill it is received successfully during a handover process. Since there are totally M signaling messages to be transmitted during the whole handover procedure, the successful handover probability is equal to ⎡ ⎤ Ni M −1 M −1 ⎣ (1 − pi )j−1 pi ⎦ . (3) P (Ai ) = Psucc = i=0
i=0
j=1
If pi is primarily determined by the BER of the wireless channel between a BS and an MS, we can express pi = ϕ(Pb (γb ), Li ), where Pb (γb ) is the bit error probability when the received bit energy-to-noise ratio is γb . Thus, If the i-th signaling message has a size of Li bits, we can re-write the above equation as pi = [1 − Pb (γb )]Li ,
(4)
where the independence of bit errors is assumed since most of radio channels are memoryless. The interleaving channel coding technique, which is widely adopted in wireless communications, also allows us to treat radio channels as memoryless ones. Consequently, the relationship between the successful
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handover probability and BER of this wireless link is derived by substituting Eq. (4) into Eq. (3) as Psucc =
M −1
the total latency associated with the handover procedure can be written as TM = (N0 + N1 + · · · + NM −1 ) · Tretx + M · Tprop ,
P (Ai )
⎫ where Tretx is the time interval of a timer between two ⎬ adjacent transmission of the same message, and Tprop is the j−1 1 − [1 − Pb (γb )]Li · [1 − Pb (γb )]Li time interval for the electromagnetic wave to travel between = ⎩ ⎭ i=0 j=1 the BS and the MS. (5) We use Doverlap to denote the overlap distance along the direction of the MS movement between two neighboring BSs, An example of the relationship in Eq. (5) is plotted in Fig. 2. and vm the velocity of the MS. Then, the following constraint It is apparent that, the successful handover probability, Psucc , has to be satisfied if a handover operation is performed decreases as the bit error probability, Pb (γb ), increases. When successfully: BER is relatively high (between 10−4 and 10−2 ), the use of Doverlap . TM < retransmission improves the successful handover probability vm significantly. With this constraint, the probability of a successful handover 1 in Eq. (5) can be rewritten as No Retransmission Retransmission ⎧ 0.9 D M −1 Ni j−1 ⎪ pi , if TM < overlap , ⎨ i=0 j=1 (1 − pi ) vm 0.8 Psucc = ⎪ ⎩ 0.7 0, others, (6) 0.6 where 0.5 M −1 (Ni ) · Tretx + M · Tprop (7) TM = 0.4 i=0
⎧
Probability of a successful handover
Ni M −1 ⎨
i=0
0.3
and
0.2
pi = [1 − Pb (γb )]Li .
0.1
0 −9 10
−8
10
−7
10
−6
10
−5
10 Bit Error Rate
−4
10
−3
10
−2
10
−1
10
Figure 2. The successful handover probability as a function of BER, where −1 M = 5, M i=0 Ni
