A New Efficient BS Scheduler and Scheduling Algorithm in WiBro Systems Juhee Kim, Eunkyung Kim, Kyung Soo Kim Mobile Telecommunication Research Division Electronics and Telecommunications Research Institute (ETRI) Daejeon, Republic of Korea
[email protected]
Abstract In this paper, we propose an efficient structure of base station (BS) scheduler for the Wireless Broadband (WiBro) system. The BS scheduler consists of an uplink scheduling block, a downlink scheduling block and a channel quality information block. Both uplink and downlink scheduling blocks operate closely with a channel quality information block to schedule downlink packets and allocate uplink/downlink radio resources. Next, we propose a new efficient scheduling algorithm for WiBro systems. The proposed algorithm utilizes the user-based scheduling to relieve the MAP overhead problem and to modify the normal proportional fair (PF) scheduling algorithm to guarantee user-based quality of service (QoS). The simulation study shows that the proposed algorithm relieves the MAP overhead problem by allocating the minimum number of user bursts in a frame and supports better fairness performance, while supporting the system throughput performance similar to that of the normal PF scheduling algorithm. Keywords packet scheduling, WiBro, OFDMA, quality of service (QoS), wireless.
1. Introduction Wireless Broadband (WiBro) System, based on IEEE 802.16, supports various IP based wireless data services, such as steaming video, FTP, e-mail, chatting, and so on [1]. To provide multiple service classes, i.e. Real-Time service, Non-Real-Time service and Best Effort service, for traffic connections, different levels of QoS requirements are negotiated between a base station (BS) and subscriber stations (SS) during every service additions. Figure 1 shows a frame structure of the WiBro system. A MAP message, which is located at the beginning of a frame, contains transmission and reception information of allocated bursts in a frame. As the number of allocated bursts is increased, the size of a MAP message is also increased
proportionally. Because a MAP message is transmitted by using the most robust MCS (Modulation and Coding Scheme), the total system throughput goes down abruptly. Therefore, a BS scheduler should be implemented carefully by considering this MAP overhead problem in the WiBro system. In this paper, we propose an efficient structure of a BS scheduler and a scheduling algorithm for the WiBro system. The proposed scheduler adapts the proportional fair (PF) algorithm [2] to the WiBro system to guarantee the QoS and fairness. Additionally, a user-based scheduling concept is also used to minimize the MAP overhead and to simplify the implementation of a BS scheduler. This paper is composed of five sections. Section 2 presents a new structure of BS scheduler in the WiBro system. In section 3, we propose an efficient scheduling algorithm that considers not only system throughput but also fairness with user QoS. Performance evaluation of the proposed scheduling algorithm is compared with that of conventional algorithms based on simulation results in section 4. Summary and conclusion is given in section 5.
2. BS scheduler structure in WiBro systems The proposed scheduler consists of a channel quality information block, an uplink scheduling block and a downlink
Figure 1. Frame structure of the WiBro system
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Figure 2. Structure of the proposed BS scheduler
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scheduling block as shown in Figure 2. A channel quality information block collects and manages measured wireless channel information, which is transmitted by each user through Fast Feedback Channel and/or reported by a BS physical layer. An uplink scheduling block allocates radio resources for the requested uplink bandwidth. The uplink scheduling block consists of two scheduling procedures, i.e., a random access scheduler and a unicast scheduler. The random access scheduler allocates radio resources for the received CDMA random access codes. The unicast scheduler allocates radio resources for active SSs according to a Bandwidth-Request from SSs or a scheduling policy, which is negotiated between a BS and SSs during every service additions. A downlink scheduling block consists of a packet classifier, packet buffers, a downlink scheduler and a protocol data unit (PDU) generator. A packet classifier determines connection identity and service priority by checking the header field of IP packets received from a wired network, and stores the packets into the classified packet buffers. A downlink scheduler decides service order, allocates downlink radio resources and serves PDUs with the selected packets by using a PDU generator. For scheduling, both uplink and downlink scheduling blocks utilize QoS information of users, and a channel status from the channel information block and a buffer status from the packet buffers.
∑r ∑∑ r
(2)
i, j
PM i (k ) = ρ i
j
DRCi (k ) / Ri (k )
i, j
i
j
where k is the service time (frame number), ρι is the user priority factor which is negotiated during registration procedure of the i-th SS. ri,j is the target service rate for the j-th connection, which is served for the i-th S. DRCi(k) is a variable reflecting current channel state and Ri(k) is the average service data rate of the SS(i). The scheduler sorts out and serves the SSs by using their PMi(k) in descending order. In contrary to a conventional packet-based scheduler, this scheduler allocates enough radio resources to the selected SS with the highest PMi(k). If there are not enough resources for all waiting traffic capacity of the selected SS, long-waited packets are served first. After the selected SS was served, the next SSs are served in the same way. Because the selected SS uses as much resource as possible, the number of bursts in a frame is reduced and the MAP overhead problem is relieved. Moreover, a processing time of the scheduler is relatively shorter than conventional packet-based schedulers especially in a heavy traffic environment.
3. Proposed scheduling algorithm
4. Performance Evaluation
Figure 3 presents the proposed scheduling algorithm. The scheduler selects service SSs with priority metrics in a frame, and allocates radio resource to the selected SSs. More detailed scheduling procedure is as follows. In the BS scheduler, first, the scheduler calculates service scheduling priority metrics (PMi) for all active SSs and decides the service order of SSs according to the calculated PMi as (1) describes.
To investigate the performance of the proposed algorithm, the simulation study has been carried out. The target system of out interest is the WiBro system using OFDMA radio transmission technology. We assume that bandwidth is 20MHz and 1024 subcarriers are used. It is also assumed that radio resource in a frame is allocated in units of subchannel, which consists of 48 data subcarriers and 6 pilot subcarriers. The MCS for data transmission is determined with the instantaneous signal-interference-ratio (SIR), which is received from the related SS. A summary of simulation parameters for system model is shown in Table 1.
S = arg max{PM i (k )} i
(1)
Table 1. A Summary of Simulation Parameters Parameters Place of TextType Styles Simulation plane Wrap-Around Number of cells 19 Cell radius[km] 1 Radio channel environment Vehicular_Macro Multipath type Pedestrian_A[3km/h] Frame duration 5msec Symbol duration Number of Downlink Symbols 27 Number of Uplink Symbols 15 FFT size 1024 Modulation scheme QPSK,16QAM,64QAM
Figure 3. Proposed scheduling algorithm
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Figure 4 shows the overhead of MAP in the WiBro system. It is shown that the portion of MAP is about 37% when 6 normal SS bursts are assigned in a frame. We notice from Figure 4 that the number of user bursts, assigned in a frame, is closely associated with the system throughput in WiBro systems. It means that the BS scheduler in WiBro systems
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System throughput [Mbps]
1.2 FCFS Proposed
1 0.8 0.6 0.4 0.2 0 2
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Number of WWW users
Figure 5. Comparison of system throughput throughput performance of the proposed scheduling scheme and the FCFS scheme
Figure 4. Proposed scheduling algorithm
should minimize the number of allocated bursts in a frame. Figure 5 compares the system throughput of the proposed scheduling scheme and the FCFS(First-Come First-Service) scheme which is a representative packet-based scheduling algorithm. From the figure, we observe the MAP overhead problem in the WiBro systems. To compare the fairness performance of the proposed scheduling and that of conventional PF scheduling, we define a fairness index F in the WiBro system. According to [3], the conventional fairness index F is shown in (3): (3)
N
(∑ Ri ) 2 F=
i =1 N
N ∑ Ri
2
i =1
where Ri is the transmission data rate for the i-th SS, and N is the number of total SSs. For F = 1, it is the fairest condition between SSs, and it is the most unfair as F = 0. In the WiBro systems, however, the conventional fairness index no longer represents the fairness condition between SSs with different QoS requirements. Accordingly, we modify the fairness index
as shown in equation (4). N
(∑ F=
N R N ∑ i i =1 R 'i
2
where R’i is the sum of minimum service rates for connections between BS and the i-th SS. In Figure 6, we see that the fairness of the proposed scheduling scheme is better than the normal PF scheduling scheme when the number of SSs is over 4. To evaluate the fairness performance, we consider real-time video data service, non-real-time data service, and best effort service. For real-time video data service, we use the 3GPP streaming video traffic model [4]. We also use the FTP model for non-real-time data service and use the world-wide web (WWW) model for the best effort service [4]. Figure 7 compares the system throughput performance of both scheduling schemes. The difference of throughput performance between the proposed scheduling and the normal PF is insignificant.
1.2
0.7 Normal PF Propos ed
Normal PF Propos ed
1 System throughput[Mbps]
0.6
0.5 Fairness Index
i =1
(4)
Ri 2 ) R'i
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0.8
0.6
0.4
0.2
0.1
0
0 2
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Figure 6. Comparison of fairness performance of the proposed scheduling scheme and the normal PF scheme
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Number of WWW users
Number of W WW us ers
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Figure 7. Comparison of system throughput performance of the proposed scheduling scheme and the normal PF scheme
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We find that the proposed scheduling scheme relieves the MAP overhead problem by allocating the minimum number of user bursts in a frame and improves the fairness performance, while supporting the system throughput performance similar to that of the normal PF scheduling algorithm.
throughput performance similar to that of the normal PF scheduling algorithm. Our simulation, also, has shown that the fairness performance is found to be much better when the proposed algorithm is used.
5. Conclusions
REFERENCES
In this paper, an efficient structure of BS scheduler has been proposed for the WiBro system. This scheduler consists of a channel quality information block, an uplink scheduling block and a downlink scheduling block. Both uplink and downlink scheduling blocks operate closely with channel quality information block to schedule downlink packets and allocate uplink/downlink radio resource. Next, we have proposed a new efficient scheduling algorithm for WiBro systems. The proposed scheduler utilizes the user-based scheduling to relieve the MAP overhead problem and modify the normal PF scheduling algorithm to guarantee user-based QoS. With the simulation result, we have shown that the proposed scheduler relieves the MAP overhead problem by allocating the minimum number of user bursts in a frame and supports better fairness performance, while supporting the system
[1] IEEE 802.16 – “Part 16: Air Interface for Fixed Broadband Wireless Access Systems”, Dec. 2001standard [2] R.Padovani A.Jalai and R.Pankaj, ”Data throughput of cdma hdr a high efficiency-high data rate personal communication wireless system”, In Proceedings of VTC2000-Spring, pages 1854-1858, July 2000 [3] H.Sirisena, A. Haider, M. Hassan, and K. Powlikowski, ”Transient fairness of optimized end-to-end window control”, Proceedings of IEEE Global Telecommunications Conferenct, pp. 3979-3983, Dec. 2003. [4] 1xEV-DV EvaluationMethodology,3GPP2 TSG-C.R1002
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