The Online Journal on Computer Science and Information Technology (OJCSIT)

Vol. (3) – No. (2)

The Opportunistic Scheduling for Mobile WiMAX Systems Ghada Sandoub 1, Rawya Rizk1, and Fayez W. Zaki 2 1 Electrical Engineering Department, Port Said University, Egypt 2 Electronics & Communication Eng. Department, Mansoura University, Egypt Abstract- Providing Quality of Service (QoS) to different service classes with real-time and non-real-time traffic integration is an important issue in WiMAX systems. In this paper, a cross-layer QoS support scheduling framework and a corresponding opportunistic scheduling algorithm with new traffic classification called Advanced Holistic Opportunistic Scheduling (ADHOS) is proposed. It provides QoS support to the heterogeneous traffic in single carrier WiMAX point-to-multipoint (PMP) systems. The uplink transmission in WiMAX system is modeled as a multi-class priority TDMA queuing system to analyze the average packet delays of different service classes. Numerical results show that the proposed scheme improves the WiMAX PMP system in terms of packet loss rate, packet delay and system throughput. Keywords- Cross-Layer Protocol Design, Opportunistic Scheduling, QoS, WiMAX. I. INTRODUCTION The IEEE 802.16, commercially known as Worldwide Interoperability for Microwave Access (WiMAX), is an emerging broadband wireless access technology to provide users with high speed multimedia services. The IEEE 802.16 standard defines specifications for Medium Access Control (MAC) and physical (PHY) layers. The main advantage of WiMAX is providing the missing link for the "last mile" connection in metropolitan area networks where DSL, Cable and other broadband access methods are not available or too expensive. It also offers an alternative to satellite Internet services for rural areas and allows mobility of the customer equipment. WiMAX consists of one Base Station (BS), central entity, and M Subscriber Stations (SS). Transmissions of data take place through two independent channels: Downlink Channel (from BS to SS) and Uplink channel (from SS to BS). Uplink channel is shared between all SSs whereas downlink channel is used only by BS. The standard supports four different flow classes for QoS and the MAC supports a request grant mechanism for data transmission in uplink direction, but it does not define a slot allocation criterion. So, scheduling module is necessary to provide QoS for each class. The scheduling is the process of resolving contention for shared resources in a network by allocating bandwidth among the users and determining their transmission order.

Reference Number: W13-C-0024

IEEE 802.16 defines the following four types of service flow with distinct QoS requirement [1]: - Unsolicited Grant Services (UGS): designed to support Constant Bit Rate (CBR) services such as voice applications. - Real-Time Polling Services (rtPS): designed to support realtime services that generate variable size data packets on a periodic basis, such as MPEG video. - Non-Real-Time Polling Services (nrtPS): designed to support non-real-time and delay tolerant services that require variable size data grant burst types on a regular basis such as FTP. - Best Effort (BE): designed to support data streams that do not require any guarantee in QoS such as HTTP. Wireless access networks have unique characteristics, which are the time-varying channel conditions and the multiuser diversity. The MAC protocol and scheduling algorithms have to be developed specially for this environment [2]. It requires across-layer MAC protocol design approach, where by Channel Specification (Cspec) carrying the estimated instantaneous channel information can be fed to the MAC layer from the physical (PHY) layer and Traffic Specification (Tspec) carrying traffic QoS related information can be fed to the MAC layer from higher layers such as the network or application layer. The Cspec feedback includes information on the estimated instantaneous Signal-to-Interference and Noise Ratio (SINR),supportable data rate R(t), Received Signal Strength Indications (RSSI) or Bit Error Rate (BER)of a link. The Tspec feedback includes information on the traffic Maximum Latency (ML) constraint, Maximum Sustained Traffic Rate (MSTR) and the instantaneous length of queues at a station [3]. Wireless scheduling has two features [4]: (i) Channel is not perfect and is subject to errors. This causes bursts of errors to occur during which packets cannot be successfully transmitted on link. The implication of this is that good scheduling algorithms need to be channel quality (state) dependent. (ii) States of different channels can asynchronously switch from "good" to "bad" within a few milliseconds and viceversa. A good scheduling algorithm should take advantage of this by giving some preference to a user whose channel is currently good.

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Recently, various opportunistic scheduling schemes have been proposed for wireless communication systems. They can exploit the variability in channel conditions and assign the priority to users based on channel state. The proposed opportunistic scheduling schemes can be classified into channel-aware only and channel-aware and queue-aware algorithms based on their functionality of scheduling algorithms [5]. Max Carrier-to-Noise Ratio Scheduling (MCS) is a channel-aware opportunistic scheduling scheme. It allocates resources to users with the best channel condition to achieve high system throughput [6]. On top of channel characteristics, traffic characteristics also play an important role in the design of an opportunistic scheduling algorithm. The Proportional Fair Scheduling (PFS) scheme attempt to trade-off among the throughput, efficiency and fairness among users by taking packet length into account [7], or estimating future channel quality [8]. Modified Largest Weighted Delay First (M-LWDF) is a modified version of the PFS scheduler that tries to meet the QoS requirement in terms of head-of-line packet delay [9]. Exponential Rule Scheduler attempts to equalize the weighted delays of all buffers when their differences become larger in a wireless system [4]. Traffic-Aided Opportunistic Scheduling (TAOS-1) is a heuristic opportunistic scheduling scheme that unifies file size information and wireless channel variations in order to reduce the completion time of file transmission [10]. The Opportunistic Deficit Round Robin (O-DRR) scheduler attempts to fulfil the delay requirements of multi-class traffic in WiMAX by using deficit counter and setting polling interval [11]. A priority-based scheduler has been proposed in [12] that a Priority Function (PRF) has been defined for each connection to be admitted into the system and is updated dynamically depending on the wireless channel quality, QoS satisfaction and service priority through a MAC-PHY crosslayer manner. However, the existing scheduling schemes have not been applied to packet scheduling in *each traffic flow. The studies have not explored the packet delay under different traffic loads and they do not concern of traffic classification. In [3], a Holistic Opportunistic Scheduling (HOS) with the features of channel-awareness, queue-awareness and traffic QoS-awareness is proposed. It is a cross-layer QoS support framework with a two-stage opportunistic scheduling scheme in WiMAX PMP system. However, the traffic classification is based on the four service classes while some applications need to have a higher priority than another in the same service class. In addition, some common performance metrics such as packet loss and throughput have not been evaluated in all the previous studies.

Reference Number: W13-C-0024

Vol. (3) – No. (2)

The main objective of this paper, is satisfying QoS requirements for all applications. An Advanced Holistic Opportunistic Scheduling (ADHOS) is proposed with new traffic classification based on different applications. All important measures are taken into account such as packet delay, packet loss rate and system throughput under different traffic loads. The rest of the paper is organized as follows. Section II presents the cross-layer QoS support framework and a corresponding two-stage opportunistic scheduling algorithm. In Section III, the proposed ADHOS scheme with the associated queuing model is presented. The performance measures and the numerical results are introduced in Sections IV, and V; respectively. Finally, the conclusion is presented in Section VI. II. CROSS-LAYER QoS SUPPORT FRAMEWORK A WiMAX system with point-to-multipoint (PMP) topology includes one Base Station (BS) and M Subscriber Stations (SSs), where M is the number of SSs. The performance of MAC protocol and the functionality of scheduling algorithms are influenced by the time-varying wireless channel in WiMAX systems. Figure 1 shows the cross-layer QoS support framework and a corresponding twostage opportunistic scheduling algorithm in WiMAX PMP system [3]. The cross-layer QoS support framework supports both UL and DL transmission. UL transmission is considered as an example to explain its functionality as follows [3]: (1) A wireless Channel Condition Estimator (CCE) is designed at the PHY layer at the BS as well as the SSm, where m=1,2,3,…,M. It not only monitors instantaneous channel condition status like the receiver’s received signal strength RSS(m) and SINR γ(m) when the BS receives signal from SSm, but also indicates long term channel condition by using the statistic parameters including Max_γ(m), Average_γ (m) and Min_ γ (m) over the execution period. Based on the information of the channel’s status provided by CCE, the forward error correction (FEC), the Symbol Mapper and the Adaptive Modulation and coding controller (AMC) at the BS select an FEC scheme, AMC scheme, and the data transmission rate for UL transmission from SSm to the BS. (2) The output of the PHY Symbol Rate Controller called AMC_Controller _Info is forwarded to MAC layer via its PHY Service Access Point (SAP). Embedded in UL-Map, the information is further broadcasted to all SSs for UL scheduling and transmission from SSm to the BS. (3) In the first stage of scheduling each SS uses the Tspec of each connection and AMC_Controller_Info to compute four scheduling parameters and determine scheduling priority (SP) for every packet in each connection. It takes the information of instantaneous wireless channel condition and the real-time traffic condition as well as the traffic QoS specification to set the Scheduling Priority (SP) for each

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The Online Journal on Computer Science and Information Technology (OJCSIT)

individual packet for the UL transmission from rtPS or nrtPS traffic flows [3]. Then SPs are sorted in descending order. The SSm takes the maximum value of the calculated scheduling priority as its Max_SP(m) to represent itself to compete with other SSs for the order of UL transmission in the next frame. SSm sends its Max_SP(m), its total BW_request and AMC_Controller_ Info embedded in a Polling_response(m) to the BS when the SSm is polled. (4) In the second stage of scheduling the BS sorts Max_SP(1),Max_SP(2),…, Max_SP(M) in descending order and selects SSm with the highest Max_SP(m) to allocate time slots according to its BW_request(m) in the next UL subframe, excluding the time slots for UGS. If BW_request(m) is less than the number of available time slots in the next UL sub-frame, the remaining time slots will be allocated to SSn which has the second highest Max_SP(n) according to its BW_request(n). This procedure will be iterated until all the available time slots in the next UL sub-frame are closed up. AMC_Controller_Info and the result of scheduling, which is the time slot allocations for SSs in the next UL sub-frame, are embedded in a DL/UL_Map. The DL/UL_Map is broadcasted by the BS to all SSs for UL transmission in the next frame. (5) The scheduler at SSm extracts the information of its allocated transmission time slots and the AMC_Controller_ Info from the DL/UL_Map and selects packets from the highest priority to the lowest priority from different connections. SSm transmits the selected packets to the BS in the allocated time slots at the data rate determined by BS receiver.

queue characteristics are essential factors in the design of opportunistic scheduling algorithms. The services are classified according to different applications. A priority coefficient is assigned for each application to minimize the packet delay and loss rate in each traffic flow. Table (1) presents some common applications with different QoS requirements at each service class [13-15]. A priority is assigned for each application according to its requirements as shown in Table (1). As BE connection can tolerate the demand of bandwidth and delay constraint, only UGS, rtps and nrtps are considered in this scheme. The scheduling scheme set the Scheduling Priority (SP) for each individual packet for the UL transmission from rtPS or nrtPS traffic flows. Since the UGS connections have been allocated with a fixed bandwidth based on their fixed bandwidth requirement specified by the framework of the IEEE 802.16d standard, the scheduling is only applied for the rtPS and nrtPS services. Table (1): Classification of Applications and their QoS Requirements Service Class

UGS

rtps

nrtps BE

Figure (1): Cross-layer QoS support framework Ш. THE PROPOSED ADHOS SCHEME Each application have different QoS requirement. So, some applications need to have a higher priority than another in the same service class. The proposed ADHOS scheme provides QoS to the heterogeneous traffic in WiMAX systems. Channel characteristics, traffic characteristics and

Reference Number: W13-C-0024

Vol. (3) – No. (2)

Applications

T1/E1 (overIP) VOIP Video Conference Streaming Video on Demand FTP Still image E-mail or Fax SMS

Bit rate (Mbps)

QoS Demands Sensitive to Bandwidth

Priority

Delay

Jitter

Loss

Low

High

High

High

J= 1

Low

High

High

Med

J= 2

5M

High

High

High

Med

J= 3

11.5M

High

Med

Med

Med

J= 4

2M 64K8M

Med

Low

Low

High

J= 5

Low

Low

Low

High

J= 6

64K

Low

Low

Low

High

J= 7

16K

Low

Low

Low

High

J= 8

1.52M 2M

The UL channel of WiMAX PMP system is modeled as a multi-class priority TDMA queuing system. The UL channel can be considered as a multiple-access communication channel shared by M SSs. The UL sub-frame consists of N consecutive time slots. Each time slot is normalized to be of unit length τ. The duration of a time frame is TF, so that TF=Nτ. The TDMA scheme for inter-SS is Max–Min Fair Sharing (MMFS) scheduling, in which station i is allocated ni time slots per frame. The TDMA scheme for inter-class is the strict priority queue scheme in which higher priority packets are queued ahead of lower priority ones. Packets of the same priority class that arrive at different slots are served on a First-Come-First-Served basis. Packets of the same priority class that arrive during the same slot are randomly ordered for transmission. Each station can transmit its packets only during its dedicated time slots, which is synchronized and

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The Online Journal on Computer Science and Information Technology (OJCSIT)

governed by the BS via DL/UL_Map. Packets arriving at each SS belong to one of the eight priority classes: UGS (priority-1and 2, highest priority), rtPS (priority 3 and 4), nrtPS (priority 5 and 6) or BE (priority 7 and 8, lowest priority). At each station, packet arrival is a Poisson arrival process so that λj is the average arrival rate of class j packets, j=1, 2,… ,8. Each packet is transmitted in different number of time slots according to its length. The number of time slots to transmit the kth arriving packet of priority class j is denoted by (j=1, 2,…,8). The transmission requests can be expressed by integer multiples of a time slot. It is assumed that the probability of collision of the request contention in the BE service is zero in the queuing model. The information of bandwidth allocation are carried by DL/UP_Map and broadcasted to all SSs. When the UL channel becomes available, any waiting priority-i packet is served before any priority-j one, if i