Hybrid Frame Structure for Improving Network Throughput in WiMAX ...

Hybrid Frame Structure for Improving Network Throughput in WiMAX Mesh Networks M. Ilker Ulutas Mehmet S. Kuran Tuna Tugcu Department of Computer Engineering Bogazici University Bebek 34342, Istanbul, TURKEY {ilkeru, sukru.kuran, tugcu}@boun.edu.tr Abstract IEEE 802.16d standard defines WiMAX Mesh mode, which uses Centralized Scheduling (CS), Distributed Scheduling (DS), and co-existence of CS/DS data subframes to allocate resources for data transmissions. The CS and CS/DS schemes are suitable for Internet traffic, but the lack of spatial reuse in over-pessimistic use of CS causes scalability problems and bandwidth limitations. In this paper a Hybrid Frame Structure (HFS), which fine tunes the nodes in the network according to their hop counts, using both CS and DS methods to overcome this problem is proposed. By exploiting the spatial reuse property of DS, HFS achieves significant network throughput increase and acceptable latencies, without changing any control message contents defined in the standard. The proposed method is implemented and evaluated using our publicly available NETLAB WiMAX Mesh Simulator (NEMMS).

1. Introduction WiMAX included only Point-to-Multipoint (PMP) mode when it was first standardized in 2002, enabling communications between one hop Subscriber Stations (SS) and the Base Station (BS). Shortly after its release, the optional Mesh mode support was added to the standard allowing multi-hop transmissions [1]. Compared to the PMP mode, extended coverage, additional routing advantages, and less interference using more efficient modulation choices over multiple hops can be achieved using the Mesh mode. The proposed method in this paper focuses on the MAC layer of WiMAX Mesh mode as defined in [1]. Recently the focus on WiMAX standardization has shifted to complete IEEE 802.16m, which amends both IEEE 802.16-2009 and IEEE 802.16j standards to fulfill the IMT-Advanced requirements, commonly referred as 4G

defined by ITU-R. IEEE 802.16m provides high data rates while IEEE 802.16j extends the PMP mode by implementing relay capabilities. However they both lack the flexibility of Mesh mode allowing SSs in the same cell to communicate with each other directly. In the Mesh mode of operation, in addition to communications between the BS and SSs, the SSs can also exchange data packets with each other using their mesh links. A WiMAX Mesh network uses Centralized Scheduling (CS), Distributed Scheduling (DS) or both at the same time (CS/DS), dividing the data subframe in two parts. According to the type of scheduling method used, network parameters can be tuned on the BS side and broadcasted to the new entering nodes. Mesh mode transmissions are done using Orthogonal Frequency Division Multiplexing (OFDM), which allows only one user on the channel at a time. Therefore, Time Division Multiple Access (TDMA) scheme is used to schedule the grants in different minislots [6]. As defined in [1], there are 256 minislots in the data subframe which can all be used for CS, DS, or both when they coexist. When both of the scheduling methods are used, MSH-CSCH-DATAFRACTION parameter, which is broadcasted in the Network Configuration Messages (MSH-NCFG), defines the number of minislots used for CS. This parameter is set before the network initializes, so the size of CS and DS subframes are fixed. The corresponding frame structure defined in the standard is shown in Figure 1. In CS, the resources are allocated in a centralized way, which is coordinated by the BS. For uplink transmissions every SS makes a request during its control message Transmission Opportunity (TO) in each scheduling cycle. A scheduling cycle is composed of CSCH-Request messages being collected by the BS followed by CSCH-Grant messages distributed throughout the SSs hop by hop. There is no request mechanism for the downlink since BS grants itself considering its corresponding queues for each SS.

Control Packets

Data Packets

NCFG OR CSCF/CSCH

CENTRALIZED DATA

DISTRIBUTED DATA

DSCH

Figure 1. WiMAX mesh mode frame structure

Compared to CS, which is referred as scheduling on the basis of superiority of the BS in [1], DS provides scheduling on the basis of equality. Coordinated Distributed Scheduling (CDS) is done using a three-way-handshake procedure, considering the extended neighborhood of the requester node by exchanging Distributed Scheduling Control Messages (MSH-DSCH). A node’s Neighborhood is defined as the set of nodes which are one hop away from itself whereas Extended Neighborhood also includes the nodes which are 2-hops away. In order to make a successful unicast transmission in a single channel Time Division Duplex (TDD) network, there must be only one receiver in the neighborhood of a transmitter and only one transmitter in the neighborhood of a receiver [10]. There is also another DS method defined in the standard, called the Uncoordinated Distributed Scheduling (UDS), which is a contention based scheduling method. UDS is not suitable for the CS/DS scheme because request and grant messages are sent in the distributed data subframe and have a chance to collide. Therefore we use only CDS in this work so it will be referred simply as DS throughout the text. This paper focuses on WiMAX Mesh mode and proposes a Hybrid Frame Structure (HFS) to improve network performance by combining CS/DS and DS frame structures in such a way that significant network throughput increase is achieved while maintaining the end to end delay within acceptable limits. HFS uses the standard control messages and scheduling methods, so it preserves the collision-free nature of CS and DS messaging. The rest of this paper is organized as follows. Section 2 presents the previous work in the literature about various scheduling methods proposed for WiMAX Mesh networks. Section 3 states the problem, followed by the detailed description of the HFS method. In Section 4, the HFS method implementation, simulation environment, and the parameters used for the simulations are discussed along with the graphical results presented comparing the WiMAX standard and HFS. Finally, Section 5 concludes this paper by outlining the results.

2. Related Work IEEE 802.16d standard defines MAC and physical layer properties of a WiMAX network, including the contents of the control messages and general guidelines for both CS and DS, but it does not specify how the scheduling should

be done. Many papers in the literature propose different scheduling methods to overcome the limitations of the CS, and some use the combined frame structure where CS and DS are used together. Wei et al. propose the Interference Aware Route Construction method in [11] which defines a blocking metric B(k). The value of B(k) is calculated cumulatively hop by hop and gets larger when there is interference between the links. The route construction algorithm minimizes B(k) for each newly connecting node, which is implemented by modifying the MSH-NCFG messages. This method provides a centralized routing tree with minimum interference and using the Interference-Aware Scheduling algorithm concurrent transmissions are enabled. A similar routing tree construction is applied in [10] where new connecting SSs select the parent node having minimal interference. As new nodes connect, the interference values keep changing and some of the SSs need to switch their parent nodes which is implemented by a slight modification to CSCF messaging on the BS side. Uniform Slot Allocation Algorithm is proposed in [9], which requires a slot demand matrix, link interference matrix (LIM) and the routing tree as inputs and gives out the scheduling matrix to be used for concurrent transmissions. LIM is assumed to carry the binary interference information of every link in the network. Kim et al. have a different approach in [7], which also proposes a CS method but considers fairness amongst SSs in the network. It defines a satisfaction index, which combines node’s weight information and resource allocation history during the satisfaction window. Then, each node calculates the collision-free schedule assuming they have complete topology information. The methods discussed above are examples of CS applications with spatial reuse which either require modifications in standard control messages or assume that every node in the network have the interference or extended neighborhood knowledge of rest of the nodes in the network, which is not possible. According to the standard SSs can learn the link quality of the nodes only in their extended neighborhood by the help of the NCFG messages. No other interference information is carried by the control messages. There are several publications which use both CS and DS for data transmissions. A Centralized Queue Aware Routing (CQAR) scheme is proposed in [8] to solve local network congestion problems. According to CQAR, each node keeps track of its potential parent nodes other than its actual parent node. When the centralized queue length exceeds a preset threshold, the centralized traffic is routed through a distributed link belonging to one of the potential parent nodes. Another approach, Combined Distributed and Centralized method [5], suggests removing the partition boundary in CS/DS scheme. By eliminating the boundary, it is possible to utilize data subframe under varying traffic

4-hop Centralized Transmission

Some Wasted Minislots

Source

1-Hop

single packet, multiple minislots should be allocated for relaying the packet to the destination. Combined with the problem stated above, the network throughput gets limited significantly. Let the network parameters Tf r be the frame duration, Cms be the total CS minislots in a frame , s be the OFDM symbols per minislot, and Bsym be the number of bits that can be sent using one OFDM symbol depending on the modulation coding used. D · Tf r ·

M ∑

(Nh · h) ≤ Cms · s · Bsym

(1)

h=1

2-Hop

3-Hop

4-Hop

Centralized Scheduling Minislot Wasted Transmission Minislot Minislot the data packet is received

Figure 2. CS wasted minislots conditions. SSs extract CS information from CSCH-Grant message and send their DS data packets in each idle minislot even in the CS data subframe.

3. Hybrid Frame Structure 3.1. Problem Definition According to the standard, the division boundary between the centralized and distributed data subframes should be predefined by the BS and broadcasted to all of the SSs. Therefore, it is fixed for every network. As a consequence, the network loses its flexibility to adapt to varying traffic conditions in real-time. Another weakness of the standard frame structure comes from the definition of the CS mechanism. WiMAX standard states that during the centralized data subframe only one node in the whole network can transmit at a given time and all of the remaining nodes should keep radio silence. This wastes some valuable bandwidth, as shown in Figure 2, which could have been used for transmissions between the nodes distant enough and will not cause any interference. As the radius of the network gets larger, the centralized scheduling mechanism starts to suffer, because for every

Equation 1 shows the CS limitation where topology dependent parameter h is the SS hop count, M is equal to the maximum hop count in the network, Nh is the number of SSs with hop count h, and it is assumed that each SS having similar traffic demands D (in bps). Right hand side of Equation 1 is the total number of bits that can be sent in one frame and left side represents the summation of all traffic demands per frame. It is clear that as h and Nh increase, the maximum capacity is reached rather quickly. In Figure 2, a centralized packet transmission of 4-hops is illustrated. The packets need to be relayed four times by the nodes in the route consuming four times the bandwidth of the size of a packet. Left part of the figure shows how a standard centralized transmission should be and the right part of the figure points out several wasted transmission opportunities. It is possible to utilize the bandwidth without interfering the ongoing centralized communication by benefiting from the spatial reuse property of DS. Since nodes need to perform a three-way-handshake for each DS request the transmissions have higher latency especially when sending multihop packets, since a three-way-handshake is required for each hop. On the other hand, using CS after a CSCH request-grant cycle is completed, all nodes (including multi-hop nodes) can transmit their packets to the BS in the same frame, according to the scheduling information. Since the BS needs to coordinate all of the SSs in the network, as network size increases, connection setup time of CS gets longer because CSCH request-grant cycle needs to collect requests from all of the SSs and then relay the grants to all of them. Compared to CS, DS has the advantage of local scheduling where the number of nodes need to be coordinated is equal to the size of its extended neighborhood, which is relatively small. Having these properties CS is more suitable for the Internet traffic, especially for the nodes closer to the BS. On the contrary, DS’s local resource allocations can be used for intranet traffic, so there is a tradeoff between latency and throughput (spatial reuse). The proposed HFS method aims to combine the useful properties of both CS and DS, enabling higher bandwidth with acceptable end-to-end delay values.

0-Hop (BS) :

1-Hop :

2-Hop :

3-Hop :

n-Hop :

NCFG OR CSCF/CSCH

NCFG OR CSCF/CSCH

Centralized Scheduling

Distributed Scheduling

Centralized Scheduling

Distributed Scheduling

DSCH

DSCH

NCFG OR CSCF/CSCH

NCFG OR CSCF/CSCH

Distributed Scheduling

DSCH

Distributed Scheduling

NCFG OR CSCF/CSCH

Distributed Scheduling

DSCH

Distributed Scheduling

DSCH

Figure 3. Proposed Hybrid Frame Structure

3.2. The HFS Method Figure 4. Topology illustration In order to overcome the weaknesses of CS mentioned in the previous section, we propose a Hybrid Frame Structure which limits the CS transmissions up to 1-hop nodes. Beyond 1-hop, nodes can only transmit using DS, even to communicate with their parent nodes. So, the data subframe becomes fully distributed for the nodes having hop count three or more. Thus the network bandwidth can be used more efficiently since the CS transmission is limited to 1-hop nodes. The proposed HFS method combines the Standard Fully Distributed (SFD) frame structure with the Standard Centralized and Distributed (SCD) frame structure as shown in Figure 3. This method benefits from the low latency transmissions of CS and exploits the spatial reuse property of the DS for higher hop SSs. WiMAX data transmissions are collision free in nature so in order to avoid collisions between the nodes in the transition region radio silence is needed for 2-hop nodes at the first part of the data subframe. The control messaging remains the same as the standard and all of the nodes transmit both CSCH and DSCH messages. At the first glance it may seem unnecessary for the nodes only using DS to transmit CSCH messages, but they carry link updates along with sponsor node information, which are required for network entry of new nodes. [1] HFS implementation can be done by modifying the standard WiMAX Mesh MAC layer in such a way that the SSs behavior changes according to their hop counts to the BS. There are three different kinds of data subframe structures that the SSs may use corresponding to their hop counts. • The BS and 1-hop SSs use the SCD frame structure. They mainly communicate with each other using the centralized data subframe since 1-hop SSs may only use the distributed data subframe to exchange packets with 2-hop SSs. Therefore, they need to save majority of the distributed data subframe for those transmis-

sions. • 2-hop SSs are in the transition region and should keep radio silence in the first part of their data subframe since they have 1-hop neighbors using CS and 3-hop nodes using DS simultaneously. Any transmission done during that period may cause collisions which violate the collision-free data scheduling of WiMAX Mesh mode. • The third type of frame structure present in the HFS is the SFD scheme, which is and used by nodes having hop count higher than two. Using SFD can be beneficial especially when intranet traffic is present between SSs having hop counts greater than two since the whole data subframe can be used for DS transmissions. HFS method only requires the existing control messages defined in the standard and no extra information is assumed to be known to schedule collision-free data packet transmissions.

4. Performance Evaluation The HFS method is implemented and evaluated on OPNET 14.5 [4] using the MAC module of our publicly available NETLAB WiMAX Mesh Simulator (NEMMS) [3]. NEMMS implements the WiMAX mesh mode as defined in IEEE 802.16d standard and provides basic scheduling algorithms for simulation purposes. The implementation of HFS method was done by modifying the BS and SS MAC modules of NEMMS. In order to evaluate the HFS method several improvements were done on NEMMS MAC such as new traffic generation options. Sleep schedule was implemented in order

40 35 30 25 20 15 10 5 0

SFC HFS

0

1

2

3

4

5

Traffic Load Per Node (Mbps)

Avrg End-to-End Delay (ms)

Figure 5. Throughput, all Internet traffic case Delay vs Network Load 2500

SFC HFS

2000 1500 1000 500 0 0

1

2

3

4

5

Traffic Load Per Node (Mbps)

Figure 6. Delay in all internet traffic case Hybrid Frame Structure Delay by Hop Count Avrg End-to-End Delay (ms)

to observe the network behavior under bursty and continuous traffic patterns. Additionally, a mechanism to adjust the percentage of Internet and intranet traffic generation was deployed, which directly affects the performance of the network. In the rest of this section the proposed HFS method is evaluated and compared with WiMAX Mesh networks using Standard Fully Centralized (SFC) and SCD frame structures. The evaluation of a moderate sized network with 25 nodes is presented here due to page limitation. In Figure 4, the topology is illustrated by shading the SSs according to their hop counts, darkest SSs being 1-Hop. As stated in the standard, nodes use the most robust modulation, 1/2 QPSK, for the TOs in the control subframe and 3/4 64-QAM is used for the transmissions in the data subframe. The number of centralized control message TOs is tuned according to the method used. In HFS, 3 TOs are reserved since HFS requires more DSCH slots, whereas SCD uses 5 and SFC uses all 9 of the TOs present for centralized scheduling messages. The rest of the TOs are used for exchanging DSCH messages in the local neighborhood of each node. Interarrival times of data packets and the length of active and idle periods of traffic generators are exponentially distributed. Rest of the important parameters used in the simulations are listed in Table 1. First simulations are done only using Internet traffic, to observe how HFS performs providing Internet connection to end users. Network load is incremented slightly in each experiment to find out the maximum achievable network throughput and end-to-end delay of successfully transmitted packets. For 100% Internet traffic case, only SFC and HFS results are presented since DS minislots in SCD would be wasted because there is no intranet traffic present. Figure 5 shows that SFC starts to fail around 18 Mbps when each node is allowed to generate 2.5 Mbps internet traffic, whereas the network using HFS can have 32 Mbps network throughput, corresponding to 4.5 Mbps connections, with acceptable end-to-end delay values. It is recommended by

Total Network Throughput (Mbps)

Total Achievable Network Throughput

Table 1. Simulation parameters Number of SSs 24 Frame Duration 10 ms Modulation & Coding Rate 3/4 64-QAM (Data) Data Packet Size 108 bytes SS Buffer Size 20,000 packets Control Subframe Length 9 TOs NCFG Holdoff Exponent 2 DSCH Holdoff Exponent 1 Constant Exponent 1 Sleep/Active Time exponential(0.7s/0.3s) Simulated Time 5 minutes

1400

1-Hop SSs 2-Hop SSs 3-Hop SSs

1200 1000 800 600 400 200 0 0

1

2

3

Traffic Load Per Node (Mbps)

4

5

Figure 7. Delay comparison of SSs in HFS

ITU-R in [2] that one-way delay of 400 ms should be the upper bound for general network planning. It also states that under 150 ms delay, most applications are not affected. In the presented graphs, 400 ms limit was used while considering the maximum achievable throughput values. The end-to-end delay of HFS, which is around 200 ms in a stable network, is rather large compared to SFC having 50 ms average end-to-end delay (Figure 6). Looking at Figure 6, when the network load is greater than 3 Mbps per SS, the delay value seems to decrease, which may seem like an improvement in performance. This misleading behavior is caused by the high packet drop rates of the overloaded SFC network. We are only interested in the regions where

Total Achievable Network Throughput

Total Network Throughput (Mbps)

60

are also in the acceptable region. Starting from 3-Hop onwards the SSs start to get relatively high latencies. HFS does not allow SSs that are multiple hops away to use CS. As a future work, this limit can be incremented or it could be made dynamic (which requires modification in standard control messages) changing according to the ratio of Internet and intranet traffic in order to observe the network performance.

HFS SCD SFC

50 40 30 20 10 0 0

2

4

6

Traffic Load Per Node (Mbps)

Figure 8. Throughput 70% Internet traffic delay does not exceed 400 ms, but the overloaded part is also depicted to observe the overall behavior. As mentioned in previous sections, HFS has a tradeoff between network throughput and delay values. Since 200 ms is the average value of all data packet delays in HFS, a more detailed graph is plotted to investigate the delay values belonging to the SSs with specific hop counts. Figure 7 shows that 1-Hop, 2Hop, and 3-Hop nodes have around 40 ms , 170 ms, and 300 ms average delay values, respectively. So 1-Hop and 2-Hop SSs can use more delay intensive applications compared to 3-Hop nodes. In order to observe the effects of intranet traffic on HFS another scenario is also simulated, having 70% Internet traffic and 30% intranet traffic. When intranet traffic is introduced HFS can use its distributed links to exploit spatial reuse and reach over 40 Mbps (with acceptable delay) network throughput where SFC and SCD performs poorly achieving around 15 Mbps throughput (Figure 8). Considering topologies having 8 SSs and 15 SSs, the end-to-end delay of a stable HFS network drops down to 90 ms and 120 ms respectively. In smaller topologies with similar node densities compared to the 24 SS case, the total network throughput drops slightly since benefit from spatial reuse decreases.

5. Conclusion This paper presents the Hybrid Frame Structure method, which combines the CS and DS schemes defined in the IEEE 802.16d standard. As the simulation results suggest, the proposed frame structure achieves significant network throughput increase by benefiting from spatial reuse applying DS. HFS method works even better when there is intranet traffic between SSs since 3-Hop and further away SSs use all of their data subframes for distributed messaging. The average end-to-end delay and delay values based on hop counts are also presented. As hop count increases, the suffered delay increases as well, which is caused by the DS. 1-Hop nodes have similar delay values compared to SFC and can use delay sensitive applications while 2-Hop nodes

6. Acknowledgments This work is partially supported by the Scientific and Technical Research Council of Turkey (TUBITAK) under grant number 108E101 and by The State Planning Organization of Turkey (DPT) under grant number DPT07K120610.

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