Kun Yang, Shumao Ou, Hsiao-Hwa Chen and Jianghua He

1. A Multihop Peer-Communication Protocol With Fairness Guarantee forIEEE 802.16-Based Vehicular Networks Kun Yang, Shumao Ou, Hsiao-Hwa Chen and Jianghua He IEEE Transactions on Vehicular Technology ,2007 Mei-jhen Chen

2. Outline • Introduction • System Model • Proposed Protocol－CEPEC (coordinated external peer-communication ) • Transmission Scheduling Within and Across Segments • Simulation Design and Results • Conclusion

3. Introduction (1/4) • Vehicles are an essential part of people’s everyday life. • Much work has been conducted to provide a common platform facilitating intervehicle communications (IVCs) or intelligent transportation systems. • IVC is necessary to realize traffic-condition monitoring, dynamic route scheduling, emergency-message dissemination, and, most importantly, safe driving. • Safety communication is essential for IVC, and dedicated short-range communications (DSRC) is a key enabling technology for it.

4. Introduction (2/4) • This is largely due to the fact that IEEE 802.11 is recommended by DSRC systems as the default protocol. • However, the contention nature of IEEE 802.11 generally renders itself hard to provide guaranteed QoS. • In this paper, authors consider a vehicular network that accesses the Internet through fixed roadside IEEE 802.16 base stations (BSs).

5. Introduction (3/4)

6. Introduction (4/4) • In this paper, we present a cross-layer protocol designed mainly for vehicle–roadside communication (VRC). • Authors name this protocol as coordinated external peer-communication (CEPEC) protocol for vehicular networks. • The major objective of CEPEC is to increase the end-to-end throughput in IEEE 802.16-enabled VRC while ensuring fairness guarantee in bandwidth usage among road sections.

7. System Model -Problem Definition (1/3) • As far as IEEE 802.16 is concerned, works have been conducted on providing Internet services using different QoS supporting scheduling algorithms. • This paper focuses on another equally important issue arising in vehicular networks, i.e., data transmission from moving vehicles on highways to peers connected to the Internet. • These peers are typically not commercial service/content providers but are individuals such as the driver/passenger’s family members or colleagues. • We call this type of communication as external peer communication (EPC).

8. System Model -Problem Definition (2/3) • This paper is largely inspired by the Controlled Vehicular Internet Access (CVIA) protocol. • Data relaying is also needed within segments and this leads to longer delay. • Select two routers in a segment: • The routers should be as close to the segment borders as possible. • They need to stay inside the segment during the active period of the segment given their high moving speeds. • The way fairness is measured is at segment level and ignores the unevenness of traffic requests from different vehicles.

9. System Model -Problem Definition (3/3) • In CEPEC, R : the physical transmission range of the vehicles and BSs.

10. System Model -Network Model • The network model employed in this paper is briefed as follows. • Each vehicle is equipped with a global-positioning system (GPS) device for time synchronization and providing vehicle positions. • The physical layer of mesh mode uses time-division multiple access (TDMA) for channel allocation. • Time-division duplex is utilized to share the channel between uplink and downlink. • A mesh frame consists of a control and a data subframe. The multiple-access scheme at the radio interface is orthogonal frequency-division multiple access.

11. Proposed Protocol－CEPEC-Overview • CEPEC is a multihop cluster-based protocol. • Each segment represents a cluster, and data delivery from vehicles to a BS is carried out on a segment-to-segment basis. LPi:All packets collected locally from segment Si. APi:aggregated with the packets received from the neighboring segment Si+1. APi = LPi + APi+1 APi+1 APi LPi

12. 1. Inactive 2. Inter- segment Packet (APi+1) Receiving 3. Local Packet (LPi) Collecting wi ai ci bi 6. Inter- segment Packet (APi) Sending 5. Next Head (SHi-1) Selection 4. Aggregated Packet (APi) Generating hi Proposed Protocol－CEPEC-Protocol Operation • In CEPEC, each segment Si normally goes through six phases as a lifecycle.

13. Proposed Protocol－CEPEC-Protocol Operation 1. Inactive phase: • A segment becomes inactive for duration wiif no time slot is allocated to it. • In CEPEC, inactive phase is employed to ensure no interference from Si to the ongoing data transmission in the neighboring segments. • The duration of widepends on the scheduling algorithms. • If a mesh frame can be shared by multiple segments, then wi can be very small. • Otherwise, wican be as long as the duration of a segment’s lifecycle.

14. Proposed Protocol－CEPEC-Protocol Operation 2. Intersegment packet-receiving phase: • In CEPEC, each Si(i≠N) needs to relay packets from Si+1 in uplink. • SN, as the farthermost segment away from the BS, does not have this phase. • After Sibecomes active, SHistarts receiving packets from SHi+1. • In TDMA-based IEEE 802.16, the AP-sending phase of SHi+1 can be synchronized to the AP-receiving phase of SHi. • A time interval of aiis reserved for this purpose. • the amount of traffic to be relayed from SHi+1 to SHi • the fairness mechanism • the number of hops to the BS and the bandwidth capacity of the BS

15. Proposed Protocol－CEPEC-Protocol Operation 3. Local-packet-collecting (LPC) phase of duration bi: • After receiving APi+1 from SHi+1, the vehicles start competing for transmission opportunities in the control subframe. • The vehicles that are successfully granted a transmission opportunity can send their data transmission requests to SHi. • Authors assume that all traffics are of same type (BE) and have the same priority. • Upon receipt of the requests, a scheduling algorithm is activated to allocate time slots to vehicles. • Vehicles in Si send their data packets directly to SHi. • All these packets received by SHi are aggregated into LPi.

16. Proposed Protocol－CEPEC-Protocol Operation 4. AP generating phase: • APi+1 and LPiare merged together to create a new AP: APi. • Only some local processing is performed in this phase, so the duration for this phase is neglected.

17. Proposed Protocol－CEPEC-Protocol Operation 5. Next head-selection phase: • Before APiis transmitted to Si−1 (or BS if the current segment is S1), SHi−1 needs to be selected. • Vehicles in Si−1 start to compete to become the SHi−1. Before competition, each vehicle calculates the segment ID it belongs to using xv: the X-coordination of vehicle v obtained from its onboard GPS xBS :the BS’s X-coordination obtained via the BS’s EAP (existence announcement packets)

18. Proposed Protocol－CEPEC-Protocol Operation • A BS periodically broadcasts the following information to all vehicles within its SvC: • its capacity C • the number of segments N • the segment length L • Otherwise, they can be informed of to vehicles by the BS when vehicles register themselves • A competing vehicle v has to satisfy the following two conditions. • C1: xv+ (ai+ bi+ ci) × Vmax< xb-i • xb-i : the X-coordinate of the segment border that is closer to the BS. • i :the segment ID, where vehicle v is located in. • Vmax : the maximum velocity of vehicles. • C2: |xCi − sv| < d • d = |xCi − sa| ∧ a ∈ Si ∧ a ≠v.

19. Proposed Protocol－CEPEC-Protocol Operation 6. Intersegment packet-sending phase: • APiis forwarded to SHi−1. • In CEPEC, duration of ciis reserved for this phase. • cishould be long enough to forward packets from both local segment and packets collected from other preceding segments.

20. Proposed Protocol－CEPEC-Fairness • Packets originated from vehicles locating at Si(i =2, . . . , N) have to go through i segments to reach the BS. • C : a fixed overall bandwidth capacity at the BS. • C/N : each segment is granted an equal portion of C. • SH allocates bandwidth to requesting vehicles in proportion to their packet-transmission requirements. • Reqi : the overall requested packet transmission in Si. • p = (C/N)/Reqi:an equal proportion of each vehicle’s requested packet is transmitted to SHi. • Reqi > C/N : saturation status. • Reqi≤ C/N : all Reqiis transmitted.

21. Transmission Scheduling Within and Across Segments • Authors use a centralized-scheduling scheme to allocate aggregate transmission time to the SHs. • the bandwidth request-and-grant mechanism specified in the 802.16 standard • Goal: to maximize throughput delivered to the BSs while ensuring bandwidth-usage fairness.

22. Transmission Scheduling Within and Across Segments-Allocation of Time to Segments • The scheduling is made with a period of nfrm frames. • tfrm : the duration of a frame. • In a schedule, each SH will be granted by the BS the transmission time of niframes for segment i. • ni= ni,i+ ni,i−1

23. Transmission Scheduling Within and Across Segments-Allocation of Time to Segments • The centralized-scheduling problem is formulated by an optimization model.

24. Transmission Scheduling Within and Across Segments-Allocation of Time to Segments

25. Simulation Design and Results • A vehicular-network simulator has been developed by us to simulate the performance of both the CEPEC protocol and the IEEE 802.16 protocol. • There is a two-lane straight highway of limited length with each lane for each direction of traffic flow. • There is more than one sublane all carrying traffic to same direction. • The highway is divided into a few segments of fixed length L starting from a BS. • These segments constitute the SvC of this BS.

26. Simulation Design and Results • Simulated vehicles randomly enter the SvC with exponentially distributed interarrival time at a rate of 100 vehicles per minute. • Vehicle speed follows a Gaussian distribution with a mean of 85 km/h and a standard deviation of 10 km/h. • Uplink data request of each vehicle follows an exponential distribution with the rate of 512 kb/s. • The capacity of the BS is 40 Mb/s. • Channel data rate varies between 5–40 Mb/s. • Transmission ranges of all vehicles and the BS are same: R = 2.4 km. • Payload of packets varies between 500 and 2200 B.

27. Simulation Design and Results • Scenario 1: payload = 500 B, N = 4. • Scenario 2: payload = 2200 B, N = 4. • Scenario 3: payload = 2200 B, N = 8. Fig 6. Overall uplink throughput

28. Simulation Design and Results • Scenario 1: payload = 500 B, N = 4. • Scenario 2: payload = 2200 B, N = 4. • Scenario 3: payload = 2200 B, N = 8. Fig 7. Overall delay

29. Simulation Design and Results • SvC size : N=4 and 8 • L1 = 2R/3, L2 = 2R√10, and L3 =1R/2 (L1 < L2 < L3) Fig 8. Lengths of segments on delay Fig 9. Lengths of segments on throughput

30. Conclusion • This paper proposes a novel communication protocol for vehicular networks that supports vehicles, on highways, communicating with peers at home or office. • The CEPEC protocol coordinates the functions of physical, MAC, and network layers to provide a fair and handoff-free solution for uplink packet delivery from vehicles to BSs. • The simulation results have showed that the proposed protocol has better performance than the standard IEEE 802.16 protocol in terms of both end-to-end throughput and packet delay.