近年來，車載隨意網路引起了相當多的關注。在車載隨意網路中有兩種主要的組成元件，分別為裝置在車輛上的On-Board Unit(OBU)和裝置在主要道路周邊的Road-Side Unit(RSU)。由於RSU的部屬需要花費相當可觀的金額，我們希望能夠盡量減少RSU部屬的數量，並且能夠維持一樣的傳輸品質與覆蓋範圍。因此，我們提出了一個媒體存取控制(Medium Access Control)層的傳輸協定。這個協定可以在相同的物理傳輸距離的前提之下，延伸虛擬的傳輸範圍，並且增加傳輸的效率與維持相同的傳輸品質。這個傳輸協定運用了機會式中繼與網路編碼來加強了在長距離時的封包傳輸的品質。透過機會式中繼，我們可以增加封包成功接收的機率：另外，經由使用網路編碼，我們可以減低因為回覆失敗而造成的效能下降。我們提出的協定不僅可以減少RSU部屬的數量，同時也可以提高傳輸的效率。從模擬的結果，我們可以看到我們提出的方法明顯的優於其他的媒體存取控制協定，其中包括了直接傳輸與經由固定的中繼點之協定。
最後，我們從分析的結果與模擬結果的比對可以確保我們提出的方法之正確性。

In recent years, Vehicular Ad Hoc Networks (VANETs), which consists of On-Board Units (OBUs) and Road Side Units (RSUs), are becoming more and more popular. Due to the high expenditure on the deployment of RSUs, it is important to minimize the number of deployed RSUs while maintaining the same coverage. To this end, we propose a MAC protocol that can extend virtual transmission range at the same physical transmission range. The protocol enhances packet transmission across a long distance by utilizing opportunistic relaying and network coding. The use of opportunistic relaying improves the packet reception probability because relay nodes opportunistically forward packets when needed. The use of network coding decreases performance degradation caused by acknowledgement loss. As a result, our proposed protocol not only can be used to decrease the number of deployed RSUs but also helps to improve performance in terms of throughput and delay. Simulation results show that our proposed protocol outperforms several MAC protocols, including direct communication and deterministic relaying (both using IEEE 802.11). We also compare the simulation results with our analysis to ensure the correctness.

Chapter 1 Introduction 1
Chapter 2 Related Work 5
Chapter 3 Preliminary 7
3.1. Opportunistic Relaying 7
3.2. Network Coding 10
Chapter 4 Protocol Design 12
4.1. Hello Messages and Neighbor Lists 14
4.2. Source 15
4.3. Relay 18
4.3.1. Relay Selection Scheme 19
4.3.2. Buffering and Linear Coding 21
4.3.3. Enhanced Relay Mechanism 22
4.4. Destination 24
4.4.1. Acknowledgement Packets 25
Chapter 5 Performance Evaluation 27
5.1. Simulation environment 27
5.2. Comparison with 802.11 29
5.3. Mathematical analysis 35
Chapter 6 Conclusions 39
Bibliography 40

[1] “Parameters for 802.11p vehicular ad hoc networks” http://dsn.tm.uni-karlsruhe.de/english/Overhaul_NS-2.php
[2] R. Ahlswede, N. Cai, S.-Y. R. Li, and R. W. Yeung, “Network information flow,” IEEE Transactions on Information Theory, vol. 46, no. 4, pp. 1204–1216, July 2000.
[3] S. Y. R. Li, R. W. Yeung, and N. Cai, “Linear network coding,” IEEE Transactions on Information Theory, vol. 49, p. 371, Feb. 2003.
[4] R. Koetter and M. Medard, “An Algebraic Approach to Network Coding,” IEEE/ACM Transactions on Networking, vol. 11, no. 5, pp. 782-795, Oct. 2003.
[5] S. Katti, H. Rahul, D. Katabi, W. H. M. Médard, and J. Crowcroft. “XORs in the Air: Practical Wireless Network Coding,” In Proc. of ACM SIGCOMM 2006, Pisa, Italy.
[6] G. J. Foschini, “On the limits of wireless communication in a fading environment when using multiple antennas,” Wireless Personal Commun., vol. 6, no. 3, pp. 311–335, 1998.
[7] S. M. Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE J. Select. Areas Commun,, vol. 16, no. 8, pp. 1451–1458, Oct. 1998.
[8] A. Sendonaris, E. Erkip, and B. Aazhang, “User cooperation diversity. part i. system description,” IEEE Trans. Commun., vol. 51, no. 11, pp. 1927–1938, Nov. 2003.
[9] A. Sendonaris, E. Erkip, and B. Aazhang, “User cooperation diversity—part II: implementation aspects and performance analysis,” IEEE Trans. Commun., vol. 51, no. 11, pp. 1939–1948, Nov. 2003.
[10] J. N. Laneman and G. W. Wornell, “Distributed space-time-coded protocols for exploiting cooperative diversity in wireless networks,” IEEE Trans. Inform. Theory, vol. 49, no. 10, pp. 2415–2425, Oct. 2003.
[11] J. N. Laneman, D. N. C. Tse, and G. W. Wornell, “Cooperative diversity in wireless networks: efficient protocols and outage behavior,” IEEE Trans. Inform. Theory, vol. 50, no. 12, pp. 3062–3080, Dec. 2004.
[12] A. Bletsas, A. Khisti, D. P. Reed, and A. Lippman, “A simple cooperative diversity method based on network path selection,” IEEE Journal on Selected Areas in Communications, vol. 23, no. 1, pp. 7-18, Jan. 2005.
[13] D. Michalopoulos and G. Karagiannidis, “Performance analysis of single relay selection in rayleigh fading,” IEEE Trans. Wireless Commun., vol. 7, no. 10, pp. 3718–3724, Oct. 2008.
[14] A. Bletsas, A. Khisti, and M. Z. Win, “Opportunistic cooperative diversity with feedback and cheap radios,” IEEE Trans. Wireless Commun., vol. 7, no. 5, pp. 1823–1827, May 2008.