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Optimal Broadcast for Multihop Wireless Network through Collision Resolution

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A Conventional wireless network uses a protocol called Chorus that improves the efficiency and scalability of broadcast service with a MAC/PHY layer that allows packet collisions. Chorus is built upon the observation that packets carrying the same data can be effectively detected and decoded, even when they overlap with each other and have comparable signal strengths. It resolves collision using symbol-level interference cancellation, and then combines the resolved symbols to restore the packet. Such a collision-tolerant mechanism significantly improves the transmission Diversity and spatial reuse in wireless broadcast. Chorus MAC-layer cognitive sensing and scheduling scheme further facilitates the Realization of such an advantage, resulting in an asymptotic broadcast delay that is proportional to the network radius. But there is a major disadvantage in it. We could not able to recognize the nodes which is in out of range (ie hidden terminals).In order to bring those hidden nodes inside a clustered network we break the mesh network into a Random network. So that we can reach all the nodes which is beyond the limit.
Keywords: Optimal broadcast, wireless ad hoc and mesh networks, collision resolution, multipacket reception, selfinterference cancellation, analog network coding,Hidden Terminals,clustered network.
Network wide broadcast is a fundamental primitive for many communication protocols in multihop wireless networks, such as route discovery and information dis-semination. An efficient broadcast protocol needs to delivers packet (or a continuous stream of packets) from the source node to all other nodes in the network, with high packetdelivery ratio (PDR) and low latency. To improve PDR in a lossy network, multiple relay nodes can forward and retransmit each packet, thereby creating retransmission diversity. To reduce latency and resource usage, however, the number of transmissions must be kept to minimum, since redundant retransmissions waste channel time, slowing down the packet’s delivery to the edge of the network. Therefore, a delicate balance needs to be maintained between PDR and delay.Chorus, based on a MAC layer that adopts CSMA with collision resolution (CSMA/CR).Chorus is built upon the key insight that packets carrying the same data can be detected and decoded, even when they overlap at the receiver with comparable strength. With Chorus, collision of the same packets from different relays can be effectively resolved. The advantage of such a collision-tolerant protocol is obvious. A novel broadcast protocol, called Chorus, based on a MAC layer that adopts CSMA with collision resolution (CSMA/CR). Chorus is built upon the key insight that packets carrying the same data can be detected and decoded, even when they overlap at the receiver with comparable strength. With Chorus, collision of the same packets from different relays can be effectively resolved. In this section, we introduce the physical-layer collision resolution in Chorus. For clarity, we start with a simple case of two-packet collision, focusing on how to detect, decode, and combine the collided packets to achieve the diversity gain. Both the spatial reuse and the transmit diversity gain in Chorus are realized via its collision resolution scheme which is based on self interference cancellation [5]. Unlike traditional transmit diversity schemes such as beam forming ,Chorus requires neither symbol time synchronization nor instantaneous channel state information. In reality, it is difficult to synchronize the independent transmitters A and B at the symbol level . The decoding succeeds as long as one packet has sufficient SNR, hence realizing the diversity offered by multiple transmitters.
At the MAC layer, Chorus adds a cognitive sensing and scheduling module to the 802.11 CSMA mechanism. Specifically, senders back off only when they sense a packet on the air that has a different identity from what they intend to transmit. Such a cognitive MAC allows Chorus to fully exploit the advantage of collision resolution, while main-taining friendliness to background traffic. In addition, the collision-resolution capability enables anonymous broad-cast at the network layer, without any topology or neighborhood information
To quantify the effectiveness of Chorus, we establish an analytical framework for its achievable SNR and bit error rate (BER), which takes into account the error-propagation effects in iterative collision resolution. We further analyze its network-level performance in terms of latency and throughput. With a joint design of CSMA/CR and broad-cast, Chorus achieves Âorp latency (r is the network radius), which is asymptotically optimal and unachievable in existing CSMA/CA-based broadcast protocols. The performance gain is relatively insensitive to network size, source rate, and link quality, and is observed for both static and mobile topologies, and in both single- and multisource broadcast scenarios. These salient properties are important, especially for information dissemination in large-scale wireless net-works, and represent the importance of exploiting PHY-layer signal processing to improve application performance.


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