• Tradeoff of power. Since one signal needs to be sent to at least two different nodes, more power is needed to send data from source to destination. On the other hand, with diversity, the transmit power at each node can be smaller than in the noncooperative mode. Thus, it is necessary to develop a power allocation algorithm such that minimum transmit power is used to maintain the user cooperative diversity.
• Tradeoff of transmission rate. In cooperative communication, a node needs to relay other nodes’ data and also transmit its own data. Thus, its transmission rate is reduced. However, due to diversity, channel coding rates can be higher, which increases the transmission rate. Thus, the actual transmission rate may not be reduced. In order to keep the transmission rate as high as possible, we need to consider tradeoff between diversity and reduced chance of transmission.
• Interference. With cooperative diversity, interference may also be increased, because the same signal is sent more than once in the network. However, diversity may reduce the transmit power level, which can compensate for the increased interference.
• Cooperation assignment scheme. This is concerned with finding other cooperative nodes for each node so that diversity can be achieved. In a one-hop infrastructure based network, cooperation assignment may be just a simple task. However, in a multihop distributed network such as WMNs, cooperation assignment is rather complicated, because it has to take into account many factors such as diversity gain, power, interference, and even fairness among nodes.
• New requirements on network nodes. Although diversity can be achieved through cooperative communications, algorithms are still needed to retrieve original data from multiplexed signals. This requires additional processing power on either transmitter or receiver. The functionality of cooperative communications may also need change of hardware in each node.
wireless
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Aug 09
Cooperative Diversity and Cooperative Communications ”Note 5”
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Aug 09
Physical Layer “note 3”
Physical layer techniques advance rapidly as communication theories, digital signal processing algorithms, RF technologies, and circuit design for wireless communications quickly evolve. These techniques mainly focus on three directions: increasing transmission rate, improving error resilience capability in a wireless environment, and enhancing reconfigurability and software controllability of radios. In order to increase the capacity of wireless networks, various high-speed physical techniques have been invented. For example, orthogonal frequency division multiplexing (OFDM) has significantly increased the speed of IEEE 802.11 from 11 Mbps to 54 Mbps. A much higher transmission rate can be achieved through Ultra-Wideband (UWB) techniques. However, UWB is only applicable to short-distance applications such as wireless personal area networks (WPANs).
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Aug 09
Characteristics “Note 2”
• Multihop wireless network: One incentive to develop WMNs is to extend the coverage range of current wireless networks without sacrificing the channel capacity. Another major objective of WMNs is to provide nonline-of-sight (NLOS) connectivity among users without direct line-of-sight (LOS) links. To meet these requirements, mesh-style multihopping is indispensable , which facilitates higher throughput without sacrificing effective radio range via shorter link distances, less interference between nodes, and more efficient frequency reuse. • Support for ad hoc networking, and capability of self-forming, self-healing, and self-organization:Ad hoc networking enhances network performance, such as flexible network architecture, easy deployment and configuration, fault tolerance, and mesh connectivity, i.e., multipoint-to-multipoint communications. Due to these features, WMNs have low upfront investment requirement, and the network can grow gradually as needed.
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Aug 09
Wireless Mesh Networks “note 1”
Wireless Mesh Networks (WMNs) are one of the key technologies which will dominate wireless networking in the next decade. They will help to realize the long-lasting dream of network connectivity anywhere anytime with simplicity and low cost. Accordingly they will play a major role within the next generation Internet. Their capability for self-organization significantly reduces the complexity of network deployment and maintenance, and thus, requires minimal upfront investment.
These networks consist of simple mesh routers and mesh clients, where mesh routers have minimal mobility and form the backbone of WMNs. They provide network access for both mesh and conventional clients. The integration of WMNs with other networks such as the Internet, cellular, IEEE 802.11, IEEE 802.15, IEEE 802.16, sensor networks, etc., can be accomplished through the gateway and bridging functions in the mesh routers. Mesh clients can be either stationary or mobile, and can form a client mesh network among themselves and with mesh routers. WMNs are anticipated to resolve the limitations and to significantly improve the performance of ad hoc networks, wireless local area networks (WLANs) wireless personal area networks, (WPANs), and wireless metropolitan area networks (WMANs). These networks deliver wireless services to a large variety of applications in personal, local, campus, and metropolitan areas.
