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Cooperative
Communications (合作式通訊) |
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Most recent wireless communication systems available are concerned
mostly with the transceiver design of a single transmitter and a single
receiver. However, the performances that can be achieved by modern cellular
systems are often limited by the reliability of the wireless medium. More
specifically, the quality of our reception may vary dramatically depending on
the location and the mobility of both the mobile and its neighboring objects.
This large variation in performance often causes interruption in
communication and a bad quality of service. As the proportion of multimedia
contents gradually increase in the wireless traffic, the reliability of the
communication link becomes more and more important. Although higher layer
networking protocols can provide a certain level of quality guarantee, the
improvement is still limited by the reliability provided through the physical
layer design. The main idea of cooperative communications is to have multiple
wireless terminals cooperate in transmitting or receiving information. The
advantage is rather intuitive: the probability that all the cooperating users
experience bad channels simultaneously is exponentially smaller than the probability
of that for a single user. This is a form of distributed spatial diversity
which is analogous to that of multiple antennas. However, there are several
challenges in this area of research. For example: (1) the cooperation
requires coordination among users that may occupy communication resources
that can otherwise be used to increase the data rate, therefore, the problem
is how to design a transmission or signaling for coordination such that the
improvement in data rate or reliability is increased even with the additional
use of communication resources. (2) Another interesting question is: Who
should we cooperate with? Ideally, every user wants to cooperate with the
user that possesses the best channel, however, the inter-user channel may not
allow one to do so and the fairness may also be sacrificed. It is interesting
to see, what the ideal tradeoffs for such systems are. (3) What are the
additional advantages that we can explore when we increase the diversity in
frequency, in time or in coding? The interesting problems to explore goes on
and on. Several
references on this subject is listed below:
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Wireless Sensor
Networks (無線感測網路) |
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Wireless sensor networks typically consists of a group of small
miniature devices that are able to sense the environment, process the
observations and communicate to coordinate a common response. This is a wide and
exciting topic that has received a new wave of interest in recent years, in
all areas of engineering. Specifically, we are interested in the
communication and signal processing aspect of this problem. Sensor networks are often assumed to be deployed in massive scale to
perform pervasive monitoring and computing of the environment, which provides
an interface for closely integrating our every-day lives with the natural
world. The major challenges of a sensor network are the limited energy
resources at each node and the scalability of the communications and the
computations. In fact, it has been shown that the point-to-point per node
throughput of a wireless network vanishes rapidly with the number of nodes in
the network. This is a pessimistic result that has hindered the development
of dense wireless networks. However, in sensor networks, the observations at
each sensor are often highly correlated and the computation is often
performed for a common purpose. Therefore, the highly redundant information contained
in the sensors’ messages can be utilized to reduce the communication cost of
the network. In this sense, there is a significant difference between
wireless sensor nodes and that of conventional wireless ad hoc networks.
Therefore, it is important to develop both energy efficient and bandwidth
efficient strategies for the sensors to achieve their common goal or to
efficiently communication their local observations in a compressed form. There are many open problems concerning wireless sensor systems.
Specifically, I am interested in topics related to distributed data
compression, data gathering strategies, distributed detection, localization
and energy-efficiency through sleep-wake algorithms. There is a vast literature on this topic, some survey
articles are listed below:
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Network
Information Theory (網路消息理論) |
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With all in mind, I am certainly interested in the fundamental theory of
communications and networking. In particular, I am interested in looking at
multi-user systems from the information-theoretic point of view. This
involves fundamental research on source coding, such as Distributed Source
Coding, Multiple Description Coding, Successive
Refinement Coding; and, also in channel coding, such as Broadcasting,
Multiple Access, Relay Transmissions. Extending from the conventional theory
that often requires long blocks of data to achieve the coding efficiency, I
am particularly interested in looking at single letter coding strategies or,
in other words, uncoded transmissions of
information. |
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