Juite Hwu

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— The use of signal processing techniques to protect wireless transmissions is proposed as a way to secure wireless networks at the physical layer. This approach addresses a unique weakness of wireless networks whereby network traffic traverses a public wireless medium making traditional boundary controls ineffective. Specifically, a randomized array(More)
Cognitive radio is a desirable technique for secondary users to utilize the spectrum gray space. In order to avoid intolerable interference to primary users, the transmission power of cognitive radios must be carefully managed. In this paper, we analyze the sum transmission power of a group of randomly distributed but fully cooperated secondary transmitters(More)
For wireless sensor networks or robotic networks, cooperative communications encoded with space-time block code (STBC) are desirable to improve their energy efficiency and reliability. However, existing STBC-encoded cooperative communication schemes usually require perfect synchronization among the transmitting sensors or robots, which is difficult and even(More)
One of the major challenges to cognitive radios is the synchronization of distributed radios onto the same spectrum white spaces which vary in time and space. In this paper, we propose a frequency-hopping spread spectrum transmission scheme which works reliably without any a priori handshaking assumption. Each cognitive radio independently detects white(More)
In this paper, we analyze the capacity of secondary spectrum access in a broadcasting system consisting of one primary transmitter and multiple primary receivers. At the cost of a small redundancy of the SINR of primary receivers, secondary users can gain a significant capacity when allowed to share the spectrum at the same time with the primary(More)
In contrast to the classical spread spectrum or data encryp-tion methods, we propose an array redundancy-based approach for wireless transmissions with inherent low probability of interception (LPI). The redundancy of transmit antenna arrays introduces some degrees of freedom for deliberate signal randomiza-tion, based on which, diversity is exploited to(More)
Channel dispersion is inevitable in asynchronous cooperative transmissions even in flat fading environment , which may greatly degrade the gain of cooperative diversity and cast doubts on their application in wireless sensor networks for energy efficiency or reliability. This paper studies the interference caused by such channel dispersion, and shows that(More)
In this paper, we analyze the allowable transmission power of cognitive radios when used as secondary users in a dynamic spectrum access network. Closed-form upper and lower bounds are derived, while accurate solutions to transmission power are available through numerical evaluation. The transmission power depends on the distance between primary and(More)
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