Browsing by Subject "Full-Duplex"

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Open Access
Full-duplex MRI for zero TE imaging
(2016-05) Salim, Maryam
In this thesis a new method for decoupling of RF transmit and receive coils in MRI is presented. A modified version of isolation concept used in the full-duplex radios in communication systems is applied to acquire MRI signal using concurrent excitation and acquisition (CEA) method. Since in MRI transmit power is many orders of magnitude larger than receive signal, a weak coupling might dominate the MR signal during CEA in MRI. In our new method, a small copy of RF transmit signal is attenuated and delayed to generate the same coupling signal which is available in the receiver coil then it is subtracted from the receive signal in order to detect the MRI signal. The proposed decoupling method is developed and implemented in two designs. First a semi-automatic controllable decoupling design which uses a programmable attenuator and coaxial cables for the purpose of time delay. After estimating the length of coaxial cables an optimization algorithm finds the amount of attenuation factor. Using this method we could achieve more than 75 dB decoupling. Second design is a fully-automatic controllable decoupling design which contains four delay and attenuator lines. In this design four fixed phase shifters are used in order to generate the same phase delay between transmit and receive coils. A genetic optimization algorithm is used to find the amount of attenuation factors of each line. It is shown that this method provides more than 100 dB decoupling between transmit and receive coils which is good enough for detecting MRI signals during excitation from tissues with very short relaxation time. This study shows feasibility of applying full duplex electronics which is used in telecommunications, to decouple transmit and receive coils for MRI with CEA, using a clinical MRI system. This device can automatically tune the cancellation circuit and it is a potential tool for recovering signal from tissues with extremely short T2 in clinical MR systems.
Open Access
User grouping in wireless networks with full duplex base stations and legacy mobile stations
(2018-08) Ünal, Deniz
Improving spectral efficiency is a key objective in next generation wireless networks. Recent advances in self-interference cancellation techniques made in-band full-duplex wireless communications possible. Unlike half-duplex systems which require orthogonal frequency or time resources to separate transmission and reception, in-band full-duplex radios utilize the channel bidirectionally and theoretically can double the ergodic capacity. However due to cost, power consumption and complexity constraints, mobile stations may not support this technology. In this work, operation of full-duplex base stations with legacy half-duplex mobile stations is considered. An inherent issue of this topology is the presence of signi cant inter-user interference between half-duplex mobile stations. In order to manage this at network level, an optimization problem is formulated for a cellular network topology. Solution methods and their corresponding sum throughput are compared with respect to the number of mobile stations. An analytic solution is presented to evaluate the throughput and full-duplex gains of random pairing method for the same scenario. Then the case of limited channel state information is evaluated and a learning strategy is introduced to extend the user pairing problem to a continuous case. Performance evaluation with 100 mobile stations show that the proposed learning strategy can reduce the overhead airtime more than 80%. A weighted random sequential algorithm which is integrated to the learning process is proposed, and its performance evaluation under random walk and random waypoint mobility cases are performed.