Yang-Seok Choi | Polytechnic Institute of NYU (original) (raw)

Papers by Yang-Seok Choi

2015 IEEE Global Communications Conference (GLOBECOM), 2014

2015 IEEE 16th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), 2015

2015 IEEE 81st Vehicular Technology Conference (VTC Spring), 2015

This paper presents a novel RF circuit architecture for self-interference cancellation in inband ... more This paper presents a novel RF circuit architecture for self-interference cancellation in inband full-duplex radio transceivers . The developed canceller is able to provide wideband cancellation with waveform bandwidths in the order of 100 MHz or beyond and contains also self-adaptive or self-healing features enabling automatic tracking of time-varying self-interference channel characteristics. In addition to architecture and operating principle descriptions, we also provide actual RF measurements at 2.4 GHz ISM band demonstrating the achievable cancellation levels with different bandwidths and when operating in different antenna configurations and under low-cost highly nonlinear power amplifier. In a very challenging example with a 100 MHz waveform bandwidth, around 41 dB total cancellation is obtained while the corresponding cancellation figure is close to 60 dB with the more conventional 20 MHz carrier bandwidth. Also, efficient tracking in time-varying reflection scenarios is demonstrated.

2003 IEEE 58th Vehicular Technology Conference. VTC 2003-Fall (IEEE Cat. No.03CH37484), 2003

Fading caused by multipath propagation and by shadowing is a significant problem for wireless loc... more Fading caused by multipath propagation and by shadowing is a significant problem for wireless local area networks (WLANs). The use of diversity can mitigate the effects of this fading. In this paper we report on the effect of using beam selection diversity for a 2.4 GHz WLAN system. Propagation measurements were made in a typical office environment using a planar antenna array capable of forming seven simultaneous 10-degree beams. The beam selection diversity receiver selected the best beam, either on a packet-by-packet basis or using a long-term average measurement. Measurement results show that the mean received signal strength can be significantly increased by use of beam selection diversity.

IEEE 60th Vehicular Technology Conference, 2004. VTC2004-Fall. 2004, 2004

Smart antennas can greatly improve the performance of wireless communication systems by providing... more Smart antennas can greatly improve the performance of wireless communication systems by providing better link quality and immunity to interference. A simple smart antenna structure is fixed beamforming and beam selection. More complex structures require combining of the signals from multiple receivers. In this paper we compare the performance of beam and antenna selection and combining techniques. Our results are based on recently reported statistical and measurement based spatial channel models and in most cases include impact of co-channel interference. We derive the probability density function (pdf) of SINR in space- and frequency-selective multiple clustered Rayleigh fading channel with finite angle spread (AS). We also analyze the outage capacity and probability of beam selection in flat fading assuming both correlated and independent fading between the antennas. Our analysis also includes the impact of erroneous beam or antenna selection. Furthermore, theoretical bounds for a hybrid of beam/antenna selection and maximal ratio combining (MRC) are presented. We show that beam selection techniques can be very effective against both multipath fading and co-channel interference. We also show that simple fixed beamforming networks such as Butler matrices are nearly as effective as more complex fixed beamforming networks.

2003 IEEE 58th Vehicular Technology Conference. VTC 2003-Fall (IEEE Cat. No.03CH37484), 2003

2003 IEEE 58th Vehicular Technology Conference. VTC 2003-Fall (IEEE Cat. No.03CH37484), 2003

We consider the capacity of multiple-inputmultiple-output (MIMO) systems with reduced complexity.... more We consider the capacity of multiple-inputmultiple-output (MIMO) systems with reduced complexity. One link end uses all available antennas, while the other chooses the "best" L out of N antennas. The selection of the optimum antenna subset requires an exhaustive search of all possible combinations, involving ( N L ) computations of determinants of size L × L, which can become prohibitively complex. In this paper, we suggest a class of fast antenna selection algorithm that are based on the correlation or mutual information between the signals at the different antenna elements. It requires less than N 2 vector multiplications and thus leads to dramatic savings on the computation time. Its performance is very close to the optimum selection procedure: the capacity penalty is less than 1bit/s/Hz for the analyzed examples. The algorithm thus offers the possibility of almost-optimum selection even in fast-changing environments.

2006 IEEE International Conference on Communications, 2006

This paper derives a maximum likelihood Doppler frequency estimator for orthogonal frequency divi... more This paper derives a maximum likelihood Doppler frequency estimator for orthogonal frequency division multiplexing (OFDM) systems in time-varying multipath channels. The proposed scheme is a frequency-domain approach that utilizes pilot subcarriers, which are commonly implemented in most practical systems. Time-varying fading causes intercarrier interference (ICI) in OFDM systems. Thus, in the proposed estimator, the effect of ICI is taken into consideration with a proper model for accurate results. The estimator can be implemented using a finite impulse response (FIR) filter bank whose coefficients can be pre-calculated and stored in order to lower the computational complexity. We evaluate various methods to improve the estimation accuracy and analyze their complexityperformance tradeoffs. We also derive the Cramér-Rao bound and provide simulation results to quantify the performance of the proposed algorithm.

Wireless Networks and Mobile Communications, 2007

A clear majority of the reported works in in-band full-duplex radio technology research focus on ... more A clear majority of the reported works in in-band full-duplex radio technology research focus on the network element side or relay type of devices. However, to fully capitalize the benefits of the full-duplex principle in, e.g., cellular systems, also the mobile devices should support simultaneous transmission and reception at same carrier. Due to the strict requirements regarding the size and cost, as well as highly limited linearity in the deployed low-cost RF components, this is an extremely challenging task. In this article, we focus on demonstrating and pushing the state-of-the-art performance boundaries of transmitter-receiver isolation in mobile full-duplex devices, building on shared-antenna based transceiver architecture where antenna reflections already substantially limit the natural isolation between the transmitter and receiver chains. Novel self-adaptive analog RF cancellation circuitry is first described, capable of tracking time-varying self-interference coupling characteristics in mobile devices over very wide waveform bandwidths. Furthermore, novel adaptive nonlinear DSP methods are also explored for final self-interference suppression at digital baseband, when operating under highly nonlinear low-cost RF components. Measurement results from a complete demonstrator implementation are provided, evidencing beyond 30 dB of active RF cancellation over a 100 MHz waveform bandwidth and highly nonlinear transmitter power amplifier (PA) operating at +20 dBm output power. Measured examples also demonstrate the good self-healing characteristics of the developed control loop, operating fully in the analog domain, against fast changes in the coupling characteristics. Furthermore, when Manuscript

2009 IEEE International Conference on Acoustics, Speech and Signal Processing, 2009

In order to meet the constantly increasing demand for ubiquitous, mobile access to the internet, ... more In order to meet the constantly increasing demand for ubiquitous, mobile access to the internet, next-generation mobile broadband communications systems based on OFDMA, such as IEEE 802.16m, require a significant performance increase over previous generation systems, such as IEEE 802.16e-2005, particularly in cell-edge and average spectral efficiency . In this paper, we address the downlink adaptive frequency reuse (AFR) and multi-user MIMO (MU-MIMO) techniques which are considered to be the most promising candidates for meeting the requirements on cell-edge and average spectral efficiency of next-generation mobile broadband systems.

IEEE Transactions on Wireless Communications, 2000

We consider the capacity of multiple-input multipleoutput systems with reduced complexity. One li... more We consider the capacity of multiple-input multipleoutput systems with reduced complexity. One link-end uses all available antennas, while the other chooses the L out of N antennas that maximize capacity. We derive an upper bound on the capacity that can be expressed as the sum of the logarithms of ordered chi-square-distributed variables. This bound is then evaluated analytically and compared to the results obtained by Monte Carlo simulations. Our results show that the achieved capacity is close to the capacity of a full-complexity system provided that L is at least as large as the number of antennas at the other linkend. For example, for L = 3, N = 8 antennas at the receiver and three antennas at the transmitter, the capacity of the reducedcomplexity scheme is 20 bits/s/Hz compared to 23 bits/s/Hz of a full-complexity scheme. We also present a suboptimum antenna subset selection algorithm that has a complexity of N 2 compared to the optimum algorithm with a complexity of N L .

IEEE Transactions on Wireless Communications, 2000

Full Duplex or Simultaneous transmission and reception (STR) in the same frequency at the same ti... more Full Duplex or Simultaneous transmission and reception (STR) in the same frequency at the same time can potentially double the physical layer capacity. However, high power transmit signal will appear at receive chain as echoes with powers much higher than the desired received signal. Therefore, in order to achieve the potential gain, it is imperative to cancel these echoes. As these high power echoes can saturate low noise amplifier (LNA) and also digital domain echo cancellation requires unrealistically high resolution analog-to-digital converter (ADC), the echoes should be cancelled or suppressed sufficiently before LNA. In this paper we present a closed-loop echo cancellation technique which can be implemented purely in analogue domain. The advantages of our method are multiplefold: it is robust to phase noise, does not require additional set of antennas, can be applied to wideband signals and the performance is irrelevant to radio frequency (RF) impairments in transmit chain. Next, we study a few protocols for STR systems in carrier sense multiple access (CSMA) network and investigate MAC level throughput with realistic assumptions in both single cell and multiple cells. We show that STR can reduce hidden node problem in CSMA network and produce gains of up to 279% in maximum throughput in such networks. Moreover, at high traffic load, the gain of STR system can be tremendously large since the throughput of non-STR system is close to zero at heavy traffic due to severe collisions. Finally, we investigate the application of STR in cellular systems and study two new unique interferences introduced to the system due to STR, namely BS-BS interference and UE-UE interference. We show that these two new interferences will hugely degrade system performance if not treated appropriately. We propose novel methods to reduce both interferences and investigate the performances in system level. We show that BS-BS interference can be suppressed sufficiently enough to be less than thermal noise power, and with favorable UE-UE channel model, capacities close to double are observed both in downlink (DL) and uplink (UL). When UE-UE interference is larger than DL co-channel interferences, we propose a simple and "non-cooperative" technique in order to reduce UE-UE interference.

IEEE Transactions on Wireless Communications, 2000

Multiple antenna systems can greatly improve the performance of wireless systems by providing bet... more Multiple antenna systems can greatly improve the performance of wireless systems by providing better link quality, capacity and immunity to interference. A very simple structure for smart antennas is a fixed beamforming network (FBN) with beam selection. In Y.S. Choi and S.M. Alamouti (2004) the performance of antenna and beam selection/combining techniques are compared. The channel model in that paper

IEEE Transactions on Communications, 2000

Smart antenna technology can greatly improve the coverage radius of wireless systems by focusing ... more Smart antenna technology can greatly improve the coverage radius of wireless systems by focusing the transmitted signal (beam) toward desired users. However, beamforming has an important side effect as it creates a region, herein referred to as the complementary region, where some users in the network cannot sense the directional signals (beams). We have referred to this as the hidden

IEEE Transactions on Communications, 2001

... In order to reduce the complexity of the estimator, we apply the theory of optimal low rank a... more ... In order to reduce the complexity of the estimator, we apply the theory of optimal low rank approximation to a minimum mean squared error ... Index Terms—Channel estimation, fast fading, multicarrier sig-nals, OFDM, time diversity, time-varying channel. I. INTRODUCTION ...

IEEE Journal on Selected Areas in Communications, 2000

MIMO (multiple input multiple output) techniques are widely employed to improve the performance o... more MIMO (multiple input multiple output) techniques are widely employed to improve the performance of wireless systems. These techniques are used to overcome multipath fading and/or improve the peak throughput of wireless systems. It is well known that there is a fundamental tradeoff between diversity gain and multiplexing gain [1]. Orthogonal space time codes such as the Alamouti code (also known

Animal Reproduction Science, 2006

The objective of the present study was to develop a procedure for isolating pure populations of r... more The objective of the present study was to develop a procedure for isolating pure populations of round spermatid(s) (RS) by Percoll density gradient from bull testes. Bull testes were de-capsulated and testicular tissues were dissociated enzymatically to recover RS. After being filtered through a 20 microm nylon mesh, the cells were centrifuged at 650 x g for 25 min through the discontinuous Percoll density gradients (20, 35, 40, 45 and 90% Percoll solution). Isolated cells were analyzed by microscopic observation for survivability and apoptosis. In Experiment 1, both microscopic observation and DNA analysis by flow cytometry showed that approximately 40% of cells collected from 35% Percoll gradient were presumptive RS, whereas in 40% Percoll gradient, mostly primary spermatocytes were observed. Experiment 2 compared the effect of 35% Percoll density isolation on the incidence of apoptosis and necrosis in fresh and frozen-thawed cells to those of untreated cells. The percentage (mean+/-S.E.M.) of necrosis in cells collected from 35% Percoll gradient was less (P<0.05) than in untreated and frozen-thawed cells from 35% Percoll gradient (11.7+/-3.1% compared with 26.3+/-2.0% and 53.5+/-1.3%, respectively), but the rate of apoptosis did not differ (1.2+/-0.49% compared with 2.5+/-0.8% and 0.9+/-0.04%, respectively). The proportional data (mean+/-S.E.M.) of live cells in Percoll treated group were greater (P<0.05) than in untreated and frozen-thawed cells from the 35% Percoll gradient (86.7+/-3.26% compared with 70.8+/-2.73% and 41.9+/-1.69%, respectively). Experiment 3 compared the development rates of embryos injected with RS isolated from fresh and frozen-thawed cells collected with the 35% Percoll gradient to those of untreated cells, and parthenotes as control. There were no significant (P>0.05) differences in the rates of cleavage and blastocyst development between untreated fresh cells and fresh cells collected from the 35% Percoll gradient (75.4 and 10.5% compared with 82.4 and 12.8%). However, there were lesser (P<0.05) cleavage and blastocyst rates in frozen-thawed cells from the 35% Percoll gradient (51.6 and 6.3%) and parthenotes (60.7 and 4.1%) were observed. These results suggest that isolation of presumptive RS by 35% Percoll density gradient is effective in eliminating apoptotic and early necrotic cells. However, the use of RS in improving the developmental potential of embryos merits further studies.

IEEE Transactions on Vehicular Technology, 2001

In this paper, we present a new maximum likelihood estimator (MLE) of carrier frequency offset fo... more In this paper, we present a new maximum likelihood estimator (MLE) of carrier frequency offset for multicarrier signals in a frequency-selective Rayleigh fading channel. The proposed MLE is able to achieve wider estimation range of frequency offset with higher accuracy than previous blind approaches by exploiting the intrinsic structure of multicarrier signals. Simulations show that the estimation accuracy is close to the Cramér-Rao bound. In order to reduce the complexity of the proposed MLE, a suboptimum technique is presented that enables simple implementation. The bit error rate performance of the suboptimum technique with two or three symbols averaged is close to the theoretical bound. An important feature of both the MLE and the suboptimum method is the capability to correct the frequency offset in a feedforward approach that makes it possible to perform fast acquisition and tracking of the frequency offset.

2015 IEEE Global Communications Conference (GLOBECOM), 2014

2015 IEEE 16th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), 2015

2015 IEEE 81st Vehicular Technology Conference (VTC Spring), 2015

This paper presents a novel RF circuit architecture for self-interference cancellation in inband ... more This paper presents a novel RF circuit architecture for self-interference cancellation in inband full-duplex radio transceivers . The developed canceller is able to provide wideband cancellation with waveform bandwidths in the order of 100 MHz or beyond and contains also self-adaptive or self-healing features enabling automatic tracking of time-varying self-interference channel characteristics. In addition to architecture and operating principle descriptions, we also provide actual RF measurements at 2.4 GHz ISM band demonstrating the achievable cancellation levels with different bandwidths and when operating in different antenna configurations and under low-cost highly nonlinear power amplifier. In a very challenging example with a 100 MHz waveform bandwidth, around 41 dB total cancellation is obtained while the corresponding cancellation figure is close to 60 dB with the more conventional 20 MHz carrier bandwidth. Also, efficient tracking in time-varying reflection scenarios is demonstrated.

2003 IEEE 58th Vehicular Technology Conference. VTC 2003-Fall (IEEE Cat. No.03CH37484), 2003

Fading caused by multipath propagation and by shadowing is a significant problem for wireless loc... more Fading caused by multipath propagation and by shadowing is a significant problem for wireless local area networks (WLANs). The use of diversity can mitigate the effects of this fading. In this paper we report on the effect of using beam selection diversity for a 2.4 GHz WLAN system. Propagation measurements were made in a typical office environment using a planar antenna array capable of forming seven simultaneous 10-degree beams. The beam selection diversity receiver selected the best beam, either on a packet-by-packet basis or using a long-term average measurement. Measurement results show that the mean received signal strength can be significantly increased by use of beam selection diversity.

IEEE 60th Vehicular Technology Conference, 2004. VTC2004-Fall. 2004, 2004

Smart antennas can greatly improve the performance of wireless communication systems by providing... more Smart antennas can greatly improve the performance of wireless communication systems by providing better link quality and immunity to interference. A simple smart antenna structure is fixed beamforming and beam selection. More complex structures require combining of the signals from multiple receivers. In this paper we compare the performance of beam and antenna selection and combining techniques. Our results are based on recently reported statistical and measurement based spatial channel models and in most cases include impact of co-channel interference. We derive the probability density function (pdf) of SINR in space- and frequency-selective multiple clustered Rayleigh fading channel with finite angle spread (AS). We also analyze the outage capacity and probability of beam selection in flat fading assuming both correlated and independent fading between the antennas. Our analysis also includes the impact of erroneous beam or antenna selection. Furthermore, theoretical bounds for a hybrid of beam/antenna selection and maximal ratio combining (MRC) are presented. We show that beam selection techniques can be very effective against both multipath fading and co-channel interference. We also show that simple fixed beamforming networks such as Butler matrices are nearly as effective as more complex fixed beamforming networks.

2003 IEEE 58th Vehicular Technology Conference. VTC 2003-Fall (IEEE Cat. No.03CH37484), 2003

2003 IEEE 58th Vehicular Technology Conference. VTC 2003-Fall (IEEE Cat. No.03CH37484), 2003

We consider the capacity of multiple-inputmultiple-output (MIMO) systems with reduced complexity.... more We consider the capacity of multiple-inputmultiple-output (MIMO) systems with reduced complexity. One link end uses all available antennas, while the other chooses the "best" L out of N antennas. The selection of the optimum antenna subset requires an exhaustive search of all possible combinations, involving ( N L ) computations of determinants of size L × L, which can become prohibitively complex. In this paper, we suggest a class of fast antenna selection algorithm that are based on the correlation or mutual information between the signals at the different antenna elements. It requires less than N 2 vector multiplications and thus leads to dramatic savings on the computation time. Its performance is very close to the optimum selection procedure: the capacity penalty is less than 1bit/s/Hz for the analyzed examples. The algorithm thus offers the possibility of almost-optimum selection even in fast-changing environments.

2006 IEEE International Conference on Communications, 2006

This paper derives a maximum likelihood Doppler frequency estimator for orthogonal frequency divi... more This paper derives a maximum likelihood Doppler frequency estimator for orthogonal frequency division multiplexing (OFDM) systems in time-varying multipath channels. The proposed scheme is a frequency-domain approach that utilizes pilot subcarriers, which are commonly implemented in most practical systems. Time-varying fading causes intercarrier interference (ICI) in OFDM systems. Thus, in the proposed estimator, the effect of ICI is taken into consideration with a proper model for accurate results. The estimator can be implemented using a finite impulse response (FIR) filter bank whose coefficients can be pre-calculated and stored in order to lower the computational complexity. We evaluate various methods to improve the estimation accuracy and analyze their complexityperformance tradeoffs. We also derive the Cramér-Rao bound and provide simulation results to quantify the performance of the proposed algorithm.

Wireless Networks and Mobile Communications, 2007

A clear majority of the reported works in in-band full-duplex radio technology research focus on ... more A clear majority of the reported works in in-band full-duplex radio technology research focus on the network element side or relay type of devices. However, to fully capitalize the benefits of the full-duplex principle in, e.g., cellular systems, also the mobile devices should support simultaneous transmission and reception at same carrier. Due to the strict requirements regarding the size and cost, as well as highly limited linearity in the deployed low-cost RF components, this is an extremely challenging task. In this article, we focus on demonstrating and pushing the state-of-the-art performance boundaries of transmitter-receiver isolation in mobile full-duplex devices, building on shared-antenna based transceiver architecture where antenna reflections already substantially limit the natural isolation between the transmitter and receiver chains. Novel self-adaptive analog RF cancellation circuitry is first described, capable of tracking time-varying self-interference coupling characteristics in mobile devices over very wide waveform bandwidths. Furthermore, novel adaptive nonlinear DSP methods are also explored for final self-interference suppression at digital baseband, when operating under highly nonlinear low-cost RF components. Measurement results from a complete demonstrator implementation are provided, evidencing beyond 30 dB of active RF cancellation over a 100 MHz waveform bandwidth and highly nonlinear transmitter power amplifier (PA) operating at +20 dBm output power. Measured examples also demonstrate the good self-healing characteristics of the developed control loop, operating fully in the analog domain, against fast changes in the coupling characteristics. Furthermore, when Manuscript

2009 IEEE International Conference on Acoustics, Speech and Signal Processing, 2009

In order to meet the constantly increasing demand for ubiquitous, mobile access to the internet, ... more In order to meet the constantly increasing demand for ubiquitous, mobile access to the internet, next-generation mobile broadband communications systems based on OFDMA, such as IEEE 802.16m, require a significant performance increase over previous generation systems, such as IEEE 802.16e-2005, particularly in cell-edge and average spectral efficiency . In this paper, we address the downlink adaptive frequency reuse (AFR) and multi-user MIMO (MU-MIMO) techniques which are considered to be the most promising candidates for meeting the requirements on cell-edge and average spectral efficiency of next-generation mobile broadband systems.

IEEE Transactions on Wireless Communications, 2000

We consider the capacity of multiple-input multipleoutput systems with reduced complexity. One li... more We consider the capacity of multiple-input multipleoutput systems with reduced complexity. One link-end uses all available antennas, while the other chooses the L out of N antennas that maximize capacity. We derive an upper bound on the capacity that can be expressed as the sum of the logarithms of ordered chi-square-distributed variables. This bound is then evaluated analytically and compared to the results obtained by Monte Carlo simulations. Our results show that the achieved capacity is close to the capacity of a full-complexity system provided that L is at least as large as the number of antennas at the other linkend. For example, for L = 3, N = 8 antennas at the receiver and three antennas at the transmitter, the capacity of the reducedcomplexity scheme is 20 bits/s/Hz compared to 23 bits/s/Hz of a full-complexity scheme. We also present a suboptimum antenna subset selection algorithm that has a complexity of N 2 compared to the optimum algorithm with a complexity of N L .

IEEE Transactions on Wireless Communications, 2000

Full Duplex or Simultaneous transmission and reception (STR) in the same frequency at the same ti... more Full Duplex or Simultaneous transmission and reception (STR) in the same frequency at the same time can potentially double the physical layer capacity. However, high power transmit signal will appear at receive chain as echoes with powers much higher than the desired received signal. Therefore, in order to achieve the potential gain, it is imperative to cancel these echoes. As these high power echoes can saturate low noise amplifier (LNA) and also digital domain echo cancellation requires unrealistically high resolution analog-to-digital converter (ADC), the echoes should be cancelled or suppressed sufficiently before LNA. In this paper we present a closed-loop echo cancellation technique which can be implemented purely in analogue domain. The advantages of our method are multiplefold: it is robust to phase noise, does not require additional set of antennas, can be applied to wideband signals and the performance is irrelevant to radio frequency (RF) impairments in transmit chain. Next, we study a few protocols for STR systems in carrier sense multiple access (CSMA) network and investigate MAC level throughput with realistic assumptions in both single cell and multiple cells. We show that STR can reduce hidden node problem in CSMA network and produce gains of up to 279% in maximum throughput in such networks. Moreover, at high traffic load, the gain of STR system can be tremendously large since the throughput of non-STR system is close to zero at heavy traffic due to severe collisions. Finally, we investigate the application of STR in cellular systems and study two new unique interferences introduced to the system due to STR, namely BS-BS interference and UE-UE interference. We show that these two new interferences will hugely degrade system performance if not treated appropriately. We propose novel methods to reduce both interferences and investigate the performances in system level. We show that BS-BS interference can be suppressed sufficiently enough to be less than thermal noise power, and with favorable UE-UE channel model, capacities close to double are observed both in downlink (DL) and uplink (UL). When UE-UE interference is larger than DL co-channel interferences, we propose a simple and "non-cooperative" technique in order to reduce UE-UE interference.

IEEE Transactions on Wireless Communications, 2000

Multiple antenna systems can greatly improve the performance of wireless systems by providing bet... more Multiple antenna systems can greatly improve the performance of wireless systems by providing better link quality, capacity and immunity to interference. A very simple structure for smart antennas is a fixed beamforming network (FBN) with beam selection. In Y.S. Choi and S.M. Alamouti (2004) the performance of antenna and beam selection/combining techniques are compared. The channel model in that paper

IEEE Transactions on Communications, 2000

Smart antenna technology can greatly improve the coverage radius of wireless systems by focusing ... more Smart antenna technology can greatly improve the coverage radius of wireless systems by focusing the transmitted signal (beam) toward desired users. However, beamforming has an important side effect as it creates a region, herein referred to as the complementary region, where some users in the network cannot sense the directional signals (beams). We have referred to this as the hidden

IEEE Transactions on Communications, 2001

... In order to reduce the complexity of the estimator, we apply the theory of optimal low rank a... more ... In order to reduce the complexity of the estimator, we apply the theory of optimal low rank approximation to a minimum mean squared error ... Index Terms—Channel estimation, fast fading, multicarrier sig-nals, OFDM, time diversity, time-varying channel. I. INTRODUCTION ...

IEEE Journal on Selected Areas in Communications, 2000

MIMO (multiple input multiple output) techniques are widely employed to improve the performance o... more MIMO (multiple input multiple output) techniques are widely employed to improve the performance of wireless systems. These techniques are used to overcome multipath fading and/or improve the peak throughput of wireless systems. It is well known that there is a fundamental tradeoff between diversity gain and multiplexing gain [1]. Orthogonal space time codes such as the Alamouti code (also known

Animal Reproduction Science, 2006

The objective of the present study was to develop a procedure for isolating pure populations of r... more The objective of the present study was to develop a procedure for isolating pure populations of round spermatid(s) (RS) by Percoll density gradient from bull testes. Bull testes were de-capsulated and testicular tissues were dissociated enzymatically to recover RS. After being filtered through a 20 microm nylon mesh, the cells were centrifuged at 650 x g for 25 min through the discontinuous Percoll density gradients (20, 35, 40, 45 and 90% Percoll solution). Isolated cells were analyzed by microscopic observation for survivability and apoptosis. In Experiment 1, both microscopic observation and DNA analysis by flow cytometry showed that approximately 40% of cells collected from 35% Percoll gradient were presumptive RS, whereas in 40% Percoll gradient, mostly primary spermatocytes were observed. Experiment 2 compared the effect of 35% Percoll density isolation on the incidence of apoptosis and necrosis in fresh and frozen-thawed cells to those of untreated cells. The percentage (mean+/-S.E.M.) of necrosis in cells collected from 35% Percoll gradient was less (P<0.05) than in untreated and frozen-thawed cells from 35% Percoll gradient (11.7+/-3.1% compared with 26.3+/-2.0% and 53.5+/-1.3%, respectively), but the rate of apoptosis did not differ (1.2+/-0.49% compared with 2.5+/-0.8% and 0.9+/-0.04%, respectively). The proportional data (mean+/-S.E.M.) of live cells in Percoll treated group were greater (P<0.05) than in untreated and frozen-thawed cells from the 35% Percoll gradient (86.7+/-3.26% compared with 70.8+/-2.73% and 41.9+/-1.69%, respectively). Experiment 3 compared the development rates of embryos injected with RS isolated from fresh and frozen-thawed cells collected with the 35% Percoll gradient to those of untreated cells, and parthenotes as control. There were no significant (P>0.05) differences in the rates of cleavage and blastocyst development between untreated fresh cells and fresh cells collected from the 35% Percoll gradient (75.4 and 10.5% compared with 82.4 and 12.8%). However, there were lesser (P<0.05) cleavage and blastocyst rates in frozen-thawed cells from the 35% Percoll gradient (51.6 and 6.3%) and parthenotes (60.7 and 4.1%) were observed. These results suggest that isolation of presumptive RS by 35% Percoll density gradient is effective in eliminating apoptotic and early necrotic cells. However, the use of RS in improving the developmental potential of embryos merits further studies.

IEEE Transactions on Vehicular Technology, 2001

In this paper, we present a new maximum likelihood estimator (MLE) of carrier frequency offset fo... more In this paper, we present a new maximum likelihood estimator (MLE) of carrier frequency offset for multicarrier signals in a frequency-selective Rayleigh fading channel. The proposed MLE is able to achieve wider estimation range of frequency offset with higher accuracy than previous blind approaches by exploiting the intrinsic structure of multicarrier signals. Simulations show that the estimation accuracy is close to the Cramér-Rao bound. In order to reduce the complexity of the proposed MLE, a suboptimum technique is presented that enables simple implementation. The bit error rate performance of the suboptimum technique with two or three symbols averaged is close to the theoretical bound. An important feature of both the MLE and the suboptimum method is the capability to correct the frequency offset in a feedforward approach that makes it possible to perform fast acquisition and tracking of the frequency offset.