Kwang-Eog Lee

Design of user clustering and precoding for downlink non-orthogonal multiple access (NOMA)

In this paper, we propose a novel user clustering algorithm for non-orthogonal multiple access (NOMA) considering the channel correlation between users and the channel gain. We also adopt sum-rate maximization approach to find an optimal precoding matrix in the multi-user multiple-input-multiple-output (MU-MIMO) setup after the user clustering. Grouping two users in a single beam to serve users in a non-orthogonal fashion raises multi-user interference (MUI). To tackle this problem, the clustering algorithm considers the channel correlation to select users aligned with same direction. Furthermore, the channel gain is also reflected in the clustering algorithm to enhance signal-to-interference-plus-noise ratio (SINR) inspired by the fact that the optimal precoding matrix is less sensitive to channel direction. Optimization algorithm over the precoding matrix is proposed based on Majorization Minimization (MM) approach for a non-convex objective function. We demonstrate the superiority of the proposed schemes comparing with conventional ones through numerical results.

Some good frequency hopping sequences with arbitrary number of slots

We propose a new construction of some favorable FH (frequency-hopping) sequences with an p/sup k/-p/sup m/ number of slots using a FSR (feedback shift register) over GF(p) and some nonlinear logic. We show that the resulting sequences satisfy the (almost) uniform symbol distribution in one period and pre-serve a reasonably good Hamming correlation property so that they are appropriate for military applications.

Performance improvement of a fast FH-FDMA system by the clipped-linear combining receiver

We investigate the performance of a fast frequency-hopped (FH) frequency division multiple access (FDMA) satellite system in an environment with partial-band jamming. First, by simulation, the bit error rate (BER) performance of the FH-FDMA system with a simple linear combining receiver is compared with that of other diversity combining receivers. Under the worst case partial-band jamming environment, the performance of the linear combining receiver is worse than that of other diversity combining receivers. We use a clipper together with the linear combiner to improve the BER performance of the linear combining receiver. As a result, the performance of the clipped-linear combining receiver becomes comparable to that of the normalized envelope detection (NED) diversity combining receiver under the condition of partial-band jamming. Particularly, the NED combining receiver is slightly outperformed by the clipped-linear combining receiver in the worst case partial-band jamming.

Reliable cluster-based common channel setup method for DSA CR networks

Due to the increasing demands for spectrum resource, a method for the efficient use of frequency for wireless communications is required. In dynamic spectrum access, secondary users (SUs) temporary use the frequency when the licensed spectrum is not occupied by the primary users. To exchange control and data messages between SUs, common channels between them should be established first. In the most of previous cognitive radio ad-hoc network researches, it is assumed that there exists a predetermined common channel. However, in real environments, it has some problems such as jamming attack and traffic concentration on the dedicated common channel. In this paper, we propose a cluster-based reliable dynamic common channel setup method for ad-hoc DSA networks. Without any predetermined channel, we dynamically setup common channels based on the each nodes channel availability. In the simulation, performance of the proposed method in terms of response failure rate, setup delay and cluster join probability is evaluated.

Development of a CSGPS/DR Integrated System for High-precision Trajectory Estimation for the Purpose of Vehicle Navigation

In this study, a carrier smoothed global positioning system / dead reckoning (CSGPS/DR) integrated system for high-precision trajectory estimation for the purpose of vehicle navigation was proposed. Existing code-based GPS has a low position accuracy, and carrier-phase differential global positioning system (CPDGPS) has a long waiting time for high-precision positioning and has a problem of high cost due to the establishment of infrastructure. To resolve this, the continuity of a trajectory was guaranteed by integrating CSGPS and DR. The results of the experiment indicated that the trajectory precision of the codebased GPS showed an error performance of more than 30cm, while that of the CSGPS/DR integrated system showed an error performance of less than 10cm. Based on this, it was found that the trajectory precision of the proposed CSGPS/DR integrated system is superior to that of the code-based GPS.

Error analysis of cooperative positioning system using two-way ranging measurements

In the cooperative positioning system (CPS), members of a group communicate each other and share their information to localize their position. The advantage of this system is that all the elements in the CPS can obtain their position collaboratively even if only a few members of the group get their position from GPS. This paper presents a low-cost CPS using two-way ranging (TWR) measurements and GPS. Using chirp spread spectrum ultra wideband (CSS-UWB) modulation scheme, precise TWR measurements with less than 1m error can be obtained and they are used to solve the position of members that cannot acquire GPS signal. This paper also analyzes the error propagation in the CPS. From the ranging covariance matrix, the location covariance matrix is derived to analyze the position error propagation. Simulations with various scenarios show that the geometry of members is the most important factor in the error propagation.

Construction of Sequences for Multi-group FH-FDMA in DRT System

This paper presents a method of constructing frequency hopping sequences for military satellite communication system which employs frequency dehop-rehop transponder(DRT). In DRT system, the dehopping and rehopping is to be performed upon a group based approach where each group has its own hopping sequence and is characterized by synchronous connection-oriented(SCO) link. The hopping patterns generated through proposed method have optimal Hamming auto-correlation property according to Lempel-Greenberger bound and assure perfect orthogonality between hopping groups i.e. no chip collisions during the period. A practical implementation of the proposed method using shift registers is also described.

Analysis on detection performance of an energy detector for several chip waveforms in a DS/SS communication

In this paper, we analyze the effect of chip waveform on the detection performance of an energy detector in a multi-user DS/SS system. Detection performance of the partial-band detection is investigated together with the full-band detection for a DS/SS signal. In order to evaluate the detection performance of the partial-band detection, bandwidth factor and energy factor are introduced. Numerical results show that the partial-band detection strategy always produces better detection performance than the full-band detection strategy with a proper selection of partial-band detection bandwidth. In addition, among the compared three chip waveforms, such as rectangular, half-sine and raised-cosine waveform, the raised-cosine waveform; produces much lower detection probability compared with other waveforms, as the number of concurrent users increases.