Yeo Hun Yun

An orthogonal cognitive radio for a satellite communication link

In this paper, we consider overlaying a secondary-user signal over a satellite communication channel occupied by a primary user. The objective is to find the jointly optimal transmitter and receiver pair of a secondary system to minimize the mean squared error (MSE) at the output of the secondary receiver, subject to zero interference induced at the output samples of the primary receiver. It is assumed that both the primary and the secondary transmitters employ linear modulation and the receivers employ linear filter front-ends. Unlike prior work, the secondary user is allowed to transmit data symbols at a fraction of the primary users symbol rate. This rate reduction technique enables the secondary user to achieve the additive white Gaussian noise (AWGN) bound or the performance close to the AWGN bound even when the excess bandwidth of the primary user is very small but positive. Numerical results show that the proposed scheme significantly improves the spectral efficiency by exploiting the cyclostationarity of the primary-user signal.

An optimal orthogonal overlay for a cyclostationary legacy signal

In this paper, the design of an overlay system is considered that operates on a channel already occupied by a legacy system. It is assumed that the legacy transmitter generates a cyclostationary signal, and that the legacy receiver is equipped with a linear filter followed by a uniform sampler. Under a zero-interference constraint at the legacy receiver, the meansquared error at the overlay receiver is minimized to find the optimal overlay system that employs linear modulation and demodulation. For the trade-off between performance and rate of the overlay system, the problem is formulated to allow a transmission rate that is a fraction of the fundamental cycle frequency of the legacy signal. The notion of a virtual legacy receiver is devised to incorporate cases with non-identical frequency bands for the transmission and reception of the overlay signal. Using the vectorized Fourier transform technique, the problem is reformulated, the solution is derived, and a necessary and sufficient condition for the existence of the optimal solution is found, all in the frequency domain. Numerical results show that the proposed scheme significantly improves the spectral efficiency by effectively exploiting the cyclostationarity of the legacy signal.

On sum capacity of continuous-time overloaded CDMA systems

In this paper, the optimal design problem of overloaded code-division multiple-access (CDMA) systems is examined. Unlike previous results for chip or symbol synchronous systems, a continuous-time, band-limited, additive white Gaussian noise channel is considered for the multiple-access communications. First, non-information theoretic results are summarized, where the total transmit power is minimized, subject to lower bounds on the signal-to-interference-plus-noise ratio at the output of the linear minimum mean-squared error receivers. Second, the sum capacity is derived and shown to be the same as that of the optimal frequency-division multiple-access system, where each users bandwidth is upper-bounded by the cycle frequency of the corresponding CDMA system. As the non-information theoretic results, the geometric procedure called multi-user constrained water-pouring leads to the optimal system that maximizes the sum rate. It is shown that orthogonal waveforms are assigned to oversized users and continuous-time equivalents of generalized Welch bound equality sequences are assigned to non-oversized users. A method to construct an optimal codebook is also proposed to be used in a CDMA signal modulator in each transmitter.

A Design of Optimal Overlay System Inducing Zero Interference to Legacy Systems

In this paper, the jointly optimal transmitter and receiver pair of an overlay system is derived to minimize the mean squared error (MSE) at the output of the linear receiver, subject to zero interference induced to legacy systems already existing in the frequency band of interest. It is assumed that the overlay and the legacy systems all employ linear modulation with the same symbol transmission rate but they are not necessarily symbol- synchronized. The symbol sequences are modeled as wide-sense stationary (WSS) colored random processes and the channels are modeled as strictly band-limited linear time-invariant systems with frequency selective impulse responses. Using the vectorized Fourier transform (VFT) technique, the optimal transmit and receive waveforms are derived in the frequency domain. Numerical results show that the proposed system significantly improves the bit error rate (BER) performance over the systems with receiver-only optimization and the systems with no transmitter and receiver optimization.

Maximum sum rate of a restricted FDMA system

In this paper, the sum-rate optimal restricted FDMA system is investigated for asymmetric-power users, when they have individual upper limits on the bandwidths in addition to a constraint on total system bandwidth. A convex optimization problem is formulated to optimally allocate the system bandwidth to the users. First, the users are classified into oversized and non-oversized users, based on the strength of their minimal power spectral densities (PSDs). Then, the maximum sum rate and the unique optimal solution in a closed algorithmic expression are derived by using the Karush-Kuhn-Tucker optimality conditions. An iterative algorithm to construct the optimal solution is also presented that provides insights into the restricted FDMA system. Interestingly, the non-oversized users end up with an equal PSD among one another, which is always less than the PSDs of the oversized users, if they exist. It turns out that the sum-rate optimal restricted FDMA system achieves the sum capacity of the band-limited multiple-access channel if and only if there is no oversized user.

Sum-Rate Optimal Multicode CDMA Systems: An Equivalence Result

In this paper, the sum rate of a multicode code-division multiple-access (CDMA) system with asymmetric-power users is maximized, given a processing gain and the power profile of users. Unlike the sum-rate maximization for a single-code CDMA system, the optimization requires the joint optimal distribution of each users power to its multiple data streams as well as the optimal design of signature sequences. The crucial step is to establish an equivalence of the multicode CDMA system to restricted frequency-division multiple-access (FDMA) and time-division multiple-access (TDMA) systems. The CDMA system has upper limits on the numbers of multicodes of users, whereas the FDMA system has upper limits on the bandwidths, and the TDMA system has upper limits on the duty cycles of users. The equivalence facilitates the complete characterization of the maximum sum rate of the multicode CDMA system and also provides new insights into single- and multicode CDMA systems in terms of the parameters of the equivalent FDMA and TDMA systems.