Sungkyu Jung

A new ML based interference cancellation technique for layered space-time codes

This paper presents a new and simple decoding algorithm for layered space time block codes such as the two independent Alamoutis codes which are also called the double space-time transmit diversity (DSTTD) system. By using group interference suppression and successive interference cancellation, we can treat DSTTD as two independent space-time block codes (STBC). We can then decode both of these STBCs through a simple maximum likelihood (ML) detector with null space-based interference cancellation. We also compare the proposed interference cancellation (IC) scheme with the conventional MMSE IC scheme. The performance of the proposed IC scheme is comparable to that of the MMSE IC scheme while the complexity reduction factor of the proposed scheme can be up to 5 compared to the MMSE IC scheme.

Interference alignment and cancellation for the two-user X channels with a relay

A novel interference alignment technique combined with interference cancellation is proposed. A new scenario of single-antenna 2-user X channel with a multiple-antenna relay is considered. The proposed interference alignment and cancellation scheme does not require any wireline links between receivers. In the proposed scheme, interference signals are not decoded. Instead, interference signals are just aligned, and the aligned signal is cancelled out to extract the desired signal. We call the proposed scheme an aligned interference cancellation technique. It is known that DoF of 4/2 is achievable for the two-user X channel using the interference alignment. In the proposed scheme, relay precoding matrices are designed to accomplish two purposes. One is to cancel out the aligned interference signals perfectly, and the other is to enable an equivalent received signal model of the Alamoutis codes. The proposed relay-aided interference alignment and cancellation technique not only makes interference alignment feasible, but also improves performance. The interference signals are aligned at a specific time slot, and the precoding matrices of the relay are designed to cancel the aligned interference. In simulations, it is verified that the diversity gain of 2 is achieved with the degrees-of-freedom (DoF) of 4/3.

Hybrid MU-MISO Scheduling with Limited Feedback Using Hierarchical Codebooks

The multiuser MIMO (MU-MIMO) systems tend to have inter-user interference in practical systems with limited feedback. Practical MU-MIMO systems with limited feedback can be divided into two classes in general. One is zero-forcing beamforming (ZFBF), and the other is per user unitary rate control (PU2RC) which is an extended version of random beamforming (RBF). To improve performance, ZFBF needs more feedback bits, and PU2RC needs a larger number of users. We propose a new hybrid MU-MISO system which has the advantages of the two MU-MIMO schemes simultaneously, and analyze the sum-rate performance and the quantization error of the hybrid scheme. The analysis shows how the number of feedback bits, the number of users, and the signal power affect the sum-rate. The throughput scaling laws are also derived in the high and the medium SNR regimes. Simulation results show that the proposed hybrid MU-MISO system performs better than either of the two MU-MIMO systems with the same number of feedback bits. Another advantage is that it has lower complexity for codeword search since it has a hierarchical codebook structure.

Post-Scheduling SINR Mismatch Analysis for Multiuser Orthogonal Random Beamforming Systems

In this letter, we analyze the post-scheduling SINR mismatch of a multiuser orthogonal random beamforming (ORBF) system. The mismatch probability is derived for a Gaussian broadcast channel with M transmit antennas and K single-antenna users. Numerical results are shown to be identical to theoretical results. The probability analysis enables us to calculate the minimum number of users to satisfy a target mismatch probability. Another way to use the mismatch probability is to control CQI mismatch handling mechanisms.

Capacity Comparison of Orthogonal and Non-Orthogonal Cooperative Relay Systems

For a single relay channel, we study the average throughput of two different amplify-and-forward (AF) protocols, which are orthogonal-AF (OAF) and nonorthogonal-AF (NAF). The NAF protocol has been proposed to overcome a significant loss of performance of OAF in the high spectral efficiency region, and it was also shown that NAF performs better than OAF in terms of the diversity-multiplexing tradeoff. However, the results have been evaluated at the asymptotically high signal to noise ratio (SNR), and the power allocation problem between the source and the relay was neglected. We examine which protocol has a better performance in a practical system operating at a finite SNR. We also study where a relay should be located with consideration of the power allocation. In this paper, we show that the performance depends on both SNR and power allocation ratio, which indicates OAF may perform better in a certain environment.

On the SINR Distribution for an Orthogonal Random Beamforming System and Its Performance

This paper presents closed-form expressions of the distribution of signal-to-interference-plus-noise ratios (SINRs) and the average bit error rate (BER) for an orthogonal random beamforming system in multiple-antenna Gaussian broadcast channels. First, we derive the statistical distribution of the SINR of the scheduled transmit signals and confirm that the approximate form of the cumulative distribution function (cdf) of the SINR is equivalent to the formula in \cite{sharif} and \cite{letaief}. We then obtain a closed-form expression for the total average BER of the system by using the approximate cdf. Our theoretical results are compared to the Monte Carlo simulation results.

Linear Degrees-of-Freedom for the

This paper analyzes the linear degrees of freedom (LDoF) for K-user M ×N MIMO interference channels with constant channel coefficients. In this correspondence, we interpret the interference alignment problem as a multivariate polynomial system in terms of properness. The properness provides a LDoF upperbound for interference alignment. With this approach, we propose a method to obtain the upperbound for total LDoF achievable by linear interference alignment. The LDoF upperbound for properness condition results have two different cases which depend on the number of users, the number of transmit antennas, and the number of receive antennas. A key contribution of this paper is that it provides a closed-form LDoF upperbound for M ×N MIMO interference channels where the LDoF of each user may be different.

M\times N

This paper presents a new and simple decoding algorithm for layered space time block codes such as the two independent Alamoutis codes which are also called the double space-time transmit diversity (DSTTD) system. By using group interference suppression and successive interference cancellation, we can treat DSTTD as two independent space-time block codes (STBC). We can then decode both of these STBCs through a simple maximum likelihood (ML) detector with null space-based interference cancellation. We also compare the proposed interference cancellation (IC) scheme with the conventional MMSE IC scheme. The performance of the proposed IC scheme is comparable to that of the MMSE IC scheme while the complexity of the proposed scheme is lower than that of the MMSE IC scheme.

MIMO Interference Channels With Constant Channel Coefficients

For a single relay channel, we compare the capacity of two different amplify-and-forward (AF) protocols, which are orthogonal AF (OAF) and non-orthogonal AF (NAF). The NAF protocol has been proposed to overcome a significant loss of performance of OAF in the high spectral efficiency region, and it was also theoretically proved that NAF performs better than OAF in terms of the diversity-multiplexing tradeoff. However, existing results have been evaluated at the asymptotically high signal to noise ratio (SNR), thus the power allocation problem between the source and the relay was neglected. We examine which protocol has better performance in a practical system operating at a finite SNR. We also study where a relay should be located if we consider the power allocation problem. A notable conclusion is that the capacity performance depends on both SNR and power allocation ratio, which indicates OAF may perform better than NAF in a certain environment.

Low Complexity ML Based Interference Cancellation for Layered Space-Time Codes

This paper analyzes the optimal degrees of freedom (DoF) for K-user M × N MIMO interference channels with constant channel coefficients. In this paper, we use the methodology of interpreting the interference alignment problem as a multivariate polynomial system. The properness of the system enables us to determine the feasibility of interference alignment. Using this methodology, we propose an algorithm to obtain the optimal DoF, and compute the closed-form optimal DoF. We show that K min(M,N) is optimal DoF if δ ≥ min(M,N), where δ = ⌊M+N over K+1⌋. If δ < min(M,N), K M+N over K+1 is optimal DoF when M+N over K+1 is an integer, and ⌈K M+N over K+1⌉ − 1 is the optimal DoF when M+N over K+1 is not an integer. A key contribution of this paper is that it provides a closed-form optimal DoF for M × N MIMO interference channels when the DoF of each user can be different.