Wooseok Nam

Capacity of the Gaussian Two-Way Relay Channel to Within

In this paper, a Gaussian two-way relay channel, where two source nodes exchange messages with each other through a relay, is considered. We assume that all nodes operate in full-duplex mode and there is no direct channel between the source nodes. We propose an achievable scheme composed of nested lattice codes for the uplink and structured binning for the downlink. Unlike conventional nested lattice codes, our codes utilize two different shaping lattices for source nodes based on a three-stage lattice partition chain, which is a key ingredient for producing the best gap-to-capacity results to date. Specifically, for all channel parameters, the achievable rate region of our scheme is within 1/2 bit from the capacity region for each user and its sum rate is within log3/2 bit from the sum capacity.

{1\over 2}

We provide achievable rate regions for two-way relay channels (TRC). At first, for a binary TRC, we show that the subspace-sharing of linear codes can achieve the capacity region. And, for a Gaussian TRC, we propose the subset-sharing of lattice codes. In some cases, the proposed lattice coding scheme can achieve within 1/2-bit the capacity and is asymptotically optimal at high signal-to-noise ratio (SNR) regimes.

Bit

In this paper, we consider a class of single-source multicast relay networks. We assume that all outgoing channels of a node in the network to its neighbors are orthogonal while the incoming signals from its neighbors can interfere with each other. We first focus on Gaussian relay networks with interference and find an achievable rate using a lattice coding scheme. We show that the achievable rate of our scheme is within a constant bit gap from the information theoretic cut-set bound, where the constant depends only on the network topology, but not on the transmit power, noise variance, and channel gains. This is similar to a recent result by Avestimehr, Diggavi, and Tse, who showed an approximate capacity characterization for general Gaussian relay networks. However, our achievability uses a structured code instead of a random one. Using the idea used in the Gaussian case, we also consider a linear finite-field symmetric network with interference and characterize its capacity using a linear coding scheme.

Capacity Bounds for Two-Way Relay Channels

Blind adaptive I/Q imbalance compensation has become more popular during the last decade due to its simplicity and need for no training data. In existing work, it has been pointed out that a statistical property of a signal, which is referred to as the properness condition, can be used for blind I/Q imbalance compensation. In this letter, the equi-absolute variance condition as well as the properness condition are used to propose two blind adaptive I/Q imbalance compensation algorithms based on the well-known LMS and RLS adaptation algorithms. The performances of the proposed algorithms are evaluated through simulations, and it is shown that the proposed algorithms provide nice convergence behaviors and high image rejections.

Nested Lattice Codes for Gaussian Relay Networks With Interference

Preamble-based cell identification (CID) schemes are derived for orthogonal frequency division multiplexing (OFDM) systems. They include the optimal schemes based on the Bayesian and the maximum likelihood (ML) approaches, a suboptimal scheme that is a simplification of the ML scheme, and a differential decoding-based scheme that does not require any channel information. The complexities and performances of these CID schemes are examined and compared to existing schemes. The differential decoding-based scheme performs like the suboptimal scheme for most practical channels of interest and outperforms existing schemes, yet it is simpler to implement than the others.

Blind Adaptive I/Q Imbalance Compensation Algorithms for Direct-Conversion Receivers

In this paper, we investigate the multipath resolution problem for direct sequence spread spectrum signals. To resolve multipath components arriving within a very short interval, we propose a new multipath super-resolution algorithm based on the iterative least-squares method. The proposed least-squares-based iterative multipath super-resolution (LIMS) algorithm exploits a triangular shaped auto-correlation function (ACF) of the pseudo-noise (PN) sequence and simplifies the least-squares parameter estimation procedure using iterative and algebraic operations. This results in an algorithm demanding low computational load with a high multipath resolution capability. It is also discussed that the LIMS algorithm can be applied for recursive multipath tracking of source localization systems, such as the global navigation satellite systems (GNSS). Simulation results show that the LIMS algorithm maintains its good performance even in a low [(C)/(N0)] or severe multipath interference conditions.

Preamble-based cell identification for cellular OFDM systems

In this paper, we consider a class of relay networks with orthogonal components. We assume transmitted signals at a node to its neighbor nodes are orthogonal, e.g., using frequency division multiplexing (FDM). We first consider a simple discrete memoryless network where there is a source, a destination, and two parallel relays between them. We characterize its capacity under a certain restriction on encoders. For general discrete memoryless relay networks with orthogonal components, we find the capacity if the channels are linear finite field channels with random erasures. For general Gaussian relay networks with orthogonal components, we show an achievable rate based on a sequence of nested lattice codes. The cut-set upper bound and our achievability are within a constant number of bits that depends only on the network topology but not on the channel gains. This is similar to the recent result by Avestimehr, Diggavi, and Tse who showed such an approximate characterization of the capacity of general Gaussian relay networks. However, our achievability uses a structured code instead of a random one.

Least-Squares-Based Iterative Multipath Super-Resolution Technique

Increasing the dwell time in two-dimensional frequency-time hypothesis testing is, in practical terms, one of the most effective ways for Assisted Global Navigation Satellite Systems (A-GNSS) and GNSS receivers to achieve higher sensitivity. In an asynchronous cellular network, however, a mobile terminal may have a non-negligible unknown clock drift rate error originating from the received cellular downlink signal. In such a case, increasing the dwell time may not necessarily result in the expected sensitivity improvement. In addition, a mobile terminal in a rich multipath environment may experience jitters in the code phase of the resolved first arrival path due to short-delay multipaths, which also degrades the sensitivity. In this paper, new decision variables using a lone or a pair of adjacent H_1 cells for code phase hypothesis testing and clock drift rate hypothesis testing are proposed to cope with the unknown code phase drift rate error and the effect of code phase jitter in a parallel acquisition system. The statistics of the proposed decision variables are analyzed in a Rayleigh fading channel, and the performances of the proposed decision variables are compared with that of the conventional decision variable.

Relay networks with orthogonal components

A new partial transmit sequence (PTS) technique for orthogonal frequency division multiplexing (OFDM) signals which does not require side information, is proposed. In the proposed method, the transmitter applies PTS to reduce the peak-to-average power ratio (PAPR) and does not send the side information on the PTS phase factors to the receiver. Based on the OFDM symbol structure having pilot subcarriers, the receiver jointly estimates the channel and PTS phase factors from the pilot subcarriers. The proposed technique inherently causes bit error rate (BER) loss depending on the PTS subgroup size. However, simulation results demonstrate that if the PTS subgroup size is carefully chosen, the proposed method can reduce the PAPR significantly with a negligible BER loss.