Inyup Kang

LTE-advanced modem design: challenges and perspectives

The commercial deployment of LTE Release 8 is gaining significant momentum all over the globe, and LTE is evolving to LTE-Advanced, which offers various new features to meet or exceed IMT-Advanced requirements. Since LTE-Advanced targets ambitious spectral efficiency and peak throughput, it poses tremendous system design challenges to operators and manufacturers, especially for mobile terminals. This article discusses modem design issues related to carrier aggregation, enhanced ICIC for HetNet, detection of eight-layer transmission, reference signals for enhanced multi-antenna support, and HARQ buffer management. We provide an overview of technical challenges and sketch the perspectives for tackling them to exploit the full benefits of the LTE-Advanced system.

Performance Limits and Practical Decoding of Interleaved Reed-Solomon Polar Concatenated Codes

A scheme for concatenating the recently invented polar codes with non-binary MDS codes, as Reed-Solomon codes, is considered. By concatenating binary polar codes with interleaved Reed-Solomon codes, we prove that the proposed concatenation scheme captures the capacity-achieving property of polar codes, while having a significantly better error-decay rate. We show that for any ε > 0, and total frame length N, the parameters of the scheme can be set such that the frame error probability is less than 2-N1-ε, while the scheme is still capacity achieving. This improves upon 2-N0.5-ε, the frame error probability of Arikans polar codes. The proposed concatenated polar codes and Arikans polar codes are also compared for transmission over channels with erasure bursts. We provide a sufficient condition on the length of erasure burst which guarantees failure of the polar decoder. On the other hand, it is shown that the parameters of the concatenated polar code can be set in such a way that the capacity-achieving properties of polar codes are preserved. We also propose decoding algorithms for concatenated polar codes, which significantly improve the error-rate performance at finite block lengths while preserving the low decoding complexity.

Near ML Modulation Classification

This paper deals with the problem of classification of digital modulation. In particular, we develop and propose a practical modulation classification scheme based on the likelihood of observations. While ML classification is well known and shows the optimal performance, its computational complexity prevents it from being easily implemented in hardware. On the contrary, our proposed scheme has low computational complexity and near optimal classification performance. Moreover, this scheme is designed to perform in fast fading channels. It is shown that our proposed classifier takes advantage of the channel variation without loosing near optimality.

Compound polar codes

A capacity-achieving scheme based on polar codes is proposed for reliable communication over multi-channels which can be directly applied to bit-interleaved coded modulation schemes. We start by reviewing the ground-breaking work of polar codes and then discuss our proposed scheme. Instead of encoding separately across the individual underlying channels, which requires multiple encoders and decoders, we take advantage of the recursive structure of polar codes to construct a unified scheme with a single encoder and decoder that can be used over the multi-channels. We prove that the scheme achieves the capacity over this multi-channel. Numerical analysis and simulation results for BICM channels at finite block lengths shows a considerable improvement in the probability of error comparing to a conventional separated scheme.

Advanced downlink MU-MIMO receiver for 3GPP LTE-A

Third Generation Partnership Project Long Term Evolution (3GPP LTE) provides potential of higher spectral efficiency by using multi-user multiple-input and multiple-output (MU-MIMO) system. Because of the coarse knowledge of channel state information at the transmitter (CSIT) under current standard, co-scheduled user equipments (UEs) may suffer large residual multi-user interference in MU-MIMO. In this paper, we investigate performances of various types of receivers in the cases where residual interference is not negligible. Interference ignoring receiver is expected to perform poorly in this scenario, and hence, we consider interference-aware receivers. Interference rejection combiner (IRC) and joint Max-Log-MAP receiver are considered as interference-aware receivers. In order to perform joint detection of the target and interference layers, UE needs to know interference modulation which current standard does not provide. Because of this, we consider modulation-estimation-based joint receiver as our advanced downlink MU-MIMO receiver. Performances of aforementioned receivers are investigated in several practically relevant cases. It is shown that modulation-estimation-based joint receiver can significantly outperform IRC or the joint receiver with assumption of fixed interference modulation.

Interference mitigation in MIMO interference channel via successive single-user soft decoding

This paper examines a practical way of mitigating interference in multi-input multi-output (MIMO) interference channel when each user is constrained to use a point-to-point code. It was recently shown that the capacity region of interference channel with point-to-point codes can be established by a combination of two schemes: treating interference as noise and joint decoding. In practice, a straightforward implementation of a joint decoding receiver could be significantly more complex than treating interference as noise. Thus, as a low-complexity approach to joint decoding, this paper proposes a successive single-user soft decoding receiver, which performs interference-aware detection with a priori log-likelihood ratio (LLR) and single-user soft decoding for every user successively. The proposed receiver is compared with a receiver treating interference as noise and a receiver performing interference-aware detection without a priori LLR and single-user decoding for the desired user only. Simulation results show that the proposed receiver significantly outperforms these existing receivers.

Polar Coding for Bit-Interleaved Coded Modulation

A polar coding scheme is proposed for reliable communication over channels with bit-interleaved coded modulation (BICM). In an ideal information-theoretic model, BICM schemes are modeled as a set of multiple binary-input channels that experience different reliabilities. This model is referred to as multichannel. The conventional scheme for encoding information over multi-channels is to encode separately across the individual constituent channels, which requires multiple encoders and decoders. As opposed to the conventional scheme, we take advantage of the recursive structure of polar codes to construct a unified compound polar code. The proposed scheme has a single encoder and decoder and can be used over multi-channels, in particular over channels with BICM. We prove that the scheme achieves the multi-channel capacity with the same error decay rate as in Arıkans polar codes. Furthermore, due to more levels of channel polarization, we obtain a better finite block length performance compared with the separated polarization scheme. This is confirmed through various simulations over the binary erasure channel and with transmissions over additive white Gaussian noise (AWGN) and fast fading channels with BICM and different modulation orders.

Achieving the Uniform Rate Region of General Multiple Access Channels by Polar Coding

We consider the problem of polar coding for transmission over m-user multiple access channels. In the proposed scheme, all users encode their messages using a polar encoder, while a multiuser successive cancellation decoder is deployed at the receiver. The encoding is done separately across the users and is independent of the target achievable rate. For the code construction, the positions of information bits and frozen bits for each of the users are decided jointly. This is done by treating the polar transformations across all the m users as a single polar transformation with a certain polarization base. We characterize the resolution of achievable rates on the dominant face of the uniform rate region in terms of the number of users m and the length of the polarization base L. In particular, we prove that for any target rate on the dominant face, there exists an achievable rate, also on the dominant face, within the distance at most (m-1)√m/L from the target rate. We then prove that the proposed L MAC polar coding scheme achieves the whole uniform rate region with fine enough resolution by changing the decoding order in the multiuser successive cancellation decoder, as L and the code block length N grow large. The encoding and decoding complexities are O(N log N) and the asymptotic block error probability of O(2-N0.5-ϵ) is guaranteed. Examples of achievable rates for the 3-user multiple access channel are provided.

2.7 A hybrid supply modulator with 10dB ET operation dynamic range achieving a PAE of 42.6% at 27.0dBm PA output power

Envelope tracking (ET) prolongs the battery life by modulating the supply of a power amplifier (PA) according to the signal envelope. With this emerging technology, the PA efficiency is greatly improved, whereas the supply modulator (SM) itself needs to provide efficient and accurate envelope tracking for the overall performance of the SM-PA combined system (PA module). The ET technique, meanwhile, has seen limited use in high-power transmission due to the reduced SM efficiency for low output power originating from the linear stage in hybrid structures [1].

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

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.