Research and development of signal processing algorithms for UMTS Long Term Evolution (LTE) requires a realistic, flexible, and standard-compliant simulation environment. To facilitate comparisons with work of other research groups such a simulation environment should ideally be publicly available. In this paper, we present a MATLAB-based downlink physical-layer simulator for LTE. We identify different research applications that are covered by our simulator. Depending on the research focus, the simulator offers to carry out single-downlink, single-cell multi-user, and multi-cell multi-user simulations. By utilizing the Parallel Computing Toolbox of MATLAB, the simulator can efficiently be executed on multi-core processors to significantly reduce the simulation time.

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Simulating the Long Term Evolution physical layer

Multiple-input multiple-output (MIMO) technology is the key to meet the demands for data rate and link reliability of modern wireless communication systems, such as IEEE 802.11n or 3GPP-LTE. The full potential of MIMO systems can, however, only be achieved by means iterative MIMO decoding relying on soft-input soft-output (SISO) data detection. In this paper, we describe the first ASIC implementation of a SISO detector for iterative MIMO decoding. To this end, we propose a low-complexity minimum mean-squared error (MMSE) based parallel interference cancellation algorithm, develop a suitable VLSI architecture, and present a corresponding four-stream 1.5 mm2 detector chip in 90 nm CMOS technology. The fabricated ASIC includes all necessary preprocessing circuitry and exceeds the 600 Mb/s peak data-rate of IEEE 802.11n. A comparison with state-of-the-art MIMO-detector implementations demonstrates the performance benefits of our ASIC prototype in practical system-scenarios.

ASIC Implementation of Soft-Input Soft-Output MIMO Detection Using MMSE Parallel Interference Cancellation

In this article, we introduce MATLAB-based link and system level simulation environments for UMTS Long-Term Evolution (LTE). The source codes of both simulators are available under an academic non-commercial use license, allowing researchers full access to standard-compliant simulation environments. Owing to the open source availability, the simulators enable reproducible research in wireless communications and comparison of novel algorithms. In this study, we explain how link and system level simulations are connected and show how the link level simulator serves as a reference to design the system level simulator. We compare the accuracy of the PHY modeling at system level by means of simulations performed both with bit-accurate link level simulations and PHY-model-based system level simulations. We highlight some of the currently most interesting research questions for LTE, and explain by some research examples how our simulators can be applied.

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The Vienna LTE simulators - Enabling reproducibility in wireless communications research

Orthogonal frequency division multiplexing (OFDM) has been widely adopted in modern wireless communication systems due to its robustness against the frequency selectivity of wireless channels. For coherent detection, channel estimation is essential for receiver design. Channel estimation is also necessary for diversity combining or interference suppression where there are multiple receive antennas. In this paper, we will present a survey on channel estimation for OFDM. This survey will first review traditional channel estimation approaches based on channel frequency response (CFR). Parametric model (PM)-based channel estimation, which is particularly suitable for sparse channels, will be also investigated in this survey. Following the success of turbo codes and low-density parity check (LDPC) codes, iterative processing has been widely adopted in the design of receivers, and iterative channel estimation has received a lot of attention since that time. Iterative channel estimation will be emphasized in this survey as the emerging iterative receiver improves system performance significantly. The combination of multiple-input multiple-output (MIMO) and OFDM has been widely accepted in modern communication systems, and channel estimation in MIMO-OFDM systems will also be addressed in this survey. Open issues and future work are discussed at the end of this paper.

Channel Estimation for OFDM

Implementation of an iterative QR decomposition (QRD) (IQRD) architecture based on the modified Gram-Schmidt (MGS) algorithm is proposed in this paper. A QRD is extensively adopted by the detection of multiple-input-multiple-output systems. In order to achieve computational efficiency with robust numerical stability, a triangular systolic array (TSA) for QRD of large-size matrices is presented. In addition, the TSA architecture can be modified into an iterative architecture that is called IQRD for reducing hardware cost. The IQRD hardware is constructed by the diagonal and the triangular process with fewer gate counts and lower power consumption than TSAQRD. For a 4 t 4 matrix, the hardware area of the proposed IQRD can reduce about 41% of the gate counts in TSAQRD. For a generic square matrix of order m IQRD, the latency required is 2m - 1 time units, which is based on the MGS algorithm. Thus, the total clock latency is only 10 m - 5 cycles.

Iterative $QR$ Decomposition Architecture Using the Modified Gram–Schmidt Algorithm for MIMO Systems

Today iterative receivers have proved their efficiency in cancelling interference within the field of wireless communications. However their complexity is often seen as a brake for their use in real systems. In this paper an efficient iterative receiver real-time implementation for a 4 times 4 MIMO system is presented. An architecture of MMSE iterative receiver for MIMO-OFDM systems is proposed to limit latency and complexity due to iterative process: MMSE equalization implementation is realized using CORDIC operators; the scheduling between MIMO detection and channel decoding is optimized and specific interleaving functions are introduced to reduce latency and accelerate the convergence process. The implemented receiver is integrated in a real-time FPGA testbench and compared in terms of complexity and performance with a non iterative solution.

FPGA Implementation of an Iterative Receiver for MIMO-OFDM Systems

This paper provides a link level study of a downlink B3G system using MIMO OFDMA techniques. A 3GPP/LTE- like scenario, in terms of frame structure and main system parameters is considered. We especially focus on the impact of iterative MIMO receiver implementation when transmitting via double Alamouti MIMO scheme. A performance analysis is carried out regarding to the channel coding scheme implementing into the iterative loop. Several scenarios have been considered (different modulation and coding schemes (MCS), real or perfect channel estimation ...) to quantify the gain provided by such iterative receivers. Keywords- MIMO-OFDMA, iterative receiver, B3G. I. INTRODUCTION

Performance Analysis of Iterative Receiver in 3GPP/LTE DL MIMO OFDMA System

In this paper a FPGA implementation of a 4 times 4 MIMO MMSE Equalizer based on the QR factorization technique is presented. Considering fast varying channels a new filter is computed at each new channel realization thanks to an efficient architecture of matrix triangularization. The QR decomposition is performed with only 6 unrolled coordinate rotation digital computer (CORDIC) operators, which are efficiently exploited to minimize the latency of computation. Soft LLR obtained in output of the equalizer are validated on an FPGA hardware bench including a 4 times 4 Rayleigh fading channel and turbo- coding.

An Efficient MMSE Equalizer Implementation for 4×4 MIMO-OFDM Systems in Frequency Selective Fast Varying Channels

This paper deals with transform domain channel estimation (TD-CE) in the MIMO-OFDM system when null carriers are placed at the edges of the spectrum in order to assure band isolation from interfering signals that occupy neighboring frequency bands. Indeed, the presence of these null carriers leads to a degradation of channel estimation because of a ?border effect? phenomenon when using a discrete fourier transform (DFT). First, the problems of the conventional DFT based channel estimator are presented as well as a method to reduce the impact of this ?border effect?. Then, the same method is used to improve the conventional discrete cosine transform (DCT) based channel estimation. Finally the efficiency of these estimators is evaluated within IEEE802.11n system context.

Transform domain channel estimation with null subcarriers for MIMO-OFDM systems

This paper deals with a novel transform domain channel estimation (TD-CE) based on discrete cosine transform (DCT) for the MIMO-OFDM system when null carriers are placed at the edges of the spectrum. After the description of the conventional DCT based method, an improved channel estimation scheme using truncated singular value decomposition is proposed. Finally the efficiency of this estimator is evaluated within 3GPP/LTE system context.

Rodrigue Rabineau

Papers

FPGA Implementation of an Iterative Receiver for MIMO-OFDM Systems

Performance Analysis of Iterative Receiver in 3GPP/LTE DL MIMO OFDMA System

An Efficient MMSE Equalizer Implementation for 4×4 MIMO-OFDM Systems in Frequency Selective Fast Varying Channels

Transform domain channel estimation with null subcarriers for MIMO-OFDM systems

Robust DCT Based Channel Estimation for MIMO-OFDM System