Hui-Ling Lou

SNR Analysis of OFDM Systems in the Presence of Carrier Frequency Offset for Fading Channels

This letter analyzes the effect of the carrier frequency offset on orthogonal frequency division multiplexing (OFDM) systems for multipath fading channels. A simple approximate expression for the average signal-to-noise ratio (SNR) is derived. This approximate expression is shown to be an upper bound of the average SNR for flat fading channels and an exact expression for the AWGN channel. The approximate average SNR expression is validated using Monte Carlo simulation for both flat fading channels and frequency-selective fading channels

Effect of carrier frequency offset on OFDM systems for multipath fading channels

The paper presents an exact analysis of the effect of the carrier frequency offset on orthogonal frequency division multiplexing (OFDM) systems for a general multipath fading channel. As is well known, the carrier frequency offset attenuates the desired signal and causes intercarrier interference, thus reducing the signal-to-noise ratio (SNR). The SNR degradation due to the carrier frequency offset is evaluated by deriving the exact SNR expression in the presence of the carrier frequency offset. The SNR analysis can be used for the design of a practical OFDM system in determining how small the frequency offset should be in order to maintain the SNR degradation to negligible levels.

Joint ml estimation of frame timing and carrier frequency offset for OFDM systems employing time-domain repeated preamble

When a preamble signal is repeated multiple times in OFDM systems, we derive joint maximum likelihood (ML) estimation of the frame timing (FT) and carrier frequency offset (CFO). Unlike conventional estimators which use correlation of adjacent repetition patterns only or some specific sets of patterns, the joint ML estimation exploits correlation of any pair of repetition patterns, providing optimized performance. To reduce the implementation complexity involved in the joint ML estimation, we also propose a near-ML estimation method that separates estimation of the FT and the CFO. The performance of the proposed methods is verified by computer simulation.

Optimal Combining Schemes for MIMO Systems with Hybrid ARQ

This paper proposes and analyzes receiver schemes for multiple-input multiple-output (MIMO) systems with hybrid automatic-repeat-request (HARQ). It is shown by means of analysis as well as computer simulations that the proposed receiver schemes have optimal decoding performance in the sense that all the relevant information is fully used. Also complexity-wise, the proposed receiver schemes are found beneficial when designing MIMO systems, because the same decoding architecture can be re-used regardless of the number of retransmissions.

On the combining schemes for MIMO systems with hybrid ARQ

Multiple-input multiple-output (MIMO) systems with hybrid automatic-repeat-request (HARQ) promises high throughput with high reliability. However, combining-scheme design for such systems faces challenges, the presence of interference and the existence of multiple signal-to-interference-and-noise power ratios (SINRs). To overcome these challenges, this paper suggests to design combining schemes with the objective of directly optimizing the log-likelihood ratio (LLR) values. Using this approach, this paper proposes several combining schemes and then analyzes them based on three key design factors: decoding performance, scalability, and memory requirement. Computer simulations under the IEEE 802.16e standard settings show the performances of the proposed combining schemes, which align well with the analysis. Based on the analyses and the simulation results, this paper suggests preferable combining schemes respectively for MIMO systems with HARQ-chase combining (HARQ-CC) and HARQ-incremental redundancy (HARQ-IR).

Estimation of Integer Carrier Frequency Offset in OFDM Systems Based on the Maximum Likelihood Principle

An algorithm is derived for the estimation of the integer part of the carrier frequency offset in OFDM systems. The algorithm employs correlation of consecutive OFDM symbols in the frequency domain, is formulated for the general case of a system using data subcarriers and/or pilots and approximates the maximum likelihood (ML) correlation-based estimator for the AWGN channel. The cases of pseudo-pilots, boosted pilots and virtual subcarriers are also examined. It is shown that, compared to previous approaches based on symbol correlation, the algorithm makes better use of the information contained in the correlated symbols, leading to a more accurate, approximate ML estimate of the integer carrier frequency offset at operating signal-to-noise ratios. Simulations indicate that, even in the case of multipath channels, the proposed algorithm is very likely to outperform other techniques.

Transceiver design for MIMO wireless systems incorporating hybrid ARQ

Hybrid ARQ, an extension of ARQ that incorporates forward error correction coding, is a retransmission scheme employed in current communications systems. The use of HARQ can contribute to efficient utilization of the available resources and the provision of reliable services in latest-generation systems. This article focuses on wireless systems using HARQ with emphasis on the multiple-input multiple-output paradigm. MIMO-HARQ offers new opportunities because of the additional degrees of freedom introduced by the multiple antennas at the transmitter and receiver. The architecture of MIMO transceivers that are based on bit-interleaved coded modulation and employ HARQ is described. Additionally, receiver implementations are presented and compared in terms of complexity, memory requirements, and performance.

Joint Maximum Likelihood Estimation of Integer Carrier Frequency Offset and Channel in OFDM Systems

For orthogonal frequency division multiplexing (OFDM) systems, the carrier frequency offset (CFO) between the carrier frequencies of the received signal and the receiver local oscillator is represented as the sum of the integer part and the fractional part by normalizing the CFO with subcarrier spacing. This paper focuses on the pilot-aided estimation of the channel gain and the integer part of the normalized CFO OFDM systems. For multipath channels, a simple estimator of the channel gain and the integer CFO is described, and the joint maximum likelihood (ML) estimator is developed. It is proved that the simple estimator becomes equivalent to the joint ML estimator when two OFDM symbols are observed. For the AWGN channel, this paper derives both the ML estimator of the integer CFO only and the joint ML estimator of the integer CFO and the phase offset. It is proved that the integer CFO estimation part of the joint ML estimator is the same as the ML estimator of the integer CFO only, and it is also shown by simulation that the ML estimator outperforms existing estimators significantly.

Performance of MIMO HARQ under Receiver Complexity Constraints

Hybrid Automatic Repeat reQuest (HARQ) schemes are becoming common in wireless systems. In addition to the channel characteristics, several system design considerations need to be taken into account when choosing the particular HARQ scheme to be used. For SISO systems, it has been shown that the coding gain of Incremental Redundancy (IR-) HARQ over Chase Combining (CC-) HARQ may be offset by adverse fading channel conditions. In this paper it is shown that, for MIMO systems, IR- HARQ may also be affected by practical complexity constraints on the implementation of the receiver that dictate the use of bit-level combining. In some cases the coding gain advantage of IR-HARQ may be offset by the inability to perform symbol combining in the receiver as opposed to the case when CC-HARQ is employed. A system compliant with the IEEE 802.16e standard that employs a Convolutional Turbo Code is used to demonstrate an example of such performance loss.

Generalized co-phasing for multiple transmit and receive antennas

We propose and study a class of transmit beamforming techniques for systems with multiple transmit and multiple receive antennas with a per-antenna transmit power constraint. The per-antenna transmit power constraint is more realistic than the widely used total (across all transmit antennas) power constraint, since in practice each transmit antenna is driven by a separate power amplifier with a maximum power rating. Under the per-antenna power constraint, from an implementation perspective, it becomes desirable to vary only the phases (as opposed to both power and phase variation) of the signals departing from the transmit antennas. We name this class of techniques generalized co-phasing and formulate an optimization problem to calculate the transmit antenna phases. Furthermore, we propose five heuristic algorithms to solve the optimization problem. All the proposed algorithms except one are optimal for the case of two transmit antennas and an arbitrary number of receive antennas. For an arbitrary number of transmit and receive antennas, simulations indicate that the proposed algorithms perform very close to the optimal solution calculated through an exhaustive search of all possible transmit phases.