Patrick Ho Wang Fung

Robust MMSE channel estimation in OFDM systems with practical timing synchronization

Robust minimum mean-square error (MMSE) channel estimation for orthogonal frequency-division multiplexing (OFDM) systems with practical timing synchronization is considered. The correlation between the channel coefficients at the different subcarriers depends upon the channel power delay profile (PDP) and the probability density function (pdf) of the timing synchronization offset. Since both of these are not known at design time, there is a need for a robust estimator that results in mean-square error performance independent of the actual channel power delay profile (PDP) and the actual timing synchronization offset pdf. We develop the notion of effective PDP, given by the convolution of the channel PDP and the pdf of the timing synchronization offset. We show that designing with a uniform PDP of the same length as the effective PDP leads to a robust solution. This follows from the robustness of the uniform PDP for MMSK estimation, a known result for which we provide an alternative derivation. We illustrate the performance of the robust estimator using a numerical example.

Performance analysis of MIMO system with serial concatenated bit-interleaved coded modulation and linear dispersion code

The upper bound on bit error rate (BER) of a multiple-input multiple-output (MIMO) system with the serial concatenated bit-interleaved coded modulation (BICM) and linear dispersion code (LDC) is derived in this paper. This bound is evaluated based on the expurgated bound of BICM and the approach of moment generation function (MGF). Compared with simulated performance of the 16QAM Alamouti code (AC) and optimal QPSK LDC, this bound is tight in the range of medium to high signal-to-noise ratio (SNR). By utilizing the bound, we obtain the optimal choice of code rate and modulation size for fixed transmission rates in BICM-LDC MIMO system.

Bit-interleaved coded modulation in linear dispersion coded MIMO system over spatially correlated Rician fading channel

A multiple-input multiple-output (MIMO) system with serially concatenated bit-interleaved coded modulation (BICM) and linear dispersion code (LDC) is investigated. LDC is a member of the family of linear space-time block codes STBC. A tight upper bound for this system over a spatially correlated Rician fading channel is derived, based on the moment generation function (MGF) approach. We utilize this bound to study the impact of the propagation environment features, such as the Rician factor, geometry of the Rician component, and transmit and receive antenna correlation, on the performance of a bit-interleaved convolutional-coded LDC MIMO system.

Multiple Carrier Frequency Offset and Channel Estimation for Distributed Relay Networks

Cooperative communication using decode and forward (DF)-based distributed relays is one important solution to achieve better connectivity and higher data rates in wireless fading channels. In distributed relay networks, each relay has an independent local oscillator which when unsynchronized results in the presence of multiple carrier frequency offsets (CFOs) at the destination. The maximum likelihood estimator (MLE) for estimating the multiple CFOs requires a two dimensional grid search involving matrix inversion for each search point. Thus, the multiple CFO and channel estimation is computationally prohibitive and challenging in distributed relay networks. In this paper, we propose a simple estimator for multiple CFO and channel estimation at the destination by exploiting the fact that the relays receive information from a common source. Computer simulations show that the mean square error (MSE) performance of the proposed estimator is close to the Cramer-Rao lower bound for small carrier frequency synchronization errors at the relays.

Semiblind channel estimation for MIMO-OFDM

A semiblind method for channel estimation in multiple-input multiple-output (MlMO) orthogonal frequency-division multiplexing (OFDM) systems is proposed. The MIMO-OFDM channel matrix is first estimated blindly up to an ambiguity matrix using the subspace identification algorithm based on the second-order statistics of the channel outputs. The ambiguity matrix is then estimated using a few training symbols. It shows that the number of training symbols required by the proposed semiblind method is significantly less than that used in the training sequence based channel estimation algorithm. The simulation results demonstrate the effectiveness of the proposed algorithm.

Initial ranging code detector for IEEE 802.16-compliant TDD OFDMA systems

Initial ranging (IR) involves detecting the binary phase shift keying (BPSK) code transmitted by the new subscriber station (SS). In this paper, we formulate a simple detector using the Neyman-Pearson theorem (NPT) for single-code detection in IEEE 802.16-compliant time division duplex (TDD) orthogonal frequency division multiple access (OFDMA) systems. Exploiting the BPSK structure in the ranging code together with the time-invariant channel, a novel method for separating multiple ranging codes is presented. The NPT-based detector is then used to detect the separated codes. The proposed detector is computationally simple and has favorable detection performance for practical implementation.

Frequency-domain timing synchronization for IEEE 802.11n communications systems

This paper addresses the problem of timing synchronization in an IEEE 802.11n-based multiple-input single-output (MISO) communications system. In IEEE 802.11n systems, cyclic delay diversity (CDD) is implemented to avoid the problems associated with the unintentional beamforming effect which occurs when scalar multiples of one signal combine destructively after being transmitted through different antennas. However, the use of CDD results in the creation of pseudo-multipaths which will degrade the performance of correlation-based timing synchronization algorithms commonly used in such systems, due to the increased delay spread of the channel. Thus, a frequency domain timing synchronization algorithm, which is robust to the increase in delay spread due to the presence of pseudo-multipaths, is proposed. The proposed algorithm achieves a higher probability of timing synchronization than correlation-based timing synchronization algorithms typically implemented.

Novel cyclic-prefix-efficient preamble design for MIMO OFDM

This paper presents a novel cyclic-prefix-free preamble structure to enable efficient transmission of training sequences for channel estimation in Multiple-Input-Multiple-Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) systems. The joint space, time and frequency domain design requirements are derived and two solutions are formulated. Simulation results confirm that the proposed preamble structure provides overhead reduction without compromising channel estimation performance.

Optimal Double Correlation Filtering for Carrier Frequency Offset Estimation in MIMO OFDM

A computationally efficient optimal double correlation (ODC) filter, which works on the fourth order statistics of received samples, is proposed for carrier frequency offset estimation in MIMO OFDM systems using repetitive pilot symbols. The integral weights applied to the double correlation coefficients together with the requirement of a single phase operation allow particularly simple hardware implementation. Theoretical analysis shows that the mean square error in the estimate reaches the Cramer-Rao bound at high signal-to-noise ratios over a wide frequency range. Simulation results further indicate that the performance of the new algorithm is comparable to that of the maximum likelihood estimation solution even for large carrier frequency offsets.

Timing synchronization for IEEE 802.11a-based non-regenerative wireless relay networks

This paper addresses the problem of timing synchronization at the destination node of a non-regenerative wireless relay network which employs the IEEE 802.11a standard for data transmission. The two-hop relay network consists of an IEEE 802.11a-compliant source and destination pair, each equipped with a single antenna, aided by a relay node equipped with two antennas, which operate in half-duplex mode. The source-to-relay link remains a point-to-point system and perfect timing synchronization can be assumed at the relay. The relay performs cyclic prefix (CP) removal and re-insertion operations on the received signal to restore the cyclic structure of the amplified and forwarded signal and also introduces transmit diversity, in the form of cyclic delay diversity (CDD), into the forwarded signal. The maximum likelihood (ML) timing synchronization cost function for the received signal at the destination node of the system is derived to show that the ML solution cannot be implemented in practice due to the high computational complexity involved. The use of correlation-based timing synchronization algorithms is then studied. In addition to the probability of achieving timing synchronization, the packet error rate (PER) performance is used as an additional measure of the performance of the different timing synchronization algorithms. It is seen that the use of a correlation-based timing synchronization algorithm does not result in significant PER performance loss compared to the PER achieved by ML methods. In addition, the introduction of CDD in the forwarded signal, which is transparent to the destination node, is shown to achieve an improvement in the PER performance over a relay which performs conventional amplify-and-forward (A & F) relaying.