LDPC based differential unitary space-frequency coding for MIMO-OFDM systems

This paper proposes a novel LDPC based differential unitary space-frequency coding (DUSFC) scheme for MIMO-OFDM systems when neither the transmitter nor the receiver has access to the channel state information (CSI). The new DUSFC strategy basically consists of coding across transmit antennas and OFDM tones simultaneously as well as differential modulation in the time-domain. It can fully exploit the inherent advantages provided by the multipath fading channels, resulting in a high degree of diversity. The state-of-the-art low-density parity-check (LDPC) codes are concatenated with our DUSFC as channel coding to improve the bit error rate (BER) performance considerably. Owing to the maximum multipath diversity and large coding advantages, LDPC-DUSFC strongly outperforms the differential unitary space-time coded OFDM techniques re- cently proposed in literature. The corresponding iterative decoding algorithm without channel estimation is finally provided to offer significant performance gain. Simulation results illustrate the merits of the proposed scheme.

An efficient training sequences strategy for channel estimation in OFDM systems with transmit diversity

This paper deals with channel estimation for orthogonal frequency-division multiplexing (OFDM) systems with transmit diversity. Space time coded OFDM systems, which can provide transmit diversity, require perfect channel estimation to improve communication quality. In actual OFDM systems, training sequences are usually used for channel estimation. The authors propose a training based channel estimation strategy suitable for space time coded OFDM systems. This novel strategy provides enhanced performance, high spectrum efficiency and relatively low computation complexity.

A Reduced Complexity Time-Delay Tracking and Channel Estimation for OFDM Systems

This paper presented an improved channel estimator for orthogonal frequency division multiplexing (OFDM) systems using joint time delay detection and channel gain estimation. The algorithm well designs an adjustment scheme using the time correlation of time delays to increase the accuracy of the time delay detection. The most attractive advantage is that the complicated matrix calculation is replaced by the search steps to estimate the channel parameters without significantly increasing the complexity of the system. The computer simulation demonstrates that the proposed algorithm can track the time delays adaptively and improve the channel estimation performance. Consequently, the better system performance will be achieved.

Performance analysis of a rate adaptive multi-band ultra-wideband system based on quadrature fractal modulation scheme

A rate adaptive multi-band ultra-wideband (UWB) system based on the quadrature fractal modulation (QFM) scheme was proposed.Exploring the use of homogeneous signals as modulating waveforms in UWB system,the signal within each 528MHz sub-band was divided into 8 different frequency bandwidths using wavelets transform and these data sequences to be transmitted were embedded into homogeneous waveforms.It is found that the use of homogeneous signals in such UWB system is quite feasible,leadings to a novel multi-rate diversity strategy.Within each 528MHz sub-band,the UWB-QFM system can provide much higher data rates than that of the UWB orthogonal frequency division multiplexing (OFDM) system.Simulation results also show that the bit error rate (BER) performance of the UWB-QFM system achieves a greatly improvement over existing UWB-OFDM system.Due to the fractal properties of the homogeneous signals,these data sequences to be transmitted can be recovered using arbitrarily short receiver signal.

OFDM Scheme Based on Wavelet Packet Transform-Oriented Graded Multi-Service

In this paper, a concept of image grading transmission is put forward to enhance data rate and to improve the usage of subcarriers in orthogonal frequency division multiplexing (OFDM). The idea originates from the wavelet packets representative of an image in which information is graded in terms of different priorities. The graded image facilitates more efficient use of adaptive subcarriers and bits allocation. The results of simulation in typical mobile environment prove that the output signal noise ratio (SNR) of the graded image excels that of the ungraded image by 1—2 dB under the same channel condition.

PERFORMANCE ANALYSIS OF CHANNEL ESTIMATION FOR LDPC-CODED OFDM SYSTEM IN MULTIPATH FADING CHANNEL

In this paper, the channel estimation techniques for Orthogonal Frequency Division Multiplexing (OFDM) systems based on pilot arrangement are studied and we apply Low Density Parity Check (LDPC) codes to the system of IEEE 802.16a with OFDM modulation. First investigated is the influence of channel es-timation schemes on LDPC-code based OFDM system in static and multipath fading channels. According to the different propagation environments in 802.16a system, a dynamic channel estimation scheme is proposed. A good irregular LDPC code is designed with code rate of 1/2 and code length of 1200. Simulation results show that the performance of LDPC coded OFDM system proposed in this paper is better than that of the convolution Turbo coded OFDM system proposed in IEEE standard 802.16a.

A COMPARISON OF TWO 2D CHANNEL ESTIMATORS FOR OFDM SYSTEM

The paper deals with two-dimensional (2D) channel estimation of Orthogonal Frequency Division Multiplexing (OFDM) system in slow fading wireless channel. We concentrate on two channel estimation schemes: Least Square (LS)+Weighted BiLinear (WBL) and LS+Linear Minimum Mean-Squared Error (LMMSE) where the first method is proposed in this paper. After theory analysis and simulation in Typical Urban (TU) channel, we find that LS+LMMSE achieves the optimal perform-ance by exploiting prior knowledge of channel whereas LS+WBL, without requiring channel knowl-edge and with only half of the computational amount of LS+LMMSE, approaches LS+LMMSE in Bit Error Ratio (BER) performance when the distance of two adjoining pilot symbols along frequency di-rection is sufficiently small. This makes LS+WBL very suitable for wideband wireless applications.