Modeling and Estimation of Wireless OFDM Channels by Using Time-Frequency Analysis

In the new generation wireless communication systems where high data rates are desired, orthogonal frequency division multiplexing (OFDM) has become the standard method because of its advantages over single carrier modulation schemes on multi-path, frequency selective fading channels. However, inter-carrier interference due to Doppler frequency shifts, and multi-path fading severely degrades the performance of OFDM systems. Estimation of channel parameters is required at the receiver. In this paper, we present a time-varying channel modeling and estimation method based on the discrete evolutionary transform that provides a time-frequency procedure to obtain a complete characterization of a multi-path, fading and frequency selective channel. Performance of the proposed method is tested on different levels of channel noise, and Doppler frequency shifts.

Multiple input-multiple output (MIMO) systems hold the potential to drastically improve the spectral effciency and link reliability in future wireless communications systems. A particularly promising candidate for next-generation fixed and mobile wireless systems is the combination of MIMO with Orthogonal Frequency Division Multiplexing (OFDM). OFDM has become the standard method because of its advantages over single carrier modulation schemes on multipath, frequency selective fading channels. In this paper, we present a time-varying channel modeling and estimation method based on the Discrete Evolutionary Transform. Performance of the proposed method is evaluated on different levels of channel noise, and Doppler frequency shifts.

In this paper, we analysis the receiver of orthogonal frequency division multiplexing (OFDM) systems over frequency selective fading channels where the multipath delay exceeds the guard interval duration and propose a novel receiver scheme. The proposed receiver consists of the Inter Symbol Interference (ISI) canceller and the turbo equalizer. At each iteration, the proposed turbo equalization algorithm performs soft Inter Carrier Interference (ICI) cancellation and frequency domain minimum mean-square error (MMSE) filtering to minimize the performance degradation caused by ICI distortion and frequency-selective channels. Simulation results of the system performance show that the proposed receiver effectively recovers the contaminated OFDM signals.

This work explores the performances of Space-Time and Space Frequency Coded Orthogonal Frequency Division Multiplexing (OFDM) with simple two branches transmit diversity scheme. The combination of multiple-antenna and orthogonal frequency division multiplexing (OFDM) provides reliable communications over frequency selective fading channels. We focus on the application of space-time block codes (STBC) and space-frequency block codes (SFBC) in OFDM systems over time-varying and frequency-selective channels. SFBC transmitter shows superior performance in fast varying channels while STBC shows better performance in frequency selective channels. A switching technique is presented that selects an appropriate transmission scheme between the STBC and SFBC assuming Rayleigh fading model.

In this paper, we analyze the Bit Error Rate (BER) performance of Golden coded Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system over Rician multipath fading channel. We also compare the performance of the MIMO-OFDM system using Golden code in Rayleigh and Rician multipath fading channels. We discuss the effects of the presence of line-of-sight (LoS) component in the multipath fading environment which renders the improvement in the overall performance of the Golden coded MIMO-OFDM. This paper discusses the performance of Golden codes in a frequency selective Rician fading channel. To deal with the frequency selective fading channel, we use the OFDM (Orthogonal Frequency Division Multiplexing) modulation. The BER performance of the Golden coded MIMO-OFDM system under several propagation conditions has been evaluated. Keywords—MIMO, OFDM, Golden STBC, Rayleigh fading, Rician fading.

Orthogonal frequency division multiplexing (OFDM) is one key technology in wireless multimedia communication. In this paper, the performance of channel estimation methods for OFDM systems based on block-type under frequency selective fading channels is discussed. Analysis of a 3G carrier model with 1 MHz bandwidth under frequency selective fading channels is made. The pilot signal estimation based on LS, MMSE, LMMSE (a simplified MMSE algorithm with low-rank approximation using singular value decomposition) criteria, and channel interpolation based on the simple linear interpolation method. Simulation indicates its good anti-multi-path performance and anti-doppler frequency shift ability.

In this paper, a transmission of JPEG2000 images using an Unequal Power Allocation (UPA) scheme and Orthogonal Frequency Division Multiplexing (OFDM) technique over a block fading — frequency selective channel is presented. Using instantaneous and average channel state information, power is assigned to each bit in the JPEG2000 bitstream based on its contribution to the decoded image quality. Moreover, the actual total power consumed for transmission is measured and compared with the total power initially assigned. Simulation results show an improvement of up to 10.5 dB in the decoded image quality when the UPA scheme is used. In addition, the simulation results demonstrate the effectiveness of the proposed UPA algorithm in frequency selective block fading channels.

Orthogonal frequency division multiplexing (OFDM) has been adopted as a standard for various high data rate wireless communication systems due to the spectral bandwidth efficiency, robustness to frequency selective fading channels, etc. However, implementation of the OFDM system entails several difficulties. One of the major drawbacks is the high peak-to-average power ratio (PAPR), which results in intercarrier interference, high out-of-band radiation, and bit error rate performance degradation, mainly due to the nonlinearity of the high power amplifier. This paper reviews the conventional PAPR reduction schemes and their modifications for achieving the low computational complexity required for practical implementation in wireless communication systems.

Orthogonal frequency division multiplexing (OFDM) has been widely regarded as an effective modulation technique for mitigating the effects of ISI in a frequency selective fading channel and for providing reliable high data rate transmission over wireless links. Adaptive antenna arrays at the base and mobile stations can achieve further increases in system's capacity and bandwidth efficiency, as well as in QoS improvement in conventional OFDM systems. The conventional adaptive antenna arrays based OFDM systems always use the sub-carriers characterized by the first largest eigenvalues to transmit the OFDM block symbols. In this paper and in contrast to previous work, we propose dynamic sub-channel allocation with adaptive beamforming for broadband OFDM wireless transmission systems. The proposed system adaptively selects the eigenvectors associated with the relatively large sub-channel eigenvalues to generate the beamforming weights at the mobile and base stations and then dynamically assigns the corresponding best sub-channels to transmit the OFDM block symbols. It is shown by simulation that, without adding much complexity, the proposed system can achieve a better performance than an adaptive antenna arrays based OFDM system without dynamic sub-channel allocation over multipath fading channels. Simulation results also reveal that the proposed system is not too sensitive to channel estimation errors.

This paper deals with low-complexity joint channel estimation and decoding for faster-than-Nyquist (FTN) signaling over frequency selective fading channels. The inter-symbol interference (ISI) imposed by FTN signaling and the frequency selective channel are intentionally separated to fully exploit the known structure of the FTN-induced ISI. Colored noise due to the faster sampling rate than that of the Nyquist signaling system is approximated by autoregressive process. A Forney style factor graph representation of the FTN system is developed and Gaussian message passing is performed on the graph. Expectation propagation (EP) is employed to approximate the message from channel decoder to Gaussian distribution. Since the inner product between FTN symbols and channel coefficients is infeasible by belief propagation (BP), we propose to perform variational message passing (VMP) on an equivalent soft node in factor graph to tackle this problem. Simulation results demonstrate that the proposed low-complexity hybrid BP-EP-VMP algorithm outperforms the existing methods in FTN system. Compared with the Nyquist counterpart, FTN signaling with the proposed algorithm is able to increase the transmission rate by over 40%, with only negligible BER performance loss.

High peak to average power ratio (PAPR) is a serious drawback for orthogonal frequency division multiplexing (OFDM) system as it reduces the efficiency of high-power amplifiers commonly used in the transmitter and causes spectral spillage. OFDM transmissions are also sensitive to frequency selective fading channels which can severely attenuate certain parts of the signal making it much more vulnerable to additive noise. This paper proposes an effective precoding technique with the dual ability to both reduce PAPR and condition the OFDM signal against frequency selective fading channels. It will be demonstrated that by advisedly creating diversity among the subcarriers of one OFDM symbol, and exploiting it with a dirty- paper-coding (DPC) approach, excellent bit-error-rate (BER) performance is achieved without reducing transmission rate. Low PAPR factor can also be obtained through a proper selection of diversity matrix that distributes the power of each modulated symbol over the OFDM block. Simulation results show that the proposed technique has not only better BER performance than the ordinary channel inversion but also similar PAPR reduction ability to the selective mapping (SLM) technique.

Orthogonal frequency division multiplexing (OFDM) transmitter diversity techniques have been shown to be efficient means of achieving near optimal diversity gain in frequency selective fading channels. For these systems, knowledge of the channel parameters is required at the receiver for diversity combining and decoding. We propose a pilot-symbol assisted channel estimator using DFT-based and linear interpolation for OFDM based systems with two antenna transmitter diversity. We then consider four transmit antenna systems and propose new channel estimation techniques, which allow estimation and separation of the channel state information of the different transmit antennas without changing the preamble structure of the system.

Orthogonal frequency division multiplexing (OFDM) systems work well if the subcarriers are orthogonal to each other. However, the orthogonality among subcarriers may not exist and result in inter-carrier interference (ICI) at the receiver. This may cause by residual carrier frequency offset, time variations due to Doppler shift or phase noise. Further developing the parallel cancellation (PC) scheme to mitigate the ICI of OFDM systems, we expand this OFDM symbol-based PC scheme into a space-frequency (SF) coded system. This new simple space- frequency parallel cancellation (SFPC) scheme is a technique that combines the SF and PC schemes together. Computer simulations indicate that OFDM systems using the SFPC scheme outperform the regular PC and SF systems in slow and fast frequency selective fading channels, specifically at a high signal-noise-ratio (SNR). Furthermore, the error floor of the SFPC-OFDM system is significantly lower than that of the PC and SF systems without increasing computational load. Additionally, the SFPC scheme is a simple multiple input and multiple output (MIMO) system.

Different attempts have been made to compare the performance of coded orthogonal frequency division multiplexing (COFDM) and a corresponding single carrier system employing a linear equalizer often by comparing the average bit error rates. The random coding technique is used to analyze the performance of these systems for signaling over piecewise constant channels. The probabilities that the cutoff rates are less than a desired rate are calculated and compared. It is shown that for low SNRs and low code rates, the cutoff rates of both systems are approximately the same. With an increasing SNR the cutoff rate for the linearly equalized system, however, converges faster to its maximum value than the cutoff rate for the COFDM system. Simulation results for the outage probability are presented for the two-ray Rayleigh fading channel with QPSK as the fundamental modulation technique. For code rates close to one or for uncoded transmission, the linearly equalized single carrier system substantially outperforms the COFDM system. For low to medium code rates COFDM and a linearly equalized single carrier system perform approximately identically.

Digital radio mondiale (DRM) is a terrestrial radio broadcasting standard to replace existing analogue AM and FM broadcasting, which is based on an orthogonal frequency division multiplexing (OFDM) technique. This paper focuses on the issue of estimating a sampling frequency offset (SFO) in OFDM-based broadcasting systems under frequency selective fading channels. In order to design a robust SFO estimation scheme and to benchmark its performance, the performance of the various conventional SFO estimation schemes is discussed and some improvements on the conventional estimation algorithms are highlighted. The simulation results show that such a design enhances the robustness of the proposed scheme against frequency selective fading.