OFDM Channel Research Articles

Optimal training signals for MIMO OFDM channel estimation in the presence of frequency offset and phase noise

We develop robust mean-square error (MSE)-optimal training signal designs for multiple-input multiple-output orthogonal frequency-division multiplexing channel estimation with frequency offset and phase noise (PN), and present analytical and simulation results for the frequency-offset and PN effects on channel estimation. The proposed designs are more advantageous for moderate-to-high values of signal-to-noise ratio (SNR), residual frequency offset, and PN level. At SNR = 10 dB, the normalized MSE reductions of our proposed training signals at normalized frequency offset |v|=0.1,0.5 are about 9 and 19 dB, respectively, for one transmit antenna, and 6 and 11 dB for two transmit antennas

Optimal training signals for MIMO OFDM channel estimation

This paper presents general classes of optimal training signals for the estimation of frequency-selective channels in MIMO OFDM systems. Basic properties of the discrete Fourier transform are used to derive the optimal training signals which minimize the channel estimation mean square error. Both single and multiple OFDM training symbols are considered. Several optimal pilot tone allocations across the transmit antennas are presented and classified as frequency-division multiplexing, time-division multiplexing, code-division multiplexing in the frequency-domain, code-division multiplexing in the time-domain, and combinations thereof. All existing optimal training signals in the literature are special cases of the presented optimal training signals and our designs can be applied to pilot-only schemes as well as pilot-data-multiplexed schemes.

Error prediction for adaptive modulation and coding in multiple-antenna OFDM systems

In this paper, the problem of packet error rate (PER) prediction is addressed in the multiple-antenna broadband OFDM context, and its impact on adaptive modulation and coding (AMC) is quantified. The analysis is based on a physical layer comprising various modulation and coding schemes, ranging from robust space–time block coding (STBC) modes to high bit rate spatial division multiplexing (SDM) modes, and also hybrid SDM–STBC schemes. For each mode the expression of several link quality metrics (LQM) enabling PER prediction in the broadband OFDM channel, such as instantaneous signal-to-noise ratio (SNR), capacity, or exponential effective SNR metrics are provided. Their advantages and limitations are investigated. Finally, their performance is benchmarked in the IEEE 802.11a/g/n context. It is shown that the choice of the LQM has a significant impact on the throughput performance of the AMC algorithm.

LDPC-based space-time coded OFDM systems: Turbo-EM receiver design with channel state information guessing algorithms

In this paper we study some turbo receiver architectures employing low-density parity check (Ldpc) codes together with orthogonal frequency division multiplexing (Ofdm) for high data rate wireless transmissions. Different demodulation schemes based on expectation-maximization (Em) algorithm are studied along with the channel impulse response (Em) algorithms. We studied differentCir guessing algorithms including the EM-based algorithms such as a space-alternating generalized expectation-maximization algorithm (Sage). It is shown that the proposed turbo-Em receiver employing a soft maximum a posteriori (Map)Em demodulator and a belief propagationLdpc decoder can perform within 1 dB from the ergodic capacity of the studiedMimo ofdm channels. Besides, we find that a suboptimum structure based on a soft interference cancellationMmse filtering demodulator exhibits negligible loss in non-correlated fadingMimo channels but suffer extra performance loss in highly correlatedMimo channels.

Iterative receiver for OFDM-SDMA system

An iterative receiver is proposed based on the EM (Expectation-Maximization) algorithm for an OFDM-SDMA (Orthogonal Frequency Division Multiplexing-Space Division Multiple Access) system. By using a few pilots in every OFDM symbol, both channel estimation and multiuser detection can be simultaneously obtained by iteration. The computer simulation results show this receiver can track channel variations and detect multiuser symbols for different number of users under time-varying multipath channels.

Characterization of indoor wireless channel at 5GHz U-NII bands

The paper reports on the experiments undertaken at the University of Wollongong to characterise fading profiles and delay parameters of an indoor wireless channels at 5 GHz U-NII bands. The measurements were undertaken at different locations around the campus with results recorded for a post-processing to calculate the Rician K-factor, the level crossing rate and the average fade duration as well as mean excess delay, rms delay spread, and the coherence bandwidth of the channel. The presented measurement results can be useful in developing a Markov chain based model of the transport channel for IEEE802.11a or HYPRLAN-2 networks. The results also indicate scenarios where the coherence bandwidth of the channel is smaller than the width of the sub-carrier OFDM channels in either of the mentioned systems.

Characterization of indoor wireless channel at 5 GHz U-NII bands

The paper reports on the experiments undertaken at the University of Wollongong to characterise fading profiles and delay parameters of an indoor wireless channels at 5 GHz U-NII bands. The measurements were undertaken at different locations around the campus with results recorded for a post-processing to calculate the Rician K-factor, the level crossing rate and the average fade duration as well as mean excess delay, rms delay spread, and the coherence bandwidth of the channel. The presented measurement results can be useful in developing a Markov chain based model of the transport channel for IEEE802.11a or HYPRLAN-2 networks. The results also indicate scenarios where the coherence bandwidth of the channel is smaller than the width of the sub-carrier OFDM channels in either of the mentioned systems.

The Phasor Representation of the OFDM Signal in the SFN Networks

The multipath is a cause of frequency notches leading to signal fading. In order to get insight into this effect the connections between individual path parameters and frequency envelope of the OFDM signal are needed. To this aim the phasor representation of the OFDM signal is proposed. The phasor representation exposes the regularity related within different paths of the signal allowing for insight into the structure of multipath OFDM channel. In this work we analyze paths with time delays also accessing OFDM symbol guard interval.

Principles of OFDM

After an introduction to the main principles of OFDM the author discusses the basic OFDM model, channel shortening, synchronization, and loading algorithms.

An investigation into time-domain approach for OFDM channel estimation

A time-domain based channel estimation for OFDM system with pilot-data multiplexed scheme is investigated. As an approximation to linear minimum mean square estimator (LMMSE), a time-domain based channel estimation is proposed where intra-symbol time-averaging and most significant channel taps selection are applied. The relation and differences of the proposed method to DFT-based LMMSE methods are discussed. The performances of the proposed method, DFT-based LMMSE method and the methods of Chini, Wu, El-Tanany and Mahmoud (see IEEE Trans. on Broadcasting, vol.44, no.1, p.2-11, 1998) and of Yeh and Lin (see IEEE Trans. on Broadcasting, vol.45, no.4, p.400-409, 1999) are evaluated in multipath fading channels. The simulation results show that proposed method achieves almost the same performance as DFT-based LMMSE method and better BER performance than the other methods while keeping less complexity.

DSP based OFDM demodulator and equalizer for professional DVB-T receivers

This work treats the design of base-band processing subsystems for professional DVB-T receivers using 1600 MIPS, fixed point DSPs. We show the results about the implementation of OFDM demodulation, channel estimation and equalization functional blocks, for the 2k/8k modes. The adoption of general purpose DSPs provides a great flexibility in the development of different configurations of the receiver. Furthermore it enables the feasibility of adaptive equalization schemes, where the quality of the channel estimation varies according to both channel characteristics and speed of the channel variations. The 16/32 bit fixed point architecture leads to a very low implementation loss, and a careful optimization of the pipeline architecture of the processor allows the receiver to obtain short processing delays. The flexibility of the software approach along with the obtained performance make the proposed implementation very interesting in professional and high-end receivers for interactive, multimedia applications of DVB-T.

Performance Analysis of COFDM for Broadband Transmission on a Fading Multipath Channel

This paper gives a unified analysis of the effects of multipath dispersion and Doppler spreading due to fast fading in OFDM systems, deriving simple closed-form expressions for intersymbol and interchannel interference in the most important cases. It shows also why sine wave carriers are in general the optimum bearers for the sub-channels. The expressions for interference are used to estimate the BER performance of coded OFDM, showing that the use of forward error correction (FEC) coding and a guard period are both essential for optimum performance. As an example it is shown that a 64 sub- channel OFDM system can achieve a data rate of 6.4 Mbit/s on a typical urban mobile radio channel.

A power-efficient single-chip OFDM demodulator and channel decoder for multimedia broadcasting

Large parts of Europe have the ability to receive high-quality digital audio broadcasting (DAB) programs, without distortion, in a mobile environment. This puts pressure on the availability of receivers, which are planned to appear on the market in 1998. For a consumer, not only audio is important: the DAB system allows any combination of audio, video, and data services to be transmitted with a total gross capacity of about 2.3 Mb/s. This paper describes a chip that makes it all possible, the ability to make low-cost receivers for audio, or a full featured video or data receiver. The 4.5-million-transistor mixed-signal device converts 40 MHz analog intermediate frequency and performs all signal processing needed to demodulate and decode a DAB signal to ISO-MPEG transport stream, including full receiver synchronization and deinterleaving using an embedded 0.5-Mb memory.

OFDM for broadcasting in presence of analogue co-channel interference

This paper treats the problems of digital TV terrestrial broadcasting in presence of co-channel interference from analogue TV services. The channel capacity is calculated for the optimum distribution of the transmitted power. An OFDM scheme is presented which is designed to be robust to the analogue TV interference. Power and number of bits per symbol are assigned to each OFDM channel to give the maximum data rate for a given signal to interference ratio. Simulations of the proposed scheme, for 8 MHz bandwidth and interference from a system G PAL signal, show that acceptable error rate can be achieved for e.g. 27 Mbit/s and 39 Mbit/s at signal to interference ratios of -6 dB and 0 dB, respectively. The proposed scheme is suitable for the next generation terrestrial TV networks which are going to coexist with the current analogue systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>