Hong-Yu Liu

QAM Symbol Error Rate in OFDM Systems Over Frequency-Selective Fast Ricean-Fading Channels

For digital data transmission using an orthogonal frequency-division multiplexing (OFDM) over fading channels, the interchannel- interference (ICI) term caused by the Doppler spread is correlated with the desired signal term. Nonetheless, many researchers have assumed the ICI term to be uncorrelated with the desired signal term. In this paper, we present the symbol-error-rate (SER) performance for the M-quadratic amplitude-modulation OFDM systems over Ricean fading using the exact correlated ICI model. The results are then compared with the uncorrelated ICI results. We find that the two results present some differences. The differences tend to increase as the Ricean factor is increased.

Effective Adaptive Iteration Algorithm for Frequency Tracking and Channel Estimation in OFDM Systems

For joint maximum-likelihood (ML) frequency tracking and channel estimation using orthogonal frequency-division multiplexing (OFDM) training blocks in OFDM communications over mobile wireless channels, a major difficulty is the local extrema or multiple-solution complication arising from the multidimensional log-likelihood function. To overcome this, we first obtain crude ML frequency-offset estimators using single-time-slot samples from the received time-domain OFDM block. These crude frequency estimators are shown to have unique closed-form solutions. We then optimally combine these crude frequency estimators in the linear-minimum-mean-square-error (LMMSE) sense for a more accurate solution. Finally, by alternatively updating the LMMSE frequency estimator and the ML channel estimator through adaptive iterations, we successfully avoid the use of a multidimensional log-likelihood function, hence obviating the complex task of global solution search and, meanwhile, achieve good estimation performance. Our estimators have mean square errors (MSEs) tightly close to Cramer-Rao bounds (CRBs) with a wide tracking range.

Error probability for orthogonal space-time block code diversity system using rectangular QAM transmission over Rayleigh fading channels

In this paper, theoretical symbol error probability (SEP) expressions are derived for orthogonal space-time block code (OSTBC) diversity systems employing arbitrary rectangular M-QAM transmission over flat Rayleigh fading channels. Independent fading between diversity channels is assumed. Channel average powers may be distinctive, identical, or mixed with both. The rectangular M-QAM results are extended to square M-QAM, M-PAM, and binary antipodal signaling. All derived expressions are in elementary forms without complicated high-order transcendental functions and unevaluated integrals and, hence, are strictly exact and can be readily simulated by the computer. Moreover, it is shown that mixed Rayleigh fading results can be readily extended to various Nakagami-m fading results. A four-transmit-antenna system with a half-rate OSTBC for 16-QAM signaling is used to demonstrate that the theoretical result is in excellent agreement with the Monte Carlo simulated result. From simulation curves, it is shown that, under the independent channel fading condition, channels with identical powers have better error rate performance than channels with distinctive powers.

A general moment generating function for MRC diversity over fading channels with Gaussian channel gains

For maximal ratio combining (MRC) diversity over correlated fading channels with Gaussian channel gains, we utilize unitary diagonalization of the channel covariance matrix to decorrelate the physical channels into uncorrelated virtual channels to obtain the moment generating function (MGF) of the received signal-to-noise ratio (SNR). The MGF thus obtained has a compact form and can be universally applied to various popular fading models. In addition to the advantage of simple derivation procedure, this general MGF can be readily modified to express various scenarios of channel power distributions as well as joint fading models. To demonstrate these advantages, we use the generalized Ricean fading as a specific example to compare our derivation and our MGF expression with an existing work in the literature. Again, we present numerical simulations for MRC reception of binary phase shift keying (BPSK) signals over Nakagami fading to compare with existing results appearing in the literature. Copyright

Correlation between ICI and the carrier signal in OFDM under Doppler spread influence

In this paper, we derive an exact expression for the power correlation between the inter-carrier interference (ICI) and the carrier signal and show that the correlation is non-negligible for normal Doppler spread values. However, the correlation tends to vanish as Doppler spread approaches infinity. Then, the symbol error rates (SERs) for M-QAM OFDM systems over frequency-selective Ricean fading channels based on both the correlated and the uncorrelated ICI models are given for comparison

Maximum-Likelihood Decoding for Nonorthogonal and Orthogonal Linear Space–Time Block Codes

It is a general consensus that an orthogonal space-time block code can achieve full diversity, and due to its orthogonal nature, the multiple-input-multiple-output (MIMO) maximum-likelihood (ML) decoding metrics can be decoupled into single-input-single-output (SISO) ML metrics based on linear processing at the receiver, thus greatly reducing the decoding complexity. In fact, nonorthogonal codes also currently exist that can achieve better symbol-error-rate performance without rate reduction and complexity increase for correlated fading channels. In this paper, we show by detailed derivations that nonorthogonal linear space-time block codes can also be decoded by ML decoupling through receiver linear processing. Our derived expressions for the decoupled ML metrics automatically contain the design information for the receiver linear processors.

Performance of Orthogonal Space-Time Block Codes With Generalized Complex Orthogonal Design for Arbitrary Rectangular QAM Over Correlative Fading Channels

We derive the symbol error probability (SEP) expressions for orthogonal space-time block codes with any form of generalized complex orthogonal design (GCOD) employing arbitrary rectangular quadratic-amplitude modulation signaling over correlative fading channels. By first decorrelating the physical branches into uncorrelated virtual branches, a general expression for the moment generating function of the received signal-to noise ratio (SNR) is obtained, from which the SEP is derived. The result can be applied to correlative fading channels with any form of Gaussian fading gains. Moreover, channels having joint fading models, as well as mixed channel powers, are considered. We also discover that, for certain GCOD codes, different information symbols may exhibit different SEPs. The above features are demonstrated by two GCOD examples used for simulations. Theoretical performance curves are compared with Monte Carlo simulated results in excellent agreement.

The adaptive MSINR algorithm to improve error rate for channel equalization

Based on the expression of the error probability for the infinite length equalizer and the error bound for the finite length equalizer for M-ary PAM channels using the unbiased decision rule, we are inspired to develop an adaptive algorithm called the maximum signal-to-interference-plus-noise ratio algorithm (MSINR) to improve error rate performance. The MSINR algorithm is found to outperform the LMS both in convergence behaviors and error rate performance. When comparing with a true minimum error rate algorithm called approximate minimum bit error rate (AMBER), it is found that AMBER yields a lower error probability as expected, but as the equalizer length is sufficiently long, the error probabilities obtained from MSINR very closely approximate the minimum error probabilities given by AMBER. Moreover, the MSINR convergence performance, both in speed and smoothness, is far superior to the AMBER.

Symbol error probability for rectangular M-QAM OFDM transmission over Rayleigh fading channels

By using the pre-averaging method, an exact closed form expression for the symbol error probability (SEP) is derived for arbitrary rectangular M-QAM signaling in OFDM systems over frequency-selective Rayleigh fading channels. In addition, the channel capacity for the QAM OFDM transmission over Rayleigh fading environment is obtained.

Effect of changing symbol rate on equalizer performance for band‐limited systems

In this work, the effect of changing symbol rate (or equivalently, symbol rate) on the equalizer performance for M-ary PAM systems is investigated. We keep increasing the symbol rate (or equivalently, keep decreasing the symbol period) until a limit is reached. An effort is made to find this limit. In other words, we are investigating the possibility of increasing the symbol rate beyond the usual Nyquist rate. Since incessantly increasing the symbol rate will eventually produce channel spectrum null (almost null) which will render difficulties for equalization, decision-feedback equalization combined with fractionally spaced sampling is used to overcome the difficulties. For a given channel bandwidth and a prescribed error probability, optimum symbol rates vs SNR are found so as to achieve maximum bit rates. Explanations are given for the resultant phenomena both analytically as well as by computer simulations. A theoretical channel model and a realistic twisted pair channel are used for demonstration. Important analytical expressions are given along with computer simulations to quantify performance results. Copyright