Some statistical characteristics, including the means and the cross-correlations, of frequency-selective Rician fading channels seen by orthogonal frequency division multiplexing (OFDM) subcarriers are derived in this paper. Based on a pairwise error probability analysis, the mean vector and the cross-correlation matrix are used to obtain an upper bound of the overall bit-error rate (BER) in a closed-form for coded OFDM signals with and without inter-carrier interference. In this paper, the overall BER is defined as the average BER of OFDM signals of all subcarriers obtained by considering their cross-correlations. Numerical examples are presented to compare the proposed upper bound of the overall BERs and the overall BERs obtained by simulations.

Time variation within an OFDM symbol causes inter-carrier interference (ICI). In this letter, frequency-domain partial response coding (PRC) is investigated to reduce ICI in the Alamouti SFBC-OFDM system. Based on the expression of the ICI power in the SFBC-OFDM system with PRC, the near-optimal weights of PRC are derived. Simulation results show that the PRC scheme can reduce ICI effectively.

Orthogonal frequency division multiplexing (OFDM) systems for mobile applications suffer from inter-carrier-interference (ICI) due to frequency offset and to time-variation of the channels and from high peak-to-average-power ratio (PAPR). In this paper, we revisit symmetric cancellation coding (SCC) proposed by Sathananthan et al. and compare the effectiveness of SCC with a fixed subtraction combining and the well-known polynomial cancellation coding (PCC) over Rayleigh fading channels with Doppler spread in terms of the signal-to-interference plus noise power ratio (SINR) and bit-error-rate (BER). We also compare SCC with subtraction combining and SCC of Sathananthan et al. with maximum ratio combining (MRC). Our results show that SCC-OFDM with subtraction combining gives higher SINR than PCC-OFDM over the flat Rayleigh fading channel and that this superiority is not maintained under multi-path induced frequency-selective fading unless diversity combining is used. A simulation result shows, however, that SCC-OFDM with subtraction combining may perform better than PCC-OFDM for a certain range of Doppler spread when differential modulation is employed. Finally, we also demonstrate that the SCC-OFDM signal has less PAPR compared to the normal OFDM and PCC-OFDM and hence may be more practical.

In orthogonal frequency-division multiplex access (OFDMA) uplink, the carrier-frequency offsets (CFOs) between the multiple transmitters and the receiver introduce inter-carrier interference (ICI) and severely degrade the performance. In this paper, based on the perfect estimation of each user's CFO, we propose two low-complexity iterative algorithms to cancel ICI due to CFOs, which are denoted as the basic algorithm and the improved algorithm with decision-feedback equalization (DFE), respectively. For the basic one, two theorems are proposed that yield a sufficient condition for the convergence of iterations. Moreover, the interference-power-evolution (IPE) charts are proposed to evaluate the convergence behavior of this interference cancellation algorithm. Motivated by the IPE chart, the procedure of DFE is introduced into the iterations, which is the basic idea of the improved algorithm. For this improved algorithm, the error-propagation effect are analyzed and suppressed by an efficient stopping criterion. From IPE charts and simulation results, it can be easily observed that the basic algorithm has the same capability of ICI cancellation as the linear optimal minimum mean square error (MMSE) method, but offers lower complexity, while the improved algorithm with DFE outperforms the MMSE method in terms of the bit-error rate (BER) performance.

In this letter, a novel pilot-aided inter-carrier interference (ICI) self-cancellation scheme is proposed for use in orthogonal frequency division multiplexing (OFDM) systems. The proposed scheme maps both modulated data symbols and pre-defined pilot symbols onto non-neighboring sub-carriers with weighting coefficients of +1 and -1. With the aid of pilot symbols, a more accurate estimation of frequency offsets can be obtained, and the ICI self-cancellation demodulation can be operated properly.

In OFDM systems, employing a cyclic prefix (CP) as the guard interval is a simple way to combat the inter-symbol interference (ISI) and the inter-carrier interference (ICI), however it reduces the transmission efficiency of the system, especially for some channels with a very long delay spread. In this paper, we consider the OFDM system with insufficient CP, much more efficient than conventional OFDM systems. First, we present the system mathematical model and give the ISI and ICI analysis. Then the signal-to-interference power ratio (SIR) performance is presented. To reduce the ISI and ICI due to the insufficient CP, we develop a minimum-mean-square-error decision feedback equalizer (MMSE_DFE). Based on the MMSE criterion, the optimum feedforward and feedback filter coefficients are derived. For time-varying channel, to avoid brute force matrix inversion in conventional schemes, we propose an adaptive LMS based solution to update the filtering coefficients by tracing the channel variation. Since the high complexity of MMSE_DFE, a reduced complexity scheme, ordered successive partial interference cancellation DFE (OSPIC_DFE), is developed. From the performance comparison between the MMSE_DFE and the OSPIC_DFE, we see that the latter is very near to the former. Finally the simulation shows these proposed methods are highly effective in combating ISI and ICI with low complexity.

Polynomial cancellation coding (PCC) was proposed to mitigate the sever inter-carrier-interference (ICI) in an orthogonal frequency division multiplexing (OFDM) system caused by frequency offset. In this paper, we consider the effectiveness of PCC under time-variant multi-path Rayleigh fading analytically and by simulations. We first consider an analytical expression of the signal-to-interference plus noise power ratio (SINR) and then derive an approximation of the bit-error-rate (BER) of the OFDM-PCC system under the assumption that ICI is well approximated by a white Gaussian noise. Since the bandwidth efficiency of OFDM-PCC is half of that of normal OFDM, we compare the BER performance of the scheme with the normal OFDM system of the same bit-rate when low, medium, and high level modulations are used. Our results show that OFDM-PCC performs well even for high modulation level under time-varying multi-path fading.

When the multipath delay difference exceeds the guard interval (GI), the performance of MIMO-OFDM transmission suffers severely from both the inter-symbol interference (ISI) from the adjacent OFDM symbols and the inter-carrier interference (ICI) within the same symbol. This paper therefore proposes a MIMO-OFDM receiver employing the low-complexity turbo equalization. The proposed receiver initially separates the data streams and suppresses ICI by linear processing. In the iterative processing, it cancels the other data streams as well as ISI and ICI. The MIMO-OFDM turbo equalizer consists of an ISI canceller, an ICI canceller, an optimal detection filter, and a MAP detector. The proposed receiver can improve the transmission performance by exploiting the log-likelihood ratio that the decoding process produces for canceling both ISI and ICI and separating of the spatially multiplexed streams. Computer simulations, which apply the wireless LAN to MIMO, demonstrate that the proposed receiver can provide excellent performance in the severe multipath channels where the delay difference is greater than GI.

We analyze the performance of code division multiple access (CDMA) systems with orthogonal frequency division multiplexing (OFDM). We obtain the probability density function (pdf) of the multiple access interference (MAI) of CDMA systems and extend the results to OFDM-CDMA systems to determine the pdf of the MAI and inter-carrier interference (ICI) in terms of the number of users, the spreading length, the crosscorrelation of spreading sequences, the number of sub-carriers and the frequency offset. We consider the synchronous downlink of cellular multi-carrier CDMA systems and derive a Gaussian approximation of the MAI and ICI. The results show that the overall effect of frequency offset varies with system loading for a given crosscorrelation. The performance of OFDM-CDMA in frequency selective fading channels is analyzed in terms of the joint probability of the fading parameter in each sub-carrier.

Time variations of the wireless channel cause Inter-Carrier Interference (ICI) between different subcarriers in an OFDM system. In a highly mobile environment this interference may become so high that it degrades up to unacceptable levels the communication channel. In this paper, firstly we obtain a simplified expression for the total ICI experienced by every subcarrier in an OFDMA system. Unlike other previous works, the result establishes an explicit and useful relation between the ICI on each subcarrier and the speed of the rest of the terminals in the system. Then, by means of a mathematical analysis we extend the scope of that expression to a hybrid system in which OFDMA and CDMA are combined. Using the good autocorrelation and cross-correlation properties provided by Gold-sequences we propose a Gold-Code-based CDMA-OFDMA transmission technique for the asynchronous uplink channel. We show that the proposed method can reduce the total ICI and potentially increase the capacity of the system in comparison to a conventional OFDMA system.

OFDM transmission performance in mobile communications suffers severe degradation caused by multipath delay difference greater than the Guard Interval (GI). This is because the excess delay results in considerable Inter-Symbol Interference (ISI) between temporally adjacent symbols and Inter-Carrier Interference (ICI) among subcarriers in the same symbol. This paper proposes a robust OFDM receiver for the scattered pilot OFDM signal that can effectively suppress both ISI and ICI by using two types of equalization and a smoothed FFT-window. In order to verify the performance of the proposed receiver, computer simulations are conducted in accordance with the scattered pilot OFDM signal format of the Digital Terrestrial Television Broadcasting (DTTB). The simulation results demonstrate that the proposed receiver shows much better performance than the conventional receiver in multipath fading environments with the delay difference greater than GI duration.

This paper investigates the impact of inter-carrier interference (ICI) due to Doppler spread on the packet error rate (PER) performance in Orthogonal Frequency and Code Division Multiplexing (OFCDM) packet wireless access employing turbo coding in a multipath fading channel, and describes the optimization of the sub-carrier spacing, Δ f, i.e., the number of sub-carriers, Nc, with an approximate 50-100 MHz bandwidth. Simulation results show that although the uncoded OFCDM in a 1-path flat Rayleigh fading channel is affected by the ICI caused by the Doppler spread when the maximum Doppler frequency, fD, becomes more than 5% of Δ f, OFCDM employing turbo coding in a 24-path Rayleigh fading channel is robust against Doppler spread and the degradation is not apparent until fD reaches more than 10% of Δ f. This is because the turbo coding gain and the frequency diversity effect compensate for the degradation due to ICI. Meanwhile, the PER performance with a larger Nc is degraded, since the effect of the error correction capability becomes smaller due to the larger variance of the despread OFCDM symbols associated with the narrower spreading bandwidth in the frequency domain. Consequently, along with the packet frame efficiency for accommodating the guard interval to compensate for the maximum multipath delay time of 1 µsec, we clarify that the optimum number of sub-carriers is approximately 512-1024 (the corresponding Δ f becomes 156.3-78.1 kHz) for broadband OFCDM packet wireless access assuming a 50-100 MHz bandwidth.