Van Duc Nguyen

Block interleaving for soft decision Viterbi decoding in OFDM systems

In this paper, frequency and time interleaving algorithms used for soft decision Viterbi decoding for OFDM systems, performed on frequency-selective and time-selective channels respectively, are investigated. To achieve optimal performance in terms of the bit error rate (BER), the size of the interleavers must be specified depending on the properties of the channel and the characteristics of the code. Under these aspects, new frequency- and time interleaving algorithms are proposed. To get the lowest BER, we have found that the frequency interleaving length should be chosen in the range of the decoding constraint length. On the other hand, the time interleaving depth should be derived from the coherence time of the channel.

Channel impulse response length and noise variance estimation for OFDM systems with adaptive guard interval

A new algorithm estimating channel impulse response (CIR) length and noise variance for orthogonal frequency-division multiplexing (OFDM) systems with adaptive guard interval (GI) length is proposed. To estimate the CIR length and the noise variance, the different statistical characteristics of the additive noise and the mobile radio channels are exploited. This difference is due to the fact that the variance of the channel coefficients depends on the position within the CIR, whereas the noise variance of each estimated channel tap is equal. Moreover, the channel can vary rapidly, but its length changes more slowly than its coefficients. An auxiliary function is established to distinguish these characteristics. The CIR length and the noise variance are estimated by varying the parameters of this function. The proposed method provides reliable information of the estimated CIR length and the noise variance even at signal-to-noise ratio (SNR) of 0 dB. This information can be applied to an OFDM system with adaptive GI length, where the length of the GI is adapted to the current length of the CIR. The length of the GI can therefore be optimized. Consequently, the spectral efficiency of the system is increased.

A spatial simulation model for shadow fading processes in mobile radio channels

This paper deals with the design of spatial simulation models for shadowing processes. Such processes are generally assumed to be lognormally distributed. The simulation model is derived from a reference model by using the sum-of-sinusoids principle. Analytical expressions are presented for the spatial autocorrelation function (ACF), probability density function (PDF), and the level-crossing rate (LCR) of both the reference and the simulation model. Two methods are discussed enabling the fitting of the simulation model to the reference model with respect to the PDF of the received signal strength as well as to a given spatial ACF with an exponential shape. The usefulness and the performance of the shadowing channel simulator is demonstrated by applying the proposed design method on two scenarios capturing the shadowing effects in suburban and urban areas.

Intercarrier and intersymbol interference analysis of OFDM systems on time-invariant channels

In order to derive the mathematical description of interference and useful power in the case of insufficient guard length of an OFDM system, the channel impulse response is truncated into two parts, the so-called two truncated channels, in mathematical analysis. The first one describes the behaviour inside the guard interval, the other outside. Assuming the channel to be wide-sense stationary with uncorrelated scattering (WSSUS), the linear characteristic of Fourier transformation allows to be obtained exactly the mathematical expressions of intercarrier interference (ICI) power, P/sub ICI/, intersymbol interference (ISI) power, P/sub ISI/, and useful power, P/sub U/, in dependence of the correlation functions and the multipath channel profiles of the two truncated channels. We consider only the case of the time-invariant channel. The simulation results show a good agreement with the theoretical results.

Interference Aware Medium Access in Cellular OFDMA/TDD Networks

In this paper a novel decentralized and interference aware medium access control (MAC) protocol combined with a dynamic subchannel selection algorithm for OFDM (orthogonal frequency division multiplexing) is presented. The protocol resolves several drawbacks of the existing radio resource allocation techniques for OFDM systems, such as the hidden and exposed node problem, and the requirement for a time-invariant channel. The proposed scheme enables the transmitter to determine the level of interference it would cause to already active links prior to any transmission. This is achieved through a busyslot signaling that exploits the channel reciprocity offered by the TDD mode. Compared to existing radio resource allocation schemes the required signaling overhead is significantly reduced. The interference awareness allows the system to avoid significant co-channel interference (CCI) and to operate with full frequency reuse. When applied to a broadband OFDM air interface, this principle can also exploit the frequency selective fading channel on the intended as well as the interference links. This results in an autonomous subcarrier allocation algorithm which can dynamically adapt to time-varying channels. Due to its decentralized nature, the algorithm can be utilized in both cellular and ad hoc networks. Simulation results for a cellular system show that the new decentralized medium access and channel assignment algorithm is capable of achieving spectral efficiencies of up to 2.25 bits/Hz/cell using 16 QAM (quadrature amplitude modulation).

Estimation of the channel impulse response length and the noise variance for OFDM systems

A new method is proposed for the estimation the channel impulse response (CIR) length and the noise variance for orthogonal frequency division multiplexing (OFDM) systems. To estimate the CIR length and the noise variance, the difference between the statistical characteristics of the additive noise and the mobile radio channel is exploited. This difference lies in the fact that the true CIR coefficients are located within the CIR length, whereas the additive noise of an estimated channel tap is uniformly distributed over the whole length of the estimated CIR. Moreover, the CIR length changes more slowly than its coefficients. An auxiliary function is established to describe these characteristics. The CIR length and the noise variance can be estimated by varying the parameters of the auxiliary function. The proposed method provides reliable information on the estimated CIR length even at signal-to-noise ratios (SNRs) of 0 dB.

Location-aware and load-balanced data delivery at road-side units in vehicular Ad hoc networks

Being composed of car electronics and road-side communication devices, vehicular ad hoc networking (VANET) is the heart of intelligent transportation system. Beside car-to-car communications, the system also communicates to centralized points in infrastructure networks. Given that data load is heavy when the density of cars is high and that they move fast, roadside electronics potentially face congestion. This study designs a novel framework finding the best route with respect to efficiency and fairness among road-side units. The introduced algorithm exploits both position data and load to make routing decisions optimally. Simulation results clearly demonstrate the soundness of our proposals.

Intercarrier and intersymbol interference analysis of OFDM systems on time-varying channels

Intersymbol interference (ISI) and intercarrier interference (ICI) for OFDM systems with insufficient guard length was analyzed (Nguyen et al. (2002)) in the case of time-invariant radio channels. This was carried out by truncating the channel impulse response (CIR) in mathematical analysis into two parts, the so-called truncated channels. The same technique is used in this paper to compute the ISI and ICI components for the case of time-varying channels, where again the WSSUS assumption (wide sense stationary uncorrelated scattering) is applied. The exact expression of the ICI power is obtained in dependence of the time correlation function of the channel and the multipath channel profile. Moreover, this ICI power can be well approximated by the sum of the ICI power caused by the insufficient guard interval effect obtained in Nguyen and ICI power caused by the effect of time variations of the channel obtained in Russell et al. (1995). The ISI power is not dependent on the time variations of the channel.

An expending ring search algorithm for Mobile Adhoc networks

Routing protocols are challenges in designing Mobile Ad-hoc networks with low energy consuming. This paper extends the expanding ring search (ERS) algorithm proposed in [3] to optimize the energy consuming caused by broadcasting redundance messages. Simulation results show that the required message packages for routing by using our algorithm is significantly reduced in comparison with that required by using the ERS method.

Channel estimation and interference cancellation for MIMO-OFDM systems

This paper proposes a new channel estimation method and a new interference cancellation scheme for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems in the presence of intersymbol interference (IS1). The proposed channel estimation method uses special training sequences (TSs) to have a desirable crest-factor of the transmitted training signal, and to prevent the influence of ISI on the channel estimation performance. By using the recommended training sequences, the ill-conditioned problem of the least square (LS) filter integrated in the proposed channel estimator can be avoided. The proposed interference cancellation scheme uses the estimated channel coefficients and the channel state information (CSI) to reproduce the interference components, which are then iteratively cancelled from the received signals. To reduced the error-floor of the demodulated symbols using for the calculations of the interference components, the so-called remodulation technique is also included in the proposed interference cancellation scheme. Simulation results show that the proposed channel estimation method outperforms conventional channel estimation methods, especially in the presence of ISI and if the signal-to-noise ratio (SNR) is larger than 15 dB. The combination of the proposed method with a space-time block code (STBC) to combat the interference influences results in an excellent system performance in terms of symbol error ratio (SER). In comparison with a STBC MIMO-OFDM system with sufficient guard interval (GI), this combination gains 1.52 dB of SNR at the same SER of 1.1 10 -6 even after performing only one iteration of interference cancellation.