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This paper deals with the estimation of time-varying Multi-Carrier Direct-Sequence Code DivisionMultiple Access (MC-DS-CDMA) fading channels using a training-aided scheme. Our approach consists in using an optimal filtering based on a linear state-space model of the fading channel system. In that case, two issues have to be investigated: 1) what kind of optimal filtering can be used? 2) how to estimate the state-space matrices? Thus, Kalman filtering can be considered. It is optimal in the H2 sense providing the underlying state-space model is Gaussian and accurate. However, as these assumptions may no longer be satisfied in real cases, we propose to study the relevance of H∞ filtering. More particularly, when an explicit AR model is used for the channel, our first solution consists in estimating the fading channel and its AR parameters by means of two-cross-coupled H∞ filters. Instead of AR model based-estimators, our second contribution is to view the channel estimation as a realization issue where the state-space matrices are estimated by using subspace methods for system identification without any a priori explicit model for the channel. [ABSTRACT FROM AUTHOR]

This paper deals with the joint symbol-channel estimation for quasi-synchronous Multi-carrier Direct-Sequence Code DivisionMultiple Access (MC-DS-CDMA) systems over Rayleigh fading channels. To solve this non-linear problem, Rao- Blackwellized particle filters have proved efficient. In this framework, our contribution is twofold. 1) Instead of using an autoregressive (AR) model which does not match the bandlimitation of the theoretical power spectrum density (PSD) of a Rayleigh channel, we suggest modeling the channel by a low-pass filtered version of the so-called stochastic sinusoidal process. It consists of sinusoids in quadrature with random magnitudes modeled as AR processes. By suitably choosing the AR parameters, this combination has the advantage of providing a stationnary process whose PSD is bandlimited and has two peaks at the maximum Doppler frequency for any AR order. 2) The estimation of the model parameters is included in the joint symbol-channel estimation process. For this purpose, a deterministic Rao-Blackewellized Particle Filter which jointly estimates the symbols and the channels is cross-coupled with a Kalman Filter which yields the AR parameters. [ABSTRACT FROM AUTHOR]

This paper is concerned with joint channel estimation and data detection for uplink multicarrier code-division multiple-access (MC-CDMA) systems in the presence of frequency fading channel. The detection and estimation, implemented at the receiver, are based on a version of the expectation maximization (EM) algorithm which is very suitable for the the multicarrier signal formats. Application of the EM-based algorithm to the problem of iterative data detection and channel estimation leads to a receiver structure that also incorporates a partial interference cancelation. Computer simulations show that the proposed algorithm has excellent BER end estimation performance. [ABSTRACT FROM AUTHOR]

The paper investigates the performance of a multi-carrier code division multiple access (MC-CDMA) system adopting a partial equalization (PE) technique at the receiver. It has been recently shown that, in spite of the fact that the PE technique has the same complexity of the well known maximal ratio combining, orthogonality restoring combining and equal gain combining techniques, the PE significantly improves the performance in terms of bit error probability (BEP). Previous works considered PE with ideal channel state information (CSI) and evaluate the PE parameter which minimizes the BEP; here we relax this assumption considering the presence of CSI errors. In particular, we analytically derive the mean BEP for the downlink of a MC-CDMA system and the optimal PE parameter, when imperfect CSI is assumed. Numerical results show that the optimum choice of the PE with ideal CSI provides significant performance improvement also in the presence of CSI errors. [ABSTRACT FROM AUTHOR]

In this paper we investigate the possibilities to generate a postamble in block-oriented OFDM transmission without compromising the effective user data rate. The idea is to complete known bits at the end of each physical layer frame to a full OFDM symbol to be used for channel estimation. A Wiener Filter is then employed to interpolate between the channel estimates obtained on the pre- and postamble. We present experimental results for an IEEE 802.11a-like system. A comparison with purely preamble-based channel estimation shows a reduced mean square channel estimation error and bit error rate at low computational complexity and only minor reduction of the net user data rate. Results of robustness tests are also discussed. [ABSTRACT FROM AUTHOR]

In this contribution 1 we address the issue of pilot design for channel estimation in multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM) systems applying beamforming at the transmitter. Specifically we consider the case when a finite set of fixed beamforming vectors is available and the selection of the optimal weight for each transmit antenna is performed exploiting long-term channel information. Capacity maximization is considered as the optimization parameter in order to achieve a proper trade-off between estimation performances and available resources for data transmission. Our analysis shows the impact of different values of pilot boost and pilot overhead on the channel estimation mean square error (MSE) and on the capacity of the system. Simulation results are given for an outdoor mobile wireless channel, different number of antennas and channel estimator filter order for a single and a multi-user transmission scenario. [ABSTRACT FROM AUTHOR]

In this paper, multiple input multiple output (MIMO) channel coefficients, jamming power, and jammer state state information (JSI) are jointly estimated to suppress the adverse effects of partial band noise jamming (PBNJ) for mlticarrier code division multiple access (MC-CDMA) systems using complex quadratic spreading sequences and cyclic delay diversity (CDD).We derive weighted least square error (LSE) and linear minimum mean square error (MMSE) channel estimators (CE) under PBNJ and investigate the constraints on cyclic delays. for the pilot symbols, hard or soft JSI is estimated to improve the channel estimates by updating the covariance matrix. On the other hand, iteratively estimated JSI is exploited to suppress jamming in the despread data symbols. Simulation results demonstrate that the proposed iterative despreading (includes JSI estimation and soft chip combining), demodulation, and decoding (IDDD) receiver with soft JSI (S-JSI) based MMSE-CE and chip-combining is very robust against jamming. [ABSTRACT FROM AUTHOR]

This paper investigates a new time domain channel estimation for MIMO OFDMA downlink systems. Compared to classical frequency domain channel estimation, time approach allows improvements in a multi-antenna system. Nevertheless, the presence of null carriers in the spectrum leads to performance degradation because of "border effect" phenomenon. An improved time domain channel estimation for MIMO-OFDMA system is proposed for downlink transmission and compared to classical channel estimation methods. Besides, this channel estimation process can be applied in any SISO and/or MIMO multicarrier transmission. [ABSTRACT FROM AUTHOR]

Copyright of Multi-Carrier Spread Spectrum 2007 is the property of Springer eBooks and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In OFDM mobile communication with high-speed vehicle, due to the effect of large multi-path Doppler shifts varying independently with respect to the paths, the time allowed to the conventional methods of estimation of the channel characteristics is severely limited without using separate estimates of the Doppler shifts. As a result, the quality of performance of the data demodulation remains low using the estimates insufficiently smoothed of the noise. This report proposes a novel method using Expectation-Maximization (EM) algorithm for simultaneously estimating the Doppler shifts and the complex amplitudes of the path-wise channel responses. The results of computer simulation show capability of proposed method much superior to the conventional one. [ABSTRACT FROM AUTHOR]

In this article, we present an explicit comparative analysis of the two low-complexity channel estimation techniques, which can be used in the pilot-assisted orthogonal frequency division multiplexing (OFDM) systems, - linear minimum mean squared error (LMMSE) and least squares (LS) criteria based. Numerical results are preceded by derivation of the optimum design form to yield both robustness and minimum possible computational complexity for the given class of algorithms. The work also investigates the effect of quantity and power of pilot subcarriers on estimation accuracy and BER performance. [ABSTRACT FROM AUTHOR]

This article investigates the influence of sampling time offset on the performance of an interleaved frequency-division multiple-access (IFDMA) uplink scheme for mobile radio transmission. It is shown that sampling time offset causes signal-to-noise ratio (SNR) degradation and leads to inter-symbol interference (ISI). In addition, an efficient algorithm is proposed which is capable to estimate sampling time offset by applying oversampling at the receiver. The proposed algorithm requires only three-times oversampling, does not use any pilot symbols, and is independent from the transmission channel and modulation alphabet. [ABSTRACT FROM AUTHOR]

In this paper we present a novel channel impulse response estimation technique for block-oriented OFDM transmission based on combining estimators: the estimates provided by a Kalman Filter operating in the time domain and a Wiener Filter in the frequency domain are optimally combined by taking into account their estimated error covariances. The frequency domain Wiener Filter smoothes Maximum-Likelihood estimates of the channel frequency response obtained by the Expectation-Maximization algorithm. Different variants of the proposed scheme are experimentally investigated in an IEEE 802.11a-like system setup. They compare favourably with known approaches from the literature resulting in reduced mean square estimation error and bit error rate. Further, robustness and complexity issues are discussed. [ABSTRACT FROM AUTHOR]

OFDM systems such as the schemes proposed for 3G evolution exhibit a great sensitivity to the effects of phase noise and channel estimation, which can introduce inter-channel interference due to the loss of orthogonality among sub-carriers. In this work channel estimation and tracking schemes are investigated, proposing to compensate jointly the effects of phase noise and of the channel. A decision feedback scheme is proposed together with an estimator based on the pilots, to improve the performance also in the presence of fast fading multipath. [ABSTRACT FROM AUTHOR]

This paper covers positioning techniques for cellular networks using generalized multi-carrier communications signals. These techniques are based on symbol-timing synchronization algorithms to find the starting point for the underlying orthogonal frequency division multiplexing symbols of the incident signals. Well known algorithms are, e.g., the Schmidl-Cox algorithm and the Minn algorithm. These algorithms are investigated w.r.t. their positioning capabilities. Furthermore, they are extended to improve the performance in multipath environments and to achieve a sufficient accuracy with weak signals from neighboring out-of-cell base stations. [ABSTRACT FROM AUTHOR]

This paper investigates the cancellation of intercellular interference for downlink (DL) MC-CDMA systems. We especially focus on two turbo-iterative receiver schemes that allow cancellation of a part of the intercell interference. After deriving the block diagram receivers and the mathematical expressions at different receiver stages for both receivers, we present several link-level simulation results considering B3G multicellular scenario in presence of one interfering cell. We demonstrate the significant performance improvements of the proposed schemes over a wide signal to interference ratio (SIR) area at half and full loads. Index Terms—Iterative receiver, intercell interference, MC-CDMA. [ABSTRACT FROM AUTHOR]

This paper handles a transmit diversity technique to provide adequate performances at the cell border in a next generation cellular downlink of a mobile communications system. The used technique is based on the Alamouti scheme, and therefore, we call the scheme cellular Alamouti technique (CAT). The CAT is exemplarily applied to an orthogonal frequency division multiplexing access (OFDMA) which is a promising candidate for future communications systems. For improving the performance at the cell borders, the CAT exploits resource management to apply the Alamouti technique and to avoid inter-cellular interference using both adjacent base stations to transmit the desired signal. The influence of the CAT to a cellular OFDMA system is investigated for different resource management strategies. Simulation results show a significant performance improvement at the cell border by using CAT. The performance depends on the resource allocations of the chosen sub-carriers. Nevertheless, CAT is even robust to high-speed scenarios. Furthermore, investigations regarding the system throughput with different modulation alphabets offer a reliable and flexible adaptation to different mobile terminal requests. [ABSTRACT FROM AUTHOR]

We consider the uplink of an MC-CDMA network and assume that the base station is endowed with a uniform linear array. Transmission takes place over a multipath channel and the goal is the estimation of the directions of arrival (DOAs) of the signal from the active users. In a multi-user scenario difficulties are due primarily to the large number of parameters involved in the estimation of the DOAs which makes this problem much more challenging than in single-user transmissions. The solution we propose allows estimating the DOAs of different users independently, thereby leading to significant reduction in the system complexity. In the presence of multipath propagation, however, estimating the DOAs of a given user through maximum likelihood methods remains a formidable task since it involves a search over a multidimensional domain. Therefore, we look for simpler solutions and discuss two alternative schemes based on the SAGE and ESPRIT algorithms. [ABSTRACT FROM AUTHOR]

In future mobile radio concepts, the cost and complexity of the mobile terminals (MT) should be low. Concerning the downlink, this demand can be fulfilled by resorting to receiver orientation (RO). In conventional RO MIMO OFDM multi-user downlinks, to the data elements to be transmitted uniquely defined complex values are assigned by modulation, such that the transmit processing can be characterized by a modulator matrix, with for instance represents the Transmit Zero Forcing (TxZF) filter. Unfortunately, such a unique assignment does not provide any transmit power reductions compared to the equivalent transmitter oriented (TO) schemes. However, dramatic transmit power reductions become possible if the conventional unique mapping of data elements is replaced by a selective assignment within a set of valid representatives and/or representative domains. The benefit of low transmit power - in addition to low receiver complexity - has to be bargained for increased signal amplitude dynamics at the receivers. In this paper we analyzethis dynamics increase and discuss a possible counter measure. [ABSTRACT FROM AUTHOR]

Rapidly time-varying channels are a major obstacle for successful data-transmission via OFDM. The resulting loss of orthogonality among neighboring subcarriers leads to intercarrier-interference, which affects channel estimation. This in turn impedes the subsequent data detection. Literature contains numerous approaches to cope with this problem working either in time- or frequency-domain. Our concern in this paper is a novel time-domain method which relies on the use of multiple directional receive antennas. Each of these antennas experiences only a fraction of the total Doppler spread of a comparable omnidirectional antenna. This not only eases channel estimation but also allows for a diversity gain due to maximum-ratio-combining. We will demonstrate that our scheme copes well with large maximum Doppler frequencies. [ABSTRACT FROM AUTHOR]

Spatial phase coding (SPC) can closely approach the performance of optimum pre-coding schemes, whereas SPC requires less complexity and overhead with respect to channel estimation and feedback. This paper analyses the effects of SPC in MC-CDMA multi-user systems. The downlink performance and complexity is evaluated and compared to that of known pre-coding schemes. It can be shown that MC-CDMA with SPC applying single as well as multi-user detection can outperform OFDM(A) with SPC. [ABSTRACT FROM AUTHOR]

Cyclic delay diversity (CDD) provides additional diversity in Rayleigh fading channels, and therefore, improves system performance. For line-of-sight (LOS) propagation, e.g. the additive white Gaussian noise channel, the implementation of CDD yields to a performance loss. Therefore, we investigate CDD for Ricean channels with different Ricean factors. To combat the SNR loss for significant LOS propagation we propose the principle of antenna power weighting. The idea is to feed different power levels into the multiple transmit (TX) antenna branches rather than distributing the TX power uniformly among the TX-antennas. We exemplarily implement CDD with antenna power weighting to the terrestrial digital video broadcasting system (DVB-T). Simulation results show that antenna power weighting significantly reduces the SNR loss in LOS propagation by the cost of a slight degradation of the SNR gain in non-LOS scenarios. [ABSTRACT FROM AUTHOR]

Most of the traditional channel-aware schedulers produce very unfair situations, whereas schedulers based on input buffers state and quality of service requirements lead to low spectrally efficient systems. In this paper a novel radio resource allocation algorithm based on combination of the CSI information as well as the input buffers is presented and applied to a multicarrier system This work has been partially carried out in the framework of the Celtic project WISQUAS (CP-2-035), and in the national Spanish code project: FIT-330220- 2005-108. In fact, the Group Orthogonal MC-CDMA and the Spread Spectrum Multicarrier Multiple Access schemes are analyzed and performances compared when applying the proposed algorithm in a typical low-mobility environment. [ABSTRACT FROM AUTHOR]

Copyright of Multi-Carrier Spread Spectrum 2007 is the property of Springer eBooks and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We examine the performance gains from two variants of adaptive coding and modulation (ACM) in an OFDM-based cellular system with realistic system parameters. In this system a channel-aware scheduler allocates to each active user a set of subchannels, upon which the users may perform link adaptation with ACM. We investigate the achievable performance gain by frequency-selective ACM, in comparison to non frequency-selective adaptation. Since the scheduler, which is aware of the channel and the traffic conditions, already exploits the multi-user diversity and tends to allocate similar subchannels to a given user, it is not obvious that frequency-selective link adaptation achieves a signifi- cant performance gain, especially when realistic ACM with limited granularity is considered. In this paper, we evaluate the gain of frequencyadaptive vs. non frequency-adaptive transmission for the uplink under realistic conditions for a 4G system proposal. [ABSTRACT FROM AUTHOR]

A new precoding technique for MC-CDMA is presented, that outperforms conventional precoding by exploiting useful interference. This is done by selectively pre-decorrelating users to destructive interference while allowing interference when it is expected to contribute to their signal. The resulting SINR improvement is achieved by making use of energy existent in the system so performance enhancement is attained without the need for increased transmitted power-per-user. This, however, comes at the expense of additional processing at the transmitter for the allocation of the dynamic precoding matrix. The proposed technique applies to the downlink of cellular MC-CDMA systems. Comparative simulations of this and other precoding methods provide evidence that the proposed method offers significant performance benefits. [ABSTRACT FROM AUTHOR]

This paper discusses the performance of the Satellite Downlink Transmitter that uses a Wavelet based Filtered Multi-Tone (WFMT) modulation. The novel WFMT modulation is proposed to improve the characteristics of the OFDM multicarrier systems. In addition, it can also be used successfully in satellite communications. In this paper, we discuss the advantages of WFMT modulation such as low sensitivity to narrowband RF interference and lower Peak-to-Average Power Ratio (PAR) in comparison with OFDM. It is shown that modern GaAs Solid-State Power Amplifiers are well suitable for transmission of multicarrier signals. An effect of non-linear distortion on power added efficiency (PAE) of the RF Power Amplifier is discussed. The effect of clipping of the WFMT signal on the error performance is described. It is shown that the error correction block-codes can significantly improve PAR characteristics of the communication system. The Power Efficiency of WFMT Satellite Transmitter is significantly higher than in case of OFDM. [ABSTRACT FROM AUTHOR]

We propose an alternative to the well-known MC-CDMA technique for downlink transmission by replacing the conventional cyclic-prefix OFDM modulation by an advanced filterbank-based multicarrier system (OFDM/OQAM). Indeed, in one hand, MC-CDMA has already proved its ability to fight against frequency selective channels thanks to the use of the OFDM modulation and its high flexibility in multiple access thanks to the CDMA component. In the other hand, OFDM/OQAM modulation confers a theoretically optimal spectral efficiency as it operates without guard interval. In this paper, we study OFDM/OQAM with CDMA combination and we compare it to the conventional MC- CDMA scheme in terms of Bit Error Rate (BER). [ABSTRACT FROM AUTHOR]

In this contribution an accurate average bit error rate (BER) formula is derived for Rayleigh-faded MC-DS-CDMA in the context of asynchronous transmissions and random spreading sequences. Our analysis is based on the characteristic function and does not rely on any assumption concerning the statistical behavior of the interference. We develop a new closed-form expression for the conditional characteristic function of the inter-carrier interference and require only a single integration for the associated BER calculation. The accuracy of the standard Gaussian approximation (SGA) techniques is also evaluated. [ABSTRACT FROM AUTHOR]

Performance of RAKE receiver in a multipath channel for direct sequence code division multiple access system (DS CDMA) can be severely impaired due to high level of multiple access interference (MAI), whose value is controlled by correlation functions of user signatures. Due to data modulation MAI becomes dependent on aperiodic user signature correlations of big shifts, which are hard to control and become even less so as multipath delay grows, leading to significant MAI increase. In this article we analyze performance of RAKE receiver for Signature-Interleaved (SI) DS CDMA in synchronous multipath Rayleigh channel in comparison with that of conventional DS CDMA. It is shown that SI DS CDMA provides better performance in terms of bit error rate (BER), especially when using specifically tailored user signatures. [ABSTRACT FROM AUTHOR]

Spectrum-overlay scenarios for wideband multi-carrier (MC) systems bring new technical challenges that must be considered during the system design. In such a scenario, the performance of the multi-carrier system is affected by the presence of possibly strong in-band interference. To improve the performance of the MC communication link in an interference corrupted environment (without increasing transmit bandwidth), the interference must be estimated and removed. In this paper, we propose a new low complexity algorithm to estimate and suppress digitally modulated interferers located in the MC spectrum. The performance of the interference suppression is evaluated in an analytical way. Further, simulations have been carried out to verify the validity of approximations in the analysis. [ABSTRACT FROM AUTHOR]

In overlay systems based on orthogonal frequency-division multiplexing (OFDM) narrow-band interference (NBI) from licensed systems transmitting in the same frequency band degrades system performance significantly. In this paper, the impact of NBI is either reduced by means of an optimized compensation matrix or by simply subtracting an estimated NBI signal from the received signal. The required estimation of the NBI signal is performed by means of a few observation subcarriers. Variations in the center frequency of the interferer are tracked by exploiting information from pilot OFDM symbols for channel estimation. Simulation results show that with both techniques system performance is improved significantly even in presence of strong and dynamically changing NBI. [ABSTRACT FROM AUTHOR]

OFDM systems super from high out-of-band radiation. Consequently, they require methods reducing those spectral out-of-band components. Because of adjustable frequency confinement, filter bank based multicarrier systems allow for a lower out-of-band radiation. This paper compares an OFDM system employing one out-of-band reduction method with a filter bank based multicarrier system (FBMC). [ABSTRACT FROM AUTHOR]

In this paper, a full-duplex joint navigation and communication system with a unified signal structure based on interleave-division multiple access (IDMA) is proposed. At the transmitter side, the navigation data and the communication data are linearly superimposed and transmitted on the same carrier frequency at the same time. Both data rates are adjustable in a wide range, including high data rates. At the receiver side, the navigation data can be separated near perfectly from the communication data. The position is estimated by means of a channel estimator rather than a delay-locked loop. No oversampling is necessary. Navigation and communication is feasible in the presence of low signal-to-noise ratios, interference, and multipath - situations where conventional navigation systems may fail. Possible applications include, but are not restricted to, indoor scenarios. Monte-Carlo simulations illustrate the performance of the navigation and the communication unit. [ABSTRACT FROM AUTHOR]

IDMA is a multiple access scheme closely related to CDMA and of great interest for future mobile networks. In contrast to orthogonal schemes like TDMA or FDMA collisions are not avoided but the interference is treated by iterative detection exploiting channel coding. By tolerating a certain amount of interference a higher spectral efficiency is achieved. The number of supportable users can be further increased by an optimized power profile. In order to decode all users with reasonable complexity, efficient detection strategies should be considered. This paper investigates a complexity reduction for parallel interference cancellation (PIC) by means of decoding only promising users in the first iterations, which means a user- and iteration-wise switching strategy. The optimal strategy is found by variance transfer charts (VTC) and confirmed by bit error rate simulations. Furthermore, a scheduling strategy for successive interference cancellation (SIC) is considered and shown to be most efficient in terms of channel decoder evaluations. [ABSTRACT FROM AUTHOR]

This paper is concerned with multi-user detection techniques for orthogonal frequency division multiplexing - interleave division multiple access (OFDM-IDMA) and IDMA with cyclic prefix (IDMACP). These two schemes have attractive features such as low-cost multi-user detection/equalization and flexible rate adaptation. We will show that significant performance improvement (i.e., the so-called multi-user gain) can be achieved by allowing concurrent multiple user transmission on common carries. [ABSTRACT FROM AUTHOR]

Copyright of Multi-Carrier Spread Spectrum 2007 is the property of Springer eBooks and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Multi-Carrier Spread Spectrum 2007 is the property of Springer eBooks and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

MIMO systems are of interest to meet the expected demand for higher data rates and lower delays in future wireless systems. In addition, multi-carrier systems are of interest to combat frequency-selective fading experienced over the larger bandwidth these future broadband systems will use. The introduction of these spatial and frequency resources adds extra dimensions and complexity for any scheduling algorithm in a multiuser system. In this paper, we investigate scheduling through utility functions implemented via genetic algorithms in order to reduce complexity. This paper extends our prior work for a single-carrier MIMO system using dirty paper coding to the multi-carrier case. Here, the users that are scheduled, the order they are encoded in, and the subcarrier frequencies they are assigned to will all affect the performance of the scheduling algorithm. We demonstrate that the genetic algorithm is still able to achieve a similar near-optimal performance relative to an exhaustive search with the same relative reduction in complexity. Additionally, with the use of OFDM, an increase in capacity is seen relative to the single-carrier case. [ABSTRACT FROM AUTHOR]

The influence of a cyclic prefix on the spectral power density of discrete-time, zero-mean, cyclo-stationary, random sequences is investigated. Analytical expressions for the time-varying autocorrelation function, the average autocorrelation function, and the spectral power density of the cyclic-prefixed sequence are provided. Application to discrete Fourier transform based orthogonal frequency division multiplexing is presented, and shown that the cyclic prefix introduces a strong ripple in the spectral power density within the main frequency band. Besides the loss of spectral efficiency caused by the cyclic prefix, this ripple may require reduction of transmit power in order to obey regulatory spectrum masks. [ABSTRACT FROM AUTHOR]

Orthogonal Frequency Division Multiple Access (OFDMA) appears today as a strong candidate in on-going standardization. Despite its attractive features, it has several drawbacks when employed in the uplink: The high peak-to-average power ratio (PAPR) inherited from Orthogonal Frequency Division Multiplexing (OFDM) and the inherent frequency diversity loss. The loss of frequency diversity can be combated by precoding. Variants of Precoded OFDMA include Spread Spectrum Multi-Carrier Multiple Access (SS-MC-MA) and the frequency-domain implementation of single-carrier (SC) FDMA called DFT-Spread OFDM. The latter transforms OFDMA into a SC transmission system, avoiding the PAPR problem. SC-FDMA with uniformly spaced carriers can be also generated by time-domain processing, a technique known as Interleaved Frequency Division Multiple Access (IFDMA). This paper analyzes OFDMA, several variants of Precoded OFDMA as well as SC-FDMA in its time- and frequency-domain implementations and compares them for uplink transmission. The spectrum and performance analysis confirm the benefits of SC-FDMA in terms of both high-power amplifier (HPA) output back-off and bit error rate (BER) performance on selective channels. [ABSTRACT FROM AUTHOR]

In this paper, we consider different types of guard intervals for OFDM systems, i.e. cyclic prefix (CP), zero padding (ZP) and known symbol padding (KSP). We compare the different OFDM systems with respect to their SNR performance. We show that CP-OFDM and ZP-OFDM have exactly the same performance, whereas KSP-OFDM has a slightly worse performance. Further, we consider data aided channel estimation for the three OFDM systems; the MSE of the estimators is compared to the corresponding Cramer-Rao bound. It turns out that in practice, channel estimation for CP-OFDM slightly outperforms the one for ZP-OFDM. The practical channel estimation techniques for KSP-OFDM perform worst. [ABSTRACT FROM AUTHOR]

Frequency-domain equalization (FDE) technique may play an important role for broadband packet transmission using MC- and DS-CDMA. The downlink performance is significantly improved with FDE; however, the uplink performance is limited by the multi-access interference (MAI). To remove the MAI while gaining the frequency diversity effect through the use of FDE, frequency-domain block spread CDMA can be used. The performance can be further improved by the use of multi-input/multi-output (MIMO) antenna technique. Recently, particular attention has been paid to MIMO space division multiplexing (SDM) to significantly increase the throughput without expanding the signal bandwidth. In this paper, we present a comprehensive performance comparison of MC- and DS-CDMA using FDE. [ABSTRACT FROM AUTHOR]