OFDM-FDMA/SDMA is currently considered as the leading transmission strategy for the downlink of beyond 3G mobile radio systems. When channel state information is available at the transmitter with sufficient accuracy, significant performance gain can be achieved through adaptive resource allocation. In this paper, a greedy method is proposed for adaptive resource allocation that enables to jointly consider the time, frequency and spatial dimensions at each allocation. A comparative assessment with a well-known disjoint allocation approach is carried out and a well-promising variant of the proposed approach has been identified.

An optimization algorithm for finding user allocation, bit and power loading in the downlink of MIMO-OFDM systems is proposed. The algorithm represents a generalization of the well-known concepts of LQ decomposition with row pivoting and water filling.

Recently, a new multiple access (MA) scheme denoted Block-Interleaved Frequency Division Multiple Access (B-IFDMA) has been proposed which can be regarded as a generalization of Interleaved Frequency Division Multiple Access (IFDMA). Compared to IFDMA, B-IFDMA provides higher flexibility, increased robustness to carrier frequency offsets and additional power savings at the mobile terminal at the expense of increased envelope fluctuations and higher complexity. In this paper, different variants of B-IFDMA are presented and their properties are discussed. Moreover, new algorithms for a low complexity implementation of B-IFDMA providing low envelope fluctuations are introduced. The complexity of B-IFDMA is shown to be lower than for OFDMA. New performance results for coded transmission over a mobile radio channel are given.

We propose a new method to efficiently carry out an interleaved subcarrier assignment in multicarrier (MC) multiple-access (MA) systems. According to the conventional approach, the assignment of the subcarriers to the different users is accomplished prior to the MC modulation that is performed over the whole set of subcarriers within the transmission bandwidth. The proposed method enables the MC modulation/demodulation of only the subcarriers assigned to a specific user, thus being much simpler to implement than the conventional approach

In this work, we study a MIMO system with a transmitter using a linear dispersion code (LDC) and a linear MMSE detector at the receiver in a Ricean flat-fading environment. The focus of our work is the analysis of the optimal transmit strategy using different types of LDC assuming that the transmitter knows only the mean channel matrix. On the one hand we consider spatial multiplexing (SM) schemes achieving high data rates, however sacrificing diversity. On the other hand, we have schemes achieving full diversity like quasi-orthogonal space-time block codes (QSTBC) or orthogonal space-time block codes (OSTBC). Depending on the LDC in use, the optimization problem is either convex or non-convex. For both these classes of LDC we derive the properties of the average normalized MSE and analyze the impact of the mean component on the MSE, the optimal transmit strategy and the optimal power allocations

We consider a multi-input multi-output (MIMO) multiple access channel (MAC) where the base station has perfect channel state information (CSI), while the mobiles have only partial CSI. The statistical CSI at the mobiles can be obtained either by implicit (during downlink transmission) or explicit (via a low rate control channel) feedback. The base applies a linear multiuser MMSE receiver and thus the optimization criteria is the average sum MSE. We optimize the average sum MSE with implicit and explicit mean- or covariance-feedback under individual power constraints. The optimal transmit directions of a user corresponds to the eigenvectors of his channel mean or correlation matrix. The optimal power allocation is characterized in terms of its beamforming range. Finally, the gain due to covariance feedback and all theoretical results are illustrated by numerical simulation.

In order to reduce the signaling overhead in adaptive OFDM transmission systems being designed for B3G and 4G wireless communications, the basic resource unit, referred to as chunk, comprises a number of adjacent sub-carriers and OFDM symbol periods. A consequent open issue is the mapping of long codewords onto limited size chunks, especially when link adaptation per chunk is carried out. This paper proposes a new method for joint coding and modulation. A data packet, independently of its length, is encoded without any segmentation into a single codeword spanning different chunks employing different signal constellations. The parameters of the underlying error correction code are chosen independently of the signal constellation and no puncturing is required. Moreover, a joint adaptation of the code length and the modulation format is proposed to optimise the number of chunks allocated for transmission

In this paper, an efficient digital implementation of multicarrier transmission scheme based on generalized discrete Fourier transform (DFT) filter banks is presented for multicarrier code-division multiple-access (MC-CDMA) systems. Generalized DFT filter banks has been traditionally discussed for very high-speed digital subscriber lines (VDSL) wireline systems, received interest also for wireless applications. The design of the generalized DFT modulated filter banks is investigated and its fast implementation is derived in the time-domain for arbitrary integer sampling rate. Through the utilization of the generalized DFT modulated filter banks, one or more subcarriers, even noncontiguous subcarriers can be easily supported by system in both uplink and downlink, which facilitates the users to access the network in a frequency-division multiple-access manner. Simulation results show that the overall bit-error-rate performance of the proposed multicarrier transmission scheme well approaches that of a single-carrier system due to the negligible intercarrier interference introduced by an appropriate design of the generalized DFT modulated filter banks.

In this paper, an algorithm for low-density parity-check (LDPC) codes with reduced complexity is presented. The complexity reduction is achieved by calculating a linear function for updating the check nodes in each iteration, instead of an exponential and logarithmic function. The parameters of the linear function are optimized by density evolution. Simulation results show that there is nearly no loss in the performance by using this approximation compared to the exact calculation of the belief propagation decoding algorithm, even for very large block lengths.

The interleaved frequency division multiple access (IFDMA) scheme is based on compression, repetition and subsequent user dependent frequency shift of a modulated signal. Multiple access is enabled by the assignment of overlapping but mutually orthogonal subcarriers to each user. In this paper it is shown that IFDMA can be regarded as unitary precoded OFDMA with interleaved subcarriers. On the other hand, IFDMA is shown to be a CDMA variant with frequency domain orthogonal signature sequences and chip interleaving. Thus, it combines the advantages of single and multi-carrier transmission such as low peak to average power ratio, orthogonality of the signals of different users even for transmission over a time dispersive channel and low complexity. Simulation results show the good performance of coded IFDMA transmission over a mobile radio channel for different data rates

The interleaved frequency division multiple access (IFDMA) scheme is a promising candidate for next generation mobile radio systems. IFDMA is based on compression, repetition and subsequent user dependent frequency shift of a modulated signal. As in OFDMA, multiple access is enabled by the assignment of overlapping but mutually orthogonal subcarriers to each user. It combines the advantages of single and multicarrier transmission such as low peak to average power ratio, orthogonality of the signals of different users even for transmission over a time dispersive channel and low complexity. In this paper, a linear low complexity frequency domain equalizer for IFDMA is presented and extended by subsequent decision feedback with initialization. Simulation results show the good performance for data transmission using IFDMA with frequency domain equalization over a mobile radio channel

In this work, we study a MIMO system employing quasi-orthogonal space-time block codes (QSTBC) at the transmitter with an MMSE receiver in a Ricean flat-fading environment. We assume that the receiver has perfect CSI and the transmitter knows only the mean channel matrix either by feedback or channel estimation. We analyze the impact of the mean component on the MSE, the optimal transmit strategy and the optimal power allocation. Finally, we derive some bounds on the error rate performance for different scenarios with the MMSE receiver.

In this paper, an interference avoiding allocation strategy is proposed to reduce the effects of interference between user terminals (UT) and between access points (AP) in a mobile network operating with time division duplex (TDD). The performance of the strategy is analysed using a simplified two cell scenario. The results show an improvement in interference levels for crossed slots but at the expense of increased interference in uncrossed slots.

Multi carrier (MC) transmission is considered the leading candidate for a new air interface for fourth generation wireless systems. In order to enable a flexible exploitation of the envisaged large transmission bandwidths, a sub-division of the available bandwidth into sub-bands through FDMA (frequency division multiple access) is seen as a very promising solution. Within each sub-band, an MC-multiple access system can be accommodated. Recently, an efficient adaptive sub-band allocation (ASBA) approach has been shown to provide a significant performance gain in such systems. A system design method is proposed that yields, for each propagation scenario, the proper number of sub-bands, such that a given performance gain can be attained through ASBA.

We study the low complexity turbo detector for cyclic prefix based single-carrier block transmission (CP-SCBT) systems with multiple receive antennas. It has the following characteristics: firstly, it can be implemented by FFT/IFFT with low complexity; secondly, along with SISO decoder, turbo detection can be applied. Simulation results show that the iterative receiver provides much better performance than the conventional CP-SCBT system. It also outperforms its coded OFDM counterpart. The most attractive advantage is that it can be implemented by FFT/IFFT without significantly increasing the complexity of the system.

In this paper, we investigate some space-time joint channel estimation (JCE) schemes in filtered multitone (FMT) based multicarrier multibranch (multiuser and/or multiantenna) systems. Each subcarrier is proposed to be processed separately due to the spectral characteristics of the FMT signals On each subcarrier, the wireless channels experienced by all transmit branches are estimated jointly at the receiver. The least square (LS) space-time JCE and weighted least square (WLS) schemes are derived. The constant amplitude zero auto-correlation (CAZAC) code based pilots are proposed for the aforementioned multicarrier applications to reduce the mean square estimation error. Simulation results are provided to validate the theoretical analysis.

In this paper, a coding scheme with M-ary Walsh-Hadamard (WH) orthogonal modulation is considered for multi-carrier (MC) transmission. A novel pilot-assisted channel estimation approach is proposed, that efficiently exploits the properties of WH codes. Firstly, simulation results on the single-user performance show the influence of the energy distribution between pilot and information symbols. Then, the impact of the channel estimation method is assessed for different system loads in a MC code division multiple access (MC-CDMA) uplink scenario.

For orthogonal frequency-division multiplexing (OFDM) systems with pilot-symbol-aided channel estimation, a straightforward way to overcome system throughput deterioration is by decreasing the power and reducing the number of pilots, which unfortunately, leads directly to the degradation of BER performance. For the purpose of improving the BER performance, an iterative joint channel estimation and symbol detection algorithm with low complexity and fast convergence is proposed. Both theoretical analysis and computer simulation show that this algorithm gives better BER performance and saves the overhead of OFDM systems.

Multi-carrier (MC) transmission is seen as the leading candidate for beyond 3G mobile communications. We focus here on the MC scheme known as filtered multi tone (FMT) modulation. We investigate its suitability for an asynchronous uplink multi-user scenario, under the hypothesis of frequency division multiple access (FDMA). In particular, we show the influence of the filter spectral characteristics and the effects of introducing some redundancy in bandwidth through non-critical sampling. Furthermore, we compare the achievable performance with that of an OFDMA system over an AWGN channel in terms of bit error rate.

The sparsity of multipath fading channel is introduced, and then the improved MMSE channel estimation for orthogonal frequency-division multiplexing (OFDM) system is presented. With computationally simple algorithm, the proposed channel estimator extracts the channel information from its noisy version accurately by utilizing the correlation of the taps of finite impulse response (FIR) filter channel model. Simulation results show that the proposed channel estimation gives much better performance than LS and DFT-based MMSE channel estimator.

A novel data multiplexing scheme is presented. This technique uses an antenna array with K transmit antennas. In. total K information bits are multiplexed in an orthogonal fashion. The special property of the multiplexing technique is that, for each symbol duration, only one out of K antennas is transmitting using BPSK modulation. Furthermore, the symbol duration is equivalent to the bit-duration due to the parallelism introduced by the K antennas. The receiver is able to detect the transmitting antenna and by using this information the demultiplexing is carried out. It is demonstrated that the proposed method achieves about the same spectral efficiency as 8PSK, but with a relaxed bit-energy-to-noise ratio of about 2.5 dB. Furthermore, the linear channel capacity gain (error-free transmitted bits) over the single antenna BPSK transmission scheme is found to be in the range of 2.5-3.

Space-time block code (STBC) has a good BER performance. In 3GPP FDD specifications, space time transmit diversity (STTD) employs STBC to maintain orthogonality between the two antennas in order to avoid self-interference in fading channels. However, the capacity of multiuser STBC systems is still severely affected by multiple access interference (MAI) and interpath interference (IPI). Fortunately, nonlinear parallel interference cancellation (NPIC) can effectively reduce MAI and IPI with simple signal processing. And for the small spreading factor systems, weighted NPIC (WNPIC) technique provides a good solution. In order to alleviate the influence of the MAI and IPI of multiuser STBC systems under the frequency-selective Rayleigh fading channel, WNPIC based space-time block code (WNPIC based STBC) scheme is proposed firstly in this paper. And then the error probability of the WNPIC based STBC is analyzed theoretically. From our analyses, RAKE based STBC can be regarded as 0th stage WNPIC based STBC whose weighted factors are one. Later, to further evaluate the proposed scheme and verify our analyses, some simulation resulted are achieved. These simulation results show us the optimum weighted factors under different user environments and the multiuser BER performance of simple (RAKE) STBC systems, NPIC based STBC, WNPIC based STBC systems.

This paper analyzes the performance of the adaptive modulation system over Nakagami-m fading channels, which utilizes four modulation modes. The optimization criterion of the setting of the modulation switching levels is to maintain the target BER which is desired by many data services. Under this condition, the performance analysis of the throughput and the transmission outage probability of the adaptive modulation system are presented. We do so by adopting the numerical analysis methods. Finally, the simulation results show that the adaptive modulation system can theoretically achieve good performance with high throughput and low transmission outage probability.

As an important space-time code, the layered space-time (LST) code has been studying widely since it was firstly proposed by Foschini in 1996. To exploit its potential, the Bell Lab Layered Space-Time (BLAST) structure of the LST was proposed by Bell Lab. There are two types of BLAST architectures: vertical BLAST (V-BLAST) and diagonally BLAST (D-BLAST). Detection algorithms of V-BLAST were proposed by Golden (see Electronics Letters, vol.35, no.1, 1999). His detection process uses linear combination nulling and successive symbol cancellation (SSC) based on inversing and ordering. However, inversing (Moore-Penrose pseudoinverse) and ordering operation for each iteration bring a huge computation complexity. To detect M transmit antenna signals, Golden's detection algorithm needs M inversing and M ordering operations. Aiming at this shortcoming, a new detection algorithm for layered space-time code is proposed. This new sub-optimal detection algorithm is based on Greville inversing process, with two inversing and one ordering process. Simulation results show us the proposed sub-optimal scheme still has a good BER performance.

This paper discusses some issues in designing space-time trellis codes of high complexity, i.e., when the total number of candidate codes is large or the total number of pairwise error events is large. By adding windows in the search domain and combining the equal states in error state trellis, the complexity of search is greatly reduced. And at the same time, we can still find some codes which have very good performance. We present some new codes of high complexity at the end of this paper and show that our codes have better performance than many codes before.

The orthogonal frequency-division multiplexing (OFDM) modulation format has been proposed in Europe as the standard for broadcasting both audio and television digital signals and for wide-band wireless communication systems (e.g., HIPERLAN II). The performance of the OFDM scheme is severely affected by the nonlinearity of the high-power amplifier at the transmitter end and by the phase noise of the oscillators. In this paper, we investigate the joint effects induced on the OFDM signal by the amplifier nonlinearity and by the phase noise. An accurate statistical description of each contribution to the signal distortion is provided. Theoretical results show perfect agreement with those obtained by simulation and they can be used to derive the OFDM system performance, without the need to run extensive simulations.

The third generation cellular communications standard (UMTS) will be based on wideband code division multiple access technology. In this paper the conventional RAKE receiver architecture is compared to the chip matched filter (CMF). Consider that performance of both RAKE and CMF is highly dependent on the channel impulse response estimate which is typically carried out by a correlation method for the RAKE. The CMF instead allows for the more powerful least squares (LS) method. Indeed, due to the presence of interference and noise, better results are obtained by considering only the coefficients of the channel impulse response with higher amplitude. Hence, two new structures, the reduced chip matched filter (R-CMF) and the selective RAKE (S-RAKE) architecture have been considered. For a typical UNITS environment we compare the performance of CMF, R-CMF and S-RAKE both in terms of signal to estimation error (SER) and bit error rate (BER) and show that R-CMF outperforms both S-RAKE and CMF.

Space-time code techniques have been widely studied in wireless communications, for they provide better signal qualities in fading channel environments and increase the system capacity. As an important space-time code, Bell Lab Layered Space-Time (BLAST) code has been paid more attention. However, the traditional detection algorithm of V-BLAST space-time code needs much time to perform linear combination nulling and successive symbol cancellation. The greater the number of transmit antennas, the greater the time delay. To overcome this disadvantage, a new scheme based on linear combination nulling and parallel symbol cancellation is proposed. The new scheme has a lower time delay. The simulation results of this new scheme are obtained by COSSAP simulation in flat Rayleigh channels. These simulation results are presented to verify the BER performance of the new scheme based on parallel symbol cancellation.