We devise a centralized node localization algorithm, which relies on linearization and employs the orthogonal matching pursuit algorithm at its core, albeit with modifications and additional steps in order to address the localisation problem. The main feature of the algorithm is that it can determine the position of more than one simultaneously transmitting unknown nodes. This renders it generic and efficient in terms of power consumption and use of time and frequency resources. Moreover, efficiency and computational complexity can be traded off for improved localization accuracy.

Motivated by applications such as battery-operated wireless sensor networks (WSN), we propose an easy-to-implement low-complexity two-way relaying scheme. In particular, we address the challenge of improving the standard two-way selective decode-and-forward protocol (TW-SDF) in terms of block-error-rate (BLER) with minor additional complexity and energy consumption. By following the principle of soft relaying, our solution is the two-way one-bit soft forwarding (TW-1bSF) protocol in which the relay forwards the one-bit quantization of a posterior information metric about the transmitted bits, associated with an appropriately designed reliability parameter. In WSN-related standards (such as IEEE802.15.6 and Bluetooth), block codes are adopted instead of convolutional and other sophisticated codes, due to their efficient decoder hardware implementation. As the second main contribution, we derive tight upper bounds on the BLER performance for both TW-SDF and TW-1bSF, when the two-way relaying network employs block codes and hard decoding. As a valuable tool for the BLER analysis, we introduce a new code-theoretic performance metric, named sphere partition function (SPF). The error probability analysis confirms the superiority of TW-1bSF. Moreover, we derive the asymptotic performance gain of TW-1bSF over TW-SDF, which further suggests that the proposed protocol is a good choice, especially when long block codes are used.

The increasing availability of low-cost hardware along with the rapid growth of wireless devices has enabled the development of Wireless Multimedia Sensor Networks (WMSNs). Multimedia content such as video and audio streaming is transmitted over a WMSN which can easily be deployed with low cost. However, enabling real-time data applications in those networks demands not only Quality of Service (QoS) awareness, but also efficient energy management. Sensor network devices have limited energy resources. The limited energy poses significant threats on the QoS of WMSNs. In this paper, to improve the efficiency of QoS-aware routing, we examine an angle-based QoS and energy-aware dynamic routing scheme designed for WMSNs. The proposed approach uses the inclination angle and the transmission distance between nodes to optimize the selection of the forwarding candidate set and extend network lifetime. Simulation results indicate that considerable lifetime values can be achieved.

A cognitive Blind Interference Alignment scheme is devised for use in macro-femto cellular networks. The proposed scheme does not require any channel state information at the transmitter or data sharing among the Macro Base Station and the Femto Access Points. It achieves transmission to femto cell users without affecting the rates of the Macro users. This is achieved by appropriately combining the supersymbols of the Macro Base Stations and the Femto Access Points. It is shown that in some scenarios the use of this scheme results to considerable rates for Femto users.

A cooperative blind interference alignment (BIA) strategy is considered for the downlink of cellular systems. The aim is to reduce intercell interference in order to protect users, especially at the cell edge. The strategy consists of appropriately splitting the available bandwidth and is shown to be well-suited to scenarios where the number of cell-edge users is considerable. For a system comprising two cells each with a base station of Ntantennas, it is shown that, compared to a previous augmented code approach where transmission to all users occurs in the same frequency band, the proposed strategy leads to better rates over a wide range of signal-to-noise ratios when the number of cell-edge users in both cells exceeds 2Nt-1.

Subject monitoring and tracking is one of the most appealing classes of applications for Wireless Sensor Networks (WSNs). Numerous inch scale nodes can sense, collect and distribute crucial information with low deployment cost. For instance, the movement of endangered species in a national park can be monitored with a WSN. However, in such applications privacy issues can jeopardize the successful deployment of the network. An adversary might trace the network traffic over the same paths and eventually locate the source in the network. In this paper, we propose a source-location privacy scheme that employs randomly selected intermediate nodes based on inclination angles. The introduced Angle-based Dynamic Routing Scheme (ADRS) is analysed and is compared with the Phantom Single-path Routing Scheme (PSRS). Simulation results demonstrate that ADRS improves the safety period and the packet latency.

We present a novel low-complexity technique that obtains the Physical-Layer Network Coding (PNC) equation coefficient vectors for the two-way relay channel when Compute-and-Forward is employed. The proposed method is based on pre-computed look-up tables that are used for all channel realizations. It is shown that the size of the look-up tables can be made small by taking into account the statistics of the channel coefficients as well as power and performance specifications. Moreover, a low-complexity algorithm is developed for efficient real-time selection of the equation coefficient vectors using the instantaneous channel coefficients and the look-up tables. Although the method may at times exclude some candidate vectors from the search space, simulation results indicate that the effect on the achievable computation rate at the relay is very small. Hence, significant complexity reduction is achieved, while the computation rate remains extremely close to the optimal value.

This paper investigates an uplink multi-cell processing (MCP) model where the cell sites are linked to a central processor (CP) via noiseless backhaul links with limited capacity. A simple compress-and-forward scheme is employed, where the base-stations (BSs) quantize the received signals and send the quantized signals to the CP using distributed Wyner-Ziv compression. The CP decodes the quantization codewords first, then decodes the user messages as if the users and the CP form a virtual multiple-access channel. This paper formulates the problem of maximizing the overall sum rate under a sum backhaul constraint for such a setting. It is shown that setting the quantization noise levels to be uniform across the BSs maximizes the achievable sum rate under high signal-to-noise ratio (SNR). Further, for general SNR a low-complexity fixed-point iteration algorithm is proposed to optimize the quantization noise levels. This paper further shows that with uniform quantization noise levels, the compress-and-forward scheme with Wyner-Ziv compression already achieves a sum rate that is within a constant gap to the sum capacity of the uplink MCP model. The gap depends linearly on the number of BSs in the network but is independent of the SNR and the channel matrix.

In this paper, the achievable rate of distributed antenna systems (DASs) in high buildings is investigated for indoor downlink high data rate wireless transmission. A DAS, where radio transmission/reception within one or several neighbouring floors is controlled by a central unit (CU) exploiting the entire system bandwidth is considered. The same frequency channels are reused by all the CUs, co-channel interference is caused. A mathematical channel model for transmission to one user per floor including direct signal propagation between floors within the building and reflection from a nearby building is built based on practical measurements. Using the model, it is shown that the indoor path loss exponent and the Nakagami channel fading parameter have a significant effect on the achievable rate performance.

The articles in this special issue focus on the technology and applications supported by virtual multiple antennas, or VMIMOs. The impetus for this has been spurred by the strong desire to understand VMIMO, which is a rapidly growing research area. VMIMO is believed to be a key technology for beyond 4th generation mobile communications technologies (B4G). It enables one to make use of all the neighboring terminals and amortize the cost of multiple antennas; hence, a large MIMO channel can be created to increase capacity significantly as well as improve error rate performance. Nevertheless, fundamental roadblocks need to be addressed in order to take full advantage of VMIMO.

Over the last few years high data rate wireless transmission has gained considerable attention in hot spot areas, including high buildings. It has been demonstrated that distributed antenna systems (DASs) improve the performance of the indoor environment by covering dead spots. In this paper, the achievable rate is investigated for indoor high data rate wireless transmission in high buildings when DASs are employed. In the system, radio transmission/reception within one or several neighbouring floors is controlled by one central unit (CU) exploiting the entire system bandwidth. In order to efficiently use the spectrum, the same frequency channels are reused among floors controlled by different CUs. The radio signals in high buildings can propagate vertically and reach the neighbouring floors. Therefore, the performance of the system is limited by cochannel interference. Direct propagation inside the building and reflection from nearby buildings are considered in the channel model. The achievable rate of the system is evaluated through analysis and simulation. The impact of representative system parameters on the achievable rate, including the number of remote antenna units (RAUs), the frequency reuse factor, the floor penetration loss, the path loss exponent, and the Nakagami fading parameter, is discussed.

Transmission with discrete constellations is considered over the two-user Gaussian interference channel. % under strong interference. The boundaries between the very strong, strong and weak interference regime are examined. Achievable regions are then found for the two-user Gaussian interference channel under strong interference for rates that can be attained using receivers of small complexity.

This paper addresses the design of optimal and near-optimal detectors in an interference channel with fading and with additive white Gaussian noise (AWGN), where the transmitters employ discrete modulation schemes as in practical communication scenarios. The conventional detectors typically either ignore the interference or successively detect and then cancel the interference, assuming that the desired signal and/or the interference are Gaussian. This paper quantifies the significant performance gain that can be obtained if the detectors explicitly take into account the modulation formats of the desired and the interference signals. This paper first describes the optimal maximum-likelihood (ML) detector that minimizes the probability of detection error for a given modulation scheme, and the joint minimum-distance (MD) detector, which is a lower-complexity approximation of the ML detector. It is then demonstrated by analysis and by simulation that in an AWGN channel, while interference-ignorant and successive interference cancellation detectors are both prone to error floors, the optimal ML and joint MD detectors are not. This paper further analyzes the performance of joint detection in a Rayleigh fading environment. It is demonstrated that the joint detector can achieve symbol error rates that have the same dependence on the received signal-to-noise ratio (SNR) as if the channel were interference free. Thus, the performance of joint detection is fundamentally limited by the SNR rather than the signal-to-interference ratio (SIR). Moreover, the joint detector enables the use of transmit diversity schemes to achieve the same diversity order as in the absence of interference. These results show that the use of interference-aware detectors can significantly alleviate the effect of interference thereby improving the achievable rates and the reliability of future wireless systems.

In the paper it is shown that, similar to the Gaussian multiple access channel (MAC) that was considered in previous work, the gap approximation can also be used for the Gaussian broadcast channel (BC) to analyze transmission of discrete constellations using superposition. The values of the gap for the Gaussian BC are different than for the Gaussian MAC and depend on the powers of the noises at the receivers. Pulse Amplitude Modulation (PAM) is employed as an example, and it is demonstrated how the gap for the single-user AWGN channel should be modified in the case of the Gaussian BC. Moreover, even when discrete constellations are employed, similar to the capacity-achieving case, it is possible to simplify the implementation of the receivers using Successive Interference Cancellation (SIC) without penalty in performance.

This paper addresses the design of optimal and near-optimal detectors for a practical interference channel scenario where the transmitters employ discrete modulation schemes. The conventional detectors, which either ignore the interference or successively detect then cancel the interference, typically assume that the desired signal and/or the interference are Gaussian. This paper proposes detectors that explicitly take into account the modulation formats of both the desired signal and the interference. The optimal maximum-likelihood (ML) detector that minimizes the probability of detection error for a given set of modulation schemes is derived first. A joint minimum-distance detector (MD) is then presented as a low-complexity approximation of the optimal ML detector. It is demonstrated by analysis and by simulation that the proposed detectors can significantly outperform their conventional counterparts. In particular, while the interference-ignorant and the successive interference cancellation detectors are both prone to error floors, the proposed optimal ML and joint MD detectors are not.

The applicability of the gap approximation is investigated in the context of Gaussian multi-user channels where suboptimal transmission schemes are employed. In this case, the assumption of all users being subject to Gaussian interference may not always hold. Considering the two-user Multiple Access Channel as an example, and assuming uncoded Pulse Amplitude Modulation (PAM), it is shown that the gap approximation can be used, albeit in a modified fashion compared to the single-user Gaussian channel. It is also demonstrated that successive interference cancellation at the receiver does not cause performance degradation when PAM is used by all transmitters, similar to the case of capacity-achieving transmit waveforms.

In the paper, we propose a conditionally biased linear estimator to estimate a deterministic vector h subjected to a linear transformation M and corrupted by zero-mean additive white Gaussian noise, w. We begin by drawing inspiration from the rank shaping idea of the Rank Shaped Least Squares (RSLS) estimator. We then incorporate the estimator into the Covariance Shaping Least Squares (CSLS) framework, which results in inheritance of attractive properties, such as achieving the Cramer-Rao Lower Bound for biased estimators (B-CRLB) and conditional dominance over the Least Squares Estimator (LSE). We further introduce a bias control parameter that not only allows elegant control of the trade-off between the bias and the MSE floor, but also helps in elucidating the common framework that binds the proposed estimator, RSLS, CSLS and LS estimators. We conclude by applying the proposed estimator for pilot-aided channel estimation in OFDM systems.

In this work an efficient low-complexity user selection algorithm for the multiuser MISO downlink wireless channel is presented. The algorithm combines Zero-Forcing linear precoding with low-complexity correlation-based Space Division Multiple Access (SDMA) and aims at maximizing the sum rate of all system users. The complexity of the algorithm is analyzed and is shown to be lower than previously proposed schemes. Moreover, simulation results indicate that the performance penalty is small.

The implementation of decision feedback equalization-based receivers for MIMO systems that employ hybrid ARQ (HARQ) is considered. It is shown how the theory of MIMO decision feedback equalization can be applied to derive optimal DFE architectures for the case of MIMO HARQ. Incremental structures are proposed that reduce the implementation complexity of the DFE and the memory requirements at the receiver without loss in performance. Moreover, sub-optimal, post-equalization combining architectures are examined that simplify the receiver at the cost of performance loss.

Hybrid Automatic Repeat reQuest (HARQ) schemes are becoming common in wireless systems. In addition to the channel characteristics, several system design considerations need to be taken into account when choosing the particular HARQ scheme to be used. For SISO systems, it has been shown that the coding gain of Incremental Redundancy (IR-) HARQ over Chase Combining (CC-) HARQ may be offset by adverse fading channel conditions. In this paper it is shown that, for MIMO systems, IR- HARQ may also be affected by practical complexity constraints on the implementation of the receiver that dictate the use of bit-level combining. In some cases the coding gain advantage of IR-HARQ may be offset by the inability to perform symbol combining in the receiver as opposed to the case when CC-HARQ is employed. A system compliant with the IEEE 802.16e standard that employs a Convolutional Turbo Code is used to demonstrate an example of such performance loss.

When the channel state information is known to both the transmitter and the receiver, opportunistic transmission over fading AWGN channels can lead to increased rates in the low SNR region. Constraints on the transmission delay limit this gain and result in non-zero outage probability. In this article, in addition to a delay constraint, it is assumed that, while the transmitter and the receiver can estimate the channel perfectly, the transmitter can not know a priori the future channel states. Two transmission schemes are examined. The first scheme performs waterfilling with virtual future channel gains, whereas the second one only allocates power when the channel gain exceeds a threshold. The performance of the two schemes is evaluated in terms of the trade-off between the attainable average rate and the probability of outage for flat i.i.d. Rayleigh fading.

For orthogonal frequency division multiplexing (OFDM) systems, the carrier frequency offset (CFO) between the carrier frequencies of the received signal and the receiver local oscillator is represented as the sum of the integer part and the fractional part by normalizing the CFO with subcarrier spacing. This paper focuses on the pilot-aided estimation of the channel gain and the integer part of the normalized CFO OFDM systems. For multipath channels, a simple estimator of the channel gain and the integer CFO is described, and the joint maximum likelihood (ML) estimator is developed. It is proved that the simple estimator becomes equivalent to the joint ML estimator when two OFDM symbols are observed. For the AWGN channel, this paper derives both the ML estimator of the integer CFO only and the joint ML estimator of the integer CFO and the phase offset. It is proved that the integer CFO estimation part of the joint ML estimator is the same as the ML estimator of the integer CFO only, and it is also shown by simulation that the ML estimator outperforms existing estimators significantly.

This letter analyzes the effect of the carrier frequency offset on orthogonal frequency division multiplexing (OFDM) systems for multipath fading channels. A simple approximate expression for the average signal-to-noise ratio (SNR) is derived. This approximate expression is shown to be an upper bound of the average SNR for flat fading channels and an exact expression for the AWGN channel. The approximate average SNR expression is validated using Monte Carlo simulation for both flat fading channels and frequency-selective fading channels

This paper analyzes the effect of phase noise on orthogonal frequency division multiplexing (OFDM) systems with a focus on time-direction (TD) differential receivers. Both Wiener phase noise and wide-sense stationary (WSS) phase noise are considered. A signal-to-noise ratio (SNR) expression for TD differential receivers is derived in the presence of phase noise and is compared to that for coherent receivers, common phase error correction (CPEC) receivers, and frequency-direction (FD) differential receivers. It is shown that Wiener phase noise affects TD differential receivers more than CPEC receivers and FD differential receivers. On the other hand, for WSS phase noise, the performance of TD differential receivers depends on the shape of the power spectral density of the phase noise and the subcarrier bandwidth. When the power of the phase noise in one subcarrier bandwidth is much smaller than the total power of the phase noise, the performance of TD differential receivers is comparable to that of FD differential receivers

In orthogonal frequency division multiplexing (OFDM) systems, the carrier frequency offset can he normalized by the subcarrier bandwidth and then he divided into the integer and the fractional part. This paper focuses on the estimation of the integer carrier frequency offset and derives approximate maximum likelihood (ML) estimators in the presence and in the absence of pilot subcarriers. Simplified approximate ML estimators with reduced complexity are also proposed. The performance of the proposed estimators is compared with that of existing estimators, and it is shown by simulation that the proposed estimators outperform existing estimators.

This paper presents blind symbol timing estimators in orthogonal frequency division multiplexing (OFDM) systems for the additive white Gaussian noise (AWGN) channel. Depending on the performance measure, different optimal estimators can be devised. The existing maximum likelihood estimator is optimal in the sense that it maximizes the probability that the symbol timing estimation error is zero, but it does not guarantee that the error will be small when the estimate is incorrect. When maintaining small error magnitude is critical, the minimum mean square error estimator can be developed with the objective of minimizing the mean square error. In some practical scenarios, the estimation error needs to be defined as the distance between the starting point of the fast-Fourier transform window and the region consisting of the cyclic prefix samples and the first useful sample. With this modified definition of the estimation error, the minimum failure probability estimator and the minimum modified mean square error estimator can be derived. The performance of the proposed estimators is compared with that of the maximum likelihood estimator using simulation.

The paper presents an exact analysis of the effect of the carrier frequency offset on orthogonal frequency division multiplexing (OFDM) systems for a general multipath fading channel. As is well known, the carrier frequency offset attenuates the desired signal and causes intercarrier interference, thus reducing the signal-to-noise ratio (SNR). The SNR degradation due to the carrier frequency offset is evaluated by deriving the exact SNR expression in the presence of the carrier frequency offset. The SNR analysis can be used for the design of a practical OFDM system in determining how small the frequency offset should be in order to maintain the SNR degradation to negligible levels.

In order to obtain high transmission rates DSL systems have to cope with stationary and non-stationary disturbances, as well as the non-ideal characteristic of the twisted pair channel. To mitigate non-stationary interference, a forward error correction (FEC) scheme is employed that consists of a Reed-Solomon outer code and interleaving. By augmenting the interleaving delay, protection against increasingly long noise bursts can he achieved. however, when radio frequency ingress (RFI) also affects the subscriber loops, the required interleaving delay may potentially increase to undesired levels. The additional delay that is required for combined impulse noise and RFI protection can be lowered by using more redundant RS codes, or by employing RFI cancellation techniques. In the latter case, additional digital cancellation is also necessary in order to mitigate the effect of RFI on the transmitted data. This paper proposes a digital RFI cancellation scheme for DMT-DSL systems that is based on information exchange between the DSL demodulator and the RS decoder at the receiver. It is shown that the technique can be seamlessly incorporated in current DMT-DSL systems, since no changes are required at the transmitter. The method can partly cancel the effect of RFI, and can help bridge the gap between the required interleaving delay when only impulse noise is present and the increased delay when RFI also affects transmission.

In addition to being designed to successfully cope with stationary noise, crosstalk, and intersymbol interference, digital subscriber loop (DSL) systems need to be shielded from nonstationary disturbances, such as impulse noise and RF interference. Currently, deployed DSL systems achieve protection against nonstationary interference using a combination of Reed-Solomon (RS) codes and interleaving. However, interleaving results in delay. Long delays are undesirable in high-rate systems that support interactive applications. In this study, it is shown that the interleaving delay of DSL systems can be significantly reduced by performing erasure decoding of the RS codewords at the receiver. Three different techniques for determining the erasures are proposed. Use of the techniques results in a reduction of the interleaving delay that is required to mitigate worst-case impulse noise by up to a factor of 2, which is verified by simulation. Moreover, the techniques do not require any changes at the transmitter and therefore guarantee compatibility with currently deployed systems.

Impulse noise is a potential impairment of very-high speed digital subscriber line (VDSL) services. VDSL systems are protected from impulse noise using a combination of Reed-Solomon coding and interleaving. Since there are two competing proposals for VDSL, the first one based on the use of QAM line code and the second one using DMT modulation, it is interesting to compare their performance in terms of immunity to impulse noise. The paper presents a comparison of the behavior of QAM-VDSL and DMT-VDSL in the presence of impulse noise in real configurations. The required interleaving delay to correct all the generated errors for both systems is investigated. It is shown that when no notches are used in the radio-amateur bands, QAM and DMT systems require similar delays in order to achieve the same level of protection against impulse noise. However, when notches are implemented, DMT-VDSL achieves higher rates than QAM-VDSL. Therefore, larger error-correcting code redundancy can be used, which results in lower end-to-end delays for DMT-VDSL systems.

DSL systems achieve high rate transmission through copper pairs traditionally used for voiceband communication. In order to attain high rates, sophisticated system designs are employed to cope with both stationary and non-stationary disturbers. In current DSL systems, protection against nonstationary interference, comprising impulse noise and radio frequency ingress (RFI), is achieved using a forward error correction (FEC) scheme consisting of a Reed-Solomon (RS) code and interleaving. However, interleaving results in increased end-to-end delay. The end-to-end delay can be reduced by taking advantage of the erasure decoding property of RS codes. The paper presents a byte-erasure technique that allows erasure decoding to be employed. The method locates the possibly erroneous bytes in each RS codeword by comparing the square distance between each received carrier symbol of the interleaved stream and the closest point of the constellation to which the symbol belongs. Thus, the interleaving delay can be reduced by a factor up to 2 when impulse noise impairs transmission. The method can also be applied to systems affected by both impulse noise and RFI, since it can distinguish between the two sources of non-stationary interference. Finally, no change is required at the transmitter, and hence compatibility with currently deployed systems is guaranteed. A DMT-VDSL system is used as a particular example of the achieved reduction of the interleaving delay.

The data that is transmitted in DSL system is subject to corruption by impulse noise, i.e., noise bursts of high energy that interfere with the transmitted symbols. As DSL data rates increase the crosstalk mitigation techniques become more sophisticated, impulse noise limits service in terms of rate or delay. Because of the highly non-stationary nature of impulse noise, a combination of interleaving and Reed-Solomon coding is currently used to shield systems from noise burst. This paper presents a modified impulse noise protection algorithm that takes advantage of the improved performance of Reed-Solomon codes when the location of the impaired bytes is known. Without changing the structure of the encoder or the interleaver, it is shown that the delay, or equivalently the overhead due to forward error correction coding, can be reduced without compromising the immunity of the system to impulses. A DMT-VDSL system is used as a particular example of the improvement achieved using byte-erasure.

A significant portion of the end-to-end delay in high-rate DSL systems is due to the impulse noise protection scheme employed in order to shield those systems against random, non-stationary noise bursts of high energy that appear on the copper lines. Systems are protected from impulse noise using a combination of interleaving and Reed-Solomon codes. In order to lower the end-to-end delay without reducing the data rate that is available to the user, one needs to decrease the interleaver depth. This paper presents a way to achieve this reduction without compromising neither the robustness to noise bursts nor the data rate of the system. The proposed algorithm relies on the inner code used by many DSL systems and uses the metric provided by the inner code decoder at the receiver. A DMT-VDSL system is used as a particular example of the achieved reduction of the end-to-end delay.

The paper presents a numerical study of the impact of impulse noise on asymmetric digital subscriber lines (ADSL). Methods for simulating the effect of impulse disturbances on a discrete multitone system are first presented, and actual measured noise bursts are then used for the simulations as if they were deterministic signals, in order to characterize their effects on ADSL systems. It is shown that, while a combination of coding, interleaving, and 6-dB margin is adequate in protecting ADSL systems from isolated impulses, an impulse train with long duration can cause a significant number of error bits in the system. In this case, a tradeoff among the number of error seconds, the maximum reach, and the coding delay must be made.