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High-resolution delay-Doppler estimation is an important requirement for automotive radar systems, especially in multi-target scenarios that require better target separation performance. Orthogonal frequency-division multiplexing (OFDM) is a promising candidate waveform for future intelligent transport networks, since it enables the integration of both radar and communication functionalities. Exploring the dual functionality enabled by OFDM, this paper presents a new cooperative method for high-resolution delay-Doppler estimation. The proposed subspace-based method exploits the combination of both the radar and received communication signals to estimate target parameters. The procedure achieves high-resolution delay-Doppler estimation for both uncorrelated, partially correlated and coherent signals, and enables a significant reduction in the required bandwidth when compared to previous approaches which do not exploit the knowledge of the communication signals. Laboratory measurements at 24 GHz and simulation results demonstrate the efficacy of the proposed method for the estimation of multiple targets.

This paper proposes a hybrid multi-user equalizer for the uplink of broadband millimeterwave massive multiple input/multiple output (MIMO) systems with dynamic subarray antennas. Hybrid subconnected architectures are more suitable for practical applications since the number of required phase shifters is lower than in fully connected architectures. We consider a set of only analog precoded users transmitting to a base station and sharing the same radio resources. At the receiver end, the hybrid multi-user equalizer is designed by minimizing the sum of the mean square error (MSE) of all subcarriers, considering a two-step approach. In the first step, the digital part is iteratively computed as a function of the analog part. It is considered that the digital equalizers are computed on a per subcarrier basis, while the analog equalizer is constant over the subcarriers and the digital iterations due to hardware constraints. In the second step, the analog equalizer with dynamic antenna mapping is derived to connect the best set of antennas to each radio frequency (RF) chain. For each subset of antennas, one antenna and a quantized phase shifter are selected at a time, taking into account all previously selected antennas. The results show that the proposed hybrid dynamic two-step equalizer achieves a performance close to the fully connected counterpart, although it is less complex in terms of hardware and signal processing requirements. published

In this paper, we propose the usage of Generalized Frequency Division Multiplexing (GFDM), a non-orthogonal multicarrier waveform for radar. We presented a novel method that cancels the effect of interference caused by the non-orthogonality of GFDM waveform in the radar processing, thus not affecting the performance of the radar. We show the viability of GFDM for radar with communications systems and the benefits of using it over Orthogonal Frequency Division Multiplexing (OFDM). Finally, we also present GFDM as a solution to mitigate inter-system interference in radar with communications (RadCom) systems, thus showing that GFDM may prove to be a better candidate than OFDM for RadCom applications. This method is validated with simulations and practical measurements at 24 GHz.

In this paper we consider a multipath channel subject to variations in time due to the presence of a Doppler drift which imposes a phase rotation on the original channel impulse response. Accordingly, a decision-directed state-space approach based on the extended Kalman filter (EKF) is proposed to track the channel variation. This approach is particularly efficient since we are able to reduce the number of training symbols used for channel tracking to be as few as two times the number of transmitters. These training symbols are required to initialize the EKF. Furthermore, using a decision-directed approach we are able to provide the EKF with state-observations without additional training symbols. Our approach is specially designed to work with hybrid analog/digital multiple-input multiple-output (MIMO) architectures.

Although millimeter-wave (mmWave) and massive multiple input multiple output (mMIMO) can be considered as promising technologies for future mobile communications (beyond 5G or 6G), some hardware limitations limit their applicability. The hybrid analog-digital architecture has been introduced as a possible solution to avoid such issues. In this paper, we propose a two-step hybrid multi-user (MU) equalizer combined with low complexity hybrid precoder for wideband mmWave mMIMO systems, as well as a semi-analytical approach to evaluate its performance. The new digital non-orthogonal multi carrier modulation scheme generalized frequency division multiplexing (GFDM) is considered owing to its efficient performance in terms of achieving higher spectral efficiency, better control of out-of-band (OOB) emissions, and lower peak to average power ratio (PAPR) when compared with the orthogonal frequency division multiplexing (OFDM) access technique. First, a low complexity analog precoder is applied on the transmitter side. Then, at the base station (BS), the analog coefficients of the hybrid equalizer are obtained by minimizing the mean square error (MSE) between the hybrid approach and the full digital counterpart. For the digital part, zero-forcing (ZF) is used to cancel the MU interference not mitigated by the analog part. The performance results show that the performance gap of the proposed hybrid scheme to the full digital counterpart reduces as the number of radio frequency (RF) chains increases. Moreover, the theoretical curves almost overlap with the simulated ones, which show that the semi-analytical approach is quite accurate.

In this paper, we propose a hybrid multiuser equalizer combined with low complexity hybrid precoder for wideband millimeter-wave massive MIMO systems. We consider a generalized frequency division multiplexing (GFDM) modulation scheme, which is a new digital non-orthogonal multi-carrier modulation, aiming to achieve higher spectral efficiency, better control of out-of-band (OOB) emissions, and obtain a reduction in peak to average power ratio (PAPR) compared to OFDM. At the transmitter side, a simple yet efficient pure analog precoder is applied. At the base station, the hybrid multiuser linear equalizer is optimized by minimizing the mean square error between the hybrid approach and the full digital counterpart. The analog phase shifters have constant amplitude being constant over the bandwidth due to the hardware constraints. The results show that the performance of the proposed hybrid scheme is very close to the full digital counterpart as the number of RF chains increases.

The high capacity demands of 5G communications require multiple input multiple output (MIMO) technologies and millimeter wave communications to enables the required multi-Gbps. The small wavelengths of millimeter wave communications allow packing a huge number of antennas in a small space, enabling massive MIMO schemes, with the associated high capacity gains. The hardware impairments inherent in low complexity hardware implementations are characteristic of these systems. This means strong nonlinear effects throughout the transmission chain that will have negative impact on the system’s performance. In this paper we study the impacts of these nonlinear effects on the overall performance of an OFDM massive MIMO communication employing a hybrid analog/digital architecture. For this purpose, we develop analytical and semi-analytical tools for performance evaluation of nonlinear effects in those systems. These tools are shown to be very accurate. They make it possible to predict the nonlinearities’ penalties in hybrid systems and/or to assess the optimal operation point for a specific nonlinear amplifier.

Most of the previous work on hybrid transmit and receive beamforming focused on narrowband channels. Because the millimeter wave channels are expected to be wideband, it is crucial to propose efficient solutions for frequency-selective channels. In this regard, this paper proposes an iterative analog&ndash, digital multi-user equalizer scheme for the uplink of wideband millimeter-wave massive multiple-input-multiple-output (MIMO) systems. By iterative equalizer we mean that both analog and digital parts are updated using as input the estimates obtained at the previous iteration. The proposed iterative analog&ndash, digital multi-user equalizer is designed by minimizing the sum of the mean square error of the data estimates over the subcarriers. We assume that the analog part is fixed for all subcarriers while the digital part is computed on a per subcarrier basis. Due to the complexity of the resulting optimization problem, a sequential approach is proposed to compute the analog phase shifters values for each radio frequency (RF) chain. We also derive an accurate, semi-analytical approach for obtaining the bit error rate (BER) of the proposed hybrid system. The proposed solution is compared with other hybrid equalizer schemes, recently designed for wideband millimeter-wave (mmWave) massive MIMO systems. The simulation results show that the performance of the developed analog&ndash, digital multi-user equalizer is close to full-digital counterpart and outperforms the previous hybrid approach.

Network densification through the deployment of small cells along the coverage area of a macro-cell, employing massive multiple input multiple output (MIMO) and millimeter-wave (mmWave) technologies, is a key approach to enhancing the network capacity and coverage of future systems. For ultra-dense mmWave heterogeneous scenarios with a massive number of users, one should address both inter- and intra-tier interferences contrary to the low-density scenarios, where the network is mainly noise limited. Therefore, this paper proposes low complex hybrid analog-digital receive and transmit beamforming techniques for ultra-dense uplink massive MIMO mmWave heterogeneous systems to efficiently mitigate these interferences. At the small cells, the hybrid analog-digital receive beamforming/equalizer is computed in a distributed fashion, where the analog processing is performed at the small cell base stations or access points and the digital part at a central unit for joint processing. To optimize the analog part of the hybrid equalizer and the precoders used at the user terminals, we consider as a metric the distance relative to the fully digital counterpart induced by the Frobenius norm. In the optimization problem, apart from the analog constraints usually considered in the homogeneous systems, we further impose the constraints inherent to the distributed nature of the access points. To cancel the inter-tier interference, the digital parts of the precoders employed at the small cell user terminals are designed so that this interference resides in a low-dimensional subspace at the macro base station. The results show that the performance of the proposed hybrid analog-digital precoder/equalizer scheme is close to the fully digital counterpart and is able to efficiently remove both inter- and intra-tier interferences.

This paper presents a new concept of high-resolution direction-of-arrival (DoA) estimation in orthogonal frequency division multiplexing (OFDM) multiple-input multiple-output (MIMO) radar with integrated communication system (RadCom) for automotive applications. High-resolution DoA estimation is an important requirement for automotive radar systems, especially in multi-target scenarios that require higher target separation performance. This paper introduces a subspace-based procedure for high-resolution DoA estimation for closely spaced targets in uncorrelated and partially correlated signals, and allows better DoA estimation in coherent signals. This procedure integrates the use of the radar signal together with the communication signal received from another user (one of the targets to be estimated).

In this paper we consider the use of a single- carrier with frequency-domain equalization (SC-FDE) scheme for the uplink in a cellular radio system. To cope with the doubly-selective nature of the channel we propose a receiver structure for the joint channel equalization and tracking. We use a low-complexity iterative frequency-domain equalizer based on the iterative block decision-feedback equalization (IB-DFE) concept for the channel equalization and an extended Kalman filter (EKF) for the channel tracking procedure. The tracking procedure works by, first, observing the channel through channel estimation based on training blocks and, then, by predicting the channel during data transmission. Alternatively, decision-directed channel estimation can be used also, resulting in an improved performance.

Most previous work on hybrid beamforming millimeter wave (mmWave) communications and massive multiple input multiple output (MIMO) systems focused on narrowband channels. However, it is expected that the mmWave channels will be wideband, and for that reason, it is important to design solutions for frequency-selective channels. Therefore, in this paper, we consider the design of a hybrid analog/digital multiuser linear equalizer for broadband mmWave massive MIMO systems, optimized using the bit error rate (BER) as a metric. For the digital part, a closed-form solution is obtained by showing that the minimum mean square error (MMSE) and minimum BER solutions are identical. The analog part, which is assumed to be the same for all subcarriers, is computed iteratively over the radio frequency (RF) chains, and it is shown that the solution obtained in a previous iteration may be updated inexpensively to obtain a solution for the current iteration. The simulation results show that the performance gap between the proposed hybrid broadband multiuser equalizer and full digital equalizer decreases monotonically and exponentially with the number of RF chains until reaching zero. Moreover, the gap decay rate is faster for sparser mmWave channels, where fewer RF chains are required to achieve the same performance of the full digital architecture.

This paper considers a hybrid two-step space-frequency receiver structure for multiuser uplink millimeter-wave (mmWave) massive MIMO based systems. We adopt constant envelope OFDM (CE-OFDM) since it is a promising modulation technique for future systems, which allows very efficient power amplification. We consider low-complexity user terminals employing a single RF chain and using an analog precoding based only on the average angles of departure of the channel. At the base station a non-linear multi-user equalizer, based on the iterative block decision feedback equalization (IB-DFE) principle, and using a hybrid analog-digital structure, is designed to efficiently remove the inter-user and inter-carrier interferences. This equalizer is designed to minimize the sum of the mean square errors of all subcarriers. Numerical results have shown that the proposed scheme outperforms the linear equalizers. Moreover, performance is already close to the full digital counterpart after only a few iterations.

This paper considers a multiuser broadband uplink massive multiple input multiple output (MIMO) millimeter-wave (mmWave) system. The constant envelope orthogonal frequency division multiplexing (CE-OFDM) is adopted as a modulation technique to allow an efficient power amplification, fundamental for mmWave based systems. Furthermore, a hybrid architecture is considered at the user terminals (UTs) and base station (BS) to reduce the high cost and power consumption required by a full-digital architecture, which has a radio frequency (RF) chain per antenna. Both the design of the UT’s precoder and base station equalizer are considered in this work. With the aim of maximizing the beamforming gain between each UT and the BS, the precoder analog coefficients are computed as a function of the average angles of departure (AoD), which are assumed to be known at the UTs. At the BS, the analog part is derived by assuming a system with no multi-user interference. Then, a per carrier basis nonlinear/iterative multi-user equalizer, based on the iterative block decision feedback equalization (IB-DFE) principle is designed, to explicitly remove both the multi-user and residual inter carrier interferences, not tackled in the analog part. The equalizer design metric is the sum of the mean square error (MSE) of all subcarriers, whose minimization is shown to be equivalent to the minimization of a weighted error between the hybrid and the full digital equalizer matrices. The results show that the proposed hybrid multi-user equalizer has a performance close to the fully digital counterpart.

Millimeter waves and massive multiple-input multiple output (MIMO) are two promising key technologies to achieve the high demands of data rate for the future mobile communication generation. Due to hardware limitations, these systems employ hybrid analog&ndash, digital architectures. Nonetheless, most of the works developed for hybrid architectures focus on narrowband channels, and it is expected that millimeter waves be wideband. Moreover, it is more feasible to have a sub-connected architecture than a fully connected one, due to the hardware constraints. Therefore, the aim of this paper is to design a sub-connected hybrid analog&ndash, digital multi-user linear equalizer combined with an analog precoder to efficiently remove the multi-user interference. We consider low complexity user terminals employing pure analog precoders, computed with the knowledge of a quantized version of the average angles of departure of each cluster. At the base station, the hybrid multi-user linear equalizer is optimized by using the bit-error-rate (BER) as a metric over all the subcarriers. The analog domain hardware constraints, together with the assumption of a flat analog equalizer over the subcarriers, considerably increase the complexity of the corresponding optimization problem. To simplify the problem at hand, the merit function is first upper bounded, and by leveraging the specific properties of the resulting problem, we show that the analog equalizer may be computed iteratively over the radio frequency (RF) chains by assigning the users in an interleaved fashion to the RF chains. The proposed hybrid sub-connected scheme is compared with a fully connected counterpart.

In order to fulfil the high capacity demands of 5G communications, massive MIMO and millimetre wave are two enabling key technologies considered for future wireless communication. Although these technologies allow huge capacity gains, the radio frequency transceivers employed in this systems induce several hardware impairments as noise quantization (due to resolution quantizers), nonlinear power amplifiers, frequency and phase offsets and in-phase and quadrature imbalance, which lead to a strong nonlinear distortion in the transmitted signal. The impact of these nonlinear effects on the overall performance of hybrid analog-digital massive MIMO millimetre wave OFDM systems is studied in this work, highlighting the importance of its knowledge to improve communication quality. Theoretical analysis of the distortion spectral signal in transmission is assessed and validated for these communication systems, using the quasi-Gaussian nature of the input nonlinear signals. The nonlinearity impact in these systems was observed for different scenarios, where the value of saturation level shows to have a strong influence on the overall system performance.

At the present time many studies have been carried out to jointly use the OFDM signal for communication and radar functions, but other waveforms have been shown to be better candidates than OFDM for communications applications. Therefore studies on evaluation of the application of these same signals to radar functions may be necessary. In this paper, we adapted the radar processing of the OFDM signal to be applied in a new Filter Bank Multicarrier (FBMC) scheme, which is shown to be much superior to the OFDM system since it presents the same bit error rate (BER) performance in multipath channels, a much higher spectral efficiency and as shown here, better performance in radar targeting. We evaluated its performance for radar applications with joint communications.

Most of the works consider hybrid fully connected architectures to overcome the constraints of millimeter wave massive MIMO systems. However, these schemes require a one-to-one connection between the RF chains and antennas. In this paper we propose a two-step broadband multiuser equalizer for hybrid sub-connected architectures, which is a more realistic approach for practical systems. The low-complexity user-terminals employ only analog precoders computed from the knowledge of the average angle of departure of each cluster, which is constant over the bandwidth. At the receiver side, we design a hybrid multi-user equalizer by minimizing the average bit-error-rate. A two-step approach is considered, where the analog part is constant over the iterations due to hardware constraints and the digital part is iterative. The analog part is also constant over all subcarriers while the digital part is computed on a per subcarrier basis. The proposed sub-connected based equalizer is compared with the fully connected counterpart. The results show that the performance of the proposed scheme is close to the fully connected one after just a few iterations performed at the digital domain.

Following the recent interest in millimeter wave (mmWave) for wireless applications and the difficulty in having the channel state information at transmitter due to the large amount of data required for feedback, a low complex uniform-based limited feedback technique is proposed, for a mmWave massive MIMO OFDM-based (Orthogonal Frequency Division Multiplexing) system. The wideband mmWave system has both transmitter and receiver equipped with a large antenna array, and employs a hybrid analog-digital precoding and combiner: the digital precoder is applied in the frequency domain and differ from subcarrier to subcarrier, while the analog precoder is frequency flat. The limited feedback strategy employs a uniform-based quantization for channel amplitudes, angle of departure and angle of arrival in time domain. After being fed back, these information is used to reconstruct the overall channel in frequency domain and the transmit antenna array, which are then used to compute the hybrid analog-digital precoders. Numerical results show that the proposed quantization strategy achieves a performance close to the one obtained with perfect full channel with a low overhead and complexity.

This work studies a digital transmission system by fiber optic. We give special relevance to the synchronism block. The fiber is a transmission medium with large bandwidth and low attenuation. These characteristics allow high speed and long distances. The transmission, instead asynchronous, it is synchronous what provides a big efficiency of 100%. However, the synchronous transmission needs a synchronizer. The objective is to study the synchronizers and their performance of output jitter Unit Interval Root Mean Square (UIRMS) versus input Signal Noise Ratio (SNR).

The aim of this manuscript is to propose a new hybrid nonlinear block space-time receiver structure for millimeter wave (mmW) systems, to efficiently separate the spatial streams. We consider a fully connected analog-digital architecture. We assume that both the transmitter and the receiver are equipped with a large antenna array and the number of radio frequency (RF) chains is lower than the number of antennas. The analog and digital parts of the equalizer are jointly optimized using as a metric the mean square error (MSE) between the transmitted data vector and its estimate after the digital equalizer. The specificities of the analog domain impose several constraints in the joint optimization. To efficiently deal with the constraints the analog part is selected from a dic-tionary based on the array response vectors. The performance results have shown that the performance of our hybrid nonlinear space-time equalizer is close to the fully digital counterpart after only a few iterations. Moreover, it clearly outperforms the linear receivers recently proposed for hybrid mmW massive MIMO architectures.

The work in this manuscript proposes to enhance the secrecy level of wireless systems through the use of a jamming technique that uses uncorrelated reciprocal channel estimations from the legitimate channel as a common random source to select discrete jamming signals in both sides of the legitimate link. After that selection, these jamming signals are combined with data at the legitimate transmitter, being posteriorly canceled at legitimate receiver. In order to improve the secrecy level of the proposed scheme, an algorithm that efficiently combines data with discrete jamming signals is also developed. The comparison of the scheme proposed in this manuscript with a jamming technique discussed in [13], allowed to verify that the proposed scheme achieves a level of secrecy that does not fall to zero with the signal-to-noise ratio (SNR) increase, instead it saturates in a positive level becoming independent of the power conditions. The results have also showed that making an efficient selection of the jamming signals that will be combined with data, the level of secrecy can be further improved.

In this paper, we consider the downlink communication of an arbitrary number of transmitters over broadcast interference channels. In order to suppress interference, we investigate a network-level linear precoding scheme dubbed precoding reuse. Such a technique is leveraged on the interference alignment (IA) principle, where the undesired signals overlap in a reduced-dimension subspace at the receivers. Precoding reuse relies on two cascaded precoders to suppress intra- and inter-cell interference and requires feedback only within a cell. We designed precoding reuse schemes for which interference suppression can be attained in a coordinated or uncoordinated fashion over a fixed number of quasi-static channel uses. We provide expressions for the parametrization of the precoders and investigate the advantages, limitations, suitable scenarios, and applications of the proposed IA scheme. Finally, we pointed out the open problems, where the proposed IA scheme and parametrization can be implemented with little effort, demanding minimum changes to an existing cellular system supporting multi-user MIMO.

The combination of millimeter wave (mmW) with massive MIMO is a promising approach to achieve the multi Gb/s required by future wireless systems. Fully digital architectures are not feasible due to hardware limitations, and thus, the design of signal processing techniques for hybrid analog-digital architectures is of paramount importance. In this paper, we propose a new hybrid iterative block space-time receiver structure for multiuser mmW massive MIMO systems. We consider low-complexity user terminals employing analog-only random precoding and a single RF chain. At the base station, a hybrid analog-digital equalizer/detector is designed to efficiently remove the multiuser interference. The analog and digital parts of the equalizer are jointly optimized using as a metric the average bit-error-rate. The specificities of the analog domain impose several constraints in the joint optimization. To efficiently handle these constraints, the analog part is selected from a dictionary based on the array response vectors. We also propose a simple, yet an accurate semi-analytical approach for obtaining the performance of the proposed hybrid receiver structure. The results show that the performance of the hybrid iterative equalizer is close to the fully digital counterpart after only a few iterations. Moreover, it clearly outperforms the linear receivers recently considered for hybrid mmW massive MIMO architectures.

Millimeter waves and massive MIMO are a promising combination to achieve the multi-Gb/s required by future 5G wireless systems. However, fully digital architectures are not feasible due to hardware limitations, which means that there is a need to design signal processing techniques for hybrid analog-digital architectures. In this manuscript, we propose a hybrid iterative block multiuser equalizer for subconnected millimeter wave massive MIMO systems. The low complexity user-terminals employ pure-analog random precoders, each with a single RF chain. For the base station, a subconnected hybrid analog-digital equalizer is designed to remove multiuser interference. The hybrid equalizer is optimized using the average bit-error-rate as a metric. Due to the coupling between the RF chains in the optimization problem, the computation of the optimal solutions is too complex. To address this problem, we compute the analog part of the equalizer sequentially over the RF chains using a dictionary built from the array response vectors. The proposed subconnected hybrid iterative multiuser equalizer is compared with a recently proposed fully connected approach. The results show that the performance of the proposed scheme is close to the fully connected hybrid approach counterpart after just a few iterations.

The knowledge of channel state information (CSI) at transmitter is crucial in precoding techniques to efficiently explore wireless multiuser multi-antenna capabilities namely to mitigate the interference and/or to separate spatial streams. For frequency division duplex systems, the CSI available is a quantized version fed back from the receivers, and the nonlinear effects associated to CSI quantization can lead to significant performance degradation. The knowledge of the statistical characteristics of those nonlinear distortion effects is important for performance evaluation purposes, as well as to design robust precoding techniques. In this paper we consider a general broadband multipath channel sampled in the frequency domain and obtain the analytical statistical characterization of the nonlinear distortion effects associated to CSI quantization. Our analytical approach is valid for non-uniform quantizers, and it also includes quantizer saturation and channel oversampling effects. It provides a detailed statistical characterization in time and frequency domains, associated to the channel impulse and frequency responses, respectively. The derived theoretical statistical characterization of quantization distortion is validated by simulation, showing its high accuracy. As an application, we consider a multicarrier transmission employing a precoding technique with reduced feedback techniques employing a small number of quantization bits per channel sample.

The exploration of the physical layer characteristics of the wireless channel is currently the object of intensive research in order to develop advanced secrecy schemes that can protect information against eavesdropping attacks. Following this line of work, in this manuscript we consider a massive MIMO system and jointly design the channel precoder and security scheme. By doing that we ensure that the precoding operation does not reduce the degree of secrecy provided by the security scheme. The fundamental working principle of the proposed technique is to apply selective random rotations in the transmitted signal at the antenna level in order to achieve a compromise between legitimate and eavesdropper channel capacities. These rotations use the phase of the reciprocal wireless channel as a common random source between the transmitter and the intended receiver. To assess the security performance, the proposed joint scheme is compared with a recently proposed approach for massive MIMO systems. The results show that, with the proposed joint design, the number of antenna elements does not influence the eavesdropper channel capacity, which is proved to be equal to zero, in contrast to previous approaches.

This paper considers a hybrid two-stage space-time receiver structure for uplink millimeter wave (mmW) based systems, where the user terminals (UTs) and the base station (BS) are equipped with a large antenna array and the number of radio frequency (RF) chains is lower than the number of antennas. In the first stage the analog transmit and receive beam-formers/precoders are computed for each UT — BS pair considering multi-user interference free links, to maximize the desired signal power for each UT. In the second one, an iterative block space-time equalizer is designed to explicitly remove the multi-user interference. The designed space-time equalizer is optimized by using as a metric the mean square error (MSE) between the transmitted data vector and its estimate after the digital equalizer. Our performance results have shown that the proposed iterative space-time equalizer outperforms the linear MMSE based receivers for hybrid mmW massive MIMO architectures.

The aim of this manuscript is to propose a new hybrid iterative block space-time receiver structure for millimeter wave (mmW) systems, to efficiently separate the spatial streams. We consider that both the transmitter and the received are equipped with a large antenna array and the number of radio frequency (RF) chains is lower that the number of antennas. The analog and digital parts of the equalizer are jointly optimized using as a metric the mean square error (MSE) between the transmitted data vector and its estimate after the digital equalizer. The specificities of the analog domain impose several constraints in the joint optimization. To efficiently deal with the constraints the analog part is selected from a dictionary based on the array response vectors. Our performance results have shown that the performance of the hybrid iterative space equalizer is close to the fully digital counterpart after only a few iterations. Moreover, it clearly outperforms the linear receivers recently proposed for hybrid mmW massive MIMO architectures.

A vertically integrated approach is presented to evaluate the performance of cognitive radio (CR) systems. The approach consists of three pillars: measurement, modeling, and emulation (MME). This integrated approach enables the reproduction of the radio environment in laboratory conditions and aims to guarantee the same performance results as one would obtain in the field. This article provides a detailed explanation for each pillar along with state-of-the-art overviews. Finally, a test bed based on the MME approach is presented.

The general PLL (Phase Lock Loop) is a device (LOOP) whose VCO (Voltage Controlled Oscillator) is able to follow (LOCK) the input signal phase (PHASE). However, the input signal can be a carrier regular sinusoidal wave, or a symbol/bit/data random stream, or a block of bits. So, in this direction, we have respectively the Carrier Synchronizer (CPLL), the Symbol Synchronizer (SPLL) and the Block Synchronizer (BPLL). The carrier synchronizer can be adapted as synthetizer of frequencies with different carriers. The symbol synchronizer recovers the clock of a synchronous system and can operate at different rhythms. The block synchronizer can produce streams of different bit rates. The three synchronizer prototypes can be adapted to different systems. Other objective is to study the output jitter UIRMS (Unit Interval Root Mean Square) versus input SNR (Signal noise ratio).

This work studies the prefilter bandwidth effects in four asynchronous sequential symbol synchronizers. We consider three prefilter bandwidths namely B1=∞, B2=2.tx and B3=1.tx, where tx is the bit rate. The synchronizer has two variants one asynchronous by both transitions at bit rate and other asynchronous by hybrid (both and positive) transitions at half bit rate. Each variant has two versions namely the manual and the automatic. The objective is to study the prefilter with the four synchronizers and to evaluate their output jitter UIRMS (Unit Interval Root Mean Square) versus input SNR (Signal Noise Ratio). A new approach is presented using a prototype operating by hybrid transitions.

The distributed antenna system concept promises to enhance the capacity and diversity of next-generation wireless communication networks, due to the inherently added micro and macro diversity. In this article we first give an overview of the main benefits of a DAS in relation to a collocated antenna system. Next we study the sum-capacity scaling of a multi-user DAS with the number of jointly processed transmit antennas in the downlink. In a practical system this scaling will have implications on the number of antennas worth jointly processing, since the costs of processing an additional antenna can be higher than the additional benefits obtained. Results show that the most important system property to attain the highest capacity gains is symmetry, and the users that attain the maximum gain are those at cell borders. They also confirm that the main DAS feature that makes possible its gains over the CAS architecture are the additional degrees of freedom/diversity provided by such an architecture, which increase the probability of finding a system state with high symmetry and of each user being near one of the transmit antennas.

This paper proposes two space-frequency schemes with a multi-user pre-filtering technique for downlink (DL) multicarrier code division multiple access (MC-CDMA) systems. We consider the use of antenna arrays at the base station (BS) and a single antenna at the mobile terminal (MT) and derive the proposed multi-user pre-filtering technique that modulates the transmitted signal to eliminate the effects of multiple access interference (MAI) and channel distortions at the mobile terminals, while maintaining low MT complexity. Two types of detectors are considered at the MT: simple despreading and single user equalizers. The performances of the proposed schemes are compared to those of other transmit signal design approaches that have been recently proposed for DL MC-CDMA, considering both typical indoor and pedestrian scenarios, and channel coding based on UMTS specifications.

Constant envelope OFDM (CE-OFDM) is a promising modulation technique for future millimeter wave (mmW) based wireless communications, since it allows low-cost and highly efficient power amplification based on strongly nonlinear amplifiers. The aim of this paper is to design a new iterative space-frequency equalizer, based on iterative block decision feedback equalization (IB-DFE) principle, for multiplexing MIMO CE-OFDM based systems. The equalizer is designed to take into account both inter-block and residual inter-carrier interferences by efficiently exploit the space-frequency diversity order inherent to these systems. The matrices for this iterative space-frequency equalizer are obtained by minimizing the overall mean square error (MSE) of all data streams at each subcarrier. Our iterative scheme significantly outperforms the linear frequency domain equalizers, recently considered for CE-OFDM, with only a few iterations.

In this manuscript, we consider the uplink of a single carrier frequency division multiple access (SC-FDMA) based system, where a set of small-cells coexist with a macro-cell using the same spectrum. To deal with both the inter-carrier interference (ICI) and inter-system interference, we combine interference alignment (IA) precoding at the small-cell transmitters with iterative decision feedback equalization at the macro-receiver. The transmitter and receiver design is performed jointly by considering both full-coordination and limited inter-system information exchange between macro- and small-cells. At the small cells side, we assume that the access points are connected through a limited capacity backhaul network to a central unit, where the separation of a quantized version of the small-cell signals is performed. For this case, the iterative feedback equalizer is designed by explicitly taking into account the impact of signal quantization. Moreover, a semi-analytical approach for the performance of the proposed schemes is provided. The results show that the proposed schemes are robust to both inter-system and inter-carrier interferences and thus are able to efficiently separate the macro- and small-cells spatial streams under limited information exchange.

Mobile traffic in cellular networks is increasing exponentially, mainly due to the use of data intensive ser- vices like video. One way to cope with these demands is to reduce the cell-size by deploying small-cells along the coverage area of the current macro-cell system. The de- ployment of small-cells significantly improves indoor cov- erage. Nevertheless, as additional spectrum licenses are difficult and expensive to acquire it is expected that the macro and small-cells will coexist under the same spec- trum. The coexistence of the two systems results in cross- tier/inter-system interference. In this context, we design several interference alignment based techniques for the downlink of heterogeneous networks, in order to cancel the interference generated from macro-cell at small-cell user terminals. More specifically, in this contribution we design interference alignment methods under different levels of inter-system coordination and the constraint that the per- formance of macro-cell system is kept close to the case where small-cell system is switched-off. Numerical results demonstrate that the proposed methods achieve close to the optimal performance with low overhead.

Fixed wireless systems represent an attractive alternative to provide broadband access to the home. However since radio spectrum is scarce and expensive an efficient usage of it is required. To reach this goal, we propose the transmission of narrowband services using part of the frequency band assigned to a broadband one. The feasibility of this proposal is based on the cyclostationary nature of the information bearing digital signals. A simple receiver that allows the coexistence these two kinds of signals is proposed, and its performance analyzed. The results show that a high interference rejection capability is achieved for the narrowband signals.

Coordinated multipoint (CoMP) from long term evolution (LTE)-advanced is a promising technique to enhance the system spectral efficiency. Among the CoMP techniques, joint transmission has high communication requirements, because of the data sharing phase through the backhaul network, and coordinated scheduling and beamforming reduces the backhaul requirements, since no data sharing is necessary. Most of the available CoMP techniques consider perfect channel knowledge at the transmitters. Nevertheless for practical systems this is unrealistic. Therefore in this study the authors address this limitation by proposing a robust precoder for a multicell-based systems, where each base station (BS) has only access to an imperfect local channel estimate. They consider both the case with and without data sharing. The proposed precoder is designed in a distributed manner at each BS by maximising the signal-to-leakage-and-noise ratio of all jointly processed users. By considering the channel estimation error in the design of the precoder, they are able to reduce considerably the impact of these errors in the system's performance. The results show that the proposed scheme has improved performance especially for the high signal-to-noise ratio regime, where the impact of the channel estimation error may be more pronounced.

Interference alignment (IA) is a promising technique that allows high capacity gains in interfering channels. On the other hand, iterative frequency-domain detection receivers based on the IB-DFE concept (Iterative Block Decision Feedback Equalization) can efficiently exploit the inherent space-frequency diversity of the MIMO MC-CDMA systems. In this paper we combine iterative IA precoding at the transmitter with IB-DFE based processing at the receiver for MC-CDMA systems. The receiver is designed in two steps: first a linear filter is used to mitigate the inter-user aligned interference, and then an iterative frequency-domain receiver is designed to efficiently separate the spatial streams in the presence of residual inter-user aligned interference at the output of the filter. The matrices for this non-linear space-frequency equalizer are obtained by minimizing the overall mean square error (MSE) of all data streams at each subcarrier. Our receiver structure is explicitly designed taking into account the residual inter-user interference, allowing both an efficient separation of the spatial streams and a reduction in the number of iterations of the IA procedure. We also propose a simple, yet accurate analytical approach for obtaining the performance of the proposed receiver structure. Our scheme achieves the maximum degrees of freedom provided by the IA precoding, while allowing an almost optimum space-diversity gain, with performance close to the matched filter bound (MFB).

Interference alignment (IA) is a promising technique that allows high-capacity gains in interference channels, but which requires the knowledge of the channel state information (CSI) for all the system links. We design low-complexity and low-bit rate feedback strategies where a quantized version of some CSI parameters is fed back from the user terminal (UT) to the base station (BS), which shares it with the other BSs through a limited-capacity backhaul network. This information is then used by BSs to perform the overall IA design. With the proposed strategies, we only need to send part of the CSI information, and this can even be sent only once for a set of data blocks transmitted over time-varying channels. These strategies are applied to iterative MMSE-based IA techniques for the downlink of broadband wireless OFDM systems with limited feedback. A new robust iterative IA technique, where channel quantization errors are taken into account in IA design, is also proposed and evaluated. With our proposed strategies, we need a small number of quantization bits to transmit and share the CSI, when comparing with the techniques used in previous works, while allowing performance close to the one obtained with perfect channel knowledge.

Interference alignment (IA) is a promising technique that allows high capacity gains in interfering channels. On the other hand, iterative block decision feedback equalization (IB-DFE) based receivers can efficiently exploit the inherent space-frequency diversity of the MIMO MC-CDMA systems. In this paper we combine IA precoding at the transmitter with IB-DFE based processing at the receiver for MC-CDMA systems. The IA precoding is applied at chip level instead of the data symbols level. The receiver is designed in two steps: first an equalizer based on zero forcing (ZF) is used to remove the aligned users' interference, then and after a whitening noise process, an IB-DFE based equalizer is designed to remove the residual inter-carrier interference. Our proposed scheme achieves the maximum degrees of freedom provided by the IA precoding, while allowing a close-to-optimum diversity gain, with performance close to the matched filter bound (MFB).

Interference alignment (IA) is a promising technique that allows high capacity gains in interfering channels. In this paper we consider iterative IA techniques for the downlink of OFDM-based (Orthogonal Frequency Division Multiplexing) broadband wireless systems with limited feedback. A quantized version of the channel state information (CSI) associated to the different links between base station (BS) and user terminal (UT) is feedback from the UT to the BS, which sends it to the other BSs through a limited-capacity backhaul network. This information is employed by each BS to perform the overall IA design. Our channel quantization method requires much less complexity than random vector quantization based techniques and requires the quantization of a fraction of the channel frequency response samples. The results have shown that a small number of quantization bits per multipath component is enough to allow performance close to one obtained with perfect channel knowledge.

In this paper we propose distributed power allocation schemes for the downlink of distributed precoded multicell MISO-OFDM systems. The precoders are designed in two phases: first the precoder vectors are computed in a distributed manner at each BS considering two criteria, distributed zero-forcing and virtual signal-to-interference noise ratio; then the system is optimized through distributed power allocation with per-BS power constraint. The proposed power allocation schemes minimize the average virtual bit error rate over all user terminals and the available subcarriers. Both the precoder vectors and the power allocation are computed by assuming that the BSs have only knowledge of local channel state information. The performance of the proposed schemes are compared against other power allocation schemes that have been recently proposed for multicell systems.

This work studies the asynchronous sequential symbol synchronizers based on pulse comparison by both transitions at rate (atx). Their performance will be compared with the reference synchronous synchronizers based on pulse comparison by both transitions at rate (stx).

In this paper, a novel and practical transmission scheme for a Multiple Input Multiple Output–Orthogonal Frequency Division Multiple Access (MIMO-OFDMA) relay-based network is proposed and evaluated. We coin the term Virtual Maximum Ratio Transmission (VMRT) for the scheme which obtains diversity and array gain at both links in a two-hop transmission, while keeping the complexity low. Besides, the requirements of Channel State Information needed at the different network elements such as the Base Station, Relay Station or User Terminals are also limited, and so the feedback. The obtained global performance by using proposed VMRT outperforms other schemes. Diversity gains larger than 6 can be easily obtained with a reduced number of relays. For this reason, VMRT is a good candidate to achieve high speed transmission or increase coverage and reliability in slow varying channels for relay-based networks. This work has been partly funded by the Spanish government with projects MACAWI (TEC 2005-07477-C02-02), MULTIADAPTIVE (TEC 2008-06327-C03-02), COMONSENS (CDS2008-00010) and CODIV (ICT-2007-215477). Publicado

In this paper we propose and asses multiuser linear precoding techniques for the downlink of distributed MIMO OFDM systems. We consider a distributed broadband wireless system where the base stations are transparently linked by optical fiber to a central unit. We further assume that both the distributed base stations and the user terminals are equipped with an antenna array. This architecture provides a high speed backhaul channel allowing an efficient joint multiuser multicell processing. The precoder is designed in two phases: first the intercell interference is removed by applying a block diagonalization algorithm. Then the system is further optimized by using a new power allocation algorithm, based on minimization of the sum of inverse signal-to-noise ratio (SNR−1) on each user terminal over the available subcarriers. The motivation to minimize the sum of SNR−1 instead of bit error rate is the fact that the first criterion achieves a closed-form solution, which is more interesting from practical point of view. The aim is to propose a practical distributed precoding technique to remove the intercell interference and improve the user’s fairness at the cell-edges. The performance of the proposed scheme is evaluated and compared with an iterative precoding scheme designed to minimize the bit error rate, extended to the proposed multiuser distributed scenario, considering typical pedestrian scenarios based on LTE specifications. Portuguese CADWIN - PTDC/EEA-TEL/099241/2008