The proliferation of large multi-antenna configurations operating in high frequency bands has recently challenged the conventional far-field, rich-scattering paradigm of wireless channels. Extra large antenna arrays usually work in the near field where the probability of having multipath tends to be low, which are far from traditional assumptions in conventional wireless communication systems. The present study proposes to analyze the spatial multiplexing capabilities of large multi-antenna configurations under line-of-sight, near field conditions by considering the use of multiple orthogonal diversities at both transmitter and receiver. The analysis is carried out using aholographicapproximation to the problem, whereby the number of radiating elements is assumed to become large while their separation becomes asymptotically negligible. This emulates the operation of a continuous aperture of infinitesimal radiating elements, also recently known as holographic surfaces. The present study characterizes the asymptotic MIMO channel as seen by extra large uniform linear and planar arrays, as well as their associated achievable rates assuming access to perfect channel state information (CSI). It is shown, in particular, that for a given distance between the receiver and the center of the array and a given signal quality, there exists an optimum dimension of the multi-antenna surface that maximizes the spectral efficiency.

One of the most relevant challenges in future 6G wireless networks is how to support a massive spatial multiplexing of a large number of user terminals. Recently, extremely large antenna arrays (ELAAs), also referred to as extra-large MIMO (XL-MIMO), have emerged as an potential enabler of this type of spatially multiplexed transmission. These massive configurations substantially increase the number of available spatial degrees of freedom (transmission modes) while also enabling to spatially focus the transmitted energy into a very small region, thanks to the properties of near-field propagation and the large number of transmitters. This work explores whether multiplexing of multiple orthogonal polarizations can enhance the system per-formance in the near-field. We concentrate on a simple scenario consisting of a Uniform Linear Array (ULA) and a single antenna element user equipment (UE). We demonstrate that the number of spatial degrees of freedom can be as large as 3 in the near-field of a Line of Sight (LoS) channel when both transmitter and receiver employ three orthogonal linear polarizations. In the far-field, however, the maximum number of spatial degrees of freedom tends to be only 2, due to the fact that the equivalent MIMO channel becomes rank deficient. We provide an analytical approximation to the achievable rate, which allows us to derive approximations to the optimal antenna spacing and array size that maximize the achievable rate.

Managing a massive number of terminals in a contention-based multiple access is challenging due to its intrinsic limited efficiency. For example, in the random access channel considered in LTE-A and 5G NR, Base Station (BS) is just aware of the collided and non-collided preambles. Several time-based protocols have been investigated to redistribute the overload under high terminal activity, thus avoiding the congestion. In this work, we explore the use of the spatial domain by means of a hierarchical codebook-based beamforming, where the BS selects the appropriate beams as a function of the number of non-collided and collided preambles. Since the activity and placement of terminals may be dynamic over time, the sequential selection of parameters can benefit from a reinforcement learning (RL) framework. We propose an algorithm that can exploit both domains, temporal and spatial, with the goal of reducing collisions and enhancing transmission delay. Our approach is able to efficiently learn whenever there is a non-homogeneous spatial distribution of terminals and adapt the spatial beams accordingly.

We consider a point to point large-scale multipleinput multiple-output (MIMO) system operating in the millimeter wave (mmWave) band and an outdoor scenario. Novel transmit and receive spatial modulation (SM) schemes are proposed for uplink (UL) and downlink (DL) data transmission phases based on a novel energy efficient hybrid user terminal architecture. The analog circuitry of the proposed hybrid architecture is divided into two stages: phase shifters and analog switches. The phase shifting stage assures high gain and overcomes the severe path-loss caused by outdoor mmWave propagation. The analog switching stage smartly allocates the antennas to be used at the phase shifting stage and combats the spatial correlation. We provide the analysis of the spectral efficiency (SE) of the UL and DL systems. Next, we propose a reduced complexity algorithm that jointly optimizes the analog beamformer and combiner design of the UL and DL circuitry to maximize the energy efficiency (EE). Finally, we compare and evaluate the performance of the proposed algorithm in terms of the SE and EE assuming both stochastic and realistic channel models.

In this paper, we consider the downlink of a multi-user multiple-input-multiple-output system operating at the millimeter wave band in an outdoor environment. In this band, receive spatial modulation (RSM) schemes have been shown to achieve good spectral efficiency-energy consumption trade-off for the single-user case by exploiting new transceiver architectures that use a single radio-frequency (RF) chain at the user terminal (UT). In this work, we propose a novel RSM scheme for multiple RF chains at the UT. With the goal of maximizing the spectral efficiency (SE), we consider analog switches at the UT that select which antennas are active. To minimize the power consumption at the BS, we include analog switches that control the ON/OFF status of RF chains such that the SE is higher than a given threshold. Moreover, we extend the RSM concept to multiple users transmission. Specifically, we propose an algorithm that jointly optimizes the number of users, set of antennas and transmit power allocated to each user to maximize the sum SE. Simulation results show that RSM outperforms conventional modulation (no spatial symbols are transmitted) in terms of spectral and energy efficiency. Moreover, the proposed algorithms tightly approach the exhaustive search and outperform the prior art in terms of performance and convergence.

Orthogonal Frequency Division Multiplexing (OFDM)-based networks rely on time synchronization to obtain their best performance. Time synchronization with neighboring nodes can be satisfied by increasing the cyclic prefix (CP) length (at the expenses of spectral efficiency reduction) We show that by optimizing the transmit pre-compensation and receive post-compensation we can meet the time synchronization constraints and keep the CP at its minimum value. This concept is applied to paired-bands Frequency Division Duplexing (FDD) systems which tend to show inefficient occupancy of the uplink (UL)-band due to the traffic asymmetry. We consider the possibility of deploying multiple Time Division Duplexing (TDD) small eNBs (SeNBs) in the unused UL spectrum. In this scenario, time synchronization with macro eNB (MeNB) and neighboring SeNBs becomes essential. Two algorithms are proposed in order to ensure orthogonality of OFDM transmissions network-wide.

In this paper, we consider the downlink of a massive multiple-input-multiple-output (MIMO) single user transmission system operating in the millimeter wave outdoor narrowband channel environment. We propose a novel receive spatial modulation architecture aimed to reduce the power consumption at the user terminal, while attaining a significant spectral efficiency and low bit error rate. The energy consumption reduction is obtained through the use of analog devices(amplitude detector), which reduces the number of radio frequency chains and analog-to-digital-converters (ADCs). The base station transmits spatial and modulation symbols per channel use. We show that the optimal spatial symbol detector is a threshold detector that can be implemented by using one bit ADC. We derive closed form expressions for the detection threshold at different signal-to-noise-ratio (SNR) regions. We derive expressions for the average bit error probability in the presence and absence of the threshold estimation error showing that a small number of pilot symbolsis needed. A performance comparison is done between the proposed system and fully digital MIMO showing that a suitable constellation selection can reduce the performance gap.

This paper develops and evaluates precoding techniques for coordinated joint transmission in visible light communications (VLC). A multi-user multiple-input single-output (MISO) setup is adopted. Transmitters are equipped with light emitting diode (LED) arrays, while receivers incorporate a single photodetector (PD). We design zero forcing (ZF) linear precoders considering, as figure of merit, the weighted sumrate for multilevel pulse amplitude modulation (PAM) and take into account the optical power constraints. We present two approaches. First, we formulate the design as a convex optimization problem and find its exact solution. Second, by considering certain approximations, we find an alternative precoder that despite being suboptimal can be computed in an almost closed-form, achieving a good trade-off between performance and complexity. Finally, we demonstrate that the proposed approaches largely outperform the conventional pseudo-inverse precoding and single-user TDMA approaches.

New short-length single-direction frame structures are proposed for 5G time division duplex (TDD) systems, where the transmit direction [i.e., either downlink (DL) or uplink (UL)] can be independently chosen at each cell in every frame. Accordingly, high flexibility is provided to match the per-cell DL/UL traffic asymmetries and full exploitation of dynamic TDD is allowed. As a downside, interference management becomes crucial. In this regard, this paper proposes a procedure for dynamic TDD in dense multiple-input multiple-output small cell networks, where the transmit direction selected per small cell (SC) is dynamically optimized together with the user scheduling and transmit precoding. We focus on the maximization of a general utility function that takes into account the DL/UL traffic asymmetries of each user and the interference conditions in the network. Although the problem is non-convex, it is decomposed thanks to the interference-cost concept and then efficiently solved in parallel. Simulation results show gains in DL and UL average rates for different traffic asymmetries and SC/user densities as compared to existing dynamic TDD schemes thanks to the proposed joint optimization. The gains become more significant when there is high interference and limited number of antennas.

This paper proposes a joint user scheduling and transmit direction selection procedure for dynamic TDD in 5G dense small cell networks, where the transmit direction (i.e. downlink (DL) or uplink (UL)) selected per small cell is dynamically optimized together with the user scheduling at every frame. We focus on the maximization of a general utility function that takes into account the DL/UL traffic asymmetries of each user and the interference conditions in the network. After relaxation of the integer variables that subsume the user scheduling and transmit direction selection, the problem is decomposed and efficiently solved in parallel at every frame. Simulation results show significant gains in DL and UL average rates for different traffic asymmetries, network densities, and user densities as compared to existing schemes for dynamic TDD.

This paper presents long-term resource provisioning schemes for dense multi-cell OFDMA-based networks, where multiple cells with possibly overlapping coverage areas compete for the same set of resources. The optimization is done over the long term, being independent of the specific users connected to each cell but dynamic enough to follow significant variations of the traffic load. In this sense, we focus on the average resource utilization (RU) and propose schemes to minimize the maximum RU of all cells. Firstly, we consider orthogonal resource usage among cells. Optimal closed-form expressions for the long-term resource provisioning are derived for this case. Secondly, we assume that resources can be reused at non-overlapping cells. In this case, the resource provisioning is solved in two steps: i) the number of resources required per cell is obtained by discretizing the optimal solution of a convex problem, and then ii) the specific resources to be utilized by each cell are determined by using graph coloring. In contrast to previous works, graph coloring can be applied to get an implementable solution for any condition of the cell loads. Simulation results show a significant reduction of the maximum RU, which translates into an increase of the served traffic and a reduction of the packet delay, as compared to static resource provisioning schemes.

Recent results have elucidated the benefits of using improper Gaussian signaling (IGS) as compared with conventional proper Gaussian signaling (PGS) in terms of achievable rate for interference-limited conditions. This paper exploits majorization theory tools to formally quantify the gains of IGS along with widely linear transceivers for multiple-input multiple-output (MIMO) systems in interference-limited scenarios. The MIMO point-to-point channel with interference is analyzed, assuming that received interference can be either proper or improper, and we demonstrate that the use of the optimal IGS when received interference is improper strictly outperforms (in terms of achievable rate and mean square error) the use of the optimal PGS when interference is proper. Then, these results are extended to two practical situations. First, the MIMO Z-interference channel (Z-IC) is investigated, where a trade-off arises: with IGS, we could increase the achievable rate of the interfered user while gracefully degrading the rate of the non-interfered user. Second, these concepts are applied to a two-tier heterogeneous cellular network where macrocells and small cells coexist and multiple MIMO Z-ICs appear.

The degrees of freedom (DoFs) of the K-user multiple-input multiple-output (MIMO) interference channel are studied when perfect, but delayed channel state information is available at the transmitter side (delayed CSIT). Recent works have proposed schemes improving the DoF knowledge of the interference channel, but at the cost of developing transmission involving many channel uses (long delay), thus increasing the complexity at both transmitter and receiver side. This paper proposes three linear precoding strategies, limited to at most three phases, based on the concept of interference alignment, and built upon three main ingredients: delayed CSIT precoding, user scheduling, and redundancy transmission. In this respect, the interference alignment is realized by exploiting delayed CSIT to align the interference at the non-intended receivers along the space-time domain. Moreover, a new framework is proposed where the number of transmitted symbols and duration of the phases is obtained as the solution of a maximization problem, and enabling the introduction of complexity constraints, which allows deriving the achievable DoF as a function of the transmission delay, i.e., the achievable DoF-delay trade-off. Finally, the latter part of this paper settles that the assumption of time-varying channels common along all the literature on delayed CSIT is indeed unnecessary.

This paper addresses distributed interference management in downlink of multi-cell multiple- input multiple-output (MIMO) time division duplex (TDD) networks. Each user is associated with a user-centric cluster of base stations (BSs), which cooperatively serve the user through CoMP joint transmission. Clusters of different users possibly share some BSs such that they may overlap, being coupled by interference and transmit power constraints at each BS. Our objective is the design of BSs clustering and precoding matrices per-user in order to maximize the weighted sum-rate of the system by controlling the interference and the power spent at BSs. In contrast to previous works where all channel matrices in the system are needed, we propose a distributed procedure whereby only channel matrices towards a limited number of candidate BSs per user are required while interference is still controlled by using the signal received from an uplink transmission. For an LTE-compliant dense deployment of 2×2 MIMO BSs/users, results show gains of 6-16% in terms of sum-rate and 49-84% in terms of 5%-tile per-user rate (depending on the maximum cluster size) as compared to distributed BS-disjoint clustering schemes.

Self-organizing network is a new capability envisioned for the near-future wireless cellular networks. Among the different use cases, here we tackle the coverage optimization with the objective of adjusting the power devoted for the pilot signal as a function of the load of the different small Node Bs (SNBs). The optimization is done with the objective of minimizing the total transmitted power for the pilot signals in the network, leading to a switch on/off SNBs solution as a particular case. In this paper we present a decentralized algorithm that is executed at UEs and SNBs, exploiting the messages reported by UEs.

Retrospective Interference Alignment is a linear precoding strategy that seizes delayed channel state information at the transmitters (CSIT), improving the degrees of freedom (DoF) that can be achieved without CSIT for both the broadcast and interference channels. However, little is known for other channel configurations. This work studies the multiple-input multiple-output interference broadcast channel (IBC) with delayed CSIT, 2 cells and 2 users per cell. Although some of the schemes designed for other channels may be applied, they do not exploit the specific topology of the IBC. In this regard, we proposes a precoding strategy that includes the concept of redundancy transmission, and takes into account that users of each cell are served by a common transmitter. Assuming that transmitters and receivers are equipped with M, N antennas, respectively, state-of-the-art achievable DoF are improved for ρ = M over N > ρA, with ρA≈ 3.5414.

Cellular networks need the densification of small eNBs (SeNBs) to face the tremendous data traffic demand growth, implying an interference increase and making transmit coordination a key enabler. This article proposes a decentralized coordinated precoding (D-CoP) for downlink (DL) weighted sum-rate maximization in dense MIMO TDD small cell networks (SCNs). Each SeNB designs its own precoding matrices based on channel state information (CSI) of the served users and knowledge of the interference-cost matrix that allows managing interference towards unintended users. A protocol is proposed to acquire the interference-cost matrix by processing the uplink (UL) received signal provided that: (1) channel reciprocity can be assumed and (2) all users participating in DL can transmit in UL with an adequate transmit filter. In contrast to existing transmit coordination techniques, D-CoP is fully scalable, avoids estimation of the interfering channels, and does not require information exchange between SeNBs. In case all parameters are perfectly acquired, an iterative algorithm is presented with demonstrated monotonic convergence when all SeNBs update its transmit precoders simultaneously. Further, the problem is reformulated in order to derive a robust D-CoP under imperfect CSI conditions. Finally, simulations in 3GPP LTE-Advanced SCNs show significant user packet throughput gains, without increasing the complexity associated to transmit coordination. Robustness to imperfect CSI and non-ideal channel reciprocity is shown through simulations.

A precise knowledge of the MIMO channel between the serving node and the user equipment (UE) is important for attaining good data rates in downlink transmissions (DL) in cellular systems. The interfering point-to-multipoint (I-P2MP) channel, consisting of multiple transmitters coexisting in the same area, where each transmitter is intended to serve multiple users, is a model that subsumes most of the scenarios that can be found in wireless cellular networks with a dense deployment of small cells (SCs). In conventional channel estimation procedures, resources allocated to each SC for training tend to be orthogonal, negatively impacting in the efficiency of the whole system. Reusing resources for training allows releasing resources for data transmission, but at the cost of degrading the channel estimation due to interference. We propose a decentralized algorithm for interference management that enforces the coordination among SCs in the design of the training sequences for de DL. Results in this work elucidate how to reuse resources for training and significantly improve the throughput of the system.

The optimal transmission scheme for a multiple-input multiple-output point-to-point channel (MIMO-P2P) is dependent on the type of received interference, which can be either proper or improper. We will show that the presence of improper interference is beneficial in terms of improving the achievable rate compared to a case with proper interference. These benefits are due to using widely linear precoder and receiver. Additionally, we investigate a cellular scenario where two types of users are coexisting, ones that are receiving proper interference while others which might be receiving improper interference, trying to devise the best transmission scheme to be adopted. We propose an improper Gaussian signaling-based scheme that allows controlling system performance and fairness in terms of bitrate through a single parameter.

This work investigates the degrees of freedom (DoF) of the K-user multiple-input single-output (MISO) interference channel (IC) with imperfect delayed channel state information at the transmitters (dCSIT). For this setting, new DoF inner bounds are provided, and benchmarked with cooperation-based outer bounds. The achievability result is based on a precoding scheme that aligns the interfering received signals through time, exploiting the concept of Retrospective Interference Alignment (RIA). The proposed approach outperforms all previous known schemes. Furthermore, we study the proposed scheme under channel estimation errors (CEE) on the reported dCSIT, and derive a closed-form expression for the achievable DoF with imperfect dCSIT.

The degrees of freedom (DoF) of the 3-user multiple input multiple output interference channel (3-user MIMO IC) are investigated where there is delayed channel state information at the transmitters (dCSIT). We generalize the ideas of Maleki et al. about Retrospective Interference Alignment (RIA) to be applied to the MIMO IC, where transmitters and receivers are equipped with (M, N) antennas, respectively. We propose a two-phase transmission scheme where the number of slots per phase and number of transmitted symbols are optimized by solving a maximization problem. Finally, we review the existing achievable DoF results in the literature as a function of the ratio between transmitting and receiving antennas ρ = M/N. The proposed scheme improves all other strategies when ρ ∈ (1/2, 31/32].

The degrees of freedom (DoF) of the 3-user multiple-input multiple-output (MIMO) interference channel (IC) with full channel state information and constant channel coefficients are investigated when (p,p+1) antennas are deployed at the transmitters and receivers, respectively. The point of departure of this paper is the work of Wang et al., which conjectured but did not prove the DoF for the antenna settings with p>1. Here the achievability of the DoF outer bound is formally proved using linear methods, thereby avoiding the use of the rational dimensions framework. The proposed transmission scheme exploits asymmetric complex signaling together with symbol extensions in time and space interference alignment concepts. While the paper deals with the p=2,3,\ldots,6 cases, providing the specific transmit and receive filters, there are also provided the tools needed for proving the achievability of the optimal DoF for p>6, whose DoF characterization is conjectured.

This paper proposes distributed inter-cluster interference management procedures for 1 Coordinated Multi-Point (CoMP) Time Division Duplexing (TDD) cellular networks, in which a set of base stations (BSs), that form a cluster, cooperate in the downlink (DL) and uplink (UL) transmissions to communicate with a group of users while causing interference to users in neighbor clusters. In order to reduce the complications associated to the estimation of the interfering channels, we describe new iterative procedures based on channel reciprocity that avoid such estimation by exploiting the received signal in the UL, provided that transmit and receive filters in DL and UL are properly designed. Simulations show that proposed techniques attain significant DL and UL spectral efficiency gains. Furthermore, an analysis of imperfect channel state information (CSI) at both ends is included, evidencing the robustness of the proposed procedures. Considerations on convergence are included.

The multiple-input multiple-output (MIMO) X channel with rank-deficient channels is considered. We characterize the total degrees of freedom (DoF) by defining an outer bound which is attained by the proposed precoding scheme. The total DoF are described in closed-form when all transmitters and receivers have M and N antennas, respectively. For an arbitrary number of antennas, the total DoF are obtained as solution of a linear programming problem. Results elucidate that, unlike point-to-point MIMO systems where DoF increase with the rank of the channel, the total DoF in a multipoint-to-multipoint MIMO system can increase up to a certain point if channels are rank-deficient. Hence, we corroborate the relevance of knowing the rank-deficiency in the MIMO X channel, a property already observed in the MIMO Interference channel.

This paper proposes a decentralized algorithm for interference coordination in LTE-A networks, based on the exchange of information (pricing) at control plane level. In our approach, every user equipment (UE) report the maximum modulation and coding scheme (MCS) that can be used within several sets (sub-bands) of available resource blocks, along with a parameter (cost) that measures the MCS degradation due to the transmission from an interfering neighbor. Through the exchange of pricing messages, the higher costs are distributed to the dominant interferers; and the selection of the MCS and operational sub-band is based on the current link conditions along with the received pricing messages. We evaluate the algorithm in a cellular area covered by a macro base station (MBS) and multiple active femtocells. We also measure the degradation due to the quantization of the exchanged cost values. The results demonstrate significantly enhanced performance as compared to the non-coordinated case.

A relay station (RS)-based cellular system deployment is considered, where multiple base stations (BS) cooperate in the BS-RS in-band transmission for the downlink. With the joint optimization of the resources allocated to the wireless backhaul and to the RS-MS access, it is possible to exploit the benefits of networked-MIMO along with combating the pathloss and shadowing effects, and thus obtain significantly enhanced spectral efficiency and coverage homogeneity. The problem is shown to be convex under conventional QoS objective functions. Suboptimal low complexity solutions are also proposed and evaluated.

We derive a linear transmit precoding scheme based on interference alignment at the transmitters and zero-forcing at the receivers, for the MIMO X channel based on the Generalized Singular Value Decomposition (GSVD) that is able to attain the outer bound on the degrees of freedom (DoF) region for any antenna configuration, except when all terminals have one antenna. Additionally, the proposed scheme is able to enhance the transmission rate in terms of high SNR offset.

Recent results elucidate the optimality of the interference alignment concept in attaining the degrees of freedom of the MIMO X channel. However, in the low-medium SNR regime optimizing the weighted sum-rate seems more meaningful. Since the MIMO X channel subsumes the MIMO interference channel, MIMO multiple access, parallel MIMO point-to-point and MIMO broadcast channel, it is desirable to have a procedure that designs the linear transmitters and receivers in all cases. In this work we have observed that an algorithm based on alternate optimization, along with the proper initialization, is able to provide MMSE precoders attaining the degrees of freedom and significant gains in terms of SNR offset with respect the conventional interference alignment solution.

This work looks into the extension of the proportional fair (PF) scheduler to the multi-user case, where the source is transmitting several messages under the superposition coding (SC) strategy in a Gaussian Broadcast channel. Jointly with the weighted sum-rate (WSR) scheduling criterion, we derive guidelines for 2-user pairing, based both on SNR and user priority. We compare superposition coding access with frequency division multiple access, while time division is obtained naturally as a result of the user pairing. Results elucidate that the 2-user SC PF always improves the single-user PF in terms of average throughput and delay reduction, and we compare its performance with the 3-user SC PF. Finally, the WSR shows throughput gains similar to the 2-user SC PF for certain configuration of the scheduler.

The two-way relay channel (TWRC) describes the communication between two terminals sharing a common relay. In this work we compare different protocols for the TWRC with a half-duplex relay (forwarding and protocol I) and transmission schemes (network coding relaying with random binning and XOR precoding), when the relay works in decode-and-forward and without channel state information at the transmitters. We provide the optimal resource allocation (in terms of phase duration, data rate and power allocation) when there is an individual or sum-average power constraint over the terminals. Closed-form solutions are derived whenever it is possible, while for others cases we show that the resource allocation can be formulated as a convex problem, which means that the solution can be obtained by low-complexity interior point methods.

Automatic repeat request (ARQ) protocols deal with the situations where the selected data rate is not supported by the current channel realization. The inappropriate selection is motivated by the limited channel state information at the transmitter (CSIT). This work looks into how the ARQ protocols are applied to the half-duplex relay-assisted transmission with decode-and-forward relays by exploiting the ACK feedback channel of the terminals. Two implementations are tackled: 1) only the source or 2) source and relay manage the retransmissions under the pure-ARQ protocol. This work presents an analytical framework for the comparison of both options. The study allows the optimization of the bitrate and amount of resource allocated for the relay transmission phases.

Under limited channel state information at the transmitter the communication suffers from the outage events produced by a data rate selection not supported by the current channel realization. Automatic repeat request (ARQ) protocols are a useful tool to deal with those outage events in delay-tolerant services. This work looks into how automatic repeat request (ARQ) protocols are applied to the half-duplex relay-assisted transmission with a decode-and-forward (DF) relay. The exploitation of ACK feedback channels from destination and relay leads to two possible implementations: 1) only-source manages the retransmission procedure or 2) both source and relay control the retransmissions. We analyze both options under a chase-combining ARQ protocol with limited number of transmission rounds, providing integral expressions for the throughput. This study allows the optimization transmission rate and the amount of resources allocated for the relay transmission.

This paper addresses the design of joint multi-cell resource allocation and scheduling with coordination among neighboring base stations (BS). Considering Nsneighboring sectors, the solution that maximizes the sum-rate in the covered area is found for TDMA and TDMA/OFDMA access. The optimum solution exhibits low complexity as it presents the useful property that its computation admits a decomposition such that each BS is responsible for the selection of the users to be scheduled in its own sector. This result is proved analytically.

Cooperative transmission has emerged as an effective tool to deal with channel impairments. In this work we study the applicability of amplify-and-forward cooperative transmission in the downlink of a TDMA cellular system where idle terminals act as relays. The base station may have a multiple antenna configuration, whereas all terminals are equipped with a single antenna. Although cooperation can be viewed as a virtual MIMO system, it further allows dealing with shadowing and pathloss. In order to improve the spectral efficiency loss incurred by the half-duplex relays, in this work we consider the reuse of the relay time slot by simultaneous transmissions from the active relays in the cell. We are able to model the interfering power received by each destination in that slot. The only system parameters adjusted to control the interference are the number of interfering relay terminals and a cell-wide .target SNR in the relay-destination link. This way, it is shown that cooperative transmission is able to provide significant gains in terms of throughput over the direct transmission under round-robin packet scheduling and uniform distribution of terminals by controlling the rate of each transmission (rate control).

This work explores ways to boost the spectral efficiency of transmission schemes based on amplify-and-forward (AF) cooperation. While AF has usually been considered to suffer from bandwidth expansion due to the increased use of transmission resources, in a multiuser environment it is possible to overcome this limitation by either spatially reusing the transmissions from different relay terminals, or by using multiple relays. These two possibilities are not mutually exclusive and are studied below in the context of TDMA transmissions for the downlink. It is shown that under mild assumptions the interference generated during the relay transmission phases can be statistically modeled. The outage events generated by this interference are mitigated by an adaptive transmission rate scheme that maximizes the system throughput. This low complex implementation of multiuser AF provides significant throughput gains over a conventional TDMA non-cooperative scenario.

Cooperative transmission has emerged as an effective tool to combat fading by improving the diversity gain. Although this technique can be viewed as a "virtual" MIMO system, when implemented under orthogonal access it does not allow multiplexing gains. In this work we have studied a possible application of the cooperative transmission in a TDMA centralized cellular system in the downlink. In order to reduce the inefficiency, we have considered the spatial reuse of one relay slot by simultaneous active relays in the cell. To this end we have modeled the interfering power received by each destination in the relay link. It is shown that cooperative transmission with amplify and forward strategy is able to provide significant throughput gains for user-relaying over the direct transmission under TDMA

This paper deals with the design of non-regenerative relays in cooperative transmission schemes. The conventional nonregenerative approach is the amplify and forward (AF) approach, where the signal received at the relay is simply amplified and retransmitted. In this paper, we propose an alternative design which maximizes the capacity for nonregenerative cooperative transmission, when channel state information is available at the relay station. Therefore, the relay units are not simple amplify and forward units, but still they are not required to demodulate or remodulate the symbols transmitted by the base station as in regenerative relaying schemes. We compare the performance so obtained with the performance for the conventional AF approach, and also with the performance of regenerative relays and direct noncooperative transmission.

An iterative game theory based algorithm is proposed to allocate the resources in cooperative schemes for the downlink. Both amplify-and-forward (AF) and decode-and-forward (DF) cooperative schemes are considered with cellular reuse of the relay slot. Multiple antennas can be used at the involved stations. Using the algorithm proposed, the simultaneous relay powers are decided in a decentralized way using mean channel level measures and mean values of noise and interference power. The cell capacity gain for the cooperative schemes using the decentralized algorithm is evaluated by means of simulation for both the AF and DF approaches.

The performance of downlink coded cooperative transmission in mobile communication systems is evaluated. Cooperative transmission is seen as a "virtual" MIMO system where multiple transmit antennas are provided by a relay station and the base station, and different space-time block coding schemes are considered. Regenerative and non-regenerative systems are considered for the cooperative schemes. The use of rate compatible puncture turbo codes as forward error correction codes allow the system to be tested under the hybrid-type II automatic repeat request protocol. Results in terms of throughput show the benefits of coded cooperation over direct transmission.

A particular property of the matched desired impulse response receiver is introduced, namely, the fact that full exploitation of the diversity is obtained with multiple beamformers when the channel is spatially and timely dispersive. This particularity makes the receiver specially suitable for mobile and underwater communications. The new structure provides better performance than conventional and weighted VRAKE receivers, and a diversity gain with no need for additional radio frequency equipment. The baseband hardware needed for this new receiver may be obtained through reconfigurability of the RAKE architectures available at the base station. The proposed receiver is tested through simulations assuming UTRA frequency-division-duplexing mode.