Energy efficiency maximization for given traffic and target packet loss probability is a main design goal for grant-free protocols supporting massive connectivity. This paper investigates the performance limits of a class of coded slotted ALOHA (CSA)based protocols over the Gaussian channel. Assuming a multipacket reception decoder with perfect estimation of the number of packets in a slot and ideal error detection, we present a novel asymptotic analytical framework that allows deriving the minimum packet loss probability limits, the conditions for an$E_{b} / N_{0}$limits. Numerical results show that CSA schemes have energy efficiency limits within few dB from the random coding bounds, with 1-2 dB improvement to irregular repetition slotted ALOHA (IRSA) ones.

In this paper, grant-free uplink communication from a large number of machine-type devices in CF-mMIMO networks is explored. A novel approach that leverages coded random access, on the device side, with combining of signals received at properly selected AP and cooperative successive interference cancellation, on the network side, is presented. Initially, an analytical framework based on stochastic geometry is developed to investigate performance of AP cooperation through signal combining under diverse AP cluster compositions. The potential gain from AP signal combining is then assessed by evaluating a genie-aided scheme, guiding the network in cluster selection for each active device. Subsequently, two practical AP selection algorithms that operate in grant-free conditions (i.e., do not require prior information regarding the active users) are proposed. Numerical results show how AP cooperation through signal combining and distributed interference cancellation can bring tangible benefits even without prior information about active users, under different SNR regimes, closing in some cases the gap to the genie-aided approach. Additionally, the results prove that AP cooperation can be used to reduce the devices’ energy consumption and the number of AP that have to be deployed by the service providers to achieve specific performance levels.

On-chip optical wireless links are attracting a great deal of interest as they can provide a possible solution to overcome the drawbacks associated with wired connections. In this paper, we propose a new approach for on-chip communication using optical wireless switches based on thin-film lithium niobate on insulator (LNOI) technology. The optical wireless switches exploit reconfigurable optical phased arrays (OPAs) both at the transmitter and at the receivers. We investigate the radiation characteristics and design criteria of the LN antenna element serving as a unit radiator in the OPAs. We then demonstrate the implementation of an on-chip optical wireless switch in a simple infinite homogeneous host medium and assuming a realistic multilayer structure configuration. Moreover, we examine the impact of various geometrical parameters and the fabrication imperfections on the device performance and provide a discussion on the design optimization. Our findings assess the feasibility of optical wireless switches based on LNOI technology for on-chip wireless communication.

A framework for the analysis of synchronous grant-free massive multiple access schemes based on the irregular repetition slotted ALOHA (IRSA) protocol and operating over the Gaussian multiple access channel is presented. IRSA-based schemes are considered here as an instance of the class of unsourced slotted random access codes, operating over a frame partitioned in time slots, and are obtained by concatenation of a medium access control layer code over the entire frame and a physical layer code over each slot. In this framework, an asymptotic analysis is carried out in presence of both collisions and slot decoding errors due to channel noise, which allows the derivation of density-evolution equations, asymptotic limits for minimum packet loss probability and average load threshold, and a converse bound for threshold values. This analysis is exploited as a tool for the evaluation of performance limits in terms of minimum signal-to-noise ratio required to achieve a given packet loss probability, and also provides convergence boundary limits that hold for any IRSA scheme with given physical layer coding scheme. The tradeoff between energy efficiency and spectrum efficiency is numerically evaluated comparing some known coding options, including those achieving random coding bounds at slot level. It is shown that IRSA-based schemes have a convergence boundary limit within few dB from the random coding bound when the number of active transmitters is sufficiently large.

An information-theoretic approach to irregular repetition slotted ALOHA (IRSA) is proposed. In contrast with previous works, in which IRSA analysis is conducted only based on quantities that are typical of collision models such as the traffic, the new approach also captures more fundamental quantities. Specifically, a suitable codebook construction for the adder channel model is adopted to establish a link with successive interference cancellation over the multi-packet reception channel. This perspective allows proving achievability and converse results for the average sum rate of IRSA multiple access schemes.

Neural Network Decoders (NNDs) have been recently considered and investigated as an alternative to the classical algorithms for decoding Polar Codes. In particular, a class of partitioned decoders has been proposed where modified Successive Cancellation (SC) or Belief Propagation (BP) algorithms exploit the neural network decoding of short sublocks. Although NNDs work as one-shot decoders with small decoding latency, the performance of these decoders, in comparison with classic approaches, still suffers from some losses; this fact limits their application to codes with short/medium length. In this article we introduce the novel Neural Successive Cancellation List (NSCL) decoder, as a partitioned version of the SC decoder operating on a list of L multiple decoding candidates that are built with the decoded sub-blocks obtained from a set of NNDs working in parallel at each partition. This decoder significantly improves the BER performance of the non-list version under the same code length. Hence, it reduces the intrinsic loss of performance given by the use of NNDs and allows the application of NN-based decoding to longer codewords.

Optical wireless (OW) communications, besides being of great interest for indoor and outdoor applications, have been recently proposed as a powerful alternative to the existing wired and wireless radio frequency (RF) interconnects in a network-on-chip (NoC). Design and analysis of networks with OW links require a careful investigation of cross-link interference, which impacts considerably the efficiency of systems that reuse the same channel for multiple transmissions. Yet, there is no comprehensive analysis of interference for OW NoCs, and the analyses of crosstalk in optical waveguide communications usually rely on synchronous data transmissions. A novel framework for the analysis of on-chip OW communications in the presence of cross-link cochannel interference and noise is proposed, where asynchronous data transmissions are considered. Self-beating of interfering signals is also considered, which was often neglected in previous literature. The bit error probability (BEP) for arbitrary number of interfering sources is derived as a function of signal-to-noise ratio (SNR), interference powers, detection threshold and pulse shaping, using both exact and approximation methods. The proposed analysis can be applied to both noise- and interference-limited cases, and enables a system designer to evaluate reuse distance between links that share the same optical carrier for simultaneous communication in NoCs.

Optical wireless (OW) links have been recently proposed as an interconnection technology for multiple processing cores operating in parallel on the same chip. OW communication is also a mature option for indoor and outdoor applications. Design and analysis of networks with optical wireless links require a careful investigation of cross-link interference which plays a key-role on the performance and efficiency of systems that reuse the same channel for multiple parallel transmissions. In this paper we analyze the bit-error rate performance of OW links for on-chip applications with cross-link cochannel interference. As a novelty with respect to known literature on crosstalk in optical communications we consider asynchronous data transmission and address the system performance in case of heavy interference. Analytical methods are used to derive error probabilities as a function of signal-to-noise ratio (SNR), crosstalk power ratio, detection threshold, pulse shaping. Both exact and tight approximation methods are considered. As shown in the results, robustness against interference increases with asynchronous transmission, RZ pulse shaping and suitable design of detection threshold. It is also shown how the proposed analysis can be used to evaluate the reuse distance between two parallel links simultaneously transmitting in the same direction.

Device-to-Device (D2D) communications enable user equipments (UEs) in proximity to each other to exchange information by taking advantage of high data-rate and low energy consumption. When D2D transmissions share the radio resources with the cellular UEs, efficient admission control (AC) and radio resource allocation (RRA) strategies play a key-role in controlling the co-channel interference and allowing quality of service (QoS) provision to UEs. This paper proposes a novel joint AC and RRA strategy that provides long-term QoS support to cellular and D2D communications. The AC algorithm derives the best set of cellular and D2D links by maximizing the revenues of the service provider under QoS constraints. The RRA algorithm assigns the available channels and transmits powers to admitted users on the short-term, in order to maximize an average weighted sum-rate under the same QoS constraints of the AC. Due to the NP-hard nature of the optimization problem, we propose an AC greedy algorithm that achieves near-optimal results for reasonable numbers of D2D links. Then, we propose a low-complexity RRA algorithm that decouples channel and power allocation. Numerical results show that the proposed joint AC and RRA strategy outperforms existing frameworks by increasing up to 40% the number of satisfied cellular and D2D links and by reducing energy consumption by more than 50%.

HTTP adaptive streaming (HAS) has emerged as the main technology for video streaming applications. Multiple HAS video clients sharing the same wireless channel may experience different video qualities as well as different play-out buffer levels, as a result of both different video content complexities and different channel conditions. This causes unfairness in the end-user quality of experience. In this paper, we propose a quality-fair adaptive streaming solution with fair buffer (QFAS-FB) to deliver fair video quality and to achieve asymptotically fair play-out buffer levels among HAS clients competing for the same wireless resources in a long-term evolution (LTE) cell. In the QFAS-FB framework, the share of radio resources is optimized according to video content characteristics, play-out buffer levels, and channel conditions. The proposed solution is compared with the other state-of-the-art strategies, and the numerical results show that it significantly improves the quality fairness among heterogeneous HAS users, reduces the video quality variations, and improves the fairness among the user's play-out buffers.

M-health services are expected to become increasingly relevant in the management of emergency situations by enabling real-time support of remote medical experts. In this context, the transmission of multiple health-related video streams from an ambulance to a remote hospital can improve the efficacy of the teleconsultation service, but requires a large bandwidth to meet the desired quality, not always guaranteed by the mobile network. In order to deliver the multiple streams over a single bandwidth-limited wireless access channel, in this paper we propose a novel optimization framework that enables to classify the available video sources and to automatically select and adapt the best streams to transmit. The camera ranking algorithm jointly works with a cross-layer adaptation strategy for multiple scalable streams to achieve different objectives and/or tradeoffs in terms of number and target quality of the transmitted videos. The final goal of the optimization is to dynamically adjust the overall transmitted throughput to meet the actual available bandwidth, while being able to provide high quality to diagnostic video sequences and lower quality to less critical ambient videos. Numerical simulations considering a realistic emergency scenario with long term evolution advanced (LTE-A) connectivity show that the proposed content/context-aware solution is able to automatically select the best sources of information from a visual point of view and to achieve optimal end-to-end video quality for both the diagnostic and the ambient videos.

In HTTP adaptive streaming (HAS) applications multiple video clients sharing the same wireless channel may experience different video qualities as result of both different video content complexity and different channel conditions. This causes unfairness in the end-user video quality. In this paper, we propose a quality-fair adaptive streaming (QFAS) solution to deliver fair video quality to HAS clients competing for the same resources in an LTE cell. In the QFAS framework the share of radio resource is optimized according to video content characteristics and channel condition. The proposed solution is compared with other state-of-the-art strategies and numerical results in terms of SSIM quality metric shows that it significantly improves the quality fairness among heterogeneous HAS users.

The design of optimized video delivery to multiple users over a wireless channel is a challenging task, especially when the objectives of maximizing the spectral efficiency and providing a fair video quality have to be jointly considered. In this paper we propose a novel cross-layer optimization framework for scalable video delivery over OFDMA wireless networks. It jointly addresses rate adaptation and resource allocation with the aim of maximizing the sum of the achievable rates while minimizing the distortion difference among multiple videos. After having discussed the feasibility of the optimization problem, we consider a “vertical” decomposition of it and propose the iterative local approximation (ILA) algorithm to derive the optimal solution. The ILA algorithm requires a limited information exchange between the application and the MAC layers, which independently run algorithms that handle parameters and constraints characteristic of a single layer. In order to reduce the overall complexity and the latency of the optimal algorithm, we also propose suboptimal strategies based on the first-step of the ILA algorithm and on the use of stochastic approximations at the MAC layer. Our numerical evaluations show the fast convergence of the ILA algorithm and the resulting small gap in terms of efficiency and video quality fairness between optimal and suboptimal strategies. Moreover, significant individual PSNR gains, up to 7 dB for high-complexity videos in the investigated scenario, are obtained with respect to other state-of-the-art frameworks with similar complexity.

M-health services are expected to become increasingly relevant in the management of emergency situations, enabling real-time support of remote medical experts. In this context, the transmission of health-related information from an ambulance to a remote hospital is a challenging task, due to the variability and the limitations of the mobile radio link. In particular, the transmission of multiple video streams can improve the efficacy of the tele-consultation service, but requires a large bandwidth to meet the desired quality, not always guaranteed by the mobile network. In this paper we propose a novel cross-layer adaptation strategy for multiple SVC videos delivered over a single LTE channel, which dynamically adjusts the overall transmitted throughput to meet the actual available bandwidth, while being able to provide high quality to diagnostic video sequences and lower (but fair) quality to less critical ambient videos. After having introduced a realistic LTE uplink scenario, including an advanced resource allocation strategy, we show through numerical simulations that the proposed solution is capable to achieve an optimal end-to-end video quality for both the diagnostic and the ambient videos.

Relay assisted communications exploit cooperative diversity to efficiently extend area coverage or improve the performance in wireless channels. We propose a mathematical framework to analyze the frame error probability (FEP) at the destination depending on distributed coding, nodes spatial distribution, and power allocation among source and relay nodes. We evaluate the impact of cooperative links characterization on the overall FEP at the destination by considering simple approximations on the link FEP which depend on the signal-to-noise ratio and diversity. We consider a number of power allocation techniques such as uniform, ideal power control, outage-optimal, and FEP-optimal. Results are given for a case study in which pragmatic space-time codes are employed as distributed coding technique. Analytical results are confirmed by simulations that are given to serve as a benchmark for the considered cases. The framework enables the system designer to allocate power and individuate the best relay position to minimize the FEP.

Radio Resource Management with Inter-cell Interference Coordination (ICIC), is a key issue under investigation for next generation wireless systems such as Long Term Evolution (LTE). Although centralized resource allocation (RA) and collaborative processing can optimally perform ICIC, the overall required complexity suggests the consideration of distributed techniques. In this paper we propose and compare a centralized RA strategy aimed at maximizing the sum-rate of a multi-cell clustered system in presence of power and fairness constraints, and a distributed RA strategy where inter-cell interference is partially coordinated through power planning schemes with preassigned power (an example is fractional frequency reuse). The distributed RA strategy reduces both signaling and feedback requirements, preserves intra-cell fairness and jointly works with a load balancing algorithm to support inter-cell fairness. We show in the results that the distributed RA with aggressive frequency reuse is able to approach the performance of the centralized RA when the number of users is large, while preserving the same level of fairness.

Low-density parity-check (LDPC) codes are gaining interest for high data rate applications in both terrestrial and spatial communications. They can be designed and studied through a bipartite graph whose characteristics affect the performance. This paper proposes a low-complexity method to improve the performance of LDPC codes by selectively removing some cycles from the associated bipartite graph. The method is based on a modified version of the breadth first search (BFS) algorithm that we call modified BFS (MBFS), which is applied to find cycles, and a greedy procedure to eliminate them. Throughout the paper we will give a detailed description of the algorithm proposed and analytically study its complexity. Simulation results show that this girth conditioning method applied to some classes of codes, whose structure allows further optimization, can lead to a significative complexity reduction and a performance improvements with respect to other methods.

A clear understanding of the behavior of error probability (EP) as a function of signal-to-noise ratio (SNR) and other system parameters is fundamental for assessing the design of digital wireless communication systems. We propose an analytical framework based on the log-concavity property of the EP which we prove for a wide family of multidimensional modulation formats in the presence of Gaussian disturbances and fading. Based on this property, we construct a class of local bounds for the EP that improve known generic bounds in a given region of the SNR and are invertible, as well as easily tractable for further analysis. This concept is motivated by the fact that communication systems often operate with performance in a certain region of interest (ROI) and, thus, it may be advantageous to have tighter bounds within this region instead of generic bounds valid for all SNRs. We present a possible application of these local bounds, but their relevance is beyond the example made in this paper.

In this paper we consider linear wireless networks with variable number of nodes. We first derive a basic cut-set bound for transport capacity of this class of networks highlighting the functional dependence on both node density and network length. After, we investigate how transport throughput scales with respect to node density delta in a wireless network with multihop communication. While in linear multihop networks without fading transport throughput does not increase by increasing node density, i.e. it scales as delta0with a gap with respect to capacity bounds, we show that a capacity increase up to log delta is achieved in the presence of fading by using, instead of a simple nearest- neighbor communication, suitable algorithms that adaptively schedule source-destination links in the networks. Finally, we compare multihop schemes with schemes based on distributed MIMO communication by discussing the relevant differences and showing that, despite the latter strategy is able to asymptotically achieve scaling law bound approaching delta, multihop schemes appear better in terms of capacity in a significant range of values of delta and when the network becomes extended.

In this paper we propose an efficient and simple method to improve system capacity in GSM/GPRS/EDGE radio systems. It exploits the power shaping concept and the parameter "TRX power reduction" already present in the standard and does not require major modifications to operators specific radio resource management algorithms. Power-shaping means assigning in advance (static allocation) a maximum power level to each channel before dynamically performing channel allocation. This is done by exploiting a set of suitably reused power profiles that partially organize the inter-cell interference in the available channels and make it partially predictable. This technique, thanks to the possibility of tightening the reuse factor, permits to increase the system capacity for both voice and data services if compared to most common reuse schemes or to the widely used concentric cells schemes. This is shown in the results where an interesting gain is achieved by power shaping with power reduction with respect to the other schemes.

In this paper we address the issue of optimizing the performance of data-link and transport protocols running on a system including multiple transmit and receive antennas with a V-BLAST architecture. More specifically, we explore the possibility of adaptively selecting which transmit antennas and which per antenna rates to use as well as the antenna transmit powers in order to maximize the rate of data packets successfully delivered through the system. This is a novel approach with respect to the recent literature which is generally focused on channel capacity optimization. We show that the use of this rate/power adaptation technique, when a data transmission protocol is running over the link, may lead to significant throughput improvements especially at small signal-to-noise ratios.

This paper addresses the issues of distributed dynamic resource allocation (DRA) in the downlink of a SDMA broadband wireless packet network with multiple access points and adaptive antennas. Packet access and downlink beamforming make the intercell interference hard to predict, and how to handle it in order to efficiently perform slot and power allocation is still an open issue. We propose here new DRA strategies for efficient allocation in a distributed environment, which jointly consider spatial compatibility of users and an intercell interference estimation based on a fraction of the worst case interference measurements. The latter is independent of the actual allocation. We consider a low mobility TDD system with synchronized base stations and we assume perfect channel knowledge. The algorithms which are introduced and compared are the distributed max-min fit (DMMF) and the distributed reverse fit (DRF), which are based on the max-min fit criterion, the DMMF combined with the novel concept of nulls preallocation, where each AP reserves some beamforming nulls for the most interfered users of neighboring cells, and the power shaping technique (PS-DRA for SDMA), which efficiently exploits static power preallocation on time slots. The numerical results show that our techniques are able to significantly reduce the gap between a greedy centralized strategy that fully coordinates all the access ports and the baseline case of random distributed allocation, previously proposed for packet access. However, distributed power control in a packet switched environment with downlink beamforming still remains a hard task: nevertheless, PS-DRA is an efficient solution for joint slot-power allocation

In this paper we propose energy efficient forwarding techniques in a fading environment which exploit channel knowledge by means of nodes cooperation. Unlike forwarding strategies based on geographic information only, channel knowledge allows to take advantage from diversity by means of selection of the best node to forward the packet, and also to achieve higher energy savings thanks to power control. We also consider the case of multi-route diversity where redundancy packets are sent over different paths. We propose several adaptive strategies which explicitly address the energy issues and we compare them in terms of total consumed energy, mean packet error probability and delay. As a reference case we consider a forwarding strategy based on pure geographic routing (without channel knowledge). The results show that a considerable improvement can be obtained with respect to the pure geographic forwarding. Our algorithms are also in the direction of a cross layer view, since routing decisions are taken adaptively through physical layer information

With the aim of optimizing the performance of a data protocol, we investigate the effects and the role played by distributed resource allocation and rate/power adaptation in a wireless SDMA/TDMA network where multiple antenna elements are used at both transmitter and receiver side, and perfect channel state information (CSI) is available at the receiver only. We consider the uplink of a multiple cell system, throughput-driven allocation and adaptation algorithms, and address the issues related to the management of intercell interference. We find that the capacity of the network is heavily affected by the presence of interference and it is important to exploit a part of the receiving antennas to mitigate strong interference and to have an estimation of the interference covariance matrix to tune allocation and adaptation algorithms. The results show that the joint use of distributed best-fit allocation algorithms with adaptive antenna selection allow substantial improvement with respect to the basic case of random access. Moreover, simple stream control strategies prevent capacity degradation when the offered load exceeds the maximum capacity.

This paper addresses dynamic resource allocation (DRA) for the downlink of a SDMA broadband wireless packet network with multiple access ports and adaptive antennas. The main issue for distributed DRA is how to manage the intercell interference, which is very difficult to be handled in an uncoordinated environment, due to packet access and downlink beamforming. Here, we investigate the performance and the feasibility of some distributed DRA strategies to gain insight on the impact of the lack of coordination among cells. We propose to estimate the intercell interference on the basis of cell load and worst case (taking into account adaptive antennas) measured interference and implement some greedy strategies suitable for a distributed implementation. We show that the gap between random allocation (previously proposed for distributed packet allocation) and a centralized algorithm can be significantly reduced, although the implementation of an efficient distributed power control still appears a hard task.

This paper proposes a novel distributed dynamic resource allocation (DRA) algorithm for the downlink of a SDMA broadband wireless packet network with multiple access ports and adaptive antennas. With the new scheme, each access port (AP) independently preallocates some beamforming nulls toward the most interfered users of the neighbor APs and broadcasts this information to the other APs. Then all APs independently and distributedly perform the allocation of their users using the distributed max-min fit (DMMF) scheme. Simulation results show a significant improvement over the DMMF without nulls preallocation and a significantly reduced gap between the DMMF and the centralized max-min fit (CMMF) algorithm, which performs the allocation by fully coordinating all the access ports. As a baseline case, the performance of a random slot allocation algorithm is also reported.

We consider distributed dynamic slot and power allocation for the downlink of a TD/SDMA broadband wireless packet network with multiple access ports and adaptive antennas. The still open issue for packet multicell SDMA is how to manage the intercell interference, which is very difficult to predict in an uncoordinated environment, due to packet access and downlink beamforming. For this reason, doing distributed allocation efficiently as well as improving the performance by means of power control results in a very hard task. We propose a greedy SDMA algorithm exploiting the power shaping technique, which is based on a static preallocation of the transmit power to each slot of the frame. This permits to obtain a partial predictability of intercell interference, allowing different levels of estimated intercell interference and available power for each slot. We show that our greedy SDMA algorithm with power shaping increases system capacity with respect to the same algorithm without power shaping and reduces the performance gap with respect to a greedy centralized strategy, thus limiting the need of coordination among cells. Both centralized and distributed algorithms are compared, as reference, to the baseline case of random allocation, previously proposed for packet access.

This correspondence presents results of code search for convolutional codes over block-fading channels (BFCs). Search criteria are based on a union bound approach which exploits the concept of generalized transfer function (GTF) of the error trellis diagram. A new asymptotic bound is derived, and the performance of the codes found using our search criterion is compared analytically and by numerical simulation with already existing codes.

We propose and investigate a new resource allocation technique for the downlink of time-division multiple access (TDMA)-based fixed broadband wireless access systems (FBWA) with full frequency reuse. This technique, named power-shaped advanced resource assignment exploits an appropriate set of power profiles which limit (or shape) the power transmitted in each slot of the frame and are suitably reused among the cells, in order to efficiently distribute the intercell and intersector interference inside the frame and to make it partially predictable. In systems where base stations assign radio resources in an uncoordinated fashion, this allows the allocation algorithm to assign time slots to users on the basis of the power required to fulfill a predefined carrier-to-interference ratio, since worst case interference can be suitably estimated; moreover, different degrees of protection against interference are provided across the slots to efficiently accommodate users with different location-dependent channel conditions. Simulation results for a typical cellular FBWA system show that this technique significantly improves the capacity with respect to other techniques recently proposed, e.g., the enhanced staggered resource assignment, even when they use power control. Moreover, an analytical framework useful to understand the concept of power shaping and to discuss some guidelines for the design of power profiles is provided.

In this paper we investigate a new dynamic resource allocation technique for TDMA and TD/CDMA cellular radio systems with full frequency reuse. Unlike traditional allocation strategies, that first allocate the time slots and then perform power control, the proposed technique first assigns in advance (static allocation) a maximum power level to each channel and then dynamically performs channel allocation and fine power control. This is done by exploiting a set of suitably reused power profiles that partially organize the intercell and intersector interference in the available slots and make it partially predictable. We show that this technique, originally proposed by the Authors for fixed broadband wireless access systems, where interference is mitigated by the use of highly directional antennas, is able to increase the capacity of systems with and without centralized or partially centralized resource management control. Moreover, it is also able to fill the efficiency gap between centralized and not centralized strategies. In our investigation a greedy algorithm for resource allocation suitable for both centralized and not centralized management is also proposed.

This paper addresses the design of bit-interleaved pragmatic space-time codes (BI-PSTC) over block fading channels (BFC). BI-PSTC are obtained with the serial concatenation of a convolutional encoder, a bit-interleaver and a space-time mapper, whereas the decoder is based on a Viterbi algorithm which uses suitably defined branch metrics. Here, a method for good BI-PSTC construction, which exploits the concept of generalized transfer function for space-time codes over BFC, is proposed. Codes for QPSK constellations achieving a considerable coding gain over known space-time codes are then derived. Different spectral efficiencies and fading rates (taken into account by the BFC model) are investigated.

This paper investigates a new resource allocation technique for TDMA-based fixed broadband wireless access systems (FBWA) with full frequency reuse where base stations assign radio resources in an uncoordinated fashion. This technique, named Power-Shaped Advanced Resource Assignment (PSARA) exploits an appropriate set of power profiles that limit (or shape) the power transmitted in each slot of the frame with the aim of helping the allocation algorithm to efficiently distribute the intercell and intersector interference inside the frame. Simulation results for a typical cellular FBWA system show that this technique significantly improves the capacity with respect to other techniques recently proposed, e.g. the Enhanced Staggered Resource Assignment (ESRA), even when they use power control. Moreover, an analytical framework useful to understand the concept of power-shaping and to discuss some guidelines for the design of power profiles is provided.

We investigate the possibility of using Markov chains to model the error process in the data blocks delivered by the physical layer of a WCDMA (wideband code division multiple access) cellular system. Starting from the results on the error statistics obtained from a suitable simulation tool, which jointly performs system and link level analysis, we first classify the users on the basis of performance level and burstiness, then we provide some guidelines for the design of Markov models in the different system and channel conditions. The performance of an ARQ (go-back N) protocol at the link layer is taken as a reference to test the reliability of the proposed models. It is shown that the perspective of using simple error models in the analysis of upper-layer protocol is feasible in many cases.

In this paper we introduce bit-interleaved pragmatic space time codes (BI-PSTC) and evaluate their performance over correlated fading channels taken into account by a block fading channel (BFC) model. More precisely, the BI-PSTC proposed here are obtained with the serial concatenation of a convolutional encoder, a bit-interleaver and a space-time mapper. A simple decoder based on a Viterbi algorithm with suitably defined branch metrics can be used, but iterative decoding can be also applied. In this paper we also discuss the diversity achievable and the performance of BI-PSTC obtained by using the classical optimal convolutional codes for AWGN channel and QPSK constellations. Performance evaluation shows that a considerable gain over known space-time codes can be achieved.

We apply dynamic slot allocation with power-shaping to TDMA-based (pure or hybrid) mobile radio systems. We show that this technique, originally proposed by the Authors for fixed broadband wireless access systems, where interference is mitigated by the use of highly directional antennas, is able to increase the capacity even in full frequency reuse cellular systems with omnidirectional antennas. By using a set of power profiles, suitably defined here for hexagonal cells with three sectors, that limit (or shape) the power transmitted in each slot of the frame, this allocation technique partially organizes the intercell and intersector interference in the available slots and makes it partially predictable. This allows an efficient use of radio resources, when the base stations assign radio resources in an uncoordinated fashion without the need of centralized strategies requiring heavy signaling load.

In this work we investigate and discuss the behavior of a wideband CDMA cellular system from the point of view of the power efficiency and the quality level at the physical layer. More precisely, special attention is given to the role played by the SIR-based closed loop power control when it tries to compensate for channel loss variations in different conditions of fading velocity. It is shown that, for a heavily loaded system in a slow fading environment, the largest amount of power is required by those users experiencing the worst quality level. This aspect should be taken into account in the management of the system resources. The results have been obtained from a suitable simulation tool which joins the system level analysis with the link level analysis and includes many features of the WCDMA air interface of UMTS.

In this paper we investigate the performance of data-link and transport protocols running on a transmission system including multiple transmit and receive antennas with a BLAST architecture. More specifically, by exploring the possibility of adaptively selecting the set of transmit antennas to improve protocol performance, we propose and discuss a novel strategy for antenna selection which is suited to the protocols considered. We show that the use of these adaptive techniques leads to an increase of the link throughput, which becomes significant even at small values of the signal-to-noise ratio if the proposed strategy is adopted. Moreover, when applied to TCP, our selection method significantly outperforms the more general channel-capacity-based selection rules.

Self intercarrier interference cancellation (ICI), a half rate repetition coding scheme, is a simple and efficient technique to reduce frequency errors in OFDM. However at higher fractional frequency offset values, normalized to subcarrier separation, the self ICI cancellation scheme can no longer provide the same performance. Further due to the simple structure of repetition code, the self ICI cancellation scheme alone cannot provide much immunity to channel interference. In this paper we combine a maximum likelihood estimation technique with that scheme to get better immunity for frequency errors at higher fractional frequency offset values. This paper also discusses the possibility of application of a bandwidth efficient coding scheme for the considered cancellation scheme, to obtain the immunity to channel interference without further reducing bandwidth efficiency.

The design of code-division multiple-access (CDMA) transmission systems for satellite communications requires an appropriate consideration of the distortion effects due to on-board nonlinear amplification. The aim of this paper is to provide an analytical framework for the evaluation of the in-band nonlinear distortion effects on the performance of CDMA systems. Both synchronous systems with orthogonal codes and asynchronous systems are considered. It is first shown that, when the users accessing the channel have the same power and their number is sufficiently large, the nonlinear distortion in the decision variables at the receiver can be simply described by a complex scale factor, which depends on the high-power-amplifier (HPA) characteristics only, and an additive noise, which is uncorrelated to the useful signal. Moreover, an analytical formulation of the bit error probability and the total degradation as a function of the output back-off and number of users is given. In the results, which are obtained for three classes of HPA models (i.e., the traveling wave tube amplifier, the solid-state power amplifier, and the amplifier with ideal predistortion), the performance and the capacity of power-limited systems is also discussed.

A pragmatic approach to space-time codes (STC) over block fading channels (BFC) is proposed. The new approach consists in using common convolutional codes to obtain STC, simplifying the encoder and the decoder. It is shown that pragmatic space-time codes (P-STC) achieve good performance, similar to that of the best known STC, and that they are suitable for systems with different spectral efficiencies and fading velocity (taken into account by the BFC model). To design P-STC we propose a search algorithm based on a new formulation of the pairwise error probability and the error enumerating function for geometrically uniform STC over BFC.

A channel adaptive scheduling algorithm for TDD/TCDMA radio interfaces was proposed by Ferracioli et al. (2000). This algorithm is investigated in this paper in order to assess its ability to improve the performance of the radio interface in the presence of fading and both real time and non real time traffic. More precisely, we estimate the maximum fading rate which gives performance improvements.

In this letter, a wireless local multipoint distribution system (LMDS) at millimeter waves for the last-mile broad-band distribution to users of interactive services is investigated. The system analyzed employs a coded orthogonal frequency-division multiplexing transmission scheme with frequency-division multiplexing (FDM) and/or time-division multiplexing techniques and adaptive carrier allocation to counteract the effects of the wireless communication channel. The idea of deploying the reverse channel (exploited by the user for interactivity purposes) to provide channel information to the broadcasting transmitter is introduced. System performance is evaluated for an urban microcellular radio system in an actual propagation environment and the choices of the optimum multiplexing technique and carrier allocation algorithm are discussed in the case of ideal feedback. It is found that a pure FDM technique combined with an adaptive carrier allocation algorithm giving priority to users having the largest path loss leads to the best performance.

In this paper the behavior of the UTRA-FDD radio interface of UMTS, based on wideband CDMA technology, is investigated at different conditions of Doppler bandwidth, network load and power control threshold. To this aim, a general simulation setup has been developed, which includes a cellular environment with hexagonal layout and uniform user distribution, a propagation channel with path-loss, log-normal shadowing and Rayleigh fast fading, a transmission system with SIR-based closed loop power control, convolutional coding with interleaving and RAKE receiver. Among the simulation results, the SIR process, the frame error probability process and the energy efficiency of the transmitters are analyzed. It is shown that power control provides a good level of fairness across the system and, even though an increasing network load reduces the fraction of users fulfilling the performance requirements, the behavior of the satisfied users remains unchanged. Moreover, the robustness to network load improves as the fading changes faster.

A general setup for simulating a WCDMA cellular system has been developed to estimate the performance, in terms of both the signal-to-interference ratio (SIR) and error burst statistic. Here, the analysis has been focused on closed loop power control in order to characterize the performance as a function of the control delay and the step size in different conditions. The simulator takes into account a hexagonal geometry with spatially uniform user distribution and a propagation channel characterized by distance attenuation, log-normal slow fading, and Rayleigh fast fading. The system is based on a wideband CDMA radio interface with RAKE receiver, convolutional coding with Viterbi decoding, and interleaving to counteract burst error occurrences. A relationship between the power control loop and the Doppler frequency trend is underlined in the numerical investigation. In particular, a power control delay increase could be related to an "equivalent" Doppler frequency increase. Furthermore, the sensitivity of the performance to the power control step size as a function of the Doppler frequency is shown.

This paper presents a theoretical characterization of nonlinear distortion effects in orthogonal frequency division multiplexing (OFDM) transmission systems. In the theoretical framework developed, it is shown that the effects on the decision variables of the in-band distortion introduced by a bandpass memoryless nonlinearity can be described by means of a complex gain and an additive Gaussian term with zero mean and suitable variance; analytical expressions for gain and variance are given. The conditions which allow this description are emphasized and discussed. As a consequence, a completely analytical procedure to evaluate error probability is also obtained and illustrated using OFDM/discrete multitone modulation (DMT) systems with rectangular pulse shaping; for the soft-envelope limiter nonlinearity, a closed form is derived. A comparison with simulation results is carried out to verify the accuracy of this method.

We investigate the performance of TCP Tahoe and NewReno on wireless links with a FEC/ARQ protocol at the link layer. The effect of link layer (LL) parameters like the LL packet size, number of LL transmission attempts, and FEC code rate on the TCP throughput is evaluated. It is shown that the retransmissions at the link layer must persist long enough to outlast the average bad state duration of the Rayleigh fading process in order to achieve performance better than TCP without an ARQ at the link layer. It is further shown that with a suitable link layer in place, TCP Tahoe offers as high a throughput as offered by other enhanced versions like NewReno in the considered wireless environment.

This paper proposes an efficient semi-analytical simulation method for the evaluation of convolutional code performance over fading channels. The major emphasis is placed on frame transmission over correlated fading channels attracting great attention in wireless communications. The simulation method exploits the analytical evaluation of suitable bounds for frame and bit error probability conditioned to the fading process samples. The most general bound is an upper bound obtained by adapting a well-known hound for the AWGN channel to the fading channel. A second simple approximated (lower) bound is also introduced to allow the construction of a very efficient estimator for frame and hit error probability. The efficiency of the proposed method with respect to the usual simulation is investigated and the results prove the possibility of an accurate and fast evaluation in the region of low error probabilities.

A high-speed indoor wireless communication system using coded orthogonal frequency-division multiplexing (OFDM) and working at 60 GHz is proposed and analyzed. An actual propagation environment consisting of a medium sized research laboratory, characterized by means of a ray-tracing technique, is considered for the analysis. In this contest the paper investigates and discusses the effects of frequency diversity, antenna sectorization, OFDM clustering, and different block coding strategies. Moreover, to characterize the communication between stationary indoor terminals at millimeter waves, a new definition of coverage is introduced. In order to evaluate the performance of the coded system in the actual environment, a suitable semianalytical algorithm is defined and applied. In the results the feasibility of a coded OFDM system for 155 Mbit/s packet transmission is checked. It is shown that all the line-of-sight (LOS) positions and 70% of the no LOS points can be covered in the scenario considered with a transmitted power of 10 dBm.

In this paper a wireless local multipoint distribution system (LMDS) at 30 GHz for the last mile broadband distribution to users of interactive services is investigated. The system we propose users a coded OFDM transmission scheme with FDM and/or TDM multiplexing techniques and adaptive carrier allocation to counteract the effects of the wireless communication channel. An urban microcell in an actual propagation environment is analyzed by means of a ray-tracing tool. Coverage probability is defined and investigated and the choices of the optimum multiplexing technique and carrier allocation algorithm are discussed. It is found that a pure FDM technique with adaptive carrier allocation and priority to users having the largest path loss leads to the best performance.

This paper proposes a new technique to reduce the non-linear distortion effects in orthogonal frequency division multiplexing (OFDM) systems. It exploits virtual carriers, which are modulated by using properly defined "dummy symbols", to obtain a transmitted signal with reduced envelope variations. With this method no additional redundancy is introduced, thus preserving the spectrum efficiency, and no changes at the receiver are required. To evaluate the dummy symbols some optimization criteria are investigated; an optimum algorithm and a low complexity iterative algorithm are proposed and compared from the point of view of the system's performance. The effectiveness of this technique is tested by considering some typical high power amplifier (HPA) models and by evaluating the performance degradation. A comparisons with other techniques proposed in the literature is also carried out.

A framework for the performance characterization of short-range communications systems is developed with the intention of investigating the feasibility of new multimedia wireless services at millimeter waves (MMWs). Both narrow- and wide-band systems are considered for mobile and/or fixed users. This paper aims at defining and evaluating proper metrics to characterize the service quality for the user and jointly takes the propagation characteristics, the transmission techniques, and the multiple access protocols into account. The definition of service-oriented metrics is emphasized. Three different real scenarios operating at MMW are investigated with a unified perspective: intelligent transport systems, wide-band local-area networks, and local multipoint distribution systems for interactive video services. The role played by the MMW band in the development of these services is discussed. In each scenario, accurate propagation analysis is carried out and suitable countermeasure techniques are pointed out in order to join suitable service-quality levels. The methodology considered is based on both analytical and semianalytical tools for performance evaluation.

Performance of small-cell wireless direct sequence code-division multiple access (DS-CDMA) systems with multilevel spreading sequences is investigated. An analytical methodology to evaluate both outage probability and mean bit error probability is presented; and its results are compared to those obtained with a semianalytical Monte Carlo based approach. Multipath propagation, the exact correlation properties of spreading codes, and both the simple correlator and RAKE receivers are taken into account in the analysis, which is oriented to asynchronous and synchronous environments. The spreading sequences investigated include multilevel sequences with complex isomorphic mapping, as well as some well-known two-level sequences. The numerical results show the impact of multilevel sequences on system performance, the improvements found in the absence of multiple access interference by using them rather than binary sequences, and the agreement between analytical and semianalytical evaluation.

The design of high quality, high speed wireless networks to connect mobile users requires large bandwidth availability. For this reason and with the aim of implementing new wireless systems, special attention has been paid to millimetre waves networks (e.g. at 60 GHz) based on short range communications. The paper presents some results concerning indoor channel characterisation. A ray tracing method has been applied to a realistic environment, with the view to define proper analytical models for propagation parameters. More precisely, large-scale received power, Rice factor, delay spread as a function of distance and small-scale variations statistics have been investigated in the presence and in the absence of room furniture: a two-state model has been proposed and checked.

This paper proposes a semi-analytical approach to characterise the performance of digital cellular systems when cochannel interference is determined by a small number of interferers. Linearly modulated signals (such as BPSK, QPSK, M-ASK, M-QAM, MSK, O-QPSK, etc.) with linear receivers are investigated by considering the probability density function of the interference at the input of the decision device. Non-linear (continuous phase) modulation signals (such as GMSK) and non-coherent detection are also investigated by means of simulation. As a consequence, a general approach to outage probability definition considering the protection ratio as a function of the interferers number and useful signal-to-noise ratio is introduced.

A completely analytical approach is presented in order to evaluate outage probability of TDMA (time-division multiple access) local radio systems using coding and diversity to combat frequency selective fading. First, starting from a multiray model for multipath propagation, performance sensitivity is tested by suitably simplifying the channel model. An analytical expression is obtained of the probability that BER may exceed a fixed value in the presence of a simplified channel model. A suitable upper bound formulation is given to express the same probability in the presence of coding and diversity techniques. Finally, some application examples are proposed both with reference to slow frequency hopping and space diversity systems.>