The proliferation of intelligent IoT devices and the growing demand for low-latency, task-aware inference have placed significant strain on communication and computation resources in next-generation (6G) wireless networks. Nevertheless, it is still an open problem to enable a semantic communication that alleviates the memory management in memory-constrained IoT networks. In this paper, we propose a modular and memory-efficient semantic communication framework for distributed task inference in IoT networks. Leveraging the information bottleneck (IB) principle, we introduce a novel sequential multi-step feature encoding scheme, wherein each encoder block is trained independently with earlier blocks held constant. This design supports flexible and reusable deployment of pre-trained encoders across heterogeneous devices with memory constraints. Building on this foundation, we formulate a multi-gateway edge inference architecture, where gateways selectively offload compressed task-relevant features to a core edge server. To handle resource limitations at the gateway level, we model a nonlinear mixed-integer optimization problem that selects encoder blocks for tasks in a manner that maximizes inference quality while satisfying delay and memory constraints. A greedy heuristic algorithm is proposed to efficiently solve the resource allocation problem. Simulation results demonstrate that our framework achieves up to 40% reduction in normalized delay compared to baseline methods, while tripling the number of concurrently supported tasks in the network.

In this paper, we investigate task-oriented communication in hierarchical in-X subnetworks, where multiple in-X devices collaboratively perform inference on a shared task by offloading their individual observations. To achieve robust and efficient collaborative inference, we propose a two-stage encoding framework tailored for this setting using information bottleneck (IB) principle. In the first stage, we focus on extracting task-relevant features from each device’s observation while mitigating the impact of channel impairments caused by wireless environment dynamics and inter-cell interference. In the second stage, a feature accumulator at the in-X access point (AP) is trained to effectively integrate features from different devices by reducing redundancy and filtering out common components across partial observations. To address the computational intractability of mutual information in our formulation for the two stages of encoding, we derive tractable variational upper bounds using a variational approximation technique. Extensive experiments on image classification tasks demonstrate that our proposed method achieves significant improvements in collaborative task inference performance and exhibits strong robustness to channel condition variations. Specifically, under challenging channel conditions, our method improves inference accuracy by up to 5%, while reducing the data load offloaded to the edge server by as much as 50%, depending on the defined resemblance index of the observations, in comparison with the baseline method.

In this paper, we introduce an innovative trajectory optimization framework for autonomous robots, designed to effectively navigate the complexities of maintaining network connectivity, avoiding collisions, and ensuring comprehensive area coverage. This framework employs GraphSAGE, a cutting-edge graph-based deep learning algorithm, notable for its adaptability to real-time environmental changes. Our approach enables autonomous robots to make strategic path-planning decisions that uphold continuous network connectivity, minimize the risk of collisions, ensuring optimal area coverage through strategic spacing and coordinated exploration. Through rigorous simulations, we have benchmarked the performance of our GraphSAGE-based method against traditional trajectory planning strategies. This research not only underscores the viability and scalability of integrating GraphSAGE into autonomous robotic systems but also marks a significant progression in autonomous navigation technologies, highlighting the capacity of graph-based deep learning algorithms to substantially improve the adaptability, performance, and operational efficiency of autonomous robots in complex network scenarios.

This paper presents a novel approach for managing interference in Vehicular Ad-Hoc Networks (VANETs) through dynamic resource allocation using Graph Isomorphism Networks (GINs). In VANETs, interference due to concurrent communications among vehicles within close proximity over the same channel can severely impact network reliability. Our proposed solution involves constructing a communication graph that represents vehicular interactions, where GINs are employed to intelligently allocate communication resources based on real-time channel conditions and vehicle movements. The GIN-driven approach adapts resource allocation dynamically, leading to optimized bandwidth utilization, minimized interference, and improved overall network efficiency. Through extensive simulations and comparisons, we showcase the superiority of our approach over traditional interference management methods. This research not only contributes to addressing interference challenges in VANETs but also underscores the potential of Graph Neural Networks (GNNs) in enabling intelligent and adaptive communication strategies in dynamic vehicular environments.

In wireless vehicular networks, dynamic bandwidth allocation (DBA) faces two main challenges: mobility and heterogeneity. Rapidly changing vehicle locations make it difficult to predict and allocate bandwidth efficiently, and the diverse communication capabilities and requirements of vehicles can make fair allocation a challenge. The solution proposed in this paper involves two algorithms. The first algorithm uses Graph Neural Networks (GNNs) to predict the connection topology of the vehicular network based on historical data. This topology can be used to prioritize vehicles for bandwidth allocation based on their Quality of Service (QoS) requirements and proximity to other vehicles. The second algorithm dynamically allocates available network resources based on demand, ensuring flexible, efficient, and reliable communication services to all vehicles. These algorithms work together to address the mobility and heterogeneity challenges while providing a fair distribution of network resources to all users. A novel approach is used to optimize resource allocation in a volatile network, ensuring that users with higher QoS requirements receive a larger share of bandwidth while maintaining fair distribution.

Federated learning (FL) is a distributed learning paradigm that has received great attention over the past several years due to its privacy-preserving property. As the models involved in FL are usually dense and overparameterized however, various studies are being conducted in gradient sparsification to reduce the high communication overhead. While many of the recently-presented schemes that are variations of top-k have shown competitive inference accuracy convergence to the baseline “vanilla” FL, they have a fixed sparsity rate throughout all of the communication rounds, which leads to an unnecessary excessive transmission of gradients as the global model converges. Furthermore, the constant-threshold gradient sparsification method called Threshold-z-, that is well-known for its dynamic rate, does not account for the ratio between the gradient and the pre-update parameter value, causing some gradients that are orders of magnitude larger than the pre-update parameter values to be neglected in the following aggregation process. In this paper, we introduce a new algorithm that addresses both of these issues, called adaptive ratio-based-threshold gradient sparsification method. Our main idea is introducing a custom gradient sparsity threshold for each local parameter based on their pre-update value and a hyperparameter denoted as Ψ. We demonstrate through image classification experiments on MNIST and CIFAR-10 datasets in both independent-and-identically-distributed (IID) and non-IID settings that under optimal Ψ, the gradient sparsity rates adapt & increase as the global model converges, while simultaneously producing inference accuracies that are competitive to vanilla FL.

Cellular traffic prediction is an essential aspect of mobile network management that uses data analytics and machine learning to forecast the volume and pattern of communication traffic generated by mobile users at a particular location and time. Graph Convolution Network (GCN) has been widely employed to model the spatial relationships between different cell towers and their neighboring counterparts. However, GCN is limited to highly regular and well-structured graphs. This paper proposes a Graph Attention Network (GAT) to capture more nuanced spatial relationships between cell towers, making it more suitable for irregular and complex graphs. Additionally, a novel graph attention mechanism is proposed that enables the creation of a dynamic graph structure, capable of capturing the evolving spatial relationships over time. Comprehensive experiments on an actual cellular traffic dataset show that the proposed technique outperforms state-of-the-art baselines on two evaluation metrics - RMSE and MAE - with a significant improvement.

We study the data collection problem in an Internet of Things (IoT) network where an unmanned aerial vehicle (UAV) is utilized to aggregate data from a set of IoT devices. We formulate the scheduling and path planning problems for the UAV. The goal of the scheduling problem is to find the sequence of nodes that the UAV will visit to complete the data collection task in the shortest possible time, ensuring that it does not run out of energy during its mission. We express this problem as a mixed-integer nonlinear problem and propose an efficient algorithm to solve the aforementioned NP-hard problem in polynomial time. Path planning problem aims to find a collision-free path for the UAV. While the state-of-the-art schemes have focused on solving the path planning problem in static environments, we study the problem in a dynamic environment with moving obstacles. We develop an algorithm that works on both static and dynamic environments. Our method combines deep reinforcement learning (RL) with graph-based global path planning algorithms to find a collision-free path for the UAV. One important advantage of our RL-based method over the existing studies is its map independency, which allows us to transform the agent’s learning from one environment to another. Via simulation studies, we show that our method is significantly effective in improving the safety of the path planning algorithms in dynamic environments.

To prolong the lifetime of the unmanned aerial vehicles (UAVs), the UAVs need to fulfill their missions in the shortest possible time. In addition to this requirement, in many applications, the UAVs require a reliable internet connection during their flights. In this paper, we minimize the travel time of the UAVs, ensuring that a probabilistic connectivity constraint is satisfied. To solve this problem, we need a global model of the outage probability in the environment. Since the UAVs have different missions and fly over different areas, their collected data carry local information on the network's connectivity. As a result, the UAVs can not rely on their own experiences to build the global model. This issue affects the path planning of the UAVs. To address this concern, we utilize a two-step approach. In the first step, by using Federated Learning (FL), the UAVs collaboratively build a global model of the outage probability in the environment. In the second step, by using the global model obtained in the first step and rapidly-exploring random trees (RRTs), we propose an algorithm to optimize UAVs' paths. Simulation results show the effectiveness of this two-step approach for UAV networks.

This paper studies the trajectory optimization problem for an aerial vehicle with the mission of flying between a pair of given initial and final locations. The objective is to minimize the travel time of the aerial vehicle ensuring that the communication connectivity constraint required for the safe operation of the aerial vehicle is satisfied. We consider two different criteria for the connectivity constraint of the aerial vehicle which leads to two different scenarios. In the first scenario, we assume that the maximum continuous time duration that the aerial vehicle is out of the coverage of the ground base stations (GBSs) is limited to a given threshold. In the second scenario, however, we assume that the total time periods that the aerial vehicle is not covered by the GBSs is restricted. Based on these two constraints, we formulate two trajectory optimization problems. To solve these non-convex problems, we use an approach based on the double Q-learning method which is a model-free reinforcement learning technique and unlike the existing algorithms does not need perfect knowledge of the environment. Moreover, in contrast to the well-known Qlearning technique, our double Q-learning algorithm does not suffer from the over-estimation issue. Simulation results show that although our algorithm does not require prior information of the environment, it works well and shows near optimal performance.

Aerial vehicles have recently attracted significant attention in a variety of commercial and civilian applications due to their high mobility, flexible deployment and cost-effectiveness. To leverage these promising features, the aerial users have to satisfy two critical requirements: First, they have to maintain a reliable communication link to the ground base stations (GBSs) throughout their flights, to support command and control data flows. Second, the aerial vehicles have to minimize their propulsion power consumption to remain functional until the end of their mission. In this paper, we study the trajectory optimization problem for an aerial user flying over an area including a set of GBSs. The objective of this problem is to find the trajectory of the aerial user so that the total propulsion-related power consumption of the aerial user is minimized while a cellular-connectivity constraint is satisfied. This problem is a non-convex mixed integer non-linear problem and hence, it is challenging to find the solution. To deal with, first, the problem is relaxed and reformulated to a more mathematically tractable form. Then, using successive convex approximation (SCA) technique, an iterative algorithm is proposed to convert the problem into a sequence of convex problems which can be solved efficiently.

In this paper, the trajectory optimization problem for a multi-aerial base station (ABS) communication network is investigated. The objective is to find the trajectory of the ABSs so that the sum-rate of the users served by each ABS is maximized. To reach this goal, along with the optimal trajectory design, optimal power and sub-channel allocation is also of great importance to support the users with the highest possible data rates. To solve this complicated problem, we divide it into two sub-problems: ABS trajectory optimization sub-problem, and joint power and sub-channel assignment sub-problem. Then, based on the Q-learning method, we develop a distributed algorithm which solves these sub-problems efficiently, and does not need significant amount of information exchange between the ABSs and the core network. Simulation results show that although Q-learning is a model-free reinforcement learning technique, it has a remarkable capability to train the ABSs to optimize their trajectories based on the received reward signals, which carry decent information from the topology of the network.

In order to face rising traffic demands and reduce the rigid requirements for antenna array design and operation, antenna placement geometry is being embraced by multiple automotive and cellular companies. Despite being supported in IMT2020 and 3GPP standards, MIMO antenna placement has been limited to two dimensional arrays, restricting antenna orientation flexibility and lack of scalability for Multi-user MIMO capabilities. This paper highlights the benefits of arbitrary placement of MIMO elements in 3D and introduces reconfigurable antenna structures for the base station as well as the UE, represented as a vehicular antenna array to expand the technology to Internet-of-Things (IoT) devices. The parameters of the patch antenna used are outlined for novel vehicular antenna structures. A 40% improvement is observed in user throughput for urban and suburban areas over typical 2D grid arrays.

Accommodating the coverage and capacity requirements of wireless cellular communication systems evolved contemporary cellular architectures to comprise two tiers: A macro-cell overlay coverage tier encompassing a small-cell tier offloading user traffic in high traffic demand areas. While two-tier cellular architectures can effectively meet coverage and capacity requirements when operating on independent frequency bands, radio spectrum scarcity entails sharing spectrum by both tiers which substantially complicates interference management for such architectures. In order to address uplink interference in two-tier cellular architectures, this paper considers the joint application of dual space-frequency block codes (SFBC) and signal alignment (SA) enabled physical network coding (PNC). When compared to designs adopting interference alignment (IA) or PNC, the proposed joint dual-SFBC with SA-PNC design substantially reduces interference and enables connecting a larger number of users. Numerical results also verify that the proposed method outperforms the SA-PNC static method without any information exchange requirement between the two tiers while achieving close to optimal performance for macro-cells.

Numerous candidate systems have emerged to address the connectivity requirements of Low Power Wireless Access (LPWA) Internet of Things (IoT) applications for massive Machine Type Communication (MTC). This paper analyzes the radio interface design of LoRaWAN systems operating in the North American 915 MHz licence-exempt frequency band. After providing a detailed overview of LoRaWAN system and connectivity structuring, the performance of LoRaWAN radio interface under the default North American mode of operation is thoroughly analyzed. Specifically, coverage, capacity and End Device (ED) throughput rates are determined for defined LoRaWAN Data Rate (DR) classes. Analysis of the LoRaWAN radio interface reveals a requirement of non-uniform ED distribution to attain maximal coverage and capacity, with the DR class achieving maximal coverage providing 1% of the peak capacity and the DRs providing 76% of the maximal capacity confined within 26% of the peak coverage area.

Two-tier cellular communication systems introduce a primary-secondary connectivity structure that improves the performance of cellular systems by offsetting the limitations of mobile User Equipment (UE) and providing superior high performing links to system Access Points (APs). This paper considers two-tier cellular systems employing vehicular-based primary nodes, with focus on enhancing the performance of primary links connecting vehicular nodes with system APs. Current vehicle mounted antenna designs do not take advantage of the relaxed energy and spacing constraints confining mobile UE antennas, thus limiting vehicle antenna gains to the elimination of vehicle penetration losses only. The employment of widely-spaced vehicular antenna arrays is proposed in this paper to enable the exploitation of high-order Multiple-Input Multiple-Output (MIMO) transmission schemes. Detailed system-level simulations are employed to compare the performance of various vehicular antenna array configurations for a wide range of deployment scenarios. When compared to narrowly-spaced antenna arrays with low-order MIMO transmission schemes, widely-spaced vehicular antenna arrays utilizing high-order MIMO transmission schemes are shown to provide substantial system coverage and capacity gains.

The uplink in a cellular network where the base station (BS) and an advanced user equipment (UE) that is not the traditional mobile device is investigated. This is proposed as a method of antenna offloading for the typical UE which suffers from size constraints. The Gram-Schmidt (GS) precoding algorithm is used to design novel precoding solutions when the receiver and transmitter can have a large antenna array (e.g. Nr= 64, NT= 32), as expected to be the case for deploying wireless backhaul links. Using the QR-decomposition on the Zero-Forcing (ZF) receiver, the proposed scheme is shown to essentially perform channel inversion with the help of the transmitter and receiver. With a moderate number of receive antenna elements, the matched filter (MF) was observed to incur inter-stream interference, while the ZF and minimum mean squared error (MMSE) receiver were more susceptible to transmitter side correlation compared to the GS precoding algorithm. When there is only channel correlation information (CCI) at the transmitter side, the GS precoding matrix is shown to be approximated by inverting the Cholesky decomposition of the CCI transmit matrix and outperforms the MF. Finally a block successive interference cancellation (SIC) detection scheme for the K-User MIMO channel is presented. It is shown for NT= 2 the GS precoder has a higher sum rate than the ZF receiver in the high signal to noise ratio (SNR) regime.

We consider a cellular network where base stations with widely different maximum power capabilities (or power classes) are deployed in a highly inhomogeneous or irregular pattern - referred to as an irregular cellular network. This paper proposes an extension of a spatially correlated shadowing model appropriate for irregular networks, that was detailed in previous work. The extended model has the capability to generate correlated lognormal shadowing in scenarios with inhomogeneous shadowing parameters. This is accomplished through computation of multiple Gaussian potential fields. A complete algorithm for the shadowing model is detailed in the paper. In addition, the model is verified through simulation, and computational complexity reduction is discussed. The proposed model enables the calculation of correlated shadowing in realistic irregular cellular networks. In addition, it forms part of a channel simulation framework which will be important in evaluating slow adaptation algorithms such as those forming part of 3GPP LTE SON.

The autonomous deployment of small Access Points (APs), commonly referred to as small cells, provides an economically feasible network densification solution to increase the capacity of cellular communication systems. However, having a small number of users with a single operator and typically employing a single Radio Access Technology (RAT) results in suboptimal spectrum utilization at small cells. To improve the performance and spectrum utilization at small cells, this paper presents a Dynamic Spectrum Access (DSA) framework that provides small cells with secondary access to spectrum primarily assigned to other RATs and operators. To avoid the shortcomings of spectrum sensing, APs broadcast spectrum occupancy information to facilitate secondary spectrum access. System-level simulations, performed for a setup of two operators employing two RATs, show substantial performance gains when implementing the proposed DSA framework.

We consider a cellular network in which base station positions, power levels and coverage areas are highly inhomogeneous or irregular - referred to in this paper as an irregular network. An adaptive algorithmic framework for resource allocation in irregular networks is discussed. Specifically, the framework is based on dynamically forming clusters of coordinating cells. As part of the overall framework, we implement an algorithm for cell outage compensation (COC) that is appropriate for irregular networks. The COC algorithm determines the base station power levels, outage user cell associations and a compensating cluster, and achieves robustness against cell outage.

More flexible spectrum access policies that take advantage of the cognitive radio capabilities of small cells are required to improve the suboptimal spectrum utilization at small cells. However, care must be taken to ensure that interference from autonomously deployed small cells is tightly controlled when developing such policies. This paper presents a dynamic spectrum access framework that enables small cells employing a specific Radio Access Technology (RAT) to access the spectrum allocated to other RATs as secondary users without degrading the performance of primary RATs. Secondary small cells opportunistically access unutilized spectrum in their coverage area and the protection of primary RATs is achieved by preventing secondary small cells from accessing spectrum when a potential interference scenario is detected. A system where HSPA and LTE-Advanced are co-deployed is considered. Simulation results show that substantial performance gains for both RATs are achieved by increasing spectrum utilization through secondary access for small cells. Furthermore, gains are achieved without degrading the performance of primary RATs.

In order to face the increasingly rising costs and reduce the required time to deploy and upgrade networks, network sharing is being embraced by many operators and multi-operator networks are being deployed worldwide. Despite being supported by current standards, spectrum sharing has not been widely adopted due to concerns over performance degradation under mismatched loading between systems. This paper addresses these concerns and studies the performance of spectrum sharing LTE-Advanced systems under different sharing scenarios. Advanced system level simulations are performed to measure and compare the performance of two LTE-Advanced systems under fixed spectrum allocation and spectrum sharing. Both matched and mismatched system loading are investigated under symmetric and asymmetric spectrum sharing scenarios. The performance gains are demonstrated and the bounds on spectrum sharing gains are determined.

In a traditional cellular network the base stations (BSs) are deployed regularly according to a pre-determined pattern modeled by hexagons. The deployment of a larger number of BSs improves the network performance. However, a dense regular deployment of BSs is prohibitively expensive and in most cases not feasible. Alternatively the network can grow in an organic fashion by the deployment of BSs according to the traffic demand. A high traffic demand in a given locality is reflected in a higher number of BSs deployed in the area. In this paper, we propose a practical framework for the resource allocation of cellular networks with an irregular BS deployment pattern. To this end, a network clustering technique is proposed which forms clusters of coordinating BSs. The coordinated resource allocation among the BSs within each cluster is devised to achieve proportional fairness. The performance of the proposed framework is evaluated with regular as well as irregular deployment of BSs. The results are compared against standard resource allocation techniques and show promising results.

This paper proposes an adaptive and scalable approach to resource allocation in cellular networks. We introduce an adaptive strategy to form clusters of coordinating base stations, referred to as scheduling cells (SCs), based on the local traffic demand. A system parameter is proposed which adjusts the level of coordination between the base stations. A proportional fair coordinated resource allocation scheme is discussed which allows informed reuse of resources within each cluster. The performance is evaluated for a system with one frequency channel. The proposed framework can adaptively form clusters of coordinating base stations in an autonomous cellular network with an irregular deployment of base stations.

In collaborative spectrum sensing, the presence of the primary user is detected at a central entity, known as the fusion center. This center collects the information from the secondary users and decides on the occupancy of the desired frequency band. In the conventional strategy, the secondary users transmit their initial observations toward the fusion center with their maximum transmit powers. In this paper, however, we consider the problem of beamforming among the secondary users with individual power constraints. Correlated shadow fading has been considered in the channel gains between the primary transmitter and the secondary users as well as the channel gains between the secondary users and the fusion center. We consider the problem of maximizing the probability of detection for a required probability of false alarm. Most previous works have considered the total power constraint while in practical scenarios, each secondary user has a limited battery lifetime. An algorithm is developed which efficiently solves the problem via second order cone programming (SOCP) in an iterative manner. An approximation of the original problem is studied which reduces the computational complexity of the iterative procedure. The Monte Carlo simulations confirm the effectiveness of the spectrum sensing framework compared to the conventional strategy.

This paper proposes an adaptive fractional frequency reuse (FFR) resource allocation scheme for downlink transmission of OFDMA cellular networks. Resource allocation is localized in the sense that in each region system resources are assigned based on local traffic demand. Furthermore, the allocated resources in each region adapt to changes in the environment (i.e. channel gains and number/mobility of active terminals). To this end, a multi-stage proportional fair scheduler is introduced. The performance of the proposed scheme is compared with a variation of the well-known FFR technique. Numerical results studying the average and cell edge spectral efficiencies show promising improvements.

This paper proposes a generalized methodology for frequency reuse in future cellular networks. These networks are equipped with a mixture of access points (APs) in different sizes and power capabilities. Some (or all) of the APs are deployed autonomously which results in an irregular deployment pattern. We first generalize the concept of a frequency reuse cluster - originally proposed for traditional cellular networks. Subsequently, adaptive clustering of APs is proposed based on which a resource allocation methodology is presented. Finally a hierarchical design framework with four levels of abstraction is introduced.

In a downlink multiuser multiple-input and multiple-output (MU-MIMO) system, a base station (BS) communicates with multiple mobile stations (MS) simultaneously in a given spectrum band. The performance of a MU-MIMO system depends on the choice of user selection, power allocation and precoding schemes. In this paper, we study user selection in Zero-Forcing (ZF) precoding based MU-MIMO systems to maximize the sum rate for high data rate applications. We derive analytical results for simultaneous transmission to two MSs with zero forcing (ZF) precoding, based on which we propose a extended low-complexity algorithm that jointly considers the noise power, the channel gain of the candidate MSs and orthogonality with respect to the selected MSs' channels. Analysis shows that the proposed scheme requires much lower complexity than current schemes. Simulation results demonstrate that the proposed algorithm outperforms existing algorithms in terms of higher throughput.

In UMTS cellular networks, Common Pilot Channel (CPICH) signals are broadcast by base stations for channel estimation and cell selection. The strength of CPICH signal determines cell coverage and pilot pollution to neighboring cells; pilot power allocation thus involves a tradeoff between coverage and interference. In this paper, we study the issue of pilot power management in closed-access High-Speed Downlink Packet Access (HSDPA) femtocell network. We minimize the total pilot power in a HSDPA femtocell network subject to instantaneous coverage requirements. We formulate a generalized optimization problem in a femtocell network and propose a suboptimal analytic solution to pilot power allocation implemented in each femtocell. Simulation results show that the proposed algorithm outperforms fixed pilot power schemes in terms of much lower allocated pilot power and interference to macro user equipments.

In this paper, we consider the problem of optimal control for throughput utility maximization in cognitive radio networks with dynamic user arrivals and departures. The cognitive radio network considered in this paper consists of a number of heterogeneous sub-networks. These sub-networks may be power-constrained and are required to operate in such a way that the average total interference received on primary channels are kept below given thresholds. We develop a control policy that performs joint admission control and resource scheduling. Through Lyapunov optimization techniques, we show that the proposed policy achieves a utility performance within O(¿) of optimality for any positive ¿. We further show that this arbitrarily closeness to optimality comes at the price of having a delay that is O(1/¿) in admitting users. We also propose constant factor approximations of the policy for distributed implementation.

We study a cognitive channel reuse approach for autonomous femtocell deployment in an OFDMA-based cellular network. The objective is to manage the uplink interference from macrocell users to femtocell access points. Macrocell users are normally far away from the base station and may cause strong uplink interference to the close-by femtocells. By cognitive interference recognition, a femtocell access point autonomously reuse the proper channel modes for interference minimization. We propose an opportunistic channel scheduling scheme that determines optimal channel and power allocation to femtocell users. Simulation results show improved uplink performance in terms of the average channel outage probability in femtocells.

In a multihop cellular network, mobile terminals are able to transmit directly to other mobile terminals allowing them to lower their maximum transmission power and use other terminals as relays to forward traffic towards the base station. However, a large amount of interference is created near the base station because all traffic either emanates or is destined to the base station making it the capacity bottleneck of the network. In an autonomous infrastructure multihop cellular network, certain mobile terminals that have a connection to the backbone network act as access points and send traffic directly onto the backbone network, as would a base station. This reduces the amount of traffic required to be handled by the base station and increases network capacity. However, access points will require transmission parameters like their pilot power to be adjusted autonomously to optimal levels. In this paper, we propose an autonomous pilot power protocol that can be used by both access points and base stations. Our simulation results show that by adjusting a parameter within the pilot power protocol, a required percentage of covered terminals can be achieved by the network without prior knowledge of the location or density of terminals. Furthermore, the pilot power protocol determines the pilot power level which is optimal in terms of SINR and power consumption that achieves the required coverage while effectively eliminating the capacity bottleneck that existed at the base station.

In this paper we review a 4G vision of deploying a cellular network whose infrastructure topology can be autonomously determined by cellular users. Intelligent spectrum management is required for cellular interference control and mobile handoff management etc. We study an optimization method that maximizes the cell size coverage for user-deployed base stations, e.g. femtocells. The algorithm periodically updates the pilot power configuration based on each femtocell's channel pass loss and the global traffic distribution in the network. Performance results are studied by simulation based on the WCDMA cellular standard.

In a multihop cellular network, the physical layer of mobile terminals is modified so that in addition to being able to transmit to base stations, mobile terminals are also able to transmit directly to other mobile terminals. This allows mobile terminals to lower their maximum transmission power and use other terminals to relay their traffic towards the base station. However, there is still a large amount of interference surrounding the base station because all traffic either emanates or is destined to the base station making it the capacity bottleneck of the network. In order to reduce the interference surrounding the base station, we propose a novel architecture called the autonomous infrastructure multihop cellular network. In this architecture, certain mobile terminals that have a connection to the backbone network will be allowed to act as access points. Access points will receive traffic from other terminals and send it directly onto the backbone network, as would a base station. This will reduce the amount of traffic required to be handled by the base station and increase network capacity. The results of our analysis and simulations show that when mobile terminals can act as access points, the SINR at the base station is higher, the power consumption is lower and the coverage is better than in a normal multihop cellular network.

We study a method of cognitive interference management in user-deployed 3G femtocell networks. Each femtocell cognitively recognizes an interference signature from the network environment and opportunistically allocates the proper channel patterns for interference minimization. Simulation results show great channel SINR improvement by implementing our opportunistic channel scheduler. The approach offers a novel view of autonomous spectrum management based on cognitive interference recognition and adaptive channel management.

It is well known that the generalized max-weight matching (GMWM) scheduling policy, and in general throughput-optimal scheduling policies, often require the solution of a complex optimization problem, making their implementation prohibitively difficult in practice. This has motivated many researchers to develop distributed sub-optimal algorithms that approximate the GMWM policy. One major assumption commonly shared in this context is that the time required to find an appropriate schedule vector is negligible compared to the length of a timeslot. This assumption may not be accurate as the time to find schedule vectors usually increases polynomially with the network size. On the other hand, we intuitively expect that for many sub-optimal algorithms, the schedule vector found becomes a better estimate of the one returned by the GMWM policy as more time is given to the algorithm. We thus, in this paper, consider the problem of scheduling from a new perspective through which we carefully incorporate channel variations and time-efficiency of sub-optimal algorithms into the scheduler design. Specifically, we propose a dynamic control policy (DCP) that works on top of a given sub-optimal algorithm, and dynamically but in a large time-scale adjusts the time given to the algorithm according to queue backlog and channel correlations. This policy does not require the knowledge of the structure of the given sub-optimal algorithm, and with low-overhead can be implemented in a distributed manner. Using a novel Lyapunov analysis, we characterize the stability region induced by DCP, and show that our characterization can be tight. We also show that the stability region of DCP is at least as large as the one for any other static policy. Finally, we provide two case studies to gain further intuition into the performance of DCP.

Recently wireless sensor networks featuring direct sink access have been studied as an efficient architecture to gather and process data for numerous applications. We focus on the joint effect of clustering and data correlation on the performance of such networks. We propose a novel cluster-based data collection scheme for sensor networks with direct sink access (CDC-DSA), and provide an analytical framework to evaluate its performance in terms of energy consumption, latency, and robustness. In our scheme, CHs use a low-overhead and simple medium access control (MAC) conceptually similar to ALOHA to contend for the reachback channel to the data sink. Since in our model data is collected periodically, the packet arrival is not modeled by a continuous random process and, therefore, our framework is based on transient analysis rather than a steady state analysis. Using random geometry tools, we study how the optimal average cluster size and energy savings vary in a response to various data correlation levels under the proposed MAC. Extensive simulations for various protocol parameters show that our analysis is fairly accurate for a wide range of parameters. Our results suggest that despite the tradeoff between energy consumption and latency, both of which can be substantially reduced by proper clustering design.

Motivated by the low utilization of the licensed spectrum across many frequency bands, sensing-based oppor- tunistic spectrum access has recently emerged as an alternative to the outdated exclusive spectrum access policy. Under this new paradigm, a secondary (unlicensed) user monitors a primary (licensed) frequency band for a given and opportunistically transmits if it does not detect any ongoing licensed operations. Evidently, selection of the parameters involves balanc- ing a tradeoff between the speed and the quality with which the secondary user senses the licensed band. With the average throughput as the performance criterion, we obtain the parameters so as to optimize the performance of the secondary user while providing the primary user with its desired level of interference protection. I. INTRODUCTION As evidenced by recent measurements, many frequency bands across the licensed spectrum are significantly under- utilized (1), (2). This finding suggests that the spectrum scarcity, as perceived today, is largely due to the inefficient fixed frequency allocations rather than the physical shortage of the spectrum and has led the regulatory bodies to consider the opportunistic access to the temporally/spatially unused licensed bands (a.k.a. the white spaces) as a means to improve the efficiency of spectrum usage. In the absence of cooperation or signalling between the primary licensee and the secondary users, spectrum availability for the opportunistic access may be determined by direct spectrum where the secondary user monitors a licensed band for a given sensing time and opportunistically transmits if it does not detect any ongoing licensed operations. This approach is particularly appealing due to its low deployment cost and its compatibility with legacy primary users and is being considered for inclusion in the upcoming IEEE 802.22 standard for opportunistic access to the TV spectrum (3). Due to their ability to autonomously detect and to react to the changes in the spectrum usage, secondary users equipped with the spectrum capability may be considered as a primitive form of the cognitive radio (4). Design of any scheme involves balancing a tradeoff between the quality and the speed of through an appropriate selection of the time. As we shall illustrate, in the context of spectrum sensing, may be fine- tuned to enhance the secondary users' perceived quality-of- service (QoS) as long as the regulatory constraint for the protection of the primary users against harmful interference is satisfied. In particular, we will obtain the optimum times at different stages of the spectrum to maximize the average throughput of the secondary user. In this paper, simple energy detection (a.k.a. radiometry) (5) is chosen as the underlying detection scheme. In general, when some information about the structure of the primary signal is available, ad hoc feature-detectors offer a better performance (6). We note, however, that the methodology employed in this paper may be applied to optimize different spectrum sensors once the quality is characterized in terms of the time. The remainder of this paper is organized as follows. The regulatory constraints on spectrum are described in the following section. Section 3 provides an overview of the energy-based spectrum sensing. The optimum times for different stages of the spectrum are derived in Section 4. Finally, this paper is concluded in Section 5.

Pilot signals are often used to ease mobile channel acquisition and detection. Cochannel signals are usually treated as interference. However, for synchronized cellular CDMA forward links, all cochannel signals from the same base station share a spatial and temporal channel signature. We propose an extended blind subspace approach to exploit this feature to enhance channel estimation. We demonstrate the performance gain by simulating two transmission schemes for MIMO cellular base stations: fixed beam and adaptive beam.

Throughput optimal scheduling policies in general require the solution of a complex optimization problem. The past literature has shown that the complexity of this optimization problem can be greatly reduced, but at the expense of memory requirement that is exponential with the number of users. In this paper, we study the stability region of a class of linear-memory scheduling policies for time varying channels, and investigate how the channel memory impacts the supportable input rates. The set of scheduling policies in this paper covers a wide spectrum of resource allocation algorithms, which allows us to study policies with different complexity levels. In particular, we are able to model a class of low-complexity scheduling policies with linear memory, which are suitable for practical implementation.

Traditionally, the frequency spectrum is licensed to users by government agencies in a rigid manner where the licensee has the exclusive right to access the allocated band. Therefore, licensees are protected from any interference all the time. From a practical standpoint, however, an unlicensed (secondary) user may share a frequency band with its licensed (primary) owner as long as the interference it incurs is not deemed harmful by the licensee. In a fading environment, a secondary user may take advantage of this fact by opportunistically transmitting with high power when its signal, as received by the licensed receiver, is deeply faded. In this paper we investigate the capacity gains offered by this dynamic spectrum sharing approach when channels vary due to fading. In particular, we quantify the relation between the secondary channel capacity and the interference inflicted on the primary user. We further evaluate and compare the capacity under different fading distributions. Interestingly, our results indicate a significant gain in spectrum access in fading environments compared to the deterministic case

Motivated by the results in (K. Navaie et al., 2006) on the self-similarity of the downlink interference in heterogenous service DS-CDMA networks, in this paper, we propose a model-based linear adaptive-predictive method to estimate the level of interference for optimizing the system throughput and minimizing the delay for non-real-time data transmission. We use a fractional Gaussian noise (fGn) model in an appropriate time-scale to represent the self-similarity in the downlink interference. The estimated interference is utilized to allocate the available power to non-real-time services. In doing so, we use a utility-based optimization scheme and dynamic programming for time-domain optimal scheduling of non-real-time traffic. Simulation results validate the fGn model and show a substantial improvement in the delay fairness and a significant increase in the average cell throughput using our proposed scheme; and confirm that the interference model is valid for a broad range of arrival rates of non-real time traffic.

Abstract Opportunistic unlicensed access to the (temporarily) unused frequency bands across the licensed radio spectrum is currently being investigated as a means to mitigate the spectrum scarcity. Such opportunistic access calls for the implementation of safeguards so that the ongoing licensed operations are not interfered with. Among different candidates, sensing‐based access, where thesecondary(unlicensed) users transmit if they sense theprimary(licensed) band to be free, is particularly appealing due to its low deployment cost and its compatibility with legacy primary systems. Incorporatingspectral awarenessfunctionality into the radio transceivers is a major step towards the realization of thecognitive radios. In this paper performance of spectrum‐sensing cognitive radios is studied under channel fading. In particular, it is shown that due to the uncertainty resulting from fading, local signal processing alone may be inadequate to meet the performance requirements. To remedy this issue, cooperation among secondary users is proposed and studied in this paper. Moreover, we characterize and study a tradeoff between local processing and cooperation, which should be balanced in order to maximize the spectrum utilization. Copyright © 2007 John Wiley & Sons, Ltd.

Traditionally, the frequency spectrum is licensed to users by government agencies in a rigid manner where the licensee has the exclusive right to access the allocated band. Therefore, licensees are protected from any interference all the time. From a practical standpoint, however, an unlicensed (secondary) user may share a frequency band with its licensed (primary) owner as long as the interference it incurs is not deemed harmful by the licensee. In a fading environment, a secondary user may take advantage of this fact by opportunistically transmitting with high power when its signal, as received by the licensed receiver, is deeply faded. In this paper we investigate the capacity gains offered by this dynamic spectrum sharing approach when channels vary due to fading. In particular, we quantify the relation between the secondary channel capacity and the interference inflicted on the primary user. We further evaluate and compare the capacity under different fading distributions. Interestingly, our results indicate a significant gain in spectrum access in fading environments compared to the deterministic case.

Spectrum scarcity is becoming a major issue for service providers interested in either deploying new services or enhancing the capacity for existing applications. On the other hand, recent measurements suggest that many portions of the licensed (primary) spectrum remain unused for significant periods of time. This has led the regulatory bodies to consider opening up under-utilized licensed frequency bands for opportunistic access by unlicensed (secondary) users. Among different options, sensing-based access incurs a very low infrastructure cost and is backward-compatible with the legacy primary systems. In this paper we investigate the effect of user collaboration on sensing- based secondary access in terms of critical parameters such as the spectrum utilization, the required signal-to-noise-ratio and the detection time. As suggested by our analysis and simulation results, collaboration may improve the opportunistic spectrum access significantly.

In the multicarrier direct-sequence code-division multiple access with repetition code system, decreasing the code rate can result in greater frequency diversity. However, to keep the same transmit bit rate, the spreading factor in each subcarrier needs to be reduced. In this paper, we use the outage probability as a criterion for these two system parameters' design. We first examine the condition on code rate for the uncorrelated fading channel assumption. The system performance is derived and the outage probability is obtained in closed forms for equal and unequal transmit bit energy scenarios. The relation between the spreading factor and the rate of the repetition code is investigated by observing the outage probability equation.

The problem of base station antenna assignment (BSAA) with minimum mobile transmit power (MTP) is studied for CDMA networks with fixed overlapping sector antenna architecture (FOSAA) where more than one co-located antenna is used to cover any space in the network. It is first noted that the non-FOSAA has limitations in switching users between in-cell sectors and also out-of-cell sectors in moderately-loaded networks. It is then shown that by employing overlapping sectors in FOSAA, we can exploit the flexibility of assigning a user to one of possibly many potential antenna to effectively support the nonuniform azimuthal traffic. It is also shown that the BSAA problem with minimum MTP is a special case of a general problem that was solved by Hanly and Yates. The process of dynamic cell sectoring is differentiated twofold as cell-breathing (CB) and cell-slicing (CS) and the latter can be viewed as being azimuthal discrete counterpart of the former radial scheme. The hybrid scheme, CB+CS, offers better performance in terms of minimum total MTP in a FOSAA system. Simulation results demonstrate the flexibility and effectiveness of the FOSAA system in nonuniform angular traffic

In this paper, we study the downlink of a multiuser system, in which antenna arrays are employed at both the transmitter (base station) and the receivers (clients). A space-time modulation technique that can be seen as two-dimensional spreading is introduced. It provides full transmit diversity for every user, and accommodates Nttimes the number of users as a single-antenna code-division multi-access (CDMA) scheme, where Ntis the number of transmit antennas. Thus multiple access is provided through spatial as well as code dimensions. In addition, the scheme forms groups of users that are orthogonal to each other. This feature translates into simplified detection strategies without loss of performance. The main detector structure of interest is a two-stage interference canceller because of its low complexity compared to other joint detectors. We will demonstrate that in conjunction with an unequal power allocation scheme, this receiver provides full diversity and suffers from only a small performance loss compared to the full-complexity maximum likelihood (ML) receiver. In a single-user multiple antenna system, the same spreading scheme and unequal power allocation yields a new approach to designing full-rate, full-diversity space-time codes having good performance with successive interference cancellers

In this paper, we show that the total downlink interference in heterogeneous wireless DS-CDMA networks follows an asymptotically self-similar (as-s) process. The as-s model is valid for the interference under certain conditions on channel variations and traffic characteristics that cover a range of practical situations. We derive these conditions and generalize earlier results, obtained for data-centric cellular networks, to heterogeneous cellular networks. Simulation results for actual cases confirm analytical results, and show that non-uniform spatial distribution of users and their soft-hand-off status do not affect the nature of this self-similar process. Furthermore, we discuss the impact of the analysis developed in this paper in designing appropriate mechanisms for controlling radio resources in such networks.

Assuming perfect channel knowledge at the transmitter, we study space-time beamforming for the downlink in a multiuser multi-input multi-output (MIMO) channel while a nonlinear interference pre-cancellation is presumed at the transmitter. The antenna arrays may be employed at both the transmitter and the receivers. The optimum transmit/receive beam vectors are obtained based on a minimum mean-squared error (MMSE) criterion and a per-user power constraint. In frequency selective fading channels, where orthogonal frequency division multiplexing (OFDM) is applied, the precoding and beamforming design is extended over space and frequency dimensions as well. In fact the proposed algorithm offers a unique method for assigning frequency bins in a MIMO-OFDMA system. The bit error rate performance of the proposed algorithm is assessed by computer simulations.

In this paper, we develop a cross-layer model for downlink interference in heterogeneous DS-CDMA wireless cellular networks. In this model, interference is described as a function of application layer parameters (traffic characteristics) and physical layer variations (channel characteristics). We show that for a heterogeneous service DS-CDMA network, downlink interference is a second-order self-similar process and thus has long-range dependence. We then use the predictive structure of total downlink interference to maximize non-realtime data throughput. We use fractional Gaussian noise (fGn) to model the self-similarity of downlink interference. In the proposed method, the base-station uses an optimal linear predictor, based on the fGn model, to estimate the level of interference. The estimated interference is then used to allocate power to users. To maximize data throughput, we use time domain scheduling. The simulation studies confirm the self-similarity of downlink interference and validate the fGn model. The simulation results also show a substantial performance improvement using the proposed predictive-adaptive scheme and confirm that the interference model is still valid after applying the proposed method.

In this paper, using the dynamic pricing platform, a novel framework for downlink resource allocation in heterogeneous cellular CDMA networks is proposed. For each user, we define a utility that is a function of channel status and delay condition of that individual user as well as network load status. The network utility is then defined as the summation of the users' achieved utilities. We solve downlink resource allocation problem through maximization of total network utility. This approach results in a suboptimal base-station assignment scheme which-unlike previous work-is network optimal instead of cell optimal. We then show that optimal base-station assignment is a multidimensional multiple-choice Knapsack problem (MMKP). Since MMKP is NP-Hard a polynomial-time suboptimal heuristic algorithm is then employed to develop an efficient base-station assignment.

In this paper, using the concept of capacity regions, an algorithm for joint rate allocation and routing scheme is proposed in order to maximize the total throughput of multihop cellular CDMA networks. The notion of infeasibility factor is defined and used to propose an adaptive scheme on top of the above algorithm to manage fundamental coverage-capacity tradeoff for the downlink. In fact, the output of our algorithm is the joint rate allocation, routing scheme and base station (relay) assignment which in conjunction with the proposed adaptive scheme leads to implementing the concept of cell breathing via allocating proper transmit powers to base stations and relays. We show that our optimization problem can be mapped to a multidimensional multichoice knapsack problem (MMKP). Since MMKP is NP-hard, a polynomial-time heuristic algorithm is then developed to solve our MMKP.

Space-time block coding (STBC) is a promising spatial diversity technique for future wireless communications systems. The combination of STBC and wideband code division multiple access (WCDMA) technology has the potential to increase users performance in wireless communication networks. In the downlink of WCDMA systems, multiple access interference (MAI), which impairs system performance, can be mitigated by chip level channel equalization. In this paper, we have proposed to combine chip level STBC (CSTBC) (I. R. S. Casella et al., 2003) and equalization pos-combining with channel estimation (EPCCE) scheme (2003) for improving diversity and system robustness to frequency selective fading channels. Additionally, we present a new iterative channel estimation (ICE) algorithm to reduce inter-antenna interference (IAI), due to multiple antennas transmissions, and improving space-time block decoding at the receiver when the number of training symbols is limited.

Space-time block coding has emerged as a promising spatial transmit diversity scheme that reduces the fading effects of multipath channels and increases the system capacity, mainly due to the its reduced decoding complexity and channel state information that is not required at the transmitter side. Recently, iterative processing has also attracted vast attention for improving the system performance at low complexity cost. We propose a new chip space-time block coding structure that employs iterative joint decision feedback equalization and decoding scheme for improving the performance of the downlink of a WCDMA system using long scrambling codes under frequency selective fading channels. Simulation results show that the presented schemes offer significant performance improvement against conventional disjoint receivers.

We study the downlink of a MIMO multi-user system when no information about the channel is assumed at the transmitter. Instead of applying direct-sequence code-division multiple access (DS-CDMA) over the codewords of conventional space-time coding (STC) schemes, a modulation technique that can be seen as two-dimensional space-time spreading (2D-STSC) is described. It is based on well-known Walsh codes, provides full transmit diversity and high spectral efficiency, and produces groups of users that are orthogonal to each other. This last point translates into simplified detection strategies without loss of performance. The main detector structure of interest is a two-stage interference canceller which employs serial interference cancellation (SIC) in the first stage. We will demonstrate that in conjunction with an unequal power allocation scheme, this receiver is able to provide full diversity and suffers from only a small performance loss compared to the full-complexity maximum likelihood receiver. The proposed scheme compares favorably with related ones in terms of spectral efficiency, bit error probability, and complexity at the receiver.

Four different digital delay locked loops for PN code tracking in DS-CDMA systems are compared in this paper. Three of them, namely coherent, noncoherent and modified tracking schemes are well known. The fourth is the new scheme that we propose here as an alternative to the noncoherent (L2 norm) DLL. The new scheme is based on L1 norms of the early and late correlator output signals that are used to form an error signal. The error signal is used to control the local PN code delay. It has been shown that L1 norm based tracking offers similar performance in terms of tracking jitter to that of L2 norm tracking and as such is a viable candidate for implementation because it is computationally less demanding when implemented on DSPs.

Beyond 3G high speed cellular systems like HDR use a high speed downlink shared channel to provide users with services which are in many cases non real time. Each base station schedules transmissions to its users in a one by one fashion and transmits at its full power. In order to maintain an acceptable degree of fairness among users we should either assign more time slots to users at the cell boundary experiencing higher interference or use other means like intercell coordination to reduce users ' received interference. This paper proposes a fixed distributed intercell coordination method where base stations in a cluster of neighboring cells transmit one by one in a round-robin fashion and further shows that this intercell coordination scheme does not necessarily benefit all users of a cell especially when there is a maximum data rate limit for some users due to hardware limitations. Thus in order to increase the efficiency of our fixed intercell coordination scheme we propose a time reuse partitioning algorithm similar to the channel reuse partitioning in narrowband cellular systems.

Nonuniform traffic can severely degrade the performance of CDMA cellular systems. To alleviate this, load balancing schemes have been proposed which let the neighboring cells accommodate the traffic of a heavily loaded cell. Most of the previous works have addressed load balancing for the uplink since it was considered to be the bottleneck of the system performance. In fact it is very likely for the uplink to become congested before the downlink in the presence of conventional symmetric services (e.g. voice). However emergence of the new highly asymmetric services has put much more demand on the downlink and there may be many cases where the downlink of a cell is heavily loaded without significant uplink-loading. This motivates us to perform downlink load balancing by assigning an appropriate base station to each user. In this paper we propose an algorithm based on space division duplex which assigns optimal base stations for the up- and downlink independently.

In this paper, the coded performance of a previously proposed space-time multiuser multiplexing scheme called the spread space-spectrum multiple access (SSSMA) (Ng, BK et al.) is investigated for the forward link MIMO system. The key feature of SSSMA is that the number of user-channels is increased by exploiting additional degrees of freedom offered by multiple antennas. The large MIMO data pipe is divided and allocated to multiple coded user-channels over the entire transmission time interval and bandwidth. Thus, unlike the orthogonal multiple access scheme, the spatial multiple access interference (MAI) exists in SSSMA and is mitigated through the use of the space-time diagonal (STD) spreading sequences. When coding is introduced, each user may employ a user-specific optimal inter-leaver. It is theoretically shown that for two transmit antennas and optimal interleaving, the full diversity criterion is satisfied for all multiuser codeword pairs, thereby suggesting that spreading is an effective means to achieve spatial diversity at high bandwidth-efficiency. With suboptimal detector such as interference-cancelling receiver based on turbo processing, it is shown that the SSSMA offers near-capacity performance that is superior to many well-known MIMO transmission schemes.

In this paper, we propose a new differential semi-blind spatial-temporal beamforming receiver for asynchronous direct sequence wideband code division multiple access (DS-WCDMA) systems. The new receiver presents improved robustness to phase variations and reduction of the effects of nonlinear distortion.

Recently, space-time block coding has emerged as a promising spatial transmit diversity scheme, mainly due to its simple decoding complexity. As it was initially proposed, space-time block coding was based on the assumption of flat multipath fading channel, where there is no intersymbol interference. In this paper. we investigate the performance of two different receiver proposals using space-time block coding for frequency selective fading channels. The first one, called here equalization pos-combining with channel estimation receiver, is based on the time-reverse space-time block coding proposed by Lindskog and Paulraj and it performs channel-estimation, linear combination and equalisation. The second one, denoted here multiple-input multiple-output joint equalization-combining receiver, is based on the scheme presented by Meshkati and Sousa and it performs channel equalisation, interference cancellation and linear combination simultaneously. Simulation results show that, increasing the signal to noise ratio, multiple-input multiple-output joint equalisation-combining can outperform equalisation pos-combining with channel estimation.

In this paper, we investigate the performance of the downlink of a wideband code division multiple access system employing space-time block coding in a time-varying multipath channel. Space-time block coding is applied at the chip level to improve the robustness of the system to fading envelope variations. Simulation results show that the presented scheme offers considerable diversity gain even for high mobility users transmitting low data rate, outperforming a system employing symbol level space-time block coding in time-variant channels.

In this paper we examine the effect of the unresolvable multipath on the performance of mobile unit that uses antenna diversity scheme for three different types of tracking algorithms. In the first algorithm, the tracking is performed on the basis of the single antenna output. The second algorithm involves RAKE like tracking, where all antenna outputs are combined before tracking. The last algorithm is based on independent tracking for each antenna. It has been shown that the best performance in terms of bit error rate at the receiver is achieved with independent tracking in the presence of unresolvable multipath.

A cellular system with overlapping sectors is proposed to enhance system performance through interference avoidance. Due to the overlapping effect, each user is allowed to select one sector out of many for operation. It is shown that the mutual interference among users in different regions within a cell can be minimized by choosing a good sector combination. The potential benefits of the proposed technique are investigated in two scenarios: orthogonal and nonorthogonal code division multiple access systems. In the former case, it is shown that when compared with the traditional nonoverlapping sectors system, one is able to reduce the number of spreading codes required to support all users by using appropriate code allocation in the proposed system. A theoretical framework based on combinatorial enumeration is developed to assess the performance of the proposed system. For the latter case, the enhancement takes the form of multiple access interference reduction. Simulation results show that its improvement is significant and can be further enhanced by increasing the number of overlapping sectors.

In this paper, a packet scheduling scheme based on real-time channel conditions and avoidance of dominant inter-cell interferers is proposed and studied. This scheme is implemented by the distribution of tags by receivers among transmitters. We focus on reverse links of a CDMA/TDM system where for every slot, each base station issues M(/spl ges/1) tags to M mobile users (MU) based on the ranked reverse link gains. The number of tags thus issued depends on the expected traffic, the number of cells and the propagation conditions in the network. Two types of tags can be issued: eg, hard and soft tags. A MU will transmit only if it receives a tag from its home base station (BS) and no hard tag from any other BS. Hence, in our scheme, MU that not only have stronger channel pins to their respective home BS but also cause relatively lower inter-cell interference are scheduled for transmissions. This scheme is hence a cooperative approach whereby good-neighbor behaviour is exhibited. Soft-tagging offers flexibility in allowing MU that cause moderate inter-cell interference to transmit under certain conditions such as longer starved time. Simulation results in shadow and a Rayleigh failing environment are presented to show that our scheme outperforms the conventional scheme by 30% in transmit bit energy when M = 10 in a 9-cell network with 100 MU uniformly distributed.

It was shown recently that semi-blind methods can be effective for separation, estimation, and classification of sources in wireless systems. These methods overcome difficulties of training algorithms (reduction of data information throughput) and blind ones (impossibility to classify separated source and multi-modal optimization due to nonlinearities). In this paper, we present a semi-blind spatial-temporal beamforming receiver that uses the recently proposed semi-blind constant modulus algorithm with channel identification (SBCMACI) to perform joint channel equalization, multipath energy combination and spatial interference cancellation for asynchronous direct sequence wide band code division multiple access (DS-WCDMA) systems. The simulation results show a significant performance improvement and reduction of required training symbols when compared against a receiver employing a training-based spatial-temporal recursive least squares (RLS) beamformer.

Using n-element arrays for transmission and reception in a radio communication link is one of the recent focuses in wireless communication research. Most of the studies are based on the assumption that the transmitter is in complete ignorance of the channel information. Radio modem designs are often carried out on the basis of average channel characteristics. However, radio communications are seldom simplex and channel information could be readily fed back to the transmitter. We present a transmission and reception theory for such an ideal situation that both the transmitter and receiver have the knowledge of the channel instantaneously and precisely, in the hope that an alternative to the complete statistical treatment can be developed.

Joint transmit beamforming, power control and the MMSE receiver are proposed for downlink channels of CDMA communication systems. Variability of the total transmit power while satisfying each user's quality of service is studied. We show that the total transmit power has a significant variation when we consider the combination of MMSE receiver and power control. When transmit beamforming is added, such variation is reduced significantly and great transmit power saving is achieved through the proposed scheme. The signature diversity concept is revealed for joint transmit beamforming and the MMSE receiver. An optimum MMSE, transmit beamforming and adaptive frequency hopping scheme is proposed to maximize the benefits of the MMSE receiver and transmit beamforming. Two sub-optimal algorithms are proposed to solve the optimization problem with reduced computational requirements.

The effects of multipath and tracking error on a RAKE receiver are analyzed. It is shown that, in the presence of unresolvable multipath, the RAKE receiver, formed by combining the same samples that are used for tracking, is less sensitive to tracking errors, and that there is a wide range of tracking errors for which such a receiver outperforms the conventional receiver. Also, the effects of how interchip interference degrades the performance of the proposed receiver are examined.

Most of the traffic carried by future wireless systems will be data-oriented. The majority of data applications have bursty and asymmetric traffic. Hybrid spread spectrum/time division multiplexing (SS/TDM) is a promising scheme for the air interface of future high bitrate wireless data networks, especially for the downlink. In SS/TDM systems, while spread spectrum transmission provides implicit path diversity, the diversity gain reduces considerably if the delay spread of the channel is less than one chip or if the arriving paths are highly correlated. This may result in unsatisfactory bit error rates. In this paper, we combine transmit antenna diversity with chip equalization to achieve both robustness against fading and interference suppression in the downlink of SS/TDM systems. We show through chip-level simulations that the combined scheme performs better than simple chip equalization. The improvement is significant at high SNR (signal to noise ratio) levels.

We study our previously proposed space-time multiple access framework (see Ng, B.K. and Sousa, E., WCNC'02, 2002) for MIMO forward link transmission in a multipath-rich environment. The primary goal is to achieve high bandwidth efficiency employing multicarrier modulation, while the number of user-channels, rather than the individual's data rate, is to be maximized. In the proposed framework, we focus on a single spreading period, in which the data from different users are multiplexed using space-time-frequency-diagonal (STFD) spreading sequences. It is shown that spectral efficiency of N users/Hz/sec can be achieved, where N is the number of transmit and receive antenna elements. Simulation results indicate that frequency diversity gain can be obtained with only linear-processing at the receiver.

We propose a channel allocation scheme called unified channel allocation (UCA) appropriate for both uniform and non-uniform traffic environment. The currently available channel allocation schemes such as fixed, dynamic and hybrid schemes are not suitable for all traffic conditions. The fixed channel allocation scheme does not perform well in non-uniform traffic environment and the dynamic scheme does not do well in uniform heavy traffic conditions. In real situations, however, the user traffic pattern changes with time and space. The UCA scheme adaptively changes its channel recommendation pattern for each cell to satisfy the current demand under all traffic conditions. We demonstrate through simulation that the proposed scheme, using the virtual channel set (VCS), performs better than the other schemes in all traffic conditions. Furthermore, the UCA scheme is a practical scheme, because it can be implemented without knowledge of the traffic fluctuations with time and space, since the underlying VCSs will adapt to traffic fluctuations.

We consider a new multiple access framework for the forward link transmission with multiple transmit and receive antennas. The primary goal is to achieve high bandwidth efficiency while the number of user-channels, rather than the individual's data rate, is to be maximized. In the proposed framework, we focus on a single spreading period, in which the data from different users are multiplexed using the space-time-diagonal (STD) spreading sequences. With a real constellation, N users/Hz/sec is achieved where N is the number of transmit and receive antenna elements. Any form of joint coding among these data over space-time cannot be considered within this period because they belong to different users. Hence, spreading over space-time becomes an effective approach for the purpose of multiplexing. Both analytical studies and simulations results show that our scheme has profound performance.

Signals designed for transmission over multiple transmit antennas are capable for achieving significant capacity gain. Traditional approaches aim at improving the single-user link with a centralized control over the set of transmit antennas. In this paper, by considering a set of independent and synchronized users communicating with the base station on the up-link, the joint signal can be viewed as space-time coded signal without a centralized control. Co-channel/inter-antenna interference presents a major impairment that limits the capacity. We propose a novel multiuser signal structure called interference-resistant modulation (IRM) to improve performance without coding nor bandwidth expansion. IRM can also be combined with fading-resistant modulation or space-time coding to yield additional gain when each user employs multiple transmit antennas. We prove that, both analytically and by simulations, the IRM with maximum-likelihood (ML) detection achieves the single-user performance asymptotically. Furthermore, to reduce the prohibitive complexity posed by ML detection, we propose a simple minimum-mean-square-error based precombining group detector and an interference cancellation scheme. It is shown that the proposed detector combined with IRM provides significant improvement over previous approaches.

A new double-filter receiver for spread spectrum wireless systems is presented. The receiver exploits the inherent diversity due to the small delay difference between the multipaths which may not be exploited by the conventional RAKE receiver. The bit error rate performances of both the coherent and the noncoherent detectors are analyzed and compared with the ideal RAKE receiver and the matched filter bound. We also examine the best and the worst cases of a three-path fading channel for the proposed receiver. Optimum pulse shapes under various channel conditions are designed. Implementation issues in code-division multiple access systems are discussed. The numerical results show that the proposed receiver achieves significant gains for a given spreading factor under a complexity constraint.

In a multiantenna system, there is a potential antenna gain against interference, in addition to the diversity gain achieved against fading. It is well known that in order to attain most of the diversity gain (against fading), the antenna elements should be placed apart with a distance (many times) greater than the wavelength=[speed of light]/[frequency] of the carrier. The results presented in this paper indicate that in order to attain most of the antenna gain (against interference) in the reverse-link of finite-bandwidth interference-limited CDMA systems, the interantenna distance should be (many times) greater than a new parameter which is defined as the chiplength=[speed of light]/[chip rate] of the spreading code.

We study the interference effects in a multirate DS-code-division multiple-access (CDMA) system. Optimum chip waveform selection with arbitrary shapes is analyzed using a time domain approach. The problem is posed as an interference minimization problem under energy and time-bandwidth constraints and prolate spheroidal wave functions are used to arrive at a solution. Various factors affecting the interference are identified and the trade-off between competing factors is analyzed. The effect of the interchip interference on the optimum chip waveform design is also quantified under a practical bandwidth constraint. We study the benefits of employing two different chip waveforms for two classes of users. We compare the performance of systems employing two different chip waveforms with that of a single-chip waveform system such as IS-95. We show that when the power imbalance is large, it is advantageous to employ two different chip waveforms for different classes of users.

We study the reverse link performance of a cellular CDMA network that employs fixed overlapping sector antenna architecture (FOSAA) in a hot-spot environment. Simulation results demonstrate the flexibility and effectiveness of the FOSAA in non-uniform angular traffic. The CDF statistics of received SIR are presented for different congestion levels in hot-spot sectors. A scheme with FOSAA can yield on the average 0.8 dB SIR increase over a scheme without FOSAA, when 60% of the mobiles is concentrated in a hot-spot sector in a conventional 3-sector cell. The performance measures such as the mobile transmit power and the antenna selection and sector coverage are also studied.

The problem of base station antenna assignment (BSAA) with minimum mobile transmit power (MTP) is studied for CDMA networks that employ fixed overlapping sector antenna architecture (FOSAA). It is noted that the non-FOSAA has limitations in switching users between in-cell sectors and also out-of-cell sectors in moderately loaded networks. It is then shown that by employing overlapping sectors in FOSAA, we can exploit the flexibility of assigning a user to one of possibly many potential antenna to effectively support the non-uniform angular traffic. It is also proven that the problem of selecting a set of antenna from a pool of overlapping antenna and assigning the users to them in FOSAA with minimum MTP is a special case of a general problem that was solved by Hanly (1995) and Yates (1995). The process of dynamic cell sectoring is differentiated two-fold as cell-breathing (CB) and cell-slicing (CS) and the latter can be viewed as azimuthal counterpart of the former radial scheme. The hybrid scheme, CB+CS, is shown to yield the optimal solution in minimum total MTP in a CDMA/FOSAA system. The performance results for the total MTP and the received signal quality are reported. As the congestion level increases, the difference in SIR performance between CB and CS schemes becomes more apparent with the latter outperforming the former. The performance results also show that on average, the CB scheme requires about 30% more power than in CB+CS, when 60% of the mobiles are concentrated in a hot-spot sector in a conventional 3-sector cell.

The space-time code has shed new insights in designing signals over multiple transmit antennas. In this paper, by considering a set of independent and synchronized users communicating with the base station in the reverse link, the joint users' signals can be analyzed within the framework of the space-time code. As multiuser transmission requires that each user's signal is confined to its own transmitting antenna, co-channel/inter-antenna interference degrades the performance significantly. We propose a novel multiuser signal structure called interference-resistant modulation (IRM) to improve the performance without coding and bandwidth-expansion. We prove that, both analytically and by simulations, the asymptotic performance of IRM with maximum-likelihood detection achieves the single-user performance. Further, to reduce the prohibitive complexity posed by ML detection, we propose a simple MMSE-based pre-combining detector. It is shown that the proposed detector combined with IRM provides significant improvement over previous approaches with the same complexity.

We analytically derive the appropriate rates and optimum transmit power levels that need to be allocated for high data rate services in downlinks of a WCDMA system with a time-slotted structure consisting of variable-length time slots and frames. It is shown that the average throughput decreases by 90% and average delay increases ten-fold in a severe shadowing environment (/spl sigma/=8 dB) compared to no shadowing. However, by introducing an outage probability of 0.05 as opposed to serving all the mobiles, the average system throughput can be increased six-fold and the average delay can be reduced by about 80% in such channel conditions. We analyze the trade-off between the throughput and delay performance of the system and the operating point of the outage. We consider two modes of transmission: a uni-access mode in which only one user is allowed to access the channel at a time, and a multi-access mode in which multiple users are allowed. We compare the performance of four scheduling schemes such as round-robin and fastest-first schemes that are appropriate for the uni-access mode, and equal-rate and equal-weight schemes that are for multi-access mode transmission. Simulation results show that the system employing the uni-access mode schemes performs better than one with multi-access mode schemes in terms of the average delay, and performs worse in terms of fair allocation of data rates.

In broadcast wireless systems, dynamic allocation of resources such as transmission power/rate, time slot and frequency is necessary. We focus on dynamic resource allocation in the forward-link in a TDM/SS system, resources include power/rate, time slot and antenna beam. The proposed algorithm is based on achievable data rates from an information theoretic point of view. After a proper allocation of beam and time subslot to each user, power is allocated to the users in different channel states in a water-filling construction to maximize the overall transmitted data rate of the network with the minimum transmitted power.

A novel combined rate, power and cell (R/P/C) control scheme that gracefully implements congestion control, is proposed and studied. We consider the reverse link in a CDMA network with a high degree of traffic fluctuations spatially with time, as would be the case in future wireless systems. Our scheme attempts to reduce the high interference power variations among cell-site antennas that can exist in a real network. The transmission rates of those users in the congested (non-congested) cells are decreased (increased) providing the required average throughput among users; hence, the proposed scheme is appropriate for delay insensitive applications. We consider the minimization of the average transmit bit energy (/spl Theta/) subject to maintaining individual target E/sub b//I/sub 0/ for each user. Two algorithms, one directly minimizing /spl Theta/ and the other indirectly using measured pilot power, are given. Both algorithms select the optimal cell-site if forward and reverse link gains are equal; however, the latter is decentralized and uses only local measurements and amenable for practical implementation. Simulation results show that a 20% reduction in transmit bit energy can be achieved on average using our schemes over the scheme with no rate control.

A new double-filter receiver for spread spectrum (SS) wireless systems is presented in this paper. The receiver exploits the inherent diversity due to the small delay difference between the multipaths which may not be exploited by the conventional Rake receiver. The bit error rate (BER) performances of both the coherent and the non-coherent detectors are analyzed and compared with the ideal Rake receiver and the matched filter bound. We also examine the best and the worst cases of a three-path fading channel for the proposed receiver. Optimum pulse shapes under various channel conditions are designed, Implementation issues in CDMA systems are discussed. The numerical results show that the proposed receiver achieves significant gains for a given spreading factor under a complexity constraint.

As the information theory has suggested that deploying multiple transmit and receive antenna would bring about a huge capacity gain, future development is driven towards designing a system that would realize this gain in practice. Here, we consider the performance of a new space-time multilayered CDMA structure. The data stream is divided into layers, each of which is modulated with an unique space-time spreading sequence. At the receiver, the linear multiuser MMSE detector is used to recover all layers. Simulation results show that the capacity grows linearly with the number of transmit and receive antenna. We investigate the issue of selecting good space-time sequences to reduce co-interference from other layers. Results show that the joint space-time orthogonal codes exhibit good performance. The proposed structure also provides better immunity against signal loss, when any antenna element experiences failure.

This paper studies the benefits of combining beamforming and power control in the downlink channel of a CDMA communication system employing a variable length orthogonal spreading code (OLSC). It is demonstrated that the high data users cause significant interference to the low data users if we do not employ beamforming. Furthermore, if we perform beamforming for both types of users, most of the gain we achieve is due to the high data users if the total power of the high data users is much higher than that of the low rate users. Thus we may reduce the computational requirement significantly if we perform beamforming for the high data users only with a small increase in total power.

We present a hybrid FDD/SDD-WCDMA system architecture for supporting symmetric and asymmetric data users, with the former served by a cellular network and the latter served by broadcast systems. The asymmetric data users employ packet-mode access to transmit demand messages in relatively short bursts and are silent at other times, whereas the symmetric data users employ circuit-mode access. This imposes a significant challenge for performance optimization since accessing asymmetric data users can degrade the performance of currently active symmetric data users by introducing extra interference variations which reduce system capacity. We study the implementation and performance of a special version of iterative SIR-based power control algorithms in which currently active symmetric data users are protected from interference by accessing asymmetric data users. Moreover, as the accessing asymmetric data users are expected to have short duty cycles on the cellular network's reverse link, significant overhead may be required for iterative power control. We thus investigate the use of space diversity schemes to help reduce the high iteration overhead and access their performance jointly with the proposed iterative SIR-based power control algorithm.

It is reported by Hanly (see IEEE Trans. Commun., vol.44, no.2, p.247-56, 1996) that in the reverse link of a CDMA macrodiversity system a remarkable L-fold capacity (throughput) increase can be attained by using L antenna elements (AEs) provided that the spread spectrum bandwidth approaches infinity. In a finite-bandwidth system, however, the increase in capacity as a result of the utilization of multiple AEs will be less than linear due to the presence of the correlated interference effects. In this paper, a spatial correlated interference analysis is presented in order to investigate the effects of the system parameters on the severity of the correlated interference. The results presented indicate that the parameter which we defined as the chiplength ([speed of light]/[chip rate]) plays a role in macrodiversity systems (in regards to the correlation effects) similar to the role of the carrier wavelength in microdiversity systems. It is observed that in systems where the size of the service region is large enough to enable inter-AE distances many times larger than the chiplength, significant (close to linear) capacity gains can be achieved by employing macrodiversity with many AEs. If, on the other hand, the service region size is not large enough with respect to the chiplength, the returns due to macrodiversity will not be as high.

Optimum chip waveform selection with arbitrary shape is analyzed and a time domain approach is employed. The problem is posed as an interference minimization problem under energy and time-bandwidth constraints and prolate spheroidal wave functions are used to arrive at a solution. We propose to employ two chip waveforms for two classes of users. A 3 dB gain is possible for one class of users without penalizing another class of users. Various factors affecting the interference are analyzed. The tradeoff between competing factors is quantified. The effect of the interchip interference on the optimum chip waveform design is also quantified under a practical bandwidth constraint.

The conceptual design of a system for Internet wireless access based on a large number of antenna sectors or fixed beams, with an overlapping structure, is proposed. The system utilizes spread spectrum modulation with time division multiplexing for transmissions from different overlapping antenna sectors. The fixed antenna beam structure is a practical approach to achieve space division multiplexing capacity gains with modest complexity. The time division multiplexing structure is shown to be a solution to the more general problem of designing sets of orthogonal signals for overlapping antenna sectors. We introduce the concept of time re-use as an analogue of frequency re-use in order to allocate time slots to clusters of antenna sectors. A frequency/time allocation strategy that depends on the terminal position within the cell and reduces interference is also introduced.

This paper presents interference analysis of a single frequency broadcast system (SFBS) in a power-controlled CDMA cellular network which operates in a low frequency band (LFB) for the reverse link and a high frequency band (HFB) for the forward link. Common to all forms of power control is the equalization of received power or received signal-to-interference ratio (SIR) at both the base station (BS) and portable (PT) disregarding the induced power distribution through the rest of the communication cell. As a result, an SFBS which operates in either the LFB or the HFB and is located somewhere inside the cell, may not benefit from power control, and is therefore susceptible to the "near-far" effects. Our emphasis is on dealing with the interference received at an SFBS receiver and on demonstrating the susceptibility of the SFBS receiver to the "near-far" problem in the LFB and the HFB. Both intracell and intercell interference scenarios are considered, with distance dependent and lognormal shadowing models adopted for propagation path loss. The end products are the cumulative distribution functions (CDFs) of the interference power received at the SFBS receiver in the LFB and the HFB for a PT uniformly distributed in a circular cell. These expressions are analytic for the distance dependent propagation model but require numerical integration when the lognormal shadowing model is introduced, and contribute toward the evaluation of outage probabilities of the SFBS receiver under various propagation conditions and location considerations.

Hybrid TDD/FDD-CDMA is an attractive technique for third generation cellular networks, in which FDD-CDMA is used to provide wide area voice and low bit-rate data services, and TDD-CDMA with limited coverage is used to provide high bit-rate asymmetric data services. In this paper, we propose two new frequency selection strategies to select the FDD-CDMA channel (forward link f/sub F/ or reverse link f/sub R/) for TDD-CDMA picocell operation in a FDD-CDMA macrocellular network. One is random selection strategy (RSS) which chooses f/sub F/ or f/sub R/ in a random fashion, and the other one is distance based selection strategy (DBSS) which applies a cell partitioning structure to divide each macrocell into an inner tier and an outer tier. With two separated carrier frequencies (f/sub F/ and f/sub R/), the inner tier of each macrocell can use one of these two frequencies, while the outer tier can use the other one. The outage performance of the hybrid TDD/FDD-CDMA network employing the above strategies is studied through computer simulations.

CDMA sectorized distributed antenna system constitutes the logical extent of the soft handoff scheme where all the users communicate with all the antennas in the service region. It has been reported that, in the reverse link, such systems, in comparison to conventional single-antenna systems, have the potential to achieve a capacity increase which is equal to the number of antenna elements used. In order to achieve this gain, the interference picked up by different antenna elements must be uncorrelated. We present a spatial correlated interference analysis for the simplest non-trivial system which has 2 antenna elements and 2 users. The effects of system parameters and user and antenna element locations on the correlated interference are investigated. This work lays down a foundational framework that may yield a comprehensive understanding of the performance of CDMA multi-antenna systems.

We explore the use of a novel duplex scheme-space division duplex (SDD) in a CDMA mixed cellular architecture for supporting symmetric data services and asymmetric data services with the former served by FDD regular cells and the latter served by SDD picocells (System I). Using per-cell capacity as our measure, the performance of System I is evaluated and compared with two other approaches (System II and III) in which System II utilizes FDD in both regular cells and picocells and System III simply utilizes FDD in a regular cellular architecture to serve both symmetric and asymmetric data users.

Power control in direct sequence code division multiple access (DS-CDMA) systems and power/rate allocation in multirate DS-CDMA based networks is an open and interesting research area which has attracted much attention. However, with a few exceptions, most researchers have emphasized centralized resource allocation algorithms for cellular systems where the base station keeps track of the requirements of the various users and is thus responsible for the management of network resources. Ad hoc wireless local area networks (WLANs), on the other hand, are generally configured as peer-to-peer networks with no centralized hub or controller. Thus resource allocation has to be conducted in a distributed fashion. We address the issue of distributed resource management for multirate DS-CDMA based multimedia WLANs by (1) presenting a distributed resource allocation protocol, known as distributed resource negotiation protocol (DRNP) that builds on the RTS/CTS bandwidth reservation mechanism provided by IEEE 802.111, and provides quality of service (QoS) guarantees through distributed control of resources in DS-CDMA based multimedia WLANs and (2) investigating the performance of various resource allocation schemes within the context of DRNP, in terms of network wide metrics such as overall throughput and blocking rates.

Power control (PC) on the reverse link of a direct-sequence code-division multiple-access system is essential to increase system capacity. Perfect PC eliminates fluctuations in the received signal level and hence reduces the required signal-to-interference ratio. However, a perfect PC algorithm tracks multipath fading accurately, which results in increasing the intercell interference level. A fixed step PC algorithm becomes almost perfect when the power command rate is too fast compared to the Doppler rate, which is the case for low-mobility users. We investigate the statistics of the intercell interference assuming users are moving slowly. These statistics are then used to find the system capacity. Three parameters that can affect the capacity are considered: the number of the fading process resolvable paths, the maximum transmitted power, and soft handoff.

The capacity of the reverse link DS/CDMA system has been investigated by many researchers. Power control is essential for such systems to increase the capacity. Power control based on equalizing the received power levels from different users was proposed. The user's bit error rate (BER) depends on its received signal to noise ratio (SNR). Since the user's required SNR to achieve a given BER depends on its velocity, low mobility users are expected to have a lower BER compared to high mobility ones. Hence, the BER performance of a high mobility user is investigated when determining the system capacity. We propose a power control algorithm based also on power level measurements but where the required threshold is determined according to the user velocity where slow users thresholds are lower than fast ones. This results in increasing the slow users BER but lower the interference they cause to other users and hence increases the system capacity. This increase in capacity is found to be about 30% for a three resolvable Rayleigh fading paths channel and a path-loss exponent of four.

In this study the relationship between the number of antenna elements (AEs) in a CDMA distributed antenna (DA) system and the yielding reverse link SIR is investigated by taking the power control dynamic range into account. In environments hostile to propagation, perfect power control may not be realized with a central antenna (CA), because this would require an impractically high dynamic range. This situation may yield a significant decrease in capacity. In such environments, the DA system is an ideal solution, since as the number of antenna elements increases, the dynamic range of the power control decreases. It is demonstrated that by using a DA system with as small as 4 AEs, a capacity increase of almost 30% is achievable, compared to the CA type. However, in a single-cell system once there are a sufficient number of antenna elements to implement perfect power control within a reasonable dynamic range, there is no need for additional antenna elements. Also, in a multi-cell system with CAs, the occasional transmissions at very high power levels in order to maintain perfect power control cause significant intercell interference. Since with the DA such situations are almost eliminated, the intercell interference is kept at a minimal level. Therefore, the DA is an ideal antenna type for both singleand multi-cell systems employing CDMA modulation.

Power control is essential for CDMA systems to increase the capacity. Power control based on equalizing the received power levels from different users was proposed. Perfect power control is hard to achieve for high mobility users. The error in the received signal is usually modeled as a lognormal variable with a standard deviation that is a function of the mobile's velocity. In a previous work, we have shown that this standard deviation is also a function of whether or not the mobile is communicating with the base station where the power is measured. In this work, we use the error statistics to model the intercell interference. We also employ adaptive rate transmission where the data transmission rate is a function of the number of users in the system and the errors in the power of the received signals. We show that the adaptive rate scheme helps to reduce the blocking probability and the average service time for light traffic conditions. However, for heavy traffic, users reduce their transmission rate and start to accumulate in the system making its performance similar to the constant rate system. Finally, we investigate the effect of imperfect power control on such an adaptive rate scheme.

The CDMA sectorized distributed antenna (SDA) is a novel antenna architecture which yields an increase in the reverse link capacity, in the order of the number of antenna elements used. In an SDA system, a power control algorithm that balances the SIR should be considered, since the conventional power-balanced power control algorithm results in considerable disparities among the SIR levels of different users. However, SIR-balancing for the SDA system is more complicated than that for the conventional central antenna systems due to macrodiversity. We use a power control algorithm which we refer to as SIR-balanced macro power control (SBMPC). SBMPC can be viewed as a special case of the power control algorithm introduced by Hanly (see IEEE Trans. Commun., vol.44, no.2, p.247-56, 1996) in the context of CDMA macrodiversity radio networks. In SBMPC, the set of equations to be solved are nonlinear (due to diversity) which makes the solutions for SIR-balancing algorithms, given in the literature, inapplicable. Therefore, we propose an iterative solution to the SBMPC algorithm which always converges. Because of the non-smooth convergence characteristics of the iterations, finding a suitable termination criterion for the iterations is a nontrivial problem. We suggest a multi-stage criterion which yields very low disparities among the SIR levels of different users for reasonably low number of iterations. Although the SBMPC algorithm and its iterative solution addressed in this paper are presented in the context of SDA systems, they may have wide applications. One such application is the power control problem in cellular systems employing macro diversity.

This paper investigates a hybrid space-time-division multiple access (S-TDMA) for broadband indoor wireless systems using sectored antennas. It is shown that portables which are located in different sectors of an indoor microcell may be able to reuse the same frequency and the same time slot. However, this requires careful scheduling of packet transmissions in order to avoid transmitting packets that would jam each other during the same time slot. It is proposed that the scheduling be performed in the base station, i.e., a central control architecture. The optimum scheduling algorithm, the one that maximizes the number of packets transmitted per frame, may be in the NP-complete class of problems, so it cannot be solved in real time. Therefore, a suboptimum algorithm, called the first fit algorithm (FFA), is proposed for frame scheduling. It was found that the capacity gain achieved by the FFA is dependent on the capture threshold, which is defined as the minimum signal-to-interference ratio required in order to achieve a given packet error rate goal. The capture threshold depends on the modulation and coding schemes. This paper investigates the performance of the FFA operating with multicarrier trellis-coded modulation. An alternative multicarrier modulation is analyzed, and the FFA performance is investigated when operating with the alternative and with the conventional multicarrier through computer simulations based on measured data which were obtained with a sectorization level of ten (using ten antenna sectors in the base station). The simulations have shown that the FFA can provide a large capacity gain when operating with multicarrier trellis-coded modulation and using differential detection. For example, while previous schemes can transmit only one packet at a time, the proposed scheme can transmit, on average, more than four packets per time slot in an open indoor location, or close to three packets per time slot in a closed indoor location with internal walls of concrete blocks.

In this paper we investigate the use of OFDM for broadband indoor wireless links as a means of dealing with the multipath propagation problem. We consider three solutions capable of simplifying the system. First, we consider differential encoding, so that phase estimation is not required. The second solution consists of the use of special coding schemes that are capable of spreading the information bits among the subcarriers, so that the BER performance is not deteriorated by badly attenuated subcarriers. Two coding schemes are compared, a Walsh coding scheme and 4-state trellis-coded modulation. As a third solution, we consider a Nyquist sampling rate OFDM. We show that the Nyquist sampling rate OFDM signal, which is supposed to be dependent on a perfect receiver timing, can be made resistant to a receiver timing mismatch by using the coding schemes and by decreasing the rolloff factor of the Nyquist pulse, with extra benefits of greater bandwidth efficiency and lower added noise.

The Steiner minimal tree (SMT) architecture is proposed for the wired-network infrastructure of wireless access networks. It is demonstrated that the wireless access networks having star or bus logical topologies can be realized within the proposed optimal SMT conduit structure. The SMT architecture results in a significant reduction in conduit length compared to the conventional star type, besides it provides more flexibility and robustness. For the systems which have logical bus topologies with centralized complexity, such as the distributed antenna (DA) systems, the SMT architecture is optimal in both cable and conduit lengths.

Microcellular and distributed antenna systems are two promising candidates for implementing personal communication systems. Antenna interconnection strategies for these systems are studied in order to determine cost-efficient as well as robust and flexible architectures in hexagonal layouts. To this end, some results from minimal networks theory are used, in particular, those dealing with the problem of Steiner trees. The significant reduction in conduit and cable lengths that the Steiner minimal tree (SMT) architecture provides over the star type, especially in large networks, is demonstrated. It is further shown that the SMT architecture also provides more flexibility and robustness compared to the star type. The suboptimal, but easy-to-construct, minimal spanning tree (MST) architecture is given as well, and it is compared to the SMT and star types.

This paper proposes a bandwidth-efficient fading-resistant transmission scheme which implements transmitter diversity using L antennas at the base station. When the antennas are spaced sufficiently far apart, the transmission from each antenna undergoes a different degree of fading. These transmissions are coordinated to mitigate the effects of Rayleigh fading, and the mobile receiver can recover the entire L-dimensional transmitted vector signal as long as the signal energy of at least one coordinate is large enough. L-dimensional fading-resistant signal constellations are generated by maximizing a figure of merit for the Rayleigh fading channel. This scheme offers a significant performance improvement over a conventional single-antenna binary phase-shift keying (BPSK) scheme when coding is ineffective due to slow fading.

New differentially coherent detectors for acquisition of direct sequence spread-spectrum signals are introduced. These detectors are alternatives to the noncoherent detectors that have been considered almost exclusively in the past. The proposed detectors are suitable for commercial code-division multiple-access (CDMA) systems which operate with a relatively large noise floor and provide a surprisingly large signal-to-noise ratio (SNR) improvement over the noncoherent detectors of approximately 5 dB. Under the random code sequence assumption, an exact analysis of the differentially coherent detection performance for both full period correlation (FPC) and partial period correlation (PPC) is carried out. The detector performance in terms of detection and false alarm probabilities for both partial and full period correlations is investigated, and the results are compared with those of classical noncoherent detection. The mean acquisition time for both single-dwell and multiple-dwell acquisition schemes are compared with their noncoherent counterparts.

We propose a wireless multimedia DS-CDMA system with dynamic power assignment (DPA) for adaptive traffic control. The system supports service groups with different quality of service (QoS) requirements specified in terms of both transmission bit rate and bit error rate (BER). The optimal received power level for each service group is obtained by minimizing a cost function consisting of the weighted sum of the blocking rates of different service groups subject to their QoS requirements. The performance is compared to a similar system with fixed power assignment (FPA) in which the received power level for each service group is assigned based only on its transmission bit rate. Simulation results show that DPA algorithm can support much higher offered traffic loads, on merit of the blocking probability. By implementing the cost function with different weighting coefficients, we can assign different blocking probabilities to different service groups to cope with the demands in the system. The DPA algorithm therefore plays the role of a "movable blocking boundary" in the system which can accommodate multimedia traffic and priority-based capacity access.

A modified multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) system has been proposed for use over slow multipath fading channels with frequency selectivity in the reverse link transmission of a cellular network. Instead of transmitting data substreams uniformly through subchannels, data substreams hop over subchannels with the hopping patterns adaptively adjusted to the channel fading characteristics. The problem of determining the optimal hopping pattern is formulated as a multiobjective optimization problem, for which an efficient algorithm, based on the water-filling (WF) principle, is designed to solve the problem practically. Simulation results show that the performance in terms of the average bit-error probability (BEP) (over all users) is better than that of single carrier RAKE receiver systems, conventional MC CDMA systems applying moderate error protection, or diversity systems with different combining techniques.

A modified multi-carrier (MC) DS-CDMA system has been proposed for use over slow multi-path fading channels with frequency selectivity in the reverse link transmission of the cellular network. Instead of transmitting data sub-streams uniformly through sub-channels, data sub-streams hop over sub-channels with the hopping patterns adaptively adjusted to the channel fading characteristics. The problem of determining the optimal hopping pattern is formulated as a multi-objective optimization problem, for which an efficient algorithm is designed to solve the problem practically. Simulation results show that the performance in terms of the average system bit error probability (BER) is better than that of a single carrier RAKE system, conventional MC CDMA systems applying moderate error protection and diversity systems with different combining techniques.

In this paper we consider different antenna sub-systems for integrated wireless access networks (IWAN). The various antenna sub-systems present alternatives to the standard architecture of a cellular system where the transmission/ reception occurs via omnidirectional or directional antennas with each antenna located at a base station. The appropriate antenna sub-system is dependent on the modulation scheme used. In this paper we consider a CDMA scheme along with various antenna sub-systems such as a distributed antenna, an antenna sub-system which we refer to as a sectorized distributed antenna, and a distributed antenna utilizing sub-carrier multiplexing. These concepts lead to a network architecture consisting of network switches, radio controllers, base stations, and the various antenna subsystems.

Non-coherent detectors for initial code synchronization (acquisition) of BPSK direct sequence spread spectrum signals on an AWGN channel are analyzed. In addition to the thermal noise, in many applications such detectors are faced with the "self-noise", due to the partial period correlations. Under the random code sequences assumption, in this paper an exact analysis of the non-coherent correlator's detection performance is carried out by using the theory of circularly symmetric random variables. The exact analysis shows that the familiar Gaussian approximation to the distribution function of the code self-noise is justified for all cases of practical interest. Furthermore, the overall detection performance was found to be determined asymptotically by the sum of the thermal and correlator's self-noise. In most cases of practical interest, this asymptotic result provides a very good approximation to the actual detection performance of a non-coherent correlator, improving the approximations devised previously.>

Considers a code division multiple access (CDMA) cellular network and proposes sub-optimal multiuser detectors which exploit a type of diversity inherent in the system and are based on the processing of signals received at multiple base stations. The authors investigate the performance of conventional matched-filter, linear, and decision-based detection schemes, using this diversity scheme, for both synchronous and asynchronous systems. The detection of user signals is based on tracking the signals at several base stations. The optimal detector would operate on all the signals received at all the base stations but would have a high degree of complexity. The authors discuss the case where the detection of a given signal is based on received signals at a small subset of base stations. They show that with judicious selection of this smaller set of base stations, good performance can be obtained without much increase in complexity.>

Proposes a multicarrier orthogonal CDMA signaling scheme for a multiple-access communication system, such as the reverse channel of a cellular network, as an alternative to the multi-user interference cancellation approach. The average variance of cross-correlations between sequences is used as a measure for sequence design. The authors search for sets of sequences that minimize the probability of symbol detection error, given that there is imperfect synchronization among the signals, that is, the signals are quasi-synchronous. Orthogonal sequences based on the Sylvester-type Hadamard matrices (Walsh functions) are shown to provide a significant improvement over the case where a Hadamard (orthogonal) matrix is chosen at random. Computer searches suggest that this set of codes is optimal with respect to the above measure. The issue of chip pulse shaping is investigated. Optimal pulses designed to minimize multiple-access interference in quasi-synchronous systems are obtained for various bandwidths and are shown to provide a large improvement over the raised cosine pulses. A multicarrier signaling scheme is introduced in order to reduce chip level synchronization offsets between the users.>

This paper describes a set of measurements performed in five existing cells in the Toronto area, in order to assess the impact of multipath propagation on the performance of the CTIA IS-54 digital standard. A sweeping correlator apparatus with a 0.1 mu s resolution and 910 MHz carrier frequency was used. Generally, the measurements exhibit multipath propagation with considerably smaller excess delays than some reported recently but are consistent with earlier results obtained in the U.S. and Europe. In view of the results, it appears that the long delay (high selectivity) problem has been somewhat over-emphasized in the past, at least for areas like Toronto. Problems associated with short delays (flat fading), on the contrary, appear to have been underestimated, although these would be typical in any dense urban environment.>

An error-correction scheme for a helical-scan magnetic recording system is presented. The system consists of a commercial VHS video cassette recorder with the front-end electronics adapted for high-speed digital recording. Error statistics have been collected and show a bursty error recording channel both within each track and from track-to-track. The proposed error-correction scheme uses interleaved Reed-Solomon codes. The interleaving function consists of a two-level interleaver performing track interleaving and byte interleaving. An efficient interleaving implementation is presented.>

Results on the modeling of interference in a radio communication network and performance measures for the link as a function of distance are presented. It is assumed that a transmitter-receiver pair in a radio network is affected by a set of interferers, using the same modulation and power, whose positions are modeled as a Poisson field in the plane. Assuming a 1/r/sup gamma / propagation power loss law, the probability distributions for the noise at the receiver are found to be the stable distributions. Results are given for the probability of symbol error and link capacity as a function of the distance between the transmitter and receiver for direct sequence and frequency hopping spread spectrum schemes. It is found that the frequency hopping schemes are inherently superior and their performance is not dependent on the synchronization of the hopping times for the different users.>

A set of computer-based modeling tools to help understand the performance and behavior of spread spectrum networks is presented. These tools allow for numerical evaluation of important network performance measures and experimentation with network parameters. A connectivity model for a direct sequence spread spectrum packet radio network which allows the computation of network throughput and can be used to study routing techniques is discussed. This model places direct sequence spread spectrum networks on an equal footing with narrowband networks which have been modeled using the collision channel and transmission range concepts. The mathematical models and algorithms used to evaluate network performance are described, and several examples are presented to demonstrate the impact of various parameters.>

A modified carrier-sense multiple-access with collision detection (CSMA-CD) protocol which is suitable for high speed channels is presented and analyzed using the equilibrium point analysis (EPA) method. With the protocol each user has a finite buffer and does not transmit until it has accumulated a given number of packets, L. When this number is exceeded it will access the channel using a conventional CSMA-CD protocol and transmit all the packets in its buffer. This protocol is very simple to implement compared with multichannel protocols and results in higher throughput compared with slotted Aloha. The disadvantage is a larger delay, which is related to the parameter L, the packet arrival process, and the number of users. The delay can be reduced by dynamically controlling the parameter L and the retransmission probabilities of the protocol. The protocol is well suited for traffic arrival processes which are bursty where each burst of packets corresponds roughly to a message. Some performance results are given assuming a Bernoulli packet arrival process.>

It is widely known that the probability of symbol error in a direct-sequence spread-spectrum multiple-access network is dependent on the carrier and spreading code chip phases of the interfering terminals with respect to the signal of interest. As a result, for a given total interference power, the performance is worse for the case of one strong interferer when compared to the case of many weak interferers where the Gaussian assumption on the multiple-access interference holds. The author assumes a random-spreading code model and shows that if there is an offset on the carrier and spreading code frequencies of the interfering signals relative to the signal of interest, the above difference in performance between the cases of a set of balanced and a set of unbalanced interferers is drastically reduced. The result is that subject to such frequency offsets one may make the Gaussian assumption on the interference power regardless of whether it is composed of one strong interferer or many weak interferers of the same total power. In a network it may be desirable to actually force the offsets by putting a slight offset on the clock oscillators of the various transmitters. It is shown that to achieve a small variation of the noise variance, the spreading code and carrier offset frequencies should be greater than the symbol rate.>

The authors obtain the optimum transmission ranges to maximize throughput for a direct-sequence spread-spectrum multihop packet radio network. In the analysis, they model the network self-interference as a random variable which is equal to the sum of the interference power of all other terminals plus background noise. The model is applicable to other spread-spectrum schemes where the interference of one user appears as a noise source with constant power spectral density to the other users. The network terminals are modeled as a random Poisson field of interference power emitters. The statistics of the interference power at a receiving terminal are obtained and shown to be the stable distributions of a parameter that is dependent on the propagation power loss law. The optimum transmission range in such a network is of the form CK/sup alpha / where C is a constant, K is a function of the processing gain, the background noise power spectral density, and the degree of error-correction coding used, and alpha is related to the power loss law. The results obtained can be used in heuristics to determine optimum routing strategies in multihop networks.>

A technique for characterizing multiuser interference and background noise in a direct-sequence spread-spectrum network is introduced, and packet error probabilities are calculated. The multiuser interference over a packet in the network is modeled as a compound Gaussian multivariate random variable for moderate to large values of the processing gain. The conditional variance is dependent on the number of users and their interference powers. The method works for any interference with statistics of the block interference having a spherically symmetric distribution. The best performance results, in terms of the expected total interference power, are obtained for the case of a large number of interferers with comparable interference powers. As the number of interferers approaches infinity, the performance is the same as that for Gaussian noise. For a small number of interferers, the block error probability curve is broader than that for Gaussian noise. For small values of SNR, the probability of error is smaller than that corresponding to Gaussian noise, and for large values of SNR it is larger. Modifications to the basic direct-sequence scheme that improve performance in the case of one strong interferer are suggested.>

The author considers the possibility of obtaining performance gains by using channel power sensing in the channel access protocol of a terrestrial spread-spectrum packet radio network as a generalization of the CSMA (carrier-sense multiple-access) protocol of narrowband networks. Channel load sensing is viewed as a method of estimating the interference at the receiver based on the interference measured at the transmitter. To do this, a bivariate probability distribution function of the interference at the two locations is required. The probability of packet success is then determined. Since it is difficult to evaluate numerically, a program was written to simulate it. The author gives simulation results for the case of a random network.>

Spreading code protocols for a distributed spread-spectrum packet radio network are presented. A distributed single-hop system (i.e. each terminal can hear all other terminals) with the users approximately synchronized and a set of prespecified spreading codes are presented. The spreading code protocol is a policy for choosing a spreading code to be used, given that a terminal has a packet to send, and a policy for monitoring spreading codes, given that a terminal is idle. A slotted system where a packet occupies a number of slots is considered, and two protocols that involve changing the spreading code of a transmission after an initial header is transmitted are presented. In one protocol, the header is transmitted on a common code, and in the other it is transmitted on a receiver-based code, the rest of the packet being transmitted on a transmitter-based code. In the receiving mode, a terminal monitors either a common code, in the first case, or a receiver-based code in the latter. Upon recognizing its own address and the source address, the receiver dynamically switches to a despreading code corresponding to the source. Throughput results are obtained for the case of geometrically distributed packet lengths.>

This paper studies the problem of designing a suitable pulse shape for teletext data transmission. The following four criteria are used: 1) Nyquist I criterion, 2) Nyquist II criterion, 3) degree of overshoots in the channel signal, and 4) robustness to sampling phase jitter. For system bandwidths less than the inverse-baud rate, it is not possible to satisfy all these criteria simultaneously; tradeoffs that have to be made are illustrated. Several candidate pulse shapes are given and a composite criterion developed. A pulse shape, which satisfies the Nyquist I criterion and is closest to satisfying the Nyquist II criterion, in a sum-of-squares-of-deviations sense, is recommended.

For a raised cosine pulse spectrum with pulse shaping fully at the transmitter, several enhanced receivers for pulse amplitude modulation are presented which recover some of the 1.76 dB SNR lost due to nonmatched filtering reception. Three receivers using concepts such as double-the-symbol-rate sampling, the partial response-type property of the pulse, and soft decisions are analyzed; they recover 0.6, 0.97, and 1.21 dB SNR, respectively. An error monitoring scheme is also proposed.

Various pulse shapes have been proposed for teletext data transmission. The effect of apportioning the different pulse shapes between the transmitter and the receiver is studied for the cases of a constraint on the power and a constraint on the peak value of the channel signal.