Search

Aerial base stations (ABSs) have shown significant potential to improve cellular networks’ performance due to their high mobility and on-demand deployment. In this paper, the trajectory optimization problem for a multi-ABS network is investigated. The objective is to maximize the minimum data rate of the cell-edge mobile users while the power constraint of the ABSs, including both propulsion and signal transmission powers, the backhaul link capacity constraint, and the collision avoidance constraint, are taken into account. To reach this goal, first, based on the modified K-means approach, the ABSs and users are partitioned into different clusters so that their associated ABS serve the users of each cluster. Afterward, the ABSs need to find an efficient trajectory optimization and resource allocation scheme to support the users. To solve this challenging problem, we convert the main trajectory optimization and resource allocation problem into three sub-problems: power allocation sub-problem, joint ABS-user association and sub-channel assignment sub-problem, and trajectory optimization sub-problem. Then, using the successive convex approximation approach, an efficient algorithm is proposed, which iteratively solves these sub-problems. Simulation results show that the proposed algorithm converges fast and improves the network's data rate while it satisfies all the required constraints.

The development of 5G system provides a unique opportunity to implement a unified wireless transmission platform that can simultaneous deliver unicast, multicast and broadcast services using the same spectrum and the same infrastructure. This can be achieved by developing a more capable point-to-multipoint transmission (PTM) system component, also called 5G Multimedia Broadcast Multicast Services (5G-MBMS). A properly designed 5G-MBMS could achieve the convergence of terrestrial mobile broadband and TV broadcasting systems.By inherently allowing dynamic spectrum allocation among different types of services, this converged system could offer significant higher spectrum utilization. In this paper, the power-based non-orthogonal multiplexing (P-NOM) technology is proposed in addition to the existing orthogonal time-division-multiplexing (TDM) scheme. Significant capacity gains could be achieved by using P-NOM in a 5G-MBMS system for delivering different types of broadcast services, and for delivering mixed unicast and broadcast services. The capacity benefits of a 5G-MBMS with P-NOM is demonstrated by both theoretical analysis and computer simulations. A key finding is that using a two-layer P-NOM can essentially deliver a broadcast service on top of a unicast network, each achieving nearly full capacity. Next, a complexity analysis will reveal that implementing P-NOM in a 5G-MBMS system does not require significant additional complexity at consumer devices. Finally, it will be shown that, different from the non-orthogonal-multiple-access (NOMA) technologies, using P-NOM in 5G-MBMS system requires little or no change to the existing radio resource allocation mechanisms in LTE and 5G, when one power layer is used for broadcast services delivery.

This paper studies the traffic monitoring problem in a road network using a team of aerial robots. The problem is challenging due to two main reasons. First, the traffic events are stochastic, both temporally and spatially. Second, the problem has a non-homogeneous structure as the traffic events arrive at different locations of the road network at different rates. Accordingly, some locations require more visits by the robots compared to other locations. To address these issues, we define an uncertainty metric for each location of the road network and formulate a path planning problem for the aerial robots to minimize the network's average uncertainty. We express this problem as a partially observable Markov decision process (POMDP) and propose a distributed and scalable algorithm based on deep reinforcement learning to solve it. We consider two different scenarios depending on the communication mode between the agents (aerial robots) and the traffic management center (TMC). The first scenario assumes that the agents continuously communicate with the TMC to send/receive real-time information about the traffic events. Hence, the agents have global and real-time knowledge of the environment. However, in the second scenario, we consider a challenging setting where the observation of the aerial robots is partial and limited to their sensing ranges. Moreover, in contrast to the first scenario, the information exchange between the aerial robots and the TMC is restricted to specific time instances. We evaluate the performance of our proposed algorithm in both scenarios for a real road network topology and demonstrate its functionality in a traffic monitoring system., Comment: IEEE International Conference on Intelligent Transportation - ITSC2021

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., Comment: to appear in IEEE GLOBECOM 2020

The 3rd Generation Partnership Project (3GPP) submitted the 5G New Radio (NR) system specifications to International Telecommunication Union (ITU) as a candidate fifth generation (5G) mobile communication system (formally denoted as IMT-2020 systems). As part of the submission, 3GPP provided a self-evaluation for the compliance of 5G NR systems with the ITU defined IMT-2020 performance requirements. This paper considers the defined 5G use case families, Ultra Reliable Low-Latency Communication (URLLC), massive Machine Type Communication (mMTC) and enhanced Mobile Broadband (eMBB), and provides an independent evaluation of the compliance of the 3GPP 5G NR self-evaluation simulations with the IMT-2020 performance requirements for connection density, reliability, and spectral efficiency for future mobile broadband and emerging IoT applications. Independent evaluation indeed shows the compliance of the 3GPP 5G NR system with the ITU IMT-2020 performance requirements for all parameters evaluated by simulations., 14 pages, 16 figures, 26 tables, IEEE Access

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 Q-learning 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., Accepted to appear in IEEE ICC 2020

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., Comment: 6 pages, 5 figures, to be presented in IEEE PIMRC 2019

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., Comment: 6 pages, 3 figures, to be presented in IEEE PIMRC 2019

Multi-radio access technology (RAT) cellular communication systems limit connected users to utilizing a single RAT even when employing multi-mode user equipment (UE) capable of utilizing multi-RATs. Single-mode access, combined with static spectrum partitioning between co-deployed RATs and independent resource allocation for employed RATs, results in suboptimal spectrum utilization in multi-RAT systems. This paper models user access in multi-RAT systems and proposes enabling multi-mode UE to simultaneously utilize multiple RATs, using multi-RAT carrier aggregation, to improve the performance and spectrum utilization of multi-RAT systems. Several realizations of multi-mode access with varying implementation requirements are presented and discussed. Detailed system-level simulations, for a system co-deploying High Speed Packet Access (HSPA) and Long-Term Evolution (LTE), are performed to investigate the gains and limitations of different user access configurations in multi-RAT systems.

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.

Throughput optimal scheduling policies in general require the solution of a complex and often NP-hard optimization problem. Related literature has shown that in the context of time-varying channels, randomized scheduling policies can be employed to reduce the complexity of the optimization problem but at the expense of a memory requirement that is exponential in the number of data flows. In this paper, we consider a Linear-Memory Randomized Scheduling Policy (LM-RSP) that is based on a pick-and-compare principle in a time-varying network with $N$ one-hop data flows. For general ergodic channel processes, we study the performance of LM-RSP in terms of its stability region and average delay. Specifically, we show that LM-RSP can stabilize a fraction of the capacity region. Our analysis characterizes this fraction as well as the average delay as a function of channel variations and the efficiency of LM-RSP in choosing an appropriate schedule vector. Applying these results to a class of Markovian channels, we provide explicit results on the stability region and delay performance of LM-RSP., Long version of preprint to appear in the IEEE Transactions on Networking

Opportunistic unlicensed access to the (temporarily) unused frequency bands across the licensed radio spectrum is currently being investigated as a means to increase the efficiency of spectrum usage. Such opportunistic access calls for implementation of safeguards so that ongoing licensed operations are not compromised. Among different candidates, sensing-based access, where the unlicensed users transmit if they sense the licensed band to be free, is particularly appealing due to its low deployment cost and its compatibility with the legacy licensed systems. The ability to reliably and autonomously identify unused frequency bands is envisaged as one of the main functionalities of cognitive radios. In this article we provide an overview of the regulatory requirements and major challenges associated with the practical implementation of spectrum sensing functionality in cognitive radio systems. Furthermore, we outline different design trade-offs that have to be made in order to enhance various aspects of the system's performance.

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.

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.

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.

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.

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.

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.

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. >

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. >

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. >

The paper describes a set of measurements performed in the Toronto area, in order to assess the impact of multipath propagation on the performance of the TIA IS-54 digital standard. Five existing cells were systematically surveyed: two in downtown Toronto, two in the suburbs, and one in a suburban/rural area. A sweeping correlator apparatus with a 0.1 /spl mu/s resolution and 910 MHz carrier frequency, and an omnidirectional antenna was used. In one of the cells, the measurements were repeated with a 60 degree beamwidth directional antenna. A fairly sophisticated thresholding technique was applied in order to reduce the impact of noise. Generally, the measurements with the omnidirectional antenna exhibit multipath propagation with considerably smaller excess delays than some reported, but are consistent with earlier results obtained in the U.S. and Europe. Sectorization was found to considerably reduce the multipath effects. Very large delay components appear to be due mostly to the combination of large transmitter-receiver distances and a large degree of shadowing, and could probably be avoided to a large extent by standard cellular engineering techniques. In view of these results, it seems that the long delay (high selectivity) problem has been somewhat overemphasized in the past, at least for areas like Toronto. Problems associated with short delays (flat fading), on the contrary, appear to have been underestimated in the case of dense urban environments. >

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 $O(\delta)$ of optimality for any positive $\delta$. We further show that this arbitrarily closeness to optimality comes at the price of having a delay that is $O(\frac{1}{\delta})$ in admitting users. We also propose constant factor approximations of the policy for distributed implementation.

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. >

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.

It is well known that for ergodic channel processes the Generalized Max-Weight Matching (GMWM) scheduling policy stabilizes the network for any supportable arrival rate vector within the network capacity region. This policy, however, often requires the solution of an NP-hard optimization problem. This has motivated many researchers to develop sub-optimal algorithms that approximate the GMWM policy in selecting schedule vectors. One implicit assumption commonly shared in this context is that during the algorithm runtime, the channel states remain effectively unchanged. This assumption may not hold as the time needed to select near-optimal schedule vectors usually increases quickly with the network size. In this paper, we incorporate channel variations and the time-efficiency of sub-optimal algorithms into the scheduler design, to dynamically tune the algorithm runtime considering the tradeoff between algorithm efficiency and its robustness to changing channel states. Specifically, we propose a Dynamic Control Policy (DCP) that operates 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 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 throughput stability region induced by DCP and show that our characterization can be tight. We also show that the throughput stability region of DCP is at least as large as that of any other static policy. Finally, we provide two case studies to gain further intuition into the performance of DCP., Submitted for journal consideration. A shorter version was presented in IEEE IWQoS 2008

Considering a multihop cellular system with one relay per sector, an effective modeling for the joint base-station/relay assignment, rate allocation, and routing scheme is proposed and formulated under a single problem for the downlink. This problem is then formulated as a multidimensional multichoice knapsack problem (MMKP) to maximize the total achieved throughput in the network. The well-known MMKP algorithm based on Lagrange multipliers is modified, which results in a near-optimal solution with a linear complexity. The notion of the infeasibility factor is also introduced to adjust the transmit power of base stations and relays adaptively. To reduce the complexity, and in order to analyze the underlying key factors in the system, the framework is restricted to a two-base-station two-relay system. In fact, the output of the proposed algorithm is the joint optimization of the routing path, and base-station selection to achieve the maximum total throughput in the system, which in conjunction with the proposed adaptive scheme leads to the implementation of the cell breathing via allocating the proper transmit power to the base-stations and relays.

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. >

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. >

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. >

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. >

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.

High data rates, low-power consumption, and low complexity will be the most important parameters in the design of the next-generation mobile terminals. In this paper we are introducing a new paradigm in the design of direct sequence spread spectrum receiver by combining analog and digital signal processing. The main difference with respect to the conventional all-digital receiver design approach is that the proposed mixed analog/digital processing results in a symbol rate sampling rather than the high-rate subchip sampling. Analog signal despreading is the key part of the proposed receiver solution, which is based on a five-port device, a passive RF square-law-type device. It is used to perform two important tasks at the same time, namely, the direct conversion and analog despreading. To achieve lower complexity, the proposed receiver uses rectangular instead of pulse-matched despreading at the cost of only a small performance degradation. Also, we propose a new noncoherent pseudonoise (PN) code tracking scheme based on error signal generated through the L1 norm. This results in comparable or even better PN code tracking performance than L2 norm circuitry, using less complex hardware. Further, we explore how this technology can be applied in the design of DS-CDMA RAKE receiver for mobile terminals. Depending on how the pilot signal is multiplexed, we propose two types of RAKE receivers. It is shown that under Rayleigh fading channel such receiver structures offer robustness and high performance, while maintaining the low complexity achievable through the five-port device.

The evolution of wireless communications is bringing the world to a different way of experiencing and producingmultimedia content. As a result, users are having an increasingly active role in the definition of networks. The chosen topic for the conference that was at the basis of this special issue was thus the pursuit of a really autonomous network infrastructure that will allow the efficient use of the spectrum and reduce energy requirements, exploring new possibilities like cognition and cooperation between users and systems. These developments bring excellent opportunities for the mobile community since new paradigms are needed to support the enormous surge of mobile data in present and future networks. The series of international symposia on Wireless Personal Multimedia Communications (WPMC) have always been a place where people from all over the world can bring their experiences together to develop knowledge about all aspects related to wireless networks

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.

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. >