Cellular Wireless Communication Research Articles

Transmitter-based processing for down-link DS/CDMA systems with multiple transmit antennas

In cellular wireless communication systems, there have been various receiver-based techniques for performance improvement. However, it may be desirable to use transmitter- based techniques to improve the down-link capacity, since the implementation complexity is less critical at a base station (BS) than at a mobile station (MS). This paper presents a transmitter- based processing for the down-link direct-sequence code-division multiple-access (DS/CDMA) systems with multiple transmit antennas. We propose a combined pre-rake/pre-decorrelating approach. This approach combines the advantage of pre-rake scheme, to achieve diversity gain and average received signal-to- noise ratio (SNR) gain, with that of pre-decorrelating scheme, to suppress multiple access interference (MAI) and multipath interference (MPI). Furthermore, to make the total transmit power the same as that without pre-rake/pre-decorrelating processing, two power normalization methods are presented. Simulation results show that the proposed schemes significantly outperform the conventional transmitter-based techniques. The effects of the number of users and the block size on the bit error rate (BER) performance are also investigated.

Performance impact of flexible power arrangement in OFDMA based cellular communication networks

In the OFDMA-based downlink of wireless cellular communication networks, the intercell interference would be a key performance-limiting factor, especially for the cell edge users. To enhance the cell edge user performance, several flexible power allocation schemes have been proposed, e.g., the so-called soft frequency reuse scheme and the partial frequency reuse scheme. This paper analyzes those schemes in a very realistic multicell setting, investigates the layer 2 resource allocation algorithms that are associated to the physical layer setting, and finally gives results of the performance evaluation.

Real World Implementation Lessons and Outcomes from the Worker Interactive Networking (WIN) Project: Workplace-Based Online Caregiver Support and Remote Monitoring of Elders at Home

The objective of this research was to determine the feasibility of and receptivity to the first computerized workplace-based direct caregiver intervention and to assess the effects on businesses, working family caregivers, and their elderly relatives. Working family caregivers, with at least one health and/or safety concern related to an elder residing alone at home during the workday, were recruited from five companies (n = 27). Caregivers received free computer access to the Worker Interactive Networking (WIN) Internet online caregiver support group and a remote elder monitoring system at home for 6 months. The remote monitoring system provided Web-based status reports and e-mail/pager alerts when individualized parameters were exceeded. Motion sensor signals were transmitted to a transponder that uploaded via wireless cellular communications to the project server, thereby not interfering with elders' telephone use. Formative qualitative analyses clarified acceptance and implementation issues. Summative quantitative evaluation determined pilot intervention effects and was conducted by external evaluators. Despite interoperability and cellular reception issues, the system was successfully deployed across four states to a variety of businesses and housing types. Positive results occurred on worker morale, productivity, and reduction of caregiver stress. Participants found it easy to learn and use. Elders did not find the technology "intrusive" or "isolating." Contrary to their expectations, managers reported no abuse of Internet access. Workers expressed a willingness to pay for a similar system in the future ranging from $10 to $130, depending on the features. They would pay the most for the option involving a geriatric nurse coach. The WIN system innovatively tailored to users' wants, and provided users customized control and personalized support. Use of the system was associated with positive outcomes. Enrollment response suggests a specific niche market for remote home monitoring, making it a manageable employee benefit.

Downlink Wireless Packet Scheduling with Deadlines

Next-generation cellular wireless communication networks aim to provide a variety of quality-of-service (QoS)-sensitive packet-based services to downlink users. Included among these are real-time multimedia services, which have stringent delay requirements. Downlink packet scheduling at the base station plays a key role in efficiently allocating system resources to meet the desired level of QoS for various users. In this paper, we employ dynamic programming (DP) to study the design of a downlink packet scheduler capable of supporting real-time multimedia applications. Under well-justified modeling reductions, we extensively characterize structural properties of the optimal control associated with the DP problem. We leverage intuition gained from these properties to propose a heuristic scheduling policy, namely, Channel-Aware Earliest Due Date (CA-EDD), which is based on a "quasi- static" approach to scheduling. The per-time-slot implementation complexity of CA-EDD is only O(K) for a system with K downlink users. Experimental results show that CA-EDD delivers up to 50 percent of performance gains over benchmark schedulers. CA-EDD achieves these performance gains by using channel and deadline information in conjunction with application layer information (relative importance of packets) in a systematic and unified way for scheduling.

Space-Time Block Coding for Uplink Single-Carrier CDMA with Joint Detection in the Frequency Domain

Single-carrier code-division multiple access (SC-CDMA), also named cyclic-prefix CDMA in the literature, is a promising air interface for the uplink of the 4G cellular wireless communication systems. It enables the high capacity intrinsically offered by CDMA by making the equalization of the multipath channels and the mitigation of the resulting interference possible at a low complexity. This paper proposes a new air interface that combines SC-CDMA with space-time block coding (STBC) across multiple transmit antennas in order to make the link more robust. Contrary to existing air interfaces that perform the STBC at the chip level, making them only applicable to the downlink, the STBC is performed at the symbol level, making it also applicable to the uplink. In order to optimally detect the different antenna and user signals, a linear joint detector optimized according to the minimum mean square error (MMSE) criterion is designed. By exploiting the cyclic properties of the channel matrices, the complexity of the joint detector is significantly reduced. Furthermore, it is shown analytically that the inter-antenna interference is canceled out at the output of the first stage of the linear MMSE joint detector, consisting of a matched filter. By space-time coding the signal through two antennas at each transmit mobile terminal, a significant gain in signal-to-noise-ratio can be achieved. However, the spatial diversity gain of the proposed system is limited by the multiuser interference (MUI), that is increasing with the user load. Higher complexity non-linear receivers are needed to better compensate the MUI and still benefit from the spatial diversity at high user loads.

Performance Analysis of an OFDMA Transmission System in a Multicell Environment

The paper deals with design and performance analysis of orthogonal frequency-division multiple-access (OFDMA)-based downlink cellular wireless communications. Due to a high degree of user mobility, the base station is assumed to have only a statistical knowledge of the users' channels. Relying on the ergodic capacities connected to the user rates, a subcarrier and power allocation that minimizes the total transmitted power is proposed. The allocation strategy requires only the knowledge of the channel statistics and the rate requirements for all users. An extension and a performance analysis of this allocation algorithm in a multicell environment working with a frequency reuse factor equal to one is also conducted. A condition for the multicell network to be able to satisfy all rate requirements is derived

Channel Holding Time in Wireless Cellular Communications with General Distributed Session Time and Dwell Time

Channel holding time is fundamental to teletraffic analysis of wireless cellular networks. This quantity depends on user's mobility which can be characterized by the dwell time, and the traffic model which is associated with the unencumbered session time. In this paper, under a general assumption on the distributions of unencumbered session time and dwell time, the characteristics of new call channel holding time and handoff call channel holding time are investigated. Analytical formulae for the distributions of new call channel holding time and handoff call channel holding time are derived

Two-dimensional Models of Wireless Cellular Communication Networks with Infinite Queues of h-Calls

Simple computational procedures of approximate calculation of twodimensional models of cellular communication networks based on a scheme of communication channel reservation with feasible arrangement of infinite queue of patient or impatient handover calls are proposed. New calls are handled by a scheme with explicit loss. Unlike the classical models of the above communication networks, here new and handover calls are assumed to be not identical in time of occupying radio channels. The results of numerical experiments show that the formulae suggested have high accuracy for micro and picocells in which the intensity of handover calls essentially exceeds intensity of new calls.

A Shaped and Reconfigurable Reflector Antenna With Low Sidelobe Level for Cellular Wireless Communications

This letter presents a shape synthesis and an array optimization of the hourglass reflector antenna. A cosecant-squared pattern in the elevation plane, with the reconfigurable beamwidth in the horizontal plane, is achieved by controlling the excitation of the feed array elements and maintaining an invariable reflector surface. Thus, a sectorial pattern configuration of four beams (with 90 beamwidth) or eight beams can be selected in the azimuth plane. Both kinds of patterns (vertical and horizontal) have proven their usefulness and advantages for base stations in wireless communication systems.

Performance analysis of an adaptive handoff algorithm based on distance information

Mobility is the most important feature of a wireless cellular communication system. Handoff, the process of changing the channel associated with the current connection while a call is in progress, is a key concept in providing this mobility. Continuation of an active call is one of the most important quality measurements in the cellular systems. Handoff makes it possible for a user to travel from one cell to another while having a seamless connection. In this study, an adaptive handoff algorithm with a dynamic hysteresis value, based on the distance between the mobile station and the serving base station, is proposed for cellular communications. Handoff probability is calculated to evaluate handoff algorithms analytically. The proposed handoff algorithm is compared with an algorithm with a fixed hysteresis value, algorithm based on absolute and relative signal strength measurements and algorithm based on distance and relative signal strength measurements. Performance is evaluated in terms of the average number of handoffs, average handoff delay, standard deviation of handoff location, average signal strength during handoff, and probability of link degradation. Numerical results and simulations demonstrate that the proposed algorithm outperforms the handoff algorithm with fixed hysteresis level, handoff algorithm using both threshold and hysteresis, and handoff algorithm based on distance and hysteresis. The effect of distance error is also discussed.

Analytically Derived Uplink/Downlink TOA and 2-D-DOA Distributions With Scatterers in a 3-D Hemispheroid Surrounding the Mobile

This paper presents the open literature's first closed-form explicit expressions of the trivariate joint and marginal distributions of a landmobile cellular wireless communication system's uplink and downlink multipaths' time of arrival and two-dimensional direction of arrival, rigorously derived via thorough mathematics based on a "geometrical" model of the three-dimensional (3-D) spatial relationships among the mobile station, the scatterers, and the base station. The scatterers are herein modeled to have a uniform 3-D spatial distribution in an aboveground hemisphere with a flat circular base (or alternatively, within a sphere) centered at the mobile

A call admission control scheme for packet data in CDMA cellular communications

In wireless cellular communication systems, call admission control (CAC) is to ensure satisfactory services for mobile users and maximize the utilization of the limited radio spectrum. In this paper, we propose a new CAC scheme for a code division multiple access (CDMA) wireless cellular network supporting heterogeneous self-similar data traffic. In addition to ensuring transmission accuracy at the bit level, the CAC scheme guarantees service requirements at both the call level and the packet level. The grade of service (GoS) at the call level and the quality of service (QoS) at the packet level are evaluated using the handoff call dropping probability and the packet transmission delay, respectively. The effective bandwidth approach for data traffic is applied to guarantee QoS requirements. Handoff probability and cell overload probability are derived via the traffic aggregation method. The two probabilities are used to determine the handoff call dropping probability, and the GoS requirement can be guaranteed on a per call basis. Numerical analysis and computer simulation results demonstrate that the proposed CAC scheme can meet both QoS and GoS requirements and achieve efficient resource utilization.

Guest editorial: Evolving technologies for 3G cellular wireless communications systems

Presents the guest editorial for this issue of the publication.

A bandwidth sharing approach to improve licensed spectrum utilization

The spectrum of deployed wireless cellular communication systems is found to be underutilized, even though licensed spectrum is at a premium. To efficiently utilize the bandwidth left unused in a cellular system, the primary system (PRI), we propose an overlaid ad hoc secondary network (ASN) architecture, with the ASN operating over the resources left unutilized by the PRI. Our basic design principle is that the ASN operates in a nonintrusive manner and does not interact with the PRI. In this article we present the ad hoc secondary medium access control (AS-MAC) protocol to enable PRI-SEC interoperation, address a number of technical challenges pertinent to this networking environment, and evaluate the performance of the AS-MAC. In a single-hop ASN the AS-MAC transparently utilizes 75 percent of the bandwidth left unused by the PRI, while in multihop ASNs, due to spatial reuse, the AS-MAC can utilize up to 132 percent of the idle PRI resources in our experiments.

Analytical derivation of finite-data-record performance of Wong-Lok-Lehnert-Zoltowski's DS-CDMA "Blind" space-time receiver

Finite-data-record performance and breakdown behaviors are analytically derived for a space-time blind rake-receiver algorithm in a previous proposed by Wong, Lok, Lehnert, and Zoltowski (WLLZ) for uplink direct sequence-code division multiple access (DS-CDMA) cellular wireless communications using only single-user-type conventional detectors with either short or long spreading sequences. WLLZ is significant because it needs no prior knowledge nor explicit estimation of 1) the fading channel's multipaths' arrival angles/delays/amplitudes/complex-phases, 2) the receiver's nominal or actual (i.e., calibrated) antenna-array manifold, and 3) the other CDMA users' signature spreading-codes. This analysis a) reveals how the multiuser access cellular system's various parameters influence WLLZs blind beamformer's output SINR. These herein-derived quantitative relationships are explicit, closed-form, and accurate to within 1/2-2 dB for the examined scenarios if WLLZ does not break down . This analysis also b) explains WLLZ's breakdown, which can occur for channels with large delay-spreads, due to a matrix rank deficiency, explicitly in closed-form and in terms of the fading-channel's delay spread and temporal variability. It also c) proposes a simple algorithmic modification to WLLZ to avert its breakdown.

Location Tracking Using Differential Range Measurements

Automatic vehicle location and automatic vehicle monitoring systems were developed originally to track fleet vehicles such as bus lines, truck lines, taxicabs, police patrol cars, and delivery vehicles. In a bus system, individuals known as “point men” were situated at certain places in the bus route to monitor the flow. In police work, a jurisdiction was subdivided into sectors to help dispatchers identify patrol vehicles in particular geographic areas. The widespread acceptance of wireless cellular and personal communication systems by the public has lead to an exponential growth in the industry. Each mobile telephone or pager is connected electronically to a fix radio infrastructure, which provides an opportunity for applying an automatic monitoring system. Thus, the potential and opportunity to automatically track a mobile station are unlimited. A major driving force behind geolocation tracking is emergency calls, E-911. A large number of emergency calls are place from mobile telephones, so it is important that one is able to achieve a position fix on these calls. Thus, there is a need to develop accurate and efficient algorithms for locating a mobile user. In this article we propose an algorithm using differential range measurements to determine a mobile's location. A differential range measurement is defined as the difference between two range measurements. A closed-form location algorithm based on the least-squares solution was developed. This estimation algorithm was shown to be tolerant to measurement errors caused by unsynchronized base station clocks.

Robust Downlink Power Control in Wireless Cellular Systems

A serious shortcoming of current downlink power control methods is that their performance may be severely degraded when the downlink channel information is known imprecisely at the transmitter. In this paper, a computationally and implementationally simple centralized downlink power control method is proposed for cellular wireless communication systems using code division multiple access (CDMA) or space division multiple access (SDMA). Our method provides a substantially improved robustness against imperfect knowledge of the wireless channel by means of maintaining the required quality of service for the worst-case channel uncertainty. In the SDMA case, the proposed technique can be straightforwardly combined with any of the existing transmit beamforming methods. Simulation results validate substantial robustness improvements achieved by our approach.

Space‐time alignment for asynchronous interference suppression in MIMO OFDM cellular communications

AbstractFuture broadband cellular wireless communication systems will be characterized by high delay spread and the need to maximize the system capacity. One method of increasing capacity, aggressive frequency reuse, subjects mobiles to high levels of interference from neighboring basestations. Receiver interference suppression algorithms that work well for conventional MIMO systems may not be appropriate for suppressing interference in multiple‐input‐multiple‐output (MIMO) orthogonal frequency division multiplexing (OFDM) cellular systems, since differing propagation delays and channel lengths can destroy the alignment of the cyclic prefixes, making the interference asynchronous. This work presents MIMO algorithms for cyclic prefix based systems that are suitable for scenarios where the interferer's cyclic prefix is not time‐aligned with the desired signal's cyclic prefix. Specifically, we present several space‐time partial equalization techniques for the alignment of cyclic prefixes for aiding in interference suppression. We use a two‐stage architecture with partial equalization for cyclic‐prefix alignment in the time domain followed by simple channel equalization in the frequency domain. We consider interference suppression in both domains and demonstrate the efficacy of the two‐stage architecture over both zero‐forcing (ZF) and per frequency bin minimum mean squared error (MMSE) equalization. Copyright © 2004 John Wiley & Sons, Ltd.

무선망에서 개선된 ZMHB 알고리즘의 성능 평가

핸드오프는 무선 셀룰러 환경에서 사용자의 이동성(Mobility)을 유지해줄 수 있는 가장 중요한 기술중의 하나이다. 핸드오프 기술은 사용자가 이동할 주변 셀에 대한 대역폭 예약과 관련이 있다. 또한, 대역폭 예약은 사용자가 이동한 새로운 영역에서 이전 영역에서와 같은 레벨의 데이터나 서비스를 받기 위해서 핸드오프 이전에 실시되어야 한다. 이러한 무선 셀룰러 환경에서 효과적으로 대역폭을 사용하기 위하여 사용자의 이동성을 예측하는 기술은 핸드오프 호의 실패율(Dropping Probability)과 핸드오프 지연(Latency)을 줄이는 효과적인 방법이다. 최근에 제시된 ZMHB 알고리즘은 기존의 알고리즘과는 달리 셀 내부의 이동 경로를 저장한 히스토리를 이용하여 사용자가 이동할 셀을 예측하였다. 그러나, 모든 사용자에 대하여 80～85% 정도의 예측 정확도만을 보인다. 본 논문에서는 ZMHB 알고리즘에서 사용하는 존(Zone)을 세분화하여 이동 패턴을 저장하고, 이를 예측에 이용하는 Detailed-ZMHB 예측 알고리즘을 제안하고 성능 평가 결과를 보인다. Handoff is one of the most important features for the user's mobility in a wireless cellular communication system. It is related to resource reservation at nearby cells. Resource reservation to the new connection point should occur prior to handoff to enable the user to receive the data or services at the new location, at the same level of service as at the previous location. For the efficient resource reservation, mobility prediction has been reported as an effective means to decrease the call dropping probability and to shorten the handoff latency in a wireless cellular environment. A recently proposed algorithm, ZMHB, makes use of the history of the user's positions within the current cell to predict the next cell. But, the prediction of the ZMHB algorithm is found to be 80∼85% accurate for regular and random movements. In this paper, we propose a new improved ZMHB mobility prediction algorithm, which is called Detailed-ZMHB that uses detailed-zone-based tracking of mo-bile users to predict user movements. The effectiveness of the proposed algorithm is then demonstrated through a simulation.

A new model for network traffic forecast based on user's mobility in cellular networks with highway stretches

AbstractWireless communications have always faced the scarceness of bandwidth and until today the cellular division has solved this problem by making radio cells always smaller. However, as cell size is reduced, more users will probably require handoffs, and signaling can overload the system. Further, the decrease in cell dimension cannot ensure the best utilization of system resources, because user density in the cell is not constant over the time. Dynamic allocation might be the solution, but it is a complex task, basically due to complexity of the models for the mobility prediction, especially in those environments where users move very fast (e.g. highways). This last one is a typical environment where high mobility of vehicles does not allow small cell sizes and thus system capacity is intrinsically bounded.In this paper, we deploy an analytical model for a cellular network operating in a high mobility environment. Such a model is capable of representing and forecasting wireless system evolution in terms of channel occupancy, starting from the current state of the mobile network and road traffic. This model has been defined by integrating a vehicular traffic model with a wireless cellular communication one. With such approach the dynamic behaviour of the most critical mobile terminals, those associated with vehicles, are described in a very precise and specific way with respect to mobility.Many results obtained with specific simulation tools and herein reported show the good behaviour of our proposal. Copyright © 2004 John Wiley & Sons, Ltd.