Enhanced high‐performance distributed coordination function for IEEE 802.11 multi‐rate LANs

To compensate for the effects of fading in wireless channels, IEEE 802.11 systems utilize a rate‐adaptation mechanism to accomplish a multi‐rate capability. However, the IEEE 802.11 distributed coordination function results in a fundamental performance anomaly in multi‐rate networks; namely, when stations with different transmission rates collide, the throughput performance of the high‐rate station is significantly degraded by the relatively longer channel occupancy time of the low‐rate station. This study resolves this problem through the use of an enhanced high‐performance distributed coordination function (EHDCF) protocol. While most existing solutions to the multi‐rate performance anomaly problem have the form of simple contention‐based protocols, EHDCF has two modes, namely a contending mode and an active mode. In the proposed protocol, new stations joining the network are assigned a contending mode, but switch to an active node (and are therefore permitted to transmit data packets) as soon as they have gained access to the channel. Having transmitted a data packet, the active node then selects the next transmission station in accordance with a probability‐based rule designed such that the high‐rate stations within the network receive a greater number of transmission opportunities than the low‐rate stations. The simulation results show that the EHDCF protocol not only yields a significant improvement in the network throughput but also guarantees the temporal fairness of all the stations. Copyright © 2009 John Wiley & Sons, Ltd.     

Analysis of a wireless MAC protocol with client-server traffic andcapture

We analyze an efficient medium access control (MAC) protocol for use in a single cell of a wireless local area network (LAN). A fixed frame structure with two periods is used; one period for making reservations using a slotted Aloha protocol and a second period for data transmission. The MAC protocol operates in a centralized manner in which a single station, the base station, accepts reservations (transmission or data requests) that are made by the remote stations, and then schedules the times in which they will transmit or receive data. We consider two different types of client-server traffic models, an open-loop and a closed-loop model. In the open-loop model, a remote station can generate reservation requests for the base station whether or not it has received a response from the base station. In the closed-loop case, the remote station waits for a response before generating a new request. The performance of the MAC protocol was analyzed exactly for both the open and closed-loop traffic models and for cases in which transmission errors and a mean-value type of radio capture model are included. In addition, different policies were considered for the transmission strategy that is used by the remote stations when they attempt to make reservations. We derive exact results for the mean throughput and waiting times as well as for the queue length distributions. We also derive an approximate Markov chain to treat a case in which a fixed-position capture model is used. Several types of behavior are illustrated through the use of numerical examples     

基于多点协作联合传输的超密集组网性能分析

超密集组网的基站高密度特性会带来严重的小区间干扰,多点协作联合传输应用于超密集组网进行干扰管理是目前的研究热点,该文对多点协作联合传输时基站密度对网络性能的影响进行了分析。首先采用随机几何方法推导了3维空间基站与用户距离的概率密度函数,为选取距离用户最近的多个基站联合传输的协作机制提供了基础;然后结合有界双斜率路径损耗模型,进行用户下行链路的干扰建模,进一步推导出用户下行链路覆盖率和网络区域频谱效率的表达式,并分析了协作基站数、基站密度等参数对网络性能的影响。数值仿真表明:协作基站数为2时就可使下行链路覆盖率增加10%,且实现2到3倍的频谱效率的增益,当协作基站数为3时,费效比更优,同时可得到多点协作下的基站密度极限使区域频谱效率最高。该文工作可为下一代移动通信网络的基站部署提供理论支持。     

A transmit power control scheme for improving performance in amobile packet radio system

The problem of a number of mobile users attempting to send data packets over a commonly shared slotted channel to a central base station is studied. A scheme for controlling the transmit power of the mobiles is proposed and shown to yield better throughputs than a conventional transmit power scheme (in which all mobiles transmit with constant power) for both nonfading and fading channels. In any given time slot, mobiles that are closer to the base station are allowed to transmit at a higher power level in an attempt to improve capture probability     

Optimal Microwave Wireless Backhaul Link Design Using a Massive MIMO for 5G HetNet-Practical Deployment Scenario

The massive multiple-input–multiple-output that enhances energy efficiency and spectral efficiency is the primary technology for fifth generation wireless networks. A move toward heterogeneous elements such as microcells, femtocells, picocells as well as remote radio heads, characterised by physical measurements, backhauls, transmission and propagation, is now underway to ensure an economical transition to the cellular network infrastructure, that is far from expensive high power mounted base stations. This adaptation presents many obstacles to network operations and co-existence. The proposed work therefore provides a design for a 28 GHz microwave wireless backhaul link for small cell base stations (SBSs) as well as the number of antennas required for the base station (BS) to achieve a target backhaul rate of 10 Gbit/s within a given transmit power of 40 dBm. In this work a distributed beamforming algorithm is formulated in a multi-cell scenario using methods from random matrix theory, under the assumption that the system dimensions are large. The design purpose is to minimize the total transmit power over all BSs according to the constraints of the signal-to-interference-and-noise ratio (SINR) as beamformers are integrated in a distributed manner. The BSs may only need to share the channel statistics in the suggested algorithm, instead of the instant channel state information. The simulation results show that the proposed algorithm strongly satisfies the target SINR constraints as the number of SBSs per cell grows high.

     

Design of an enhanced access point to optimize TCP performance in Wi-Fi hotspot networks

In the last years, the number of Wi-Fi hotspots at public venues has undergone a substantial growth, promoting the WLAN technologies as the ubiquitous solution to provide high-speed wireless connectivity in public areas. However, the adoption of a random access CSMA-based paradigm for the 802.11 MAC protocol makes difficult to ensure high throughput and a fair allocation of radio resources in 802.11-based WLANs. In this paper we evaluate extensively via simulations the interaction between the flow control mechanisms implemented at the TCP layer and the contention avoidance techniques used at the 802.11 MAC layer. We conducted our study considering initially M wireless stations performing downloads from the Internet. From our results, we observed that the TCP downlink throughput is not limited by the collision events, but by the inability of the MAC protocol to assign a higher chance of accessing the channel to the base station. We propose a simple and easy to implement modification of the base station’s behavior with the purpose of increasing the TCP throughput reducing useless MAC protocol overheads. With our scheme, the base station is allowed to transmit periodically bursts of data frames towards the mobile hosts. We design a resource allocation protocol aimed at maximizing the success probability of the uplink transmissions by dynamically adapting the burst length to the collision probability estimated by the base station. By its design, our scheme is also beneficial to achieve a fairer allocation of the channel bandwidth among the downlink and uplink flows, and among TCP and UDP flows. Simulation results confirm both the improvement in the TCP downlink throughput and the reduction of system unfairness.     

Redundancy concepts to increase transmission reliability in wireless industrial LANs

Wireless LANs are an attractive networking technology for industrial applications. A major obstacle toward the fulfillment of hard real-time requirements is the error-prone behavior of wireless channels. A common approach to increase the probability of a message being transmitted successfully before a prescribed deadline is to use feedback from the receiver and subsequent retransmissions (automatic repeat request-ARQ-protocols). In this paper, three modifications to an ARQ protocol are investigated. As one of these modifications a specific transmit diversity scheme, called antenna redundancy, is introduced. The other modifications are error-correcting codes and the transmission of multiple copies of the same packet. In antenna redundancy the base station/access point has several antennas. The base station transmits on one antenna at a time, but whenever a retransmission is needed, the base station switches to another antenna. The relative benefits of using FEC versus adding antennas versus sending multiple copies are investigated under different error conditions. One important result is that for independent Gilbert-Elliot channels between the base station antennas and the wireless station the antenna redundancy scheme effectively decreases the probability of missing a deadline, in a numerical example approximately an order of magnitude per additional antenna can be observed. As a second benefit, antenna redundancy decreases the number of transmission trials needed to transmit a message successfully, thus saving bandwidth.     

Capture models for mobile packet radio networks

The probability q_i of successful reception in a nonfading mobile radio channel with i contending mobiles transmitting to a central base station is studied for a number of different capture and spatial distribution models. It is shown that a generalized capture model can be used to estimate q_i's for a simplified example system which uses noncoherent frequency shift keying modulation. This model can be applied to other systems as well. An example of the use of the q_i 's in the throughput evaluation of a finite population slotted ALOHA system is given. In most practical systems, the mobiles cannot get arbitrarily close to the base station. The effect of this constraint on q_i is examined. Finally, the dependence of the capture probability for a test mobile on its distance from the base station is obtained     

Throughput of inhibit sense multiple access with propagation delays

A large number of terminals transmitting data packets over a common radio channel to a central base station is studied. In inhibit sense multiple access (ISMA), the base station broadcasts a busy signal when an incoming packet is being received, to inhibit other terminals from colliding transmissions. This busy signal arrives at each terminal with a propagation delay, proportional to the distance between the base station and the terminal. This leads to unfairness in the probability of successfully transmitting a data packet, since nearby terminals have more up-to-date information on the actual channel status than remote terminals. This unfairness is additional to the advantage that nearby terminals have because of the capture effect, which is also considered. The paper applies nonstationary Poisson processes to describe the random arrivals of data packets at the central receiver. It is shown that the probability of a successful attempt to transmit a packet decreases, approximately linearly with the distance between the transmitter and the central receiver. The total throughput is also assessed, and it is found that the assumption of a fixed propagation delay adopted in CSMA studies by others gives too optimistic results for ISMA. Moreover, the analysis suggests a subtle change of the fixed delay approximation that enhances its accuracy, without adding complexity. The effect of propagation delays in random access radio networks are of increasing importance, since newly developed systems are designed to transmit with increasingly high bit rates     

Power allocation and relay selection for network-coded D2D communication underlay heterogeneous cellular networks

Underlay device-to-device (D2D) communication is an attractive technology enabling nearby cellular users to communicate with each other directly in order to increase data rate and spectral efficiency. The current cellular heterogeneous networks consist of macrocell base stations and small cell base stations with different transmit powers and coverage areas. Femtocell is the most popular small cell which is expected to be utilized in dense and ultra-dense scenarios in the future. Network coding in relay-assisted multi-hop communications improves achievable transmission rate and coverage of D2D communications. In this paper, two-hop random linear network coding network in cooperative D2D communication (RLNC-CDC) is considered. We propose to use femtocell base station (FBS) as a relay. We assume that the D2D pair and relay operate in the frequency band which is allocated to femtocell network. Therefore, there would be interference from the relay node and the D2D communication on the femtocell network users. To reduce the interference, the sum of transmit powers of the D2D pair and selected relay FBS should be minimized in a way that the highest transmission rate for the D2D pair is achieved. The constraints on the bounds of transmit powers of the D2D and relay node as well as the minimum required transmission rate for D2D communication are considered and the optimum solution is obtained. Simulation results indicate that the proposed RLNC-CDC achieves higher data rate and smaller outage probability than the direct D2D transmission.     

Software‐only TDOA/RTT positioning for 3G WCDMA wireless network

A hybrid location finding technique based on time difference of arrival (TDOA) with round‐trip time (RTT) measurements is proposed for a wideband code division multiple access (WCDMA) network. In this technique, a mobile station measures timing from at least three base stations using user equipment receive—transmit (UE Rx—Tx) time difference and at least three base stations measure timing from the mobile station using RTT. The timing measurements of mobile and base stations are then combined to solve for both the location of the mobile and the synchronization offset between base stations. A software‐only geolocation system based on the above mobile/base stations timing measurements is implemented in Matlab platform and the performance of the system is investigated using large‐scale propagation models. Copyright © 2007 John Wiley & Sons, Ltd.     

改进的多信道S-ALOHA暂态性能分析办法及其应用

海量机器类终端(或MTC终端)同步入网时,其业务呈现瞬时突发性,这使得基于齐次或复合泊松假设的多信道S-ALOHA稳态性能分析办法难以直接应用。该文以第i个随机接入时隙内第j次进行随机接入的用户数Mi(j)作为状态变量,提出了一种沿Mi(j) 的j方向迭代进行多信道S-ALOHA暂态性能分析的办法及其近似形式。该迭代办法可建立第i个随机接入时隙内第j次进行随机接入的用户数与第x个随机接入时隙内新到用户数的直接关系(其中xi),也可给出接入时延概率密度函数、概率分布函数和均值的求解办法。以3GPP MTC业务参考模型进行数值仿真,验证了所提迭代办法及其近似形式的有效性。相关研究可为承载网络的优化设计提供参考。     

多串口转GPRS组网方式的基站监控应用

在无有线传输资源的移动基站中,基站内智能设备通过多串口转GPRS传输方式实现了对基站的远程监控,提高了监控覆盖率。GPRS技术使用方便,成本较低,可以稳定地传送数据,有利于大规模网络的建立。     

拉远站传输接入方案探讨

目前4G网络建设进入高速发展期,随着移动网络建设速度的加快,拉远基站凭借成本低,适应环境的能力强,工程建设方便的优势,在工程建设中得到了越来越多的应用.本文对如何为大量建设的拉远基站提供快速有效的传输接入方式提出了几种解决方案,对于将来的4G拉远基站的大规模建设提供了参考模型.     

The capacity achievable with a broadband CDMA microcell underlay toan existing cellular macrosystem

Broadband code division multiple access (B-CDMA) using direct sequence spread spectrum can be used as an overlay to an existing analog or narrowband digital cellular system to provide increased capacity and new data services. In order to achieve significant capacity, it has been shown that both transmit and receive notch filters should be used at the base station. This paper addresses whether the B-CDMA overlay concept can be applied to creating a CDMA microcell underlaying an existing analog macrocell. It is shown that indeed high capacity can be achieved in the microcell on both forward and reverse links, largely independently of the separation between microcell and macrocell bases. Furthermore, in the forward link the effect of neighboring base stations is shown to be negligible. In order to achieve maximum capacity, it is found that transmit and receive notch filters at the microcell base station are invaluable at small separations between micro and macrocells. It is also shown that key parameters which must be properly controlled are the powers of the CDMA base and mobile transmitters relative to their analog counterparts     

Some Message Variables and the MUM Effect

A separate group of subjects scaled 55 messages with regard to pleasantness for a potential recipient (N = 53), desire to communicate (N = 55), obligation to communication (N = 57), and the probability with which they would communicate each (N = 56). These ratings were used to test the hypothesis that message transmission is a function of felt obligation and desire to transmit, and that felt desire and obligation to transmit, in turn, are functions (linear for desire; U-shaped for obligation) of message pleasantness for the recipient. Results indicated that transmission is a function of felt desire and obligation over all levels of message pleasantness (r= 97; p <.001); that the determinants of obligation and desire are qualitatively different for good news versus bad news messages, i.e., message pleasantness can account for desire (r=.85; p <.01) and obligation (r=.75; p <.01) among good news messages, but neither message pleasantness nor other inferred relevant parameters of messages, namely locus of cause of message, urgency of message, or ability to empathize with the recipient, could account for desire and obligation among bad news messages. Finally, greater variability was found for bad news messages than good news messages on rated probability of transmitting, felt obligation, and desire to transmit. This greater variability suggested greater norm ambiguity with respect to the handling of bad news than of good news messages.     

区域覆盖下的最优中继部署与功率分配

在大型建筑底层及偏远地区等,传统的蜂窝网系统没有提供可靠的无线覆盖。该文采用中继方式延伸基站的无线覆盖至目标区域。主要研究最优的中继部署与功率分配最小化目标区域的最大中断概率。功率分配是指在总功率受限的情况下,最优化基站和中继的发送功率。通过分析可以得到最优的中继部署,基站和中继发送功率的比值不随总功率的增加而变化。针对优化问题的非凸性质,提出了一种获得局部最优解的算法。通过仿真可得给出算法的性能与穷搜索算法的性能基本重合。由于给出算法的复杂度较低,更适用于实际的系统。     

Dynamic Cooperative Base Station Selection Scheme for Downlink CoMP in LTE-Advanced Networks

Coordinated Multi-Point (CoMP) transmission is a technique proposed to enhance the spectral efficiency and system throughput in an interference limited cellular networks. In CoMP joint processing (JP) scheme multiple base stations (BSs) are coordinately transmit data streams to each user. As more than two base stations are involved, abundant spatial resources are exploited and more backhaul spectrum for JP cooperation is required. The backhaul architecture for CoMP JP is crucial to provide low latency, unlimited capacity, less power consumption, and perfect synchronization among the BSs. However, satisfying all these constraints is impossible as the number of cooperative BSs increases for each user. In this paper, a dynamic cooperative base station selection scheme is proposed to reduce the backhaul load for CoMP user by selecting the appropriate number of coordinated BSs from the CoMP cluster to ensure the certain quality of service (QoS). In particular, for cell edge user the number of cooperative BSs per user has been selected in order to achieve reduced overhead and the allocation of backhaul capacity is performed under the max–min fairness criterion. Simulation results show that the proposed selection scheme achieves significant performance improvement than other transmission modes in terms of the average sum rate per backhaul use and minimal total power consumption.     

Power allocation and temporal fair user group scheduling for downlink NOMA

Non-Orthogonal Multiple Access (NOMA) has been proposed as a new radio access technique for cellular networks as an alternative to OMA (Orthogonal Multiple Access) in which the users of a group (pairs or triples of users in a group are considered in this paper) are allowed to use the wireless channel simultaneously. In this paper, for downlink single-input single-output SISO-NOMA, a heuristic power allocation algorithm within a group is first proposed which attempts to ensure that the users of a group benefit from simultaneous transmission equally in terms of achievable throughput. Moreover, a user group scheduling algorithm is proposed for downlink NOMA systems by which a user group is to be dynamically selected for transmission while satisfying long term temporal fairness among the individual contending users. The effectiveness of the proposed power allocation method along with the temporal fair scheduling algorithm for downlink NOMA is validated with simulations and the performance impact of the transmit power and the coverage radius of the base station as well as the number of users are thoroughly studied.

     

Decoupled Active/Passive Base Stations for Second Generation Cellular Networks

The effect of electromagnetic radiations from wireless cellular telephony systems on human health has been intensely discussed. As a result, and despite the lack of evidence on any adverse effects, recent regulations make network planning even more difficult. Base stations, even those covering urban areas, tend to be placed at suburban locations (in this paper we cite some examples of related local regulations). In these cases, mobile stations inside urban locations must raise their power transmission level to reach suburban antenna placements. Therefore, users may complain of a higher radio exposure. In this work we propose a feasible solution to reduce electromagnetic activity from mobile communications equipment and to cope with restrictive placement regulations. To accomplish this goal, a suburban base station is divided into an active part (transmitter) that is kept at the suburban location, and a passive part (receiver), which is moved inside the urban area. Thereby, the power received from the base station can be considered negligible inside the urban area (since the active part is far away), and mobile terminal/stations transmit lower power levels (since passive receivers are close to them). We justify the solution in the context of the globally available GSM networks, both analytically and with empirical and simulation tests. The results in this paper reveal that our approach reduces electromagnetic levels and improves uplink quality, which reaches typical levels of urban base stations.