An interference and latency aware uplink scheduling mechanism in IEEE 802.16j networks

IEEE 802.16j spreads out the coverage of WiMAX networks and strengthens wireless signal transmission using relay technology. To take advantage of relaying in IEEE 802.16j networks, an efficient scheduling schedule with quality of service (QoS) provision for multiple link transmissions is necessary, especially when link interference exists. In this paper, we propose an uplink scheduling mechanism in the transparent mode of IEEE 802.16j, which enables multiple devices to transmit without interference. The maximum latency of each connection has been considered in order to optimize the violation and transmission rate. An interference detection task is first carried out, and then a resource allocation algorithm and a dynamic frame adjustment method are developed. Two simulation experiments were conducted with different interference levels. The results demonstrate that under a fixed QoS type of connection, when the total number of connections goes up to 360 and 420 and the maximum latency violation rate approaches 20%, the average uplink transmission rate of the proposed mechanism can achieve 6.67 and 7.92 Mbps, which apparently outperform regular relay scheduling schemes with rate of 4 and 3.91 Mbps, respectively.     

Software architecture design,implementation, and evaluation of DCF-based MAC protocol on a low-power transceiver

The carrier-sensing multiple access with collision avoidance (CSMA/CA) protocol is the most well-known medium access control (MAC) protocol for wireless networks. Both the distributed coordination function (DCF) defined in IEEE 802.11 and the MAC layer defined in IEEE 802.15.4 are based on the CSMA/CA protocol. Nevertheless, these two standards have quite different carrier-sensing mechanisms. Different to continuous carrier sensing in DCF, an IEEE 802.15.4 node only senses the channel once just after a backoff. Sensing-once mechanism can reduce the computation loading on the CPU. However, it significantly increases the probability of failure transmission because a node is not fully aware of channel activity. This paper first proposes a software architecture integrating proper hardware features for designing a DCF-based MAC protocol and then successfully implements it on a low-power transceiver. In addition, this paper conducts experiments in a star topology network to compare the performance of the above DCF-MAC protocol with the IEEE 802.15.4 MAC protocol. Experimental results show that, without continuous sensing, the IEEE 802.15.4 network suffers a high transmission failure probability as the network size increases. Consequently, the proposed DCF-based MAC protocol outperforms the IEEE 802.15.4 MAC protocol in terms of packet loss probability and throughput.     

无线城域网OFDM系统信道估计算法

OFDM技术是无线城域网IEEE802.16a标准中物理层面的主流调制技术,今建立符合IEEE802.16a标准的OFDM信道估计模型,阐述其导频基本信道估计方法的原理及特点,分析比较了LS/MMSE/LMMSE/SVD四种算法的MSE和BER性能特点.仿真结果说明,LMMSE算法的均方误差和误码率性能最好,且计算复杂度相对较低,易于物理实现.     

Performance analysis of IEEE 802.11 DCF and 802.11e EDCA based on queueing networks

The authors propose a new analytical model based on BCMP closed queueing networks in order to evaluate the performance of IEEE 802.11 DCF MAC protocol when all nodes are in the transmission range of each other, that is, a single hop wireless ad hoc network. By the proposed model, some performance metrics such as saturation and non-saturation throughput, distributions of channel access delay and the number of packets in the MAC buffer are derived. An extension of the proposed model is used for the analysis of IEEE 802.11e EDCA and the same performance metrics are evaluated for this protocol. Analytical results on IEEE 802.11e prove that differentiation in service is possible and channel share for each service type may be well assigned by tuning the MAC protocol parameters. Simulation results show consistency with our analytical results.     

Simulation-based performance analysis and improvement of orthogonal frequency division multiplexing - 802.11p system for vehicular communications

In this study, the physical layer (PHY) of the upcoming vehicular communication standard IEEE 802.11p has been simulated in vehicle-to-vehicle situation through two different scenarios. IEEE 802.11p wireless access in vehicular environment defines modifications to IEEE 802.11 to support intelligent transportation systems applications. The standard is being considered as a promising wireless technology for enhancing transportation safety and provides safety-related services like collision avoidance and emergency breaking. At first, this includes data exchange between high-speed vehicles and between the vehicles and the roadside infrastructure in the licensed ITS band of 5.9 GHz. Performance analysis of PHY model has been evaluated into different propagation conditions (AWGN, Ricean and Rayleigh fading). In particular, bit error rate (BER) and signal to noise ratio for all the data rates have been estimated. Simulation results reveal that our system can efficiently mitigate inter-symbol interference and inter-carrier interference introduced by multi-path delay spread in our high mobility environment but against frequency-selective fading BER values are on to increase. To overcome this problem, the authors propose to use a different value of guard interval (3.2 μs). Our initial results indicate that the performance with the larger cyclic prefix outperforms the performance of the initial value in our mobile channel profiles. Moreover, the authors investigated in which way the Doppler spread affects the performance with regard to the transmission distance.     

Performance of impulse radio direct sequence ultra-wideband system with variable-length spreading sequences

An adaptive transmission scheme using variable-length spreading sequences (VLSS) is proposed. The system is based on an impulse direct sequence ultra-wideband (DS-UWB) approach for wireless communication where the number of active users is variable. In contrast to the conventional DS-UWB system, which employs a fixed length spreading code, the proposed system changes the length of spreading code according to the system load adaptively, which is proven to be able to reduce the inter-chip interference, inter-symbol interference and multiple-access interference and thus improve the system performance in terms of bit error rate. The proposed system is simulated in multipath channels with a high level of multipath fragments, modelled by the standard IEEE 802.15.3a indoor channel model 1 (CM1) and channel model 2 (CM2). Numerical results demonstrate that when RAKE receivers are employed, the proposed VLSS DS-UWB system outperforms the conventional system by appropriately allocating the spreading sequences.     

Measurements of radio channels and bit error rate estimation of IEEE802.16 standard in semi-rural environment at three frequencies in the 2?6 GHz frequency band

Simultaneous measurements with 10 MHz bandwidth at 2.5, 3.5 and 5.8 GHz were performed in a rural/semi-rural environment in the UK. The measurements were processed to generate power delay profiles to estimate the root mean square delay spread of the channel. The frequency range of the orthogonal frequency division multiplexing (OFDM) symbol bandwidth that has dropped below a predefined level, termed herein as average fade bandwidth, and the corresponding level crossing are employed to quantify the severity of frequency selectivity. The channel data were used to estimate the bit error rate, for the 256 carrier-OFDM IEEE802.16 standard using a frequency domain channel simulator especially designed for the study. It was found that the performance of quasi-stationary wireless broadband systems depends mainly upon the frequency selectivity and the channel coding rate with 1/2 rate coding giving a superior performance to 3/4 rate coding. Puncturing was found to weaken the capability of forward error correction coding in the presence of series of deep fades in the channel transfer function.     

Frequency domain simulator for mobile radio channels and for IEEE 802.16-2004 standard using measured channels

Two radio channel simulators based on the time variant frequency transfer function have been implemented using digital signal processing techniques in SIMULINK. The first simulator uses a two ray channel model to determine the number of taps per coherent bandwidth of the channel by comparing the bit error rate (BER) with the tapped delay line model for differential quadrature phase shift keying. The results show that ten taps per coherent bandwidth are appropriate for a very close approximation of the channel. The second simulator utilises real channel measurements to estimate the BER for the IEEE 802.16 -2004 wireless metropolitan area network orthogonal frequency division-multiplexing standard with 256 carriers. Channel measurements at three frequencies in the 2-6-GHz frequency band in rural/semi rural environment demonstrate the simulator and relate the BER performance of the standard to the frequency selectivity of the channel.     

Directional Antenna Intelligent Coverage Method Based on Traversal Optimization Algorithm

Wireless broadband communication is widely used in maneuver command communications systems in many fields, such as military operations, counter-terrorism and disaster relief. How to reasonably formulate the directional antenna coverage strategy according to the mobile terminal dynamic distribution and guide the directional antenna dynamic coverage becomes a practical research topic. In many applications, a temporary wireless boardband base station is required to support wireless signal communications between many terminals from nearby vehicles and staffs. It is therefore important to efficiently set directional antenna while ensuring large enough coverage over dynamically distributed terminals. The wireless broadband base station mostly uses two rotatable conical-polarized directional antennas with a coverage angle of 80 degrees. In this paper, we study this directional antenna coverage problem and propose a new solution by using three-dimensional coordinate transformation, provides wireless signal coverage schemes for point-to-point and point-to-region, determined the required horizontal rotation angle and pitch rotation angle of the directional antenna intelligent coverage, which lays the foundation for the performance of the wireless broadband communication in the maneuver command communication system.     

Design of Intra‐BS handoff in multi‐hop IEEE 802.16 broadband wireless networks

Abstract

In IEEE 802.16‐based multi‐hop networks, transmissions using a multi‐hop high‐rate relay path can potentially achieve better throughput and delay than using a single‐hop low‐rate direct path. The establishment of relay or direct connection and the handoffs between different connections for mobile subscribers are thus key design issues. This paper studies connection setup and Intra‐BS handoff operations in IEEE 802.16‐based multi‐hop networks, where each connection employs adaptive modulation and coding scheme at the physical layer against wireless fading channel. Both centralized and decentralized designs are considered. In the centralized design, the base station coordinates connection establishments and handoffs for multi‐hop IEEE 802.16‐based subscribers. In the decentralized design, the MMR‐enhanced relay stations and mobile subscribers are required to participate in the connection and handoff management. Simulation results show that the two designs can significantly improve the good put, the packet transmission delay and network throughput. The results also highlight that the centralized design outperforms decentralized design in terms of throughput and delay.     

IEEE 802.11b based ad hoc networking and its performance in mobile channels

It is widely believed that IEEE 802.11 standard is aimed mainly for fixed indoor wireless local area networks and is not suited for mobile applications, even though the IEEE 802.11b systems may work in either infrastructure mode or ad hoc mode. The impact of node mobility on ad hoc network performance has already been studied intensively, but these studies mostly do not consider temporal fluctuations of the mobile wireless channel due to the Doppler shift. An investigation of the mobility impact on the performance of IEEE 802.11b ad hoc systems with Rician/Rayleigh fading under different node velocities is presented. A comprehensive and in-depth analysis of the impacts of a multitude of different signal distortions on an IEEE 802.11b system performance is also presented. Specifically, the authors study the bit-error rate performances with respect to node velocities for different modulation schemes. The simulation results show that, owing to its extremely low implementation and deployment cost, the current IEEE 802.11b standard has its potential to be deployed in a mobile ad hoc environment if the line-of-sight path between transmitter and receiver exists.     

Differentiated cooperative multiple access for multimedia communications over fading wireless networks

The quality of service (QoS) support for multimedia communications faces a big challenge in a fading wireless network. On one hand, conventional automatic repeat request (ARQ) schemes are not effective for small-scale fading channels with correlated errors due to consecutive retransmission failures. On the other hand, large-scale fading due to propagation loss or shadowing severely limits transmission range. A novel differentiated cooperative medium access control (MAC) protocol, called DC-MAC, is proposed to enhance the QoS support for multimedia communications while supporting service differentiation based on the IEEE 802.11e architecture. By enabling cooperative ARQ, the retransmission is initiated from an appropriate transmission queue of an appropriate relay node instead of the original source. Since unnecessary and useless retransmissions may intensify the node contention and degrade the system performance contrarily, a novel negative acknowledgement feedback mechanism is introduced for loss distinguishing and channel estimation such that cooperative retransmission will be employed only when necessary and only by competent nodes. Extensive simulations are conducted on the OPNET platform to analyse the performances of DC-MAC under both small-scale and large-scale fading. Simulation results show that the proposed scheme significantly improves the performances of both multimedia applications and best-effort data applications in terms of throughput, delay and coverage with moderate user contention.     

Design and theoretical analysis of throughput enhanced spatial reuse distributed coordination function for IEEE 802.11

The IEEE 802.11 distributed coordination function (DCF) employs a carrier sensing mechanism, a simple and effective mechanism to mitigate collisions in wireless networks. But the carrier sensing mechanism is inefficient in terms of shared channel use because an overcautious channel assessment approach is used to estimate interference at a receiver. A DCF node simply blocks its transmission when it senses that the channel is busy. However, in many cases this channel assessing node?s own transmission may not generate enough interference to disrupt the ongoing transmission at the receiver. This overcautious channel assessment unnecessarily blocks transmission attempts, and thus degrades the overall network throughput. To avoid this unnecessary blocking, the authors propose a spatial reuse DCF (SRDCF), which utilises location information and transmission parameters to make accurate channel assessments and to permit concurrent transmissions by adjusting the transmission power. SRDCF also resolves the contention between opportunistic concurrent transmissions with a secondary backoff counter. Consequently, the proposed scheme improves the overall network throughput because of more concurrent transmissions. The authors theoretically analyse the performance enhancement of SRDCF over the original IEEE 802.11 DCF by using a Markov chain model and verify it through simulations.     

Enhancing enhanced distributed channel access function based on cross-layer information for multirate wireless networks

IEEE 802.lie provides guaranteed quality of service (QoS) by proving different transmission priorities. IEEE 802. lie improves the media access control layer of IEEE 802.11 to satisfy the different QoS requirements by introducing two new channel access functions: the enhanced distributed channel access (EDCA) and the hybrid coordination function-controlled channel access. The available bandwidth and transmission rate may be easily affected by the signal quality, because the communication channel in a wireless environment operates in a random time-variation manner. Generally, a station using a low transmission rate will occupy the communication channel for a long time and degrade system performance, which causes bandwidth waste and unfairness; thus the guaranteed QoS for stations with higher transmission rates cannot be provided. An enhancing EDCAF (E DCAF) is proposed that consolidates the cross-layer concept and the IEEE 802.1 le EDCAF protocol. After simulation experiments, E DCAF obviously improves performance, especially in throughput and fairness. E DCAF scheduling also allows the different QoS requirements to be processed efficiently and flexibly.     

Virtual multiple input multiple output in multiple high-altitude platform constellations

Multiple-input multiple-output (MIMO) technology, which makes use of the spatial dimension by utilising multiple antennas at the transmitter and receiver, has proved an efficient solution for providing higher data throughput and/or link reliability in wireless systems. In this study, the authors investigate a virtual MIMO (V-MIMO) technique, based on a constellation of multiple high-altitude platforms (HAPs), providing broadband wireless access to high-speed trains under predominantly line-of-sight (LOS) propagation conditions. They analyse the performance of transmit diversity based on space?time block coding (STBC), in particular Alamouti and extended Alamouti schemes, using fixed wide-lobe receive antennas, and compare it to the reference receive diversity scheme based on best HAP selection that requires highly directional and steerable antennas. Simulation results for different diversity schemes are based on the calculation of carrier to interference-plus-noise ratio (CINR) along representative railway lines in a realistic terrain configuration. The CINR levels obtained are mapped onto transmission modes specified by standards developed for two wireless systems operating at distinct frequency bands, the DVB-S2 assumed for operation in mm-wave bands and the IEEE 802.16e (mobile WiMAX) for operation at 3.5 GHz. Simulation results show that the use of transmit diversity schemes in a multiple HAP constellation decreases the link outage and also increases the average spectral efficiency, but requires a marked increase in transmit power, especially for the system operating in mm-wave bands.     

Direction finding in IEEE802.11 wireless networks

A novel direction-finding method for stations of IEEE802.11 wireless local area networks is presented in this paper. The method uses a switched beam array for determining the direction of arrival of the incident electromagnetic field in a time efficient way and associates certain medium access control (MAC) layer functions with different radiation patterns of the switched antenna array, in order to determine the proper orientation of directional beams on both entities of a communication link. The application of the proposed method to an IEEE802.11 wireless network is presented and it is depicted how the method improves the network performance without requiring any modifications to the existing MAC protocol.     

无线局域网上IP语音传输的容量分析

由于无线媒介的易变性，IP语音在无线局域网上传输受到极大的限制。通过对无线局域网媒体接入层机制的分析，考虑了碰撞概率因素，提出了采用马尔可夫链模型来推导在IEEE 80211b/a/g标准下单个接入点同时支持IP语音用户最大容量的方法，计算出针对不同IP语音编码标准如G711, G729和G723.1下的用户容量上限。并在NS2仿真环境下进行结果验证。     

Quality of service supporting downlink scheduling scheme in worldwide interoperability for microwave access wireless access systems

IEEE 802.16 is a standardisation for a broadband wireless access in metropolitan area networks (MAN). Since the IEEE 802.16 standard defines the concrete quality of service (QoS) requirement, a scheduling scheme is necessary to achieve the QoS requirement. Many scheduling schemes are proposed with the purpose of throughput optimisation and fairness enhancement, however, few scheduling schemes support the delay requirement. In this study, the authors propose a new scheduling scheme reflecting the delay requirement. Specifically, the authors add the delay requirement term in the proportional fair scheduling scheme and the scheduling parameters are optimised with respect to the QoS requirement. Therefore the QoS requirement is achieved without the excessive resource consumption.     

Ergodic Capacity Evaluation of Multi-Hop Decode-and-Forward MIMO-OFDM Relaying Network

Spatial diversity plays a significant role in wireless communication systems, including the Fourth Generation (4G) and Fifth Generation (5G) systems, and it is expected to be a fundamental part of the future wireless communication systems as well. The Multiple-Input Multiple-Output (MIMO) technology, which is included in the IEEE 802.16j standard, still holds the most crucial position in the 4G spectrum as it promises to improve the throughput, capacity, spectral, and energy efficiency of wireless communication systems in the 2020s. This makes MIMO a viable technology for delay constrained medical and health care facilities. This paper presents an approximate closed-form expression of the ergodic capacity for the Decode-and-Forward (DF) protocol MIMO-Orthogonal Frequency Division Multiplexing (OFDM) relaying network. Although MIMO-OFDM is highly valuable for modern high-speed wireless communication systems, especially in the medical sciences, its performance degrades in multi-hop relay networks. Therefore, in this paper, an approximate closed-form expression is derived for an end-to-end ergodic capacity of multi-hop DF MIMO-OFDM wireless communication system has been presented. Monte-Carlo simulations are conducted to verify the performance of the presented analysis regarding the capacity (bits/s/Hz) for different SNR-dB values for single, 2 × 2, 4 × 4, and multi-hop DF MIMO-OFDM systems. The presented results provide useful insights for the research on the end-to-end ergodic capacity evaluation.     

Energy efficiency modelling for IEEE 802.11a distribution coordination function system without finite retry limits

Use of IEEE 802.11 wireless local area networks (WLANs) as an extension to existing wired networks, offering both mobility and portability in a residential or office environment, is growing at an unprecedented rate. One of the critical limitations of current WLANs is the limited energy storage of mobile devices, and the design of energy-efficient protocols for WLANs has therefore become an area of intensive research. An analytical framework to study the energy consumption and energy efficiency of IEEE 802.11a WLANs is proposed. The energy consumption by considering the interactions between IEEE 802.11a PHY and MAC layers is modelled. Simulation results demonstrate that the theoretical model is accurate in predicting the energy efficiency over a wide range of scenarios. In addition, the effects of different PHY and MAC layer parameters on energy efficiency of IEEE 802.11a WLANs are investigated, as are the effects of different parameters on energy efficiency.