Shifted Known Symbol Padding for Efficient Data Communication in a WLAN Context

To allow for a computationally efficient equalization scheme for the frequency-selective transmission channels encountered in wireless local area network (WLAN) applications, cyclic prefix (CP) block transmission schemes have been proposed, such as single-carrier CP (SC-CP) and multi-carrier CP (MC-CP) transmission, also known as orthogonal frequency division multiplexing (OFDM). In this letter, however, we focus on the known symbol padding (KSP) transmission scheme. In this scheme known padded sequences can be exploited for synchronization as well as for channel estimation. However, to simultaneously allow for low-complexity frequency-domain equalization and accurate channel estimation within the KSP context, a modified KSP scheme is proposed, namely shifted KSP (S-KSP). Comparing different block transmission schemes in the WLAN context, the S-KSP scheme is shown to offer a very good performance.     

A dual-driven adaptive video system over WLAN

This paper proposes a dual-driven adaptive (D2D-Adap) video server system over wireless local area network (WLAN). In the system, the status of the wireless channel is monitored at the sender side, And at the receiver side. the packet loss rate, delay etc. are measured and fed back to the sender. The status information from both sides is used to adapt the encoding and transmission rate of the video in the server. The adaptation scheme in the proposed system can estimate the network channel situation accurately and quickly and can provide better video communication service over WLAN. The simulation and test results show that the proposed system can respond to the changes of the wireless channel quickly and therefore provide smoother video to clients.     

无线实时多播系统中混合纠删方案性能分析与评估

为了对无线实时多播系统中纠删方案的性能进行精确的分析与评估,提出基于GE(Gilbert-Elliot)信道模型与实际实验数据的精确分析与统计评估方法。提出一种分析混合纠删方案性能的闭式表达式,并设计基于WLAN的实时多播验证与评估系统,在此基础上,采用极大似然估计方法对实验数据进行统计评估。结果表明,GE模型能精确匹配实际删除错误信道,且待评估纠删方案的理论分析结果与实际测试结果基本一致,验证了该方法的精确性,提出的方法对精确分析评估各种纠删方案在无线实时多播系统中的性能具有参考和借鉴意义。     

Effects of Carrier Frequency Offset and Channel Estimation Errors on the Performance of MIMO-OFDM Systems in Spatially Correlated Channels

As an effective technique for combating multipath fading and for high-bit-rate transmission over wireless channels, orthogonal frequency division multiplexing (OFDM) is extensively used in high-rate wireless communication systems, such as, the wireless local area network (WLAN) and the digital television terrestrial broadcasting (DTTB) systems, to support high performance bandwidth-efficient multimedia services. Multiple antennas and transmit or receive diversity, multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM), can be used to improve error performance and capacity of wireless systems. In this paper, we consider the effects of carrier frequency offset and channel estimation errors on the performance of MIMO-OFDM systems in spatially correlated channels. Theoretical calculations and computer simulations are done to analyze the performance degradation of MIMO-OFDM systems in spatially correlated channels due to carrier frequency offset and channel estimation errors, and the theoretical and simulated results match well.     

Measurement-Based Achievable Throughput Estimation in IEEE 802.11a WLANs

Knowledge of dynamically changing achievable throughput is important in the design of an adaptive QoS-provisioning system over WLANs. In this letter, we introduce a novel measurement-based throughput estimation scheme and its practical realization for IEEE 802.11a WLAN environment. By utilizing traffic statistics measurements at the wireless AP (access point) in a timely manner, the proposed scheme estimates idle (i.e., remaining) channel time and converts it into available min/max throughput guidelines. Prototype realization in a Linux-based testbed verifies that the proposed scheme can estimate the available throughput with precision.     

The Effect of Terminal Movement on the Performance of IEEE 802.11 g Wireless LAN Systems in Simulated Radio Channels

The effect of terminal movement on the performance of the IEEE 802.11 g wireless LAN (WLAN) system is evaluated using a measurement set-up including a radio channel simulator. The evaluation is based on laboratory measurements of WLAN PC cards in different simulated radio environments. In the measurements, two different radio channel models are used; the exponential channel model and the UMTS vehicular channel model. The measurement results indicate promising operation of IEEE 802.11 g WLAN systems as such. However, the use of different packet sizes has a significant effect on the system behaviour. With large packets the terminal is more likely to experience channel estimation errors than with small packets. This is due to the fact that the IEEE 802.11 g receiver estimates the channel only once per frame, and uses this estimation over the entire frame. Based on the measurement results we suggest a modification to the medium access control (MAC) layer operation that overcomes this problem: the use of optimized fragmentation.     

Joint Synchronization and Channel Estimation for OFDM Systems

1Introduction Orthogonal Frequency Division Multiplexing(OFDM) has become a popular technique for transmis-sion of signals over wireless channels . OFDMhas beenadoptedinseveral wireless standards such as Digital Au-dio Broadcasting ( DAB) , Digital Video Broadcasting(DVB-T) ,the IEEE 802 .11a[1]Local Area Network(LAN) standard and high performance LAN type 2(HIPERLAN/2)[2]standard. OFDMis also being pur-sued for Dedicated Short-Range Communications(DSRC) for road side to …     

Power-Efficient Spatial Reusable Channel Assignment Scheme in WLAN Mesh Networks

Interference has strong effect on the available bandwidth of wireless local area network (WLAN) based mesh networks. The channel assignment problem for multi-radio multi-channel multihop WLAN mesh networks is complex NP-hard, and channel assignment, routing and power control are tightly coupled. To mitigate the co-channel interference and improve capacity in multi-channel and multi-interface WLAN mesh networks, a power-efficient spatial reusable channel assignment scheme is proposed, which considers both channel diversity and spatial reusability to reduce co-channel interference by joint adjusting channel, transmission power and routing. In order to assign channel appropriately, an efficient power control scheme and a simple heuristic algorithm is introduced to achieve this objective, which adjust the channel and power level of each radio according to the current channel conditions so as to increase the opportunity of channel spatial reusability. The proposed channel assignment scheme also takes load, capacity and interference of links into consideration. Simulation results show the effectiveness of our approach and demonstrate that the proposed scheme can get better performance than other approaches in terms of throughput, blocking ratio, energy consumption and end-to-end delay.     

用于直接序列扩频系统的信道估计算法及改良

对于无线通信接收机设计说，信道估计单元一直是一个关键的模块。在WLAN（无线局域网）接收机的设计中，室内环境下无线信道的复杂性和不可预测性成为系统研究和分析的焦点。如何获得精确的信道估计，从而计算出准确的Rake接收机和均衡器的抽头系数，是关系着整个系统性能的重要因素。介绍了传统信道估计的方法和过程，并针对其缺点展开讨论，并提出了两种改良的信道估计算法。该算法能够大大地缩小估计误差，提高系统性能。     

A decision theoretic approach for channel ranking in crowded unlicensed bands

Interference is disruptive to the operation of wireless sensor networks (WSNs) in unlicensed bands as wireless systems proliferate on the spectrum. The design of a spectrum sharing scheme for WSNs to enable coexistence with geographically collocated heterogeneous wireless systems having multiple parallel interfering channels is a persistent challenge. In this context, interference identification and channel ranking in terms of spectrum access opportunities are addressed in this paper. The goal is to develop a low complexity channel ranking algorithm from channel energy measurements at sensors when a packet-reception-ratio to signal-to-interference-and-noise-ratio (PRR-SINR) interference model is unavailable at network initialization phase. The interference characterizing estimators, temporal occupancy and strength level of a channel, are proposed for interference identification. The effectiveness of the estimators is tested on a sensor platform at 2.4 GHz ISM band under interference from WLAN. Subsequently, the impact of the interference estimators on a channel quality from a receiver perspective is determined with a decision theoretic approach. The estimators are weighted according to their influence on the fitness of a channel and channel ranking is established. The proposed channel ranking achieves a significant gain over heuristic channel ranking (HCR) and gives an accurate interference profile of the channels.     

Joint TOA/AOA Positioning Scheme with IP-OFDM Systems

In this paper, a new positioning scheme with impulse postfix OFDM (IP-OFDM) systems is proposed for estimating the positional information of a user within a wireless local area network. In the proposed scheme, the channel information is estimated by the IP of the IP-OFDM symbol in the time domain, and then the round-trip time and the angle of signal arrival from a wireless terminal to an access point equipped with a uniform linear antenna array are investigated for time-of-arrival (TOA) and angle-of-arrival (AOA) estimation for a user. The position of the wireless terminal is finally estimated by combining the estimated TOA and AOA information. The performance of the proposed scheme is evaluated with computer simulations, and the error characteristics are analyzed with respect to the TOA error, and the AOA error influence on the positioning error. According to the simulation results, the positioning error of the proposed scheme can be reduced to some centimeters for high signal-to-noise ratio (SNR). The major influence on the positioning error comes from the TOA error in low SNR, while the AOA error has more influence on the positioning error in high SNR.     

Channel estimation in space and frequency domain for MIMO-OFDM systems

Multiple-input multiple-output (MIMO) systems can be combined with orthogonal frequency division multiplexing (OFDM) systems to improve the capacity and quality of wireless communications. In this article, a channel estimation technique in both space and frequency domain for MIMO-OFDM systems is proposed. It is shown that the proposed scheme with space-frequency pilot tones achieve optimal minimum mean square error (MMSE) channel estimation. Simulation results indicate that the proposed method achieves good performance.     

Fairness-aware radio resource management for medical interoperability between WBAN and WLAN

Wireless body area networks (WBANs) in the industrial, scientific, and medical (ISM) bands have been increasingly adopted for various medical applications. Due to the shared nature of the ISM bands, when a WBAN coexists with a wireless local area network (WLAN), performance of WBAN may significantly degrade because of asymmetric attributes between WBAN and WLAN such as transmit power and response time. In this paper, we propose a novel channel access protocol for achieving effective channel sharing in the aspect of efficiency and fairness, which adaptively controls the contention window size of WLAN based on the delay information of WBAN. Our extensive simulation results for real-time electrocardiogram (ECG) monitoring show that the proposed scheme can guarantee the required quality of service of WBAN while insignificant degradation of WLAN performance.     

高速铁路场景下环境反向散射辅助的无线传输方案

将反向散射技术引入到高铁场景的无线通信系统中,提出一种反向散射辅助无线传输方案（Backscatter Aided Wireless Transmission,BAWT）,并针对BAWT和传统无线传输方案（Direct Wireless Transmission,DWT）进行了收发信机设计,包括信道估计以及信号检测。与传统无线传输方案相比,证明了在固定的火车天线和不变的轨道情况下,BAWT可以获得信道统计信息为信道估计提供助力。通过仿真对比两种方案的信道估计均方误差,BAWT方案相较DWT方案的信道估计准确性明显提高;对比BAWT、传统中继技术以及DWT方案,当信噪比为20 dB时,对应的BAWT方案和中继技术、DWT方案的误码率分别为1%和2%和12%,BAWT方案误码检测性能获得明显提升。此外,针对广域环境下的环境反向散射通信,推导了高铁场景下使用蜂窝移动通信信号作为RF信源的无线链路预算,反向散射信号在车厢内的覆盖范围内可以满足接收机的灵敏度要求,从链路预算角度论证了BAWT方案实现的可行性。     

Semiblind channel estimation for pulse-based ultra-wideband wireless communication systems

This paper proposes an H/sub /spl infin// based semiblind channel estimation algorithm for pulse-based ultra-wideband (UWB) wireless communication systems. In the proposed scheme, sparsely inserted periodic pilot symbols are exploited to adapt to not only the time-varying channel fading and noise processes but to their changing statistics and potential external disturbances, such as interference. While the existing optimal filtering-based channel estimation schemes, which are optimized mostly for traditional narrowband or wideband systems, require a priori knowledge of the channel and noise statistics, the proposed scheme does not. By further making full use of the channel characteristics unique in UWB systems, the proposed method is thus especially useful for robust operation in the highly frequency-selective UWB indoor channels for which the channel statistics are environment-dependent, and the noise processes do not necessarily satisfy the white Gaussian distribution in the presence of potential narrowband and multiuser interferences. Performance gain of the proposed scheme over the least square method, an existing technique that could also be applied to UWB channels with unknown statistics, and the Wiener filter-based algorithm is also provided.     

An iterative singular vectors estimation scheme for beamforming transmission and detection in MIMO systems

A novel iterative singular vector estimation scheme has been proposed for a beamforming transmission and detection in wireless multiple input multiple output (MIMO) systems. Two singular channel matrix vectors, which correspond to the largest singular value, are iteratively obtained at the transmitter and the receiver without estimating the channel coefficients. The proposed singular vectors estimation strategy has advantages over the conventional MIMO channel estimation schemes in terms of both frame-error-rate performance, bandwidth efficiency and computation complexity.     

Interference Suppression and Receiving Performance Improvement for Local Cooperation with Channel Estimation Error

In the next-generation wireless communication systems, the local cooperation between different units may be deployed to satisfy communication requirements. In this case, the interference suppression between different units and the receiving performance improvement for each single unit should be considered. Multiple schemes have been utilized to solve these problems. However, these schemes ordinarily require sufficiently accurate information of channel, if this accuracy can not be maintained (e.g., channel estimation error can not be ignored), these schemes may not obtain the satisfactory performances. To overcome this disadvantage, in this paper, a novel scheme is proposed. The proposed scheme has several characteristics: (i) a low-complexity extra estimation is implemented to acquire more information of channel estimation error; (ii) with the help of channel estimation error information, each unit can separately execute a two-step process for interference suppression and receiving performance improvement; (iii) no exorbitant information interaction and high-overhead algorithm are needed between multiple units. Through characteristic analysis and numerical results, it is found the proposed scheme can achieve the satisfactory effect in the locally cooperative network.

     

Efficient Non-Pilot-Aided Channel Length Estimation for Digital Broadcasting Receivers

Channel estimation and equalization techniques are crucial for the ubiquitous broadcasting systems. Conventional receivers for most broadcasting or wireless standards preset the channel length to the maximal expected duration of the channel impulse response for the adopted channel estimation and equalization algorithms. The excessive channel length often significantly increases the implementational complexity of the wireless receivers and leads to the redundant information which would induce the additional estimation errors. Moreover, such a scheme does not allow the dynamic memory allocation for variable channel lengths. This could further increase the power consumption and reduce the battery life of a mobile device. The knowledge of the actual channel length would, in principle, help the system designers decrease the complexity of the channel estimators using maximum likelihood (ML) and minimum-mean-square-error (MMSE) algorithms. In this paper, we address this important channel length estimation problem and propose a novel autocorrelation-based algorithm to estimate the channel length without the need of pilots or training sequence. The associated threshold for the channel length estimation depends on the sample size, the signal-to-noise ratio and the leading/last channel coefficients. In addition, we provide the mean-square analysis on the effectiveness of the proposed non-pilot-aided channel length estimator through Monte Carlo simulations.      

基于OFDM技术的多模式可配置同步系统结构

OFDM技术较强的抗多径能力和较简化的接收机设计方案,使得越来越多的无线网络在OFDM技术的框架下共存,这使得多模式终端成为支持多网融合泛在通信的重要解决方案。从OFDM系统的同步技术着手,将定时同步和频偏估计方案进行整合,提出了多模式可配置的同步系统的概念,并给出了支持WLAN、LTE和DVB-T等标准的多模式可配置同步系统结构。     

WRAN系统中基于自适应内插的信道估计算法

提出了一种基于自适应复系数内插的信道估计算法,改善了无线区域网络（WRAN）系统对抗动态多径时延的能力。WRAN是第一个采用认知无线电技术改善频谱效率的宽带接入标准,系统在下行链路中采用了正交频分复用（OFDM）调制技术,而信道估计技术对于采用相干解调的OFDM系统十分重要。传统的OFDM信道频域响应（CFR）估计算法通常采用实系数频域内插的方式,在对抗WRAN系统长多径时延信道时,不能有效地工作。该文在研究实系数FIR内插变换域响应的基础上,提出了一种复系数内插算法。为了同时适用于短时延信道,提出了一种低复杂度、自适应匹配信道最大多径时延的算法。通过仿真,验证了该算法能够对抗更大的多径时延,提高信道估计的精度,改善系统误码性能。