Spectral efficiency improvement for LMDS systems using adaptive techniques in fading channels

Adaptive modulation, error control coding and microdiversity are well-known techniques that have been applied to wireless communication systems to overcome the fading process experienced by the transmitted signal. In this paper, we investigate the throughput returns due to the employments of various combinations of adaptive modulation, adaptive coding, and microdiversity in a broadband fixed wireless access cellular system.The results indicate that using adaptive modulation, coding, and maximal ratio combining (MRC) yields significant enhancements in spectral efficiency of the system. Second order selection combining has almost the same performance of MRC.     

ICRN功率分配与中继选择联合优化

针对工业认知无线电网络中出现的无线干扰和冲突严重,特别是工业现场的金属环境和移动特性造成多径与阴影衰落,从而使得传输可靠性难以保证的问题,引入感知信道概率及信道可用度概念,提出一种联合优化中继选择与功率分配的算法。该算法给出3种认知中继选择方案,分别是信道增益最大准则、最近准则及调和平均准则,并在源节点和所选中继节点之间进行最优功率分配,以最小化网络中断概率。仿真结果表明,与平均功率分配算法相比,所提算法能更好地降低系统中断概率,提高传输可靠性。     

Nonlinear L2-by-3 transform for PAPR reduction in OFDM systems

Orthogonal frequency division multiplexing (OFDM), the multicarrier modulation technique with high bandwidth efficiency and robustness against multipath fading, is used in several high-speed broadband communication systems including digital video broadcasting (DVB), digital audio broadcasting (DAB), asymmetric digital subscriber line (ADSL), wireless local area network (WLAN IEEE 802.11a,g), high performance radio local area network (HIPERLAN 2), and worldwide interoperability for microwave access (WiMAX IEEE 802.16). However, the transmit signal in OFDM system has a high peak-to-average power ratio (PAPR), one of the major drawbacks of multicarrier transmission, and therefore high dynamic range is required in both the digital to analog converter (DAC) and the power amplifier (PA) for proper operation. Otherwise, the nonlinearities of PA cause out-of-band distortion and increase in bit error rate (BER). Therefore, the reduction in PAPR in OFDM system is desirable in order to obtain power efficiency and increase BER performance. In this paper, a new parametric PAPR reduction technique, L₂-by-3 transform from sliding norm transform (SNT) family is proposed. Based on its parameter, a significant PAPR reduction is obtained. This paper presents the analysis of power saving and computational complexity of the proposed method L₂-by-3 and comparison to two other methods; selected mapping and partial transmit sequence.     

基于无线能量采集的多跳网络安全传输中的路径选择

针对具有无线能量采集技术的多跳传输网络,提出了全双工中继网络在有多个窃听者和多个功率节点场景下进行多跳多路径传输时的路径选择问题。首先,所提系统模型在有多个窃听者和功率节点的场景下进行信息传输,将源发射端和接收端之间的最佳路径作为数据传输路径。然后,为了增强系统性能,中继节点对功率节点发出的射频信号进行能量采集并利用所获能量来进行下一跳的数据传输。最后,推导出了所提路径选择方法在瑞利衰落信道下端到端中断概率的精确表达式,蒙特卡洛仿真和理论分析曲线拟合较好,说明了理论推导的正确性,以及所提方法具有性能上的优越性。     

Power Control for Cellular Communications with Time‐Varying Channel Uncertainties

Power control in a code‐division multiple access (CDMA) based cellular network is a challenging problem because the communication channels change rapidly because of multipath fading. These rapid fluctuations cause detrimental effects on the control efforts required to regulate the signal‐to‐interference plus noise ratios (SINRs) to the desired level. Thus, there is a need for power‐control algorithms that can adapt to rapid changes in the channel gain caused by multipath fading. Much of the previous work has either neglected the effects of fast fading, assumed that the fading is known, or assumed that all the link gains are known. In this paper, we model the effects of fast fading and develop practical strategies for robust power control based on SINR measurements in the presence of the fading. We develop a controller for the reverse link of a CDMA cellular system, and use a Lyapunov‐based analysis to prove that the SINR error is globally uniformly ultimately bounded. We also utilize a linear prediction filter that utilizes local SINR measurements and estimates of the Doppler frequency that can be derived from local SINR measurements to improve the estimate of the channel fading used in the controller. The power‐control algorithm is simulated for a cellular network with multiple cells, and the results indicate that the controller regulates the SINRs of all the mobile terminals (MTs) with low outage probability. In addition, a pulse‐code‐modulation technique is applied to allow the control command to be quantized for feedback to the transmitter. Simulation results indicate that the outage probabilities of all the MTs are still within the acceptable range if at least 3‐bit quantization is employed. Comparisons to a standard algorithm illustrate the improved performance of the predictive controller.     

A high-speed wireless LAN

WLAN systems face technical problems similar to those encountered in outdoor wide-area radio-based systems, including the limited available bandwidth and fading noise due to multipath interference and blockage. The goal in WLAN system design is to transmit at the maximum information rate with an acceptable probability of error and minimum equipment complexity, power, and cost. Competing approaches use either infrared radiation or radio waves in the microwave or millimeter-wave bands. We have developed a radio solution at millimeter-wavelength frequencies, where the spectrum is sufficient to accommodate link speeds of hundreds of Mbps. Using a test bed with burst-mode transmission capability and an experimental 40-GHz radio, we have demonstrated a picocellular approach with a range of approximately 10 meters and link rates up to 185 Mbps. In addition, we have built a prototype modem with a raw link rate of 54 Mbps for use in a high-speed indoor WLAN demonstrator     

Robust progressive image transmission over OFDM systems using space-time block code

A joint source-channel coding (JSCC) scheme for robust progressive image transmission over broadband wireless channels using orthogonal frequency division multiplexing (OFDM) systems with spatial diversity is proposed for the application environments where no feedback channel is available such as broadcasting services. Most of current research about JSCC focuses on either binary symmetric channels (BSC) or additive white Gaussian noise (AWGN) channels. To deal with fading channels in most previous methods, the fading channel is modeled as two state Gilbert-Elliott channel model and the JSCC is normally aimed at the BER of bad channel status, which is not optimal when the channel is at good status. By using diversity techniques and OFDM, the frequency selective fading effects in broadband wireless channels can be significantly decreased and we show that subchannels in OFDM systems approach Gaussian noisy channels when the diversity gain gets large; as a result, the system performance can be improved in terms of throughput and channel coding efficiency. After analyzing the channel property of OFDM systems with spatial diversity, a practical JSCC scheme for OFDM systems is proposed. Simulation results are presented for transmit diversity with different numbers of antennas and different multipath delay and Doppler spread. It is observed from simulations that the performance can be improved more than 4 dB in terms of peak signal-to-noise ratio (PSNR) of the received image Lena and the performance is not very sensitive to different multipath spread and Doppler frequency.     

基于多用户调度的闭环MIMO系统中断性能分析

针对无线资源的匮乏,结合多用户分集与多用户调度技术,提出一种基于多用户调度的闭环多输入多输出跨层算法.该算法是采用跨层理论的方法,以包错误率为服务目标,利用调度技术,自适应调制,自动重传技术与MIMO技术的有机结合,提高系统的中断性能.推导了系统平均包错误率,平均传输次数与中断概率的闭合表达式,分析了天线配置,移动用户数以及重传次数对系统性能的影响;仿真结果表明,该系统的中断概率明显减小,中断性能提高.     

Throughput analysis of the IEEE 802.11 distributed coordination function considering erroneous channel and capture effects

This paper presents a performance study of the distributed coordination function (DCF) of 802.11 networks considering erroneous channel and capture effects under non-saturated traffic conditions employing a basic access method. The aggregate throughput of a practical wireless local area network (WLAN) strongly depends on the channel conditions. In a real radio environment, the received signal power at the access point from a station is subjected to deterministic path loss, shadowing, and fast multipath fading. The binary exponential backoff (BEB) mechanism of IEEE 802.11 DCF severely suffers from more channel idle time under high bit error rate (BER). To alleviate the low performance of IEEE 802.11 DCF, a new mechanism is introduced, which greatly outperforms the existing methods under a high BER. A multidimensional Markov chain model is used to characterize the behavior of DCF in order to account both non-ideal channel conditions and capture effects.     

Conservative estimates of blocking and outage probabilities in CDMA networks

The outage probability is a key performance measure for real-time traffic in wireless networks, often considered as even more critical than the blocking probability. The blocking and outage probabilities do not have closed-form expressions as they depend in a too complex way on the traffic characteristics (call duration, bit rate requirement), the radio conditions (fading, shadowing, noise, interference) and the considered admission and outage policies. We derive upper bounds for the blocking and outage probabilities for general traffic characteristics and radio conditions under the mild assumption that the admission and outage policies satisfy a certain monotonicity property. These conservative estimates may depend on call characteristics like the bit rate requirement and the trajectory of the mobile. The results are applied to the uplink and the downlink of CDMA networks.     

DS-CDMA downlink systems with fading channel employing the generalized receiver

In this paper, we investigate a generalized receiver (GR) constructed on the basis of the generalized approach to signal processing (GASP) in noise employing by direct-sequence code-division multiple access (DS-CDMA) downlink wireless communication system with multipath fading. Transmitted signaling technique is based on using the orthogonal unified complex Hadamard transform spreading sequences. The use of GR allows us to maintain the orthogonality between users and reduce the multipath fading effect and interference from other users. A general multipath-fading model is assumed. Bit-error rate (BER) performance of system is evaluated by means of the signal-to-interference-plus-noise ratio (SINR) at the output of GR employed by DS-CDMA downlink wireless communication system. Using the orthogonal unified complex Hadamard transform spreading sequences as the transmitted signaling technique, we obtain that SINR at the GR output is independent of the phase offsets between different paths. If the Walsh–Hadamard (WH) spreading sequences are used as the transmitted signaling technique, the SINR at the output of GR employed by the same system is a function of squared cosine of path phase offsets. As a result, the BER performance of the last DS-CDMA downlink wireless communication system is worse in comparison with that of the first system. Comparative analysis between the BER performance of DS-CDMA downlink wireless communication systems employing both the GR and the Rake receiver, which consists of a bank of correlation receivers, with each individual receiver correlating with a different arriving multipath component, shows us a superiority of the first system over the second one both at high and low SINRs.     

OFDM超分辨率联合TOA/AOA定位

对无线信号到达时间（TOA）和到达角度（AOA）的精确估计是室内无线定位的关键。正交频分复用（OFDM）是适合多径衰落环境的一种高速传输技术。提出一种联合TOA和DOA的二维定位方法用于OFDM信号定位。算法首先通过信道估计得到信道的频率响应（CFR）,对CFR采用超分辨率算法估计信道时延。首径（first arrival path,FAP）的时延估计就是接收信号的TOA估计。将天线阵列接收信号通过DFT变换到频域,利用同样的算法估计信号各径的AOA/DOA。最后联合TOA和AOA在二维平面上确定目标位置。由于信号是多径、多载波信号,对各载波的DOA/AOA估计在载波间进行处理,再结合各径脉冲响应的幅值,选出属于首径的DOA/AOA估计。在多径环境下仿真表明算法有效且定位精度能够满足实际需要,尤其适合环境中只有一个基站或接入点的情况。     

Performance analysis of IEEE 802.11 WLANs with rate adaptation in time-varying fading channels

The IEEE 802.11 supports multiple transmission bit rates by using different modulation and coding schemes. Due to different bit error characteristics and transmission efficiencies of the rates, stations may benefit from an adaptive use of them for a varying channel condition, called rate adaptation. The accuracy of rate adaptation is expected to be highly affected by a time varying nature of typical radio channels due to multipath fading. This paper presents an analytic model of the IEEE distributed coordination function (DCF) with the automatic rate fallback (ARF) rate adaptation algorithm, which is the most widely used one in the 802.11 market, under time-correlated Rayleigh fading. The key idea behind the approach is to exploit the first-order Markovian approximation of Rayleigh fading channels, based on which transmission failure probabilities are obtained depending on the current and previous transmission status. By using those probabilities, the ARF process of a station is modeled as a Markov chain, then, the rate distribution obtained by solving the Markov chain is fed to a DCF model. The proposed DCF model is described in a per-station manner, thus enables the analysis of heterogeneous channel conditions and medium access control (MAC) configurations among stations.     

Demonstration of 60 GHz millimeter-wave short-range wireless communication system at 3.5 Gbps over 5 m range

Millimeter-wave communication has received considerable attention for use in new-generation broadband wireless communication because of the scarcity of microwave band spectrum resources. The unlicensed frequency band at 60 GHz is suitable for indoor short-range broadband communication. In this paper, we introduce a 60 GHz millimeter-wave short-range wireless communication demo system adhering to the IEEE 802.11ad standard and targeting high-definition video streaming transmission. The system uses single-carrier transmission with frequency-domain equalization. The hardware prototype consists of a 65-nm CMOS radio-frequency front-end and a baseband transceiver for the physical layer functions. Important baseband technologies such as carrier synchronization, phase noise compensation, frequency-domain equalization, and low-density parity-check decoding based on probability calculation are also discussed. The system performance is demonstrated in an experiment on high-definition video transmission, where a data rate of 3.52 Gbps is achieved with a quadrature phase-shift keying signal over a distance of 5.109 m.     

Distributed space-time-frequency coding for cooperative OFDM systems

A distributed space-time-frequency (STF) coding scheme is proposed for cooperative OFDM (C-OFDM) systems with three terminals over quasi-static frequency-selective Rayleigh fading channels. The outage probability is derived and its tight closed-form lower bound is presented. Asymptotic analysis indicates that the proposed scheme can achieve both spatial and multipath (frequency) diversity. The theoretical analysis of the proposed STF coded scheme is further implemented by the distributed group STF block coding (D-GSTFBC) scheme based on the subcarrier grouping technique. Simulation results confirm the previously introduced theoretical analysis.     

信道预测天线选择的空时分组码物理层安全增强

针对延时发射天线选择(TASD)/正交空时分组码(OSTBC)无线通信系统,基于最小均方误差(MMSE)信道预测器,提出一种系统物理层安全增强方案。将MMSE信道预测方案应用于TASD/OSTBC无线通信系统,构成信道预测发射天线选择(TASP)/OSTBC无线通信系统,并对其推导瑞利块衰落信道上安全中断概率、非零安全容量概率以及渐近安全中断概率的解析表达式。在此基础上,分析主信道收发天线数、窃听者天线数和信道的归一化延时等参数对系统物理层安全性能的影响。数值计算和仿真结果表明,采用TASP可以提高OSTBC编码无线通信系统的物理层安全性能。     

Reconfigurable method of smart antenna beam shaping in indoor fading environment using adaptive algorithms: An exhaustive experimental study

Adaptive algorithms for beam shaping of a phased array antenna and multiple‐input multiple‐output (MIMO) system gaining importance in today's advanced wireless networks to mitigate interference effects and distortion in the receiving signal due to multipath, small scale, and large scale fading effects. This article deals with the development of reconfigurable field programmable gate array (FPGA)‐based hardware for smart antenna system to explore parameter dependencies, drawbacks, and relative performance comparison of popular adaptive beamforming and interference suppression algorithms. These are least mean square, recursive least squares (RLS), and sample matrix inversion (SMI) used in real‐time under laboratory environment where the existing wireless channel between transmitters and receivers is linear time‐varying in nature due to presence of secondary sources giving rise to small‐scale fading. For this at first, we propose a novel received signal strength indicator‐based procedure to measure the radiation pattern of the antenna under an echoic indoor environment on a reconfigurable and portable FPGA system named wireless open‐access research platform (WARP), controllable by generic programming codes over a user‐friendly MATLAB interface. For better performance, the SMI algorithm was modified to increase block size rather than block shifting in general SMI. Later a comparative study was performed under varying conditions to observe the best utilization of three adaptive algorithms in beam shaping. In all cases, SMI performs the best with less beam shaping error and faster convergence, validating its use in a real‐time fading environment.     

Nakagami-m信道下多播中继系统网络编码性能分析

在Nakagami-m衰落信道下,分析多播中继系统中网络编码的中断概率性能.分别采用复数域网络编码(complex fiel dnetwork coding,CFNC)和放大转发网络编码(amplify-and-forward network coding,AFNC)两种传输策略,推导了高信噪比时的中断概率近似表达式,得到两种传输策略下的系统分集增益均为2m,并通过仿真结果加以验证.仿真结果还表明,CFNC在系统中断概率、和速率和误符号率性能上均优于AFNC和无中继直接传输策略.     

Performance of PRMA in a mobile radio channel

The performance of the packet reservation multiple access (PRMA) protocol in a real mobile radio channel that suffers from propagation impairments including multipath fading, shadowing and distant-dependent path loss, is studied by simulation. Significant performance degradations, measured by the number of simultaneous conversations supportable in a cell subject to a packet dropping probability of 1%, are observed. Additionally, receiver capture is also found to be ineffective in increasing this supportable number of simultaneous conversations. A modified PRMA protocol is then introduced, and shown to be capable of improving performance through minimizing premature loss of reservations and avoiding additional collisions that arise due to the channel errors.     

Cross-layer radio resource management in integrated WWAN and WLAN networks

This paper proposes a mobility adaptive network selection scheme in the context of wireless wide area network (WWAN) and wireless local area network (WLAN) radio access technologies (RATs) that supports both real-time (RT) and non-real-time (NRT) service classes. Physical layer information based call admission control (CAC) is considered for the two RATs to enforce service specific QoS requirements. The effectiveness of the cross-protocol-layer information for radio resource management (RRM) in integrated WWAN and WLAN networks is assessed analytically for individual service classes in a multi-service environment using the theory of Markov chains. The impact of non-uniform user and mobility distributions due to the existence of hotspot in the macro-cell area and the effect of network selection parameter measurement errors on the RRM performance are also evaluated. Numerical results show that the proposed network selection scheme minimizes the rate of unnecessary vertical handoffs, thereby providing stable communication without degrading the call blocking probability and call outage probability performance metrics.