隧道中UHF窄带无线电波传播信道的特性

本文以在五个不同隧道中的实验数据为基础，分析了窄带ＵＨＦ无线电波在隧道中的传播特性，给出了表征空直隧道传播损耗的距离因子，并讨论了隧道中的行人、车辆及隧道弯曲度对传播特性的影响。研究表明，接收信号的慢衰落基本服从对数正态分布，快衰落在直遂道中满足Ｒｉｃｉａｎ分布，在弯曲隧道中Ｒａｙｌｅｉｇｈ分布并不能很好地模拟快衰落的分布。     

Using a pair of phased antennas to improve UHF reception/transmission in tunnels

UHF propagation inside tunnels improves considerably if a pair of phased antennas is used as a fixed antenna system, instead of a single antenna. In this article, we have analyzed the causes of signal deterioration in a tunnel due to slow fading and fast fading. We obtained simple relations for their calculation, which are valid for a rectangular tunnel. We then demonstrated that the pair of phased antennas can reduce fast fading. The measurements performed on pilot projects in actual tunnels confirmed that the pair of phased antennas is capable of reducing fast fading, which is usually the real limiting factor for transmission in tunnels.     

Improvements in W-CDMA: principles and experimental results

This article first reviews the channel structure and spreading code assignment for the physical layer and transport channel multiplexing along with a sophisticated rate-matching scheme that accommodates composite transport channels with various levels of quality of service (QoS) on one physical channel. Then, the key technologies of wideband ds-cdma (w-cdma) wireless access are presented and the results of experiments pertaining to these technologies are evaluated. Flexible system deployment is possible by employing inter-cell asynchronous operation with a three-step fast cell search method. The signal-to-interference power ratio (sir) measurement based fast transmit power control (tpc) guarantees the minimum transmit power according to the channel load and the changes in the link conditions due to fading. Furthermore, various diversity technologies are described such as pilot symbol-assisted (psa) coherent Rake combining, antenna diversity, site diversity (soft/softer handover), and transmit diversity in the forward link that are effective in decreasing the required transmit power, which results in increases system capacity. This article also presents link-capacity enhancing techniques such as using an interference canceller (IC) and adaptive antenna array diversity (AMD) receiver/transmitter. Experimental results are presented for an actual multipath fading channel that indicate the potential of the IC and aaad transceiver to decrease the mobile transmit power in the reverse link and interference from high rate users with high transmit power in the forward link.     

Low complexity per-antenna rate and power control approach for closed-loop V-BLAST

Previous studies have shown that per-antenna rate and power control can greatly increase the data throughput of vertical Bell Labs layered space-time (V-BLAST), while an extra transmit antenna selection can provide additional diversity advantage. We combine the transmit antenna selection with power and rate control for each antenna. We derive a simple criterion for minimum bit-error rate (BER) or minimum total transmit power when the data throughput is constant over time. Zero-forcing and zero-forcing successive interference cancellation detections are considered. For practical implementation, we also present a fast algorithm that gives near-optimal performance with very low complexity. Simulation results show that the proposed closed-loop BLAST outperforms the open-loop V-BLAST significantly in terms of BER performance, especially when the antennas exhibit strong fading correlations.     

一种移动终端多天线方向图重构快速选择算法与基于滑动时间窗的加速实现

 多输入多输出(MIMO)系统中,天线选择技术可以在降低复杂度的同时,有效地提高系统的性能.但对于天线安装空间受限的小型终端,天线的数目将受到很大限制,本文结合方向图可重构技术,考虑基于线性接收机的空间复用系统,通过在已选择的发射天线上采用等功率分配等增益传输以减少反馈信息量.在此条件下,推导出空间相关衰落信道下自适应发射天线选择的统计容量公式下限,然后,在此基础上提出基于方向图重构的发射天线快速选择方法以最大化该容量值.此外,在实现中提出了一种基于滑动时间窗的梯度更新估计模型,缩短了算法所需时间.     

Low-density parity-check codes for space-time wireless transmission

Irregular low-density parity-check (LDPC) codes have shown exceptionally good performance for single antenna systems over a wide class of channels. In this paper, we investigate their application to multiple antenna systems in flat Rayleigh fading channels. For small transmit arrays, we focus mainly on space-time coding with 2/sup p/-ary LDPC codes, where p equals the number of encoded bits transmitted by the transmit antenna array during each signaling interval. For large transmit arrays, we study a layered space-time architecture using binary LDPC codes as component codes of each layer: We show through simulation that, when applied to multiple antenna systems with high diversity order, LDPC codes of quasi-regular construction are able to achieve higher coding gain and/or diversity gain than previously proposed space-time trellis codes, space-time turbo codes, and convolutional codes in a number of fading conditions. Extending the work of density evolution with Gaussian approximation, we study 2/sup p/-ary LDPC codes on multiple antenna fading channels, and search for the optimum 2/sup p/-ary quasi-regular codes in quasi-static fading. We also show that on fast fading channels, 2/sup p/-ary irregular LDPC codes, though designed for static channels, have superior performance to nonbinary quasiregular codes and binary irregular codes specifically designed for fast fading channels.     

Linear precoding for space-time coded systems with known fadingcorrelations

We design an optimal linear precoder for a space-time coded system assuming knowledge of only the transmit antenna fading correlations. Assuming a flat fading channel and a maximum-likelihood receiver, we show that the linear precoder transmits power on the eigenmodes of the transmit antenna correlation matrix. The power allocation on the eigenmodes is a form of waterpouring policy. Simulation results are presented to show performance improvement on a space-time coded system     

Optimized Transmission for Fading Multiple-Access and Broadcast Channels With Multiple Antennas

In mobile wireless networks, dynamic allocation of resources such as transmit powers, bit-rates, and antenna beams based on the channel state information of mobile users is known to be the general strategy to explore the time-varying nature of the mobile environment. This paper looks at the problem of optimal resource allocation in wireless networks from different information-theoretic points of view and under the assumption that the channel state is completely known at the transmitter and the receiver. In particular, the fading multiple-access channel (MAC) and the fading broadcast channel (BC) with additive Gaussian noise and multiple transmit and receive antennas are focused. The fading MAC is considered first and a complete characterization of its capacity region and power region are provided under various power and rate constraints. The derived results can be considered as nontrivial extensions of the work done by Tse and Hanly from the case of single transmit and receive antenna to the more general scenario with multiple transmit and receive antennas. Efficient numerical algorithms are proposed, which demonstrate the usefulness of the convex optimization techniques in characterizing the capacity and power regions. Analogous results are also obtained for the fading BC thanks to the duality theory between the Gaussian MAC and the Gaussian BC.     

Transmit Antenna Selection with Optimal Power Allocation and Channel Estimation Error

In this paper, we consider the capacity of transmit antenna selection with optimal power allocation and channel estimation error over uncorrelated Rayleigh fading channels. We assume that one transmit antenna is chosen for transmission and the maximal ratio combining (MRC) is applied at the receiver. We derive the exact capacity expression. Numerical result to validate our analysis is also presented.     

移动通信系统中空时格码的改进设计

本文根据空时格玛在快瑞利衰落信道下的成对错误概率上界，得出不同发射天线的码设计具有独立性，进而提出一种改进的快瑞利衰落信下空时格码的设计准则，当信道衰落快慢介于准静态和快瑞利衰落之间时，空时格码的设计方法也可做相应的改进，分析及仿真结果表明，由改进方法设计的空时格码比由传统方法得出的具有更优的性能。     

WCDMA中变步长功率控制过程

本文提出了一种可应用于WCDMA中的变步长功率控制策略,这种功控策略根据TPC命令的"历史",将功率控制过程划分为Markov状态,分析信道衰落步长的概率密度,每一状态对应一个确定的步长值,从而可以确定功率控制步长集中的步长数,我们选用概率密度最大的数值,同时考虑硬件可以分辨的最小步长在0.5dB左右,确定步长集.再用仿真的方法,确定状态和步长的对应关系.发射机依据当前的TPC命令决定应该增大还是减小发射功率,依据当前TPC命令和"历史"上的TPC命令选择合适的功率步长,进而确定发射功率.     

基于导频符号辅助的天线选择分集方案

在大多数散射环境下,天线分集是减小多径效应的一种实际、有效的技术,在分析Alamouti两发一收发射分集方案的基础上,介绍了一种基于导频符号辅助的发射天线选择分集方案.该方案通过引入反馈和导频符号辅助技术,在发射端对两个发射天线进行天线选择,给出的仿真结果表明,运用分集技术可大大改善无线通信系统的性能,该方案性能优于Alamouti两发一收发射分集方案,并且发送功率要低于Alamouti两发一收方案.     

Efficient signal processing techniques for exploiting transmitantenna diversity on fading channels

A class of powerful and computationally efficient strategies for exploiting transmit antenna diversity on fading channels is developed. These strategies, which require simple linear processing at the transmitter and receiver, have attractive asymptotic characteristics. In particular, given a sufficient number of transmit antennas, these techniques effectively transform a nonselective Rayleigh fading channel into a nonfading, simple white marginally Gaussian noise channel with no intersymbol interference. These strategies, which we refer to as linear antenna precoding, can be efficiently combined with trellis coding and other popular error-correcting codes for bandwidth-constrained Gaussian channels. Linear antenna precoding requires no additional power or bandwidth and is attractive in terms of robustness and delay considerations. The resulting schemes have powerful and convenient interpretations in terms of transforming nonselective fading channels into frequency- and time-selective ones     

分布式残余频偏信道中STBC-OFDM的发射功率优化

在分布式残余频偏信道中,考虑多径瑞利衰落,针对采用判决反馈检测的两发射天线STBC-OFDM链路,提出了一种发射功率优化方法:根据平均信道功率增益、残余频偏方差以及噪声方差的大小,以最小化平均误比特率下界为目标,设置两分布发射天线的发射功率,仿真结果表明:相比于传统的各天线满功率发射方法,所提方法能够在节省发射功率的同...     

End‐to‐end security‐reliability analysis of multi‐hop cognitive relaying protocol with TAS/SC‐based primary communication,total interference constraint and asymmetric fading channels

In this paper, we analyze the tradeoff between outage probability (OP) and intercept probability (IP) for a multi‐hop relaying scheme in cognitive radio (CR) networks. In the proposed protocol, a multi‐antenna primary transmitter (PT) communicates with a multi‐antenna primary receiver (PR), using transmit antenna selection (TAS) / selection combining (SC) technique, while a secondary source attempts to transmit its data to a secondary destination via a multi‐hop approach in presence of a secondary eavesdropper. The secondary transmitters such as source and relays have to adjust their transmit power to satisfy total interference constraint given by PR. We consider an asymmetric fading channel model, where the secondary channels are Rician fading, while the remaining ones experience the Rayleigh fading. Moreover, an optimal interference allocation method is proposed to minimize OP of the primary network. For the secondary network, we derive exact expressions of end‐to‐end OP and IP which are verified by Monte Carlo simulations.     

Spatial multiplexing with transmit antenna and constellation selection for correlated MIMO fading channels

We consider spatial multiplexing systems in correlated multiple-input multiple-output (MIMO) fading channels with equal power allocated to each transmit antenna. Under this constraint, the number and subset of transmit antennas together with the transmit symbol constellations are determined assuming knowledge of the channel correlation matrices. We first consider a fixed data rate system and vary the number of transmit antennas and constellation such that the minimum margin in the signal-to-noise ratio (SNR) is maximized for linear and Vertical Bell Laboratories Layered Space-Time (V-BLAST) receivers. We also derive transmit antenna and constellation selection criteria for a successive interference cancellation receiver (SCR) with a fixed detection order and a variable number of bits transmitted on each substream. Compared with a system using all available antennas, performance results show significant gains using a subset of transmit antennas, even for independent fading channels. Finally, we select a subset of transmit antennas to maximize data rate given a minimum SNR margin. A lower bound on the maximum outage data rate is derived. The maximum outage data rate of the SCR receiver is seen to be close to the outage channel capacity.     

SBR image approach for radio wave propagation in tunnels with andwithout traffic

We propose a deterministic approach to model the radio propagation channels in tunnels with and without traffic. This technique applies the modified shooting and bouncing ray (SBR) method to find equivalent sources (images) in each launched ray tube and sums the receiving complex amplitude contributed by all images coherently. In addition, the vector effective antenna height (VEH) is introduced to consider the polarization-coupling effect resulting from the shape of the tunnels. We verify this approach by comparing the numerical results in two canonical examples where closed-form solutions exist. The good agreement indicates that our method can provide a good approximation of high-frequency radio propagation inside tunnels where reflection is dominant. We show that the propagation loss in tunnels can vary considerably according to the tunnel shapes and the traffic inside them. From the results we also find a “focusing” effect, which makes the power received in an arched tunnel higher than that in a rectangular tunnel. Besides, the deep fading that appears in a rectangular tunnel is absent in an arched tunnel. The major effect of the traffic is observed to be the fast fading due to the reflection/obstruction of vehicles. Additional considerations, such as time delay, wall roughness, and wedge diffraction of radio wave propagation in tunnels are left for future studies     

A low-complexity port selection and power allocation scheme in distribution MIMO systems

Information theoretical results have shown that Distributed Antenna Systems (DAS) can obtain higher capacity than Co-located Antenna Systems (CAS). In this paper, we investigate a downlink port selection and power allocation scheme in Distributed Multiple-Input Multiple-Output (D-MIMO) systems, where Distributed Antenna (DA) ports randomly locate in the cell. The contribution of this paper can be summarized as two parts. Firstly, we analyze how antenna correlation affects power allocation in D-MIMO systems. Secondly, based on large scale fading and antenna correlation, a low-complexity port selection and power allocation scheme is proposed. In the proposed scheme, we take both large scale fading and antenna correlation into consideration. Moreover, User Equipment (UE) only needs to feedback the rank of transmit antenna correlation matrix, which will not increase system complexity too much. Simulation results verify the capacity improvement based on the proposed power allocation scheme.     

System Performance of Transmit Diversity Methods and a Two Fixed-Beam System in WCDMA

Downlink transmit diversity modes for WCDMA together with a two fixed-beam antenna array system are compared relative to the single antenna sectorized system in a radio network simulator. The transmit diversity methods investigated are: space-time transmit diversity and closed-loop mode I transmit diversity. Frequency selective (COST 259) and flat fading channels are considered and their impact to speech-only and data-only services is evaluated. A third service, which highlights the system performance of the various advanced antennas, is also investigated.The results in this investigation point out that the diversity gain in flat fading channels is substantial. In frequency-selective fading, the benefits of fixed beam systems is encouraging, whereas transmit diversity methods (especially Space-Time Transmit Diversity) is unsatisfactory.     

System Performance of Transmit Diversity Methods and a Two Fixed-Beam System in WCDMA

Downlink transmit diversity modes for WCDMA together with a two fixed-beam antenna array system are compared relative to the single antenna sectorized system in a radio network simulator. The transmit diversity methods investigated are: space-time transmit diversity and closed-loop mode I transmit diversity. Frequency selective (COST 259) and flat fading channels are considered and their impact to speech-only and data-only services is evaluated. A third service, which highlights the system performance of the various advanced antennas, is also investigated.The results in this investigation point out that the diversity gain in flat fading channels is substantial. In frequency-selective fading, the benefits of fixed beam systems is encouraging, whereas transmit diversity methods (especially Space-Time Transmit Diversity) is unsatisfactory.