Performance of the complex sphere decoder in spatially correlated MIMO channels

The use of multiple antennas at both transmitter and receiver is a promising technique for significantly increasing the capacity and spectral efficiency of wireless communication systems. In particular, spatial multiplexing techniques provide a means of increasing the data rate of the system without having to increase the transmitter power or the bandwidth. In recent years, special attention has been paid to the sphere decoder (SD) to detect spatially multiplexed signals. It provides optimal maximum likelihood (ML) performance with reduced complexity, compared to the maximum likelihood detector (MLD). An analysis of the performance of the SD in the presence of spatially correlated multiple-input multiple-output (MIMO) channels is presented. Analytical and simulation results show that, compared to suboptimal linear and nonlinear MIMO detectors, the SD suffers a complexity increase when correlation exists between the antennas at the transmitter or the receiver. In addition, a novel low-complexity channel ordering technique is introduced to reduce the complexity of the SD     

相关MIMO信道下一种特征波束成型方案的性能研究

提出了相关多输入多输出信道下的一种空时分组码、空分复用与波束成型相结合的新方案．该方案利用了发射分集和空分复用所带来的分集和复用增益，并基于接收机反馈的信道相关矩阵设计了一种波束成型器．仿真结果表明，这种方案的性能较传统的方案在高信噪比时能获得较大的改善．     

提高MIMO系统容量的检测算法研究

为了满足通信容量的需求，在发射端和接收端设置多元素天线阵列构成多输入多输出系统，可以显著提高频谱的利用率，基于贝尔实验室提出的非线性迫零检测算法，提出了反向迫零检测算法．理论和仿真证明，在相同信噪比条件下，反向迫零检测算法获得的系统容量高于非线性迫零检测以及线性迫零检测算法．与奇异值分解算法相比，该算法虽然得到的系统容量有所减小，但是由于所需的运算量相对较小，因而易于实现．     

Optimal transmission expansion under different market structures

The debate is still on regarding the rationale for transmission expansion in deregulated power markets. Transmission planning with social welfare as the objective would lead to a different optimal expansion plan compared to the planning deemed optimal by individual merchants acting on market forces with profits as their objective. This issue remains to be resolved amid varying rationales and conflicting interests. Fundamental formulations and analyses for long-term network expansion are presented under the paradigms of (i) system-wide social cost minimisation under central planning, and (ii) profit maximising objectives of merchant entrants covering a range of market structures. Distinct planning outcomes under centralised (regulated monopoly) and decentralised (merchant transmission) network managements, and the relation between these optimal solutions have been quantified analytically. These fundamental formulations would be useful for detailed policy studies, and in the formulation of practical regulatory frameworks in the context of transmission sector market liberalisation. Simple and clear examples are presented to illustrate the theoretical results.     

Increasing the noise immunity of data transmission over digital communication channels under Nakagami fading conditions

The action of fadings on the noise immunity of digital communications in a channel with Nakagami signal fading when radio waves propagate through a medium with random nonuniformities (in particular, in satellite communications when the radio waves pass through the troposphere), is investigated. The gain in noise immunity from the use of intermittent data transmission compared with existing methods of reception is estimated.     

Channel capacity of dual-branch diversity systems over correlated Nakagami-m fading with channel inversion and fixed rate transmission scheme

Closed-form expressions are derived for the channel capacity of dual-branch maximal ratio combining, equal gain combining, selection combining, and switch and stay combining (SSC) diversity systems over correlated Nakagami-m fading for the channel inversion with fixed rate transmission scheme. Since some of the final capacity expressions contain infinite series, the series are truncated and upper bounds on the truncation errors are presented. An expression is also derived that can be used to numerically determine the optimum adaptive switching threshold for the capacity of a dual-branch SSC system over correlated Nakagami-m fading channels. A closed-form expression for the optimum adaptive switching threshold is derived, however, for the case of independent branches. The corresponding expressions for Rayleigh fading are obtained as a special case of Nakagami-m fading. Finally, numerical results are presented, which are then compared to the capacity results that the authors previously obtained for the rate adaptation with constant power transmission scheme.     

On two-tier healthcare system under capacity constraint

In the healthcare industry, to reduce the waiting time for patients, policy makers may allow private hospitals (or called the toll system) to enter the market. However, when the total healthcare capacity in the market is limited (e.g. the number of medical staff or equipment is limited), the entrance of the toll system may offer higher salaries to attract medical staff from public hospitals (or called the free system) and reduce its capacity. Then whether or not introducing toll system in the system can reduce the waiting time becomes an issue. In this paper, we investigate the impact of capacity constraint on a two-tier healthcare system. The results show that when the total capacity is tight enough, the two-tier healthcare system often yields less social welfare than the one-tier free system; and when the total capacity is sufficient (the demand does not exceed the total capacity), the two-tier healthcare system improves the social welfare. Specially, we find under certain conditions the capacity constraint can improve social welfare. In addition, if the capacity constraint has a negative effect on the two-tier system’s performance, the government can set an appropriate upper limit for the toll system’s capacity to remove the negative effect.     

Quality of service constrained throughput optimisation for joint adaptive transmission with automatic repeat request over block-fading channels

An optimisation framework for wireless link adaptation which maximises the system throughput subject to a packet-level quality of service (QoS) constraint is presented. The authors consider joint adaptive variation of the transmitted power level, transmission data rate and packet-error-rate (PER) at the physical layer to improve the throughput performance of the selective-repeat automatic repeat request (SR-ARQ) protocol over block-fading channels. Specifically, for discrete-rate coded M-QAM schemes, the authors present a power and rate adaptation algorithm that guarantees a target PER constraint. The proposed framework also facilitates optimising the throughput performance for delay constrained wireless applications, which imposes a limit on the number of retransmissions for the ARQ. In particular, a link adaptation scheme is presented, which guarantees a target packet-loss rate (PLR) when a truncated SR ARQ protocol at the data-link layer is employed. Numerical results indicate that the proposed adaptation schemes compared with other adaptive schemes noticeably enhance the system throughput. Also, in the case of truncated ARQ, the results illustrate a fundamental trade-off between the delay and PLR QoS metrics achieved at the maximum throughput performance.     

基于海面散射的莱斯MIMO信道容量研究

通过相关莱斯信道模型对海面散射水声多入多出(MIMO)信道容量进行了研究。从Helmholtz-Kirchhoff积分和Fresnel近似出发,得到海面散射信号的空间相关性,由于信道的协方差矩阵不可以表示成发射相关矩阵和接收相关矩阵的Kronecker积,通过Monte Carlo仿真研究了莱斯因子、接收信噪比和空间相关性对MIMO信道容量的影响。从对outage capacity和信道容量的CCDF(Complementary Cumulative Distribution Function)的仿真计算结果可以看出,莱斯衰落的MIMO信道容量并不总是小于瑞利衰落MIMO信道容量,当信道的空间相关性较大和接收信噪比较小时,由于信道的衰落作用将起主要作用,对于较小的收发阵元数,以非衰落的直达信号为主的莱斯MIMO信道容量将大于瑞利衰落的MIMO信道容量。     

空间相关性对水声多输入多输出系统信道容量的影响

通过三维多径相关模型,以垂直收发阵为例研究了信道参数如声线角度扩展范围、到达方向和收发阵元间距对水声MIMO信道容量的影响。如果考虑静止的收发阵,且不考虑信道中散射体的运动时,MIMO系统的信道容量由收发阵元信号的空间相关性决定,当空间相关性较小时,增加收发阵元数可以带来较大的信道容量增益,而当空间相关性较大时,随着收发阵元数的增加信道容量将趋于饱和。     

Performance of zero-forcing detectors over MIMO flat-correlated Ricean fading channels

The bit error rate (BER) performance of the zero-forcing (ZF) receiver over transmit-correlated flat Ricean fading channels is investigated. In particular, for a multiple-input multiple-output (MIMO) channel with M transmit and N receive antennas, an approximation for the average BER of each substream is derived. Then the system performance in receive-correlated flat Ricean fading channels is addressed. In this case, it is shown that the performance, when N ? M, is the same as that of transmit-correlated flat Ricean fading channels. A closed-form expression for the optimum transmit correlation coefficient, which achieves the maximum capacity (i.e. uncorrelated case), is also derived. As a result, a significant capacity gain is achieved by exploiting the knowledge of the Ricean channel. Extensive simulations are presented to validate and demonstrate the performance gain with different system parameters.     

Information theoretic capacity of Gaussian cellular multiple-access MIMO fading channel

Higher spectral efficiency can be achieved by exploiting the space dimension inherent to any wireless communication system using multiple receiver and multiple transmitter antennas (MIMO). There are several results that provide closed form solutions for acellular system with a single antenna at each base station and each user terminal. Results are also available for the single cell case with MIMO. A cellular system with multiple antennas at the transmitter and the receiver nodes has not been investigated to obtain a closed form solution for the capacity limit. The main information theoretic theorems are not directly applicable to this system because of the form of the channel matrix of such a system. In this paper we extend the well known Wyner's model to a MIMO cellular system. It is observed that the achievable rate is bound by an upper limit and lower limit corresponding to two extreme fading conditions: channel with Rayleigh fading and with no fading. The analytical results are verified using Monte Carlo simulations. The analysis provides the insight that for a cellular system, increasing the number of transmitting antennas is not beneficial to increase the achievable rate, and this is reflected in the results obtained.     

一种低复杂度的下行MIMO多用户分组策略

在研究了多用户MIMO线性处理技术和多用户选择方法的基础上,提出了多用户分组的思想。为了降低分组过程的复杂度,提出了基于最小奇异值和信道向量的内积的分组评价准则,并且给出了一种低复杂度的快速分组算法。仿真结果显示所提出的算法可以明显提升系统性能：随着系统中激活用户数量的增加,逐渐接近单用户点对点MIMO通信系统的性能;在一个具有4个发射天线的基站的系统中,当发射信噪比达到15dB,分组数量达到10时,采用了快速分组策略后,块对角化和规格化反转方法的各态历经性信道容量分别提升4bits／Hz和3bits／Hz以上。     

On the competition of different mechanisms of sintering under SHS-disintegration

Theoretical estimates of changes in the strength of SHS products that take place during their sintering and cooling are proposed. The possibility of passing from the mechanism of diffusion sintering to the mechanism of sintering by compaction of the material under the action of external mechanical loads has been taken into account. It has been shown that the competition between these mechanisms of sintering under cooling of products is responsible for the nonmonotonic character of the dependence of the strength of product on the delay time. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 6, pp. 151–156, November–December, 2007.     

Quasi-superactivation of classical capacity of zero-capacity quantum channels

One of the most surprising recent results in quantum Shannon theory is the superactivation of the quantum capacity of a quantum channel. This phenomenon has its roots in the extreme violation of additivity of the channel capacity and enables reliable transmission of quantum information over zero-capacity quantum channels. In this work we demonstrate a similar effect for the classical capacity of a quantum channel which previously was thought to be impossible. We show that a nonzero classical capacity can be achieved for all zero-capacity quantum channels and it only requires the assistance of an elementary photon–atom interaction process – stimulated emission.     

Outage-constrained capacity of spectrum-sharing channels in fading environments

Cognitive radio technology has been recently proposed for sharing and utilising the spectrum in order to satisfy the increasing demands for spectrum access. In this radio technology, secondary users may be granted access to the spectrum bands occupied by a primary user as long as the interference power, inflicted on the primary receiver as an effect of the transmission of the secondary user, is deemed unharmful. In this paper the authors assume that the successful operation of the primary user requires a minimum rate to be guaranteed by its channel for a certain percentage of time and obtain the interference-power constraint that is required to be fulfilled by the secondary user. Considering the input transmit-power constraint, on average or peak power, for the secondary user, the authors investigate the capacity gains offered by this spectrum-sharing approach when only partial channel information of the link between the secondary's transmitter and primary's receiver is available to the former. In particular, the lower bounds on the capacity of a Rayleigh flat-fading channel with two different transmission techniques, namely channel inversion and optimum rate allocation with constant power transmission, are derived. Closed-form expressions for these capacity metrics are provided, and numerical simulations are conducted to corroborate the theoretical results.     

Error performance of general order selection in correlated nakagami fading channels

A procedure for determining the probability distribution of the rth order statistic, Gr:L, r=1, 2, ==, L, among a set of L correlated Nakagami diversity branch gains G ₁, G ₂, , GL has been described in David and Nagaraja (2003) and Elkashlan et al. (2008). The results are used to evaluate the bit error rate (BER) of general order selection (GOS), a diversity method in which the rth order branch is selected for transmission, over correlated Nakagami fading branches. GOS can be used to improve system throughput and provide various levels of services, both of which are highly desirable in high-speed communication systems. Numerical and simulation results are presented and used to illustrate the effects of fading correlation on the BER associated with the rth order gain branch.     

Effects of diversity schemes and correlated channels on OWC systems performance

The salient features and advantages of free-space optical (FSO) system are very appealing for different applications in a number of communication network sectors. In spite of the advantages and diverse applications of FSO communication, its extensive use is hindered by the atmospheric turbulence-induced fading in real-life scenarios. Spatial diversity technique is one of effective means of mitigating turbulence-induced fading and, consequently, improves the system performance. In this paper, we study the spatial diversity schemes for mitigating turbulence-induced fading in the FSO communication systems using the bit error rate (BER) as a performance metric. The schemes considered are based on orthogonal space–time block codes and repetition codes (RCs). We derive simple approximate closed-form expressions for the error probability of the log-normal FSO links with intensity modulation and direct detection (IM/DD). Furthermore, we also investigate the effects of spatial correlation between the transmit apertures on the system performance. We achieve this using the exponential model for determining the correlations between the apertures. We observe that the proposed BER expressions are able to quantify the effects of spatial diversity schemes and spatial correlations on the system.     

Performance analysis of dual-diversity receivers over correlated generalised Gamma fading channels

The performance of dual-branch equal-gain combining (EGC) and maximal-ratio combining receivers operating over a composite correlated fading environment, modelled by the generalised Gamma (GG) distribution, is analysed. The moments of the output signal-to-noise ratio are derived in closed form for both types of receivers, and by employing the Pade approximants method, the average bit error probability is studied for a great variety of modulation schemes. Furthermore, based on the statistic of the product of two correlated GG random variables, a tight union upper bound for the outage probability of the EGC is obtained, whereas for the special case of Weibull fading a simpler bound is derived in closed form. The proposed mathematical analysis is complemented by various, numerically evaluated performance results, whereas simulations verify the correctness of the proposed analysis.