Spatial, polarization, and pattern diversity for wireless handheldterminals

This paper examines the antenna diversity configurations that improve the performance in handheld radios. Experiments using spatial, polarization, and pattern diversity were conducted for both line-of-sight (LOS) and obstructed outdoor and indoor multipath channels that experienced Ricean fading. Antenna separation, polarization, and pattern were varied independently to the extent possible. Envelope correlation, power imbalance, and diversity gain were calculated from the measurements. Diversity performance is measured by diversity gain, which is the difference in signal-to-noise ratio (SNR) between the output of a diversity combiner and the signal on a single branch, measured at a given probability level. Diversity gain increases with decreasing envelope correlation between the antenna diversity branches. However, diversity gain decreases as the power imbalance between diversity branches increases because a branch that has a weak signal has only a small contribution to the combined signal. Diversity gain values of 7-9 dB at the 99% reliability level were achieved in non-line-of-sight (NLOS) channels for all diversity configurations even with very small antenna spacings. The use of polarization diversity reduced polarization mismatches, improving SNR by up to 12 dB even in LOS channels     

Branch correlation measurements and analysis in an indoor Rayleigh fading channel for polarization diversity using a dual polarized patch antenna

Spatial diversity techniques are advantageous if the received signals of the diversity branches are sufficiently uncorrelated. Theoretical study using the power unbalance between the two branches employing Laplacian direction of arrival (DOA) distribution is carried out. It is shown that for low value of cross-polarization power ratio (XPR) the correlation has a negligible effect. In order to validate the theoretical results indoor NLOS measurements were carried out. The measurements were done to study the cross-correlation between two received signals of orthogonal polarization using a dual linear polarized patch antenna. The antenna has been designed, built and measured. The results of the channel measurements are given. It is concluded that received signals with orthogonal polarization in indoor Rayleigh fading channels have no significant envelope correlation.     

室内MIMO系统采用极化分集时的信道容量

典型的室内传播信道为莱斯衰落信道,可将其分解为LOS传播信道和散射信道.本文假设散射信道为瑞利衰落信道,并根据天线的极化特性,给出了存在极化分集时的LOS传播信道.基于此,本文提出了室内MIMO系统采用极化分集时的信道模型.研究表明:极化分集可以有效提高MIMO信道容量.     

Optimal maximal ratio combining with correlated diversity branches

It is proved that a maximal ratio combiner operating on correlated branches and weighting the branch signals as though they were independent is optimal. It is also proved that performance measures of maximal ratio combining operating with correlated Rayleigh or Ricean fading input branches are identical to performance measures of an equivalent diversity system operating with independent and, in general, unbalanced inputs     

Receive antenna selection for unitary space-time modulation over semi-correlated Ricean channels

Receive antenna selection for unitary space-time modulation (USTM) over semi-correlated Ricean fading channels is analyzed (this work generalizes that of Ma and Tepedelenlio-glu for the independent and identically distributed (i.i.d.) Rayleigh fading case). The antenna selection rule is that the receive antennas with the largest signal powers are chosen. For single antenna selection, we derive the maximum likelihood decoding for the correlated Ricean case. We also derive the Chernoff bound on the pairwise error probability for the high signal to- noise ratio (SNR) region and obtain the coding gain and diversity order. Our results show that even when there are transmitter side correlations and a line of sight component, receive antenna selection with USTM preserves the full diversity order if the USTM constellation is of full rank. We also give an approximation to the distribution function of a quadratic form of non-zero mean complex Gaussian variates (from Nabar et al.) at the high SNR region. Based on this approximation, a closed-form expression for the coding gain is also obtained and compared with that of the i.i.d. Rayleigh case. We also analyze the case of multiple receive antenna selection and derive the coding gain and diversity order. We show that USTM constellations, which have been proposed for the i.i.d. Rayleigh channel, can be used with the correlated Ricean channel as well.     

一种新的提高极化分集性能的预处理方法

极化分集的两分支信号间往往存在严重的接收功率差,从而制约了其分集性能。该文提出一种新的改善极化分集性能的预处理方法,可以在任意极化的入射场条件下保证两分集分支的接收功率相等,并给出了相应的理论分析及其物理解释。在典型的信道环境中,通过与两种典型的极化分集方案的性能比较,证明了该方法的优越性。     

Level crossing rate and average fade duration of MRC and EGC diversity in Ricean fading

The average level crossing rate and average fade duration of the output signal of a maximal ratio combiner (MRC) and equal gain combiner (EGC), operating on independent Ricean fading input branch signals, are derived. Exact, closed-form results are obtained for MRC diversity, while precise expressions for EGC diversity are presented with an infinite series method. The results are valid for an arbitrary number of independent, identically distributed diversity branches, isotropic scattering, and a specular component perpendicular to the line of motion of the mobile.     

Polarization Diversity System for Mobile Radio

Conventional space diversity reception at typical elevated base locations requites separation of 30λ for broadside incidence and even more for in-line incidence and is therefore difficult to implement. A polarization diversity system for mobile radio is proposed. This is a two-branch receiver diversity system with the advantage that the base station antennas can be spaced as closely as desired. An experimental program has been carried out to obtain the statistical properties of vertically and horizontally polarized electromagnetic waves in a suburban environment at 836 MHz. It was observed that signals of both polarizations were Rayleigh plus log normal, where one is uncorrelated and other is correlated irrespective of base or mobile antenna spacings. The local means of the two signals were highly correlated and were with ± 3 dB for almost 90 percent of the time. Variation of base transmitter heights appeared to have little effect on the ratio of the local means of the two signals. The analysis and experiment demonstrated the feasibility of providing two diversity branches at UHF by polarization diversity.     

Asymptotic BEP and SEP of Quadratic Diversity Combining Receivers in Correlated Ricean Fading, Non?Gaussian Noise, and Interference

In this paper, we study the asymptotic behavior of the bit-error probability (BEP) and the symbol-error probability (SEP) of quadratic diversity combining schemes such as coherent maximum-ratio combining (MRC), differential equal-gain combining (EGC), and noncoherent combining (NC) in correlated Ricean fading and non-Gaussian noise, which in our definition also includes interference. We provide simple and easy-to-evaluate asymptotic BEP and SEP expressions which show that at high signal-to-noise ratios (SNRs) the performance of the considered combining schemes depends on certain moments of the noise and interference impairing the transmission. We derive general rules for calculation of these moments and we provide closed-form expressions for the moments of several practically important types of noise such as spatially dependent and spatially independent Gaussian mixture noise, correlated synchronous and asynchronous co-channel interference, and correlated Gaussian interference. From our asymptotic results we conclude that (a) the asymptotic performance loss of binary frequency-shift keying (BFSK) with NC compared to binary phase-shift keying (BPSK) with MRC is always 6 dB independent of the type of noise and the number of diversity branches, (b) the asymptotic performance loss of differential EGC compared to MRC is always 3 dB for additive white Gaussian noise but depends on the number of diversity branches and may be larger or smaller than 3 dB for other types of noise, and (c) not only fading correlation but also noise correlation negatively affects the performance of quadratic diversity combiners.     

Multiple-antenna capacity in the low-power regime

This paper provides analytical characterizations of the impact on the multiple-antenna capacity of several important features that fall outside the standard multiple-antenna model, namely: (i) antenna correlation, (ii) Ricean factors, (iii) polarization diversity, and (iv) out-of-cell interference; all in the regime of low signal-to-noise ratio. The interplay of rate, bandwidth, and power is analyzed in the region of energy per bit close to its minimum value. The analysis yields practical design lessons for arbitrary number of antennas in the transmit and receive arrays.     

一种径向偏振光的产生方法

为了产生径向偏振光,采用组合半波片和组合线偏振片在腔外对线偏振光做极化整形的方法,进行了理论分析和实验验证。为检测该径向偏振光产生系统的性能,采用旋转检偏器法对输出光束的偏振分布进行了检测,并用经典斯托克斯参量测量法计算了偏振纯度,最后基于马赫-曾德尔干涉的原理,检测了径向偏振光对称区域的线偏振相位关系。结果表明,当组合线偏振片为4个分块时,获得了偏振纯度为80.5%的径向偏振光,并检测出光斑对称区域的线偏振相位差为π。这一结果对在低成本条件下产生高纯度的径向偏振光是有帮助的。     

偏振正交外差光信噪比监测技术研究

传统偏振方法监测光信噪比的准确性易受偏振模色散和非线性双折射影响,提出了一种新的偏振正交外差监测方法以克服传统偏振监测方法的缺点.该方法通过将信号分成两个支路并控制偏振态达到相对偏振态正交,用偏振正交的两支路信号外差混频消去信号,通过测量保留下来的差拍噪声实现对光信噪比的监测.仿真试验证实,该方法对偏振模色散和非线性双折射引起的信号偏振态变化不敏感.     

MIMO multichannel beamforming: SER and outage using new eigenvalue distributions of complex noncentral Wishart matrices

This paper analyzes MIMO systems with multichannel beamforming in Ricean fading. Our results apply to a wide class of multichannel systems which transmit on the eigenmodes of the MIMO channel. We first present new closed-form expressions for the marginal ordered eigenvalue distributions of complex noncentral Wishart matrices. These are used to characterize the statistics of the signal to noise ratio (SNR) on each eigenmode. Based on this, we present exact symbol error rate (SER) expressions. We also derive closed-form expressions for the diversity order, array gain, and outage probability. We show that the global SER performance is dominated by the subchannel corresponding to the minimum channel singular value. We also show that, at low outage levels, the outage probability varies inversely with the Ricean A*-factor for cases where transmission is only on the most dominant subchannel (i.e. a singlechannel beamforming system). Numerical results are presented to validate the theoretical analysis.     

Comment on “estimate of channel capacity in Rayleigh fadingenvironment”

Lee (see ibid., vol.39, p.187, no.8, 1990) proposed a very simple and elegant model for estimating the capacity of slowly fading channels. A small error in his paper, however, has led to an inexact picture. We correct that inaccuracy. As a consequence, the loss in capacity of a Rayleigh fading channel with respect to the additive white Gaussian noise (AWGN) channel turns out to be negligible for all values of the signal-to-noise ratio that are of interest. Furthermore, we extend the analytical results to the situation with M-branch diversity and Ricean fading channels. The lower moments of the associated distributions, i.e., of the χ²-distribution (Nakagami with integer m-parameter) and Rice distribution agree when the parameters of the distribution are suitably chosen. With that choice of parameters, the capacities of the M-branch diversity and of the Ricean fading channel become almost identical     

Performance analysis of diversely polarized antenna arrays

The performance of direction finding systems utilizing diversely polarized antenna arrays is investigated. The Cramer-Rao bound (CRB) is used to evaluate the accuracy of the estimated directions of arrival. The resolving power of the array is evaluated by analyzing the performance of a likelihood ratio detector designed to determine whether the signals observed by the array originate from a single source or from two closely spaced sources. Numerical examples and analysis of some special cases provide insights into the improvement in direction finding accuracy and in the resolving power of the array due to polarization diversity. The case of a signal in the presence of an interferer is studied in some detail. The CRB and the probability of detection are evaluated as functions of the spatial separation and the polarization difference between signal and interferer, signal-to-noise ratio, and signal-to-interference ratio. Performance comparisons are made between uniformly polarized and diversity polarized arrays. It is shown that improved direction finding performance can be obtained by using polarization diversity, with no increase in hardware complexity     

Switched diversity on microcellular Ricean channels

The performances of switched dual diversity systems operating on independent and correlated Ricean fading channels are analyzed using a discrete time model. The average bit error rate (BER) of the discrete time switched diversity system using binary noncoherent frequency shift keying (NCFSK) on slow, nonselective Ricean fading channels is derived. A closed form expression that gives the optimum switching threshold in a minimum error rate sense is derived for the case of independent branch signals. Results for the optimum switching threshold for the case of correlated branch signals, obtained numerically, are also presented. Results using selection diversity combining are obtained for comparison. The effects of fading severity on both the BER and on the optimum switching threshold are investigated. The Ricean fading model may be used to model both the microcellular radio environment and the mobile satellite fading channel. Hence, the results of the paper are useful for both of these areas     

Base station polarization diversity reception for mobile radio

Base station polarization diversity reception in which signals are received by dual polarization (ex. ±45° polarization) is discussed. A theoretical analysis is presented on correlation coefficient ρ between diversity branches, and received signal level decrease L caused by polarization difference at the base and mobile station. The generalized expressions of ρ and L are then derived. Measurements were also carried out at 900 MHz in an urban area. Consequently, it was found that the ρ and L values are expressed by three factors, ρ is lower than 0.6 and L is smaller than 2.5 dB. It is concluded that this polarization diversity reception can be used as an effective diversity reception.     

Dual polarized staggered PRT scheme for weather radars: analysisand applications

Weather radars require long unambiguous ranges, high Nyquist sampling rates and good clutter suppression. In addition, polarization diversity is also used extensively, switching polarization of the transmit pulse on a pulse by pulse basis. This paper presents a dual polarized staggered pulse repetition scheme that can be used for Doppler and polarization diversity measurements in a weather radar. Processing algorithms for radar returns under this scheme are also presented. This pulsing scheme was implemented on the CP-2 radar operated by the National Center for Atmospheric Research. Experimental observations of convective storms using dual polarized staggered pulse repetition scheme in the Colorado front range were made and the results of the data analysis are presented     

An Empirical Model for Nonstationary Ricean Fading

Ricean fading is common in dense urban cellular networks and, as a mobile moves through that environment, the K-factor of the Ricean fading will change. This paper presents a statistical model for dense urban vehicular nonstationary Ricean fading, where the K-factor gradually changes due to movement through changing surroundings. This model is empirical and is based on K-factor fluctuations that are observed in dense urban cellular radio channel measurements. The K -factor is modeled using a random process with a distribution that is fit to the measured K-factor values. An autoregressive (AR) model is also used to ensure that the autocorrelation of the simulated K-factor process matches the empirical data. The nonstationary Ricean fading envelope that is generated using this model is verified by comparing it with the fading envelope that is observed in the measurements.     

Error Performance of Rotated Phase Shift Keying Modulation over Fading Channels

In this paper, the modulation diversity is used to improve the performance of M-PSK modulation over fading channels. Modulation diversity can be achieved by rotating the signal constellation and using component interleaving. We derive symbol error probability expressions for rotated uncoded M-PSK over Ricean fading channels and obtain optimal rotation angles for M-PSK (M = 2, 4, 8). We show that rotated signal constellations with component interleaving improve the performance of M-PSK significantly as compared to the unrotated one over Rayleigh and Ricean fading channels. For example, when the ratio of the direct path power to the multipath signal power, K is 0 and 10, 8 and 1.5 dB gains are obtained, respectively, at a symbol error probability of 10⁻³ for 8PSK modulation. We also show that as K gets larger, the gain obtained by the rotation rapidly decreases. We develop a new asymmetric 8PSK signal constellation obtained from two QPSK signal constellations that are optimally rotated by different angles. This asymmetric 8PSK and also the rotated 8PSK signal constellation together with component interleaving are applied to four-state trellis-coded schemes. Simulation results show that these new schemes provide good performance improvements over the original TCM schemes and previous relevant works over Rayleigh and Ricean fading channels.