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Along with the increase in operating frequencies in advanced radio communication systems utilised inside tunnels, the location of the break point is further and further away from the transmitter. This means that the near region lengthens considerably and even occupies the whole propagation cell or the entire length of some short tunnels. To begin with, this study analyses the propagation loss resulting from the free-space mechanism and the multi-mode waveguide mechanism in the near region of circular tunnels, respectively. Then, by conjunctive employing the propagation theory and the three-dimensional solid geometry, a general analytical model of the dividing point between two propagation mechanisms is presented for the first time. Moreover, the model is validated by a wide range of measurement campaigns in different tunnels at different frequencies. Finally, discussions on the simplified formulae of the dividing point in some application situations are made. The results in this study can be helpful to grasp the essence of the propagation mechanism inside tunnels. [ABSTRACT FROM AUTHOR]

An improved segmented polynomial-based pre-distortion method and a novel memory polynomial-based pre-distortion method with two-dimensional indexing are proposed in digital baseband power amplifier pre-distortion systems. Through computer simulations and corresponding analysis, the feasibility and reliability of the proposed methods are verified with three different polynomial formats in the evaluation of relative mean square error, the convergence speed, the random access memory storage units and the power spectral density. [ABSTRACT FROM AUTHOR]

When a high-resolution algorithm is applied in a ship-borne radar, its non-linearity and distributional characteristics before high-resolution processing determine the distributional characteristics of background clutter after high resolution and detector design afterwards. Because background noise before high-resolution has physical significance, the statistical model of first-order Bragg lines and second-order components of sea clutter is put forward. Then, by using higher order cumulative quantity's statistical verification of actual measured data, it is concluded that the background noise before high resolution conforms to normal distribution in a ship-borne radar. The non-linearity of the high-resolution algorithm means that the background noise after high-resolution processing conforms to non-normal distribution. Non-normal-distributed clutter mainly includes Weibull, lognormal, and K-clutter. Rayleigh clutter can be seen as a special case of Weibull clutter. These clutter types have different statistical characteristics and can be discriminated by recognition of clutter characteristics. The numerical domain's distribution after high-resolution processing is determined by an improved, minimum entropy, clutter characteristics recognition method based on the AIC rule, namely, a two-parameter domain-scanning method. This identification method has a higher recognition rate. It is verified that the background noise after high-resolution by pre-whitened-constrained-MUSIC conforms to lognormal distribution. [ABSTRACT FROM AUTHOR]

The problem of user selection and resource allocation for the downlink of secure multiuser MISO-OFDMA systems overheard by multiple multi-antenna eavesdroppers is investigated. Based on zero forcing beamforming and artificial noise (AN), a physical-layer security model is proposed. Furthermore, the original problem is formulated as an AN aided secrecy rate maximisation problem, which maximises the sum of the secrecy rate by jointly designing user selection and the AN covariance. However, the formulated problem is non-convex and hard to optimise in general. It is shown that the problem can be handled by performing an exhaustive search in which a sequence of semi-definite relaxation programs are involved, leading to a tractable user selection and resource allocation method. [ABSTRACT FROM AUTHOR]

An integrative wideband direction of arrival (DOA) estimation method is proposed based on a novel data model using the concept of sparse representation. The new method makes full use of the frequency distribution of a received signal to generate the over-complete dictionary. The method requires no prior information of signal number. No spectral decomposition or focusing is necessary for either coherent or incoherent signals. [ABSTRACT FROM AUTHOR]

Presented is the statistical analysis of radio wave propagation in a high-speed railway cutting scenario, derived from 930 MHz measurements taken along the 'Zhengzhou-Xi'an' high-speed railway of China. The crown width and bottom width of the cutting are well-covered by the proposed models. The Ricean distribution offers a good fit and the K-factor is found to be lognormal, with a mean value of 1.88 dB and standard deviation of 3.29 dB. [ABSTRACT FROM AUTHOR]