Ultra-reliable communication (URC) is one of the distinctive features of the upcoming 5G wireless communication, characterized by packet error rates going down to 10-9. In this paper, we analyze the tail of the cumulative distribution function of block fading channels in the regime of extremely rare events, i.e., the ultra-reliable (UR) regime of operation. Our main contribution consists of providing a unified framework for statistical description of wide range of practically important wireless channel models in the UR regime of operation. Specifically, we show that the wireless channel behavior in this regime can be approximated by a simple power law expression, whose exponent and offset depend on the actual channel model. The unification provides a channel-agnostic tool for analyzing and performance optimization of radio systems that operate in the UR regime. Furthermore, the unified model is particularly useful in the emerging measurement campaigns for empirical characterization of wireless channels in the regime of low outages. Finally, the asymptotic analysis can serve as an underlying building block for designing more elaborate, higher-layer technologies for URC. We showcase this by applying the power law results to analyze the performance of receiver diversity schemes and obtain a new simplified expression for maximum ratio combining.

This paper introduces three key features in geometry-based stochastic channel models in order to include massive MIMO channels. Those key features consists of multiuser (MU) consistency, non-stationarities across the base station array and inclusion of spherical wave modelling. To ensure MU consistency, we introduce the concept of “user aura”, which is a circle around the user with radius defined according to the stationarity interval. The overlap between auras determines the share of common clusters among users. To model non-stationarities across a massive array, sub-arrays are defined for which clusters are independently generated. At last, we describe a procedure to incorporate spherical wave modelling, where a cluster focal point is defined to account for distance between user and cluster.

Long haul transport can be considered an obvious deployment scenario for communication between vehicles travelling in convoy, where environments will be comparable to that of a rural highway. Therefore this paper addresses antenna array techniques applicable to such circumstances, which provide little scattering to the propagation channel and little support for diversity. Only beamforming is realistically suited to increase the capacity in the vehicle to vehicle link. However, the possibility to enable multi user channels is limited in such environments since the propagation channels are highly correlated when more than two vehicle to vehicle links are in close proximity. The deliberate introduction of delay spread through wide spaced and combined multiple antenna topology thus creating an artificial wideband fading channel is proposed in this paper, which results in de-correlated vehicle to vehicle links. Measurements and experimental results are presented that demonstrate the means by which uncorrelated wideband channels between separate vehicle to vehicle links can be formed to enable multi user capabilities specific to vehicles on a rural highway.

Preliminary results on the use of the vehicle to vehicle MIMO channel in a rural highway environment are presented. This is looked at both in terms of the available spatial multiplexing through singular value decomposition and also angular distribution within the channel. Results indicate a strong predominant line of sight link in general while instances of scattering from other vehicles will cause changes in the Doppler spectrum as well as beamforming jitter.

Wireless transmission based on cooperation promises to bring performance improvements in multi-node networks. Several cooperative schemes have appeared in the literature, and in most cases have been assessed under simplified assumptions for the channel model. Experimental data for cooperative transmission are difficult to obtain because they should provide simultaneous characterization of at least three links. In this paper we present measurements that involve two access points (APs) and two user terminals, each of which is equipped with 4 antennas, in an indoor office environment. The data indeed allow for the simultaneous characterization of the multiple input-multiple output (MIMO) links from the APs to the user terminals and between the user terminals. We use these measurements to practically assess several schemes for cooperative transmission.

A cooperative scheme enabling spatial reuse, namely cooperative spatial reuse (CSR), is proposed as a cooperative extension of the current TDMA-based MAC in wireless networks. In the CSR, a cooperative group is formed by the links that are willing to do spatial reuse. In the group, every cooperating link contributes its time slots for spatial reuse among the cooperating participants. Following the cooperation principle, a link joins the group only if it can benefit. Otherwise, the link will stop doing CSR and switch back to the TDMA-based MAC. In this work, we focus on the transmit beamforming techniques to enable CSR on MISO (Multiple Input Single Output) links. We compared the CSR scheme using zero-forcing (ZF) transmit beamforming, namely ZF-CSR, to the TDMA-based MAC using maximum ratio combining (MRC) transmit beamforming, namely MRC-TDMA. The numerical results of a simulated two 2 x 1 MISO links scenario show the great potential of CSR to substantially increase the capacity and energy efficiency.

In this paper, we look at multiple antenna systems and code information on the communication medium itself to improve the link security. In the case of single data stream transmission, the medium refers to the response across the receive sensor array. In multiple data stream transmission, the medium is the set of virtual orthogonal channels that result from the singular value decomposition of the channel transfer matrix. We code phase information on the orthogonal channels by rotating the received signal constellation. This does not impair the signal detection of differential-phase coded signals on the intended receiver. However the rotation at an eavesdropper location is different from the one on the intended receiver even at a distance of lambda/10 (6 mm), and therefore security is enhanced.

In this paper we present measurements that involve two access points (APs) and two user terminals, each of which is equipped with 4 antennas, in an indoor office environment. The data allow for the simultaneous characterization of the multiple input-multiple output (MIMO) links from the APs to the user terminals and between the user terminals, and are, to the best of our knowledge, the first of their kind. We show that the links between user terminals are impaired by body shadowing, and have therefore much lower gains than the direct links to the APs. Moreover we investigate the statistics of the links, and their correlation. Based on these measurements, we investigate the performance of cooperative relaying in terms of rate improvement (for this analysis the data from only one AP are used). We show that a significant rate improvement can be achieved if the relaying node is free to set the power level at which it transmits, and if it is free to select which of its antennas is used to perform the relaying.

Interference caused by other users in a multi-user environment degrades the system performance significantly. Conventionally, user separation in a multi-user environment is achieved by signal separation in time, frequency or code. The use of multiple elements antenna (MEA) systems can apply separation of the users in space by targeting the transmitted power to the user of interest. Inevitably in that case, the separation is not ideal and there is interference on the other users in the systems. Moreover in a wideband transmission, frequency selective fading causing inter symbol interference (ISI) is a big challenge for point to point wireless communications. In this paper, we study the feasibility of applying the time reversal technique (TR) in multiple input single output (MISO) systems to alleviate the effect of ISI. The ability of TR to focus the signal on the receiver of interest in a multi-user environment is also considered. The studies are supported by measured complex channel impulse responses with 10 MHz bandwidth centered at 2.14 GHz. For an 8/spl times/1 MISO system, using TR the root mean square delay spread is reduced by a factor of 2. To evaluate the capability of the TR in reducing the ISI, simulation of the bit error rate (BER) was made. The irreducible BER of the TR-MISO system was shown to be lower than that of the SISO system by at least an order of magnitude. By using TR and assuming that two users are communicating simultaneously with the same BS the two signals have 17 dB of isolation.

Attaching cables to mobile terminals for accurate measurements is progressively influencing the radiation characteristics with the diminishing size of the terminals. Simulations show the large influence of cables and their exact curvature on the radiation properties of mobile terminals that are small with respect to the wavelength. These results are confirmed by measurements in an anechoic chamber. As corrections for these almost irreproducible influences are hardly feasible, we propose to omit conducting cables completely. Instead, optical fibres are used for transparent transport of the RF signals. Off-the-shelf system components allow for two channels in a box of size comparable to modern small terminals. When driven at an appropriate level this set-up shows ample dynamic range for mobile radio channel sounding.

This paper describes radio channel and BER investigations performed in various scenarios typical to the TETRA direct mode operation (DMO), in a forest area. The data from radio channel sounding measurements was analysed in terms of average fade duration and envelop CDF. The same data was used to evaluate the unprotected BER on a TETRA communication link. The BER level distribution was compared to the distribution obtained when simulating a pedestrian speed TETRA DMO radio channel. The dynamics of the mobiles can have a strong influence on the radio link quality. As a general tendency, higher practical BER values can be expected as compared to the simulations when using the simple TETRA radio channel model.

Linear antenna arrays have an inherent angular variance, which makes them difficult to use for angular channel characterisation. Through operation in the virtual Doppler domain at the base station (BS)-an invariant convolution between environment Doppler response and antenna array factor can be exploited. This enables the use of a deconvolution procedure, providing increased resolution in the Doppler and angular spectrum. The Doppler domain description also results in a subtraction property for the Doppler spread. Thus providing a super resolved statistical dispersion measure for channel characterisation. Furthermore the statistics of the received amplitude and phase response along the array are given.

When measuring antenna diversity branch envelope signals, uncorrelated noise-like signals are corrupting the desired signals (described by Rician/Rayleigh fading processes). This causes a higher degree of envelope decorrelation between the total antenna branch signals. This has a high importance in system design when estimating the diversity gain and the resulting link quality and cell layout. An analysis is presented, showing the total power cross-correlation being a 2nd order function of the signal-to-noise ratio (SNR) and Rician K-factor. For the Rayleigh fading case the total auto-correlation function is presented, which is useful in estimating the necessary spacings in a space diversity system.

An analogy between the temporal and angular domain propagation channel descriptions is presented, as well as dispersion metrics and channel classifications. This analysis is relevant when considering angular fixed and dynamic (adaptive beam) base station antennas.

Typical DECT and UMTS radio environments have been investigated with high gain directional base station (BS) antennas. Experiments have been performed as preparation for field tests with an adaptive BS antenna testbed. The dispersion of the radio environment leads to a distortion of the perceived BS antenna sidelobe levels, though still providing increase in effective gain. The spatial selectivity of directive antennas leads to a reduction in time dispersion by about a factor of two, while the cross polarization discrimination is increased a few dB. Quantitative measures of the spatial environment spreading are given, so a numerical comparison between different environments is possible.

Uncommon DECT diversity combiners are compared with more usual DECT combiners. Soft combining provides good performance and switched pattern diversity seems to have a good performance potential in moderate spatially dispersive environments.>

The authors describe cross-correlation properties for compact urban base station antenna configurations, nearly all of which result in very low envelope cross-correlation coefficients of about 0.1 to 0.3. Specifically, polarization diversity systems are examined for their potential in improving link quality when hand-held terminals are involved. An expression is given for the correlation function of compound space and polarization diversity systems. Dispersion and envelope dynamic statistics are presented for the measured environments. For microcell applications, it is found that systems such as GSM having a bandwidth of 200 kHz or less can use narrowband cross-correlation analysis directly.>