Search

This paper proposes a two dimensional (2-D) statistical channel model for Terahertz (THz) chip-to-chip wireless communication in desktop size metal enclosures. This model differs from traditional statistical channel models as it models both traveling and resonant waves that exist inside metal enclosures. Based on the cavity environment and the statistical properties of the channel inside the metal cavity, the geometrical model which describes propagation in resonant cavity as a superposition of LoS, single bounced (SB), double bounced (DB), and multi-bounced (MB) rays is proposed. Based on the geometrical model, a parametric reference model is proposed. Furthermore, the path loss model that captures signal strength variation in a resonant cavity is proposed. Frequency correlation functions (FCF) and power delay profiles (PDP) for different possible chip-to-chip communication scenarios are derived and compared with the measured ones. The results show a good agreement between the simulated and measured statistics.

This paper investigates the capability of millimeter-wave (mmWave) channel sounders with phased arrays to perform super-resolution parameter estimation, i.e., determine the parameters of multipath components (MPC), such as direction of arrival and delay, with resolution better than the Fourier resolution of the setup. We analyze the question both generally, and with respect to a particular novel multi-beam mmWave channel sounder that is capable of performing multiple-input-multiple-output (MIMO) measurements in dynamic environments. We firstly propose a novel two-step calibration procedure that provides higher-accuracy calibration data that are required for Rimax or SAGE. Secondly, we investigate the impact of center misalignment and residual phase noise on the performance of the parameter estimator. Finally we experimentally verify the calibration results and demonstrate the capability of our sounder to perform super-resolution parameter estimation.

Time-division multiplexed (TDM) channel sounders, in which a single RF chain is connected sequentially via an electronic switch to different elements of an array, are widely used for the measurement of double-directional/MIMO propagation channels. This paper investigates the impact of array switching patterns on the accuracy of parameter estimation of multipath components (MPC) for a time-division multiplexed (TDM) channel sounder. The commonly-used sequential (uniform) switching pattern poses a fundamental limit on the number of antennas that a TDM channel sounder can employ in fast time-varying channels. We thus aim to design improved patterns that relax these constraints. To characterize the performance, we introduce a novel spatio-temporal ambiguity function, which can handle the non-idealities of real-word arrays. We formulate the sequence design problem as an optimization problem and propose an algorithm based on simulated annealing to obtain the optimal sequence. As a result we can extend the estimation range of Doppler shifts by eliminating ambiguities in parameter estimation. We show through Monte Carlo simulations that the root mean square errors of both direction of departure and Doppler are reduced significantly with the new switching sequence. Results are also verified with actual vehicle-to-vehicle (V2V) channel measurements., 11 pages, submitted to IEEE Transaction on Wireless Communications. arXiv admin note: text overlap with arXiv:1805.06611

Outdoor to indoor penetration loss is one of the crucial challenges faced at millimeter-wave frequencies. This paper presents the results from 28-GHz channel sounding campaigns performed to investigate the impact of this phenomenon on the wireless propagation channel characteristics in small cell and fixed wireless access scenarios. The measurements are performed with a real-time channel sounder equipped with phased array antennas that allow beam-forming and electronic beam steering for directionally resolved measurements. Thanks to the short measurement time and the excellent phase stability of the system, we obtain both directional and omnidirectional channel power delay profiles without any delay uncertainty. We compare the measured path loss, delay spread, and angular spread for indoor and outdoor receiver locations for two different types of buildings. We find that the penetration loss strongly depends on the angle of incidence, and the scatterers on the outside of the building strongly impact how much power is coupled into the building. Based on the results, we provide statistical models for path loss, delay spread, and angular spread.

Wireless body area networks (BANs) have many important applications such as wearable communication devices and Internet of Things. Wireless propagation in on-body channels has been measured and modeled in the past. However, a crucial element that is usually ignored is the impact of the body size of the user—a 50-kg person obviously creates a different on-body channel than a 150-kg person. The status-quo “one-size-fits-all” approach to channel characterization in BAN is thus incomplete. In this paper, we provide a detailed description of a propagation measurement campaign that employs a self-developed $4 \times 4$ ultrawideband multiple-input-multiple-output array channel sounding system to perform BAN channel sounding both in an anechoic chamber and indoor laboratory environments. A total of 60 human subjects were investigated in our work. These human subjects had widely varying body mass index (BMI) values, which were grouped into three different categories, i.e., 20 per BMI category. Various propagation properties such as path gain, frequency-decay factor, shadowing gain, rms delay spread, amplitude fading, and spatial correlation are extracted for each on-body channel under consideration. A comparison of statistics among the BMI categories reveals considerable differences, emphasizing the fact that the aforementioned propagation properties are BMI dependent. Parameters such as path gain showed a monotonic decrease across the BMI categories with values ranging from 1–2 to almost 13 dB in some channels. This paper proposes a propagation channel model for the BMI-dependent parameters and validates that it can reproduce the measured channel capacities.

The IEEE 802.11ac amendment enhances WLAN throughput by exploiting the spatial diversity of the antennas in a multi-user multi-input multi-output downlink transmission. Still, network resources remain under-utilized in uplink transmission due to single-user communication. In this paper, we propose an access point-controlled MAC protocol (A-MAC) that enables simultaneous transmissions from multiple STAs in uplink. The protocol uses the EDCA channel access technique to initiate multi-user transmission and the OFDMA method to transmit multiple RTSs simultaneously. It also introduces the explicit channel sounding technique by using dedicated OFDM subcarrier blocks for each user. Performance measurement shows that network throughput of the A-MAC is 150% higher than that of a single uplink transmission, thanks to the availability of concurrent multiple RTS transmissions in the uplink. The proposed protocol shortens the backoff time by up to 50% for all traffic categories due to concurrent multiple transmissions and thus enhances the battery life of the nodes. We observe that although the smaller backoff window of high-priority traffic category enhances the network throughput, higher intensity of high-priority traffic drives the network faster to saturation. Furthermore, better network stability and fairness among different traffic categories can be achieved when the dominant traffic has low priority.

Pilot contamination in the uplink (UL) puts asymptotic limits on the downlink (DL) spectral efficiency in time-division duplex (TDD) massive multiple-input multiple-output (MIMO) systems relying on TDD channel reciprocity. Pilot contamination can be induced by the UL pilot reuse across different neighboring cells. In this paper, we show that receiver front-end impairments also contribute to pilot contamination. In particular, we consider a TDD multi-user (MU) massive MIMO orthogonal frequency-division multiplexing (OFDM) system, where either time-division multiplexed (TDM) pilots or frequency-division multiplexed (FDM) pilots are utilized for UL channel sounding. We endeavor to characterize the impacts of the residual timing offsets (TOs) and the carrier frequency offsets (CFOs) on the DL performance of the massive MIMO-OFDM system. Closed-form expressions of the asymptotic DL MU spectral efficiencies are derived in the presence of both TOs and CFOs under different scenarios. Our analytical results reveal how the residual TOs and CFOs destroy the orthogonality among the UL training sequences from different users and give rise to pilot contamination. Specifically, we show that the DL spectral efficiencies become bounded even when the number of antennas goes toward infinity. Furthermore, to alleviate the impacts of TOs and CFOs, we propose different pilot decontamination methods based on our analyses for both the TDM pilots and the FDM pilots. Numerical simulation results corroborate our analyses and designs.

Fifth generation wireless systems are expected to employ multiple antenna communication at millimeter wave (mmWave) frequencies using small cells within heterogeneous cellular networks. The high path loss of mmWave as well as physical obstructions make communication challenging. To compensate for the severe path loss, mmWave systems may employ a beam alignment algorithm that facilitates highly directional transmission by aligning the beam direction of multiple antenna arrays. This paper discusses a mmWave system employing dual-polarized antennas. First, we propose a practical soft-decision beam alignment (soft-alignment) algorithm that exploits orthogonal polarizations. By sounding the orthogonal polarizations in parallel, the equality criterion of the Welch bound for training sequences is relaxed. Second, the analog beamforming system is adapted to the directional characteristics of the mmWave link assuming a high Ricean K-factor and poor scattering environment. The soft-algorithm enables the mmWave system to align innumerable narrow beams to channel subspace in an attempt to effectively scan the mmWave channel. Thirds, we propose a method to efficiently adapt the number of channel sounding observations to the specific channel environment based on an approximate probability of beam misalignment. Simulation results show the proposed soft-alignment algorithm with adaptive sounding time effectively scans the channel subspace of a mobile user by exploiting polarization diversity., Comment: 12 pages, 9 figures

In this work we study the capacity of multi-user multiple-input multiple-output (MU-MIMO) downlink channels with codebook-based limited feedback using real measurement data. Several aspects of MU-MIMO channels are evaluated. Firstly, we compare the sum rate of different MU-MIMO precoding schemes in various channel conditions. Secondly, we study the effect of different codebooks on the performance of limited feedback MU-MIMO. Thirdly, we relate the required feedback rate with the achievable rate on the downlink channel. Real multi-user channel measurement data acquired with the Eurecom MIMO OpenAir Sounder (EMOS) is used. To the best of our knowledge, these are the first measurement results giving evidence of how MU-MIMO precoding schemes depend on the precoding scheme, channel characteristics, user separation, and codebook. For example, we show that having a large user separation as well as codebooks adapted to the second order statistics of the channel gives a sum rate close to the theoretical limit. A small user separation due to bad scheduling or a poorly adapted codebook on the other hand can impair the gain brought by MU-MIMO. The tools and the analysis presented in this paper allow the system designer to trade-off downlink rate with feedback rate by carefully choosing the codebook.

We investigate the reliability of radio channel simulators in predicting channel responses throughout a well-specified environment. Indoor environments for which the geometric layout and material properties of surfaces are known lend themselves to such site-specific simulation. We assess the performance of this approach by comparing its predictions with measurements in a specific static environment. The good agreement on path loss, Ricean K-factor and RMS delay spread, over the set of paths measured and simulated, suggests that a well-designed radio simulator can be used reliably to predict system behavior. Typically, wireless channel models obtained through this or similar techniques do not capture the temporal variability in the channel response due to people movement in the environment. We treat the time-varying part of the channel response using stochastic processes. Using channel sounding experiments for several typical office scenarios, we show that autoregressive processes can be used to model the time-varying tap gains for several different motion scenarios.

We develop in this paper an Alamouti-type space- time (ST) block code that employs orthogonal M-ary FSK (MFSK) modulation. It is demonstrated that every transmitted symbol in the proposed ST-MFSK scheme can be treated as an implicit pilot symbol, and consequently the receiver can attain close to ideal coherent bit-error performance even in the absence of an explicit channel sounding signal. In addition to employing the popular block-wise static fading model for performance evaluation, we also study the implications of a more realistic fading model whereby the fading gains are allowed to vary from one subinterval to another within a ST code block. It was discovered that an iterative channel estimation strategy has to be adopted to mitigate the cross-talk associated with this refined channel model. The iterative channel estimator, in conjunction with a companion minimum mean square error detector, or a companion maximum likelihood detector, enable the receiver to obtain a bit-error performance similar to that obtained under the block-wise static fading model. In comparison with differential ST-MPSK, the two approaches yield similar error performance in a static fading environment. However, the error performance of differential ST-MPSK deteriorates rapidly as the fade rate increases whereas the performance of ST-MFSK is almost independent of the fade rate. In terms of implementation complexity and delay of the receiver, the proposed coherent ST- MFSK scheme and differential ST-MPSK are very similar, as both approaches employ digital filters of similar lengths. The findings in this paper suggest that despite its larger bandwidth requirement, ST-MFSK is a very attractive alternative space-time signaling format.