Hector Carrasco

On Adaptive BICM with Finite Block-Length and Simplified Metrics Calculation

In this paper we present a novel performance analysis of bit-interleaved coded modulation (BICM) over AWGN channel for different modulation schemes considering a finite block length and suboptimal metrics calculation. Using the known BICM capacity analysis as well as the Gallager bound, we show that among many known modulations, only three M-ary quadrature amplitude modulation (QAM) are necessary to ensure the maximization of the channel capacity. We analyze the maximum throughput and the switching thresholds of the system, and we compare the theoretical analysis with simulation results of practical strong codes, i.e., turbo codes. We show that if the modulation and code are well adapted, the optimal throughput decreases by less than 1% when suboptimal metrics are used. We show also, that the turbo codes throughput is within 1.5 dB of the new bound, and the SNR switching thresholds, are shifted by 1.5 dB with respect to the theoretical analysis.

Wireless Access Channels with Near-Ground Level Antennas

In this work we present an empirical study of the added propagation losses that may be associated with providing fixed wireless service from near-ground base-stations to homes in a suburban environment. We present results for various types of environments, classified according to the existence of obstructions in the propagation path and the choice of outdoor-outdoor or outdoor-indoor service. Our results indicate that while on average the additional path-losses associated with lowering the base antenna are relatively small, the variance of these losses will increase at near-ground level, particularly in obstructed links. This has as a result that the power margin required for high availability of a near-ground base antenna may be quite significant.

An Empirical Study of the Achievable Rates of Several Indoor Network-MIMO Techniques

This paper presents an empirical study of the achievable data rates of network multiple-input multiple-output (MIMO) techniques including zero-forcing (ZF), zero-forcing dirty paper coding (ZF-DPC) and dirty paper coding (DPC) using actual 4-by-4 indoor wireless channel measurements at 3.5 GHz. Their performances are contrasted with those of conventional techniques, in which either the base stations are not coordinated (NC), or their interference is avoided using frequency division (FD) multiplexing. The measurements were taken in aisle-to-office and large unobstructed hall scenarios. The study of these results reveals that, at high signal-to-noise ratios (SNRs), DPC and ZF-DPC can yield more than a three-fold increase in attainable data rates when compared to NC and FD. The gains obtained using ZF are smaller, but still significant. At low SNRs the system is noise-(rather than interference-) limited, and only DPC exhibits gains. The evaluations in this paper also show that collaborative systems such as DPC can benefit from interference-prone environments to yield increased transmission capacity. With regard to the propagation channel, the classical log-normal plus Rayleigh/Ricean fading model, with parameters fitted to the scenario type, was found to be good at predicting the statistics of the achievable data rates of all the strategies considered.

Empirically based study of channel characteristics with near-ground base-stations

Based on outdoor-indoor measurements we study the channel characteristics of wideband multiple-input-multiple-output (MIMO) wireless access with near-ground base-stations (BS). These provide broadband service to indoor nomadic users in suburban residential environments. We compare near-ground channel characteristics with those of a lamp-post BS for line-of-sight (LOS) and non-line-of-sight (NLOS) conditions. Essentially path-loss models, spectral efficiencies for 2×2 and 4×4 MIMO systems, delay spreads and coherence bandwidth are evaluated. We found that in general lowering the BS array has a detrimental effect on path-loss and efficiencies. Up to 50% less efficiency was observed in the NLOS channel, and path-losses are moderately greater for both LOS and NLOS conditions. Our investigation also shows differences in the power delays profiles when varying the array height.

Subwavelength resolution of conical Fresnel zone lens

Compared to the bulky and heavy ordinary lens the Fresnel zone (FZ) plate has the advantage of being thin, light and easy to manufacture. The ordinary lens, however, has a broadband and effective focusing. For a better FZ lens efficiency the next techniques are employed: subzone phase-correcting, curved (3D) zone surface or both put together. Here the second technique is illustrated in a Conical Fresnel Zone (CFZ) lens of Metal rings (Soret-type CFZM.υ.α lens), and both techniques are applied in a CFZ lens of Dielectric rings (Wood-type CFZD.υ.α lens) rings, where υ is the number of rings and α is the cone half-opening angle.

Double-band backfire antenna for low-terahertz frequencies

Double-band operation in the long backfire antenna (BFA) is revealed and examined numerically. The studied low-terahertz BFA radiates efficiently in two near-by frequency bands around 216 GHz and 236 GHz. At these resonant frequencies the BFA input gain is 19.9 dBi and 21.9 dBi, respectively.

Empirical gains achievable with low altitude remote radio heads in wireless urban links

We present empirical results on the achievable gains stemming from the use of wireless remote radio heads (RRH) in a typical urban environment. Our work is based on simultaneous path-loss measurements of the base station and RRH links to outdoor street level users. We statistically characterize the increase in received power, when a RRH is added to improve the coverage achieved by a base station. We consider diverse expected coverage areas for the mobile terminal, evaluating the effect of RRH position with respect to the intended users. We also compare the power gains that would be obtained in practice from combining the signals from the base with those of the RRH, using schemes such as selection combining and maximum ratio combining. We conclude that under practical conditions, the benefits of using RRHs will depend very strongly on the existence of line-of-sight links between the RRH and the intended users. For RRHs placed at low heights, below the clutter, only users in a street-canyon position with respect to the RRH will obtain a significant benefit. Our data also shows that the gains in signal-to-noise ratio achieved when using maximum ratio combining are only marginally better than those of the much simpler selection combining.

Effect of pedestrian traffic on fade statistics of fixed wireless links in public spaces

This study presents an empirical characterisation of the wireless channel observed by static users in settings that correspond to populated open public spaces, where wireless services are increasingly being deployed. Link stability is analysed in terms of the temporal K-factor of the Ricean distribution. Second-order statistics were parameterised in terms of the Doppler shift, coherence time and level crossing rates. The authors found that the volume of pedestrian traffic had a very significant effect on the temporal K-factor, overshadowing the influence of line-of-sight or lack of it in the link. On the other hand, the volume of pedestrian traffic in the environment had no detectable effect on the second-order statistics of the channel.