Based on measurements for a street-canyon-type femto-cell, we compare the downlink spectral efficiencies of various intra-cell interference mitigation systems. This includes zero-forcing (ZF), regularized ZF (RZF), and dirty-paper coding (DPC). As a reference for comparison, we consider subsectorization and time-division multiple access (TDMA). Outof-cell interference, treated as additive Gaussian noise, is included in two modes: 1) all cells transmit simultaneously and 2) neighboring cells transmit in alternated time slots. We find that ZF, RZF, and DPC can offer increases in spectral efficiency over subsectorization and TDMA by factors of ~4 under interference-limited conditions. This contrasts with the estimated gains of only ~2 when using a standard 3GPP mode instead of our measured data. This difference is due to the greater inter-cell isolation that characterizes our street-canyon-type test environment. We find that RZF achieves over 80% of the DPC's spectral efficiency under virtually all operating conditions.

We present an empirically based model for the gain of an indoor antenna array. This corresponds to the re-parametrization of the Greenstein-Erceg model, as applied to 5 m-50 m narrowband outdoor-indoor links. Our model is applicable to a few adjacent tones in an OFDM femtocell system such as LTE. We find that as much as 14 out of 16 dB of antenna-gain are attainable even in rich indoor scattering conditions. At the same time, no larger fade margin is required when using the array vs. the omnidirectional antenna. To provide a reference for the observed results we consider a simple propagation model, which is analyzed theoretically and via simulation. This model is found to match our empirical results very well.

We present empirical results on the achievable cellular system throughput gains stemming from the use of remote radio heads (RRH) in an urban environment. Our work is based on simultaneous path-loss measurements of the base station (BS) and RRH links to outdoor street-level users. We calculated the increase in received power, when a RRH is added to improve the coverage achieved by a BS. We consider diverse settings for the RRH and diverse expected coverage areas for the mobile station (MS), evaluating the effect of RRH height and position with respect to the intended users. We also compare the power gains that would be obtained in practice from combining schemes such as selection combining (SC) and maximum ratio combining (MRC). 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 MRC are only marginally better than those of the much simpler SC.

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.

Americanae nace como un proyecto conjunto que surge dentro de la Red Europea de Información y Documentación sobre América Latina (REDIAL), y que ha afrontado la Biblioteca de la Agencia Española de Cooperación Internacional para el Desarrollo (AECID). Esta nueva biblioteca virtual hace más accesibles los libros digitales de tema americanista a los investigadores y usuarios interesados de cualquier parte del mundo.

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.

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.

We analyze the throughput achievable by a truncated Hybrid ARQ protocol (HARQ) using incremental redundancy (IR) when transmitting over a block-fading channel whose state is unknown at the transmit end. We allow the transmission rates to vary, optimize them, and show that such a variable-rate HARQ-IR provides gains with respect to a fixed-rate transmission in terms of increased throughput and decreased average number of transmissions.

We present a summary of results based on several years of measurements for urban fixed wireless (FW) channels. Our results are aimed at providing reference data for practical applications in FW services. We discuss temporal stability and fade dynamics of outdoor-outdoor single-input single-output (SISO) channels with and without spatial diversity, a relevant aspect when considering the types of modulations that such a channel can support. Our main findings are that nearby vehicular traffic is the main source of temporal variations, but that in most practical situations temporal fades are very shallow. Spatial fades are quite significant and are stable over time. Diversity has the twofold effect of mitigating these fades and increasing temporal stability. We also present practical results on outdoor- indoor multiple-Input multiple-output (MIMO) channels. Based on empirical data we contrast spectral efficiencies for diverse types of systems, ranging from those that only use multiple antennas for beamforming to those that take full advantage of channel state information. We find that for low signal to noise ratio (SNR), beamforming achieves in practice spectral efficiencies comparable to that of MIMO and that it retains a significant proportion of such spectral efficiency at higher SNR, with the benefit of considerably lower complexity.

In this paper we develop closed form approximations for the probability density function (PDF) of the reliability metrics (L-values) in bit-interleaved coded modulation (BICM). The expressions are valid for M2-ary quadrature amplitude modulations (M2-QAM) with binary reflected Gray mapping when the metrics are calculated using the so-called max-log approximation. Based on the developed expressions, we also propose two simple Gaussian mixture approximations that are analytically tractable. We apply our developments to efficiently calculate the BICM capacity, and to develop bounds on the coded bit-error rate when a convolutional code is used. The coded performance of a hybrid automatic repeat request (HARQ) based on constellation rearrangement is also evaluated.

MIMO systems are usually associated with high scattering isotropic propagation while the use of directive antennas is associated with free space conditions. We found outdoor-indoor channels to be in between these two extremes, in the sense that we observed directivity - and - MIMO gain, for the same ensemble of channels. Our observation is based on measurements with directive (8 dB) and dipole antennas. Median MIMO capacities were found to be about 80% of the ideal (Rayleigh i.i.d.), at 5 dB signal to noise ratio (SNR), for both types of antennas. Using properly aimed directive antennas, the SNR was found on average to be 5.4 dB above that obtainable with dipoles, somewhat less than the 7 dB antenna gain difference. Thus, isotropic propagation, which would have negated directivity gains, cannot be justified in general. We empirically established that aiming for largest received power is the best array pointing strategy with directive antennas. Combining MIMO processing and angular search resulted on average in gains of 70% over the median capacities obtained with dipoles. Therefore it may in some cases be convenient to arrange subgroups of antennas for beamforming, and then process the thus reduced number of radio channels for MIMO gain.

We present an empirically based comparative study of spectral efficiency for a variety of transmission systems applicable to a fixed or repositionable wireless environment, in the context of Wi-Fi, WiMAX or MuniNet systems. A narrowband 4times4 multiple input multiple output (MIMO) channel sounder was constructed and a series of outdoor to indoor measurements were carried out, in multiple locations and with different array configurations. The channel measurements were used to compute the efficiency of different systems that could be deployed in such scenarios, ranging from a full MIMO system with perfect channel state information (CSI) at both ends to simple diversity schemes such as classical beamforming. We show comparisons of efficiency for the different transmit/receive configurations operating in a representative variety of locations. Our results indicate that for low values of signal to noise ratio (SNR), in the range of 5 dB, such as found in strong interference scenarios, simple schemes can achieve median spectral efficiencies as high as 80% of that of MIMO with complete CSI.

We present an empirically based comparative study of spectral efficiency for a variety of transmission systems applicable to a fixed or repositionable wireless environment, in the context of Wi-Fi, WiMAX or MuniNet systems. A narrowband 4times4 Multiple Input Multiple Output (MIMO) channel sounder has been constructed and a series of outdoor to indoor measurements have been carried out, in multiple locations and with different array configurations. The channel measurements have been used to compute the spectral efficiency of different systems that could be deployed in such scenarios, ranging from a full MIMO system with perfect Channel State Information (CSI) at both ends to simple diversity schemes such as classical beamforming. We show comparisons between the different transmit/receive configurations operating in a representative variety of indoor receive locations. Our results indicate that for low values of Signal to Noise Ratio (SNR, in the range of 5dB), simple schemes can achieve capacities as high as 80% of that of MIMO with complete CSI.

Flexibility in a supply chain is usually conceived as a consequence of the amplification of production, typically known as the bullwhip-effect. In this article we modeled pull, push and hybrid inventory management methods, considering an AR(1) stationary stochastic demand process. We show that for these cases, the production amplification results to be highly influenced by the flexibility policies implemented in the supply chain. In this context, we demonstrate that flexibility (measured by the adjustment capability between production and demand rates, and the ordering responsiveness) should be viewed as one of the main causes - and not a consequence - of the bullwhip effect. This is relevant for chain managers since controlling the flexibility of the chain will also let them control the amplification. We also found an analytical relation between amplification and adjustment degree, showing that the amplification acceptability constraint introduced by Muramatsu can be accomplished by controlling the flexibility dimension described before. Weak and strong flexibility conditions are proposed. We show that the weak condition ultimately does not depend on the demand autocorrelation coefficient, whilst in the strong condition the ordering responsiveness, measured by lead time, may be modulable by the adjustment degree, both in push and hybrid methods. Thus, the supply chain manager is enabled to increase or decrease the lead time just increasing or decreasing the adjustment, respectively, always satisfying the Muramatsu criteria.

In general, MIMO systems are associated with high scattering isotropic propagation while the use of directive antennas is associated with free space or narrow angular spread conditions. We found outdoor-indoor channels to be in between these two extremes, in the sense that we observed directivity - and - MIMO gain, for the same ensemble of channels. Our observation is based on measurements with directive (8 dB gain) and dipole antennas. Median MIMO capacities were found to be 80% of the ideal (Rayleigh i.i.d.), at 5 dB signal to noise ratio (SNR), with no significant difference resulting from the choice of antenna. On the other hand, using properly aimed directive antennas, the SNR was found on average to be 5.4 dB above that obtainable with dipoles, somewhat less than the 7 dB antenna gain difference. Thus, isotropic propagation, which would have negated directivity gains, cannot be justified in general. The combination of MIMO processing and angular search with directive antennas resulted on average in gains of 69% over the median capacities obtained with dipoles. Since MIMO and directivity can combine effectively, it may in some cases be convenient to arrange subgroups of antennas for beamforming, and then process the thus reduced number of radio channels for MIMO gain.

In this paper we develop formulas for the probability density function (PDF) of the reliability metrics in bit- interleaved coded modulation (BICM) for arbitrary M2-ary quadrature amplitude modulation (M2-QAM) with Gray mapping. The advantage of this approach over an entirely general one proposed previously, is that the resulting formulas are obtained without resorting to any algorithmic steps. We also propose Gaussian mixture approximations that are analytically tractable, and we analyze their accuracy when evaluating uncoded bit error rate (BER) and BICM capacity.

The temporal behavior of fixed wireless (FW) links is usually described in terms of fade depth distribution, level crossing rates and average fade duration. This knowledge is needed for the optimization of transceiver parameters such as modulation format, frame length, automatic gain control dynamics, etc. We present analytic and empirical results on the level crossing rate (LCR) and average fade duration (AFD) in FW links. We show that under certain conditions the LCR and AFD do not depend on the shape of the Doppler spectrum and that with a proper choice of parameters, expressions that have been justified for mobile wireless links are also applicable to FW. Results of extensive measurements validate the assumptions that sustain our model

Diversity techniques can be very effective against fading. For fixed wireless terminals, antenna diversity increases spatial coverage by reducing the likelihood of all antennas being at a faded location. At the same time, temporal fade depth is reduced, as time variations are normally uncorrelated for the antennas. The analysis of temporal fade in fixed wireless applications must take into account that the standard assumption made in mobile applications, namely equal average power across the diversity branches, is no longer justified, as a given antenna may be at a faded location for an extended time. In this paper we quantify the benefits of diversity receivers for fixed wireless links in terms of reductions in temporal fluctuation and outages, including the effect of power unbalance among diversity branches which is typical for these links. We specifically consider the case where the time varying multipath fade components are independent but identically distributed random processes for all branches, while due to spatial fades, the time invariant components may be different. A statistical description that includes the average level crossing rate (LCR) of the output signal of a maximal ratio combiner is derived in closed form. Simplified expressions, applicable for the case of links with high Ricean X-factors, as occurs in fixed wireless, are derived for both maximal ratio and equal gain combining. It was found that the combiner output statistics essentially depend on the value of sum of the branch K-factors, with the number of branches having virtually no effect. Extensive measurements at 3.5 GHz are used to compare, and found to match well, the theoretical expressions derived in this work. The results are useful for link design, by providing theoretical tools and empirical data for the calculation of the fade margin reduction and fade duration that results from the use of diversity combiners in the case of fixed wireless links with unequal power across the diversity branches.

We present a statistical model for the dynamics of the envelope of the received signal in narrowband urban fixed wireless links, with applications to the performance evaluation of diversity receivers. The model is based on a straightforward analytical treatment based on simplifying assumptions. The assumptions are validated by an extensive empirical study of a broad range of urban settings representative of realistic deployment scenarios for fixed wireless service. The results are useful for system designers and for the efficient selection of error control mechanisms that need to adjust to the dynamics of the propagation channel. We find that in practice, the autocovariance of the combiner branch input signals are similar and also similar to the autocovariance of the output of the diversity combiners.