Outdoor-to-indoor signal propagation poses significant challenges to millimeter-wave link budgets. To gain insight into outdoor-to-indoor millimeter-wave at 28GHz, we conducted an extensive measurement campaign consisting of over 2,200 link measurements in West Harlem, New York City, covering seven highly diverse buildings. A path loss model constructed over all measured links shows an average of 30dB excess loss over free space at distances beyond 50m. We find the type of glass to be the dominant factor in outdoor-to-indoor loss, with 20dB observed difference between grouped scenarios with low-and high-loss glass. Other factors such as the presence of scaffolding, tree foliage, or elevated subway tracks, as well as difference in floor height are also found to have a 5–10dB impact. We show that for urban buildings with high-loss glass, outdoor-to-indoor downlink capacity up to 400Mb/s is supported for 90% of indoor customer premises equipment by a base station up to 40m away. For buildings with low-loss glass, such as our case study covering multiple classrooms of a public school, downlink capacity over 2.8/1.4Gb/s is possible from a base station 57/133m away within line-of-sight. We expect these results to help inform the planning of millimeter-wave networks targeting outdoor-to-indoor deployments in dense urban environments, as well as provide insight into the development of scheduling and beam management algorithms.

Millimeter-Wave (mmWave) has great potential to provide high data dates given its large available bandwidth, but its severe path loss and high propagation sensitivity to different environmental conditions make deployment planning particularly challenging. Traditional slope-intercept models fall short in capturing large site-specific variations due to urban clutter, terrain tilt or foliage, and ray-tracing faces challenges in characterizing mmWave propagation accurately with reasonable complexity. In this work, we apply machine learning (ML) techniques to predict mmWave path loss on a link-to-link basis over an extensive set of 28 GHz field measurements collected in a major city of USA, with over 120,000 links from both urban and suburban scenarios, with over 40 dB variation for links at similar distances. Either raw environmental profile (terrain+clutter) of each link or 8 selected expert features are used to either directly predict path loss via regression-based approaches or predict the best performing option out of a pool of theoretical/empirical propagation models. Our evaluation shows that Lasso regression provides the best path loss prediction with a performance (RMSE 8.1 dB) comparable to the per-site slope-intercept fit (RMSE 8.0 dB), whereas model selection method achieves 8.6 dB RMSE, both are significantly better than the best a posteriori 3GPP model (UMa-NLOS, 10.0 dB).

In the US, people spend 87% of their time indoors and have an average of four connected devices per person (in 2020). As such, providing indoor coverage has always been a challenge but becomes even more difficult as carrier frequencies increase to mmWave and beyond. This paper investigates the outdoor and outdoor-indoor coverage of an urban network comparing globally standardized building penetration models and implementing models to corresponding scenarios. The glass used in windows of buildings in the grid plays a pivotal role in determining the outdoor-to-indoor propagation loss. For 28 GHz with 1 W/polarization transmit power in the urban street grid, the downlink data rates for 90% of outdoor users are estimated at over 250 Mbps. In contrast, 15% of indoor users are estimated to be in outage, with SNR <−3 dB when base stations are 400 m apart with one-fifth of the buildings imposing high penetration loss (∼ 35 dB). At 3.5 GHz, base stations may achieve over 250 Mbps for 90% indoor users if 400 MHz bandwidth with 100 W/polarization transmit power is available. The methods and models presented can be used to facilitate decisions regarding the density and transmit power required to provide high data rates to majority users in urban centers.

Outdoor-to-indoor (OtI) signal propagation further challenges link budgets at millimeter-wave (mmWave). To gain insight into OtI mmWave at 28 GHz, we conducted an extensive measurement campaign consisting of over 2,000 link measurements in West Harlem, New York City, covering seven highly diverse buildings. A path loss model constructed over all links shows an average of 30 dB excess loss over free space at distances beyond 50 m. We find the type of glass to be the dominant factor in OtI loss, with 20 dB observed difference between clustered scenarios with low- and high-loss glass. Other factors, such as difference in floor height, are found to have an impact between 5--10 dB. We show that for urban buildings with high-loss glass, OtI data rates up to 400 Mb/s are supported for 90% of indoor users by a base station (BS) up to 49 m away. For buildings with low-loss glass, such as our case study covering multiple classrooms of a public school, data rates over 2.8/1.4 Gb/s are possible from a BS 68/175 m away when a line-of-sight path is available. We expect these results to be useful for the deployment of OtI mmWave networks in dense urban environments and the development of scheduling and beam management algorithms.

The wireless industry is moving towards developing smart cellular architectures that dynamically adjust the use of the network elements according to the service demand, and automating their operations in order to minimize both capital expenditure (CAPEX) and operation expenditure (OPEX). This involves developing efficient and unsupervised radio access network (RAN) planning, which has a direct impact on the system performance and CAPEX. This intelligent cellular planning aims at providing the base stations (BSs) configurations (e.g., coverage, user associations and antenna radiation pattern) that minimize the number of deployed BSs and meet the requirements in terms of coverage and capacity. The cellular planning optimization problem has been shown to be complex and non-scalable. Moreover, most of the existing cellular planning techniques result in an over or under provisioning architecture. Motivated by the above, we propose in this paper a novel and efficient unsupervised planning process. We make use of statistical machine learning (SML) to solve the problem at hand. The core idea of SML is that the planning parameters are treated as random variables. The parameters that maximize the corresponding joint probability distribution, conditioned on observations of users' positions, are learned or inferred using Gibbs sampling theory and Bayes' theory. To apply this theory to the planning problem, we make significant efforts to properly formulate the problem to be able to incorporate the constraints into the inference process and extract the planning parameters from the inferred model. Through several numerical examples, we compare the performance of the proposed approach to clustering-based and optimization-based existing planning approaches, and demonstrate the efficacy of our approach. We also demonstrate how our approach can leverage existing cellular infrastructures into the new design.

Accurate path gain models are critical for coverage prediction and radio frequency (RF) planning in wireless communications. In many settings irregular terrain induces blockages and scattering making it difficult to predict the path gain. Current solutions are either computationally expensive or slope-intercept fits that do not capture local deviations due to terrain variation, leading to large prediction errors. We propose to use machine learning to learn path gain based on terrain elevation as features. We implement different neural network architectures with dense and convolutional layers that could include effects difficult to describe with traditional models (e.g. back scatter). We test our framework on an extensive set of measured path gain data and consistently predict with 5 dB Root Mean Squared Error, an 8 dB improvement over traditional slope-intercept solutions.

Delivering Gbps high user rate over long distances (~1 km) is challenging, and the abundant spectrum available in millimeter wave band cannot solve the challenge by its own due to the severe path loss and other limitations. Since it is economically challenging to deploy wired backhaul every few hundred meters, relays (e.g., wireless access points) have been proposed to extend the coverage of a base station, which has wired connection to the core network. These relays, deployed every few hundred meters, serve the users in their vicinity and are backhauled to the base station through wireless connections. In this paper, the wireless-relayed backhaul design has been formulated as a topology-bandwidth-power joint optimization problem, and the influence of path loss, angular spread, array size, and RF power limitation on the user rate has been evaluated. It has been shown that for a linear network deployed along the street at 28 GHz, when high joint directional gain (50 dBi) is available, 1 Gb/s user rate within cell range of 1 km can be delivered using 1.5 GHz of bandwidth (using single polarization antennas). The user rates drop precipitously when joint directional gain is reduced, or when the path loss is much more severe. When the number of RF chains is limited, the benefit of larger arrays will eventually be surpassed by the increased channel estimation penalty as the effective beamforming gain saturates owing to the channel angular spread.

Great increase in wireless access rates might be attainable using the large amount of spectrum available in the millimeter wave (mmWave) band. However, higher propagation losses inherent in these frequencies must be addressed, especially at ranges beyond 100 m and in non-line-of-sight (NLOS) settings. In contrast to the interference limited legacy cellular systems, where using more bandwidth is favorable, to use wider bandwidth for mmWave channels in noise limited settings may be ineffective or even counterproductive when accounting for channel estimation penalty. In this paper, we quantify the maximum beneficial bandwidth for mmWave transmission in some typical deployment scenarios where pilot-based channel estimation penalty is considered assuming a minimum mean square error channel estimator at the receiver. We find that, under I.I.D. block fading model with coherence time Tcand coherence bandwidth Bc, for transmitters and receivers equipped with a single antenna, the optimal (rate maximizing) signal-to-noise-ratio is a constant that only depends on the product BcTc, which measures the channel coherence and equals the average number of orthogonal symbols per each independent channel coefficient. That is, for fixed channel coherence BcTc, the optimal bandwidth scales linearly with the received signal power. Under 3GPP urban micro NLOS path loss model with coherence time Tc= 5 ms and coherence bandwidth Bc= 10 MHz, using 52-dBm equivalent isotropic radiated power (EIRP) at the transmitter and 11-dBi antenna gain at the receiver, the maximum beneficial bandwidth at 28 (resp. 39) GHz is less than 1 GHz at a distance beyond 210 (resp. 170) m with maximum throughput about 200 Mbps, and less than 100 MHz beyond 400 (resp. 310) m with maximum throughput about 20 Mbps. At EIRP of 85 dBm, corresponding to the FCC limit of 75 dBm per 100 MHz, 1 Gbps rate can be delivered using 1-GHz bandwidth up to 860 (resp. 680) m.

Theory of normal mode propagation in a line-of-sight street scenario is extended to include propagation into buildings through coupling to a diffuse indoor field. Signal strength predictions are in close agreement with measurements, producing 2 dB and ≤ 3.5 dB root-mean-square model-data difference, in line-of-sight and outdoor-indoor scenarios, respectively. The full 3-D field model predicts actual signal directions and antenna correlations as a function of range, important for evaluating performance of directional antennas and spatial diversity. Only minimal description of the environment is needed, i.e., street width and representative building wall properties, without any interior details.

Owing to its low spectral decay rate, conventional orthogonal frequency-division modulation (OFDM) is unsuitable for supporting asynchronous access, user-specific subcarrier spacing and symbol period, and spectrum aggregation, all of which are envisioned for fifth-generation (5G) cellular networks. This has spurred recent work on designing new multi-carrier waveforms for 5G. In this paper, a unified signal model for OFDM is presented, which encompasses variants employing either a cyclic prefix (CP) or a zero postfix (ZP) as the guard interval technique, and either windowing or filtering to increase the spectral decay rate of the transmitted signal; both windowing-based and filtering-based receivers are also accommodated. Sufficient conditions on the channel impulse response and any windowing/filtering are given for the avoidance of inter-symbol and intra-symbol interference. Numerical results are then presented to show that, in a multiuser system operating asynchronously and with user-specific subcarrier spacing and symbol period, filtering and windowing yield substantial and comparable rate gains over standard OFDM when implemented at both transmitter and receiver; these gains can be realized with no performance degradation relative to standard OFDM (at SNRs typically of interest) when the system operates synchronously and all users have identical subcarrier spacing and symbol period. Further, CP-OFDM and ZP-OFDM have near-identical rate-versus-SNR performance.

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.

For wireless systems in which randomly arriving devices attempt to transmit a fixed payload to a central receiver, we develop a framework to characterize the system throughput as a function of arrival rate and per-device data rate. The framework considers both coordinated transmission (where devices are scheduled) and uncoordinated transmission (where devices communicate on a random access channel and a provision is made for retransmissions). Our main contribution is a novel characterization of the optimal throughput for the case of uncoordinated transmission and a strategy for achieving this throughput that relies on overlapping transmissions and joint decoding. Simulations for a noise-limited cellular network show that the optimal strategy provides a factor of four improvement in throughput compared with slotted ALOHA. We apply our framework to evaluate more general system-level designs that account for overhead signaling. We demonstrate that, for small payload sizes relevant for machine-to-machine (M2M) communications (200 bits or less), a one-stage strategy, where identity and data are transmitted optimally over the random access channel, can support at least twice the number of devices compared with a conventional strategy, where identity is established over an initial random-access stage and data transmission is scheduled.

Propagation theory is developed to get analytic expressions for both guided and diffusely scattered 3-D fields in common indoor environments, with access points (APs) placed along a hallway and mobile nodes in the rooms. Resulting predictions agree with measurements of both path loss and single-user multiple-input-multiple-output link capacity. In particular, the model reproduces the observed reduction in normalized channel capacity with increasing range. This effect, occurring in a common indoor environment, can be beneficial. Interfering APs, which tend to be further away, occupy channel dimensions corresponding to low-order hallway modes, causing total interference to concentrate in a small subset of the channel dimensions, in a manner reminiscent of interference alignment. The channel for the desired signal has a greater number of effective degrees of freedom (EDOFs) due to a shorter distance from its serving AP. The disparity in the number of EDOFs between the signal and the interference leads to higher rates, which can be further increased through water filling applied individually to each link, with no interlink coordination. The resulting rates are on the order of 30% greater than would be anticipated in range-independent scattering models.

We consider a wireless time-slotted random access channel where user arrivals are characterized by a Poisson process. Each user comes with a fixed payload, which has to be transmitted in the slot in which it arrives. If the transmission is successful, the user leaves the system, else it is dropped. The receiver and users are assumed to have knowledge of the arrival rate λ, but they are not aware of the actual number of users simultaneously attempting to communicate during a given time slot. In contrast to a conventional slotted ALOHA-based strategy where the channel is partitioned into orthogonal subchannels and each user communicates on a randomly chosen subchannel, we propose a novel strategy whereby users transmit simultaneously over the entire channel resource and the receiver jointly decodes the transmissions. Under the proposed strategy, neither users nor the receiver have prior knowledge of the active user set. Our analysis concretely demonstrates that the proposed strategy is optimal in terms of maximizing the average throughput among all uncoordinated strategies. Numerical results show that the proposal provides an order of magnitude throughput improvement compared to slotted ALOHA in a single-cell environment under a 10% maximum outage constraint.

Heterogeneous cellular networks are composed of a mixture of macro and small (pico) cells embedded in the macro network. A greedy strategy is proposed to maximize network capacity. Picos placement and either macro or pico transmit power are optimized jointly, as picos are deployed. The scaling of area spectral efficiencies (ASE) as pico densities are increased is studied. Median ASEs grow linearly with density and are comparable to cell-splitting given fixed pico power -20dB EIRP relative to the macro. The effectiveness of the pico at such low power is due in part to the improvement in area coverage of an omni vs. a 3 sectored antenna.

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.

The growing popularity of Machine-to-Machine (M2M) communications in cellular networks is driving the need to optimize networks based on the characteristics of M2M, which are significantly different from the requirements that current networks are designed to meet. First, M2M requires large number of short sessions as opposed to small number of long lived sessions required by the human generated traffic. Second, M2M constitutes a number of battery operated devices that are static in locations such as basements and tunnels, and need to transmit at elevated powers compared to the traditional devices. Third, replacing or recharging batteries of such devices may not be feasible. All these differences highlight the importance of a systematic framework to study the power and energy optimal system design in the regime of interest for M2M, which is the main focus of this paper. For a variety of coordinated and uncoordinated transmission strategies, we derive results for the optimal transmit power, energy per bit, and the maximum load supported by the base station, leading to the following design guidelines: (i) frequency division multiple access (FDMA), including equal bandwidth allocation, is sum-power optimal in the asymptotically low spectral efficiency regime, (ii) while FDMA is the best practical strategy overall, uncoordinated code division multiple access (CDMA) is almost as good when the base station is lightly loaded, (iii) the value of optimization within FDMA is not significant in the regime of interest for M2M.

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.

Diffusion theory is found to provide a simple yet accurate expression for the average received radio power loss indoors in non-line-of-sight propagation. Effective specific absorption is the sole parameter. Agreement with extensive indoor power measurements is found at all ranges. Use of the standard power-law to describe the data is found to lead to power exponents that vary from near 2 at short ranges to around 11 at 100 m. Extending the diffusion model to account for variation in scatterer density provides a stochastic mechanism explaining the log-normal nature of observed average power variations, usually termed "shadow fading". The standard deviation of power variation is found to increase with range (as r1/2at short ranges), in agreement with measurements and unlike accepted models. Exponential absorption in diffusion is determined to have a large impact on the signal-to-interference-and noise behavior, materially affecting conclusions on the value of increasing cell density as a method of increasing wireless network capacity.

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.

In current wireless base station solutions it is becoming common to physically separate baseband units and radio subsystems. In many wireless technologies this architecture requires allocation of significant transport network resources. In this paper a low-latency baseband signal compression scheme is presented. The compression scheme significantly lowers the transport data rate while maintaining low levels of signal distortion, thus resulting in a lower-cost transport network. Considering the importance of packet-based networks, a number of additional novel compression schemes are proposed. They are optimized for transport networks that implement a quality-of-service (QoS) mechanism and/or multi-link transmission. The compression schemes are parameterized such that a smooth trade-off between the required signal quality and compression performance can be achieved through operator choice of the suitable parameter values. An attractive feature of these schemes is that they can be applied to different wireless technologies, with appropriate parameter settings, without disrupting the present architecture. The proposed solutions will lead to a cost-effective implementation of collocated and distributed network-centric baseband processing, coordinated multi-point (CoMP) and/or distributed antenna system (DAS) which are critical topics for the entire wireless telecommunications industry and infrastructure.

We consider a flat fading multiantenna downlink system with a large number of users where the objective is to deliver equal rates to nonoutage users with a low complexity. We show that in the limit of a large number of users, a zero-forcing beamforming strategy combined with a low complexity user grouping algorithm based on a semi-orthogonal user selection achieves asymptotically optimal performance, with respect to an upper bound that can be achieved when no interference is present among users.

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 compare the performance of half-duplex relays in downlink cellular system against a baseline system without relays. We simulate the performance of (i) a collaborative power addition scheme, where the relay boosts the received power (P-CPA) at the mobile locations, and (ii) a CPA scheme with power control (PC-CPA) at the base station and relays. Evaluations are done in the context of a 19-cell, 57-sector set-up in which each of the served users must be delivered a message. The user messages are taken to have the same size and 90% of users in the network must be served. Improvements over the baseline due to relay deployments are measured in terms of increase in common rate of users as well as power savings in terms of reduction in peak or average power transmitted by base stations. In the CPA schemes with base stations and relays transmitting at full power, the peak power saving is 1.46 dB, alternately, the throughput improvement over a 1 bit/sec/Hz baseline rate is 21%. In the PC-CPA scheme, the peak power saving is 2.6 dB and the average total power in the system can be reduced by 3 dB.

In this paper we present a new paradigm for multiuser MIMO downlink communications called Rate Approximation with significant impact on the upcoming LTE standard. Rate Approximation combines flexibility and robustness needed for reliable communications with the terminal in the downlink and taking advantage of the channel state information. The scheme responds to two major developments in the recent literature: One observation is that it is not so important to approximate the channel but rather the rate itself. The second observation is that a fixed codebook at the transmitter is often better when simultaneously the channel state information is more accurate. Both observations are incorporated in the new scheme where the transmit and feedback codebook are strictly separated and user rates are brought to the base station subject to a controlled uniform error. The new metric is amenable to further numerical optimization. Analysis and simulations show the superior performance of the scheme and, furthermore, a strong impact on upcoming cooperative schemes is expected.

This paper studies the up-link of a multi-user Multiple-Input Multiple-Output (MIMO) system under a fairly general channel model, subsuming a number of situations of relevant practical interest. Concerning the available prior information, we consider the case of Prior Channel State Information at the Transmitter (PCSIT) wherein only a statistical channel characterization is available before transmission, while we assume perfect Channel State Information at the Receiver (CSIR). Under this assumptions, we characterize the sum-capacity-achieving input covariance matrix for such a system and we also examine the low-power regime in terms of both minimum energy contrast and multiple access slope region, validating our theoretical findings through a set of numerical results.

We present a new robust feedback and transmit strategy for multiuser MIMO downlink communication systems, termed Rate Approximation (RA), and analyze its performance. The new scheme combines flexibility and robustness needed for reliable communications with the user terminal, under a limited feedback constraint. The scheme responds to two major developments in the recent literature: One observation is that it is not so important to approximate the channel but rather the rate itself. The second observation is that a fixed codebook at the transmitter is often better when simultaneously the channel state information is more accurate. Both observations are incorporated in the new scheme where the transmit and feedback codebook are strictly separated and user rates are brought to the base station subject to a controlled uniform error. The analysis provides two astonishing results. First, under perfect channel state information at the transmitter RA outperforms zeroforcing beamforming (ZFBF) for a large fraction of the practically relevant signal-to-noise ratio range for the considered operating point. Second, under a limited feedback constraint the quantization error scaling of RA is shown to be doubly exponentially in the number of feedback bits, whereas the quantization error of ZFBF with random vector quantization was shown by Jindal to scale only exponentially. Simulations sustain our analytic results showing the superior performance of the new scheme.

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.

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.

Single-user, multiuser, and network MIMO performance is evaluated for downlink cellular networks with 12 antennas per site, sectorization, universal frequency reuse, scheduled packet-data, and a dense population of stationary users. Compared to a single-user MIMO baseline system with 3 sectors per site, network MIMO coordination is found to increase throughput by a factor of 1.8 with intra-site coordination among antennas belonging to the same cell site. Intra-site coordination performs almost as well as a highly sectorized system with 12 sectors per site. Increasing the coordination cluster size from 1 to 7 sites increases the throughput gain factor to 2.5.

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.

It has been demonstrated that base station cooperation can reduce co-channel interference (CCI) and increase cellular system capacity. In this work we consider another approach by dividing the system into microcells through denser base station deployment. We adopt the criterion to maximize the minimum spectral efficiency of served users with a certain user outage constraint. In a two-dimensional hexagon array with homogeneous microcell structure, under the proposed propagation model denser base station deployment outperforms suboptimal cooperation schemes (zero-forcing) when the density increases beyond 3 - 12 base stations per km2, the exact value depending on the rules of outage user selection. However, close- to-optimal cooperation schemes (zero-forcing with dirty-paper- coding) are always superior to denser deployment. Performance of a hierarchial cellular structure mixed with both macrocells and microcells is also evaluated.

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.

The 17 papers in this special issue focus on MIMO systems and applications: field experience, practical aspects, limitations and challenges. The papers are summarized here.

In this paper we present a multiuser MIMO experimental system and the corresponding indoor measurement results that demonstrate power of distributed and coherently-coordinated downlink transmit antennas using zero-forcing (ZF) beamforming. We present experimental performance under different conditions and communication scenarios. We consider different transmit power levels, number of transmit antennas, resolution of channel state information (CSI) quantizer as well as CSI feedback channel data rates. We demonstrate significant gains even in the presence of practical impairments.

We investigate optimum zero-forcing beamforming in multiple antenna broadcast channels with per-antenna power constraints. We show that standard zero-forcing techniques, such as the Moore-Penrose pseudo-inverse, considered mainly in the context of sum-power constrained systems are suboptimal when there are per-antenna power constraints. We formulate convex optimization problems to find the optimum zero-forcing beamforming vectors. Our results indicate that optimizing the antenna outputs based on the per-antenna constraints may improve the rate considerably when the number of transmit antennas is larger the number of receive antennas. Having more transmit antennas gives rise to additional signal space dimensions that may be exploited effectively to reduce transmit power at particular antennas with limited power budget.

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

The wireless communication channel between vehicular nodes in an ad-hoc network was assessed in a comprehensive measurement and modeling effort. Nodes were equipped with roof mounted azimuthally omnidirectional antennas, and measurements were taken at both short (line-of-sight) and long ranges, at a center frequency of 2.5 GHz. The median MIMO capacity with 8 transmitters and 10 receivers (8 times10) was found to be about five times the corresponding 1times1 SISO capacity, and three times the corresponding 1times10 SIMO capacity. Wideband soundings of such channels were conducted, where the median rms delay spread was found to be 0.6 mus. Measurements, spatial correlation scales, an empirical model of pathloss, and cross- polarization properties of peer-to-peer channels in a rural area are reported.

We compare the performance between beamforming and spatial multiplexing showing in which downlink scenarios the higher performance of spatial multiplexing justify its complexity. We compute performance using readily measurable parameters such as angle spread (AS), antenna separation and signal to noise ratio (SKIR). Firstly, a semi-analytical approach relates these measurable parameters with parameters that theoretically characterize beamforming optimality such as the spatial correlation matrix first two eigenvalues and SNR. Secondly, the achieved spectral efficiency is given for beamforming and spatial multiplexing as a function of antenna separation, AS and SNR. Also, a "practical" region is given where beamforming achieves at least 90% of the spectral efficiency of spatial multiplexing.

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 quantify the ultimate performance limits of inter-cell coordinatation in a cellular downlink network. The goal is to achieve fairness by maximizing the minimum rate in the network subject to per base power constraints. We first solve the max-min rate problem for a particular zero-forcing dirty paper coding scheme so as to obtain an achievable max-min rate, which serves as a lower bound on the ultimate limit. We then obtain a simple upper bound on the max-min rate of any scheme, and show that the rate achievable by the zero-forcing dirty paper coding scheme is close to this upper bound. We also extend our analysis to coordinated networks with multiple antennas.

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.

The capacity advantage of multiple input-multiple output (MIMO) channels lays in the decomposition of the channel into several spatial sub-channels. Knowledge of the channel at the transmitter can further increase the system capacity. This effect has been theoretically demonstrated. Here we illustrate the potential of channel state information (CSI) at the transmitter using actual channel measurements of a system with 4 transmitters and 4 receivers in an indoor environment, under line-of-sight and non line-of-sight conditions, considering both infinite and finite allowable modulation sizes. We show that CSI at the transmitter is more beneficial in the case of low signal to noise ratio and in cases where the channel richness is limited. In terms of achievable spectral efficiency, 60% of the theoretical capacity can be obtained without advanced coding techniques, as long as eigen mode transmission is used, and the performance is similar for all power and rate allocation schemes that we investigate

Multiple antenna techniques for spatial multiplexing (SM) and spatial division multiple access (SDMA) are efficient ways to increase spectral efficiency. We characterize downlink fixed wireless systems using a few essential parameters—including number of antennas per link, number of sectors per cell, and frequency reuse plan—and study their impact on the system spectral efficiency and user peak rates through simulation. Characterizing the system in this way provides insight into the fundamental performance of fixed wireless systems and tradeoffs between SM and SDMA that are independent of specific air interface implementations. We conclude that for a given number of transmit and receive antennas, SM is efficient for increasing user peak rates, but SDMA is more efficient for increasing system spectral efficiency.

Abstract Coexistence of different access technologies, hierarchical cellular deployment, a wide variety of data services, requirements for transparent operation across different technologies, adaptivity to varying network conditions and mobility and quality of service (QoS) constraints introduce a number of challenges in the design of future generation systems and the specification of new air interfaces, such as efficiency and flexibility in the utilization of spectrum, dynamic resource allocation and exploitation of the multiuser diversity and reconfigurable interference management and inter‐cell coordination. In this paper, three critical issues for the design of next generation systems are addressed: (i) duplexing, (ii) scheduling and resource allocation and (iii) interference and inter‐cell coordination. A number of research directions are presented, which constitute promising potential candidates for next generation systems specification. Copyright © 2005 John Wiley & Sons, Ltd.

This paper presents a generalized multiple-input multiple-output (MIMO) channel modeling technique for link level and system level simulations. The model combines the correlation approach and wave superposition approach to achieve both accuracy and efficiency. The spatial, temporal, and frequency dispersions of the MIMO channels are implicitly modeled based on any given statistics. Polarizations, channel transitions, uplink and downlink channels, and keyhole-pinhole channels are modeled as optional modules. Theoretical justifications as well as experimental verification of the model are also presented. The proposed model can be applied to system simulations for MIMO as well as other adaptive antenna applications.

Narrowband multiple-input-multiple-output (MIMO) measurements using 16 transmitters and 16 receivers at 2.11 GHz were carried out in Manhattan. High capacities were found for full, as well as smaller array configurations, all within 80% of the fully scattering channel capacity. Correlation model parameters are derived from data. Spatial MIMO channel capacity statistics are found to be well represented by the separate transmitter and receiver correlation matrices, with a median relative error in capacity of 3%, in contrast with the 18% median relative error observed by assuming the antennas to be uncorrelated. A reduced parameter model, consisting of 4 parameters, has been developed to statistically represent the channel correlation matrices. These correlation matrices are, in turn, used to generate H matrices with capacities that are consistent within a few percent of those measured in New York. The spatial channel model reported allows simulations of H matrices for arbitrary antenna configurations. These channel matrices may be used to test receiver algorithms in system performance studies. These results may also be used for antenna array design, as the decay of mobile antenna correlation with antenna separation has been reported here. An important finding for the base transmitter array was that the antennas were largely uncorrelated even at antenna separations as small as two wavelengths.

In this paper, we discuss some of the most basic architectural superstructures for wireless links with multiple antennas: M at the transmit site and N at the receive site. Toward leveraging the gains of the last half century of coding theory, we emphasize those structures that can be composed using spatially one dimensional coders and decoders. These structures are investigated primarily under a probability of outage constraint. The random matrix channel is assumed to hold steady for such a large number of M-dimensional vector symbol transmission times, that an infinite time horizon Shannon analysis provides useful insights. The resulting extraordinary capacities are contrasted for architectures that differ in the way that they manage self-interference in the presence of additive receiver noise. A universally optimal architecture with a diagonal space-time layering is treated, as is an architecture with horizontal space-time layering and an architecture with a single outer code. Some capacity asymptotes for large numbers of antennas are also included. Some results for frequency selective channels are presented: It is only necessary to feedback M rates, one per transmit antenna, to attain capacity. Also, capacity of an (M,N) link is, in a certain sense, invariant with respect to signaling format.

Multiple-input-multiple-output (MIMO) systems have the potential to achieve very high capacities, depending on the propagation environment. Capacity increases as signal correlation decreases. We present the measurements of a MIMO system under strong and weak line-of-sight conditions. The system capacity decreases as the distance from the transmitter increases. Indeed the transmitter correlation increases as the distance increases. The receiver correlation is lower than the transmitter correlation under both propagation conditions.

This paper presents analysis for spatial and temporal variations of multiple-input-multiple-output (MIMO) channels at the mobile. The channel study is based on the narrowband measurements at 2.11 GHz in Manhattan, New York with 16 transmitting antennas and 16 receiving antennas. Doppler spread and angle of arrival (AOA) are derived from the temporal correlation of field components. The results show that the AOA at the mobile is not uniformly distributed. The restricted AOA distribution results in approximately twice the correlation distance and correlation time than the predicted values from the Jakes (1974) model. The measured median coherence time is at least a few seconds for stationary channels, and 90 ms at a mobile pedestrian speed of 3 km/hr. These observations hold for both vertically and horizontally polarized antennas. The measured median RMS angular spread at the mobile is 22.5/spl deg/ for horizontally polarized antennas and 25.5/spl deg/ for vertically polarized antennas.

Narrowband MIMO measurements using 16 transmitters and 16 receivers at 2.11 GHz were carried out in Manhattan. High capacities were found for full as well as smaller array configurations, all within 80% of the fully scattering channel capacity. It was found that two streets which might be considered near LOS, had different capacities, one about 45% of and the other about 75% of the fully scattering channel capacity.

The paper presents a multiple-input-multiple-output (MIMO) channel modeling technique for link level and system level simulations. The classical approach of wave superposition is adopted to generate MIMO channels that are statistically consistent in delay, frequency and angle domains. The paper proposes an efficient way to generate four dimensional MIMO channel transfer matrices based on standard channel statistics such as power delay profile (PDP) and power azimuth spectrum (PAS).

There are ever increasing demands for additional capacity in wireless communications to handle voice, data, and wideband Internet applications. These demands are constrained by the bandwidth that was allocated to wireless communications. The spectral efficiencies in present day wireless systems hover around 1 bit/s/Hz. Bell-labs Layered Space-Time (BLAST) is a communication technique for achieving very high spectral efficiencies in highly scattering environments using multiple transmit and receive antennas. A measurement campaign was undertaken to assess the BLAST gains in spectral efficiency in the suburban outdoor environment for stationary subscribers. The measurements employed directive antennas to better control interference from adjacent cells. The measurements were performed over a narrow band at 2.44 GHz with five transmitting and seven receiving antennas, respectively. Extensive calibration methods, assisted by simulations, were developed to assure accurate results for the BLAST capacities of the measured remote subscriber sites. Initial results indicate that BLAST capacities of C/sub B//spl ges/38 bits/s/Hz at 20% of the measured locations and C/sub B//spl ges/24 bits/s/Hz at 50% of these locations are feasible, for reasonable link parameters and negligible interference.

We analyze the behavior of a multiple element antenna system in an indoor environment based on the measurements taken in the Lucent Bell Labs building in Crawford Hill, NJ, with a system of 12 transmitters and 15 receivers. In particular, we investigate the capacity behavior with respect to the polarization of the transmitting/receiving elements and the distance between the transmitting and the receiving arrays. The analysis of the power rolloff versus distance clearly demonstrates the different propagation characteristics of the horizontally versus the vertically polarized electric fields. Under strong line-of-sight (LOS) conditions (hallway environment), the power of the horizontally polarized waves falls off faster with distance than that of the vertically polarized fields. Also, the cross-polarization coupling is about -15 dB. Under nonline-of-sight (NLOS) conditions (labs), both polarizations display similar rolloff behavior with distance and the cross-polarization coupling is about 0 dB. There is a power loss of at least 15 dB under NLOS conditions relative to LOS conditions. The average received signal power affects the system capacity. In the hallway, horizontally polarized systems achieve lower capacities than their vertically polarized counterparts. Also the achievable capacity in the labs is much lower than that in the hallway, because of the lower average received power. The comparison of single polarization systems to hybrid polarization systems shows that combined polarization systems perform better in terms of achievable capacity. Therefore, there lies an advantage in using both electric field polarizations. However, under strong LOS conditions the channel itself inherently limits the capacity behavior of the system.

Previous studies have shown that single-user systems employing n-element antenna arrays at both the transmitter and the receiver can achieve a capacity proportional to n, assuming independent Rayleigh fading between antenna pairs. We explore the capacity of dual-antenna-array systems under correlated fading via theoretical analysis and ray-tracing simulations. We derive and compare expressions for the asymptotic growth rate of capacity with n antennas for both independent and correlated fading cases; the latter is derived under some assumptions about the scaling of the fading correlation structure. In both cases, the theoretic capacity growth is linear in n but the growth rate is 10-20% smaller in the presence of correlated fading. We analyze our assumption of separable transmit/receive correlations via simulations based on a ray-tracing propagation model. Results show that empirical capacities converge to the limit capacity predicted from our asymptotic theory even at moderate n = 16. We present results for both the cases when the transmitter does and does not know the channel realization.

Multielement system capacities are usually thought of as limited only by correlations between elements. It is shown here that degenerate channel phenomena called "keyholes" may arise under realistic assumptions which have zero correlation between the entries of the channel matrix H and yet only a single degree of freedom. Canonical physical examples of keyholes are presented. For outdoor environments, it is shown that roof edge diffraction is perceived as a "keyhole" by a vertical base array that may be avoided by employing instead a horizontal base array.

The ever growing need for higher capacity in wireless systems has fueled the interest in exploiting the spatial dimension-through the use of antennas arrays-to improve the utilization of the available radio spectrum. As a result, a large number of space-time techniques have been proposed wherein arrays are used to mitigate interference and enhance signal levels. More recently, information theory has shown that, with spatial data multiplexing, very large spectral efficiencies can be attained in multipath channels using transmit and receive antenna arrays. In this paper, the system benefit of using transmit and receive arrays in multicell scenarios is evaluated as a function of both the propagation environment and the number of antennas. Our results confirm the potential for very large system spectral efficiencies associated with the use of transmit and receive arrays, in particular in interference-limited rich-multipath conditions wherein the ability to perform interference mitigation-leading to tighter spectral reuse-and spatial data multiplexing grows with the number of antennas. In environments free of multipath, the potential is smaller but still very significant, associated with interference mitigation and signal enhancement only, since spatial data multiplexing is no longer possible.

We present the results of propagation measurements at 2.485 GHz for fixed wireless loops. Path loss measurements were performed and characterized at 43 subscriber locations around a base station antenna located on top of Crawford Hill in Holmdel, NJ. This suburban location is characterized by rolling hills, foliage, and terrain blockages. Temporal and horizontal motion path loss fluctuations were found to be uncorrelated, each characterized by a different Ricean distribution. Lower r.m.s. delay spreads were obtained with directive subscriber antennas than with omni-directional antennas. No substantial gain loss (less than 2 dB) of subscribers' directive antennas was observed. The effects of trees, with foliage, surrounding the base station upon the path loss and the ratio of scattered power to specular power are also examined. The distance exponent of path loss versus distance (about 1.5) was observed to be less than free-space. Diffraction loss from hilltop trees, shadowing the base station, are suspected to be the cause. This loss decreases as the remote moves further away and comes out of the shadow. Scattered power from directions other than line-of-sight was observed to be as high as one half of the specular contribution when tree scattering near the base station was significant.

At Bell Laboratories, he studied indoor microwave propagation and modeling, packet reservation multiple access for wireless systems and optical WDM networks.He became Manager, Voice Research Department, at Motorola Codex, involved in the implementation integrated voice and data packet systems.On returning to Bell Laboratories he led a multi-disciplinary team to create a software tool for Wireless System Engineering (WiSE), now in widespread use in Lucent Technologies.He received the Distinguished Member of Technical Staff award and is Director of the Wireless Communications Research Department.He is interested in microwave propagation measurements and models, intelligent antennas, third generation wireless system and space time systems achieving high capacities using transmit and receive antenna arrays.He has published over eighty papers and has twelve patents.He is a Fellow of the IEEE. He is editor for the IEEE Transactions on Communications and the IEEE Transactions on Wireless.

We demonstrate an experimental evaluation of unsupervised (i.e., blind) channel deconvolution that is based on the multi-user kurtosis (MUK) maximization criterion. We focus on the narrowband case and compare the performance of the MUK algorithm against the conventional trained linear MMSE and the V-BLAST algorithm. These results constitute, to the best of our knowledge, the first over-the-air demonstration of blind techniques for multiple-transmitter/multiple-receiver systems.

A statistical space-time model for indoor wireless propagation based on empirical measurements is compared with results from the deterministic ray-tracing simulation tool WiSE for the same environment. Excellent agreement is found in terms of the distributions of arrival times and angular spread for both modeling approaches. The WiSE package is also use to synthesize MIMO channel matrices and determine the theoretical capacity, available in the tested environments. It is found that, for narrowband channels, the spatial clustering of the multipaths limits the capacity gains associated with increased array size.

Recent breakthroughs in the application of information theory have shown great capacity increase by deploying multiple transmitting and multiple receiving antennas as compared to conventional single antenna systems. This work presents an experimental verification of the theoretical capacity prediction and a summary of measurement results for 16 fixed wireless locations in suburban New Jersey.

In BLAST (Bell Labs' Layered Space Time) systems, very high spectral efficiency can be achieved by employing antenna arrays at both transmit and receive sides. Coding for these array systems is an interesting topic, as such, has seen intensive research. We study coding architectures constructed from conventional codes including convolutional codes and Reed Solomon codes. Our main interest is in the performance and complexity trade-offs involved in the design of coding/decoding and signal detection algorithms. We show that iterative detection and decoding (IDD) can significantly improve the performance of coded BLAST. In some cases, IDD allows for very simple detection algorithms to be used at the receiver front end. Therefore, it may in fact reduce the overall receiver complexity. Our results again demonstrate that coded V-BLAST (vertical-BLAST) is a promising architecture to achieve the great potential of BLAST with limited complexity.

Recent information-theory results have shown the enormous capacity potential of wireless techniques that use transmit and receive antenna arrays. As a result, a number of layered space-time (BLAST) architectures have been proposed wherein multiple data streams are transmitted in parallel and separated at the receiver on account of their distinct spatial signatures. While extremely promising, all analysis of BLAST to date were restricted to the context of a single-user link. In this paper, the system-level benefit of using BLAST in multicell scenarios is evaluated in comparison with other directive- and adaptive-array techniques.

We demonstrate the capacity growth of multiple-element antenna arrays (MEAs) in a realistic propagation environment using WiSE, an experimental ray tracing tool. WiSE is used to construct the channel response for MEAs operating at 1.9 GHz for two situations: (a) (16-16)-MEAs located inside an office building and (b) (4, 4)-MEAs located in an outdoor fixed wireless loop. We define effective degrees of freedom (EDOFs) as parallel spatial modes of transmission for an MEA. We quantify the increase in both the number of EDOFs and capacity with transmit power, received SNR, and antenna spacing. More EDOFs are present when receiving MEAs are physically closer to the transmitting MEA, regardless of the scattering effect.

The accuracy with which simulation techniques can be used to predict the performance of an experimental high speed M-QAM testbed in typical indoor wireless environments is studied in this paper. Ray tracing techniques were used to produce impulse responses for specific transmit and receive antenna locations where actual measurements were taken using a highly versatile equalized M-QAM testbed operating at 10 Mbps, 20 Mbps and 30 Mbps. The results obtained using a simulation of the testbed plus the ray-traced channel are compared with those obtained with experimental field trials. The paper presents the results of such comparisons for a total of 2,322 trials, where a trial is defined as a simulation run plus a field test for a given data rate and antenna location. The study shows that when quantization noise is factored into the simulation, the performance predicted using the simulation approach closely matches the experimental data.

We investigate robust wireless communication in high-scattering propagation environments using multi-element antenna arrays (MEAs) at both transmit and receive sites. A simplified, but highly spectrally efficient space-time communication processing method is presented. The user's bit stream is mapped to a vector of independently modulated equal bit-rate signal components that are simultaneously transmitted in the same band. A detection algorithm similar to multiuser detection is employed to detect the signal components in white Gaussian noise (WGN). For a large number of antennas, a more efficient architecture can offer no more than about 40% more capacity than the simple architecture presented. A testbed that is now being completed operates at 1.9 GHz with up to 16 quadrature amplitude modulation (QAM) transmitters and 16 receive antennas. Under ideal operation at 18 dB signal-to-noise ratio (SNR), using 12 transmit antennas and 16 receive antennas (even with uncoded communication), the theoretical spectral efficiency is 36 bit/s/Hz, whereas the Shannon capacity is 71.1 bit/s/Hz. The 36 bits per vector symbol, which corresponds to over 200 billion constellation points, assumes a 5% block error rate (BLER) for 100 vector symbol bursts.

Different attempts have been made to compare the performance of coded orthogonal frequency division multiplexing (COFDM) and a corresponding single carrier system employing a linear equalizer often by comparing the average bit error rates. The random coding technique is used to analyze the performance of these systems for signaling over piecewise constant channels. The probabilities that the cutoff rates are less than a desired rate are calculated and compared. It is shown that for low SNRs and low code rates, the cutoff rates of both systems are approximately the same. With an increasing SNR the cutoff rate for the linearly equalized system, however, converges faster to its maximum value than the cutoff rate for the COFDM system. Simulation results for the outage probability are presented for the two-ray Rayleigh fading channel with QPSK as the fundamental modulation technique. For code rates close to one or for uncoded transmission, the linearly equalized single carrier system substantially outperforms the COFDM system. For low to medium code rates COFDM and a linearly equalized single carrier system perform approximately identically.

This paper presents simulation studies of a class of autonomous, dynamic channel assignment (ADCA) protocols applied to a fixed wireless access (FWA) system utilizing TDMA as the multi-access technique. The terms ADCA refers to a call admission/rejection policy where initial time-slot assignment is based only on measurements of the total interference power. The measurement is carried out both by the base-station and the terminal prior to making a time-slot assignment. Once assigned, a time-slot may be replaced if conditions deteriorate. The FWA system uses narrow steerable antenna beams at the base-stations to point at the desired terminal during the assigned time-slot, thereby minimizing the interference generated by transmitters supporting other terminals during the same time-slot. It is shown that with this simple scheme high capacity can be achieved.

A wireless office service (WOS) system is a virtually self-contained cellular system for providing wireless service in an office or campus environment. It co-exists with, and uses the same technology and spectrum as the surrounding public wireless system. For such systems to be viable, it is necessary to automate their RF (radio frequency) engineering process, and even to make them "self-engineering". We devise procedures and algorithms to achieve this self-engineering at system-startup, and then validate them through simulations. Although designed for in-building or campus environments, with suitable modifications these procedures and algorithms may be applicable to more general scenarios.

A wireless office service (WOS) system is a virtually self-contained cellular system for providing wireless service in an office or campus environment. It co-exists with, and uses the same technology and spectrum as the surrounding public wireless system. These systems are assuming increasing importance with increasing demand for consistent services from a single phone while in all environments: home, office, and at large. For these systems to be viable, it is necessary to automate their RF (radio frequency) engineering process, and even to make them "self-engineering" or "self-adapting". We discuss procedures to achieve this self-engineering during system operation. Although designed for in-building or campus environments, with suitable modifications, these procedures may be applicable to more general scenarios.

Engineers designing and installing outdoor and indoor wireless communications systems need effective and practical tools to help them determine base station antenna locations for adequate signal coverage. Computer-based radio propagation prediction tools are now often used in designing these systems. We assess the performance of such a propagation tool based on ray-tracing and advanced computational methods. We have compared its predictions with outdoor experimental data collected in Manhattan and Boston (at 900 MHz and 2 GHz). The comparisons show that the computer-based propagation tool can predict signal strengths in these environments with very good accuracy. The prediction errors are within 6 dB in both mean and standard deviation. This shows that simulations, rather than costly field measurements, can lead to accurate determination of the coverage area for a given system design.

Indoor wireless systems will be used in a large variety of office, factory and residential environments. Thus, adequate guidelines for radio port placement are needed to ensure satisfactory performance at the lowest cost. These guidelines must be derived from a large body of site-specific propagation data. However, collecting a statistically significant database through measurements is a daunting task. Alternatively, this database can be generated by using propagation models, validated by measurements. Several models exist for the statistical characterization of microwave propagation within buildings, However, statistical models do not provide site-specific information. We propose a hybrid model in which ray tracing is used to predict, at any given location, the local mean of the received power and the delay profile. Variations about the mean values can then be captured via a statistical description matched to the local environment. We describe an efficient 3-D ray tracing algorithm which accounts for all (transmitted as well as reflected) rays reaching the receiver location after an arbitrary number of reflections. We include the effects of the angle of incidence, the material dielectric constant and the antenna patterns. The predicted values for the local means of the received power are then compared against measurements to establish the accuracy of this approach.

A 10-cell, 10-channel line-of-sight (LOS) microcellular environment with Poisson arrivals and exponentially distributed holding times is stimulated. The performance of cellular systems for personal communications networks (PCNs) is evaluated. Call blocking, bumping, and dropping probabilities, as affected by fast fading, power control, and traffic load, for LOS, 900-MHz microcellular environments are discussed. It is established that DRA (dynamic resource allocation) using local information only does not result in a performance penalty. In fact, in the cases of interest, it performs significantly better than fixed channel allocation, increasing capacity by about 20%, or even as much as 60% with power control. It is shown that fast fading results in a limited increase, about 2-3 times the blocking probability, while the use of power control translates directly, and consistently, into a significant increase in traffic handling capacity.>

The authors present a voice-packet reconstruction scheme in which the pitch and amplitude of a packet reconstructed according to the pitch detection method of D.J. Goodman et al. (1986) are modified to accommodate amplitude and slight fundamental frequency changes that might have been contained in the missing packet. Audible clicks in pitch-detection-reconstructed speech are therefore significantly reduced, and the perceived quality of reconstructed speech is improved. Preliminary subjective tests indicate that for a given perceptual quality (mean opinion score), the number of missing packets for this technique may be about twice what can be tolerated when using the pitch detection method only.>

Simulation work is reported indicating that packet reservation multiple access (PRMA) allows a variety of information sources to share the same wireless access channel. Some of the sources, such as speech terminals, are classified as periodic and others, such as signaling, are classified as random. Packets from all sources contend for access to channel time slots. When a periodic information terminal succeeds in gaining access, it reserves subsequent time slots for uncontested transmission. Both computer simulations and a listening test reveal that PRMA achieves a promising combination of voice quality and bandwidth efficiency.>

Adaptive equalization is used to ensure that the outage probability is less than 10/sup -3/ for a target bit error rate of 10/sup -4/ in buildings with RMS delay spread of up to 100 ns. A time-division multiple-access system with four-level quadrature amplitude modulation point-to-point links strikes the right balance between flexibility and complexity. It is shown that such a system can support rates of at least 1 Mb/s.>

The results of indoor multipath propagation measurements using 10 ns, 1.5 GHz, radarlike pulses are presented for a medium-size office building. The observed channel was very slowly time varying, with the delay spread extending over a range up to about 200 ns and rms values of up to about 50 ns. The attenuation varied over a 60 dB dynamic range. A simple statistical multipath model of the indoor radio channel is also presented, which fits our measurements well, and more importantly, appears to be extendable to other buildings. With this model, the received signal rays arrive in clusters. The rays have independent uniform phases, and independent Rayleigh amplitudes with variances that decay exponentially with cluster and ray delays. The clusters, and the rays within the cluster, form Poisson arrival processes with different, but fixed, rates. The clusters are formed by the building superstructure, while the individual rays are formed by objects in the vicinities of the transmitter and the receiver.

The performance ofM-QAM for indoor radio communications is evaluated via a realistic model for indoor multipath propagation [1]. It is found that the outage (BER-4) is about 0.3 percent at 1 Mbit/s and 20 percent at 4 Mbits/s. Two-antenna predetection diversity reduces the outage from 1.2 to 0.04 percent at 2 Mbits/s. Increasing the signaling pulse rolloff factor from 0.5 to 1.0 reduces the outage by not more than 35 percent.

A new transmultiplexer design is presented in which reliability is achieved by adopting a tree structure. A working principle based upon analytic signals results in a modular structure. The usage of FIR filters designed to have coefficients which are sums of a limited and prescribed number of powers of two together with an efficient, low-sensitivity interpolating structure results in a very efficient design. When these interpolators are used to implement all processing functions required, a highly modular design is obtained which is based solely upon the elementary all-pass cell. Simulation results for a four-channel transmultiplexer are presented together with a performance evaluation for the translation between two 30 channel TDM primary groups and one 60 channel FDM supergroup and for the translation between one 24 channel North American TDM unit and two 12 channel FDM groups. The design is found to have the reliability and modularity attributes which can be obtained from a tree structure, and at the same time its efficiency compares favorably with that of transform-based transmultiplexers.