Mandar N. Kulkarni

Tractable Model for Rate in Self-Backhauled Millimeter Wave Cellular Networks

Millimeter wave (mmWave) cellular systems will require high-gain directional antennas and dense base station (BS) deployments to overcome a high near-field path loss and poor diffraction. As a desirable side effect, high-gain antennas offer interference isolation, providing an opportunity to incorporate self-backhauling, i.e., BSs backhauling among themselves in a mesh architecture without significant loss in the throughput, to enable the requisite large BS densities. The use of directional antennas and resource sharing between access and backhaul links leads to coverage and rate trends that significantly differ from conventional UHF cellular systems. In this paper, we propose a general and tractable mmWave cellular model capturing these key trends and characterize the associated rate distribution. The developed model and analysis are validated using actual building locations from dense urban settings and empirically derived path loss models. The analysis shows that, in sharp contrast to the interference-limited nature of UHF cellular networks, the spectral efficiency of mmWave networks (besides the total rate) also increases with the BS density, particularly at the cell edge. Increasing the system bandwidth does not significantly influence the cell edge rate, although it boosts the median and peak rates. With self-backhauling, different combinations of the wired backhaul fraction (i.e., the fraction of BSs with a wired connection) and the BS density are shown to guarantee the same median rate (QoS).

Modeling and Analyzing Millimeter Wave Cellular Systems

We provide a comprehensive overview of mathematical models and analytical techniques for millimeter wave (mmWave) cellular systems. The two fundamental physical differences from conventional sub-6-GHz cellular systems are: 1) vulnerability to blocking and 2) the need for significant directionality at the transmitter and/or receiver, which is achieved through the use of large antenna arrays of small individual elements. We overview and compare models for both of these factors, and present a baseline analytical approach based on stochastic geometry that allows the computation of the statistical distributions of the downlink signal-to-interference-plus-noise ratio (SINR) and also the per link data rate, which depends on the SINR as well as the average load. There are many implications of the models and analysis: 1) mmWave systems are significantly more noise-limited than at sub-6 GHz for most parameter configurations; 2) initial access is much more difficult in mmWave; 3) self-backhauling is more viable than in sub-6-GHz systems, which makes ultra-dense deployments more viable, but this leads to increasingly interference-limited behavior; and 4) in sharp contrast to sub-6-GHz systems cellular operators can mutually benefit by sharing their spectrum licenses despite the uncontrolled interference that results from doing so. We conclude by outlining several important extensions of the baseline model, many of which are promising avenues for future research.

Coverage and rate trends in dense urban mmWave cellular networks

The use of dense millimeter wave (mmWave) cellular networks with highly directional beamforming stands as an intriguing solution to the current spectrum congestion problem. Due to significantly different propagation characteristics at such high frequencies, however, the coverage and rate trends differ drastically from conventional microwave networks. This paper aims to gain insights into the coverage and rate performance of mmWave cellular networks in major metropolitan cities. Our results confirm that, unlike conventional cellular networks, mmWave networks operating at 73 GHz carrier frequency are pre-dominantly noise-limited. Though larger system bandwidth leads to higher peak rates, it does not improve the cell edge rates. It is observed that dense base station (BS) deployment is the key to achieve both better coverage and rates in mmWave cellular networks. Further, based on actual building locations, we show the inadequacy of existing blockage models and validate a better blockage model.

Downlink and Uplink Cell Association With Traditional Macrocells and Millimeter Wave Small Cells

Millimeter wave (mmWave) links will offer high capacity but are poor at penetrating into or diffracting around solid objects. Thus, we consider a hybrid cellular network with traditional sub-6 GHz macrocells coexisting with denser mmWave small cells, where a mobile user can connect to either opportunistically. We develop a general analytical model to characterize and derive the uplink and downlink cell association in the view of the signal-to-interference-and-noise-ratio and rate coverage probabilities in such a mixed deployment. We offer extensive validation of these analytical results (which rely on several simplifying assumptions) with simulation results. Using the analytical results, different decoupled uplink and downlink cell association strategies are investigated and their superiority is shown compared with the traditional coupled approach. Finally, small cell biasing in mmWave is studied, and we show that unprecedented biasing values are desirable due to the wide bandwidth.

A Comparison of MIMO Techniques in Downlink Millimeter Wave Cellular Networks With Hybrid Beamforming

Large antenna arrays will be needed in future millimeter wave (mmWave) cellular networks, enabling a large number of different possible antenna architectures and multiple-input multiple-output (MIMO) techniques. It is still unclear which MIMO technique is most desirable as a function of different network parameters. This paper, therefore, compares the coverage and rate performance of hybrid beamforming enabled multiuser (MU) MIMO and single-user spatial multiplexing (SM) with single-user analog beamforming (SU-BF). A stochastic geometry model for coverage and rate analysis is proposed for MU-MIMO mmWave cellular networks, taking into account important mmWave-specific hardware constraints for hybrid analog/digital precoders and combiners, and a blockage-dependent channel model which is sparse in angular domain. The analytical results highlight the coverage, rate, and power consumption tradeoffs in multiuser mmWave networks. With perfect channel state information at the transmitter and round robin scheduling, MU-MIMO is usually a better choice than SM or SU-BF in mmWave cellular networks. This observation, however, neglects any overhead due to channel acquisition or computational complexity. Incorporating the impact of such overheads, our results can be re-interpreted so as to quantify the minimum allowable efficiency of MU-MIMO to provide higher rates than SM or SU-BF.

Rate analysis and feasibility of dynamic TDD in 5G cellular systems

In conventional applications of time division duplex (TDD) in cellular systems, the time resource split between uplink (UL) and downlink (DL) is fixed across all base stations (BSs) in the network. This leads to under utilization of BS resources when there is a mismatch between the expected and experienced UL/DL traffic in a given cell. A dynamic split that varies in each cell is desirable, but is challenging due to the high interference experienced by UL receivers in one cell from DL transmissions in adjacent cells. This paper analyzes the performance of UL users in dynamic TDD enabled next generation cellular networks using a stochastic geometry framework. The analysis highlights the trade-off between spectral efficiency and resource utilization for dynamic TDD. With appropriate interference mitigation, dynamic TDD offers a significant gain in data rates as compared to static TDD, which is higher when the BSs are lightly loaded and/or the fraction of UL users is low.

Performance analysis comparison of transmit antenna selection with maximal ratio combining and orthogonal space time block codes in equicorrelated Rayleigh fading multiple input multiple output channels

In this study, bit error rate (BER) performance of multiple input multiple output (MIMO) systems employing transmit antenna selection with maximal ratio combining (TAS/MRC) and orthogonal space time block codes (OSTBC) is analysed and compared. Analysis has been done for several modulation schemes in equicorrelated Rayleigh fading channels. Novel infinite series expressions for BER of TAS/MRC are proposed in this work. The authors observe that the existing literature on performance analysis of TAS/MRC in correlated channels implicitly or explicitly assume no correlation on the transmitter side. In this work the authors overcome this shortcoming. An alternate closed-form expression for BER performance of OSTBC MIMO systems is derived. This closed-form expression is computationally efficient than existing expressions. Monte Carlo simulations were performed to validate the analytical results. It has been observed that TAS/MRC outperforms OSTBC in terms of BER performance in equicorrelated Rayleigh fading channel. As a case study, performance comparison of cognitive radio (CR) links employing OSTBC and TAS/MRC MIMO systems is investigated in order to show the usefulness of the present analytical study. Even though TAS is the optimum transmit antenna technique in terms of BER performance, OSTBC seems to be a promising alternative for more realistic scenarios in CR systems.

A tractable model for rate in noise limited mmWave cellular networks

The use of millimeter wave (mmWave) spectrum for future cellular systems can be made possible with the use of directional antenna arrays and dense base station deployments. MmWave broadband networks exhibit fundamentally different behaviors compared to conventional sub-3 GHz cellular systems. Prominently, interference and path loss models and the corresponding effect on rate need to be re-examined. We propose a general and tractable model to capture and analyze the key distinguishing features of mmWave cellular networks, and characterize the user rate distribution in such networks. The proposed model and analysis are validated by simulations using real building locations in a region of New York in conjunction with empirically measured mmWave path loss models. Using both the proposed model and simulations, it is shown that unlike interference-limited nature of 4G cellular networks, mmWave cellular networks often tend to be noise-limited, and coverage heavily relies on a user being able to received sufficient power from the serving BS. Further, the cell edge rates are shown to be limited mostly by the base station density and are not necessarily improved by increasing the bandwidth of the system.

Coverage and rate trends in moderate and high bandwidth 5G networks

Higher frequency bands (>6 GHz) look promising to meet the proposed 5G data rates, given the large amount of available spectrum in these bands. However, a rigorous understanding of some fundamental tradeoffs like network densification, sectorization, and bandwidths has only begun to be investigated at millimeter wave (mmW) bands. In this work, we investigate the coverage and rate performance of cellular networks with sectorized access points (APs) operating at high frequency bands, using tools of stochastic geometry. We observe that sectorizing the APs can significantly improve the data rates and thus can be used in conjunction with network densification, in order to achieve the 5G data rate requirements. However, the increased data rates come at the expense of increased interference in the network. We investigate the interference effects on a typical moderate (200 MHz) bandwidth network at 28 GHz and a high (2 GHz) bandwidth network at 72 GHz carrier frequency, with 4 sector APs and validate the trends observed with the help of detailed system-level simulations using METIS-like scenarios.

Performance of Dynamic and Static TDD in Self-Backhauled Millimeter Wave Cellular Networks

Initial deployments of millimeter wave (mm-wave) cellular networks are likely to be enabled with self-backhauling. In this paper, we propose a random spatial model to analyze uplink (UL) and downlink (DL) signal to interference plus noise ratio distribution and mean rates corresponding to different access-backhaul and UL–DL resource allocation schemes in a self-backhauled mm-wave cellular network with Poisson point process (PPP) deployment of users and base stations (BSs). In particular, we focus on heuristic implementations of static and dynamic time division duplexing (TDD) for access links with synchronized or unsynchronized access-backhaul (SAB or UAB) time splits. We propose PPP approximations to characterize the distribution of the new types of interference encountered with dynamic TDD and UAB. These schemes offer better resource utilization than static TDD and SAB, however, potentially higher interference makes their choice non-trivial and the offered gains sensitive to different network parameters, including UL/DL traffic asymmetry, user load per BS or number of slave BSs per master BS. One can harness notable gains from UAB and/or dynamic TDD only if backhaul links are designed to have much larger throughput than the access links.