Felipe Gomez-Cuba

On the Analysis of Scheduling in Dynamic Duplex Multihop mmWave Cellular Systems

With the shortage of spectrum in conventional cellular frequencies, millimeter-wave (mmWave) bands are being widely considered for use in next-generation networks. Multihop relaying is likely to play a significant role in mmWave cellular systems for self backhauling, range extension and improved robustness from path diversity. However, designing scheduling policies for these systems is challenging due to the need to account for both adaptive directional transmissions and dynamic time-division duplexing schedules, which are key enabling features of mmWave systems. This paper considers the problem of joint scheduling and congestion control in a multihop mmWave network using a Network Utility Maximization (NUM) framework. Interference is modeled with an exact model and two auxiliar simplified models: actual interference (AI), with a graph-based calculation of the Signal to Interference plus Noise Ratio (SINR) depending on dynamic link activity and directivity, as well as upper and lower bounds computed from worst-case interference (WI) and interference free (IF) approximations. Throughput and utility optimal policies are derived for all interference models (AI, WI and IF) with both deterministic Maximum Weighted and randomized Pick and Compare scheduling algorithms, jointly with decentralized Dual Congestion Control. Results are evaluated with numerical simulations, using accurate mmWave channel and beamforming gain approximations based on measurement campaigns.

Millimeter Wave Receiver Efficiency: A Comprehensive Comparison of Beamforming Schemes With Low Resolution ADCs

In this paper, we study the achievable rate and the energy efficiency of analog, hybrid, and digital combining (AC, HC, and DC) for millimeter wave (mmW) receivers. We take into account the power consumption of all receiver components, not just analog-to-digital converters (ADCs), determine some practical limitations of beamforming in each architecture, and develop performance analysis charts that enable comparison of different receivers simultaneously in terms of two metrics, namely, spectral efficiency (SE) and energy efficiency (EE). We present a multi-objective utility optimization interpretation to find the best SE-EE weighted tradeoff among AC, DC, and HC schemes. We consider an additive quantization noise model to evaluate the achievable rates with low resolution ADCs. Our analysis shows that AC is only advantageous if the channel rank is strictly one, the link has very low SNR, or there is a very stringent low power constraint at the receiver. Otherwise, we show that the usual claim that DC requires the highest power is not universally valid. Rather, either DC or HC alternatively results in the better SE versus EE tradeoff depending strongly on the considered power consumption characteristic values for each component of the mmW receiver.

Scaling laws for Infrastructure Single and multihop wireless networks in wideband regimes

With millimeter wave bands emerging as a strong candidate for 5G cellular networks, next-generation systems may be in a unique position where spectrum is plentiful. To assess the potential value of this spectrum, this paper derives scaling laws on the per mobile downlink feasible rate with large bandwidth and number of nodes, for both Infrastructure Single Hop (ISH) and Infrastructure Multi-Hop (IMH) architectures. It is shown that, for both cases, there exist critical bandwidth scalings above which increasing the bandwidth no longer increases the feasible rate per node. These critical thresholds coincide exactly with the bandwidths where, for each architecture, the network transitions from being degrees-of-freedom-limited to power-limited. For ISH, this critical bandwidth threshold is lower than IMH when the number of users per base station grows with network size. This result suggests that multi-hop transmissions may be necessary to fully exploit large bandwidth degrees of freedom in deployments with growing number of users per cell.

Dynamic time-domain duplexing for self-backhauled millimeter wave cellular networks

Millimeter wave (mmW) bands between 30 and 300 GHz have attracted considerable attention for nextgeneration cellular networks due to vast quantities of availavery high-dimensional antenna arraysble spectrum and the possibility of very high-dimensional antenna arrays. However, a key issue in these systems is range: mmW signals are extremely vulnerable to shadowing and poor high-frequency propagation. Multi-hop relaying is therefore a natural technology for such systems to improve cell range and cell edge rates without the addition of wired access points. This paper studies the problem of scheduling for a simple infrastructure cellular relay system where communication between wired base stations and User Equipment follow a hierarchical tree structure through fixed relay nodes. Such a systems builds naturally on existing cellular mmW backhaul by adding mmW in the access links. A key feature of the proposed system is that TDD duplexing selections can be made on a link-by-link basis due to directional isolation from other links. We devise an efficient, greedy algorithm for centralized scheduling that maximizes network utility by jointly optimizing the duplexing schedule and resources allocation for dense, relay-enhanced OFDMA/TDD mmW networks. The proposed algorithm can dynamically adapt to loading, channel conditions and traffic demands. Significant throughput gains and improved resource utilization offered by our algorithm over the static, globally-synchronized TDD patterns are demonstrated through simulations based on empirically-derived channel models at 28 GHz.

Improving third-party relaying for LTE-A: A realistic simulation approach

In this article we propose solutions to diverse conflicts that result from the deployment of the (still immature) relay node (RN) technology in LTE-A networks. These conflicts and their possible solutions have been observed by implementing standard-compliant relay functionalities on the Vienna simulator. As an original experimental approach, we model realistic RN operation, taking into account that transmitters are not active all the time due to half-duplex RN operation. We have rearranged existing elements in the simulator in a manner that emulates RN behavior, rather than implementing a standalone brand-new component for the simulator. We also study analytically some of the issues observed in the interaction between the network and the RNs, to draw conclusions beyond simulation observation. The main observations of this paper are that: i) Additional time-varying interference management steps are needed, because the LTE-A standard employs a fixed time division between eNB-RN and RN-UE transmissions (typical relay capacity or throughput research models balance them optimally, which is unrealistic nowadays); ii) There is a trade-off between the time-division constraints of relaying and multi-user diversity; the stricter the constraints on relay scheduling are, the less flexibility schedulers have to exploit channel variation; and iii) Thee standard contains a variety of parameters for relaying configuration, but not all cases of interest are covered.

Unified Capacity Limit of Non-Coherent Wideband Fading Channels

In non-coherent wideband fading channels, where energy rather than spectrum is the limiting resource, peaky and non-peaky signaling schemes have long been considered species apart, as the first approaches asymptotically the capacity of a wideband AWGN channel with the same average SNR, whereas the second reaches a peak rate at some finite critical bandwidth and then falls to zero as bandwidth grows to infinity. In this paper, it is shown that this distinction is in fact an artifact of the limited attention paid in the past to the product between the bandwidth and the fraction of time it is in use. This fundamental quantity, called bandwidth occupancy, measures average bandwidth usage over time. For all signaling schemes with the same bandwidth occupancy, achievable rates approach to the wideband AWGN capacity within the same gap as the bandwidth occupancy approaches its critical value, and decrease to zero as the occupancy goes to infinity. This unified analysis produces quantitative closed-form expressions for the ideal bandwidth occupancy, recovers the existing capacity results for (non-) peaky signaling schemes, and unveils a tradeoff between the accuracy of approximating capacity with a generalized Taylor polynomial and the accuracy with which the optimal bandwidth occupancy can be bounded.

Optimal link scheduling in millimeter wave multi-hop networks with space division multiple access

In this paper, we introduce a model for Multiple-Input Multiple-Output (MIMO) Space Division Multiple Access (SDMA) into the analysis of a multi-hop millimeter wave network under the classic Network Utility Maximization (NUM) framework with Maximum Back Pressure scheduling (MBP). We show that the proof of convergence of MBP remains valid when we allow the scheduler to select multiple links to the same receiver in the same frame. Conventional MBP with a single link per receiver is traditionally implemented using the Maximum Weighted Matching (MWM) algorithm over the network graph. Under our modification, the problem becomes a Maximum Weighted Partition of the graph. Message Passing (MP) algorithms are efficient and have been successfully applied to graph partitioning problems in the past, so we use one to approximate the optimal MBP scheduling. Through simulation over a randomized mmWave picocell, we compare the MWM reference without SDMA, the efficient MP approximation, and the exact optimal MBP scheduler with SDMA (obtained by brute force). Simulations show that by leveraging SDMA in multi-hop mmWave network scheduling, a 50% capacity increase is obtained on average.

Capacity scaling in noncoherent wideband massive SIMO systems

This paper studies noncoherent wideband systems with a single antenna transmitter and a multiple antenna receiver with many elements, under signaling with peak-to-average power ratio constraints. The analysis considers the scaling behavior of capacity and achievable rates by letting both the number of antennas and the bandwidth go to infinity jointly. In contrast to prior work on wideband single input single output (SISO) channels without a-priori channel state information, it is shown that a sufficiently large number of receive antennas can make up for the vanishingly small SNR at each antenna. In particular, it is shown that when bandwidth grows sufficiently slowly with the number of antennas, the capacity scaling with an increasing number of receive antennas is the same as the optimal coherent capacity scaling. If the bandwidth grows faster than a certain threshold, however, the additional bandwidth does not help because a finite transmit power is spread over an excessively large bandwidth.

A Microeconomic Approach to Data Trading in User Provided Networks

In this paper, we consider a novel cellular network connection paradigm, known as user-provided network (UPN), where users share their connectivity with others. Motivated by the recently launched traded data plans, where Wireless Service Providers allow users to sell and buy leftover data capacities (caps) from each other, we have developed a dynamic pricing scheme for such a network. Optimal prices for the volume of data traded are derived from market equilibrium. The proposed pricing scheme provides a global data market reference price, trading price for local data markets, considering user connectivity requirements, and price recovery. An analysis of UPN based on microeconomics is presented to provide insights into how traffic dynamics affect data volume demand and supply and thus, trading prices. The stability of the market equilibrium is also analyzed. Numerical results show that our scheme incentivizes user participation in UPNs by dynamically adapting the price to data volume demand and supply, and also provides high returns to both buyers and sellers.

Bandwidth occupancy of non-coherent wideband fading channels

Peaky and non-peaky signaling schemes have long been considered species apart in non-coherent wideband fading channels, as the first approaches asymptotically the linear-in-power capacity of a wideband AWGN channel with the same SNR, whereas the second reaches a nearly power-limited peak rate at some finite critical bandwidth and then falls to zero as bandwidth grows to infinity. In this paper it is shown that this distinction is in fact an artifact of the limited attention paid in the past to the product between the bandwidth and the fraction of time it is in use. This fundamental quantity, that is termed bandwidth occupancy, measures average bandwidth usage over time. The two types of signaling in the literature are harmonized to show that, for any type of signals, there is a fundamental limit-a critical bandwidth occupancy. All signaling schemes with the same bandwidth occupancy approach the capacity of wideband AWGN channels with the same asymptotic behavior as the bandwidth occupancy grows to its critical value. For a bandwidth occupancy above the critical, rate decreases to zero as the bandwidth occupancy goes to infinity.