Research Activities Progress Report

Russell FordPh.D. Candidate, Dept. Computer Science, NYU-Poly

Advisor: Sundeep Rangan, Ph.D.Sept. 20, 2013

Overview• Conference papers

– Opportunistic Third-Party Backhaul for Cellular Wireless Networks (R. Ford, C. Kim, S. Rangan)

• Accepted to Asilomar 2013– Joint Interference – User Association Optimization for Cellular Wireless

Networks (C. Kim, R. Ford, S. Rangan)• Submitted to CCNC 2014

• 5G mmWave cellular– Currently investigating cellular mesh networks employing highly-

directional antenna arrays

Opportunistic 3rd-Party Backhaul• Rapid deployment of small-cell technology

– Predict dense networks of small-cell base stations• Addresses coverage problem• Multi-tier or heterogeneous network• $22 billion industry by 2016 (73% operator-deployed public area cells)

– Operator-deployed base stations• Urban micro and picocells• Require dedicated or leased-line backhaul connection between BS and

transport/core network• Usually “open access”

– Many more residential femtocells deployed by end-users• Backhaul links are user-provided broadband connections• Closed access

Problems• Backhaul is becoming the main bottleneck and cost-driver

– Wireless capacity will be plentiful but each cell needs a wired connection

• How to incentivize third-party backhaul providers?– Many residential femtocells with underutilized wireless and backhaul capacity– Cellular operators may benefit from opportunistically assigning users to 3rd-party links– Need to compensate third-party providers for backhaul– Understanding the market model: What price should be offered to maximize operator utility?

• What are the gains from leveraging 3rd-party backhaul?– In terms of operator utility/revenue and average overall and cell edge rate to mobiles– Compare to scaling operator-deployed network and provisioning dedicated backhaul– Intuition: Operators must pay for peak rate on dedicated links but only for average rate on 3rd-

party links

• How to allocate resources efficiently?– Cell selection/user assignment problem – Many constraints: Wireless capacity, maximum backhaul rate, cost of backhaul, QoS to users– Assigning mobiles to cells with the highest SNR is generally suboptimal

Optimization Formulation

ρij : spectral efficiency if MS i connects to BS jzij : interference from BS j to MS iwij : fraction of bandwidth allocated to MS i by BS jrij : rate from BS j to MS i

• Utility is a function of the rates to Mobile Stations in ΓMS

• Backhaul cost function (C = 0 for operator BSs)

• Base Station and Mobile Station rate constraints

• Single path constraint

• Problem is for the operator to maximize their net utility:

is a vector of unknowns:

Dual Decomposition Algorithm• Problem is non-convex but can find approximate solution close to optimal

– First, relax the single path constraint and run optimization on multipath problem– Then truncate to single path with highest utility

• Maximize utility for relaxed MS problem using Augmented Lagrangian method– Unconstrained dual form

– Lagrangian with dual parameters μ

– Solve numerically by iteratively updating μ’s(compute θ’s through gradient ascent)

– Augmenting (smoothing) function Φ results in faster convergence

Simulation Setup

• 2-Tier Network Model – Assumption: Femto adoption on par with WiFi– WiFi AP locations obtained from WiGLE.net– Macrocell site locations from Opencellid.net– Also use stochastic models prescribed by industry

white papers

• Methodology:1) Generate network using real-world macrocell (operator-deployed) sites and a random sampling of some fraction of femtocell locations (also randomly drop mobiles)2) Apply standard LTE urban and suburban propagation models to compute path loss3) Run user assignment algorithm for various prices4) Compute gains and compare to macrocell-only case

Cells (blue) and WiFi BS locations (green) in lower Manhattan

Results (1 of 2)

• Left: Distribution of mobile rates for a set of prices• Average rate decreases as price increases• Approaches macro-only case as p →∞

• Right: Additional backhaul rates required for increased utility• Gains from deploying additional macrocells (green) • Gains from 3rd-party femto offload (red)• Note: 3rd-party gains require negligible CapEx/OpEx compared to• Conclusion: Operator will gain as long as cost of 3rd-party backhaul is less than

cost of dedicated backhaul links

Results (2 of 2)

• Average overall (left) and cell edge (right) rates for urban scenario

Future Work• Assess open mmWave mesh networks

– Large swaths of spectrum allocated for “unlicensed” use in the 60 GHz range– Potential for large gains (1000x) by employing directional antennas

• A case for revisiting ad-hoc networking– Networks are power-limited (instead of interference-limited like current cellular),

so relaying becomes important – Opportunity for end-users to buy/sell capacity (may permit game-theoretic

analysis)