R2D2: Embracing Device-to-Device Communication in Next Generation Cellular Networks Tarun Bansal*, Karthik Sundaresan+, Sampath Rangarajan+ and Prasun Sinha* Speaker: Zhixue Lu* *Ohio State University and +NEC Labs America

Device-to-Device (D2D) Communication Normally smartphones communicate with each other through cellular base station Without D2D With D2D

D2D Traffic Applications (b) Peer-to-Peer: Public Safety when traditional infrastructure is not available Benefit: No interference (a) Base Station Assisted: Localized communication and Machine-to-Machine (M2M) communication Benefit: Service provider helps with security, neighbor discovery and ensuring Quality-of-Service Our focus is on integration of Base Station Assisted D2D Traffic with existing cellular communication

Our analysis for sectored deployments: D2D Benefits Spatial Reuse of Resources Multiple D2D transmissions per cell [Janis 2009, Doppler 2009, Lee 2013] Our analysis for sectored deployments: Very little benefit from additional spatial reuse D4 D1 D2 D1 D2 D4 Cell divided into 3 sectors with each sector covering 120o Not possible to schedule multiple D2D transmissions in the same sector. D3 D3

D2D Benefits (contd.) Offloading Fewer time slots taken [Janis 2009, Doppler 2009, Lee 2013] Time slot 1 Time slot 2 Time slot 1 Without D2D With D2D

Benefit of D2D in sectored deployments (Static Channel Allocation) classified as cellular Spatial Reuse Benefit Offload Benefit Tries to schedule multiple D2D transmissions per sector Offload benefit is significant (1 time slot instead of 2) Additional Spatial Reuse benefit (due to D2D) is not significant in sectored deployments due to small sector size

Identifying third D2D Benefit: Flexible Load Use as flexible load Resources need to be fixed in both Uplink and Downlink directions In practice, UL-DL traffic distribution varies both in space and time e.g. residential vs. commercial, morning vs. evening D2D can go on either on the Uplink or Downlink resources Use resources that would have been otherwise wasted Uplink Flows Downlink Flows D2D flows time time frequency frequency

How do we maximize these two benefits? D2D Benefits Offloading (1 time slot instead of 2) Use as flexible traffic (Place on either DL or UL resource) How do we maximize these two benefits?

Static vs. Dynamic Channel Allocation Interior Channels Too much traffic: Borrow channels from neighboring sectors Exterior Channels Two co-located sectors can have the same channel Static Channel Allocation Dynamic Channel Allocation leverages spatial traffic variations

Challenges in Incorporating D2D with Dynamic Channel Allocation UL or DL communications (directional) can still go simultaneously D2D (omnidirectional) may cause interference to coexisting transmissions in sectored deployments (not considered before) Determining interference from D2D transmission requires knowledge of path loss between all users (costly). D1 D2 D3 Similarly, D2D transmission cannot coexist with UL or D2D D2D presents a new challenge: Transmissions may cause interference if co-located sectors are using the same resource.

D2D Interference (Dynamic Channel Allocation) classified as cellular Lower throughput with D2D due to collisions among neighboring sectors Omnidirectional nature of D2D makes it non-trivial to schedule transmissions

Objective How to do dynamic resource allocation in multi-cell deployments with D2D traffic? How to schedule UL, DL and D2D transmissions while avoiding interference? Previous work only looks at resource allocation without D2D traffic, OR Scheduling with D2D in single cell deployments with no sectorization.

R2D2 Contributions A light-weight scalable solution (R2D2) that works at two different time-scales Phase 1: Allocate resources to each base station at coarser time scale (cross-sectors) Phase 2: Allocate resources to each flow independently at each base station at finer time scale (co-located sectors) while avoiding interference

R2D2 Contributions Practical: Works in sectored deployments with multiple cells Proposed multiple scheduling algorithms with provable guarantees Showed using simulation results that proposed algorithms perform close to optimal in practice

Phase 1: Cross-Base Station Resource Allocation J M L Cell\ Historical Demand DL UL D2D Cell J X Cell L Cell M Resources Available Cell\ Resources Allocated DL UL Cell J X Cell L Cell M R2D2 Phase 1 Algorithm Input Output

Phase 1: Cross-Sectors Resource Allocation Objective: Allocate UL and DL resources across 3 sectors in each of the directions Proposed algorithm satisfies following properties: Flexibly places the D2D traffic on UL and DL resources Localized and Scalable Ensure no interference across sectors belonging to different base stations See Paper for more constraints and detailed solution

Phase 2: Per-frame Scheduling Each sector was a part of different interfering set in Phase 1: Same resource may get assigned to co-located sectors D2 D3 D1

Phase 2: Per-frame Scheduling (contd.) Schedule UL, DL and D2D transmissions such that total throughput is maximized while avoiding interference Assign a time-frequency resource block to each flow in the three sectors Challenge: Path loss information between devices is unknown

Phase 2 Contributions See Paper for detailed solutions Scheduling algorithms at each Base Station: A greedy polynomial algorithm with approximation ratio of ½ A faster greedy polynomial algorithm with approximation ratio of ¼ More challenging since D2D can go on either DL resources or UL, but not both A greedy polynomial algorithm with approximation ratio of 1/3 Time-Divisioned System Frequency-Divisioned System See Paper for detailed solutions

Simulation Setup Simulation with 19 base stations and 57 sectors Modeled practical parameters including path loss, shadowing Other algorithms simulated R2D2 Low Complexity D2D Dynamic Genie (Optimal, knows path loss information between all users) No D2D Dynamic (Uses dynamic resource allocation and classifies all D2D traffic as Cellular) Existing Algorithm Static (Uses static resource allocation and does not use D2D as flexible traffic, Janis et al., IJCNS 2009)

Results with variation in D2 traffic R2D2 Low Complexity performs within 5% of optimal R2D2 Low Complexity gives a throughput of 2.5x and 4.9x compared to existing solution and No D2D, respectively Benefits coming from Intelligent resource allocation during Phase 1 while placing D2D traffic flexibly Interference-free scheduling during Phase 2 within 5% 2.5x 4.9x

Conclusions Investigated the problem of incorporating D2D communication in next generation cellular deployments with sectorization Identified a new benefit of D2D communications: Flexibility Towards that, proposed a solution that works at two different time granularities that ensures scalability Synergy between the two phases makes R2D2 light weight while avoiding all interference Proposed multiple algorithms with provable approximation ratios for resource allocation and scheduling Thank You