Cellular traffic offloading onto network-assisted device-to-device connections
IEEE Communications Magazine
While operators have finally started to deploy fourth generation broadband technology, many believe it will still be insufficient to meet the anticipated demand in mobile traffic over the coming years. Generally, the natural way to cope with traffic acceleration is to reduce cell size; and this can be done in many ways. The most obvious method is via pico-cells, but this requires additional capital (CAPEX) and operational (OPEX) investment to install and manage these new base stations. Another
... pproach, which avoids this additional CAPEX/OPEX, involves offloading cellular traffic onto direct device-todevice (D2D) connections whenever the users involved are in proximity. Given that most client devices are capable of establishing concurrent cellular and WiFi connections today, we expect the majority of immediate gains from this approach to come from the use of the unlicensed bands. However, despite its huge commercial success, WiFi-based direct connectivity may suffer from stringent session continuity limitations, excessive user contention, and cumbersome manual setup/security procedures. In this article, we detail our vision of integrating managed D2D communications into current cellular technology to overcome the limitations of WiFi. We also quantify the estimated network performance gains from offloading cellular traffic onto D2D connections. Our analysis is based on an advanced system-level simulation toolkit which captures the relevant details of the network environment and on a detailed characterization of dynamic D2D communications based on stochastic geometry. We conclude that D2D communications provide a significant boost to network capacity as well as user energy efficiency and quality of service perception. Introduction Industry has recently completed the fourth generation (4G) of mobile broadband standards offering decisive improvements in all aspects of wireless system design. However, with the predicted explosion in both types and numbers of wireless devices  it is commonly believed that despite novel 4G technologies, mobile broadband networks will still face a capacity crunch in the near future. The most expedient way of boosting network capacity is by increasing cell densities across the network (i.e. shrink cell sizes and increase their numbers in congested areas). This improves network capacity by increasing the frequency reuse per unit area and the average data rate per transmission  (smaller cells yield shorter radio links and thus higher data rates). However, greater cell densities imply increased interference management complexities and CAPEX/OPEX for the mobile broadband operator. Hence, industry will not be able to leverage the full potential of the "small cell revolution" until changes are made to the way we approach wireless content delivery. Currently, the lion's share of expected mobile traffic growth comes from peer-to-peer (P2P) services that commonly involve clients in close proximity. This presents an excellent opportunity for clients to offload their traffic onto direct device-to-device (D2D) radio links (which are generally shorter and lower-to-the-ground than standard "small cell" connections). If the mobile broadband operators were to encourage this form of offloading by providing assistance with device discovery, D2D connection establishment, and service continuity, it could reduce the network load without the cost of additional infrastructure while creating the potential for new service revenue. From the client's perspective the benefits are clear; D2D communication promises higher data rates, lower transfer delays, and better power efficiency . These potential benefits along with the growing number of services and applications that could leverage user proximity have led academia and industry to aggressively pursue research and standardization of D2D communications over the past couple of years. The potential applications of D2D in cellular networks are numerous  and include local voice service (offloading calls between proximate users), multimedia content sharing, gaming, group multicast, context-aware applications, and public safety. However, depending on client mobility patterns, some services are better suited for proximity-based communication than others  . For example, if D2D peers are non-stationary, the quality of the link may change dramatically over short periods of time, thus making it difficult to guarantee service. In these cases, the best candidates for network offloading are delay-tolerant services, i.e. those whose traffic can be queued until either the D2D link recovers or a path switch to the infrastructure network completes (e.g. video-on-demand or file transfers). However, if both clients are (semi-)stationary, many other P2P services, such as cooperative streaming and social gaming, can be offloaded onto D2D links with good results. We recently completed an advanced, in-depth characterization of D2D communication with the goal of fully understanding the performance gains and resolving any impediments to them. In this article, we reveal our most important findings starting with a technical discussion of several options for networkassisted D2D and their potential implementation within 3GPP's Long Term Evolution (LTE) technology. We continue by quantifying the predicted gains of these solutions via both system-level simulations and mathematical analysis based on stochastic geometry.