Gorrepati Rakesh received his MS in Electronics and Communication Engineering (ECE). His research work was supervised by Dr. Sachin Chaudhari. Here’s a summary of Gorrepati Rakesh’s MS thesis, Opportunistic Use of Successive Interference Cancellation in Reverse TDD HetNets as explained by him:
Increasing the spectral efficiency of the cellular networks has been and continues to be one of the major factors from 1G to 5G and beyond. In this regard, cellular operators have turned towards heterogeneous cellular networks (HetNets) to improve the area spectral efficiency of the networks. One of the significant challenges in HetNets is the underlying co-channel interference that is experienced by the users. Though the network throughput increases due to a better area spectral efficiency of a HetNet, there is a possibility that high interference will make few link capacities close to zero when users regard interference as noise (IAN). In other words, the dynamic nature of these cochannel interference links is also a problem due to its threat of causing outage or reduction in quality of service (QoS) to the user equipments (UEs). Instead of trying to avoid interference, nodes in the network can also exploit it if they are equipped with such capabilities. In this work, successive interference cancellation (SIC) technique is investigated to reduce the cross- tier interference in a reverse time division duplexing (RTDD) two-tier HetNet. In this work, the sum link capacity expressions for an RTDD HetNet, where the users use SIC to remove the cross-tier interference, is derived. The derived sum link capacity expression for SIC is compared with that of IAN to derive a switching condition on using SIC. This opportunistic use of SIC (which is referred to as switching in this thesis) ensures a minimum sum link capacity (proportional to the desired signal strengths) independent of the interference links, thereby solving the QoS problem. A network operator can also remove the interference dependency by splitting the available resources between the tiers. However, we have proven that the maximum sum link capacity that can be achieved by orthogonal resource allocation schemes is the same as the minimum that is ensured by switching. System-level simulations are done to show the gain in the overall system capacity of switching compared to only using either IAN or SIC for randomly paired UEs. The system capacity for IAN, SIC, and switching can be further improved by optimizing the selection of UEs across the tier (user pairing) that use the same frequency resources. In this regard, the user pairing scheme that should be used for IAN, SIC, and switching cases is formulated as a discrete optimization problem. This optimization problem also encompasses the cases where all the UEs do not need to be paired. The Hungarian algorithm is used to solve this optimization problem after some prepossessing done to the cost matrix. Through system-level simulations, the gains in system capacities for Hungarian paired UE using IAN, SIC, and switching is shown compared to their random counterparts.
The above-discussed results assume that the UE has the necessary demand for the supported down- link (DL) and uplink (UL) data rates. In the case of any traffic asymmetries, the subframe allocation can be dynamically adapted in a synchronized TDD HetNet. However, the inability of a synchronized RTDD HetNet’s frame structure questions the usefulness of these results when there is an asymmetric data demand (DL≫UL). In such cases, the UE does not need to operate at its full capacity in UL. Hence, it would throttle either the transmission rate or power. Rate expressions for rate and power throttling are derived for cases where UE uses SIC or regards IAN. System-level simulations are performed to com- pare the overall system throughput of IAN and SIC for different traffic asymmetries (DL:UL ratios). It is shown that if UEs does rate throttling, then using SIC would improve the overall achievable rates compared to IAN, and vice-versa for power throttling.