6 Contributions and publications
The main contributions of this study can be summarized as follows:
1. Evaluating the potential of recovery mechanisms and link adaptation on the dynamic TDD 5G system.
Dynamic TDD allows to optimally accommodate the traffic, but also brings challenges in terms of interference variability, since the interfering source may change at each time slot. Although the use of interference suppres-sion receivers has been proven to mitigate the drawback introduced by dynamic TDD, other mechanisms are required to cope with the unstable interference. HARQ and OLLA are introduced to the 5G system design to evaluate their potential in dealing with the unpredictable interference.
Results show that recovery mechanisms at lower layers are beneficial for the system. On the other hand, OLLA shows limited gain but it does not come at the expenses of increased overhead or delay.
2. Introducing a RRM module to provide support for FD technology into the envisioned 5G system.
Given the recent advances in the transceiver design, FD communication is positioned as a potential technology component for 5G systems. There are three different types of FD communication: when both the BS and the UE are FD capable (bidirectional FD); when only the BS is able to exploit such technology for data transmission (BS FD); and when only the BS is FD capable and uses the technology for relaying user data (relay FD). The module introduced allows for the evaluation of the first two FD types. It is composed by two submodules. Firstly, the optimal transmission direction is extracted per each node. Secondly, the decision of which user or pair of users is going to be scheduled is obtained. By separating functionali-ties, the number of operations to perform can be reduced and hence the processing time and the system complexity.
3. Identifying the constraints that limit the achievable gain of FD in 5G small cell networks.
The performance of bidirectional FD and BS FD is evaluated in the 5G ultra-dense small cell system, considering dynamic TDD as the baseline system performance. An exhaustive analysis is carried out, identifying the constraints that affect the gain that FD can provide over dynamic TDD.
In addition, an evaluation of the interaction of FD with higher protocols, such as TCP, is provided. The overall analysis is carried out assuming
Background and Thesis Overview
ideal self-interference cancellation, thus providing an upper bound of the achievable FD gain in indoor small cell networks.
The evaluation of the potential of FD in 5G indoor small cell networks shows that this technology brings limited gains over dynamic TDD. Conse-quently, identifying other applications where FD could provide larger im-provements is required.
4. Evaluating the potential of FD to accelerate the discovery procedure in D2D communication.
Direct D2D communication is becoming popular given its potential to of-fload traffic from the infrastructure and to allow for a faster communica-tion among devices. However, prior to the exchange of data, devices must discover their peers. This procedure, known as device discovery, should be completed in 10 milliseconds to meet the requirements imposed by 3GPP [13]. The potential of FD technology to speed up such process and to satisfy the latency requirements is evaluated. The analysis shows that the use of interference cancellation receivers is a requirement to achieve low latency, and in that case, FD technology is able to further reduce the discovery time.
5. Designing a protocol supporting FD technology to provide fast discov-ery in D2D communication.
Given the potential of FD in accelerating the discovery phase in D2D com-munication, a protocol to perform such procedure is proposed. A tech-nique to estimate the number of neighboring devices alongside a signaling exchange mechanism to reduce the network interference are introduced.
Furthermore, several approaches on how to use the signaled information are proposed and evaluated. The study compares the performance of HD transmission mode with FD communication, proving that FD technology achieves lower latency, achieving the 3GPP requirements in most of the evaluated scenarios.
In addition to these contributions, exhaustive development of the sim-ulators was performed during the entire period of the studies. The C++
event-driven simulator used to extract the results for Part II and III of the dissertation was originally implementing the LTE system, and it was modi-fied to include the envisioned 5G design [41]. The main contributions to the simulator development are listed below:
6. Contributions and publications
• 5G processing delay
The control and data transmission timing, processing delay and the pro-posed frame structure defined in the concept [41] were implemented in the simulator to extract realistic results.
• HARQ and link adaptation
An asynchronous and non-adaptive implementation of the HARQ mech-anism was implemented in the simulator. In terms of link adaptation, an scheme to obtain the Modulation and Coding Scheme (MCS) for transmis-sion based on the average of the latestkSINR samples in logarithmic scale was implemented, as well as a static and a dynamic OLLA mechanism.
• Full duplex transmission
The procedure for the signal transmission and reception in the simula-tor was modified to allow for both HD and FD transmission modes. The modification was included in both control and data parts, although in this dissertation such functionality is only exploited in the data part.
• Multi-user functionality
The limitation of a single user per small cell did not allow for evaluating the FD case where the AP is the only node which can exploit simultaneous transmission and reception. The multi-user functionality affected the entire simulator, e.g., the generation of data flows or the RLC data aggregation and acknowledgment transmission.
• User and transmission direction scheduler
Introducing the multi-user functionality required a mechanism to decide the transmission mode, the transmission direction in case of HD and the scheduled user(s).
• Ideal RLC control transmission
The RLC acknowledgment transmission occupied a single Transmission Time Interval (TTI) and the whole bandwidth. Since the generated over-head was unrealistic, an ideal RLC acknowledgment transmission through the control channel was implemented.
The post-processing of the data obtained from the simulator was per-formed with Matlab to extract and present the results. Finally, Part IV of the thesis is carried out with an own Matlab simulator, developed entirely and only for the purpose of the last study in D2D communication.
The following publications were authored or co-authored in relation with this study:
Background and Thesis Overview
Part I:
• Improving Link Robustness in 5G Ultra-Dense Small Cells by Hybrid ARQ.
Gatnau, Marta; Catania, Davide; Frederiksen, Frank; Cattoni, Andrea Fabio;
Berardinelli, Gilberto; Mogensen, Preben. IEEE 11th International Sympo-sium on Wireless Communications Systems (ISWCS), 2014.
• Dynamic Outer Loop Link Adaptation for the 5G Centimeter-Wave Con-cept. Gatnau, Marta; Catania, Davide; Frederiksen, Frank; Cattoni, Andrea Fabio; Berardinelli, Gilberto; Mogensen, Preben. IEEE 21th European Wire-less Conference, 2015.
• The Potential of Flexible UL/DL Slot Assignment in 5G Systems. Cata-nia, Davide; Gatnau, Marta; Cattoni, Andrea Fabio; Frederiksen, Frank;
Berardinelli, Gilberto; Mogensen, Preben. IEEE 80th Vehicular Technology Conference (VTC) Fall, 2014.
• Flexible UL/DL in Small Cell TDD Systems : A Performance Study with TCP Traffic. Catania, Davide;Gatnau, Marta; Cattoni, Andrea Fabio; Fred-eriksen, Frank; Berardinelli, Gilberto; Mogensen, Preben. IEEE 81st Vehic-ular Technology Conference (VTC) Spring, 2015.
Part II:
• Full Duplex Communication Under Traffic Constraints for 5G Small Cells.
Gatnau, Marta; Catania, Davide; Berardinelli, Gilberto; Mahmood, Nurul Huda; Mogensen, Preben. IEEE 82nd Vehicular Technology Conference (VTC) Fall, 2015.
• Can Full Duplex Boost Throughput and Delay of 5G Ultra-Dense Small Cell Networks?. Gatnau, Marta; Berardinelli, Gilberto; Mahmood, Nurul Huda; Mogensen, Preben. IEEE 83rd Vehicular Technology Conference (VTC) Spring, 2016.
• Impact of Transport Control Protocol on Full Duplex Performance in 5G Networks. Gatnau, Marta; Berardinelli, Gilberto; Mahmood, Nurul Huda;
Mogensen, Preben. IEEE 83rd Vehicular Technology Conference (VTC) Spring, 2016.
• On the Potential of Full Duplex Performance in 5G Ultra-Dense Small Cell Networks. Gatnau, Marta; Fleischer, Marko; Berardinelli, Gilberto; Mah-mood, Nurul Huda; Mogensen, Preben; Heinz, Helmut. IEEE European Signal Processing Conference (EUSIPCO), 2016.
• Analyzing the Potential of Full Duplex in 5G Ultra-Dense Small Cell Net-works. Gatnau, Marta; Berardinelli, Gilberto; Mahmood, Nurul Huda;