PhD Dissertation Assessment (

1 dr hab. inż. Sławomir Hausman, prof. uczelni

Instytut Elektroniki

Wydział Elektrotechniki, Elektroniki, Informatyki i Automatyki Politechnika Łódzka

ul. Wólczańska 211/215 90-924 Łódź

Name of the PhD Student: mgr inż. Anna Maria Łukowa

Title of the dissertation: " Coordinated Radio Resource Management in 5G Systems with Time Division Duplex"

Supervisor: prof. dr hab. Inż. Krzysztof Wesołowski

Institution: Wydział Informatyki i Telekomunikacji Politechniki Poznańskiej

PhD Dissertation Assessment (

Recenzja dysertacji doktorskiej)

THE FORMAL BASIS

The assessment of the dissertation was commissioned by dr hab. inż. Andrzej Jaszkiewicz – Dean of the Faculty of Computing and Telecommunications of the Poznan University of Technology (Wydział Informatyki i Telekomunikacji Politechniki Poznańskiej) by the formal letter DIiT-63-01/20 of 2020.02.20. The corresponding dissertation was submitted by mgr inż. Anna Maria Łukowa in fulfillment of the requirements for the doctoral degree.

TOPIC, SCOPE, GOALS, HYPOTHESES, CLAIMS

The emergence of fifth-generation (5G) mobile networks promises to fundamentally change how citizens, companies, and government agencies will interact with technology in general (not only telecommunication technologies. 5G will enable the next generation of consumer and industrial solutions, such as autonomous vehicles, smart grid technology, wireless factories, Cyber-Physical Systems (IoT), e-Health, remote surgery, environmental monitoring, real-time Augmented Reality (AR), advanced robotics, and precision agriculture systems. These application areas pose diverse traffic performance requirements, which can be broadly categorized into three general traffic classes:

enhanced mobile broadband (eMBB), massive machine-type communications (mMTC), and ultra- reliable and low-latency communications (URLLC). 5G network RAN specification exploits both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) solutions, to achieve physical layer flexibility in supporting the various traffic classes. Until recently, the uplink and downlink transmission slots of TDD were fixed and synchronized across the system, which limited the flexibility of the scheme. In contrast, flexible per-cell TDD schemes would allow for fast switching between uplink and downlink, increasing network capacity. In particular, using TDD, which can dynamically allocate time slots, it would be possible to achieve improved usage of the frequency resources in the case of UL/DL imbalance. Some researchers have recently reported experimental results [E. Zeydan,

2 O. Dedeoglu and Y. Turk, “Experimental Evaluations of TDD-Based Massive MIMO Deployment for Mobile Network Operators,” in IEEE Access, vol. 8, pp. 33202-33214, 2020, doi:

10.1109/ACCESS.2020.2974277.], revealing that TDD-based massive MIMO has higher cell throughput than FDD-based MIMO deployments. TDD seems to be a better choice for systems exploiting massive MIMO regarding reduced complexity associated with channel characteristics estimation when compared with FDD. To summarise, TDD probable benefits include fast and flexible switching, low latency, and efficient massive MIMO. With all its benefits, flexible FDD poses some challenges, the major one being inter-cell interference. Also, TDD can potentially adversely affect one of the 5G critical advantages, i.e., low user plane latency, the more so that guard time must be increased with distance. These problems, which motivated the author of the dissertation, can be mitigated, e.g., by transmission direction UL/DL switching on the transmission slot basis. The author concentrates on three groups of solutions which are represented by the following three claims, as they are phrased in Introduction:

1. Inter-cell coordinated scheduling can significantly improve the overall system performance as compared to uncoordinated scheduling. The additional gains can be achieved with IC-aware scheduling which leverages IC capabilities of the receivers.

2. Joint cell clustering and scheduling can effectively reduce the coordinated scheduling search overhead and at the same time does not limit the system flexibility. Furthermore, the system performance with flexible TDD can benefit from joint scheduling, MIMO rank and rate coordination. In particular, the exploitation of the interference relations allows to design an effective, low-complexity scheme for MIMO rank coordination.

3. User-specific beamforming together with advanced receivers can be efficiently used for interference mitigation with none or very limited cooperation between the base station nodes.

In the light of the state of the art, the claims were relevant both at the time the PhD project commenced and at the time when this review is being prepared. The author decided to focus her research on small-cell scenarios.

CONTENT

The dissertation comprises 195 pages and follows the standard structure of a PhD thesis in engineering/applied science. The front matter consists of the Title page, Abstract (in English and in Polish), Acknowledgements, Contents, List of Figures, List of Tables, and List of Abbreviations. The body matter is split into eight chapters, including Introduction and Summary of the results and the Conclusions. The back matter is an Appendix entitled “System level simulator description”.

Chapter 1 - Introduction is well written and provides an excellent background for understanding the following chapters. It includes three, clearly formulated theses/claims, which were relevant not only at the time when the PhD project commenced but also when this review is being prepared. All three claims are justified with the objective of improving the performance of 5G mobile cellular systems and are backed up by the state-of-the-art knowledge in the field. Introduction clearly indicates the scope of the dissertation, its purpose, and its research aims.

The central part of the thesis consists of chapters 2-7, each of them addressing a specific aspect of the problem under study.

3 Chapter 2 - 5G features explains flexible Time Division Duplex and various 5G frame configurations as well as the concepts of radio resource management, Multiple-Input Multiple-Output technology.

Chapter 3 - Background literature study provides a review of the present progress and potential future developments in the methods of radio resource management and interference mitigation. The concept of self-backhaul systems is also presented. In general, (also beyond this chapter) the dissertation excels in the discussion of the literature and the state-of-the-art analysis.

Chapter 4 - Multiple-input multiple-output is the first of the four chapters encompassing the author’s original contribution to the field. In the beginning, linear (Minimum Mean Square Error MMSE, and Interference Rejection Combining - IRC), and non-linear (Successive Interference Cancellation - SIC) interference mitigation techniques are explained. Then, the simulation of achievable data rates using the techniques for the case with perfect and erroneous channel estimation are presented.

Chapters 5 - 7 are devoted to the investigation of three different centralized/coordinated scheduling approaches. Each of the chapters contains an in-depth theoretical background, description of the system model, performance evaluation, and conclusions. The achievements presented in this part of the thesis are supported by peer-reviewed journal and conference papers co-authored by the author.

Chapter 5 - Centralised scheduling with flexible time division duplex is devoted to a new, original method which allows for significant interference, provided that it can be reduced at the reception site. This requires the use of interference cancellation or suppression, e.g., using IRC or SIC techniques, as described in Chapter 4. The stress in Chapter 5 is on scheduling coordination between interfering cells. To this end, two new algorithms are proposed, and their performance is evaluated, namely: a) IC-aware joint scheduling based on cell-clustering, and b) SPARK (Successively PAired RanK allocation) rank coordination. These results have been summarised in the following peer- reviewed publications:

[1] Lukowa, V. Venkatasubramanian, “Performance of strong interference cancellation in flexible UL/DL TDD systems using coordinated muting, scheduling and rate allocation,” Proceedings of IEEE Wireless Communications and Networking Conference (WCNC), pp. 1 - 7, April 2016.

[2] Lukowa, V. Venkatasubramanian, “On the Value of MIMO Rank Coordination for Interference Cancellation-based 5G Flexible TDD Systems,” Proceedings of EEE Conference on Standards for Communications and Networking (CSCN), pp. 1 - 7, November 2016.

[3] Lukowa, V. Venkatasubramanian, “Centralized UL/DL Resource Allocation for flexible TDD Systems with Interference Cancellation,” IEEE Transactions on Vehicular Technology, vol. 68, no. 3, pp. 2443 - 2458, March 2019.

Chapter 6 - Coordinated scheduling with MIMO beamforming concentrates on coordinated beamforming schemes in application to dynamic TDD systems. This and the next chapter are the most advanced and intricate part of the research presented in the dissertation, both in terms of mathematical modeling and simulations. As a result of this investigation, the performance gain of standalone and partially distributed scheduling to reduce cross-link interference has been evaluated.

As a reference, the author used the upper-bound performance of fully centralized scheduling (which might be impractical because of its complexity). The author arrives at valuable quantitative conclusions, such as “The results showed that centralized scheduling employing advanced receivers capable of interference cancellation can improve both the uplink and the downlink performance by

4 over 47% and 36%, respectively, as compared to centralized scheduling without IC.” These results have been presented in the following peer-reviewed publications:

[4] A. Lukowa, V. Venkatasubramanian, “On the Performance of 5G Flexible TDD Systems with Coordinated Beamforming,” 2018 IEEE Eleventh International Workshop on Selected Topics in Mobile and Wireless Computing (STWiMob), pp. 175 - 180, October 2018.

[5] _ A. Lukowa, V. Venkatasubramanian, “Coordinated User Scheduling in 5G Dynamic TDD Systems with Beamforming,” 2018 IEEE 29th Annual International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC): Recent Results Papers, pp. 596 - 597, September 2018.

[6] A. Lukowa, V. Venkatasubramanian, “Delay-Aware Heuristic-Based Scheduling for 5G Flexible TDD Systems with Beamforming” in Proceedings of IEEE Vehicular Technology Conference (VTC Spring), pp. 1- 7, June 2019.

[7] A. Lukowa, V. Venkatasubramanian, "Minimalizacja interferencji w systemach 5G z ształtowaniem wiazki,"

Przeglad Telekomunikacyjny, June 2019.

Chapter 7 - Coordinated scheduling for dynamic in-band self-backhaul system investigates the problem which is relevant to dense 5G networks typical of future millimeter frequency band deployments. The author considers centralized scheduling exploiting SIC receivers for wireless relaying using integrated access and backhaul. Such schemes may be beneficial in terms of significantly reduced deployment costs of the backhaul infrastructure. Detailed mathematical models have been proposed in the dissertation for performance evaluation of the suggested schemes, specifically, using backhaul and uplink multiple access channels. The author proves quantitively significant performance gains of this approach, compared to the scheme without interference coordination. These results have been presented in the following publications:

[8] A. Lukowa, V. Venkatasubramanian and P. Marsch, “Dynamic Self-Backhauling in 5G Networks,”

Proceedings of IEEE Annual International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), pp. 1 - 7, September 2018. Paper awarded with PIMRC Best Student Paper Award.

[9] A. Lukowa, V. Venkatasubramanian, “Dynamic In-Band Self-Backhauling with Interference Cancellation,”

Proceedings of IEEE Conference on Standards for Communications and Networking (CSCN), pp. 1 - 7, October 2018.

[10] A. Lukowa, V. Venkatasubramanian, “Dynamic In-band Self-Backhauling for 5G Systems with Inter-cell Resource Coordination,” International Journal of Wireless Information Networks, vol. 26, no. 4, pp. 319 - 330, December 2019.

Chapter 8 — Summary of the results and the conclusions – briefly recapitulates the novel ideas and solutions, as well as simulation results presented in the dissertation.

GENERAL REMARKS

1. The aims of the research have been clearly defined. As mentioned above, the author formulates three verifiable theses/claims. In the light of state of the art, they were relevant and of practical importance. The three claims have been verified by the results of extensive computer simulations based on mathematical models explained in detail in the dissertation.

2. The main achievements of the research are supported by peer-reviewed journal and conference papers co-authored by Ms Anna Łukowa.

3. The author uses appropriate research methods and techniques (mathematical models, simulation methods, etc.) for the research topic under consideration. I want to emphasize a

5 comprehensive investigation of the topic and the ability to interpret results and develop conclusions. The dissertation excels in the discussion of the literature and the state-of-the-art analysis, which is well prepared. The number of references is notable (200) as for a PhD dissertation, and their selection is suitable both for the background/introductory knowledge and for the state-of-the-art analysis in the discussed field.

4. The dissertation is well structured, comprehensible – with unity and connectedness, and is prepared with excellent editing skills. The text is written in concise, clear, almost error-free English with relevant terminology and unambiguous wording. All the figures are carefully prepared and clearly annotated.

CRITICAL REMARKS

Considering the high standard o the dissertation in all its aspects, I have only a few minor critical remarks.

1. In the statement: “Furthermore, in the case of sectorised deployment, the UE has to be within the aperture of the base station antenna panel.”, what was most probably meant is the antenna beamwidth, not aperture.

2. Bearing in mind Ms Anna Łukowa’s excellent command of English, I will not mention minor editing errors, such as “far distant” in the sentence: “The role of self-backhaul nodes is thus to opportunistically route the data around obstacles, as well as improve coverage to far distant users.”

3. The author decided to focus her research on small-cell scenarios, which influenced the choice of radio wave propagation models. Finally, the urban microcell street canyon model is used. I think that the usage of this specific model could have been justified in more detail.

4. Since the first reference to the simulation methodology described in Appendix A is in section 5.8, it is not clear what simulation tools were used to obtain the results presented in Fig. 4.7-4.13.

CONCLUSION

In my opinion, the dissertation by Ms Anna Maria Łukowa fully complies both with Polish and international standards for PhD dissertations in the field of applied computer science and telecommunications in particular. It meets the requirements of art. 13 Sec. 1 of the Polish Bill on Academic Degree of March 14, 2003 with later amendments (Ustawa o stopniach naukowych i tytule naukowym oraz o stopniach i tytule w zakresie sztuki z dnia 14 marca 2003 r z późniejszymi zmianami). Specifically, it is clear that Ms Łukowa has presented original solutions to a scientific problem. She has demonstrated general theoretical knowledge in the area of modern cellular systems, including its fifth generation. She clearly has the ability to independently conduct scientific research. The research methodologies have been described in detail, which follows the good practice of science – reproducibility, and replicability. In the conclusion of the assessment, I strongly recommend that the dissertation be subjected to public defense. Moreover, I propose that the dissertation be distinguished (Polish: doktorat z wyróżnieniem).

Łódź, 15 July 2012 Sławomir Hausman