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Ultra-Densification for Future Cellular Networks: Performance Analysis and Design Insights
KTH, School of Electrical Engineering and Computer Science (EECS), Communication Systems, CoS, Radio Systems Laboratory (RS Lab).ORCID iD: 0000-0002-7559-8911
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The traffic volume in wireless communication has grown dramatically in the lastdecade. Recent predictions indicate such data storm will be even more violent in theshort run. Potential solutions for accommodating the rapid traffic growth can besummed up into three categories: broadening the available bandwidth, improvingthe spectral efficiency, and densifying the infrastructure. In this thesis, we focuson the densification dimension which has been proven to be the most effectiveone in the past. The current gain of network densification mainly comes from cellsplitting, thereby serving more user equipment (UE) simultaneously. This trendwill decelerate as the base station (BS) density gets closer to or even surpassesthe UE density which forms an ultra-dense network (UDN). Thus, it is crucialto understand the behavior and design operations of ultra-densification in futurenetworks.

An important question for future system design and operating strategy is whichelement is more effective than others. To this end, we start from comparing the effectivenessof densification with spectrum expansion and multi-antenna systems interms of meeting certain traffic demand. Our findings show that deploying more BSsprovides a substantial gain in sparse network but the gain decreases progressively ina UDN. Meanwhile, even with the same area throughput, different combinations ofindividual throughput and UE density lead to different requirements for resources.The diminishing gain appearing in UDNs makes us curious to know if there existsa terminal on the way of densification. Such uncertainty leads to the study onthe asymptotic behavior of densification. By incorporating a sophisticated boundeddual-slope path loss model and practical UE densities in our analysis, we present theasymptotic behavior of ultra-densification: the coverage probability and area spectralefficiency (ASE) have non-zero convergences in asymptotic regions unless theUE density goes to infinity (full load). Our results suggest that network densificationcannot always improve the UE performance or boost the network throughput.

Next, we shift our focus to the operations of UDNs. We first study BS cooperationsin two UDN scenarios: homogeneous and heterogeneous UDNs which aredistinguished by BS types. In both cases, the cooperation rules become more complicatedthan those in traditional networks. Either channel state information (CSI) orextra delay information needs to be acquired in order to obtain cooperation gains.At last, we investigate the feasibility of applying random beamforming to initialaccess in millimeter-wave (mmWave) UDNs. To our surprise, the simple methodcan provide sufficient performance in both control and data plane, comparing withthe existing schemes. Therefore, it may be unnecessary to develop complex algorithmsfor initial access in future dense mmWave networks. The findings indicatethat UDN may complicate network operations while it may also facilitate the use ofsimple schemes. Our work provides insights into the understanding of the networkdensification and thus paves the way for the operational design of future UDNs.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. 59
Series
TRITA-EECS-AVL ; 2018:88
National Category
Communication Systems
Research subject
Telecommunication
Identifiers
URN: urn:nbn:se:kth:diva-239175ISBN: 978-91-7873-017-9 (print)OAI: oai:DiVA.org:kth-239175DiVA, id: diva2:1264028
Public defence
2018-12-07, Ka-Sal C (Sal Sven-Olof Öhrvik), Electrum, Kungl Tekniska högskolan, Kistagången 16, Kista, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20181119

Available from: 2018-11-19 Created: 2018-11-19 Last updated: 2018-11-19Bibliographically approved
List of papers
1. Tradeoff between spectrum and densification for achieving target user throughput
Open this publication in new window or tab >>Tradeoff between spectrum and densification for achieving target user throughput
2015 (English)In: IEEE Vehicular Technology Conference, IEEE , 2015Conference paper, Published paper (Refereed)
Abstract [en]

Dense deployment which brings small base stations (BS) closer to mobile devices is considered as a promising solution to the booming traffic demand. Meanwhile, the utilization of new frequency bands and spectrum aggregation techniques provide more options for spectrum choice.Whether to increase BS density or to acquire more spectrum is a key strategic question for mobile operators. In this paper, we investigate the relationship between BS density and spectrum with regard to individual user throughput target. Our work takes into account load-dependent interference model and various traffic demands. Numerical results show that densification is more effective in sparse networks than in already dense networks. In sparse networks, doubling BS density results in almost twofold throughput increase. However, in dense networks where BSs outnumber users, more than 10 times of BS density is needed to double user throughput. Meanwhile, spectrum has a linear relationship with user throughput for a given BS density. The impact of traffic types is also discussed. Even with the same area throughput requirement, different combination of user density and individual traffic amount leads to different needs for BS density and spectrum.

Place, publisher, year, edition, pages
IEEE, 2015
Keywords
Densification, Individual user throughput, Spectrum, Frequency bands, Mobile devices, Individual traffic, Interference modeling, Linear relationships, Mobile operators, Numerical results, Spectrum Aggregation, Traffic demands, Throughput
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-175100 (URN)10.1109/VTCSpring.2015.7146138 (DOI)000371404700552 ()2-s2.0-84940399382 (Scopus ID)9781479980888 (ISBN)
Conference
81st IEEE Vehicular Technology Conference, VTC Spring 2015, 11 May 2015 through 14 May 2015
Note

QC 20151211

Available from: 2015-12-11 Created: 2015-10-09 Last updated: 2018-11-20Bibliographically approved
2. Technical rate of substitution of spectrum in future mobile broadband provisioning
Open this publication in new window or tab >>Technical rate of substitution of spectrum in future mobile broadband provisioning
2015 (English)In: 2015 IEEE International Symposium on Dynamic Spectrum Access Networks, DySPAN 2015, IEEE conference proceedings, 2015, p. 297-300Conference paper, Published paper (Refereed)
Abstract [en]

Dense deployment of base stations (BSs) and multi-antenna techniques are considered as key enablers for future mobile networks. Meanwhile, spectrum sharing techniques and utilization of higher frequency bands make more bandwidth available. An important question for future system design is which element is more effective than others. In this paper, we introduce the concept of technical rate of substitution (TRS) from microeconomics and study the TRS of spectrum in terms of BS density and antenna number per BS. Numerical results show that TRS becomes higher with increasing user data rate requirement, suggesting that spectrum is the most effective means of provisioning extremely fast mobile broadband.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2015
Keywords
densification, multi-antenna, Spectum, stochastic geometry
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-185918 (URN)10.1109/DySPAN.2015.7343923 (DOI)000380544200043 ()2-s2.0-84960327036 (Scopus ID)9781479974528 (ISBN)
Conference
IEEE International Symposium on Dynamic Spectrum Access Networks (DySPAN),Stockholm,Sept.29-Oct.2, 2015
Note

QC 20160429

Available from: 2016-04-28 Created: 2016-04-28 Last updated: 2018-11-20Bibliographically approved
3. On the asymptotic behavior of ultra-densification under a bounded dual-slope path loss model
Open this publication in new window or tab >>On the asymptotic behavior of ultra-densification under a bounded dual-slope path loss model
2017 (English)In: European Wireless 2017 - 23rd European Wireless Conference, Institute of Electrical and Electronics Engineers (IEEE), 2017, article id 8011320Conference paper, Published paper (Refereed)
Abstract [en]

In this paper, we investigate the impact of network densification on the performance in terms of downlink signal-to-interference (SIR) coverage probability and network area spectral efficiency (ASE). A sophisticated bounded dual-slope path loss model and practical user equipment (UE) densities are incorporated in the analysis, which have never been jointly considered before. By using stochastic geometry, we derive an integral expression along with closed-form bounds of the coverage probability and ASE, validated by simulation results. Through these, we provide the asymptotic behavior of ultra-densification. The coverage probability and ASE have non-zero convergence in asymptotic regions unless UE density goes to infinity (full load). Meanwhile, the effect of UE density on the coverage probability is analyzed. The coverage probability will reveal an U-shape for large UE densities due to interference fall into the near-field, but it will keep increasing for low UE densites. Furthermore, our results indicate that the performance is overestimated without applying the bounded dual-slope path loss model. The derived expressions and results in this work pave the way for future network provisioning.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2017
Keywords
Bounded path loss model, Dual-slope path loss model, Network densification, Stochastic geometry
National Category
Communication Systems
Identifiers
urn:nbn:se:kth:diva-216320 (URN)2-s2.0-85030693243 (Scopus ID)9783800744268 (ISBN)
Conference
23rd European Wireless Conference, EW 2017, Dresden, Germany, 17 May 2017 through 19 May 2017
Note

QC 20171023

Available from: 2017-10-23 Created: 2017-10-23 Last updated: 2018-11-20Bibliographically approved
4. Cooperative transmissions in ultra-dense networks under a bounded dual-slope path loss model
Open this publication in new window or tab >>Cooperative transmissions in ultra-dense networks under a bounded dual-slope path loss model
Show others...
2017 (English)In: 2017 European Conference on Networks and Communications (EuCNC), Institute of Electrical and Electronics Engineers (IEEE), 2017Conference paper, Published paper (Refereed)
Abstract [en]

In an ultra-dense network (UDN) where there are more base stations (BSs) than active users, it is possible that many BSs are instantaneously left idle. Thus, how to utilize these dormant BSs by means of cooperative transmission is an interesting question. In this paper, we investigate the performance of a UDN with two types of cooperation schemes: Non-coherent joint transmission (JT) without channel state information (CSI) and coherent JT with full CSI knowledge. We consider a bounded dual-slope path loss model to describe UDN environments where a user has several BSs in the near-field and the rest in the far-field. Numerical results show that non-coherent JT cannot improve the user spectral efficiency (SE) due to the simultaneous increment in signal and interference powers. For coherent JT, the achievable SE gain depends on the range of near-field, the relative densities of BSs and users, and the CSI accuracy. Finally, we assess the energy efficiency (EE) of cooperation in UDN. Despite costing extra energy consumption, cooperation can still improve EE under certain conditions.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2017
Keywords
bounded path loss model, cooperative transmissions, multi-slope path loss model, Ultra-dense networks
National Category
Communication Systems
Identifiers
urn:nbn:se:kth:diva-221121 (URN)10.1109/EuCNC.2017.7980732 (DOI)2-s2.0-85039946526 (Scopus ID)9781538638736 (ISBN)
Conference
2017 European Conference on Networks and Communications, EuCNC 2017, Oulu, Finland, 12 June 2017 through 15 June 2017
Note

QC 20180115

Available from: 2018-01-15 Created: 2018-01-15 Last updated: 2018-11-20Bibliographically approved
5. Cooperation Strategies for Partly Wireless C-RAN
Open this publication in new window or tab >>Cooperation Strategies for Partly Wireless C-RAN
2018 (English)In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 22, no 6, p. 1248-1251Article in journal (Refereed) Published
Abstract [en]

This letter presents a variant of cloud radio access network (C-RAN) architecture, which we term partly wireless C-RAN (PW-CRAN). It is characterized by extra radio nodes connected through the existing remote radio heads. Because of the newly connected nodes and the consequent additional delay, the operation of PW-CRAN requires different approaches compared with that of the typical C-RAN. Specifically, the effect of delayed channel state information on the suitability of network cooperation should be identified. To tackle this problem, we introduce two representative cooperation strategies and evaluate the performance with regard to the delay via simulations. Numerical results suggest that it is better to exclude the extra nodes from cooperation if they incur excess delay. Furthermore, whether the delay is deemed excessive depends on the interference environment. Hence, we provide quantified guidelines on the cooperation strategy of PW-CRAN.

Place, publisher, year, edition, pages
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2018
Keywords
Cloud radio access network, cooperation strategy, fronthaul link, delay, imperfect channel state information
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Communication Systems
Identifiers
urn:nbn:se:kth:diva-231721 (URN)10.1109/LCOMM.2018.2807814 (DOI)000435175800035 ()2-s2.0-85042172488 (Scopus ID)
Note

QC 20180815

Available from: 2018-08-15 Created: 2018-08-15 Last updated: 2018-11-20Bibliographically approved
6. Reducing Initial Cell-search Latency in mmWave Networks
Open this publication in new window or tab >>Reducing Initial Cell-search Latency in mmWave Networks
Show others...
2018 (English)In: INFOCOM 2018 - IEEE Conference on Computer Communications Workshops, IEEE conference proceedings, 2018Conference paper, Published paper (Refereed)
Abstract [en]

Millimeter-wave (mmWave) networks rely on directional transmissions, in both control plane and data plane, to overcome severe path-loss. Nevertheless, the use of narrow beams complicates the initial cell-search procedure where we lack sufficient information for beamforming. In this paper, we investigate the feasibility of random beamforming for cell-search. We develop a stochastic geometry framework to analyze the performance in terms of failure probability and expected latency of cell-search. Meanwhile, we compare our results with the naive, but heavily used, exhaustive search scheme. Numerical results show that, for a given discovery failure probability, random beamforming can substantially reduce the latency of exhaustive search, especially in dense networks. Our work demonstrates that developing complex cell-discovery algorithms may be unnecessary in dense mmWave networks and thus shed new lights on mmWave system design.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2018
National Category
Communication Systems
Identifiers
urn:nbn:se:kth:diva-239139 (URN)10.1109/INFCOMW.2018.8406924 (DOI)2-s2.0-85050687161 (Scopus ID)
Conference
IEEE INFOCOM 2018 - IEEE Conference on Computer Communications Workshops (INFOCOM WKSHPS)
Note

QC 20181120

Available from: 2018-11-16 Created: 2018-11-16 Last updated: 2018-11-20Bibliographically approved
7. Fast and Reliable Initial Access with Random Beamforming for mmWave Networks
Open this publication in new window or tab >>Fast and Reliable Initial Access with Random Beamforming for mmWave Networks
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Millimeter-wave (mmWave) communications rely on directional transmissions to overcome severe path loss. Nevertheless, the use of narrow beams complicates the initial access procedure and increase the latency as the transmitter and receiver beams should be aligned for a proper link establishment. In this paper, we investigate the feasibility of random beamforming for the cell-search phase of initial access. We develop a stochastic geometry framework to analyze the performance in terms of detection failure probability and expected latency of initial access as well as total data transmission. Meanwhile, we compare our scheme with the widely used exhaustive search and iterative search schemes, in both control plane and data plane. Numerical results show that, compared to the other two schemes, random beamforming can substantially reduce the latency of initial access with comparable failure probability in dense networks. We show that the gain of the random beamforming is more prominent in light traffics and low-latency services. Our work demonstrates that developing complex cell-discovery algorithms may be unnecessary in dense mmWave networks and thus shed new lights on mmWave network design.

National Category
Communication Systems
Identifiers
urn:nbn:se:kth:diva-239142 (URN)
Note

QC 20181120

Available from: 2018-11-16 Created: 2018-11-16 Last updated: 2018-11-20Bibliographically approved

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