CTN Issue: October 2014

All ten of the July, 2014 top ten most popular (in terms of online views through IEEE Xplore) articles published in ComSoc periodicals are about 5G [Top10, 2014], so it seems covering 5G is a judicious choice for this month’s CTN.

The IEEE Communications Magazine had two special issues on 5G this year, entitled “5G Wireless Communication Systems: Prospects and Challenges” with Part 1 in February, 2014 and Part 2 in May, 2014.  Each part included nine technical articles along with a guest editorial by the guest editors: John Thompson (University of Edinburgh), Xiaohu Ge (Huazhong University of Science and Technology), Hsiao-Chun Wu (Louisiana State University), Ralf Irmer (Vodafone), Hong Jiang (Alcatel-Lucent Bell Labs), Gerhard Fettweis (Technical University of Dresden), and Siavash Alamouti  (independent entrepreneur, formerly Vodafone).  These eighteen articles give a comprehensive view of the exciting technologies envisioned for 5G, and their associated technical challenges.

The May, 2014 CTN column highlighted three of the articles from Part 1 (February, 2014) of the IEEE Communications Magazine focus on 5G, and in this month’s CTN, we highlight three of the articles from Part 2 (May, 2014).  All three of these articles are among the July, 2014 top ten most popular articles. 

Three papers on 5G from the May, 2014 issue of the IEEE Communications Magazine

Our first focus paper is by Shanzhi Chen and Jian Zhao (Datang Telecom Technology & Industry Group), entitled “The Requirements, Challenges, and Technologies for 5G of Terrestrial Mobile Telecommunication”.  The paper addresses the dramatic increase in traffic that 5G systems will be expected to handle, the shift to coordinated outdoor/indoor cell coverage, and summarizes the various emerging technologies envisioned to support the increased traffic load.

Our second focus paper is by Volker Jungnickel (Fraunhofer Heinrich Hertz Institute), Konstantinos Manolakis (Fraunhofer Heinrich Hertz Institute), Wolfgang Zirwas (Nokia Solutions and Networks), Berthold Panzner (Nokia Solutions and Networks), Volker Braun (Alcatel Lucent Bell Labs), Moritz Lossow (Deutsche Telekom AG, Innovation Labs), Mikael Sternad (Uppsala University), Rikke Apelfröjd (Uppsala University), and Tommy Svensson (Chalmers University of Technology), entitled “The Role of Small Cells, Coordinated Multipoint, and Massive MIMO in 5G”.  The paper summarizes the techniques for increasing the spectral efficiency and cell edge coverage through the use of small cells, joint transmission coordinated multipoint, and massive MIMO.

Our third and final focus paper is by Mohsen Nader Tehrani (University of Waterloo), Murat Uysal (Özyegin University), and Halim Yanikomeroglu (Carleton University), entitled “Device-to-Device Communication in 5G Cellular Networks: Challenges, Solutions, and Future Directions”.  The paper investigates a proposed two-tier cellular network architecture with the macrocell tier enabling base station to device communications, and a device tier for device to device communications.

Additional resources on 5G

We briefly highlight several other sources of information about 5G.  First, the June, 2014 issue of the IEEE Journal on Selected Areas in Communications includes eighteen articles on the theme of“5G Wireless Communications”, including a guest editorial “What Will 5G Be?” by guest editors Jeffrey G. Andrews (University of Texas at Austin), Stefano Buzzi (University of Cassino), Wan Choi (Korea Advance Institute of Science and Technology), Stephen V. Hanly (Macquarie University), Angel Lozano (Universitat Pompeu Fabra), Anthony C. K. Song (Huawei), and Jianzhong Charlie Zhang (Samsung Research America).

The ComSoc Emerging Technology Subcommittee on “5G Mobile Wireless Internet”, currently chaired by Latif Ladid, is focused on “exploring and elucidating all facets of the next generation of 5G Mobile Wireless Internet technologies” [5GMWI].  The subcommittee coordinates meetings, workshops, and conferences, and maintains a mailing list for its members. 

The ComSoc “5G Training and Certification” initiative [5GT&C] is a recent (January, 2014) project led by Ekram Hossain (University of Manitoba), Mischa Dohler (King’s College London), and Rath Vannithamby (Intel Labs).  The project coordinates 5G training at IEEE-sponsored workshops and conferences, and is in the process of developing a 5G certification program.

The upcoming IEEE Global Communications Conference (GlobeCom) in Austin, TX (December 8-12, 2014) features a full-day International Workshop on Emerging Technologies for 5G Wireless Cellular Networks, which will be “a venue to brainstorm on and to identify the emerging concepts, technologies, and analytical tools for 5G cellular networks” [5GWorkshop].

As these publications, committees, trainings, certifications, and workshops demonstrate, the 5G revolution has begun, and the IEEE Communications Society is playing a leading role.

1. The Requirements, Challenges, and Technologies for 5G of Terrestrial Mobile Telecommunication

Authors: Shanzhi Chen and Jian Zhao (Datang Telecom Technology & Industry Group)
Title: “The Requirements, Challenges, and Technologies for 5G of Terrestrial Mobile Telecommunication”
Publication: May, 2014 IEEE Communications Magazine

The article opens with a list of characteristics of 5G traffic, including the frequently mentioned traffic increase (78% compounded yearly growth rate for 2012-2016), indoor/hotspot dominance (70% data traffic indoors), uplink/downlink asymmetry (currently 6:1), anticipated increase in the number of subscribers (due to machine-to-machine applications), and the need to simultaneously reduce energy consumption.  The authors identify the key requirements facing 5G as i) achieving 1000-fold mobile data capacity, ii) reducing energy consumption, iii) leveraging new spectrum (above 3 GHz), and  iv) reducing costs for terminals and (macro and micro) base stations.

The article then discusses six possible technology areas that may play a role in meeting this requirements:

1. Small cells
2. Coordination across heterogeneous cell layers
3. Flexible design of control and user planes
4. TDD and FDD solutions for asymmetric traffic
5. Hybrid topologies for D2D and relay connections
6. Signal processing and integrated circuit (IC) technologies

1. Small cells will be designed for indoor/hotspot traffic.  The requirements of (small) cells for indoor traffic are significantly different than the requirements of (macro) cells for outdoor traffic since i) indoor user mobility is due to walking and the required cell radius is on the order of tens to hundreds of meters, ii) the indoor channel characteristics are distinct from those found outdoors, and iii) the required transmission power of small cells is orders of magnitude smaller than that required of macro cells.  These differences should be exploited in several aspects of designing small cells including i) the selection of physical layer parameters (the cyclic prefix length and subcarrier spacing), ii) the antenna element spacing in MIMO designs, iii) the ability to save energy by dynamically turning off small cells when not in use, and iv) the ability to use cognitive radio principles in dynamic spectrum selection.

2 and 3. The coexistence of small cells for indoor / hotspot traffic with traditional macro cells requires coordination of traffic and spectrum resources across these heterogeneous layers, and may include the need for a flexible design of the control and user planes.  The envisioned flexible design of these planes will facilitate user equipment being simultaneously connected with small cells and macro cells, as in coordinated multipoint (CoMP). 

4. The increasing asymmetry between downlink and uplink traffic volumes will require flexible time-division duplexing (TDD) and frequency-division duplexing (FDD) solutions.  FDD technologies like FDD carrier aggregation and supplemental downlink  (SDL) are dependent upon spectrum regulation and allocation.  TDD solutions are more flexible through semi-dynamic slot allocation.  

5. The traditional star topology of mobile users connecting to a common base station is expected to evolve in 5G to allow multihop connections.  These may include connections from small cell to small cell and from device to device, in addition to the traditional cell to device connection.  An anticipated challenge of device to device (D2D) connections is how to best allocate spectrum between D2D-dedicated channels, and channels shared by D2D communications and the base station.

6.  Finally, the authors highlight recent advances in both signal processing and integrated circuits that will improve performance of base stations and user equipment.  Active antenna array technology integrates radio frequency (RF) components such as power amplifiers directly onto antenna elements.  The advanced signal processing required by 5G standards will necessitate more advanced integrated circuits.

2. The Role of Small Cells, Coordinated Multipoint, and Massive MIMO in 5G

Authors: Volker Jungnickel (Fraunhofer Heinrich Hertz Institute), Konstantinos Manolakis (Fraunhofer Heinrich Hertz Institute), Wolfgang Zirwas (Nokia Solutions and Networks), Berthold Panzner (Nokia Solutions and Networks), Volker Braun (Alcatel Lucent Bell Labs), Moritz Lossow (Deutsche Telekom AG, Innovation Labs), Mikael Sternad (Uppsala University), Rikke Apelfröjd (Uppsala University), and Tommy Svensson (Chalmers University of Technology)
Title: “The Role of Small Cells, Coordinated Multipoint, and Massive MIMO in 5G”
Publication: May, 2014 IEEE Communications Magazine

The authors focus on three key technologies for achieving higher spectral efficiency in 5G: interference mitigation (through coordinated multipoint), network “densification” (through small cells), and massive MIMO.   They outline an architecture where these three technologies work together, as outlined below.

Joint transmission coordinated multipoint (JT CoMP) involves simultaneous downlink transmissions from a small number of base stations to an end user.  The authors identify recent demonstrations of JT CoMP validating its technical feasibility, but which also identify the need for refinements to improve performance.  The authors identify two key areas for improvement as i) clustering and user selection, and ii) feedback compression and channel prediction. 

First, JT CoMP involves both identifying which user(s) should receive joint transmissions, as well as which collection of base stations is best-suited to serving the selected users.  Early results identified this optimal selection problem as NP-hard, and as such efficient heuristics are suitable.  The authors propose a “cover-shift” concept for forming overlapping cooperation areas (CAs), particularly for the cell-edge users who are particularly vulnerable to inter-cell interference.  The authors also suggest a “successive user grouping” heuristic for user selection wherein users are successively added to a JT CoMP group provided the addition of the user does not reduce the CoMP performance gain of the group.

Second, the performance gains of JT CoMP are known to be particularly sensitive to channel estimation errors arising from feedback quantization and delay.  To address this concern, the authors have investigated feedback compression and channel prediction techniques.  Feedback compression is achieved by combining user clustering, tap selection, and adaptive channel quantization; their implementation reduced the channel state information (CSI) feedback overhead by a factor of 15 relative to the LTE reference case.  Furthermore, the authors have implemented a recently developed channel prediction incorporating Doppler effects which have demonstrated improvements in terms of mean squared error of channel predictions relative to conventional techniques.

In the last two main sections of the paper, the authors outline a proposed integrated 5G design incorporating small cells, JT CoMP, and massive MIMO, and present some initial performance results.  The synergy between JT CoMP and massive MIMO is due to the fact that massive MIMO helps to “localize” the interference, which facilitates improvements in coordinated transmissions, as the participating base stations have a reduced need for coordination.  Their initial results are for the combined use of small cell architecture with JT CoMP, and are based on ray tracing simulations in the Munich area.  By extrapolating their results to incorporate anticipated massive MIMO antenna designs, the authors predict this architecture capable of supporting 1000 times what can be supported with LTE.

3. Device-to-Device Communication in 5G Cellular Networks: Challenges, Solutions, and Future Directions

Authors: Mohsen Nader Tehrani (University of Waterloo), Murat Uysal (Özyegin University), and Halim Yanikomeroglu (Carleton University)
Title: “Device-to-Device Communication in 5G Cellular Networks: Challenges, Solutions, and Future Directions”
Publication: May, 2014 IEEE Communications Magazine

While the conventional cellular architecture consists of connections from base stations to user equipment, 5G systems may well rely upon a two-tier architecture consisting of a macrocell tier for base station to device communication, and a second device tier for device to device (D2D) communications.  Such architectures are a hybrid of conventional cellular and ad hoc designs.  The authors first taxonomize possible D2D architectures and outline three technical challenges in D2D, namely, security, interference management, and resource allocation.  Second, the authors describe possible pricing models to incentivize users to let their devices serve as relays for other communications. 

The D2D architecture taxonomy consists of four designs:

1. Device relaying with operator controlled link establishment (DR-OC)
2. Direct D2D communication with operator controlled link establishment (DC-OC)
3. Device relaying with device controlled link establishment (DR-DC)
4. Direct D2D communication with device controlled link establishment (DC-DC)

Observe the base station (operator) is only involved in the first two designs, and in fact the fourth design is a purely ad hoc connection (i.e., not requiring a cellular network). 

The first technical challenge is security.  The parties sending and receiving the data must be assured their data is not accessible to the relay, and the relay must be assured the data it is handling is benign.  The authors distinguish between closed and open access designs, where the user of a closed access device explicitly allows the device to relay for a specific list of trusted sources.  There is an existing body of research on security issues for D2D designs, including routing, key management, and attack identification.

The second technical challenge is interference management.  D2D designs must carefully manage channels designated exclusively for D2D connections, as well as channels jointly used by both D2D connections and connections with the base station.  The existing literature on D2D interference management addresses channel assignment via game theory, D2D admission control, D2D power control, and D2D relay selection. 

The third technical challenge is resource allocation, and the possible solutions are specific to the D2D design.  Under the DR-OC and DC-OC designs, for example, the base station can (partially) manage the relay and channel selections.  Under the DR-DC and DC-DC designs, however, there is no centralized supervision on either relay selection or channel management.  

Besides the technical challenges, there is the very practical problem of incentivizing users to lend their devices to serve as relays for the traffic of others, especially since these connections will consume bandwidth, storage, and battery power on the relay.  Again, the appropriate class of solutions depends upon the D2D design. 

For DR-OC designs, the operator may incentivize users to lend their devices as D2D relays by offering a bill reduction in proportion to the amount of data relayed that month.  By designing an appropriate utility function for each device based on the bandwidth consumed and the bandwidth relayed, as well as a revenue function for the network operator, the authors demonstrate an incentive compatibility between the operator and the users. 

For DC-OC designs, the challenge is to incentivize users to use D2D communications instead of free WiFi or Bluetooth connections, which can be cast as a spectrum trading problem within the context of auction theory.  The authors discuss various auction mechanisms including the Bertrand game, and present an example to demonstrate the increase in operator revenue.

Finally, for DR-DC and DC-DC designs, the operator is external from the connection, and as such the two parties involved should agree on a pricing scheme (or simply not charge for the relay).  Cited studies demonstrate the performance improvement achievable when altruistic users offer their devices to serve as relays.

References

1. [Top10, 2014] IEEE Communications Society, “The July 2014 list of the ten most popular articles published in ComSoc periodicals viewed online, based on PDF views through IEEE Xplore”, Available at http://www.comsoc.org/topten.
2. [ComMag, Feb. 2014] IEEE Communications Magazine, “5G Wireless Communications Systems: Prospects and Challenges (Part 1)”, February 2014.
3. [ComMag, May 2014] IEEE Communications Magazine, “5G Wireless Communications Systems: Prospects and Challenges (Part 1)”, May 2014.
4. [JSAC, June 2014] IEEE Journal on Selected Areas in Communications, “Special Issue on 5G Wireless Communications”, June 2014.
5. [5GMWI] IEEE Communications Society Emerging Technical Subcommittee on 5G Mobile Wireless Internet, Available at http://www.comsoc.org/about/committees/emerging.
6. [5GT&C] IEEE Communications Society 5G Training and Certification, Available at http://www.ieee-5g.org.
7. [5GWorkshop] International Workshop on Emerging Technologies for 5G Wireless Cellular Networks (at IEEE Globecom 2014), December 8, 2014, Available at http://wcsp.eng.usf.edu/5g/2014/.

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