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

CTN Issue: October 2014 IEEE Communications Magazine

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.

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