Page 56 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 6 – Wireless communication systems in beyond 5G era
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ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 6




          to signal processing on relay nodes. Furthermore,    channel  capacity.  Numerical  evaluations  are
          since the cost is higher than other relay methods,   conducted  to  verify  the  effectiveness  of  the
          it is not suitable for deploying a large number of DF   proposed massive analog relay MIMO system.
          nodes. Intelligent Reflecting Surfaces (IRS) is also a   The remainder of this paper is organized as follows.
          remarkable solution. The IRS is a reflector that can   In  Section  2,  a  system  model  of  the  proposed
          switch the reflection direction of radio waves using   massive  analog  relay  MIMO  is  described.  In
          e.g., meta-surface technologies [14]. However, IRS   Section 3, expansion to multi-hop relay is explained,
          cannot  significantly  improve  the  coverage  and   and Section 4 describes numerical analysis. Finally,
          capacity  since  reflection  directions  are  not    Section 5 concludes and summarizes the paper.
          controlled instantaneously. In this paper, we focus
          on  the  Amplifier-and-Forward  (AF)  relay  method   2.   MMWAVE MASSIVE ANALOG RELAY
          with active beam-forming capability. Since the AF
          method  only  amplifies  signals,  delay  is  almost       MIMO
          negligible.  Moreover,  its  cost,  size,  and  power   This section describes architecture of the proposed
          consumption of nodes are relatively small, so that it   mmWave  massive  analog  relay  MIMO  that  can
          is suitable to install a large number of AF nodes.    generate MIMO channels artificially by using a large
          There  have  been  several  studies  on  AF  relay   number of analog RSs.
          methods  for  mmWave  communications  [15][16].      2.1  System model of massive analog relay
          The performance of mmWave communications with
          a full-duplex AF relay was investigated in [15] by   Fig. 1 shows the system architecture consisting of
          considering  the  effect  of  beam  width  and  self-  one  source  (BS),  one  destination  (UE),  and  a
          interference.  In  [16],  the  basic  characteristics  of   massive number of RSs. Because of the compact size,
          mmWave MIMO communications with multiple AF          low cost, and low power consumption of analog RSs,
          relays were evaluated under a fixed topology, and it   it is feasible to massively and non-invasively deploy
          shows potential of the method. When applying the     them  in  the  target  environments  to  enhance  or
          AF  relay  to  mmWave  mobile  communications,       enable mmWave communications.
          adaptive  beam-forming  is  required  to  improve
          communication quality. The conventional AF relay
          only  performs  amplification  and  frequency
          conversion  but  adaptive  beam-forming  has  not
          been  considered.  Moreover,  relay-node  selection
          from massive relay nodes is also important in the
          case of mobile communications.

          In  this  paper,  we  propose  an  mmWave  massive
          analog   relay   MIMO     system   for   mobile
          communications by extending the authors’ previous
          work  in  [17].  A  large  number  of  dynamically
          controllable AF Relay Stations (RSs) are deployed to
          relay  signals  for  NLOS  User  Equipment  (UE).             Fig. 1 – System model for mmWave massive
          Adaptive  beam-forming  in  each  RS  is  assumed.                    analog relay MIMO
          A dynamic  RS  selection  algorithm  based  on  UE
          location  is  introduced  to  generate  an  artificial
          multipath channel to improve capacity of mmWave
          MIMO  communications.  The  RSs  should  have  a
          function of adaptive beam switching for dynamic RS
          selection and a function of beam tracking to follow
          the UE’s mobility. We start from the formulation in
          single-hop  relay,  and  then  extend  it  to  multi-hop
          relays to improve mmWave capacity. The full search
          of optimized route for multi-hop relays requires a
          huge computational cost for beam selection due to
          the  massive  number  of  RSs.  Hence,  a  fast-routing           Fig. 2 – mmWave analog RS with
          algorithm is introduced in this paper to maximize                       beam-forming





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