Cache-aided mmWave-based Connectivity Solutions

Imagine a scenario in an extended reality (XR) or immersive digital environment. A large group of users is submerged in a network-based immersive  application – either educational, industrial, gaming, public safety or social networking – which runs on high-end eye-wear. The solution maneuvers heavy multimedia traffic and every user in the operating environment can enjoy a realistic experience.

A comfortable and immersive user experience in this type of a scenario is already supported by the latest intelligent high-end head-mounted devices and software where spatial audio and high-rate streaming (4K-and-beyond) create a sensation of nearly physical presence in a virtual world.

In real life, the immersive experience places extremely high performance requirements for wireless networks. Associate  Professor Antti Tölli is now  developing  an  innovative  physical layer connectivity framework which combines directional mmWave connectivity,  use of affordable in-device memory and multipoint coded caching (CC) techniques.

“In  our  emerging  vision,  cache-aided  mmWave-based  connectivity solutions will be able to manage extreme peak rates while ensuring the reliability of increasingly demanding XR applications,” Tölli says. “The storage capacity of mobile devices is  constantly  increasing  and  therefore  becoming  one  of  the  cheapest network resources, readily available closer than we think. With our innovative solution, the realization of immersive digital environments in demanding dynamic wireless settings may become affordable in the near future.”

Tackling wireless bottlenecks

One  of  the  key  paths  towards  the  intelligent  use  of  network  resources  and  device  memory  departs  from  so-called  bottleneck areas which suffer from limited wireless connectivity. “Advanced  content  distribution,  wireless  delivery  and  proactive  caching  algorithms  can  dramatically  increase the system performance in such areas,” Tölli says. “In an immersive scenario, coded and collaborative caching is especially beneficial  because  the  location-dependent multimedia content becomes a dominant part of the wireless data delivery.”

However, current techniques  of  data  placement  in  cache  memories  cannot  yet  handle  a  dynamic  scenario  where  the  users  move  freely  and  where  the  immersive  virtual  environment is captured based on the exact location and tilt angle of each user’s headset. “We need to acquire large-scale detailed information of the environment in the proximity of each headset,” Tölli notes. “Each user’s streamed data is unique and also highly  location  and  time  dependent,  and  the  delivery  delays  must be lower than the human perception thresholds.”

The magic of in-device memory

In  actual  fact,  a  large  part  of  the  rich  multimedia  content  for  rendering  a  certain  viewpoint  is  common  among  several  users. “If we can identify and select the common location-specific content, it offers opportunities for efficient use of pooled in-device memory resources,” Tölli says. “We can then employ intelligent  cache  placement  and  network  coded  multicast  content delivery mechanisms which make the pooled memory an additional degree of freedom to be exploited, in addition to wireless resources.”

In  practice,  the  cache  memory  of  the  users  could  be  opportunistically filled, for instance, with multimedia content which represents  the  fixed  structures  of  the  virtual  world.  In  the  memory placement, content which is relevant to the identified system-wide  bottleneck  areas  with  poor  connectivity,  is  preferred and incentivized,” Tölli says. “We foresee that, ultimately,  a  significant  portion  of  the  multimedia  content  rendered  on the eyewear would originate from the local cache available conveniently at the user device.”

Less cache involvement, on the contrary, is needed closer to the  access  points  where  the  wireless  connectivity  is  fluent.  “We  are  exploring  various  cache  placement  opportunities  to  establish appropriate schemes for distributing files across the selected cache memories,” Tölli says. “When we combine dynamic and proactive cache replacement strategies with intelligent multiantenna content delivery, common coded content can  be  delivered  to  maximize  the  quality  of  experience  of  all  the users, given the limited total memory across the devices.” 

The  remaining  requested  content  across  the  network  can  be  delivered e.g. via multipoint multicast beamforming of the network  coded  content.  On  the  other  hand,  part  of  content  can  also be exchanged directly employing nearby device-to-device connections  (D2D).  After  the  cache  (re-)placement  has  been  carried out, the network can potentially offload the requested content onto the D2D connections. For this purpose, the caches of the nearby users are utilized. “These hybrid scenarios are highly useful,” Tölli says. “The network can then decide whether it shares the content directly, possibly to multiple users simultaneously, using unicast or multicast beamforming, or whether it allocates the request to a short-range D2D connection.”

Challenges of mmWave links

As software companies are ramping up, larger audiences consume extended reality applications. With the constantly growing usage intensity and unconstrained user mobility, a dynamic  full  immersion  scenario  causes  major  capacity  and  quality  challenges for existing networks. Field tests in the current 5G networks have proven 5G capability for a Gbps access link on mmWaves.  “Immersive  digital  experience  in  multi-user  environments requires extremely high area capacity and ultra-low over-the-air  latencies,  which  are  not  yet  fully  achieved  with  5G,”  Tölli  notes. “Communication  over  mmWave  links  is  by  default unreliable, subject to random blockage. Solving these challenges  will  become  critical  as  we  strive  to  enable  future  dense and dynamic immersive networks.”

The  considered  immersive  viewing  application,  in  all  of  its  complexity, offers valuable results for other fields of technol-ogy too including autonomous driving, rural network connectivity  and  remote  surgery,  to  mention  a  few.  The  algorithms,  system design solutions, and performance evaluation frameworks  that  Tölli  and  his  team  innovate,  will  be  able  to  direct  cellular operators, telecommunication research organizations, equipment vendors, and mobile software companies, among others, towards affordable future solutions beyond 5G.

Industry relevance of the solutions is highlighted in an on-going  project  with  Nokia  Bell  Labs  with  funding  from  Finnish  Research  Impact  Foundation.  “The  goal  is  to  validate  selected technical solutions in the early phase of 6G system development,  through  proof-of-concept  systems  and  demonstrations, jointly with our industry partner,” Tölli says.

Although  he  immerses  himself  into  the  challenges  of  virtual  content  delivery  during  working  hours,  Tölli  has  managed  to  keep  his  home  and  three  sons  detached  from  virtual  worlds.  As a fervent outdoor enthusiast, he is not likely to replace the real world with a digital twin soon, either. “Fresh air and Finnish forests  are  much  too  compelling  and  hard  to  replicate,”  Tölli  notes.  “My  ultimate  immersion  happens  in  the  middle  of  nowhere – with my feet on pedals or skis.”


Antti Tölli graduated with a doctoral degree in 2008. In his research, he focuses especially on multi-antenna communications and related signal processing challenges. He has also accumulated strong experience in industry.