Queen's University Belfast

For nearly 50 years, mobile networks have relied on cellular network topologies, where geographical areas are divided into smaller areas known as cells, each served by a single radio tower called a base station. Through the successive generations of wireless technology, this approach to network design has undoubtedly proven its worth, although it is not without its limitations. Reliability, interference, latency and coverage are all challenges which have dogged mobile networks throughout the years. As we continue to push the boundaries (no pun intended!) on the number of users a single cell can serve (purportedly up to 10 million per square kilometre in 6G), coupled with ambitious user-experienced data rates (1 Gbps in 6G) and nearly negligible latency (~100μs in 6G), a rethink of how we approach the design of mobile networks is undoubtedly long overdue. Enter cell-free access.

Cell-Free Access: How does it work?

Unlike traditional mobile networks, cell-free networks, upon which cell-free access is built, eliminate cell boundaries. In cell-free architectures, base stations are replaced by access points (APs) that are densely distributed across the entire geographical area. Each mobile user or user equipment (UE), as they are referred to in 5 and 6G, is served by all of the APs in the surrounding area, as shown in Figure 1.

Cell-Free Access: Cell-Free Networks vs. Traditional Mobile Networks

Cell-free networking offers several advantages over traditional mobile networking (disclaimer: this is not an exhaustive list!). First of all, we can look forward to increased reliability as a result of increased densities of APs. Shorter distances between mobile users and APs will naturally lead to better signal quality and coverage (reducing the impact of deleterious factors such as path loss and shadowing). The fact that a mobile user can be connected to multiple APs at any given instance means that even if one AP encounters interference or obstruction, the other APs can maintain connectivity with the mobile user.

Driven by applications such as smart cities, industrial IoT, remote healthcare, intelligent transportation and extended reality, cell-free access is a hot topic in wireless research, garnering significant interest from academia and industry alike

Secondly, a significant reduction in interference. As cell-free networks have no predefined cells, APs can work together in a coordinated manner to serve all users in their area of influence. Thirdly, more responsive and detailed user experiences. Closer proximity to APs and multiple simultaneous connections will pave the way for even lower latency and massive data throughputs compared to traditional mobile networks. This is closely coupled with our fourth benefit, which is ubiquitous coverage. With high densities of APs, seamless and uniform coverage will become the norm, helping to fill in so-called 'not' spots, which have been the nemesis of traditional mobile networks.

Lastly, but maybe most importantly, with sustainability and green credentials high on everyone’s agenda, cell-free networks will be more environmentally friendly. This can be achieved in many ways, but probably the simplest is optimizing energy usage by intelligently choosing the best-serving APs. Of course, this can be taken one step further, whereby APs can be activated and deactivated in response to user demands, resulting in even greater energy savings.

Cell-Free Access: The Challenges

Cell-free access doesn’t come without its challenges. For example, dense deployments of APs may be costly in terms of hardware including the new fronthaul and backhaul links required to connect everything together. Connecting a mobile user simultaneously to many APs will require very tight synchronization between APs (e.g. to support advanced antenna techniques such as beam forming). This can be complex and difficult to achieve, especially where APs are spread out over wide geographical areas. Probably the greatest adversary of cell-free access is scalability. As cell-fee networks scale, so too do the resources and management required to maintain network operation.

Cell-Free Access: Where are we now?

Driven by applications such as smart cities, industrial IoT, remote healthcare, intelligent transportation and extended reality, cell-free access is a hot topic in wireless research, garnering significant interest from academia and industry alike. Within the Centre for Wireless Innovation at Queen’s University Belfast in the United Kingdom, we are working to develop a new generation of scalable, low-cost, and eco-friendly technologies that will underpin future cell-free access. This includes multiple antenna technologies, transmission protocols, novel signal processing techniques, and optimal resource allocation strategies. We hope to be among the first to demonstrate this exciting new networking paradigm and make it a core technology in 6G and the generations of wireless which come after.