There’s much talk about small cells these days – on industry news sites, at conferences, in corporate announcements. This is not surprising given that mobile traffic doubles every year and, according to Cisco, will grow at a compound annual growth rate (CAGR) of 78 percent from 2011 to 2016. Wireless network architecture has relied greatly on macrocells, but with the advent of the small cell era, how will small cells shape the wireless landscape? To answer, consider the following:
1- Coverage: cellular networks have relied heavily on macrocells with antennas generally mounted above the roofline. This resulted in the honeycomb topology. Small cells are close to street level, only removed from ground by a few meters. The coverage profile for small cells will be drastically different. New tools and processes are required to design and plan small cell deployments. The relative orderly design of macrocells is replaced by a plan where small cells are located in high-traffic areas.
2- Load-balancing: Today’s wireless networks associate users with the best serving base station which is determined by best signal quality (which incidentally closely tracks the power level). Small cells have widely varying coverage area and consequently traffic will vary widely. How will the network distribute traffic between small cells and macrocells to relief congestion and maximize performance? This is why load-balancing techniques are very important. Optimizing the network performance by shifting traffic between cells will be of great importance in heterogeneous networks.
3- Interference management: Spectrum is limited, so, should small cells use the same or different spectrum from macrocells? If they are using the same spectrum, how should we coordinate frequencies between the macro cell layer and small cell layer? More specifically, how can this be done with minimal human intervention? There are techniques such as fractional frequency reuse (FFR) and Enhanced Inter-Cell Interference Coordination (eICIC) that address this question.
4- Backhaul: How do we connect the small cells to the core network, and more specifically how do we do that at low enough cost to have a business case? The backhaul challenge is in providing consistent, reliable, stable and sufficient capacity on the backhaul link at a reasonable cost point. A number of techniques are being developed to address the backhaul challenge with re-purposed solutions as well as novel techniques designed specifically for small cells.
5- Mobility management: Mobile subscribers are on the move. Users are moving in and out of the coverage range of large and small cells alike. How will the network deal with mobility when large and small cells are present? Mobility and call reliability is required while the network minimizes interference and overhead signaling.
6- Downlink-uplink symmetry: In macrocells, the downlink path (base station to mobile) and the uplink path (mobile to base station) are roughly equivalent. But this is not the case in small cells, where the path is different as is signal quality and interference. This happens in co-channel small cell deployments where the much higher power of the macro base station shifts the handover boundary closer to the small cell resulting in high uplink interference to the small cell base station. Consider the implications: if the downlink and uplink are symmetric, as in the case of macrocells, it is straight forward to identify the best serving base station for every user. But the view is muddled in small cells. This has wide-reaching implications on interference, mobility and overall user experience.
These are some of the main differences between macrocells and small cells. But there are more differences aside from technological issues. For instance, one area of immense importance is how to acquire small cell locations which starts with identifying the location of traffic hotspots and includes negotiating lease space of public asset infrastructure such as poles and securing municipal permits. This process will be radically different from that for macrocells. For this and the other reasons outlined above, I believe that small cells are a disruptive technology that are set to reshape how wireless networks are architected, built, and operated.