The FYI on SON, C-RAN, and HetNet

Given that frequency spectrum is a limited and extremely valuable resource, mobile operators must become more efficient in using spectrum, while at the same time managing the demand for higher capacity and lower OPerating EXpenses (OPEX). The exponential increase in capacity demands make it impractical, if not impossible, for mobile operators to meet future capacity demands by acquiring more spectrum. Mobile operators need new methods and technologies to meet the needs of their customers using their existing spectral resources at a lower cost per bit per MHz of spectrum.

There are a lot of different terms flying around to describe new approaches to wireless networking. For example, Self-Organising Networks (SON), Cloud Radio Access Networks (C-RAN), and Heterogeneous Networks (HetNet) are all terms associated with newer wireless networking technologies, and they will become more and more familiar as mobile operators roll out small cells, LTE-A, and 5G networks.

SON technology enables network elements like base stations and access points to collectively configure and optimise themselves automatically and autonomously without user intervention. When small cell technology appeared, interest in SON intensified, because mobile network operators realised they needed to automate the commissioning and configuration of the small cells: it simply wasn’t going to be efficient and cost effective to dispatch technicians to commission every small cell.

The Holy Grail (which won’t be achieved for years) is to be able to implement SON in the overall RAN, so the whole network organises and optimises itself. In a SON-capable RAN, mobile operators could power up a cell site, and the site would automatically determine its RF frequencies, power levels, neighbour lists and other operating parameters that are normally configured manually by the operator or their contractors.

One of the major challenges in implementing SON is sensing the state of the surrounding network. In some scenarios, a SON capable base station must passively determine the configuration and state of its neighbouring base stations while in other instances information can be queried from its neighbours. Traditional base stations only receive uplink signals –transmission from mobile devices to the network. However, to coordinate and optimise themselves with the overall network, base stations must also be able to receive downlink signals in order to determine signal levels and other parameters of its neighbours. For Frequency Division Duplexed (FDD) systems, SON-capable base stations need to have frequency-agile receivers that can listen to the downlink as well as the uplink or receivers also dedicated to the downlink. With Time Division Duplexed systems (TDD), SON capable base station will need to allocate certain time instances where it can receive downlink transmissions from neighbouring sites.

Coordination

C-RAN, which is often used as an acronym for cloud RAN, or centralised RAN, may be most appropriately referred to as a coordinated RAN. The terms cloud or centralised RAN are often used because the C-RAN is an evolution of the RAN that places all BaseBand Units (BBUs) in a centralised location, which is often envisioned to be the cloud with the functionality of the BBU virtualised into common processing platforms. As with the virtualisation of the core network into the cloud, one of the primary motivations for co-locating and virtualising the RAN is to lower the overall cost of the RAN through better efficiencies and higher usage of processing resources, while also reducing maintenance and operational costs associated with fewer BBU locations. From a technical performance perspective one the major advantages of centralising the BBU functions is that it is much easier to coordinate functions between BBUs, which leads to the term coordinated RAN.

While some of the concepts behind the C-RAN have been around for several years, the practicality of implementing C-RAN really began to gain momentum with the advent of LTE technology, because of the flatter non-hierarchical architecture as well as LTE’s X2 logical interface between base stations which facilitates information sharing among BBUs. C-RAN continues the evolution of the RAN, from the highly integrated 2G/3G base stations to the more distributed base stations popularised with LTE, to the centralised BBU concepts envisioned for LTE-A and even 5G. In the early days of the RAN, base band processors and radio heads were integrated into the same units, but with advancements in semiconductors, software, protocols and transport technology, it became more practical to separate the BBU from the radio head because one BBU shelf with multiple processing elements could drive several radio heads. In the early distributed base station architecture, you simply have the radio head separated from the BBU, but in a C-RAN the BBUs have the ability to talk to each other. With a C-RAN, there is coordination between BBUs – they share information and make decisions based on what the other BBUs are doing or will do.

Having this cooperation between BBUs allows for technologies such as Coordinated Multiple Point (CoMP) transmission and reception which are targeted for LTE-A. With CoMP the RAN is able to transmit and receive signals to and from the same user device from multiple antenna sites in a coordinated fashion. The decision on which antenna sites should be used at any instant of time is based on a variety of factors such as signal levels, signal quality, overall network quality, overall network throughput, power consumption, traffic loading, traffic distribution, etc. By coordinating the transmissions and reception of signals, the overall interference level of the network is lowered which allows for higher throughputs, and essentially more capacity, using the same spectral resources. Not coincidentally these same topics of coordinated transmission-reception and the C-RAN architecture are also being discussed for the continued evolution to 5G mobile networks.

HetNet

The heterogeneous network, or HetNet as it is often referred to, is simply a wireless network comprised of different types of base stations and wireless technologies. HetNets include macro base stations, small cells, distributed antenna systems (DAS), and even WiFi access points. All of the major mobile operators today deploy HetNets to a certain degree in their networks, especially in the major metropolitan areas, such as London, Birmingham and Leeds, where user densities and capacity needs cannot be met through only traditional macro base stations. As capacity demands continue to escalate across all types of markets, the need for HetNets will also grow and the coordination of the various types of elements in the HetNets will become critical to making HetNets a viable solution to the mobile operators across their entire network.

As mentioned previously, SON is one technology that is crucial for ensuring small cells and macro cells not only co-exist but also dramatically improve the overall mobile user experience. In the very near future, it is expected that SON technology will evolve to also include DAS. The mitigation of interference between macro-cells and small cells, as well as the improvement of performance at the edges of these cells, is being addressed through technologies such as eICIC (enhanced Inter-Cell Interference Coordination). Inter-cell interference coordination was first introduced in LTE to help mitigate interference between LTE cells, especially at the cell edge where cells from different LTE signals would overlap. The enhanced version, eICIC, is targeted for LTE-A and specifically addresses reducing interference between small cells and macro-cells where the small cell coverage area is completely overlapped by the macro-cell.

Technologies such as LTE WLAN interworking and LTE link aggregation address coordination between the LTE RAN and WiFi access points operating in the HetNet. With LTE WLAN interworking the RAN provides information to the UE device such that the UE device can determine which wireless link should be used for data traffic depending upon various factors such as the required service level, link quality and application. The RAN can also provide rules and policies from the network operator that can be used by the UE in its decision making process. In LTE link aggregation, the RAN will have more control over which radio access technology is used for data sessions and will have the ability to route traffic either over the LTE air link or the WiFi air link.

The C-RAN concept helps to increase throughput and capacity over the air link while SON helps to reduce the operational costs of commissioning and optimising the RAN. HetNets increase the overall capacity of the network by deploying small cells and DAS to address targeted high capacity and high density needs while allowing the macro cells to cover large areas with high mobility needs. Coordinated use of WiFi provides the operator with a controlled method of off-loading traffic to unlicensed spectrum while meeting user expectations on coverage and service level quality.