Electronic Design
What’s the Difference Between SON, C-RAN, and HetNet?

What’s the Difference Between SON, C-RAN, and HetNet?

Get ready for next-generation mobile wireless by gaining an understanding of SON, C-RAN, and HetNet technologies.

Lance Uyehara, Senior Manager of Design Engineering, TE Connectivity

These days, lots of different terms fly around to describe new approaches to wireless networking. In this article, we’ll look at self-organizing networks (SONs), C-RAN, and HetNet. All of these terms are associated with newer wireless-networking technologies, and will become more and more familiar as mobile operators roll out small cells, LTE-A, and 5G networks.

SON

SON technology enables network elements like base stations and access points to collectively configure and optimize themselves automatically and autonomously without user intervention. Traditionally, each cellular base station needed to be manually configured by a skilled technician, which is still true for macrocell sites today. In addition, optimizing the radio access network (RAN) required a substantial amount of drive-testing and hand-tweaking of base-station parameters. This was expensive and difficult enough when all base stations were macros servicing large areas, but it became an unmanageable scenario with the deployment of small cells.

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When small-cell technology appeared, interest in SON intensified because mobile-network operators realized 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. SON technology allows a base station to configure and optimize itself, sometimes in cooperation with its neighboring base stations. A recent and more common example of SON is the residential femtocell, which can be automatically configured in 30 to 45 minutes after being plugged in by the home consumer.

The Holy Grail (which won’t be achieved for years) is to be able to implement SON in the RAN, so that the whole network organizes and optimizes 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, neighbor lists, and other operating parameters that are normally configured manually by the operator or its contractors. SON also has the potential to improve the network’s reliability and availability. For example, if a cell site goes down in a SON-capable network, the cell sites around it organize themselves to provide coverage in the areas previously supported by the failed cell site.

One of the major challenges facing the implementation of SON concerns sensing the state of the surrounding network. In some scenarios, a SON-capable base station must passively determine the configuration and state of its neighboring base stations, while in other instances information can be queried from its neighbors.

Traditional base stations only receive uplink signals—transmission from mobile devices to the network. However, to coordinate and optimize themselves with the overall network, base stations must also be able to receive downlink signals to determine signal levels and other parameters of its neighbors. 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 (TDD) systems, a SON-capable base station will need to allocate certain time instances where it can receive downlink transmissions from neighboring sites.

C-RAN

C-RAN, which is often used as an acronym for cloud RAN, or centralized RAN, may be most appropriately referred to as a coordinated RAN. The terms cloud or centralized RAN are often used because the C-RAN is an evolution of the RAN that places all baseband units (BBUs) in a centralized location, typically envisioned to be the cloud with the functionality of the BBU virtualized into common processing platforms. 

As with the virtualization of the core network into the cloud, one of the primary motivations for co-locating and virtualizing 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 major advantage of centralizing the BBU functions is that it’s much easier to coordinate functions between BBUs, thus the term coordinated RAN. 

While some 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. That’s due to the flatter non-hierarchical architecture as well as LTE’s X2 logical interface between base stations, which facilitates information sharing among BBUs. If base stations are geographically distributed, mobile operators require a large number of high-speed links between these locations to enable the X2 interface, which can require leasing fiber, often from a competitor. It’s much easier for BBUs to share information when they’re in the same building or even in the same equipment rack.

C-RAN continues the evolution of the radio access network, from the highly integrated 2G/3G base stations, to the more distributed base stations popularized with LTE, to the centralized BBU concepts envisioned for LTE-A and even 5G. In the early days of the RAN, baseband processors and radio heads were integrated into the same units. However, with advances 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’s coordination between BBUs—they share information and make decisions based on what the other BBUs are doing or will do.

Having such cooperation between BBUs enables technologies like Coordinated Multiple Point transmission and reception (CoMP), 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. Coordinating the transmissions and reception of signals lowers the overall interference level of the network, 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, 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 Wi-Fi access points. All major mobile operators today deploy HetNets to a certain degree in their networks, especially in major metropolitan areas such as New York, Los Angeles, and Chicago, where user densities and capacity needs can’t be met by using only traditional macro base stations.

As capacity demands continue to escalate across all types of markets, the need for HetNets will also grow. Coordinating the various elements in the HetNets will become critical to making the HetNet a viable solution to mobile operators across their entire networks. 

As mentioned previously, SON is one technology that’s crucial for ensuring small cells and macrocells not only coexist, but also dramatically improve the overall mobile user experience.  In the very near future, it’s expected that SON technology will evolve to also include DAS.

The mitigation of interference between macrocells and small cells, as well as the improvement of performance at the edges of these cells, is being addressed through technologies such as enhanced inter-cell interference coordination (eICIC). Inter-cell interference coordination was first introduced in LTE to help reduce interference between LTE cells, especially at the cell edge where cells from different LTE signals would overlap.  The enhanced version, eICIC, targets LTE-A and specifically addresses reduction of interference between small cells and macrocells in a small-cell coverage area that’s completely overlapped by the macrocell.

Technologies such as LTE WLAN interworking and LTE link aggregation address coordination between the LTE radio access network and Wi-Fi access points operating in the HetNet. With LTE WLAN interworking, the RAN provides information to the user-equipment (UE) device so that the UE device can determine which wireless link should be used for data traffic (depending on various factors such as the required service level, link quality, and application). Also, the UE, during its decision-making process, can use network-operator rules and policies provided by the RAN. 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 Wi-Fi air link.

What’s Driving HetNet, SON, and C-RAN?

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 makes 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 xisting spectral resources at a lower cost per bit per megahertz of spectrum.

The C-RAN concept helps increase throughput and capacity over the air link, while SON helps reduce the operational costs of commissioning and optimizing the radio access network. 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 macrocells to cover large areas with high mobility needs. Coordinated use of Wi-Fi provides the operator with a controlled method of offloading traffic to unlicensed spectrum while meeting user expectations on coverage and service-level quality.

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.

SON, C-RAN, and HetNet are all emerging technologies, representing visions of what mobile networks of the future will look like. By understanding how each of these technologies fits into the overall RAN picture, we’re better able to grasp and implement these advances as they become available.

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