Silicon SoCs driving HetNet evolution
Joachim Hallwachs, DesignArt Networks —
SoCs have the cost-improving versatility to see the RAN build-out through to HetNets.
As mobile data consumption explodes and the demand for 3G/4G services increases, the radio equipment market is moving toward a Heterogeneous mobile Network (HetNet). Analyzing the HetNet infrastructure demonstrates how fully integrated, all-in-one System-on-Chip platforms can deliver any type and form factor of equipment needed for the HetNet radio access layer, as well as a new breed of unified wireless mobile access solutions.
The dramatic shift from low-capacity telephony services to high-capacity data services is driving the evolution of a Heterogeneous mobile Network (HetNet). This architecture requires the rapid build-out of a more densely deployed distributed Radio Access Network (RAN).
HetNet comprises compact, cost-effective equipment targeting small and large cell service access connected to high-performance centralized processing and control locations through various types of cell site backhaul equipment. Four critical equipment segments serve the HetNet access infrastructure market:
- Cost-effective and compact radio access equipment
- Cell site backhaul equipment
- Distributed Antenna Systems (DAS)
- Centralized processing and control equipment
These equipment segments utilize highly integrated System-on-Chip (SoC) silicon platforms capable of delivering uniform high-capacity service performance while maintaining the low total cost of ownership essential for successfully operating data-centric 3G and 4G services.
Two deployment approaches are available in the market today: existing distributed base station infrastructure and low-cost, compact, all-in-one radio access equipment. Both of these options allow small and large cells to be deployed using different cell site backhaul methods. In addition, both options can be combined with DAS to bring antenna installations even closer to subscribers, especially inside larger buildings.
HetNet architecture layers and radio access equipment sectors
Deployment costs and subsequent operational and maintenance costs of the HetNet infrastructure must be kept below that of existing RAN deployment practices. This facilitates a successful transition toward a data-centric mobile business model.
However, exploding mobile data capacity consumption requires the deployment of more cell capacity in existing locations and additional cells in new types of in- and outdoor locations. Small cells are deployed in these new cell sites, while large cells are typically deployed in existing cell sites, for example, on towers and buildings.
The mobile infrastructure equipment evolution has two components: the cost optimization of existing base station equipment and deployment practices and the addition of new types of cell sites with new deployment practices. The terms for the two practices are distributed base station deployments and compact, all-in-one base station deployments.
To reduce existing base station deployment costs, especially in heavily loaded metropolitan areas, operators have already tried implementing a distributed base station architecture, in which Remote Radio Heads (RRH) installed on towers and buildings are connected via a standardized dark fiber interface to centralized baseband units. The distribution transport for this architecture is dark fiber using protocols like Common Public Radio Interface (CPRI).
The deployment of all-in-one base station equipment initially started indoors, where operators are now deploying femtocells as coverage and offload solutions. The expanding femtocell model includes small business, large enterprise, and private and public hot-spot applications and is projected to extend to form metropolitan radio access layers called Metrozones. All-in-one base stations require IP-based wired or wireless Metro Ethernet transport.
DAS solutions connect to antenna ports on third-party base stations combined with RRH or all-in-one equipment such as picocells to further distribute antenna mounting locations.
SoCs in distributed all-in-one equipment
The distributed RAN comprises cost-effective in- and outdoor small cell equipment and compact, passively cooled large cell equipment connected to a high-performance centralized processing and control layer.
The all-in-one equipment segment fundamentally relies on the use of highly integrated SoC platforms, driven largely by a requirement for passive cooling and minimal enclosure size, which are directly related to equipment cost. These requirements cannot be achieved through discrete processing and component layers, thus leading to the need for a fully integrated multilayer SoC architecture.
As the all-in-one model expands from residential femtocells to enterprise and hot-spot applications, underlying SoC platforms must be able to scale up to support tens or even hundreds of subscribers. In the context of 3G and 4G data capacity, this requirement demands the use of a multilayer multicore SoC architecture.
Multilayer multicore SoC platforms include multiple task-optimized processing cores at each processing layer. These cores handle all RAN protocol layers in a single-chip product, providing scale and data capacity as required, as well as optimized cost and power consumption to fit the total system heat dissipation budget for low-cost, passively cooled in- and outdoor equipment.
As the all-in-one deployment model expands to outdoor infrastructure, the use of higher RF output power becomes another system requirement. This adds an extra layer of digital processing to the multilayer SoC architecture, as power amplifier linearization becomes a mandatory requirement to minimize power amplifier heat dissipation – again in the context of cost-effective, passively cooled outdoor equipment with minimal enclosure size and weight.
Furthermore, the use of SoC-based all-in-one equipment as part of the operator-installed outdoor RAN infrastructure adds another full set of requirements. All infrastructure-grade equipment must support two concurrent service generations, be highly reliable and in-field software upgradable, and provide full support of carrier-grade Operation, Administration, Maintenance, and Provisioning (OAM&P) tools. Related SoC requirements include dual-protocol operation (3G and 4G) and non-service-affecting methods for in-service monitoring, diagnostic, and alarming procedures.
Concurrent multiprotocol support requires the digital front-end layer to support multiple carriers, operating 3G and 4G protocols in different frequencies just like single RAN RRH equipment.
SoCs in radio head equipment
The two basic options for distributed in- and outdoor radio and antenna deployments are RRH as part of a distributed base station system and Remote Antenna Units (RAUs) as part of a DAS.
RRH equipment is installed and managed as an integral part of the serving operators’ mobile radio access infrastructure. RRH equipment connects to the digital baseband layer of a baseband unit through a standardized transport and management interface such as CPRI. Due to high capacity and stringent timing requirements, CPRI uses dark fiber connectivity, which enables digital signal transmission over extended distances where dark fiber facilities are available at suitable costs.
Using optical fiber DAS, solutions distribute RF signals over potentially long distances to distribution units located throughout a given building, campus, or metropolitan area. Distribution units in turn aggregate signals from several RAUs using cost-effective indoor cabling options such as thin coax or CAT5 cabling over limited distances.
Active DAS equipment is a stand-alone equipment segment that provides building owners and operators with a means to distribute a dominant service signal throughout a building or service area, independent of any particular base station vendor or mobile service technology.
RAUs and RRH equipment provide digital-to-analog signal conversion, integrating digital RF control and signal processing layers with analog power amplifier subsystems of varying RF output power suitable for in- or outdoor deployments on small or large cell layers. Both types of equipment support several mobile service technologies and can employ digital RF samples compression methodologies, thereby decreasing cell site backhaul/transmission capacity for cost reduction.
These signal processing capabilities use the same processing techniques already discussed for all-in-one outdoor equipment. All signal processing related to the antenna array is carried out on the baseband processing layer, as in all-in-one equipment. Samples compression (not required in all-in-one equipment) can be carried out in the multicore DSP layer of all-in-one SoC platforms, presenting a fully software-defined solution for samples compression.
Moreover, an active DAS solution’s main unit and distribution unit equipment can employ scalable multicore baseband processing SoCs to provide distributed samples decompression technology and flexible logical RAU management options, with future in-field software upgradability to more frequent spectrum reuse, for example, as part of an in-field software-defined system capacity evolution (virtual picocells).
In all of these use cases, the SoC platforms suggested must abide by the same principles previously discussed as part of all-in-one equipment – low system-level cost and heat dissipation.
R&D efficiency improvements
As integrated multilayer, multicore SoC platforms for all-in-one equipment come to market, equipment vendors can choose SoC platforms that integrate single RAN digital front-end and signal processing layers. Fully integrated all-in-one SoCs can provide a good R&D platform for RRH or RAU equipment.
A major benefit of using all-in-one SoC platforms in this context is the enormous flexibility of radio head equipment, which is in-field software upgradable to process additional layers of all-in-one equipment, including, but not limited to, the entire Base Transceiver Station (BTS) functionality. Thus, from a hardware and software development standpoint, there would be little to no difference between a 23 dBm femtocell and a 23 dBm RAU – or a 40 W RRH and a 40 W Micro BTS, for that matter.
This creates a single R&D framework with the option of 100 percent software, RF subsystem, and hardware subsystem reuse across an entire product portfolio, providing a significant improvement in R&D cost efficiency across the life cycle for the HetNet access layer.
See Table 1 for a summary of common SoC processing layers for HetNet products.
Cell site backhaul technology
Another critical aspect of the evolving HetNet architecture is related to cell site service or signal backhaul. As the HetNet architecture differentiates two deployment practices, new technology and equipment will be required for cell site backhaul connecting radio access and antenna distribution equipment to a centralized service processing and control layer. These include:
- All-in-one BTS: Wired and wireless IP-based backhaul equipment
- RRH: Dark fiber transmission such as CPRI
- DAS: Proprietary fiber long haul and small coax or CAT5 for short-range distribution
Wireless backhaul can be much more cost-effective and flexible for deploying radio access equipment in new sites such as lampposts or traffic lights, for which any wire-line backhaul would require untimely construction permits and the very high costs of per-site construction and installation.
As a fundamental difference between the two deployment practices, only all-in-one equipment can utilize wireless backhaul with capacity and timing requirements that can be met by almost all wireless backhaul technologies, including emerging high-capacity Point-to-Multi-Point (PMP) and Non-Line-Of-Sight (NLOS) backhaul technology operating in frequency bands below 6 GHz.
Compact all-in-one equipment requires several types of backhaul solutions, including microwave long haul of compact rural Macro BTS sites, high-capacity short haul of suburban Micro BTS equipment, and new solutions for sub-6 GHz PMP/NLOS backhaul in Metrozone deployments.
Future-proof all-in-one SoC platforms are built to spec for 4G. How else could they be used as development platforms for carrier-grade, in-field software-upgradable 4G products? Hence, future-proof all-in-one SoC platforms provide baseband processing up to or above the ITU-mandated 1 Gbps of downlink capacity for 4G services. With a small enhancement, such SoC platforms can be extended to support more than 1 Gbps of full-duplex baseband processing capacity, making them an ideal SoC platform for unified wireless HetNet backhaul products.
A reference design example for the HetNet access layer
A single future-proof all-in-one SoC platform can support any distributed in- and outdoor service access product, including all-in-one BTS, RRH, or DAS equipment. If dimensioned to support future-proof ITU 4G specs, this SoC platform provides an ideal solution for gigabit-plus unified wireless cell site backhaul products. Figure 1 depicts a reference design based on a fully integrated all-in-one SoC platform.
The evaluation kit for the DAN3000 doubles as a complete reference design and comes preloaded with optional base station, radio head, or backhaul software packs and integrated development environments for all product design, simulation, implementation, test, and certification cycles. Programming on embedded multicore processing layers is based entirely on industry standard C/C++ language and related development tools. No FPGA, DSP, or assembly language coding is required for any software layer of a complete all-in-one BTS product.
An SoC platform such as this gives OEMs and system integrators a single R&D platform for any type of HetNet access or backhaul equipment, representing a huge cost improvement over traditional development frameworks still in use today.
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