SDN Radio Router Technology for Fronthaul Networks

By Grant Henderson

November 29, 2018

Any baseband resource may be routed to any remote radio unit, regardless of vendor and over any protocol interface, with a software-defined network (SDN) radio router.

More than one million new mobile broadband subscribers will be added each day for the next six years, according to the Ericsson Mobility Report 2017. With the promises of 5G wireless communications, not only will more people be connected, but also many devices, machines, cities and infrastructures will also be connected. Gartner forecasts that 20.4 billion connected things will be in use worldwide by 2020. This exponential growth in subscribers and internet of things (IoT) will place unprecedented pressure on existing wireless infrastructure.

The practice of providing wireless coverage and capacity using dedicated wireless infrastructure in venues is almost as old as the mobile industry itself. With the expected mobile data trafﬁc growth, mobile operators are looking at innovative ways to cost effectively scale their networks to meet the expected demands. In recent years, we have seen virtualization adopted in the core network to drive scalability, ﬂexibility, and agility. This same type of virtualization is now being expanded into the radio access network (RAN). However, today’s fronthaul network is still static and hardware-dependent.

In addition, the 5G RAN will represent a fundamental change. 5G will disaggregate the RAN into functional blocks that can be executed in different physical locations and employ a ﬂexible, service-oriented protocol stack with centralized network control. As a result, today’s closed, proprietary, point-to-point fronthaul networks will no longer support the needs and future evolution of networks.

Dali Wireless SDN Radio Router radio over IP technology plays a key role by transforming the fronthaul network from a point-to-point to a multipoint-to-multipoint network that is driven by software. It allows any baseband resource to be routed to any remote radio unit, regardless of vendor, over any protocol interface — analogous to how IP data packets are routed in the data network.

The following information explains the patented SDN Radio Router radio over IP technology and how it enables a multipoint-to-multipoint fronthaul network and open RAN. It illustrates how the technology can effectively facilitate rapid network expansion, densiﬁcation and migration to the next generation networks while protecting operators’ investments in legacy networks.

Transforming Fronthaul

To achieve greater utility and cost efﬁciency, and to effectively scale to meet the demands of LTE and 5G, the radio over IP router transforms the traditional two-tier fronthaul architecture, which employs a point-to-point connection between the baseband unit (BBU) and the remote radio unit (RRU) to a three-tier multipoint-to-multipoint network that is driven by software.

By adding a tier that implements advanced radio signal routing, aggregation and translation capabilities to the fronthaul network, the technology enables shared infrastructure, dynamic capacity allocation and, more importantly, an open RAN. This three-tier architecture creates a more competitive and innovative ecosystem where mobile operators have the option to select the best-in-class RAN elements that not only meet their current network requirements, but also the varying 5G applications and deployment scenarios in the most cost-effective and efﬁcient way.

This is accomplished by enabling interoperability between new entrant vendors and existing deployed RAN elements. A three-tier architecture, enabled by SDN radio-over-IP router technology, enables a cost-effective multiple-operator solution and enables network modernization without incurring the step function cost of rip and replace.

The additional tier, which consist of the fronthaul radio router and edge radio router, enable a multipoint-to-multipoint fronthaul network that is fundamental in enabling open RAN. The fronthaul radio router and edge radio router are part of the Dali Matrix Virtual Fronthaul Interface (vFI) technology.

Figure 1. A multipoint-to-multipoint fronthaul network enables ﬂexible routing of baseband resources to any remote radio units.

The fronthaul radio router interfaces with the baseband units and supports translation among different transport protocols including CPRI, eCPRI, RoE and Ethernet. It also aggregates multiple baseband streams and routes or forwards the data to the RRUs on the edge directly or via an edge radio router. An edge radio router receives the speciﬁc content from the fronthaul radio router and then aggregates and routes the data to speciﬁc RRUs on the edge. In the case of a third-party RRU, the edge radio router can also translate among different transport protocols (see Figure 1. The three primary features (protocol translation, fronthaul aggregation and fronthaul routing) have roles to play.

Protocol Translation

The RAN market today is highly consolidated with only a few vendors remaining. Although the interface between the RAN and the core network is standardized, open and interoperable, the fronthaul or intra-RAN interfaces remain non-interoperable. In part, this is because the BBU-RRU connection is traditionally part of the interworking of the base station. Currently, the widely used interface between the BBU and RRU is the Common Public Radio Interface (CPRI). Optical ﬁbers have been used almost exclusively because of the demanding CPRI data rate. However, to support the centralization of the baseband resources and the fronthaul throughput requirements of 5G, use of CPRI as a transport protocol has its limitations.

Figure 2. A fronthaul radio router and an edge radio router, based on Dali’s SDN Radio Router technology, enable interoperability between the central unit (CU) and distributed unit (DU), thus unbundling the radio access network (RAN).

Dali’s SDN Radio Router technology helps mobile operators break through the highly monopolized RAN market by allowing interworking between central units (CUs) and distributed units (DUs) supplied from different vendors, thus unbundling the RAN (see Figure 2). The fronthaul radio router and edge radio router support multiple fronthaul transport protocols and mapping or translation among different protocols including CPRI, eCPRI, Radio over Ethernet (RoE), Ethernet and Next Generation Fronthaul Interface (NGFI). This unique translation capability provides mobile operators the ﬂexibility to connect any baseband element to any remote radio unit regardless of vendors and fronthaul transport protocols, hence creating an architecture where mobile operators have the ability to select the best RAN components for its independent applications, in the most cost-effective way.

Fronthaul Aggregation

On the northbound interface, the fronthaul radio router accepts multiple baseband streams and multiplexes or aggregates them onto higher data rate streams. On the southbound interface, the fronthaul radio router interconnects with the edge radio routers located at multiple buildings of a campus or large enterprise venues for content aggregation and delivery to multiple RRUs for broadcasting.

Figure 3. The Fronthaul radio router aggregates multiple fronthaul streams onto high data rate serial stream and the edge radio router receives the speciﬁc content and routes the data to speciﬁc remote radio units (RRUs) on the edge.

By connecting directly to BBUs or virtual base stations (vBSs), the fronthaul radio router creates a fully digital transport platform. It incorporates secure, FPGA hardware that enables network equipment providers (NEPs) to implement proprietary management information base (MIB) extensions to link to their baseband unit or virtual base station (see Figure 3).

Fronthaul Routing

Another function of the fronthaul radio router and edge radio router is to connect multiple operator resources and route the data content to particular RRUs based on trafﬁc requirements at the RRU locations, hence enabling shared fronthaul infrastructure. This is especially beneﬁcial for neutral-host operators and mobile operators with shared towers or radio heads. The fronthaul radio router and the edge radio router exchange information about destination addresses using a routing protocol similar to an IP routing protocol. The fronthaul radio router and edge radio router build up a routing table listing the preferred routes between any interconnected devices.

Figure 4. Multipoint-to-multipoint network enables any CU resources to be aggregated and routed to any DU.

With a traditional point-to-point fronthaul network, the source and destination are always the same because the network is hardwired. With Dali’s SDN Radio Router technology, an intelligent logical layer supersedes the hardwired physical network. Irrespective of the underlying physical transport network, be it star, daisy-chain or hybrid, a logical network is formed that enables signals to travel from any source to any destination — a multipoint-to-multipoint network. The technology essentially enables trafﬁc from any base station, BBU, vBS or CU to be routed to any RRU or distributed units (DUs) (see Figure 4).
The edge radio router receives the overall content from the fronthaul radio router or multiple fronthaul radio routers (each belonging to a different MNO) at the access site, and then aggregates the data (in a case of multiple-operator installation) and routes the desired content to the appropriate RRUs.

Fronthaul aggregation, routing and protocol translation are the key features that enable a multipoint-to-multipoint network. A multipoint-to-multipoint network enables any vendor’s baseband element to be connected to any vendor’s RRU, regardless of transport protocol, thereby creating an environment where an interoperable and open RAN can become a reality.
The following information illustrates how a multipoint-to-multipoint fronthaul network supports mobile operators and venue owners’ wireless network objectives through various deployment scenarios.

Deployment Scenarios

In venues such as hotels, hospitals, airports, multiple-dwelling units, universities, stadiums, metros, trains and tunnels, having a single system in place that supports multiple mobile operators, frequency bands and technologies is critical. Not only for the mobile subscriber’s experience, but also for the building owner’s and the mobile operator’s ability to reduce capital and maintenance cost with shared infrastructure.

Figure 5. Shared infrastructure for multiple operators for a single venue or multiple venues.

With a multipoint-to-multipoint fronthaul network, multiple sectors and base station resources from multiple operators can be digitally processed and aggregated. The fronthaul radio router, which also acts as an edge radio router, translates and aggregates the signal, and routes content to multiple RRUs where the signal is converted back to RF for antenna distribution.
This is applicable for a single venue or multiple venues within a 10-kilometer to 20-kilometer radius (governed by maximum allowed round trip delay that is technology-dependent). With a multipoint-to-multipoint network, additional base station resources, either different sectors or different mobile operators can be added incrementally without changes to the existing distribution network making scaling much easier (see Figure 5).

To further improve the business case, having the ability to allocate capacity in a multiple-use facility or a city area is also important because mobile trafﬁc is not static. A multipoint-to-multipoint network eliminates the need to provision every destination point with the peak capacity. This reduces capital, lowers operational expenditure and increases the utilization of

Figure 6. Dynamic capacity allocation during and after the event in a stadium.

valuable base station resources and spectrum assets. Fewer base stations are required to serve a set of venues because it is now possible to dynamically allocate wireless capacity on demand where and when it is needed.

Let’s consider a stadium and its nearby restaurants. During an event, trafﬁc shoots up in the stadium. When the event ends, trafﬁc increases in the nearby buildings (restaurants) and decreases in the stadium. With Dali Matrix vFI, capacity can be dynamically routed on-demand to match the usage pattern of the mobile subscribers (see Figure 6).

Centralized RAN Architecture

In a centralized RAN architecture, the base station resources are located in a ﬁber center or a data center far away from the

Figure 7. Centralized RAN, enabled by the fronthaul radio router and edge radio router, enables cost-effective densiﬁcation and rural coverage.

venues where the RRUs are located. In this architecture, the fronthaul radio router is located at the ﬁber center or data center, and it translates and aggregates multiple baseband streams and then routes the data content to the edge radio routers located at the DU site. Edge radio routers then route the desired content to the RRUs at the venue (see Figure 7). A multipoint-to-multipoint fronthaul network enables a centralized architecture and supports both high-functional split and low-functional split between CUs and DUs, enabling an elegant migration to 5G while providing maximum investment protection for RRUs already deployed.

Densiﬁcation

In an urban area like a downtown center, where capacity is critical and space is limited, a Dali vFI multipoint-to-multipoint fronthaul network enables multiple mobile operators’ base station resources to be concentrated in a data center. This is critical in some markets where real estate can be expensive and difﬁcult to obtain.

To minimize interference, the aggregated baseband resources are simulcasted to multiple RRUs to create one virtual cell. By changing simulcast ratio, these virtual cells can shrink or expand based on the capacity demands of the mobile subscribers.
Not only does this architecture enable mobile operators or even sectors to be added incrementally in the ﬁber center or data center to easily densify the network, it enables cell sizes to dynamically change to accommodate the ﬂuctuations in mobile trafﬁc.

Rural Coverage

In rural areas where coverage is the key requirement, a Dali vFI multipoint-to-multipoint network enables mobile operators’ capacity to be dynamically multicast over multiple rural sites. The ability to centralize multiple mobile operators’ base stations resources in a local data center and deliver the signal to multiple rural areas over long distances increases the revenue per square mile of such systems.

Functional Split

Figure 8. The ability for Dali’s SDN Radio Router technology to support multiple fronthaul protocols and translate between the different protocols such as CPRI, RoE, eCPRI, and Ethernet is vital.

Currently, the industry standard for fronthaul transport is based on synchronous serial transport protocols like CPRI or OBSAI. However, to support the centralization of the baseband resources and the throughput requirements of 5G, use of CPRI as a transport protocol has its limitations. New transport protocols based on high functional split (e.g., option 2 per 3GPP TR 38.801 standard) are less demanding on the fronthaul transport infrastructure and can tolerate longer delays, which makes it ideal for long hauls between national and local data centers. Therefore, the ability for Dali’s SDN Radio Router technology to support multiple fronthaul protocols and translate among the different protocols such as CPRI, RoE, eCPRI, and Ethernet becomes vital (see Figure 8).

The fronthaul radio router and edge radio router effectively provide the link between the centralized BBUs and the RRUs, irrespective of the level of baseband processing incorporated at the DU. A multipoint-to-multipoint fronthaul network that can support various functional splits enables end-to-end network slicing required to support the different service levels, performance, spectrum use, latency and power that various 5G applications require. In addition, it can provide investment protection for existing RRUs.

Depending on the CU-DU functional split, the fronthaul radio router can be located at the CU or DU side of the RAN. In a case of a high-level protocol split (e.g., option 2) that is ideal for long distance transport, the fronthaul radio router is located at the local data center. It receives Ethernet streams (<1 Gbps) from multiple mobile operators’ national data centers. It then aggregates and translates them to I/Q data based transport protocol (CPRI, eCPRI, or RoE), and delivers that content to edge radio routers located at the venue, which then route the signals to RRUs.

An edge radio router placed at the venue can also directly receive signals from multiple data centers, belonging to multiple operators, and then aggregate, translate and route content to RRUs inside the venue. Deployed in this manner, an edge radio router provides the combined functionality of a fronthaul and edge radio router.

Low-level functional split, which is ideal for shorter distance transport and where ﬁber is available, is also supported. With low-level functional split (option 7 or 8), the fronthaul radio router receives multiple CPRI streams, aggregates them at the data center, and then routes speciﬁc content to speciﬁc macro towers location or venues using CPRI or RoE transport proto- col.
In this case, the fronthaul transport data rate is between 10 Gbps to 200 Gbps depending on the amount of information required at the macro towers or venue sites. At the macro tower or venue location, an edge radio router receives the overall content, demultiplexes data, and forwards the desired signals to speciﬁc RRUs.

Figure 9. In a centralized RAN architecture with low-level protocol split, multiple low data rate streams can be multiplexed to support higher data rates

A centralized RAN architecture with low-level protocol split that needs to support higher data rates to the macro towers or venues can be supported as well. The fronthaul radio router can receive multiple low data rate streams from the pool of BBUs, and aggregates/multiplexes these data rates to a higher data rate serial stream (10 Gbps each; 25 Gbps each in the future). Multiple high data rate streams can then be multiplexed using integrated CWDM or DWDM (coarse or dense wavelength-division multiplex) optical transceivers, and be sent to the edge radio router at the venue over a pair of optical ﬁber links, achieving aggregated transport rates of 100-200 Gbps (or higher in the near future) (see Figure 9).

A multipoint-to-multipoint fronthaul network that supports various functional splits will not only ensure that mobile operators have a 5G ready network but offers a network that protects operators’ investment in legacy 2G, 3G and 4G infrastructure.

MIMO Scaling

Multiple-input multipoint-output (MIMO) requires a dedicated link for every antenna or a dedicated wavelength on a ﬁber. This becomes problematic as mobile operators invest in massive MIMO to increase data rates and capacity. Multiple data streams are required currently to transfer data simultaneously for MIMO. Today’s point-to-point network might be enough for a 2×2 MIMO or a 4×4 MIMO; however, massive MIMO proposed for 5G with 16×16, 32×32 and higher will pose severe demands on the fronthaul transport. In addition, for higher orders of MIMO, a high functional split is required such that the data can be transported from the vBBU to the venue before the individual MIMO streams are generated. This again highlights the importance of a multipoint-to-multipoint network that can aggregate and multiplex multiple data streams onto a single, higher data rate stream, and can support high or low functional splits.

Finally, with a multipoint-to-multipoint network, mobile operators can easily scale to meet the demands of massive MIMO by adding more capacity and more MIMO paths without changing the fronthaul distribution network.

Supports Various Use Cases of 5G

5G is the next generation wireless technology. Not only does it promise enhanced mobile broadband (eMBB) experience, it also enables massive machine type communications (mMTC) and ultra-reliable and low-latency communications (URLLC). To provide the different levels of service, performance, spectrum use, latency and power that the various applications require, a 5G ready fronthaul network must be ﬂexible enough to support multiple operators, standards, protocols, vendors, frequency bands, and applications. A Dali vFI multipoint-to-multipoint network is the answer.

A multipoint-to-multipoint fronthaul network enables a mix and match of base stations, vendors, and mobile operators so that the most economic and most effective network infrastructure can be chosen to best handle the requirements of 5G. In addition, as new services and new applications are enabled, a multipoint-to-multipoint fronthaul network enables a phased roll-out approach. This offers mobile operators the ﬂexibility to add new applications and services incrementally without changes to the fronthaul distribution network.

Backward Compatible

Supporting legacy 2G, 3G and 4G networks is just as important as modernizing the networks in preparation for 5G. Having a fronthaul network in place that is backward-compatible is vital as mobile operators have invested billions into their existing infrastructure. It is imperative that the fronthaul network can enable mobile operators to maximize the use of their existing infrastructure while helping them seamlessly migrate to the next-generation networks incrementally without rip-and-replace. This again illustrates the importance of a fronthaul network that can support multiple vendors, multiple fronthaul protocols and multiple technologies like 2G, 3G and 4G.

Conclusion

The rigid nature of the fronthaul until this point has been an Achilles heel to a cost-effective and capacity-efﬁcient network. A Dali multipoint-to-multipoint fronthaul network acts like a catalyst that enables operators to truly reap the beneﬁts of an end-to-end virtualized and open RAN. The ability to mix and match RAN elements from any vendors, over any protocol interface provides mobile operators with the utmost ﬂexibility to deliver the board scope of applications with varying requirements in 5G.
Ultimately, networks are only as strong as their weakest link. Since the advent of software deﬁned networking (SDN), until now, this has arguably been the fronthaul. By embracing vRAN and extending the virtualization to the fronthaul, operators can create resilient, future-proof networks that will not only maximize the usage of their existing 2G, 3G and 4G infrastructure, but also will serve as the foundation for 5G and network expansion for generations yet to come.

Grant Henderson is chief marketing officer at Dali Wireless. His experience includes executive management, marketing, product management, and international business development and strategy. Dali Wireless provides wireless infrastructure, such as its digital wireless signal routing solution, to handle exponential growth in mobile data traffic. Visit www.daliwireless.com.