Strategies for Future Mobile Backhaul Networks in the era of LTE-A, Small Cells and Cloud-RAN

This article tries to summarize various technologies and media types used for mobile backhaul and explore the impact of LTE-A, Small Cells and Cloud-RAN on Mobile Backhaul/Fronthaul Strategies.

Mobile backhaul has evolved significantly over the last few decades. During initial deployment of 2G/GSM networks in 1990s, backhaul was provided using TDM (E1/T1 circuits). Introduction of 3G NodeBs was supported by ATM circuits in the beginning and then the industry settled at IP/Ethernet backhaul. The result was a hybrid or dual backhaul approach with both TDM and IP/Ethernet. With the introduction of LTE eNodeBs over the last five years or so, the backhaul traffic mix has been tilting more and more towards All IP/Ethernet.

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Similarly from the choice of medium perspective, there have been three dominant media types:

  1. Microwave (offering ease of installation and lower CAPEX)
  2. Copper (utilizing existing telephony infrastructure)
  3. Fiber (providing the scale of bandwidth necessary for LTE and even LTE-A, thought as the most future proof medium).

As the penetration of different mobile access technologies has evolved, we have seen fiber replacing copper access in many areas, however microwave has maintained its share because of costs involved in laying fiber to each remote cell site. This distribution has also been impacted by geography, terrain and economical context of various countries and regions.

Below is an indicative graphic to represent the complexity of mobile backhaul that this mix of technologies and media types has induced over the last several years. It is not meant to be comprehensive from solution availability point of view but rather cover some of the more famous technologies which are being used for mobile backhaul.

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Increasing requirements for LTE-Advanced

LTE advanced is a combination of enhancements over LTE including features like CA (Carrier Aggregation), CoMP (Coordinated Multipoint) and eICIC (enhanced inter-cell interference coordination). Each of them brings the benefits of increased throughput and/or coverage but introduces new challenges for backhaul.

  • Peak bandwidth requirement for a large macro site aggregating multiple 3G & LTE cells/carrier can easily go in several Gbps which most of currently deployed backhaul solutions cannot support.
  • Performance measure including latency, jitter and frame loss thresholds is significantly stringent.
  • Coordination aspect of LTE-A requires phase synchronization in addition to currently supported frequency synchronization.

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Solutions like Carrier Ethernet, IP/MPLS, (N)GPON are developing features to support these requirements but the economics of increased costs because of these features is still not very clear.

Multiple Categories of Small Cells require different backhaul strategies

Small Cells working as a component of Heterogeneous Network (HetNet) are being lauded as a solution to increase capacity within the constraints of available spectrum. Small Cells have been around for quite some time and have been deployed to achieve different purposes.

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  1. Increasing coverage using Femto cells for 2G/3G/4G and IBS (In-building solutions) using DAS (Distributed Antenna Systems) for areas of bad coverage.

Utilizing fixed broadband access is a common and viable solution here.

  1. Increasing Capacity and Offloading Macro sites:
    • Carrier Wi-Fi offload using 802.11n/ac
    • Use of Metro / Micro / Pico Cells

Depending upon available and planned capacity, either a dedicated backhaul solution can be built or existing fixed broadband can be used.

  1. Increasing Capacity and enhancing Customer Experience using:
    • Seamless Carrier Wi-Fi with 802.11u & Hotspot 2.0
    • Coordinated small cells with Macro via eICIC or CoMP

This category of small cells requires special attention for backhaul planning because of requirement of similar features as mentioned for LTE-A above.

How Cloud RAN impacts the backhaul networks

Cloud RAN (radio access network) is tipped as the disruptive technology which will change the status quo of mobile industry. Enabled by 3GPP distributed architecture of BBU (baseband unit) and RRU (remote radio unit), Cloud RAN starts with BBU centralization to aggregate multiple RRUs with the benefit of resource sharing and increased utilization.

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This distribution introduces a new category of fronthaul which is being standardized on CPRI (common packet radio interface). CPRI is a standard developed with cooperation of all major mobile network vendors including Ericsson, Huawei, NSN, Alcatel-Lucent and NEC. It transports raw and uncompressed data between BBU and RRU and requires very high performance characteristics, e.g. latency of 5μs, jitter of ±0.002 ppm and Round Trip Delay Accuracy of ± 16.276ns.

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Examples of bit-rates required for a single cell. One site can have several cells/RRUs depending upon configuration.

Most of the current backhaul technologies cannot meet these performance criteria or the exponential bandwidth per site.

Alternatives for CPRI front-haul

  • Dark Fiber is a viable solution only if plenty of fiber resources are available for each cell site.
  • DWDM/OTN can be a suitable solution for macro sites where multiple RRU (CPRIs) can be aggregated to offer high scalability. But it introduced very high cost and negates the CAPEX & OPEX benefits gained by BBU centralization.
  • CWDM is significantly less expensive than DWDM/OTN however it has limited scalability, OAN and protection options.
  • Ethernet (Compressed) with 1:2 or 1:4 ratio makes it viable to transport CPRI over Carrier Ethernet like solutions, however it’s not proven yet whether it will meet the performance specs of CPRI
  • Radio (Compressed/uncompressed) over millimeter radio is being developed as a viable alternative of wired solutions which is attractive for shorter distances and distributed small cells who are coordinated with each other via BBU centralization.
  • WDMPON/NGPON2 are potential solutions to be deployed in point-to-multipoint architecture which utilized the existing (G/E)PON deployments. Important concern is cost economics for WDMPON and solution maturity for NGPON2.

To summarize, the future mobile backhaul network should provide reduced complexity and OPEX. They have to support for legacy (2G) as well as the bandwidth scalability for LTE-Advanced and small cells. Efficient and cost effective support of CPRI fronthaul will be the major inhibitor in adoption of Cloud-RAN.

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