Small Cells, Big Impact

Why Thinking Small Spells Success

Thanks to their versatility and low deployment costs, small cell technologies are playing an increasingly vital role in wireless carriers’ 4G/LTE upgrade strategies. Cellular network operators’ interest in small cells has grown as they upgrade their existing macro base station infrastructures with software-defined radios (SDRs). SDRs can simultaneously support existing 2G/3G services and 4G/LTE services. SDRs also allow carriers to gradually phase in advanced 4G services such as Time Division Duplex LTE (TD-LTE) and Advanced LTE (LTE-A) which make more efficient use of the spectrum within an allocated channel, enabling higher subscriber densities and data rates.

The strategic deployment of additional micro/nano/pico cell clusters is becoming an increasingly common part of any program to upgrade a network’s existing macrocell infrastructure for 4G/LTE service. These locally-networked arrays of small cells are used to enhance coverage and capacity in crowded urban environments and indoor venues. Both these new small cells and LTE-capable macro cell nodes are equipped with Ethernet ports which allow them to connect to IP-based backhaul networks which are dramatically less expensive to deploy and operate than traditional SONET/SDH-based infrastructures.

In this article, I will show how mobile carriers are using small cell networks and TDD-based air interfaces to expand their network’s coverage and capacity in an incremental, cost-effective manner. This will include a discussion of the challenges involved with supporting TD-LTE’s stringent timing requirements across packet-based backhaul networks and other issues carriers encounter as they develop their 4G/LTE migration strategies.

A Three-Pronged Approach

Subscribers’ growing demand for advanced mobile data services represents both an opportunity and
a challenge for wireless network operators. Upgrading networks to support 4G/LTE will allow them to serve new customers and offer advanced mobile services. Meanwhile, limited capital and the need to support their existing customer base makes so-called "forklift upgrades" impractical. For these and other reasons, most carriers employ a three-pronged approach, which uses a combination of:

• SDR-enabled base stations
• Small cell networks
• IP-based backhaul links

As we will see, this combination of technologies allows operators to upgrade their network incrementally, expanding coverage and adding extra capacity where they are most needed. Base stations and small cell networks equipped with SDRs allow carriers to roll out new LTE services while continuing to support subscribers’ existing 3G handsets. SDR-enabled base stations also reduce cost of backhaul infrastructure by using packet-based multi-Gigabit Ethernet links to connect to the radio access network (RAN). These IP-based backhaul connections deliver 10X or more bandwidth to base stations than traditional T1/E1 technologies at a much lower cost.

Small Cells: A Critical Element of Any Deployment Model

Early successes with using small cells to enhance the coverage and capacity of 3G networks have made it clear that they will play a key role in most carriers’ strategies for migration to 4G/LTE. Inexpensive to purchase and deploy, small cell networks are the most cost-effective way to deliver the bandwidth needed to support advanced mobile data services in heavily congested areas.

Small cells also make it possible to improve cellular coverage in challenging environments or bring wireless service to places considered impractical until now. Thanks to their smaller footprint and lower power requirements, small cells can be quickly deployed on the tops or sides of buildings to reach deep into so-called urban canyons. They can also be mounted on top of lampposts, traffic signals, and other existing free-standing elements of an urban streetscape. (See Figure 1.) Similar strategies can be used to extend cell coverage indoors to shopping malls, stadiums, and even large office buildings.

Figure 1. Small cells can be deployed to boost capacity in congested locations and provide coverage in areas such as urban canyons, where macrocells cannot reach. These low-power, compact devices are equipped with integrated networking capabilities and can be housed in a variety of specialized packages which allow them to be easily integrated into an urban landscape.

Once in place, small cell networks’ IP-based backhaul links add much-needed data capacity to existing 3G radio access networks (RANs) and lay the groundwork for a smooth migration to 4G/LTE service. But while backhaul reinforcement may be carriers’ prime motivation for deploying small cells, several customer-facing issues are also driving their adoption.

Smart phones, tablets, "phablets", and other net-centric devices are changing the way people work, play, and shop. All these devices require fast, dependable wireless data services to deliver the Quality of Experience (QoE) subscribers have come to expect. As more subscribers routinely use web-enabled mobile devices for watching movies, placing video calls, and cloud computing, their bandwidth and QoE requirements are rapidly outstripping the capacity of carriers’ existing cell-tower infrastructures. To meet this demand, carriers will need to supplement their existing networks with small cells and make them a major element of their new build-outs.

Small Cell Adoption Accelerates

The resulting demand is shaping both the strategies carriers are using to migrate to 4G/LTE service, and the rate at which it occurs. A recent Infonetics study⁷ provided some insight on cellular operators’ plans when it asked them "What percent of your mobile traffic (voice, data, video, Internet) will be handled in the future (2013 and later) by traditional macrocells, outdoor small cells, and indoor small cells?" The poll revealed that by the end of 2013, carriers expect that conventional macrocells will only be handling 71% of their networks’ traffic, a significant drop from the 90% level recorded in 2011. Meanwhile, they think outdoor small cells will account for 11% of total capacity (a 5x increase since 2011), with indoor small cells handling the remaining 8%.

Ethernet Backhaul Slashes CapEx & OpEx, Raises New Challenges

The SDRs used in both macro cell upgrades and small cell deployments include Ethernet connections, enabling them to make use of high-capacity Ethernet backhaul networks. Since there is no "one size fits all" solution for all small cell deployment scenarios, mobile operators and transport providers will need a toolkit of solutions for small cell backhaul which includes the ability to transmit Ethernet packets over copper, fiber, and RF (air) links. (See Figure 2.)

Figure 2. Microcell backhaul networks leverage IP/MPLS backhaul to enable a consistent deployment model and simplified operations. They use combinations of copper, fiber, and wireless links to accommodate the diverse environments they are required to serve.

Unlike the E1/T1 connections of the past, Ethernet does not have a native synchronization capability. In addition, TD-LTE and LTE-Advanced require Phase/Time-of-Day synchronization in addition to the frequency synchronization that was available using E1/T1 backhaul. Luckily packet synchronization based on the IEEE1588-2008 Precision Time Protocol (PTP) has matured to the degree that nanosecond accuracy can now be delivered over fiber, copper, and RF with just incremental cost impact.

LTE Migration Challenges

Thanks to its ability to support more subscribers and make more efficient use of a limited slice of spectrum, LTE-A is expected to become the dominant air interface for advanced 4G networks. The higher capacities, superior utilization factor and lower cost of subscriber handsets afforded by TD-LTE technologies helps operators achieve a better return from their CapEx and OpEx expenditures. In a similar manner, IP-based backhaul networks will cut infrastructure costs by supporting call handoff between local cell nodes and concentrating traffic for transport across low-cost Ethernet links.

Achieving these savings does, however, require more sophisticated base stations and backhaul links that can meet the precise timing requirements of the LTE-A. Both FDD and TDD systems need precise frequency synchronization between base stations but TD-LTE and LTE-Advanced also require very precise and stable phase alignment. Tight phase alignment is necessary to insure that the data packets contained in each channel created by the time-slotted air protocol arrive on time and don’t interfere with the data in adjacent channels. In TD-LTE and LTE-A systems, phase accuracy is measured with respect to the point in time in which the inter-channel guard band separating usage periods occurs.

Although the table shown in Figure 3 shows the minimum requirements for each advanced mobile air interface, it should be noted that most of the systems listed there deliver improved performance/throughput when they use timing signals with higher time/phase and frequency accuracy. Improved timing precision also yields dividends in terms of higher systems and network margins that enable reliable service in spite of residual link or systems impairments that frequently occur in real-world situations.

Figure 3. Frequency and phase accuracy requirements for 3G and 4G cellular systems.

LTE Migration Solutions

While 3G macro base stations previously received accurate frequency synchronization from legacy E1/T1 TDM connections and Time of Day (ToD) through GPS/GNSS satellite receivers, E1/T1 connections are being decommissioned everywhere to reduce recurring access operating costs and deliver more bandwidth at lower cost required for 4G. Many of the current mobile base stations use GNSS/GPS as their primary timing source. The 1pps signal derived from the satellite’s timing data is used to manage the ToD calculation for next second rollover as well as to synthesize the fundamental source radio frequencies. But GPS is often neither a cost-effective nor reliable timing technology because there is often no unobstructed line of sight to the satellites, and there are increasing concerns about jamming and spoofing of GPS/GNSS — especially for small cells that are located at street level.

Fortunately, the IEEE 1588v2 Precision Timing Protocol (PTP) provides a practical, cost-effective alternative for creating accurate timing domains across IP/Ethernet-based backhaul networks. PTP carries time of day information (using timestamps) directly within the data packets. The packets carrying the timestamps flow along with the rest of the data traffic in the network from grand master clock equipment that generates the timestamps (also known as primary reference clocks) all the way to base station equipment where these timestamps are used to recover the original time using the IEEE 1588v2 protocol.

While frequency synchronization can be achieved without hardware assist by the network itself (albeit at the expense of hour-long acquisition times), ToD cannot be delivered without help from the network. PTP’s enhanced capabilities include several mechanisms that can correct for the large packet delay variations often found in IP switches and routers through a defined hierarchy of clocks. The standard defines Boundary Clocks (BC), as well as Transparent Clocks (TC) to correct for such packet delay variations.

A node that implements BC has multiple network connections and can accurately bridge synchronization from one network segment to another and regenerates the timing based on the timestamps that it receives. TCs simply correct for any packet delays and delay variations between ingress and egress port on a packet-by-packet basis on the fly, providing "cut-through" timestamp correction. This enables both macro base stations and small cell networks to support nanosecond-level frequency and time-of-day synchronization across the cellular system’s Ethernet backhaul network.

Carriers today can build IP-based backhaul networks using PTP-capable switches, routers and other networking equipment which cost a fraction of earlier PDH-based products. This is made possible by highly-integrated silicon solutions which can add PTP capabilities to the Ethernet backhaul ports of base stations, and microcells as well as switches, routers, and other standard carrier-class networking equipment.⁸

Conclusions

Small cells help wireless carriers improve the capacity and coverage of their 3G networks while enabling a cost-effective incremental upgrade path to 4G/LTE services. Small cell networks can be used to add capacity in congested areas and help extend the network’s coverage to urban canyons and indoor environments where traditional macrocell infrastructures cannot reach. Multi-Gigabit Ethernet backhaul networks over fiber and air (microwave or millimeter-wave) comprise the third element of the upgrade strategy, providing the high-speed data capacity necessary to satisfy subscribers’ growing hunger for wireless multimedia services.

Underlying this evolutionary path is the IEEE 1588v2 PTP standard which enables Ethernet backhaul networks to deliver the precise timing synchronization between cells required to support 4G TD-LTE
and LTE-Advanced services.

About the Author
Uday Mudoi is a product marketing director for Vitesse Semiconductor. He has more than 15 years experience in the communications and semiconductor industries. For more information, email[email protected] or visit www.vitesse.com.

References:
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2. Cisco Visual Networking Index: Global Mobile Data Traffic Update, 2012-2017, February 6, 2013 —http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/…

3. "China Mobile Commits $6.7B to TD-LTE Capex in 2013" — Tammy Parker, Fierce Broadband March 18, 2013 — http://www.fiercebroadbandwireless.com/story/china-mobile-commits-67b-td…

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9. "Trends in the commercial development of TD-LTE", Benson Wu, DIGITIMES Research, November 14, 2012 — http://www.digitimes.com/Reports/Report.asp?datepublish=2012/11/14&pages…

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