SMALL CELLS: AN INTRODUCTION Strategic White Paper Small cells is an umbrella term for a genre of low-powered wireless access points capable of using both licensed and unlicensed spectrum. Small cells enable wireless users to enjoy high-performance data and voice communications at home, in the office, in public areas, and wireless hotspots. As a vital tool to increase network capacity, small cells will soon reshape the wireless landscape indefinitely. There are three primary market segments for small cells: household femto cells, enterprise cells, and metro cells. In the case of metro cells, the application is further delineated by placement indoor or outdoor. Small cells improve coverage and, perhaps more importantly, they increase capacity by offloading traffic from the macro wireless network. With data demands growing substantially, wireless operators are now intending to leverage small cells for just this purpose. According to a Bell Labs study, as much as 80 percent of traffic can be offloaded to small cells during peak times mitigating the need to add expensive macro cell sites or perform cell splits.
Table of contents Home femto cells / 1 Enterprise cells / 1 Metro cells / 2 Heterogeneous networks / 3 Summary / 5
Figure 1. Small cells represent a range of solutions Home Cell Enterprise Cell Enterprise Cell Metro Cell indoor Metro Cell outdoor Home femto cells Femto cell typically refers to a range of appliances that package the radio and baseband components into a consumer-oriented product for the small office, home office (SOHO). They are generally intended to be self-installed and utilize a consumer s broadband connection for connectivity to the mobile operator s core network. Because they operate within the same spectrum as the macro cell network, their deployment is within the same geographic area as the wireless provider is licensed to operate within. Home cells typically radiate at low power (20 mw to 100 mw) using an omnidirectional antenna and are designed to only support a small number of users usually four in a home environment or 4 to 8 in a SOHO environment. Like all small cells, users experience increased service quality and connection speeds because of increased signal to noise plus interference (SINR) ratios. This provides faster, more reliable data connections and highest voice quality with improved coverage. Additionally, battery life of mobile devices is generally extended because the user equipment only needs to transmit a short distance at a very low power. From a mobile operator s perspective, several additional key elements are needed to complete the small cells application. A centralized gateway is needed to aggregate the often thousands of femto cells before connection to the core wireless network. A security gateway and Ethernet-based timing source is needed to ensure privacy and synchronization. And, generally, some sort of operational system is required to interface customer relations management teams if things go awry. Because small cells must integrate with the greater wireless network, substantial back office work is required ahead of their introduction to the network at large. The market for home femto cells is normally driven by coverage rather than capacity. Consumers experiencing poor coverage are often directed toward femto cells as a solution. For the value proposition, in most cases, customer satisfaction and the ability for the operator to reduce churn at little to no expense if remarketing, costs are factored in. Enterprise cells In-building network performance is often a challenge for mobile operators. Indoor hotspotsthat have limited in building penetration and poor coverage and increased data demand are some of the reasons why small cells will be required. The analyst 1
firm In Stat predicts that by 2015 there will be 160.3 million active small cells worldwide; of that number 139.5 million will be in the home and enterprise environment. Enterprise cells target institutions that require in-building and/or outdoor coverage. While much of the enterprise segment is dominated by offices or campuses that need improved indoor coverage and capacity, applications for outdoor coverage can include malls, transportation centers and stadiums. Enterprise cells usually operate at higher power than the home cells (100 mw to 250 mw) and have higher capacity; often having 8 to 16 users, but up to 32 users is not uncommon. The enterprise cell uses the customer s site, power, and Internet connection, but it is not always owned by the customer. Due to the complexity of many enterprise locations, the operator usually conducts a radio frequency (RF) analysis to determine best placement for the cells and installation of the network. Otherwise, enterprise cells work similarly to home femto cells, but with additional capabilities, such as support for grouping and operation using open and/or semi-open access modes that have controlled mobile access lists. Lastly, in many cases, enterprise cells can provide a lower cost of ownership per site compared with traditional in-building solutions (such as distributed antenna systems) while enabling improved performance and capacity. When used as a landline replacement, they can also reduce overall costs for the small business owner. Metro cells Like other small cells, metro cells operate in licensed mobile operator spectrum. They are similar to enterprise cells except they usually deliver higher power (1 W to 10 W), support a greater number of users, and are often hardened to operate in an outside plant. They provide a flexible, lower cost alternative to expanding macro cells or performing cell splitting even as network capacity is increased at the expense of additional engineering and outside plant construction. Consistent with all small cells, SINR is substantially improved, which greatly improves the air link performance, while offloading the macro network and greatly increasing its capacity. Relative to other capacity improvements that can be made (for example, carrier aggregation, addition of spectrum, multiple-input multiple-output [MIMO]), small cells, and metro cells in particular, offer substantially more capacity. They essentially multiply wireless network capacity by creating an underlayment of small, coverage-specific serving areas. This gain is repeated with the addition of each subserving area. Working together with the macro base station, gains of up to tenfold can be estimated. Figure 2. Gains of metro cells Options LTE-A 3G and 4G options MORE SPECTRUM SPECTRAL EFFICIENCY SPATIAL EFFICIENCY ADD CARRIERS CARRIER AGGREGATION eicic CoMP MIMO METRO CELLS AAA MACRO ~ 2X MORE SPECTRUM IN PIPELINE UP TO 2X GAIN GAIN=N METROS (MAX N ~ 10) 2X 2X gain when R10 standards become available 10X gain available now 2
While all small cells can be expected to connect to backhaul and the network using Ethernet, metro cells in particular can also be equipped to connect using the common public radio interface (CPRI), which is the native connection used between base station transceivers and modem units. Depending upon venue, there can be some advantages to this, although this type of connectivity does rely on the use of dark fiber which can be prohibitively expensive when it must be leased. Typical options for outdoor small cells vary by: Type of antenna it can be omnidirectional or directional Power levels can vary from milliwatts to several watts, but less than the macro base station Number of users can typically vary from 16 to 64 Physical size and mounting options might include pole or wall mount Backhaul or network connections are usually made using Ethernet, although the backhaul itself can be microwave, millimeter wave, fiber to the building (FTTB), very high speed digital subscriber line (VDSL), and even dark fiber when using CPRI Figure 3. Outdoor small cells Heterogeneous networks As with any radio frequency system, correct placement of small cells usually starts with an RF analysis and network design along with an expected set of requirements regarding performance outcomes. To integrate outdoor small cells a detailed analysis of current capacity demands and potential growth must be done. This study will then point to the type of small cell that is required to satisfy the network need; for example, antenna type, power required, number of users to be served, and site availability. A completely self-contained small cell has the radio and baseband unit (modem) integrated with an interface to traditional backhaul, such as a microwave link using Ethernet. A second architecture exists using a distributed configuration. In this configuration only the radio is in the small cell, whereas the baseband unit is located separately. In the distributed configuration, high-speed fiber link(s) carrying CPRI is used to connect the baseband units to the radio-only small cells. The choice between the types of small cell architectures will depend upon many factors. If the coverage area to be served is relatively small, a low power small cell with an omnidirectional antenna might be used. If demand is forecasted to be in a highly targeted region (hotspot), a directional antenna would be more effective. 3
Figure 4. Heterogeneous network 3G MACRO LTE MACRO SMALL CELLS AND WI-FI In addition to power concerns and antenna placement, a detailed engineering analysis must also take into consideration the impact to the macro network. Interference is one case in point. With specific regard to home and enterprise networks, it is customary to share band plans with the macro network. Given that these small cells typically operate at low power and usually within enclosed spaces, the risk to interference with the macro network or other network user equipment is slight. However, as the coverage area increases through power, or small cells are placed in public or partially public venues, it may be desirable to operate at a specific frequency within the band or another band to avoid interference. Because Long Term Evolution (LTE) is designed with frequency reuse in mind, these constraints are largely restricted to 3G (Code Division Multiple Access [CDMA] or Wideband Code Division Multiple Access [W-CDMA]). Handovers are another topic for consideration in heterogeneous networks (HetNets). Generally, small cells intended to address coverage issues will also terminate voice calls. With 3G networks, small cells generally terminate to a common security and routing gateway ahead of the core voice (3G mobile switching center [3G MSC]) and packet (serving gateway support node [SGSN]) networks. This makes soft handovers more complex, as the small cells are not directly attached to a radio node controller (RNC), but are negotiated between the gateway and RNC instead. In the case of LTE, the small cells often connect to the evolved packet core (EPC), making the job simpler. Fortunately, their limited geographic coverage means that small cells connectivity is best restricted to instances of low-speed mobility, which aligns well to the needs of data users, while termination of highly mobile and often high-velocity voice is best left to the macro network unless it is critical. Given certain regulatory considerations, a decision must be made as to whether voice is actually terminated on the small cells. 4
Summary Small cells for wireless networks are proving to be an important tool to dramatically increase capacity, improve coverage as well as the subscriber experience. They also provide an opportunity for operators to differentiate their quality of service, be it in the home, office, or public venue. With spectrum limited, small cells also provide a cost- effective means for utilizing it to its maximum potential. 5
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