Unlicensed Spectrum

Wi-Fi, an unlicensed wireless technology, has experienced huge success due to high throughput rates, ease of use for consumers, extensive deployment by businesses, widespread availability in public places, and large amounts of available spectrum.

For mobile operators, Wi-Fi can offload data traffic, relieving some stress from capacity demands. To make offload work more effectively, the industry is working to more tightly bind Wi-Fi functionality with cellular operation, as discussed below in more detail under “Wi-Fi Integration and Data Offload.”

Wi-Fi uses spectrum efficiently because its small coverage areas result in high-frequency reuse and high data density (bps per square meter). Less efficient are white-space unlicensed networks, sometimes called “super Wi-Fi,” that have large coverage areas, because the throughput per square meter is much lower. While white-space networks may be a practical broadband solution in rural or undeveloped areas, they face significant challenges in urban areas that already have mobile and fixed broadband available.34 See the section on “White Space Networks” in the appendix for further details.

Advocates argue that unlicensed spectrum unleashes innovation and that government should allocate greater amounts of unlicensed spectrum. Although Wi-Fi has been successful, the core elements that make unlicensed spectrum extremely successful are also the source of inherent disadvantages: local coverage and its unlicensed status. Local coverage enables high data density and high frequency reuse but makes widespread continuous coverage almost impossible. Similarly, unlicensed operation facilitates deployment by millions of entities but results in overlapping coverage and interference.

Wi-Fi cannot replace networks built using licensed spectrum. The two are complementary and helpful to each other, as summarized in Table 1

Table 1: Pros and Cons of Unlicensed and Licensed Spectrum

Unlicensed Pros	Unlicensed Cons	Licensed Pros	Licensed Cons
Easy and quick to deploy	Potential of other entities using same frequencies	Huge coverage areas	Expensive infrastructure
Low-cost hardware	Difficult to impossible to provide wide-scale coverage	Able to manage quality of service	Each operator has access to only a small amount of spectrum

Some operators, such as Republic Wireless, offer a “Wi-Fi first” capability under which devices always attempt to use a Wi-Fi connection and fall back to a cellular connection only if no Wi-Fi is available. Such cellular backup is essential because Wi-Fi, due to low- power operation in many bands, is inherently unsuited for providing continuous coverage. The sharp drop-off in signal strength makes coverage gaps over large areas inevitable, especially outdoors.

Figure 1: Propagation Losses of Cellular vs. Wi-Fi

A capability being discussed for Release 13 is LTE operating in unlicensed bands. Carrier aggregation would combine a licensed carrier with an unlicensed 20 MHz carrier in the 5 GHz band as a supplemental channel. LTE uses channels differently than Wi-Fi, so engineers are evaluating how LTE could be a fair neighbor in unlicensed bands and how it could meet varying regulatory requirements for unlicensed bands in different parts of the world.

LTE operating in unlicensed bands could eliminate handoffs to Wi-Fi, possibly creating a more seamless user experience. Under heavy load, LTE is spectrally more efficient than Wi-Fi, since it uses more sophisticated over-the-air scheduling algorithms.

A capability being discussed for Release 13 is LTE operating in unlicensed bands. Carrier aggregation would combine a licensed carrier with an unlicensed 20 MHz carrier in the 5 GHz band as a supplemental 

Harmonization | 5G Bands

Spectrum harmonization delivers many benefits, including higher economies of scale, better battery life, improved roaming, and reduced interference along borders.

As regulators make more spectrum available, it is important that they follow guidelines such as those espoused by 4G Americas

1.       Configure licenses with wider bandwidths.

2.       Group like services together.

3.       Be mindful of global technology standards.

4.   Pursue harmonized/contiguous spectrum allocations.

5.       Exhaust exclusive use options before pursuing shared use.

6.       Because not all spectrum is fungible, align allocation with demand.

Emerging technologies such as LTE benefit from wider radio channels. These wider channels are not only spectrally more efficient, they also offer greater capacity. Figure 1 shows increasing LTE spectral efficiency obtained with wider radio channels, with 20 MHz on the downlink and 20 MHz (20+20 MHz) on the uplink showing the most efficient configuration.

Figure 1: LTE Spectral Efficiency as Function of Radio Channel Size

Of some concern in this regard is that spectrum for LTE is becoming available in different frequency bands in different countries. Roaming in many cases is based on GSM or HSPA on common regional or global bands.

The organization tasked with global spectrum harmonization, the International Telecommunication Union, periodically holds World Radiocommunications Conferences (WRC).

Harmonization occurs at multiple levels:

q  Allocation of radio frequencies to a mobile service in the ITU frequency allocation table.

q  Establishment of global or regional frequency arrangements, including channel blocks and specific duplexing modes.

q  Development of detailed technical specifications and standards, including system performance, RF performance, and coexistence with other systems in neighboring bands.

q  Assignment for frequency blocks with associated technical conditions and specifications to appropriate operators and service providers.31

Figure 2 shows the harmonization process.

Figure 2: Spectrum Harmonization

5G Bands

As radio technology progresses, it can handle higher frequencies, and it occupies greater bandwidth. 1G systems used 30 kHz radio carriers, 2G in GSM uses 200 kHz carriers, 3G in UMTS uses 5 MHz carriers, and 4G in LTE uses carriers of up to 100 MHz through carrier aggregation.

Although 5G research and development is in its infancy, to achieve the 10 Gbps or higher throughput rates envisioned for 5G will require radio carriers of at least 1 GHz, bandwidths available only at frequencies above 5 GHz. Researchers globally are studying high-frequency spectrum options. For example, the 5G organization Mobile and wireless communications Enablers for the Twenty-twenty Information Society (METIS) has published a report on spectrum needs that evaluates the following frequency bands:

q  380 to 5925 MHz (current systems)

q 5.925 MHz to 40.5 GHz

q  40.5 GHz to 95 GHz

q  95 GHz to 275 GHz (representing the upper limits of mmWave bands)

Higher frequencies are well suited for ultra-dense small-cell deployments, but longer propagation is also possible using antenna arrays and beamforming. For example, Samsung has demonstrated 2 km line-of-sight transmission at 28 GHz.

Spectrum Developments 4G LTE

Spectrum continues to challenge the industry. Given this limited resource, the industry is:

q  Deploying technologies that have higher spectral efficiency.

q  Adapting specifications to enable operation of UMTS-HSPA and LTE in all available bands.

q  Designing both FDD and TDD versions of technology to take advantage of both paired and unpaired bands.

q  Designing carrier aggregation techniques in HSPA+ and LTE-Advanced that bond together multiple radio channels (both intra- and inter-frequency bands) to improve peak data rates and efficiency.

q  Deploying as many new cells (large and small) as is economically feasible.

Although all of these industry initiatives greatly expand capacity, they do not obviate the need for additional spectrum.

The FCC released a report in October 2010 that projected U.S. spectrum requirements19 and concluded that 275 MHz of additional spectrum would be needed within five years and 500 MHz of additional spectrum within 10 years. This forecast assumes ongoing increases in spectral efficiency from improving technologies.

An important aspect of UMTS-HSPA and LTE deployment is for infrastructure and mobile devices to accommodate the expanding number of available radio bands. The fundamental system design and networking protocols remain the same for each band; only the frequency- dependent portions of the radios must change. As other frequency bands become available for deployment, standards bodies adapt UMTS-HSPA and LTE for these bands as well.

The 1710-1770 uplink was matched with the 2110-2170 downlink to allow for additional global harmonization of the 1.7/2.1 GHz band. These new spectrum bands were reserved or allocated harmoniously across North, Central, and South America. The Advanced Wireless Services (AWS) band, at 1710-1755 MHz (uplink) with 2110-2155 MHz (downlink) in the United States, is affording operators additional deployment options and could eventually provide a means for LTE roaming in the Americas. Multiple U.S. operators are deploying LTE in 1.7 GHz, as is Rogers in Canada. In addition, many countries in Latin America have allocated this band for mobile broadband.

In 2012, the FCC changed the rules on the Wireless Communications Service (WCS) band, which constitutes 30 MHz of spectrum at 2.3 GHz, making 20 MHz available for mobile broadband.20

The forthcoming 2.6 GHz frequency band in Europe, Asia, Latin America, and other parts of the world will also be a common band for LTE deployment. Unfortunately, different band plans in different parts of the world will complicate roaming on this band. Globally, operators are also refarming spectrum by limiting the spectrum used for 2G (cellular and Personal Communications Service [PCS] bands) to create space for 3G and 4G deployments.

Recently, the FCC repurposed 40 MHz of Mobile Satellite Services (MSS) spectrum for terrestrial use in what it termed the “AWS-4 band.”

Unfortunately, the process of identifying new spectrum and making it available for the industry is a lengthy one, as shown in Figure 1.

Figure 1: Spectrum Acquisition Time

The biggest opportunity areas for new spectrum in the United States currently are incentive auctions of TV-broadcasting spectrum at 600 MHz, government spectrum from 1755 to 1850 MHz and 1695 to 1710 MHz managed by the National Telecommunications and Information Administration (NTIA) to be auctioned as part of AWS-3, and the proposed “small-cell” band from 3550 to 3650 MHz and its possible extension to 3700 MHz.

In June 2013, the Obama Administration announced new Administration spectrum initiatives, including directing Federal agencies to enhance the efficiency of their spectrum use, making greater capacity available for commercial networks, and funding research into spectrum sharing.

Table 1: United States Current and Future Spectrum Allocations

Frequency Band	Amount of Spectrum	Comments
700 MHz	70 MHz	Ultra-High Frequency (UHF)
850 MHz	64 MHz	Cellular and Specialized Mobile Radio
1.7/2.1 GHz	90 MHz	Advanced Wireless Services (AWS)-1
1.9 GHz	140 MHz	Personal Communications Service (PCS)
2000 to 2020, 2180 to 2200 MHz	40 MHz	AWS-4 (Previously Mobile Satellite Service)
2.3 GHz	20 MHz	Wireless Communications Service (WCS)
2.5 GHz	194 MHz	Broadband Radio Service (Closer to 160 MHz deployable.)
	FUTURE
600 MHz	Up to 120 MHz	Incentive auctions
1695-1710 and 1755 to 1780 MHz.	65 MHz	AWS-3. 1755 to 1780 MHz to be combined with  2155 to 2180 MHz. Spectrum sharing.
3.55 to 3.70 GHz	100 or 150 MHz	Small-cell band with spectrum sharing
Above 5 GHz	Multi GHz	Anticipated for 5G systems in 2020 or later timeframe

Bandwidth Management

To manage bandwidth, one can either attempt to reduce demand or increase capacity. With operators in some countries or markets shifting pricing for data usage from unlimited to tiered, users have become more conscious of how much data they consume. Application (app) developers have also provided tools for managing bandwidth by, for instance, allowing users to specify the maximum size of emails to automatically download. Nevertheless, average usage keeps growing.

Three factors determine wireless network capacity, as shown in Figure 1: the amount of spectrum, the spectral efficiency of the technology, and the size of the cell. Because smaller cells serve fewer people in each cell and because there are more of them, small cells are a major contributor to increased capacity.

Figure 1: Dimensions of Capacity

 Given the relentless growth in usage, mobile operators are combining  multiple approaches to increase capacity, as per the dimensions just discussed:

q  More spectrum. Spectrum correlates directly to capacity, and more spectrum is becoming available globally for mobile broadband. In the U.S. market, the FCC National Broadband Plan seeks to make an additional 500 MHz of spectrum available by 2020. Multiple papers by Rysavy Research and others10 argue the critical need for additional spectrum.

q  Unpaired spectrum. LTE TDD operates in unpaired spectrum. In addition, technologies such as HSPA+ and LTE permit the use of different amounts of spectrum between downlink and uplink. Additional unpaired downlink spectrum can be combined with paired spectrum to increase capacity and user throughputs.

q  Supplemental downlink. With downlink traffic five to ten times greater than uplink traffic, operators often need to expand downlink capacity rather than uplink capacity. Using carrier aggregation, operators can augment downlink capacity by combining separate radio channels.

q  Spectrum sharing. Policy makers are evaluating how spectrum might be shared between government and commercial entities. Although a potentially promising approach for the long term, sharing raises complex issues, as discussed further in the section “Spectrum Developments.”

q  Increased spectral efficiency. Newer technologies are spectrally more efficient, meaning greater aggregate throughput using the same amount of spectrum. Wireless technologies such as LTE, however, are reaching the theoretical limits of spectral efficiency, and future gains will be quite modest, allowing for a possible doubling of LTE efficiency over currently deployed versions. See the section “Spectral Efficiency” for a further discussion.

q  Smart antennas. Through higher-order MIMO and beamforming, smart antennas gain added sophistication in each 3GPP release and are the primary contributor to increased spectral efficiency (bps/Hz).

q  Uplink gains combined with downlink carrier aggregation. Operators can increase network capacity by applying new receive technologies at the base station (for example, large-scale antenna systems) that do not necessarily require standards support. Combined with carrier aggregation on the downlink, these receive technologies produce a high-capacity balanced network, suggesting that regulators should in some cases consider licensing just downlink spectrum.

q Small cells and heterogeneous networks. Selective addition of picocells to macrocells to address localized demand can significantly boost overall capacity, with a linear increase in capacity q  relative to the number of small cells. HetNets, which also can include femtocells, hold the promise of increasing capacity gains by a factor of four and even higher with the introduction of interference cancellation in devices. Distributed antenna systems (DAS), used principally for improved indoor coverage, can also function like small cells and increase capacity. Actual gain will depend on a number of factors, including number and placement of small cells11, user distribution, and any small-cell selection bias that might be applied.

q  Wi-Fi offload. Wi-Fi networks offer another means of offloading heavy traffic. Wi-Fi adds to capacity because it offloads onto unlicensed spectrum. Moreover, since Wi-Fi signals cover only small areas, Wi-Fi achieves both extremely high frequency reuse and high bandwidth per square meter across the coverage area.

q  Higher-level sectorization. For some base stations, despite the more complex configuration involved, six sectors can prove advantageous versus the more traditional three sectors, deployed either in a 6X1 horizontal plane or 3X2 vertical plane

Strategies to manage demand include:

q  Off-peak hours. Operators could offer user incentives or perhaps fewer restrictions on large data transfers during off-peak hours.

q  Quality of service (QoS). Through prioritization, certain traffic, such as non-time- critical downloads, could occur with lower priority, thus not affecting other active users.

Figure 2 demonstrates the gains from using additional spectrum and offload. The bottom (green) curve is downlink throughput for LTE deployed in 20 MHz, with 10 MHz on the downlink and 10 MHz on the uplink, relative to the number of simultaneous users accessing the network. The middle (purple) curve shows how using an additional 20 MHz doubles the throughput for each user, and the top (orange) curve shows a further possible doubling through aggressive data offloading onto Wi-Fi.

Figure 2: Benefits of Additional Spectrum and Offload

Given a goal of increasing capacity by a factor of 1,000, 50X could roughly be achieved through network densification; 10X through more spectrum, including higher frequencies such as mmWave; and 2X by increases in spectral efficiency.