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.

Wireless vs. Wireline

Wireless technology is playing a profound role in networking and communications, even though wireline technologies such as fiber have inherent capacity advantages. Relative to wireless networks, wireline networks have had greater capacity and historically have delivered faster throughput rates.

While wireless networks can provide a largely equivalent broadband experience for many applications, for ones that are extremely data intensive, wireline connections will remain a better choice for the foreseeable future. For example, users streaming Netflix movies in high definition consume about 5 Mbps. Typical LTE deployments use 10 MHz radio channels on the downlink and have a spectral efficiency of 1.4 bps/Hertz (Hz), providing LTE an average sector capacity of 14 Mbps. Thus, just three Netflix viewers could exceed sector capacity. In the United States, there are approximately 1,100 subscribers, on average, per cell site8, hence about 360 for each of the three sectors commonly deployed in a cell site. 

In dense urban deployments, the number of subscribers can be significantly higher. Therefore, just a small percentage of subscribers can overwhelm network capacity. For Blu-ray video quality that operates at around 16 Mbps or Netflix 4K streaming that runs at 15.6 Mbps, an LTE cell sector could support only one user.

Even if mobile users are not streaming full-length movies in high definition, video is finding its way into many applications, including education, social networking, video conferencing, business collaboration, field service, and telemedicine.

Over time, wireless networks will gain substantial additional capacity through  the methods discussed in the next section, but they will never catch up to wireline. One can understand this from a relatively simplistic physics analysis:

q  Wireline access to the premises or to nearby nodes uses fiber-optic cable.

q  Capacity is based on available bandwidth of electromagnetic radiation. The infra- red frequencies used in fiber-optic communications have far greater bandwidth than radio.

q  The result is that just one fiber-optic strand has greater bandwidth than the entire usable radio spectrum to 100 GHz, as illustrated in Figure 1.

Figure 1: RF Capacity vs. Fiber-Optic Cable Capacity

A dilemma of mobile broadband is that it can provide a broadband experience similar to wireline, but it cannot do so for all subscribers in a coverage area at the same time. Hence, operators must carefully manage capacity, demand, policies, pricing plans, and user expectations. Similarly, application developers must become more conscious of the inherent constraints of wireless networks.

Mobile broadband networks are best thought of as providing access to higher-capacity wireline networks. The key to improving per-subscriber performance and bandwidth is reducing the size of cells and minimizing the radio path to the wireline network, thus improving signal quality and decreasing the number of people active in each cell. These are the motivations for Wi-Fi offload and small-cell architectures.

Optimization phase | Network Planning

Once the planning and implementation are complete, it is very common practice to run drive tests for the planned sites. Drive tests verify the predictions made by the planning tools, and the results from the drive tests are compared against the results from the simulations. Fine tuning is performed after the drive tests to ensure that the deviation in the results between simulation and drive tests is minimal.

A part of the drive test, parameters like reference signal receive power (RSRP), reference signal received quality (RSRQ), or SINR the UL and DL throughputs at different points of the cell are noted. The results are then compared with the SNR predictions made by the planning tool and deviations are noted and tuned wherever required.

Coverage Planning

Coverage planning targets for the complete service area are tested to ensure there are no coverage holes (i.e., the UE never experiences a no-service condition within the entire service area). Coverage plans, however, do not take into consideration any quality of service that the user experiences within a cell or site. The end aim is to provide the count of the resources or eNodeBs and cells that are required for the complete service area.

Some of the most important aspects that need to be considered as a part of the coverage planning are:

1.       The eNodeB transmitting power and the type of cell that is being planned. The eNodeB transmitting power is the key for any coverage planning, and the transmitting power will vary based on the cell size. For example, a macro cell will have a transmission power of 10 watts per port (40 watts per cell in cases of MIMO cells). The DL coverage cell radius should be derived based on the transmission power of the antenna added with the gains (antenna gain, diversity gain, etc.) with the assumption of path loss (receiver loss, propagation loss, etc.). Cell radius calculation will be covered in detail in the link budget calculation section.

2.       The eNodeB receiver sensitivity. In the uplink, in order to calculate the cell radius, one of the most important parameters that the operator relies on is the receiver sensitivity of the eNodeB. The eNodeB receiver sensitivity is a deciding factor for the maximum allowed path loss between the UE and the eNodeB in the uplink direction, beyond which the eNodeB cannot differentiate accurately between signal and noise. Better receiver sensitivity of the eNodeB will directly result in a larger cell radius (coverage radius) in the uplink. The 3GPP 36.141 defines the test for deriving the reference sensitivity of a receiver.

The specification also requires that a receiver sensitivity of less than -100.8 decibel milliwatts (dbm) is acceptable. However, many vendors have a receiver sensitivity value of around -102 dbm or better.

3.       UE receiver sensitivity and transmission power. Similar to the eNodeB receiver sensitivity, UE receiver sensitivity is an important factor in determining the DL cell radius for coverage planning. Typically for a macro cell, the UL cell radius will be a limiting factor in comparison with the DL cell radius simply because of the difference in the transmission powers. In LTE category 2 UE and onward, the maximum uplink transmit power is 23 db.

4.       Terrain. Terrain is an important consideration for any site planning and will impact the absorption or attenuation capability of a site. For example, a site with irregular heights will not have linear loss and is subjected to shadow areas or reflection, whereas a site with fairly regular height will have a more predictable linear loss. Similarly, the indoor to outdoor ratio of a site also makes a difference when it comes to cell radius calculation (i.e., the penetration losses for an indoor user is higher compared with that for an outdoor user); therefore, planning an urban cell will be subject to more losses due to a higher percentage of indoor to outdoor users in comparison with a rural cell.