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.
