12-07-2013, 02:00 PM
An Introduction to Millimeter-Wave Mobile Broadband Systems
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ABSTRACT
Almost all mobile communication systems
today use spectrum in the range of 300 MHz–3
GHz. In this article, we reason why the wireless
community should start looking at the 3–300
GHz spectrum for mobile broadband applications.
We discuss propagation and device technology
challenges associated with this band as
well as its unique advantages for mobile communication.
We introduce a millimeter-wave mobile
broadband (MMB) system as a candidate nextgeneration
mobile communication system. We
demonstrate the feasibility for MMB to achieve
gigabit-per-second data rates at a distance up to
1 km in an urban mobile environment. A few
key concepts in MMB network architecture such
as the MMB base station grid, MMB inter- BS
backhaul link, and a hybrid MMB + 4G system
are described. We also discuss beamforming
techniques and the frame structure of the MMB
air interface.
INTRODUCTION
Mobile communication has been one of the most
successful technology innovations in modern history.
The combination of technology breakthroughs
and attractive value proposition has
made mobile communication an indispensable
part of life for 5 billion people. Due to the
increasing popularity of smart phones and other
mobile data devices such as netbooks and ebook
readers, mobile data traffic is experiencing
unprecedented growth. Some predictions indicate
that mobile data will grow at 108 percent
compound annual growth rate (CAGR) [1] with
over a thousandfold increase over the next 10
years. In order to meet this exponential growth,
improvements in air interface capacity and allocation
of new spectrum are of paramount importance.
MILLIMETER WAVE SPECTRUM
UNLEASHING THE 3–300 GHZ SPECTRUM
Almost all commercial radio communications
including AM/FM radio, high-definition TV, cellular,
satellite communication, GPS, and Wi-Fi have
been contained in a narrow band of the RF spectrum
in 300 MHz–3 GHz. This band is generally
referred to as the sweet spot due to its favorable
propagation characteristics for commercial wireless
applications. The portion of the RF spectrum
above 3 GHz, however, has been largely unexploited
for commercial wireless applications. More
recently there has been some interest in exploring
this spectrum for short-range and fixed wireless
communications. For example, unlicensed use of
ultra-wideband (UWB) in the range of 3.1–10.6
GHz frequencies has been proposed to enable high
data rate connectivity in personal area networks.
The use of the 57–64 GHz oxygen absorption band
is also being promoted to provide multigigabit data
rates for short-range connectivity and wireless local
area networks. Additionally, local multipoint distribution
service (LMDS) operating on frequencies
from 28 to 30 GHz was conceived as a broadband,
fixed wireless, point-to-multipoint technology for
utilization in the last mile.
MILLIMETER-WAVE PROPAGATION
FREE-SPACE PROPAGATION
Transmission loss of millimeter wave is accounted
for principally by free space loss. A general
misconception among wireless engineers is that
free-space propagation loss depends on frequency,
so higher frequencies propagate less well than
lower frequencies. The reason for this misconception
is the underlying assumption often used
in radio engineering textbooks that the path loss
is calculated at a specific frequency between two
isotropic antennas or λ/2 dipoles, whose effective
aperture area increases with the wavelength
(decreases with carrier frequency). An antenna
with a larger aperture has larger gain than a
smaller one as it captures more energy from a
passing radio wave. However, with shorter wavelengths
more antennas can be packed into the
same area. For the same antenna aperture areas,
shorter wavelengths (higher frequencies) should
not have any inherent disadvantage compared to
longer wavelengths (lower frequencies) in terms
of free space loss [5]. In addition, large numbers
of antennas enable transmitter and receiver
beamforming with high gains. For example, a
beam at 80 GHz will have about 30 dB more gain
(narrower beam) than a beam at 2.4 GHz if the
antenna areas are kept constant.
DOPPLER AND MULTIPATH
The Doppler of a wireless channel depends on
the carrier frequency and mobility. Assuming a
rich scattering environment and omnidirectional
antennas, the maximum Doppler shift for carrier
frequency of 3–60 GHz with mobility of 3–350
km/h ranges from 10 Hz to 20 kHz. The Doppler
shift values of incoming waves on different
angles at the receiver are different, resulting in a
phenomenon called Doppler spread. In the case
of MMB, the narrow beams at the transmitter
and receiver will significantly reduce angular
spread of the incoming waves, which in turn
reduces the Doppler spread. In addition, as the
incoming waves are concentrated in a certain
direction, there will be a non-zero bias in the
Doppler spectrum, which will be largely compensated
by the automatic frequency control (AFC)
loop in the receiver. Therefore, the time-domain
variation of an MMB channel is likely to be
much less than that observed by omnidirectional
antennas in a rich scattering environment.
CONCLUSION
Millimeter-wave spectrum with frequencies in the
range of 3–300 GHz can potentially provide the
bandwidth required for mobile broadband applications
for the next few decades and beyond.