1. technology and computing

Advanced technologies for 4G:
Mobile broadband multimedia
everywhere
by Vish Nandlall, Ed Sich, Wen Tong, and Peiying Zhu
Nortel’s Wireless Technology Lab (WTL) is working on numerous innovations that will fundamentally rewrite the economic equation for
wireless access infrastructures and pave the way for ubiquitous deployment of networks that support true broadband. From the industry’s
most compact 4G base station, to new antenna designs, sophisticated
scheduling algorithms, advanced beamforming, and mobile multi-hop
relay, the WTL continues to pioneer breakthroughs that will dramatically lower the cost per bit of over-the-air transmission, while improving
spectral efficiency and boosting data rates, capacity, coverage, reach,
and throughput. For operators of all kinds – whether traditional 3G and
2G operators, or new entrants – these technologies form the underlying pillars for evolving to and rolling out high-performance, multimedia
4G wireless networks and, ultimately, for supporting an affordable true
broadband experience anywhere.
ven though the fourth-generation (4G) wireless story has not
yet been written in stone – 4G is
still being defined – the industry is clearly moving aggressively to a 4G world.
Both the pace of 4G adoption and
the rate of standards development are by
4
Nortel Technical Journal, Issue 6
far faster than all previous generations of
wireless. Whereas it took nearly 10 years
for 3G to roll out and become standardized, 4G activities are proceeding on an
accelerated three-year cycle.
For instance, the first instantiation of
4G – the rollout of Worldwide
Interoperability for Microwave Access
(WiMAX) systems – is already well
under way in the networks of many
wireless operators, and several new
entrants are planning nationwide and
even continent-wide WiMAX networks.
At the same time, standardization and
technology initiatives for other 4G technologies are proceeding rapidly. The 4G
evolution of the UMTS standard, called
the Long Term Evolution (LTE) project,
is now being standardized within the 3rd
Generation Partnership Project (3GPP).
And the 3GPP2 has standardized Ultra
Mobile Broadband (UMB) to improve
CDMA for 4G applications and
requirements.
As this pace continues, many in
the industry consider the year 2010 to
be the inflection point for 4G mobile
broadband – the point at which 4G will
replace 2G and 3G infrastructures. Some
analyst reports put the size of the 4G
market at about $60 billion by 2015.
Changing the wireless experience
The drive to 4G is being fueled by the
promise of target peak data rates of
approximately 1 Gigabit per second
(Gbit/s) for low mobility (such as
nomadic/local wireless access) and approximately 100 Megabits per second
(Mbit/s) for high mobility (such as
vehicular mobility). These data rates are
part of the International Telecommunication Union (ITU) long-term vision
(2010-2015) for 4G, as outlined in the
ITU’s IMT-Advanced requirements.
(IMT stands for International Mobile
Telecommunications.) At these speeds,
downloading a 600-MByte full-length
movie would take about 45 seconds,
compared to more than 20 minutes in a
3G network.
Fourth-generation wireless is not
solely about data rates, and it is certainly
not just “faster 3G.” Today’s cellular 2G
and 3G networks offer excellent quality
for mobile voice and narrowband data in
the form of text messaging and emails,
and even rudimentary web browsing and
video transmission. By contrast, 4G networks will change the game entirely.
This next wireless generation – the
first to be based on an all-IP network
and support all types of traffic on a
single converged infrastructure – will
substantially alter the definition and
experience of mobile communications.
The 4G user experience will be characterized by seamless, high-bandwidth
network connectivity, with access to any
application regardless of device and location. High-quality and reliable delivery
of unified communications sessions, virtual-reality experiences, real-time video
streaming, high-speed data, multimedia
messaging, and high-definition mobile
television will all become commonplace
and affordable for wireless subscribers.
The transition to 4G will also be critical for supporting what is fast becoming a hyperconnected communications
environment. More and more devices
and machines, many of them mobile,
are becoming IP-enabled and connected to the network, far exceeding the
number of humans using the network.
As this trend continues, we will see an
exponential growth in the number and
types of users, applications, and services,
and a greater drive toward much richer
experiences and more extensive mobile
lifestyles.
The plethora of new, rich mobile applications and services that can be made
possible with 4G is, not surprisingly,
attracting many players – both existing
and new. Traditional wireless carriers
are being joined by new entrants that
are acquiring non-traditional spectrum
and moving into the broadband wireless
space, including operators such as cable
and satellite providers that are extending their infrastructures into the wireless
world. A recent example is Rogers Communications and Bell Canada, which
are pooling their broadband resources
to build a new national wireless network, called Inukshuk, in Canada. This
network aims to deliver access to voice,
video streaming, and Internet data via
mobile devices to customers in some 40
urban centers, as well as in 50 rural and
remote communities.
At the same time, non-traditional
Internet and IT companies like Google,
Yahoo, Microsoft, and Apple are coming to the wireless arena with entirely
new business models. 2G operators also
are keenly interested in accelerating the
transition to 4G and evolving to true
broadband using technology that allows
them to leapfrog the incremental step to
3G without having to rip out their 2G
equipment.
4G technology challenges
All of these existing players and new
entrants have their eyes on opportunities brought on by the transition to
4G, with its unprecedented jump in
bandwidth, capacity, and multimedia
capability. (The table on page 19 shows a
comparison of wireless and wireline performance metrics.) As 4G technologies
and standards mature, the industry will
take a giant step closer to realizing the
long-held vision of a single, converged,
packet-based “fat pipe” that will carry all
wireless multimedia services with high
quality and reliability, will scale easily to
accommodate subscriber growth, and
will provide users with whatever bandwidth they need, wherever they need it,
simply and cost-effectively. Most importantly, 4G promises a “true” broadband
experience, where mobile services will
be delivered with enough capacity and
transparency that users will be unaware
of the underlying network.
This next era in the wireless industry
is an exciting one from a technology
perspective. The capabilities of 4G will
not be achieved through a series of incremental improvements in capacity, spectral efficiency, and throughput. Rather,
the disruptive shift to a 4G architecture
affords the opportunity to rethink and
revamp nearly every function in the
wireless access network, from the radio
and antennas to the point of backhaul
and every stop in between. Put simply,
this next wireless network will be
designed differently.
Interestingly, there are as many different schools of thought on 4G as there
are providers. For some, 4G means highcapacity metro hot spots. For others, 4G
is about enabling cheaper voice service,
or achieving a better experience when
viewing or uploading YouTube clips.
Still others are looking to 4G for mobile
video, broadband for the extended enterprise or home, or as a digital subscriber
line (DSL) replacement. Nortel believes
that if technology is built to solve only
one of these scenarios, 4G will not be
successful in the long term. Rather, 4G
is about enabling all of these scenarios,
by developing a common technology
base to support all future 4G configurations, standards, and deployment
strategies.
Nortel’s commitment and approach
to developing fundamental enabling
technologies was demonstrated with
its pioneering efforts with OFDM and
MIMO (orthogonal frequency division multiplexing, and multiple-input
multiple-output antenna processing
Nortel Technical Journal, Issue 6 5
technology), which have become the
foundation for all 4G technologies.
Beginning some eight years ago, Nortel
researchers achieved many industry
firsts in OFDM-MIMO. (For more on
Nortel’s OFDM-MIMO leadership, see
page 26 in Issue 2 of the Nortel Technical Journal.) As well, almost a year ago,
Nortel became the first in the industry
to complete live calls using MIMO
advanced antenna technology to deliver
high-bandwidth multimedia over all of
the major 4G technologies – WiMAX,
LTE, and UMB.
Now, researchers in Nortel’s Wireless
Technology Lab (WTL) are building a
set of fundamental technologies that will
establish a flexible base upon which not
only to achieve the promise of mobile
multimedia and high bandwidth, speed,
and capacity across all 4G technologies,
but also to significantly alter the economic paradigm for mobility solutions.
In these efforts, the WTL team is ensuring that all innovations meet a common
set of requirements. Specifically, the
team is developing disruptive technologies that deliver a number of important
benefits.
Provide dramatically lower OpEx
and CapEx: A top priority for operators
is to provide subscribers with mobile
broadband multimedia services that are
both affordable and ubiquitous. Operators are faced with rising consumer pressure for mobile services delivered at the
same prices that they pay for similar services in the wireline domain. Consumers
are increasingly pushing the market, expecting to be able to do much more with
their mobile devices than ever before,
but with no increase in service price. For
instance, consumers have come to expect
a growing richness of functionality from
the network and their devices, so that
they can upload photos and movies,
surf the Internet, engage in social networking by interacting with friends and
colleagues via instant messaging, and so
on – and they expect the price of these
services to fall over time. Increasingly,
they also expect to be able to access these
services everywhere they roam.
6
Nortel Technical Journal, Issue 6
Jim MacFie,
advisor,
standards
management
and tools, inresearcher,
Nortel’s
Harpreet
Panesar,
radio
frequency
hardware technology
Strategic Standards
group, power
addresses
a Global
examines
a high-efficiency
amplifier
(PA)Standards
prototypeCollaboration
developed for
(GSC) meeting
in France.
future
base station
PA systems.
This requirement leads to an interesting technology challenge: how to
provide a significant leap in capability
and performance, while at the same
time driving down cost and complexity
to keep network capital and operating
costs equal to, or preferably lower than,
those of today’s 3G networks. Certainly,
OFDM-MIMO brings a significant improvement in spectral efficiency. Nortel
is also leveraging new silicon technologies (taking advantage of the ongoing
progression of Moore’s Law) and advances in materials technologies, which
will be important from the standpoint of
lowering overall cost of ownership.
Support a wide variety of spectrum
bands: Spectrum is a key challenge for
4G systems, because of both the differing bandwidth requirements and
the spectrum bands available globally.
Whereas previous wireless generations
– 1G, 2G, and 3G – all globally operate
roughly in two major bands [the 800
to 900 Megahertz (MHz) band and the
1.7 Gigahertz (GHz) to 2.1 GHz range
allocated for Personal Communications
Services (PCS)], new spectrum is spread
across several additional bands – specifically, in parts of the 400 to 700 MHz
range and in the 2.3, 2.5, 3.5, and 4.4
GHz bands. Unlike previous generations
of wireless, 4G systems therefore will not
lend themselves to a single product in a
globally coordinated frequency band. As
a result, 4G products must be deployable in different markets and adjustable
to a wide range of different frequency
bands. Moreover, these new frequencies
have completely different over-the-air
transmission characteristics compared to
traditional bands, raising new challenges
in ensuring a high quality of service.
Develop cost-effective high-performance cell-site solutions: 4G will
demand extremely high levels of performance and capability from the cell-site
equipment. Multi-branch antennas, for
instance, will be required at the cell site
to provide the greater coverage and
range needed to support the growth in
both subscribers and capacity. Adding
of their starting points – whether 2G,
2.5G, or 3G – operators will be seeking
a migration path to 4G that is costeffective and does not require a “rip
and replace” of existing equipment. 4G
solutions, therefore, need to co-exist
with today’s infrastructures to enable
operators to preserve their spectrum and
extend their existing investments.
Enabling 4G technologies
Art Fuller, digital signal processing hardware technology researcher,
evaluates digital-to-analog converter technology for use in future wireless
systems for 4G and beyond.
antennas, however, raises significant issues with cabling from the base station
to support those antennas. In conventional systems, such additions would
place an impractical burden on the cell
tower itself. There is an opportunity,
therefore, to devise new cell-site architectures and technologies that integrate
the base station with the antenna for
placement at the tower top, thereby
reducing and even eliminating the need
for extra cabling.
Enable higher capacity for hot-spot
deployment: In a 4G-enabled wireless
environment, the amount of highbandwidth, high-speed data traffic is
expected to soar, especially for the high
concentrations of users in dense urban
environments and in-building office
scenarios. This will require the development of technologies that provide higher
throughput to users in these areas and
ensure high quality of service at lower
costs.
Support VoIP applications: Voice,
which has been the raison d’être of cel-
lular and wireless communications since
the beginning of the industry, is also
undergoing a transformation. With 4G,
voice transmission will, for the first time,
be carried over the same infrastructure as
all other traffic. No longer will separate
parallel circuit-switched and packetswitched networks be required for voice
and data. Instead, all traffic will be carried on a single all-IP wireless network.
Even more, 4G air interfaces, such as
LTE or UMB, can deliver VoIP capacity
that is three times greater than that of
3G interfaces. To reach these capacity
levels, however, and to meet and even
exceed the quality and reliability that
users have come to expect in the 2G and
3G worlds, several difficult technical
challenges need to be met to accommodate the different traffic behaviors and to
deliver real-time voice and video. Here,
innovations are required in the area of
digital signal processing and scheduling
algorithms.
Provide a cost-effective 4G migration path from 2G and 3G: Regardless
These requirements form the backdrop
for Nortel’s 4G research efforts, where
teams are focusing on developing the
core technologies that will ultimately allow operators to come at 4G from many
different angles while delivering a “true”
broadband experience. In particular,
Nortel is developing:
• new base station and terminal radio
technologies;
• advanced antenna designs, integration
strategies, and configurations;
• sophisticated digital signal processing
techniques; and
• solutions for mobile multi-hop relay.
Base station technologies
The focus on new base station technologies is a major research effort, primarily
because the base station represents the
majority (upwards of 70 percent) of the
costs – both CapEx and OpEx – of the
entire wireless infrastructure.
Industry’s most compact 4G
platform: In the area of base stations,
researchers are focusing on several innovations – including a simpler, more
cost-effective base station architecture,
use of advanced silicon technology, a
new network timing and synchronization solution, and more efficient power
amplifiers. These innovations have
culminated in the first 4G technology
platform, and the industry’s most compact and efficient base station solution
for WiMAX.
Nortel’s new Base Transceiver Station
(BTS) 5000 represents a major departure
from traditional base station design,
offering a dramatically smaller footprint
that gives operators more flexible
continued on page 10...
Nortel Technical Journal, Issue 6 7
Technology innovations behind Nortel’s WiMAX base station:
The industry’s most compact and efficient 4G platform
by Steve Beaudin, David Choi, Brian Lehman, and Brad Morris
“Where’s the rest of it?” is becoming an
increasingly common reaction to Nortel’s
new WiMAX Base Transceiver Station
(BTS). Compared to the “fridge-sized”
base stations that have been the norm
for some time, the BTS 5000 is the
most compact the industry has seen yet
– supporting three sectors per cell site
with two transmit and two receive paths
per sector, packaged in just 6 rack units
(Us) of a standard 19-inch rack. [One U
is 1.75 inches (44.45 mm) high.]
The WiMAX BTS 5000 is the industry’s first 4G platform – a three-sector,
full-power, fully multiple-input multipleoutput (MIMO)-compatible unit that
offers operators a dramatically smaller
footprint, significantly lower CapEx and
OpEx, and a radically higher fivefold
increase in data capacity.
The BTS design represents a new
architectural approach, one that takes
advantage of not only technology and
materials-packaging advances, but also
the generational jump from 3G to 4G
technology – a once-in-a-decade
opportunity to rethink the way products
are designed and technology is used.
Specifically, the evolution from code
division multiple access (CDMA) to
orthogonal frequency division multiple
access (OFDMA) has enabled a fundamental shift in the architecture of base
stations by changing the capacity growth
strategy. In the CDMA world, because
the signal characteristics over the air are
fixed, capacity growth is achieved by
adding radios (increasing bandwidth by
increasing the number of carriers) and
adding modems (to boost data rates).
By contrast, with OFDMA, capacity growth is achieved by changing the
modulation parameters to increase the
bandwidth of a single carrier – a strategy
that requires a single radio with flexible
bandwidth.
8
Nortel Technical Journal, Issue 6
Embracing this architectural challenge,
Nortel researchers focused on innovations
in the following key areas:
• architecture and packaging;
• MIMO algorithms and antennas;
• channelization;
• network timing and synchronization; and
• power amplifier memory-correction.
Architecture and packaging
Analytical design resulted in an evolution
from a base station system with five or
more modules to a simpler 4G WiMAX
base station with just two key modules:
a MIMO radio module and an integrated
digital module.
As shown in Diagram A, the current
CDMA macro-cell BTS requires five basic
elements: radio, timing, core, control, and
modem modules. This architecture follows
a hierarchical approach of multiple radios
and modems, and expansion is achieved
by plugging additional radios and modems
into the core. The core provides a switching function, directing data transmissions
between the radios and modems. The core
then connects to a control module, which in
turn connects to the network.
By contrast, as shown in Diagram B,
Nortel’s WiMAX BTS embraces a different
approach, providing a direct one-to-one
mapping of radio to modem. Through several key innovations, the team integrated
the core, timing, and control modules into
a single integrated digital module. This
integration dramatically shrinks the size of
Diagram A. Basic architecture of a CDMA BTS
Modem
Timing
module
Radio
Modem
Core
Radio
.
.
.
Modem
.
.
.
Control
module
Radio
Network
Diagram B. Basic architecture of a WiMAX BTS
Network
Integrated
digital
module
MIMO
radio
a base station, and facilitates a simple fiber
interconnect, which eliminates the need for
backplane infrastructure.
MIMO algorithm and antenna
MIMO is a spatial diversity technique that
increases coverage or data capacity by
either transmitting the same data on different antennas (Matrix A) or different data
on different antennas (Matrix B). Nortel
implements a two-antenna MIMO BTS
with two transmitters and two receivers per
sector on the radio frequency (RF) module.
All digital processing functions to generate
MIMO Matrix A/B and OFDM waveforms
are implemented in the single-board modem, which can support up to 30 Megabits
per second (Mbit/s).
The knowledge and patented technology developed by the WTL for its MIMO testbed were transferred directly to the WiMAX
product, including several thousand lines of
prototype code that captured Nortel’s key
innovations in OFDM uplink-receive MIMO
maximum-likelihood detection decoding
and channel estimation. The code was supported with detailed link-simulation suites,
and the transfer was accelerated through
the selection of compatible processor components in both the testbed and product.
MIMO relies on sophisticated antennas,
and Nortel’s world-class competency in
antenna design and propagation modeling
was applied to early MIMO proof-of-concept trials, channel models, system linkbudget simulations, and antenna selection.
Channelization technology
The evolution to OFDM places new demands on the radio channelization function.
One key challenge is the need to costeffectively support, in a single configurable
device, the wide array of band classes that
are specified in the WiMAX (IEEE 802.16e)
profiles.
Channelization enables the radio to find
and extract a specific channel from the
radio spectrum, while filtering out adjacent
channels or interferers, and includes supporting functions for power measurement,
quadrature error correction, diversity, and
transport formatting for data exchange
with the modem. In older systems, analog
filters were able to extract one channel.
As technology evolved, digital filters were
introduced and eventually were able to
extract several channels.
WiMAX, however, calls for a dramatic
increase in flexibility. Building on the patented common-rate channelization technology developed to support the two dominant
implementations of CDMA (IS-2000 and
UMTS), designers utilized advanced
multi-rate signal processing concepts,
and implemented a unified, efficient field
programmable gate array (FPGA) device
that enables a single WiMAX radio to support 12 defined bandwidths. In this way, a
single radio deployment can be configured
to simultaneously meet the initial and future
growth requirements of customers, including those in emerging global markets with
disparate spectrum allocations.
Network timing and synchronization
OFDM technology can increase the on-air
data rate by delivering improved resistance
to many of the impairments in the wireless
channel, including multi-path fading. This
benefit, however, requires more stringent
timing and frequency accuracies, compared to earlier technologies.
Synchronization is obtained with a
Global Positioning System (GPS) satellite constellation, and precision timing is
typically delivered with a crystal oscillator,
which must continue to operate in “holdover mode” if satellite visibility is temporarily lost. Because crystals age and are affected by changes in temperature (causing
frequency variations, or drifting), they are
normally subjected to a costly manufactur-
ing process that prematurely ages them.
This process improves the crystal’s initial
insensitivity to temperature changes and
thus produces extremely low frequency
variations. In operation, one or more tiny
heated enclosures (called ovens) inside
the crystal package are used to moderate
environmental temperature variations.
Researchers decided to try solving the
problems of crystal drift, inaccuracy, aging,
and residual temperature variation with an
intelligent algorithm. The resulting control
system, which earned researchers several
patents, successfully disciplined the crystal
to produce highly accurate timing and
synchronization performance. In the end,
the control system was powerful enough to
enable the selection of a much-lower-cost
crystal and a smaller, simpler thermal package with only a single oven.
This technology delivered a dramatic
reduction in the size of the network timing
and synchronization subsystem, collapsing a separate timing module the size of
a bread box into a subsystem the size of
a deck of cards. This subsystem, built onboard the WiMAX integrated digital module,
delivers a dramatic cost reduction of some
80 percent compared to a previousgeneration timing module.
Power amplifier memory-correction
Even with today’s significant advances in
pre-distortion techniques and increased
transistor efficiencies, RF power amplifiers (PAs) still consume significant cost,
volume, and energy, and require considerable manufacturing expertise. The move
to multiple transmitters in a single radio to
support MIMO has heightened the need to
reduce transmitter size.
The most expedient approach to
reducing transmitter size is to increase
power efficiency, which reduces not only
the circuit size but also the attracted
power supply and cooling requirements.
Nortel Technical Journal, Issue 6 9
Nortel’s WiMAX base station
continued
Building smaller, more compact, more
efficient power amplifiers, however, tends
to force design trade-offs – specifically with
respect to memory effects, which is an undesirable but inherent phenomenon of PAs.
Briefly, the behavior of a PA is based on
the signal history it has received. How long
the PA remembers this history can pose a
problem in the form of signal distortion or
interference. Certainly, one ideal in designing PAs is to achieve close to zero memory,
but reaching this goal requires the use of
various complex techniques and expensive
hardware.
To counteract this memory phenomenon with signal processing techniques,
designers integrated into the WiMAX radio
module an innovative memory-correction
technology, initially developed and patented by Nortel for an envelope-tracking
power amplifier. This memory-correction
algorithm enabled the power transistors to
be “pushed harder,” or driven further into
saturation, increasing efficiency while still
achieving the required linearity. Designers combined the new memory-correction
technology with a proven technology called
baseband pre-distortion, developed by
Nortel’s wireless business unit.
Together, these two technologies minimize the impact of memory, allowing Nortel
to push the transmitters to the compression point, while maintaining excellent
emissions performance. Critically, Nortel’s
WiMAX radios meet U.S. FCC emissions
specifications with no guard band. For
example, a 10-MHz carrier can be inserted
into a 10-MHz frequency block and meet
the emissions specifications at the corners
of the frequency block. This key performance capability maximizes spectrum
usage and eases deployment.
The power amplifier technology, along
with all of the BTS innovations, was
designed-in upfront to achieve best-in-class
manufacturability, compliance testing, and
10
Nortel Technical Journal, Issue 6
time to market. For example, researchers worked closely with the manufacturing-testset and test-development
teams to transfer the designer test
environments and tools from the early
system integration efforts, in order to
seed the factory testset, speed the
transition from design to product, and
shorten time to market.
Flexible 4G technology enablers
Far from incremental improvements,
these innovations constitute a significant technology disruption in the wireless arena. While these innovations
are targeted initially for WiMAX deployments, they are also applicable to all
emerging 4G technologies, whether
Long Term Evolution (LTE), Ultra
Mobile Broadband (UMB), or beyond.
The fact that all 4G technologies are
based on OFDMA enables both these
and tomorrow’s innovations to be highly flexible technology enablers – ones
that will help ensure that 4G systems
deliver the high capacity and the OpEx
and CapEx reductions that operators
need in order to offer broadband multimedia wireless everywhere.
Steve Beaudin is Manager of WiMAX
RF System Development, in Nortel’s
Carrier Networks organization.
David Choi is Leader of WiMAX
Systems Development, in Nortel’s
Carrier Networks organization.
Brian Lehman is Manager of Wireless
DSP Algorithms and Technology, in the
CTO Office.
Brad Morris is Advisor, Signal
Processing Hardware Technology, in
the CTO Office.
deployment options, along with a significant reduction in cell-site operations
costs. As well, the BTS 5000 can already
operate in 12 different frequency bands
(more bands by far than any other base
station in the industry) giving it unparalleled flexibility for deployment in different markets and for various carriers. The
BTS 5000, in fact, is well-positioned to
set a new benchmark for performance
and size and to become the standard
against which all future base station
technologies will be measured. These
innovations are detailed on page 8.
Miniature band reject filters: Nortel
is developing miniature band reject
filter (MBRF) technology, which is a
key enabler in mobile terminals for allowing operators and new entrants with
non-traditional spectrum, such as that
used for satellite transmission, to deploy
WiMAX.
Where satellite signals and terrestrial
signals are adjacent spectrally, there is a
high likelihood that the more powerful
near signal (i.e., terrestrial) can easily
interfere with the far signal (satellite), by
“leaking” unwanted transmit signals into
the adjacent band. Preventing this effect
requires filters that can separate the
adjacent bands effectively.
Researchers are meeting this need by
designing a very small device, about the
size of a fingernail, that can provide the
steep transition band required to give
WiMAX terminals the filtering capability they need for use in the satellite spectrum. The MBRF will allow WiMAX
terminal vendors to build terminals that
meet the stringent out-of-band requirements in certain frequency allocations,
such as in the 1.5-GHz band that is adjacent to the Global Positioning System
(GPS) satellite band, and the 2.3-GHz
band that is adjacent to the Satellite
Digital Audio Radio Service.
Innovations in antenna design
4G systems are fundamentally based
on a multiple antenna technology
called multiple-input multiple-output
(MIMO). MIMO allows the creation of
multiple parallel data streams between
High-performance Hex-MIMO antenna technology
for cost-effective 4G
by David Adams and Andy Jeffries
In recent years, there have been many
advances in the development of advanced spatial division multiple access
(SDMA) and multiple-input multipleoutput (MIMO) algorithms for broadband wireless access. While these
techniques offer the potential for substantial improvements in capacity and
coverage, they also require increased
signal processing capability in the base
transceiver station (BTS) and, equally
important, add complexity to the radio
frequency (RF) and antenna systems
at the cell site – issues that have the
potential to significantly impact deployment, operating, and hardware costs.
Researchers in Nortel’s Wireless
Technology Lab (WTL) are investigating a wide range of antenna configurations based on various combinations of
SDMA and MIMO. One configuration
that is receiving considerable attention
for its ability to overcome these practical overheads, while still delivering
valuable capacity and coverage gains
over conventional 2G and 3G systems,
is an advanced antenna concept called
Hex-MIMO.
As shown in the diagram, a HexMIMO configuration essentially partitions each sector of a conventional trisector antenna area into two, achieving
six-sector (or hex-sector) coverage
around the cell site. Hex-MIMO
employs two carefully shaped fixed
dual-polar beams within a sector, each
of which carries two-branch MIMO.
Depending on the scenario, Hex-MIMO
can offer both downlink and uplink
capacities in excess of 2.5 times those
of a conventional deployment.
Nortel’s Hex-MIMO solution is based
on several innovations, specifically:
• a unique combination of SDMA and
MIMO that leverages the performance
gains of both techniques, while deliver-
ing a solution that is sufficiently low-cost,
easy to implement, and cost-effective to
operate for widespread deployment;
• the integration of the antenna and
beamformer, which reduces the number
of RF feeder cables and needs only three
antennas (instead of the usual six) per
cell site; and
• the use of new low-cost lightweight
materials and technologies to make the
antenna simple, compact, and costeffective to manufacture and deploy.
Unique SDMA-MIMO combination
Hex-MIMO is based on a carefully
designed combination of both SDMA
and MIMO techniques.
• SDMA partitions each sector using
narrow beams achieved through spatial
beamforming. In this way, spectrum resources can be reused for each sector.
A two-part division would therefore, in
principle, double the capacity.
• Adding MIMO transmission to each
sector further increases capacity and
coverage. MIMO techniques are now being introduced into the marketplace as
part of Nortel’s WiMAX product portfolio.
Initial implementations will be in the form
of two-branch MIMO, which employs antennas with two orthogonal polarizations
(± 45 degrees) in a single-column sector
antenna.
In using SDMA, researchers chose to
disregard the common industry perception that SDMA should be used only for
higher-order antenna designs (creating
three, four, or even more divisions in
each sector) where high concentrations
of users, such as in dense urban areas
or military uses, warrant the cost of the
added complexity and larger antennas.
These more advanced forms of SDMA
include options to steer adaptive beams
toward multiple users and/or null out
interference. Instead, the team worked
to determine the optimum trade-offs – in
antenna size and system complexity relative to performance – that would be
key to the effective use of SDMA tech-
Hex-MIMO antenna configuration
Two-beam
dual-polar
MIMO
per sector
Nortel Technical Journal, Issue 6 11
Hex-MIMO
continued
niques for sub-1-GHz spectrum, for
example.
In doing so, they overcame several
difficult challenges, including the design
of the antenna array. Array antennas are
required to support SDMA and these antennas typically require half-wavelengthspaced columns to provide good control
of radiation patterns. The larger the
number of antenna columns within the
array, the greater the performance improvements, but at the cost of increased
antenna width. So, while eight columns,
for example, may be an ideal SDMA antenna from a performance perspective, it
Shown above is a prototype of Nortel’s
low-cost lightweight (LCLW) modular
antenna design for use in a Hex-MIMO
solution. Each pair of dual-polar elements (the smaller unit at bottom) is an
antenna building block that provides flexibility in antenna configuration. A single
column of elements would form the core
of a dual-polarized sector antenna suitable for two-way MIMO, while an array of
directly adjacent columns forms an array
antenna suitable for SDMA-MIMO.
This design uses a combination of
flat-plate antenna technology, slot antenna designs, and plated plastics to reduce
weight and costs. Such antenna designs
are important enablers for the widespread deployment of cost-effective 4G
antenna solutions, such as Hex-MIMO.
12
Nortel Technical Journal, Issue 6
is less than ideal from the point of view of
deployment, since the antenna width, and
therefore area, often has a direct bearing
on the site leasing costs, as well as the
wind loading stresses the antenna will
experience.
These issues become increasingly difficult to solve as the RF carrier frequency
drops and the antenna size increases.
To address these issues, the team combined SDMA and MIMO techniques and
shaped the beams in a way that reaches
a compromise between these various
considerations. The resulting solution,
which limits the width of the resulting
aperture to only 50 percent of the width of
conventional full-sector antennas, is being actively pursued by the research team
for a variety of potential 4G (WiMAX and
LTE) applications.
Antenna-beamformer integration
The use of fixed beams allows a low-loss
passive RF beamformer to be integrated
into the antenna and avoids the need for
any phase-calibrated cabling. For configurations where the active elements of the
radios are located remotely from the antenna, this beamforming approach offers
the advantage of reducing the number
of RF feeder cables required at the cell
site to a single pair per beam rather
than a pair per array column. For active
mastheads, the cables can be short or,
alternatively, can be avoided altogether
and replaced by optical fiber through integration of the electronics and antenna.
The antenna-beamformer combination
can also be configured to provide coverage equivalent to that of a conventional
tri-sectored deployment, able to support
both existing 2G/3G and SDMA-MIMO 4G
systems from a single antenna aperture.
This coverage gives 2G/3G operators the
opportunity to consolidate their masthead
hardware and future-proof their services
by deploying the Hex-MIMO antenna
now, ready for 4G SDMA-MIMO when
required.
Low-cost lightweight antenna
technologies
As SDMA systems become increasingly
common, the need for antenna technologies tuned to the requirements of
these systems will become ever greater.
Because antennas to support SDMA
are inevitably larger and more complex
than conventional designs, controlling
and ideally reducing their weight and
cost becomes increasingly important.
Larger SDMA antenna configurations and
higher operating frequencies also require
increasing levels of masthead integration, where the conventional boundaries
between antenna and active RF systems
can be questioned and new concepts
explored.
For instance, the research team has
been exploring a number of new low-cost
lightweight (LCLW) antenna technologies
that can be used for both passive standalone antennas and integrated active
ones.
Using a combination of Nortel’s flatplate antenna technology, slot antenna
designs, and plated plastics to reduce
weight and costs, the WTL team has
developed a modular antenna design that
employs a half-wavelength-wide dualpolar element capable of meeting both
narrow- and wide-spaced array antennas.
This modular design is suited to both
active and passive sector and array
solutions.
The basic element is a cavity-backed
slot that is excited by probes fed from
an integral feed network. Two of these
elements make up a pair of orthogonally
polarized slot-cavity combinations. These
dual-polar elements are fabricated in
pairs (or more) to create an antenna
building block that is small enough to
both keep manufacturing costs down
and provide flexibility in the configuration of the resulting antenna. A single
column of such elements would then be
the core of a dual-polarized sector antenna suitable for two-way MIMO, while
an array of directly adjacent columns
then forms an array antenna suitable for
SDMA-MIMO (see photo, page 12).
This antenna technology and
others like it currently under development at Nortel are important enablers to
the widespread deployment of SDMAMIMO technologies and are key to the
deployment of affordable and ubiquitous
4G wireless broadband multimedia
services.
David Adams is Advisor, Advanced Antenna Technology, in Nortel’s
Wireless Technology Lab.
Andy Jeffries is Senior Manager,
Advanced Antenna Technology, in
Nortel’s Wireless Technology Lab.
One technology focus at Nortel is the design of miniature band reject filters
(MBRFs) using proprietary surface acoustic wave (SAW) technology. MBRFs
will be key enablers in mobile terminals and handsets for allowing operators and new entrants with non-traditional spectrum to deploy WiMAX.
multiple transmit and receive antennas.
By exploiting the multi-path phenomenon to differentiate among the multiple
parallel signal paths between MIMO
antennas, MIMO technology achieves
a multifold user throughput gain and
multiple aggregated capacity increase
compared to current 3G macro-cellular
networks.
Nortel has long held a leadership position in MIMO-based techniques, and
MIMO has essentially become the de
facto standard for antenna operation in
4G systems.
The adoption of multiple antenna
technology means that 4G systems will
require new antenna designs that enable
a range of deployment scenarios depending on operators’ needs. For instance,
incumbent operators with many existing
cell sites would most likely be interested
in antenna deployments that increase
the capacity at each site. By contrast,
new entrants deploying new cell sites
would be looking to maximize capital
investments by building as few sites as
possible, but equipping them with antennas that increase range. As well, because of the typically high costs involved
with leasing cell sites, all players are de-
manding antennas that are compact and
require a smaller footprint than previous
generations.
To meet this range of needs, researchers are building on their experience and
leadership in MIMO technology to develop several advanced technologies and
configurations, as well as researching the
use of advanced lightweight materials
for more flexible, compact, and robust
products.
Adaptive antenna technology: One
of the technologies Nortel is developing
enables antennas to be directed at each
user and ensures that the transmission
and reception of every packet is optimized for that user. This technology involves adaptive beamforming techniques
based on spatial division multiple access
(SDMA). It also requires the development of sophisticated algorithms that
steer a beam toward the target user, effectively optimizing the transmission of
each packet delivered to each user. As
well, the team is devising sophisticated
schedulers for assigning users and determining priorities for transmission.
Earlier research by Nortel demonstrated
that OFDM-MIMO with multi-beam
continued on page 16...
Nortel Technical Journal, Issue 6 13
Closed-loop MIMO beamforming:
Taking MIMO transmission the “extra mile”
by Caroline Chan and Wen Tong
Nortel researchers are developing a 4G
solution that will extend the capabilities
of multiple-input multiple-output (MIMO)
transmission and cost-effectively deliver
even higher data rates to mobile users
in highly cluttered propagation environments, such as dense urban and indoor
areas, as well as increase MIMO signal
strength for greater reach in rural areas.
Called closed-loop MIMO beamforming, this solution enables intrinsic
integration of MIMO transmission with an
advanced beamforming technique based
on Eigenvectors.
(An Eigenvector forms the complex
mathematical equations that deal with
transforming the shape and orientation
of objects. In the wireless world, it is
antenna beams that are transformed
to enable them to direct radio signals
around obstacles, such as buildings, that
would normally block the signal path.)
In this way, MIMO signals can be concentrated into narrow, and therefore more
powerful, virtual beams that can, in turn,
be focused on an individual user and optimized for each user’s reception conditions.
“Closed loop” refers to the feedback
loop created by the continuous communication of channel information between
the mobile and base station, thus enabling
the base station to direct the virtual beam
to a particular user. This contrasts with an
“open loop,” whereby signal transmissions
are spread more broadly.
Nortel is a strong proponent of closedloop MIMO beamforming because of its
potential to achieve higher performance,
while supporting a wide range of antenna
configurations for field deployments and
providing operators with greater flexibility
and significantly reduced costs.
According to both Nortel studies and
analysis conducted within the WiMAX
Forum, incumbent operators that are considering deployment of a cell-site overlay
could potentially realize a 65-percent user
throughput gain at 95-percent tile coverage, and a 40-percent aggregated sector
capacity gain. For greenfield operators, this
technology could provide a 30-percent cellcount reduction and a link budget gain of
5 dB in the downlink.
Importantly, this technology complements and can be deployed in the company of other advanced 4G techniques, such
as mobile multi-hop relay (see page 16) to
further enhance reach and throughput.
Conceptual view
Nortel is developing a number of key technologies, including specialized algorithms
and schedulers, to give 4G base stations
Closed-loop MIMO beamforming: Architectural view (downlink transmission)
OFDMA
transmit
Mobile terminal
MIMO channel
OFDMA
receive
OFDMA
transmit
Packet
OFDMA
receive
OFDMA
transmit
Codebook
and search
OFDMA
transmit
Codebook
Codebook index feedback via uplink
14
Nortel Technical Journal, Issue 6
Pre-coder
Pre-coder
Base station
Packet
the ability to dynamically track the continuously changing locations and directions of
mobile terminals, and to enable the cell-site
antennas to create narrow beams with the
focus they need to target signals at
individual user devices.
These capabilities are performed by
three key elements: a pre-coder, a codebook database, and orthogonal frequency
division multiple access (OFDMA) transmitters/receivers. All three elements are
housed both in the 4G base station and in
the 4G-capable device (such as a WiMAX
terminal). The job of the pre-coder is to apply a mathematical “weight” to each packet,
and then feed the differently weighted
packets to each OFDMA receiver or transmitter, which uses this information to form a
narrow beam. The pre-coded beamforming
weights are stored in a codebook – a database that contains the information required
to translate channel information sent by the
different devices into standard “terminology” understood by all network elements.
The accompanying diagram presents a
conceptual architectural view of these elements, and highlights the steps involved
in downlink transmission – from the base
station to the terminal.
The first step begins on the device side
(far right of diagram). Before the network
transmits any packets, the device must
instruct the base station on where to focus
the virtual beam. To do this, the device
searches its codebook for the appropriate codeword (or pre-coded beamforming
weight) and then sends this value – called
a codebook index – to the base station via
closed-loop feedback signaling (on a separate channel from that used for user data).
These pre-coded weights are then stored
in the base station’s codebook.
When the network transmits the packet,
the packet is fed into the base station’s precoder, which retrieves the codebook index
and applies the pre-generated weights to
each packet. These weights are calculated
specifically to ensure that the MIMO signal
energy that is about to be emitted from the
four-branch antenna can be best “added
up” on the user device side for each packet
over the scattering MIMO channel environment, hence delivering MIMO signal quality
that will form the optimum targeted beam.
Since the MIMO channel changes over
time (as the mobile user changes position),
the pre-coder weights must be adjusted as
the channel changes, and as often as the
user changes position, in order to ensure
the best-quality transmission for every
packet.
The weighted packet is then mapped
onto the four-branch OFDMA transmitters,
which perform Inverse Fast Fourier Transform (IFFT) to generate OFDM signals. The
packet is then fed into the antenna element, which sends the formed beam over
the air to the targeted terminal.
Beamforming for a MIMO signal requires MIMO channel information, which
is carried through the MIMO pilot signal
at the base station. The device receiver
extracts the MIMO pilot signal and derives
the beamforming weights that were applied
at the base station. In a rich-scattering,
non-line-of-sight environment, Eigen-based
beams – virtual beams in the signal space
that can navigate around obstacles (represented by the white “kidney” shapes in
diagram) – constitute the best beamforming
solution. The virtual beams are generated
on both the transmit and receive sides. The
weights of the beams can be computed by
using the Eigen-decomposition technique
based on the MIMO channel matrix. Another advantage of this technique is that it
minimizes the level of overhead (the codebook index feedback) over the air interface.
On the device side, the antennas collect
the beamformed MIMO signals and feed
them into the OFDMA receivers, which
demodulate the OFDM signal using FFT.
The device pre-coder then applies the
codeword to form the best receive beam.
Deployment considerations
With respect to cost, footprint, and ease
of deployment, the ideal base station
configuration is one that provides closedloop MIMO beamforming while keeping
the user device simple and low-cost.
This challenge can be met with an advanced remote radio head cell-site architecture that provides four-branch transmit antennas and supports 4x2 downlink
closed-loop MIMO, with two receive
antennas on the user device side. The
ideal antenna structure is two spatially
separated cross-polarized antennas with
3 to 18 RF signal wavelength separation.
Another attractive antenna configuration
is a single, compact, two-column crosspolarized antenna with 0.7 RF signal
wavelength separation.
Nortel researchers are meeting this
need with several innovations, including
development of a compact remote radio
head solution and low-cost lightweight
antennas that leverage Nortel’s flatplate antenna technology, slot antenna
designs, and plated plastics to reduce
weight and costs (see page 12).
While practical deployments of
closed-loop MIMO beamforming are
expected in the 2009/2010 timeframe,
Nortel is working aggressively within the
various standards bodies to help drive
industry direction and standardization.
In fact, Nortel is a leading technology
contributor in this area within the WiMAX
Forum and other standards development
organizations.
Caroline Chan is Leader, WiMAX
Network Product Line Management,
Carrier Networks.
Wen Tong is Leader of Nortel’s Wireless
Technology Lab.
Nortel Technical Journal, Issue 6 15
technology can provide 10 times higher
capacity on the downlink than current
3G baseline system deployments – and
at one-tenth the cost. The team is now
working with developers in the Nortel
wireless business unit to deploy this
technology into future products.
Hex-MIMO: Another technology
being actively pursued by our research
teams for a variety of 4G applications is
a concept called Hex-MIMO, a
configuration that uses compact, lightweight antenna arrays to provide six
sectors per cell site versus the typical
three, delivering valuable capacity and
coverage gains in urban hot-spot deployments. This concept, described on
page 11, is based on both MIMO and
SDMA techniques. Depending on the
scenario, a Hex-MIMO solution can
offer downlink and uplink capacities in
excess of 2.5 times that of conventional
deployments.
Closed-loop MIMO beamforming: For dense urban indoor and rural
environments, researchers are working
on antenna solutions that utilize fourbranch receive and closed-loop MIMO
transmission, also called closed-loop
Eigen beamforming (see page 14). Essentially, Eigen beamforming concentrates radio signals and aims them at
the targeted user in the complex propagation clutter environment, enabling
high-quality signals to be delivered to
users who are located in areas of weak or
poor radio reception, such as indoors.
Here, designers are developing a new
algorithm and scheduler that can determine the optimum mode of transmission for each user’s reception conditions.
Researchers are also working within the
WiMAX Forum to bring this technology into the WiMAX Forum profile and
allow interoperability among all types of
mobile devices.
Cable reduction technology: The
WTL has developed innovative cable
reduction technology that allows multiple antenna signals to travel between
the top and bottom of the tower along a
single RF cable. For traditional cellular
systems, this technology enables a single
16
Nortel Technical Journal, Issue 6
The many advantages of mobile multi-hop
relay technology
by Hang Zhang and Peiying Zhu
Nortel’s Wireless Technology Lab
(WTL) is researching and developing
key technologies and architectures
that will enable mobile multi-hop relay (MMR), which is set to become a
crucial capability in 4G networks for
extending service coverage, enhancing throughput, reducing the need for
and cost of additional base stations,
and lowering overall CapEx and OpEx
costs for operators.
The MMR concept centers on a
new network node called a relay station (RS). An RS (or several of them)
is positioned between the base station
and the mobile device (e.g., cell phone,
personal computer, or laptop) and acts
as a “hop” for wireless signals as they
travel between the base station and the
mobile.
The diagram highlights the advantages of MMR technology across different cell-site deployments – from indoor
office buildings, to dense urban areas
with high concentrations of mobile
users, to less-concentrated rural and
suburban areas. As well, relay stations
can be configured with various levels
of functionality – from the simplest RS
that physically relays data between the
base station and terminal (under the
control of the base station), to a highly
configurable RS with the added intelligence necessary for performing such
functions as resource management,
scheduling, and security, effectively
acting as a “mini” base station. With
this flexibility, the RS can be deployed
in many different scenarios, some of
which follow.
To enhance capacity: As shown in
scenario A in the diagram, an RS can
be used to enhance cell capacity and
increase throughput. Today, in large
cells that cover suburban areas, wireless signals can lose strength as they
propagate to the edges of the cell, leading to weak transmission and therefore
lower data rates and throughput for users
positioned at these edges. An RS can be
positioned at the edges of each cell to
boost the strength of the wireless signal
and provide ubiquitous and uniform highspeed access.
To extend the network: Relay stations can also be used to extend network
coverage (scenario B) and provide
service to mobile users who are out of
range of a cell site. Rather than deploy
additional costly base stations, antennas,
and RF feeder cables, a series of lowercost relays can be used to reach beyond
the cell-site coverage area, with signals
hopping from relay to relay.
To eliminate coverage holes: Often
in cellular and wireless networks, “holes”
in coverage exist among two or more adjacent cell sites (scenario C). These coverage holes are especially pronounced
within buildings and in dense urban environments, where signals may be blocked
by physical obstacles such as buildings,
or may be degraded while traveling
around corners. Positioning relay stations
at these points of blockage would allow
signals to reach mobiles that otherwise
would receive either poor or no service in
these spots.
To boost uplink throughput for
mobiles: Wireless networks have always
experienced link-budget imbalances for
transmission on the downlink and uplink
paths. Downlink transmission – which is
the direct data path from the base station to the mobile station (MS) or mobile
device – is generally of a higher power
than that of the uplink – the reverse path
from the terminal to the base station – because of the MS limitations in maximum
transmission power. As a result, the uplink presents the more difficult challenge
with respect to throughput, particularly
Mobile multi-hop relay scenarios
A. To enhance capacity
BS
B. To extend the network
RS
BS
C. To eliminate coverage holes
RS
RS
D. To boost uplink throughput for mobiles
BS
Up
k
lin
wn
Do
k
lin
Radio
RS
k
lin
RS
BS
BS
BS
Up
E. To simplify handover operation to a group of mobiles
BS
BS
RS
BS
Base station
RS
Relay station
Nortel Technical Journal, Issue 6 17
Mobile multi-hop relay
continued
when user devices are farther away from
the base station. To help boost the uplink
throughput for the MS, relay stations can
be positioned between the MS and the
base station to shorten the distance for
the uplink traffic and boost throughput
(scenario D).
To simplify handover operation to
a group of mobiles: In this particularly
exciting application (scenario E), which
is attracting much early interest in the
wireless community, the RS can be deployed as an agent for a group of mobile
devices.
For instance, a small RS could be
mounted in a vehicle – a car, bus, or
train – that is carrying several wireless
subscribers who are using their handsets, laptops, or BlackBerry devices.
Today, those devices would each have to
communicate directly with the base station, with each individual mobile device
exchanging requests for handover and
receiving assignments as the vehicle
moves across the cell boundary. This
individual handover process consumes
a significant amount of bandwidth and
results in less bandwidth available for MS
data.
Instead, an onboard RS would move
with the mobiles, thereby keeping the
relative location of the RS to MS unchanged, and the mobiles would communicate directly with the RS. The RS, then,
would initiate and manage handover
when the vehicle crosses a cell boundary,
performing the procedure only once and
making handover transparent to the mobile devices. This procedure is especially
beneficial for fast-moving vehicles, where
handover is performed more frequently.
To enable these benefits across all
types of scenarios, WTL researchers are
developing several new enabling technologies. These technologies are currently
being driven into key standards-setting
18
Nortel Technical Journal, Issue 6
bodies, and discussions regarding precise implementation schemes are under
way.
Notably, Nortel is the leading contributor to the IEEE 802.16j standard
for WiMAX implementations, with more
than 100 contributions submitted. IEEE
802.16j is currently in the final stages of
standardization. Nortel is also actively
participating in the WiMAX Forum,
which is specifying MMR for the WiMAX
profile.
Across all of these efforts, the WTL
continues to embrace a design approach that will ensure the technology
innovations under way are applicable
not just for a single 4G standard, such
as WiMAX, but also for all current and
future 4G technologies. With MMR,
Nortel researchers are ensuring that
the associated technologies are flexible
enough to be easily extended to other
4G air interfaces, such as Long Term
Evolution (LTE) and Ultra Mobile Broadband (UMB).
Hang Zhang is Advisor, MAC Layer,
Advanced Wireless Access Systems, in
Nortel’s Wireless Technology Lab.
Peiying Zhu is Leader of Advanced
Wireless Access Systems, in the
Wireless Technology Lab.
RF cable to carry transmit, receive-main,
receive-diversity, and control and synchronizing signals, as well as DC power,
bringing cost reductions and greater
deployment flexibility. This signal combining (or multiplexing) technique can
be done in the time, frequency, phase, or
code domains. For a MIMO-based system, this technology innovation is key to
enabling multiple transmit and receive
signals to travel on a single RF cable,
providing cost-effective connectivity
between the base station and antenna.
Remote radio head: Another critical requirement in the antenna domain
is ease of deployment. Because there is
typically little spare room on the cell-site
towers to accommodate additional large
antennas and their associated RF cables,
next-generation antennas must be much
more compact, while still offering higher
capacity and throughput. Our team is
designing a solution that integrates the
antenna with the necessary electronics
for deployment on the tower top. This
solution – a remote radio head – essentially puts a small, lightweight, highly reliable carrier-grade base station on top of
the tower. The small size of the remote
radio head brings new levels of flexibility
for operators deploying 4G networks.
Signal processing
Digital signal processing (DSP) software
– in the form of sophisticated scheduling algorithms – controls and optimizes
the transmit and receive signals to and
from the user’s mobile device. This
optimization is a difficult challenge in
over-the-air environments, where channel quality cannot be guaranteed and is
much more vulnerable to undesirable
channel fading, delay, and jitter than
traffic in wireline environments. To date,
signal processing in the wireless access
portion of the network has focused on
meeting the individual traffic characteristics of separate circuit-switched and
packet-switched cellular voice and data
networks.
A 4G wireless access system that
supports all types of traffic (voice,
data, video) on a single converged IP
Comparison of wireless and wireline access technologies
Wireless technologies
Wireline technologies
2G
3G
4G
ADSL
Cable
EDGE
1xEV-DO HSPA
Rev. A
(Rel.6)
WiMAX
UMB
LTE
ADSL2+
DOCSIS
3.0
Commercial availability
Now
Now
2008
2007/beyond
2009
2009/10
Now
Now
Channel bandwidth
(MHz)
0.200
1.25
5
1.25, 2.5,
5, 10, 20,
3.75, 7, 8.75
1.25 to
20
1.4, 1.6,
3, 5, 10,
15, 20
–
–
FDD/TDD
FDD
FDD
FDD
TDD (2:1)
FDD
FDD
–
–
DL
0.384
3.1
10.8
31.68
37.25
29.4
28
171.25
UL
0.384
1.8
5.76
5.13
19.5
10.55
3.5
122.88
Aggregate per
carrier-sector
throughput (Mbit/s)
DL
0.16
0.95
2.6
7.88
8.1
7.95
–
–
UL
0.16
0.45
1.5
3.03
4.0
3.75
–
–
Spectral efficiency
(bits/sec/Hz/
carrier-sector)
DL
0.1
0.76
0.52
1.28
1.62
1.59
–
–
UL
0.1
0.36
0.3
0.79
0.8
0.75
–
–
Performance metric
Peak data rate
(Mbit/s)
This table compares the capabilities and several key
performance metrics for the different generations of
wireless access and the two dominant wired access
technologies. The metrics for both the 3G and 4G
technologies are based on the following assumptions.
Frequency division duplex (FDD) assumes a 5-MHz
bandwidth in the downlink (DL), and 5 MHz in the uplink
(UL). Time division duplex (TDD) is based on a total
network and directs this multimedia
traffic through a single interface brings
new DSP-related challenges. Not only
must the software overcome the inherent
over-the-air transmission characteristics,
but it must also meet the different traffic behaviors and requirements of new
types of services and a variety of user
devices. For example, to ensure that
users are provided with optimal transmission and reception for a particular
service – whether real-time voice, realtime high-bandwidth streaming video,
or lower-priority data, for instance – the
scheduler must now determine priority for multiple types of traffic, change
the coding modulation depending on
the channel conditions, and change the
10-MHz bandwidth. For 4G, the peak data rate in the
downlink is based on 64 quadrature amplitude modulation (QAM), and in the uplink, 16 QAM. For 3G, the
downlink is 16 QAM and the uplink is based on quadrature phase-shift keying (QPSK). Spectrum reuse for
both 3G and 4G is equal to 1, and signaling overheads
are included.
MIMO transmission modes according
to both the robustness and throughput
needed for each service and user type.
To meet these challenges, Nortel is
working to develop a new scheduling
algorithm, along with new adaptive
coding modulation schemes and radio
resource management software. These
elements are critical enablers for supporting Voice over IP and delivering a
high quality of service.
Mobile multi-hop relay
Mobile multi-hop relay is another technology that Nortel is promoting, both
in the industry and in the key standards
bodies. This technology will enhance
coverage and capacity (primarily in
urban environments) for 4G deployments, such as WiMAX.
In a multi-hop relay architecture,
communication between the base stations and mobile terminals can be extended and improved through a number
of intermediate relay stations. These
relays enable signals to “hop” between
them, without having to communicate
back to the base station. In this way,
relays can be used in a variety of deployments to provide many advantages.
For instance, relays can help extend
the network, allowing operators to effectively reduce the number of cell sites
needed for coverage – a solution that is
especially attractive to new entrants. Relays can also be deployed to fill “holes”
Nortel Technical Journal, Issue 6 19
or shadowing caused by environmental
obstacles (such as buildings, or corners
within buildings), which can lead to
weak signal reception and thus lower
data rates. (For more information on the
many advantages of mobile multi-hop
relay, see page 16.)
To realize the benefits of multi-hop
relay technology, researchers have developed several innovative technologies,
and have brought these into the IEEE
802.16j standard, which is currently in
the final stages of standardization. In
fact, Nortel holds numerous patents in
this area and was a key contributor to
the 802.16j standard.
The advanced wireless technologies
that Nortel is pursuing – including
the base station, antenna, digital signal
processing, and mobile multi-hop relay
innovations discussed in this article
– will serve to significantly raise the bar
for overall wireless network throughput,
performance, and cost, surpassing those
of even a baseline 4G implementation.
In fact, the wireless infrastructure that
our teams are making possible is expected to enable operators to achieve up
to three times higher throughput, with
a corresponding reduction in CapEx
and OpEx. Together, these technology
enhancements will help operators create the more powerful and cost-effective
infrastructures they will need not only to
meet the intensifying subscriber demand
for affordable “true” broadband services,
but also to deliver a simpler and much
richer user experience, with the ability
to move seamlessly across the traditional
IT, wired, and wireless worlds.
Technology roadmap
Beyond these innovations, Nortel’s longterm technology roadmap represents a
continuation of this steep performance
trajectory, with progression on all infrastructure fronts. Indeed, while the
different 4G air interfaces, notably
WiMAX and LTE, meet different market needs and are proceeding down separate evolution paths, Nortel’s vision is
ultimately to deliver a single converged
platform that will support both of these
20
Nortel Technical Journal, Issue 6
technologies. This convergence is made
possible by the fact that all 4G technologies are built upon the same foundations
– OFDM and MIMO.
Nortel has long embraced the philosophy of developing common architectures, technologies, and platforms that
will underpin and support all flavors of
4G and easily adapt to support other 4G
technologies that appear in the future.
With WiMAX and LTE following
different evolution paths, a similar evolution of technologies to support them is
expected over the next five years. For instance, we expect to see the evolution of
duplexing schemes, such as time division
duplex (TDD) and frequency division
duplex (FDD). We also expect the development of new hot-spot technologies
and new network elements, including
nodes that support relay technology and
femto cells. All are the focus of various
research projects within the WTL.
In addition, Nortel is continuing its
leading role in major wireless standards
activities. (For more on Nortel’s standards activities, see page 60.)
For instance, Nortel was the early
promoter of the OFDM-MIMO air
interface as the 4G technology of choice.
Nortel has been one of the key contributors in the LTE area, in both the study
phase and the standards development
phase. Here, researchers are applying
their years of experience in OFDMMIMO technology to ensure that LTE
will deliver best-in-class performance
levels. Nortel is also working within the
3GPP, 3GPP2, and WiMAX Forum to
ensure internetworking and seamless
operation across LTE and all existing
wireless technologies.
With respect to LTE, standards work
is scheduled to be completed by the
end of 2008, with a complete set of
specifications developed by the Technical Specification Group-Radio Access
Network (TSG-RAN), TSG-SA (Services and System Aspects), and TSG-CT
(Core Network and Terminals). These
specifications will cover the end-to-end
LTE system – from the air interface to
network evolution. Already, Nortel has
launched LTE design initiatives and is
working with lead customers to conduct
early trials.
Another exciting technology revolution in the 4G landscape is occurring in
the end-user mobile terminal and handset industry. Many vendors in the consumer electronics mass market are recognizing the tremendous potential of 4G
connectivity and are quickly developing
open devices, open source software,
and open applications, which will give
subscribers easy access to the richness of
broadband multimedia services and
facilitate a true broadband experience.
As technology innovations such as
those being spearheaded by the WTL
progress and standards mature, the
1-Gbit/s (low mobility) and 100-Mbit/s
(high mobility) access speeds envisioned
by the ITU will become a reality. Soon,
it will be second nature to expect that
the same advanced broadband multimedia and communications-enabled
capabilities offered in the wireline environment will also be available and
affordable in the wireless world, providing users with a ubiquitous true broadband experience.
Vish Nandlall is Chief Architect in
Nortel’s Carrier Networks organization.
Ed Sich is Leader of RF Technology in
Nortel’s Wireless Technology Lab.
Wen Tong is Leader of Nortel’s Wireless
Technology Lab. He was also recently
awarded the title of Nortel Technical
Fellow.
Peiying Zhu is Leader of Advanced
Wireless Access Systems in Nortel’s Wireless
Technology Lab.