Advanced technologies for 4G

Advanced technologies for 4G:Mobile broadband multimediaeverywhereby Vish Nandlall, Ed Sich, Wen Tong, and Peiying Zhu

Nortel’s Wireless Technology Lab (WTL) is working on numerous in-novations that will fundamentally rewrite the economic equation for wireless access infrastructures and pave the way for ubiquitous deploy-ment 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 dramatical-ly lower the cost per bit of over-the-air transmission, while improving spectral effi ciency 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 underly-ing 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-genera-tion (4G) wireless story has not yet been written in stone – 4G is

still being defi ned – the industry is clear-ly moving aggressively to a 4G world.

Both the pace of 4G adoption and the rate of standards development are by

far faster than all previous generations of wireless. Whereas it took nearly 10 years for 3G to roll out and become standard-ized, 4G activities are proceeding on an accelerated three-year cycle.

For instance, the fi rst 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 tech-nologies are proceeding rapidly. Th e 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 infl ection point for 4G mobile broadband – the point at which 4G will replace 2G and 3G infrastructures. Some

4 Nortel Technical Journal, Issue 6

Nortel Technical Journal, Issue 6 5

analyst reports put the size of the 4G market at about $60 billion by 2015.

Changing the wireless experienceThe 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 ap-proximately 100 Megabits per second (Mbit/s) for high mobility (such as vehicular mobility). These data rates are part of the International Telecommu-nication 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 net-works 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 char-acterized by seamless, high-bandwidth network connectivity, with access to any application regardless of device and loca-tion. High-quality and reliable delivery of unified communications sessions, vir-tual-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 criti-cal for supporting what is fast becom-ing a hyperconnected communications environment. More and more devices and machines, many of them mobile,

are becoming IP-enabled and con-nected 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 ap-plications 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 extend-ing their infrastructures into the wireless world. A recent example is Rogers Com-munications and Bell Canada, which are pooling their broadband resources to build a new national wireless net-work, 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 com-ing 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 challengesAll of these existing players and new entrants have their eyes on opportuni-ties 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 per-formance 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 band-width they need, wherever they need it, simply and cost-effectively. Most impor-tantly, 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 incre-mental improvements in capacity, spec-tral 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 dif-ferent schools of thought on 4G as there are providers. For some, 4G means high-capacity 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 enter-prise 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 configura-tions, 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 divi-sion multiplexing, and multiple-input multiple-output antenna processing


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 Techni-cal 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 eco-nomic paradigm for mobility solutions. In these efforts, the WTL team is ensur-ing that all innovations meet a common set of requirements. Specifically, the team is developing disruptive technolo-gies 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. Opera-tors are faced with rising consumer pres-sure for mobile services delivered at the same prices that they pay for similar ser-vices in the wireline domain. Consumers are increasingly pushing the market, ex-pecting 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 net-working 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.

This requirement leads to an inter-esting 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 im-provement in spectral efficiency. Nortel is also leveraging new silicon technolo-gies (taking advantage of the ongoing progression of Moore’s Law) and ad-vances 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 dif-fering 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 – specifi-cally, 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 deploy-able 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-per-formance cell-site solutions: 4G will demand extremely high levels of perfor-mance 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

Jim MacFie, advisor, standards management and tools, in Nortel’s Strategic Standards group, addresses a Global Standards Collaboration (GSC) meeting in France.

Harpreet Panesar, radio frequency hardware technology researcher, examines a high-efficiency power amplifier (PA) prototype developed for future base station PA systems.


antennas, however, raises significant is-sues with cabling from the base station to support those antennas. In conven-tional systems, such additions would place an impractical burden on the cell tower itself. There is an opportunity, therefore, to devise new cell-site archi-tectures 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 high-bandwidth, 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 develop-ment 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 packet-switched networks be required for voice and data. Instead, all traffic will be car-ried 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 accommo-date 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 migra-tion path from 2G and 3G: Regardless

of their starting points – whether 2G, 2.5G, or 3G – operators will be seeking a migration path to 4G that is cost- effective 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 technologiesThese requirements form the backdrop for Nortel’s 4G research efforts, where teams are focusing on developing the core technologies that will ultimately al-low 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 technolo-gies 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 in-novations – including a simpler, more cost-effective base station architecture, use of advanced silicon technology, a new network timing and synchroniza-tion solution, and more efficient power amplifiers. These innovations have culminated in the first 4G technology platform, and the industry’s most com-pact 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

Art Fuller, digital signal processing hardware technology researcher, evaluates digital-to-analog converter technology for use in future wireless systems for 4G and beyond.

continued on page 10...


“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 indus-try’s first 4G platform – a three-sector, full-power, fully multiple-input multiple-output (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 funda-mental 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, capac-ity 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.

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 packagingAnalytical 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 switch-ing 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 sev-eral key innovations, the team integrated the core, timing, and control modules into a single integrated digital module. This integration dramatically shrinks the size of

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

Diagram A. Basic architecture of a CDMA BTS

Modem

Modem Core

Modem Controlmodule

Network

Radio

Radio

Radio

.

.

.

.

.

.

Diagram B. Basic architecture of a WiMAX BTS

Integrateddigitalmodule

NetworkMIMO radio

Timingmodule


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 differ-ent 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 mo-dem, which can support up to 30 Megabits per second (Mbit/s).

The knowledge and patented technolo-gy developed by the WTL for its MIMO test-bed 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 sup-ported with detailed link-simulation suites, and the transfer was accelerated through the selection of compatible processor com-ponents 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-con-cept trials, channel models, system link-budget simulations, and antenna selection.

Channelization technologyThe evolution to OFDM places new de-mands on the radio channelization function. One key challenge is the need to cost- effectively 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 sup-porting 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 pat-ented common-rate channelization technol-ogy 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 sup-port 12 defined bandwidths. In this way, a single radio deployment can be configured to simultaneously meet the initial and future growth requirements of customers, includ-ing 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, com-pared to earlier technologies.

Synchronization is obtained with a Global Positioning System (GPS) satel-lite constellation, and precision timing is typically delivered with a crystal oscillator, which must continue to operate in “hold-over mode” if satellite visibility is temporar-ily lost. Because crystals age and are af-fected 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 pack-age with only a single oven.

This technology delivered a dramatic reduction in the size of the network timing and synchronization subsystem, collaps-ing a separate timing module the size of a bread box into a subsystem the size of a deck of cards. This subsystem, built on-board the WiMAX integrated digital module, delivers a dramatic cost reduction of some 80 percent compared to a previous- generation timing module.

Power amplifier memory-correction Even with today’s significant advances in pre-distortion techniques and increased transistor efficiencies, RF power ampli-fiers (PAs) still consume significant cost, volume, and energy, and require consider-able 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.


Building smaller, more compact, more efficient power amplifiers, however, tends to force design trade-offs – specifically with respect to memory effects, which is an un-desirable 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 design-ing 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 phenom-enon with signal processing techniques, designers integrated into the WiMAX radio module an innovative memory-correction technology, initially developed and pat-ented 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. Design-ers 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 mini-mize the impact of memory, allowing Nortel to push the transmitters to the compres-sion 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 perfor-mance 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

time to market. For example, research-ers worked closely with the manufac-turing-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 signifi-cant technology disruption in the wire-less arena. While these innovations are targeted initially for WiMAX deploy-ments, 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 high-ly 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 multi-media 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.

Nortel’s WiMAX base stationcontinued

deployment options, along with a sig-nificant 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 unparal-leled flexibility for deployment in differ-ent 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 al-lowing 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 capabil-ity they need for use in the satellite spec-trum. The MBRF will allow WiMAX terminal vendors to build terminals that meet the stringent out-of-band require-ments in certain frequency allocations, such as in the 1.5-GHz band that is ad-jacent 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

In recent years, there have been many advances in the development of ad-vanced spatial division multiple access (SDMA) and multiple-input multiple-output (MIMO) algorithms for broad-band wireless access. While these techniques offer the potential for sub-stantial 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 deploy-ment, operating, and hardware costs.

Researchers in Nortel’s Wireless Technology Lab (WTL) are investigat-ing a wide range of antenna configura-tions based on various combinations of SDMA and MIMO. One configuration that is receiving considerable attention for its ability to overcome these practi-cal 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 Hex-MIMO configuration essentially parti-tions each sector of a conventional tri-sector 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-

High-performance Hex-MIMO antenna technology for cost-effective 4G

by David Adams and Andy Jeffries

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 cost- effective to manufacture and deploy.

Unique SDMA-MIMO combinationHex-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 re-sources 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 be-ing 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 an-tennas with two orthogonal polarizations (± 45 degrees) in a single-column sector antenna.

In using SDMA, researchers chose to disregard the common industry percep-tion 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 rela-tive to performance – that would be key to the effective use of SDMA tech-


Two-beam dual-polarMIMOper sector

Hex-MIMO antenna configuration

Hex-MIMOcontinued

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 ele-ments (the smaller unit at bottom) is an antenna building block that provides flex-ibility in antenna configuration. A single column of elements would form the core of a dual-polarized sector antenna suit-able 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 anten-na designs, and plated plastics to reduce weight and costs. Such antenna designs are important enablers for the wide-spread deployment of cost-effective 4G antenna solutions, such as Hex-MIMO.


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 an-tennas typically require half-wavelength-spaced columns to provide good control of radiation patterns. The larger the number of antenna columns within the array, the greater the performance im-provements, but at the cost of increased antenna width. So, while eight columns, for example, may be an ideal SDMA an-tenna from a performance perspective, it

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 dif-ficult to solve as the RF carrier frequency drops and the antenna size increases. To address these issues, the team com-bined 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 be-ing actively pursued by the research team for a variety of potential 4G (WiMAX and LTE) applications.

Antenna-beamformer integrationThe 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 config-urations where the active elements of the radios are located remotely from the an-tenna, 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 inte-gration of the electronics and antenna.

The antenna-beamformer combination can also be configured to provide cover-age 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 technologiesAs SDMA systems become increasingly common, the need for antenna tech-nologies 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 integra-tion, 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 stand- alone antennas and integrated active ones.

Using a combination of Nortel’s flat-plate 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 dual-polar 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


multiple transmit and receive antennas. By exploiting the multi-path phenom-enon 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 po-sition 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 depend-ing 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 an-tennas that increase range. As well, be-cause 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, research-ers are building on their experience and leadership in MIMO technology to de-velop 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 opti-mized for that user. This technology in-volves adaptive beamforming techniques based on spatial division multiple access (SDMA). It also requires the develop-ment of sophisticated algorithms that steer a beam toward the target user, ef-fectively optimizing the transmission of each packet delivered to each user. As well, the team is devising sophisticated schedulers for assigning users and de-termining priorities for transmission. Earlier research by Nortel demonstrated that OFDM-MIMO with multi-beam

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 opera-tors and new entrants with non-traditional spectrum to deploy WiMAX.

continued on page 16...

building block that is small enough to both keep manufacturing costs down and provide flexibility in the configura-tion of the resulting antenna. A single column of such elements would then be the core of a dual-polarized sector an-tenna 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 develop-ment at Nortel are important enablers to the widespread deployment of SDMA-MIMO technologies and are key to the deployment of affordable and ubiquitous 4G wireless broadband multimedia services.

David Adams is Advisor, Advanced An-tenna Technology, in Nortel’s Wireless Technology Lab.Andy Jeffries is Senior Manager, Advanced Antenna Technology, in Nortel’s Wireless Technology Lab.

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 environ-ments, such as dense urban and indoor areas, as well as increase MIMO signal strength for greater reach in rural areas.

Called closed-loop MIMO beam-forming, 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 con-

centrated into narrow, and therefore more powerful, virtual beams that can, in turn, be focused on an individual user and opti-mized for each user’s reception conditions.

“Closed loop” refers to the feedback loop created by the continuous commu-nication 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 closed-loop 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 con-sidering deployment of a cell-site overlay could potentially realize a 65-percent user throughput gain at 95-percent tile cover-age, and a 40-percent aggregated sector capacity gain. For greenfield operators, this technology could provide a 30-percent cell-count reduction and a link budget gain of 5 dB in the downlink.

Importantly, this technology comple-ments and can be deployed in the compa-ny of other advanced 4G techniques, such as mobile multi-hop relay (see page 16) to further enhance reach and throughput.

Conceptual viewNortel is developing a number of key tech-nologies, including specialized algorithms and schedulers, to give 4G base stations

Closed-loop MIMO beamforming: Taking MIMO transmission the “extra mile”

by Caroline Chan and Wen Tong


Closed-loop MIMO beamforming: Architectural view (downlink transmission)

Base station

MIMO channelOFDMAtransmit

OFDMAtransmit

Packet

OFDMAtransmit

OFDMAtransmit

Pre

-co

de

r

Codebook

Mobile terminal

Codebook and search

Codebook index feedback via uplink

Packet

Pre

-co

de

rOFDMAreceive

OFDMAreceive

the ability to dynamically track the continu-ously 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 code-book database, and orthogonal frequency division multiple access (OFDMA) trans-mitters/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 ap-ply a mathematical “weight” to each packet, and then feed the differently weighted packets to each OFDMA receiver or trans-mitter, which uses this information to form a narrow beam. The pre-coded beamforming weights are stored in a codebook – a data-base that contains the information required to translate channel information sent by the different devices into standard “terminol-ogy” understood by all network elements. The accompanying diagram presents a conceptual architectural view of these ele-ments, 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 appropri-ate 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 sepa-rate 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 pre-coder, 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 environ-ment, 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 Trans-form (IFFT) to generate OFDM signals. The packet is then fed into the antenna ele-ment, which sends the formed beam over the air to the targeted terminal.

Beamforming for a MIMO signal re-quires 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 (rep-resented 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. An-other advantage of this technique is that it minimizes the level of overhead (the code-book 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 considerationsWith respect to cost, footprint, and ease of deployment, the ideal base station configuration is one that provides closed-loop MIMO beamforming while keeping the user device simple and low-cost. This challenge can be met with an ad-vanced remote radio head cell-site archi-tecture that provides four-branch trans-mit 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 cross-polarized 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 flat-plate 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.


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 re-lay (MMR), which is set to become a crucial capability in 4G networks for extending service coverage, enhanc-ing 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 sta-tion (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 advan-tages of MMR technology across differ-ent 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 intel-ligence 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, wire-less signals can lose strength as they

propagate to the edges of the cell, lead-ing 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 high-speed access.

To extend the network: Relay sta-tions 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 lower-cost 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 ad-jacent cell sites (scenario C). These cov-erage holes are especially pronounced within buildings and in dense urban envi-ronments, 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 sta-tion 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 – be-cause of the MS limitations in maximum transmission power. As a result, the up-link presents the more difficult challenge with respect to throughput, particularly


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, light-weight antenna arrays to provide six sectors per cell site versus the typical three, delivering valuable capacity and coverage gains in urban hot-spot de-ployments. 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 beamform-ing: For dense urban indoor and rural environments, researchers are working on antenna solutions that utilize four-branch receive and closed-loop MIMO transmission, also called closed-loop Eigen beamforming (see page 14). Es-sentially, Eigen beamforming concen-trates radio signals and aims them at the targeted user in the complex propa-gation 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 deter-mine the optimum mode of transmis-sion for each user’s reception conditions. Researchers are also working within the WiMAX Forum to bring this technol-ogy 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 mul-tiple 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


Mobile multi-hop relay scenarios

Radio

A. To enhance capacity

BS

C. To eliminate coverage holes

BS BS

BS

B. To extend the network

BS RSRS

RS

RS

E. To simplify handover operation to a group of mobiles

BS

D. To boost uplink throughput for mobiles

BS

Uplink

Uplink

Down

link

RS

BS RS

BS Base station

Relay stationRS

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 de-ployed 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 hand-sets, laptops, or BlackBerry devices. Today, those devices would each have to communicate directly with the base sta-tion, 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 un-changed, and the mobiles would commu-nicate 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 mo-bile 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 technol-ogies. These technologies are currently being driven into key standards-setting

bodies, and discussions regarding pre-cise implementation schemes are under way.

Notably, Nortel is the leading con-tributor 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 ap-proach 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 Broad-band (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 syn-chronizing signals, as well as DC power, bringing cost reductions and greater deployment flexibility. This signal com-bining (or multiplexing) technique can be done in the time, frequency, phase, or code domains. For a MIMO-based sys-tem, 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 criti-cal 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 – essen-tially puts a small, lightweight, highly re-liable 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 schedul-ing 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 chan-nel 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 character-istics 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

Mobile multi-hop relaycontinued


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 traf-fic behaviors and requirements of new types of services and a variety of user devices. For example, to ensure that users are provided with optimal trans-mission and reception for a particular service – whether real-time voice, real-time high-bandwidth streaming video, or lower-priority data, for instance – the scheduler must now determine prior-ity for multiple types of traffic, change the coding modulation depending on the channel conditions, and change the

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 sup-porting Voice over IP and delivering a high quality of service.

Mobile multi-hop relay Mobile multi-hop relay is another tech-nology 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 deploy-ments, such as WiMAX.

In a multi-hop relay architecture, communication between the base sta-tions and mobile terminals can be ex-tended 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 deploy-ments to provide many advantages. For instance, relays can help extend the network, allowing operators to ef-fectively reduce the number of cell sites needed for coverage – a solution that is especially attractive to new entrants. Re-lays can also be deployed to fill “holes”

Comparison of wireless and wireline access technologies

Commercial availability

Channel bandwidth (MHz)

FDD/TDD

Spectral efficiency (bits/sec/Hz/carrier-sector)

Wireless technologies Wireline technologies

Aggregate per carrier-sector throughput (Mbit/s)

2G

EDGE

DL

UL

DL

UL

3G

1xEV-DORev. A

HSPA (Rel.6)

4G

WiMAX UMB LTE

ADSL

ADSL2+

Cable

DOCSIS3.0

Performance metric

Peak data rate(Mbit/s)

DL

UL

Now Now 2008 2007/beyond 2009 2009/10 Now Now

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 FDD FDD TDD (2:1) FDD FDD

0.384 3.1 10.8 31.68 37.25 29.4 28 171.25

0.384 1.8 5.76 5.13 19.5 10.55 3.5 122.88

0.16 0.95 2.6 7.88 8.1 7.95

0.16 0.45 1.5 3.03 4.0 3.75

0.1 0.76 0.52 1.28 1.62 1.59

0.1 0.36 0.3 0.79 0.8 0.75

–

–

–

–

–

–

–

–

–

–

–

–

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

10-MHz bandwidth. For 4G, the peak data rate in the downlink is based on 64 quadrature amplitude modula-tion (QAM), and in the uplink, 16 QAM. For 3G, the downlink is 16 QAM and the uplink is based on quadra-ture phase-shift keying (QPSK). Spectrum reuse forboth 3G and 4G is equal to 1, and signaling overheads are included.


launched LTE design initiatives and is working with lead customers to conduct early trials.

Another exciting technology revolu-tion in the 4G landscape is occurring in the end-user mobile terminal and hand-set industry. Many vendors in the con-sumer electronics mass market are recog-nizing 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 multi-media and communications-enabled capabilities offered in the wireline envi-ronment will also be available and affordable in the wireless world, provid-ing users with a ubiquitous true broad-band 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.

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 de-veloped 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 ex-pected 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 cre-ate 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 long-term technology roadmap represents a continuation of this steep performance trajectory, with progression on all infra-structure fronts. Indeed, while the different 4G air interfaces, notably WiMAX and LTE, meet different mar-ket needs and are proceeding down sepa-rate evolution paths, Nortel’s vision is ultimately to deliver a single converged platform that will support both of these

technologies. This convergence is made possible by the fact that all 4G technolo-gies are built upon the same foundations – OFDM and MIMO.

Nortel has long embraced the phi-losophy of developing common architec-tures, 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 evo-lution of technologies to support them is expected over the next five years. For in-stance, we expect to see the evolution of duplexing schemes, such as time division duplex (TDD) and frequency division duplex (FDD). We also expect the de-velopment 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 stan-dards 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 contribu-tors in the LTE area, in both the study phase and the standards development phase. Here, researchers are applying their years of experience in OFDM-MIMO 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 Techni-cal Specification Group-Radio Access Network (TSG-RAN), TSG-SA (Ser-vices 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

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Advanced technologies for 4G