LTE Takes Shape              Technical White Paper                              V2.1                     February 2012    ...
LTE Takes ShapeContentsExecutive Summary                                                           31    Introduction     ...
LTE Takes ShapeExecutive SummaryUMTS Long Term Evolution (LTE) has emerged as the favored next step on the roadfor both 3G...
LTE Takes Shape1 IntroductionLess than a decade after the first deployments of WCDMA-based 3G systems, thecellular communi...
LTE Takes Shape             2 LTE and the NGMN             2.1 LTE History and Development             The idea of a “long...
LTE Takes Shape                 2.2 LTE In-Brief                 The standard’s designers wished to provide – amongst othe...
LTE Takes Shape(After: NGMN Alliance, “Next Generation Mobile Networks Beyond HSPA and EVDO”               More recently, ...
LTE Takes Shape             The physical problem becomes worse as frequency increases, and is intensified by             t...
LTE Takes Shape3.2.1 CoverageIn the case of coverage, Shannon establishes a minimum ratio between the energyper bit of a s...
LTE Takes ShapeFigure 5: Cooper’s Law demonstrates that spectrum re-use is the most potent method forincreasing network ca...
LTE Takes Shape              4.1 A Logical Conclusion              Although femtocells may seem like a radical idea, in ma...
LTE Takes Shapeof granularity cannot be attained using a macrocell-type network, but is exactlywhat femtocells are good at...
LTE Takes Shape4.5 Improving the RF Environment for All UsersA traditional concern about femtocells is the feeling that it...
LTE Takes ShapeFitting femtocells into such an architecture presented two major challenges: Iubinterfaces often include pr...
LTE Takes Shape              Such developments point semiconductor makers not just to lower price points, but             ...
LTE Takes Shape6 ConclusionLTE excels when providing extra coverage and capacity in targeted metropolitanhotspots and, in ...
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White Paper: LTE Needs Small Cells


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UMTS Long Term Evolution (LTE) has emerged as the favored next step on the road for both 3GPP-based networks and for operators using a variety of cellular standards around the world. Among its many attractions are improved capacity, reduced latency, easier integration with packet-based networks that use internet protocol (IP), and lower cost-per-bit.

We conclude that LTE will benefit from the use of femtocells from the outset.

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White Paper: LTE Needs Small Cells

  1. 1. LTE Takes Shape Technical White Paper V2.1 February 2012 Mindspeed Ltd Upper Borough Court Upper Borough Walls Bath BA1 1RG UK +44 1225 469744© Mindspeed Technologies, Inc. 2012
  2. 2. LTE Takes ShapeContentsExecutive Summary 31 Introduction 42 LTE and the NGMN 5 2.1 LTE History and Development 5 2.2 LTE In-Brief 6 2.3 The NGMN Concept 63 Why Are Traditional Macrocell Networks Inadequate for LTE? 7 3.1 The Great Indoors 7 3.2 Shannon Meets Cooper 8 3.2.1 Coverage 9 3.2.2 Data rate 9 3.2.3 Cooper’s Law 94 The Role of the Femtocell/Home eNodeB 10 4.1 A Logical Conclusion 11 4.2 Fitting the Usage Model 11 4.3 Femtocells from the Start 12 4.4 Superior Overall Network Capacity 12 4.5 Improving the RF Environment for All Users 135 What Will It Take? 13 5.1 Standardization 13 5.2 Equipment Cost 14 5.3 Intelligence “at the Node” 156 Conclusion 167 References 16Mindspeed Technologies, Inc Page 2 of 16
  3. 3. LTE Takes ShapeExecutive SummaryUMTS Long Term Evolution (LTE) has emerged as the favored next step on the roadfor both 3GPP-based networks and for operators using a variety of cellularstandards around the world. Among its many attractions are improved capacity,reduced latency, easier integration with packet-based networks that use internetprotocol (IP), and lower cost-per-bit.With “headline” data rates in excess of 300Mbit/s, and the ability to co-existgracefully with existing cellular systems, LTE represents an attractive revenuegenerating opportunity for operators. It can be used to target premium services atusers who need and can afford them, and to relieve capacity problems in areaswhere existing networks are congested. It is also intended to achieve a dramaticreduction in cost-per-bit of transmission to the operator – translating revenue intoprofit.Whatever the advantages, realizing LTE networks looks set to present significantchallenges. In seeking to move ever closer to the theoretical information-bearinglimits of the wireless spectrum, LTE uses wider channel bandwidths, advancedcoding and OFDMA (orthogonal frequency division multiple access) modulationmethods that require unprecedented signal processing power. Also built-in fromthe outset is the use of techniques such as multiple input-multiple output (MIMO),that combine signals from several antennas to enable more effectivecommunication.The nature of LTE points to a fundamental shift in the architecture of the networkitself, with smaller cells, closer to the user, being a key element in the mix.Several factors mean that this trend is likely to go further, faster than haspreviously been expected. First, the physics of radio communication makes itdifficult to attain higher and higher performance from a system that places largebasestations at a significant distance from the handset or user equipment (UE). Themost fundamental “laws” of communications, established sixty years ago by ClaudeShannon and Ralph Hartley, mean that the full benefits of LTE can only be gainedby using cells of a much smaller size than are currently employed.Moreover, usage patterns and user expectations are also evolving. More and morecellular communication takes place indoors. In this situation, using a macrocellnetwork for high-speed data transmission has been compared to “trying to fill acup from a fire hose spraying through an open window”.These drivers mean that architectures based on small cells serving few users(femtocells) will be much more than a convenient revenue-generating add-on forLTE operators: they will be the foundation of the network. But such a deploymentmodel also emphasizes the fact that a femtocell is much more than just a scaled-down macrocell. It requires a high degree of intelligence, so that it can deliver lowinstallation and operating costs. Additionally, the lower unit cost will benefit CapEx– especially for high-volume consumer deployments.We conclude that LTE will benefit from the use of femtocells from the outset.Mindspeed Technologies, Inc Page 3 of 16
  4. 4. LTE Takes Shape1 IntroductionLess than a decade after the first deployments of WCDMA-based 3G systems, thecellular communications industry has moved on to the next set of newdevelopments. Hot on the heels of the “original” UMTS specification have comeHSDPA (high-speed downlink packet access), HSUPA (high speed uplink packetaccess) and more recently HSPA+. LTE is the next in line, and is closely linked withthe emerging concept of the next-generation mobile network (NGMN).This paper argues that realizing the undoubted potential of LTE requires the use ofinnovative, fine-grained network architectures based on small cells (femtocells).The femtocell concept has a broad reach. It can address the networkingrequirements of enterprises, delivering converged services with high quality at lowcost. It can be used by operators as part of a more traditional networkinfrastructure enabling “metropolitan femtocells” or “hot zones” that improvenetwork capacity in a highly targeted fashion.An even more radical change is the idea of a home basestation. As the namesuggests, this is a low-cost consumer product for use in the home and has givenrise to the (slightly misleading, since it implies home use only) 3GPP terminologyfor a femtocell: Home eNodeB.Having established the need for femtocells, we argue that for LTE deployments,femtocell implementations will precede or even replace the roll-out of macrocell-based systems.In section two we look briefly at the drivers behind the development of LTE, andanalyze some of the benefits expected in its deployment, for both operators andend users. We also describe the genesis of the NGMN concept.The document then moves on to look at the most pressing challenges in LTEimplementation, and examines why traditional macrocell-based architecturescannot deliver the technology’s promise.Section four explains how small-cell architectures can be used to solve theproblems of macrocell-based implementations, and how femtocells may beinstrumental in attaining progress towards the NGMN.In section five, we look at some of the key technologies and developments recentlyput in place to enable the cost-effective deployment of fine-grained femtocell-typenetworks.Mindspeed Technologies, Inc Page 4 of 16
  5. 5. LTE Takes Shape 2 LTE and the NGMN 2.1 LTE History and Development The idea of a “long term evolution” (LTE) for the UMTS standard was initiated in 2004 i. The stated intention was to develop a universal terrestrial radio access network (UTRAN) which would provide a “framework for the evolution of the 3GPP radio-access technology towards a high-data-rate, low-latency and packet- optimized radio-access technology” ii. In short, cellular operators wanted to be able to offer a broadband internet experience that would rival that on offer from fixed- line providers. An LTE feasibility study initiated in March 2005 was concluded in September 2006 with the selection of OFDMA modulation for the downlink direction and Single Carrier Frequency Division Multiple Access (SC-FDMA) for the uplink. The standard also called for the use of multiple antenna systems, with the intention of increasing capacity and providing spatial diversity. iii In the meantime, of course, 3GPP has continued its evolutionary work on WCDMA with Releases 6,7 and 8 of the standard iv. Probably the most significant development along the way has been the introduction of HSPA/HSPA+, and the inclusion of higher order modulation (HOM) techniques such as 16QAM and 64QAM. Specifications for LTE are encapsulated in 3GPP Release 8: 3GPP recently announced peak theoretical downlink date rates for LTE of up to 326Mbit/s v over two 20MHz channels with four-by-four MIMO.Figure 1: LTE basestation concepts have already been demonstrated; femtocells will bedeployed in networks from the outset As noted earlier, while in other technologies femtocells are a more recent addition to the standards, LTE considers femtocell architectures from the outset, in 3GPP Release 8 vi. Mindspeed Technologies, Inc Page 5 of 16
  6. 6. LTE Takes Shape 2.2 LTE In-Brief The standard’s designers wished to provide – amongst other things – high data rates (both aggregate and as seen by the individual user) and high spectral efficiency. As a consequence they chose a combination of OFDMA, MIMO and higher-order modulation for its implementation. In addition to its high spectral efficiency, the standard has been planned to facilitate deployment in many different frequency bands with very little change to the radio interface. It is resistant to interference between cells, and spreads transmission efficiently over the available spectrum. Just as importantly, it co-exists well with other existing types of modulation, meaning that it can be deployed in the same location as existing services such as GSM or UMTS. Increasing modulation density is another key to providing the higher data rates promised by LTE. Depending on the signal-to-noise ratio (SNR) of a given channel it is possible to increase the number of bits-per-symbol that can be transmitted by a given system. While QPSK was used in pre-Release 5 WCDMA, successive versions have added 16QAM (HSDPA) and even 64QAM (HSPA+ in downlink). LTE continues this trend. Finally, it is worth noting that LTE is part of a wider plan to evolve the entire mobile network towards a flattened, all-IP architecture. This involves an evolved core network architecture termed the Evolved Packet Core (EPC) and LTE as the air interface. This combination is dubbed the Evolved Packet System (EPS), and is intended to provide an evolution path for a broad range of 2G and 3G communications systems. 2.3 The NGMN Concept The NGMN concept has been brought to prominence largely via the activities of the NGMN Alliance, established in 2006 to ensure that operators’ needs are represented in the evolution of network technology and standards. Now with over 50 members, the Alliance has enunciated both general requirements for evolved architectures – such as the need to deliver services “cost-effectively” without excessive IPR burdens – and very specific aspirations, such as those related to network latency and data rates.Figure 2: The NGMN Alliance has set out key requirements for evolved networks Seamless mobility 6 Reliability Low latency 5 4 3 Total cost of ownership Spectral efficiency 2 1 0 Simplicity End-to-end throughput Interworking Quality of service Integrated network Security Mindspeed Technologies, Inc Page 6 of 16
  7. 7. LTE Takes Shape(After: NGMN Alliance, “Next Generation Mobile Networks Beyond HSPA and EVDO” More recently, the NGMN Alliance acknowledged LTE and the wider EPS as a system that conforms to its aspirations for the evolved network. Although the organization remains technology agnostic, this step represents a powerful endorsement of LTE as it stands, while simultaneously increasing the relevance of the Alliance’s inputs and aspirations as the standard is realized. 3 Why Are Traditional Macrocell Networks Inadequate for LTE? LTE has been carefully designed from the ground up to provide many benefits. So why are traditional macrocells, that cover kilometers and cost tens of thousands of dollars, unable to deliver its promise? The answer to this question takes in a complex combination of factors, from operators’ motivations for deploying, to users’ requirements and the economics of service delivery. Amongst the most critical factors are the limitations imposed by the fundamentals of wireless communication. 3.1 The Great Indoors One of mobile operators’ biggest challenges today is to improve mobile coverage in an environment that is simultaneously demanding and potentially lucrative – inside buildings. Statistics show that up to 57% of mobile minutes are now clocked-up either at home or at work vii. This figure looks set to rise, with some estimates predicting figures of up to 75% by 2011. Statistics also consistently demonstrate that one of the major sources of churn for operators is poor coverage: a problem that is at its worst indoors. For mainly voice-based services, in the past the answer to this problem was to deploy large macrocell basestations with sufficient transmit power to overcome losses through walls. However, this approach became inadequate at the higher data rates and frequencies used in 3G networks, as a combination of sheer distance and losses through physical structures (walls) combined to prevent the service behaving as promised.Figure 3: the percentage increase in number of macrocell sites required to attainadditional coverage depth(Source: Femto Forum.Assumes a 2dB/m internal penetration loss, 5,000 sites and €225k NPV per site) Mindspeed Technologies, Inc Page 7 of 16
  8. 8. LTE Takes Shape The physical problem becomes worse as frequency increases, and is intensified by the use of more advanced modulation and coding schemes to enhance data rates. The additional attenuation reduces throughput for those users indoors, but there is another effect too: if the traditional macrocell allocates more power to reach the indoor user, this increases the interference for other users and/or cells. Such realities inevitably have a quantifiable, negative impact on cell capacity, making it impossible to deliver the 10x performance improvement compared to 3G that are a fundamental requirement of both the NGMN vision and 3GPP’s ambitions for LTE. Indeed, the NGMN Alliance specifically cites improved indoor data rates as a key quality metric for future network developments. It is these physical phenomena that have caused the macrocell approach to indoor coverage to be compared to “trying to fill a cup in the living room by aiming a fire hose on the street through the front window”. It is a brute force approach that is doomed to failure. The Femto Forum has attempted to quantify the problem, in a presentation by Prof Simon Saunders. He concluded that, for a small network of 5,000 initial sites and €225,000 net present value per site, achieving a meter of extra coverage depth in- building (which is hardly dramatic) via an increased macrocell deployment would come at a cost of €290 million. For LTE, therefore, this problem is critical – particularly so in the majority of places where the standard is likely to be deployed. This is because the in-building attenuation makes achieving 64QAM – a key factor in enabling higher data rates – unlikely. In fact, even 16QAM is borderline. Moreover, attenuation at the 2.5GHz frequencies targeted for LTE is even worse than is the case at the 1.8GHz used by current 3G systems.Figure 4: maximum throughput requires use of the highest order modulation, no retries,perfect scheduling and adequate MIMO channel decorrelation 3.2 Shannon Meets Cooper Even neglecting the need to enhance in-building coverage, macrocell approaches are also limited by two of the most fundamental “laws” of communications theory: Shannon’s Law viii, and a newer observation, made by Dr Martin Cooper, and sometimes called “Cooper’s Law” ix. Shannon’s Law establishes an upper limit on the coverage and/or capacity of a communications link: two parameters that are, in fact, two sides of the same coin. Mindspeed Technologies, Inc Page 8 of 16
  9. 9. LTE Takes Shape3.2.1 CoverageIn the case of coverage, Shannon establishes a minimum ratio between the energyper bit of a signal and the noise spectral density in the channel being used.Mathematically, the relationship is expressed as: Eb/N0 >-1.6dBIf the ratio of energy per bit to noise dips below -1.6dB, communication cannot beguaranteed, no matter what advanced coding or modulation regime is put in place.Modern systems – such as HSPA+ and LTE – come close to the -1.6dB fundamentallimit.The energy per bit at the receiver depends on the transmitter power, the path loss(dependent upon frequency, physical separation, obstructions and terrain), and thesize of the antennas at each end. The noise spectral density depends on physicalfundamentals, the receiver performance, and the amount of interference in thechannel.As a result, an increase in transmission frequency or bit rate in a system at, or near,the theoretical limit can only be attained by boosting transmitter power, reducingcell size, or using MIMO (effectively creating additional, parallel, channels to givepotentially up to N times the Shannon capacity between the two points).3.2.2 Data rateThe same kind of argument establishes the maximum data rate attainable from agiven channel: that is, it establishes the capacity limit. In particular, the maximumcapacity in bits per second (C) is given by C = B log2(1+S/N)Where B is the bandwidth in hertz, and S and N are signal and noise powerrespectively. Obviously using a more powerful signal increases the information-carrying capacity of the channel. But when more users attempt to access the samechannel or those closely adjacent - particularly by simply "turning the volume up" –the noise environment can become significantly worse, negating the positiveeffects of boosting signal strength. Eliminating such conflict is a big component inthe efficiency gains offered by femtocells.3.2.3 Cooper’s LawCoopers Law observes that the number of radio frequency conversations which canbe concurrently conducted in a given physical area has doubled every 30 monthssince Marconi’s earliest radio transmissions. Just as important as the bareassertion, however, is the analysis of the driving forces behind the progression.This 30-month doubling has yielded a one-million-fold overall increase in capacityin the last 45 years alone. Of this, it is estimated that 25x is due to using morespectrum; 5x is due to better modulation techniques; and 5x is down to frequencydivision. But by far the biggest factor, some 1600x, is due to spectrum re-use: theeffects of confining the area needed for individual interchanges within smaller andsmaller cells.Mindspeed Technologies, Inc Page 9 of 16
  10. 10. LTE Takes ShapeFigure 5: Cooper’s Law demonstrates that spectrum re-use is the most potent method forincreasing network capacity Coopers Law - contributors to improved network capacity Spectrum re-use More spectrumFrequency division Better modulation 0 200 400 600 800 1000 1200 1400 1600 Factor by which capacity has improved Seen from this point of view, it becomes clear that decreasing the average distance between the basestation and the user is the most potent weapon available both for increasing coverage and improving capacity: and is the only way of defeating the absolute limits imposed by Shannon’s Law. 4 The Role of the Femtocell/Home eNodeB If the traditional macrocell-based architecture is unable to offer the benefits of LTE, what kinds of networks will be deployed instead? With complex modulation, error correction and antenna diversity schemes, LTE is, after all, a demanding standard to implement. Future handsets will inevitably be dual-mode or more, offering a combination of GSM, EVDO, HSPA and LTE. Equally, most operators will have existing HSPA, EVDO or GSM networks in place when they come to roll out LTE. The question therefore arises: why deploy at all, if you can only match the performance of an already-existing network? The answer lies in the deployment of femtocells, or Home eNodeBs, as 3GPP calls them (somewhat confusingly, since they are just as useful in enterprise environments or the carrier network as in the home). Such small basestations are already being deployed to enhance 3G capacity and coverage. By installing a large number of smaller cells, operators can overcome the problems of signal attenuation and indoor coverage, and make use of the powerful ability of spectrum re-use to increase network capacity. In the home environment, femtocells need to include fully automated OA&M functionality: in effect they must be self-configuring so that they can be installed by the consumer. Similar kinds of capabilities are required in carrier-class equipment, to reduce OA&M costs and meet the NGMN goal of lower cost-of-ownership overall. Mindspeed Technologies, Inc Page 10 of 16
  11. 11. LTE Takes Shape 4.1 A Logical Conclusion Although femtocells may seem like a radical idea, in many ways they are simply a logical conclusion both of the NGMN aspirations and the LTE evolution path. The key requirements in this regard are reduced end-to-end latency, the associated need to simplify the network, and the move towards an architecture that is more “friendly” to Internet Protocol (IP) communication. In terms of latency, 3GPP has stated aims for HSPA+ of better than 50ms round-trip time, better than 500ms packet call set-up time and better than 100ms dormant- to-active control plane latency. At LTE the UE-to-basestation round-trip figure should be as low as 10ms. In order to achieve that, HSPA+ and LTE will use a “flattened” architecture, with many of the traditional RNC functions integrated into the NodeB (the basestation).Figure 6: HSPA+ and LTE will move away from the traditional hierarchical carrier networkarchitecture towards a “flatter” structureSource: “UMTS Evolution from 3GPP Release 7 to Release 8”, 3G Americas June 2008/Seymour, JP, “HSPA+ Performance Benefits." HSPA+ Seminar, CTIA, 2007 The wider EPS network architecture will follow a similar path. As a result, the eNodeB (whatever its size) will support not only the air interface but also radio resource control, user plane ciphering and Packet Data Convergence Protocol (PDCP). Effectively, femtocell-like capabilities are inherent in this new network model from the outset. In fact, the NGMN Alliance has been even more explicit about the need for femtocells. From the outset, the organization envisaged at least four basestation types, including one “optimised for size and cost and not capacity, with variants for home, office, and mobile installations”. Its 2006 white paper also stated: “The NGMN RAN shall be designed in a way that it allows a large-scale deployment of cost- optimised plug-and-play NGMN-only indoor radio equipment at a price level of commercial quality WLAN components.” 4.2 Fitting the Usage Model Femtocells are not only inherent in the architecture of the NGMN and LTE, they also provide an excellent fit with the manner in which evolved networks are expected to be used. The transition from 2G to 3G saw a change in the nature of cellular services, from voice-only to converged voice-and-data. LTE offers more data, at faster rates-per- user and lower cost-per-bit; but it does not offer the same kind of shift in the user paradigm. As a consequence, LTE’s power will be in offering improved coverage and capacity in a highly targeted fashion – either to help operators surmount capacity problems, or to allow them to offer premium services to small groups of end-users. This level Mindspeed Technologies, Inc Page 11 of 16
  12. 12. LTE Takes Shapeof granularity cannot be attained using a macrocell-type network, but is exactlywhat femtocells are good at.4.3 Femtocells from the StartOne very important difference between LTE and 3G is that LTE will be rolled outwith femtocells already established as a product.Dr. Peter Meissner, Operating Officer of the NGMN Alliance, encapsulated thechange when commenting on the Alliance’s partnership with the Femto Forum:“Adding femtocells to existing cellular networks provides operators with numerousbenefits, but rolling out a completely new mobile technology designed to includefemtocells introduces fundamental and powerful advantages. The partnershipbetween the Femto Forum and the NGMN Alliance will offer operators a completelydifferent approach to network rollout than exists today.”Not least of the advantages of a femtocell architecture is that it allows operators toput their infrastructure investment exactly where they know a return is most likely:in targeted locations where they have identified a demand for service.Industry analyst ABI has picked up on these benefits. In the words of senior analystNadine Manjaro:“We will see some macro network deployment of LTE, but not to the same extentthat we saw with previous technologies. I think a large portion of it will be deployedvia femto and picocells alone, with macro deployments following later.”4.4 Superior Overall Network CapacityOn a technical level, femtocell architectures deliver more than improved coverageand capacity across an individual channel. A typical macrocell might provide anaggregate bandwidth of 30Mbit/s, designed to be shared between 100 users;typical smaller cells provide only slightly less total bandwidth – perhaps 10-20Mbit/s – but share this between 20 users at most. Moreover, the superior radiopropagation environment within the femtocell makes it more likely that optimumdata rates can be attained for any given user. Effectively the femtocell architecturewins twice; it provides more theoretical bits-per-second-per-user when fullyloaded; and it offers each user a better chance of actually attaining the highestdata rates.The impact on individual users is substantial. But the analysis of Cooper’s Lawshows that the benefits, in terms of increased overall capacity, are to be seenacross the network. In Cooper’s Law, it is the overall capacity of the network that isenhanced. This is instinctively obvious when comparing the aggregate bandwidthof a macrocell network, which might include thousands of 30Mbit/s basestations,with a femtocell network. The latter could conceivably encompass hundreds ofthousands of basestations: perhaps even millions, for a service that reachedsubstantial residential penetration in a large developed nation.Mindspeed Technologies, Inc Page 12 of 16
  13. 13. LTE Takes Shape4.5 Improving the RF Environment for All UsersA traditional concern about femtocells is the feeling that it must be difficult tomanage RF issues in an environment of very many small cells. In fact, femtocellsinherently ease many of the interference issues that create problems in today’snetworks.One of the biggest challenges in RF management is surmounting the “near-far”problem. Terminals further from the basestation need to turn up their transmissionpower. Their transmissions represent potential interference to all of the otherterminals within the cell, which may respond in turn by turning up their ownpower. The bigger the cell size, the more terminals are likely to be operating farfrom the basestation, and the worse the problem.Femtocells directly cure this difficulty, because their presence means that mostterminals are operating “very near” the basestation, and therefore transmit lesspowerfully. This reduces interference for all users in the network, as everyonebenefits from less noise. Additionally, and perhaps surprisingly, because power canbe reduced, it also has a positive effect on terminal battery life.Moreover, the self-same structural attenuation that limits the in-building coverageand capacity of macrocell networks is a positive benefit for femtocells. The walls ofa building provide an effective isolation barrier, reducing interference, not justbetween terminals, but also between cells.It is obvious that femtocells deliver increased capacity for the users connected tothat femtocell: what is less obvious but very significant is that, by reducinginterference, they increase efficiency for all users of the network, even those onthe macrocell.Even better for a diversity-based technology such as LTE, an indoor environmentrepresents a rich source of multi-path signals – allowing the benefits of MIMO tobe maximized. As a result, in an indoor environment LTE can work at its highestmodulation rates and greatest spectral efficiency.5 What Will It Take?A common misconception is that femtocells are little more than “stripped-down”macrocells. In fact, nothing could be further from the truth. Developers havealready identified – and begun to implement – the key capabilities required tosuccessfully integrate femtocells into LTE networks.5.1 StandardizationRadio standards and planning are the first issues that spring to the minds of manywhen they think of femtocell deployments. But just as important is the task ofintegrating large numbers of femtocells into the core network.For reasons of both cost and convenience, the femtocell must include a highdegree of self-configuring capability: for home use it must offer the same level ofplug-and-play functionality that consumers expect from a WiFi access point. Avariety of backhaul strategies exist, including the use of an existing networkconnection and the public internet.Operators’ cellular radio access networks, in contrast, comprise hundreds ofbasestations connected to a single radio network controller or basestationcontroller (RNC or BSC). The interface between the NodeB and RNC/BSC is via the3GPP Iub standard (TS 25.434), running the ATM protocol over dedicated leasedlines.Mindspeed Technologies, Inc Page 13 of 16
  14. 14. LTE Takes ShapeFitting femtocells into such an architecture presented two major challenges: Iubinterfaces often include proprietary elements; and the installed base of RNCs arenot designed to handle many thousands of NodeBs.3GPP has now adopted an approach, effectively as a reference architecture, thatovercomes all of these concerns. It defines a new concentrator-type networkelement, the Home NodeB Gateway (HNB-GW) that can aggregate traffic fromthousands of NodeBs into the core network. Communication between femtocelland HNB-GW is via a new standard interface, Iuh, that implements securityfunctions, control signaling and a new application protocol (HNBAP) designed toease HNB deployment.This approach fits seamlessly into current mobile network operators’ radio accessnetworks (RANs) by supplementing or replacing their current RNCs with theconcentrator element. As we have already seen, the Home NodeB itself musthandle the radio resource management functions formerly residing in the RNC.5.2 Equipment CostWe have already observed that implementing LTE represents an extremelydemanding set of signal processing tasks. Providing the necessary processingpower, with low energy consumption, at an appropriate price point, requires a newgeneration of semiconductor devices.Vendors such as Mindspeed are already coming to market with solutions to thischallenge. The company is shipping an LTE design that supports both TDD and FDDmodes, includes OFDMA downlink and SC-FDMA uplink, and offers 2 x 2 MIMO andadaptive antenna systems (AAS).The use of a programmable multi-core device has several advantages: it is aproven, system-level solution that ensures fast time-to-market; at the same time,its programmability allows OEMs and ODMs to customize and add value quicklyand easily; it delivers the required processing power, with significantly lowerenergy consumption than an equivalent FPGA; and it suffers none of the long leadtimes and non-recurring engineering costs of a full ASIC implementation.Mindspeed Technologies, Inc Page 14 of 16
  15. 15. LTE Takes Shape Such developments point semiconductor makers not just to lower price points, but also to a system-level proposition that allows customers and infrastructure makers to focus on their own value-add – as is the case in many consumer markets today. 5.3 Intelligence “at the Node” As we have seen, femtocell LTE architectures will necessarily need to push intelligence out towards the node. Because they will need to be simple to install – often by end users – they will need to be (in the words of the Femto Forum) “self- planning, self-configuring, self-optimizing, self-tuning and self-testing”. In short, they will be the first genuinely self-organizing networks (SONs). All of this is not as much of a logical leap as it may seem. As long as the femtocell can be equipped with some way of knowing about its surroundings, it can be provided with sufficient intelligence to configure itself. Indeed, the adaptive nature of OFDMA itself means that the network can make adjustments on an on-going basis. Such technology is already in use in 3G femtocells, which are equipped with a UE “sniffer” (or network listen) mode to determine the nature of the radio environment in which they are operating. In this case, the device configures itself into UE mode, and synchronizes to the surrounding network. It can thus determine which scrambling codes will minimize interference, and how much transmitter power it needs to deploy to size the cell. By “knowing” about adjacent macrocells, it can also ensure that hand-off can be implemented effectively.Figure 7: Mindspeed’s femtocell reference designs includeassociated “sniffer” functionality to reduce OA&M costs The same principles will apply with LTE femtocells. As well as other femtocells and the macrocellular LTE network (if any) they will also need to be able to “see” adjacent HSPA macrocells, and even the surrounding 2G network, to ensure hand- off if no higher-generation network coverage is available. Mindspeed Technologies, Inc Page 15 of 16
  16. 16. LTE Takes Shape6 ConclusionLTE excels when providing extra coverage and capacity in targeted metropolitanhotspots and, in particular, indoors. Networks based on macrocells cannot deliversufficient benefits to warrant deployment for such purposes: they are tooexpensive, and in many cases are hamstrung by fundamental limitations of physicsand communications systems.Femtocells provide many of the answers, a fact that is recognized by organizationssuch as the NGMN Alliance, which has stated: “The NGMN RAN shall be designed ina way that it allows a large scale deployment of cost-optimized plug-and-playNGMN-only indoor radio equipment at a price level of commercial quality WLANcomponents.”Now it is up to infrastructure makers and their suppliers to take up the challenge ofdeveloping and implementing the technology needed to make LTE a success.Consumer market economies of scale can be exploited to bear down on CapExcosts; intelligence at the node and SON technology will contribute to dramaticallyreduced OpEx: and standardization efforts have been accelerated to ensure theinteroperability that will make it all possible.With these elements in place, it will be possible to write the next chapter in thestory of “the incredible shrinking cell”.7 Referencesi 3Gpp TR 25.912 v 7.2.0 Northstream 2007viii Technologies, Inc Page 16 of 16