White Paper: LTE Needs Small Cells

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
www.mindspeed.com

© Mindspeed Technologies, Inc. 2012

LTE Takes Shape

Contents
Executive Summary 3
1 Introduction 4
2 LTE and the NGMN 5
2.1 LTE History and Development 5
2.2 LTE In-Brief 6
2.3 The NGMN Concept 6
3 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 9
4 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 13
5 What Will It Take? 13
5.1 Standardization 13
5.2 Equipment Cost 14
5.3 Intelligence “at the Node” 15
6 Conclusion 16
7 References 16

Mindspeed Technologies, Inc Page 2 of 16

LTE Takes Shape

Executive Summary
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.
With “headline” data rates in excess of 300Mbit/s, and the ability to co-exist
gracefully with existing cellular systems, LTE represents an attractive revenue
generating opportunity for operators. It can be used to target premium services at
users who need and can afford them, and to relieve capacity problems in areas
where existing networks are congested. It is also intended to achieve a dramatic
reduction in cost-per-bit of transmission to the operator – translating revenue into
profit.
Whatever the advantages, realizing LTE networks looks set to present significant
challenges. In seeking to move ever closer to the theoretical information-bearing
limits of the wireless spectrum, LTE uses wider channel bandwidths, advanced
coding and OFDMA (orthogonal frequency division multiple access) modulation
methods that require unprecedented signal processing power. Also built-in from
the outset is the use of techniques such as multiple input-multiple output (MIMO),
that combine signals from several antennas to enable more effective
communication.
The nature of LTE points to a fundamental shift in the architecture of the network
itself, 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 has
previously been expected. First, the physics of radio communication makes it
difficult to attain higher and higher performance from a system that places large
basestations at a significant distance from the handset or user equipment (UE). The
most fundamental “laws” of communications, established sixty years ago by Claude
Shannon and Ralph Hartley, mean that the full benefits of LTE can only be gained
by using cells of a much smaller size than are currently employed.
Moreover, usage patterns and user expectations are also evolving. More and more
cellular communication takes place indoors. In this situation, using a macrocell
network for high-speed data transmission has been compared to “trying to fill a
cup 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 for
LTE operators: they will be the foundation of the network. But such a deployment
model 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 low
installation 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.


LTE Takes Shape

1 Introduction
Less than a decade after the first deployments of WCDMA-based 3G systems, the
cellular communications industry has moved on to the next set of new
developments. Hot on the heels of the “original” UMTS specification have come
HSDPA (high-speed downlink packet access), HSUPA (high speed uplink packet
access) and more recently HSPA+. LTE is the next in line, and is closely linked with
the emerging concept of the next-generation mobile network (NGMN).
This paper argues that realizing the undoubted potential of LTE requires the use of
innovative, fine-grained network architectures based on small cells (femtocells).
The femtocell concept has a broad reach. It can address the networking
requirements of enterprises, delivering converged services with high quality at low
cost. It can be used by operators as part of a more traditional network
infrastructure enabling “metropolitan femtocells” or “hot zones” that improve
network capacity in a highly targeted fashion.
An even more radical change is the idea of a home basestation. As the name
suggests, this is a low-cost consumer product for use in the home and has given
rise to the (slightly misleading, since it implies home use only) 3GPP terminology
for 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, and
analyze some of the benefits expected in its deployment, for both operators and
end users. We also describe the genesis of the NGMN concept.
The document then moves on to look at the most pressing challenges in LTE
implementation, and examines why traditional macrocell-based architectures
cannot deliver the technology’s promise.
Section four explains how small-cell architectures can be used to solve the
problems of macrocell-based implementations, and how femtocells may be
instrumental in attaining progress towards the NGMN.
In section five, we look at some of the key technologies and developments recently
put in place to enable the cost-effective deployment of fine-grained femtocell-type
networks.


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 be
deployed 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.


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


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 attain
additional coverage depth

(Source: Femto Forum.
Assumes a 2dB/m internal penetration loss, 5,000 sites and €225k NPV per site)


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.


LTE Takes Shape

3.2.1 Coverage
In the case of coverage, Shannon establishes a minimum ratio between the energy
per bit of a signal and the noise spectral density in the channel being used.
Mathematically, the relationship is expressed as:
Eb/N0 >-1.6dB
If the ratio of energy per bit to noise dips below -1.6dB, communication cannot be
guaranteed, 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 fundamental
limit.
The energy per bit at the receiver depends on the transmitter power, the path loss
(dependent upon frequency, physical separation, obstructions and terrain), and the
size of the antennas at each end. The noise spectral density depends on physical
fundamentals, the receiver performance, and the amount of interference in the
channel.
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, reducing
cell size, or using MIMO (effectively creating additional, parallel, channels to give
potentially up to N times the Shannon capacity between the two points).

3.2.2 Data rate
The same kind of argument establishes the maximum data rate attainable from a
given channel: that is, it establishes the capacity limit. In particular, the maximum
capacity 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 power
respectively. Obviously using a more powerful signal increases the information-
carrying capacity of the channel. But when more users attempt to access the same
channel or those closely adjacent - particularly by simply "turning the volume up" –
the noise environment can become significantly worse, negating the positive
effects of boosting signal strength. Eliminating such conflict is a big component in
the efficiency gains offered by femtocells.

3.2.3 Cooper’s Law
Coopers Law observes that the number of radio frequency conversations which can
be concurrently conducted in a given physical area has doubled every 30 months
since Marconi’s earliest radio transmissions. Just as important as the bare
assertion, however, is the analysis of the driving forces behind the progression.
This 30-month doubling has yielded a one-million-fold overall increase in capacity
in the last 45 years alone. Of this, it is estimated that 25x is due to using more
spectrum; 5x is due to better modulation techniques; and 5x is down to frequency
division. But by far the biggest factor, some 1600x, is due to spectrum re-use: the
effects of confining the area needed for individual interchanges within smaller and
smaller cells.


LTE Takes Shape

Figure 5: Cooper’s Law demonstrates that spectrum re-use is the most potent method for
increasing network capacity

Cooper's Law - contributors to improved network capacity

Spectrum re-use

More spectrum

Frequency 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.


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 network
architecture towards a “flatter” structure

Source: “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


LTE Takes Shape

of granularity cannot be attained using a macrocell-type network, but is exactly
what femtocells are good at.

4.3 Femtocells from the Start
One very important difference between LTE and 3G is that LTE will be rolled out
with femtocells already established as a product.
Dr. Peter Meissner, Operating Officer of the NGMN Alliance, encapsulated the
change when commenting on the Alliance’s partnership with the Femto Forum:
“Adding femtocells to existing cellular networks provides operators with numerous
benefits, but rolling out a completely new mobile technology designed to include
femtocells introduces fundamental and powerful advantages. The partnership
between the Femto Forum and the NGMN Alliance will offer operators a completely
different approach to network rollout than exists today.”
Not least of the advantages of a femtocell architecture is that it allows operators to
put 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 analyst
Nadine Manjaro:
“We will see some macro network deployment of LTE, but not to the same extent
that we saw with previous technologies. I think a large portion of it will be deployed
via femto and picocells alone, with macro deployments following later.”

4.4 Superior Overall Network Capacity
On a technical level, femtocell architectures deliver more than improved coverage
and capacity across an individual channel. A typical macrocell might provide an
aggregate 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 radio
propagation environment within the femtocell makes it more likely that optimum
data rates can be attained for any given user. Effectively the femtocell architecture
wins twice; it provides more theoretical bits-per-second-per-user when fully
loaded; and it offers each user a better chance of actually attaining the highest
data rates.
The impact on individual users is substantial. But the analysis of Cooper’s Law
shows that the benefits, in terms of increased overall capacity, are to be seen
across the network. In Cooper’s Law, it is the overall capacity of the network that is
enhanced. This is instinctively obvious when comparing the aggregate bandwidth
of a macrocell network, which might include thousands of 30Mbit/s basestations,
with a femtocell network. The latter could conceivably encompass hundreds of
thousands of basestations: perhaps even millions, for a service that reached
substantial residential penetration in a large developed nation.


LTE Takes Shape

4.5 Improving the RF Environment for All Users
A traditional concern about femtocells is the feeling that it must be difficult to
manage RF issues in an environment of very many small cells. In fact, femtocells
inherently ease many of the interference issues that create problems in today’s
networks.
One of the biggest challenges in RF management is surmounting the “near-far”
problem. Terminals further from the basestation need to turn up their transmission
power. Their transmissions represent potential interference to all of the other
terminals within the cell, which may respond in turn by turning up their own
power. The bigger the cell size, the more terminals are likely to be operating far
from the basestation, and the worse the problem.
Femtocells directly cure this difficulty, because their presence means that most
terminals are operating “very near” the basestation, and therefore transmit less
powerfully. This reduces interference for all users in the network, as everyone
benefits from less noise. Additionally, and perhaps surprisingly, because power can
be reduced, it also has a positive effect on terminal battery life.
Moreover, the self-same structural attenuation that limits the in-building coverage
and capacity of macrocell networks is a positive benefit for femtocells. The walls of
a building provide an effective isolation barrier, reducing interference, not just
between terminals, but also between cells.
It is obvious that femtocells deliver increased capacity for the users connected to
that femtocell: what is less obvious but very significant is that, by reducing
interference, they increase efficiency for all users of the network, even those on
the macrocell.
Even better for a diversity-based technology such as LTE, an indoor environment
represents a rich source of multi-path signals – allowing the benefits of MIMO to
be maximized. As a result, in an indoor environment LTE can work at its highest
modulation 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 have
already identified – and begun to implement – the key capabilities required to
successfully integrate femtocells into LTE networks.

5.1 Standardization
Radio standards and planning are the first issues that spring to the minds of many
when they think of femtocell deployments. But just as important is the task of
integrating large numbers of femtocells into the core network.
For reasons of both cost and convenience, the femtocell must include a high
degree of self-configuring capability: for home use it must offer the same level of
plug-and-play functionality that consumers expect from a WiFi access point. A
variety of backhaul strategies exist, including the use of an existing network
connection and the public internet.
Operators’ cellular radio access networks, in contrast, comprise hundreds of
basestations connected to a single radio network controller or basestation
controller (RNC or BSC). The interface between the NodeB and RNC/BSC is via the
3GPP Iub standard (TS 25.434), running the ATM protocol over dedicated leased
lines.


LTE Takes Shape

Fitting femtocells into such an architecture presented two major challenges: Iub
interfaces often include proprietary elements; and the installed base of RNCs are
not designed to handle many thousands of NodeBs.
3GPP has now adopted an approach, effectively as a reference architecture, that
overcomes all of these concerns. It defines a new concentrator-type network
element, the Home NodeB Gateway (HNB-GW) that can aggregate traffic from
thousands of NodeBs into the core network. Communication between femtocell
and HNB-GW is via a new standard interface, Iuh, that implements security
functions, control signaling and a new application protocol (HNBAP) designed to
ease HNB deployment.
This approach fits seamlessly into current mobile network operators’ radio access
networks (RANs) by supplementing or replacing their current RNCs with the
concentrator element. As we have already seen, the Home NodeB itself must
handle the radio resource management functions formerly residing in the RNC.

5.2 Equipment Cost
We have already observed that implementing LTE represents an extremely
demanding set of signal processing tasks. Providing the necessary processing
power, with low energy consumption, at an appropriate price point, requires a new
generation of semiconductor devices.
Vendors such as Mindspeed are already coming to market with solutions to this
challenge. The company is shipping an LTE design that supports both TDD and FDD
modes, includes OFDMA downlink and SC-FDMA uplink, and offers 2 x 2 MIMO and
adaptive antenna systems (AAS).
The use of a programmable multi-core device has several advantages: it is a
proven, system-level solution that ensures fast time-to-market; at the same time,
its programmability allows OEMs and ODMs to customize and add value quickly
and easily; it delivers the required processing power, with significantly lower
energy consumption than an equivalent FPGA; and it suffers none of the long lead
times and non-recurring engineering costs of a full ASIC implementation.


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 include
associated “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.


LTE Takes Shape

6 Conclusion
LTE excels when providing extra coverage and capacity in targeted metropolitan
hotspots and, in particular, indoors. Networks based on macrocells cannot deliver
sufficient benefits to warrant deployment for such purposes: they are too
expensive, and in many cases are hamstrung by fundamental limitations of physics
and communications systems.
Femtocells provide many of the answers, a fact that is recognized by organizations
such as the NGMN Alliance, which has stated: “The NGMN RAN shall be designed in
a way that it allows a large scale deployment of cost-optimized plug-and-play
NGMN-only indoor radio equipment at a price level of commercial quality WLAN
components.”
Now it is up to infrastructure makers and their suppliers to take up the challenge of
developing and implementing the technology needed to make LTE a success.
Consumer market economies of scale can be exploited to bear down on CapEx
costs; intelligence at the node and SON technology will contribute to dramatically
reduced OpEx: and standardization efforts have been accelerated to ensure the
interoperability that will make it all possible.
With these elements in place, it will be possible to write the next chapter in the
story of “the incredible shrinking cell”.

7 References

i
ftp://ftp.3gpp.org/workshop/Archive/2004_11_RAN_Future_Evo/
ii
http://www.3gpp.org/Highlights/LTE/lte.htm
iii
http://www.3gpp.org/Highlights/LTE/lte.htm
iv
http://www.3gpp.org/specs/releases-contents.htm
v
3Gpp TR 25.912 v 7.2.0 http://www.3gpp.org/ftp/specs/archive/25_series/25.912/
vi
http://www.3gpp.org/specs/releases-contents.htm
vii
Northstream 2007
viii
http://www.stanford.edu/class/ee104/shannonpaper.pdf
ix
http://www.arraycomm.com/serve.php?page=Cooper


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