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Using the 60GHz band for LTE backhaul An analysis of the benefits and technical challenges
Mark Barrett
CMO Blu Wireless Technology Ltd
www.bluwirelesstechnology.com
Using the 60GHz for LTE backhaul An analysis of the benefits and technical challenges
White paper
Mark Barrett, CMO, Blu Wireless Technology Ltd
Introduction
The rise of 4G LTE mobile technology
is presenting significant opportunities
for operators, equipment vendors and
chip developers, but delivering the
technology in a cost effective way also
presents a significant technological
challenge too.
The 4G LTE standard, which currently
provides throughput rates of 30Mbps
(100Mbps peak) and is predicted to
deliver up to 1Gbps in future 3GPP
releases, enables operators to sell
premium rate packages to a rapidly
growing number of subscribers.
But operators therefore also need to
increase the backhaul capacity to cope
with the higher throughput speeds.
And, in order to control avoid repeating
the near crippling costs that were seen
in the 3G rollouts, operators need to
increase capacity inline with
subscriber numbers and locations.
Predicted data flow
The speed at which we create and
consume data is increasing. Cisco
predicts 1.4 zettabytes (1021 bytes) will
flow over global fixed and mobile
networks by 2017. And every day, in
North American alone, 1.3 exabytes
(1018 bytes) of IP traffic will flow.
Today’s average backhaul capacity is
35 Mbps per cell and this needs to
increase to 1 Gbps per cell in just 5
years to support this mobile data
growth.
But data rates that can be achieved on
deployed networks will be a function of
capacity and, as cells hit capacity
limits, operators will be forced to add
smaller and smaller cell sites known
as micro and picocells.
Indeed, it is predicted that 72,000
small cells will be rolled out in London
alone by 2015 and the market is
forecast to reach $2.7 billion by 2017.
Fig 1: Backhaul market projections for
60GHz assume current cost of $8k+/link.
Low cost mesh backhaul using the 60GHz
band will bring this to <<$2k
Connecting the small cells
Small cells are particularly relevant in
urban deployments, where there is an
exceptionally high density of users.
But, this increased user density
requires more cells per square mile…
and therefore increased costs.
A city’s existing street furniture – such
as lamp posts – means there is a high
degree of freedom in where these
base stations can be placed. Indeed,
the only real limitation is a high
bandwidth data link to the core
network.
This backhaul requirement has
traditionally been serviced through a
combination of fibre optic and licensed
point-to-point (P2P) microwave radio
links, operating at selected bands from
6 to 38GHz. But current wireless sub-6
GHz NLOS (non line of sight) and
60/80 GHz LOS systems do not
deliver the necessary capacity at
acceptable cost points and the high
link cost of licensed bands has acted
as a significant economic brake on the
roll out of these P2P links.
A typical license costs in the region of
$200 per year in the US and roughly
10x this figure in Europe. Furthermore,
licensed microwave equipment costs
are typically in the order of $10,000-
20,000 per unit. High capacity optical
fiber can provide an ideal backhaul
connection but fiber is often not
available at the locations where small
cells are required and laying new fiber
requires costly and disruptive
construction works … hardly ideal if
you’re trying to grow the network in
line with subscriber number / demand.
60GHz cuts this cost
In urban areas the distance between
small cells will be no more than a 2-
300 metres. This means that a high
throughput, low interference wireless
networking standard capable of mesh
networking could be used to link the
base stations instead, significantly
reducing the cost of administering
backhaul.
In August 2013 the US
communications regulator, the FCC,
backed the use of the unlicensed
60GHz (57-64GHz) band for backhaul
applications. The exceptional
bandwidth (7GHz) that this provides,
combined with its oxygen absorption
that limits transmissions to a maximum
of 1 km means this is able to provide
the throughput that meets current and
future needs with virtually no
interference, even within a high
density mesh network.
The FCC’s new Part 15 rules permit
an increase in permitted power for
outdoor operations and could,
according to the FCC, “provide
wireless broadband network
connectivity over distances up to a
mile at data rates of 7 Gb/s, potentially
relieving the need and expense of
wiring facilities or using existing
facilities with less capability”.
The new rules could enable significant
benefits to the mobile operators,
reducing the cost of setting up a
network and simplifying the process of
adding capacity to the network in line
with demand.
It will also enable a huge market for
such equipment and drive innovation
in this multi gigabit band.
The 60GHz band in more depth
The 60GHz band dates back to the
1990s, when the FCC adopted rules
for unlicensed operations over a 7
GHz wide band; the 57-64 GHz band.
This is a very wide bandwidth, making
this spectrum very desirable for high-
capacity uses, both as networking
equipment indoors – streaming lag-
free HD video from a Blu-ray player or
tablet to the television – and point-to-
point (P2P) fixed operations outdoors -
providing broadband access to
adjacent structures in commercial
facilities to extend the reach of fiber
optic networks.
The FCC has now ruled that
outdoor operations between fixed
points can use a higher power –
increased from +40 dBmi up to a
maximum of +82 dBmi with +51 dBi
gain antennas. To benefit, equipment
needs to use highly directional
antennas and, because this increase
could cause interference to other
users and networks, the FCC has
linked the maximum power
permitted to the antenna beam width.
In short, the rule change permits
outdoor devices to deliver this high-
speed data over longer distances and
further cuts the cost of deploying a
network to significantly less than $2k
per link.
Fig 2: Urban mesh backhaul links
between small cells located on street
furniture in London
Technological challenges
1) TDD vs FDD
60GHz backhaul equipment already
exists, however, these generally use
FDD (frequency division duplex),
which not only requires complex
diplexor filters to be implemented, but
also requires separation of transmit
and receive frequencies. This leads to
potential inefficiencies in the use of
available frequencies as the guard
band can consume a significant
portion (1-2 GHz) of the useable
frequency band.
Diplexors also add significant loss in
both the transmit and receive paths
with 2 to 4 dB loss being typical. For a
single P2P (point to point)
transmission this wouldn’t be a
problem, but backhaul requires
multiple adjacent links and therefore
frequency re-use is essential if we’re
to avoid interference and further
increase capacity in very dense
network deployments.
In contrast TDD (time division duplex)
technologies, like those used in WiFi
and, the 60GHz version of WiFi also
known as WiGig, allow the complete
band to be used for both send and
receive and the up / downlinks can be
dynamically adjusted to match the
current traffic profile where downlink
data traffic to the user device usually
dominates.
2) Modulation schemes
WiGig, based on the IEEE 802.11ad
standard, has the capability of
delivering from 1 to 7 Gbps of data
within a 2 GHz wide channel through a
flexible combination modulation types
(BPSK, QPSK, 16QAM and 64QAM,
access modes) single carrier and
OFDM / advanced channel coding
using LDPC. These modes are
dynamically selecting during link
setup. In comparison, the typical
backhaul needs for a LTE small cell
base station of less than 1 Gbps can
typically be accommodated by using a
relatively low order modulation such as
QPSK which can deliver > 2Gbps.
This is both more robust than the
higher order (up to 1024 QAM)
schemes used in narrow channel
modems and has the future capability
of extending the data rate through
deployment of higher order modulation
modes in future versions.
Whilst WiGig can be used ‘out of the
box’ for 2Gbps QPSK links, backhaul
applications require the flexibility of
trading data rate and operational
distance. One other option, therefore,
is to increase the radio link budget
through the use of reduced channel
bandwidth. For example, for each
halving of the channel bandwidth the
receiver sensitivity is improved by an
additional 3 dB. This is illustrated in
the figure overleaf. Here, a 1000 Mbps
full duplex link (2 Gbps QPSK) is
further scaled by ½ and ¼ in order to
illustrate the trade offs of range versus
data rate under different rain fade
conditions.
A flexible baseband architecture
allows this scaling of frequency
channel bandwidth thus enabling this
increase in range and to cope with
differing operator scenarios.
Max EIRP = 40 dBmi (FCC 15.255). Link availability 99.99%
802.11ad WiFi QPSK SC modem, 20% overhead and 1:1 full duplex data stream
Fig: Estimated range for 60GHz backhaul communications under ideal and real world
(London / New York) urban situation
3) Data packetisation
Data packetisation for LTE backhaul is
particularly challenging. Unlike
standard P2P networks, the wireless
mesh networks used to transmit data
between points add an extra level of
complexity and each small cell needs
to know if it is merely relaying (via
60GHz or other backhaul connection
mechanisms) data or transmitting it via
the mobile network’s base station to
the mobile smartphone.
This data packetisation is controlled
via the MAC function. This is slightly
different to the standard WiGig MAC
and to implement WiGig technology for
LTE backhaul, the baseband platform
requires the flexibility to cope with both
requirements.
One proposal is to use the OpenFlow
as a MAC framework to define an
industry standard backhaul API.
However, for the time being there is no
fixed standard, and as a result we’re
co-operating with several equipment
vendors and operators to ensure our
IP complies universally.
4) Phased Array antennas
The new FCC rules stipulate a narrow
antenna beam as low as a 0.4 degree
beam width and, as small cells for LTE
backhaul transmit over distances of
0
50
100
150
200
250
300
350
400
0 mm rain/hr O2+Rain a2en: 15
dB/km
London : Region E 22mm rain/hr
O2+Rain a2en: 25 dB/km
NYC: Region K 42mm rain/hr
O2+Rain a2en: 30 dB/km
Range (m
)
250 Mbps
500 Mbps
1000 Mbps
several hundred meters, even a
position change of a fraction of degree
will have a negative effect on the link
performance and therefore the quality
of the mobile network.
Furthermore, LTE backhaul small cells
will be positioned on lampposts and
other street furniture and are therefore
at the mercy of both the elements and
accidents… this means, there is one
final key challenge facing the
operators when deploying 60GHz: how
to easily install, align and provide in
life adjustments for backhaul links.
Electronic antenna steering using
phased array antenna (PAA)
technology provides an ideal solution
to these problems. Originally designed
for military applications, this is now a
mature technology and 60 GHz PAA
technology is now becoming available
at cost points compatible with the
commercial constraints for small cell
backhaul. PAA technology also fits
well with the emerging use of Self-
optimizing networks (SON) as SON
could utilize PAA to dynamically steer
antenna connections and thus re-
configure network coverage for very
high capacity hotspots.
The market and its key players
1) Operators
All major mobile operators – from
Europe’s Orange, EE and Vodafone
through Japan’s KDDI to the US’
Clearwire, Sprint and Verizon – are all
actively assessing the 60GHz
backhaul technology and the FCC’s
recent ruling will likely speed this take
up.
2) Equipment vendors
In addition, products are beginning to
hit the market from vendors – such as
NEC’s Pasolink. Smaller vendors are
also producing interesting solutions
technology, for example Sub10
Systems showcased their 60 GHz
backhaul equipment on Vodafone’s
Mobile World Congress stand and
Siklu’s millimetre wave backhaul
equipment is worthy of note.
The key question facing all operators
and equipment vendors is how to
reduce the total cost of ownership
(TCO) of backhaul equipment. The
current generation of 60 GHz backhaul
products typically uses expensive
discrete RF, analog and digital signal
processing components based on a
combination of FPGA and high-end
programmable DSP devices. With the
continued development of WiGig
highly integrated 60GHz devices will
begin to reach the market in high
volumes from 2014 onwards. The
HYDRA baseband core from Blu
Wireless will be integrated within
several of these devices chips, which
in turn will significantly reduce the
equipment costs for 60 GHz backhaul
applications.
3) Chip vendors
According to ABI Research a total of, 2
billion WiFi chips (2.4 and 5.8 GHz)
will be shipped in 2013. ABI further
estimate that by 2018 WiFi chips
incorporating 60GHz will add a further
1.5 billion units per annum to this
market. Thus, by using these
economies of scale, the potential for
cost reduction of 60 GHz backhaul
equipment is very considerable.
Clearly, the intersection of increased
demand for cost effective backhaul for
LTE mobile networks and the reducing
cost of 60 GHz technology as being
deployed in the WiFi market offers the
potential to deliver backhaul at
performance points and TCO
compatible with operator
requirements.
4) Baseband IP
In addition to the very small number of
tier-1 vendors, who have had the
resources to develop 60GHz WiFi
technologies in house, a large number
of WiFi chip manufacturers are set to
enter the 60GHz market using silicon
IP from companies like Blu Wireless to
cut the cost and time of market entry.
For further information on Blu
Wireless’s 60GHz silicon IP and how
to implement it for backhaul
applications visit
bluwirelesstechnology.com/backhaul
Blu Wireless Technology Ltd
The Engine Shed, Station Approach
Temple Meads, Bristol, UK
www.bluwirelesstechnology.com