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

info@bluwirelesstechnology.com

www.bluwirelesstechnology.com