Real-Time Demonstration of an Optically Powered Radio Head for Low-Power Small Cells with 94 dB End-to-End Budget

B. Schrenk, T. Zemen

AIT Austrian Institute of Technology, Dept. Digital Safety&Security / Optical Quantum Technology, Donau-City Strasse 1, A-1220 Vienna, Austria. Corresponding author: bernhard.schrenk@ait.ac.at

Abstract End-to-end analogue down- and uplink radio transmission with real-time signal processing is

experimentally demonstrated using a technologically lean and energy-conscious remote radio head.

Powering through the fronthaul at an optical feed of 290 mW enables a centralised power supply.

Introduction

In the upcoming years we anticipate a

continuous increase in wireless data rates.

Hence, small cell techniques increasing network

density have an important role to play1. This

entails that baseband processing is off-loaded to

the CO while simple remote radio head (RRH)

technology remains at the cell site. However, as

load conditions in wireless networks are shifting

rapidly with the mobility of its users, RRHs that

are distributed across the network infrastructure

need to be dynamically taken into operation to

optimize the interplay of macro- and small cells.

Such a scenario is unfavourable for practical

deployment: even in case of strongly localized

cells, each site needs to be supplied with data

and energy, thus being subject to high cost.

In this work we experimentally demonstrate a

remotely powered RRH with low-cost VCSEL

transmitter and energy-conscious RF

amplification. End-to-end real-time transmission

of OFDM radio signals with PRBS and HDTV

payload is shown over a radio and fronthaul loss

budget of 84 and 10 dB, respectively.

Low-Power Remote Radio Head

In the proposed radio access scheme

conventional high-power RRHs of macro-cells

are assisted by additional low-power distributed

RRHs during on-peak, especially in overloaded

spots where the user density is peaking

regularly. With the introduction of numerous

small cells associated to fewer macro-cells the

complexity of these additional RRHs becomes a

critical parameter since cost attributed to RRH

subsystems is shared among a smaller number

of network users. Simplicity is therefore of

paramount importance and can be facilitated by:

(1) Analogue radio-over-fibre transmission as

an attractive solution to handle high CPRI-

equivalent data rates at the fronthaul2 while also

assuring lean small-cell RRHs based solely on

low-noise amplification (LNA) and an linear

optical transceiver. (2) Energy reclamation at

the RRH fronthaul interface allows to centralize

power supply units of RRHs at the CO.

Down- and uplink performance of a small-cell

RRH has been evaluated in the setup shown in

Fig. 1. At the CO the downlink I/Q data signal is

up-converted to a RF carrier at 2.05 GHz. A

post-amplified Mach-Zehnder modulator (MZM)

transmits the signal at 1547.72 nm with a power

of 10 dBm over the 4 km SMF-based fronthaul.

At the RRH the signal is boosted by energy-

conscious LNA towards the radio equipment

(RE) where real-time block error ratio (BLER)

estimation is performed on a payload size of

9024 bits/packet. OFDM with an IEEE 802.11a

compliant format having a modulation bandwidth

of 20 MHz and 64 subcarriers was employed.

The real-time DSP at transmitter and receiver

performs OFDM (de-)modulation with insertion

of pilots, synchronization sequences and cyclic

prefix. Moreover convolutional coding with code

generator polynom (1718, 1338) and code rate 1/2 is employed. The DSP stack was

implemented using the NI USRP-2953R

software-defined radio and was fed by either

PRBS for real-time BLER measurement or by

Fig. 1: Experimental setup and DSP stack for evaluating a low-power RRH that is supplied with energy through its fronthaul.

RRH Pump

α

LNA

Radio pathattenuator REVCSEL

PIN

small-cell RRHCO

Opticalfronthaul

PIN

Buffer

PRX

PIN LNAMixer

IQ

IQ

MZMSOA

IQ

PTX

IQ

BPF

4-stage LNA

MixerD/A

D/A

A/D

A/D

A/D

A/DVideoClient

D/A

D/A

MAC/IP

MAC/IP

RF RF

PRBSRe

al-t

ime

DSP

BLER

Re

al-t

ime

DSP

BLER

PRBS α

α

VideoServer

RX-DSP TX-DSP

Ch

ann

el C

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Mo

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Map

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Inve

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FFT

Cyc

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Pilo

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Seq

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nce

In

sert

ion

MA

C/P

RB

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D/A

Cyc

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Pilo

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trac

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n

Ch

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Car

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Fre

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ency

O

ffse

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om

pen

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on

Ch

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Tim

e Sy

nch

ron

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ion

A/D

MA

C/P

RB

S

Sym

bo

l Dem

app

ing

high-definition video traffic.

In case of uplink transmission, the RE radio

signal at 1.93 GHz is received at the RRH with a

multi-stage LNA. The small-signal gain of a 4-

stage LNA element is 45 dB at a low supply bias

of 8.3 mA at 1.6V.3 A low-cost multi-mode

vertical cavity surface emitting laser (VCSEL) is

used as energy-efficient uplink transmitter at the

RRH. The VCSEL has a low threshold current of

1.6 mA and a light-current slope of 60 µW/mA.

The linearity of the RRH was evaluated in

terms of IM3 distortion through LNA block and

opto-electronics. In case of downlink the power

consumption of the RRH can be minimized by

off-loading LNA stages to the CO. However, the

non-linear MZM transfer function will reduce the

spurious-free dynamic range (SFDR). Figure 2a

compares different combinations for this

partitioning, such as both LNA stages before the

optical transmitter at the CO (■,□), both stages

after the optical receiver at the RRH (●,○), one

stage at CO and RRH (▲,Δ), and 3 stages after

the optical receiver (+,×). The baseline

MZM+PIN link without extra amplifier (♦,◊) is

also shown as a reference. The IM3 products

significantly enhance by 10 dB and more once a

single LNA stage is placed before the MZM

(Δ,□) while clipping effects reduce the

achievable RF launch of the RRH to less than

-20 dBm. In the optimal case that all 3 LNAs are

placed after the PIN diode a launch of -6 dBm

at a SFDR > 40 dB can be achieved at a power

consumption of 11 mW (● in Fig. 2b). The input

IP3 point is 6.7 dBm (×,+ in Fig. 2a). For uplink

direction high RF input power levels beyond -58

dBm at the RRH input lead to a SFDR reduction

for nonlinear VCSEL-based optics (Δ in Fig. 2b),

which amounts to 6 dB and more with respect to

the case of having only the uplink LNA (◊).

Degradation due to LNA noise at very low RF

input levels can also be observed.

In order to supply the RRH with energy, the

RRH is optically powered by the CO through a

dedicated MMF-based fronthaul link using a 980

nm pump. A stack of 3×3 PIN diodes in

photovoltaic mode provides opto-electronic

feeding of the RRH. The consumption for all

subsystems including opto-electronic conversion

amounts to 6.9 (downlink), 8.3 and 10.6 mA

(uplink) at a supply of 1.6V. With the given V/I

characteristics of 1×N PIN stacks (Fig. 2c), a

feed of 290 mW is required to scavenge the

required energy for full LNA gain. It shall be

noted that remote powering been demonstrated

for up to 40W using double-clad fibre.4

End-to-end Radio Transmission Performance

Figure 3 shows the end-to-end downlink BLER

performance for real-time PRBS transmission.

The reception penalty for 16QAM-OFDM

subcarrier modulation with respect to BPSK is

4.5 dB at a BLER of 10-1 (Fig. 3a). Logarithmic

iso-BLER for (b) QPSK and (c) 16QAM

subcarrier modulation is shown as function of

the received power at RE and RRH. Ideally the

iso-BLER curves correspond to a constant

compound power level comprising the arithmetic

sum of received RF power at RE and received

optical power at RRH. However, saturation

effects occur at the RRH and are specifically

pronounced in case of higher-order 16QAM

modulation (see region I, Fig. 3c). The optimal

point of operation at the optical fronthaul can be

determined by excess noise: In case of QPSK

modulation the difference in reception penalty

between low delivered optical signal levels of -4

and -2 dBm is 4.8 dB at a BLER of 10-2 (see

points II, III). However, this improvement at a

higher signal feed has to be offset by the 2 dB of

required optical excess power. The observation

of the compound power is therefore adequate to

evaluate compatible transmission path losses in

each domain. When increasing the optical feed

further to 0 dBm the RF reception performance

improves by another 2 dB (point IV), however,

the 2 dB higher feed erodes any improvement in

terms of loss budget. Off-loading of bandwidth-

hungry services that would otherwise congest

the macro-cell is evidenced by real-time video

Fig. 2: (a,b) Linearity of the RRH. (c) V/I supply budget after energy reclamation at the fronthaul interface of the RRH.

-80

-70

-60

-50

-40

-30

-20

-10

0

-40 -30 -20 -10 0

RF

Ou

tpu

t P

ow

er

at R

RH

[d

Bm

]

RF Input Power at MZM [dBm]

MZM + PIN fund

Datenreihen6

IP3 Carr

IP3 IM3

IP2 IM2

Datenreihen3

Datenreihen4

Datenreihen11

MZM+PIN+2LNA fund

2LNA + MZM + PIN im3

2LNA + MZM + PIN fund

Datenreihen17

1LNA + MZM + PIN + 1LNA fund

MZM+PIN + 3LNA

MZM+PIN +3LNA, im3

Poly. (Datenreihen11)

(a)

2 before

2 before

1 + 1

IP3

DOWNLINK

SFD

R

-105 -95 -85 -75 -65 -55 -45

0

10

20

30

40

50

-2 -1 0 1 2 3

min

. (SF

DR

, No

ise

Bac

kgro

un

d)

[dB

]

LNA partitioning relative to optics

MZM + PIN fund

Datenreihen6

IM2

IP2 IM2

Datenreihen4

Datenreihen1

Datenreihen5

Poly. (IM2)

CO

eo o

e

RRH CO

eo o

e

RRH

CO RRH

MZM + PIN

LNA + MZM + PIN + LNA

(b) RF Input power [dBm]

UPLINK

LNA + o/e(VCSEL+PIN)

LNA only

5

7

9

11

13

0 0.5 1 1.5 2

RR

H S

up

ply

Cu

rre

nt

[mA

]

RRH Supply Voltage [V]

1xPIN, 780er pump direkt

2xPIN, 780er pump hinter 50/50

Datenreihen4

op point I

op point V

Poly. (Datenreihen4)

3 PINs 120 mW total

1 PIN

2 PINs40 mW total

safe operating areaoperation

current

op

era

tio

nvo

lta

ge

(c)

40 mW

streaming at a 720p resolution (Fig. 1). Neither

lost frames nor artefacts have been observed for

end-to-end transmission between video server

at the CO and video client at the RE.

The compatible downlink loss budgets can

be found according to the RRH power transfer

characteristics that relate optical feed to

launched radio signal (Fig. 2a). An optimization

of the compatible RF loss budget would favour a

high delivered optical power for the RRH, which

is explained by the square-law relation between

the generation rate of photoelectrons and the

optical field incident on the photodetector. For

an optical signal feed of 2 dBm a radio path loss

budget of 83.8 dB is found for QPSK subcarrier

modulation and a BLER of 10-2. This

corresponds to a line-of-sight reach of ~500m.5

The error floors found in the uplink BLER (Fig.

4a) are determined by the optical reception

sensitivity of the fronthaul receiver at the CO.

16QAM subcarrier modulation is negatively

affected by non-linear effects and partition noise

arising at the low-cost VCSEL. The penalty with

respect to BPSK amounts to 5.6 dB at a BLER

of 10-1. The logarithmic iso-BLER for QPSK (Fig.

4b) shows a similar compatible fronthaul loss

budget of ~10 dB. The optimal point of upstream

operation can be found at the cross-section

where neither optical nor RF noise is imposing a

limitation for the compound sensitivity. For a

targeted BLER of 10-1 a received RF power of

-95.8 dBm and an optical budget of >10 dB can

be obtained (point V). The typically high RE

launch of 10 dBm and more renders the uplink

loss budget as less critical.

Conclusions

An optically powered RRH was proposed. Real-

time analogue OFDM transmission including

error-free video streaming was demonstrated

despite the use of a low-cost VCSEL for opto-

electronic conversion at the RRH. The

compatible radio loss budget of 84 dB suits

short-reach applications, for which the RRH can

be electrically supplied by scavenging an optical

power of 290 mW at the fronthaul.

Acknowledgements

This work was supported by the EC through the FP7

Marie-Curie CIG grant WARP-5 (n° 333806).

References

[1] V. Jungnickel et al., “The Role of Small Cells,

Coordinated Multipoint, and Massive MIMO in 5G,” IEEE

Comm. Mag., vol. 52, no. 5, p. 44 (2014).

[2] X. Liu et al., “Efficient Mobile Fronthaul via DSP-Based

Channel Aggregation,” JLT, vol. 34, no. 6, p.1556 (2016)

[3] B. Schrenk et al., “Fully-Passive Remote Radio Head for

Uplink Cell Densification in Wireless Access Networks,”

Phot. Technol. Lett., vol. 27, no. 9, p. 970 (2015).

[4] J. Sato et al., “40-Watt Power-Over-Fiber Using a

Double-Clad Fiber for Optically Powered Radio-Over-

Fiber Systems,” Proc.OFC, We3F6, Los Angeles (2015).

[5] A. Tahat et al., “Analysis of Propagation Models at 2.1

GHz for Simulation of a Live 3G Cellular Network,” Proc.

Wireless Advanced, p. 164, London (2011).

Fig. 3: (a) Downlink BLER and iso-BLER (logarithmic) as function of received RF power at RE and received optical power at

the RRH for (b) QPSK and (c) 16QAM subcarrier modulation.

0.001

0.01

0.1

1

-96 -94 -92 -90 -88 -86

Blo

ck e

rro

r ra

tio

Received RF power at RE [dBm]

Datenreihen6

Datenreihen7

Datenreihen2

bpskfit

Datenreihen5

Log. (Datenreihen2)

4 dBmBPSKQPSK16QAM

OFDMModulation

-1 dBm

PRX at RRH

(4km SMF)

(b2b)

(a)

SMF b2bfronthaul:

OFDM-QPSK

(b)

II

III

IV SMF b2b

fronthaul:

OFDM-16QAM

(c)

I

Fig. 4: (a) Uplink BLER as function of received RF power at RRH. (b) Iso-BLER (logarithmic) for QPSK subcarrier modulation.

0.001

0.01

0.1

1

-99 -97 -95 -93 -91 -89

Blo

ck e

rro

r ra

tio

Received RF power at RRH [dBm]

BPSKQPSK16QAM

-6 dBm

PRX at CO

OFDMModulation

-16.5 dBm

(a)

PRX at CO

-16.5 dBm

-6 dBm

16QAMOFDM-QPSK

(b)

MMF b2b

fronthaul:

V