real-time demonstration of an optically powered radio head...
TRANSCRIPT
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: [email protected]
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
od
er
Mo
du
lati
on
Map
pin
g
Inve
rse
FFT
Cyc
lic P
refi
x In
sert
ion
Pilo
t an
d S
ync
Seq
ue
nce
In
sert
ion
MA
C/P
RB
S
D/A
Cyc
lic P
refi
x R
emo
val
FFT
Pilo
t Ex
trac
tio
n
Ch
ann
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eco
der
Car
rier
Fre
qu
ency
O
ffse
t C
om
pen
sati
on
Ch
ann
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stim
atio
n
Tim
e Sy
nch
ron
isat
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