OPTICAL ARCHITECTURES FOR
MOBILE BACK- AND FRONTHAULING
Thomas Pfeiffer, Frank Schaich - Alcatel-Lucent Bell Labs Stuttgart
OFC/NFOEC wireless backhauling workshop - Los Angeles, 5.3.2012
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Backhauling or fronthauling ?
metro cell
EPC : Evolved Packet Core
BBU : Baseband Unit
RAN : Radio Access Network
cloud RAN
(200 m diam.)
centralized
BBU
CPRI
fronthaul
macro cell
IP backhaul
(1 km diam.)
IP backhaul
EPC
conventional RAN
core network
IP backhaul or CPRI fronthaul ?
=
conventional RAN or cloud RAN ?
… most likely both of them
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Choice of transmission technology : optical only
Fiber transmission systems
• protocol :
IP, CPRI, others
• multiplexing :
TDM, WDM, TWDM, …
• topology :
ptp, ptmp, ring
• architecture:
dedicated ? overlay ? shared with FTTx ?
(digital; RoF not considered here)
direct or over PON, Ethernet, …
Metrics
• technical metrics :
- bandwidth (scaleability, user statistics), latency, jitter
- environmental factors : temperature, humidity, mechanical
- location factors : availability of local powering, footprint, accesseability
• economic metrics :
- infrastructure : ownership, availability of dark fibers, digging cost, leasing cost, opportunity for sharing
- location factors : power supply and power consumption, rights of way
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Backhauling and fronthauling bandwidth in LTE
IP peak bandwidth per site
CPRI bandwidth per site
100000
total
1000
total
Mbps
Gbps
3 sectors
MHz
3 sectors
typ. for macro cell
100
10000
MHz
typ. for macro cell
100
100
40
40
20
20
10
10
10
1000
* 8/15
in case of
WCDMA
1
0,1
100
1
# antennas / sector
1
10
IP backhauling = variable bitrate
- antennas may be grouped (e.g. beamforming) :
each group counts as one single element
# antennas / sector
10
CPRI fronthauling = constant bitrate
- each antenna counts separately (individual streams)
- 8B/10B can be removed for transport over Ethernet
- user traffic statistics apply :
- shown above are achievable peak rates on air i/f
- avged. values may be less by an order of mag
- compression can be applied to reduce to 1:3
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Impact from traffic statistics
Backhaul and fronthaul network dimensions and architecture shall account for
traffic statistics
• traffic statistics per cell
 statistical multiplex gain on IP backhaul
• variations of total cell traffic over the day
 load sharing (pooling gain) in cloud RAN
taken from Alcatel-Lucent Technology lightRadioTM White Paper „Economic analysis“ (2011)
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Latency in LTE : limited by synchronous UL HARQ
The allowed RRH
eNB transmission time is limited to <<1 msec
It comes at the expense of a reduced processing time in the eNB
UE
UE
eNB
n
RRH
eNB
round trip time
(10 µsec / km)
+ transport system
processing time
n
3 msec fixed delay
defined by LTE standard
eNB processing
1. PHY: UL frame decoding
2. MAC: ACK/NACK creation
3. PHY: DL frame creation
n+4
n+4
reduced time for
eNB processing
n+8
n+8
t [ms]
t [ms]
t [ms]
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t [ms]
t [ms]
IP backhaul by 10G-PON : urban area, macro cells
Serving area around traditional CO
• 32 macro cells, backhauled by single dedicated 10G-PON
- peak rate = 10Gbps per site;
 sufficient even for extreme loads
- average rate = 320 Mbps per site
 can be increased by using multiple 10G-PONs, WDM-stacked
- link length = 20 km
 reaches any site within the area over realistic cable routes
Central Office
CO
macro cell:
diam. 1 km
Router
serving area:
diam. 6 km
max. 20 km
eNBs
1
2
1
ONT
OLT
4
power
splitter
8
power
splitter
Possible migration towards serving from consolidated Super-CO
•
via WDM stacking : hybrid WDM/TDM long reach 10G-PONs
(cf. PIEMAN, MUSE, SARDANA for example architectures + upcoming NGPON2 standardisation for specs (tbd) )
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IP backhaul (ct‘d) : urban area, macro + metro cells
serving area:
diam. 6 km
macro cell:
diam. 1 km
metro
cells
CO
Scenario: serving area around CO with
- 32 macro cells: 10G peak / 320M avge.
(3 sectors * 8 antennas * 100 MHz)
 10G-PON, 1:32 split
(XGPON1 or XGPON2)
- 16 metros per macro : 1.7G peak / 26M avge.
 8 x GPON, 1:64
split each
(1 sector * 4 antennas * 100 MHz)
(stacking via low cost WDM)
low cost WDM-PON by cyclic wavelength allocation within 40 nm band
 cf. Pöhlmann, Pfeiffer: ECOC 2011, paper We.9.C.1
Central Office
GPON #1 … #8
Router
10G
PON
OLTs
max. 20 km
WDM1r
(diplexer)
1
2
1
l 10G +
lj GPON
4
power
splitter
cyclic
AWG
eNBs
8
hybrid splitter:
10G - power splitter
GPON - cyclic AWG
10G-PON
dipl
exer
power
splitter
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macro (32 x)
1
2
GPON
16
metro (8 x 64)
macro area
Centralized processing : variants and benefits
• BBU clustering : move BBU hardware from BTS into common central space
- simplified hardware
at antenna sites (footprint, electrical power) and in BBU (indoor specs)
- „zero latency“ links between BBUs allow for implementing CoMP and ICIC algorithms
• BBU pooling : share hardware elements between multiple colocated BBUs
- additional benefit : ease of load-sharing between clusters
• Either variant requires CPRI links to remote antenna sites
- transmission bandwidths easily reach levels that render TDM-PON unattractive
- small split factors (1:2 or 1:4)
- constant bitrate, i.e. no statistical multiplex gain
- strict latency limits (<<1 msec) require zero framing/buffering etc. delays
- most viable solutions employ ptp-links via
- fiber, if available …
- wavelength : ptp-WDM overlay on TDM-PON or „pure“ WDM-PON
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CPRI fronthaul via WDM overlay on LR-PON
macro BS
fronthauling
X/GPON overlay
guard
guard
GPON
DS
*
FH-DWDM
metro BS
fronthauling
XGPON1
(ACCORDANCE project)
DWDM Tunsbles
MCO … Metro Central Office
RN … remote node
EDFA
US
MCO 2
eNB
L1
RN
L1
RN
1400
1450
1500
1550
1600
FH-DWDM
1350
GPON
1300
10-20km
32...64 subs
OLT
XGPON1
1260
MCO 1
SMCO
l/nm
wate
rpeak
L2
RN
R-,BONUs
L3
RN
R-,BONUs
L1
RN
R-,BONUs
L1
RN
10-20km
MCO 4
metro mux
MCO 3
street level coupler
fixed/mobile coupler
Metro Cell TRX (CWDM)
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Macro Cell TRX (DWDM)
(to scale)
* ... being stripped before
fixed-services ONUs
Enable BBU pooling, but not via CPRI : alternatives
IP backhauling
EPC
PDCP
RLC
BIP
MAC
PHY
RF
classical eNB
variable
split within L2
core
network
EPC
PDCP
RLC
MAC
MAC
≥ BIP
central unit (cluster)
PHY
RF
slim eNB
variable
split within L1
core
network
EPC
PDCP
RLC
MAC
PHY
PHY
e.g. 0.2 * BCPRI
extended RRH
CPRI fronthauling
core
network
EPC
PDCP
RLC
MAC
PHY
CPRI
CPRI
BCPRI
fixed
central unit (cluster)
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RRH
RF
- simpler remote unit
- possible pooling gains
fixed
central unit (cluster)
RF
increased optical link bandwidth
core
network
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Back-Up
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Conventional Approach
Example XG-PON1 upstream, 4 Wavelength Subbands SB1 – SB4
SB2
SB1
1260nm
1265nm
SB4
SB3
1270nm
1275nm
1280nm
Wavelength band is separated in four subbands for wavelength stacking
Randomly distributed DFB laser wavelengths in the 20nm band
1260nm
1265nm
1270nm
1275nm
1280nm
DFB laser wavelength can be tuned by heating or cooling by ≈ 0.08nm/K. Tuning range up to 3nm.
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TWDM 40/10G with ultra-low cost WDM upstream
(ALU proposal, ECOC 2011)
WSDM (wavelength set division multiplexing)
Operational principle:
- cyclic optical filter at Rx, 50 or 100 GHz grid
- narrow range Tx tuneability instead of full band
- accomplished by integrated heater stripe (no TEC)
- otherwise conventional transmitter technology
wavelength sets
WS4
WS3
WS2
WS1
I1
50 GHz
heating
CO
feeder
RN
wavelength set
OLT 1
OLT 4
ONU 2
wavelength sets
…
OLT 2
OLT 3
ONU 1
Cyclic
WDM
…
Downstream : 4 x 10G TDM
DWDM channels, 100GHz spacing, 1575-1580nm band
- OLT : l-stabilised DFB transmitter
- ONU : FP based tunable filter
Upstream : 4 x 2.5G TDMA
wavelength sets, 50GHz grid, 1260-1280nm band
- OLT : filtered with cylical AWG
- ONU : partially tunable DFB with integrated heater
…
ONU n
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