1. OPTICAL ARCHITECTURES FOR MOBILE BACK- AND FRONTHAULING
Thomas Pfeiffer, Frank Schaich - Alcatel-Lucent Bell Labs Stuttgart OFC/NFOEC wireless backhauling workshop - Los Angeles,

2. Backhauling or fronthauling ?
EPC : Evolved Packet Core BBU : Baseband Unit
RAN : Radio Access Network
core network
metro cell (200 m diam.)
EPC
centralized BBU
IP backhaul
CPRI fronthaul
macro cell (1 km diam.)
conventional RAN
cloud RAN
IP backhaul or CPRI fronthaul ?
= conventional RAN or cloud RAN ? … most likely both of them

3. Choice of transmission technology : optical only
Fiber transmission systems
protocol : IP, CPRI, others (digital; RoF not considered here) direct or over PON, Ethernet, …
multiplexing : TDM, WDM, TWDM, …
topology : ptp, ptmp, ring
architecture: dedicated ? overlay ? shared with FTTx ?
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

4. Backhauling and fronthauling bandwidth in LTE
IP peak bandwidth per site
typ. for macro cell
CPRI bandwidth per site
* 8/15 in case of WCDMA
typ. for macro cell
IP backhauling = variable bitrate
antennas may be grouped (e.g. beamforming) : each group counts as one single element
- user traffic statistics apply : shown above are achievable peak rates on air i/f avged. values may be less by an order of mag
CPRI fronthauling = constant bitrate - each antenna counts separately (individual streams) - 8B/10B can be removed for transport over Ethernet
compression can be applied to reduce to 1:3

5. 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)

6. 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 n
Orig. TX
n+4
n+8
NACK
1st RTX UE
eNB
t [ms]
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
Orig. TX
n+4
n+8
NACK
1st RTX UE
eNB
reduced time for eNB processing
t [ms]
RRH
round trip time (10 µsec / km) + transport system processing time

7. 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
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) )
max. 20 km
power splitter
Central Office
eNBs 1 4
Router 2 8
OLT
ONT CO
serving area: diam. 6 km
macro cell: diam. 1 km

8. IP backhaul (ct‘d) : urban area, macro + metro cells
Scenario: serving area around CO with
32 macro cells: 10G peak / 320M avge.  10G-PON, 1:32 split (3 sectors * 8 antennas * 100 MHz) (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 CO
serving area: diam. 6 km
macro cell: diam. 1 km
metro cells
max. 20 km
hybrid splitter: 10G - power splitter GPON - cyclic AWG
Central Office
eNBs 1 2 8
metro (8 x 64)
macro (32 x)
10G-PON
GPON
macro area 16
dipl exer
power splitter
l 10G + lj GPON 4
Router
WDM1r (diplexer)
GPON #1 … #8
10G PON
cyclic AWG
OLTs

9. 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

10. CPRI fronthaul via WDM overlay on LR-PON (ACCORDANCE project)
MCO … Metro Central Office
RN … remote node

11. Enable BBU pooling, but not via CPRI : alternatives
IP backhauling
core network
EPC
classical eNB
PDCP RLC RF
MAC
PHY
BIP variable
increased optical link bandwidth
split within L2
core network
EPC
central unit (cluster)
PDCP RLC
MAC
slim eNB RF
PHY
≥ BIP variable
split within L1
core network
EPC
central unit (cluster)
PDCP RLC
MAC
PHY
extended RRH RF
e.g. 0.2 * BCPRI fixed
CPRI fronthauling
core network
EPC
central unit (cluster)
PDCP RLC
MAC
PHY
CPRI
RRH RF
BCPRI fixed
- simpler remote unit - possible pooling gains

13. Back-Up

14. Conventional Approach Example XG-PON1 upstream, 4 Wavelength Subbands SB1 – SB4
1260nm nm nm nm nm
Wavelength band is separated in four subbands for wavelength stacking
Randomly distributed DFB laser wavelengths in the 20nm band
1260nm nm nm nm nm
DFB laser wavelength can be tuned by heating or cooling by ≈ 0.08nm/K. Tuning range up to 3nm.

15. TWDM 40/10G with ultra-low cost WDM upstream (ALU proposal, ECOC 2011)
wavelength sets
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
Downstream : 4 x 10G TDM DWDM channels, 100GHz spacing, nm band
- OLT : l-stabilised DFB transmitter
- ONU : FP based tunable filter
Upstream : 4 x 2.5G TDMA wavelength sets, 50GHz grid, nm band
- OLT : filtered with cylical AWG
- ONU : partially tunable DFB with integrated heater