1. Multi-tier Topologies in Future Wireless Broadband Networks
IEEE Presentation Submission Template (Rev. 9)
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IEEE C /0004
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Kerstin Johnsson, Shilpa Talwar, Nageen Himayat, S. Yeh
Intel Corporation
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San Diego, CA, USA
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For discussion in the Project Planning Adhoc
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2. Trend towards personal and mobile devices
Device Perspective
Macro- BS (1km)
Micro- BS (500m)
Pico-BS (300m)
Communications evolution lags computing
Femto-AP
(100m)
Soft-AP
(WiFi/ WiMax)
MID
(Mobile Hotspot/ Mobile PAN)
Personalization
Mobility
Mainframe
We started this process by looking at this area through the view of the 2010 user – here is the question we decided that user would face
- As a likely technology use leader you may well already seeing the beginnings of this
Laptop
Netbook
Desktop
Trend towards personal and mobile devices

3. Multi-tier Networks Spectrum Utilization
Overlay multiple tiers of cells, macro/pico/femto, potentially sharing common spectrum
Client can be viewed as center of additional tier (see client co-op)
Tiers can be heterogeneous (WiFi)
WiFi-AP
(Offload macro-BS)
Macro-BS
Femto-AP
(Indoor coverage & offload macro-BS)
Pico-BS
(Areal capacity)
Client Cooperation
Wired backhaul
Relay
Wireless backhaul
Coverage Hole

4. Advantages of Multi-tier Networks
Significant gains in Areal Capacity through aggressive spectrum reuse and use of unlicensed bands
Cost structure of smaller cells (Pico & Femto) is more favorable
Indoor coverage is improved through low cost femto-cell
Small Cell Scenario
Areal capacity gain* from spectral efficiency improvement and increase spatial reuse
Sparse FAP deployment
Dense FAP deployment
Public
Private
FAP Tx Power
0 dBm 34
152
10 dBm 36 38
144
154
20 dBm 35 39
135
Capacity gain comes from:
Spectral Efficiency Improvement
Spatial reuse of frequency
Ex: 150x ~ 1.5(average SE gain) x 100 FAP’s
*Areal capacity gain = (System Capacity with Femto-APs deployed) / (System Capacity without Femto-APs)
Source: Johansson at al, ‘A Methodology for Estimating Cost and Performance of Heterogeneous Wireless Access Networks’, PIMRC’07.
Significant savings in Cost per Bit from Multi-tier Networks

5. Challenge: Inter-tier Interference
Need to protect control & data signals from inter-tier interference
Femto-cells cause significant INT to macro-users and other femto-cells
Power control improves interference only slightly
Macro and femto on diff carriers prevents INT, but lowers throughput and significantly decreases trunking efficiency and RRM flexibility
Simple FFR on macro & femto reduces INT; but more sophisticated FFR and/or Femto-Free Zones (FFZ) required to fully protect macro-users
Tx Scheme
Max FAP Tx Pwr
Outdoor Outage (%)
Indoor Outage (%)
50% Outdoor rate (Mbps)
50% Indoor rate (Mbps)
No FAP 25
31.8
0.06
0.05
Co-channel (10MHz)
-10dBm
38.2
8.6
6.4
0dBm
61.9
2.2
14.3
Tx Scheme
Outdoor Outage (%)
Indoor Outage (%)
50% Outdoor rate (Mbps)
50% Indoor rate (Mbps)
FFR + NO FAP on 10 MHz
3.0
17.0
0.07
0.03
FFR Macro on 5 MHz, Femto on diff 5 MHz
0.2
0.06
10.7
FFR + FFZ + 0dBm FAP power on 10 MHz
0.5
11.3
Interference reduction for Control Signals remains unresolved

6. Challenge: Mobility
Handovers across small cells can be highly inefficient
Intelligent handover mechanisms required to determine when intra- or inter-tier handover is beneficial
Example 1: If a macro-user moves into the coverage area of a high data rate femto-cell, the first instinct is to handover. However, the benefit/cost of handover depends on the user’s mobility.
Example 2: Although a femto-user may have better channel quality to the macro-ABS, it may benefit from remaining associated to the femto-ABS if the femto load is significantly less.
handoff

7. Challenge: Network Management
Scalability is key feature in multi-tier networks
Self-organization and management across tiers will be crucial to maintaining low OPEX and quick network response
May facilitate network management to merge network elements
Need to consider new network elements
Example: what is the optimum middle ground between consumer owned & deployed private femto-AP (low cost) versus operator owned & deployed public pico-BS?

8. Summary & Recommendations
Multi-tier networks promise significant improvements in total network, average user, and indoor user throughput
However, to realize these gains, next generation standard should develop protocols to control interference across network tiers, perform handover intelligently, and manage network elements efficiently.