1. MITP 413: Wireless Technologies Week 2
Michael L. Honig
Department of ECE
Northwestern University
April 2004

2. Infrastructure vs. Ad-hoc Topologies
Base station
or Access Point
Single-hop peer-to-peer
Multi-hop peer-to-peer
Infrastructure (cellular, LANs)
Ad hoc (wireless LAN/PAN)

3. Infrastructure Configuration
Single Cell
Station A
Station B
Station C
Access Point
Access Point is analogous to a cellular Base Station
System referred to as Infrastructure Basic Service Set (BSS)

4. Peer-to-Peer Configuration
Single Cell
Station C
Station A
A Service Set is a Collection of stations
Station B
Mobile devices are referred to as Stations.
Each Station can communicate directly with another Station.
System referred to as Independent Basic Service Set (IBSS)

5. Extended BSS (EBSS) Configuration
Distribution System (DS)
BSS
Distribution System is a wired network (L2/L3 devices) connecting BSS
Connect APs via a wired network
DS consist of Layer 2, Layer 3 devices
System referred to as Extended BSS
APs have a BSSID
System has a SSID
Basic Service Area

6. Ad Hoc Networks Used originally for military tactical applications
Enables rapid deployment
Multihop provides reliability with unpredictable propagation, node failures
802.11: Terminal searches for a beacon signal from an AP or another terminal. If no beacon, then the terminal provides a beacon for the next terminal. The network is “self-configuring”.
Coverage may be limited to maximum radio link distance (single-hop).
Can be connected to a wired network through a proxy server.
Typically terminals operate in dual-mode (ad hoc or infrastructure).
Multihop routing requires “neighborhood” information.
Multihop increases coverage, but adds delay.
Power control, synchronization are relatively difficult.

7. Cellular Frequency AssignmentsB A C E D G A C B F D G A C B E F D G C B E F D G A C B E D G A C B F G A D
cell cluster

8. Co-Channel Cells A A
i=3, j=2
N=i2+ij+j2=19 A A A A A

9. Co-Channel Reuse Ratio2 1 5 . 4 R 3 7
From hexagonal geometry D 6 2 1 5 . 4 3 7 6

10. First Tier Interference
Cell Site-to-Mobile Interference
(Downlink)
Mobile-to-Cell Site Interference
(Uplink)

11. First Tier Co-Channel CellsD
First Tier 1 R
Interfering Cell

12. SIR vs. Frequency Reuse

13. Worst Case InterferenceD
D+R
D-R D R
D-R
D+R D

14. Sectorization (120o) Cell Site-to-Mobile Interference (Downlink)
Two Interferers in
First Ring per Sector
120o
120o
120o
Cell Site-to-Mobile Interference
(Downlink)
Mobile-to-Cell Site Interfaces
(Uplink)

15. 60o Sectorization Cell Site-to-Mobile Interference (Downlink)
One Interferer in
First Tier per Sector
60o
60o
60o
60o
60o
60o
Cell Site-to-Mobile Interference
(Downlink)
Mobile-to-Cell Site Interfaces
(Uplink)

16. 60o Sectorization: Worst Case Interference2 .
D R . R . M D . 1

17. Growing by Splitting Cell 4 Into
Cell Splitting 2 2 1 5 1 5
(4)
(2)
(3)
(6)
(7)
(5)
(1) 4 1 1 3 7 3 7 6 3 6 3
Growing by Splitting Cell 4 Into
Cells of Small Size

18. Zone Microcells Any channel can be assigned to any zone.
Base
Station
Zone Selector
Microwave or
fiber optic link
Any channel can be assigned to any zone.
No handoff between zones.
Radiation localized, improves S/I.
Highways, urban corridors.

19. Zone Microcells: Co-Channel Distance. D DZ RZ R
3 hexagons (zones) per cell
N=7
DZ/RZ = 2(D/R)
(less interference with same
frequency reuse)

20. Erlang B Curves

21. Channel Allocation
Objective: equalize grade of service (blocking probability) over coverage area  Allows increase in subscriber pool.
Fixed Channel Assignment (FCA): channels assigned to each cell are predetermined.
Separate channels within a cell to avoid adjacent-channel interference
Nonuniform FCA: distribute channels among cells to match averaged traffic load over time.
Channel borrowing: borrow channels from neighboring cell
Temporary: high-traffic cells return borrowed channels
Static: channels are non-uniformly distributed and changed in a predictive manner to match anticipated traffic
Dynamic Channel Assignment (DCA): channels are assigned to each call from the complete set of available channels
Must satisfy S/I constraint
Channels returned to pool after call is completed
Can be centralized (supervised by MSC) or distributed (supervised by BS)
Distributed DCA used in DECT

22. FCA vs. DCA FCA DCA Low complexity Better under heavy traffic
Sensitive to changes in traffic
Variable grade of service
Higher probability of outage
Suitable for macro-cellular systems (e.g., cellular)
Low call setup delay
Requires careful frequency planning
Centralized assignment
Moderate/High complexity
Must monitor channel occupancy, traffic distribution, S/I (centralized)
Better under light/moderate traffic
Insensitive to changes in traffic
Stable grade of service
Low probability of outage (call termination)
Suitable for micro-cellular systems (e.g., cordless)
Moderate/high call setup delay
No frequency planning
Assignment can be centralized or distributed