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

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)

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)

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)

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

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.

Cellular Frequency Assignments B 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

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

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

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

First Tier Co-Channel Cells D First Tier 1 R Interfering Cell

SIR vs. Frequency Reuse

Worst Case Interference D D+R D-R D R D-R D+R D

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)

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)

60o Sectorization: Worst Case Interference 2 . D + 0.7 R . R . M D . 1

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

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.

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)

Erlang B Curves

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

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