Channel Assignment

Role of standardizing agency Deciding CH BW Numbering channels Designating channel types

(traffic, access, paging, etc)

Strategies Fixed channel assignment Dynamic channel assignment Hybrid channel assignment Channel borrowing strategies

Governing Factors in grouping the channels

Co-channel Interference

Adjacent channel intereference

CHANNEL ASSIGNMENT

• Fixed Channel Assignment (FCA) – each cell is assigned a fixed number of channels – channels used for both handoff and new calls

• Reservation Channels with FCA – each cell reserves some channels for hand off calls

• Channel Borrowing – a cell may borrow free channels from neighboring cells

• Dynamic Channel Assignment

7C29822.67-Cimini-9/97

Channel Assignment Strategies Fixed Channel Assignments

Each cell is allocated a predetermined set of voice channels.

If all the channels in that cell are occupied, the call is blocked, and the subscriber does not receive service.

Variation includes a borrowing strategy: a cell is allowed to borrow channels from a neighboring cell if all its own channels are occupied.

This is supervised by the Mobile Switch Center: Connects cells to wide area network; Manages call setup; Handles mobility

Channel Assignment Strategies Dynamic Channel Assignments

Voice channels are not allocated to different cells permanently.

Each time a call request is made, the serving base station requests a channel from the MSC.

The switch then allocates a channel to the requested call based on a decision algorithm taking into account different factors: frequency re-use of candidate channel and cost factors.

Dynamic channel assignment is more complex (real time), but reduces likelihood of blocking

Mobility support

1. Maintain the roaming connectivity (mobility management, routing...)

2. Maintain the continuation of com- munication in progress when crossing cells (i.e., handoff).

Handoff in cellular networks

- Handoff denotes the process of changing the channel associated with a connection in progress as a mobile move to the coverage of a new cell.

f1 f2

Movement direction

Handover/handoff Reasons for handover

Moving out of range Load balancing

Cell, BSC (base station controller), MSC (mobile switching center)

Handover scenarios Intra-cell handover (e.g., change frequency due

to narrowband interference) Inter-cell, intra-BSC handover (e.g., movement

across cells) Inter-BSC, intra-MSC handover (e.g., movement

across BSC) Inter MSC handover (e.g., movement across

MSC)

Four Types of Handoff

MSC MSC

BSC BSCBSC

BTS BTS BTSBTS

MS MS MS MS

12 3 4

Handoffs important task in any cellular radio system must be performed successfully,

infrequently, and imperceptible to users. identify a new base station channel allocation in new base station high priority than initiation request (block

new calls rather than drop existing calls)

Handoff trigger

f1 f2

A B C D E

A B C D E

Signal strength from BS_2

BS_1 BS_2

Signal strength fromBS_1

Decisionpointtime

Ref: - T.S. Rappaport, “Wireless communications, principles & practice”, ISBN 0133755363, Prentice-Hall, 1996, pp31-37.

Handoff threshold selection

Handoff margin =Handoff threshold - Minimum acceptable

signal to maintain the call

too small: Insufficient time to complete handoff before call is

lost More call losses

too large: Too many handoffs Burden for MSC

Styles of Handoff Network Controlled Handoff (NCHO)

in first generation cellular system, each base station constantly monitors signal strength from mobiles in its cell

based on the measures, MSC decides if handoff necessary mobile plays passive role in process burden on MSC

Mobile Assisted Handoff (MAHO) present in second generation systems mobile measures received power from surrounding base stations

and report to serving base station handoff initiated when power received from a neighboring cell

exceeds current value by a certain level or for a certain period of time

faster since measurements made by mobiles, MSC don’t need monitor signal strength

Mobile Controlled Handoff

Types of handoff

Hard (FDMA and TDMA systems)(both BS and carrier frequency are changed)

Soft (CDMA systems)(BS (thus PN) is changed but not the carrier frequency, its smoother as MS was already processing the signals of this new BS thus it takes place by selecting appropriate receiver.)

Softer (CDMA systems)(between sectors of same BS thus neither BS nor Carrier frequency is changed but only PN is changes)

Hard handoff - (break before make) FDMA, TDMA mobile has radio link with only one BS at anytime old BS connection is terminated before new BS connection is made.

Soft handoff (make before break) CDMA systems mobile has simultaneous radio link with more than one

BS at any time new BS connection is made before old BS connection is

broken mobile unit remains in this state until one base station

clearly predominates

Performance issues during handoff

• Call dropping probability: Probability that a handoff call is dropped.

• Handoff processing time: Time used to complete a handoff process.

• Handoff frequency: Depends on cell size and handoff trigger schemes.

• Unnecessary handoff events: Handoff that could have been avoided. Depends on handoff trigger schemes.

Prioritizing handoff Dropping a call is more annoying than line busy Guard channel concept

Reserve some channels for handoffs Waste of bandwidth But can be dynamically predicted

Queuing of handoff requests There is a gap between time for handoff and time to

drop. Better tradeoff between dropping call probability and

network traffic. Reduce the burden for handoff

Cell dragging Umbrella cell

Cellular networks: Examples

Marco cell

Base-station

Mobile user

Cellular networks: Examples

Small cells

Umbrella Cells

Roaming All cellular systems provide a service called

roaming. This allows subscribers to operate in service areas other than the one from which service is subscribed.

When a mobile enters a city or geographic area that is different from its home service area, it is registered as a roamer in the new service area.

Registration MSC polls for unregistered mobiles Mobiles respond with MINs MSC queries mobile’s home for billing info

Calls MSC controls call and bills mobile’s home

Trunking theory

A system which has limited number of resources is known as a trunk system (telephone, mobile, etc)

Trunking theory is about serving a large number of users in a trunk system and finding various performance measures.

Two types of trunk systems are used Blocked Calls Cleared (Erlang-B) Blocked Calls Delayed (Erlang-C)

Various terms Service rate

(inverse of holding time; number of requests served in unit time)

1- Erlang (a channel is said to have 1-erlang traffic if it is busy all the time)

Traffic intensity (it is a ratio of request rate to service rate measured in erlang)

Blocked/lost call (an arrived request which cannot be completed, as all resources are busy)

Request Rate (avg. number of requests per unit time)

Holding time (avg. duration of a call)

(Ability of a user to get service in busiest hour. In telephone system, it is measured as blocking probability of a call.)

In cellular mobile systems dropping of calls should also be considered in defining GOS.

Trunking efficiency : measure of no. of users which can be offered a particular GOS with a particular configuration of channel.

HO calls have greater priority over new calls in cellular systems

GOS = Pbnew+ a.Pbho ; a is any suitable constant

Grade of Service

Trunking Theory Each user generates a traffic intensity of Au Erlangs.

Au=H, where H is the average duration of a call and is the average number of call requests per unit time for each user

A=UAu, where U no. of users and A is the total offered traffic intensity

Ac=UAu/C, where C is the number of channels Offered traffic >= the traffic carried by the trunked system

First type of trunked system Blocked calls cleared: no queuing for call requests, no

setup time M/M/m/m: memory-less, Poisson arrival, exponential

service, finite channels. Erland B formula GOS

KA

blockingC

k

kCAC

0

!

!

)Pr(

Blocked Calls Delayed Blocking calls are delayed until channels are

available, queuing

Erlang C

1

0 !)1(!

)0Pr(C

k

k

CAC

C

kA

CA

Adelay

Blocked Calls delayed system

P [call delayed] = Erlang-C formula

P [wait>t|call delayed]= e-(C-A)t/H

P[wait>t]= P [wait>t|call delayed].P[call delayed]

Average delay = ∫(0 to ∞)P[wait>t]dt

= P[call delayed] H/(C-A)

Approaches to Increasing Capacity Frequency borrowing

frequencies are taken from adjacent cells by congested cells

Cell splitting cells in areas of high usage can be split into

smaller cells Cell sectoring

cells are divided into a number of wedge-shaped sectors, each with their own set of channels

Microcell Zoning antennas move to buildings, hills, and lamp posts

Cell Splitting Subdivide a

congested cell into smaller cells

Each with its own base station, reduction in antenna and transmitter power

More cells more clusters higher capacity

Achieves capacity improvement by essentially rescaling the system.

Cell splitting from radius R to R/2

Cell Splitting Example

Power reduction is 16 times and 12 dB for half cell radius for a propagation factor of 4.

Antenna downtilting

Umbrella cell Example 3.8

Sectoring In basic form, antennas are omni-directional Replacing a single omni-directional antenna at base station

with several directional antennas, each radiating within a specified sector achieves capacity improvement by essentially rescaling the system.

less co-channel interference, number of cells in a cluster can be reduced, more clusters, larger capacity

Sectoring Examples Only two co-channel

cells S/I improvement 7.2dB Capacity 12/7 Intracell handoff Trunking efficiency low Urban area not good Example 3.9

Repeater Extend coverage range Directional antenna or distributed antenna systems

Micro Cell Zone

Superior to sectoring, any base station channel may be assigned to any zone by the base station

Same channel No handoff Only the active

zone

Micro Cell Zone Concept Large control base station is replaced by several

lower powered transmitters on the edge of the cell.

The mobile retains the same channel and the base station simply switches the channel to a different zone site and the mobile moves from zone to zone.

Since a given channel is active only in a particular zone in which mobile is traveling, base station radiation is localized and interference is reduced.

Example For N =7, D/R = 4.6

gives SIR = 18 dB

In zoning concept, Dz/Rz should be 4.6 for SIR = 18 dB D/R is 3.

N =3 , down from 7

2.33 times increase in capacity