Fundamentals of Cellular Engineering

Prepared By :

Pratik R. Detroja

Outline

Introduction to cellular system

Concept of frequency reuse

Channel Assignment Strategies

Handoff Process and Generation

Handoff Priority

Enhancing Capacity And Cell Coverage : The key Trade-off

Cell Splitting

Sectoring

Microcell Zone Concept

Reference

Introduction

Goals of a Cellular System : • High capacity

• Large coverage area

• Efficient use of limited spectrum

Reuse of radio channel

Enable a fix number of channels to serve an arbitrarily large number of users by reusing the channel throughout the coverage region

What is cell ?

Each cellular base station is allocated a group of radio channels within a small geographic area called a cell.

Neighboring cells are assigned different channel groups.

By limiting the coverage area to within the boundary of the cell, the channel groups may be reused to cover different cells.

Consider a cellular system which has a total of S duplex channels.

Each cell is allocated a group of k channels, K < S

The S channels are divided among N cells.

The total number of available radio channels, S = kN

The N cells which use the complete set of channels is called cluster.

The cluster can be repeated M times within the system. The total number of channels, C, is used as a measure of capacity.

C = MkN = MS

The capacity is directly proportional to the number of replication M.

The cluster size, N, is typically equal to 4, 7, or 12.

The frequency reuse factor is given by 1/N.

Frequency reuse

Hexagonal geometry has

• exactly six equidistance neighbors

• the lines joining the centers of any cell and each of its neighbors are separated by multiples of 60 degrees.

Only certain cluster sizes and cell layout are possible.

The number of cells per cluster, N, can only have values which satisfy

22 jijiN

Frequency reuse (Cont.)

Channel Assignment Strategies

Goal is to minimize interference & maximize use of capacity.

One of the Channel assignment strategies is

1. Fixed Channel Assignment :

Channels are divided in sets.

A set of channels is permanently allocated to each cell in the network. Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference.

Any call attempt within the cell can only be served by the unused channels in that particular cell. The service is blocked if all channels have used up

Most easiest to implement but least flexibility.

An modification to this is ‘borrowing scheme’. Cell (acceptor cell) tha has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls.

Borrowing can be done in a few ways: borrowing from the adjacent cell which has largest number of free channels, select the first free channel found, etc.

To be available for borrowing, the channel must not interfere with existing calls. The borrowed channel should be returned once the channel becomes free.

FCA(cont.)

2. Dynamic Channel Allocation (DCA) :

Voice channels are not allocated to any cell permanently. All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system.

Each time a call request is made, the serving BS requests a channel from the MSC. It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell, the reuse distance of the channel and other cost functions ⇒ increase in complexity

Centralized DCA scheme involves a single controller selecting a channel for each cell. Distributed DCA scheme involves a number of controllers scattered across the network.

For a new call, a free channel from central pool is selected based on either the co-channel distance, signal strength or signal to noise interference ratio.

DCA(cont.)

Handoff :

when a mobile unit moves from one cell to another while a call is in progress, the MSC must transfer (handoff) the call to a new channel belonging to a new base station

very important task → often given higher priority than new call

It is worse to drop an in-progress call than to deny a new one

Handoff operation :

• identifying a new base station

• re-allocating the voice and control channels with the new base station.

What is Handoff ?

Handoff

Handoff Process

Handoff Threshold

Minimum usable signal for acceptable voice quality (-90dBm to -100dBm)

Handoff margin cannot be too large or too small.

If is too large, unnecessary handoffs burden the MSC

If is too small, there may be insufficient time to complete handoff before a call is lost.

usable minimum,, rhandoffr PP

Handoff must ensure that the drop in the measured signal is not due to momentary fading and that the mobile is actually moving away from the serving base station.

Running average measurement of signal strength should be optimized so that unnecessary handoffs are avoided.

• Depends on the speed at which the vehicle is moving.

• Steep short term average -> the hand off should be made quickly

• The speed can be estimated from the statistics of the received short-term fading signal at the base station

Dwell time: the time over which a call may be maintained within a cell without handoff.

Handoff Process (cont.)

Handoff Generation

Handoff measurement

1st Generation Cellular (Analog FM → AMPS)

• Received signal strength (RSS) of RVC measured at base station & monitored by MSC

• A spare Rx in base station (locator Rx) monitors RSS of RVC's in neighboring cells

Tells Mobile Switching Center about these mobiles and their channels

• Locator Rx can see if signal to this base station is significantly better than to the host base station

• MSC monitors RSS from all base stations & decides on handoff

• 10 secs handoff time

Handoff Generation (Cont.) 2nd Generation Cellular w/ digital TDMA (GSM, IS-136)

• Mobile Assisted HandOffs (MAHO)

• important advancement

• The mobile measures the RSS of the FCC’s from adjacent base stations & reports back to serving base station

• if Rx power from new base station > Rx power from serving (current) base station by pre-determined margin for a long enough time period → handoff initiated by MSC

• 1 to 2 seconds handoff time

IS-95 CDMA spread spectrum cellular system • Mobiles share the channel in every cell.

• No physical change of channel during handoff

• MSC decides the base station with the best receiving signal as the service station

Dropped call is considered a more serious event than call blocking.

Channel assignment schemes therefore must give priority to

handover requests.

A fraction of the total available channels in a cell is reserved only

for handover requests. However, this reduces the total carried

traffic. Dynamic allocation can improve this.

It reduces rate of handoff failure

It is desirable from user’s point of view

Prioritizing Handoff

1. Guard channels concept

2. Queuing handoff requests

Handoff Priority

A Cell

New Calls

Handoff Calls From

neighboring cells

Common Channel Pool

Call completion

Handoff out To neighboring

cells

Wireless Cellular System Traffic in a cell

Guard Channel Method

A fraction of available channels is reserved exclusively for handoff requests

It has disadvantage of reducing total carried traffic

It Offers efficient spectrum utilization when dynamic channel assignment strategies by minimizing number of required guard channels

It reduces number of blocked handoffs

It reduces system capacity

Handoff dropping less desirable than new call blocking!

Handoff call has Higher Priority: Guard Channel Scheme GCS: g channels are reserved for handoff calls.

g trade-off between Pb & Pd

Here, New call blocking probability, Pb

Handoff call dropping probability, Pd

Guard Channel Scheme

When a new call (NC) is attempted in an cell covered by a base station (BS), the NC is connected if an idle channel is available in the cell. Otherwise, the call is blocked

If an idle channel exists in the target cell, the handoff call (HC) continues nearly transparently to the user. Otherwise, the HC is dropped

Loss Formulas

New call blocking probability, Pb : Percentage of new calls rejected

Handoff call dropping probability, Pd : Percentage of calls forcefully

terminated while crossing cells

Guard Channel Scheme (Cont.)

Queuing Handoff Requests

First, Put handoff requests in a queue

Then Serves handoffs on a FCFS basis

It reduces number of failed handoffs

It reduces system capacity

Queuing Handoff Requests (Cont.)

Possible due to time interval elapsed when the signal level drops below to threshold until minimum signal level

Decrease probability of forced termination due to lack of available channels

Tradeoff between decrease in probability of forced termination and total traffic

The delay time and queue size is determined from traffic pattern

Queuing does not guarantee zero probability of call termination since large delays will signal level to drop min

The Trade-off :

Enhancing Capacity And Cell Coverage

range (km)

Throughput/cell

(Mbps) 802.11b

Noise

Limited

Interference

Limited

A-MAS

Benefit Technical Interpretation Gain vs. noise, fading, ... expands envelope to

right Interference mitigation (+ gain) expands it

upwards

Economic Interpretation Coverage improvements reduce CapEx, OpEx (esp. backhaul, sites) Capacity improvements reduce delivery cost, spectrum requirements

The number of channels available to customers (equivalently, the channel density per square kilometer) could be increased by decreasing the cluster size.

It might be that an increase in channel density is required only in specific parts of the system to support an increased demand in those areas.

Cell-splitting is a technique which has the capability to add new smaller cells in specific areas of the system.

Sectoring is basically a technique which can increase the SIR without necessitating an increase in the cluster size.

Microcell zone

Trade-off

Introduction

Why cell shape is hexagonal…?

Cont…

Why Cell Splitting , Sectoring and Microcell zone…..?

As users increases per cell the channel capacity decreases

Techniques needed to provide extra channel

Cell Splitting, Sectoring and Microcell zone increases the capacity

Cell Splitting

In base stations where usage of cellular network is

high, these cell split into smaller cell

Cont..

A new cell site must be constructed when the cell is

split

Each with its own base station and a corresponding

reduction in antenna height

Such that the radio frequencies are reassigned, and

transmission power is reduced

The process of subdividing a congested cell into

smaller cell leads to increase in capacity

Cell splitting is one of the easy and less costly solution

when increasing the capacity of cellular network

Cell Sectoring

Sectorization consist of dividing an omnidirectional

(360 degree)view of cell site into non overlapping

slices called sectoring

To overcome some limitations like co-channel

interference, cell sectoring is done

There are 2 methods for cell sectoring

1) 60 degree

2) 120 degree

Cont…

Replacing a single omnidirectional antenna at base

station with several directional antenna achieves

capacity improvement by essentially rescaling the

system

Advantages

It reduces interference which increases capacity

It enables to reduce the cluster size and provides an additional freedom in assigning channels

Limitations

Increased number of antennas at each base station

Loss of traffic

Since sectoring reduces the coverage area of a particular group of channels, the number of handoffs increases as well

Microcell

As the splitting of cell idea evolves, the usage of

smaller cell becomes efficient and it leads the

creation of microcell

The aim of creating a microcell are increasing the

capacity of cellular networks in areas where

population is very high

Microcell Zone Concept

By the use of sectorization technique, we can increase

the system performance but there will be a large

increment of handoffs which results in the increment of

load on the switching and control link elements of the

mobile system

So a Microcell Zone Concept is introduced which

leads to an increased capacity without any

degradation in trunking efficiency caused by

sectoring

Large control base system is replaced by several lower

powered transmitters on the edge of the cell

Cont…

The mobile retains in the same channel and the base

station simply switches to a different zone site and the

mobile moves from zone to zone

A channel is active only in a particular zone in which

mobile is travelling , base station radiation is localized

and interference is reduced

Reference

Ray, S; Pawlikowski, K; Sirisena, H; , ”Handover in Mobile WiMAX Networks: The State of Art and Research Issues,” IEEE Commun. Surveys & Tutorials , vol.PP, no.99, pp.1-24, 2010

Channel assignment strategies; Srilasak , Wongthavarawat, Limmongkol; wireless Innovation & security Lab., Nat. Electron,.& Comput. Technol. Center, Pathymthani, Thailand..

Adaptive cell sectoring using fixed overlapping sectors in CDMA networks; alagan S. Anpalagan elvino S. Sousa; Department of electrical and computer engineering; University of toronto.

Evolved universal terrestrial radio access (E-UTRA), physical channelsand modulation.3GPP TR 36.211, V.8.5.0, 2008.