Fundamentals of Cellular Engineering

Prepared By :

Maulik Patel

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

N=i2+ij+ j 2

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 user moves from one cell to the other, to keep the communication between the user pair, the user channel has to be shifted from one BS to the other without interrupting the call

• when a MS moves into another cell, while the conversation is still in progress, the MSC automatically transfers the call to a new FDD channel without disturbing the conversation. This process is called as handoff.

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

Once a signal level is set as the minimum acceptable for good voice quality (Prmin), then a slightly stronger level is chosen as the threshold (PrH)at which handoff has to be made.

A parameter, called power margin, defined as

Δ = PrH − Prmin

If Δ is too small, then there may not be enough time to complete the handoff and the call might be lost even if the user crosses the cell boundary.

If Δ is too high o the other hand, then MSC has to be burdened with unnecessary handoffs. This is because MS may not intend to enter the other cell.

Handoff Generation

Handoff for first generation analog cellular systems• 10 secs handoff time• is in the order of 6 dB to 12 dB

• Handoff for second generation cellular systems, e.g., GSM• 1 to 2 seconds handoff time• mobile assists handoff• is in the order of 0 dB to 6 dB• Handoff decisions based on signal strength, co-channel

interference, and adjacent channel interference.

• 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

CommonChannel

Pool Call completion

Handoff outTo 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

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

The Trade-off :

Enhancing Capacity And Cell Coverage

range (km)

Throughput/cell

(Mbps) 802.11b

Noise Limited

Interference Limited

A-MASBenefit

Technical Interpretation 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

Cell Description Why cell shape is hexagonal…?

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

As users increases per cell the channel capacity decreases

Techniques needed to provide extra channel

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

Such that the radio frequencies are reassigned, and transmission power is reduced

Each with its own base station and a corresponding reduction in antenna height

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

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 load and switching 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

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

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

Influence of the Handoff Process on the Channel Holding Time Distribution for Cellular Systems.Doiningo Lara-Rodriguez. Center for Resenrcli and Advanced Studies. Electricill Engineering

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

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.

Thank You