Chapter 3

The Cellular Concept - System DesignFundamentals

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I. Introduction

Goals of a Cellular System High capacity Large coverage area Efficient use of limited spectrum

Large coverage area - Bell system in New York City had early mobile radio Single Tx, high power, and tall tower Low cost Large coverage area - Bell system in New York City had 12

simultaneous channels for 1000 square miles Small # users Poor spectrum utilization

What are possible ways we could increase the number of channels available in a cellular system?

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Cellular concept Frequency reuse pattern

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Cells labeled with the same letter use the same group of channels.

Cell Cluster: group of N cells using complete set of available channels

Many base stations, lower power, and shorter towers Small coverage areas called “cells” Each cell allocated a % of the total number of

available channels Nearby (adjacent) cells assigned different channel

groups to prevent interference between neighboring base

stations and mobile users

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Same frequency channels may be reused by cells a “reasonable” distance away reused many times as long as interference between same

channel (co-channel) cells is < acceptable level As frequency reuse↑ → # possible simultaneous

users↑→ # subscribers ↑→ but system cost ↑ (more towers)

To increase number of users without increasing radio frequency allocation, reduce cell sizes (more base stations) ↑→ # possible simultaneous users ↑

The cellular concept allows all mobiles to be manufactured to use the same set of freqencies

*** A fixed # of channels serves a large # of users by reusing channels in a coverage area ***

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II. Frequency Reuse/Planning

Design process of selecting & allocating channel groups of cellular base stations

Two competing/conflicting objectives:1) maximize frequency reuse in specified area

2) minimize interference between cells

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Cells base station antennas designed to cover specific cell

area hexagonal cell shape assumed for planning

simple model for easy analysis → circles leave gaps actual cell “footprint” is amorphous (no specific shape)

where Tx successfully serves mobile unit

base station location cell center → omni-directional antenna (360°

coverage) not necessarily in the exact center (can be up to R/4

from the ideal location)

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cell corners → sectored or directional antennas on 3 corners with 120° coverage. very commom Note that what is defined as a “corner” is

somewhat flexible → a sectored antenna covers 120° of a hexagonal cell.

So one can define a cell as having three antennas in the center or antennas at 3 corners.

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III. System Capacity

S : total # of duplex channels available for use in a given area; determined by: amount of allocated spectrum channel BW → modulation format and/or standard

specs. (e.g. AMPS)

k : number of channels for each cell (k < S) N : cluster size → # of cells forming cluster S = k N

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M : # of times a cluster is replicated over a geographic coverage area

System Capacity = Total # Duplex Channels = C

C = M S = M k N (assuming exactly MN cells will cover the area)

If cluster size (N) is reduced and the geographic area for each cell is kept constant: The geographic area covered by each cluster is smaller, so

M must ↑ to cover the entire coverage area (more clusters needed).

S remains constant. So C ↑ The smallest possible value of N is desirable to maximize

system capacity.

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Cluster size N determines: distance between co-channel cells (D) level of co-channel interference A mobile or base station can only tolerate so much

interference from other cells using the same frequency and maintain sufficient quality.

large N → large D → low interference → but small M and low C !

Tradeoff in quality and cluster size. The larger the capacity for a given geographic area,

the poorer the quality.

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Frequency reuse factor = 1 / N each frequency is reused every N cells each cell assigned k ≒ S / N

N cells/cluster connect without gaps specific values are required for hexagonal geometry

N = i2 + i j + j2 where i, j 1≧ Typical N values → 3, 4, 7, 12; (i, j) = (1,1), (2,0),

(2,1), (2,2)

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To find the nearest co-channel neighbors of a particular cell (1) Move i cells along any chain of hexagons, then (2)

turn 60 degrees and move j cells.

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IV. Channel Assignment Strategies

Goal is to minimize interference & maximize use of capacity lower interference allows smaller N to be used → greater

frequency reuse → larger C

Two main strategies: Fixed or Dynamic Fixed

each cell allocated a pre-determined set of voice channels calls within cell only served by unused cell channels all channels used → blocked call → no service

several variations MSC allows cell to borrow a VC (that is to say, a FVC/RVC

pair) from an adjacent cell donor cell must have an available VC to give

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Dynamic channels NOT allocated permanently call request → goes to serving base station → goes

to MSC MSC allocates channel “on the fly”

allocation strategy considers: likelihood of future call blocking in the cell reuse distance (interference potential with other cells

that are using the same frequency) channel frequency

All frequencies in a market are available to be used

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Advantage: reduces call blocking (that is to say, it increases the trunking capacity), and increases voice quality

Disadvantage: increases storage & computational load @ MSC requires real-time data from entire network related

to: channel occupancy traffic distribution Radio Signal Strength Indications (RSSI's) from all

channels

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V. Handoff Strategies

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 new voice and control channel frequencies 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

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Minimum useable signal level lowest acceptable voice quality call is dropped if below this level specified by system designers typical values → -90 to -100 dBm

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Quick review: Decibels

S = Signal power in WattsPower of a signal in decibels (dBW) is Psignal = 10 log10(S)

Remember dB is used for ratios (like S/N)dBW is used for Watts

dBm = dB for power in milliwatts = 10 log10(S x 103)dBm = 10 log10(S) + 10 log10(103) = dBW + 30

-90 dBm = 10 log10(S x 103)10-9 = S x 103

S = 10-12 Watts = 10-9 milliwatts-90 dBm = -120 dBW

Signal-to-noise ratio:N = Noise power in Watts

S/N = 10 log10(S/N) dB (unitless raio)

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choose a (handoff threshold) > (minimum useable signal level) so there is time to switch channels before level

becomes too low as mobile moves away from base station and

toward another base station

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Handoff Margin △ △ = Phandoff threshold - Pminimum usable signal dB

carefully selected △ too large → unnecessary handoff → MSC loaded down △ too small → not enough time to transfer → call dropped!

A dropped handoff can be caused by two factors not enough time to perform handoff

delay by MSC in assigning handoff high traffic conditions and high computational load on MSC

can cause excessive delay by the MSC no channels available in new cell

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Handoff Decision signal level decreases due to

signal fading → don’t handoff mobile moving away from base station → handoff

must monitor received signal strength over a period of time → moving average

time allowed to complete handoff depends on mobile speed large negative received signal strength (RSS) slope →

high speed → quick handoff statistics of the fading signal are important to

making appropriate handoff decisions → Chapters 4 and 5

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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

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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

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MSC no longer monitors RSS of all channels reduces computational load considerably enables much more rapid and efficient handoffs imperceptible to user

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A mobile may move into a different system controlled by a different MSC Called an intersystem handoff What issues would be involved here?

Prioritizing Handoffs Issue: Perceived Grade of Service (GOS) – service

quality as viewed by users “quality” in terms of dropped or blocked calls (not

voice quality) assign higher priority to handoff vs. new call request a dropped call is more aggravating than an occasional

blocked call

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Guard Channels % of total available cell channels exclusively set

aside for handoff requests makes fewer channels available for new call

requests a good strategy is dynamic channel allocation (not

fixed) adjust number of guard channels as needed by demand so channels are not wasted in cells with low traffic

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Queuing Handoff Requests use time delay between handoff threshold and

minimum useable signal level to place a blocked handoff request in queue

a handoff request can "keep trying" during that time period, instead of having a single block/no block decision

prioritize requests (based on mobile speed) and handoff as needed

calls will still be dropped if time period expires

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VI. Practical Handoff Considerations

Problems occur because of a large range of mobile velocities pedestrian vs. vehicle user

Small cell sizes and/or micro-cells → larger # handoffs

MSC load is heavy when high speed users are passed between very small cells

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Umbrella Cells Fig. 3.4, pg. 67 use different antenna heights and Tx power levels

to provide large and small cell coverage multiple antennas & Tx can be co-located at single

location if necessary (saves on obtaining new tower licenses)

large cell → high speed traffic → fewer handoffs small cell → low speed traffic example areas: interstate highway passing thru

urban center, office park, or nearby shopping mall

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Cell Dragging low speed user w/ line of sight to base station (very strong

signal) strong signal changing slowly user moves into the area of an adjacent cell without handoff causes interference with adjacent cells and other cells

Remember: handoffs help all users, not just the one which is handed off.

If this mobile is closer to a reused channel → interference for the other user using the same frequency

So this mobile needs to hand off anyway, so other users benefit because that mobile stays far away from them.

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Typical handoff parameters Analog cellular (1st generation)

threshold margin △ ≈ 6 to 12 dB total time to complete handoff ≈ 8 to 10 sec

Digital cellular (2nd generation) total time to complete handoff ≈ 1 to 2 sec lower necessary threshold margin △ ≈ 0 to 6 dB enabled by mobile assisted handoff

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benefits of small handoff time greater flexibility in handling high/low speed

users queuing handoffs & prioritizing more time to “rescue” calls needing urgent

handoff fewer dropped calls → GOS increased

can make decisions based on a wide range of metrics other than signal strength such as also measure interference levels can have a multidimensional algorithm for

making decisions

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Soft vs. Hard Handoffs Hard handoff: different radio channels assigned

when moving from cell to cell all analog (AMPS) & digital TDMA systems (IS-136,

GSM, etc.) Many spread spectrum users share the same

frequency in every cell CDMA → IS-95 Since a mobile uses the same frequency in every cell, it

can also be assigned the same code for multiple cells when it is near the boundary of multiple cells.

The MSC simultaneously monitors reverse link signal at several base stations

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MSC dynamically decides which signal is best and then listens to that one Soft Handoff passes data from that base station on to the PSTN

This choice of best signal can keep changing. Mobile user does nothing for handoffs except

just transmit, MSC does all the work Advantage unique to CDMA systems

As long as there are enough codes available.

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VII. Co-Channel Interference

Interference is the limiting factor in performance of all cellular radio systems

What are the sources of interference for a mobile receiver?

Interference is in both voice channels control channels

Two major types of system-generated interference:1) Co-Channel Interference (CCI)2) Adjacent Channel Interference (ACI)

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First we look at CCI Frequency Reuse

Many cells in a given coverage area use the same set of channel frequencies to increase system capacity (C)

Co-channel cells → cells that share the same set of frequencies

VC & CC traffic in co-channel cells is an interfering source to mobiles in Several different cells

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Possible Solutions?1) Increase base station Tx power to improve radio

signal reception? __ this will also increase interference from co-channel

cells by the same amount no net improvement

2) Separate co-channel cells by some minimum distance to provide sufficient isolation from propagation of radio signals? if all cell sizes, transmit powers, and coverage patterns

≈ same → co-channel interference is independent of Tx power

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co-channel interference depends on: R : cell radius D : distance to base station of nearest co-channel cell

if D / R ↑ then spatial separation relative to cell coverage area ↑ improved isolation from co-channel RF energy

Q = D / R : co-channel reuse ratio hexagonal cells → Q = D/R = 3N

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Fundamental tradeoff in cellular system design: small Q → small cluster size → more frequency

reuse → larger system capacity great But also: small Q → small cell separation →

increased co-channel interference (CCI) → reduced voice quality → not so great

Tradeoff: Capacity vs. Voice Quality

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Signal to Interference ratio → S / I, ____________

S : desired signal power Ii : interference power from ith co-channel cell

io : # of co-channel interfering cells

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Approximation with some assumptions

Di : distance from ith interferer to mobile

Rx power @ mobile ( ) niD

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n : path loss exponent free space or line of sight (LOS) (no obstruction) →

n = 2 urban cellular → n = 2 to 4, signal decays faster

with distance away from the base station having the same n throughout the coverage area

means radio propagation properties are roughly the same everywhere

if base stations have equal Tx power and n is the same throughout coverage area (not always true) then the above equation (Eq. 3.8) can be used.

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Now if we consider only the first layer (or tier) of co-channel cells assume only these provide significant interference

And assume interfering base stations are equidistant from the desired base station (all at distance ≈ D) then

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What determines acceptable S / I ? voice quality → subjective testing AMPS → S / I 18 dB (assumes path loss exponent ≧

n = 4) Solving (3.9) for N

Most reasonable assumption is io : # of co-channel interfering cells = 6

N = 7 (very common choice for AMPS)

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Many assumptions involved in (3.9) : same Tx power hexagonal geometry n same throughout area Di ≈ D (all interfering cells are equidistant from the

base station receiver) optimistic result in many cases propagation tools are used to calculate S / I when

assumptions aren’t valid

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S / I is usually the worst case when a mobile is at the cell edge low signal power from its own base station & high

interference power from other cells more accurate approximations are necessary in those cases

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4 4 42( ) 2( ) 2

S R

I D R D R D

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N =7 and S / I ≈ 17 dB

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Eq. (3.5), (3.8), and (3.9) are (S / I) for forward link only, i.e. the cochannel base Tx interfering with desired base station transmission to mobile unit so this considers interference @ the mobile unit

What about reverse link co-channel interference? less important because signals from mobile antennas (near

the ground) don’t propagate as well as those from tall base station antennas

obstructions near ground level significantly attenuate mobile energy in direction of base station Rx

also weaker because mobile Tx power is variable → base stations regulate transmit power of mobiles to be no larger than necessary

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HW1:

1-9, 1-11, 1-18, 3-5, 3-7