Cellular Wireless NetworksCellular Wireless Networks

An Important TechnologyAn Important TechnologyAn Important TechnologyAn Important Technology“So called Face of 21st century communication”

Cellular telephony is one of the fastest growing technologies on the planet.

A new mobile generation has appeared every 10th year since ─ the first 1G analog system (NMT/AMPS) was introduced in 1981 ─ the 2G (GSM) digital system started to roll out in 1992─ the 2G (GSM) digital system started to roll out in 1992─ the 3G (WCDMA/UMTS) digital system appeared in 2001─ the 4G technology and 4G LTE smartphones/tablets appeared in 2011

W 5G f il f d d b i l d i ld lik l b f h Were a 5G family of standards to be implemented, it would likely before the year 2020 as analysts estimate.

AbbreviationsNMT=Nordic Mobile Telephone // in Nordic countriesAMPS=Advanced Mobile Phone System // developed by Bell Labs in USAGSM=Groupe Spécial Mobile Global System for Mobile CommunicationsWCDMA=Wideband Code Division Multiple AccessWCDMA Wideband Code Division Multiple AccessUMTS=Universal Mobile Telecommunications System LTE = Long Term Evolution

Beyond VoiceSt t t i f tiStore contact informationSend/receive short text messagesSend/receive emailSend/receive picturespE-commerceMake task/to-do listsKeep track of appointmentsCalculatorCalculatorSend/receive video clipsGet information from the internetPlay gamesGPS servicesIntegrate with other devices (PDA’s, MP3 Players, etc.)

SmartphonesSmartphonesA modern smartphone coming out these days can easily incorporate an impressive host of electronic features including

A d l & hi f SIM d A dual-core processor & chips for two or more SIM cards WiFi interface for wireless Internet connectivity Touch screen for augmented reality GPS for geo-location trackingg g One or more Camera with CMOS sensor for video recording and mobile video

conferencing Bluetooth for connectivity with other wireless devices Java platform for developing and running software applications Java platform for developing and running software applications A sophisticated communication technology such as spread spectrum for cellular

bandwidth access

Smartphones keep getting smarter and smarter and they could evolve in the not-too-distant future to incorporate a variety of new sensors for measuring temperature, ambient light, air quality, levels of hazardous chemicals, carbon monoxide build-ups, and all other types of data that will enable a distributed sensing revolution that is set to sweep the world.

Early Cellular NetworksEarly Cellular Networks

Cellular system developed to provide mobile telephony: y p p p ytelephone access “anytime, anywhere.”

First mobile telephone system was developed andFirst mobile telephone system was developed and inaugurated in the U.S. in 1945 in St. Louis, MO.

This was a simplified version of the system used today.

First Mobile Telephone SystemFirst Mobile Telephone System

One and only onehigh power base station with which allusers communicate.

E ti CNormal

Telephone Entire Coverage Area

TelephoneSystem

Wired connectionWired connection

Terminologies:Terminologies: Base Station and Mobile StationBase Station and Mobile StationTerminologies: Terminologies: Base Station and Mobile StationBase Station and Mobile Station

•• Base station (BS)Base station (BS) downlink– Access point (AP)

•• Mobile station (MS)Mobile station (MS)– SS (Subscriber station)

uplinkSS (Subscriber station)

– MT (mobile terminal)– MN (mobile node)

BS MSA logical channel used by a mobile•• Downlink frequencyDownlink frequency fDL

– Forward link– BSMS

A logical channel used by a mobile phone consists of two physical

frequencies: a downlink frequency fDLand an uplink frequency fUL.

•• Uplink frequency Uplink frequency fUL– Reverse link

MSBS

Channel Duplex Distance = fDL - fUL

– MSBS

Terminologies: Cell and SectorTerminologies: Cell and SectorTerminologies: Cell and SectorTerminologies: Cell and Sector

•• CellCell– Coverage area of a BS

•• SectorSector– Partial area of a cell that is

served by a directional antenna

Terminologies:Terminologies: TessellationTessellationTerminologies: Terminologies: Tessellation Tessellation • A group of small shapes tessellate an area if they cover this

area without any gaps or overlapsarea without any gaps or overlaps.• Three regular polygons that easily tessellate:

– Equilateral triangleq g– Square– Regular Hexagon

TrianglesSSquares

Hexagons

Circular Coverage AreasCircular Coverage Areas

Original cellular systems were developed assuming that base station antennas are omnidirectional i e they transmit in allstation antennas are omnidirectional, i.e., they transmit in all directions equally. Users located far away from the base station receive weak signals or may not be covered. Ideally, a base station has a circular coverage area.

Circles Don’t TessellateCircles Don t Tessellate

• Ideally base stations have identical, circular coverage areas.y , g• Problem: Circles do not tessellate.

• The regular polygon closest to a circle that tessellates well isThe regular polygon closest to a circle that tessellates well is the hexagon.

• Thus, early researchers started using hexagons to represent the coverage area of a base station, i.e., a cell.

Thus the Name CellularThus the Name Cellular

• With hexagonal coverage area, a cellular network is drawn as shown below:

B St ti=Base Station

• Since the network resembles cells from a honeycomb, the name cellular was used to describe the resulting mobile telephone networktelephone network.

Terminologies: HandoffTerminologies: HandoffTerminologies: HandoffTerminologies: Handoff

•• HandoffHandoff– MS changes its serving BS due to movement or radio

channel variation

Architecture of Cellular NetworksArchitecture of Cellular NetworksBSC

BS

Wire-line WAN ..BS

MSC ..BSC

BSC

BSMSC

BSC..BS

BSBS .BS

BSBSBS

MS = mobile stationAir-Um

MS BS = base stationBSC = base station controllerMSC = mobile switching center

Cellular Network

BSCBS

Wire-line WAN..BS

MSC

.BSC

BS

BSC

MSC

..BSBSC

.BS

BSBSBS = base station

BSC = base station controller

MSC = mobile switching center

• Handoff Dropping– the cell does not have a free channel and declines to

accept the incoming handoff request; the call is disconnected.

• New Call Blockingg– the cell does not have a free channel and declines to

accept a new call generated inside the cell.H d ff t i i it llHandoff requests are given priority over new calls

BS2 BS1

Cellular Networks

BS1 BS2 BS3 BS4LAN/WAN

of base BS1 BS2 BS3 BS4of base stations

mobile

Channel ReuseChannel Reuse

Example of Frequency ReuseExample of Frequency ReuseTo prevent interference, cells that are near each other use different sets of radio frequencies. A radio frequency can be used in two different cells if they are separated by a sufficient distance called the channel reuse distanceseparated by a sufficient distance, called the channel reuse distance.

Cells using the same frequencies have the same color

I ll l t l di t d h l i t fIn cellular systems, a large reuse distance reduces co-channel interference while a small reuse distance can provide coverage to more users.

Adjusting Cell SizeAdjusting Cell Size

Channel Reuse: Macro Micro and Pico CellChannel Reuse: Macro, Micro and Pico Cell

Macro (Rural Town) BS1 BS2 BS3

Micro(City)

Pico(Downtown)

BS gets 2 types of requests – new call request and handoff request

Cellular Networks

GSM (Global System for Mobile Communication)GSM (Global System for Mobile Communication)GSM (Global System for Mobile Communication)GSM (Global System for Mobile Communication)In spite of the deployment of 3G and 4G, the 2G GSM technology is still In spite of the deployment of 3G and 4G, the 2G GSM technology is still considered a good model for illustrating the basic concept of cellular networks.considered a good model for illustrating the basic concept of cellular networks.

GSM Frequency BandsGSM Frequency Bands• GSM 850 &1900 mainly in USA, Canada and most of South America• GSM 900 & 1800 mainly in Europe, Middle East, Africa, most of Asia

The GSM-850 and GSM-1900 BandsIn the United States, regulatory requirements determine which area can use which

band.band.GSM-850 uses 824–849 MHz for uplink transmission and 869–894 MHz for

downlink. There are 124 channels numbered 128 to 251. GSM-1900 uses 1850–1910 MHz for uplink transmission and 1930–1990 MHz for

d li k Th 299 h l b d 512 t 810downlink. There are 299 channels numbered 512 to 810. •• Channel Separation in GSM 1900Channel Separation in GSM 1900 = 0.2 MHz •• Channel duplex distance in GSM 1900Channel duplex distance in GSM 1900 = 80 MHz

Uplink DownlinkControl

1850 1910 1930 1990

Uplink Downlink

Ch 570 Ch 720 Ch 720Ch 570

Control Gap

GSM-1900

Cellular Networks

• Dual band phonesDuplex distance = fDL - fUL

Cellular Networks

p‐ 900 & 1800 or 850 &1900

• Tri band phones

GSM Band Duplex Distance No of Channels

1900 80 MHz 299

1800 95 MHz 374

900 45 MH 124‐ 1800 & 850 &1900

• Quad band phones

900 45 MHz 124

850 45 MHz 124

‐ 900 & 1800 & 850 &1900 3G Cellular TechnologyUniversal Mobile Telecommunications System (UMTS)UMTS i 3G t h l b d th I t ti l M bil T l i tiUMTS is a 3G technology based on the International Mobile Telecommunications IMT-2000 standard.

UMTS is also referred to as 3GSM emphasizing the combination of the 3G natureUMTS is also referred to as 3GSM, emphasizing the combination of the 3G nature of the technology and the GSM standard which it was designed to succeed.

Cellular Networks

4G Cellular Technology

Cellular Networks

4G Cellular Technology4G is the fourth generation of mobile telecommunications technology succeeding 3G. In addition to usual voice and other 3G services, the 4G system provides mobile ultra-broadband Internet access for example tosystem provides mobile ultra-broadband Internet access, for example to smartphones, tablets and laptops with USB wireless modems.

Th L T E l ti (LTE) t d d i id d t b 4GThe Long Term Evolution (LTE) standard is considered to be 4G technology and LTE smartphones have been available since 2011.

On Optimal Call Admission Control in Cellular Networks

R. Ramjee, D. Towsley and R. Nagarajanj y g j

Handoff & Termination of a callHandoff & Termination of a call

As the Mobile Subscriber moves from one cell to another, the ,active call needs to be allocated a channel in the destination cell.

This event termed the handover or handoff must be transparentThis event, termed the handover or handoff, must be transparent to the end subscriber.

f h d i i ll h il bl h l h ll iIf the destination cell has no available channels, the call is terminated; this disconnection in the middle of a call is highly undesirable.

The Trade-off: handoff vs new callThe Trade-off: handoff vs new call

One of the goals of the network designer is to keep the handoff g g pblocking probability at a low value.

On the other hand reserving channels for handoff traffic couldOn the other hand, reserving channels for handoff traffic could increase blocking for new calls.

S h i d ff b h Q S hSo there is a trade-off between the two QoS measures, the handoff blocking probability and the new call blocking probability.p y

Call Admission Control (CAC)Call Admission Control (CAC)Call Admission Control (CAC) Call Admission Control (CAC) CAC can be defined as the process of managing the arriving traffic CAC can be defined as the process of managing the arriving traffic based on some predefined criteria.based on some predefined criteria.

CAC is usually used to:CAC is usually used to:Preserve the signal quality of active callsPreserve the signal quality of active calls

Reduce the call dropping probability Reduce the call dropping probability

Give priority to some classesGive priority to some classes

Maximize revenueMaximize revenue

Achieve fair resource sharingAchieve fair resource sharing

Guarantee transmission rateGuarantee transmission rate

The Guard Channel PolicyThe Guard Channel Policy

The notion of guard channels was introduced as a call admission mechanism to give priority to handoff calls over new callsmechanism to give priority to handoff calls over new calls.

In this policy, a set of channels called the guard channels are permanently reserved for handoff callsreserved for handoff calls.

Another version of this policy is the Fractional guard channel policy which effectively reserves a non-integral number of guard channels for y g ghandoff calls by rejecting new calls with some probability that depends on the current channel occupancy.

Guard Channel (GC) PolicyGuard Channel (GC) Policy

Guard Channel PolicyConsider a cellular network

if (NEW CALL) thenif (NumberOfOccupiedChannels < T)

with C channels in a given cell. The Guard Channel policy reserves a subset of these channels C T for admit call;

elsereject call;

if (HANDOFF CALL) then

these channels C -T for handoff calls. If the channel occupancy is equal to or greater than a certain if (HANDOFF CALL) then

if (NumberOfOccupiedChannels < C)

admit call;

threshold T, the Guard Channel policy rejects all new calls until the channel occupancy goes below the else

reject call;occupancy goes below the threshold T.

Analytical Model for the Guard Channel PolicyThe analytical model is based on the cell decomposition approach. In this approach, we assume that the cellular network is homogeneous and therefore we can examine a single network cell in isolation and model the impact of

i hb i ll th ll d id ti b i t tneighboring cells on the cell under consideration by appropriate parameters.

Notations:

The arrival process of new and handoff calls is Poisson with rate λ1 and λ2respectively.

λ1 = arrival rate of new calls λ2 = arrival rate of handoff callsLet λ = λ1 + λ2 and λ2 = λ

The channel holding time in this cell for both type of calls is exponentially distributed with mean 1/. Notice that the memoryless property allows us to assume that all new calls and old (handoff) calls have the same average duration for using the channel in this cell. Notice also that the authors have combined the following two different events into a single event represented by the parameter :following two different events into a single event represented by the parameter :1- Call termination event2- Mobile migration event , i.e., handoff of a mobile to a neighboring base station.

State Transition Rate DiagramState Transition Rate Diagram Guard Channel PolicyGuard Channel PolicyState Transition Rate Diagram State Transition Rate Diagram -- Guard Channel PolicyGuard Channel Policy

The Guard channel policy rejects all new calls til th h l b l th h ld

λ λ λ αλ αλ αλ αλuntil the channel occupancy goes below threshold

0 1 2 T T+1 CT+2 C-1

2μ Tμ μ (T+1)μ (T+2)μ (C-1)μ Cμ

1/ = average channel holding time λ1 = arrival rate of new calls λ2 = arrival rate of handoff callsλ = λ1 + λ2 λ 2 = λ

Balance of Flow Equations

j = (j+1) j+1 0 ≤ j ≤ T // using vertical cutsets

j-T = (j+1) T ≤ j ≤ C // i ti l t t j T j = (j+1) j+1 T ≤ j ≤ C // using vertical cutsets

Normalization Condition: 10

C

nn

Let = /. Solving the above equations, we get

0!

j

j

j 0 ≤ j ≤ T

Tjj

1

0!

j

Tjj

j

T ≤ j ≤ C

T

j

C

Tj

Tjjjo

jj0 1 !!

1

where

Blocking Probabilities for the Guard Channel Policy

The handoff blocking (dropping) probability is given by

B h(C,T) = C 0!

C

TCC

The new call blocking probability is given by

C

C

TjjTCBn ),(

j

0!

j

j

j 0 ≤ j ≤ T

Tjj

0!

j

jj

j T ≤ j ≤ C

Fractional Guard Channel (FGC) PolicyFractional Guard Channel (FGC) PolicyFractional Guard Channel (FGC) PolicyFractional Guard Channel (FGC) Policy

Fractional Guard Channel PolicyIn the Fractional Guard Channel li ll d

if (NEW CALL) thenif (random(0,1) (NumberOfOccupiedChannels))

policy, new calls are accepted with a certain probability that depends on the current channel occupancy. When the systems is

admit call;else

reject call;

if (HANDOFF CALL) th

occupa cy. W e t e syste s sin state j, 1 j C, a new call is accepted with probability

( j) = j. if (HANDOFF CALL) thenif (NumberOfOccupiedChannels < C)

admit call;else

( j) j

reject call;

/* random(0,1) returns a uniformly generatedrandom number in the interval [0,1] */

State Transition Rate DiagramState Transition Rate Diagram Fractional Guard ChannelFractional Guard Channel

λ 0 +λ

State Transition Rate Diagram State Transition Rate Diagram –– Fractional Guard Channel Fractional Guard Channel PolicyPolicy

λ 1+λ λ +λ λ +λ λ +λ λ +λ

0 1 2 T T+1 CT+2 C 1

λ1 0 +λ2 λ1 1+λ2 λ1 T-1+λ2 λ1 T+λ2 λ1 T+1+λ2 λ1 C-1+λ2

0 1 2 T T+1 CT+2 C-1

2μ Tμ μ (T+1)μ (T+2)μ (C-1)μ Cμ μ μμ ( )μ ( )μ ( )μ μ

1/ = average channel holding time λ1 = arrival rate of new calls λ2 = arrival rate of handoff calls

Redraw diagram using l b i i l ti

λ2 = arrival rate of handoff callsj = probability of accepting new calls in state j

λ = λ1 + λ2 λ 2 = λalgebraic manipulation

on next slideλ 2 = λj = + (1-) j-1

State Transition Rate DiagramState Transition Rate Diagram Fractional Guard ChannelFractional Guard Channel

For each state we have a randomization parameter which denotes the probability of accepting a new call

State Transition Rate Diagram State Transition Rate Diagram –– Fractional Guard Channel Fractional Guard Channel PolicyPolicy

γ1λ γ2λ γTλ γT+1λ γT+2λ γC-1λ γCλ

denotes the probability of accepting a new call

0 1 2 T T+1 CT+2 C-1

2μ Tμ μ (T+1)μ (T+2)μ (C-1)μ Cμ

Th l ti f th1/ = average channel holding time

The solution of the above diagram is

similar to the case of the basic Guard

λ1 = arrival rate of new calls λ2 = arrival rate of handoff callsλ = λ1 + λ2 λ 2 = λ

Channel Policy and is given in the paper.

j = probability of accepting new calls in state jj = + (1-) j-1

Limited Fractional Guard Channel (LFGC) PolicyLimited Fractional Guard Channel (LFGC) PolicyLimited Fractional Guard Channel (LFGC) PolicyLimited Fractional Guard Channel (LFGC) Policy

Limited Fractional Guard Channel PolicyIn the Limited Fractional Guard if (NEW CALL) then

if (NumberOfOccupiedChannels < T) then

admit call;

Channel Policy:

• When the system is in state T, new calls are accepted with a

else if (NumberOfOccupiedChannels == T) AND (random(0,1) )

admit call;

probability .

• When the system is in state T+1 through state C, hand off

ll d b ;else

reject call;

if (HANDOFF CALL) then

calls are accepted but new calls are rejected.

• When the system is in state 0 th h t t T 1 b th

if (NumberOfOccupiedChannels < C)

admit call;else

through state T-1, both handoff and new calls are accepted.

reject call;

Limited Fractional Guard Channel (LFGC) PolicyLimited Fractional Guard Channel (LFGC) PolicyLimited Fractional Guard Channel (LFGC) PolicyLimited Fractional Guard Channel (LFGC) PolicyIn state T, new calls are accepted with probability .

λ λ λ (α+(1- α) β)λ αλ αλ αλ

0 1 2 T T+1 CT+2 C-1

2μ Tμ μ (T+1)μ (T+2)μ (C-1)μ Cμ

1/ = average channel holding time λ1 = arrival rate of new calls λ2 = arrival rate of handoff callsλ = λ1 + λ2 λ 2 = λ = probability of accepting new calls in state T

Comparison between GC and LFGCComparison between GC and LFGCλ λ λ (α+(1- α) β)λ αλ αλ αλ

0 1 2 T T+1 CT+2 C 1

λ λ λ (α+(1- α) β)λ αλ αλ αλ

0 1 2 T T+1 CT+2 C-1

2μ Tμ μ (T+1)μ (T+2)μ (C-1)μ Cμ

For GC in state T, new calls are blocked, i.e., = 0.

μ μμ ( )μ ( )μ (C )μ μ

For LFGC in state T, new calls are accepted with probability > 0. ba

bilit

y GC uses discrete values of T and gives discrete points on the desirable

ocki

ng p

robpo s o e

performance curves.

LFGC allows smoother fi t i f th

desirable point

C-T10 11 12 13

blofine tuning for the

tradeoff between new calls and handoff calls.

The Three Optimization ProblemsThe Three Optimization Problems

In this paper, the authors consider the optimal admission control policies for three problems based on the two QoS measures, the handoff blocking probability and the new call blocking probability.

MINOBJ: Minimize a linear objective function of the two blockingMinimize a linear objective function of the two blockingMINOBJ: Minimize a linear objective function of the two blocking Minimize a linear objective function of the two blocking

probabilities.probabilities.

MINBLOCK: Minimize the new call blocking probability given a certain Minimize the new call blocking probability given a certain number of channels and subject to a hard constraint on the handoff blocking number of channels and subject to a hard constraint on the handoff blocking probability.probability.

MINC: Minimize the number of channels subject to hard constraints on the Minimize the number of channels subject to hard constraints on the

new and handoff call blocking probabilities.new and handoff call blocking probabilities.

Problem MINOBJ: Finding an admission control policy that minimizes a linear objective function of the new and handoff call blocking probabilities.

Among all call admission control policies , an optimal policy for the MINOBJ problem is of the threshold type (i.e., a Guard Channel policy).

Problem MINBLOCK: Given C channels, minimize the new call blocking probability B n(C) such that the handoff blocking probability satisfies the constraint B h(C) Thresholdh

The Limited Fractional Guard Channel (LFGC) Policy is optimal for the MINBLOCK problem. The paper gives the details of Algorithm MINB that minimizes the new call blocking probability subject to the constraint on the handoffminimizes the new call blocking probability subject to the constraint on the handoff call blocking probability.

Problem MINC: Minimize the number of channels C such that the following two t i t ti fi dconstraints are satisfied

B h(C) Thresholdh and B n(C) Thresholdn

The Limited Fractional Guard Channel (LFGC) Policy is optimal for the MINC e ed c o Gu d C e ( GC) o cy s op o e NCproblem. The paper gives the details of Algorithm MIN that finds the minimum number of channels that satisfies the above two constraints.