ties434 radioverkot ja resurssihallinta radio networks and...

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University of Jyväskylä, Department of Mathematical Information Technology

TIES434

Radioverkot ja

resurssihallinta

Radio Networks and Resource

Management

Prof. Tapani Ristaniemi


Details

8 cu’s

Lectures & exercises

Wednesdays 12:15 - 14:00

Fridays 8:15 - 10:00

CHECK Korppi for lecture rooms

Exercises: during some lecture hours, will be announced beforehand.

Lectures given also by Ari Viinikainen (GSM-related) and couple of

visiting lectures from industry.

Goal: a student will get familiar with

Wireless standards (GSM,EDGE,WCDMA,HSPA,LTE,WiMAX)

Basics of radio their network planning and resource management

Radio channel characteristics

Radio interface techniques

Other wireless systems (WLAN,broadcast networks,positioning systems,

…)

Theory and practice

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Details

Material:

Holma, Toskala: WCDMA for UMTS, 4th edition, Wiley&sons, 2007

Dahlman et. al. : 3G Evolution - HSPA and LTE for Mobile Broadband

Lecture notes

For other aspects: collection of scientific articles

Lecture notes: http://users.jyu.fi/~riesta/TIES434_lectureX.pdf

X=1,2,3,…

Passing the course by final exam: 17.12.10, 21.1.11, 11.2.11

Compulsory project work (based on articles)

Attendance to lectures and exercises is recommended, but not

compulsory. It is possible to get considerable improvement in

grading by actively participating the exercise sessions.

Those who have already completed the courses of ”Wireless

communications (Langattomant järjestelmät)” and ”Radio network

planning (Radioverkkosuunnittelu)” will have some extra work to be

done.


Contents (subject to change)

Week 36: Introductions & cellular concept

Week 37: GSM + EDGE (lectured by Ari Viinikainen)

Week 38: WCDMA basics + WCDMA PHY

Week 39: WCDMA link performance + dimensioning

Week 40: WCDMA RRM + RNP basics

Week 41: Advanced RNP + HSPA

Week 42: LTE basics + radio interface

Week 43: LTE PHY + access procedures

Week 44: LTE SAE + performance of 3-4G Evolution

Week 45: Radio Channel characteristics

Week 46: … Continues …

Week 47: Satellite Communication

Week 48: Broadcast systems

Week 49: WLANs

Week 50: recap

http://users.jyu.fi/~riesta/TIES434_lectureX.pdf



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Cellular concept


Motivation: limited radio spectrum

Many users vs. the same media: sharing the

media among users is mandatory multiplexing

By multiplexing we essentially understand division

of available channel into several subchannels in

time/frequency/code/space dimensions.

s2

s3

s1f

tc

k2 k3 k4 k5 k6k1

f

tc

f

t

c

channels ki

4



Considering a mobile communication system, one

could build up a system consisting only one

serving base station, which would divide the

wireless channel among the mobiles by the

means of either time, frequency or code

multiplexing.

However, this would immediately result in a major

disadvantage: large coverage areas would be

possible only via increased transmission power,

which would be problematic for mobile handheld

devices.



The underlying idea of cellular concept is to introduce geometrically smaller areas; called cells among which the functionality of radio network is divided.

In cellular system, each cell is characterized byone base station which serve the users within thatgeographical area.

What is needed more to make inter-cellconnections possible is a fixed (backbone) network which connects different base stations.

Radio network coverage refers to sum of individual cells’ coverage.

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Resource reuse

In cellular system, each cell have to have their own group of channels according to some multiplexing principle. Otherwise, inter-cell interferences would occur.

On the other hand, radio signal attenuates as a function of distance, which means that same group of resources (time slot / frequency band / code) can be utilized again if the cells are far apart.

This is because the interfering signal would then be weak enough until it reaches the other cell no significant interference between the cells.


Resource reuse

Resource reuse defines how often the same resource can be used. For example, frequency reuse factor defines the distance between the cells using the same frequency for communication.

In practice, resource reuse could be optimized as follows:

With a given set of frequencies, the network allocates resources such that

1. Frequency reuse factor is maximized (maximum capacity)

2. Mutual interferences between the same frequency bands at different cells is minimized (target quality in each cell is reached)

Notice: cellular concept is not limited to any

particular access technology or system

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Pros and cons of cellular concept

Higher capacity

Less transmission

power

Localized

interference

Robustness

No technological

challenges in

deployment

Massive

infrastructure

More complex

mobility

management

Resource planning

and management

Advantages Disadvantages


Reuse factor

3D

Q NR

f2f7

f1f6

f5

f4

f3

f2f7

f1f6

f5

f4

f3

f2f7

f1f6

f5

f4

f3

f2f7

f1f6

f5

f4

f3

Reuse

distance

D

Cell

range

R

Example: Cluster size N=7

For each cell, a set of

frequencies is allocated.

Cells that use the same set of

frequencies are denoted as co-

channel cells, and the

interference received from co-

channel cells is called co-

channel interference.

A set of cells can be

clustered, and the cluster can

be repeated. One cluster should

contain all the available

frequencies.

Reuse distance (D) depends

on the cluster size (N).

Reuse factor is defined as:

for hexagonal cells

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Examples of frequency reuse

N=7 N=4 N=3

Co-channel cells (the first tier of cell shown)

NR

RR

R

DQ 321

)2

34()3( 22

Hexagonal cell shapes are

conceptual and gives a

simplistic model of radio

coverage, but it has been

universaly adopted due to

simple analysis of a system.


Resource assignment stretegies

For efficient utilization of the radio resources, a variety of channel assignment strategies can be developed. Fixed channel assignment (FCA)

Each cell is allocated a pre-determined set of channels. Any call attempt within the cell can only be served by the unused channels in that cell. If all the channels are occupied, the call is blocked.

Dynamic channel assignment (DCA)No permanent channel assignments, but each time a call request is made, the serving base station requests a channel from the network controller. Accordingly, the controller allocates such a channel which is not in use in that cell or any other cell which is within the reuse distance D. DCA can reduce the blocking probability, but the implementation is more complex.

Hybrids of FCA/DCA may also be definedPart of the channels are permanently allocated, but the rest of the channels may be allocated dynamically. For example in GSM/GPRS, certain channels reserved purely for voice (i.e., GSM) and the rest for GPRS. Another example is in UTRA TDD, where hybrid DCA/FCA can be utilized between uplink and downlink channels.

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Interference

Interference limits the system capacity and the performance of

a single radio link. The higher is the interference level, the

lower is the system capacity and the poorer is the quality of

communication links.

Unlike thermal noise which can be overcome by increasing the

signal-to-noise ratio (SNR), co-channel interference can’t be

overcome just by increasing transmission power. This is

because an increase of transmission power increases co-

channel interference, too.

In cellular system, when the size of each cell is approximately

the same*, co-channel interference in independent from the

transmission power and becomes a function of the cell radius

and reuse distance.

* Cell size becomes approximately the same given that the

same power is used at the base station and the environment

is similar in each cell.


Signal to interference ratio (SIR)

Suppose the strength of the information bearing signal at the mobile

receiver is denoted as S and the co-channel interference as I.

Signal-to-interference ratio (SIR) is a common measure defining the

quality of the signal:

where Ij is the co-channel interference received from jth co-channel

cell.

For simplicity, if transmission power (P) of each base station is

assumed to be the same, then SIR can be written as

where d0 is the distance from the serving cell and dj from the jth co-

channel cell.

1

J

j

j

S S

II

0 0

1 1

n n

J Jn n

j j

j j

Pd dS

IPd d

n is so called path loss exponent. It depends on the

environment: in free space n=2, in dense urban n=4.

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SIR example

Take a simple

scenario, where r is

the distance from

the serving cell,

and D is the

distance from the

first tier of

interfering co-

channel cells (equi-

distant cells).

SIR can be now

expressed as:

6

)/(

6

n

n

n rD

D

r

I

S

r

6 co-channel cells

when hexagonal cell

structure

6

3

6

)/(n

n NRD

I

S

Worst case SIR

when r = R


Worst case SIR

N=7nnnnn

nnnnn

n

QQQQQ

DRDRDRDRD

R

I

S

)1()2/1()2/1()1(2

1

)()2/()2/()(2

Here worst case estimate gave 1-2 dB smaller SIR than

the equidistant model.

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Mobility

Mobility within a cellular network is guaranteed with handovers.

A handover (or handoff) refers to a situation where all radio

resources of a connection are handed to another base station.

A handover decision can be based either on power level or quality

measurements according to some handover function.

Parameters needed in handover process

Triggering method (power level, quality level, load balancing or

other reasons)

Measurement period of the triggering quantity

Thresholds for making handover decision

Target cell selection process

Queing of HO request


Grade-of-service

Cellular systems accomodate a large number of users in a limited radio spectrum. A large number of users share a small number of channels in a cell.

Each user is allocated a channel from a pool of available channels when demanded.

Channel allocation exploits the statistical behavior of users so that a fixed number of channels can accomodate a large random number of users.

The ability of users to access a system is measured by grade-of-service (GOS). GOS is typically given as the likelihood that the call attempt is blocked (that is, no available channel free at the time of call request and/or thereafter.)

To measure GOS we need to know the traffic intensity.

The unit for traffic intensity is Erlang (Erl). 1 Erlang means that one channel is occupied for an hour, that is “3600 call seconds”.

For example, if an average call length of 90s, the traffic generated by one user is 25 mErl (90s/3600s), and the total traffic for, say 100 users is, 2.5 Erl.

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Cell capacity

For circuit-switched traffic (such as regular voice call), required number of channels of a cell can be defined using Erlang formulas.

Suppose each user in a system generates a traffic intensity of Eu

Erlangs given by Eu = µH, where H is the average duration of a call and µ is the average number of call requests per unit time.

For a system of U users the total traffic intensity is thus

E = UEu

Definitions: H is also called channel holding time, 1/Uµ is also called an call inter-arrival time and E is also called total offered traffic.

Assuming that there exists M traffic channels, the probability that a call is blocked (or delayed if call queuing is allowed) is given by

M

i

i

M

B

iE

MEp

0

!/

!/

Erlang B formula (without queuing) Erlang C (with queuing)

1

0 !!

!M

i

iM

M

C

i

E

EM

M

M

E

EM

M

M

E

p


Example #1

Suppose one user generates 0.1 Erlangs of traffic and there are 10

channels and 40 users. Thus, we have E = UEu =40 * 0.1 Erl = 4

Erl, and blocking probability equals pB = 0.0053.

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Example #2

Suppose you have to figure out the number of GSM base stations

which is required to support a certain area.

An example plan for the situation:

25 000 inhabitants in the area & the operator has a 40 % market share

10000 potential customers

Every user is assumed to generate 25 mErl traffic

System requirements have to be dimensioned for 250 Erl traffic

Operator has a total of 40 carrier frequencies in use

Frequency reuse: How many carrier frequencies can be used per

cell ?

• SIR requirement for the system 14 dB

• Suppose the propagation exponent for

the area is n = 3.

• From the figure we see that the

required cluster size N equals 10.

• This means that 40/10 = 4 carrier

frequencies can be used in each cell.


… Example #2

How many cells are needed ?

In GSM each carrier frequency can support 8 users. Hence, 4 carrier

frequencies result in a total of 32 traffic-channels (lets reserve 2 for

signaling purposes)

30 traffic channels can support 20.30 Erl of traffic with 1% blocking

probability (see the figure)

Hence, 250 Erl / 20.30 Erl/cell = 12.3 cells 13 cells are required.

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Methods for capacity and quality enhancement: Sectoring

Sectoring means that a base

station is equipped with directive

antennas rather than omni-

directional antenna.

This means that one base station

has now more than one logical

cells.

The benefit of sectoring is the

reduced co-channel interference.

For example, when hexagonal cell

structure and Z sectors per cell is

assumed, the number of

interfering co-channel cells is

approximately 6/Z. This will give a

SIR gain !

From mobility point of view, more

intra-cell handovers are needed.


Methods for capacity and quality enhancement: Antenna downtilt

The vertical radiation pattern of a base station antenna can be directed towards the ground in order to reduce the power of co-channel interference.

By this way, most of the radiated energy can be focused more clearly towards the users within the cell’s coverage area, instead of focusing the maximum power towards the horizon.

Parameters such as the shape of vertical radiation pattern, antenna height and cell coverage area have an affect to the selected downtilt angle.

Mechanical downtilt relies on physical

movement of an antenna whereas

electrical downtilt relies on relative

phase shifts of different antenna

elements in an antenna array.

phase shift

phase shift

phase shift

phase shift

phase shift

signal

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Methods for capacity and quality enhancement: Antenna heights

First of all, antenna height affects to the path loss

characteristics of the signal. Namely, lower antenna

height increases the path loss, that is, low antenna

small cell. However, with a lower antenna position, the

signal is more likely shadowed by obstacles (from mobile

point of view).

On the other hand, higher antenna positions improve the

coverage from the serving cell (i.e., S is higher), but

simultaneously co-channel interference (i.e., I) for the

others will increase.

Hence, in the final deployment of a cellular network,

antenna heights have to be optimized together with

resource reuse distance.


Other methods for capacity and quality enhancement

Frequency hopping

Frequency hopping is based on the usage of several carrier frequencies

or frequency bands in a pseudo-random manner

It reduces co-channel (and adjacent channel) interference through

interference averaging

Overlay-underlay concept

Intelligent utilization of frequencies between small and bigger cells.

Overlay frequencies for coverage, underlay frequencies for capacity

Power control

Transmission power is controlled

to reduce interference levels.

Discontinuous transmission (DTX)

Transmission occurs only during active connection (speech)

This also reduces the average interference

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Exercises

Task 1. Derive the reuse factor Q for hexagonal cells with

a cluster size N=4 and N=3.

Task 2. Consider Example #2 once again, having now

different environments in consideration: n=2, n=2.5 and

n=3.5. How many cells are required for each case ?

Task 3. Plot the figures for Erlang C formulas for

10,20,30,40 and 50 channels. How the figures are

different from Erlang B figures ?

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