TIES434 Radioverkot ja resurssihallinta Radio Networks and...
Transcript of 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
University of Jyväskylä, Department of Mathematical Information Technology
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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University of Jyväskylä, Department of Mathematical Information Technology
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.
University of Jyväskylä, Department of Mathematical Information Technology
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
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University of Jyväskylä, Department of Mathematical Information Technology
Cellular concept
University of Jyväskylä, Department of Mathematical Information Technology
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
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University of Jyväskylä, Department of Mathematical Information Technology
Motivation: limited radio spectrum
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.
University of Jyväskylä, Department of Mathematical Information Technology
Motivation: limited radio spectrum
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.
University of Jyväskylä, Department of Mathematical Information Technology
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
University of Jyväskylä, Department of Mathematical Information Technology
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.
University of Jyväskylä, Department of Mathematical Information Technology
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.
University of Jyväskylä, Department of Mathematical Information Technology
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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University of Jyväskylä, Department of Mathematical Information Technology
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
University of Jyväskylä, Department of Mathematical Information Technology
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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University of Jyväskylä, Department of Mathematical Information Technology
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
University of Jyväskylä, Department of Mathematical Information Technology
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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University of Jyväskylä, Department of Mathematical Information Technology
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 !!
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i
iM
M
C
i
E
EM
M
M
E
EM
M
M
E
p
University of Jyväskylä, Department of Mathematical Information Technology
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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University of Jyväskylä, Department of Mathematical Information Technology
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.
University of Jyväskylä, Department of Mathematical Information Technology
… 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.
University of Jyväskylä, Department of Mathematical Information Technology
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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University of Jyväskylä, Department of Mathematical Information Technology
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.
University of Jyväskylä, Department of Mathematical Information Technology
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 ?