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re use OF THE FREQUENCY FOR BETTER UTILIZATION
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3.2 FREQUENCY RE USE
AJAL.A.J Assistant Professor –Dept of ECE,
Federal Institute of Science And Technology (FISAT) TM MAIL: [email protected]
The key characteristic of a cellular network is the ability to re-use frequencies to increase both coverage and capacity.
Cellular ConceptCellular Concept
The limited capacity of the first mobile radio-telephone services was related to the spectrum used…not much sharing and a lot of bandwidth dedicated to a single call.– good coverage
– interference: impossible to reuse the same frequency
The cellular concept addressed many of the shortcomings of the first mobile telephones– Frequency reuse
– Wasted spectrum allocated to a single user
In 1968, Bell Labs proposed the cellular telephony concept to the FCC
It was approved and then the work began!– FCC allocated spectrum (took away TV UHF channels 70-83) in the
825-845 MHz and 870-890 MHz bands
Cellular ConceptCellular Concept developed by Bell Labs 1960’s-70’s
areas divided into cells
a system approach, no major technological changes
a few hundred meters in some cities, 10s km at country side
each served by base station with lower power transmitter
each gets portion of total number of channels
neighboring cells assigned different groups of channels, interference minimized
hexagon geometry cell shape
What is a Cell?• Cell is the Basic Union in The System
– defined as the area where radio coverage is given by one base station.
• A cell has one or several frequencies, depending on traffic load. – Fundamental idea: Frequencies are reused, but not in
neighboring cells due to interference.
Cellular Concept
Limited number of frequencies => limited channels Single high power antenna => limited number of users Smaller cells => frequency reuse possible => more number of users
Base stations (BS): implement space division multiplex– Each BS covers a certain transmission area (cell)
– Each BS is allocated a portion of the total number of channels available
– Cluster: group of nearby BSs that together use all available channels
Mobile stations communicate only via the base station– FDMA, TDMA, CDMA may be used within a cell
As demand increases (more channels are needed)– Number of base stations is increased
– Transmitter power is decreased correspondingly to avoid interference
Cellular Concept
Cell size:– 100 m in cities to 35 km on the country side (GSM) – even less for higher frequencies– Umbrella cell: large cell that includes several smaller cells
• Avoid frequent handoffs for fast moving traffic
Cell shape:– Hexagonal is useful for theoretical analysis– Practical footprint (radio coverage area) is amorphous
BS placement:– Center-excited cell: BS near center of cell
• omni-directional antenna– Edge-excited cell: BSs on three of the six cell vertices
• sectored directional antennas
Cellular Concept
Advantages:– higher capacity, higher number of users– less transmission power needed– more robust, decentralized– base station deals with interference, transmission area etc. locally
Problems:– fixed network needed for the base stations– handover (changing from one cell to another) necessary– interference with other cells: co-channel, adjacent-channel
Important Issues:– Cell sizing– Frequency reuse planning– Channel allocation strategies
Bottom line: Attempt to maximize availability of channels in an area
• 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
Cell characteristics• Implements space division multiplex: base station
covers a certain transmission area (cell)• Mobile stations communicate only via the base
station• Advantages of cell structures:
– higher capacity, higher number of users– less transmission power needed– more robust, decentralized– base station deals with interference, transmission area etc. locally
• Problems:– fixed network needed for the base stations– handover (changing from one cell to another) necessary– interference with other cells
• Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies
Shape of Cells Square
Width d cell has four neighbors at distance d and four at distance d
Better if all adjacent antennas equidistant Simplifies choosing and switching to new antenna
Hexagon Provides equidistant antennas Radius defined as radius of circum-circle
Distance from center to vertex equals length of side Distance between centers of cells radius R is R Not always precise hexagons
Topographical limitations Local signal propagation conditions Location of antennas
2
3
Cellular Geometries
Cell Footprint
Cellular Network ArchitectureCellular Network Architecture
MobileSwitching
Center
PublicTelephonenetwork
and Internet
MobileSwitching
Center
Wired network
Frequency ReuseFrequency Reuse
Adjacent cells assigned different frequencies to avoid interference or crosstalk
Objective is to reuse frequency in nearby cells– 10 to 50 frequencies assigned to each cell
– transmission power controlled to limit power at that frequency escaping to adjacent cells
– the issue is to determine how many cells must intervene between two cells using the same frequency
Frequency ReuseFrequency Reuse
each cell allocated a group k channels– a cluster has N cells with unique and disjoint channel
groups, N typically 4, 7, 12
total number of duplex channels S = kN
Cluster repeated M times in a system
Total number of channels that can be used (capacity)– C = MkN = MS
Smaller cells higher M higher C
+ Channel reuse higher capacity
+ Lower power requirements for mobiles
– Additional base stations required
– More frequent handoffs
– Greater chance of ‘hot spots’
Frequency planning
f1
f2
f3
f2
f1
f1
f2
f3
f2
f3
f1
f2
f1
f3f3
f3f3
f3
f4
f5
f1
f3
f2
f6
f7
f3
f2
f4
f5
f1
f3
f5f6
f7f2
f2
f1f1 f1
f2
f3
f2
f3
f2
f3h1
h2
h3g1
g2
g3
h1
h2
h3g1
g2
g3g1
g2
g3
3 cell cluster
7 cell cluster
3 cell clusterwith 3 sector antennas
Figure Frequency reuse patterns
Frequency planning• Frequency reuse only with a certain
distance between the base stations• Standard model using 7 frequencies:
f4
f5
f1
f3
f2
f6
f7
f3
f2
f4
f5
f1
FREQUENCY REUSE
• The allotted frequency spectrum for mobile communication
• during the 1 G is in the range of (800- 900 )Mhz
• but ,Using this small bandwidth, thousands of subscribers need to be served over a large area.
• To manage this situation, a technique called ‘’frequency reuse ‘’ is adopted.
FREQUENCY REUSE
Cellular Networks
• Propagation models represent cell as a circular area• Approximate cell coverage with a hexagon - allows easier• analysis• Frequency assignment of F MHz for the system• The multiple access techniques translates F to T traffic channels• Cluster of cells K = group of adjacent cells which use all of the systems
frequency assignment
Frequency reuse conceptFrequency reuse concept
ALPHABETICAL REPRESENTATION
Wrong implementation
END Result of Wrong implementation
Correct implementation
31
Freq reuse Advantage
Freq reuse: Several cells in coverage area that use same set of frequencies
Cochannel cells: Those cells using same freqsCochannel interference: The mobile in cell A also gets signals from other Co channel cells
Cluster1
Cluster2
Cluster3
Cellular Concepts
In this case N=19 (i=3, j=2)
Frequency Reuse
Power of base transceiver controlled Allow communications within cell on given frequency Limit escaping power to adjacent cells Allow re-use of frequencies in nearby cells Use same frequency for multiple conversations 10 – 50 frequencies per cell
E.g. The pattern consists of N cells K total number of frequencies used in systems Each cell has K/N frequencies Advanced Mobile Phone Service (AMPS) K=395, N=7
giving 57 frequencies per cell on average
Characterizing Frequency Reuse D = minimum distance between centers of cells that use
the same band of frequencies (called cochannels) R = radius of a cell d = distance between centers of adjacent cells (d = R) N = number of cells in repetitious pattern
Reuse factor Each cell in pattern uses unique band of frequencies
Hexagonal cell pattern, following values of N possible N = I2 + J2 + (I x J), I, J = 0, 1, 2, 3, …
Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, … D/R= D/d =
N3N
35
Distance
DR
R
36
4
1
3
6
5
27
4
1
3
6
5
27
4
1
3
6
5
27
4
1
3
6
5
27
4
1
3
6
5
27
D
R
Example of frequency reuse factor or pattern 1/4
The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation).
38
Reuse Ratio : q
NqR
D3
where:D: Distance between the centres of cellsR: Radius of the cellq: Reuse ratioN: Cluster size
Assuming hexagonal shape cells of equal size
39
Example
RND 3
For N = 7 and R 5 km
573 D
The minimum distance at which the same frequency can be reused is approximately 4.6 times R, which is in this case 22.91 km
91.225583.4 D
FrequencyReusePatterns
N=7, 32 cells, R=1.6km, in total 336 channels
So what is FREQUENCY REUSE ?
• After allotting the available total bandwidth among a set of cells , the same frequency band will be used in another set of cells .
• This kind of reuse can be adopted until the entire area to be covered is exhausted .
• Note• Seven cells in a set is the most frequently
used configuration.this configuration usually operates with a cell diameter of 1-3 km range
Cellular architecture
One low power transmitter per cell
Frequency reuse–limited spectrum
Cell splitting to increase capacityA
B
Reuse distance: minimum distance between two cells using same channel for satisfactory signal to noise ratio
Measured in # of cells in between
Reuse distance 2 – reuse pattern
One frequency can be (re)used in all cells of the same color
Reuse distance 3 – reuse pattern
Signal-to-interference ratio S/I or SIR
50
Capacity calculation—FDMA
n: capacity (number of total users)m: number of cells to cover the areaN: frequency reuse factor (# cells/cluster)B: bandwidth per userW: total available bandwidth (spectrum)
B
W
N
mn
Problem In an FDMA system calculate
frequency re use factor ?
• m= 70
• W= 100
• n=10
• B=100
• N = ? cells/cluster
52
Capacity calculation—FDMA
In the previous example, • m=20, • W=25 MHz, • N=4, and • B=30 KHz.
166,430
25000
4
20
B
W
N
mn
53
Capacity calculation—TDMA
• n: capacity (number of total users)• m: number of cells to cover the area• N: frequency reuse factor (# cells/cluster)• B: bandwidth per user• W: total available bandwidth (spectrum)
• Nu: number of time slots per carrier
uNB
W
N
mn
54
Capacity calculation—TDMA (contd.)
Assuming again, • m=20, • W=25 MHz, • N=4, • B=200 KHz,• Nu=4.
500,24200
25000
4
20 uN
B
W
N
mn
55
Capacity of CDMA
n: number of users
W: total bandwidth
R: data rate
Sr: signal to noise ratio
rSR
Wn
56
Capacity per cell (CDMA)
Assume:
W=1.25MHz=1,250,000 Hz
R=9600 bps
Sr should be larger than 3dB => 2 times
usersSR
Wn
r
6529600
1250000
System architecture
• A set of seven cells will be controlled by a BSC base station controller
• The terminal station located in every cell will have access to the BSC.
• 7 base terminal stations will have one BSC
• For every frequency reuse , 1 BSC is required.
The elements that determine frequency reuse are the reuse distance and the reuse factor
Scenario
• For 100 BTS we require more than 14 BSC’s
• All BSC are connected to a Mobile Switching centre.
• Here mobile stations are connected to BTS by wireless means
• All Base stations are connected to their respective BSC’sby cables.
• In modern communication , fibre optic cables are used to connect BTS
Fig: Advanced cellular mobile communication system Architecture
@ 3G
Cell Planning (1/3)• The K factor and Frequency Re-Use Distance
K = i2 + ij + j2
K = 22 + 2*1 + 12
K = 4 + 2 + 1
K = 7i
j
1
2
34
5
6
7
Frequency re-use distance is based on the cluster size K
The cluster size is specified in terms of the offset of the center of a cluster from the center of the adjacent cluster
D = 3K * R
D = 4.58R
1
2
35
6
7
D
R
Cell Planning (2/3)
A3A1
A2G3
G1
G2C3
C2
B3B1
B2
F3F1
F2
D3D1
D2E3
E1
E2
G3G1
G2
F3F1
F2
C3C1
C2
A3A1
A2B3
B1
B2
E3E1
E2
D3D1
D2
7-cell reusepattern
Frequencyreuse
C1
Cell Planning (3/3)• Cell sectoring
– Directional antennas subdivide cell into 3 or 6 sectors
– Might also increase cell capacity by factor of 3 or 6
• Cell splitting– Decrease transmission
power in base and mobile
– Results in more and smaller cells
– Reuse frequencies in non-contiguous cell groups
– Example: ½ cell radius leads 4 fold capacity increase
BTS• BTS acts as an interface between the Mobile unit & BSC
• BTS connects mobile units by wireless means to the BSC
BTS Antenna• BTS has got an antenna usually at an elevation location.
• Invariantly , Roof tops,
• Small steel towerare used for erecting the BTS antenna.
Omni directional coverage= coverage in all directions
• Compact power supply systems (some even chargeable batteries) are used at the base terminal stations.
• BTS is usually located at the centre of the cell area for omni directional coverage.
INDIAN ANALOGY
Bandwidth vs mobility
• The wireless interface at BTS and wired topologytopology between BTS- BSC & BSC – MSC necessitates certain translation in protocols.
• Upto the stage of BTS , air interface protocols are used. After BTS , the transmission media happen to be wired one , mostly fibre optic communication.
• Hence at the BTS or BSC stage , there is a need for the translation of these protocols
Illustration – of translation requirement
• In mobile communication at the mobile user’s unit the speech signal is connected into 13 kbps digitized voice. This conversion is is required to have an air interface with bandwidth efficiency.
• But the backbone network through which the voice has to travel in certain applications can provide 64 kbps.
Contd…
Contd…
• PCM digitization is adopted in fixed telephone network which make use of these backbone network.
• When mobile unit converts the analog information into 13kbps digital information , the BSC converts the same into 64 kbps information.
[[6868]]
Frequency reuse revisited: cluster size and reuse distanceFrequency reuse revisited: cluster size and reuse distance
DD
frequency group A
frequency group B
Cluster with cluster size N
Co-channel cells
[[6969]]
The geometry of a hexagonal cellThe geometry of a hexagonal cell
Unit scale is distance between neighboring cell centers.
For cell radius
To find the distance from the origin, , of point , convertconvert axis:
[[7070]]
The geometry of a hexagonal cell The geometry of a hexagonal cell [continue] [continue]
– So,
– Using this equation to locate co-channel cells, we start from a reference cell and move i hexagons along the u-axis and then j hexagons along the v-axis.
– The distance between co-channel cells in adjacent clusters is
– The number of cells in a cluster, N, is hence since i and j can only take integer values.
– The frequency reuse factorfrequency reuse factor, Q, is defined by
Frequency Reuse Factor
Effective reuse of resources can highly enhance the system capacity Frequency reuse factor (FRF) K defines frequency
reuse pattern
With a smaller frequency reuse factor (FRF), more available bandwidth can be obtained by each cell
Previous Frequency Reuse Schemes
With the usage of FRF-1, the most user terminals (UTs) are afflicted with heavy Inter-cell interference (ICI) Especially near the cell edge
The conventional method to figure out this problem is by increasing the FRF mitigate the ICI efficiently but decrease on available bandwidth
The most representative approaches improving cell-edge performance while retaining spectrum efficiency Soft Frequency Reuse (SFR) scheme Incremental Frequency Reuse (IFR) scheme
Soft Frequency Reuse (SFR) Scheme
Incremental Frequency Reuse (IFR) Scheme
Enhanced Fractional Frequency Reuse (EFFR)
FREQUENCY REUSE SCHEME
Soft frequency Reuse
CCU: cell-centre usersCEU: cell-edge users
Soft Frequency Reuse (SFR) Scheme CCU: cell-centre users
CEU: cell-edge users
Limitations of SFR
How to define the borderline to divide cell area for CCUs and CEUs is a key issue Generally, there are more CEUs than CCUs in a cell
since the outer surface area is much larger than the inner part CEUs have maximum one third of the entire bandwidth to utilize,
which results in lower spectrum efficiency
More ICI could happen even in a low traffic-load situation, while there are still subchannels in idle and underutilized in the system The resource allocation via the SFR scheme starts always from
the first subchannel up
Incremental Frequency Reuse (IFR) Scheme (1)
The only difference between the IFR design and the classical reuse-1 Classical reuse-1: allocate resources always from
the first subchannel IFR: start dispensing resources from different points
Incremental Frequency Reuse (IFR) Scheme (2)
IFR scheme can overcome the low spectrum reuse efficiency problem and the more ICI at low loading traffic problem
IFR scheme only performs better when just fewer traffic exists in a system When the loading factor is greater than 0.3, it is
inferior to the SFR scheme
Enhanced Fractional Frequency Reuse (EFFR) (1)
Enhanced Fractional Frequency Reuse (EFFR) scheme intends to retain the advantages of the both approaches
Concept Define 3 cell types for directly contiguous cells in a cellular system Reserves for each cell-type a part of the whole frequency band named
Primary Segment The Primary Segments among different type cells should be
orthogonal The Primary Segment of each cell will be further divided into
reuse-3 part: cannot be reused by directly neighboring cells reuse-1 part: is at the same time a part of the Secondary Segments
belonging to the other two cell-types
HYBRIDS
Enhanced Fractional Frequency Reuse (EFFR) (2)
Power Allocation and SINR Estimation Transmission Power Allocation
Any cell-type is not allowed to use the reuse-3 subchannels dedicated to the other two cell types The power allotted to the reuse-3 subchannels can be tripled
Signal-to-Interference-Ratio (SINR) Estimation A cell acts on the Secondary Segment as a guest, and occupying
secondary subchannels is actually reuse the primary subchannels belonging to the directly adjacent cells
Reuse on the Secondary Segment should conform to two rules: monitor before use resource reuse based on SINR estimation
Each cell listens on every secondary subchannel all the time Before occupation, a cell makes SINR evaluation and chooses resources
with best estimation values for reuse If all available secondary resources are either occupied or not good
enough to a link, this cell will give up scheduling resources
Worst Case InterferenceWorst Case Interference
S/I ~ R-4 /[2(D-R)-4 + 2(D+R)-4 + 2D-4]
In CDMA reuse patterns are not required.
Subscriber in every cell can use the same frequency at the same time. Subscriber is discriminated from another by the assignment of a unique code to every conversation.
In GSM freq. Reuse pattern of 7 is used.
Frequency Reuse @ CDMA
Problems– Propagation path loss for signal power: quadratic or higher in distance – fixed network needed for the base stations– handover (changing from one cell to another) necessary– interference with other cells:
• Co-channel interference: Transmission on same frequency
• Adjacent channel interference:Transmission on close frequencies
Solution: Topology of Different Areas
100
100
100
20
60 100
100
60 60
20
20
20
20
20
20
60
40 20
20
Town
Rural
Suburb
Highway