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Chapter 2: The Cellular System
Graduate ProgramGraduate Program Department of Electrical and Computer Engineering
Goal of the Chapter
In cellular system, the available radio spectrum is limited E.g., because of regulatory issues Hence the number of simultaneous call supported is limited Hence, the number of simultaneous call supported is limited
How to achieve high capacity (or support simultaneous calls) at the same time covering very large areas?calls) at the same time covering very large areas? Frequency reuse by using cells?
Overview system design fundamentals on cellular y gcommunication Cell formation and the associated frequency reuse, handoff, and
power controlpower control
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 2
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 3
Cellular System - Architecture
R di tRadio tower
Mobile Switching
PSTNTelephoneNetwork
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 4
Mobile SwitchingCenter
Cellular System ….
High capacity is achieved by limiting the coverage of each base stations to a small geographic region called a cell Single high power transmitter (large cell) are replaced with many Single, high power transmitter (large cell) are replaced with many
low power transmitters (small cells)
A portion of the total number of channels is allocated to peach cell or base station Available group of channels are assigned to a small number of
neighboring base stations called clusterneighboring base stations called cluster Near by base stations are assigned d/t groups of channels to
minimize interference
Same channels (frequencies/timeslots/codes) are reusedby spatially separated base stations Reuse distance and frequency reuse planning?
Sem. II, 2010/11
Reuse distance and frequency reuse planning?
Wireless Communications - Ch. 2 – Cellular System 5
Cellular System ….
A switching technique called handoff enables a call to proceed from one cell to another
As demand (or # of users) increases the number of base As demand (or # of users) increases, the number of base stations may be increased to provide additional capacity Smaller cells, e.g., Microcells, Picocell, Femtocell Also cell sites in trucks to replace downed cell towers after natural
disasters, or to create additional capacity for large gatherings (football games, rock concerts)
Transmission power reduction => interference decreases
Typical power transmitted by the radios in a cell systemBase station: Maximum Effective Radiated Power (ERP) is Base station: Maximum Effective Radiated Power (ERP) is 100W, or up to 500 W in rural areas
Mobile station: Typically 0.5 W. For CDMA, transmit power is lowered when close to a BS
Sem. II, 2010/11
lowered when close to a BS
Wireless Communications - Ch. 2 – Cellular System 6
Forward and Reverse Channels
Forward Voice Channel (FVC): Used for voice transmission from BS to MS
Reverse Voice Channel (RVC): Used for voice transmission from MS to BS
C C ( CC) f Forward Control Channel (FCC): Used for initiating a call from BS to MS
R C t l Ch l (RCC) U d f i iti ti ll Reverse Control Channel (RCC): Used for initiating a call from MS to BS
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 7
Anatomy of a Cellular Call
A cell phone, when turned on, (though not yet engaged in a call) scans the group of FCC to determine the one with the strongest signalstrongest signal
It monitors the channel until it drops below the usable thresholdthreshold
It then scans for another channel with the strongest signal
Control channels are defined and standardized throughout Control channels are defined and standardized throughout the service area
Typically the control channels use up to 5% of the total number of channels
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 8
A Call TO a Mobile User
The MSC dispatches the request to all the base stations The Mobile Identification Number (MIN) is broadcast as a paging
message over all FCC throughout the service area.g g
The MS receives the paging message from the BS it is monitoringg
It responds by identifying itself over the RCC
The BS conveys the handshake to the MSCy The MSC instructs the BS to move to an unused voice
channel
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 9
A Call TO a Mobile User. . .
The BS signals the MS to change over to unused FVC and RVC
A data message (called ‘alert’) is transmitted over the FVC to instruct the mobile to ring
f f f All of these sequences of events occur in just few seconds, and are not noticeable to the user
Whil th ll i i th MSC dj t th While the call is in progress, the MSC adjusts the transmitted power in order to maintain the call quality
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 10
A Call FROM a Mobile User
A call initiation request is sent to the RCC
Along with this, the MS transmits its MIN, Electronic Serial Number (ESN) and the phone number of the called party
The MS also transmits the Station Class Mark (SCM) which findicates the maximum transmitter power level for the
particular user
Th BS f d th d t t th MSC hi h lid t th The BS forwards the data to the MSC, which validates the data and makes connection to the called party through the PSTN
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 11
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 12
Cell Shape – Why hexagon?
• The hexagonal shape is a simplistic assumption
(a) is theoretical coverage area and (b) measured coverage area h d bl d ll i di t i l t th iwhere red, blue, green, and yellow indicate signal strength, in
decreasing order Footprint: Actual radio coverage and obtained experimentally
Actual shape is a random that depends on the environment Circular (theoretical): If path loss was a decreasing function
of distance say 1/dn where d is the distance b/n BS & MS
Sem. II, 2010/11
of distance, say, 1/d , where d is the distance b/n BS & MS
Wireless Communications - Ch. 2 – Cellular System 13
Cell Shape – Required
Geometric shape that approximates the theoretical shape?theoretical shape?
Shape whose non-overlapping and repetitive placement covers an entire region?eg o
Possible shapes Triangles, squares, g , q ,
hexagons Which one to choose?
Has dead zonesHas dead zones
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 14
Cell Shape . . .
RR
RR
aT = 33/2 R2/4aR = 2R2 aH= 33/2 R2/2
Hexagonal cell is conceptual, however, it is universally g p , , yadopted for most theoretical treatment because:
Hexagons are a geometric shape that approximates a circle (for O i di ti l di ti )Omni-directional radiation)
Using a hexagon geometry, fewest number of cells can cover the entire geographic region
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 15
Cell Shape . . .
When using hexagon to model coverage areas Center-excited cell: Base station (BS) depicted as being in the
center of the cell Omni-directional antenna is used
Edge-excited cell: on three of the six cell verticesS t d di ti t i d Sectored direction antenna is used
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 16
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 17
Frequency Reuse – Example
Assume a city of 10 Million mobile users Let every user is allocated a radio spectrum for analog speech of 4
kHz bandwidth Thus the required bandwidth is 4 kHz * 10 Million users = 40 GHz!
Clearly impractical!N th i ibl i di t i i No other services possible using a radio transmission
Most of the spectrum will be unused most of the time
Cellular radio systems rely on intelligent allocation and reuse of channels through out the coverage area Available group of channels are assigned to a cluster Available group of channels are assigned to a cluster Same group of frequencies are reused to cover another cell
separated by a large enough distance, i.e., there is a tradeoff
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 18
Frequency Reuse – Example
Example: Consider a cluster of 7 cells
Same color labeled cells use the same frequencyfrequency
Frequency reuse factor is 1/7 since each cell
t i thcontains one-seventh of the total available channels
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 19
Geometry of HexagonsU
Vy
U
x
D
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 20
Geometry of Hexagons …
Axes U and V intersect at 60o
Unit distance is the distance between cell centersIf ll di t i t f h i R th If cell radius to point of hexagon is R, then 2Rcos 30o = 1 or R = 1/√3 (Normalized radius of a cell)
To find the distance of a point P(u,v) from the origin, use X-To find the distance of a point P(u,v) from the origin, use XY to U-V coordinate transformation
0
0
222
30i30cosuxyxr
21
22
0
)(
30sin
uuvvr
uvy
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 21
Geometry of Hexagons …
Using these equations, to locate the co-channel cells, start from a reference cell and move i-hexagons along the U-axis and i hexagons along the U axis and j-hexagons along the V-axis
The distance, D, between co-channel cells in adjacent clusters is given by
22 jijiD
The number of cells in a cluster, N, is given by22 jijiN
where i and j are non-negative integers There are only certain cluster sizes and layouts possible
T i l l f N 1 3 4 7 12
Sem. II, 2010/11
Typical values of N are 1, 3, 4, 7, 12, ……
Wireless Communications - Ch. 2 – Cellular System 22
Example
Re-use Coordinates Number of Cells in the cluster
Normalized repeat distance
i j N Di j N D1 0 1 1
1 1 3 1 7321 1 3 1.732
2 1 7 2.646
2 2 12 3 4642 2 12 3.464
1 3 13 3.606
3 2 19 4 3593 2 19 4.359
1 4 21 4.583
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 23
Locating Co-Channel Cells: Example N=7, i=2 & j=1
V To find out the
nearest co-channel neighbors of a
BS1
neighbors of a particular cell, do the following
M i ll i th U
U Move i cells in the U
direction Then turn 60 degree
t l k i d
BS1
BS1
counter clockwise and move j cells in the V direction
BS1
1/3
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 24
Locating Co-Channel Cells: Example N=19, i=3, j=2
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 25
Re-use Factor
For Hexagonal cells, the re-use distance is i b
Dgiven by:
RNRD 3
Where R = cell size and N = cluster size
Re use factor is
R
Re-use factor is defined as:BS1
BS1 DBS1N
RDq 3
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 26
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 27
Cell Capacity and Reuse
Consider a cellular system Which has S duplex channels available for re-use Each cell allocated a group of k channels Each cell allocated a group of k channels Let the S channels be divided among N cells (unique and disjoint)
then, kNS
Cluster: N cells, which collectively use the complete set of available frequencies
If a cluster is replicated M times in the system, the total number of duplex channels, C, as a measure of capacity is
SMNkMC
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 28
Cell Capacity and Reuse . . .
If cluster size N is reduced while cell size is kept constant More clusters are required to cover area of interest, i.e., So capacity is directly prop to replication factor for fixed area
CM So capacity is directly prop. to replication factor for fixed area
However, small cluster size means co-channel cells are located much closer togetherlocated much closer together Results in larger co-channel interference May result in lower Quality of Service (QoS)
Conversely, large cluster size indicates that co-channel cells are far from each other Less co channel interference and frequency utilization Less co-channel interference and frequency utilization
The value of N is a function of how much interference a mobile or BS can tolerate
Sem. II, 2010/11
mobile or BS can tolerate
Wireless Communications - Ch. 2 – Cellular System 29
Cell Capacity and Reuse: Example 1
Assume that: 50 MHz is available for
forward channels GSM is deployed Each channel is 200 kHz In GSM TDMA is used so In GSM, TDMA is used so
that 8 simultaneous calls can be made on each channel
How large is k? How many forward calls
can be madecan be made simultaneously for the deployment containing 28
Sem. II, 2010/11
cells as in the figure?
Wireless Communications - Ch. 2 – Cellular System 30
Cell Capacity and Reuse: Solution
Solution: There are 50 MHz / 0.2 MHz = 250 channels per cluster With N = 4 then k = 250/4 = 62 5 With N 4, then k 250/4 62.5
With 62.5 channels, 8(62.5) = 500 simultaneous calls can be made in each cell
There are 28 cells on the cell map in Figure, so the total forward calls is 28(500) = 14×103 calls can be made simultaneously
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 31
Cell Capacity and Reuse: Example 2
Suppose 33 MHz BW allocated to particular FDD cellular system, where two 25 KHz simplex channel to provide full-duplex for voice/dataduplex for voice/data
Compute the number of channels per cell if a system uses Four-cell reuse
S ll Seven-cell reuse Twelve-cell reuse
Solution: Given that Total BW = 33 MHz, channel BW = 25 KHz x 2 = 50 KHz/duplex
channel S = 33,000/50 = 660 channels
For N = 4, k = 660 / 4 ≈ 165 channels For N = 7, k = 660 / 7 ≈ 95 channels
Sem. II, 2010/11
For N = 12, k = 660 / 12 ≈ 55 channels
Wireless Communications - Ch. 2 – Cellular System 32
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 33
Channel Assignment Strategies
Which channels should be assigned to a cell? Channel assignment strategies can be classified as either
fixed or dynamicfixed or dynamic
Within a cluster, separate channels in as much as possibleWithin a cluster, separate channels in as much as possible This reduces adjacent channel interference
A scheme for increasing capacity and minimizing g p y ginterference is required
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 34
Fixed Channel Assignment
Each cell is assigned a fixed number of voice channels Any call attempt within the cell can only be served by the unused
channels in that particular cell
If all the channels in the cell are in use, the call is blocked I.e., the user will not get service
Simple, but a busy cell will run out of channels before a neighboring cell Service variations of fixed assignment strategy exit System performance will be limited by the most crowded cell
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 35
Fixed Channel Assignment …
In a variant of the fixed channel assignment, a cell can borrow channels from its neighboring cells if its own channels are fullchannels are full MSC supervises such procedures and ensures that the borrowing
of a channel does not disturb any call in the donor cell
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 36
Dynamic Channel Assignment
In dynamic channel assignment (DCA), channels are not assigned to cells permanently Each basestation can change the channels it uses Each basestation can change the channels it uses
When a call request is made, the BS requests a channel from the MSCfrom the MSC MSC only allocates the channel after verifying that the channel is
not presently in use
To ensure a required QoS, the MSC allocates a given frequency if that frequency is not currently used in The cell or The cell, or In any other cell which falls within the limiting reuse distance, i.e.,
channels in neighboring cells must still be different
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 37
Dynamic Channel Assignment . . .
The MSC allocates a channel taking into account The likelihood of future call blocking The frequency usage of the candidate channel The frequency usage of the candidate channel The reuse distance of the channel Other cost functions
DCA reduces the likelihood of blocking, thus increasing the capacity of the system
DCA strategies require the MSC to collect real-time data on channel occupancy and traffic distribution on a continuous basiscontinuous basis DAC requires more careful control as it gives extra load to
the MSC
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 38
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 39
Handoff
The process of transferring a call, which is in progress from one channel or BS to another is called handoff or handover
Handoff is required when a MS moves into a different cell MSC facilitates the transfer
In general handoff involves In general, handoff involves Identifying the new BS Allocation of voice and control channels in the new BS
Prioritize handoff requests over call initiation requests when allocating unused channels in a cell site
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 40
Handoff Region – Power Strength
P1(x)BS-1 BS-2P2(x)
By looking at the variation of signal strength from either base station, it is possible to decide on the optimum area where handoff can take place
Sem. II, 2010/11
handoff can take place
Wireless Communications - Ch. 2 – Cellular System 41
Handoff
Handoffs must be performed Successfully As infrequently as possible and As infrequently as possible, and Must be imperceptible to the user
To meet these requirements, a minimum usable signalTo meet these requirements, a minimum usable signal level must be specified for acceptable voice quality at the base station
If the received power drops too low prior to handoff the call will be If the received power drops too low prior to handoff, the call will be dropped so that users complain about dropped calls
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 42
Handoff Region . . .
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 43
Handoff Margin
Consider the following two power levels Pr,min. usable be the minimum received power in dB, below which a
call cannot be received A handoff has to be initiated much prior to this point
Pr,handoff be a higher threshold in dB at which the MSC initiates the h d ff dhandoff procedure Handoff is made when the received signal at the BS falls below the
threshold
Define handoff margin in dB as ∆ = Pr,handoff − Pr,min. usable
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 44
Handoff Margin …
How much margin is needed to handle a mobile at driving speeds?
The margin ∆ should not be too large or too small The margin ∆ should not be too large or too small The handoff threshold power is selected such that it is slightly
greater than the minimum usable signal power for an acceptable voice qualityvoice quality
If ∆ is too large, it may lead to unnecessary handoffs which may burden the MSCmay burden the MSC The call may be headed over to the neighboring BS when the MS
is well inside the home cell
If ∆ is too small, there may be insufficient time to complete a handoff before a call is lost due to weak signal conditions
Sem. II, 2010/11
g
Wireless Communications - Ch. 2 – Cellular System 45
Handoff Margin … A ll d l h h th i i A call drop can also happen when there is an excessive delay by the MSC in assigning a channel E.g., during high traffic conditionsg g g
To effect handoff, it is important to ensure that the mobile is actually moving away from the serving base station The measured signal level drop may be due to momentary fading In order to ensure this, BS monitors signal level for a certain
period of time before a handoff is initiated p The length of monitoring depends on the speed of mobile units Where to get information about the mobile speed?
At high mobile speeds, handoff needs to happen quickly In GSM, handoff is typically within 1-2 seconds In AMPS this was 10 seconds (higher potential for dropped calls!)
Sem. II, 2010/11
In AMPS, this was 10 seconds (higher potential for dropped calls!)
Wireless Communications - Ch. 2 – Cellular System 46
Handoff Margin – Example
Assume that A mobile moving at a speed of v = 35 mps (~125 Kph) Path-loss exponent n = 4 Path loss exponent n 4 Cell radius of 500 meters (the distance at which the call is
dropped) 2 second handoff 2 second handoff
What is the required handoff margin?
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 47
Handoff Margin - Solution
Assume the mobile is driving directly away from the BS So distance d changes by 70 meters in two seconds
Consider the received power at the two times Consider the received power at the two timesPr,min. usable = 0 − 10nlog10dPr,handoff = 0 − 10nlog10(d−70)
Taking the difference of the 2nd and the 1st equations,∆ = 10nlog10d − 10nlog10(d − 70) = 10n log10(d/(d − 70))
Taking that the call is dropped at d = 500 meters, we have∆ = 40 log10(500/430) = 2.6 dB
Note: In this example, the propagation equation used is for “large scale path loss” only, which changes slowly
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 48
Handoff Strategies
1. MSC controlled Used in the 1st generation analog cellular systems Signal strength measurements are made by the BS and Signal strength measurements are made by the BS and
supervised by the MSC A spare receiver in each BS, called the location receiver, is used to
determine signal strengths of mobile users which are indetermine signal strengths of mobile users which are in neighboring cells (and appear to be in need of handoff)
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 49
Handoff Strategies ….
2. Mobile-assisted hand-off (MAHO) Used in the 2nd generation systems MSs measures the received power from surrounding BSs and MSs measures the received power from surrounding BSs and
report the results to home BS Handoff is initiated when the received power at the MS from the
neighboring BS begins to exceed the home BS by a certain levelneighboring BS begins to exceed the home BS by a certain level for a certain period of time
The MAHO performs at a much faster rate, and is particularly suited for micro cellular environmentssuited for micro cellular environments
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 50
Handoff Strategies ….
Intersystem handoff When a mobile user moves from one cellular system to a different
cellular system controlled by a different MSCy y It may become a long-distance call and a roamer Compatibility between the two MSCs need to be determined
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 51
Handoff Strategies - Prioritizing Handoffs
Having a call abruptly terminated while in the middle of a conversation is more annoying than being blocked occasionally on a new call attemptoccasionally on a new call attempt
Concept of guard channels A fraction of the total available channel is reserved for handoffA fraction of the total available channel is reserved for handoff
requests, which then are not offered to mobiles making new calls It may reduce the total carried traffic However it offers efficient spectrum utilization when dynamic However, it offers efficient spectrum utilization when dynamic
channel assignment strategies are used
Queuing of handoff requestsg q Does not guarantee a zero probability of forced termination
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 52
Handoff Strategies - Practical Handoff Considerations
How to handle the simultaneous traffic of high speed and low speed users while minimizing the handoff intervention from the MSC?from the MSC? Using microcells to increase capacity also increases burden on
MSC
Another practical limitation is the abilit to obtain ne cell Another practical limitation is the ability to obtain new cell sites, particularly in an urban environment
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 53
Handoff Strategies - Umbrella Cell
By using different antenna heights (often at the same building or tower) and different power levels, “large” and “small” cells are co-located at a single locationsmall cells are co located at a single location
Minimizes the number of handoffs for high speed users and provides additional microcell channels for pedestrian users
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 54
Handoff Strategies – Hard Handoff
Hard handoff: The channel in the source cell is released only when the channel in the target cell is engaged I e assign different radio channels during a handoff I.e., assign different radio channels during a handoff
For 1st generation analog systems, if takes about 10 seconds and the value for ∆ is on the order of 6dB to 12dB
For 2nd generation digital systems, typically requires only 1 or 2seconds, and ∆ usually is between 0 dB and 6 dB
In 2nd generation systems, handoff decision is also based on a co-channel and adjacent channel interference levels
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 55
Handoff Strategies – Soft Handoff in CDMA
The channel in the source cell is retained and used for a while in parallel with the channel in the target cell
Used in CDMA system In CDMA, users share the same channel in every cell Consequently, handoff does not mean a physical change in theConsequently, handoff does not mean a physical change in the
assigned channel, rather that a different base station handles the radio communication task
B i lt l l ti th i i l f i l By simultaneously evaluating the receiver signals from a single subscriber at several neighboring base stations, the MSC may actually decide which version of the user’s signal is best at any moment in timemoment in time
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 56
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y
Co-channel interference Adjacent channel interference Power control for reducing interferences Power control for reducing interferences
Trunking and grade of service Improving capacity
Sem. II, 2010/11
p g p y
Wireless Communications - Ch. 2 – Cellular System 57
Interference
Interference is a major limiting factor in the performance of cellular radio It limits capacity thereby increasing the number of dropped calls It limits capacity thereby increasing the number of dropped calls
Interference are difficult to control in practice largely due to random propagation effectsrandom propagation effects
Sources of interference include Another mobile in the same cell or in a neighboring cellg g Other BSs operating in the same frequency band Any cellular (e.g., from competing cellular carriers) or non-cellular
system which inadvertently leaks energy into the cellular frequencysystem which inadvertently leaks energy into the cellular frequency band
…
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 58
Interference – Effects
Interference in the voice channels causes crosstalk A subscriber hears interference in the background due to an
undesired transmission
Interference in the control channels causes error in digital signaling which causessignaling which causes Missed calls Blocked calls Dropped calls
Interference is more severe in urban areas, due to the t RF i fl d th l b f bgreater RF noise floor and the large number of base
stations and mobiles
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 59
Interference – Types
There are two major types of Interferences: Co-channel Interference (CCI) Adjacent channel Interference (ACI) Adjacent channel Interference (ACI)
CCI is caused due to the cells that reuse the same frequency setfrequency set These cells using the same frequency set are referred to as co-
channel cells
ACI is caused due to signals that are adjacent in frequency
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 60
Co-Channel Interference – First-tier Interference
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 61
Co-Channel Interference – First-tier Interference
First-tier co-channel BSs
D1D 1
D2D5
D6
S i B
D3D4
Serving Base Station
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 62
Co-Channel Interference …
Unlike thermal noise, CCI cannot be overcome by increasing the carrier power of a transmitter This is because any increase in the transmitter power also increases This is because, any increase in the transmitter power also increases
the interference to other co-channel cells
Instead, co-channel cells must be physically separated by a p y y p yminimum distance to provide sufficient isolation due to propagation To reduce CCI the co-channel cells must be sufficiently separated To reduce CCI the co-channel cells must be sufficiently separated
Co-channel interference is a function of The radius of the cell R and The radius of the cell, R, and The distance to the center of the nearest co-channel cell, Di
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 63
Co-Channel Interference …
For a hexagonal geometry, the co-channel reuse ratio, Q is related to the cluster size
D
It determines the spatial separation relative to the coverage
NRDQ 3
It determines the spatial separation relative to the coverage distance of the cell
N small gives Q smallg Provides a larger capacity (i.e., can re-use more), but higher CCI
N large means Q largeB tt t i i lit d t ll l l f h l Better transmission quality due to a small level of co-channel interference but small capacity
Hence there is capacity vs interference tradeoff
Sem. II, 2010/11
Hence there is capacity vs. interference tradeoff
Wireless Communications - Ch. 2 – Cellular System 64
Co-Channel Interference …
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 65
Signal-to-Interference Ratio
Signal-to-interference ratio (S/I) for a mobile which monitors a forward channel is
SS
Where S: desired signal power, Ij: interference caused by the jth co-
m
jjII
1
jchannel cell, and m: first-tier co-channels cells
The average received power at a distance d from the transmitting antenna is approx bytransmitting antenna is approx. by
or n
or ddPP
)log(10)()(
00 d
dndBPdBPr
Where Po is the received power at a close-in reference distance in the far-field and n is the path-loss exponent
od 0d
Sem. II, 2010/11
The path loss exponent, n, ranges between 2 and 6
Wireless Communications - Ch. 2 – Cellular System 66
Signal-to-Interference Ratio …If D i th di t f th ith i t f th i d If Di is the distance of the ith interferer, the received power is proportional to
If transmit power of each BS is equal & n is the same
niD )(
If transmit power of each BS is equal & n is the same throughout the coverage area, S/I for a mobile is approx. as
nRS
To simplify, assume all first-tier interferers are equidistance
m
i
niDI
1)(
To simplify, assume all first tier interferers are equidistance
mN
mR
D
IS
nn3
This relates S/I to the cluster size, and in turn determines the overall capacity of the system
mmI
Sem. II, 2010/11
Puts a limits on how low we may set N
Wireless Communications - Ch. 2 – Cellular System 67
Signal-to-Interference Ratio …
For a hexagonal cluster of cells with the MS situated at the edge of the cell Rthe edge of the cell
nn
NRD
IS 3
61
61
DD
As long as all cells are of the i S/I i
RI 66 D
DD
same size, S/I is independent of the cell radius, R
D D
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 68
Signal-to-Interference Ratio - Example 1
Design parameters: Desired S/I = 15dB Path-loss exponent n = 4 Path loss exponent n 4 Assume that there are six co-channel cells in the first tier and all of
them are at the same distance from the mobile
What is the required re-use factor and cluster size that should be used for maximum capacity?
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 69
Signal-to-Interference Ratio – Example 1 …
Six co-channel cells in the first tiertier
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 70
Signal-to-Interference Ratio - Example 1 …
Let’s try for N= 4. The co-channel re-use ratio is
D
• Let’s try: N= 7D 58.4
And the signal-to-interference ratio is
46.3RD
dBISR
66.185.73
58.461
58.4
4
ratio is
• Which is greater than the desired
dBIS 8.132446.3
61 4
Smaller than the desired 15 dB
We m st mo e to the ne t re se
• Hence, N=7 can be used
• The frequency reuse We must move to the next reuse distance
The frequency reuse factor = 1/7
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 71
Example 2 - Repeat Example 1 for n = 3
Solution Let’s try for a seven-cell reuse pattern, i.e. N= 7. Like the previous
examplep
Which is smaller than the desired 15 dB, hence we need to use larger N
dBISand
RD 05.1204.1658.4
6158.4 3
larger N Let us try N=12
dBSandD 5615360061006 3
Since this is greater than 15 dB, N=12 can be used
N t 3 i t i l l f b b
dBI
andR
56.153600.66
00.6
Note: n=3 is typical value for sub-urban area
Exercise: Try for n=2, which represents rural area!
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 72
Summary - Re-Use Factor for n=2, n=3, and n=4
30
20
25 Path loss n= 2Path loss n = 3Path loss n=4
10
15
IR in
dB
N=12N=7
0
5
SI
0 2 4 6 8 10 12 14 16 18 20-5
0
Cluster Size N
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 73
Cluster Size, N
Worst Case Calculation of S/I
The MS is at the cell boundary
The approximate S/I is The approximate S/I is given by:
nnn
n
RDDRDR
IS
222
S
1 nnn QQQI
12212
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 74
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y
Co-channel interference Adjacent channel interference Power control for reducing interferences Power control for reducing interferences
Trunking and grade of service Improving capacity
Sem. II, 2010/11
p g p y
Wireless Communications - Ch. 2 – Cellular System 75
Adjacent Channel Interference (ACI)
Results from signals that are adjacent in frequency to the desired signal Due to imperfect receiver filters that allow nearby frequencies to Due to imperfect receiver filters, that allow nearby frequencies to
leak
Near-far effect: The adjacent channel interference is jparticularly serious. This occurs when:
When an interferer close to the BS radiates in the adjacent h l hil th b ib i f f th BSchannel, while the subscriber is far away from the BS The BS may not discriminate the desired mobile user from the “bleed
over” caused by the close adjacent channel mobile
Or, an interferer which is in close range to the subscriber’s receiver, is transmitting while the receiver receives from the BS
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 76
Near-Far Effect - Interferer Close to BS
• One solution is power control, i.e., reducing the power level transmitted by mobiles close to the BS
SubscriberInterferer
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 77
Near-Far Effect - Interferer Close to MS
SubscriberInterferer
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 78
Adjacent Channel Interference …
ACI can be reduced by Careful filtering Careful channel assignment Careful channel assignment
The frequency separation between each channel in a cell should be made as large as possibleshould be made as large as possible Assign non-adjacent channels within each cell’s channel group
Example: Assign S = 50 channels into groups for N = 7.p g g p Solution
There are about k = 50/7 ≈ 7 channels per cellF 1 f d h l {1 8 15 22 29 36 43 50} For group 1, use forward channels {1, 8, 15, 22, 29, 36, 43, 50}
For group i, i = 2, . . . 7, let the channels for group i consist of {i, i+7, i + 14, i + 21, i + 28, i + 35, i + 42}
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 79
Adjacent Channel Interference …
Example: The frequency separation between each channel in a cell should be made as large as possible while assigning themassigning them
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 80
Adjacent Channel Interference …
If a subscriber is at a distance d1 and the interferer is d2from the base station, then SIR (prior to filtering) is:
n
n
dd
IS
2
1
Example Suppose a subscriber is at d1 = 1000m from the BS and an
dj t h l i t f i t d 100 f th BSadjacent channel interferer is at d2 = 100m from the BS Assume: Path-loss exponent is n = 3 The signal-to-Interference ratio prior to filtering is then
dBdd
IS
n
3010100
1000 33
2
1
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 81
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y
Co-channel interference Adjacent channel interference Power control for reducing interferences Power control for reducing interferences
Trunking and grade of service Improving capacity
Sem. II, 2010/11
p g p y
Wireless Communications - Ch. 2 – Cellular System 82
Power Control to Reduce Interference
In practice, power levels transmitted by every subscriber are under constant control by the serving BS Each MS transmits with the smallest power necessary Each MS transmits with the smallest power necessary
In power control1. Reduces the transmit power level of MSs close to the BS since a1. Reduces the transmit power level of MSs close to the BS since a
high TX power is not necessary in this case2. MSs located far away must transmit with larger power than those
nearbynearby
Advantages of power control Reduces out-of-band interference Prolongs battery life and Even reduces even co-channel interference on the reverse
channel
Sem. II, 2010/11
channel
Wireless Communications - Ch. 2 – Cellular System 83
Power Control to Reduce Interference …
However, power control requires well control Controlling a mobile means communication from the BS to the
mobile to inform it whether to increase or decrease its power, p ,which incurs overhead
In CDMA systems, every user in every cell share the same radio channel means a tight power control is required The “near-far problem” is even more of a problem in CDMA Need to reduce the co-channel interferenceNeed to reduce the co channel interference Reduced interference leads to higher capacity
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 84
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service
Basic definitions Bl k d ll l d Blocked calls cleared
Blocked calls delayed
Improving capacity
Sem. II, 2010/11
p g p y
Wireless Communications - Ch. 2 – Cellular System 85
Trunking
Trunking: How to accommodate a large number of users in a limited radio spectrum?
T ki f t h i fi d d ll b f Trunking refers to sharing a fixed and small number of channels among a large and random user community
Each user demands access from a pool of channel Each user demands access from a pool of channel infrequently & at random times A channel is allocated on a per call basis and a channel is returned
to the pool up on termination of a call So a dedicated channel for each user is not required If U be number of users and C be number of channels, for any C < y
U, possibility of more requests than channels
Trunking exploits statistical behavior of users so that a fixed b f h l d t l d
Sem. II, 2010/11
number of channels accommodate a large, random user
Wireless Communications - Ch. 2 – Cellular System 86
Trunking …
Trunking accommodates large & random users: By providing access to each user on demand from a pool of
available channels
When a user requests service and if all channels are in use
1. The user is blocked, or denied access to the system, y2. In some systems, a queue may be used to hold the requesting
users until a channel becomes available
Upon termination of the call the previously occupied channel is Upon termination of the call, the previously occupied channel is immediately returned to the pool
Designing a trunked system, that can handle a givenDesigning a trunked system, that can handle a given capacity at a specific “grade of service”, requires trunking and queuing theories
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 87
Trunking – Definition of Terms . . .
Setup time: The time required to allocate a radio channel to a requesting user Users request may be blocked or have to wait Users request may be blocked or have to wait
Blocked Call: A call that cannot be completed at the time of request due to congestion Also called lost call => lost revenue, e.g., pick hours, holidays, …
Holding time: Average call duration in seconds, denoted H Depends on users and operator's tariff
Request (or call) rate: Average number of calls per unit time, denoted λ seconds-1denoted λ seconds Typically taken to be at the busiest time of day Depends on type of users community: Office, residential, call center,
Sem. II, 2010/11
…
Wireless Communications - Ch. 2 – Cellular System 88
Trunking – Definition of Terms . . .
Traffic Intensity: A measure of channel time utilization Is the average channel occupancy measured in Erlang, denoted by A
Load: Traffic intensity across the entire trunked radio system Load: Traffic intensity across the entire trunked radio system Measured in Erlang
Erlang: A “unit” of measure of usage or traffic intensity Erlang: A unit of measure of usage or traffic intensity A channel kept busy for one hour is defined as having a load of one
Erlang
Grade of Service (GoS): Measure of congestion (or ability of a user to access a trunked system) during the busiest hour Typically given as likelihood that a call is blocked called Erlang B or Typically given as likelihood that a call is blocked, called Erlang B or The likelihood of a call experiencing a delay greater than a certain
amount of time, called Erlang C
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 89
Trunking …
Average arrival rate, λ: Average number of MSs requesting service (call request/time)
Average hold time H Average hold-time, H Average duration of a call (or time for which MS requires service)
An average traffic intensity offered (generated) by each user An average traffic intensity offered (generated) by each user
Example 1: If a user makes on average two calls per hour
)(ErlangsHAu
Example 1: If a user makes on average two calls per hour, and that a call lasts an average of 3 minutes
ErlangA 10min32 ErlangAu 1.0min3
min60
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 90
Trunking …
Example 2: In a cell with 100 MSs average of 30 requests are generated in an hour with average holding time of 6 minutes6 minutes
The arrival rate: sec/360030 requests
Offered load is: ErlangsCallSeconds
SecondsCallsAu 3360*
360030
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 91
Trunking …
The total offered traffic intensity for U users Note: A is not necessarily the traffic carried by the trunked system
uUAA
In a C channel trunked system, if traffic is distributed equally among channels, then traffic intensity per channel
AUA
I E l 1 th t th 100 d 20
CA
CUAA u
C
In Example 1, assume that there are 100 users and 20 channels Then A = 100(0.1)= 10 and Ac = 10/20 = 0.5( ) c
Note: Ac is a measure of the efficiency of channels utilization
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 92
Trunking and GoS
Offered traffic is not necessarily the traffic carried by the trunked system, only that is offered to the system Maximum possible carried traffic is the total number of channels C Maximum possible carried traffic is the total number of channels, C,
in Erlangs
AMPS system is designed for a GOS of 2% blockingy g g Channel allocations for cells are designed so that 2 out of 100 calls
will be blocked due to channel occupancy during the busiest hour
What do we do when a call is offered (requested) but all channels are full?
Blocked calls cleared? Offers no queuing for call requests Erlang B Blocked calls cleared? Offers no queuing for call requests, Erlang B Blocked calls delayed? Erlang C
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 93
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service
Basic definitions Bl k d ll l d Blocked calls cleared
Blocked calls delayed
Improving capacity
Sem. II, 2010/11
p g p y
Wireless Communications - Ch. 2 – Cellular System 94
Trunking – Blocked Calls Cleared1 C ll i l t f ll P i di t ib ti1. Calls arrival request follows a Poisson distribution2. Memoryless arrivals of requests
I e all users including blocked users may request a channel at I.e., all users, including blocked users, may request a channel at any time
3. The probability of a call durations (or a user occupying a channel) is exponentially distributedchannel) is exponentially distributed I.e., longer calls are less likely to occur
4. There are “infinite number of users” and “finite channels” Rather than a finite number U of users each requesting Au traffic,
set the total offered traffic as a constant A, and then let U and Au 0 in a way that preserves A = UAuu y p u
These assumptions leads to the Erlang B formula Also known as the “blocked calls cleared formula”
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 95
Trunking – Erlang B Formula
The probability of an arriving call being blocked is:
GOS!][ C k
c
rCA
blockingP
Where C: number of trunked channels and A: total offered traffic
!
][
0
C
k
kr
kA
g
Erlang B is a measure of the GOS for a trunked system which provides no queuing for blocked calls
Setting the desired GOS, one can derive Number of channels needed The maximum number of users we can support as A = UA or The maximum number of users we can support as A = UAU or The maximum AU we can support (and set the number of minutes
on our calling plans accordingly)
S C
Sem. II, 2010/11
Since C is very high, it is easier to use table or graph
Wireless Communications - Ch. 2 – Cellular System 96
Erlang B Formula - Table Form
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 97
Erlang B Formula - Graphical Form
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 98
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service
Basic definitions Bl k d ll l d Blocked calls cleared
Blocked calls delayed
Improving capacity
Sem. II, 2010/11
p g p y
Wireless Communications - Ch. 2 – Cellular System 99
Trunking – Blocked Calls Delayed
Instead of clearing a call, put it in a queue and have it wait until a channel is available First-in first-out line: Calls will be processed in the order received First in, first out line: Calls will be processed in the order received
There are two things to determine here 1. The probability a call will be delayed (enter the queue), and1. The probability a call will be delayed (enter the queue), and 2. The probability that the delay will be longer than t seconds
The first is no longer the same as Erlang Bg g It goes up, because blocked calls aren’t cleared, they “stick
around” and wait for the first open channel
Meaning of GOS The probability that a call will be forced into the queue AND it will
wait longer than t seconds before being served (for some given t)
Sem. II, 2010/11
g g ( g )
Wireless Communications - Ch. 2 – Cellular System 100
Trunking - Blocked Calls Delayed …
Additional assumptions:1. The queue is infinitely long: Translates to infinite memory2 No one who is queued gives up/hangs up (rather than wait)2. No one who is queued gives up/hangs up (rather than wait)
The probability of an arriving call not having an immediate access to a channel (or being delayed) is given by Erlangaccess to a channel (or being delayed) is given by Erlang C Formula
]0[cAdelayP
1
0 !)1(!
]0[ C
k
kc
r
kA
CACA
delayP
It is typically easiest to find a result from a chart
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 101
Trunking - Calls Delayed …
Once it enters the queue, the probability that the delay is greater than t (for t > 0) is given as
AC
GOS: The marginal (overall) probability that a call will be
t
HACdelaytdelayPr exp]0[
GOS: The marginal (overall) probability that a call will be delayed AND experience a delay greater than t is then
delaytdelayPdelayPtdelayP rrr ]0|[]0[][
t
HACdelayP
yyyy
r
rrr
exp]0[
]|[][][
The average delay for all calls in a queued system
ACHdelayPD r ]0[
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 102
AC
Erlang C Formula - Graphical Form
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 103
Trunking - Example 1
Consider a system with 100 cells Each cell has C = 20 channels Each cell has C 20 channels Generates on average λ = 2 calls/hour The average duration of each call (H) = 3 Minutes
f f How many number of users can be supported if the allowed probability of blocking is 2%?
S l ti Solution: From Erlang B Chart, total carried traffic = 13 Erlangs Traffic intensity per user AU = λH = 0.1 Erlangsy p U g The total number of users that can be supported by a cell = 13/0.1
= 130 Users/cell Therefore, the total number of users in the system is 13,000
Sem. II, 2010/11
Therefore, the total number of users in the system is 13,000
Wireless Communications - Ch. 2 – Cellular System 104
Trunking - Example 2
Consider a system with 100 cells, each cell has C = 20 channels Generates on average λ = 2 calls/hour Generates on average λ 2 calls/hour The average duration of each call (H) = 3 Minutes
How many number of users can be supported if theHow many number of users can be supported if the allowed probability of blocking is 0.2%?
Solution Again from Erlang B Chart, total carried traffic = 10 Erlangs Traffic intensity per user AU = λH = 0.1 Erlangs The total number of users that can be supported by a cell = 10/0 1 The total number of users that can be supported by a cell = 10/0.1
= 100 Users/cell Therefore, the total number of users in the system is 10,000
W t l b f
Sem. II, 2010/11
We support less number of users
Wireless Communications - Ch. 2 – Cellular System 105
Trunking - Example 3C id t ith Consider a system with Total number of channels = 20 Probability of blocking = 1%
How shall we use this set of channels? Approach 1: Divide 20 channels into 4 trunks of 5 channels each
Traffic capacity of one trunk (5 channels) = 1 36 Erlangs Traffic capacity of one trunk (5 channels) = 1.36 Erlangs Traffic capacity of four trunks (20 channels) = 5.44 Erlangs
Approach 2: Divide 20 channels into 2 trunks of 10 channels eachT ffi it f t k (10 h l ) 4 46 E l Traffic capacity of one trunk (10 channels) = 4.46 Erlangs
Traffic capacity of two trunks (20 channels) = 8.92 Erlangs Approach 3: Use the 20 channels as they are
Traffic capacity of one trunk (20 channels) =12.0 Erlangs
Better to make a large pool instead of dividing Allocation of channels in a trunked radio system has a major impact
Sem. II, 2010/11
y j pon overall system capacity
Wireless Communications - Ch. 2 – Cellular System 106
Trunking - Example 4
Given An urban area has a population of 2 million residents Three competing trunked mobile networks (system A B and C) Three competing trunked mobile networks (system A, B, and C)
provide cellular service in this area System A has 394 cells with 19 channels each System B has 98 cells with 57 channels each System B has 98 cells with 57 channels each System C has 49 cells each with 100 channels
Each user averages 2 calls per hour at an average call duration of 3 minutes3 minutes
Required The number of users that can be supported at 2% blocking? Assuming that all three trunked systems are operated at maximum
capacity, compute the percentage market penetration of each cellular provider
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 107
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity and coverage
Cell splitting Sectoring Microcell zoning and use of repeaters
Sem. II, 2010/11
Microcell zoning and use of repeaters
Wireless Communications - Ch. 2 – Cellular System 108
Improving Capacity
A network may need to expand because of Increase in traffic or demand for service Or because of a change in the environment (e g a new building) Or because of a change in the environment (e.g., a new building)
As traffic increases, the channels originally assigned to each cell will be congested
System designers have to provide more channels per unit coverage area
Common techniques Common techniques Cell splitting, sectoring, microcell zoning, and use of repeaters
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 109
Cell Splitting
Cell splitting: Process of subdividing a congested cell into smaller cells (called microcells), where each cell has Its own BS (increase in BSs deployed) and Its own BS (increase in BSs deployed) and Reduction in the transmitter power and antenna height
Splitting the cells reduces the cell size and thus moreSplitting the cells reduces the cell size and thus more number of cells have to be used More number of cells = > more number of clusters => more
channels => higher capacitychannels => higher capacity
Cell splitting allows a system to grow by replacing large cells by small cells without new spectrum usage Additional channels per unit area are created
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 110
Cell Splitting . . .Large cell (low density)
S ll ll
• Depending on traffic patterns, the smaller cells may be Small cell
(high density)cells may be activated/deactivated in order to efficiently
lluse cell resources
• The co-channel re-
Smaller
use factor D/R is unchanged
O l i h cell (higher density)
• Only increases the number of channels per unit area
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 111
Cell Splitting - Example 1
Suppose the radius of cell is reduced by half To cover the entire area, four times microcells are required
What is the required transmit power for these new cells? What is the required transmit power for these new cells?
We have: nWe have: Power at the boundary of un-split cell:
Power at the boundary of a new microcell:
ntuu RPP
nRPP )2/( Power at the boundary of a new microcell:
Where Ptu : transmitted power for un-split cell, Pmc : transmitted
tmcmc RPP )2/(
power from for microcell
For same CCI performance Pu = Pmc impliesn
tutmc PP 2/
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 112
tutmc
Cell Splitting - Example 1 . . .
For n = 4; (a typical suburban area)
16tu
tmcPP
Thus, the transmit power must be reduced by 12dB in order to fill in the original coverage area with microcells,
16tmc
order to fill in the original coverage area with microcells, while maintaining the S/I requirement
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 113
Cell Splitting - Example 2
7 Cell 4 Cell Cl ster ClusterCluster
Smaller Cells
7 Cell Cluster 12 Cell
ClusterCluster
Typical city cellular radio cell plan – different cell sizes and clusters Combination of cell size and cluster size to increase capacity
Sem. II, 2010/11
Combination of cell size and cluster size to increase capacity
Wireless Communications - Ch. 2 – Cellular System 114
Cell Splitting - Example 3 Suppose a congested service area is Suppose a congested service area is
originally covered by 5 Cells Each with 80 Channels
Capacity = 5*80 = 400After Splitting: After Splitting: Let We now have 20 cells to cover the region
2/RRnew
New Capacity = 20*80 = 1600
In general, the relationship in capacity between cell litti d b ib dditi b dsplitting and subscriber addition can be expressed as
Where C : network capacity after “n” times cell splitting and C:CC n
n 4
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 115
Where Cn : network capacity after n times cell splitting and C: Network capacity before cell splitting
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity
Cell splitting Sectoring Microcell zoning and use of repeaters
Sem. II, 2010/11
Microcell zoning and use of repeaters
Wireless Communications - Ch. 2 – Cellular System 116
Cell SectoringS t i di ti l t t f th t l Sectoring uses directional antennas to further control interference and frequency reuse
As opposed to cell splitting, where D/R is kept constant while decreasing R, in sectoring keeps R untouched and reduces the D/R ratio
Capacity improvement is achieved by reducing the number of cells per cluster thus increasing frequency reuse
Sem. II, 2010/11
cells per cluster, thus increasing frequency reuse
Wireless Communications - Ch. 2 – Cellular System 117
Cell Sectoring . . .
In order to do this, it is necessary to reduce the relative interference without decreasing the transmitter power
CCI is reduced by replacing single omni-directional antenna by several directional antennas, each radiating within a specified sectorwithin a specified sector
A directional antenna transmits to and receives from only a fraction of the total number of co-channel cells Thus CCI is reduced
CCI reduction factor depends on the amount of sectoringp g A cell is normally partitioned into three 120⁰ sectors or six 60⁰
sectors
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 118
Cell Sectoring . . .
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 119
Cell Sectoring . . .
Assume 7 cell reuse and 1200 sector
Number of interference Number of interference in the first tier reduces from 6 to 2 Significant compared to
omni-directional case
Sectored groups g p
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 120
Cell Sectoring . . .
For a 7-cell cluster, the MS will receive signals from only 2 other clusters (instead of 6 in an omni-directional antenna)
For worst case, when mobile is at the edge of the cell
nn
n
RDDRSIR
)70( nn RDD )7.0(
Interfering co-channel cells @ D distanceDesired channel
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 121
Interfering co-channel cells @ D distanceDesired channel
Cell Sectoring – Problems
Increased number of antennas at each BS
Decrease in trunking efficiency due to sectoring Dividing the bigger pool of channels into smaller groups
Increased number of handoffs (sector-to-sector)
Good news: Many modern BSs support sectoring and related handoffs without the help of the MSC
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 122
Cell Sectoring – Modern BSs
13
21-11 3
2120o
1-21-3
CCISector in use
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 123
Overview
Cellular system Cell shape
F Frequency reuse Cell capacity and reuse Channel assignment strategies Channel assignment strategies Handoff Interference and system capacityy p y Trunking and grade of service Improving capacity
Cell splitting Sectoring Microcell zoning and use of repeaters
Sem. II, 2010/11
Microcell zoning and use of repeaters
Wireless Communications - Ch. 2 – Cellular System 124
Microcell Zone Concept
The problems of sectoring, i.e., increased handoff, can be addressed by the Microcell Zone concept
A cell is divided into microcells or zones Each microcell (zone) is connected to the same base
station via fiber microwave link or coaxialstation via fiber, microwave link, or coaxial Each zone uses a directional antenna
As a MS travels from one zone to another it retains the As a MS travels from one zone to another, it retains the same channel, i.e., no handoff
The BS simply switches the channel to the next zone sitep y
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 125
Microcell Zone Concept …
Let each cell be divided into three zones
Zone Selector
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 126
Microcell Zone Concept …
While the cell maintains a particular coverage area, the CCI is reduces because: The large central BS is replaced by several low power transmitters The large central BS is replaced by several low power transmitters Directional antennas are used
Decreases CCI improvesDecreases CCI improves Signal Quality Capacity
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 127
Microcell Zone Concept …
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 128
Microcell Zone Concept …
Example: Suppose the desired S/I = 18 dB, Path loss exponent n = 4 Path loss exponent n 4,
How much capacity increase can occur if we use Microcell zoning with 3 zones per cell?
Solution To achieve S/I 18 dB we need N 7 To achieve S/I = 18 dB, we need N=7
Now we create 3 zones within a cell The cluster size has been reduced to N = 3 A capacity increase factor of 7/3 = 2.33
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 129
Repeaters for Range Extension
Useful for hard-to-reach areas Within buildings or basements Tunnels Tunnels Valleys
Radio transmitters, called repeaters, can be used to provide coverage in these areas
Repeaters are bi-directional Receive signals from BSs Amplify the signals Re-radiate the signalsg
Problem: received noise and interference is also reradiated!
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 130
Repeaters for Range Extension …
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 131
Summary
Concepts such as handoff, frequency reuse, trunking efficiency, and frequency planning are covered
Capacity of cellular system is a function of many things Capacity of cellular system is a function of many things, E.g., S/I that limits frequency reuse, which intern limits the number
of channels within the coverage area
Trunking efficiency limits the number of users that can access a trunked radio system
Capacity can be improved by cell splitting sectoring and Capacity can be improved by cell splitting, sectoring, and the zone microcell techniques
Sem. II, 2010/11 Wireless Communications - Ch. 2 – Cellular System 132