cellular communications cellular basics. spectrum reuse earlier systems: single central transmitter...
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CELLULAR COMMUNICATIONS
Cellular Basics
Spectrum Reuse
Earlier systems: single central transmitter
Cover wide area Single channel per user 25kHz for sufficient audio quality and
guard interval 40 users in 1MHz, 400 users for 100MHz Modern systems have millions of
subscribers
Spectrum Reuse
Several transmitters, each having only certain coverage area
Cell==coverage area
Reuse same spectrum in many transmitters
Cells
Cells
Often shown as hexagonal shapes In reality, very irregular boundaries Signal strength decreases
gradually=>no exact cell edges Some cell areas may overlap Allocate different spectrum to adjacent
cells Can overlap without causing interference
Cells
Clusters
Cells with different spectrum grouped together as cluster
Often clusters of size 7
Cluster: set of different frequencies used in group of cells
Cluster is repeated by linear shifti steps along one directionj steps in the other direction
How many different frequencies does a cluster contain?
Theoretical Network Planning
Honeycomb (hexagonal) cell structure
Reuse Distance
Distance between cell centers = × Cell Radius
Reuse distance
distance between the centers of two co-channel cells
u2 2
R = i + j + 2ij3
3 Rcosp
whereR is Cell RadiusRu is Reuse Distance
and cos(p /3) = 1/2
3
Cluster Radius
Radius of a cluster
cu
2 2
R = R3
= i + j + ij
3 R
Cluster Size C: number of channels needed for (i,j) grid
is proportional to surface area of cluster
Surface area of one hexagonal cell is
R2S =
3 3
2 R
Surface area of a (hexagonal) cluster of C cells is
uR R
2u
S = CS = 3 3
2R3{ }
Combining these two expressions gives uR = R 3C
Possible Cluster Sizes
We have seen
uR = R 3C
and also
u2 2
R = i + j + ij 3 R
Thus:
C = i + j + ij2 2
with integer i and j .
C = 1 i = 1, j = 0 } Cluster size for CDMA netC = 3 i = 1, j = 1C = 4 i = 2, j = 0C = 7 i = 2, j = 1 } Usual cluster sizes for TDMAC = 9 i = 3, j = 0 } cellular telephone netsC = 12 i = 2, j = 2
·
Cluster size C = i2+ ij + j2 = 1, 3, 4, 7, 9, ...
· Cellular Telephony Chose C to ensure acceptable link quality at cell boundary
Typical Cluster Sizes
Reuse distance 2 – reuse pattern
One frequency can be (re)used in all cells of the same color
Reuse distance 3 – reuse pattern
Design Objectives for Cluster Size
•High spectrum efficiency
many users per cell
Small cluster size gives much bandwidth per cell
• High performance
Little interference
Large cluster sizes
The effect of decreasing cell size•Increased user capacity
•Increased number of handovers per call
•Increased complexity in locating the subscriber
•Lower power consumption in mobile terminal:· Longer talk time,· Safer operation
•Different propagation environment, shorter delay spreads•Different cell layout,
· lower path loss exponent, more interference· cells follow street pattern· more difficult to predict and plan· more flexible, self-organizing system needed (cf. DECT vs. GSM)
Cells
Macrocells 10km, sparsely populated area
Microcells 1km, densely populated area
Picocell 200m, particular buildings, streets
Reuse distance 3 – reuse pattern
Fixed and Dynamic assignment Fixed frequency assignment: permanent
certain frequencies are assigned to a certain cell
problem: different traffic load in different cells Dynamic frequency assignment:
temporary base station chooses frequencies depending on
the frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference
measurements
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Increasing Capacity
Add new channels Dynamic channel allocation – frequencies
can be taken from adjacent cells by congested cells
Cell splitting – cells in areas of high usage can be split into smaller cells
Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels (typical: 3)
Microcells – antennas move to buildings, hills, and lamp posts
Cell sectorization
Use directional antennas
Collocate cell antenna at the cell edges
Reduce cost
Handoff/Handover
Maintain call while moving
Basic Network Architecture
Basic Architecture
Base Station Controller (BSC) Control each base station Manage hand-off of a call from one base
station to other Mobile Switching Center(MSC)
Manages setup and tear down of calls to and from mobile subscribers
Home Location Register (HLR) HLR subscriber database including location
Network
Base Transceiver Station (BTS) Antenna Tower Radio
transceivers Power Supply Link to BSC (land
lines or microwave)
Setting up calls/registration
Make a call originated from mobile handset Allocate resources (channel)
Receive a call Locate the current cell
After the telephone is switched on Contact base station Register to use a network
Registration
Authenticate (e.g. for billing) Authentication Center (AuC)
Store my location HLR for “home” subscribers VLR for “visiting”/roaming subscribers
Mobile communicates with the network to update status/location
Network keeps last known location
Receiving a calls
Network should send a notification to a mobile
Network send to the area where mobile is located
Mobile listen to a “paging” channel Examine each message on the paging
channel and compares number with his own
Respond if match
Paging channel
Always listening to the paging channel drains the battery
Divide paging channel into 10 subgroups according to a last digit of mobile phone number
Mobile has to listen only 1/10 of time Longer call setup time
Random Access Channel(RACH) Respond to call /paging channel
message Initiate a call “Access” message Request a channel/slot/resources for
further communications Slotted ALOHA
Handover(EU)/Handoff(US)
Mobile monitor signal strength Network knows about availability of
channels Mobile monitors strength of signal from
current and adjacent cells and sends this information to network
When signal drops below certain level, network reserved new channel at adjacent cell
Mobile switch channel, network shuts down old channel
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Handoff Region
BSi
Signal strength due to BSj
E
X1
Signal strength due to BSi
BSjX3 X4 X2X5 Xth
MS
Pmin
Pi(x) Pj(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.
Types of Handoffs Hard handoff
A hard handoff is a “break before make” connection. MS is linked to no more than one BS at any given
time. Hard handoff is primarily used in FDMA and TDMA.
Soft handoff It isn't a “break before make” transition. The call can be carried on both cells simultaneously. Soft handoff is used in CDMA.
Handoff Decisions
Decision-making process of handoff may be centralized or decentralized
Three different kinds of handoff decisions Network-Controlled Handoff Mobile-Assisted Handoff Mobile-Controlled Handoff
Umbrella Cells
Operation Support Systems
Network Management Systems Service Delivery Service Fulfillment, including the
Network Inventory, Activation and Provisioning
Service Assurance Customer Care Billing