fundamenatals of cellular enginering
DESCRIPTION
this ppt useful for knowing all basic level of mobile communication technologies.TRANSCRIPT
Fundamentals of Cellular Engineering
Prepared By :
Maulik Patel
Outline
Introduction to cellular system
Concept of frequency reuse
Channel Assignment Strategies
Handoff Process and Generation
Handoff Priority
Enhancing Capacity And Cell Coverage : The key Trade-off
Cell Splitting
Sectoring
Microcell Zone Concept
Reference
Introduction
Goals of a Cellular System :• High capacity• Large coverage area• Efficient use of limited spectrum
Reuse of radio channel
Enable a fix number of channels to serve an arbitrarily large number of users by reusing the channel throughout the coverage region
What is cell ? Each cellular base station is allocated a group of radio
channels within a small geographic area called a cell.
Neighboring cells are assigned different channel groups.
By limiting the coverage area to within the boundary of the cell, the channel groups may be reused to cover different cells.
Consider a cellular system which has a total of S duplex channels.
Each cell is allocated a group of k channels, K < S
The S channels are divided among N cells.
The total number of available radio channels, S = kN
The N cells which use the complete set of channels is called cluster.
The cluster can be repeated M times within the system. The total number of channels, C, is used as a measure of capacity.
C = MkN = MS
The capacity is directly proportional to the number of replication M.
The cluster size, N, is typically equal to 4, 7, or 12.
The frequency reuse factor is given by 1/N.
Frequency reuse
Hexagonal geometry has
• exactly six equidistance neighbors
• the lines joining the centers of any cell and each of its neighbors are separated by multiples of 60 degrees.
Only certain cluster sizes and cell layout are possible.
The number of cells per cluster, N, can only have values which satisfy
N=i2+ij+ j 2
Frequency reuse (Cont.)
Channel Assignment Strategies
Goal is to minimize interference & maximize use of capacity.
One of the Channel assignment strategies is
1. Fixed Channel Assignment : Channels are divided in sets.
A set of channels is permanently allocated to each cell in the network. Same set of channels must be assigned to cells separated by a certain distance to reduce co-channel interference.
Any call attempt within the cell can only be served by the unused channels in that particular cell. The service is blocked if all channels have used up
Most easiest to implement but least flexibility.
An modification to this is ‘borrowing scheme’. Cell (acceptor cell) tha has used all its nominal channels can borrow free channels from its neighboring cell (donor cell) to accommodate new calls.
Borrowing can be done in a few ways: borrowing from the adjacent cell which has largest number of free channels, select the first free channel found, etc.
To be available for borrowing, the channel must not interfere with existing calls. The borrowed channel should be returned once the channel becomes free.
FCA(cont.)
2. Dynamic Channel Allocation (DCA) :
Voice channels are not allocated to any cell permanently. All channels are kept in a central pool and are assigned dynamically to new calls as they arrive in the system.
Each time a call request is made, the serving BS requests a channel from the MSC. It then allocates a channel to the requested cell following an algorithm that takes into acount the likelihood of future blocking within the cell, the reuse distance of the channel and other cost functions ⇒ increase in complexity
Centralized DCA scheme involves a single controller selecting a channel for each cell. Distributed DCA scheme involves a number of controllers scattered across the network.
For a new call, a free channel from central pool is selected based on either the co-channel distance, signal strength or signal to noise interference ratio.
DCA(cont.)
Handoff :
• When a user moves from one cell to the other, to keep the communication between the user pair, the user channel has to be shifted from one BS to the other without interrupting the call
• when a MS moves into another cell, while the conversation is still in progress, the MSC automatically transfers the call to a new FDD channel without disturbing the conversation. This process is called as handoff.
Handoff operation :
• identifying a new base station
• re-allocating the voice and control channels with the new base station.
What is Handoff ?
Handoff
Handoff Process
Once a signal level is set as the minimum acceptable for good voice quality (Prmin), then a slightly stronger level is chosen as the threshold (PrH)at which handoff has to be made.
A parameter, called power margin, defined as
Δ = PrH − Prmin
If Δ is too small, then there may not be enough time to complete the handoff and the call might be lost even if the user crosses the cell boundary.
If Δ is too high o the other hand, then MSC has to be burdened with unnecessary handoffs. This is because MS may not intend to enter the other cell.
Handoff Generation
Handoff for first generation analog cellular systems• 10 secs handoff time• is in the order of 6 dB to 12 dB
• Handoff for second generation cellular systems, e.g., GSM• 1 to 2 seconds handoff time• mobile assists handoff• is in the order of 0 dB to 6 dB• Handoff decisions based on signal strength, co-channel
interference, and adjacent channel interference.
• IS-95 CDMA spread spectrum cellular system• Mobiles share the channel in every cell.• No physical change of channel during handoff• MSC decides the base station with the best receiving signal as
the service station
Dropped call is considered a more serious event than call blocking.
Channel assignment schemes therefore must give priority to
handover requests. A fraction of the total available channels in a cell is reserved
only
for handover requests. However, this reduces the total carried
traffic. Dynamic allocation can improve this.
It reduces rate of handoff failure
It is desirable from user’s point of view
Prioritizing Handoff
1. Guard channels concept
2. Queuing handoff requests
Handoff Priority
A Cell
New Calls
Handoff Calls From
neighboring cells
CommonChannel
Pool Call completion
Handoff outTo neighboring
cells
Wireless Cellular System Traffic in a cell
Guard Channel Method
A fraction of available channels is reserved exclusively for handoff requests
It has disadvantage of reducing total carried traffic
It Offers efficient spectrum utilization when dynamic channel assignment strategies by minimizing number of required guard channels
It reduces number of blocked handoffs
It reduces system capacity
Handoff dropping less desirable than new call blocking!
Handoff call has Higher Priority: Guard Channel Scheme
GCS: g channels are reserved for handoff calls.
g trade-off between Pb & Pd
Here, New call blocking probability, Pb
Handoff call dropping probability, Pd
Guard Channel Scheme
When a new call (NC) is attempted in an cell covered by a base station (BS), the NC is connected if an idle channel is available in the cell. Otherwise, the call is blocked
If an idle channel exists in the target cell, the handoff call (HC) continues nearly transparently to the user. Otherwise, the HC is dropped
Guard Channel Scheme (Cont.)
Queuing Handoff Requests
First, Put handoff requests in a queue
Then Serves handoffs on a FCFS basis
It reduces number of failed handoffs
It reduces system capacity
The Trade-off :
Enhancing Capacity And Cell Coverage
range (km)
Throughput/cell
(Mbps) 802.11b
Noise Limited
Interference Limited
A-MASBenefit
Technical Interpretation noise, fading, ... expands envelope to right Interference mitigation (+ gain) expands it
upwards
Economic Interpretation Coverage improvements reduce CapEx, OpEx (esp. backhaul, sites) Capacity improvements reduce delivery cost, spectrum requirements
The number of channels available to customers (equivalently, the channel density per square kilometer) could be increased by decreasing the cluster size.
It might be that an increase in channel density is required only in specific parts of the system to support an increased demand in those areas.
Cell-splitting is a technique which has the capability to add new smaller cells in specific areas of the system.
Sectoring is basically a technique which can increase the SIR without necessitating an increase in the cluster size.
Microcell zone
Trade-off
Cell Description Why cell shape is hexagonal…?
Why Cell Splitting , Sectoring and Microcell zone…..?
As users increases per cell the channel capacity decreases
Techniques needed to provide extra channel
Cell Splitting
In base stations where usage of cellular network is high, these cell split into smaller cell
Cont..
A new cell site must be constructed when the cell is split
Such that the radio frequencies are reassigned, and transmission power is reduced
Each with its own base station and a corresponding reduction in antenna height
The process of subdividing a congested cell into smaller cell leads to increase in capacity
Cell splitting is one of the easy and less costly solution when increasing the capacity of cellular network
Sectoring
Sectorization consist of dividing an omnidirectional (360 degree)view of cell site into non overlapping slices called sectoring
To overcome some limitations like co-channel interference cell sectoring is done
There are 2 methods for cell sectoring
1) 60 degree
2) 120 degree
Cont… Replacing a single omnidirectional antenna at base
station with several directional antenna achieves capacity improvement by essentially rescaling the system
Advantages
It reduces interference which increases capacity
It enables to reduce the cluster size and provides an additional freedom in assigning channels
Limitations
Increased number of antennas at each base station
Loss of traffic
Since sectoring reduces the coverage area of a particular group of channels, the number of handoffs increases as well
Microcell
As the splitting of cell idea evolves, the usage of smaller cell becomes efficient and it leads the creation of microcell
The aim of creating a microcell are increasing the capacity of cellular networks in areas where population is very high
Microcell Zone Concept
By the use of sectorization technique, we can increase the system performance but there will be a large increment of handoffs which results in the increment of load on the switching and control link elements of the mobile system
So a Microcell Zone Concept is introduced which leads to an increased capacity without any degradation in load and switching caused by sectoring
Large control base system is replaced by several lower powered transmitters on the edge of the cell
Cont… The mobile retains in the same channel and the base
station simply switches to a different zone site and the mobile moves from zone to zone
A channel is active only in a particular zone in which mobile is travelling , base station radiation is localized and interference is reduced
Reference
Evolved universal terrestrial radio access (E-UTRA), physical channelsand modulation.3GPP TR 36.211, V.8.5.0, 2008.
Channel assignment strategies; Srilasak , Wongthavarawat, Limmongkol; wireless Innovation & security Lab., Nat. Electron,.& Comput. Technol. Center, Pathymthani, Thailand..
Influence of the Handoff Process on the Channel Holding Time Distribution for Cellular Systems.Doiningo Lara-Rodriguez. Center for Resenrcli and Advanced Studies. Electricill Engineering
Ray, S; Pawlikowski, K; Sirisena, H; , ”Handover in Mobile WiMAX Networks: The State of Art and Research Issues,” IEEE Commun. Surveys & Tutorials , vol.PP, no.99, pp.1-24, 2010
Adaptive cell sectoring using fixed overlapping sectors in CDMA networks; alagan S. Anpalagan elvino S. Sousa; Department of electrical and computer engineering; University of toronto.
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