Cellular Concept-Lec IV_3
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Transcript of Cellular Concept-Lec IV_3
The Cellular Concept
Presentation Credits
• The material for these slides has been taken from T. S. Rappaport’s Book: “Wireless Communications, Principles and Practice (2nd Edition)”. I would like to thank the author and publishers for making the figures for this book easily accessible online.
• Content for some slides and snapshots of figures have been taken from video lectures on Wireless Communications by Dr Ranjan Bose,Dept of Electrical Engineering,IIT,Dehli
2/12/2009 2NUST-SEECS
Learning Objectives
• We know that– Cell capacity and Reuse
• Improving Capacity & Coverage in Cellular Systems
– Power Control to reduce interference
– Cell Splitting
– Sectoring
– Microcell Zone Concept
– Repeaters for Range Extension
N and C
2/12/2009 3NUST-SEECS
Power Control to Reduce Interference
• In practical systems, power level of every subscriber is under constant control by the serving BS
• Power control not only reduces interference levels but also prolongs battery life
• In CDMA spread spectrum systems, power control is a key feature to ensure maximal utilization of system capacity
• Reduced Interference leads to high capacity
2/12/2009 4NUST-SEECS
Improving Capacity• As demand for service increases, system designers
have to provide more channel per unit coverage area
• Common Techniques are: Cell Splitting, Sectoring and Microcell Zoning
• Cell Splitting increases the number of BS deployed and allows an orderly growth of the cellular system
• Sectoring uses directional antennas to further control interference
• Micro cell Zoning distributes the coverage of cell and extends the cell boundary to hard-to-reach areas
2/12/2009 5NUST-SEECS
Cell Splitting• Cell splitting is the process of subdividing a congested cell into
smaller cells with
– their own BS
– a corresponding reduction in antenna height
– a corresponding reduction in transmit power
• Splitting the cell reduces the cell size and thus more number of cells have to be used
• For the new cells to be smaller in size the transmit power of these cells must be reduced.
• More number of cells more number of clusters more channels high capacity
2/12/2009 6NUST-SEECS
Cell Splitting
Cells are split to add channels with no new spectrum usage
2/12/2009 7NUST-SEECS
Cell Splitting-Power Issues• Suppose the cell radius of new cells is reduced by half
• What is the required transmit power for these new cells??
Pr[at old cell boundary]=Pt1R-n
Pr[at new cell boundary]= Pt2(R/2)-n
where Pt1 and Pt2 are the transmit powers of the larger and smaller cell base stations respectively, and n is the path loss exponent.
• So,
Pt2 = Pt1/2n
2/12/2009 8NUST-SEECS
Cell Splitting• Pt2 = Pt1/2
n
• If we take n=3 and the received powers equal to each other, then
Pt2
=Pt1
/8
In other words, the transmit power must be reduced by 9dBin order to fill in the original coverage area while maintaining the S/I requirement
2/12/2009 9NUST-SEECS
Cell Splitting
In practice not all the cells are split at the same time. This means that different
size cells will exist simultaneously.
In such situations, special care needs to be taken to keep the distance between
co-channel cells at the required minimum, and hence channel assignments
become more complicated.
2/12/2009 10NUST-SEECS
Cell Splitting
2/12/2009 11NUST-SEECS
Cell Splitting
2/12/2009 12NUST-SEECS
Cell Splitting
2/12/2009 13NUST-SEECS
Cell Splitting
2/12/2009 14NUST-SEECS
When there are two cell sizes inthe same region, one cannot use original transmit power for all new cells or the new transmit power for all original cells
Larger transmit power for all->some channels used by smaller cells would not be sufficiently separated from co-channel cells
Smaller transmit power for all->some parts of larger cells left un-served
Channels in the old cell must be broken down into two channel groups, one for smaller cell and other for larger cell
The larger cell is usually dedicated to high speed traffic so that handoffs occur less frequently
Practical Handoff Considerations• Umbrella Cell Approach
– Mobile Assisted Handover. Advantage is reduced load on MSC.
• What is the Guard channel concept?
– Reservation of a fraction of available channels exclusively for handoff requests from ongoing calls.
Cell Splitting
•Two channel group sizes depend on the stage of splitting process•At the beginning of splitting process, there will be fewer channels in small power groups•With increasing demand,smaller groups will require more group•Splitting continues until all channels in area are used in lower power group•Entire system by that time is rescaled to have smaller radius per cell
Antenna down tilting is used to focus energy from BS toward ground, to
limit radio coverage of newly formed microcells
Cell Splitting
2/12/2009 17NUST-SEECS
Cell Splitting• Suppose original congested area is
originally covered by
– 5 cells
– Each with 80 channels
• Capacity=5x80=400 users
• After cell splitting,R new =R/2
• We now have 24 cells
• New capacity=24x80=19200 users
• For n=4,Transmit Power of New BS is 12 dB lower than original
2/12/2009 18NUST-SEECS
Sectoring
• As opposed to cell splitting, where D/R is kept constant while decreasing R,sectoring keeps R untouched and reduces the D/R
• Capacity improvement is achieved by reducing the number of cells per cluster, thus increasing frequency reuse
• In this approach first SIR is improved using directional antennas, the capacity improvement is achieved by reducing the number of cells in a cluster this increasing frequency reuse
2/12/2009 19NUST-SEECS
Sectoring• The CCI may be decreased by replacing the single omni-
directional antenna by several directional antennas, each radiating within a specified sector
2/12/2009 20NUST-SEECS
2/12/2009 21NUST-SEECS
Sectoring
Sectoring• A directional antenna transmits to and receives from only a
fraction of total of the co-channel cells. Thus CCI is reduced
2/12/2009 22NUST-SEECS
Problems with Sectoring
• Increases the number of antennas at each BS
• Decrease in trunking efficiency due to sectoring(dividing the bigger pool of channels into smaller groups)
• Increase number of handoffs(sector-to sector)
• Good news:Many modern BS support sectoring and related handoff without help of MSC
2/12/2009 23NUST-SEECS
The Micro Zone Cell Concept
Microcell Zone Concept• The Problems of sectoring can be addressed by Microcell Zone
Concept
• A cell is conceptually divided into microcells or zones
• Each microcell(zone) is connected to the same base station(fiber/microwave link)
– Doing something in middle of cell splitting and sectoring by extracting good points of both
• Each zone uses a directional antenna
• Each zone radiates power into the cell.
• MS is served by strongest zone
• As mobile travels from one zone to another, it retains the same channel,i.e no hand off
• The BS simply switches the channel to the next zone site
2/12/2009 25NUST-SEECS
The Zone Cell Concept
Microcell Zone Concept• Advantages:
– Reduced Interference (Zone radius is small and directional antennas are used).
– No loss in trunking efficiency (all channels are used by all cells).
– No extra handoffs.
– Increase in capacity (since smaller cluster size can be used).
2/12/2009 27NUST-SEECS
Microcell Zone Concept
• Illustration of extent of Capacity Increase by an example– Suppose the desired S/I=18 dB with path loss exponent of n=4
– For a system of N=7,a D/R of 4.6 was shown to achieve this
– How much capacity increase can occur if we use Microcell Zoning of 3 zones/cell???
2/12/2009 28NUST-SEECS
In zone microcell system, transmission at any instant is confined to one zoneTherefore,D z/R z=4.6
Each hexagon represents a zone and 3 hexagons represent a cell
Zone radius=One hexagon radius
Capacity of system related to distance between co-channel cells and not zones.Shown as D
Microcell Zone Concept
2/12/2009 29NUST-SEECS
Value of co channel reuse is 3
D/R = 3 corresponds to N=3
Reduction in cluster size from N=7 to N=3
Increase in capacity is 7/3=2.33 times
Repeaters for Range Extension• Useful for hard to reach areas
– Buildings
– Tunnels
– Valleys
• Radio transmitters called Repeaters can be used to provide coverage in these area
• Repeaters are bi-directional
– Rx signals from BS
– Amplify the signals
– Reradiate the signals
• Received noise and interference is also reradiated.
2/12/2009 30NUST-SEECS
Assignment#04
• Attempt the following Exercise problems at the end of the chapter and submit neat and clean solutions.
Problems: 3.7, 3.10, 3.11, 3.12, 3.13, 3.15.
• Submission Date: Thursday 19 February 2009,At the start of Class
• You are advised to attempt the problems yourself and not to submit solutions copied blindly from your friend’s assignments.