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Improving Capacity in Cellular Systems

• Methods for improving capacity in cellular systems– Cell Splitting: subdividing a congested cell into smaller cells.

– Sectoring: directional antennas to control the interference and frequency reuse.

– Coverage zone : Distributing the coverage of a cell and extends the cell boundary to hard-to-reach place.

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Cell Splitting

• Cell splitting is the process of subdividing a congested cell into smaller cells (called microcells), each with its own base station and a corresponding reduction in antenna height and transmitter power.

• Cell splitting increases the capacity of a cellular system since it increases the number of times that channels are reused.

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• Example to cell splitting:

– In the figure of the following slide

• Base stations are placed at the corner of the cells, and the area served by base station A is assumed to be saturated with the traffic (i.e., the blocking of the base station A exceeds acceptable rates).

• New base stations are therefore needed in the region to increase the number of channels in the area and to reduce the area served by the single base station.

• The smaller cells were added in such a way as to preserve the frequency reuse plan of the system.

• For example, the microcell base station G was placed half way between two larger stations utilizing the same channel set G.

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• Split congested cell into smaller cells.– Preserve frequency reuse plan.– Reduce transmission power.

microcell

Reduce R to R/2

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• For the new cells to be smaller in size, the transmit power of the cells must be reduced.

• The transmit power of the new cells with radius half that of the original cells can be found by examining the received power Pr at the new and old cell boundaries and setting them equal to each other.

• This is necessary to ensure the frequency reuse plan for the new microcells behaves exactly as for the original cells.

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• In our example transmission power reduction from to • Examining the receiving power at the new and old cell boundary

where and are the transmit power of the larger and smaller cell base stations, respectively, and n is the path loss exponent

• If we take n = 4 and set the received power equal to each other

• The transmit power must be reduced by 12 dB in order to fill in the original coverage area.

1tP 2tP

ntr RPP 1]boundary cell oldat [

ntr RPP )2/(]boundary cellnew at [ 2

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2t

t

PP

1tP 2tP

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HW

• Explain the problems met due to cell splitting. To be collected at 25 Friday, November.

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Sectoring• The co-channel interference in a cellular system may be decreased

by replacing a single omni-directional antenna at the base station y several directional antennas, each radiating within a specified sector.

• By using directional antennas, a given cell will receive interference and transmit with only a fraction of the available co-channel cells.

• The technique for decreasing co-channel interference and thus increasing system capacity by using directional antennas is called sectoring.

• A cell is normally partitioned into there 1200 sectors or six 600 sectors

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1200 sectoring

600 sectoring

20Interfering co-channel cells are reduced from 6 to 2 using 1200 directional antennas

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• Interference Reduction

position of the mobile

interference cells

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Example: Decrease in trunking efficiency for constant N

Let N = 7, each cell has C = 100 channels, and users who make calls with λ = 0.01 per minute with average holding time 3 minutes.

a) For blocked-calls-cleared and a GOS of 2%, what is the number of users which can be supported in this cell?

b) Next, using 120 degree sectoring, and otherwise identical system parameters, what is the number of users

which can be supported in this cell? What percentage reduction in capacity does sectoring with constant N cause?

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Solution: a) For C = 100 and GOS= 0.02, from Figure 2.6 in text book, I read total traffic as A=99. Thus with Au = 0.01(3) =0.03, we could support U = 99/0.03 = 3300 users.

b) For the sectoring case, C = 33.3 in each sector, and from Figure 2.6, total traffic is A = 24. So we could support U = 24/0.03= 800 per sector, or 2400 total in the cell. The number of users has reduced by 28%.

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Software tools may be used for Erlang B formula

Busy Hour Traffic (in Erlangs) is the number of hours of call traffic there are during the busiest hour of operation of a telephone system.

Blocking is the failure of calls due to an insufficient number of lines being available.  E.g. 0.03 mean 3 calls blocked per 100 calls attempted.

Lines is the number of lines in a trunk group.

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Microcell Zone Concept• Antennas are placed at the outer edges of the cell• Any channel may be assigned to any zone by the base station• Mobile is served by the zone with the strongest signal.

• Handoff within a cell– No channel re-

assignment

– Switch the channel to a different zone site

• Reduce interference– Low power transmitters

are employed

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• Sectoring increases the number of handoffs (results in an increased load on switching and control link elements )

• As an example; shown below; each of the three zone sites are connected to a single BS and share the same radio equipment

• The antennas are placed at the outer edge of the cell

• As a mobile travels in this zone, it will be served by the strongest signal

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• Microcell Zone Concept• The handoff is not required (MS retains

the same frequency channel, BS simply switches the channel into a different zone site)

• Co-channel interference reduced (large central BS replaced by several low powered transmitters), and thus improves the signal quality

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HW Due to 25, Friday, November