Cellular concept

possible radio coverage of the cell

idealized shape of the cell

cell

segmentation of the area into cells

GSM: cellular network

– use of several carrier frequencies– not the same frequency in adjoining cells– cell sizes vary from some 100 m up to 35 km depending on user

density, geography, transceiver power etc.– hexagonal shape of cells is idealized (cells overlap, shapes

depend on geography)– if a mobile user changes cells

handover of the connection to the neighbor cell

The Cellular Concept

Cluster of 7 cells

Cells

Frequency Reuse Concept

• Consider a cellular system which has total of S duplex channels available for use,

• each cell has k no. of channels (k<S),• S channels are divided among N total no.

of cells into unique and disjoint channel groups such that each cell has same number of channels,

• So total no. of available radio channels can be expressed as:

S = kN

Cont…..

• N cells collectively use complete set of available frequencies termed as cluster.

• If a cluster is replicated M times within the system, the total number of duplex channels C can be used as a measure of capacity and is given by

C = MkN = MS

19-cell reuse example (N=19)

Figure 3.2 Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., I = 3, j = 2). (Adapted from [Oet83] © IEEE.)

The factor N is called the cluster size and is given N=i2+ij+j2

A

B

C

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C

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CA

B

C

A F

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D

E

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D E

To find the nearest co-channel neighbor of a particular cell, one must do the following:

• move i cells along any chain of hexagons and then

• turn 60 degrees counter-clockwise and move j cells.

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i

j

i=1, j=2 , N=1+2+4=7

Handoffs – the basics

Handoff Strategies•Handoffs must be performed successfully, to meet this requirement system designers must specify an optimum signal level at which to initiate a handoff.

•Threshold is at which handoff is made is slightly stronger signal level than the minimum usable signal for acceptable voice quality at the base station receiver.

•This margin is given by = Pr handoff - Pr minimum usable

Note: The time over which a call may be maintained within a cell, without handoff, is called the dwell time.

Role of MSC in Handoffs

• First, thing is should be not so large that there is handoff burden on MSC and not too small that the MSC don’t get sufficient time for handoff and call is lost. Therefore, should be chosen properly.

• Second, MSC has to wait in case of when there is high traffic allowing the channels to get free, which can lead to excessive delay.

Cont….

• In first generation cellular systems, signal strength is measured by base stations and supervised by the MSC. The locator receiver is used to scan and determine signal strengths of mobile users which are in neighboring cells and reports all RSSI (Received Signal Strength Indication) values to the MSC. The locator receiver is controlled by MSC

Cont…..

• In second generation systems, decisions are mobile assisted. In MAHO (mobile assisted handoff), every mobile station measures the received power of these measurements to the serving base station.

• The advantage of MAHO in comparison with locator receiver is that the MAHO enables the call to be handed over between base stations at a much faster rate.

Prioritizing Handoffs

1. Guard Channel Concept : It is the method whereby a fraction of the total available channels in a cell is reserved exclusively for handoff requests from ongoing calls which may be handed off into the cell. Disadvantage: Reduces the no. of channels to the originating calls, thereby reducing the carried traffic.

Cont…..

2.Queuing: Queuing is another method to decrease the probability of forced termination of a call due to lack of available channels.Queuing is possible due to fact that there is a finite time interval between the time the received signal level drops below the handoff threshold and the time the call is terminated due to insufficient signal level.

The delay time and size of queue is determinedfrom the traffic pattern of the particular servicearea.

Practical Handoff Considerations

• High Speed Vehicles: In this case, the change of cell is so fast that the handoff becomes typical, increasing the microcells to provide the capacity in turn increases the burden on MSC.

• Pedestrian User: In this case, the may never need a handoff during a call or may require rarely which is not the issue for MSC.

Cont…..

• Cellular Concept : It is difficult for cellular service provider to obtain new physical cell site locations, whereas it is better for cellular providers to install additional channels and base stations at the same physical location of an existing cells.

• Umbrella Approach: Using different antenna heights and small cells which are co-located at a single location is called Umbrella approach, which is used to provide the large area coverage to high speed user, also minimizing the handoffs and small area coverage to users traveling at low speed by providing additional microcells for pedestrian.

• The speed of each user may be estimated by the base station or by MSC

Umbrella Cells

Interference

NR

DQ 3

R - the radius of the cell

D - the distance between centers of the nearest co-channel cells

Q - the co-channel reuse ratio

Smaller N is less capacity

Co-channel cells for 7-cell reuse

Example

Q:A particular FDD cellular system uses two 25kHz simplex channels to provide full duplex voice and control channels. The total band allocated for the system is 40 MHz. Compute the number of channels available per cell if the system uses (a) 3-cell reuse (b) 4-cell reuse (c) 12-cell reuse. If 2MHz of the allocated spectrum is dedicated to control channels, determine the distribution of voice and control channels in each cell in each of the three systems.

Solution

• Total bandwidth = 40 MHz• Channel bandwidth = 25 kHz x 2 simplex

channels = 50 kHz duplex.• Total available channels = 4000/50 = 800

channels.– (a) For N = 3, total number of channels per cell =

800/3 = 267– (b) For N = 4, total number of channels per cell =

800/4 = 200– (c) For N = 12, total number of channels per cell =

800/12 = 67

Cont…..

• A 2 MHz spectrum of control channels means that 2000/50 = 40 control channels out of 800 channels available.– (a) 40/3 13 control channels. Among 267

channels, 254 for voice and 13 for control.– (b) 40/4 = 10 control channels. Among 200

channels, 190 for voice and 10 for control.– (c) 40/12 3 control channels. Among 67

channels, 64 for voice and 3 for control.

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Two-Dimensional Cell Clusters

• Regular geometric shapes tessellating a 2D space: Square, triangle, and hexagon.

• ‘Tessellating Hexagon’ is often used to model cells in wireless systems:– Good approximation to a circle (useful when antennas

radiate uniformly in the x-y directions).– Also offer a wide variety of reuse pattern– Simple geometric properties help gain basic

understanding and develop useful models.

Coverage Patterns

Cellular Coverage Representation

Geometry of Hexagons

Hexagonal cell geometry and axes

Geometry of Hexagons (Cont’d)– axes u,v intersect at 60o

– unit scale is distance between cell centres– if cell radius to point of hexagon is R – then 2Rcos30o = 1 or

R =1

3To find the distance of a point P(u,v) from the origin

use x - y to u - v co- ordinate transformations:

r 2 x2 y2

x ucos30o

yv usin 30o

r (v2 uv u2 )1

2

Geometry of Hexagons (Cont’d)• Using this equation to locate co-channel cells,

we start from a reference cell and move i hexagons along the u-axis then j hexagons along the v-axis. Hence the distance between co–channel cells in adjacent clusters is given by:

• D = (i2 + ij + j2)1/2

– where D is the distance between co–channel cells in adjacent clusters (called frequency reuse distance).

– and the number of cells in a cluster, N is given by D2

• N = i2 + ij + j2

Hexagon Reuse Clusters

3-cell reuse pattern (i=1,j=1)

4-cell reuse pattern (i=2,j=0)

7-cell reuse pattern (i=2,j=1)

12-cell reuse pattern (i=2,j=2)

19-cell reuse pattern (i=3,j=2)

Relationship between Q and N

Proof

Cell ClustersReuse coordinates Number of

cells in re-use pattern

Normalised reuse

distance i j N SQRT(N) 1 0 1 1 1 1 3 1.732 1 2 7 2.646 2 2 12 3.464 1 3 13 3.606 2 3 19 4.359 1 4 21 4.583

since D = SQRT(N)

Co–channel Cell Location

– Method of locating co–channel cells – Example for N=19, i=3, j=2

Cell Planning Example• Suppose you have 33 MHz bandwidth available,

an FM system using 25 kHz channels, how many channels per cell for 4,7,12 cell re-use?– total channels = 33,000/25 = 1320– N=4 channels per cell = 1320/4 = 330– N=7 channels per cell = 1320/7 = 188– N=12 channels per cell = 1320/12 = 110

• Smaller clusters can carry more traffic• However, smaller clusters result in larger co-

channel interference

Remarks on Reuse Ratio

Co-channel Interference with Omnidirectional Cell Site

Propagation model

Cochannel interference ratio

Worst-case scenario for co-channel interference

Worst-case scenario for co-channel interference

Reuse Factor and SIR

Remarks

• SIGNAL TO INTERFERENCE LEVEL IS INDEPENDENT OF CELL RADIUS!

• System performance (voice quality) only depends on cluster size

• What cell radius do we choose?– Depends on traffic we wish to carry (smaller cell

means more compact reuse or higher capacity)– Limited by handoff

Adjacent channel interference

• So far, we assume adjacent channels to be orthogonal (i.e., they do not interfere with each other).

• Unfortunately, this is not true in practice, so users may also experience adjacent channel interference besides co-channel interference.

• This is especially serious when the near-far effect (in uplinks) is significant– Desired mobile user is far from BS– Many mobile users exist in the cell

Near-Far Effect

Near-Far Effect (Cont’d)

Reduce Adjacent channel interference

• Use modulation schemes which have small out-of-band radiation (e.g., MSK is better than QPSK)

• Carefully design the receiver BPF • Use proper channel interleaving by assigning

adjacent channels to different cells, e.g., for N = 7

Reduce Adjacent channel interference (Cont’d)

• Furthermore, do not use adjacent channels in adjacent cells, which is possible only when N is very large. For example, if N =7, adjacent channels must be used in adjacent cells

• Use FDD or TDD to separate the forward link and reverse link.

Key Definitions for Trunked Radio

Erlang B Trunking GOS

Erlang B

Erlang C

Cells are split to add channels with no new spectrum usage

Cell Splitting increases capacity

Sectoring improves C/I

Sectoring improves C/I

In-building deployment is the next great growth phase

The Zone Cell Concept

Zone Cell Concept