Cellular structures

Overview

Objectives

• After completion of this chapter the student is able to:
• Identify the two main cell types and name the characteristics
• Explain the use of several cell layers in a network
• Name the different factors that need to be taken into account during cell planning.

Contents

• Cell types
• Hierarchical cell structure
• Cell structure planning
• Signal contours
• Cell overlap regions
• Cell growth/expansion
• Site selection

Cell types

What is a cell?

A cell is a base transceiver service area as seen by the mobile station (MS). A cell uses a specific set of frequencies.

Main cell types

The two main cell types are:

• Omni cells:

An omni cell is a cell where the antenna transmits omni-directional.

The coverage area of an omni cell is in principle a

hexagon/circle, but in reality a rough pattern.

• Sector cells:

A sector cell is a cell where the antenna transmits directional.

Examples of sector cell types are:

- 2-sector cells (e.g. for highways)

- 3-sector cells.

The following figure shows examples of different cell types.

Sector vs. omni cells

Advantages of sector cells are (compared to omni cells):

• Increased coverage area per site (by the use of higher gain antennas)

• Possibility of mechanical tilting antennas (to reduce unwanted

interference)

• Simpler antenna mounting (reduced clearance to prevent

interaction with other antennas).

Disadvantages of sector cells are:

• More equipment required at each site

• Greater environmental impact (more antennas)

• Longer frequency re-use distance for a given C/I

• Increased cell handovers.

Hierarchical cell structure

Cell layers

The hierarchical cellular structure may be composed of three different

layers:

• A lower cell layer with rather small cells (micro-cells)

• A middle cell layer with medium size cells (macro-cells)

• An upper cell layer with large cells (umbrella-cells).

Lower cells

Lower cells cover areas that are small compared to middle cells. Lower cells increase capacity and coverage quality. They are located at areas where subscribers have communities of interest (hot spots) as well as areas where signal penetration is difficult (dead spots).

Lower cells typically serve slow or stationary MSs, and they can provide in-building coverage.

Lower cells are characterized by the following:

• Small radius: a few hundred meters, from about 100 m up to 1 km

• Antennas are typically deployed below roof level: antenna heights are kept low, 7–10 m Above Ground Level (AGL),

mounted on street lights and flag poles

• Frequency reuse is very extensive due to low transmit power and

the fact that buildings are used as isolation.

Middle cells

The middle cells can be compared to the standard cells in a non-hierarchical cellular network. Cells of the middle layer usually serve medium fast MSs.

Upper cells

Cells from the upper layer cover areas that are large compared to middle cells. In general, upper or umbrella-cells serve the fast moving MSs.

Enhanced handover algorithms

The co-existence of non-hierarchical and hierarchical networks with 3–layers is possible. The handover between all different cell types in respective layers is supported.

Concentric cells

Concentric cells represent a two layered system, where both layers are available within one radio cell with only one BCCH. This creates an inner and outer coverage area. The frequencies of the inner layer can be re-used more frequently.

Cell structure planning

Homogeneous structure

A homogeneous cell structure provides identical cell coverage across a market area based upon a regular cell grid. Practically this is impossible to achieve. However, it is desirable to design a cell structure as homogeneous as possible.

The following figure shows an example of a homogeneous cell structure.

The following figure shows an example of inhomogeneous cell structure.

Homogeneous structure advantage

Homogeneous cell structure imply:

• Reliable coverage

• Simple frequency planning

• Easy calculation of traffic loads

• Reliable handovers.

Good cell structures

Good cell structures can be planned by keeping the following points in mind:

• Use as homogeneous a cell structure as possible (no abrupt changes in cell size, e.g. at the edge of towns)

• Avoid random pointing of antenna direction - the front lobe of any BTS directional TX antenna should illuminate only the back lobe of its co-channel counterpart

• Define cell boundaries firmly - avoid areas with many equally good servers, resulting in many handovers and many interferers.

• Sufficient overlapping zones (related to the speed of mobiles)

• Avoid cell boundaries across traffic hot spots (e.g. along roads)

• Keep all antenna heights about the same

• Choose individual BSS parameters (especially handover

parameters).

Changing cell structures

Once a BTS is located through site establishment, and good coverage can be achieved, there is no guarantee that the cell will maintain its original coverage.

Cells are living elements within the network because:

• New buildings and structures may be erected within the coverage

area

• Existing buildings and structures may be demolished

• Trees and vegetation may cause seasonal (i.e. deciduous)

variations in coverage and create longer term LOS obstructions.

Signal contours

Propagation prediction

The prediction of the propagation loss (or fading) is based upon the log-normal distribution with mean µ and standard deviation  s.

The propagation prediction model (such as Okumura-Hata) provides the signal level in terms of dBm. This is the mean value, e.g. µ = -88 dBm.

Signal level variation

The signal level follows a log-normal distribution N(µ,s). This means that the percentage of signal strength above or equals µ dBm is equal 50%.

If µ = –88 dBm and s = 8 dB, the percentage of signal strength above a given threshold value (e.g. -102 dBm) is the P(xX), where X = (-102 + 88)/8 = -14/8 = -1.75. From the cumulative distribution table for standard normal distribution, this corresponds to a probability of 95.99%.

This implies that the signal at a given distance can only be guaranteed with a reliability of 95.99% to be -102 dBm or higher.

Signal contour

The signal contour refers to a cell boundary along which the signal level is, with a certain probability (e.g. 95%), higher than a certain predefined threshold value (e.g. –102 dBm).

The signal contour for a specified receiver sensitivity must be plotted around the cell site to define the coverage area. This contour is a statistical boundary.

Example

The following figure shows a typical plot of a signal contour area.

If the MS travels along the signal contour boundary, for 95% of all the locations it is expected to receive a signal that is above -102 dBm.

Cell overlap regions

Overview

Since cells do not actually have a hexagon shape, and their boundaries are not well defined, there is always an overlap coverage region between them.

Coverage probability

The two -102 dBm 95% signal contours for cell site 1 and 2 coincide in the intersection points A and A’ (see figure below). In these points the probability that at least one of the two signals is -102 dBm or better equals:

P(x  -102dBm) = 1- (1- 0.95)·(1- 0.95) = 99.75%,

assuming that the distributions for both signals are independent.

In the shaded area this probability will even be higher.

The following figure shows an example of a cell overlap region.

Handover

To have a seamless handover, a region overlapping with neighboring cells is necessary at the cell boundary:

• Handover neighbor cell signal must be stronger than the serving

cell by approximately the handover margin (e.g  3-5 dB)

• Handover requires a period of time to carry out.

• Hysteresis is included to avoid repeated ping ponghandovers.

To avoid this, handover parameters do specify:

- apower budget difference between old and new cell during

a certain time (default 6 dB).

- aminimum time between an unsuccessful handover and a

retry (default 5 sec). Re-tries after unsuccessful handovers

are only carried out after this time period.

Speed of MS

The speed of the MS also plays a role for the handover:

• The transition between two coverage areas can become too rapid

e.g. at a hill top.

Remedy: install a small cell site on the hill top

• On the urban highway many handovers per call would occur

(every few seconds at 100 km/h).

Remedy: plan a linear cell structure.

Cell growth/expansion

Reasons for growth

Network growth can be required for the following reasons:

• Extension of coverage area

A new coverage area needs to be added

• Capacity increase

The traffic density in an existing cell has grown

• Coverage quality increase

For instance, existing outdoor coverage needs to be upgraded to indoor coverage.

Example

The following figure shows that three new cells are added to the existing structure because of high traffic demand in these specific areas.

The following figure shows even further growth of the cell structure.

Possible consequences of growth

Integration of each new BTS or even each TRX has to be carefully planned into the greater system.

In all cases, the existing cells adjacent to the growth area will be affected in the following aspects:

• Changes in cell size and shape

• Changes in BSS parameters (e.g. handover parameters)

• Updates in neighbor lists

• Frequency allocation

• Interference performance (the interference relations will change

drastically).

Capacity increase and coverage quality

If the number of available channels is fixed, the basic cellular principle requires that capacity increase is achieved by reusing frequencies more often over a certain coverage area. Hence, more sites (cells) are needed within the existing area.

In such cases, increasing the capacity is accomplished by reducing the cell sizes in areas of high demand:

• This requires the creation of new small cells within the overall

cluster pattern

• Frequency reuse must not infringe on rules determining

frequency allocation for the large pattern

• Some coverage quality improvement can be expected as well.

Increasing cell density

Increasing the cell density in a coverage area can be achieved by:

• Adding more sites in the coverage area

• Cell splitting (sectorization).

Adding sites

The following figure shows increase in cell density by adding more sites.

Sectorization:

The following figure shows increase in cell density by sectorizing old sites.

Cell splitting example

Assume a cluster with 7 cluster cell pattern with 4 frequencies assigned to each. Somewhere between the cells marked with an “A”, a hot spot is growing.

This scenario is shown in the following figure.

To cope with the increased local demand, a growth cell His added

in that area as shown here.

The new cell is equipped with one frequency. The capacity of the two co-channel large cells A is reduced by about 25% because one of the original frequencies cannot be used without violating the D/R requirement.

By allowing restricted reuse of this frequency in the large A cells, most of the capacity is bought back that would otherwise have been lost.

The partitioning of frequencies into groups, one for large cell coverage and one for small cell coverage, is called cell overlaying.This is the principle of concentric cell structures.

Site selection

What is a site?

A site is the position where antennas are located.

A site may serve

• An omni cell, and as such it is called an omni site, or

• Two or more sector cells. In this case, a site is referred to as a

sector site.

Link power budget process

The coverage Quality Of Service (QOS) requirements determine the maximum allowable path loss in the power link budget in order to guarantee a minimum indoor/outdoor field strength.

The maximum path loss fixes the average distance between sites. The search radius for the site locations needs to be determined.

The QOS requirements determine the size and the separation of the search areas.

Initial site selection strategy

The proper strategy allows for a minimum site search time along with the desired QOS. Strategies are:

• Strategy 1:

- The operator supplies the sites

- which of these sites are likely candidates?

- is the distance between the sites acceptable?

• Strategy 2:

- New sites chosen by RF planning

• Strategy 3:

- A mix of the above.

Finding realistic search areas

The process of finding realistic areas includes:

1. Finding realistic search areas is an iterative loop process, by shifting sites while maintaining a given average distance in order to meet the QOS requirements. The process will result in a list of realistic search areas. If the cell size is very small, i.e. less than 1 km in diameter, the searching radius of a site is likely to be less than 150 m.

2. Performing two types of surveys:

• External site surveys: is the physical site criterion satisfied?

• Radio survey: is the RF criterion satisfied?

Sites can often not be acquired at the ideal location (a reserve in power is necessary to cope with this problem).

Site acquisition

Once a site has been selected, the acquisition process can begin. This is a costly process (time-wise) and requires special skills.

Accurate data for RF planning

The propagation predictions are only as good as the accuracy and detail of the tools and inputs used:

• Terrain data

• Clutter detail.

Propagation models need to be matched to the level of detail used. Good clutter definitions must be established.

• Accurate clutter data with detailed building heights produces

accurate predictions but is very expensive

• Detailed radio measurements provide more accurate overall

predictions but take a lot of time to perform.

An effective solution is one survey per clutter class region, plus more surveys for problem areas.

Collocation

When considering collocation with another system, a site is defined as an electrical isolation area. Major problems encountered are:

• Interference

• Intermodulation.

Collocation is usually a cooperation between existing and new system owners. Second and subsequent tenants on a shared site must be Site selection prepared to accept less than ideal antenna locations and/or antenna heights.