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Coverage and Capacity Improvement Techniques
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With the increase in demand of Mobile Communication, number of channels became insufficient and congestion increased. Cellular design techniques were needed to provide more channels per unit coverage area. Service Providers started using techniques such as cell splitting, sectoring, and microcell zone concepts which helped to expand the capacity of cellular systems. Cell splitting allows an orderly growth of the cellular system. Sectoring uses directional antennas to further control the interference and frequency reuse of channels. The zone microcell concept distributes the coverage of a cell and extends the cell boundary to hard to reach places. While cell splitting increases the number of Base Stations in order to increase capacity, sectoring and zone microcells rely on Base Station antenna placements to improve capacity by reducing co-channel interference. These three popular capacity improvement techniques will be explained in detail.

Cell Splitting

Cell splitting is the process of subdividing a congested cell into smaller cells, each with its own Base Station and a corresponding reduction in antenna height and transmitter power. The cluster size is kept unchanged. i.e.a service provider having 12 as cluster size initially will continue with 12 cell reuse pattern even after employing cell splitting technique.

As we know that, $Q = \frac{D}{R} = \sqrt{3 \times N}$, since cluster size is not changed, D/R ratio has to be maintained constant.

This means, as radius of each cell decreases, the minimum distance after which the frequency channels can be reused also decreases. Hence in a given geographical area, the number of times that we are reusing the same set of frequencies also increases. Thus, Cell splitting increases the capacity of a cellular system due to increase in number of times that channels are reused.

As the cell size decreases, now there are more number of cells in a given geographical area which results in more number of clusters. By defining new cells which have a smaller radius than the original cells and by installing these smaller cells (called microcells) between the existing cells, capacity increases due to the additional number of channels per unit area.

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Figure 21: Illustration of Cell splitting

New Base Stations have to be designed such that it’s new reduced transmitted power should not be interfering in the adjacent cells. The amount of power required by new BS can be evaluated as follows:

Proof :

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Figure 22: Illustration of Cell Splitting from radius R to R/2 and R/4

As shown in the illustration, let us assume that the original cell radius was R and after cell splitting the new radius is R/2. If the original unsplitted cells transmitted and received power is $P_{t1}$ and $P_{r1}$, we can say that,

$$P_{r1} = P_{t1} \times R^{-n}-----(1)$$

Also, if $P_{t2}$ and $P_{r2}$ are notations given to the new splitted cell’s transmitted and received power, then

$$P_{r2} = P_{t2} \times (\frac{R}{2})^{-n}-----(2)$$

Rescaling is done in such a way that the power received by the MS should always be the same during the worst case scenarios also. Hence at any instant $P_{r1} = P_{r2}$

Equating 1 and 2,

$$P_{t1} \times R^{-n} = P_{t2} \times (\frac{R}{2})^{-n}$$

$$P_{t2} =P_{t1} \times 2^{-n}$$

i.e

$$P_{t2} = \frac{P_{t1}}{2^n}$$

If n=4 , then we see that the transmitted power by the newer Base Station should 1/16 times that of the original Base station.

In practice, not all cells are split at the same time. It is often difficult for service providers to find real estate that is perfectly situated for cell splitting. Therefore, different cell sizes 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.

Advantages

  • Because cells are smaller, system capacity increases. Also, less power is used by mobiles and Base Stations.

Disadvantages

  • Handoffs become more common. To prevent handoffs and dropped calls, umbrella cells are needed for high speed traffic.
  • Many new base stations are needed, increasing system complexity and load of MSC.
  • Since all channels are not splitted simultaneously, special care have to be taken for proper allocation of the channels.

Sectoring

In the simplest way, Cell sectoring is a process in which all omnidirectional antennas at the Cell Sites are replaced by directional antennas. The coverage area of each directional antenna is called as a Sector. There can be 3 or more sectors in a cell. Each Sector will be allotted a group of channels which are unique (different from the other sectors in the same cell). In this approach, first the SIR is improved using directional antennas, then capacity improvement is achieved by reducing the number of cells in a cluster, thus increasing the frequency reuse. However, in order to do this successfully, it is necessary to reduce the relative interference without decreasing the transmit power.

When directional antennas are used, the co-channel interference reduces as lesser number of co-channel cells are now interfering on the desired mobile signal. By using directional antennas, a given cell will receive interference from only a fraction of the available co-channel cells. The factor by which the co-channel interference is reduced depends on the amount of sectoring used. A cell is normally partitioned into three $120^°$ sectors or six $60^°$ sectors as shown in Figure 23.

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Figure 23: Types of Sectoring

When sectoring is employed, the channels used in a particular cell are broken down into sectored groups and are used only within a particular sector.

Assuming seven-cell reuse, for the case of $120^°$ sectoring, the number of interferes in the first tier is reduced from six to two. This is because only two of the six co-channel cells receive interference with a particular sectored channel group. Referring to Figure 24 , consider the interference experienced by a mobile located in the right-most sector in the center cell labeled “5”. There are three co-channel cell sectors labeled “5” to the right of the center cell, and three to the left of the center cell.Out of these six co-channel cells, only two cells have sectors with antenna patterns which radiate into the center cell, and hence a mobile in the center cell will experience interference on the forward link from only these two sectors.

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Figure 24: Illustration of how 120-degree sectoring reduces number of cochannel interfering cells

The resulting S/I for this case can be found to be 24.2 dB as follows

$$\frac{S}{I} = \frac{R^{-n}}{\sum_{i=1}^{i_0} D^{-n}}$$

Let us assume that the two cells are at a distance od D+R and D. Since there are only two interfering cells with 120-degree sectoring, the equation transforms into

$$\frac{S}{I} = \frac{R^{-4}}{R^{-4}[D/R +1)^{-4} + (D/R)^{-4}]}$$

$$\frac{S}{I} = \frac{R^{-4}}{R^{-4}[(\sqrt{3N}+1)^{-4} + (\sqrt{3N})^{-4}]}$$

$$\frac{S}{I} = \frac{1}{[(\sqrt{3N}+1)^{-4} + (\sqrt{3N})^{-4}]}$$

Assuming cluster size N=7, S/I=303.29 i: e 24.81 dB, which is far above the minimum requirement of 18 dB. This S/I improvement allows the wireless engineer to then decrease the cluster size N in order to improve the frequency reuse, and thus the system capacity.

The improvement in S/I implies that with $120^°$ sectoring, the minimum required S/I of 18 dB can be easily achieved with seven-cell reuse, as compared to 12-cell reuse for the worst possible situation in the unsectored case i.e. the service provider has now to distribute its available pool of channels among 7 cells rather than 12 cells. Thus per cell will now be allotted more number of channels. As discussed earlier also, this is the advantage of using smaller cluster size. In capacity improvement techniques in which cluster size changes, there is a method to calculate increase in capacity when compared to the previous conventional technique.

$\text{Capacity Increase} =\frac{\text{Older cluster size}}{\text{Newer cluster size}}$

Thus, increase in capacity for a 120 degree sectoring technique as compared to the omnidirectional antenna technique is 12/7 or 1.714.

Advantages

  • It improves S/I ratio.
  • It reduces interference which increases capacity.
  • It enables to reduce the cluster size and provides an additional freedom in assigning channels.

Disadvantages

  • More number of antennas are required per cell site.
  • Since sectoring reduces the coverage area of a particular group of channels, the number of handoffs increases.
  • Particularly in dense urban areas, the directional antenna patterns are somewhat ineffective in controlling radio propagation. This causes loss of traffic due to decreased trunking efficiencies.
  • Because sectoring uses more than one antenna per base station, the available channels in the cell must be subdivided and dedicated to a specific antenna. This breaks up the available trunked channel pool into several smaller pools, and decreases trunking efficiency.

Micro Cell Zone Concept

The major disadvantage of sectoring was that more number of handoffs were required. This increased the load on the switching and control link elements of the mobile system. Hence, another technique was proposed called as the Micro Cell zone concept which minimizes the handoff requirements.

In this concept, a cell is divided into smaller areas called as zones.Each zone is served by a trans receiver called as zone site. The zone sites are connected to a single Base Station and share the sameradio equipment. The zones are connected by coaxial cable, fiber optic cable or microwave link to the Base Station. As a mobile travels within the cell, it is served by the zone with the strongest signal. This approach is superior to sectoring since antennas are placed at the outer edges of the cell, and any channel may be assigned to any zone by the BS.

As a mobile travels from one zone to another within the cell, it retains the same channel. Thus, unlike in sectoring, a handoff is not required at the MSC when the mobile travels between zones within the cell. The BS simply switches the channel to a different zone site. In this way, a given channel is active only in the particular zone in which the mobile is traveling, and hence the Base Station radiation is localized and interference is reduced. The channels are distributed in time and space by all three zones and are also reused in co-channel cells in the normal fashion. This technique is particularly useful along highways or along urban traffic corridors.

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Figure 25: Conceptual illustration of Micro cell zone concept

The advantage of the microzone cell technique is that while the cell maintains a particular coverage radius, the co-channel interference in the cellular system is reduced since a large central base station is replaced by several lower powered transmitters (zone transmitters). These zone sites are mounted on the edges of the cell. Decreased co-channel interference improves the signal quality and also leads to an increase in capacity without the degradation in trunking efficiency caused by sectoring. As mentioned earlier, an S/I of 18 dB is typically required for satisfactory system performance.

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Figure 26: Illustration to find the minimum safe distance in a micro cell arrangement

As shown in figure 26 , here three hexagons together represent one cell. R is the approximate radius of the cell. $R_z$ is the radius of each zone. D is the minimum safe distance after which co-channel cell can exist. $D_z$ is the zonal distance.

Following the general formula,

$$\sqrt{3N} = \frac{D}{R}-----(1)$$

Referring to the geometrical figure, D spans over radius of 2 cells and crosses sides of two hexagons. The side of a hexagon is equal to its radius. Hence, D=3R. Substituting in 1.

$$\sqrt{3N} = \frac{3R}{R}$$

$$\sqrt{3N} = 3$$

$$N = \frac{9}{3} = 3$$

Thus, it is proved geometrically that the cluster size can be reduced to 3 with the help of microcell zone concept.The exact worst case S/I of the zone microcell system can be estimated to be 20 dB. Thus, in the worst case, the system provides a margin of 2 dB over the required signal-to-interference ratio while increasing the capacity by 2.33 times over a conventional seven-cell system using omnidirectional antennas.

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