Thursday, March 7, 2019

4 Things you need to Improve Coverage and Capacity in cellular system

The improvement in wireless technology increased the number of cellular users to a massive level. This results in more wireless services but the number of channels assigned to a cell is not enough to support the user in some areas like cities which are usually more populated.

To overcome this problem cellular design techniques are needed like cell splitting, Sectoring, Microcell and Repeaters for range extension to support the required number of users in such areas. However, these techniques increase the number of Base Station and sometimes also increased the number of the cluster in the coverage area.

  1. Cell Splitting 
  2. Sectoring
  3. Microcell Zone concept
  4. Repeaters

1. Cell Splitting 

This technique is used to divide a Congested cell into a smaller cell or microcell, each with its own base station. To do this we need to lower the antennas height and transmitting power. I know a question will arise in your mind that

How can cell splitting increase improve coverage and capacity in a cellular system?
The answer is cell splitting increases the number of times that channels are reused.

How?
New cells of a shorter radius were installed between the existing cells, which have its own base station installed on the corner of the cell that increases the number of channel per unit area.

Now let’s make it more clear using an example. Take a look to below image, imagine if the radius of each cell was cut in half. This will increase the number of cells approximately four times of the original cell number and it will also increase the number of clusters which increase the number of capacity and channels. Remember that during adding smaller cell also take care frequency reuse plan.



Cell-splitting-concept-in-cellular-communication-Micro-zone
Micro-zone Cell concept

Transmitting Power for new cell

As we know that the size of the new cells is smaller, so the transmitting power also will be reduced. The power of the original cell will be cut in half as cell divided by half. You can find the transmitted power of the original cell by examining the received power (Pr) at original cell boundaries. It will help to maintain the frequency reuse plan.

Practical Scenario of cell splitting

The practical scenario of cell splitting is a bit different then it is mention above or you read somewhere on the internet. All cells cannot be split at the same time because a cell in real life is not of the same size. Even it is hard for network engineers to find the real estate of the cell to split it further. That is why network engineers take too careful to keep the distance between co-channel. Also increasing base station in a region increase the ratio of handover which is normally controlled using an umbrella approach.
A question arises can we use one large power for all newly define microcell?
The answer is simply No.
Why?
When smaller transmit power is used then many parts in the larger cell will remain left Pockets. And in the second case when a lager transmits power is used then some channel in a cell will be difficult to separate from their co-channels. In such scenario channel of old divide into two groups. One group served smaller cell and the second group is used by the larger cell for handoff purpose. The process of cell splitting continues when required more channel and one stage comes that all coverage area will be served by smaller cells.





2. Sectoring

Unlike cell splitting, Sectoring is another way to increase both capacity and coverage of the cellular system. Sectoring does not require changing cell size but it will reduce the number of clusters. As the number of cluster decreases the frequency reuse factor will increase.

To make this possible relative interface should be reduced without affecting the transmitted power. One way to reduce co-channel interface effectively is to replace Omni-directional antennas into directional antennas. The replaced directional antenna will radiate in its own specified sector which results in less interference as compared to Omni-directional antennas.

So we can define sector as
The technique used to increase system capacity and decrease co-channel interference by using directional antenna is known as sectoring.

How many Sectors does a cell have?

Normally cells are divided into three sectors (120 or 60 degrees). You can see the image below as an example where a cell is divided into three (3) and six (6) sectors.
Cell-sectors-in-cellular-system
Cell Sectoring
Remember that in sectoring, channels are dedicated to the specified sector and used within a particular sector. Now let’s understand it with an example. Take a look to below image, you will find a cell label “5” at the center of the image. It has also 3 co-channels on both left and right side center cell label “5”. Now you can see only two sectors radiate in the direction of the center cell. So this means out of 6 sectors, the center cell will experience interference on the forward link from only two left side sectors. In a practical scenario, interference can be further improved by down-tilting the antennas.
Cell-Sectoring-concept-in-cellular-system
Sector Antenna Direction view
To summarize, sectoring improves coverage and capacity in a cellular system by radiating in a specific direction using directional antennas and reducing the number of clusters. However, sectoring increases the number of antennas.

The Draw Backs of Sectoring

Sectoring has also drawbacks which include an increasing number of the antenna. Also decreasing coverage area in sectoring will increase the number of handoffs. This increases the burden on the Mobile Switching Center. However, handoffs are not the real problem because several modern base stations are allowed to control handovers in a cell from one sector to another without interrupting MSC.

The major problem is the loss of traffic due to decreased trunking efficiency. Sectoring uses more than one antenna and have a dedicated specific number of channels. This breaks up results, decreasing trunking efficiency.




3. Microcell Zone Approach

This is another pretty awesome technique is used to increase capacity and improve coverage of the cellular system. As we learn sectoring increase the number of handoffs which puts an extra burden on MSC. Microcell zone technique is then presented to overcome handoffs issue. This approach is based on a microcell for seven cell reuse.

In this proposed scheme, the cell is made from the combination of many zones and the single base station. Normally 3 zones are used which shown in below image.

Microzone-cell-concept-in-cellullar-system
Microcell


These zones are connected to a single base station through Co-axial cable, Optical fiber, and a Microwave link. Mobile users received the strongest signal within the cell. Antennas in the microcell are installed at the outer edge of the cell.

Handoffs are avoided by switching channel between zone site. This means whenever a user moves from one zone to another zone within the cell, the user receives the same channel. Co-channel are reduced by replacing a large central base station to the small lower powered transmitter on the edge of the cell. The reduction of interference also increased capacity and trunking efficiency is not affected in the microcell.

4. Repeaters

In many cases, a large building, mountains or any other obstacle make it hard for the service provider to provide coverage efficiently in such areas. For the solution of such problems, a device is used known as a repeater.




Repeater Concept in Cellular system
Repeaters
A repeater is actually a bidirectional radio transmitter. This means that it send and receive simultaneously from a particular base station. A repeater can be installed anywhere accordingly because of its size. When repeater received a signal from the base station, it amplifies and radiates in the required coverage area.

Remember that noise and interference in the original signal also radiate along with the newly generated signal.

Note that repeater does not add capacity to the system. It is only the way to provide coverage to the area block by obstacles. It is also used to provide inside the buildings.

Reference Book:
1) Wireless Communications- Principles and Practice by Theodore S Rappaport
2) Mobile Cellular Communication Gottapu Sasibhushana Rao

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