What is dwell time in wireless communication

The time period for which a call is maintained in a cellular region is called dwell time.

There is some basic factor that influences the dwell time are :

• propagation
• Interference
• The distance between the subscriber and the base station

As the mobile moves from the serving base station (BS), there will drop the signal level. Therefore, the base station generally monitors and measure the signal level before initiating for continuing with the call in progress.

Umbrella cell approach in mobile communication

The cell with low traffic speed is called as micro-cells and large high-speed traffic called macro-cells.

A hexagonal cell method is one of the closest approximation of a circle device. It is being typically used for the system.

The smaller cell is grouped and assumed to be under a large cell. This method called as an umbrella cell concept.

It can be used to provide a large area to high-speed users while small area coverage to users that travel at low speeds.

Some advantages of using an umbrella cell concept in cellular technology 

• Umbrella cell provides a large coverage area to high-speed users.
• Minimize the number of handoff for high-speed users.
• Speed estimation can be performed by slop of short term average calculation of receive power.
• If a high-speed user in large umbrella cell is near the base station and if its velocity is decreasing then the BS stands for  can decide whether to hand the user into the co-located micro-cell without the intervention of the mobile switching center (MSC stands for).
• Provide additional micro-cell channel for pedestrian user.

Umbrella cell concept

Cell splitting

Cell splitting is a method of subdividing cell into a smaller sized cell. In this cell, the parent cell that was originally congested is called as macro cells and the smaller cell called microcells.

The most important benefits for cell splitting is that it increases the cellular capacity of the system where the frequency reuse techniques can be efficiently implemented

Each microcell has a base station antenna. The radius of the microcell is half a radius of the macro cell.

In cell splitting, if the increased number of a cell will increase the number of clusters over the coverage area and will increases the number of channel per unit area so that the capacity of the system is increased.

For the new cell to be a smaller size, the transmitted power must be decreased. in this process, the large macro cell is dedicated to high traffic. In macrocell, the number of hands offs will be less than and also have the call process can be smoothly carried in larger cells.

While in Cell sectoring is one another method to increase the capacity of the system. It keeps the radius of the cell constant and  also decreases the co-channel reuse ratio D/R to reduce the cluster size N, so it is a major difference between them 

This post also gives some advantages and disadvantages of cell splitting to know more details about this concept.

Advantages of cell splitting :

• It improves the S/N ratio.
• It reduces interference which increases capacity.
• It reduces the cluster size.
• System capacity gradually expands as demand.

Disadvantages of cell splitting :

• Handoff is more frequent.
• Channel assignment becomes difficult.
• All cell are not simultaneously so special care have to be taken for proper allocation of the problem.

Cell sectoring

Cell sectoring is replacing with an omnidirectional type of the antenna at the base station by several types of a directional antenna. It is done mainly to reduce factors such as a co-channel interface. 

Some important points on cell sectoring :

• Cell sectoring is another method to increase capacity. It keeps the radius of the cell constant and decreases the co-channel reuse ratio D/R to reduce the cluster size N. 
• Cell sectoring is a method of decreasing the co-channel interference and enhancing system performance by using a directional antenna.
• The size of clusters in a particular service area can be reduced because the cell sectoring increases the signal to interference ratio (SIR).
• So in cell sectoring process enhancing the system performance by using a directional antenna and reducing the co-channel interference value. here the reduction in the co-channel interference is dependent on the amount of sectoring used.
• So here in cell sectoring process generally cell divided in 120 ^{₀  &}  60 ^₀ sectors.
• A cell is in sectoring is 120 ^₀  then hexagon cell consists of three sectors and if the sector is 60 ^₀  then the hexagonal type cell consists of six sectors. 
• The single to noise ratio improvement allows the cellular provider to decreases the cluster size N in order to improve the frequency reuse and thus, the system capacity. 

120 ^₀  and 60 ^₀  sectoring

Now let us talk about some advantages and disadvantages of cell splitting to know more details about this concept.

Advantages of cell sectoring :

• Better S/I ratio.
• Reduces interference.
• Increases capacity.
• Reduces cluster size.
• More freedom in assigning a channel

Disadvantages of cell sectoring :

• Increases number of antennas per base station.
• A decrease in trunk efficiency.
• Loss of traffic.
• Increased number of handoffs.

Hexagonal geometry cell

Having studied the cellular topology and the concept of employing cellular architecture to increases the cellular capacity, serve subscriber demand so we will consider to characteristics the interference in a cellular topology.

A hexagonal cell is one of the closest approximation of a circle to shown in the figure. It is being typically used for the system.

So now let us talk about why we have to choose hexagonal shape summarized below :

• Basically hexagonal is close approximation the circular radiation pattern in an omnidirectional base station antenna.
• Hexagonal allow easy and manageable analysis in a cellular system.
• In a circular pattern, adjacent circles can have gaps in between or can create overlapping regions.
• Hexagonal geometry is used then only less number of a cell can cover the entire market.

Hexagonal cell

What is interference in communication

The performance of a cellular system is highly affected by interference. Interference is the main problem in increasing capacity. basically, there are two types of interference in the cellular system.

1. Co-channel interference
2. Adjacent channel interference

Co-channel interference :

In the frequency reuse method, several cells use the same set of frequencies in the given service area. The cell using the same set of the channel are called co-channel cells. The interference between signals from these cells is called as co-channel interference.

While the order to reduce co-channel interference, the co-channel interference cell are actually separated by the smallest distance to provide isolation.

The minimum distance that allows the same frequency to be reused depends on factor like a number of co-channel cells near the center cell.

Now let us discuss how to find frequency reuse distance D  in co-channel interference  

D= √3N  R 

N =  i²  + i j +  j²

Here we have to calculate different values of D  for given values of N.

D=3 , N=3 (i=1, j=1)

D=4.58R, N=7(i=1, j=2)

D= 6R, N=12(i=2, j=2)

If all the cell transmits the same power, then N increases and the frequency reuse distance D increases. This increases D reduces the probability interference can occur.

The ratio D/R is called the co-channel reuse ratio. It is expressed as,

                                Q = D/R = √3N

A small value of Q provides large capacity as the cluster size N is small. A large Q, i.e a large value of D/R indicates that the spatial separation between the co-channel cell relative to the coverage distance of a cell is increased.

   

Adjacent channel interference :       

While the interference resulting from the channel that is adjacent in frequency to the desired channel is called adjacent channel interference.

If the user that is using an adjacent channel and is transmitting in close range to subscriber receiver then while the receiver tries to receive a base station on the desired channel the problem can be critical. It is called the near-far effect.

Be proper filter and channel assignment the adjacent channel interference can be minimized. As each cell is given only part of the available channels a cell needs to assign channels that are adjacent in frequency.

In adjacent channel interference is frequency reuse when the frequency reuse is high there are more chances for interference because of neighboring channels and because of separation concept. 

Let us now talk about the basic source of interference :

• Any other mobile in the same cell.
• A call in always progress in the neighboring of a cell.
• In most of the other base station operating on the same frequency.
• Any non-cellular system which leaks energy into the cellular frequency band.

Channel assignment strategies

The channel assignment strategies can be classified as fixed or dynamic depending on the system performance in managing cell calls when a mobile user is handed off from one cell to another.

Fixed channel assignment : 

In the fixed channel assignment method each cell is assigned a predetermined set of voice channel.

Any effort that is done to call a number within the cell can be serviced by the unused channels in that particular cell.

In case if all the channels in that cell are used, then the cell is blocked. such a situation the subscriber does not  get service.

Borrowing techniques are used from an adjacent cell if all of its channels are occupied. This method is called as borrowing strategies.

Dynamic channel assignment :

In dynamic channel assignment strategy there is no permanent allocation of channels. Every time a call request is done, the BS full form (base station) requests a channel from MSC full form is mobile switching center.

Mobile switching center only allocates the channel after verifying that the channel is not currently in use in the cell or any other cell that comes within the minimum restricted distance of cellular frequency reuse to avoid co channel interference.

Dynamic channel assignment strategy reduces the probability of blocking and increases the trunking capacity of the system. These strategies need MSC to collect real time data. 

Cellular frequency reuse

What is cellular frequency reuse ?

A group of radio channel is allocated a separate circular base station to be used within a small geographic area called cell.

The design method of selecting and allocating channel groups for all cellular base station within a system called as frequency reuse.

Here we have to use hexagonal geometry is used in the design of cellular system.

we have to better to understand frequency reuse concept in cellular system having S duplex channel then, and if n channel is less than S (n<S) and is S channel are divided among N cell into unique and disjoint channel groups with each having the same number of channels, then the total number of available radio channel is define as : 

            S = nN

Total number of duplex channel  C can be defined as :      

             C = mnN = mS

The capacity of cellular system is directly proportional to the number of times a cluster repeated in a fixed service area, the factor N is called as the cluster size 

For maximum capacity, the smallest possible value of N is desired. 1/N is called as the frequency reuse factor as each cell within the cluster is only assigned 1/N times of the total available channels.

The geometry of hexagon is such that the number of cell per cluster N can have values that satisfy the equation, 

N = i² +ij + j²

Where i and j are non negative integer
So here we have to calculate the possible cluster size above this equation :
i=1, j=1 so N=3
i=2, j=0 so N=4
i=2, j=2 so N=12
i=2, j=3 so N=19

Basic cellular system

Before we learn about the cellular system first let we check it out what is the cellular system. In a cellular system, replaced the single high power transmitter with many numbers of low power transmitters (Base station) each signal providing coverage to only a fraction of service area (Cell). Each base station is allocated a portion of the total number of channels and nearby BS full form (base station) are assigned different group of channels. Basically hexagonal is close approximation the circular radiation pattern in an omnidirectional base station antenna.

A basic cellular system can be comprised of three units :

1. Mobile unit
2. Cell site
3. A mobile telephone switching office

• Mobile unit :

It comprises a control unit, a transceiver, and an antenna system.

• Cell site :

It is an interface between the mobile unit and the full form of MTSO. It comprises a control unit, antennas, power plant, and data terminals.

• Mobile telephone switching office :

It is the central co-coordinating unit for all cell sites. It comprises of the cellular processor and the cellular switch.

One of the most important points is that the MTSO is responsible for controlling the call processing operation and handling the billing activities of the subscriber.

In general cellular, the switch can be analog or digital form. It can connect the mobile subscriber to the other mobile subscriber to the nationwide telephone network.

Cellular system

What is cellular system

In a cellular system, replaced the single high power transmitter with any numbers of low power transmitters (Base station) each signal providing coverage to only a fraction of service area (Cell). Each base station is allocated a portion of the total number of channels and the nearby base station (BS) are assigned different group of channels.

The neighboring base station is assigned a different group of channels that the interference between the base station is minimized in the cellular group.

In a Basic cellular system, replaced the single high power transmitter with any number of low power transmitters (Base station) each signal providing coverage to only a fraction of service area (Cell).

Some advantages and disadvantages of the cellular system :

Advantages of the cellular system :

• High capacity
• Reduced in  process
• Less transmission power
• Reduced set up times
• It reduced the interference which increases the system capacity
• It improved the S/N ratio
• Reduced the cluster size
• More robust against the failure of single components

Disadvantages of the cellular system :

• Handover is needed 
• Good infrastructure needed
• Frequency planning should be good
• Increases the number of an antenna in each base station