Cellular Wireless Networks

Cellular Communication

• Cellular technology is the foundation of mobile wireless communication systems.
• This can accommodate a large number of users over different areas which are not easily served by wired networks.

Cellular System

• The principle elements of a cellular system is as shown below

Terms needs to be familiarized..

• Base Station: A fixed station in a cellular system used for radio communication with mobile station.
• Mobile Station: A station in the cellular system intended for use while in motion at unspecified locations.
• Control Channel: Radio channel for transmission of call set up, call request, call initiation and other control purposes.
• Public Switched Telephone Network(PSTN): it provides connection to phones located in individual homes
• Mobile Switching Centers: Switching center which connects the cellular base stations and to the mobiles and to the public switched telephone network PSTN.
• Forward Channel: Radio channel for transmission of information from the base station to mobile.
• Reverse Channel: Radio Channel for transmission of information from mobile to base station.
  • Transceiver: Device capable of simultaneously transmitting and receiving radio signals.
  • SIM: Subscribers Identity Module
  • MIN: Mobile Identification Number, which is subscribers telephone number.

• The cell: a geographical area covered by Radio Frequency (RF) signals.

CELL?

• Larger area divided into small no. Of areas

• • • Shape is hexagonal
    • Each with its own base station and set of frequencies.
  • It consists of base station, mobile station and Mobile Telecommunication Switching Office (MTSO).

Operation of Cellular System

• Each base station is connected to a MTSO.
• Multiple base stations can be connected to a single MTSO.
• MTSO connects calls between mobile units.
• It is also connected to Public Switched Telecommunication Networks (PSTN).
• MTSO makes connection between a fixed land line subscriber through the PSTN and mobile subscriber through Cellular Network.

Operation of Cellular System

• • Two types of channels are available between the mobile unit and base station.
• Control Channels
• Traffic Channels
  • Control channels are used for call setup, call request, call initialization and other control purposes.
  • Traffic channels are used for transformation of information between users.

Cell Structure- Options

Why hexagonal cell ?

• A regular cell shape is needed for systematic design and adaptation for future growth.
• The hexagon has largest area compared to a square and equilateral triangle.
• Thus it can cover the entire region with the fewest number of cells.
• Cluster: group of cells where no channels are reused. The number of cells per cluster, N, can only have values which satisfy

7-cell cluster

Principles of cellular networks

• The cellular concept makes the use of replacing of single, high power transmitter with many low power transmitters providing coverage to a small portion of the service area.
• In cellular system an area is divided in to a number of cells, each one is served by a base station.
• Each cellular base station is allocated a portion of the total number of channels or frequencies available to the entire system.

• Adjacent channels are assigned different group of channels(frequencies) to avoid interference between the base stations.
• Cells which are sufficiently distant from each other can use the same frequency band.

Advantages

• Solves the problem of spectral congestion and user capacity.
• Reuse of radio channel in different cells.
• Offer very high capacity in a limited spectrum without major technological changes.

Frequency Reuse

• Cells labeled with the same letter use the same group of channels
• seven groups of channel from A to G
• footprint of a cell – actual radio coverage
• omni-directional antenna v.s. directional antenna

• For a hexagonal geometry co-channel reuse ratio Q

• A small value of Q provides large capacity since cluster size(N) is small.
• A large value of Q improves the transmission quality – smaller level of co-channel interference
• A tradeoff must be made between these two objectives
• The frequency reuse factor is given by 1/N

FREQUENCY REUSE

• Each cell is assigned a part of the available frequency spectrum.
• same part of the frequency spectrum more than once. This is called frequency reuse.

Capacity of a cellular system

C= capacity

M= number of clusters (3) N= no of cells in a cluster (7)

S=total number of available channels(eg. 49)

K= allotted channel to a cell (eg. 7)

• S=KN
• C = MKN = MS

 

• Consider a cellular system which has a total of S duplex channels.
• Each cell is allocated a group of k channels, k  S .
• The S channels are divided among N cells.
• The total number of available radio channels

S = kN

• The N cells which use the complete set of channels is called cluster.
• The cluster can be repeated M times within the system. The total number of channels, C, is used as a measure of capacity

C = MkN = MS

• The capacity is directly proportional to the number of replication M.
• The cluster size, N, is typically equal to 3, 7, or 12.
• Small N is desirable to maximize capacity.
• The frequency reuse factor is given by 1/ N

Call Stages

• Figure illustrates the steps in a typical call between two mobile users within an area controlled by a single MTSO:
• Mobile unit initialization: mobile unit scans and selects the strongest setup control channel used for this system (a).
• Mobile-originated call: A mobile unit originates a call by sending the number of the called unit on the preselected setup channel (b).
• Paging: The MTSO then attempts to complete the connection to the called unit, sending a paging message to certain BSs depending on the called mobile number (c).

Common Control Channels (CCCHs):

• Paging Channel (PCH):- { forward link channel }
• Provides paging signals from the base station to all mobiles in a cell.
• It gives notification of an incoming call to a mobile which originates from PSTN.
• It provide ASCII text messages to all subscribers, as a part of the SMS features of GSM.

• Call accepted: The called mobile unit recognizes its number on the setup channel being monitored and responds to that BS, which sends the response to the MTSO.
  • The MTSO sets up a circuit between the calling and called BSs (d).
• Ongoing call: While connection is maintained, the mobile units exchange voice or data signals, through respective BSs and MTSO (e).
• Handoff: If a mobile unit moves out of range of one cell and into the range of another during a connection, the traffic channel has to change to one assigned to the BS in the new cell (f).

Handoff

• When a mobile unit moves from one station to another, the transfer of call should be done between cells.
• This is because, the frequencies of different cells are different.
• Handoff is the process of changing the assignment of a mobile unit from one base station to another as the mobile unit moves from one cell to another.

Handoff

• Hand off process is as shown below

Handoff

• Base station monitors signal strength throughout the call.
  • If the signal strength goes below a certain level, the base station assumes that the mobile is on the move and initiates the procedure of hand off.
  • MSC request the base station covering the adjacent cell to monitor the traffic.
  • The information is passed back to the MSC which decides when hand off is necessary.
  • Then a retune command is given to the mobile and it sends information to the new base station.

Handoff 

Hand off

• The margin between “Hand off Threshold” and Minimum acceptance signal to maintain the call is known as Δ.

• Δ cannot be too small and cannot be too large.
  • If Δ is too large, unnecessary handoffs burden the MSC
  • If Δ is too small, there may be insufficient time to complete handoff before a call is lost.

• The time over which a call may be maintained within a cell without Handoff is known as Dwell Time.

Handoff Types

• Soft hand off:
  • The connection to the new cell is made before the connection with the existing cell is broken.
  • Mobile connects with more than one cell simultaneously. Used in CDMA
• Hard Hand off:
  • Connection with the existing cell is broken before connection with new cell is established.
  • Used in FDMA or TDMA

Roaming

• Roaming is a service which allows subscriber to operate in service areas which are different from the home location where the service is registered.
• When a mobile enters a new geographical area it is registered as a roamer in the new service area.

Roaming involves the following steps

• When mobile enters a new area, network will identify the home network. If there is no agreement between the home and host network, access will be denied.
• The host network contacts the home network and requests service information about the roaming device.
• The host network begins to maintain a temporary subscriber record for the device.
• Home network updates its information to indicate that the mobile is on the host network so that any information send to that device can be correctly routed.

Improving capacity in Cellular System

Coverage techniques area is increased using the following

• Cell Splitting
• Cell Sectoring
• Repeaters for Range Extension
• Micro Cell Zone Concept

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 transmitter power.

• Smaller cells are called as micro cells. and
• This increases the number of times a channel is reused.
• Capacity is increased due to the additional number of channels per unit area.

Cell Splitting

• Cell Splitting is illustrated as shown below

Cell Splitting

• For the new cells to be smaller in size, the transmitted power of these cells must be reduced.

• For n = 4, and if received powers equals each other, then

Cell Sectoring

• Sectoring is another way to increase the system efficiency by decreasing the ratio of D/R.
• Sectoring is done by using directional antennas.
• A cell is normally split into three 120 degree sectors or six 60 degree sectors.
• This is illustrated as shown in the figure.

Cell Sectoring

• Sectoring reduces the co channel interference

Cell Sectoring