Cellular Networks

Cellular Network is formed of some cells, cell covers a geographical region, has a base station analogous to 802.11 AP which helps mobile users attach to network and there is an air-interface of physical and link layer protocol between mobile and base station. All these base stations are connected to Mobile Switching Center which connects cells to wide area net, manages call setup and handles mobility.

There is certain radio spectrum that is allocated to base station and to a particular region and that now needs to be shared. There are 2 techniques for sharing mobile-to-base station radio spectrum are:

  1. Combined FDMA/TDMA:
    It divide spectrum in frequency channel and divide each channel into time slots.
  2. Code Division Multiple Access (CDMA):
    It allows reuse of same spectrum over all cells. Net capacity improvement. Two frequency bands are used one of which is for forward channel (cell-site to subscriber) and one for reverse channel (sub to cell-site).

Cell Fundamentals –
In practice cells are of arbitrary shape(close to a circle) because it has the same power on all sides and has same sensitivity on all sides, but putting up two three circles together may result in interleaving gaps or may intersect each other so in order to solve this problem we can use equilateral triangle, square or a regular hexagon in which hexagonal cell is close to a circle used for a system design.
Co-channel reuse ratio is given by:



DL/RL = Square root of (3N)
Where, 
DL = Distance between co-channel cells
RL = Cell Radius
N = Cluster Size 

The number of cells in a cluster N determines the amount of co-channel interference and also the number of frequency channels available per cell.

Cell Splitting –
When number of subscribers in a given area increases allocation of more channels covered by that channel is necessary, which is done by cell splitting. A single small cell midway between two co-channel cells is introduced.

Need for Cellular Hierarchy –
Extending the coverage to the areas that are difficult to cover by a large cell. Increasing the capacity of the network for those areas that have a higher density of users. Increasing number of wireless devices and the communication between them.

Cellular Hierarchy –

  1. Femtocells:
    Smallest unit of the hierarchy, these cells need to cover only a few meters where all devices are in the physical range of the uses.
  2. Picocells:
    Size of these networks is in the range of few tens of meters, e.g., WLANs.
  3. Microcells:
    Cover a range of hundreds of meters e.g. in urban areas to support PCS which is another kind of mobile technology.
  4. Macro cells:
    Cover areas in the order of several kilometers, e.g., cover metropolitan areas.
  5. Mega cells:
    Cover nationwide areas with ranges of hundreds of kilometers, e.g., used with satellites.

Fixed Channel Allocation –
For a particular channel the frequency band which is associated is fixed.
Total number of channels is given by

Nc = W/B 
Where, 
W = Bandwidth of the available spectrum, 
B = Bandwidth needed by each channels per cell, 
Cc = Nc/N where N is the cluster size 

Adjacent radio frequency bands are assigned to different cells. In analog each channel corresponds to one user while in digital each RF channel carries several time slots or codes (TDMA/CDMA). Simple to implement as traffic is uniform.

Global System for Mobile (GSM) Communications –
GSM uses 124 frequency channels, each of which uses an 8-slot Time Division Multiplexing (TDM) system. There is a frequency band which is also fixed. Transmitting and receiving does not happen in the same time slot because the GSM radios cannot transmit and receive at the same time and it takes time to switch from one to the other. A data frame is transmitted in 547 micro seconds, but a transmitter is only allowed to send one data frame every 4.615 micro seconds, since it is sharing the channel with seven other stations. The gross rate of each channel is 270, 833 bps divided among eight users, which gives 33.854 kbps gross.

Control Channel (CC) –
Apart from user channels there are some control channels which is used to manage the system.

  1. The broadcast control channel (BCC):
    It is a continuous stream of output from the base station’s identity and the channel status. All mobile stations monitor their signal strength to see when they moved into a new cell.
  2. The dedicated control channel (DCC):
    It is used for location updating, registration, and call setup. In particular, each base station maintains a database of mobile stations. Information needed to maintain this database and is sent on the dedicated control channel.

Common Control Channel –
Three logical sub-channels:

  1. Is the paging channel, which the base station uses to announce incoming calls. Each mobile station monitors it continuously to watch for call it should answer.
  2. Is the random access channel that allows the users to request a slot on the dedicated control channel. If two requests collide, they are garbled and have to be retried later.
  3. Is the access grant channel which is the announced assigned slot.


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