A Cellular Network is formed of some cells. The cell covers a geographical region and has a base station analogous to 802.11 AP which helps mobile users attach to the network and there is an air interface of physical and data link layer protocol between mobile and base station. All these base stations are connected to the Mobile Switching Center which connects cells to a wide-area net, manages call setup, and handles mobility.
There is a certain radio spectrum that is allocated to the base station and to a particular region and that now needs to be shared. There are two techniques for sharing mobile-to-base station radio spectrum:
- Combined FDMA/TDMA: It divides the spectrum into frequency channels and divides each channel into time slots.
- Code Division Multiple Access (CDMA): It allows the reuse of the same spectrum over all cells. Net capacity improvement. Two frequency bands are used one of which is for the forwarding channel (cell-site to subscriber) and one for the 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 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 cluster N determines the amount of co-channel interference and also the number of frequency channels available per cell.
Cell Splitting
When the 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.
Cell Splitting
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. An increasing number of wireless devices and the communication between them.
Cellular Hierarchy
- Femtocells: The 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.
- Picocells: The size of these networks is in the range of a few tens of meters, e.g., WLANs.
- Microcells: Cover a range of hundreds of meters e.g. in urban areas to support PCS which is another kind of mobile technology.
- Macrocells: Cover areas in the order of several kilometers, e.g., cover metropolitan areas.
- 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. The 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 that is also fixed. Transmitting and receiving do 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 microseconds, but a transmitter is only allowed to send one data frame every 4.615 microseconds 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.
- 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 move into a new cell.
- 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 is sent to the dedicated control channel.
Common Control Channel
Three logical sub-channels are:
- Is the paging channel, that the base station uses to announce incoming calls. Each mobile station monitors it continuously to watch for calls it should answer.
- 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.
- Is the access grant channel which is the announced assigned slot.
Advantages of Cellular Networks
- Mobile and fixed users can connect using it. Voice and data services also provided.
- Has increased capacity & easy to maintain.
- Easy to upgrade the equipment & has consumes less power.
- It is used in place where cables can not be laid out because of its wireless existence.
- To use the features & functions of mainly all private and public networks.
- Can be distributed to the larger coverage of areas.
Disadvantages of Cellular Networks
- It provides a lower data rate than wired networks like fiber optics and DSL. The data rate changes depending on wireless technologies like GSM, CDMA, LTE, etc.
- Macrophage cells are impacted by multipath signal loss.
- To service customers, there is a limited capacity that depends on the channels and different access techniques.
- Due to the wireless nature of the connection, security issues exist.
- For the construction of antennas for cellular networks, a foundation tower and space are required. It takes a lot of time and labor to do this.