1. Introduction to mobile telecommunication

1.1. Telecommunication:

Telecommunication is the assisted transmission of signals over a distance for purpose of communication ,it is an important part of the world economy and the telecommunication industry revenue has been placed at just under 3 percent of the gross world product.

Telecommunication over a phone line is called point-to-point communication because it is between one transmitter and one receiver. Telecommunication through radio broadcasts is called broadcast communication because it is between one powerful transmitter and numerous receivers.

1.2. Communication system :

For efficient exchange of data (information) any communication system must enhance the following properties:

-Deliver as much information as possible from source to destination .

-Deliver information in shortest time .

-Reduce errors in delivery of information(error detection/ correction)

1. 3 Analog and digital communication :

Communications signals can be either by analog signals or digital signals. These are analog communication systems and digital communication systems. For an analog signals the signal is varied continuously with respect to the information. In a digital signal the information is encoded as a set discrete values (for example, a set of ones and zeros)

1.4 Basic Communications System Elements :

A basic telecommunication system consists of three primary units :

1. Source (transmitter): that takes information and converts it to a signal.

2. Transmission medium: also called the "physical channel" that carries the signal.

3.Destination (receiver): receives the signal from the channel and converts it back into information.

1.5 Transmission Media :

Signals are transmitted between telecommunication devices in the form of electromagnetic energy which is propagated through transmission media. Transmission media can be divided into two broad categories : guided and unguided.

1.6 Modes of Transmission in wireless:

1. simplex communication system:

· Communication is possible in one direction only.

Example: Television & Radio

2. Half-duplex communication system:

· Two way communication on the same channel .

· At any time a user can only transmit or receive.

Example: Police radio .

3. Full-Duplex communication system:

· Simultaneous two – way communication

· Two simultaneous but separate channels are provided for communication to and from the terminals.

Example: GSM mobile radio communication.

1.7 Duplex Transmission:

1. frequency Division Duplex (FDD):

Transmission and reception take place in different frequency ranges. The distance between the Uplink UL and Downlink DL frequency range is designated as duplex distance.

2. Time Division Duplex (TDD):

Transmission and reception take place in the same frequency band. Uplink UL and Downlink DL transmission take place at different times. There is fast switching between UL and DL transmission, so that the user has the impression of simultaneous transmission and reception .

1.8 MultipleAccessTechniques:

1. Frequency Division Multiple Access (FDMA):

Frequency division is the starting point for all wireless communications because all communications within agiven cell must be separated by frequency to avoid their mutual interference. Frequency Division Multiple Access (FDMA) divides the assigned frequency range into multiple frequency channels to support multiple conversations.A given call takes place on one pair of frequencies,with one transmission in the forward direction and another for transmission in the reverse direction.

2. Time Division Multiple Access (TDMA):

Time division multiple access (TDMA) is achannel access method for shared medium (usually radio) networks . It allows several users to share the same frequency channel by dividing the signal into different timeslots . The users transmit in rapid succession, one after the other, each using his own timeslot.

3. Code Division Multiple Access (CDMA):

Code Division Multiple Access employs spread-spectrum technology and a special coding scheme ( where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. The codes in principle have orthogonal properties.

4. Orthogonal Frequency Division Multiple Access (OFDMA):

Orthogonal Frequency Division Multiple Access(OFDMA) is amulti-user version of the Orthogonal frequency –division multiplexing (OFDMA) digital modulation scheme which will be discussed in details later.Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users as shown in the illustration below. This allows simultaneous low data rate transmission from several users.

1.9 Circuit Switching [CS] and Packet Switching[PS]:

There are two ways of transmitting data through a network: circuit switching and packet switching.

· Circuit Switching [CS] : is a type of communication in which a dedicated channel (or circuit) is established physically and doesn‘t change for the duration of transmission. The most common circuit-switching network is the telephone system, which links together wire segments to create a single unbroken line for each telephone call.

· Packet Switching [PS] : is the other method communications method, which divides message into packets and sends each packet individually.The packets may take different routes and arrive out of order. Upon getting to their destination, the individual packets get put back into order by a packet assembler. The internet is based on a packet switching protocol, TCP/IP.

Circuit-Switching systems are ideal for communications that require data to be transmitted in real-time. Packet-Switching systems are more efficient if some amount of delay is acceptable.