PRESENTATION ON :

• PULSE CODE MODULATION

• MULTIPLE ACCESS TECHNIQUES

• GSM

MOBILE COMMUNICATIONS

Mobile phones send and receive radio signals with any number of cell site base stations fitted with microwave antennas. These sites are usually mounted on a tower, pole or building, located throughout populated areas, then connected to a cabled communication network and switching system. The phones have a low-power transceiver that transmits voice and data to the nearest cell sites, normally not more than 8 to 13 km (approximately 5 to 8 miles) away. When the mobile phone or data device is turned on, it registers with the mobile telephone exchange, or switch, with its unique identifiers, and can then be alerted by the mobile switch when there is an incoming telephone call

MOBIE COMMUNICATIONS :

Pulse code modulation

Analog voice data must be translated into a series of binary digits before they can be transmitted.With Pulse Code Modulation (PCM), the amplitude of the sound wave is sampled at regular intervals and translated into a binary number.The difference between the original analog signal and the translated digital signal is called quantizing error. PCM uses a sampling rate of 8000 samples per second and each sample is an 8 bit sample resulting in a digital rate of 64,000 bps (8 x 8000).

Steps involved in converting a analog signal to digital signal :

PCM consists of four steps to digitize an analog signal:1. Filtering2. Sampling3. Quantization4. Binary encoding

SPECFICATIONS OF PCM :

• Voice Frequency range 0- 4 KHz• Sampling the Voice Signal @ 8 KHz

(Double the Max. Frequency as per sampling theorem) i.e. 8000s/sec

• Sampling time period Ts=1sec/8000• Ts= 125 microsecond• Available for sampling each channel, when we have N

total channels=125/N• In PCM, Time frame=125microsec ;time available per

chl=125/32 =3.9microsec.

MULTIPLE ACCESS TECHNIQUES

A cellular system divides any given area into cells where a mobile unit in each cell communicates with a base station. The main aim in the cellular system design is to be able to increase the capacity of the channel i.e. to handle as many calls as possible in a given bandwidth with a sufficient level of quality of service. There are several different ways to allow access to the channel. These includes mainly the following:

1) Frequency division multiple-access (FDMA) 2) Time division multiple-access (TDMA) 3) Code division multiple-access (CDMA)

FREQUENCY DIVISION MULTIPLE ACCESS (FDMA) :

This was the initial multiple-access technique for cellular systems in which each individual user is assigned a pair of frequencies while making or receiving a call.

TIME DIVISION MULTIPLE ACCESS (TDMA) :Time division multiple access (TDMA) is

a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot.

CODE DIVISION MULTIPLE ACCESS (CDMA) :

CDMA is a “spread spectrum” technology, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal.

CDMA systems offer services to wide band and narrow band systems. Wide band systems supports conventional cellular voice, text and MMS services, but can also carry data at high speeds, allowing mobile operators to deliver higher bandwidth applications including streaming and broadband Internet access.

• Multiple Access in CDMA:o Each user is assigned a unique PN code.o Each user transmits its information by spreading with

unique code.o Direct Sequence spread spectrum is used.o Users are separated by code not by time slot and freq slot.

GSM

GSM is the GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS. It is a cellular network in which the mobiles are connected to it by searching for cells in the immediate vicinity. The GSM system was designed as a second generation (2G) cellular phone technology. One of the basic aims was to provide a system that would enable greater capacity to be achieved than the previous first generation analogue systems. GSM achieved this by using a digital TDMA (time division multiple access approach). By adopting this technique more users could be accommodated within the available bandwidth.

GSM ARCHITECURE

The main components of a GSM system are: 1.Mobile Station (MS) 2.Base Station Subsystem (BSS) 3.Network Switching Subsystem(NSS)

MOBILE STATION :A Mobile Station consists of two main elements: The mobile equipment or terminal. The Subscriber Identity Module (SIM).

Base Station Subsystem (BSS) : The BSS connects the Mobile Station and the NSS. It is in charge of the transmission and reception. The BSS can be divided into two parts: •The Base Transceiver Station (BTS) or Base Station. •The Base Station Controller (BSC).

BASE STATION SUBSYSTEM

BSC

BTS

The network switching subsystem has the following five important components.• Mobile Switching Center (MSC)• Home Location Register (HLR)• Visitor Location Register (VLR)• Authentication Center (AUC)• Equipment Identity Register (EIR) 

Network Switching Subsystem(NSS) :

NSS

MSC

HLR

AUC

SERVICES PROVIDED BY GSM:

Speech or voice calls are obviously the primary function for the GSM cellular system. To achieve this the speech is digitally encoded and later decoded using a vocoder. Besides speech and voice calls, GSM supports different services like : 1. SM S 2 . Vo i c e M a i l 3 . Adv i c e o f Cha rge 4 . Roa m i ng 5 . Ca l l ho l d 6 . Ca l l wa i t i ng 7 . Con f e r e nce ca l l

ADVANTAGES OF GSM :

1. Less signal deterioration inside buildings. 2. Ability to use repeaters.

3. Talk time is generally higher in GSM phones due to the pulse nature of transmission.

4 .The availability of  SIM allows users to switch networks and handsets at will, aside from a subsidy lock. 5. GSM covers virtually all parts of the world so international roaming is not a problem. 6. The much bigger number of subscribers globally  creates a better  network effect for GSM handset makers, carriers and end users.

CONCLUSION :

The communication development and the increase of living standard of people are directly related to the more use of cellular mobile radio- the high end sophisticated technology enables everyone to communicate anywhere with anybody. The mobile telephony industry is rapidly growing and that has become back bone for business success and efficiency and the part of modern life style all over the world. The features and benefits in the GSM systems are superior speech quality, low terminal, operational and service costs, a high level security and providing of international roaming support. CDMA is radically new concept in wireless communication. It has gained wide spread international acceptance by cellular radio system operators as an upgrade that will dramatically increase both their system capacity and the quality.