Lecture 8 B 0 Cellular Telephone Networks 2

Lecture 8 (B 0: Cellular Telephone Networks 2 nd semester 1439 -1438

Medium Access Control Techniques • When stations use a common link, we need a multipleaccess protocol to coordinate access to the link.

Channelization • Channelization is a multiple access method in which the available bandwidth of a link is shared in time, frequency or through code, between different stations.

FDMA • In frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands. • Each station is allocated a band to send its data. • The allocated bands are separated from one another by small guard bands.

TDMA • In time-division multiple access (TDMA), the stations share the bandwidth of the channel in time. • Each station is allocated a time slot during which it can send data.

CDMA • Data from all stations are transmitted simultaneously and are separated based on coding theory. • In TDMA and FDMA the transmissions from different stations are clearly separated in either time or frequency. • In case of CDMA, the transmission from different stations occupy the entire frequency band at the same time. Multiple simultaneous transmissions are separated by using coding theory. Each bit is assigned a unique m-bit code or chip sequence.

First Generation System • The first generation was designed for voice communication using analog signals. • One example is Advanced Mobile Phone System (AMPS) used in North America. • It is one of the leading analog cellular systems in North America. • It uses FDMA to separate channels in a link.

AMPS • Bands: AMPS operates in the 800 -MHz ISM band. • The system uses two separate analog channels: • one forward (base station to mobile station) communication, the band between 869 and 894 MHz. • one for reverse (mobile station to base station) communication, the band between 824 and 849 MHz.

AMPS • AMPS uses FDMA to divide each 25 -MHz band into 30 KHz channels, some of which are used for control. • AMPS has a frequency reuse factor of 7 • AMPS uses FM and FSK for modulation.

Second Generation • To provide higher-quality (less noise-prone) mobile voice communications, the second generation of the cellular phone network was developed. • The second generation was mainly designed for digitized voice

Second Generation • Shifting from analog to digital enabled several improvements in systems performance. • 2 G systems Besides providing • • improved voice quality improved system capacity Security against eavesdropping and fraud Enabled new applications, prime among these was the Short Messaging Service (SMS) .

D-AMPS Digital AMPS (D-AMPS) is essentially a digital version of AMPS and it is backward compatible with AMPS. Band D-AMPS uses the same bands and channels as AMPS.

D-AMPS Transmission • Each voice channel is digitized using a very complex PCM and compression technique. • Three digital voice channels are combined using TDMA. • The resulting digital data modulates a carrier using QPSK. • The result is a 30 -k. Hz analog signal. • Finally, the 30 -k. Hz analog signals share a 25 -MHz band using FDMA.

GSM • The Global System for Mobile Communication (GSM) is a European standard that was developed to provide a common second-generation technology for all Europe. • Bands GSM uses two bands for duplex communication. Each band is 25 MHz in width, shifted toward 900 MHz

GSM • Eight users are multiplexed on a single 200 k. Hz wide frequency channel by assigning different time slots to each user.

GSM • Each voice channel is digitized and compressed to a 13 Kbps digital signal. • 8 slots are multiplexed together creating a frame. • 26 frames are combined to form a multiframe • For medium access control, GSM combines both TDMA and FDMA

IS-95 CDMA • IS-95 is based on CDMA/DSSS and FDMA medium access control technique Bands and Channels • IS-95 uses two bands for duplex communication. • The bands can be the traditional ISM 800 -MHz band or the ISM 1900 -MHz band. • Each band is divided into 20 channels separated by guard bands.

Third Generation • The third generation of cellular telephony refers to a combination of technologies that provide a variety of services. • the third generation can provide both digital data and voice communication. • Using a small portable device, a person • should be able to talk to anyone else in the world with a voice quality similar to that of the existing fixed telephone network. • A person can download and watch a movie, can download and listen to music • can surf the Internet or play games • can have a video conference, and can do much more.

Any Questions?