Cellular Telephone Networks Lecture 8 (B0: Cellular Telephone Networks 2nd semester 1439-1438

Medium Access Control Techniques When stations use a common link, we need a multiple- access protocol to coordinate access to the link. Basic idea of these approaches can be explained in simple terms using the cocktail party theory. In a cocktail party people talk to each other using one of the following modes: FDMA: When all the people group in widely separated areas and talk within each group. TDMA: When all the people are in the middle of the room, but they take turn in speaking. CDMA: When all the people are in the middle of the room, but different pairs speak in different languages.

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. Each station also uses a bandpass filter to confine the transmitter frequencies. To prevent station interferences,

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 for 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. ; this means only one-seventh of these 395 traffic channels are actually available in a cell.

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. 2G 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. This means that in a cell, one telephone can use AMPS and another D-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-kHz analog signal. Finally, the 30-kHz 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 200kHz wide frequency channel by assigning different time slots to each user.

GSM Each voice channel is digitized and compressed to a 13Kbps 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.

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