Radio Telecommunications Systems Cellular Radio Principle Basic cellular

Radio & Telecommunications Systems Cellular Radio Principle • Basic cellular system • Propagation 1

Radio & Telecommunications Systems • Basic Cellular System • consists of three parts: (Fig. 1) • mobile unit • cell site • mobile telephone switching office (MTSO) 2

Radio & Telecommunications Systems Fig. 1 Cellular System 3

Radio & Telecommunications Systems 4

Radio & Telecommunications Systems 5 • Mobile units unit contains • control unit • transceiver • antenna system • Cell site provides interface between the MTSO and the mobile units, it has • control unit • radio cabinets • antennas • power plant

Radio & Telecommunications Systems 6 • MTSO is the switching office, coordinating element for all cell sites, contains • cellular processor • cellular switch • interfaces with telephone company zone offices • controls call processing • handles billing activities • each mobile unit can only use one channel at a time for its communication link; channel is not fixed, can lie any one in the entire band assigned by the serving area

Radio & Telecommunications Systems 7 Propagation • incident angle of the direct wave is q 1 and the incident angle of the reflected wave is q 2, q 1 is also called the elevation angle (Fig. 2) • C is inversely proportional to R 4 C µ R -4 = a R -4 • where C = received carrier power R = distance measured from the transmitter to the receiver a = is a constant

Radio & Telecommunications Systems Fig. 2 Mobile radio transmission 8

Radio & Telecommunications Systems 9 Fading • antenna of the mobile unit is lower than its typical surroundings • multi-path waves are generated • at the mobile unit • the sum of the multi-path waves causes a signalfading phenomenon (Fig. 3)

Radio & Telecommunications Systems Fig. 3 Typical fading signal 10

Radio & Telecommunications Systems 11 • signal fluctuates about 40 d. B (10 d. B above and 30 d. B below the average signal) • if the mobile unit moves fast, the rate of fluctuation is fast • Rayleigh fading is also called multi-path fading in the mobile radio environment (Fig. 4) • multi-path waves bounce back and forth due to the buildings and houses • summed together and become an irregular way fading structure

Radio & Telecommunications Systems Fig. 4 Multi-path fading 12

Radio & Telecommunications Systems Frequency Re-Use • Reuse channels • Reuse distance • Cochannel interference • Cell splitting 13

Radio & Telecommunications Systems 14 Concept of Frequency Reuse Channels • radio channel consists of a pair of frequencies • one for each direction of transmission • for full-duplex operation • a particular radio channel F 1 • used in one geographic zone to call a cell C 1 • coverage radius R • can be used in another cell • with the same coverage radius at a distance D away (Fig. 1)

Radio & Telecommunications Systems Fig. 1 D/R ratio Where q is called the cochannel interference reduction factor 15

Radio & Telecommunications Systems 16 Frequency reuse system • users in different geographic locations (different cells) • simultaneously use the same frequency channel • increase the spectrum efficiency • if the system is not properly designed • serious interference may occur • interference due to the common use of the same channel • called co-channel interference

Radio & Telecommunications Systems 17 Frequency reuse distance • minimum distance which allows the same frequency to be reused depend on • number of co-channel cells in the vicinity of the center cell • type of geographic terrain contour • antenna height • transmitted power at each cell site

Radio & Telecommunications Systems • the frequency reuse distance D can be determined from D = (3 K)1/2 R • where K is the frequency reuse pattern • R is the radius of cell • assume all the cell sites transmit the same power • if K increases • the frequency reuse distance D increases • increases D => reduces cochannel interference 18

Radio & Telecommunications Systems 19 • a large K is desired, however • the total number of allocated channels is fixed • when K is too large • number of channels assigned to each of K cells becomes small • smallest value of K is 3, obtained by setting • i = 1, j = 1 then • K = i 2 + ij +j 2 (see Fig. 2 a-2 d)

Radio & Telecommunications Systems Fig. 2 a 4 -cell reuse pattern 20

Radio & Telecommunications Systems Fig. 2 b 7 -cell reuse pattern 21

Radio & Telecommunications Systems Fig. 2 c 12 -cell reuse pattern 22

Radio & Telecommunications Systems Fig. 2 d 19 -cell reuse pattern 23

Radio & Telecommunications Systems 24 Cochannel Interference Reduction Factor • to find the minimum frequency reuse distance in order to reduce cochannel interference • co-channel interference is a function of a parameter q defined as: q=D/R • where q is the cochannel interference reduction factor • the ratio q increases, cochannel interference decreases

Radio & Telecommunications Systems Operation of Cellular Systems • Call Process • Handoff Procedure 25

Radio & Telecommunications Systems 26 Call Processing • Mobile unit initialization: power ON • user activates the receiver of the mobile unit • the receiver scans set-up channels • selects the strongest and locks on for a certain time • each site is assigned a different set-up channel • Mobile originated call • user places the called number into an originating register in the mobile unit • request for service is sent on a selected set-up channel obtained from a self-location scheme

Radio & Telecommunications Systems 27 • cell site receives it, and in directional cell sites, selects the best directive antenna for the voice channel to use • cell site sends a request to the mobile telephone switching office (MTSO) • MTSO selects an appropriate voice channel for the call • the cell site acts on it through the best directive antenna to link the mobile unit • the MTSO also connects the wire-line party through the telephone company zone office

Radio & Telecommunications Systems 28 • Network originated call • land-line party dials a mobile unit number • telephone company forwards the call to the MTSO • MTSO sends a paging message to certain cell sites based on the mobile unit number • each cell site transmits the page on its own set-up channel • mobile unit recognizes its own identification on a strong set-up channel, locks onto it, and responds to the cell site • mobile unit also follows the instruction to tune to an assigned voice channel and initiate user alert

Radio & Telecommunications Systems 29 • Call termination • mobile user terminates the call • a particular signal transmits to the cell site • both sides free the voice channel • mobile unit resumes monitoring pages through the strongest set-up channel

Radio & Telecommunications Systems 30 Handoff procedure • during the call, two parties are on a voice channel • mobile unit moves out of the coverage area of a particular cell site, the reception becomes weak (Fig. 1) • present cell site requests a handoff • the system switches the call to a new channel in a new cell site without either interrupting the call or alerting the user • call continues as long as the user is talking

Radio & Telecommunications Systems CS: Cell Site Fig 1 Occurrence of handoff 31

Radio & Telecommunications Systems 32 • Mobile Assisted Handoff (MAHO) • the mobile receiver is capable of monitoring the signal strength of the setup channels of the neighboring cells while serving a call • Soft Handoff • applied to CDMA systems • all cells use the same radio carrier, change from one code to another code

Radio & Telecommunications Systems Mobile and Base Stations Structure of a Mobile Station Structure of a Base Station 33

Radio & Telecommunications Systems 34 Structure of a Mobile Station • Most portable phones are divided into two parts: (Fig. 1) • RF part • handles the receiving, transmitting, and modulation tasks • digital part • takes care of the data processing, control, and signaling functions

Radio & Telecommunications Systems Structure of a Mobile Station 35

Radio & Telecommunications Systems Fig 1 Block diagram of a mobile station 36

Radio & Telecommunications Systems 37 • antenna combiner • couples the receiving and the transmitting paths onto the single antenna connector or a fixed antenna • receiver • contains the front end, a receiving filter network, and a mixer to down-convert the input signal onto an IF that is eventually converted into the data domain by the ADC • equalizer • due to multipath propagation and other reflections, the signals arriving at the receiver are distorted • the equalizer compensate distortions

Radio & Telecommunications Systems • demodulator • extracts the bit stream from the IF • demultiplexer • sorts the received information from the different time slots and frames • channel codec • Channel coding is necessary to reduce probability of errors 38

Radio & Telecommunications Systems 39 • speech codec • compresses digitized speech coming from the ADC before being encoded • minimizes bandwidth requirement • control and signaling unit • performs all the control functions of the mobile station

Radio & Telecommunications Systems • multiplexer • assigns each individual burst to a time slot within a numbered frame • modulator • imparts information onto the IF carrier • transmitter • a mixer up-converts the modulated IF signals • an amplifier increases the level of the signal • output filters limit the bandwidth of the output to its assigned channel 40

Radio & Telecommunications Systems 41 • synthesizer • provides the internal timing references for the bit and frame clock as well as for the RF sources in the transmitter and the receiver • voltage controlled oscillator (VCO) provides a stable operating frequency

Radio & Telecommunications Systems Structure of a Base Station • The general structure of a base station consists of • base station control function (BCF) • one to sixteen transceivers (TRX) 42

Radio & Telecommunications Systems Fig 2 Block diagram of a base station 43

Radio & Telecommunications Systems 44 • receiver • contains the receiving filter which blocks frequencies other than the desired receiving band • the signals are down-converted to an IF or directly to the baseband frequency, where the signals are sampled and quantized with an ADC • equalizer • compensates for the influences coming from the mobiles • demodulator • extracts the bit stream from the equalized signal and passes it to the demultiplexer

Radio & Telecommunications Systems 45 • channel codec • detect errors that have been introduced into the RF path and correct them • speech codec • compresses digitized speech to minimize bandwidth requirement • signaling unit • the logical interface for the control messages between the network and the mobile stations

Radio & Telecommunications Systems 46 • control unit • performs all the internal control tasks of the base station • multiplexer • maps the single bursts onto the single time slots bound for the individual mobile stations • modulator • modulates the digital signals onto the radio frequency carrier

Radio & Telecommunications Systems 47 • transmitter • contains the output filters to band-limit the signals • controls the output level depending on the base station's power class • synthesizer • provides the necessary frequencies for the different entities in the BTS (Base Transceiver Station) • usually synchronized with the clock from the BSC (Base Station Controller) • alternatively, it is possible to have a local clock reference in each base station • poor system performance (high hand-off failure rate) & low reliability due to sync problem

Radio & Telecommunications Systems Multiple Access Methods • Frequency Division Multiple Access • Time Division Multiple Access • Code Division Multiple Access 48

Radio & Telecommunications Systems 49 Frequency Division Multiple Access • multiple access scheme for land mobile communication systems (Fig. 1 - 3) • analog cellular systems use FDMA • assigned system bandwidth is divided into bands with its bandwidth of Wch • guard space to prevent spectrum overlapping • each user sends a call request to the BS • BS assigns one of the unused channels to the user • channel is used exclusively by that user during a call • when the call is terminated, the channel is reassigned to a different user

Radio & Telecommunications Systems Fig. 1 Basic Concept of FDMA system - spectrum 50

Radio & Telecommunications Systems Fig. 2 Call initiation and holding model 51

Radio & Telecommunications Systems Fig. 3 FDMA channel assignment 52

Radio & Telecommunications Systems Time Division Multiple Access • enables users to access the assigned bandwidth on a time basis (Fig. 4 -6) • each channel occupies the whole system bandwidth • occupies only a fraction of the time, called slot, on a periodic basis • one frame consists of Nch slots • frame length is T second 53

Radio & Telecommunications Systems Fig. 4 Concept of TDMA operation 54

Radio & Telecommunications Systems 55 • Uplink • each terminal transmits information using an assigned slot in each frame • each terminal has to transmit its slot exactly in the assigned slot timing to prevent signal collisions • Downlink • all the slot signals are transmitted by the BS

Radio & Telecommunications Systems Fig 6 Call initiation and holding model in TDMA 56

Radio & Telecommunications Systems Fig 7 Slot assignment in TDMA 57

Radio & Telecommunications Systems 58 Code Division Multiple Access • all the transmitted signals other than the desired signal are regarded as cochannel interference (CCI) signals (Fig. 7 - 9) • at the CDMA receiver • the desired signa 1 can be picked up by • taking correlation between the received signal and a code used at the transmitter (code#1)

Radio & Telecommunications Systems Fig 7 Configuration of CDMA system 59

Radio & Telecommunications Systems 60 • spreading code sequence for the receiver and that used at the transmitter are synchronized • the resultant signal spectrum becomes the same as that for the source signal • signal bandwidth of the interference signals still remains the same bandwidth even after taking correlation

Radio & Telecommunications Systems • all the terminals share the whole system bandwidth • each terminal signal is discriminated by the code • when each user sends a call request to the BS • BS assigns one of the spreading codes to the user 61

Radio & Telecommunications Systems Fig 8 Call initiation and holding model 62

Radio & Telecommunications Systems Fig 9 Channel assignment in CDMA 63

Radio & Telecommunications Systems 64

Radio & Telecommunications Systems 65

Radio & Telecommunications Systems 66 • Reference • Seiichi Sampei, “Applications of Digital Wireless Technologies to Global Wireless Communications, ” Prentice-Hall, 1997 • Siegmund M. Redl, et. Al. , “An Introduction to GSM, ” Artech House, 1995 • Lee, William C. Y. , “Mobile cellular telecommunications : analog and digital systems, ” Mc. Graw-Hill, 1995 • http: //www. radiodesign. com/cellwrks. htm