Introduction to Wireless Communication Systems ECE 476/ECE 501C/CS 513 Winter 2003

Transcription

1 troductio to Wireless Commuicatio ystems ECE 476/ECE 501C/C 513 Witer 2003 eview for Exam #1 March 4, 2003 Exam Details Must follow seatig chart - Posted 30 miutes before exam. Cheatig will be treated very seriously. You are allowed to brig oe 8 1/2 by 11 ich sheet of paper (writig o both sides, but hadwritte). Calculators will be eeded. 100 poits, 90 miutes. oughly 50/50 short aswer/multiple choice versus problems. Format of questios - ome multiple choice or true/false, some short aswer, some "lists" (give x reasos why, x reasos why ot, x techologies that ca be used), some problems (some, but ot all, will be like the homework). Questios will focus o major priciples ad topics. There is o value is askig about trivial details. The goal is to see how much studets have a commad of the major cocepts. You will be provided a copy of Erlag B Fig. 3.6 (will ot eed Fig. 3.7) Exam Topics Chapter 1 - troductio Kow defiitios from Table 1.4, page 10 Types of spectrum allocatios ole of base statio ad mobile switchig ceter Cellular phoe call timig Chapter 2 - Moder Wireless ystems 1G, 2G, 2.5G, 3G ad differeces betwee them Geeral priciple of CDMA Beefits of digital over aalog Names of techologies ad the "geeratios" where they fit. No specific details o bit rates, badwidths, etc. Do ot eed to kow techology topics we have ot covered yet (like modulatio methods) WLL's, WLAN's, PAN's Chapter 3 - Cellular Fudametals Early mobile radio vs. the cellular cocept Frequecy euse/plaig - basic cocepts; problems o clusterig, total capacity of a system, frequecy reuse Chael assigmet strategies - basic cocepts eview for Exam #1, Page 1 of 6

2 Hadoff strategies - AMP methods, 2 d geeratio ad MAHO, guard chaels, grade of service, queueig of hadoff requests, umbrella cells, soft hadoffs; problems o hadoff thresholds ad hadoff margis Co-chael terferece (CC) - sources of iterferece, how to reduce CC; problems o / vs. cluster size ad miimum acceptable / Adjacet Chael terferece (AC) - why it is caused, what are sources of AC, how to cotrol AC (higher quality filters, allocate chaels with spacig, power cotrol); o problems Trukig ad Grade of ervice - what is trukig, offered traffic itesity, GO; problems o GO ad cell chael capacities (othig o Erlag C) Cell plittig - problems o improved / vs. reduced GO ectorig - geeral cocept, problems o trukig ad improved / Chapter 4 - Large-scale path loss Differeces betwee large-scale ad small scale path loss Free space propagatio - basic formula, depedece o certai parameters, comparisos with referece distace (d 0 ); computig received power i Watts ad dbw ad dbm. eflectios - how reflectios occur, why sigals costructively or destructively iterfere; 2- ray model, differece i distace, differeces i phase, how sigals iterfere Diffractio - basic cocept, why eve a uobstructed lie-of-sight path may still have diffractio loss, o problems or detailed questios catterig - basic cocept but o detailed questios or problems No questios o path loss models Chapter 5 - Fadig mall scale fadig - Basic problem of multipath, how severe fadig ca be, itersymbol iterferece Doppler shift ad the depedece o agle of arrival, doppler spread ad coherece time Model of mobile radio chael, MC filter passbad Multipath chael parameters - mea excess delay, M delay spread, maximum excess delay, coherece badwidth Flat vs. frequecy selective fadig low vs. fast fadig Nothig o fadig sigal distributios (Nothig from Chapter 6 Modulatio Techiques for Mobile adio) ample questios 1. hort aswer. a. What mechaism is used to try to prevet a hadoff from occurrig whe a mobile is ot at the edge of a cell? Moitor received sigal stregth over a sufficiet period of time to average out fadig. eview for Exam #1, Page 2 of 6

3 b. (5 poits) f a complete coverage area was redesiged usig larger cluster sizes, what will most likely be the effect o adjacet chael iterferece? Why? AC will be reduced, because fewer chaels will be allocated to each cell, so chaels ca be spaced out more. 2. (10 poits) Give three drawbacks (egatives) to usig the sectorig to improve cellular system capacity. (1) More ateas, more costly. (2) More hadoffs (3) Fewer chaels per sector - decreased trukig efficiecy (decreased ability to share chaels betwee users sice chael groups are broke ito smaller pieces). (4) The real power of sectorig is to be able to use smaller cluster sizes for the same / ratio. Oce a cluster is implemeted, however, it caot be easily chaged. This meas system capacity caot be expaded easily without a complete redesig of the system. 3. (10 poits) Give four of the fudametal architectural differeces betwee AMP ad -95 PC. (Do ot give differeces i bit rates, chael badwidths, etc.) (1) Aalog vs. digital (2) 800 MHz vs 1900 MHz (3) FDMA vs. CDMA (4) traight voice versus PC (5) Voice oly i AMP, voice ad data i -95 (6) Hard hadoff vs. soft hadoff (7) Mobile assisted hadoff for (5 poits) How ad why does multipath degrade the quality of a received radio sigal? Multiple versios of a sigal arrive at a receiver havig traveled differet distaces alog differet paths, ad, hece, arrive with differet phases. catterig ad reflectios cause these multiple sigals. These sigals add together ad their phases ca costructively or destructively iterfere with each other. They ca also cause itersymbol iterferece. 5. (15 poits) Give the followig system characteristics for large-scale propagatio, fid the referece distace d 0. eceived power at d 0 = 10 W, eceived power at 5 km = 18 dbm, Path loss expoet = = 3.5 Usig Watts: - 18 dbm = 10 log (Pr(d 0 )/.001) Pr(d 0 ) =.001( ) = x10-5 Watts Pr(d) = Pr(d 0 )( d 0 /d) 3.5 d 0 = d P ( ) ( ) r d Pr d0 = x10 10 = 110 meters Usig db: 10 W => 40 dbm -18 dbm = 40 dbm + 10 log (d 0 /d) eview for Exam #1, Page 3 of 6

4 58 ( 35 d 0 = d 10 ) = 110 meters 6. (25 poits) Give are the followig characteristics of a cellular system Clusters of 12 cells, implex chael BW = 30 khz, Average call duratio = 6 miutes Total capacity allocated for voice chaels = 36 MHz Cotrol chaels use other spectrum ot part of the 36 MHz Average umber of calls per hour = 1, Total umber of subscribers = 48,000 ubscribers uiformly distributed across all cells Geographic coverage area is covered by exactly 7 clusters a. (20 poits) What is the GO for this system cofiguratio? 50 chaels per cell, 47.6 Erlags per cell, GO 6~8% (depedig o how the chart is read) b. (5 poits) What limit should be set o the total umber of subscribers if the cellular system provider eeded their GO to be 0.5%? For C=50, a GO of 0.5% is provided for 35~37 Erlags Total umber of users who would geerate 35~37 Erlags per cell = ~ (15 poits) A cellular system has a sigal to iterferece ratio of 15 db, which is uacceptable. Assume hexagoal cells ad that this 15 db value is approximately computed by cosiderig cochael iterferece from 6 other trasmitters. The system has the followig characteristics. Cell radius = 8 km, Trasmitter power = 10 W, Omidirectioal ateas What would be the resultig / (i db) if the followig modificatios were made to the system? educed cell radius = 5 km, creased trasmitter power = 100 W, 120 sectored ateas ( D ) Q ( 3N ) ( 3N ) 2 ice = = = =, / does ot chage whe cell radius or i0 i0 i0 i0 trasmitter power are modified. Cell radius is a chage i, but D will also chage. The D/ ratio oly chages whe cluster size is modified. May possible combiatios of N ad exist, ad the specific values are ot importat to solve the problem. Ultimately, the umerator of the / ratio stays costat ad all that chages is i 0. olved usig db: eview for Exam #1, Page 4 of 6

5 ( D ) = 15 db = 10log = 10 log 6 10 log( D ) = log(6) = db New = 10 log ( D ) 10log(6) ( D ) 10log(3) = = 18.01dB olved without usig db: 1.5 = 15 db => 10 = = ( D ) = ( D ) New = = log(63.25) = 18.01dB = ( D ) (Twice old / so just adds 3 db) 8. (10 poits) Describe two problems with hadoffs i cellular systems ad a solutio that helps to fix each problem. Fast ad slow movig mobiles umbrella cells Dropped hadoffs guard chaels ad queueig of hadoffs, also soft hadoffs Cell draggig moitor received sigal stregth from multiple base statios, try to track geographic locatio of mobile, MAHO 9. (25 poits) Give are the followig characteristics of a cellular system Omidirectioal base statio ateas Total capacity allocated for voice chaels = MHz Cotrol chaels use other spectrum ot part of the MHz implex chael BW = 30 khz Average call duratio = 5 miutes Average umber of calls per hour = 2 Total umber of subscribers = 30,240 Geographic area completely covered by exactly 168 cells ubscribers uiformly distributed across all cells Maximum allowed blockig probability = 1% c. (20 poits) What is the maximum allowable cluster size that ca be used by this system to still achieve the required grade of service? Oly cosider cluster sizes allowable by hexagoal geometry (N = 3, 7, 12, 13, etc.) MHz/60 khz = 252 chaels per cluster A u = (1/12 hour per call)(2 calls per hour) = 1/6 erl per user eview for Exam #1, Page 5 of 6

6 30240/168 = 180 users per cell 180*(1/6) = 30 erl per cell N=12: 252/12 = 21 chaels per cell, fewer chaels tha erlags - ot possible N=7: 252/7 = 36 chaels per cell, at 30 erl per cell GO is about 4% which is too large N=3: 252/3 = 84 chaels per cell, GO is very small N=3 is the largest cluster size that ca be used to meet the GO. d. (5 poits) Oce this maximum cluster size has bee determied, what other measure of quality would likely be used to see if the system desig is acceptable? You would also eed to check the / ratio. ice the cluster size is as small as possible, the cochael iterferece sources would be close ad could create co-chael iterferece levels that are too high. 10. (15 poits) Give is a direct sequece spread spectrum CDMA system has the followig requiremet. 14 db uch a small / requiremet is possible because of the properties of the pseudo-oise codes that allow a sigal to be detected i the presece of a large amout of iterferece. Assumig that iterferig sources are oly cosidered to be those from the same cell, what is the maximum umber of users that ca be trasmittig simultaeously ad still meet the above requiremet? igal has power, iterferece sources also ca have power, so =. / = / = 1/ > 10^(-1.4) => < 10^1.4, < iterferig users trasmittig plus the desired sigal trasmitter => 26 total users eview for Exam #1, Page 6 of 6