Solution 2 Discussion:

Transcription

1 Mobile Couicatios Solutio 2 Discussio: Solutio 2.: Multiplexig For ultiplexig, SDMA, TDMA, FDMA, ad CDMA were preseted i the lecture. a.) What liits the uber of siultaeous users i a TDM/FDM syste copared to a CDM syste? What happes to the trasissio quality of coectios if the load gets higher i a cell, i.e., how does a additioal user ifluece the other users i the cell? I TDM/FDM systes, we have tie slots o certai carrier sub-frequecies, which ca be assiged to sigle users. Whe all slots o all carriers are assiged, o ew users ca be accepted we have a fixed axiu uber of users. Dyaics like re-assiget are ot specified for these ethods. Withi CDM systes, we have dyaics itegrated, givig a soft liit of the uber of users: ust choose loger codes for a higher uber of users. (With the disadvatage of lower data rates ) But: each ew user icreases the oise level for all other users; i cotrast to TDM/FDM systes, user ifluece each other, ad if the SNR becoes too bad, devices farer away fall out of receptio rage. b.) Explai the ter iterferece i the space, tie, frequecy, ad code doai. What are coutereasures i SDMA, TDMA, FDMA, ad CDMA systes? Iterferece ad coutereasures are: SDMA: Iterferece is overlappig of cells. Just leave a protective distace betwee base statios ad/or devices. TDMA: Iterferece is siultaeous trasissio of several statios. Sychroizatio ad tie gaps betwee tie slots are coutereasures. FDMA: Iterferece eas trasissio o the sae carrier frequecy. Coutereasures are protective gaps o the frequecy bad. CDMA: Iterferece is sedig with correlated codes. Thus orthogoal or quasiorthogoal codes have to be used (i.e. the gap i this exaple is i code orthogoality). Also, like i TDMA, we eed sychroizatio betwee the statios to have a coo tie uderstadig o the begiig of a code word. c.) SDMA eas to build up cells. But usig cells eeds cell plaig. If you have a cluster size o, how ay differet carrier frequecies do you eed at least? f f f Cluster size o eas (see lecture): i a patter o cells, all carrier frequecies are used. Thus, the iiu uber of carrier frequecies for this case is 3, oe for each cell. f f

2 Mobile Couicatios d.) Now assue that for your cluster of size 3 you have 9 carrier frequecies f < < f 9. Give oe possible assiget of frequecies to the cells of a cluster if assuig that all cells should have the sae capacity. Are there soe rules you should cosider i assigig the frequecies to the cells? Could you thik about probles whe ot cosiderig such rules? Possible assiget: Cell f f 4 f 7 Cell 2 f 5 f 8 Cell 3 f 6 f 9 Whe assigig carrier frequecies to cells, it is best to axiize the distace betwee the frequecies i the sae cell. This is to deal with cell-iteral iterferece of chaels due to effects like e.g. Doppler Effect. Thus, havig a gap betwee the carrier frequecies is the sae solutio which is chose whe leavig gaps betwee carrier frequecies theselves. Thus, probles iside a sigle cell are iiized. Solutio 2.2: CDMA ad Spread Spectru Techology Spread spectru techology is used i ay wireless etworks, e.g. Bluetooth or WLAN. a.) What are the ai beefits of a spread spectru syste? How ca spreadig be achieved? Copared to FDMA ad TDMA, what correspods to the guard spaces? How ca spread spectru systes beefit fro ultipath propagatio? Spread spectru techology offers robustess agaist iterferece, robustess agaist ultipath propagatio, protectio agaist tappig, lowerig the sedig power, ad (whe usig it for ediu access coordiatio), ore dyaics i the axiu uber of siultaeous users. Spreadig ca be doe by frequet chage of the carrier frequecy (FHSS), icreased sybol rate by trasferrig chippig sequeces istead of sigle bits (plai CDMA as well as DSSS), or aybe by other approaches like trasferrig the chips i parallel o differet sub-carriers. As writte above, guard spaces i this case is the orthogoality of codes. Multipath propagatio ca give beefit because a receiver ca cobie all sigal powers of differet paths ad by this icrease the sigal stregth. b.) Let S, T, X, X be orthogoal chip sequeces of legth ad followig characteristics:. S T = 0 2. S S = 3. S S = 2 X i X : =. Proof the 4. If there is exactly oe i with X i = S ad o with X = S, the holds X S = 5. If there is exactly oe i with X i = S ad o with X = S, the holds X S = 6., S { X,..., X } X S = 0 S

3 Mobile Couicatios Two chip sequeces S = (s,, s ) ad T = (t,, t ) with s i, t i {-, } are called orthogoally, if S T = s t = S T = s ( t ) = s t = = = 0 2 = S S = s = 3. = = S S = s ( s ) = 2 s = = = - 4. Let be X = S ad X i { S, S} for i = 2,,. X i = (x i,, x i ) X = xi,..., xi S X = S X s = i = = = i x i = s x = S X i i 3

4 Mobile Couicatios = 5. aalogous 6. aalogous = S X + S X 2 = S S i c.) Give are the followig chip sequeces: A: ( ) B: ( ) C: ( ) D: ( ) The statios wat to trasit the followig bits: A:, B: 0, C: 0, D: -. What is the resultig chip sequece? A trasits its iverted chip sequece, B ad C trasit the chip sequece, D trasits othig: A B C D d.) A statio receives the chip sequece ( ). Which statios have set, which bits were trasitted? How does the situatio chage, if there is soe oise i the eviroet, e.g. ( )? What happes if you have o power cotrol, i.e. the statios ca sed with differet power? Without oise: X = (-, +, -3, +, -, -3, +, +) Statio : (-, -, -, +, +, -, +, +) X =, i.e. a 0 was trasitted Statio 2: (-, -, +, -, +, +, +, -) X = -, i.e. a was trasitted Statio 3: (-, +, -, +, +, +, -, -) X = 0, i.e. othig was trasitted Statio 4: (-, +, -, -, -, -, +, -) X =, i.e. a 0 was trasitted 4

5 Mobile Couicatios Chaged situatio with oise: X = (-2, 0, -2, +2, -2, -2, +2, +2) Statio : (-, -, -, +, +, -, +, +) X =.25, i.e. the 0 is detected Statio 2: (-, -, +, -, +, +, +, -) X = -0.75, i.e. the is detected Statio 3: (-, +, -, +, +, +, -, -) X = -0.25, i.e. othig was trasitted Statio 4: (-, +, -, -, -, -, +, -) X = 0.75, i.e. a 0 was trasitted Doe here to iterpret the oly-siilar sigal is ust a roudig to the ext valid value. The give oise adds to the sigal as a trasissio of oe ore statio oly usig a code which is ot exactly orthogoal to the others. This shows: if the oise level is low eough, the result still ca be iterpreted correctly. But, if o power cotrol would exist, a statio could overtoe the others. Icreasig the ow trasissio power while the others keep the forer oe eas: icreasig the oise level for the other statios. O this reaso power cotrol is very iportat is such systes. Solutio 2.3: Mediu Access Cotrol Beside ultiplexig, there are lot of developets of ediu access schees for rado access i wireless etworks. a.) Recall the proble of hidde ad exposed terials. What is the effect of such terials if Aloha, slotted Aloha, reservatio Aloha, or MACA is used? I Aloha, hidde statio is a serious proble. Statios start sedig their data, ad because of rutie to the satellite, it eeds log tie till the satellite has repeated the iforatio ad set it dow to all statios. Thus there ca be lots of hidde statios we will lear about oly very late. O the other had, exposed statios do t exist a potetial seder does ot liste to the ediu before sedig, thus he caot be exposed. For Slotted Aloha, it is the sae as for traditioal Aloha. For Reservatio-Aloha, both probles do t exist. Because we work with reservatios, all other statios kow about ours at well at their ow sedig ties. Oly i the reservatio phase we ca have probles with placig reservatios, but because we will recogize if soeoe else tried to do the sae reservatio as we, the other statios are ot really hidde to us. MACA was desiged to avoid both, hidde ad exposed statios. (Note: exposed statios i priciple are avoided because a statio which ca hear the RTS but ot the CTS could iterpret it as ot ifluecig the receiver. But i reality, also a statio which caot hear the CTS but hears a trasissio begi after a RTS has to wait, because each seder also becoes a receiver eve if oly for a ACK after fiishig its trasissio.) Nevertheless, if we have chagig topology (i.e. obile devices) or asyetric coectios, a statio ca iss the RTS/CTS essages ad sed without kowig that soethig is destroyed, see below. 5

6 Mobile Couicatios b.) How ca MACA still fail i case of hidde/exposed terials? Thik of obile statios ad chagig trasissio characteristics. Whe statios ove, ew chaels (i the eaig of i receptio rage of aother statio ) ca arise all the tie ad old oes ca break dow. Furtherore, due to icrease or weakeig of sigals based o propagatio characteristics, the sae coectio betwee two statios i both directios ca have copletely differet sigal stregth. It is possible that oe statio does ot receive the CTS of aother oe, but still ca disturb the other statio with its sedigs. O such reaso the echais is oly suitable for ore or less deteriistic propagatio characteristics ad static devices. Ad-hoc etworks ca t ake use of it. c.) What are the advatages of a fixed TDM patter copared to rado, dead drive TDM? Copare the efficiecy i the case of several coectios with fixed data rates or i the case of varyig data rates. Now explai why traditioal obile phoe systes use fixed patters, while coputer etworks geerally use rado patters. I the future, the ai data beig trasitted will be coputer-geerated data. How will this fact chage obile phoe systes? Havig fixed TDM systes, all devices ca be kept siple oly sychroisatio is ecessary to ake all statios work correctly. I dead drive approaches, e.g. with reservatios, lot ore coplexity has to be added to the statios. For several coectios with fixed data rates, fixed TDM is uch ore effective look at GSM, DECT, : siple, power-efficiet devices with guarateed fixed rate. But for variable data rate we eed ore dyaics which is the case for Iteret traffic. So i future also telephoy systes eed to switch over to ore dyaic schees. 6