Page 1 of 5 Examinaion Mobile & Wireless Neworking (192620010) April 12, 2017 13.45 16.45 Noes: Only he overhead shees used in he course, 2 double-sided shees of noes (any fon size/densiy!), and a dicionary are allowed as reference maerial. Use of he book by Bear and Salling, he reader, or any oher maerial is no allowed. Use of a calculaor is allowed bu no really needed. Use of PDA, lapop, mobile phone ec. is no allowed. Please swich off your mobile phone. Indicaions like [10] a quesions mean ha you can obain 10 poins for ha quesion. Abbreviaions ACK - ACKnowledgemen AODV - Ad-hoc On-demand Disance Vecor CDMA - Code Division Muliple Access CSMA/CA - Carrier Sense Muliple Access wih Collision Avoidance CTS - Clear To Send CW - Conenion Window Des - Desinaion DIFS - DCF Iner-Frame Space e-nodeb - evolved Node B FDD - Frequency Division Duplex FDM - Frequency Division Muliplexing FDMA - Frequency Division Muliple Access IEEE - Insiue of Elecrical and Elecronics Engineers LAN - Local Area Nework LPL - Low Power Lisening LTE - Long Term Evoluion NodeB - Node B OFDM - Orhogonal Frequency Division Muliplexing QAM - Quadraure Ampliude Modulaion RREQ - Roue REQues RTS - Reques To Send - Receiver SIFS - Shor Iner-Frame Space TDD - Time Division Duplex - Transmier
Page 2 of 5 1 General / Wireless Transmission [9] a) Explain, in your own words he hidden erminal problem. [2] b) Which problem is solved by (space) diversiy? Why is he disance beween wo anennas used for his ype of diversiy ypically half he wavelengh of he frequency band used? [2] c) Explain he basic principles of OFDM in your own words. In your answer, also explain why OFDM sysems suffer less from Iner-Symbol Inerference (compared o oher sysems wih he same daa rae), and explain he essenial difference beween OFDM and radiional FDM. [3] d) Suppose we have a sysem ha uses OFDM wih 1000 subcarriers of 10 khz in a 10 MHz band. Wha will be he symbol ime of he sysem? Why? Assuming he use of 16 QAM, wha would be he raw bi rae of he sysem? [2] 2 Medium Access Conrol [11] In he picure below he operaion of 4 differen Preamble Sampling Proocols is shown. 1.Basic LPL 2.Shor Preambles Des Info Long Preamble Daa Daa 3.Shor Preambles Daa Sar Daa 4.Shor Preambles Early ACK ACK Daa 5.Shor Preambles Transmiing Receiving Idle Lisening Sleeping
Page 3 of 5 a) Explain he operaion of proocol 1, as shown in he picure (Basic Low Power Lisening). Explain he behavior of he ransmier (), he receiver (), and he overhearers. [3] b) How long should he lengh of he preamble be? Is here a relaion beween he lengh of he preamble and he ime beween wo successive lisen periods a he receiver? If yes, which? If no, wha oher consideraions play a role in he choice of he preamble lengh? [2] c) Explain how and why he behavior of he ransmier for proocol 4 (Shor Preambles Early ACK) is differen from proocol 1 2, and 3. [2] Fas Channel-Dependen Scheduling may increase he hroughpu of a wireless sysem compared o a regular scheduling mehod (such as Round Robin). A he same ime, here is a risk ha i decreases fairness of he sysem. d) Explain how Fas Channel-Dependen Scheduling may increase he hroughpu of a sysem, compared o regular scheduling. [2] e) Explain how Fas Channel-Dependen Scheduling could decrease fairness of a sysem. Explain in your own words how he Proporional Fair Scheduling varian ries o avoid his decrease of fairness. [2] 3 Cellular Sysems [15] a) Wih reasoning explain he logical relaionship beween each pair of he following words: 1) Cell area & paging procedure [1] 2) Cell area & co-channel inerference [1] 3) Sof-handover & rae of handover [1] 4) TDD/FDD & inerference [1] 5) Cell breahing & FDMA/CDMA [1] 6) Nework capaciy & reuse facor [1] b) Wha are he wo main differences beween NodeB and enodeb? [2] c) Why is here a difference beween uplink and downlink ransmission in LTE? [2] d) Wha does he concep of a Bearer in LTE refer o? [1] e) An area of 1300 square Km! is covered by a cellular sysem using a 7-cell reuse paern. Each cell has a circular radius of 4 Km. For he covered area he specrum of 40 MHz is allocaed wih a full duplex channel bandwidh of 60 khz. Assume a grade of service of 2% for an Erlang B sysem is specified. Compue he following. (Round numbers o he lower ineger): 1) The number of cells in he service area, [1] 2) The number of channels per cell, [1] 3) Traffic inensiy of each cell, [1] 4) The maximum carried raffic. [1]
Page 4 of 5 4 Wireless LAN [11] The IEEE 802.11 sandard for Wireless LAN does allow is ransmiers o choose from differen modulaion schemes, resuling in differen available bi raes. Typically adaping he modulaion ype dynamically allows for exploiing he curren channel qualiy beer, and can improve he hroughpu compared o a sysem wih a fixed modulaion ype. a) Design and describe a (hroughpu-increasing) mechanism where a ransmier dynamically adaps he modulaion ype wihou requiring he exchange of exra packes or fields in packes (compared o he packe ha have been discussed in he course). [3] b) Describe how he design in he previous subsecion could be improved by exchanging addiional informaion beween receiver and ransmier. [2] In he following exercises, we explore he IEEE 802.11 CSMA/CA access mechanism. We make he following assumpions: 1 slo = 20 µs; SIFS = 10 µs; DIFS = 2 slos + SIFS; CWmin = 7 slos; CWmax = 255 slos; ransmission of a complee daa frame akes 1000 µs; ransmission of a complee acknowledgemen frame, an RTS frame or a CTS frame akes 250 µs; propagaion delay is negligible, no ransmission errors occur, and RTS/CTS is used. Now, suppose wo saions A and B wan o ransmi a daa frame o an access poin. The access poin and all oher saions do no have any daa o ransmi. c) How large is he probabiliy ha a collision occurs beween he RTS messages from A and B if saion A and B sar he access procedure a exacly he same ime, when he medium is idle? Explain your answer. [2] d) Wha is he collision probabiliy if saion A and B sar he access procedure a exacly he same ime, and he medium is sill busy (used by some hird saion) a his ime? Explain your answer. [2] e) Now, suppose A has successfully sen is RTS packe o he access poin because i has chosen a smaller number of slos from is conenion window han B. Wha is he probabiliy ha he CTS packe reurned by he access poin is colliding wih he RTS packe of B. Explain your answer. [2] 5 Ad-hoc neworks [10] a) In an aricle of he reader, i is menioned ha an imporan problem of mulihop broadcasing in ad-hoc neworks is he broadcas sorm problem. Three causes are menioned for his problem: redundan ransmissions, synchronizaion of ransmissions, and lack of feedback from he medium. Explain for each of hese causes how i affecs he broadcasing, and how i conribues o he broadcas sorm problem. [3] b) Explain wha he principles are of proacive and reacive rouing proocols and under wha circumsances one is beer han he oher. [2] c) In AODV, why does a roue reques (RREQ) packe conain a broadcas_id field? Wha is i used for? [2] d) Suppose ha a source node has jus sen a RREQ message o find a pah o a cerain desinaion (which we will refer o as RREQ 1). Now, he source node wans o simulaneously find a pah o a differen desinaion, and sends ou a new RREQ (RREQ 2) wih a differen Des_Addr field. For each of he following fields in he RREQ message, describe how he value in RREQ 2 relaes o he value in RREQ 1 (e.g., is he same, is larger, is differen, is no relaed), and explain why: [3] - Broadcas_ID - Source_Seq# - Des_Seq# ----- end of exam -------
Page 5 of 5 Erlang B able