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

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1 Introuction to Wireless ommunication Systems EE 476/EE 5/S 53 Winter 3 Review for Exam # April 5, 3 Exam Details Must follow seating chart - Poste 3 minutes before exam. heating will be treate very seriously. You are allowe to bring one 8 / by inch sheet of paper writing on both sies, but hanwritten. alculators will be neee. points, 9 minutes. At the en of this hanout is inclue a copy of the formulas page that will be inclue with the exam. Roughly 5/5 short answer/multiple choice versus problems. Format of questions - Some multiple choice or true/false, some short answer, some "lists" give x reasons why, x reasons why not, x technologies that can be use, some problems some, but not all, will be lie the homewor. Scope of exam: Lectures 7 through plus Sprint PS presentation Exam Topics Lecture 7 - Digital Moulation Diagram of the complete wireless transmission an receiving system. Analog Moulation how AM an FM wor, general comparisons between the two. Digital Moulation - performance avantages over analog, performance factors to consier, power efficiency vs. banwith efficiency, symbol rates vs. bit rates onstellation iagrams, spacing between points as an inication of error rates BPSK, DPSK, QPSK, how QPSK has twice the banwith efficiency of BPSK FSK not MSK or GMSK ombine Techniques - M-ary PSK, QAM Sprea Spectrum Moulation - Avantages, properties of PN coes, irect sequence, frequency hopping, processing gain, power control Guarantee question: Be able to setch a signal given carrier frequency, bit rate, an moulation scheme. It may not be a QAM question as was given in homewor an in the attache samples from previous semesters.

2 Lecture 8 - Diversity an hannel oing Why iversity helps in multipath faing environments Types of spatial iversity - selection, scanning, equal gain, maximal ratio Time iversity an the RAKE receiver in conjunction with DMA Traeoff between coing rates an error rates Balance between error correction an error etection with retransmissions Interleaving - Why use, how use, interleaving elays Lecture 9 - Voice oing, Multiple Access, GSM, an IS-95 FDMA, TDMA, DMA - How they wor, comparisons Power control SDMA - how it enhances the above, how it wors, how sectoring is a form of SDMA GSM Responsible for all etails given in lecture notes, history, FDMA/TDMA format, SIM IS-95 Responsible for all etails given in lecture notes, history,.5 MHz channels, use of DMA Lecture 8., OFDM, UWB, etc. 8. Two operating moes an coorination functions use, 8./a/b/g an physical layers for each, hien an expose noe problems, SMA/D versus SMA/A, security, use of RTS/TS, SIFS/DIFS/etc. OFDM Definition an benefits UWB Definition an benefits Sprint PS Lecture on course web site Wor being one in aition to stanars Diversity in the receiver. How significant of an enhancement can be gaine by terminal performance enhancements, how better receiver performance translates in system capacity savings. Sample questions from previous exams: Note: Some topics on previous exams were not covere this semester or are not to be on this exam. In some cases, these are struc out, but not necessarily in all cases. In other cases, material for this semester was new an no questions from previous exams involve these topics. Stuy tip: Given that you have been given these sample questions, unerstan the material very well. Exam questions are liely to be a bit more ifficult.

3 . 3 points Answer the following multiple choice questions with the BEST of the four answers. a. 3 points Given the following statements. I. Waveform encoing prouces lower bit rates than vocoing. II. Vocoing uses more of the nowlege of the voice than waveform encoing. III. Hanoffs are not an important issue in a hoc networs. A I an II are true. II an III are true. B I an III are true. D All are true. b. 3 points Raise osine filters are use for the following purpose. A To convert analog signals to igital signals. B To provie both error correction an error etection functionality. To limit the interference between ajacent bits in a bit stream. D To ecoe QPSK signals. c. 3 points Which of the following is NOT always an avantage of igital moulation over analog moulation. A Ability to aapt to faing environments. B Better security. Better signal quality elivere to the en user. D Ability to support ata traffic.. 3 points Given the following statements. I. Data rates for IEEE 8. LAN s are higher than for HIPERLAN. II. IS-95 an AMPS are examples of secon generation cellular systems. III. Interleaving is use to counteract the effects of large-scale faing. A I an II are true. All are true. B I an III are true. D All are false. e. 3 points A RAKE receiver is always use in conjunction with which moulation scheme. A Sprea Spectrum Moulation B Quarature Phase Shift Keying Frequency Shift Keying D Quarature Amplitue Moulation 3

4 f. 3 points The processing gain of a irect sequence sprea spectrum system relates to what performance characteristic? A Signal to noise ratio. B Power efficiency. Signal banwith sprea. D Banwith efficiency. g. 3 points Given the following statements. I. AM signals use more banwith than FM signals to prouce the same quality. II. oherent etection requires the use of a reference carrier signal. III. Pulse shaping is implemente to reuce intersymbol interference. A II an III are true. Only II is true. B All are false. D All are true. h. 3 points Which is NOT a major ifficulty in implementing applications in wireless environments? A Limite evice isplay capabilities. B Limite evice power capabilities. Aaptability of current Internet protocols. D Lac of stanars to support wireless applications. i. 3 points Given the following statements about st, n, an 3 r generation cellular systems. I. GSM uses a combination of TDMA an FDMA. II. GSM is bacwars compatible with AMPS. III. The main ifference between n an 3 r generation systems is a conversion from analog to igital signals. A I an II are true. Only I is true. B II an III are true. D All are false. j. 3 points Given the following statements. I. Multiple inepenent signals must be able to be receive for iversity to be able to overcome faing. II. IS-95 is base on frequency hoppe sprea spectrum. III. Selection iversity uses the receive signal with best signal-to-noise ratio. A Only I is true. I an III are true. B II an III are true. D All are true. 4

5 Answers D A A D Question a. II an III are true. b. To limit the interference between ajacent bits in a bit stream. c. Better signal quality elivere to the en user.. All are false e. Sprea Spectrum Moulation f. Signal banwith sprea g. II an III are true. h. Lac of stanars to support wireless applications. i. Only I is true. j. I an III are true. Question. 5 points Give three of the most important issues in the complexity of converting an existing AMPS system to an IS-95 system. - Analog to igital - Timing an synchronization of igital signals - FDMA to DMA - Power control on mobiles - Narrowban to wieban signals - Defining the capacity of the system base on performance characteristics instea of numbers of channels. Question 3 3. points The coing rate of an error control scheme is change from.75 to.5. a. 5points How oes this change the power efficiency? Why? Power efficiency is increase, since the same power level will now prouce a better effective error rate more error correction bits to correct errors. b. 5points How oes this change the banwith efficiency? Why? Banwith efficiency eteriorates, since the given banwith is now spent sening more error control bits an less source ata bits. 5

6 Question points Two error control schemes are to be compare, one that uses error correction without retransmissions an another that uses error etection with retransmissions. System esigners have one their best to provie equivalent performance for both systems. These are the system parameters. hannel banwith 3 Hz Spectral efficiency.5 bps/hz Error correction coing rate.5 Error etection coing rate.6 a 5 points In general, for what types of traffic is the best approach to use error correction without retransmissions? Time sensitive an elay sensitive traffic oes not wor well with retransmissions. Examples woul be voice or vieo traffic. b points What is the maximum source ata rate actual ata NOT incluing error control bits for the error correction scheme? Raw ata rate 3 Hz.5 bps/hz 45 bps Source ate rate.5 source bits for every pacet4 bps.5 bps c points When using error etection with retransmissions, one out of every 4 pacets must be retransmitte. What is the maximum source ate rate for the error etection an retransmission scheme? Out of the 4 original pacets, 6% is source ata. In aition, a 5 th pacet is sent which is all repeate ate no new source ata So, out of 5 pacets,.64/5.576 is the proportion of source ata 45 bps bps 6

7 Question 5 5. points A QAM scheme is use with the following signal constellation. Given in the table are the coorinates of each point an the corresponing symbols for each point Symbol I value Q value This moulation scheme is use to transmit the following bit stream with the following system parameters. Bit stream: Symbol rate 5, symbols per secon arrier frequency MHz a 5 points What is the bit rate of this system? Exam a 5, symbols/sec3 bits/symbol.5 Mbps b 5 points Show the transmitte signal. Be sure to label both axes. Use a sine function as the base signal. 7

8 Signal constellation points are as follows System parameters: Symbol time /5, microsecons Perio of sinusoial signal /MHz.5 microsecons 4 signal perios in one symbol time Depening on the source function, sine or cosine, the plots will ifferent. The sine function is require. Results using the sine function: Time secons 6 8 x points Answer the following multiple choice questions with the ONE BEST ANSWER of the four answers. a. 3 points For which type of mobile raio propagation impairment oes iversity provie the most benefit? A Free space path loss B Diffraction Reflections 8

9 D Scattering b. 3 points A RAKE receiver taes avantage of which type of iversity? A Space iversity B Frequency iversity Time iversity D Polarization iversity c. 3 points Given the following statements. I. Free space propagation has a larger path loss exponent than for a two-ray moel. II. A path loss of " B per ecae" means that B of signal strength is lost every meters. III. The property of raio propagation that allows a signal to be receive even behin a large obstruction is calle scattering. A Only I is true. All are false. B Only III is true. D I an II are true.. 3 points Which oes NOT have as its main goal the improvement of the quality of a receive igital signal? A Aing error control coing bits. B Using raise cosine filters to shape igital pulses. Implementing multiple antennas in receivers. D Using non-coherent moulation instea of coherent moulation. e. 3 points By increasing the coing rate of a signal say, from.33 to.5, which of the following occurs? Remember: Only give ONE answer. A More symbols per secon are transmitte. B More bits per secon are transmitte. More errors can be correcte. D Fewer errors can be correcte. f. 3 points Which type of iversity technique may NOT necessarily use the signal with the best receive quality as part of its signal reception process? A Maximal ratio combining B RAKE receiver Equal gain iversity D Selection iversity 9

10 g. 3 points Given the following statements. I. Mean excess elay is the average amount of time that multipath components arrive after the minimum elay time. II. A flat faing channel can also be a fast faing channel. III. Delay sprea an coherence banwith are relate. A I an II are true. I an III are true. B All are true. D Only III is true. h. 3 points Which system characteristic oes NOT have a major effect on the faing characteristics of a signal? A Spee of movement of a mobile. B Direction of movement of a mobile. Signal transmitte power. D Signal banwith. i. 3 points Which type of multiple access technique is not possible in an analog cellular system? A Space Division Multiple Access B Frequency Division Multiple Access Time Division Multiple Access D ellular Frequency Reuse Question Text Answer a Reflections b Time iversity c All are false. D Using non-coherent moulation instea of coherent moulation. e D Fewer errors can be correcte. f B RAKE receiver can use any form of iversity g B All are true. h Signal transmitte power i Time Division Multiple Access 7. 5 points Answer the following short answer questions. a. 5 points Show a sample signal constellation for 8-ary PSK. ircle with 8 points space out evenly every 45 egrees.

11 Q I a. 5 points Show a sample signal constellation for 6-ary QAM. Square with 6 points, 4 in each quarant of the I/Q plot. Q I b. 5 points How an why oes multipath egrae the quality of a receive raio signal? Multiple versions of a signal arrive at a receiver having travele ifferent istances along ifferent paths, an, hence, arrive with ifferent phases. Scattering an reflections cause these multiple signals. These signals a together an their phases can constructively or estructively interfere with each other. They can also cause intersymbol interference. c. 5 points The coverage area of a cell is efine to be the area within which the average signal strength is above a certain threshol. Why in practice woul the shape of this coverage area not normally be a circle? What factors woul cause this noncircular shaping? ontours of the terrain an obstructions lie large builings create places where signal strength is much worse. overage area is relate to large scale signal propagation. 8. points A pacet format is create for a wireless system that ha the same amount of pacet ata as error control coing. Then it was ecie to enlarge the pacet to have twice as much error control coing as before, an no more ata as before. a. 5points What is the coing rate for this new pacet format? part ata, parts coing - coing rate of /3.

12 b. 5points Describe how maing this moification woul change the balance between banwith efficiency an power efficiency, as compare to the original pacet format. Decrease banwith efficiency in orer to improve power efficiency. Banwith efficiency ecreases because now only /3 of the banwith is being use for real ata. Power efficiency increases because now the error rate will be less because of more coing for the same amount of power. 9. points A QAM scheme is use with the following signal constellation. Given in the table are the coorinates of each point an the corresponing symbols for each point Symbol I value Q value

13 This moulation scheme is use to transmit the following bit stream with the following system parameters. c 5 points What is the bit rate of this system? 5, symbols/sec3 bits/symbol.5 Mbps Bit stream: Symbol rate 5, symbols per secon arrier frequency MHz 5 points Show the transmitte signal. Be sure to label both axes! One symbol microsecons sqrt at 35 at 7 at 9 at Using Sine Function:.5 Signal using a Sine Function.5 Signal Amplitue Using a osine Function: Time Secons x -6 Review for Exam #, Page 3 of 6

14 .5 Signal Using a osine Function Signal Amplitue Time Secons x points A given wireless environment has an average bacgroun noise power of.5 x -3. The system is first esigne with the following signal. Signal Bit rate bps. Signal amplitues in volts + for a binary "" an - for a binary "". arrier frequency GHz Then it is ecie to change the signal accoring to the following parameters. Signal Symbol rate symbols per secon. Signal amplitues in volts +3 for a binary "", + for a binary "", - for a binary "", an -3 for binary "". arrier frequency GHz. Assume all symbols are equally liely in both cases. a. points What is the average signal energy per bit for BOTH signals? Formulas an tables that might be helpful are attache at the en of the exam. Signal : E s Ac Ts, A c volt or - volt, T s /, E s.5 x -3 for both symbols, so E s.5 x -3, one symbol equals one bit so E b.5 x -3 Signal : E s.5 x -3 for + an -, Es 4.5 x -3 for +3 an -3, so average E s.5 x -3, two bits per symbol, so E b.5 * E s, so E b.5 x -3 b. points What is the average bit error probability for BOTH signals? Approximate values are sufficient Review for Exam #, Page 4 of 6

15 Given what was presente in class, stuents woul be expecte to assume that Prob{bit error for BPSK & QPSK} Q E N b Signal : Prob{bit error} Qsqrt*.5x -3 /.5x -3 Q.75 Signal : Prob{bit error} Qsqrt*.5x -3 /.5x -3 Q3.6.8 approx Note that this is not really correct for Signal, but stuents were not expecte to now this an instructor forgot. Since the signal constellation points are space the same in both cases, the bit error rates shoul be roughly the same, not much less for signal. c. 3 points What are two benefits to using this secon signal? Given the answers stuents were expecte to obtain - lower bit error probability, ouble the bit rate.. points What is the most important rawbac to using this secon signal? Higher signal energy an power is require..5 times as much average energy per bit. Review for Exam #, Page 5 of 6

16 Review for Exam #, Page 6 of 6 Relevant Formulas M S M N N i + i j + j S/N log S/N B P hanoff threshol P minimum usable signal B Q D / R : co-channel reuse ratio hexagonal cells Q D / R N i N i N i Q i R D I S n n n n GOS Pr [bloce call] A A!! L P G G P r t t r 4 π λ n r r P P h h h h r t r t h h r t [ ] ] cos[ t f m t A t S c c AM π + + t f c c FM m t f A t S η η π π cos W f W A m f f β arson s rule: f < B T < β f + f m η p E b / N o bps/hz B R B η mean excess elay P P τ τ τ τ RMS elay sprea τ τ σ τ Avg Prob{bit error for BPSK & QPSK} N E Q b For sinusois, s c s T A E E E s b, where is the number of bits per symbol.

ISHIK UNIVERSITY Faculty of Science Department of Information Technology Fall Course Name: Wireless Networks

ISHIK UNIVERSITY Faculty of Science Department of Information Technology 2017-2018 Fall Course Name: Wireless Networks Agenda Lecture 4 Multiple Access Techniques: FDMA, TDMA, SDMA and CDMA 1. Frequency