Final Exam (ECE 407 Digital Communications) Page 1 Final Exam (ECE 408/508 Digital Communications) (05/05/10, Wed, 6 8:30PM) Name: Bring calculators. 2 ½ hours. 20% of your final grade. Question 1. (20%, 2pts each) Multi-choice questions (Only one is correct) (1) Compare CDMA, TDMA and FDMA. Which of the following is correct? A. CDMA uses both time slots and frequency bands to deliver data. B. CDMA can achieve certain security by assigning unique codes to a pair of users. C. TDMA is based on time sharing and thus causes larger communication delay than FDMA/CDMA. D. FDMA is a major cell phone working mode today. (2) On Cell Splitting, which of the following describes its major feature? A. Cell splitting uses directional antennas to achieve better signal quality. B. Cell splitting uses higher base-station tower to reach a longer distance. C. Cell splitting divides a cell into multiple smaller cells to reduce base station manufacturing cost. D. Cell splitting can make a cell serve more users after dividing it into smaller cells. (3) On cell sectoring, which of the following is NOT correct? A. Cell sectoring does not improve voice quality. It s mainly for system cost reduction purpose. B. Cell sectoring uses multiple directional antennas. C. Cell sectoring makes certain range of users receive better signals. D. Cell sectoring does not divide a cell into smaller ones. (4) Why do wireless companies use multiple cells instead of using just one base station to cover a city? A. It is very difficult to design a high tower to cover a large city. The signals will not be good. B. By using cells, we could reuse frequency bands. This helps to improve capacity. C. By using cells, we could adapt to varied communication traffic in different cells. D. All of the above. (5) On cellular network interference, what could be the possible source(s)? A. From the base stations of co-channel cells. B. From nearby cell phone due to near-far effect. C. Both A and B. D. From the Internet backbone that uses TCP/IP. (6) On channel encoders, how do we generate codewords? A. We could use generator matrix to calculate codes. B. We could use interleaver to mix up all bits. C. We could use DSP chips to filter noise and thus generate clear codes. D. We could use network packets to encapsulate each code. (7) What is code gain?
Final Exam (ECE 407 Digital Communications) Page 2 A. It is the difference between signal-to-noise ratios for coded /uncoded cases. B. It is actually hamming distance between two codes. C. It is the signal amplitude improvement between coded and uncoded cases. D. It is the length differences between original and coded bits. (8) Which of the following is NOT the feature of digital communication systems? A. Digital comm. uses encoder to change original bits for better error detection. B. Digital comm. can resist interference better than analog system. C. Digital comm.. uses signal space analysis to achieve digital modulation. D. Digital comm. does not use analog signals for long distance transmission. (9) On digital modulation, which of the following is NOT correct? A. More points in constellation diagrams mean that each shape of carrier signals can carry more bits. B. More points in constellation diagrams mean modem hardware is more complex. C. Digital modulation uses decision space to determine the correct codes for a distorted signal. D. Digital modulation is used in campus LAN (local area network). (10) On communication network layers, which of the following defines transport layer? A. Transport layer takes care of bit error detection. B. Transport layer takes care of end-to-end reliable transmission. C. Transport layer runs routing protocols. D. Transport layer does signal processing. Question 2. (10%, 1pt each) True/False questions (1) The 3 rd -generation cellular networks uses analog communications while 4 th -generation uses digital communications. (2) Handoff occurs when a cell phone passes through the overlap of two cells. (3) Shannon Theorem specifies the relationship between medium bandwidth and data rate. (4) Nyquest Theorem points out that the sampling frequency should be larger than double signal maximum frequency. (5) CDMA can only use perfectly orthogonal codes. (6) Physical Layer takes care of signal processing. However, it doesn t understand packets.
Final Exam (ECE 407 Digital Communications) Page 3 (7) Data Link layer runs TCP/IP protocols. (8) Digital signals can travel for a longer distance than analog signals. (9) Two cell phones can directly communicate with each other without going through the third device if they are very close. (10) A analog signal can be decomposed into quadrature and in-phase components, but it cannot be decomposed into more than 2 components. Question 3. (10%) Fill out the following 10 blanks. (1 point each) (1) The best metric to measure the signal quality under noise /interference (using Watts or db) is called (2) The component in ADC that reduces the number of levels of signal amplitudes before the signal is encoded is called (3) The difference between 2 codewords should be measured by (4) The task of finding the best path of passing through a series of routers should be carried out in communication system Layer. (5) The size of antenna should be similar to a signal parameter called (6) The distance between the centers of 2 co-channel cells is called (7) In the constellation diagram, we can divide it into parts in order to determine the original correct signal
Final Exam (ECE 407 Digital Communications) Page 4 for a received distorted signal. Each part is called a (8) A generator matrix for an encoder consists of an Identity Matrix and a (9) The network interface card (NIC) runs both physical layer and (10) The distance between the centers of two neighboring cells with radius R is Question 4. (5%) Explain the functionalities of the following mote components. Memory Called Digital board Sensing Chip Microcontroller Radio Transceiver Power Source (Batteries) Antenna What does the microcontroller do? List 3 examples on its tasks. What does the radio transceiver do? Besides AA batteries, can you think of any other power sources? List 2 examples.
Final Exam (ECE 407 Digital Communications) Page 5 Question 5. (5%) Please use Dijkstra s algorithm to figure out the best routing path from Source A to any of the remaining routers. (Fill out the table) Step Set N={ } Router B Router C Router D Router E Router F (Cost, predecessor) 0 {A} 1, A 3, A 1 Question 6. (5%) Suppose we have a 4-byte data sequence: 01101100 11001010 00010011 10110011 Please calculate the 2-D parity bits (suppose each row is one byte). Thus you would have 4 row parity bits and 8 column parity bits. Question 7. (5%) Finish the following digital communication system architecture.
Final Exam (ECE 407 Digital Communications) Page 6 Voice analog signals Channel encoder ADC Voice analog signals Source Decoder Question 8. (5%) For the following frequency domain diagram, please answer two questions: (1) Which function block does it come from (in a digital communication system)? Refer to Question 7. (2) What causes the overlap between the main component (in the original point) and its next one? Question 9. (5%) For the following 4-point constellation diagram, which correct signal does the distorted signal correspond to? Draw the decision space and explain your answer. 2 1 Distorted signal 4 3
Final Exam (ECE 407 Digital Communications) Page 7 Question 10. (5%) Please decompose the following signal into in-phase and quadrature components. And mark it in the constellation diagram. F(t) = 5 cos (1000t +2) Question 11. (5%) For the following generator matrix, please calculate the codewords for a data sequence [ 1 1 0 1 ]. Question 12. (5%) Deduce the following signal-to-interference ratio (also draw a diagram): (where N I is # of co-channels; K is path loss; D is frequency reuse distance; R is cell radius). k S ( D / R) I N I
Final Exam (ECE 407 Digital Communications) Page 8 Question 13. (5%) If there are 9 clusters in a city area. The cluster size is 12. The wireless system is allocated 880 different channels from the FCC (Federal Commission Committee). Calculate the system capacity. Question 14. (5%) Use an example to show how hamming distance can be used to deduce the correct code (i.e. original code from the sender) for a received, damaged code. Question 15. (5%) Use a Trellis diagram example to explain how we generate convolutionary codes.