Introduction to the 3rd Generation Cellular Systems

Thursday, November 11, 2004

Prelim

姓名: 學號:

  1. Close book.
  2. 30 formulas are allowed.
  3. You have 140 minutes (14:10-16:30).
  4. Unjustified answers earn no points.
  5. Good luck.

Problem 1: [40] Explanations

  1. [2] Is it possible to increase the system capacity (number of users per unit area) by using sectoring scheme only in GSM system? If it is possible, explain how. If it is not possible, explain why.
  2. [2] Is49 a valid reuse factor? Justify your answer.
  3. [2] Describe the main purpose of open loop power control in WCDMA FDD mode. Why open loop power control is far too inaccurate?
  4. [2] Describe the main purpose of outer loop power control in WCDMA FDD mode. Explain the reason(s) why outer loop power control is implemented in RNC.
  5. [2] Describe the main purpose of inner loop power control in WCDMA FDD mode. Explain the reason(s) why inner loop power control is operated with relatively high frequency.
  6. [2] Describe the principles of absolute threshold handover scheme in IS-95 by drawing a diagram using Window_Add, Window_Drop, monitored set, candidate set, and active set.
  7. [2] Describe the main difference between soft-handover and softer-handover.
  8. [2] Describe the main aspects of a simplex and a half-duplex communications system.
  9. [2] Describe the main difference of handover mechanisms between the 1st and the 2nd generation cellular networks.
  10. [2] For a received signal , what is the RMS (root mean square) delay spread?
  11. [2] In practical handover, we suffer from problem of simultaneous traffic of high speed and low speed mobiles. Why is this a problem? Describe one of the possible solutions.
  12. [2] Describe the operations of the closed-loop transmit diversity in WCDMA by drawing a figure.
  13. [2] Describe the operations of the TSTD (time switched transmit diversity) scheme in WCDMA downlink.
  14. [2] How do we differentiate between a wideband system and a narrowband system? (from both the time domain and the frequency domain points of view).
  15. [2] Describe the cause of co-channel interference. Describe the cause of adjacent channel interference.
  16. [2] Define . Explain the effects when Δ is too large and too small, respectively.
  17. [2] Describe one of the drawbacks of the fixed channel assignment scheme. Describe one of the drawbacks of the dynamic channel assignment scheme.
  18. [2] Describe the two possible occasions for handoff.
  19. [2] In designing a base station for pico-cell, which technology (FDD or TDD) is preferred? Explain the possible reasons.
  20. [2] Draw the block diagram to illustrate the operations of non-linear quantization.

Problem 2. [15] Reuse Factor

Prove that all the possible reuse factors are given by i2+i·j+j2, where both i and j are integers,i≥0,j≥0.

Problem 3. [10] Huffman Coding

A discrete memoryless source has an alphabet of five symbols with their probabilities for its output as given below:

Symbol / S0 / S1 / S2 / S3 / S4
Probability / 0.55 / 0.15 / 0.15 / 0.10 / 0.05

Among several different Huffman codes of this source, construct the one with the largest code-word length variance.

Problem 4. [10] Erlang B Formula

An urban area has a trunked mobile network (operator A) which provides cellular service in this area. Operator A has 394 cells with 60 channels each. Find the number of users that can be supported at 2% blocking probability if each user averages 2 calls per hour at an average call duration of 3 minutes. Assume that the trunked system is operated at maximum capacity.

Problem 5. [15] Evaluation of Co-channel Interference

In an urban propagation environment, cells are often approximated by squares as shown in the following figure. Each cell is of area R2 [km2]. Assume fixed channel assignment with two buffers between co-channel squares (in other words, the repetitious pattern is of 9 square cells). We further assume that each base station is located at the center of each cell.

Assuming that each channel has 8 co-channel interferers as shown in the above figure and that the propagation attenuation exponent is n=4.

(a)[5]Determine the location for the worst case SIR of the mobile in the central cell.

(b)[10]What is the EXACT“worst case” SIR (in dB) of the mobile in the central cell?

Problem 6. [10] Co-channel Cells

For a reuse factor of N=19, draw all the co-channel cells for the designated cell shown in the following figure.