question

CSIS 7304 Midterm Examination

THE UNIVERSITY OF HONG KONG

FACULTY OF ENGINEERING DEPARTMENT OF COMPUTER SCIENCE

CSIS 7304 The Wireless Internet and Mobile Computing (Midterm Examination)

Date: July 12, 2006 Time: 7:00pm – 9:00pm

Question Points Score 1 16 2 25 3 18 4 20 5 21

Total 100

Student Name: ________________________________________ Student Number: ______________________________________ Candidates may use any calculator which fulfils the following criteria: (a) it should be self-contained, silent, battery-operated and pocket-sized; (b) it should have numeral-display facilities only and should be used only for the purpose of calculation; (c) it should not have any printing device, alphanumeric keyboard, or graphic display; and (d) it should not contain any recorded data or program. It is the candidate’s responsibility to ensure that the calculator operates satisfactorily and the candidate must record the name and type of the calculator on the front page of the examination scripts. Lists of permitted/prohibited calculators will not be made available to candidates for reference, and the onus will be on the candidate to ensure that the calculator used will not be in violation of the criteria listed above. Answer ALL questions. This examination question/answer booklet has 6 pages. Copyright ©2006 by Dr. Ka-Cheong Leung. All rights reserved.


1. (16 points) Discuss two means to mitigate narrowband interference. Compare the complexity of these solutions.

1) Dynamic frequency selection (DFS): Senders can sense the medium for interference and choose a frequency range with no or little interference. Network operators can also use this scheme to dynamically assign frequencies to cells in mobile phone systems.

DFS has a relatively low complexity as the selected frequencies do not change between two successive assignments and during a call.

2) Direct sequence spread spectrum (DSSS): Data is XORed with a chipping sequence resulting in a spread signal. The signal is spread over a large spectrum, and, thus, narrowband interference only destroys a small fraction of the signal.

DSSS has high complexity as it requires more powerful receivers to extract the original signal from the mixture of spread signals.

3) Frequency hopping spread spectrum (FHSS): The sender and receiver stay on one channel for a certain time and then hop to another channel of a different frequency. Slow hopping schemes transmit several symbols per frequency. Fast hopping schemes transmit a symbol over several frequencies. These schemes may avoid frequencies with interference most of the time with a certain probability.

FHSS has medium complexity as time synchronization of devices is necessary.

Remark: Any 2 of the above 3 approaches.

2 + 1 1 + 1 1 1

2

1 + 1 1 + 1 1 1

2 + 1 2 1

1 1

Max. 6 1 1

Max. 6

1 1


2. (a) (8 points) Give four advantages of cellular systems with small cells.

1) Higher capacity / Supporting a higher number of users / Allowing frequency reuse

2) Less transmission power needed

3) Interference / transmission dealt with by base stations locally

4) More robust against component failures / due to decentralization

2

2

2

2 (b) (6 points) Give three problems encountered by cellular systems with small cells.

1) Complex infrastructure needed to connect all base stations

2) Handover needed when a mobile station moves from one cell to another

3) Frequency planning needed to avoid interference between transmitters using the same frequencies

2

2

2

(c) (5 points) Discuss how is space division multiplexing (SDM) typically realized

and combined with frequency division multiplexing (FDM)? Explain your answer.

Typically, each cell realizing SDM holds a certain number of frequency bands. Neighbouring cells in the same cluster apply FDM and use disjointed set of frequencies / are not allowed to use the same frequencies so as to minimize interference.

1 1 2 1

(d) (6 points) How does dynamic channel allocation (DCA) influence the

frequencies available in other cells?

A system applying DCA dynamically allocates frequencies depending on the current load. It can react upon sudden increase in traffic by borrowing capacity from other cells. However, the “borrowed” frequency must be blocked in neighbouring cells.

1 1 2 + 1 1


3. (a) (4 points) Distinguish between frequency division duplex (FDD) and time division duplex (TDD).

In FDD, mobile station to base station (uplink) and base station to mobile station (downlink) are separated using different frequencies.

In TDD, uplink and downlink are assigned with different time slots using the same frequency.

2

2 (b) (4 points) Discuss how guard spaces are realized between users in CDMA.

The guard space between a pair of users in CDMA systems is the orthogonality between their spreading codes. The lower the correlation between any pair of spreading codes is, the better is the user separation.

2 1 1

(c) (10 points) In GSM, the periodic pattern of 26 slots occurs in all TDMA frames

with a traffic channel (TCH). 24 out of 26 physical slots are used for user data transmission. Each normal burst used for data transmission carries up to 114 bits of user data and is repeated every 4.615 ms. Compute the maximum data rate (in 3 significant figures) for user traffic supported by a TCH.

Maximum user data rate required Total number of bits carried per 26 physical slots Total time elapsed per 26 physical slots

615.42611424

⋅⋅

=

8.22≈ kbps

2M 2M

1A + 1A 1A + 1A 1A + 1A

=


4. (a) (6 points) Give the full names and abbreviations of all key components of the operation subsystem (OSS) in a GSM system.

1) Operation and maintenance center (OMC)

2) Authentication center (AuC)

3) Equipment identity registry (EIR)

1 + 1

1 + 1

1 + 1 (b) (14 points) What multiplexing schemes are used in GSM and for what

purpose?

1) Space division multiplexing (SDM): Operators design the cell layout, place base stations and reuse frequencies according to certain cluster patterns.

2) Frequency division multiplexing (FDM): Regulation authorities assign channels to operators. Operators assign channels to base stations. Base stations assign a certain channel to a terminal during data transmission.

3) Time division multiplexing (TDM): Base stations assign a time slot or several time slots to a terminal for transmission.

2 + 1 2

2 1 + 1 1

2 2


5. (21 points) Discuss in detail five common handover types that can occur in a combined UMTS/GSM network when a user equipment (UE) moves within a 3G UMTS network and from a 3G UMTS network into a 2G GSM network.

1) Intra‐node B, intra‐RNC handover: A UE moves from one antenna of a node B to another antenna of the same node B. The node B performs softer handover (combining and splitting of data streams).

2) Inter‐node B, intra‐RNC handover: A UE moves from one node B to another node B. The responsible radio network controller (RNC) supports soft handover by combining and splitting data.

3) Inter‐RNC handover: A UE moves from one node B to another node B with different RNCs. The internal inter‐RNC handover, which is a soft handover, is not visible for the core network (CN) so that the previously responsible RNC acts as a serving RNC (SRNC) and the newly responsible RNC becomes a drift RNC (DRNC). The external inter‐RNC handover, which is a hard handover, occurs when a relocation of the controlling RNC takes place.

4) Inter‐MSC handover: A UE moves from one MSC to another MSC so that a hard handover of the connection is performed.

5) Inter‐system handover: A UE moves from a 3G UMTS network into a 2G GSM network resulting in a hard handover.

Remark: 5 handover types (10 points); further discussion (max. 11 points)

2 1 1

2 1 1

2 1 + 1 1 + 1 1 1 + 1 1 + 1

2 + 1 1

2 + 1 1

END OF PAPER

question

Documents