8/6/2019 hwk2-sol

1/3

CS 898T Mobile and Wireless Networks - Homework 2 - Name:_________________

Assigned: Wednesday, June 22Due: Wednesday, July 6 (in class)

Please show all work on a separate sheet attached to this sheet.For each question write key points.

1. 1) (3 points) Describe the problems when CSMA/CD is applied towireless networks.

2) (6 points) Describe how CSMA/CA solve the above problems.

Ans: 1) Two problems occur:o Hidden and exposed terminals - Carrier sensing may fail to detectanother terminal or dectect a terminal outside the interferencerange.

o Near and far terminals - The local signal might drown out the remotetransmission.2) When a station is ready for transmission, it sends a request to send

(RTS) frame to the receiver and waits to receive a clear to send (CTS)frame from the receiver. As a result, all stations within the rangewill refrain from transmitting a data frame. Once CTS is received, thesender can send packets. In this way, the CTS frame can be heard bythe hidden terminals and the medium for future use by other sendingterminal is reserved. The exposed terminal won't react to RTS anddoesn't receive CTS because the exposed terminal is not the receiver.The near and far terminals could be solved in the similar way.

2. 1) (3 points) What is FHSS? Which 802.11 standard uses it?2) (3 points) What is DSSS? Which 802.11 standard uses it?3) (3 points) What is OFDM? Which 802.11 standard uses it?

Ans: 1) Frequency Hopping Spread Spectrum (FHSS) is a transmission technologyused in wireless transmissions where the data signal is modulated witha narrowband carrier signal that "hops" in a random but predictablesequence from frequency to frequency as a function of time over awide band of frequencies. The signal energy is spread in time domainrather than chopping each bit into small pieces in the frequencydomain. This technique reduces interference because a signal from a

narrowband system will only affect the spread spectrum signal if bothare transmitting at the same frequency at the same time. Ifsynchronized properly, a single logical channel is maintained.

In the original 802.11 standard released in 1997, FHSS is used.

2) Direct Sequence Spread Spectrum (DSSS) is a transmission technologyused in wireless transmissions where a data signal at the sendingstation is combined with a higher data rate bit sequence, or chippingsequence (code), that divides the user data according to a spreadingfactor (ratio). The chipping code is a redundant bit pattern for eachbit that is transmitted, which increases the signal's resistance tointerference. If one or more bits in the pattern are damaged during

transmission, the original data can be recovered due to the redundancyof the transmission.

8/6/2019 hwk2-sol

2/3

The original 802.11 standard released in 1997 and 802.11b use DSSS.

3) Orthogonal Frequency Division Multiplexing, an FDM modulationtechnique for transmitting large amounts of digital data over a radiowave. OFDM works by splitting the radio signal into multiple smallersub-signals that are then transmitted simultaneously at differentfrequencies to the receiver

802.11a and 802.11g use OFDM.

3. 1) (3 points) How does freqency reuse enhance celluar networkcapacity?

2) (2 points) Besides the number of users, what other major factorinfluences the decision on cluster size?

3) (3 points) A celluar system uses frequency spectrum 1800 MHzto 1840 MHz for uplink channels and 1860 MHz to 1900 MHz fordownlink channels respectively. Each channel takes 200 KHzand can be shared by 8 users. Each user needs one uplink and

one downlink channel. How many users can be supported withoutfrequency resue in this celluar system?

Ans: 1) In the cellular model, adjacent cells have different frequenciesand thus frequency reuse is possible without causing interference.This enables the same frequencies to be reused across differentcells, separated by the resue distance.

2) It is observed that the number of users supported decreasedwith increase in cluster size. But the reuse distance is smallerfor lower cluster size, and hence interference is higher. Hence,interference should also be considered in determining the optimalcluster size.

3) There are (1840 - 1800)/0.2 = 200 uplink channels and

(1900 - 1860)/0.2 = 200 downlink channels.200 x 8 = 1600 users

4. 1) (3 points) Consider an area of 3600 square Km covered by acellular network. If each user requires 20 KHz for communcation,and the total available spectrum is 60 MHz, how many users canbe supported without frequency reuse?

2) (4 points) If cells of area 36 square Km are used, how many userscan be supported with cluster sizes of 3 and 7?

Ans: 1) 60 MHz /0.02 MHz = 3000 users2) Total number of cells is 3600/36 = 100 cellso In the cluster size of 3 each cell 3000/3 = 1000 users.The total number of users are 100 x 1000 = 100,000 users.

o In the cluster size of 7 each cell 3000/7 users.The total number of users are 3000/7 x 1000 = 42857 users.

5. 1) (2 points) What is a good code for CDMA?2) (6 points) Consider a sender A wants to send the data bit 1 with

key = 100101 and a sender B wants to send the data bit 0 withkey = 010110. Assume we code a binary 0 as -1, a binary 1 as +1.Both signals are transmitted at the same time. The noise tothe transmitted signal is (0, +1, -1, 0, 0, -1).

a) What signal is received by a receiver?b) What can the receiver detect for sender A and B respectively?

8/6/2019 hwk2-sol

3/3

Ans: 1) A good code for CDMA should have a good autocorrelation and should beorthogonal to other codes.

2) As = +1 * (+1, -1, -1, +1, -1, +1) = (+1, -1, -1, +1, -1, +1)Bs = -1 * (-1, +1, -1, +1, +1, -1) = (+1, -1, +1, -1, -1, +1)

S = As + Bs + N = (+2, -2, 0, 0, -2, +2) + (0, +1, -1, 0, 0, -1)

= (+2, -1, -1, 0, -2, +1)Ar = (+2, -1, -1, 0, -2, +1) * (+1, -1, -1, +1, -1, +1)

= 2 + 1 + 1 + 0 + 2 + 1 = 7 > 0 => 1Br = (+2, -1, -1, 0, -2, +1) * (-1, +1, -1, +1, +1, -1)

= -2 - 1 + 1 + 0 - 2 - 1 = -5 < 0 => 0

a) (+2, -1, -1, 0, -2, +1) is received by a receiver.b) The data sent by A can be recognized as 1.

The data sent by B can be recognized as 0.

6. 1) (3 points) List three different 3G mobile systems and indicate

the service provider and in which countries or areas they aredeployed.2) (6 points) Survey at least three 3G mobile phones.

a) Find which 3G mobile phone systems they can work with?b) Find which operating system they are using?

Ans: 1) o W-CDMA (FOMA) is provided by NTT DoCoMo in Japan.o TD-SCDMA is provided by China Telecom, China Mobile, or China Unicomin China.

o CDMA2000 1xEV-DO is provided by Verizon Wireless and Sprint PCSin North America.CDMA2000 1xEV-DV is provided by Sprint PCS in in North America.

Refer to the pages such as http://www.3gnewsroom.com/.

2) Motorola A1000: W-CDMA, SymbianNokia 7600: W-CDMA, SymbianLG 8120: W-CDMA, Symbian

Refer to the pages such ashttp://www.3gnewsroom.com/3g_mobile_phone_review/index.php.