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First Semester M.Tech. Degree Examination, June 2009Branch : Electronics and Communication (2008 Scheme)

Stream – Microwave and TV EngineeringTMC 1002 : ADVANCED DIGITAL COMMUNICATION

Time : 3 Hours Max. Marks : 100

Answer any five questions : (5×20=100 Marks)

1. For a two user synchronous CDMA system, received signal is given by

)t(n)t(gbE)t(gbE)t(r 222111 ++= for Tto ≤≤ and (b1, b2) = ( ± 1, ± 1),

g1(t) and g2(t) are signature waveforms. The noise process n(t) is zero mean Gaussian

and white with spectral density 2

No.

a) If each user employs multiuser MMSE detector, find the linear transformationmatrix A° for two users.

b) Show that MMSE detector approaches decorrelating detector as N0→ 0. 20

2. a) Explain with block diagram a system for acquisition of a frequency hoppedspread spectrum signal. 10

b) A CDMA system is designed based on DS spread spectrum with a processinggain of 1000 and binary PSK modulation. Determine the number of users, if

each user has equal power and desired level of performance is dB5.10J

E

0

b = . 10

3. In line of sight microwave radio transmission, the signal arrives at the receiver viatwo propagation paths, Received signal isr(t) = S(t) +α S(t – T) +n(t)Where s(t) is the transmitted signal, α is the attenuation (α <1)and n(t) is AWGN.

Reg. No. : ....................................

Name : .........................................

P.T.O.

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a) Determine output of demodulator at t = T and t = 2T that employs a filtermatched to s(t). 10

b) Determine probability of error for a symbol by symbol detector if the transmittedsignal is binary antipodal and the detector ignores the ISI. 10

4. For a set of M orthogonal signal waveforms Sm(t), 1≤ m≤ M, 0≤ t≤ T all of whichhave same energy E.

Determine a new set of M waveforms as

∑=

−=′M

1kk )t(S

M

1)t(Sm)t(mS

for 1≤ m≤ M, 0≤ t≤ T. Show that the M signal waveforms { })t(Sm′ have equal

energy given by ME)1M(E −=′ and are equally correlated with correlation

coefficient.

1M

1dt)t(S)t(S

E

1S n

T

0mmn −

−=′′′

= ∫ 20

5. Compare FDMA, TDMA and CDMA in terms of information rate that each methodachieves in an ideal AWGN channel of bandwidth W for K users. 20

6. A multipath fading channel has a multipath spread of Tm = 1 sec. and a doppler

spread Hz01.0B =α . The total channel bandwidth at bandpass available for signaltransmission is w = 5 Hz. The signal designer selects a pulse duration T = 10 Sec.

a) Determine coherence bandwidth and coherence time.

b) Is the channel frequency selective. Explain.

c) If a wideband signal is used for transmission and a RAKE receiver fordemodulation, how many taps would you use in RAKE receiver. 20

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