two primary resources employed in communication systems are and .
the three commonly used method of multiplexing are ， and .
this random process x (t) =cos(2πfct). the frequencies can be 100, 200, . . . , 600 hz，then:
which one of the following functions can be the autocorrelation function of a random process?
the process x (t) is defned by x(t) = x, where x is a randomvariable unifrmly distrbuted on [- 1, 1] , the autocorelation function and the power spectral density are :
two random processes x(t) and y(t) arejointly wide-sense stationar, then
we have proved that when the input to an lti system is stationary, the output is also stationary. if we know that the output process is stationary, can we conclude that the input process is necessarily stationary?
considering the properties of the autocorrelation function rx(τ) of a random process x(t).
considering the properties of the sine wave plus narrowband noise.
thermal noise is a stationary, zero-mean gaussian process.
if two processes are statistically independent, they are orthogonal.
for stationary processes, means, variances and covariance are independent of time.
in amplitude modulation (am), if the carrier wave is over-modulated, envelop detection cannot be used to recover the message signal.
for the same modulating wave, the tranission bandwidth of double sideband-suppressed carrier (dsb-sc) modulation is the same as that of amplitude modulation (am).
am, dsb-sc, ssb, vsb can use coherent demodulation.
both am and nbfm have same tranission bandwidth, they are linear modulation schemes.
angle modulation can provide better discrimination against noise and interference than am modulation.
among these modulation schemes including am，dsb，ssb and vsb , is easiest to demodulation.
fm can improve noise performance via increasing tranission bandwidth.
coherent detection of double sideband-suppressed carrier (dsb-sc) signal and envelope detection of frequency modulation (fm) signal both have the threshold effect.
am can improve noise performance via increasing tranission bandwidth.
the envelop of a pm or wideband fm signal is constant,whereas the envelop of an am or narrowband fm signal is depent on the message signal.
then the power of the modulated signal w;
then the frequency deviation of the signal is hz
then the deviation ratio is
by carson’rule, the tranission bandwidth of s(t) is khz
an fm signal is modulated by m(t)=sin(2000πt), kf=10000. the bandwidth b of the baseband signal m(t) is __hz.
if a =2， what type of modulation does this correspond to ? if a =0， what type of modulation does this correspond to ?
in a voice tranission system, the sampling frequency is 8000hz, and it is used to multiplex 12 independent voice inputs based on an 8-bit pcm word. the bit duration is about (ms, microsecond)
dm requires a sampling rate much higher than the nyquist rate.
for both m-law and a-law, the signal-to-noise (snr) of low-level and high-level signals improve with the increasing m and a.
performance of a pcm system is only affected by quantization noise.
in the sampling process, if the sampling period is larger than the nyquist interval, then the sampled signal can be completely described by the sample values.
one of the important advantages of pulse-code modulation (pcm) is the robustness to channel noise and interference.
basic operations performed in the tranitter of a pulse code modulation system include , , and .
specify the nyquist rate and the nyquist interval for each of the following signals: (1) g(t) = sinc(200t) (2) g(t) = (200t) (3) g(t) = sinc(200t) (200t)
twelve different message signals, each with a bandwidth of 10 khz, are to be multiplexed and tranitted. determine the minimum bandwidth required for each method if the multiplexing / modulationmethod used is (1) fdm, ssb (2) tdm, pam
the signal s(t) shown in figure is tranitted through an awgn channel with noise psd n0/2. suppose a matched filter with impulse response h(t) is used at the receiver, the matched filter h(t) should be:
the signal s(t) shown in figure is tranitted through an awgn channel with noise psd n0/2. suppose a matched filter with impulse response h(t) is used at the receiver, the matched filter output should be:
the signal s(t) shown in figure is tranitted through an awgn channel with noise psd n0/2. suppose a filter with impulse response h(t) is used at the receiver, the filter output should be:
the signal s(t) shown in figure is tranitted through an awgn channel with noise psd n0/2. suppose a matched filter with impulse response h(t) is used at the receiver (coefficient k=1), the peak pulse signal-to-noise ratio is:
in a voice tranission system, the sampling frequency is 8000hz, and it is used to multiplex 12 independent voice inputs based on an 8-bit pcm word. the bit duration is tb= (ms). assuming the use of an ideal nyquist channel, it follows that the minimum tranission bandwidth is (khz).
inter-symbol interference can be eliminated by a receiver filter which is matched to the catenation of the tranitter filter and the channel.
correlative-level coding uses a partial-response signaling method to avoid inter-symbol interference (isi).
error propagation is a drawback with correlative-level coding, and is unavoidable.
for a digital communication system which can tranit 4800 bits per second with 16-psk modulation, the symbol rate is ________ symbols per second.
for an m-ary psk, as the number of states, m, is increased, ________________.
comparing qpsk and bpsk tranitted over awgn channel, _______________.
in qam, _______________________ of the carrier are varied.
bandwidth efficiency of 8-psk is __________ bit/s/hz.
for m-ary psk signal, as m increased, the bandwidth efficiency will be ____________.
for 16-qam, the statement that _____________________ is correct.
_______________ is a type of psk.
the two basic operations at the dpsk tranitter are ____________.
for awgn channel, qpsk with gray mapping can achieve the same bit error rate as a coherent bpsk in white gaussian noise, but the bandwidth efficiency of bpsk is twice that of qpsk.
for awgn channel, 16-psk requires less signal energy than the 16-fsk for the fixed noise power spectrum density and symbol error rate.
a digital communications system with 8-psk modulation tranits 3600 bits per second, its symbol rate is ________ symbols per second.