Quiz 3 Flashcards
Spatial separation
allows frequency re-use
- ex: wifi (can see neighbors wifi but not out of SLO)
what allows frequency re-use
Polarization separation
TDMA
Orthogonal Waveforms (has to be completely 90 degrees)
achieving high thruput observations
more orthogonal degrees of freedom enable more capacity (thruput)
each degree of freedom _________
imposes constraints
ex: freq accuracy, guard bands, polarization accuracy, timing accuracy
constraints add _________ and reduce ________
to system complexity; flexibility
Central limit thm
all the world’s Gaussian, and all constituents merely add up to noise
(gaussian plot/bead thing)
WSS
wide sense stationary process
in WSS and PSD, mean and variance are
time-independent
in WSS and PSD, autocovariance depends only on
lag between t1 and t2
Power Spectral Density describes ___________
power distribution with respect to frequency
sample PDFs
(on iphone)
what is erf(x)?
error function
- the delta function between two limits
what is the Q function?
the right tail of the PDF
what is erfc(x)?
complementary error function
erfc = 1 - erf
- area under the two tails of zero mean Gaussian PDF
Noise is modeled as AWGN (what is AWGN?)
additive white Gaussian noise
- present in all freqs
-
PDF, CDF, and Q functions are most useful for __________
calculating probabilities
FSK
frequency domain (phase modulation)
BPSK
time domain (phase modulation)
Graphs of BPSK, QPSK, 8- 16- 64- ary
on ipad
high order modulation is limited by ______
SNR
Shannon’s Law
C = Blog2(1+SNR)
C: capacity
B: bandwidth
SNR: signal to noise ratio
for Shannon’s law, high bandwidth means high _________
data through
log2(M) =
log10(M)/log10(2)
higher data rates require
higher SNR’s
constraints of BER
- no weather margin
- no implementation margin
- JPL typical 2 dB
OFDMA
- orthogonal freq division multiplexing
- data is spread across all carriers or channels
- based on idea that we can have ortho freqs
- dont need a guard band
what is special about OFDMA?
robust/immune to fading & multipath (time delay)
DSSS
- discrete spread spectrum signal
- low data rate signal spread over wide-band
- inverse to get og signal
negative about DSSS
- a lot of bandwidth to spread low data
advantages about DSSS
send signals at lower power
- doesn’t require exquisite timing alignment of TDMA
- not susceptible frequency effects of FDMA
- guard band
CDMA
- code division multiple access
- spread spectrum techniques like DSSS
- choose orthogonal codes to spread data
codes are orthogonal when ______
dot product is zero
how to maximize capacity?
maximize SNR or maximize bandwidth or BOTH
free space path loss eqn
Lp = (4piR/lambda)^2
FSK requires _______ _____ than BPSK to achieve the same BER
greater SNR
coherent reception will have ______ signal level than non-coherent
greater
- FSK coherent BER curve is lower than FSK non-coherent
DPSK is _____
non-coherent
DPSK performs a little _____ than coherent BPSK
worse
- DPSK curve will be a little higher than BPSK
QPSK sends _______ at the _____
two bits; same time
- each bit gets half the power
QPSK has ___ energy per bit as BPSK
same
QPSK and QAM are ______
the same thing
advantage of QPSK over BPSK?
QPSK allows you to double the data rate
maximizing receiver output SNR in AWGN
leads to minimizing detection error probability
error probability depends on
noise probability density function
what does BER stand for
bit error rate
mega
10^6
log(10)
1 dB
10log(10)
10 dB
10log(2)
3 dB
10log(4)
6 dB
10log(100)
20 dB
10log(1000)
30 dB
10log(10000)
40 dB
