EC312 Homework 21
Name: ________________________
Read: (1) Lecture 21 Notes
(2) Frenzel, Principles of Electronic Communications Systems, 3rd Ed, Ch. 11-5 (p.414-420)
1. a) If I am transmitting a Frequency Hopping Spread Spectrum (FHSS) signal and the range of the
FSSS frequency band is 40 kHz to 60 KHz, and if it is evenly divided into 20 sub-bands how
much of an impact would an enemy have that is jamming 52 kHz? Would I still be able to
reasonably communicate?
Communication will be impacted a bit, but you would receive 95% of your comms.
b) What if this enemy was eavesdropping on 52 kHz instead, would she obtain enough of my
communication in a coherent way that I should be concerned?
No, she would only be receiving ~5% of the information and in a scrambled order.
2. a) If the bit rate of an FHSS system is 20 kbps, and the dwell time is 5 s on a frequency range
of 25 MHz to 55 MHz, how many frequency shifts occur during the transmission of each bit?
Rb = 20kbps;
Tb= 1/ 20X10^3= 50x10^-6
50us/bit x ( frequency shifts/5us) = 10 shifts/bit
b) What type of modulation scheme is used for FHSS.
FSK
3. a) What type of modulation scheme is used for Direct Sequence Spread Spectrum (DSSS)?
BPSK
b) Compare chip time and bit time and describe how they relate to one another. How does this relationship affect
the Bandwidth utilized?
The chip time
must be smaller than the bit time
The bandwidth is dependent on the rate of the data, or
in DSSS.
,. So the bandwidth of the spread spectrum signal is much
larger than the bandwidth of the original signal.
The ratio
is called the spreading factor or processing gain, and determines to a certain extent the upper limit of the
total number of users supported simultaneously by a base station. In our book they define G= BW/fb- which is the same
formula and the higher the gain, the greater the ability to fight interference.
4. a) The following pseudorandomnoise bits are generated to mix with a binary bit in DSSS:
10100111
The receiver obtained the following bit sequence:
01111000
What bit was being transmitted during this interval?
10100111
+ 01111000
Binary 1
11011111
b) What may have happened during the transmission of this signal to generate some uncertainty? Why are you
still confident in the value you came up with?
One of the frequencies may have been jammed causing a bit error. Still 7 out of 8 were ones, so I am confident
that binary 1 was the intended value.