Lecture 10 2007 Handouts

8/7/2019 Lecture 10 2007 Handouts

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Stanford University Department of Aeronautics and Astronautics

Communication -Communication - 22AA236AAA236A

24 October 200724 October 2007

James Cutler

http://ssdl.stanford.edu/

Copyright 2007, Duplication of material for commercial without written permission is

prohibited.


Announcements


Communication Subsystem






2m Band (140MHz)


SatCom Bands

Table 13-12


Other Interesting Bands

• We mentioned amateur bands.

– Rules governing amateur use• For example, no encryption on data

– It’s good to be a HAM (take the test, technician no code).

– Need to have an amateur use

• Industrial, Scientific, and Medical (ISM) bands – 900 MHz, 1.8 GHz, 2.4 GHz, 5.8 GHz

– Originally reserved internationally for non-commercial use of RF

– Recently shared with license-free error-tolerant communicationsapplications (ie wireless LANs)




Bandwidth (BW)

• In networking:

– Bandwidth is the data rate. – How fast can we move bits? 100

Mbps, 1Gbps.

• In radio communication:

– BW is the amount of spectrum usedto transmit data.

– What is the frequency range of your carrier signal?

• Fast signals have high bandwidth

• Slow signals have low bandwidth

http://en.wikipedia.org/wiki/Image:Bandwidth.svg


Spectrum Analyzer and Bandwidth

http://www.projects.ncassr.org/sdr/spectrum_analyzer/index.html


RF Channel Capacity

• Is there a limit to how much we can send?

– YES! – Claude Shannon, 1948

– Invention of information theory

http://en.wikipedia.org/wiki/Shannons_theory

–“A Mathematical Theory of Communication”, Bell System Journal,

1948




Shannon Capacity

• C – channel capacity (bps)

• B – bandwidth in HZ

• S – signal power over bandwidth

• N – noise power over bandwidth

)1(log2 N

S

BC W +=

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/iandm/part8/page1.html


Example 1

• Voice channel – telephony

– Bandwidth = 3.3KHz

– SNR is 100 ( in dB? )

Kbps97.21)1001(log3300 2 =+


Example 2

• DSL

– Bandwidth ~ 1MHz

– SNR is 20dB

Mbps6.6)1001(log10 2

6=+






AM – Amplitude Modulation


Digital Modulation Techniques

• OOK - On/Off Keying

• PSK - Phase Shift Keying

• FSK - Frequency Shift Keying• MSK - Minimum Shift Keying






QPSK Encoder Example


FM – Frequency Modulation


FSK – Frequency Shift Keying

1 0 1 0 1 0

f c1 f c2

BW = ~2R

http://www-personal.engin.umd.umich.edu/~richarpc/ch2_5_modems.ppt





How much data?


Common Parameters

• Bandwidth of the channel (Hz)

• Signal Power

• Noise Power

SNR RkT

G L LG PL

N

E

s

r a st l

o

b==

The Link Equation

E b" N

o= P + L

l +G

t + L

s+ L

a+G

r+ 228.6 "10logT

s"10log R

)1(log2 N

S BC W

+=


How good is our communication system?

• We can estimate and measure SNR.

– But this is analog

– What does this mean in the digital world?

• What is the bit error rate (BER)?

– How often do we see errors in our data?

– Combine SNR, noise model, modulation, and forward error correction to getBER.

• Typical BER

– Good link: 10-9 or 10-10

– Let’s say we have 10-5

• 1 error every 100000 bits

• At 9600bps, that’s 1 error every 10s

• At 1200bps, that’s 1 error every 83s.






AA236 Guidelines

• Traditional use of FSK (AFSK/GMSK)

– 1200 or 9600 bps

– HAM radio compatible

• Antenna tuning and matching work

• Power constraints

– 1W on satellite

• Link margin VERY VERY important for our missions

– Balloon mission

– 1U Cubesat, Katysat estimated on 3dB of margin

– Link budget page empty

• Much room (and need) for innovation – Better antennas

– Better radios/modems

Documents

Lecture 10 2007 Handouts