Lecture 10 2007 Handouts

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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.

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Announcements

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Communication Subsystem

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2m Band (140MHz)

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SatCom Bands

Table 13-12

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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)

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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

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Spectrum Analyzer and Bandwidth

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

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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

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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

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Example 1

• Voice channel – telephony

 – Bandwidth = 3.3KHz

 – SNR is 100 ( in dB? )

 Kbps97.21)1001(log3300 2 =+

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Example 2

• DSL

 – Bandwidth ~ 1MHz

 – SNR is 20dB

Mbps6.6)1001(log10 2

6=+

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Communication Subsystem

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AM – Amplitude Modulation

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Digital Modulation Techniques

• OOK - On/Off Keying

• PSK - Phase Shift Keying

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

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QPSK Encoder Example

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FM – Frequency Modulation

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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

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Communication Subsystem

How much data?

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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 

+=

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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.

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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