RE: Wireless Networks ppt

Wireless Networking
Wireless Networking.ppt (Size: 224 KB / Downloads: 40)
Class Communication
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Wireless Networking
Wireless networks are everywhere – cellular phone networks, wireless LANs, Bluetooth
This class is intended to cover a very wide spectrum of topics related to wireless networking, including the physical layer, the MAC layer, and the network layer.
After taking this class, you should be able to
Understand basic wireless communication theory (BPSK, CDMA, OFDM, RS code, etc)
Learn to implement wireless communication transmitters/receivers with GNU Software Defined Radio
Understand the design of wireless networks (802.11 network, cellular phone network, wireless sensor network, etc)
How this course is designed
This class is designed for CS majors who are interested in wireless networks.
There are two groups of people studying wireless networks.
The signal processing approach. Typically focusing on signal processing and deriving the channel capacity. Focusing on physical layer and cellular phone networks.
The computer science approach. Typically treat the physical layer as a black box and focusing on MAC layer and network layer. Wireless LANs, wireless sensor networks.
Physical Layer
Physical layer design goal: send out bits as fast as possible with acceptable low error ratio
Some simple schemes:
There is a wire between A and B. If A wants to send a bit `1’, he connects the wire to the positive end of a battery. Otherwise he disconnects it from the battery.
Or A can hold a radio, if `1’, he sends at frequency f1 and if `0’ he sends at frequency f2.
Or there is an optical fiber between A and B and if `1’ A lit up a light and if `0’ A does nothing.
Wireless communications
The fundamental fact is that if the sender sends a sine wave, the receiver will receive a sine wave at the same frequency. But with
A different phase
A new amplitude
How do you design communication schemes based on that?
BPSK
The simplest transmission scheme is BPSK, which is also widely used.
Convert your information bits to a {1,+1} square waveform. Let it be I(t). Multiply I(t) with cos(2 \pi ft), and send out.
This is the basic idea. But to make it work, more work has to be done.
The Transmitted Signal
So what you actually send is I(t)cos(2\pi ft), where I(t) is bandlimited to BHz.
In 802.11g network, each channel has 22MHz bandwidth. What should B be?
Assume you are given a bandwidth 2BHz centered at fHz. It means that all components higher than (f+B)Hz and all frequency lower than (fB)Hz will be (or should be) cutoff.
Receiver
The receiver receives r(t) = AI(t) cos(2 \pi ft + \phi). Here, just for now, assume the receiver somehow magically finds the value of \phi and set it to be 0 (we will talk about this shortly). So he multiplies r(t) with cos(2 \pi ft), and gets AI(t)/2 + AI(t)cos(4 \pi ft)/2.
You apply the LPF again to get rid of the highfrequency components (AI(t)cos(4 \pi ft)/2), and what is left will be proportional to I(t). 

