Wireless Networking

Class organization Class web page 9/12/2018 Class organization Class web page http://www.cs.fsu.edu/~zzhang/CIS5930_Spring_2009.htm Academic honor code Programs you submitted must be your own work While discussions of class materials and assignments are allowed, copying of solutions is strictly prohibited 9/12/2018 CDA3100 CDA3100

9/12/2018 Class Communication This class will use class web site to post news, changes, and updates. So please check the class website regularly Please also make sure that you check your emails on the account on your University record 9/12/2018 CDA3100 CDA3100

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.

How this course is designed Actually, both of these typical approaches are limited. Wireless medium and techniques are very different from wired medium and techniques. In wired medium, like Ethernet, what you send is likely what will be received. Limited noise, limited interference, large bandwidth. In wireless medium, what you send may be very different from what will be received. Substantial noise, substantial interference, limited bandwidth. Well, this is why it is so interesting!

How this course is designed Only focusing on physical layer won’t be sufficient for computer networks where traffic is random. Simply treating it as a black box will lead to suboptimal solutions. What we need is a cross-layer approach. This will require you to understand everything – from physical layer to network layer at least. This is why this course will take a non-traditional approach and will cover physical layer, MAC layer, and network layer, all in details.

How this course is designed This is a very challenging task (after all, this is a graduate level course! ) Can we achieve this? To computer science majors, the MAC layer and the network layer are more familiar. The challenge is the physical layer. We will have to spend significant amount of time on the physical layer. I will teach the physical layer in a non-conventional way. Books about the physical layer are usually written by the signal processing people, and may be alien to the computer science majors. Our goal will be to understand the physical layer. We don’t have to do things such as deriving channel capacity. We will also learn to implement the physical layer with GNU Software Defined Radio.

Materials that will be used in the class ``Fundamentals of Wireless Communication,’’ by David Tse and Pramod Viswanath, downloadable at http://www.eecs.berkeley.edu/~dtse/book.html GNU Software Defined Radio tutorial, by Dawei Shen, downloadable at http://www.nd.edu/~jnl/sdr/docs/tutorials/   Other useful resources (not required): ``Computer Networks,'' by Andrew S. Tanenbaum, Prentice Hall, 4th edition, 2003 ``Principles of Wireless Networks: A Unified Approach,’’ by Kaveh Pahlavan and Prashant Krishnamurthy,  Prentice Hall, 1st edition, 2002.

Projects Physical layer projects will be implemented by GNU SDR (C++ and Python). Upper layer projects will be implemented by C/C++. Projects will be in teams with maximum 3 persons. To work with GNU SDR, at least one of your team members should have access to a Linux machine as root.

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.

Bandwidth Bandwidth in wireless medium is limited. Check http://www.ntia.doc.gov/osmhome/allochrt.pdf 802.11g network, each channel has 22MHz bandwidth. Channel 1 is centered at 2.412GHz. The 2.412GHz is the f in the cos(2 \pi ft). What is bandwidth?

Bandwidth In very simple terms, it is how fast your signal can change. If you have an unlimited bandwidth, your signal can change infinitely fast. The frequency spectrum is shared, so you can use only a part of it. Means that your signal cannot change infinitely fast. I(t) changes infinitely fast at the transition points from -1 to +1 or from +1 to -1.

Baseband signals Roughly speaking, a signal can be represented as the summation of a series of sine waves (Fourier Transformation ). So you have to pass the bit stream to a Low Pass Filter to filter out the high frequency components. The filtered signal is called the *baseband signal*.

Low Pass Filter The simplest LPF is a RC circuit. In our projects, we will use RRC filter, or, the Root Raised Cosine filter. We will discuss it in details. So suppose you feed the bit stream to the RRC filter and gets the *baseband* signal, denoted as I(t).

The Transmitted Signal So what you actually send is I(t)cos(2\pi ft), where I(t) is band-limited 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 (f-B)Hz will be (or should be) cut-off.

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 high-frequency components (AI(t)cos(4 \pi ft)/2), and what is left will be proportional to I(t).

A simplified wireless communication scheme

Complex representations You have to get used to representing the received signal as complex numbers. That is, r(t) =Re(t) + jIm(t). Why? Because if you will multiply the r(t) with both cos(2\pi ft) and sin(2\pi ft), and both will be sent to a LPF. The one corresponding to cos(2\pi ft) is regarded as the real part and the one corresponding to sin(2\pi ft) is regarded as the imaginary part. You will see why this is convenient later.

Here is an issue How to recover the original bits? I(t) is no longer the simple, clean square waveform. Solution: sample I(t) at time instants and if the samples are taken correctly, you can get the correct bits. We will talk about this in details.

Here is another issue The oscillators are not perfect! The sender and receiver use local oscillators to generate cos(2\pi ft). There will be a slight difference between the sender and the receiver frequency. So, the receiver has to track the frequency difference, as well as the phase difference. Will be discussed later.

More issues Multi-path. In wireless communications, signals travel multiple paths to reach the destination. If you send I(t), the receiver will receive \sum a_i I(t-\tau_i). Solution 1. Ignore it. Valid if the symbol rate is low. 2. Use equalization. 3. OFDM. Will be discussed in details.

GNU Software Defined Radio http://www.gnu.org/software/gnuradio/ A tool ideal for computer science majors to practice with wireless communications. You write signal processing blocks in C++, and connect the signal processing blocks with Python. In Project 1, You will be asked to write signal processing blocks.

The code for a simple BPSK transmitter and receiver http://www.cs.fsu.edu/~zzhang/CIS5930_Spring_2009_files/OSMR_chest_snd.py