EECS 473 Advanced Embedded Systems
Lecture 14 Wireless in the real world Team status updates Team
Alert (Home Alert)
Team Fitness (Fitness watch) Team Glasses Team Mouse (Control in
hand) Team WiFi (WiFi localization) Guest talks One this Thursday
11/12 One on 11/24
Senior engineer who builds all kinds of things including power
supplies One on 11/24 National Instruments, board-level issues One
TBA, but more on software side. Recall part of homework score is
attendance at guest talks. If you have a conflict, let me know and
Ill find makeup something you can do Last time Covered: Messages
Medium Source encoding (compression)
Channel encoding (error correction) Modulation Medium A bit on the
FCC Today Review last time Wireless range FCC (again)
Antennas Broadcast and receive power FCC (again) Bandwidth and
Shannons limit A quick overview of packets and bandwidth Review:
Communication theory
What are each of these boxes? Source Encoder Channel Modulator
Decoder De- modulator Review: Channel encoding/decoding
What is a block code? What is a Hamming(7,4) code? How does this
figure relate? What is a convolutional code? What makes it
different than a block code? Channel encoding (error correction)
involves sending a lot of extra bits along with theuseful data
(maybe 2x or 3x total!). Why is this helpful when trying to send a
lot of data quickly? Review: Modulation Review: Modulation Draw the
message 0110 using the following constellations: So, who cares?
Noise immunity
Looking at signal-to-noise ratio needed to maintain a low bit error
rate. Notice BPSK and QPSK are least noise-sensitive. And as M goes
up, we get more noise sensitive. Easier to confuse symbols! I
believe this chart is wrong.Seewhich looks more like Id epect. On
to Antennas and transmission power
Antennas receive power differently depending on where the power is
coming from. An isotropic antenna is one that receives power
equally well from all directions. These dont exist. Real antennas
focus their effort more in some directions than others. A narrow
antenna, like a dish, will be focused in a very narrow range
(radiation angle) Others, like a traditional dipole (the most
common antenna) tend to have less narrow of a range. Antennas
Nothing is free here.
If you have a narrow beam, you get some great gain in that beam but
get loss in the other directions. This can be good. Think about
body-area networks or Bluetooth headphones Toroidal radiation
pattern Safety issue is the point here.Can send radiant power away
from yourself. Dish antenna radiation pattern Figures from
antenna-theory.com (if you couldnt tell) Radio power Radio signals
are generally measured in Watts
However embedded systems generally measure power in mW Typically mW
for WiFi It is often easiest to deal with power on a log scale. So
we use dBm where Basically just dB but scaled to mW. Much of this
(including graphics) from Aside: dB, dBm, dBi dB itself is a
unit-less value
Generally a ratio between two thing On a log scale. dBm a single
value where the ratio is to 1mW. So 20dB means a 100 to 1 ratio
20dBm means 100mW (100 times 1mW) Well also see dBi when looking at
antennas. Thats the power ratio of an antenna to an isotropic
antenna (that completely non-directional antenna) You might see
dBd, which is compared to a lossless dipole antenna.Its 2.15dB
lower than dBi. Vendors generally use dBi (cause its bigger) and
thus so will we. Power received vs. power sent.
The Friis Transmission Formulatells us how much power well
receive.It is: Where: Pt is the radiated power Pr is the received
power Gt is the gain of the transmitting antenna Gr is the gain of
the receiving antenna is the wavelength R is the distance between
antennas However, many of those terms arent easily available from
real spec. sheets. Instead we do some algebra and get the following
equation for range in km: Where f is the frequency in MHz, pt and
pr are in dBm and gt and gr are in dBi. Example You are running an
IEEE b network and you are currently using wireless devices with
the following specifications: Tx power: Mbps Rx sensitivity: 11
Mbps Antenna gain: 2 dBi (both) 802.11b is at 2.4GHz. Notes: We are
looking at 63mW of broadcast power. If we had dish antennas pointed
at each other with a gain of 25dBi, wed have ( )/20=275km! Note
that this assumes an unobstructed line-of-sight signal with no
significant interference. Sometimes realistic, often not. Looking
at a real antenna (ANT-WSB-ANF-09)
9dBi Gets there by radiating in a toroid Spread evenly along the
ground (half power bandwidth is 360) Doesnt go up or down at all.
Half power BW is at 10 Image taken from: en. wikipedia
Image taken from:
en.wikipedia.org/wiki/File:United_States_Frequency_Allocations_Chart_2003_-_The_Radio_Spectrum.jpg
United States Partial Frequency Spectrum
Image taken from:
en.wikipedia.org/wiki/File:United_States_Frequency_Allocations_Chart_2003_-_The_Radio_Spectrum.jpg
OK, so all 2.4 GHz things have on 50MHz of bandwidth
What does that mean? It limits how much data we can send. To really
understand that in a meaningful way, lets look at the theoretic
limitations. Shannons limit. Shannons limit First question about
the medium:
How fast can we hope to send data? Answered by Claude Shannon
(given some reasonable assumptions) Assuming we have only Gaussian
noise, provides a bound on the rate of information that can be
reliably moved over a channel. That includes error correction and
whatever other games you care to play. Taken from a slide by Dr.
Stark ShannonHartley theorem
Well use a different version of this called the Shannon-Hartley
theorem. C is the channel capacity in bits per second; B is the
bandwidth of the channel in hertz S is the total received signal
power measured in Watts or Volts2 N is the total noise, measured in
Watts or Volts2 Adapted from Wikipedia. Comments (1/2) This is a
limit.It says that you can, in theory, communicate that much data
with an arbitrarily tight bound on error. Not thatyou wont get
errors at that data rate.Rather that its possible you can find an
error correction scheme that can fix things up. Such schemes may
require really really long block sizes and so may be
computationally intractable. There are a number of proofs. IEEE
reprinted the original paper in 1998 More than we are going to do.
Lets just be sure we can A) understand it and B) use it. Comments
(2/2) What are the assumptions made in the proof?
All noise is Gaussian in distribution. This not only makes the math
easier, it means that because the addition of Gaussians is a
Gaussian, all noise sources can be modeled as a single source. Also
note, this includes our inability to distinguish different
voltages. Effectively quantization noise and also treated as a
Gaussian (though it aint) Can people actually do this? They can get
really close. Turbo codes, Low density parity check codes. Examples
(1/2) C is the channel capacity in bits per second; B is the
bandwidth of the channel in Hertz S is the total received signal
power measured in Watts or Volts2 N is the total noise, measured in
Watts or Volts2 If the SNR is 20 dB, and the bandwidth available is
4 kHz what is the channel capacity? Part 1: convert dB to a ratio
(its power so its base 10) Part 2: Plug and chug. 20dB=
*log2(101)=26.63kbits/sec Adapted from Wikipedia. Examples (2/2) C
is the channel capacity in bits per second; B is the bandwidth of
the channel in Hertz S is the total received signal power measured
in Watts or Volts2 N is the total noise, measured in Watts or
Volts2 If you wish to transmit at 50,000 bits/s, and a bandwidth of
1 MHz is available, what S/R ration can you accept?
50k=1MHz*log2(1+N).N= Or -14.5dB.More noise than power by about a
factor of 30! Adapted from Wikipedia. Summary of Shannons
limit
Provides an upper-bound on information over a channel Makes
assumptions about the nature of the noise. To approach this bound,
need to use channel encoding and modulation. Some schemes (Turbo
codes, Low density parity check codes) can get very close. IEEE is
a standard which specifies the physical layer and media access
control for low-rate wireless personal area networks (LR-WPANs).
Many embedded wireless protocols are built on top of this
(including Zigbee) From: An Introduction to IEEE STD 802.15.4 by
Jon T. Adams
The Synchronization header (SHR) contains a preamble sequence (32
bits, or 4 octets) to allow the receiver to acquire and synchronize
to the incoming signal and a start of frame delimiter that signals
the end of the preamble. The PHY header (PHR) carries the frame
length byte, which indicates the length of the PHY Service Data
Unit (PSDU). The SHR, PHR and PSDU make up the PHY Protocol Data
Unit (PPDU). The PSDU contains the MAC Header (MHR), which has two
frame control octets, a single octet Data Sequence Number, good for
reassembling packets received out of sequence, and 4 to 20 octets
of address data. The MAC Service Data Unit (MSDU) carries the
frames payload and has a maximum capacity of 104 octets of data.
Finally, the MPDU ends with the MAC Footer (MFR), which contains a
16-bit Frame Check Sequence. From: An Introduction to IEEE STD by
Jon T. Adams Putting it all together Acknowledgments and
sources
A 9 hour talk by David Tse has been extremely useful and is a basis
for me actually understanding anything (though Im by no means
through it all) A talk given by Mike Denko, Alex Motalleb, and Tony
Qian two years ago for this class proved useful and I took a number
of slides from their talk. An hour long talk with Prabal Dutta
formed the basis for the coverage of this talk. Some other sources:
-- A nice set of questions that get at some useful calculations.
all the path loss/propagation models in one place very nice
modulation overview. A very nice overview of everything wireless
for the applied engineer.Wish Id found it sooner! Im grateful for
the above sources.All mistakes are my own. Additional
sources/references
General Modulation
https://fetweb.ju.edu.jo/staff/ee/mhawa/421/Digital%20Modulation.pdf
https://www.nhk.or.jp/strl/publica/bt/en/le0014.pdf (ASK) Other: An
Introduction to IEEE STD by Jon T. Adams
