Spread Spectrum and CC2420
Gang Zhou
<[email protected]>
Computer Science Department
University of Virginia
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Group Presentation
Gang Zhou
Outline
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Spread Spectrum Theory
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CC2420 Radio
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Definition of Spread Spectrum, DSSS and FHSS
Basic Principle of DSSS
Performance in the Presence of Interference
Main features
IEEE 802.15.4 Frame Format
Modulation and Spreading Process
Tinyos and MicaZ
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
TinyOS Module Wrapping
MicaZ Hardware
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Group Presentation
Gang Zhou
Definition of Spread Spectrum
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SS is a transmission technique in which a pseudo-noise
code, independent of the information data, is employed
as a modulation waveform to “spread” the signal energy
over a bandwidth much greater than the signal
information bandwidth. At the receiver the signal is
“despread” using a synchronized replica of the pseudonoise code.
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A transmission technique
Data independent PN code
Spread the energy in transmitter side
Despread the energy in receiver side
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Group Presentation
Gang Zhou
DSSS and FHSS
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DSSS
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FHSS
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A pheudo-noise sequence pnt generated at the modulator,
is used in conjunction with an M-ary PSK modulation to
shift the phase of the PSK signal pseudorandomly.
A pseudo-noise sequence pnt generated at the modulator
is used in conjunction with an M-ary FSK modulator to shift
the carrier frequency of the FSK signal pseudorandomly.
We focus on DSSS since MICAZ uses DSSS.
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Group Presentation
Gang Zhou
Basic Principle of DSSS

Despreading: the received baseband signal rxb is multiplied
Input:
Spreading:
with
the PN
pnr: rate
 Multiply
Binary
data
dtsequence
with
dt with
PN sequence
symbol
pntRs = 1/Ts
 The
If pneffect
Pseudo-noise
oft, multiplication
code
the recovered
pnt with ischip
to
data
spread
rate
is Rproduced
= 1/T
baseband
oninteger
drbandwidth
. To of Rs)
c the
c (an
r = pn
R
multiply
to bandwidth
despread.of Rc.
s of dt toisthe
 If pnr ≠ pnt, no despreading action.
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Group Presentation
Gang Zhou
Performance in the presence of interference
Received signal: rxb = txb + i = dt . pnt + i
dr = rxb . pnt = dt . pnt . pnt + i . pnt
pnt . pnt = +1 for all t
dr = dr + i . pnt
Hence: The data signal is reproduced
The interference signal is spreaded, that is, energy spreaded. 6
Group Presentation
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The essence behind the interference rejection capability
of a spread spectrum system is that:
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The useful signal (data) gets multiplied twice by the PN
sequence
But the interference signal gets multiplied only once
The spread spectrum signal has a lower power density
than the directly transmitted signal.
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
Gang Zhou
Compared with the direct signal, the spread spectrum
signal looks like noise signal, weak and energy spreaded.
The spread spectrum signal is resistant to:
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
Interference
Multipath fading
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Group Presentation
Gang Zhou
Outline

Spread Spectrum Theory




CC2420 Radio




Definition of Spread Spectrum, DSSS and FHSS
Basic Principle of DSSS
Performance in the Presence of Interference
Main features
IEEE 802.15.4 Frame Format
Modulation and Spreading Process
Tinyos and MicaZ


TinyOS Module Wrapping
MicaZ Hardware
8
Group Presentation
Gang Zhou
Main features
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DSSS baseband modem with 2 Mchips/s and 250kbps
effective data rate
Q-QPSK with half sine pulse shaping modulation
128 (RX) + 128 (TX) byte data buffering
Low current consumption (RX: 19.7 mA, TX: 17.4 mA)
Low supply voltage (2.1 – 3.6 V) with on-chip voltage
regulator
Programmable output power
16 available frequency channels (IEEE 802.15.4 standard)
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Fc = 2450 + 5 (k-11) MHz, k = 11, 12, …, 26
Hardware MAC encryption
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Group Presentation
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Gang Zhou
High sensitivity (-94 dBm)
High adjacent channel rejection (39 dB)
High alternate channel rejection (46 dB)
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Group Presentation
Gang Zhou
IEEE 802.15.4 Frame Format
Synchronization Header:
Preamble
sequence is 4 bytes of 0x00 (length configurable)
The Start of Frame Delimiter is set to 0xA7 (content configurable)
A synchronization header is always transmitted
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Group Presentation
Gang Zhou
IEEE 802.15.4 Frame Format
Length Field:
Means
the number of bytes in the MPDU
The most significant bit is reserved, so the maximum value is 127
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Group Presentation
Gang Zhou
IEEE 802.15.4 Frame Format
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Group Presentation
Gang Zhou
IEEE 802.15.4 Frame Format
Frame Check Sequence:
FCS
is calculated over MPDU
FCS is automatically generated and verified by hardware, if enabled.
The FCS polynomial is: x16 + x12 + x5 + 1
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Group Presentation
Gang Zhou
IEEE 802.15.4 Frame Format
Acknowledge Frame:
If
AUTOACK is enabled, an acknowledge frame is transmitted for all
incoming frames accepted by the address recognition with the
acknowledge request flag set and a valid CRC.
The acknowledge frame is transmitted 12 symbol (hardware switch
time) periods after the last symbol of the incoming frame.
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Group Presentation
Gang Zhou
Modulation and Spreading Process
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The general modulation process (3 steps)
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Step 1: Each byte is divided into two symbols, 4 bits each. The
least significant symbol is transmitted first.
Step 2: Each symbol is mapped to one out of 16 pseudo-random
sequences, 32 chips each.
Step 3: The chip sequence is then transmitted at 2 MChips/s,
with the least significant chip transmitted first for each symbol.
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Group Presentation
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Gang Zhou
Step 2: Each symbol is mapped to one out of 16 pseudo-random
sequences, 32 chips each.
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Group Presentation
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Gang Zhou
Step 3: The chip sequence is then transmitted at 2 MChips/s, with
the least significant chip transmitted first for each symbol.
 The chip sequences are modulated onto the carrier using Q-QPSK
with half-sine pulse shaping.
 Even-indexed chips are modulated onto the in-phase (I) carrier.
 Odd-indexed chips are modulated onto the quadrature-phase (Q)
carrier.
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Group Presentation
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Gang Zhou
A question: Does CC2420 uses strict DSSS?
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I guess the answer is no! (I may be wrong.)
My reason:
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In CC2420, chip sequences are used to represent the data,
while in DSSS, PN codes are used to multiply the data, rather
than to replace the data.
Then another question:
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How to despread the data out from the received signal?
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Group Presentation
Gang Zhou
Outline

Spread Spectrum Theory




CC2420 Radio




Definition of Spread Spectrum, DSSS and FHSS
Basic Principle of DSSS
Performance in the Presence of Interference
Main features
IEEE 802.15.4 Frame Format
Modulation and Spreading Process
Tinyos and MicaZ


TinyOS Module Wrapping
MicaZ Hardware
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Group Presentation
Gang Zhou
TinyOS Module Wrapping
Generic Comm
AM
Control
(Freq,Power,etc )
Backoff
Encoding
Data
CC2420RadioC
CC2420Control
CC2420RadioM
Hardware Specific
Read/Write CC2420
Registers/Commands
HPLCC2420M
HPLCC2420C
RandomLFSR
High Speed Timer
Transfer to/from
TXFIFO/RXFIFO
SpiByte
MicaZ (AVR)
Telos (TI MSP430)
HPLCC2420C
TimerC
HPLCC2420M
TimerM
HPLUSART0M
MSP430TimerM
HPLCC2420C
HPLCC2420M
HPLCC2420FIFOM
HPLUARTM
TimerC
TimerM
HPLClock
MSP430ClockM
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Group Presentation
Gang Zhou
MicaZ Hardware
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IEEE 802.15.4/ZigBee compliant RF transceiver
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2.4 to 2.4835 GHz, a globally compatible ISM band
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Direct sequence spread spectrum radio which is resistant to RF
interference and provides inherent data security
250 kbps data rate
Runs TinyOS 1.1.7 and higher, including Crossbow’s reliable mesh
networking stack software modules
Plug and play with all of Crossbow’s sensor boards, data acquisition
boards, gateways, and software
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Group Presentation
Gang Zhou
References
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MPR/ MIB User’s Manual, www.xbow.com
IEEE 802.15.4 Standard, http://www.ieee802.org/15/pub/TG4.html
Ir. J. Meel, Spread Spectrum (SS), www.denayer.be
Joe Polastre, IEEE 802.15.4: Platforms, Progress, and TinyOS,
http://webs.cs.berkeley.edu/retreat-6-04/joep-nest-2004springretreat-802154.ppt
Thanks! Any question?
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