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Spread Spectrum and CC2420

Spread Spectrum and CC2420. Outline. Spread Spectrum Theory Definition of Spread Spectrum, DSSS and FHSS Basic Principle of DSSS Performance in the Presence of Interference CC2420 Radio Main features IEEE 802.15.4 Frame Format Modulation and Spreading Process Tinyos and MicaZ

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Spread Spectrum and CC2420

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  1. Spread Spectrum and CC2420

  2. Outline • Spread Spectrum Theory • Definition of Spread Spectrum, DSSS and FHSS • Basic Principle of DSSS • Performance in the Presence of Interference • CC2420 Radio • Main features • IEEE 802.15.4 Frame Format • Modulation and Spreading Process • Tinyos and MicaZ • TinyOS Module Wrapping • MicaZ Hardware

  3. Definition of Spread Spectrum • 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 pseudo-noise code. • A transmission technique • Data independent PN code • Spread the energy in transmitter side • Despread the energy in receiver side

  4. DSSS and FHSS • DSSS • 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. • FHSS • 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.

  5. Basic Principle of DSSS • Input: • Binary data dt with symbol rate Rs = 1/Ts • Pseudo-noise code pnt with chip rate Rc = 1/Tc (an integer of Rs) • Spreading: • Multiply dt with PN sequence pnt • The effect of multiplication is to spread the baseband bandwidth Rs of dt to the bandwidth of Rc. • Despreading: the received baseband signal rxb is multiplied with the PN sequence pnr: • If pnr = pnt, the recovered data is produced on dr. To multiply is to despread. • If pnr ≠ pnt, no despreading action.

  6. 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.

  7. The essence behind the interference rejection capability of a spread spectrum system is that: • 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. • Compared with the direct signal, the spread spectrum signal looks like noise signal, weak and energy spreaded. • The spread spectrum signal is resistant to: • Interference • Multipath fading

  8. Outline • Spread Spectrum Theory • Definition of Spread Spectrum, DSSS and FHSS • Basic Principle of DSSS • Performance in the Presence of Interference • CC2420 Radio • Main features • IEEE 802.15.4 Frame Format • Modulation and Spreading Process • Tinyos and MicaZ • TinyOS Module Wrapping • MicaZ Hardware

  9. Main features • 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) • Fc = 2450 + 5 (k-11) MHz, k = 11, 12, …, 26 • Hardware MAC encryption

  10. High sensitivity (-94 dBm) • High adjacent channel rejection (39 dB) • High alternate channel rejection (46 dB)

  11. 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

  12. 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

  13. IEEE 802.15.4 Frame Format

  14. 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

  15. 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.

  16. Modulation and Spreading Process • The general modulation process (3 steps) • 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.

  17. Step 2: Each symbol is mapped to one out of 16 pseudo-random sequences, 32 chips each.

  18. 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.

  19. A question: Does CC2420 uses strict DSSS? • I guess the answer is no! (I may be wrong.) • My reason: • 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: • How to despread the data out from the received signal?

  20. Outline • Spread Spectrum Theory • Definition of Spread Spectrum, DSSS and FHSS • Basic Principle of DSSS • Performance in the Presence of Interference • CC2420 Radio • Main features • IEEE 802.15.4 Frame Format • Modulation and Spreading Process • Tinyos and MicaZ • TinyOS Module Wrapping • MicaZ Hardware

  21. Generic Generic Comm Comm AM AM Backoff Backoff Encoding Encoding Data Data Control Control CC2420RadioC CC2420RadioC ( ( Freq,Power,etc Freq,Power,etc ) ) CC2420Control CC2420Control CC2420RadioM CC2420RadioM Hardware Specific Hardware Specific Read/Write CC2420 Read/Write CC2420 Registers/Commands Registers/Commands HPLCC2420M HPLCC2420C RandomLFSR RandomLFSR High Speed Timer High Speed Timer Transfer to/from Transfer to/from TXFIFO/RXFIFO TXFIFO/RXFIFO SpiByte SpiByte MicaZ (AVR) Telos (TI MSP430) TinyOS Module Wrapping

  22. MicaZ Hardware • IEEE 802.15.4/ZigBee compliant RF transceiver • 2.4 to 2.4835 GHz, a globally compatible ISM band • 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

  23. References • 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-2004-springretreat-802154.ppt Thanks! Any question?

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