Spread Spectrum and CC2420

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?