Direct Sequence Spread Spectrum
Physical Layer Specification
IEEE 802.11
Prepared by Jan Boer, Chair DS PHY
Lucent Technologies WCND Utrecht
What is DSSS?
•	Signal symbol is spreaded with a sequence

•	Wider Bandwidth
•	Less power density
  
11 chip BARKER sequence
•	Good autocorrelation properties
•	Minimal sequence allowed by FCC
•	Coding gain 10.4 dB 
DSSS benefits
•	10 dB coding gain:
–	Robust against interferers and noise (10 dB suppression)
•	Robust against time delay spread
–	Resolution of echoes
IEEE 802.11 DSSS PHY characteristics
•	2.4 GHz ISM band (FCC 15.247)
•	1 and 2 Mb/s datarate (DBPSK and DQPSK modulation)
•	Symbolrate 1MHz
•	Chipping rate 11 MHz with 11  chip Barker sequence
•	Multiple channels in 2.4 to 2.4835 GHz band
PLCP Frame Format
 
PLCP synchronization
•	128 scrambled 1 bits
•	needed for o.a.
–	gain setting
–	energy detection
–	antenna selection
–	frequency offset compensation
Start Frame Delimiter
•	16 bit field (hF3A0)
•	used for
–	bit synchronization
Signal Field
•	Rate indication
–	h0A 1Mb/s DBPSK
–	h14  2Mb/s DQPSK
•	Other values reserved for future use (100 kb/s quantities)
Service Field
•	Reserved for future use
•	h00 signifies 802.11 compliant
Length Field
•	Indicates number of octets to be transmitted in MPDU
•	Used for
–	End of frame detection
–	MPDU CRC sync
CRC field
•	CCITT CRC-16
•	Protects Signal, Service and Length Field
Data Scrambler
•	ALL bits transmitted by the DSSS Phy are scrambled
•	Purpose
–	Whithening the spectrum
–	DC blocking (Barker sequnce is asymetric)
DBPSK Modulation
DQPSK Modulation
Transmit Spectrum Mask
DSSS Channels
Clear Channel Assessment
•	Three methods:
–	CCA mode 1: Energy above threshold
–	CCA mode 2: Carrier sense only
–	CCA mode 3: Carrier sense with energy above threshold
•	Energy detection function of TX power
–	Tx power > 100 mW: -80 dBm
–	Tx power > 50mW  :  -76 dBm
–	Tx power < = 50mW: -70 dBm
•	Energy detect time : 	15 ms
•	Correct PLCP header --> CCA busy for full (intended) duration of of frame as indicated by PLCP Length field 
DSSS Specification Summary
•	Slottime				20 ms 
•	TX to Rx turnaround time		10 ms
•	Rx to Tx turnaround time		5   ms  
•	Operating temperature range	
•	type 1:     0  -  40 °C
•	type 2:  -30  -  70 °C
•	Tx Power Levels
•	1000 mW	USA
•	100 mW	Europe
•	10 mW/MHz	Japan
•	Minimum Transmitted Power	1 mW
•	Tx power level control		required above 100 mW
DSSS Specification Summary (cont)
•	Tx Center Frequency Tolerance		+/- 25 ppm
•	Chip Clock Frequency Tolerance		+/- 25 ppm
•	Tx Power On Ramp			2 ms 
•	Tx Power Down Ramp			2 ms
•	RF Carrier suppression			15 dB
•	Transmit modulation accuracy		test procedure
•	Rx sensitivity				-80 dB
                                           	@ 0.08FER (1024 Bytes)
•	Rx max input level				-4 dB
•	Rx adjacent channel rejection		>35 dB
                                	@ > 30 MHz separation 
				between	channels
PLCP Transmit Procedure
PLCP Receive Procedure